TRIBOLOGY OF MINIATURE SYSTEMS
TRIBOLOGY SERIES Advisory Board W.J. Bartz (Germany, F.R.G.) R. Bassani (Italy) B. Bri...
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TRIBOLOGY OF MINIATURE SYSTEMS
TRIBOLOGY SERIES Advisory Board W.J. Bartz (Germany, F.R.G.) R. Bassani (Italy) B. Briscoe (Gt. Britain) H. Czichos (Germany, F.R.G.) D. Dowson (Gt. Britain) K. Friedrich(Germany, F.R.G.) N. Gane (Australia)
VOl. 1
Vol. VOl. VOl. VOl. Vol. Vol. VOI.
2 3 4
5 6
7
a
Vot. 9 Vol. 10 VOl. 11 Vol. 12 Vol. 13
W.A. Glaeser (U.S.A.) M. Godet (France) H.E. Hintermann(Switzerland) K.C. Ludema (U.S.A.) G.W. Rowe (Gt. Britain) T. Sakurai (Japan) W.O. Winer (U.S.A.)
Tribology -A Systems Approach to the Science and Technology of Friction, Lubricationand Wear (Czichos) ImpactWear of Materials (Engel) Tribology of Naturaland ArtificialJoints (Dumbleton) Tribology of Thin Layers (Iliuc) Surface Effectsin Adhesion, Friction,Wear, and Lubrication(Buckley) Frictionand Wear of Polymers (Bartenevand Lavrentev) MicroscopicAspects of Adhesion and Lubrication(Georges, Editor) IndustrialTribology -The PracticalAspects of Friction, Lubrication and Wear (Jones and Scott, Editors) Mechanicsand Chemistry in Lubrication (Dorinsonand Ludema) Microstructureand Wear of Materials (Zum Gahr) Fluid Film Lubrication- Osborne Reynolds Centenary (Dowson et al., Editors) InterfaceDynamics(Dowson et al., Editors) Tribology of Miniature Systems (Rymuza)
TRIBOLOGY SERIES, 13
TRIBOLOGY OF MINIATURE SYSTEMS Zyg munt Rymuza Institute of Design of Precise and Optical Instruments, Warsaw University of Technology, Warsaw,Poland
ELSEVIER Amsterdam- Oxford- New York -Tokyo
1989
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat25 P.O. Box 211,lOOOAE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655 Avenue of the Americas New York, NY 10010
ISBN 0-444-87401-7 WOI. 13) ISBN 0-444-41677-3 (Series) Elsevier Science Publishers B.V., 1989 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./Physical Sciences & Engineering Division, P.O. Box 1991, 1000 BZ Amsterdam, The Netherlands. Special regulations for readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified.
No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the materials herein. Printed in The Netherlands
The t r i b o l o g y of m i n i a t u r e s y s t e m s i s q u i t e d i f f e r e n t from t h e t r i b o l o n o f l a r g e machinery. T h i s book i s i n t e n d e d t o c o v e r b o t h t h e b a s i c c o n c e p t s of t h e t r i b o l o g y o f m i n i a t u r e s y s t e m s and some areas of i t s p r a c t i c a l a p p l i c a t i o n . The a u t h o r ' s a i m i s t o q i v e a compact s u r v e y o f t h e s p e c i f i c problems e n c o u n t e r e d i n t h i s d i s c i p l i n e and p r e s e n t a volume which w i l l a l s o b e u s e f u l
in
s o l v i n g p r o f e s s i o n a l e n g i n e e r i n g problems i n t h e f a s t - g r o w i n q f i e l d of p r e c i s i o n e n g i n e e r i n g and m i c r o t e c h n o l o g y ( m e c h a t r o n i c s )
.
The s u i t a b i l i t y of v a r i o u s m a t e r i a l s and l u b r i c a n t s f o r t h e t r i b o l o g i c a l s y s t e m s i n m i n i a t u r e mechanisms i s d i s c u s s e d . The t r i b o l o g i c a l p r o p e r t i e s and t h e f r i c t i o n and wear p r o p e r t i e s which o c c u r i n s u c h s y s t e m s a r e a n a l y s e d . S p e c i f i c l u b r i c a t i o n problems
are d i s c u s s e d i n d e t a i l ; i n p a r t i c u l a r , t h e u s e of s p e c i a l t r i b o l o g i c a l c o a t i n g s t o s o l v e many d i f f i c u l t l u b r i c a t i o n problems and t o o b t a i n h i g h w e a r r e s i s t a n c e of t h e r u b b i n g e l e m e n t s i s considere d . The s p e c i a l i n v e s t i g a t i o n t e c h n i q u e s u s e d t o c h a r a c t e r i z e m i n i a t u r e t r i b o l o g i c a l s y s t e m s and t h e i r e l e m e n t s ( e . q . l u b r i c a n t s ) a r e p r e s e n t e d . The t r i b o l o q i c a l a s p e c t s of many of t h e most common a s s e m b l i e s found i n m i n i a t u r e mechanism and e l e c t r o m e c h a n i s m design a r e a n a l y s e d and some p r a c t i c a l s u g g e s t i o n s are p u t f o r w a r d f o r t h e r a t i o n a l d e s i g n of such s y s t e m s . A l s o s p e c i a l t r i b o l o g i c a l problems such a s t h o s e m e t i n computer t e c h n o l o q y , b i o e n g i n e e r i n g
etc., a r e discussed. The book i s i n t e n d e d f o r t r i b o l o g i s t s ( b o t h s e a s o n e d r e s e x c h ers and newcomers) s t u d y i n q t h e problems of t h i s s p e c i f i c b r a n c h o f t r i b o l o g y and also f o r p r a c t i c i n q e n q i n e e r s a c t i v e i n t h e d e s i g n , m a n u f a c t u r e and e x p l o i t a t i o n of v a r i o u s m i n i a t u r e s y s t e m s . T h i s volume s h o u l d a l s o be u s e f u l i n u n i v e r s i t i e s , b o t h € o r l e c t u r e r s and s t u d e n t s . The a u t h o r would b e cjlad t o r e c e i v e any comments o r remarks r e a d e r s may have a b o u t t h e book. T h i s monograph, a s t h e f i r s t p r e s e n t a t i o n on a n academic l e v e l of o u r p r e s e n t knowledqe o f t h e t r i b o l o g i c a l b e h a v i o u r of m i n i a t u r e s y s t e m s , i s b a s e d on t h e r e s u l t s o f much t r i b o l o g i c a l and o t h e r m u l t i d i s c i p l i n a r y r e s e a r c h which would have b e e n i m p o s s i b l e t o u n d e r t a k e w i t h o u t t h e h e l p o f many c o l l e a g u e s a c t i v e i n t h e same f i e l d .
VI
The a u t h o r i s g r a t e f u l t o P r o f . H .
C z i c h o s and P r o f . G. G l a s e r
f o r t h e i r k i n d r e v i e w s o f t h e o u t l i n e of t h e book and f o r t h e i r h e l p f u l s u g g e s t i o n s . Thanks a r e a l s o d u e t o t h e reviewer, Dip1.-Phys. H.
T i s c h e r . The a u t h o r would l i k e t o t h a n k P r o f . W. T r y l i f i s k i , h i s
eminent u n i v e r s i t y t e a c h e r , f o r t h e i n t e r e s t h e showed i n t h e work and f o r h i s v a l u a b l e comments. Many c o l l e a g u e s were k i n d enough t o d i s c u s s w i t h t h e a u t h o r t h e problems c o n s i d e r e d i n t h i s volume a n d / o r to make a v a i l a b l e v a r i o u s i t e m s c i t e d i n t h e t e x t . These a r e : Doz. M. A j a o t s , I n g . F . A u b e r t , Dip1.-Ing.
G . Bankmann, D r . M.K. B e r n e t t , I n g . A. Braun, E r r , DOZ. F. F r a n e k , Dr.-Ing. J . G e h r i g , D r . H . E . Hintermann, I n g . A . Huber, Dip1.-Phys. M. Huck, D r . A. Kropiewnick i , D r . A. Maciszewski, D r . M. Massin, I n q . M. M a i l l a t , D r . K.L. M i t t a l , Dip1.-Ing. E . N a j a s e k , A. R a s t a w i c k i M Eng., Dip1.-Chem. A. R e n f e r , Dr.-Ing. G . RBgnault, P r o f . A. RUSS, M r . W . Stehr, D r . M. T i l l w i c h , M r . M. Treboux. Thank you v e r y much f o r t h i s help. The a u t h o r i s g r a t e f u l t o t h e f o l l o w i n g c o p y r i g h t owners f o r p e r m i s s i o n t o r e p r o d u c e some o f t h e f i g u r e s and t a b l e s i n t h i s book: t h e American S o c i e t y of L u b r i c a t i o n E n g i n e e r s , p u b l i s h e r of "ASLE T r a n s a c t i o n s " , B u t t e r w o r t h S c i e n t i f i c L t d . , p u b l i s h e r of " T r i b o l o g y I n t e r n a t i o n a l " , C a r l Hanser V e r l a g , p u b l i s h e r o f " T r i b o l o g i e und S c h m i e r u n g s t e c h n i k " , E l s e v i e r S e q u o i a , p u b l i s h e r of "Wear" , t h e I n s t i t u t e of E l e c t r i c a l and E l e c t r o n i c s E n g i n e e r s Inc. , p u b l i s h e r o f " I E E E T r a n s a c t i o n s on Magnetics" , P e n t o n P u b l i s h i n g I n c . , p u b l i s h e r o f "Machine Design", S p r i n g e r - V e r l a g , p u b l i s h e r o f t h e volumes " T r i b o l o g i e : Reibung-Verschleiss-Schmierung" , and Vogel-Verlag XG Wirrzburg, p u b l i s h e r o f "Maschinenmarkt". Many t h a n k s a r e a l s o due t o t h e s t a f f of E l s e v i e r S c i e n c e Publishers f o r t h e i r c a r e i n bringing t h i s p r o j e c t t o completion.
Dip1.-Ing.
F.
Warsaw, January 1989
Zygmunt Rymuza
VI I
CONTENTS List of abbreviations used for materials 1.
2.
3.
4.
5.
6.
Introduction 1 Materials 6 6 2.1. Metals 9 2.2. Sintered metals 2.3. Minerals, ceramics, composites 15 2.4. Polymeric materials 20 Unfilled polymers 20 2.4.1. 2.4.2. Filled polymers 28 Lubricants 33 33 3 . 1 . Introduction 3.2. Oils 33 3.3. Greases 54 68 3.4. Solid lubricants Unlubricated systems 73 73 4.1. Metallic systems 4.2. Polymeric systems 83 4.2.1. Metal-polymer systems 83 4.2.2. Polymer-polymer systems 109 4.3. Other systems 131 Lubricated systems 149 5 . 1 . Metallic systems 149 5.1.1. Solid metals 149 Sintered metals 164 5.1.2. 5 . 2 . Polymeric systems 173 5.2.1. Metal-polymer systems 173 5.2.2. Polymer-polymer systems 197 203 5 . 3 . Other systems Lubrication problems 211 211 6 . 1 . Introduction 6.2. Preventing oil from spreading or creeping 6.2.1. Introduction 2 1 2 Fundamentals 212 6.2.2. 6.2.3. Methods 214 6.2.4. Coatings (epilames) 2 1 6
212
VIII 6.2.5.
Self-coating (autoepilamizing) 2 2 7 Coating (epilame) technology 230 6.3. Estimation of optimum volume of oil deposit 237 6.4. Lubricant durability 243 253 6.5. Lubrication under extreme conditions 6.6. Lubrication of polymeric systems 261 7. Special tribological coatings 269 269 7.1. Introduction 269 7.2. Anti-friction coatings 7 . 3 . Anti-wear coatings 285 302 8. Experimental techniques 8.1. Introduction 302 8.2. Friction 302 Introduction 302 8.2.1. 8.2.2. Oscillating motion of the rubbinq element 303 8.2.3. Unidirectional motion of the rubbing element 312 325 8.3. Wear 8.4. Thermal effects 331 8.5. Quality of lubricants and coatings (epilames) 333 8.5.1. Introduction 333 Lubricity of lubricants 334 8.5.2. 8.5.3. Physicochemical properties of lubricants 336 8.5.4. Effects of interactions in lubricant-rubbing elements-ambient systems 369 8.5.5. Properties of coatings and epilames 379 8.6. Cleaning 381 8.7, Special investigations 392 9. Tribological aspects of Eine mechanism assemblies 404 9 . 1 . Introduction 404 9 . 2 . Typical plain bearings 404 435 9 . 3 . Special bearings 9.4. Rolling bearings Q41 9 . 5 . Guides 448 9.6. Gears and transmissions 453 9.7. Couplings: clutches and brakes 463 467 9.8. Contacts, brushes 1 0 . Special tribological problems 481 11. Closing comments 508 6.2.6.
References 509 Subject Index 558
IX
LIST OF ABBREVIATIONS USED FOR MATERIALS ABS
Acrylonitrile /butadiene/
CF
Carbon f i b r e
DMPS
Dimethy l p o l y s i l o x a n e Ethylene/tetrafluoroethylene copolymer F l u o r i n a t e d e t h y l e n e / p r o p y l e n e copolymer
ETFE FEP GF
Glass f i b r e
HDPE
High d e n s i t y p o l y e t h y l e n e
LDPE
Low d e n s i t y p o l y e t h y l e n e
PA
Polyamide
PA1
P o l y amide- i m i d e
s t y r e n e copolymer
PAM
Polyacrylamide
PBTP
Poly ( b u t y l e n e t e r e p h t h a l a t e )
PC
Polycarbonate
PCA
Polycaproamide
PCTFE
PEI
Polychlorotrifluoroethylene Polyethylene Polyetheretherketone Polyetherimide
PESU
P o l y e t h e r s u l f one
PETP
Poly (ethylene terephthalate)
PFA
Perfluoroalkoxyethylene
PE PEEK
PI
Polyimide
PMMA
Poly (methyl m e t h a c r y l a t e )
PMP
Polymethy l p e n t e n e
P NP
P e n t a p l a s t (Penton)
P OM
Polyoxymethy l e n e
POM c
P o l y o x y m e t h y l e n e ( p o l y a c e t a l ) copolymer
POM h
P o l y o x y m e t h y l e n e homopolymer
PP
Polypropylene
PPO
Poly (phenylene o x i d e )
PPS
Poly (phenylene s u l p h i d e )
PPSU
P o l y s u If o n e
PS PSZ
Polystyrene Partially stabilized zirconia
PTFE
Polytetraf luoroethylene
PUR
Polyurethane
X
PVC
Po l y ( v i ny 1 bu t y r a 1) Poly ( v i n y l c h l o r i d e ]
PVDC
Poly ( v i n y l i d e n e c h l o r i d e )
PVDF
Poly ( v i n y l i d e n e f l u o r i d e )
SAN
S t y r e n e / a c r y l o n i t r i l e copolymer
SR
S i l i c o n e rubber
UHMWPE
Ultrahigh-molecular-weight polyethylene
PVB
O t h e r d e s i g n a t i o n s of m a t e r i a l s ( i n c l u d i n g l u b r i c a n t s , s o l v e n t s , e t c . ) , are r e g i s t e r e d t r a d e m a r k s .
1
1, INTRODUCTION The t r i b o l o g y of m i n i a t u r e s y s t e m s i s a s p e c i a l a r e a o f t r i bology s c i e n c e b e c a u s e t h e r u b b i n g e l e m e n t s a r e u s u a l l y of v e r y s m a l l dimensions. The e n e r g y d i s p o s i t i o n i n t h e c o n t a c t i n g a r e a i s q u i t e d i f f e r e n t from t h a t i n l a r g e machines. The t r i b o l o g i c a l p r o p e r t i e s o f m i n i a t u r e s y s t e m s t h e r e f o r e d i f f e r from t h o s e composed of e l e m e n t s of l a r g e dimensions ( r e f . 1 ) . The t r i b o l o g y o f m i n i a t u r e systems i s r e l a t i v e l y p o o r l y d e v e l oped. According t o t h e B u n d e s a n s t a l t f t i r M a t e r i a l p r U f u n y (BAM) i n B e r l i n , p u b l i c a t i o n s i n t h i s f i e l d are o n l y a b o u t 1 . 3 % o f t h e t o t a l t r i b o l o g i c a l w o r l d p u b l i c a t i o n s p e r y e a r ( r e f . 2 ) . The r a p i d growth o f p r e c i s i o n e n g i n e e r i n g i n r e c e n t y e a r s (and t h e dynamic p r o g r e s s which can be f o r e s e e n f o r t h e coming y e a r s ) i n t h e p r o d u c t i o n o f m e c h a n i c a l and e l e c t r o m e c h a n i c a l d e v i c e s f o r v a r i o u s a p p l i c a t i o n s which embody m i n i a t u r e t r i b o l o g i c a l s y s t e m s h a s n o t been matched i n t h e development o f t h e t r i b o l o g y s c i e n c e of s u c h s y s t e m s . T r i b o l o g i c a l s y s t e m a n a l y s i s b a s e d on t h e g e n e r a l d e s c r i p t i o n o f s t r u c t u r e , i n p u t and o u t p u t can be a p p l i e d t o m i n i a t u r e s y s t e m s
i s d e f i n e d by t h e s e t of i t s e l e m e n t s , t h e i r p r o p e r t i e s and t h e i n t e r a c t i o n s between t h e e l e ments. The i n p u t o f t h e t r i b o l o g i c a l s y s t e m i s d e s c r i b e d when l o a d , ( r e f . 2 ) . The s t r u c t u r e of a s y s t e m
s l i d i n g s p e e d , motion form ( s l i d i n g , r o l l i n g , c o n t i n u o u s , s t a r b s t o p ( i n t e r m i t t e n t ) , e t c . ) , s l i d i n g t i m e , a m b i e n t c o n d i t i o n s and d i s t u r b a n c e s i f any a r e g i v e n . The o u t p u t i s t h e u s e f u l e n e r g y , i n f o r m a t i o n o r m a s s s i g n a l t r a n s f e r r e d t o t h e o t h e r s y s t e m f r o m t h e anal y s e d t r i b o l o g i c a l system. The t r i b o l o g i c a l s y s t e m i s a l s o c o n n e c t e d w i t h t h e environment by t h e t r i b o l o g i c a l l o s s e s i . e . e n e r g y , f r i c t i o n and m a s s l o s s e s (wear) which i n f l u e n c e i t s s t r u c t u r e a n d f u n c t i o n . The f u n c t i o n i s d e s c r i b e d by t h e r e l a t i o n s h i p s between t h e o u t p u t and i n p u t q u a n t i t i e s . The m i n i a t u r e t r i b o l o g i c a l s y s t e m i s a c o n w i n a t i o n of rubbinc; e l e m e n t s of s m a l l or v e r y s m a l l d i m e n s i o n s , u s u a l l y less t h a n 5 mm, used i n s m a l l mechanisms f o r t h e t r a n s m i s s i o n of i n f o r m a t i o n o r
s m a l l e n e r g y q u a n t i t i e s ( d o s e s ) . The d i s p o s a b l e e n e r g y i n s u c h mechanisms is u s u a l l y s t r i c t l y l i m i t e d . The t e c h n i c a l f u n c t i o n o f t h e t r i b o l o g i c a l system i s d e s c r i b e d by d e f i n i n g t h e r e l a t i o n s h i p s between t h e e n e r g y f l o w s ( s t r e a m s ) , which are t h e c a r r i e r s of i n -
f o r m a t i o n , i n t h e o u t p u t and i n p u t o f t h e s y s t e m . The r e q u i r e m e n t t h a t t h e energy l o s s e s i n t h e m i n i a t u r e t r i b o l o g i c a l system b e a s s m a l l a s p o s s i b l e i s u s u a l l y t h e m o s t i m p o r t a n t a n d d i f f i c u l t cond i t i o n f o r a d e s i g n e r t o f u l f i l l . A l s o m a s s losses ( w e a r ) must b e s m a l l s i n c e t h e y o f t e n e x e r t a c o n s i d e r a b l e e f f e c t on t h e f u n c t i o n i n g of t h e s y s t e m and on i t s d u r a b i l i t y . The s e t of t h e s y s t e m ' s e l e m e n t s c o n s i s t s o f two r u b b i n g p a r t s ( e l e m e n t s ) , wear d e b r i s and any l u b r i c a n t u s e d . The p r o p e r t i e s of e l e m e n t s a r e d e t e r m i n e d by t h e i r g e o m e t r i c a l and m a t e r i a l f e a t u r e s . The c o n s t r u c t i o n a l s h a p e s of t h e r u b b i n g e l e m e n t s o f m i n i a t u r e t r i b o l o g i c a l systems a r e u s u a l l y
cylinders or p l a t e s
( r e f s . 3 - 6 ) . The
e x t e r n a l m a c r o s t r u c t u r e d e s c r i p t i o n c o n s i s t s of nominal dimensions and t h e i r t o l e r a n c e s . The m i c r o s t r u c t u r e i s g i v e n by t h e s u r f a c e r o u g h n e s s o f t h e e l e m e n t s . The most c h a r a c t e r i s t i c f e a t u r e of t h e dimensions systems is t h e i r s m a l l nominal dimensions ( a l s o less t h a n 1 mm) and v e r y h i g h r a t i o s of t h e t o l e r a n c e s t o t h e n o m i n a l d i m e n s i o n . F o r d i m e n s i o n s i n t h e r a n g e 1-5 mm, t h e r e l a t i v e t o l e rance u n i t according t o the IS0 standard is about 0.16-0.40,
de-
c r e a s i n g r a p i d l y as t h e n o m i n a l d i m e n s i o n i n c r e a s e s . F o r d i m e n s i o n s < 1 mm t h e r e l a t i v e t o l e r a n c e u n i t i n c r e a s e s r a p i d l y a s t h e n o m i n a l
d i m e n s i o n d e c r e a s e s and when t h e n o m i n a l d i m e n s i o n i s 0 . 1 mm it i s a b o u t 4.5
( r e f . 3 ) . The r e l a t i v e c l e a r a n c e s i n m i n i a t u r e j o u r n a l
b e a r i n g s when t h e n o m i n a l d i a m e t e r s a r e < 5 mm may e x c e e d 1 0 % . I n t h e polymeric b e a r i n g bushes o f t e n used i n such b e a r i n g s , t h e r a t i o of t h e w a l l t h i c k n e s s t o t h e nominal b e a r i n g d i a m e t e r c a n r e a c h 1 0 0 % . S i m i l a r p r o p o r t i o n s a r e found i n b e a r i n g s w i t h a s i n t e r e d
porous bush. The r u b b i n g s u r f a c e s o f t h e e l e m e n t s o f m i n i a t u r e t r i b o l o g i c a l s y s t e m s are u s u a l l y v e r y smooth. T h i s i s e s p e c i a l l y t h e case w i t h s u r f a c e s made o f s t e e l (e.9. j o u r n a l s u r f a c e s ) which are o f t e n r o l l e r b u r n i s h e d t o Ra < 0 . 1 6 ,um. T y p i c a l materials used i n rubbing e l e m e n t s are: m e t a l s b r a s s , bronze; m i n e r a l s a g a t e ; and polymers
-
-
-
steel,
corundum ( r u b y , s a p p h i r e ) , c h a l c e d o n y ,
polyamides (PA)
,
polyacetals
(POM). S p e c i a l
o i l s and g r e a s e s ( a l s o s o l i d l u b r i c a n t s ) a r e a p p l i e d a s l u b r i c a n t s . These m a t e r i a l s must n o t m i g r a t e from t h e l u b r i c a t e d s y s t e m s i n c e t h e l u b r i c a t i o n o f t e n o c c u r s o n l y once d u r i n g t h e assembly p r o c e s s ("for-life'' lubrication) with, e.g.
one d r o p o f o i l . The a g e i n g re-
s i s t a n c e and c h e m i c a l i n t e r t n e s s o f s u c h l u b r i c a n t s m u s t b e h i g h . The p r e s e n c e o f w e a r d e b r i s i n t h e f r i c t i o n area o f a m i n i a t u r e t r i b o l o g i c a l s y s t e m h a s an i m p o r t a n t e f f e c t on i t s f u n c t i o n
3
and on t h e t r i b o l o g i c a l p r o c e s s e s d u r i n g i t s o p e r a t i o n . The wear d e b r i s can r e s u l t i n l a r g e displacements of t h e rubbing elements ( r e l a t i v e t o t h e nominal d i m e n s i o n s o f e l e m e n t s o r c l e a r a n c e s ) which can l e a d t o s e r i o u s f a i l u r e s . The i n t e r a c t i o n between t h e a f o r e m e n t i o n e d e l e m e n t s o f t r i b o l o g i c a l s y s t e m s i s v e r y complex and h a s been r e l a t i v e l y l i t t l e i n v e s t i g a t e d . The i n t e r a c t i o n s between t h e e l e m e n t s can b e a n a l y s e d
on t h r e e c o n c e p t u a l p l a n e s : t h e m e c h a n i c a l work p l a n e , t h e t h e r m a l p l a n e and t h e m a t e r i a l p l a n e ( r e f . 2 ) . Although t h e f l o w o f mechani c a l energy i n t h e m i n i a t u r e t r i b o l o g i c a l system i s r e l a t i v e l y s m a l l , b e c a u s e o f t h e v e r y s m a l l d i m e n s i o n s o f t h e e l e m e n t s and t h e s m a l l a r e a s of c o n t a c t , t h e i n t e n s i t y of t h e e n e r g y stream ( f l u x ) c a n b e v e r y h i g h . Energy d i s s i p a t i o n o c c u r s m a i n l y a t t h e t r a n s f o r mation i n t o thermal energy, s i n c e t h e s t o r e d s t r a i n e n e r g y a s s o c i a t e d w i t h t h e deformation of elements i s r e l a t i v e l y small. Because of t h e e n e r g y ( f r i c t i o n a l ) l o s s e s , t h e t h e r m a l e n e r g y i s m a i n l y s t o r e d by t h e r u b b i n g e l e m e n t s and wear p a r t i c l e s , s i n c e t h e s m a l l s i z e of t h e r u b b i n g e l e m e n t s h i n d e r s t h e h e a t t r a n s f e r from t h e f r i c t i o n a r e a . T h i s i s e s p e c i a l l y t h e c a s e f o r m i n i a t u r e s y s t e m s w i t h e l e m e n t s m a n u f a c t u r e d from polymers o r m i n e r a l s . The mass t r a n s f e r between t h e e l e m e n t s i s c o n d i t i o n e d by t h e thermal energy flow p r o c e s s s i n c e t h i s energy flow a c t i v a t e s t h e t r i b o p h y s i c a l p r o c e s s e s . Such p r o c e s s e s p l a y an i m p o r t a n t r o l e p a r t i c u l a r l y i n t h e m i n i a t u r e t r i b o l o g i c a l systems w i t h polymeric elements ( r e f . 6 ) . Even a v e r y s m a l l m a s s t r a n s f e r c a n have an import a n t e f f e c t on t h e s y s t e m ' s f u n c t i o n , s i n c e b e c a u s e o f t h e s m a l l a r e a o f c o n t a c t t h e v a r i a t i o n s i n t h e l i n e a r d i m e n s i o n s of t h e rubb i n g e l e m e n t s c a n be v e r y l a r g e . One o f t h e m o s t i m p o r t a n t p r o p e r t i e s of l u b r i c a t e d m i n i a t u r e s y s t e m s i s t h e s t r o n g e f f e c t of t h e a g e i n g o f t h e l u b r i c a n t on t h e t r i b o l o g i c a l p r o c e s s e s . The e f f e c t of t h e a n t i - m i g r a t i o n c o a t i n g s ( e p i l a m e s ) on t h e l u b r i c a n t s h o u l d a l s o be t a k e n i n t o c o n s i d e r a t i o n . The a f o r e m e n t i o n e d s e t of i n p u t q u a n t i t i e s s i g n i f i c a n t l y a f f e c t s t h e o p e r a t i o n of m i n i a t u r e t r i b o l o g i c a l s y s t e m s . The c o n t a c t p r e s s u r e s c a n b e v e r y h i g h . Although t h e a b s o l u t e l o a d s are n o t o f t e n h i g h , t h e s p e c i f i c loads may be v e r y h i g h i n d e e d b e c a u s e of t h e h i g h c u r v a t u r e o f t h e c o n t a c t i n g s u r f a c e s and t h e v e r y s m a l l d i m e n s i o n s and a r e a s of c o n t a c t . T h i s means t h a t i n a c t u a l p r a c t i c e t h e r e a l i s t i c c o n t a c t p r e s s u r e s c a n sometimes be more t h a n s e v e r a l t h o u s a n d MPa. The s l i d i n g s p e e d i s u s u a l l y n o t h i g h e r t h a n 0 . 2 m / s e v e n though t h e a n g u l a r s p e e d s o f j o u r n a l s are h i g h b e c a u s e j o u r n a l
4
d i a m e t e r s are s m a l l . The motion c h a r a c t e r i s t i c s d i f f e r w i d e l y f r o m one s y s t e m t o a n o t h e r . The e l e m e n t s o f t e n r o t a t e , a l t h o u g h l i n e a r r e c i p r o c a t i n g s l i d i n g i s a l s o o f t e n u s e d , and i n some mechanisms t h e r e may be start-stop
( i n t e r m i t t e n t ) and o s c i l l a t i n g m o t i o n w i t h impact l o a d s .
The motion d e s c r i p t i o n c o n s i s t s o f t h e t r a j e c t o r y o f t h e moving e l e m e n t , s p e e d v a r i a t i o n s and f r e q u e n c y o f movements. T h i s i s a very important element i n t h e s e t d e s c r i b i n g t h e i n p u t of a miniat u r e t r i b o l o g i c a l system. The o p e r a t i n g t e m p e r a t u r e r a n g e i s o f t e n v e r y w i d e . F o r example, t h e m i n i a t u r e t r i b o l o g i c a l systems used i n a i r c r a f t i n s t r u m e n t a t i o n
are exposed t o t e m p e r a t u r e v a r i a t i o n s from - 6 0 to +12OoC. The atbient h u m i d i t y c a n a l s o v a r y and t h e i n s t r u m e n t s o p e r a t i n g i n t h e chemical o r m e t a l l u r g i c a l i n d u s t r y f o r example are e x p o s e d t o a c o r r o s i v e atmosphere. I n s t r u m e n t s w i t h m i n i a t u r e t r i b o l o g i c a l s y s t e m s a r e o f t e n u s e d i n vacuum c o n d i t i o n s i n s p a c e . The s l i d i n g d i s t a n c e o r s l i d i n g time of m i n i a t u r e r u b b i n g e l e ments o f t e n h a s t o b e v e r y l o n g e.g. 1 0 years o f e x p l o i t a t i o n . The v i b r a t i o n s t o which t h e mechanisms of some i n s t r u m e n t s are exposed ( e . g . some of t h o s e u s e d i n v e h i c l e s ) a f f e c t t h e t r i b o l o g i c a l p r o cesses i n t h e s e s y s t e m s . The d u s t i n e s s o f t h e atmosphere and t h e
p r e s e n c e of r a d i a t i o n have a s i g n i f i c a n t i n f l u e n c e on t r i b o l o g i c a l p r o p e r t i e s , p a r t i c u l a r l y i n t h e case of l u b r i c a t e d m i n i a t u r e s y s tems. The q u a n t i t i e s d e s c r i b i n g t h e losses o f t h e t r i b o l o g i c a l s y s t e m a r e t h e e n e r g y l o s s e s due t o f r i c t i o n ( u s u a l l y d e s c r i b e d by t h e f r i c t i o n c o e f f i c i e n t ) and t h e mass l o s s ( w e a r r a t e ) . An i m p o r t a n t r e q u i r e m e n t f o r m i n i a t u r e t r i b o l o g i c a l s y s t e m s i s t h e s t a b i l i t y of t h e f r i c t i o n c o e f f i c i e n t d u r i n g t h e p e r i o d o f e x p l o i t a t i o n and a s h o r t running-in
time ( s t a b i l i z i n g t h e f r i c t i o n c o e f f i c i e n t and
wear i n t e n s i t y ) . The v a r i a t i o n s i n t h e f r i c t i o n c o e f f i c i e n t s h o u l d
be v e r y s m a l l s i n c e it is o f t e n v e r y i m p o r t a n t t o e l i m i n a t e t h e s t i c k - s l i p e f f e c t s during s l i d i n g . The i n s t r u m e n t a t i o n u s e d i n t r i b o l o g i c a l s t u d i e s o f m i n i a t u r e s y s t e m s p r e s e n t s s p e c i a l r e q u i r e m e n t s . The s m a l l d i m e n s i o n s o f t h e rubbing e l e m e n t s , v e r y s m a l l a b s o l u t e t r i b o l o g i c a l l o s s e s observed under o f t e n extreme c o n d i t i o n s make t h e t r i b o l o g i c a l s t u d i e s of such s y s t e m s v e r y d i f f i c u l t and time a b s o r b i n g . The d e v i c e s which u s e m i n i a t u r e t r i b o l o g i c a l s y s t e m s a r e v e r y v a r i e d ; r a n g i n g from h o u s e h o l d d e v i c e s , computer p e r i p h e r a l s , m e d i c a l i n s t r u m e n t s , and image and sound r e c o r d e r s , t o t h e a p p a r a t u s
5
i n s p a c e c r a f t o r s a t e l l i t e s . T h i s book c o n c e r n s t h e t r i b o l o g y o f m i n i a t u r e systems b u t d o e s n o t a t t e m p t d e t a i l e d d e s c r i p t i o n s of part i c u l a r t r i b o l o g i c a l systems. It i s a s y n t h e s i s of t h e r e s u l t s of i n v e s t i g a t i o n s of c e r t a i n s y s t e m models and a t t e m p t s t o g e n e r a l i z e , on t h e academic l e v e l , t h e problems of t h i s s p e c i f i c area o f t r i b o l o g y . The d i s c u s s i o n t h e r e f o r e d o e s n o t c o n c e n t r a t e on p a r t i c u l a r s y s t e m s b u t p r e s e n t s a s y n t h e s i s of t h e r e s e a r c h c a r r i e d o u t so f a r , and as s u c h s h o u l d be h e l p f u l i n s o l v i n g p r a c t i c a l problems i n any m i n i a t u r e mechanism used i n modern p r o f e s s i o n a l o r h o u s e h o l d instruments. T h i s book is not a m o r p h o l o g i c a l e n g i n e e r i n g e n c y c l o p e d i a ; r a t h e r it p r e s e n t s t h e s t a t e o f t h e a r t o f m i n i a t u r e t r i b o l o g i c a l s y s t e m s .
The m a t e r i a l s and c o m b i n a t i o n s of materials u s e d i n s y s t e m s o p e r a t i n g under u n l u b r i c a t e d c o n d i t i o n s , s y s t e m s which u s e a s p e c i a l i n s t r u m e n t l u b r i c a n t ( d e s c r i b e d i n d e t a i l i n t h e book) or i n which t h e rubbing elements a r e c o a t e d ( a n t i - f r i c t i o n , anti-wear c o a t i n g s ) a r e d i s c u s s e d . The l u b r i c a t i o n problems of m i n i a t u r e t r i b o l o g i c a l s y s t e m s and t h e u s e of a n t i - m i g r a t i o n c o a t i n g s
( e p i l a m e s ) , are a l s o
a n a l y s e d . The methods and i n s t r u m e n t a t i o n u s e d i n t h e i n v e s t i g a t i o n s a r e a l s o p r e s e n t e d . Moreover, t h e a c t u a l p a r t s ( v a r i o u s t y p e s o f b e a r i n g s , g e a r s , c o u p l i n g s , e l e c t r i c a l c o n t a c t s e t c . ) used i n m i n i a t u r e mechanisms o r e l e c t r o m e c h a n i s m s a r e a n a l y s e d from a t r i b o l o g i c a l p o i n t o f view: a l s o some s p e c i a l c a s e s o f t h e problems of t r i b o l o g i c a l s y s t e m s i n v a r i o u s areas ( e . g . c o m p u t e r s , m e d i c a l d e v i c e s ) a r e a l s o d i s c u s s e d . I t i s i n t h e a u t h o r ’ s aim t h a t t h i s book s h o u l d be a b a s i c r e f e r e n c e book t o a s s i s t i n s o l v i n g t h e t r i b o l o g i c a l problems o f v a r i o u s s m a l l and micro-mechanisms. T h i s i s a c h i e v e d by p r e s e n t i n g t h e l a t e s t d e v e l o p m e n t s i n t h e s c i e n c e o f m i n i a t u r e t r i b o l o g i c a l systems through t h e s t r u c t u r a l , g e n e r a l i z e d d i s c u s s i o n o f r e s e a r c h r e s u l t s i n t h i s v e r y i m p o r t a n t and growing a r e a of t h e t r i b o l o g y s c i e n c e .
6
2 , MATERIALS
The metals u s e d i n t h e m a n u f a c t u r e o f t h e s l i d i n g e l e m e n t s o f m i n i a t u r e mechanisms a r e s t e e l s , b r a s s e s , b r o n z e s , b a b b i t t s , cadm i u m , aluminium and s i l v e r - b a s e d a l l o y s , e t c . F r e e c u t t i n g s t e e l
( a low c a r b o n s t e e l , a v e r a g e c o n t e n t 0 . 1 % C , r a i s e d p h o s p h o r u s c o n t e n t ) is o f t e n u s e d f o r s h a f t s , p i v o t p i n s e t c . , and i s f u r n i s h e d i n t h e form o f s t r a i g h t b a r s made t o v e r y c l o s e t o l e r a n c e s . I t
i s r a t h e r b r i t t l e and u n s u i t a b l e f o r cold-working p r o c e s s e s . High for pivot pins, leafed p i n i o n s h a f t s wherever l o n g e r s e r v i c e l i f e i s r e q u i r e d . E l e m e n t s c a r b o n (1%C ) b r i g h t s t e e l i s used e . g .
made o f h i g h c a r b o n s t e e l are, as a r u l e , s u b j e c t e d t o h e a t t r e a t ment. Here s p e c i a l o i l - q u e n c h e d s t e e l ( t h e r e f o r e o n l y s l i g h t l y deformed by h e a t t r e a t m e n t ) i s w i d e l y u s e d ( r e f . 3 ) . S t a i n l e s s s t e e l w i t h a chrome c o n t e n t o f between 1 2 and 1 4 % i s used f o r e l e m e n t s whose r u b b i n g s u r f a c e s a r e r e q u i r e d t o be c o r rosion-resistant
e.g.
i n a damp, t r o p i c a l c l i m a t e . Because s u c h
s t e e l i s n o t q u i t e r u s t l e s s and i t s r u s t - p r o o f i n g improves w i t h s u r f a c e q u a l i t y , t h e e l e m e n t s have t o b e c a r e f u l l y p o l i s h e d . F o r a c i d ambient c o n d i t i o n s , a u s t e n i t i c s t a i n l e s s s t e e l h a s t o be employed. Mild s t e e l ( c a r b o n c o n t e n t below 0 . 2 % ) and f r e e c u t t i n g s t e e l s can be u s e d i n s t e a d o f s t a i n l e s s ( e . g . chrome and chrome-nickel
s t e e l s ( r e f . 3 ) ) when modern h a r d non-porous c o a t i n g s a r e a p p l i e d (see C h a p t e r 7 . 3 ) . Simple c a r b u r i z a t i o n or c y a n i d i n g o f m i l d s t e e l , o r n i t r i d i n g o f n i t r i d i n g s t e e l s improves t h e i r h a r d n e s s and w e a r resistance. Leaded wrought h i g h - t e n s i l e b r a s s e s c o n t a i n i n g 5 8 % c o p p e r and 2 % l e a d , o r 6 3 % c o p p e r and 1 . 5 % l e a d , a r e e a s i l y m a c h i n a b l e and are
good b e a r i n g m a t e r i a l s ( r e f s . 3 , 7 , 8 ) . They a r e b r i t t l e i n c o l d c o n d i t i o n s . Basic 6 3 Cu
-
b r a s s i s d u c t i l e and p e r f e c t l y s u i t a b l e
f o r c o l d working c o n d i t i o n s . For c a s t i n g e l e m e n t s , b r a s s e s composed of 6 0 % c o p p e r , 1.5% l e a d and t h e r e s t z i n c , a r e u s e d . C a s t bronzes
( a v e r a g e c o n t e n t : 85% c o p p e r , 5% t i n , 5 % z i n c , 5 %
l e a d ) a r e good b e a r i n g m a t e r i a l s . The c o m p o s i t i o n o f some t y p e s o f cast bronzes i s given i n T a b l e 2 . 1
(ref. 9)
.
7 TABLE 2.1 COMPOSITION OF CAST BRONZES
ALLOY No.
(ref. 9)
AOOITIONAL COMPONENTS i n % cu
89-91 87-89 85-87 84-87 78-81
75-79 69-77
Sn
Zn
Pb
9-11
0.5 0.5 0.5 1.o 3.0 3.0 3.0
1.0 1.0 1.0
11-13 13-15 9-11 9-11 7-9 3.5-5.5
4-6 8-1 1 13-17 18-23
- -P S Fe 0.2 0.2 0.2 0.25 0.25 0.25 0.25
0.4 0.4 0.2 0.1 0.05
0.05 0.05 0.05
0.05 0.05
- --
A l l o y s 1 and 2 a r e n o t v e r y i n t e r e s t i n g a s b e a r i n g m a t e r i a l s i n comparison w i t h a l l o y 3 . These b r o n z e s a r e n o t i m p a c t - r e s i s t a n t b u t they are corrosion-resistant
and t h i s i s t h e i r main a d v a n t a g e . The
a l l o y s w i t h a r e l a t i v e l y h i g h l e a d c o n t e n t (numbers 4 t o 7 ) , demons t r a t e v e r y good t r i b o l o g i c a l p r o p e r t i e s and can be used a t temper a t u r e s f o r b a b b i t t s (see below) and a t h i g h s p e c i f i c p r e s s u r e s (10-20 MPa) i n r u b b i n g a g a i n s t s t e e l p i v o t s (hardened t o 50-60 HRC)
.
T h e i r d i s a d v a n t a g e i s low c o r r o s i o n r e s i s t a n c e . Alloy 4 i s t h e most c o r r o s i o n - r e s i s t a n t of t h e s e a l l o y s . Lead- o r t i n - b a s e d a l l o y s ( b a b b i t t s ) a r e v e r y good b e a r i n g mat e r i a l s . They can be d i v i d e d i n t o t h r e e groups: h i g h - t i n a l l o y s ( t i n 2 808,
l i t t l e o r no l e a d ) : h i g h - l e a d a l l o y s ( a b o u t 80% l e a d
and 1-128 t i n ) : and a l l o y s w i t h i n t e r m e d i a t e p e r c e n t a g e s of t i n and l e a d . Apart from l e a d and t i n , t h e s e a l l o y s c o n t a i n antimony and copper. The a p p l i c a t i o n of such a l l o y s i s l i m i t e d t o low o r medium l o a d s . These a l l o y s a r e n o t s u s c e p t i b l e t o c o r r o s i o n . Cadmium-based a l l o y s ( 9 8 % cadmium, 2% n i c k e l o r 98% cadmium, 1 % s i l v e r , 1% c o p p e r ) have a s t r u c t u r e c o n s i s t i n g of a s o f t m a t r i x c o n t a i n i n g h a r d e r c r y s t a l s o f i n t e r m e t a l l i c compounds. The cadmiumbased a l l o y s a r e c h a r a c t e r i z e d by a low c o e f f i c i e n t of f r i c t i o n , low wear and a h i g h l o a d - c a r r y i n g c a p a c i t y . They a r e n o t a s wearr e s i s t a n t a s l e a d bronze f o r example b u t t h e y c a n be u s e d w i t h an unhardened s h a f t . The b e a r i n g c l e a r a n c e may be g r e a t e r t h a n f o r a b a b b i t t b u t less t h a n f o r a l e a d bronze b e a r i n g . Aluminium-based a l l o y s (aluminium w i t h 6-7% t i n and a s m a l l amount o f copper and n i c k e l , aluminium w i t h 6-7% t i n and 1.5-2.5% s i l i c o n ) a r e s i m i l a r t o b a b b i t t s a s regards t h e i r wear-resistant p r o p e r t i e s but t h e y a r e a l s o e x t r e m e l y c o r r o s i o n - r e s i s t a n t . The disadvantage o f s u c h m a t e r i a l s i s t h e i r h i g h c o e f f i c i e n t of e x p a n s i o n .
8
They may b e u s e d a s l i n i n g s on a s t e e l b a s e . S i l v e r - b a s e d a l l o y s are used f o r e l e c t r o - d e p o s i t i o n of a t h i n
mm) i s d e p o s i t -
l a y e r on a s t e e l s u p p o r t . The s i l v e r l a y e r ( 0 . 3 - 0 . 5
e d w i t h a n i n t e r m e d i a t e l a y e r of c o p p e r or n i c k e l . A 0.02-0.03
mm
f i l m of l e a d and indium i s t h e n d e p o s i t e d o n t o p o f t h e s i l v e r , a n d t h e indium d i f f u s e d i n t o t h e l e a d by h e a t t r e a t m e n t a t 1 8 O O C . N i c k e l i n t h e t e c h n i c a l l y p u r e s t a t e i s s u i t a b l y h a r d and c o r I t c a n b e u s e d on s h a f t s , p i v o t p i n s , g e a r s , and
rosion-resistant.
f o r working i n h i g h l y c o r r o s i v e media s u c h a s h o t w a t e r . N i c k e l s i l v e r (10 t o 3 0 % n i c k e l , 55 t o 63% c o p p e r and t h e r e m a i n i n g p e r c e n t a g e o f z i n c ) i s c o r r o s i o n - and w e a r - r e s i s t a n t . M a g n e t i c a l l y s o f t m a t e r i a l s w i t h h i g h wear r e s i s t a n c e are b a s e d on Fe
-
-
A 1 o r Fe
Si
l i s t e d i n Table 2.2.
Pe
A 1 a l l o y s ( r e f . 10). Some s u c h a l l o y s are
-
Ni,
Fe
-
Co
-
N i o r Fe
-
C r a l l o y s are
c o r r o s i o n - r e s i s t a n t magnetic materials (see Table 2 . 2 )
.
T A B L E 2.2 COMPOSITION OF WEAR-RESISTANT ALLOYS BASED ON IRON
( a ) AND CORROSION-RESISTANT
(b) MAGNETICALLY SOFT
COMPONENTS % ALLOY
No.
a)
1
2
3 4
b)
Al
Cr
Re
Si
15.8-16.4 15.7-16.1 5.2-5.6
1.7-2.5 1.7-2.1
2.1-2.5
-
Ce,
5.2-5.6
1
Ni
Mo
co
21.5-22.5
2.8-3.2
35.5-37.0
-
2
3
9.4-9.8 9.4-9.8 Cr
-
15.5- 16.5 13.1-14.0
For e l e m e n t s working a t c r y o g e n i c t e m p e r a t u r e s , t h e a l l o y s b a s e d on i r o n and w i t h a c o m p o s i t i o n of 29-31% N i l
12-13.5% C r ,
5-6% Mn, 4-5% M o , 2.7-3.2% T i , 0.9-1.3% A 1 c a n b e employed. The cobalt-chromium-molybdenum
a l l o y s a r e u s e d as materials f o r a r t i f i -
c i a l j o i n t s i n t h e human body ( r e f . 1 1 ) . The o t h e r wear- and corrosion-resistant
a l l o y s u s e f u l f o r a p p l i c a t i o n s i n t h e human body
are l i s t e d i n T a b l e 2 . 3 ( r e f . 1 0 ) .
9 TABLE 2.3 WEAR- AN0 CORROSION-RESISTANT ALLOYS U S E 0 I N SURGERY
1
COMPONENTS % ALLOY
No. Ni 1
L
15-17 Rest
Cr
19-21 39-41
Co
39-41
-
Mo
6.4-7.4
C 0.07-0.12
Mn
1.8-
Ti
Al
-
-
-
3.3-3.8
Fe Rest
-2.2
-
0.6
A l l o y 1 i s u s e d as m a t e r i a l f o r m a n u f a c t u r i n g e l e m e n t s of a r t i f i c i a l j o i n t s and a l l o y 2 f o r s u r g i c a l s c a l p e l s .
2 2 I
I
S I NTERED METALS
S i n t e r e d metals a r e u s e d f o r m a n u f a c t u r i n g s l i d i n g e l e m e n t s imp r e g n a t e d e . g . w i t h o i l . The p o r o s i t y o f s i n t e r e d m a t e r i a l s i s about 20-30%. The two t y p i c a l k i n d s of powders u s e d f o r s i n t e r i n g are iron ( c h e a p ) and b r o n z e . S i n t e r e d m a t e r i a l s c a n b e p u r e o r may have g r a p h i t e , l e a d , c 3 p p e r , e t c . added. The p r e s e n c e o f g r a p h i t e c o u p l e d w i t h o x i d a t i o n and breakdown o f t h e o i l , c a n l e a d t o r a p i d caki n g . F i n e l y d i v i d e d c o p p e r a c t s as a c a t a l y s t f o r t h e o x i d a t i o n o f lubricating oil. The i r o n - b a s e d s i n t e r e d m a t e r i a l s a r e w e a r - r e s i s t a n t b u t less c o r r o s i o n - r e s i s t a n t . The p u r e i r o n powder i s o f t e n s i n t e r e d i n t h e form o f b e a r i n g e l e m e n t s . The wear r e s i s t a n c e of s u c h s i n t e r e d ma-
t e r i a l i s improved by t h e a d d i t i o n o f 1 . 5 % g r a p h i t e . The p e r l i t i c s t r u c t u r e of i r o n - g r a p h i t e m a t e r i a l is optimum f o r a wear resista n c e of a s i n t e r e d m a t e r i a l ( r e f . 1 2 ) . Copper ( 0 . 5 t o 2 0 % , u s u a l l y 5 - 9 % ) i s added t o i r o n s i n t e r e d m a t e r i a l s which are t o b e impreg-
n a t e d w i t h o i l . The c o p p e r a d d i t i o n improves t h e t r i b o l o g i c a l p r o p e r t i e s o f i r o n - g r a p h i t e a l l o y s , m a i n l y by i n c r e a s i n g t h e microh a r d n e s s o f t h e m a t e r i a l . The optimum a d d i t i o n o f c o p p e r t o i r o n g r a p h i t e c o m p o s i t e s is 5 - 6 % . F o r e l e m e n t s working w i t h l i m i t e d l u b r i c a t i o n , s i n t e r e d i r o n materials c o n t a i n i n g 4 - 1 5 % g r a p h i t e and 4 - 1 2 % c o p p e r are recommended. I r o n - c o p p e r m a t e r i a l s w i t h 4 0 - 7 0 %
copper are c o r r o s i o n - r e s i s t a n t w i t h a n t i - f r i c t i o n p r o p e r t i e s s i m i -
l a r t o b r o n z e s . The a d d i t i o n o f 3-5% of l e a d is a d v a n t a g e o u s . The i r o n - c o p p e r - g r a p h i t e - p h o s p h o r u s materials a r e wear-resista n t a t h i g h l o a d s b e c a u s e t h e a d d i t i o n of p h o s p h o r u s i n c r e a s e s t h e p l a s t i c i t y l i m i t a n d as a r e s u l t t h e p o r e s i n t h e f r i c t i o n r e g i o n
10
a r e n o t c l o s e d when t h e s h a f t r u b s a g a i n s t a s i n t e r e d b e a r i n g b u s h . The optimum c o m p o s i t i o n i s a n i r o n - b a s e d s i n t e r e d m a t e r i a l w i t h a d d i t i o n of 1 . 5 % c o p p e r , 1%g r a p h i t e and 0 . 5 % p h o s p h o r u s ( r e f . 1 2 ) . The a d d i t i o n o f 4 - 8 % ZnS and 1-3% o f g r a p h i t e m a t e r i a l t o t h e i r o n
i s a l s o b e n e f i c i a l . The s i n t e r e d i r o n - g r a p h i t e m a t e r i a l s c o n t a i n i n g 1 0 - 1 5 % molybdenum are s u i t a b l e f o r e l e m e n t s w o r k i n g u n d e r h i g h l o a d s . Such s i n t e r e d m a t e r i a l h a s a c o e f f i c i e n t of f r i c t i o n o f 0.06-0.16
a t a m b i e n t t e m p e r a t u r e , and o n l y 0.06-0.25
t u r e of 4OO0C
a t a tempera-
(ref. 12).
The i r o n - b a s e d
s i n t e r e d m a t e r i a l s containing f l u o r i d e s (6-9%
CaF2) c a n b e used a t h i g h l o a d s , h i g h t e m p e r a t u r e s and i n a vacuum, and c a n a l s o work w i t h o u t l u b r i c a n t . S t a i n l e s s s t e e l powders, u s e d f o r s i n t e r i n g e l e m e n t s r u b b i n g i n a c o r r o s i v e a t m o s p h e r e o r l i q u i d and t o which b o r o n ,
sulphur o r
carbon have been added, are w e a r - r e s i s t a n t and have a l o w c o e f f i c i e n t o f f r i c t i o n . The i r o n - b a s e d m a t e r i a l s c o n t a i n i n g chromium (max. 2 0 % ) , n i c k e l (max. (Table 2 . 4
,
ref.
lo%),
c o b a l t (max. 10%) a n d o t h e r e l e m e n t s
1 2 ) are w e a r - r e s i s t a n t
a t high temperatures. A l s o
t h e a d d i t i o n of l e a d o r powders of a l l o y s b a s e d on n o n - f e r r o u s m e t a l s c a n improve t h e t r i b o l o g i c a l p r o p e r t i e s o f i r o n - b a s e d s i n t e r e d
materials . TABLE 2 . 4 COMPOS I T I ON OF COMPLEX I RON-BASED S I NTERED MATERIALS
I COMPOSITION
L
-
MATER I AL
Fe
co
30-60 Base
0.8-1.4
Base
5.0-10.0
Ease Base
1.0 0.6-2.0
5-10
Copper-based
Cr -
1-5
Ni
Mn
0.03-0.25
0.2-10.0
0.20-10.0
0.4-4.0
12)
%
-
16-20
-
(ref.
-
0.1-0.5 S i 0.1-0.25 B one or more e l e m e n t s such a s C r , Mo, W, Nb, V, Ta, S i 1-2 BN 0.5-5.0 MO 6-11 CO
s i n t e r e d materials are o f t e n u s e d . T i n b r o n z e s
w i t h a n optimum c o n t e n t o f 9 - 1 1 % t i n a r e a t y p i c a l s i n t e r e d m a t e -
r i a l u s e d f o r m i n i a t u r e b e a r i n g b u s h e s . Bronze g r a p h i t e m a t e r i a l s w i t h o n l y a s m a l l amount of g r a p h i t e i s s u i t a b l e f o r h i g h s p e c i f i c
11
l o a d s and t h a t w i t h a l a r g e amount of g r a p h i t e i s b e s t s u i t e d t o h i g h s l i d i n g s p e e d s and s m a l l l o a d s . M a t e r i a l s c o n t a i n i n g l e a d c a n work w i t h o r w i t h o u t oil. B r o n z e s o r b r o n z e - g r a p h i t e m a t e r i a l c a n b e g r e a t l y improved by a d d i n g t i t a n i u m , n i c k e l , l e a d , z i n c , c o b a l t , i r o n o r aluminium. B r o n z e - g r a p h i t e w i t h 9.5-10.58
l e a d , 1.75% graph-
i t e , 1%i r o n and 0.5-1.5% o t h e r a d d i t i o n s i s a v e r y good b e a r i n g m a t e r i a l ( r e f . 1 2 ) . S i n t e r e d b r a s s e s are r a r e l y u s e d . They are manu f a c t u r e d u s i n g powders c o n t a i n i n g 10-50% z i n c . I n some cases 1 . 5 % l e a d and 0.25-0.8% phosphorus a r e i n t r o d u c e d . Such m a t e r i a l s have a h i g h e r p l a s t i c i t y and b e t t e r w o r k a b i l i t y . N i c k e l - and c o b a l t - b a s e d s i n t e r e d m a t e r i a l s a r e s u i t a b l e f o r elements o p e r a t i n g under extreme c o n d i t i o n s . Nickel-based a l l o y s c o n t a i n i n g i r o n or copper a r e v e r y c o r r o s i o n - r e s i s t a n t .
Nickel with
3 0 % c o p p e r , c a l l e d monel, i s c o r r o s i o n - r e s i s t a n t i n a l k a l i s , water, a 1 5 % s o l u t i o n o f H C 1 and H C 1 s a l t s . I r o n - n i c k e l a l l o y s c o n t a i n i n g
5 0 % o r more n i c k e l are s u f f i c i e n t l y c o r r o s i o n - r e s i s t a n t
i n seawater
( r e f . 1 3 ) , a l k a l i s and some a c i d s o l u t i o n s . N i c k e l - b a s e d s i n t e r e d materials a l s o c o n t a i n , a p a r t from g r a p h i t e , b e r y l l i u m , b o r o n , molybdenum, S i c , B 4 C , T i c , WC, MoB2, Z r B 2 . Cobalt-based s i n t e r e d m a t e r i a l s show a d e c r e a s e i n t h e f r i c t i o n c o e f f i c i e n t w i t h i n c r e a s e i n tgllperriture. The a l l o y i s s t r e n g t h e n e d by i n t r o d u c i n g 5 - 1 % Pd and 3-7% T i c o r 5-20% Ag ( r e f . 1 2 ) . The i m p o r t a n c e of n i c k e l , c o b a l t and t h e i r a l l o y s as a n t i - f r i c t i o n materials f o r l o n g t e r m u s e a t h i g h e r t e m p e r a t u r e s i s due t o t h e i r h a r d n e s s (HRC 5 2 - 5 8 ) . The c o b a l t - b a s e d m a t e r i a l s c a n r u b a g a i n s t s t a i n l e s s and m i l d s t e e l s . Aluminium-based s i n t e r e d m a t e r i a l s a r e c h a r a c t e r i z e d by t h e i r low s p e c i f i c w e i g h t , low c o s t and c o r r o s i o n r e s i s t a n c e . The compos i t i o n o f some aluminium-based s i n t e r e d m a t e r i a l s i s g i v e n i n Tab l e 2.5 ( r e f . 1 2 ) . Copper a d d i t i o n a l l o w s aluminium-based s i n t e r e d materials w i t h l a r g e p o r e s t o b e made. Mg and N i improve t h e m e c h a n i c a l r e s i s t a n c e of t h e a l l o y . 0.1-0.5% S i and 4 % Sn i n c r e a s e t h e wear r e s i s t a n c e of Al-based m a t e r i a l s . The a d d i t i o n of 0 . 1 - 5 . 0 % Mg improves t h e a n t i - f r i c t i o n a l p r o p e r t i e s o f aluminium-based s i n t e r e d m a t e r i a l s . Al-based s i n t c r e d m a t e r i a l s c o n t a i n i n g Cu o r Mg a r e s u i t a b l e f o r t h e m a n u f a c t u r e o f b e a r i n g e l e m e n t s working a t low s p e c i f i c l o a d s . Al-based s i n t e r e d m a t e r i a l s i m p r e g n a t e d w i t h s o l i d l u b r i c a n t s such a s MoS2 a r e u s e d f o r t h e m a n u f a c t u r e o f b e a r i n g e l e m e n t s which r u b i n a vacuum o r i n a i r w i t h o u t l u b r i c a t i o n . The w e a r , t h e r m a l and c o r r o s i o n r e s i s t a n c e o f Al-based s i n t e r e d m a t e r i -
a l s c o n t a i n i n g Mn, Mg, S i , C u , C r , N i and Fe c a n be improved by t h e
TABLE 2.5. COMPOSITION OF SOME ALUMINIUM-BASE0 SINTEREO MATERIALS ( r e f . 12)
ALLOY No.
1
2
3 4 5
6
7 8 9
3.0-4.0 -
0.5-0.8 -
0.25 4.4 0.5-0.6 0.5-10.0
0.5 0.3-2.0
0.6 0.8 5.0-50.0
0.5-10.0
0.2-2.0
5.0-25.0
10.0 0.3-5.0
11
G4.4
13 14
-
Si
cu
10
12
%
COMPOS I T ION
2.0-10 .o 10.0 Max. 10.0
1 .o
0.3-3.0 2.5
0.2
-
-
Max. 2.0
2.5-2.8 -
-
0 -9
-
-
Pb
1.15-2.25
-
Ni
Other i n g r e d i e n t s
0.3 A1203 P a r t i c l e s o f 52 Co, 28 Mo, 17 C r , 3 S i a l l o y
1 .O-4.0
-
-
-
Sn
2.0-8.0 10.0 Max. 10.0
-
3.45 1 .oo-25.00
1 .oo -
0.3-2.0
Sb p o r e s f i l l e d w i t h PI
0.5-5.0 MoS2 , 0.5-5.0 C 0.5-5.0 MoS 2 4.0 C 3.0 C 0.5-5.0 S i c 0.4 Mn , 5.6 Zn and powder o f
35 Mo , 1-4 Zn,
10 S i a l l o y (Co-based) max.
10-50 C 5-20 PbO
1 Mg, Mn, S i
13 a d d i t i o n o f 2-50% BN ( r e f . 1 4 ) . T i t a n i u m - b a s e d s i n t e r e d m a t e r i a l s c a n work a t l o w l o a d s r u b b i n g a g a i n s t b r a s s or b r o n z e . I n h i g h l o a d s y s t e m s t i t a n i u m - b a s e d mate-
r i a l s c o n t a i n chromium. D i f f u s i o n m a n u f a c t u r e d T i - b a s e d a l l o y s cont a i n i n g chromium a r e h i g h l y w e a r - r e s i s t a n t . W e a r - r e s i s t a n t s i n t e r e d materials w o r k i n g a t e l e v a t e d t e m p e r a t u r e s a r e b a s e d o n e l e m e n t s of g r o u p s I V - V I
of t h e Periodic Table.
F e r r o t i t a n i u m s ( F e , 16-20% C r , 2-17% M o , max. 5 % W , max. 0.2% N i l 0.5-0.8% C ,
1 . 0 % Mn and 5-80% T i c o r WC) are w e a r - r e s i s t a n t .
Among
t h e s i n t e r e d m a t e r i a l s w i t h h i g h wear, t h e r m a l a n d c o r r o s i o n res i s t a n c e are t h o s e based on t i t a n i u m , z i r c o n , a n d h a f n i u m b o r i d e s c o n s i s t i n g o f 87.5-70% b o r i d e s a n d 12.5-30% metal powder of t h e
same g r o u p . I r o n - b a s e d s i n t e r e d materials c o n t a i n i n g powders o f carbides o f t h e h i g h - m e l t i n g m e t a l s , g r a p h i t e a n d powders o f t h e c h a l c o g e n m e t a l s o f g r o u p s VB a n d V I B o f t h e ? e r i o d i c T a b l e a n d a l -
so powders of l a n t a n i d e s a r e w e a r - r e s i s t a n t 13OO0C
a t temperatures up t o
(ref. 15).
The a p p l i c a t i o n s of t h e s i n t e r e d a l l o y m a t e r i a l s d e s c r i b e d h e r e d e p e n d o n t h e i r p r o p e r t i e s . W i t h mixed l u b r i c a t i o n a t l o w l o a d s and
s m a l l s l i d i n g s p e e d s , t h e i r o n - b a s e d a l l o y s may b e u s e d (Fe Fe
-
(3-78)
graphite -0.8%) S)
-
g r a p h i t e , Fe ( 0 . 4 - 1 % ) S, F e
. Under
-
5% Cu
-
-
(1-38) g r a p h i t e , F e
( 2 . 5 - 3 % ) Cu
-
1.5% g r a p h i t e
-
-
30%CU,
(1-2%)
-
0.3-
b o u n d a r y l u b r i c a t i o n c o n d i t i o n s , s i n t e r e d a l l o y ma-
t e r i a l s also c o n t a i n e l e m e n t s which a c t as s o l i d l u b r i c a n t s . I n a vacuum, s i n t e r e d b e a r i n g materials s u c h a s t h e n i c k e l - b a s e d
alloys
w i t h a n a d d i t i o n o f WS2, MoS2, CaF2 and BN powders a r e u s e f u l . I n a vacuum a n d a t v e r y l o w t e m p e r a t u r e s , s i n t e r e d material c o n s i s t i n g o f 6 3 % c o p p e r , 30% PTFE and 1 0 % WS2 h a s a v e r y l o w f r i c t i o n c o e f f i c i e n t ( 0 . 1 4 a t -195OC a n d 0 . 0 5 a t 23OoC)
( r e f . 1 2 ) . Some o f t h e
s i n t e r e d a l l o y m a t e r i a l s a p p l i c a b l e a t e l e v a t e d t e m p e r a t u r e s are l i s t e d i n Table 2.6
llOOoc
,
( r e f . 1 2 ) . A t t e m p e r a t u r e s u p t o 1000
or
s i n t e r e d m a t e r i a l s b a s e d on chromium, w o l f r a m , n i c k e l and
niobium can be used. I n c o r r o s i v e c o n d i t i o n s , s i n t e r e d materials manufactured u s i n g powders o f i r o n - o r n i c k e l - b a s e d
a l l o y s a r e a p p l i c a b l e . The i r o n -
b a s e d a l l o y s u s u a l l y c o n t a i n N i l C r , C , B , Mn a d d i t i o n s and n i c k e l - b a s e d a l l o y s C r , Mo e l e m e n t s . The i r o n b a s e d a l l o y c o n t a i n i n g 1 5 . 0 % M o and 6 % CaF2 c a n r u b a g a i n s t s t a i n l e s s s t e e l a t s p e c i f i c loads of up t o 2.5 MPa a n d w i t h a s l i d i n g s p e e d of 0.083 m / s
a r g o n w i t h N a v a p o u r s a t 350°C and u n d e r n e u t r o n r a d i a t i o n (0.56-10''
2
neutrons/mm )
(ref. 16).
in
TABLE 2.6 SINTEREO METALS APPLICABLE AT ELEVATED TEMPERATURES
1
1
C
Cr
B
1
Mo
Sn
1
(ref.
12)
Other elements
Ni
.
I
0.8-1.4 <0.1
1.5
0.1-0.25
16-20
-
1.5-3.5
-
10.0-15.0
-
-
-
0.6-2.0 0.07-0.8
13-32
- I
-
3.04-10.0
13-20 3-20
-
0.5-20.0
14.0-10.0
2-15
1.5-3.5 or Sb
I
Without l u b r i c a t i o n
B i , As
6-15 c 0 , l - 5
-
cu
-
10-18
1 10-12
10-20 CUO, 1-6 PbO 1-5 Si02, 1-25 Fe 1-2 T i , 3 - 4 2 0 - 2 2 co
0.05-7.05
I
0-50 W S 2 or MoS2 25
I
-
Thermal r e s i s t a n t
2-10 CaF2
-
10.0
-
Max. 0.29 Mn, 0.1-0.5 S i
Corrosion-resistant a t elev a t e d temperatures t o 4OO0C
0.5-5.0 I
COMMENTS Anti-seizure add i t i ves
us -
7-16% CaF2, SrF2 o r BaF2 B4C,
-
A t 5O-37O0C, s p e c i f i c p r e s sures 0.2-1.0 MPa and s l i d i n ! speeds 4-36 m/s Temperature range 100-600°C 6500C
I n a i r o r vacuum a t max.600°1 40O-45O0C
20-7OO0C O x i d a t i o n r e s i s t a n t a t 500-1ooooc A t l o n g work a t s p e c i f i c p r e i s u r e t o 1 MPa and ternperaturc 650°C
I
15
2,3,
MINERALS,
CERAMICS,
COMPOSITES
These m a t e r i a l s are w i d e l y u s e d b e c a u s e of t h e i r s p e c i a l t r i b o l o g i c a l p r o p e r t i e s , r e s i s t a n c e t o c o r r o s i o n and t h e r m a l s h o c k . Corundum ( s a p p h i r e and
ruby)
-
A1203
-
i s o f t e n u s e d . Some of t h e
p r o p e r t i e s of s a p p h i r e and r u b y a r e l i s t e d i n T a b l e 2 . 7 .
The p r o -
p e r t i e s of n a t u r a l and a r t i f i c i a l s t o n e s a r e p r a c t i c a l l y t h e same. Ruby i s corundum t o which chromium o x i d e h a s b e e n added t o make i t red. TABLE
2.7.
PROPERTIES OF CORUNDUM,
AGATE AND SITALL
PROPERT I ES
CORUNDUM
Compos i t i on
A'2°3
AGATE
S ITALL
- 46%
Si02
96.3-98.9 %
SiOz
R 0
0.18-0.97 %
A1203- 13
2 3
Fe203FeO CaO MgO Cr203R20 Dens i t y , mg/mm 3 Hardness, Mohs, MPa E l a s t i c i t y modulus, MPa Compressive s t r e n g t h , MPa
f thermal exCoefficient pansion, 10- /K
g
D i e l e c t r ic constant Water a b s o r p t i o n ,
%
Chemical r e s i s t a n c e
Compressive s t r e n g t h i n radioactive water a t 5OO0C, MPa
-
2.6 3.9 5.5-7 9 4 (0.9-1.O) ' 1 0 4 (1.85-2.75)*10 (7.8-8.9) ' 104 (3.5-3.9) lo5 2.06 * l o 3 (8.75-11 .9)*lo2
2.85 7.5 0.98-1 D3
5-6.7 7.5-10
7.0-9.0
J
1.4-1.38
Res i s t a n t against acids 3nd NaOH. Nonresistant 3 g a i n s t HF s t 3OO0C
i e s i s t a n t agai n s t acids e x c e p t HF. 'artially resist a n t a g a i n s t NaOH
3 9.2 8.6 7.0 1.0
0.5
7.8.10
0.02 l e s is t a n t igainst sul>huric acid (99.8%) and K 1 (99%)
17.65
Corundum i s h i g h l y r e s i s t a n t t o c h e m i c a l a c t i o n e v e n a t h i g h t e m p e r a t u r e s and i n t h e p r e s e n c e of r a d i o a c t i v e water vapcur ( r e f . 7 ) .
16
The f l u o r i n e - c o n t a i n i n g a c i d s a t t a c k it a t t e m p e r a t u r e s above 3OO0C, however. The h a r d n e s s of t h e m a t e r i a l i n f l u e n c e s t h e s m o o t h n e s s o f t h e s u r f a c e and t h e c o e f f i c i e n t of f r i c t i o n . The v e r y h i g h e l a s t i c i t y modulus i s , b e s i d e s h a r d n e s s , t h e c h a r a c t e r i s t i c f e a t u r e of corundum. Agate ( T a b l e 2 . 7 )
is a
c a l and c h e m i c a l p r o p e r t i e s .
OW c o s t m i n e r a l w i t h i n f e r i o r mechani-
S i t a l l is a crystalline glass material
w i t h s i m i l a r p r o p e r t i e s t o a g a t e . I t can b e used as a s u b s t i t u t e f o r a g a t e i n t h e m a n u f a c t u r e o f low l o a d e d m i n i a t u r e s l i d i n g e l e ments. S p i n e l c o n s i s t s o f a m i x t u r e of magnesium and aluminium o x i d e s (MgO, A 1 2 0 3 ) a n d i s n o t a s h a r d as corundum ( 9 o n t h e Mohs' s c a l e ) . The c h e m i c a l , o p t i c a l , e l e c t r i c a l and m e c h a n i c a l p r o p e r t i e s of s p i n e l are c o m p a r a b l e t o t h o s e of corundum, b u t t h e p r i c e is s o m e what l o w e r . S p i n e l i s u s e d p a r t i c u l a r l y when l a r g e p a r t s h a v e t o b e made, a s it p o s s i b l e t o o b t a i n l a r g e r c r y s t a l s of rough s y n t h e t i c s p i n e l t h a n of corundum. P a r t i a l l y s t a b i l i z e d z i r c o n i a (PSZ) ( Z r O Z
-
-
9 % MgO) seems t o b e a n a t t r a c t i v e t r i b o l o g i c a l m a t e r i a l .
The h a r d e s t m a t e r i a l s , s u c h a s diamond ( u s e f i i l when s t i c k - s l i p e f f e c t s must b e e l i m i n a t e d ( r e f . 1 7 ) ) , b o r o n n i t r i d e o r b o r o n c a r b i d e , c a n be a p p l i e d when h i g h wear r e s i s t a n c e i s needed i n e x t r e m e c o n d i t i o n s ( r e f s . 1 8 , 1 9 ) . J a s p e r , g l a s s and g a b b r o h a v e b e e n found ( r e f . 2 0 ) t o be v e r y u s e f u l a s m a t e r i a l s f o r t h e r u b b i n g e l e m e n t s o f b e a r i n g s o p e r a t i n g i n a vacuum when t h e c o u n t e r f a c e i s manufact u r e d from a p o l y m e r i c c o m p o s i t e . Ceramic m a t e r i a l s s u c h a s s i n t e r e d A 1 2 0 3 ,
Zr02,
Si3N4, S i S i C ,
SiALON a r e v e r y i n t e r e s t i n g a s s p e c i a l b e a r i n g m a t e r i a l s b e c a u s e o f t h e i r r e s i s t a n c e t o wear and h e a t ( r e f s . 2 1 - 2 d ) .
Some p r o p e r t i e s o f
t h e aforementioned ceramic materials a r e l i s t e d i n T a b l e 2.d. c h a r a c t e r i s t i c p r o p e r t y o f SiALON i s i t s u l t r a - h i g h
The
resistance to
oxidation a t elevated temperatures. I t i s a l s o corrosion-resistant i n l i q u i d metals: A l l C u , F e , Zn and s t e e l , i n H2S04 and H C 1 a c i d s , i n borax, a l k a l i s , e t c . T h i s m a t e r i a l i s a l s o h i g h l y wear-resistant. The r e a c t i o n - s i n t e r e d ,
fine-grain,
s o l i d s i l i c o n carbide Purbide
REFEL ( P u r e Carbon C o ; ) c a n work up t o
140OOC.
High d e n s i t y f e r r i t e s w i t h a r e s i d u a l p o r o s i t y of 0 . 5 % o r l e s s have a h i g h r e s i s t a n c e t o wear and e l e m e n t s made o f s u c h m a t e r i a l s
a r e c h a r a c t e r i z e d by t h e p r e c i s i o n w i t h which t h e y c a n be cut, ground o r p o l i s h e d ( r e f . 7 ) . Such m a t e r i a l s a r e v e r y u s e f u l i n t h e manuf a c t u r e o f t a p e r e c o r d e r h e a d s . An example of t h e h i g h l y wear-res i s t a n t m a t e r i a l u s e d i n t a p e r e c o r d e r h e a d s i n HAVAR, made by
17
Hamilton, which has a good permeability and retains virtually no detectable magnetism after high magnetic field exposure (ref. 29). TABLE 2.8 SOME PROPERTIES OF CERAMIC MATERIALS I
S i 3N4
MATERIALS
REACT I ON S I NTERED
PROPERTIES Dens i t y , 3 mg/mm
S i 3N4 HOT PRESSE
2.6
SiSiC REACT I ON S I NTEREO
3.2
3.0
S i ALON
REACT I ON S I NTEREO
3.2
3'2 HOT PRESSE
3.9
Tensi l e s t r e n g t h , MPa
260
690
300
430
480
Modulus o f e l a s t i c i ty,GPa
220
310
350
300
365
Coe f f i c i en t o f thermal expansion,
10-6/K Thermal cond u c t iv i t y , W/m.K Maximum app l i c a t i o n tern p e r a t u r e , OC
3.2
3.2
16
(20°C)
14.2 (1200°C)
25 (20OC)
14
(1200°C
4.5 (20OC) 40 (lOOO°C
200
3.2 25 (20°C)
7 (10OO0C
9.0
26 (20°C) 5 (12OOOC
1500
1500
1400
1400
1750
Wei b u l 1 ' s rnodul us
10
15
10
15
10
Opened porosity, %
20
0
0
0.01
0
Composite materials of various structures are very interesting for use in bearings, especially for making unlubricated bearing elements. Composites are usually formed by a combination of metal, ceramic materials and polymers with various geometrical features and properties. The metallic glasses such as Fes7Co18B14Si1, Fe40Ni40P14B6 Fe76Cr4C12Pd demoqstrate very good tribological properties (refs. 30, 31). The composites of borosilicate glass reinforced withgraphite fibres are also of interest (ref. 32). The metallic glasses, graphite-fibre-reinforced ceramic or glass materials can be applied where there are high temperatures and a corrosive environment, and in these conditions they show better tribological properties than metal or polymer matrix materials. Composite materials based on crystalline glass sitall are sintered using, for example, Sitall-3 (manufactured on the base of Si02,
18 A1203,
1000
-
Ba203, MgO and f l u o r i d e s and w i t h a c o m p r e s s i v e s t r e n g t h o f 1 2 0 0 MPa
( r e f . 3 3 ) ) c o n t a i n i n g 30% m e t a l ( n i c k e l , c o p p e r ,
t i n , o r b r o n z e ) . The b e s t wear and a n t i - f r i c t i o n p r o p e r t i e s when r u b b i n g a g a i n s t s t e e l i n a vacuum, w e r e e x h i b i t e d by a c o m p o s i t e c o n t a i n i n g b r o n z e powder ( r e f . 3 3 ) . A n o t h e r p o s s i b i l i t y i s t o s i n t e r sitall-based
c o m p o s i t e s w i t h a p o r o s i t y o f 2-39,
impregnated w i t h
PTFE o r c o n t a i n i n g 30% c o p p e r and i m p r e g n a t e d w i t h PTFE. Such m a t e r i a l s are c o r o s s i o n - r e s i s t a n t
and c a n work i n a g a s , l i q u i d o r
vacuum e n v i r o n m e n t . The c o e f f i c i e n t o f f r i c t i o n i n a i r i s 0.25-0.31, i n a vacuum 0.2-0.25
and t h e maximum s p e c i f i c l o a d s are between 8 a n d
li) MPa ( r e f . 12). Ceramic c o m p o s i t e s s u c h a s cermets ( m e t a l bounded ceramics e . g . chromium bonded t u n g s t e n c a r b i d e ) , c a n be u s e d a s d r y b e a r i n g m a t e r i a l s working a t t e m p e r a t u r e s o v e r SOOOC ( r e f . 3 4 )
. Generally,
b o t h t h e c o e f f i c i e n t o f f r i c t i o n and t h e wear r a t e a r e v e r y h i g h , and so i s t h e c o s t : t h e r a n g e o f a p p l i c a t i o n s i s v e r y l i m i t e d . Hard carbon ( g r a p h i t e ) o r carbon ( g r a p h i t e ) - m e t a l m a t e r i a l s are very u s e f u l a s b e a r i n g m a t e r i a l s a t t e m p e r a t u r e s o f up t o 350OC ( t y p i c a l c o m p o s i t e s ) . However, t h e s e m a t e r i a l s a r e r e l a t i v e l y weak a n d b r i t -
t l e which makes b e a r i n g s d i f f i c u l t t o f i t i n t o t h e i r h o u s i n g s and l i m i t s t h e l o a d c a p a c i t y t o a b o u t 1.5 MPa, a l t h o u g h i m p r e g n a t i o n by m e t a l s can double t h i s f i g u r e ( r e f . 3 4 ) . C a r b o n - g r a p h i t e s a r e s e l f - l u b r i c a t i n g , h e a t - r e s i s t a n t and i n e r t i n t h e p r e s e n c e o f a l l b u t t h e s t r o n g e s t o x i d i z i n g a g e n t s . The maximum working t e m p e r a t u r e s f o r c a r b o n g r a p h i t e s a r e a b o u t 4 0 0 ,
65U
and 750OC i n a i r , w a t e r v a p o u r and c a r b o n d i o x i d e r e s p e c t i v e l y . They c a n b e u s e d i n e l e m e n t s o p e r a t i n g i n t h e vacuum and c o l d o f o u t e r s p a c e and c a n be s a f e l y b r o u g h t i n t o c o n t a c t w i t h f o o d s , beve r a g e s , d r u g s o r m e d i c a l a p p l i a n c e s and c a n b e s t e r i l i z e d w i t h o u t damage. C a r b o n - g r a p h i t e s c a n a l s o b e i m p r e g n a t e d w i t h m o l t e n metals, c e r a m i c s , r e s i n s , l u b r i c a n t s e t c . The s i l i c o n i z e d g r a p h i t e s a r e g r a p h i t e m a t e r i a l s w i t h s u r f a c e which c a n b e c h e m i c a l l y c o n v e r t e d t o s i l i c o n c a r b i d e . Such m a t e r i a l s have t h e wear r e s i s t a n c e o f s i l i c o n c a r b i d e w h i l e r e t a i n i n g some o f t h e l u b r i c i t y o f g r a p h i t e . C a r b o n - g r a p h i t e m a t e r i a l s c a n b e e a s i l y machined o r molded t o s i z e ; t h e y have b e e n s u c c e s s f u l l y u s e d t o make b e a r i n g b u s h e s , d e m n s t r a t i n g v e r y good t r i b o l o g i c a l p r o p e r t i e s ,
including i n conditions of
w a t e r and g a s l u b r i c a t i o n ( r e f s . 3 5 , 3 6 ) . A n i n i a t u r e b e a r i n g made o f Purebon P-9 m a t e r i a l
( P u r e Carbon Co.) w i t h a b o r e d i a m e t e r o f mm h a s b e e n a p p l i e d t o a w a t t - h o u r meter f o r w h i c h t h e l i f e expectancy i s 30 y e a r s (without l u b r i c a t i o n ) , s l i d i n g speed
0.523f0.007
19
below 0 . 0 1 m / s
and t e m p e r a t u r e r a n g e - 5 5 O t o 5OoC
( r e f . 3 ' 7 ) . Some
properties of carbon-graphites a r e l i s t e d i n Table 2.9. TABLE 2.9 PROPERTIES OF SOME CARBON-GRAPHITES
MATERIAL
MANUFACTURER PROPERTIES Dens i t y , mg/mm 3
PUREBON
PUHEBON
p-9
P-692
PURE CARBON C O .
1.75
1.85
Compressive strength, MPa
150
220
Modulus o f e l a s t i c i t y , GPa
14
21
I EK-200
WITH
RINGSDORF-WERKE GmbH BAD GOOESBERG (F.R.G.
1.70 100
9.5
1.90 250
14
C o e f f i c i e n t o f thermal expansion,
10-6/K Thermal c o n d u c t i v i t y , W m - l Temperature 1 i m i t ,
K-l
4.3
6.5
3.7-4.6
5.7
12.3
12.3
25.2
25.2
OC
Neut r a 1 a tniosphe r e O x i d i z i n g atmosphere
800
300
300 300
40 0
200
Metal-based c o m p o s i t e s a r e c h a r a c t e r i z e d by t h e i r h a r d s t r u c t u r e f i l l e d w i t h various a n t i - f r i c t i o n materials. S i n t e r e d i r o n , c o p p e r , aluminium, b r a s s o r b r o n z e , C r - C o a l l o y - b a s e d m a t e r i a l s w i t h 1 0 - 1 5 % g r a p h i t e a r e a p p l i c a b l e o v e r a wide t e m p e r a t u r e r a n g e ( - 2 0 0 t o 6OO0C f o r i r o n - b a s e d m a t e r i a l ) and d e m o n s t r a t e a s u f f i c i e n t l y low c o e f f i c i e n t of f r i c t i o n ( a s low a s 0 . 0 5 ) . M a t e r i a l s
w i t h porous b r o n z e , Cu
-
Nil
Cu
-
Pb l a y e r s a r e m a n u f a c t u r e d u s i n g
s t e e l b a c k i n g . The s i n t e r e d p o r o u s l a y e r i s i m p r e g n a t e d w i t h a low
m e l t i n g p o i n t a l l o y , PTFE or l u b r i c a n t . A c t u a l l y , PTFE and l e a d i m p r e g n a t e d i n t o s i n t e r e d p o r o u s b r o n z e ( G l a c i e r DU) h a s p r o b a b l y found a w i d e r r a n g e of a p p l i c a t i o n s t h a n a n y o t h e r b e a r i n g m a t e r i a l
(see r e f . 3 4 ) . Porous s t e e l m a t e r i a l s impregnated w i t h e . g . A g - C u - Zn - Cd a l l o y ( r e f . 1 2 ) a r e l o a d - r e s i s t a n t when o p e r a t i n g i n a vacuum a g a i n s t s t a i n l e s s o r n i t r i d e d s t e e l . The optimum p o r o s i t y of such c o m p o s i t e s i s 3 0 - 4 0 % . The c o e f f i c i e n t of f r i c t i o n i s 0.04-0.05 when l u b r i c a t e d e l e m e n t s rub i n a i r a t 5 0 - 7 0 MPa.
20
2,4,
POLYMERIC MATERIALS
2.4.1.
UNFILLED POLYMERS
U n f i l l e d bearing (anti-friction) polymers are principally thermplastic
materials s u c h a s polyamides, p o l y a c e t a l s , p o l y t e r e p h t h a l a t e s e t c . The most i n t e r e s t i n g o f them, a s f a r as t h e molding of b e a r i n g e l e ments i s c o n c e r n e d , w i l l be d i s c u s s e d h e r e . Polyamides ( P A ) a r e c r y s t a l l i n e polymers w i t h a h i g h d e g r e e o f c r y s t a l l i n i t y , which i s i m p o r t a n t f o r good t r i b o l o g i c a l p r o p e r t i e s . Polyamide 6 h a s v e r y good t r i b o l o g i c a l p r o p e r t i e s b u t it i s s e n s i t i v e t o m o i s t u r e . A t room t e m p e r a t u r e (2OoC) and 6 5 % r e l a t i v e hum i d i t y , t h e m a t e r i a l c o n t a i n s a b o u t 3 % water ( r e f . 3 8 ) . The o t h e r polyamides ( P A 6 6 , PA 6 1 0 , PA 11, PA 1 2 ) a r e more r e s i s t a n t t o t h e p r e s e n c e of m o i s t u r e and e l e m e n t s d e m o n s t r a t e h i g h e r d i m e n s i o n a l s t a b i l i t y . The p r i n c i p a l p r o p e r t i e s and some t r a d e m a r k s of t h e main t y p e s o f polyamides a r e g i v e n i n T a b l e 2.10. TABLE 2.10 MAIN TYPES OF POLYAMIDES
MATERIAL
PROPERTIES
PA 6
PA 66
4KULON M,K ULTRAMID B ORGAM I DE 3EETLE GR I LON
ZYTEL 101 ULTRAMI 0 A TECHNYL A MARANYL A D URETHAN
1.13-1.15 80
1.13-1.15
PA 6 1 0 ZYTEL 3 1 TECHNYL OP ULTRAMID S
PA 1 1
R I LSAN
DO
80
60
85
1500
2000
1400
60
34 ( a t 1% def o r mat i on)
Coefficient of thermal expansion, 1 0 - 5 1 ~ Thermal c o n d u c t i v i t y , W/m K Speci f i c h e a t , J/g.K Maximum o p e r a t i n g temperature o f beari n g , OC Surface f r e e 2 mJ/m energy, S o l u b i l i t y parameter 103J0.5,J .5
1.07-1.09 60
8-10 0.28
1.9
8-10 0.24 1.7
75
70
50
46
30
26
20 ( a t 1 % def o rmat i on) 9 0.22 1.6
60
VESTAM I D G R I LAM I D
--
_ I _ -
Dens; t y , mg/mm 3 Tensi l e strength,MPa Tensi l e e l o n g a t i o n , % Modulus o f e l a s t i c i t y , MPa Compress i ve s t r e n g t h , MPa
PA 12
1.04-1.05 300-330
1.01-1.02 60 250-300
1150
1100
50
50
13
11
50
0.29 1 .8
50
0.33 1.7 50
35 27
25
25.5
21
P o l y a c e t a l s (POM) a r e h i g h l y c r y s t a l l i n e , t h e y a r e s t r o n g and r i g i d , and have good m o i s t u r e , h e a t , and s o l v e n t r e s i s t a n c e . The homopolymers are h a r d e r , have h i g h e r r e s i s t a n c e t o f a t i g u e , a r e more r i g i d , and d e m o s t r a t e h i g h e r t e n s i l e and f l e x u r a l s t r e n g t h w i t h g e n e r a l l y lower e l o n g a t i o n . The copolymers remain s t a b l e i n long-term, h i g h - t e m p e r a t u r e s e r v i c e and o f f e r e x c e p t i o n a l resistance t o t h e e f f e c t s o f immersion i n water a t h i g h t e m p e r a t u r e s . N e i t h e r type resists s t r o n g a c i d s , b u t
t h e copolymer is v i r t u a l l y unaffect-
e d by s t r o n g b a s e s ( r e f s . 3 9 , 4 0 ) . The t y p i c a l p r o p e r t i e s o f p o l y a c e t a l s are l i s t e d i n T a b l e 2 . 1 1 . TABLE 2.11 TYPICAL PROPERTIES OF POLYACETALS, POLYCARBONATES AND POLYTEREPHTHALATES
MATERIAL
POM h
POM c
OELRl N
HOSTAFORM C ULTRA FORM CELCON (KEMATAL) TARNOFORM
PROPERTIES D e n s i t y , mg/mm 3 T e n s i l e s t r e n g t h , MPa Tensile elongation, % Modulus o f e l a s t i c i t y , MPa Compressive s t r e n g t h , MPa C o e f f i c i e n t o f thermal expansion, 10-51~ Thermal c o n d u c t i v i t y , W/m.K S p e c i f i c heat, J/g.K Maxi mum opera t i ng tempe. r a t u r e o f b e a r i n g , OC S u r f a c e f r e e energy, 2 mJ/m S o l u b i l i t y parameter, lo3J0.5;1 -5
PC -LEXAN MA KROL0 N MERLON S I NVET
PETP
PBTP
ARNITE A HOSTADUR K FR-PET RYNITE
UALOX CELANEX POCAN 0 E R OT ON ARNITE T HOSTAOUR B
1.37 74
1.31-1.32
-1.4; 70 25-75
1.41 60 60-75
I .20
65 90
50 1 goo
3500
2800
2300
125
110
85
10.0
0.2:
0.30
1.5
1.5
57 150-300 1900
85
7
7
4-4
0.20 1.2
0.24 1.05
0.23 1.1
90
80
100
55
50
43
40
33
47
44
21
23
20
21
21
P o l y c a r b o n a t e ( P C ) (see Table 2 . 1 0 )
,
low-crystalline material,
h a s h i g h i m p a c t s t r e n g t h o v e r a wide t e m p e r a t u r e r a n g e . The materi a l i s c h a r a c t e r i z e d by good d i m e n s i o n a l s t a b i l i t y even w i t h humid-
i t y changes o r d u r i n g s h o r t p e r i o d s i n b o i l i n g w a t e r . i s u n a f f e c t e d by g r e a s e s , o i l s and a c i d s , b u t s o l u b l e h y d r o c a r b o n s , e s t e r s and k e t o n e s . I t i s w e a t h e r p r o o f , composition is observed a f t e r a long p e r i o d ( 2 y e a r s )
Polycarbnate i n chlorinated b u t some de(refs. 12, 41).
22
P o l y e t h y l e n e t e r e p h t h a l a t e (PET?) and po l y b u t y l e n e tere&thalate (PBTP)
( T a b l e 2.10) a r e c o m p e t i t o r s o f p o l y a s e t a l s . They c r y s t a l -
l i z e r a p i d l y , flow r e a d i l y , and have h i g h c r e e p r e s i s t a n c e and low m o i s t u r e a b s o r p t i o n . T h e i r main advantage i s t h e i r t o u g h n e s s . They a r e c h e m i c a l l y r e s i s t a n t t o a broad r a n g e of media such as aliphatic hydrocarbons, g a s o l i n e , carbon t e t r a c h l o r i d e , p e r c h l o r o e t h y l e n e , o i l s , f a t s , a l c o h o l s , g l y c o l s , e s t e r s , e t h e r s and d i l u t e a c i d s and b a s e s . PETP and PBTP a r e a t t a c k e d , however, by s t r o n g a c i d s and bases. The p o l y o l e f i n s s u c h as p o l y e t h y l e n e and p o l y p r o p y l e n e have
s i m i l a r chemical s t r u c t u r e s b u t t h e y d i f f e r i n t h e i r c r y s t a l l i n e s t r u c t u r e , and c o n s e q u e n t l y i n t h e i r t r i b o l o g i c a l p r o p e r t i e s . Polye t h y l e n e s (PE) a r e c l a s s i f i e d i n t o t h r e e c a t e g o r i e s a c c o r d i n g t o d e n s i t y : l o w , medium and h i g h . A f o u r t h t y p e is ultrahigh-molecularweight p o l y e t h y l e n e (UHMWPE) w i t h a m o l e c u l a r w e i g h t of o v e r 3 1 0 0 0 0 0 . Low-density PE (LDPE) i s v e r y tough and f l e x i b l e b u t d e
m o n s t r a t e s r e l a t i v e l y l o w h e a t r e s i s t a n c e ( T a b l e 2 . 1 2 ) . For b e a r i n g a p p l i c a t i o n s , it can be added t o o t h e r polymers t o improve t h e i r
--
t r i b o l o g i c a l p r o p e r t i e s . High-density PE ( H D P E ) i s more r i g i d and d e m o n s t r a t e s a d e q u a t e h i g h impact s t r e n g t h , e s p e c i a l l y a t l o w t e m p e r a t u r e s . I t is u s e f u l f o r b e a r i n g s f i l l e d w i t h v a r i o u s substances
(see Chapter 2 . 4 . 2 )
. Both
m a t e r i a l s demonstrate e x c e l l e n t i n e r t -
n e s s , n e a r - z e r o m o i s t u r e a b s o r p t i o n , good e l e c t r i c a l p r o p e r t i e s , a low c o e f f i c i e n t o f f r i c t i o n and e a s e of p r o c e s s i n g . TABLE 2.12
T Y P I C A L PROPERTIES OF POLYETHYLENES. POLYPROPYLENE AND POLYPHENYLENE OXIDE I
HDPE
I
UHMWPE
PP
H 0s TALE N , LUPOL E N , VESTOL EN A, ALATHON IOSTALEN PP 'ESTOLEN P, POLYETHYLENE. ENJAY, MARLEX, DYLAEI, T E N I T E , ALKATENE, BAYLON,HOSTALEN GUR, IOVOLEN 'ROPATHENE, SUPROLEN, 1000 RCH , M I L L 1 ON H I -2EX IARLEX PP, E N I T E PP, ARLONA
-2
0.910-0.925
4- 16
t i c i t y , MPa
3
4
0.941-0.965
0.928-0.941
20-40
27-40
90-800
20-1000
200-500
100-300
400-1200
140-750
NORY L
5 0.905
1.06-1.10
35
50-70
10-20
50-60
1100
2400-2600
I
23 TABLE 2.12
I
(continued) 2
1 Compressive s t r e n g t h , MPa Coefficient o f thermal ex an sion, 10- / K Thermal conducti v i t y , W/m.K S p e c i f i c heat, J/g.K Maximum o p e r a t i n g temperature of b e a r i n g , OC Surface f r e e energy, mJ/m Solubi 1 i t meter, l o ! J ~ ~ ! ~ i .!
y -
38-48
20-25
1
8
40
I
50
35
I
6-7
6-12
3.3-3
.a
0.35 2.4
30 33
Ultrahigh-molecular-weight PE has entirely different properties from those of conventional polyethylenes (ref. 4 2 ) . It is also available with a crosslinked structure - which is obtained by a process of irradiation - giving the material outstanding thermoplastic heat resistance and strength; UHMWPE is wear-resistant at a low coefficient of friction and chemical-resistant. The material can be satisfactorily used for the rubbing elements of artificial joints in the human body (ref. 11) Polypropylene (PP) is unsuitable for rubbing elements but its chemical and electrical properties can be put to good use in the mechanical parts of electrical devices. Specially stabilized PP can also be used for elements exposed to oxidative conditions and UV radiation. Polyphenylene oxide (PPO) is characterized by having the lowest water absorption of the engineering thermoplastics, outstanding dimensional stability and excellent mechanical and thermal properties (see Table 2 . 1 2 ) . PPO is resistant to acids, bases and detergents but the material is attacked by many halogenatedor aromatic hydrocarbons. PPO rubbing against other thermoplastics demonstrates good friction properties but its wear resistance is not high (refs. 4 3 ,
.
44).
The styrene group of thermoplastics (Table 2.13) is characterized by medium price, hardness, rigidity and thoughness and dimensional stability even at low temperatures. Materials such as ABS (Acrylonitryle-butadiene-styrene copolymer) and SAN (Styrene-acrylonitrile copolymer) have enough strength, impact resistance and w a r
24
resistance to be used for certain elements in bearings (mechanism frames). Styrene copolymer parts are practically unaffected by water, salts, most inorganic acids, food acids and alkalis, especially under stress. Polystyrene (PS) is rigid and brittle and has low heat resistance. It is soluble in most aromatic and chlorinated solvents but insoluble in alcohols such as methanol, ethanol, normal heptane and acetone. Only polystyrene which has been modified by, for example, silicone is interesting as a bearing material(see Chapter 2 . 4 . 2 ) . TABLE 2.13 TYPICAL PROPERTIES OF ABS,
SAN AN0 POLYSTYRENE
AB S
MATERIAL
IMPACT NOVOOUR, BAKELITE TYBRENO, AFCORYL.
PROPERTIES
3 1.01-1.05 mg/mm 40 s t r e n g t h , MPa elongation, % 5-20 of elasticity, MPa 2300 Compressive s t r e n g t h , MPa 50-60 C o e f f i c i e n t o f thermal 10-51~ expans i on, 5.3 Thermal c o n d u c t i v i t y , W/m.K 0.17 1.4 S p e c i f i c heat, J/g*K Maximum o p e r a t i n g temperature o f bearin 50 Surface f r e e energy, 2 mJ/m S o l u b i l i t y parameter lo3J0.5;1.S
Density, Tensile Tensi l e Modulus
8;
PS
SAN
TERLURAN, ABSON, A B S , CYCOLAC ABS, LUSTRAN, VESTOOUR, URTAL, RAVIKRAL
LURAN TYR'L SAN
HOSTYREN POLYSTYROL VESTYRON LUSTREX FOSTARENE 0 I STRENE
1.04
1.04-1.10
5-70
1.04-1.06 40-50 5-25
1.5-3.7
35-70 1 .O-2.3
I 40 0 -2400
2500-260 0
2800-3900
2800-4200
55-85
95-200
I .02-1.05
35-45
5.6
4.6
0.17 1.4
0.17
50
60-80
3.5-4.0
80-110 3-5
1.4 50
50
50
43
45
19
18
Fluoropolymers (Table 2 . 1 4 ) such as polytetrafluoroethylene [PTFE), fluorinated ethylene propylene (FEP) perfluoroalkoxyethylene (PFA), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), ethylene tetrafluoroethylene (ETFE) demonstrate high chemical inertness, high- and low-temperature stability, low friction (especially PTFE) and excellent electrical properties. Their resistance to wear and creep is low, but these characteristics can be
TABLE 2.14 TYPICAL PROPERTIES OF FLUOROPOLYMERS
MATERIAL
TEFLON HALON FLUON POLY FLUON THIOKOL TFE HOSTAFLON
PROPERTIES Density, mg/mm 3 T e n s i l e strength, MPa T e n s i l e elongation, % Modulus o f e l a s t i c i t y , MPa Compressive strength, MPa C o e f f i c i e n t o f thermal expansion,
PT FE
10-5/K
Thermal c o n d u c t i v i t y , W/m.K S p e c i f i c heat, J/g.K Maximum o p e r a t i n g temperature o f bearing, Surface free energy, mJ/m2 Solubi 1 i t y parameter, 103~~.5ni’*5
OC
FEP
2.13-2.30 23 300 350-700 8-12
2.12-2.17 20 300
5.5-8.4
4.6-5.8
0.26 1.05
0.21 1.1
50 19 13
-I
TEFLON FEF rEFLON PFA
2.12-2.17 28 300
3 50
45
6.7
0.27
KYNAR
1.75-1.78 35-50 100-300 1100 70 8.0-8.5
0.1-0.14
PCTFE
ETFE
KEL-F HALON HOSTAFLON C2 PLASKON PCTFE
TEFZEL
2.13 37 125 1300 30-50 4.8-15 (below and above 6OoC) 0.27 0.9 60 31 15.5
1 .70
45
275 830 5.2
0 -25 60
N Ln
26
improved by compounding t h e r e s i n s w i t h i n o r g a n i c f i b r e s , o r p a r t i c u l a t e materials (see C h a p t e r 2 . 4 . 2 )
.
H e a t - r e s i s t a n t polymers ( T a b l e 2.15) a r e a p p l i c a b l e f o r b e a r i n g elements o p e r a t i n g a t e l e v a t e d temperatures. Polyimide ( P I ) , polyamide-imide
( P A I ) and p o l y e t h e r i m i d e
( P E I ) a r e t h e most h e a t - and
f i r e - r e s i s t a n t polymers known. They a r e f o r m u l a t e d a s t h e r m o s e t s and t h e r m o p l a s t i c s . P o l y i m i d e s a r e a v a i l a b l e m o s t l y as l a m i n a t e s and s h a p e s , molded p a r t s , and s t o c k s h a p e . P o l y i m i d e p a r t s c a n o p e r a t e c o n t i n o u s l y i n a i r a t a b o u t 30U°C
but s e r v i c e temperature
f o r i n t e r m i t t e n t e x p o s u r e c a n r a n g e from c r y o g e n i c t o a s h i g h a s 50OoC. The m a t e r i a l i s w e a r - r e s i s t a n t ,
has a l o w c o e f f i c i e n t o f fric-
t i o n and i s i n a f f e c t e d by e x p o s u r e t o d i l u t e a c i d s , aromatic and a l i p h a t i c hydrocarbons, ester, e t h e r s , a l c o h o l s , Freons, h y d r a u l i c f l u i d s , f u e l s and k e r o s e n e : however, it i s a t t a c k e d by d i l u t e alkal i s and c o n c e n t r a t e d i n o r g a n i c a c i d s . Polyamide-imide r e s i n s c a n be molded i n t o complex, p r e c i s i o n p a r t s i n c o n v e n t i o n a l i n j e c t i o n molding machines. P A 1 h a s h i g h c r e e p - r e s i s t a n c e and d i m e n s i o n a l s t a b i l i t y even a t e l e v a t e d temper a t u r e s and l o a d e d . I t h a s good r a d i a t i o n and c h e m i c a l . r e s i s t a n c e ( e . g . a g a i n s t a l i p h a t i c and a r o m a t i c h y d r o c a r b o n s , h a l o g e n a t e d solv e n t s , a c i d s and b a s e s o l u t i o n s ) . I t i s a t t a c k e d , however, by h i g k - t e m p e r a t u r e c a u s t i c m a t e r i a l s , steam and some a c i d s . PA1 a b s o r b s m o i s t u r e : f o r example, a t 50% r e l a t i v e h u m i d i t y and 2U°C i t absorbs a b o u t 1%m o i s t u r e (by w e i g h t ) i n
1 0 0 0 h; molded PA1 p a r t s m u s t b e
d r i e d t o a c h i e v e maximum perfomance a t high t e m p e r a t u r e s ( r e f . 4 0 )
.
P A 1 h a s v e r y good t r i b o l o g i c a l p r o p e r t i e s .
( i n t r o d u c e d i n 1 9 8 2 ) i s plyetherimide ( P E I ) . P E I i s a n amorphous e n g i n e e r i n g t h e r m o p l a s t i c w i t h o u t s t a n b A r e l a t i v e l y new m a t e r i a l
i n g p r o c e s s a b i l i t y and e x c e l l e n t , s t a b l e e l e c t r i c a l p r o p e r t i e s over
a wide r a n g e o f t e m p e r a t u r e s and f r e q u e n c i e s . High h e a t and c r e e p r e s i s t a n c e , h i g h s t r e n g t h and a h i g h modulus o f e l a s t i c i t y a r e c h a r a c t e r i s t i c s o f t h i s m a t e r i a l . Molded o r e x t r u d e d p a r t s made o f PEI c a n b e machined u s i n g c o n v e n t i o n a l t e c h n i q u e s and c a n be bonded t o g e t h e r o r t o d i s s i m i l a r materials u s i n g u l t r a s o n i c , adhesive o r s o l v e n t methods. P E I i s r e s i s t a n t t o m i n s r a l a c i d s , a l i p h a t i c hydroc a r b o n s and a l c o h o l s , m i n e r a l - s a l t s o l u t i o n s , d i l u t e d bases a n d f u l l y h a l o g e n a t e d h y d r o c a r b o n s b u t i t i s a t t a c k e d by p a r t i a l l y hal o g e n a t e d s o l v e n t s s u c h a s methylene c h l o r i d e and t r i c h l o r o e t h a n e and by s t r o n g b a s e s . P o l y s u l f o n e s ( p o l y p h e n y l e n e s u l f o n e (PPSU) and p o l y e t h e r s u l f o n e (PESU)) o f f e r t h e h i g h e s t performance p r o f i l e s of a n y o f t h e t h m -
TABLE 2.15 TYPICAL PROPERTIES
O F HEAT-RESISTANT POLYMERS PPS TPX
RYTON
(R-4;
PPS + 40% GLASS FIBRES)
PROPERT I ES Dens i t y , m g / m 3 Tensile strength,
MPa
Tensile elongation,
0.83 30
20°C 25OoC
%
20% 25OoC Modulus o f e l a s t i c i t y ( f l e x u r a l ) , MPa
Compressive s t r e n g t h , MPa C o e f f i c i e n t o f thermal expansion, Thermal c o n d u c t i v i t y , W/m.K S p e c i f i c h e a t , J/g.K
15
1.67 120 1.25
PI
PA I
TORLON VESPEL (PI + 15% ;RAPHITE)
PE I -
PPS u
PESU
ULTEM
UDEL
V I CTREX
1.51 65 40
4.5
1.4 190 65
15
1.2
105 7-8
1.24 70
1.37 85
50-100
40-80
22 2ooc 25OoC
lo-'/,
Maximum a d m i s s i b l e b e a r i n q t e m p e r a t u r e , S o l u b i l i t y parameter, 103J0.5~7~1.5
1500
6.5
0.17
11700
145 2.2 0.3
11000 6200
4.7
5000 3500
2
0.9
500
3.1 0.2
2.18 OC
120
250 20.5
320
19.5
300
200
-
2500 (tensile modulus) 95 ' 3.1 0.26 1.5 ( a t 15OoC) 180
2700
0.13-0.1E
200
28 p l a s t i c s p r o c e s s a b l e on c o n v e n t i o n a l i n j e c t i o n and e x t r u s i o n mchine r y . The p o l y s u l f o n e p a r t s remain t r a n s p a r e n t a t s e r v i c e t e m p e r a t u -
res a s h i g h a s 230°C.
C o n t i n u o u s u s e i n a i r o r i n steam a t r a t e d
t e m p e r a t u r e s d o e s n o t c l o u d , c r a z e , or o t h e r w i s e d e s t r o y t h e i r t r a n s p a r e n c y . Thermal s t a b i l i t y and o x i d a t i o n r e s i s t a n c e a r e e x c e l l e n t a t s e r v i c e t e m p e r a t u r e s o f a b o u t 20OoC.
Creep of p o l y s u l f o n e s
i s e x c e p t i o n a l l y low a t e l e v a t e d t e m p e r a t u r e s and under c o n t i n o u s
-
load
c r e e p a t 1 2 O o C i s l e s s t h a n t h a t of p o l y a c e t a l o r h e a t - r e -
s i s t a n t ABS a t room t e m p e r a t u r e ( r e f . 4 0 ) . P o l y s u l f o n e s d e m o n s t r a t e h y d r o l i t i c s t a b i l i t y which, combined w i t h h e a t r e s i s t a n c e , r e s u l t s i n e x c e p t i o n a l r e s i s t a n c e t o b o i l i n g water and steam. An i m p o r t a n t d i s a d v a n t a g e o f p o l y s u l f o n e s , however, i s t h e i r a b s o r p t i o n o f ultrav i o l e t r a y s , which g i v e s them poor w e a t h e r r e s i s t a n c e . P o l y p h e n y l e n e s u l p h i d e (PPS) i s a v a i l a b l e i n v a r i o u s g l a s s f i b r e and m i n e r a l / g l a s s - f i b r e - r e i n f o r c e d g r a d e s f o r i n j e c t i o n o r compression molding and i n powder form f o r s l u r r y c o a t i n g , f l o c k i n g and e l e c t r o s t a t i c s p r a y i n g . O u t s t a n d i n g c h e m i c a l r e s i s t a n c e and high-temperature
and e x c e l l e n t d i m e n s i o n a l s t a b i l i t y w i t h a low a n d
p r e d i c t a b l e s h r i n k a g e r a t e make t h i s m a t e r i a l v e r y u s e f u l i n p r e c i s i o n e n g i n e e r i n g a p p l i c a t i o n s . The w e a r - r e s i s t a n c e and r e l a t i v e l y
low c o e f f i c i e n t of f r i c t i o n a r e s a t i s f a c t o r y ( r e f . 4 5 ) . Polymethyl p e n t e n e (PMP) d e m o n s t r a t e s good c h e m i c a l and h e a t r e s i s t a n c e ( u p t o 170°C and f o r s h o r t p e r i o d s up t o 2 2 O O C ) . I t i s t r a n s p a r e n t i n t h i c k s e c t i o n s b e c a u s e i t s c r y s t a l l i n e a n d amorphous p h a s e s have t h e same r e f r a c t i o n i n d e x . E l e m e n t s m a n u f a c t u r e d from PMP a r e h a r d and s h i n y . They s h o u l d n o t b e exposed t o l o n g t e r m U V radiation. 2.4.2.
FILLED POLYMERS
F i l l e d polymers a r e i n t e r e s t i n g b e c a u s e f i l l i n g improves t h e i r m e c h a n i c a l and t r i b o l o g i c a l p r o p e r t i e s . T h e r m o p l a s t i c and t h e r m o s e t polymers a r e f i l l e d w i t h r e i n f o r c i n g s u b s t a n c e s o r w i t h i n t e r n a l l u b r i c a n t s ( o r w i t h b o t h kinds of f i l l e r ) .
or b e a d s , m i n e r a l s o r carbon i s a v e r y e f f e c t i v e way t o s i g n i f i c a n t l y improve t h e i r m e c h a n i c a l and t h e r m a l p r o p e r t i e s . The t y p i c a l c o n t e n t o f r e i n f o r c i n g s u b s t a n c e i s 2 0 t o 4 0 % by w e i g h t . G l a s s f i b r e r e i n f o r c e m e n t generall y improves t h e m e c h a n i c a l p r o p e r t i e s o f p l a s t i c s by a f a c t o r o f two o r more ( r e f s . 4 0 , 4 6 ) . The g l a s s f i b r e improves t h e c r e e p - and w e a r - r e s i s t a n c e of p o l y m e r i c m a t e r i a l b u t g l a s s f i l l e r s i n c r e a s e R e i n f o r c i n g polymers w i t h g l a s s f i b r e
29
t h e wear o f t h e m a t i n g s u r f a c e and t h e c o e f f i c i e n t of f r i c t i o n . The l u b r i c a t i o n o f g l a s s - r e i n f o r c e d p l a s t i c s i s a v e r y e f f e c t i v e way t o o f f s e t t h e n e g a t i v e wear e f f e c t s a n d t o improve t h e i r t r i b o l o g i c a l p r o p e r t i e s (see Chapter 5 . 2 )
.
Carbon f i b r e p r o v i d e s t h e h i g h e s t s t r e n g t h modulusl heat-deflect i o n t e m p e r a t u r e , and c r e e p and f a t i g u e - e n d u r a n c e v a l u e s . The t h e r mal c o n d u c t i v i t y i n c r e a s e s and t r i b o l o g i c a l p r o p e r t i e s a r e s i g n i f i c a n t l y improved (low c o e f f i c i e n t o f f r i c t i o n ; s o f t e r with g l a s s
-
-
a s compared
c a r b o n f i b r e s have less e f f e c t on t h e wear o f t h e m a t -
i n g s u r f a c e ) . Another u s e f u l p r o p e r t y o f c a r b o n - f i b r e - r e i n f o r c e m e n t o f polymers i s t h e i r low volume r e s i s t i v i t y and s u r f a c e r e s i s t i v i t y . Carbon f i b r e s a l s o d i s s i p a t e s t a t i c c h a r g e e f f e c t i v e l y . The d i s a d vantage of t h e i r a p p l i c a t i o n i s t h e h i g h e r c o s t . T y p i c a l p r o p e r t i e s o f some g l a s s - r e i n f o r c e d t h e r m o p l a s t i c p o l y m e r s a r e l i s t e d i n Table 2.16.
The i m p o r t a n t e f f e c t s o f g l a s s f i b r e
a d d i t i o n a r e less water a b s o r p t i o n ( e s p e c i a l l y less mold s h r i n k a g e and t h e r m a l e x p a n s i o n .
h i g h f o r PA 6 ) ,
and
TABLE 2.16 TYPICAL PROPERTIES OF 30% GLASS FIBRE
MATE R I A L
-
REINFORCED THERMOPLASTICS
PA 6
POM h
1.37 150
1.63 140 2
PC
PBTP
AB S med i um impact
1.52 135 2-4
1.28 100 3-4
PROPERT 1 ES D e n s i t y , mg/mm’ Tensi le s t r e n g t h , MPa Tensile elongation, % F l e x u r a l modulus o f e l a s t i c i t y , MPa Compress i ve s t r e n g t h ,MPa C o e f f i c i e n t o f thermal expansion, 10-5/~ Thermal c o n d u c t i v i t y , W/m. K S p e c i f i c heat, J/g-K Maximum o p e r a t i n g ternper a t u r e o f b e a r i n g , OC
3-4 8000 150
1.43 130 4-6 8200 150
9500 120
9500 165
1.7
2.4
1.3
1.2
0.34 1.4
0.37 0.9
0.25
0.29
110
100
To improve t r i b o l o g i c a l p r o p e r t i e s
1 .6
1.45 125
3 8200
165 1.4
1.2
1.3 115
7500 100
PPSU
100
60
200
w e a r - r e s i s t a n t thermoplastics
:an a l s o b e i n t e r n a l l y l u b r i c a t e d i n a v a r i e t y of ways. The t y p i c a l and optimum l e v e l of l u b r i c a t i n g
f i l l e r v a r i e s depending o n t h e
t y p e o f f i l l e r and polymer, as f o l l o w s ( r e f . 4 0 ) : PTFE 1 5 - 2 U % ; s i l i c o n e 1 - 5 % ; PTFE/silicone 1 5 - 2 0 % ; g r a p h i t e 10%; MoS2 2-5%. L u b r i c a n t s c a n b e used w i t h o r w i t h o u t g l a s s - o r c a r b o n f i b r e r e i n f o r c e ments. PTFE ( o r sometimes P E ) p a r t i c l e s d i s p e r s e d i n t o a thermo-
30 p l a s t i c r e s i n g r e a t l y improve wear c h a r a c t e r i s t i c s and l e a d t o a d e c r e a s e i n t h e f r i c t i o n c o e f f i c i e n t and t e m p e r a t u r e o f t h e r u b b i n g s u r f a c e s . S i l i c o n e o i l d r o p s (2-10 w) d i s p e r s e d i n a b a s e polymer m i g r a t e t o t h e r u b b i n g s u r f a c e by d i f f u s i o n and b e c a u s e o f exclusion from t h e m a t r i x . The r e s u l t o f t h i s m i g r a t o r y a c t i o n i s t h e c o n t i nuous g e n e r a t i o n o f s i l i c o n e f i l m a s s u r i n g boundary o r mixed l u b r i c a t i o n . G r a p h i t e powder and MoS2 improve t r i b o l o g i c a l p r o p e r t i e s s i g n i f i c a n t l y l e s s t h a n P T F E / s i l i c o n e l u b r i c a n t s . An i m p o r t a n t a r e a of a p p l i c a t i o n f o r g r a p h i t e - l u b r i c a t e d
t h e r m o p l a s t i c s i s i n compo-
n e n t s o p e r a t i n g i n aqueous e n v i r o n m e n t s . S p e c i a l a t t e n t i o n must b e p a i d t o s u c h p o l y m e r i c m a t e r i a l s a s PTFE ( a n d o t h e r f l u o r o p o l y m e r s ) , PE and p o l y u r e t h a n e s ( P U R )
. The
use of t h e s e m a t e r i a l s i s i n e f f e c t i v e w i t h o u t f i l l i n g ( r e i n f o r c e ment) b e c a u s e o f h i g h wear r a t e s and p o o r m e c h a n i c a l p r o p e r t i e s . The e f f e c t o f v a r i o u s f i l l e r s on t h e wear r a t e o f PTFE i s p r e s e n t e d ( r e f . 4 7 ) . PTFE f i l l e d w i t h b r o n z e a n d g r a p h i t e (com-
i n Table 2.17
p o s i t e G l a c i e r DQ1) h a s a p a r t i c u l a r l y l o w wear r a t e . G r a p h i t e a s a f i l l e r ( G l a c i e r DQ2) i s u s e f u l f o r b e a r i n g e l e m e n t s o p e r a t i n g i n l i q u i d s which a r e c o r r o s i v e t o w a r d s b r o n z e . A r e c e n t i n n o v a t i o n i s t h e u s e o f polymer f i l l e r s (e.9. Rulon J c o m p o s i t e ) . Such compsites c a u s e less damage t o s o f t m e t a l s h a f t s ( e . g . b r a s s , aluminium) t h a n PTFE c o n t a i n i n g h a r d f i l l e r s ( e . g . g l a s s f i b r e , a s b e s t o s f i b r e ) AL c o m p o s i t e s
(refs. 4d-50).
(Pampus F l u o r o p l a s t ) a r e f i l l e d w i t h
g l a s s , b r o n z e o r c a r b o n . The d r y c o e f f i c i e n t o f f r i c t i o n o f f i l l e d PTFE r a n g e s from 0 . 1 t o 0.25 depending on t h e s l i d i n g c o n d i t i o n s ( r e f s . 5 0 - 5 2 ) . R e l a t i v e l y l a r g e r u n n i n g c l e a r a n c e s of t h e b e a r i n g s a r e r e q u i r e d because of t h e high thermal expansion c o e f f i c i e n t . TABLE 2.17 EFFECT OF VARIOUS FILLERS ON WEAR RATE OF PTFE
FILLER
CONTENTS
COMMENTS
% 1
2
3 7000-14000
Glass f i b r e
22.2
5.5
G l a s s f i b r e + MoS2 Carzon o r g r a p h i t e
12.2+2.3 26.3
5.0
23.8+2.3
4.0
Bronze
+
MoS2
....................
4.0
4 p=0.233 MPa ( f o r f i 1 l e d PTFE) , P= 4.7 N ( f o r unf i 1 l e d PTFE) ; v=0.75 m / s , Hardness o f m a t i n g s u r f a c e HRC 18-22
..............................................
31 T A B L E 2.17
(continued)
Dry wear o f most PTFE c o m p o s i t e s i s a s s i s t e d by t h e t r a n s f e r of w e a r d e b r i s t o t h e m a t i n g s u r f a c e . I n t h e p r e s e n c e o f w a t e r and most o t h e r l i q u i d s t h i s t r a n s f e r d o e s n o t t a k e p l a c e a n d wear r a t e s may i n c r e a s e ( r e f . 5 3 ) . C o n s e q u e n t l y s u c h materials are s u s c e p t i b l e t o f l u i d c o n t a m i n a t i o n . PTFE c o m p o s i t e s may a l s o e x h i b i t h i g h wear r a t e s under c o n d i t i o n s o f a b r a s i v e w e a r , when a b r a s i v e p a r t i c l e s a r e p r e s e n t and p r e v e n t t h e t r a n s f e r o f wear d e b r i s t o t h e m a t i n g surface. F i l l i n g HDPE w i t h r e l a t i v e l y h a r d m a t e r i a l s r e s u l t s i n less wear. The wear r a t e c o m p o s i t i o n depends on t h e k i n d of f i l l e r a n d properties
( s u c h a s d i s p e r s i t y ) , p e r c e n t a g e c o n t e n t and s u r f a c e
t r e a t m e n t ( r e f s . 5 1 , 5 4 ) . Some wear r a t e d a t a o f f i l l e d HDPE s l i d i n g a g a i n s t s t e e l s u r f a c e ( R = 0 . 6 3 pm) are l i s t e d i n T a b l e 2.18 a ( r e f . 5 1 ) . The optimum p a r t i c l e s i z e f o r i r o n f i l l e r i s 3 0 0 - 6 0 0 Qm; f o r A 1 2 0 3 it i s 2 0 p m . The maximum c o n t e n t o f f i l l e r s h o u l d b e no more t h a n 25% ( b y v o l u m e ) . T A B L E 2.18 WEAR I N T E N S I T Y
OF
HDPE C O M P O S I T I O N S RUBBING A G A I N S T S T E E L ( R a = 0.63 ,urn)
The u s e o f Pb30rl and CuO a s f i l l e r s f o r LDPE or HDPE i s v e r y e f f e c t i v e ( r e f . 4 7 ) . The e f f e c t o f t h e a d d i t i o n o f b o t h f i l l e r s
32
(Pb304 (CuO) is more effective than the addition of a single filler. Filling LDPE gives better results than filling HDPE. The main role of filler in this case is the formation of a transferred film of polymer on the counterface. Polyurethanes (PUR) can be filled with typical fillers (e.g. glass fibre: Urafil, Urasar by Fiberfil Inc.) and can also be modified by the introduction of active epoxide oligomers (ref. 55). The best addition was found to be the oligomer based on allilphenolformaldehyde resin. The modified polyurethane composites can best be used as friction materials, because of their high coefficient of friction (d.9-1.0). Fibre reinforced thermosetting resins offer improved properties as well as cost/performance benefits; such composite materials are strong, demonstrate dimensional stability awl are lighter than most metals. More than 9 5 % of reinforced thermoset parts are based on polyester and epoxy resins; others are phenolics and silicones (ref. 40). Phenolic composites have good tribological properties, particularly those incorporating solid lubricants (e.g. Railko PX) or impregnated with oil. The materials can be used in bearings or gears operating at low loads and sliding speeds, although they are more lolerant of abrasive and fluid contamination (refs. 4 8 , 5 0 ) . High temperature stable polymeric composites such as polyimides, e.g. Vespel or Devaplas, have good tribological properties at elevated temperatures. Materials reinforced with glass or carbon-fibre and containing PTFE as a lubricant, such as polyphenylene sulphide, polyvinylidene fluoride, polyetheretherketone (PEEK), polytetrafluorethylene, polyethersulfone and perfluoroalkoxy-modified tetrafluoroethylene are wear-resistant at high temperatures ( 200-3OO0C) (ref. 5 6 ) . Other polymeric materials with good tribological properties are those which contain grease or are impregnated with oil. Some of the commercially available materials of this kind are oil-filled polyamides (Nylasint, Oilon, Sovietic SAM 3 , SAM 4 andSAM 5 (ref. 57) containing mineral oil and other lubricants) and oil-filled polyacetals (Railko PV 80, SAY 6, SAM 7 ) . When rubbing against steel, these materials demonstrate a low coefficient of friction (ca. 3 . 2 ) and wear intensity (under 0.2 pm/km) when the temperature of the rubbing surfaces ranges from 0 to 12OoC (ref. 57).
33
3 , LUBR I CANTS 3,1, INTRODUCTION It is important to choose the right lubrication for miniature rubbing elements and this is not always straightforward. The requirements f o r instrument lubricants, particularly oils, are as follows : 1) Long-term durability, since fine mechanisms are usually for life lubricated, 2 ) High lubricity, resulting in low coefficient of friction and wear - usually at high specific loads up to 2 5 0 0 MPa and frequently l o w or very lob7 sliding speeds, 3 ) Ageing-resistance should be assisted by low-evaporation, chemical inertness (e.g. against epilame coatings and rubbing materials) and non-migration (high surface tension) from the operating or stored miniature couple during several years of use, since small or very small doses of oils (as little as g ) are introduced during the assembly process of rubbing elements, 4) Low variations of viscosity during ambient temperature changes over a wide range (e.g. from - 5 0 to +2OO0C), 5 ) Resistance to the effect of special environments: climate, gases, vacuum, microorganisms, radiation, industrial or polymer gases or vapours, dust, etc., 6 ) Fulfilling occasional special requirements such as high or very low electrical conductivity when applied to electrical instruments. Oils, greases and solid lubricants are used to lubricate the rubbing elements of instruments. Oils are the most satisfactory lubricants because of their ability to improve the tribologicalproperties of lubricated systems, but their use is sometimes limited by migration and also temperature or other special ambient conditions.
3,2,
OILS
Natural and synthetic oils are used to lubricate miniature elements. Natural oils are mineral-fatty (e.g. neat's-foot oil) oil blends or mineral oils containing many special additives (sometimes
34
as many as 3 0 0 (refs. 5a, 59)) to improve their properties. The traditional clock oils are usually based on mixtures of neat's-foot oil and mineral oil (Fig. 3.1, ref. 60).
NePt'S
0
20
40
- foot
Oil 60
80
100%
Mineral o d
F i g . 3 . l . M i x i n g m i n e r a l and f a t t y o i l s t o produce c l a s s i c c l o c k o i l s . 1 - nonspreading o i l s w i t h good l u b r i c a t i n g p r o p e r t i e s , 2 - o i l s d e m o n s t r a t i n g r e l a t i v e l y good l u b r i c i t y and ageing r e s i s t a n c e used f o r l u b r i c a t i o n o f s t e e l - m i n e r a l systems, 3 - o i l s w i t h good chemical s t a b i l i t y b u t r e l a t i v e l y low l u b r i c i t y , 4 o i Is d e m o n s t r a t i n g r e l a t i v e l y h i g h a g e i n g r e s i s t a n c e and medium l u b r i c i t y b u t h i g h spreading.
-
Oils 1 (Fig. 3.1) have relatively good chemical stability and lubricity but they spread easily. Oils 2 are sufficiently chemically stable, have better lubricity and less tendency to spread; they can be used for the lubrication of, e.g. steel-brass or steel-bronze pivot bearings. Oils 3 demonstrate a lower chemical stability but good lubricity and they are nonspreading. They are useful for lubricating steel-mineral systems. Oils 4 have high lubricity and are nonspreading but are chemically unstable and can be used to lubricate elements made of materials that do not havb a catalytic effect on o i l (e.9. minerals). Such oils contain many stabilizing additives, the most important being, antioxidants. Some commercially available, traditional clock oils are listed in Table 3.1. In general, the disadvantages of traditional clock oils are poor ageing-resistance, great viscosity variations as a function of temperature, narrow temperature range of application and rela-
TABLE 3.1 EXAMPLES OF TRADITIONAL CLOCK OILS
MANUFACTURER
'ROPE RT I ES
01 L
v i scos i t y , (20'~) 2 mm / s , (50OC) ~ pI p i c a t i on temperature range,
a t 100°C,
SORTE 1
100 30
- 18
OC
Pour p o i n t , OC T o t a l a c i d number (TAN), mg KOH/g S a p o n i f i c a t i o n number, mg KOH/g Surface t e n s i o n , mN/m o r s p r e a d i n g (brass, 24 h) ( r e l a t i v e l y ) Evaporation rate, % Age i ng r e s i s t a n c e Appl i c a t i o n ( m a t e r i a 1 s of 1 u b r i ca t e d elements)
- 5 days
VEB TECHNISCHE WACHSE, JENA ( G . D. R)
D r . TILLWICH GmbH, HORB-AHLDORF (F.R.G.)
3
SORTE
SORTE 5
U.S.S.R. MBP-12
-
~~
a
MOEBIUS ET FILS ALLSCHW I L (SWITZERLAND)
b - 16 h a t 105OC,
0.1 160 34.5
l.Sa
-32 0.1
I
1.4a
-
GOST 7934.1-74
1 2.3b 27d Steel, Jewels, Brass, A
2
1.5
80 -31 0.15
b
d
-20 0.70
- 15
50 -20
0.20
74
14d grass, A l , netal s
(4 h a t 5OoC),
61-70 19-22 -10
33 - 18
33.7
Yetals, minerals ( jewe 1 s ) c
80 -13 0.25 180
142
127 31 -15 80 -27 0.22 96
-
31 33 1 .Sb 2.1 9d S i n t e r e d M e t a l s , m i n e r a 1s
0.2c l e t a 1s , ni nerals
I V i s c o s i t y r i s e (%) a t Baader's t e s t , 95OC, Cu. 02, 840 h.
W
ln
36 t i v e l y h i g h e v a p o r a t i o n r a t e s . M o d i f i c a t i o n s such as d e c r e a s i n g t h e p o u r p o i n t o r t h e a d d i t i o n o f MoS2 o r g r a p h i t e t o l u b r i c a t e o f h i g h l y loaded r u b b i n g c o u p l e s a r e p o s s i b l e (Moebius o i l s 8032,8036 (8043) and 8034 (8042) r e s p e c t i v e l y ) . S p e c i a l l y r e f i n e d m i n e r a l o i l s c o n t a i n i n g many a d d i t i v e s a r e a l s o u s e d t o l u b r i c a t e m i n i a t u r e s y s t e m s . Such p a r a f f i n i c o i l s a r e s u f f i c i e n t l y chemically s t a b l e , b u t t h e i r l u b r i c i t y is r e l a t i v e l y poor and t h e y have a s t r o n g t e n d e n c y t o s p r e a d . The i n s t r u m e n t mine r a l o i l s c a n b e a p p l i e d t o t h e l u b r i c a t i o n of i n s t r u m e n t s o p e r a t i n g a t low t e m p e r a t u r e s b u t t h e o p e r a t i o n t e m p e r a t u r e s h o u l d n o t v a r y much b e c a u s e of g r e a t v i s c o s i t y c h a n g e s . The p r o p e r t i e s o f s e v e r a l i n s t r u m e n t m i n e r a l o i l s are p r e s e n t e d i n T a b l e 3 . 2 .
Theoils
D-2 and D-5 a r e from t h e M i c r o g l i s s g r o u p ( b y Moebius e t F i l s ) developed e s p e c i a l l y f o r h i g h p r e s s u r e c o n t a c t s . They c o n t a i n e x t r e m e
p r e s s u r e (E.P.)
and w e a r - r e d u c i n g a d d i t i v e s b a s e d on a molybdenum
o r g a n i c compound c o m p l e t e l y sol.uhle i n m i n e r a l o i l s . They h a v e i m proved o x i d a t i o n s t a b i l i t y as compared w i t h t h e t r a d i t i o n a l c l o c k o i l s . The o i l s o f g r o u p D a r e a p p l i e d w i t h a n o i l e r , w h i l e t h e o i l s g r o u p L a r e s u i t a b l e f o r immersion p r o c e d u r e s . The o i l s of g r o u p L contain an a d d i t i o n a l anticorrosive agent. Pressure r e s i s t a n c e ( t e s t e d using a f i v e - b a l l apparatus) f o r t h e Microgliss group of oils D
+
L i s 230-350 N as compared w i t h 80-250 N f o r t r a d i t i o n a l
clock o i l s ( r e f . 6 1 ) . S y n t h e t i c o i l s a r e v e r y i n t e r e s t i n g l u b r i c a n t s w i t h many a p p l i c a t i o n s i n p r e c i s i o n e n g i n e e r i n g . The main a d v a n t a g e of them i s t h e i r h i g h c h e m i c a l s t a b i l i t y , b u t t h e i r l u b r i c i t y is poor a n d t h e y g e n e r a l l y have a s t r o n g t e n d e n c y t o s p r e a d ( e s p e c i a l l y t h e o i l s b a s e d on p o l y s i l o x a n e s ) . The main g r o u p s of s y n t h e t i c i n s t r u m e n t o i l s a r e t h o s e b a s e d on e s t e r s , p o l y e t h e r s ( e v e n t u a l l y w i t h f r e e a l c o h o l g r o u p s ) , p o l y g l y c o l s and p o l y s i l o x a n e s . Ester-based instrument o i l s generally demonstrate r e l a t i v e l y low c h e m i c a l s t a b i l i t y ( a s compared w i t h o t h e r s y n t h e t i c o i l s ) and t h e y c o n t a i n s t a b i l i z e r s . The main a d v a n t a g e of such o i l s i s t h a t t h e y have good l u b r i c i t y and a r e n o n s p r e a d i n g . The f i r s t s y n t h e t i c i n s t r u m e n t o i l i n t r o d u c e d i n 1928/1929 w a s b a s e d on o r g a n i c phosphate (tricresylphosphate).
T h i s o i l , w i t h good l u b r i c i t y and h i g h
s u r f a c e t e n s i o n , decomposes i n t h e p r e s e n c e o f m o i s t u r e . F r e e phosphate a c i d is a very corrosive agent, attacking steel ( i r o n ) surf a c e s i n p a r t i c u l a r . T h i s kind o f o i l is s u b j e c t t o v i s c o s i t y changes d u e t o t e m p e r a t u r e v a r i a t i o n s ( i n t h e same way a s t r a d i t i o n a l c l o c k o i l s ) . O t h e r e s t e r - b a s e d o i l s a r e more c h e m i c a l l y
TABLE 3.2 INSTRUMENT MINERAL OILS
B MANUFACTURER
OIL
PROPERTIES
v i scos i t y , 2 mm / s ,
(20'~)
(5OoC) ~ p p l i c a t i o nt e m p e r a t u r e range,
I
EXXON
ESSO AVIATION INSTRUMENTAL OIL
MOEB I US ET FILS, ALLSCHW I L
D-2
D-5
(SWITZERLAND)
11
TEY $ ::E
J ENA (G.D.R.)
POLAND, U.S.S.R.
MWP
10.8
6.3-8.5
(3OoC) OC
-50
-50 100
80
Pour p o i n t , O C T o t a l a c i d number (TAN), mg KOH/g Saponi f i c a t i on numbe r, mg KOH/g S u r f a c e t e n s i o n , mN/m Evaporation rate, %
- 57
-32
-18
-60
0.05 20.7 32.1 5 days
19.6 34.5
23.8
a t 100°C
0.83
-- 42
--
s t a b l e and have good t r i b o l o g i c a l p r o p e r t i e s . Some of t h e ester-based i n s t r u m e n t o i l s a r e l i s t e d i n T a b l e 3.3. The o i l S i l b e r C is a p p l i c a b l e t o t h e l u b r i c a t i o n of m i n e r a l e l e m e n t s , b u t S i l v e r B can b e u s e d f o r l u b r i c a t i n g metals ( s t e e l , b r a s s , A l - a l l o y s ,
sintered
m e t a l s ) and m i n e r a l s b u t n o t p o l y m e r i c e l e m e n t s . The o i l s b a s e d upon U . S .
M i l i t a r y S p e c i f i c a t i o n MIL-L-GO85
A a r e e s p e c i a l l y useful
f o r l u b r i c a t i n g miniature b a l l bearings. E t h e r o i l s a r e a g e i n g - r e s i s t a n t b u t t h e i r p r o p e r t i e s change a f t e r long-term e x p o s u r e t o s u n l i g h t . Rubbing e l e m e n t s l u b r i c a t e d w i t h e t h e r o i l s must n o t be h i g h l y l o a d e d . The o i l s have a v e r y low e v a p o r a t i o n r a t e and a r e n o n s p r e a d i n g . Examples o f s u c h o i l s a r e E l g i n M56 a and Iy56 b , w i t h viscosities a t 2OoC of 9 0 and 120 mm 2 / s 0
r e s p e c t i v e l y and a -30 C pour p o i n t . The maximum o p e r a t i n g temperat u r e however i s 80-100°C. -resistant.
P o l y e t h e r o i l s are o x i d a t i o n - and h e a t -
T h e i r v i s c o s i t y c h a r a c t e r i s t i c s as a f u n c t i o n of t e m -
p e r a t u r e are s i m i l a r t o t h o s e o f m i n e r a l o i l s . T h e i r e v a p o r a t i o n
r a t e s a r e l o w b u t s u c h o i l s have h i g h p o u r p o i n t s , i n t h e r e g i o n of -3OOC. Polyphenylether o i l s demonstrate e s p e c i a l l y high thermal s t a b i l i t y , h i g h o x i d a t i o n and r a d i a t i o n r e s i s t a n c e , c h e m i c a l i n t e r t n e s s and r e l a t i v e l y good l u b r i c i t y ( r e f s . 62-64). F l u o r i n a t e d 2 p o l y a l k y l e t h e r w i t h a v i s c o s i t y of 15 mm / s a t 4OoC c a n r e s i s t gamma-ray r a d i a t i o n 0 . 8 MeV a t a d o s e o f 1 0 0 J ( r e f . 64). Fluorinated p o l y e t h e r o i l s have been s a t i s E a c t o r i l y a p p l i e d t o t h e l u b r i c a t i o n of s m a l l motor b e a r i n g s o p e r a t i n g a t a t e m p e r a t u r e o f a b o u t 15U°C f o r s e v e r a l thousand h o u r s ( r e f . 65). The f l u o r i n a t e d p o l y e t h e r
o i l s Krytox 143 AB and K r y t o x 143 AC (Du P o n t p r o d u c t s ) have a 2 v i s c o s i t y of 85 and 270 mm / s ( a t 33OC) r e s p e c t i v e l y . They c a n b e u s e d i n t h e t e m p e r a t u r e r a n g e s -40 - +232OC and -30
-
+288OC re-
s p e c t i v e l y and a r e non-f lammable. P o l y a l k y l g l y c o l e s ( r e f . 66) w i t h e t h e r and a l c o h o l g r o u p s (alkylaryloxydibutyleneglycoles) a r e u n s a p o n i f i a b l e and d e m o n s t r a t e e x c e l l e n t c h e m i c a l s t a b i l i t y . They a r e used a s b a s e m a t e r i a l i n t h e Moebius Synta-A-Lube
s e r i e s of i n s t r u m e n t o i l s . They c o n t a i n
1 . 0 8 - 1 . 8 % of a h i g h e r m o l e c u l a r
synthetic carboxylic acid with
e t h e r t o improve t h e l u b r i c i t y . O t h e r a d d i t i v e s s u c h as a n t i o x i d a n t s , a c o p p e r d e a c t i v a t o r and p r e s s u r e r e s i s t a n c e EP a d d i t i v e g u a r a n t e e t h e i r u s e f u l n e s s . The o t h e r i m p o r t a n t a d v a n t a g e s o f s u c h o i l s a r e r e m a r k a b l e a d h e r e n c e ( n o n s p r e a d i n g ) and e x t r e m e l y low e v a p o r a t i o n r a t e s . The m o s t r e c e n t f o r m u l a o f t h e Synta-?--Lube series of o i l s a l s o a s s u r e s good t r i n o l o g i c a l p r o p e r t i e s i n m i n i a -
TABLE 3.3 ESTER
-
BASED INSTRUMENT OILS
MANUFACTURER
OIL
O r . T I LLW I CH GmbH, HORB-AHLDORF (F.R.G.)
SILBER
PROPERTIES
v i scos i t y , (20'~) 2 mm / s , (50°C) A p p l i c a t i o n temperature range,
c
I
-5 80
-15 0.27
198 0.3 30
I
ESSO
AEROSHELL FLUID 12?
UNlVlS P l p
NUODEX I N C . PISCATAWAY, NEW JERSEY
ANDEROL L401
26
27
24
-50 120
-60 120
-54
90
-44
-57
75
33 (40°C)
29
Pour p o i n t , OC T o t a l a c i d number (TAN), mg KOH/g S a p o n i f i c a t i o n number, mg KOH/g E v a p o r a t i o n r a t e , % (16 h a t 105OC) Ageing r e s i s t a n c e ( v i s c o s i t y r i s e , %) (Baader's t e s t 95OC, Cu, 02, 840 h )
SILBER B
110
99 OC
VEL*
SHELL
19
-35
-20 100 -35 0.30 173 1.8
149 -63
0.25 225 1.9
3
2.2
I
* VEL
o i l demonstrates e s p e c i a l l y h i g h s u r f a c e t e n s i o n (33.3 mN/rn)
,
a - MIL-L-6085A
w W
40 ture systems operating in humid atmospheres (see Chapter 6.5. , Fig. 6.ld). The typical properties of the Moebius synthetic oils are listed in Table 3.4. TABLE 3.4 MOEBIUS ET FILS SERIES OF SYNTHETIC OILS WITH ETHER AN0 ALCOHOL GROUPS
OIL
GF-pxSYNTA-A-LUBE
PROPERT I E S
Dens i t y , mg/mrn 3 v i scos i t y , rnm2/s
0.895
0.888
90 20 SYNTA v I sco LUBE 0.920
9030 S Y NTA FR I GO LUBE
0.908
:
11 - 5 31 - 5 150 625 3800
8OoC 50OC 20oc ooc - 20%
8.9 24
-
-3OOC -4OOC
Maximum o p e r a t i n g , t e m p e r a t u r e , Pour p o i n t , O C S a p o n i f i c a t i o n number, nig KOH/g S u r f a c e t e n s i o n a t 25OC, mN/m Contact angle, 0 : o n ruby on steel Evaporation rate, % (5 days a t 100OC)
OC
266 1450 13500
98 390 2000 5800
-
70
70 -40
14.5
45
6.5 16
58
-
80 -40 0.8 34.8
180 380 2250 9000
60
0.8
-43 0.8
33.8
33.0
22-25 15-19
22-25 15-18
24-29 20-24
14-17
0.5
0.5
0.4
0.5
-50 0.9 32.7 19-23
Synthetic oil based on polysiloxanes are very useful as lubricants because of their high chemical stability, heat resistance, wide range of operating temperature and low evaporation rate. The disadvantages of such oils are low surface tension (spreading) and poor lubricity. The basic chemical formula of polysiloxanes is as f01lows :
CH3
R
CH3
CH 3 I
I
- Cf13 - Si I
CH3
R
CH3
By taking various n numbers, the length of the macromolecule chain can be controlled and as a result the viscosity can be changed without changing the chemical structure of the substance. When R groups are CH3 or phenyl C6H5 radicals, dimethylpolysiloxane and
41 methylphenylpolysiloxane are obtained respectively. In methylalkylpolysiloxane the group R is replaced with the group CHJ
.
I
I
- 0 - S i -
O - S i -
I
I
R
(CH2)
I CH3
The fluorinated CH3 I
O - S i -
CH3 I
or chlorinated
- Si I
I
C2H4
CH 2C1
I
CF3
polysiloxanes demonstrate better tribological and anti-migration properties than dimethyl-, methyl-, phenyl- or methylalkylpolysiloxane (refs. 6 7 - 6 9 ) . Polymeric materials in particular can be very effectively lubricated with polysiloxanes (ref. 70). Several instrument oils based on polysiloxanes are listed in Table 3.5. Mineral base oils containing dispersed polymer particles are also of interest. The clock oil XU 4 3 0 (see Table 3 . 1 ) with PTFE particles ( < 5 um, specific surface 2000 m 2 /kg) and fluorinated alkenylalkylether as dispersal agent, demonstrated very good tribological properties when used to lubricate a steel-diamond tribological system operating in a vacuum 4 Pa (ambient temperature 10°C) (ref. 1 7 ) . We must include among the special lubricating liquids those natural substances which demonstrate extremely high lubricity, reduce effectively friction and wear; they are useful when the lifetime of a tribological system is short and loads are very high. Prostheses are lubricated with synovial fluid, which also demonstrates excellent lubricity in typical metal, mineral and polymeric tribological systems (refs. 11, 71). Another natural lubricant is formed by the microbes of Bacillus mucilaginosus; it is a high-polymeric substance containing 95% polysaccharides and 5 % protein, used e.g. to prepare a surface for calibration of glass vessels. This sticky, dark coloured liquid density 0.976-0.985 mg/mm3 in-
-
TABLE 3.5 INSTRUMENT OILS BASE0 ON POLYSILOXANES
MANUFACTUREd
I
O r . TlLLWlCH GrnbH, HORB-AHLOORF (F.R.G.)
SILICONUL n PROPERT I ES
I
SILICONDL V500
I
I
I
OOW CORNING
I
GENERAL
I KLUBER
I
MUNCHEN (F. R . G)
U.S.S.R.
(UN I S I L )
510
2 V i s c o s i t y , mm / s :
5OoC
82
20oc ooc -6OOC Pour p o i n t , OC A p p l i c a t i o n t e m p e r a t u r e range, OC
Evaporation rate, %
368
52 (38OC) 50 (25OC
46 5
I
1026
2
-6 7 -49 180 180 a t 105OC, 16h
0.1
0.2
-73 -68 204
300
I
1
-70 upon GOST 7934.1 a t maximum operating temperature d u r i n g
’’
I
4.5
I
3.5
1
2.2
43 creased the maximum admissible load of a metallic steel-steel tribological system by 3-4 times as compared with the best of the normal lubricants (ref. 72). The properties of the base oil are improved by the use of special additives. They are soluble o r uniformly dispersed throughout the carrier medium. The most important are antioxidants, which minimize the formation of resins, varnishlacidsf sludge and polymers. These are substances such as phenols, amines, organic sulphides, zinc dithiophosphate. The friction modifiers (fatty acids, fatty amines, fats) and anti-wear additives (zinc dialkyldithiophosphate, tricresyl phosphate) are very important for improving the lubricity of an oil. Oil-soluble fluorinated telomers,especially fatty acids and their amine salts, are very effective as anti-wear additives, forming a reaction film containing iron fluoride (ref. 73). The viscosity index improvers such as polyisobutylenes, polymethacrylate and polyacrylates, and extreme pressure additives such as sulphurized fats, olefins, chlorinated hydrocarbons, lead salts of organic acids, and amine phosphates also improve the oil. The friction reducers improve boundary lubrication and oil-film retention (even after draining); they also reduce frictional losses (ref. 7 4 ) . The complex soluble molybdenum compounds are very useful for improving ageing- and wear-resistance and can also act as extreme pressure additives (ref. 75). These additives, used instead of MoS2 in the Moebius instrument oils of Microgliss group D + L based on stabilized mineral oils, ensure very good load carrying qualities in the oil. For the best effect, other additives such as corrosion inhibitors (zinc dithiophosphates, sulphurized terpenes, phosphosulphurized terpenes, sulphurized olefins), rust inhibitors (amine phosphates, fatty acids etc.), and dispersants to prevent and retard sludge formation, must be added very carefully, bearing in mind which additives have already been put in (refs. 6 2 , 76). Instrument oils are usually very complex substances and to produce them requires a great deal of experience; there are only a few specialized producers, and these are mostly small firms. Instrument oils are normally used for the lubrication of metal and mineral rubbing elements, but special oils are manufactured for the lubrication of polymeric elements. They demonstrate high oxidation stability and are chemically inert to almost all the thermoplastic polymers commonly used as materials for miniature rubbing elements (ref. 7 ’ 7 ) . These synthetic oils are based on e.g. alkylaryloxybuthyleneglycols (Moebius oils), partially fluorinated poly-
44
e t h e r s o r p o l y s i l o x a n e a l c o h o l ( D r . T i l l w i c h GmbH o i l s ) . The p r o p e r t i e s o f some o f t h e s e o i l s a r e l i s t e d i n T a b l e 3.6.
The a p p l i c a t i o n
o f a n t i - m i g r a t i o n c o a t i n g s ( e p i l a m e s ) i s a d v i s a b l e when t h e s e o i l s a r e t o b e used (see C h a p t e r 6 . 2 ) . F i x o d r o p KlLI
Examples o f s u c h e p i l a m e s a r e
(Moebius) a n d A n t i s p r e a d M2/20U
(Dr.
T i l l w i c h GmbH).
The o i l f o r t h e l u b r i c a t i o n o f a m i n i a t u r e s y s t e m o p e r a t i n g u n d e r e x t r e m e c o n d i t i o n s must b e c h o s e n c a r e f u l l y ( s e e a l s o C h a p t e r 6 . 5 ) . When t h e t e m p e r a t u r e i s v e r y low, s a y below -2OoC,
t h e most i m p r t a n t
factor which s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n i s t h e rise i n v i s c o s i t y o f t h e o i l w i t h d e c r e a s i n g t e m p e r a t u r e . The o i l s b a s e d on p o l y s i l o x a n e s a r e a p p l i c a b l e a t low t e m p e r a t u r e s (see T a b l e 3 . 5 ) , and s o a r e some e s t e r - b a s e d and m i n e r a l o i l s ( T a b l e s 3.3 a n d 3 . 2 ) . Compounds s u c h a s RCOOCaHl1
and RCH2OCOC8Hl1
i n t h e molecule, demonstrate little m
having r a d i c a l s R as
e i n v i s c o s i t y as a f u n c t i o n
o f t e m p e r a t u r e , a n d t h e y h a v e a v e r y low p o u r p o i n t ( f r o m - 7 3
to
-5OOC) and r e l a t i v e l y low v i s c o s i t y ( r e f . 7 6 ) . The compound w i t h t h e summarized f o r m u l a C 1 5 H 2 6 0 2
h a s a p o u r p o i n t o f -66OC
r a t i o o f t h e v i s c o s i t i e s a t -40°C
and 100°C
and a
n o t h i g h e r t h a n 55.
T h e s e compounds seem v e r y p r o m i s i n g as b a s e l i q u i d s f o r low-temper a t u r e i n s t r u m e n t o i l s . O i l s o b t a i n e d by t h e r e a c t i o n o f a - o l e f i n s C6 - C l0
o r t h e i r mixture with c y c l i c aromatic
h y d r o c a r b o n s C6-Cl0
o r t h e i r m i x t u r e w i t h h y d r o c a r b o n s C6-Cl0
having a l k y l g r o u p s , have 2 a pour p o i n t of a b o u t -5lOC and a v i s c o s i t y h i g h e r t h a n 5 mm / s a t 9 9 O C and l o w e r t h a n
2 8 0 0 0 mm / s a t -4OOC
(ref. 79).
High t e m p e r a t u r e i n s t r u m e n t o i l s m u s t b e e f f e c t i v e f o r l u b r i c a t i n g m i n i a t u r e s y s t e m s when t h e o p e r a t i n g t e m p e r a t u r e i s h i g h e r t h a n 150OC. T h i s r e q u i r e m e n t c a n b e f u l f i l l e d b y o i l s b a s e d o n p l y s i l o x a n e s and p o l y e t h e r s . Some o i l s b a s e d o n p o l y s i l o x a n e s c a n be
used when t h e t e m p e r a t u r e r e a c h e s 230-250°C
( s e e T a b l e 3 . 5 ) . The
p r i n c i p a l problem w i t h s u c h o i l s i s f i n d i n g a d d i t i v e s t o s t a b i l i z e them a t h i g h t e m p e r a t u r e s , w h e r e c l a s s i c a l a n t i o x i d a n t s s u c h a s a l k y l p h e n o l s do n o t work. The t h e r m a l l y s t a b l e a d d i t i v e s (e.g. mtal-silicone-organic
compounds) w e r e found t o improve t h e o x i d a t i o n 0
r e s i s t a n c e of p o l y s i l o x a n e s a t t e m p e r a t u r e s o v e r 2 0 0 C o v e r lotimes ( r e f . 8 0 ) . Such a d d i t i v e s a r e u s e d i n t h e S o v i e t o i l s l i s t e d i n T a b l e 3.5.
F l u o r i n a t e d o i l s s u c h a s V e r s i l u b e F 5 0 , FS 1265 ( T a b l e 3 . 5 ) ,
K 7132 (Table 3.6) are useful lubricants because of their g o d l u b r i c i t y even a t
TABLE 3.6 SPECIAL INSTRUMENT OILS FOR LUBRICATION
OF POLYMERIC MINIATURE SYSTEMS
OIL
MANUFACTURER
PROPERT I E S Viscosity,
2 mm / s ,
I
OOC
20oc
I
4OoC Pour p o i n t , OC A p p l i c a t i o n t e m p e r a t u r e range,
OC
S u r f a c e t e n s i o n , mN/m Evaporation rate, % Ageing r e s i s t a n c e (Baader's t e s t D I N 51554), %
K 7132 mv
K 7132
K 2363
10000
K 2363 21000
O r . TlLLWlCH GmbH, HORB-AHLDORF
1250 434 123 (50°C)
49776 10328 4890
820 510
350
K 4563 2400
1
(F.R.G.)
24000 20700 14100
9015
4200 2400 1500
9024
MOEBIUS ET FILS, ALLSCHWIL (SWITZERLAND)
1450 266
625 150
31 (at
-44
- 26
-40
-20
200
200 28
-41 -30 27 120 120 20.6 20.9 a t 1050C. 16h -33
-
-62 -40 120 21
9027
-40 - 29 70
45 5OoC)
7600 1040 130
-40
33.8
-20
-18
-7
90
80
34.8
35.5
a t 1OOOC, 5 days
0.31
0.5
1
2
1 1.'" I
( a f t e r 12 days)
0.4 2
46 high temperatures. Polyether o i l s a r e t h e m o s t s t a b l e a t higher t e m p e r a t u r e s b u t t h e aforementioned c o n s t r a i n t s on t h e i r u s e s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n . The e l e m e n t s o f m i n i a t u r e s y s t e m s l u bricated with these high temperature o i l s should be epilamized with f l u r o p o l y m e r - b a s e d e p i l a m e s (see C h a p t e r 6 . 2 ) . I t i s p r a c t i c a l l y i m p o s s i b l e t o u s e t h e above-mentioned i n s t r u -
ment o i l s when t h e o p e r a t i n g t e m p e r a t u r e i s o v e r 25OoC. I t i s poss i b l e t h a t t h e problem o f t h e o i l l u b r i c a t i o n o f m i n i a t u r e s y s t e m s working a t h i g h e r t e m p e r a t u r e s may b e s o l v e d by u s i n g s y n t h e t i c some f l u o r i n a t e d esters
s i l a h y d r o c a r b o n s t h e r m a l l y s t a b l e t o 37OoC,
( s t a b l e t o 30OoC) , p a r t i c u l a r l y esters p r o d u c e d u s i n g f l u o r i n a t e d a l c o h o l s and t r i e t h y l a c e t i c a c i d , f l u o r i n a t e d p o l y a l k y l e t h e r s , o r f l u o r t r i a z i n e compounds t h e r m a l l y s t a b l e t o 34OoC ( r e f . 8 1 ) . A l k y l n a p h t h a l e n e s a r e p r o m i s i n g components f o r h i g h t e m p e r a t u r e o i l s b e c a u s e t h e y h a v e h i g h c h e m i c a l s t a b i l i t y and g e n e r a l l y improve o i l s i n u s e under extreme c o n d i t i o n s ( r e f . 8 2 ) . O i l s o p e r a t i n g i n a vacuum s h o u l d h a v e a v e r y low e v a p o r a t i o n
r a t e . The p o l y e t h e r o i l s a r e o f most i n t e r e s t i n t h i s r e s p e c t , b u t o i l s based on p o l y s i l o x a n e s o r esters a r e a l s o u s e f u l ( r e f s . 9 , 83, 84, 85; see a l s o C h a p t e r 6 . 5 ) . The o i l t4n-605 l i s t e d i n T a b l e 3 . 5 c a n b e u s e d t o l u b r i c a t e m i n i a t u r e s y s t e m s o p e r a t i n g i n a vacuum P a ) a n d low o r h i g h t e m p e r a t u r e c o n d i t i o n s .
( t o 1.33
A e o r s h e l l F l u i d 1 2 and A n d e r o l 4 0 1 D ( T a b l e 3 . 3 ) , m e e t i n g t h e MIL-L-6085
A s t a n d a r d , are s u i t a b l e f o r t h e l u b r i c a t i o n o f s p a c e
i n s t r u m e n t a t i o n . The i n s t r u m e n t o i l "Gold" ( D r . T i l l w i c h GmbH Horb-Ahldorf
,
F .R.G.)
has s i m i l a r properties
t e m p e r a t u r e r a n g e of - 7 0 ,
, with
,
an application
+8OoC, and i s u s e f u l f o r t h e l u b r i c a t i o n
of m e t a l , m i n e r a l and p o l y m e r i c
( e x c e p t ABS) m i n i a t u r e r u b b i n g el-
m e n t s . The a f o r e m e n t i o n e d XU 430 o i l f i l l e d w i t h PTFE p a r t i c l e s has been used e f f e c t i v e l y i n s p a c e a s a l u b r i c a n t i n a steel-diamond t r i b o l o g i c a l system i n a F o u r i e r ' s i n t e r f e r o m e t e r
(refs. 17, 86).
O i l which w i l l b e e x p o s e d t o r a d i o a c t i v i t y m u s t b e c a r e f u l l y
c h o s e n . The p o l y e t h e r - b a s e d o i l s a r e most s u i t a b l e when gamma-radia t i o n a n d h i g h t e m p e r a t u r e s o c c u r , f o r example i n t h e i n s t r u m e n t a t i o n of n u c l e a r i n s t a l l a t i o n s
( r e f s . 9 , 63, 6 4 ) . Such o i l s a r e a l s o
s a f e when e x p l o s i v e s u b s t a n c e s a r e p r e s e n t
( f o r example i n oxygen
h a n d l i n g pumps). I n t r o p i c a l climates w h e r e m i c r o o r g a n i s m s a r e p r e s e n t a n d t h e h u m i d i t y is h i g h , t h e u s e of h u m i d i t y - i n e r t o i l s s u c h a s t h e new Moebius Synta-A-Lube
9 0 1 0 , 9 0 2 0 , 9030 ( s e e T a b l e 3 . 4 )
T i l l w i c h GmbH K 4563 o i l ( s e e T a b l e 3 . 6 )
,
or Dr.
i s recommended. The MCT
47 and MPT s e r i e s o f o i l s from t h e U.S.S.R. 6.5)
,
( r e f . 8 7 , see a l s o C h a p t e r
c o n t a i n a c t i v e f u n g i c i d e s making them r e s i s t a n t a g a i n s t m i c r o -
o r g a n i s m s . F o r s y n t h e t i c o i l s u s e d i n c o n d i t i o n s of h i g h h u m i d i t y o r i n t h e p r e s e n c e of w a t e r , t h e h y d r o l y s i s o f complex esters a n d a d d i t t i v e s i s a s e r i o u s t h r e a t ( r e f . 8 8 ) , a s it d e c r e a s e s t h e a n t i - f r i c t i o n and a n t i - w e a r a c t i o n of t h e o i l s . The o i l s u s e d f o r t h e l u b r i c a t i o n o f e l e c t r i c a l c o n t a c t s , s u c h
as c o n n e c t o r s , s w i t c h e s or p r i n t e d c i r c u i t b o a r d s (see C h a p t e r 9 . 8 ) must d e m o n s t r a t e e x c e p t i o n a l l y good l u b r i c i t y , h i g h o x i d a t i o n - r e s i s t a n c e , non-migration
from t h e c o n t a c t s and a l s o e l e c t r i c a l re-
s i s t i v i t y higher than 1 5 M Q . m .
O i l c a n n o t improve t h e r e s i s t a n c e
of c o n t a c t s ( r e f s . 8 9 - 9 1 ) . The o i l s h o u l d n o t p o l y m e r i z e b e c a u s e o f t h e c a t a l y t i c e f f e c t of m e t a l s ,
i n p a r t i c u l a r p l a t i n u m . When p o l y -
m e r i c materials a r e used, t h e o i l c a n n o t p l a s t i c i z e t h e material. The p o l y e t h e r o i l s s u c h as 0s-124
(by Monsanto) w e r e f o u n d t o a t -
t e n u a t e t h e r e s i s t a n c e of p a l l a d i u m c o n t a c t s and t h e y c a n o p e r a t e i n a c o r r o s i v e atmosphere ( r e f s . 6 4 , 92).The s p e c i a l s y n t h e t i c o i l s f o r e l e c t i c a l c o n t a c t s m a n u f a c t u r e d by DODUCO K G d e m o n s t r a t e v e r y good p r o p e r t i e s ( r e f s . 9 0 , 9 3 ) . The p r o p e r t i e s o f t h e s e o i l s a r e l i s t e d i n Table 3.7.
The p r o b l e m o f t h e p r o p e r l u b r i c a t i o n of elec-
t r i c a l c o n t a c t s i s , however, f a r from s o l v e d ( r e f . 9 4 ) . TABLE
3.7
SPECIAL DODUCONTA - OILS FOR LUBRICATION OF ELECTRICAL CONTACTS MANUFACTURED BY DODUCO KG, D r . F. OURRWACHTER, PFORZHEIM (F.R.G.)
E=-2
610
PROPERT I ES
V i s c o s i t y , mPa.s 2ooc
8OoC Pour p o i n t , O C Flash p o i n t , OC R e s i s t i v i t y a t 20°C, GRVmm Temperature c o e f f i c i e n t o f r e s i s t i v i t y (0-80 'C), 10-2K-1 App 1 i c a t i o n
405
235
32 -35 230
21
-40
-55
-60
238
270
5
60
200
220 100
2 ;ene r a 1, i i g h wear-decreasing iroperties
L
47
21
1 Contact 1oad 0.1-5N. metallic and p o l y m e r i c contact
Contact 1oad 0.1-2N. noble metals
Contact load l e s s t h a n 0.2N, ga 1v a n i zed metals
An i m p o r t a n t p r o b l e m i s t h e damping o f o s c i l l a t i o n (or i m p a c t s p r o d u c i n g n o i s e ) i n m e a s u r i n g i n s t r u m e n t s , q u a r t z - c l o c k mechanisms
48
e t c . The p r o p e r t i e s of a h i g h v i s c o s i t y o i l s u i t a b l e f o r t h i s p u r p o s e a r e l i s t e d i n T a b l e 3 . 8 . T h i s o i l c a n a l s o b e u s e d t o lubricate p o l y m e r i c m i n i a t u r e s y s t e m s . The u s e of K 7132/10000 a n d K 2363/2100 ( T s b l e 3 . 6 ) o i l s i s a l s o recommended. The e s t e r - b a s e d o i l A n d e r o l 4 0 1 D ( T a b l e 3.3) c a n be used as a shock a b s o r b e r f l u i d . TABLE 3.8 PROPERTIES OF SPECIAL DAMPING INSTRUMENT OIL D 861 ( D r . -AHLDORF, F.R.G)
TlLLWlCH GmbH, HORB-
PROPERTIES
2 V i s c o s i t y , mrn / s ,
4565 1375 5 23 -3 1
OOC
20oc 40OC
Pour p o i n t , OC A p p l i c a t i o n temperature range,
- 20
OC
90 S u r f a c e t e n s i o n , mN/m Evaporation rate, 105OC/16h, % Ageing r e s i s t a n c e (Baader's test, Base f l u i d
34.6
1 .14
DIN 51554),%
1.2 Polyoxyalkylene
O p e r a t i n g c o n d i t i o n s must b e t a k e n i n t o c o n s i d e r a t i o n when sel e c t i n g a n o i l , s i n c e most m i n i a t u r e s y s t e m s o p e r a t e u n d e r boundary o r mixed l u b r i c a t i o n . When hydrodynamic l u b r i c a t i o n i s r e q u i r e d , t h e v i s c o s i t y o f t h e o i l needed c a n b e c a l c u l a t e d ( s e e C h a p t e r 9.2). When t h e s l i d i n g s p e e d i s h i g h e r and t h e c o n t a c t p r e s s u r e i s l o w , a lower o i l v i s c o s i t y c a n b e c h o s e n . When t h e s y s t e m u n d e r c o n s i d e r a t i o n w i l l b e o p e r a t i n g u n d e r boundary o r mixed l u b r i c a t i o n , t h e f o l l o w i n g f o u r groups o f f a c t o r s s h o u l d b e t a k e n i n t o consideration: t h e s t r u c t u r e o f t h e t r i b o l o g i c a l s y s t e m and e x t e r n a l l o a d ( s l i d i n g speed, c o n t a c t p r e s s u r e , n a t u r e o f t h e motion
-
see C h a p t e r 1 ) ;
ambient c o n d i t i o n s ( t e m p e r a t u r e , humidity, e t c . ) ; " t i m e f a c t o r s " ( p e r i o d o f o p e r a t i o n o f t h e system i n r e l a t i o n t o p e r i o d s o f idlen e s s ) ; and s p e c i a l r e q u i r e m e n t s f o r t h e o i l ( r e f . 9 5 ) . G e n e r a l recommendations f o r t h e s e l e c t i o n o f a n i n s t r u m e n t o i l f o r t h e l u b r i c a t i o n of a m i n i a t u r e t r i b o l o g i c a l s y s t e m a r e l i s t e d i n T a b l e 3.9 ( b a s e d o n r e f . 9 5 ) . The m o s t i m p o r t a n t c r i t e r i a i n t h e s e l e c t i o n of a n i n s t r u m e n t o i l are a l w a y s t h e l u b r i c i t y , a g e i n g r e s i s t a n c e a n d
l i t t l e change i n v i s c o s i t y as a f u n c t i o n o f t e m p e r a t u r e v a r i a t i o n s . The a b i l i t y o f t h e o i l t o d e c r e a s e f r i c t i o n and wear canbe estimated from t h e a n a l y s e s o f l u b r i c a t e d s y s t e m s g i v e n i n C h a p t e r 5 , by cons u l t i n g t h e m a n u f a c t u r e r o r , i d e a l l y by c a r r y i n g o u t t r i b o l o g i c a l i n v e s t i g a t i o n s on t h e a c t u a l t r i b o l o g i c a l s y s t e m t o b e l u b r i c a t e d .
T A B L E 3.9 R E L A T I O N S H I P S BETWEEN T H E P R O P E R T I E S OF INSTRUMENT O I L S AND O P E R A T I N G C O N D I T I O N S I N M I N I A T U R E SYSTEM ( r e f . 9 5 ) P R O P E R T I E S OF 011
VISCOS I T Y r) mm2/ S
3t OPERATING CONDITIONS
Contact pressure, MPa t o 100 over 100 S l i d i n g speed, m/s t o 0.02 over 0.02 Shearing f o r c e , mN t o 10 a v e r 10 ournal diameter ( i n j o u r n a l b e a r i n g s ) , nnn r a d i a l clearance
ro 20 + + + + +
consumption 2 (see Chapter 6 . 3 ) verage o p e r a t i n g temperature,
OC
+ +
!
,
AGE I NG EVAPORAR E S I S T A N C E T I O N RAT1
- -OVER TO OVER OOC)
20 -
).3
iIGH
0.3 -
LOW
TO 1
6 -
;PREAD I NG
-
DVER .O 1 -
3VER
+ +
+
+
+
+ +
+
+
+
+ +
+
+
+
+
+
+
2 - -
1.5 3.5 +
+
+
+
+
+
+
+
+ +
+
+ +
+
L I F E T l ME years
- ro LOW
I_
+
+
LUBR I C I T Y
%
OVER 2
-
+ + +
+
+
+
+ +
+
4-
+ +
loads, number o f g ( a c c e l e r a t i o n o f
I””‘
+
+
+
R a t i o o f period of actual o p e r a t i o n (use) t o period o f e x p l o i t a t i o n ( t o t a l l i f e t i m e )
---
+
--
+
+
+
+
+ +
I
+
+
-- -
+
Oils with very low surface tension (say below 30 mN/m) have a tendency to migrate from the miniature system. The use of anti-migration coatings (epilamizing) is then called for (see Chapter 6.2). The autoepilamizing oils can also be considered (Chapter 6.2.5). Another possibility is a special lubrication procedure involving the addition of a special oil to the last rinse during the cleaning of the microelements. This oil can adhere to the lubricated surfaces as a partly plastic lubricant layer and in this way a lasting lubricating effect is obtained (ref. 96). The oil, Miracle-Plastic, consists of two liquids which are added to the last two rinsing baths. The first liquid is a mineral oil dissolved in solvent (higher boiling point benzine). The second liquid is silicone oil (polydimethylsiloxane). Silicone oil is non-soluble in the retained lubricant, and consequently, remains on the lubricated surfaces after the two films dry one on top of the other. One actually produces the lubricating properties and the other one the protective layer, which reduces wear and tear of the functional film. Soviet o i l s of the OKB-122 series, (see Chapter 5.4) are suitable for this kind of two-layer lubrication (refs. 97, 9 8 ) and give a relatively good nonspreading effect: they are a mixture of mineral and silicone oils. In comparison with conventional lubrication methods, this method saves manufacturing time but the whole element becomes covered with a thin oil film. Oils such as polyphenyl ethers can be applied to the cleaned surfaces (of e.g. electrical contacts, magnetic media - see Chapters 9 . 8 and 10 respectively) by immersion and withdrawal at room temperature from a e.g. 0.5% solution in l,l,l-trichloroethane. The solvent quickly evaporates, leaving a thin residual film of lubricant. Non-spreading oil can be obtained by the addition of small particles of magnetic material such as magnetite Fe304 to a typical oil. Magnetite powder with granules measuring 15-30 nm is often used in magnetic fluids such as those used in seals and hardly affects the anti-wear properties of the oil (liquid). A drop of oil containing 10% (by weight) of Fe304 and 1-5% (by weight) of a surface active agent for stabilizing the mixture in a magnetic field, does not spread (refs. 99, 100). The contact angles of instrument oils treated in this way and placed on a steel surface (Ra=i).32 um) were ca. 22, 24, 23 and 33O for the dimethyl-, methylphenyl-, methylchlorophenyl- polysiloxanes and fluorinated polyether respectively and were constant for a long period (about 72 h). The drop of oil was kept in a magnetic field with a strength of 300 mT with the
51 v e c t o r of f i e l d i n t e n s i t y p e r p e n d i c u l a r t o t h e s u r f a c e o f t h e s t e e l p l a t e on which t h e d r o p was p l a c e d . T r i b o l o g i c a l t e s t s c a r r i e d o u t w i t h t h e 5 - b a l l a p p a r a t u s i n a magnetic f i e l d w i t h a s t r e n g t h o f 500 mT a t a t e m p e r a t u r e o f 20OoC and w i t h l u b r i c a t i o n by p o l y e t h e r o i l c o n t a i n i n g magnetic p a r t i c l e s , have shown
t h a t the operating
p e r i o d b e f o r e s e i z u r e was t w i c e a s long a s f o r t h e same o i l where t h e r e was no magnetic f i e l d . The t h e r m a l decomposition o f t h e polyether o i l
+
magnetic p a r t i c l e s
b e g i n s a t a b o u t 20OoC.
+
s u r f a c e a c t i v e a g e n t composition
Drops of s y n t h e t i c o i l s based on plysiloxanes
o r p o l y e t h e r s which a r e f i l l e d w i t h magnetic p a r t i c l e s and p l a c e d i n magnetic f i e l d do n o t s p r e a d when t h e s u b s t r a t e ( e . g . r u b b i n g element) i s made o f s t e e l o r g l a s s ( r e f . 9 9 ) . The i n c o m p a t i b i l i t y o f some o i l s and t h e m a t e r i a l s used f o r t h e elements i n m i n i a t u r e system i s a complex problem. The c a t a l y t i c e f f e c t and r a p i d o x i d a t i o n of o i l s i n t h e p r e s e n c e of c o p p e r , e s p e c i a l l y when wear d e b r i s i s p r e s e n t , is t h e main problem and t h i s can be s t u d i e d by u s e o f B a a d e r ' s t e s t f o r example ( s e e Chapters 6.4 and 8 . 5 . 4 ) .
The e f f e c t o f copper on o i l d e g r a d a t i o n t h r o u g h c a t a -
l y t i c o x i d a t i o n i s due t o t h e a b i l i t y o f i t s i o n s t o d i s s o l v e i n o i l more r e a d i l y t h a n i o n s o f o t h e r m e t a l s ( r e f . 1 0 1 ) . The e f f e c t s of i r o n , N i
-
C r on t h e o x i d a t i o n p r o c e s s a r e a l s o c o n s i d e r a b l e ,
although 1Ji
-
C r and aluminium have l e s s e f f e c t on o i l . d e g r a d a t i o n
t h a n copper o r i r o n because N i forms A 1 2 0 3 ,
-
C r a p p e a r s a s an a l l o y a n d a l m i n i u m
which has an a m p h o t e r i c c h a r a c t e r and r e a c t s w i t h
s t r o n g a c i d s a s a b a s e ( r e f . 1 0 1 ) . I n g e n e r a l , t h e harmful e f f e c t s caused by s i n g l e m e t a l s d e c r e a s e when o t h e r m e t a l s a r e p r e s e n t . The c a t a l y t i c d e t e r i o r a t i o n o f i n s t r u m e n t o i l s i s v e r y danger when v i r g i n s u r f a c e s of m e t a l s a r e produced b e c a u s e of t h e i n t e n s i v e wear ( r e f . 1 0 2 ) . When f r e t t i n g i s e x p e c t e d t h e t r i b o c o r r o s i v e e f f e c t s and any complex t r i b o c h e m i c a l r e a c t i o n s ( r e f . 103) r e s u l t i n g i n t h e o i l d e t e r i o r a t i n g s h o u l d be a n a l y s e d . S t u d i e s of f r e t t i n g i n s t e e l - s t e e l systems when m i n e r a l and s y n t h e t i c o i l s were used show t h a t t h e r e i s 1 e s s . t r i b o c o r r o s i o n when m i n e r a l o i l s ( i n p a r t i c u l a r p a r a f f i n i c ) a r e used
(ref. 104).
The h i g h e r t r i b o c o r r o s i o n a t f r e t t i n g when p o l y s i l o x a n e o r p l y s i l o x a n e o r p o l y e t h e r o i l s a r e used is a t t r i b u t e d t o t h e r i g i d molecules w i t h h i g h mesomerism and t h e poor l u b r i c i t y o f such o i l s . The t r i b o c o r r o s i o n a t f r e t t i n g i s h i g h e r when t h e boundary l a y e r of t h e l u b r i c a n t i s n o t formed o r t h e a d s o r p t i o n o f m o l e c u l e s i s poor. E s t e r o i l s can b e p l a c e d between m i n e r a l and p o l y s i l o x a n e o r poly-
e t h e r o i l s . Increasing t h e v i s c o s i t y of t h e o i l decreases t h e t r i -
52
bocorrosion. This effect is particularly characteristic of mineral oils. The use of polyether or polysiloxane oils for lubrication of zinc phosphated steel elements increases the tribocorrosion as compared with the tribocorrosion of unlubricated systems or those lubricated with mineral oils. Oils containing MoS2 are no use since an increase in the quantity of MoS2 in the oil increases the tribocorrosion. When mineral and ester based oils operate under boundary lubrication conditions in the presence of oxygen, products of oxidation are produced which react with an iron surface at temperatures considerably lower than boundary (or mixed) lubrication temperatures (ref. 1 0 5 ) . These processes can lead to the formation of polymrized oxidation sludge, cause corrosive wear and influence reactions of other surface-active species. These effects, although generallydeleterious, could conceivably be beneficial in some circumstances, for example under extreme pressures (refs. 105, 106). When polymeric systems are lubricated, the complex tribophysical and tribochemical effects at the interface during rubbing cause the oil to deteriorate (refs. 77, 107, 103, see also Chapter 6.6). The manufacturer will usually indicate which instrument oils are compatible with the various polymers. The special oils used for lubricating polymeric miniature systems are compatible with most polymers used in practice. However, the ester oils are incompatible with nitriles; for example, the use of ABS material is not recommended when such oils are used. The solubility parameters of the oil and polymsr material used should be different. The solubility parameter of oil can be roughly determined using eqn. ( 8 . 4 4 ) or by multiplying the dielectric constant of the oil by 6.73 (ref. 109) where the solubility parameter is expressed in 1 0 ~ ~ ~ 1 ~ m - 3The 12. dielectric constant of an oil is very important when selecting a lubricant for a particular metal-polymer or polymer-polymer system (see Chapter 5.2). The method of determining the dielectric constant of instrument oils is described in Chapter 8.5.3 and in ref. 110. Values of the dielectric constant as a function of temperature for several instrument oils are presented in Figs. 3.2 and 3.3. Variations in the dielectric constant can be used as a control parameter for estimating the ageing of instrument oils (ref. 110). Minerals oils and other combustible oils are incompatible with high-pressure oxygen: special fluorinated lubricants must be used in breathing apparatus (ref. 76). Sometimes, for example during the manufacture of f o o d s t u f f s , p h a r m a c e u t i c a l s and chemicals, even the
53
smallest l e a k o f o i l may be u n a c c e p t a b l e . I n s u c h c a s e s t h e u s e o f a p r o c e s s f l u i d as a l u b r i c a n t may b e c o n s i d e r e d ; t h i s c a n eliminate t h e need f o r s e a l s and g l a n d s . The a n t i s t a t i c a g e n t s i n c o r p o r a t e d i n t o p o l y e t h y l e n e o r polyamide f i l m s u s e d i n packing have been shcwn t o have a d v e r s e e f f e c t s on c o n t a c t w i t h p r e c i s i o n m i n i a t u r e b e a r i n g s and t h e i r l u b r i c a n t s ( r e f . 111).
w
Temperature,
OC
F i g . 3.2. D i e l e c t r i c c o n s t a n t (& o f i n s t r u m e n t m i n e r a l (4,6) and c l a s s i c c l o c k Moebius 8000, 2 - Moebius 8030, 3 - XU 120, Is (1,2,3,5) v s . temperature. 4 - Moebius 0-5, 5 - XU 430, 6 - MWP.
oi
-
M i n e r a l s o i l s and c l a s s i c c l o c k o i l s based on m i n e r a l and f a t t y o i l m i x t u r e s d e m o n s t r a t e s l i g h t t o x i c i t y and a r e n o n - c o r r o s i v e when p u r e . D i e s t e r - b a s e d o i l s a r e s l i g h t l y t o x i c and a r e s l i g h t l y c o r r o s i v e t o n o n - f e r r o u s metals w h i l e s l i g h t l y t o x i c complex ester a r e c o r r o s i v e t o some n o n - f e r r o u s metals when h o t . T y p i c a l plysiloxanes
a r e n o n - t o x i c and n o n - c o r r o s i v e w h i l e c h l o r i n a t e d p o l y s i l o x a n e s a r e n o n - t o x i c b u t a r e c o r r o s i v e t o f e r r o u s metals i n t h e p r e s e n c e of w a t e r ( r e f . 7 6 ) . P o l y g l y c o l s and polyphenyl e t h e r s e x h i b i t low t o x i c i t y and a r e n o t c o r r o s i v e t o metals.
54
\
'0
40
60
80
I00
Temperature, O C F i g . 3.3. D i e l e c t r i c c o n s t a n t (E) o f i n s t r u m e n t s y n t h e t i c o i l s v s . temperature. 1 - S i l b e r K 7132 mv, 2 - D i e s t e r o i l Nycolube 1 1 B (Nyco S.A., P a r i s , France), 3 - Synta-A-Lube 9010, 4 - K u n s t s t o f f i j l K 2363 b l a u , 5 - OKEi 122-16 ( s i l i c o n e mineral o i 1, U.S.S.R.).
-
F o r h i g h s l i d i n g s p e e d s and r e l a t i v e l y low c o n t a c t p r e s s u r e s , g a s e s a r e sometimes u s e d i n s t e a d o f o i l s f o r hydrodynamic l u b r i c a t i o n . The low v i s c o s i t y o f g a s e s makes a v e r y low f r i c t i o n c o e f f i c i e n t p o s s i b l e (see C h a p t e r 9 . 3 ) . Gas l u b r i c a t i o n c a n b e u s e d a t h i g h e r o r l o w e r t e m p e r a t u r e s t h a n o i l o r g r e a s e . I t s main advant a g e s a r e t h e r i g i d i t y it b r i n g s t o h i g h s p e e d p r e c i s i o n b e a r i n g s s u c h as t h o s e i n d e n t a l d r i l l s and p r e c i s i o n g r i n d i n g s p i n d l e s , and t h e f a c t t h a t i t a v o i d s s e a l i n g and c o n t a m i n a t i o n p r o b l e m s when a s u i t a b l e p r o c e s s gas i s a v a i l a b l e t o a c t a s t h e gas l u b r i c a n t .
G r e a s e s a r e d i s p e r s i o n s of o r g a n i c o r i n o r g a n i c t h i c k e n e r ( s ) i n o i l . Greases are i n t e r e s t i n g l u b r i c a n t s f o r m i n i a t u r e s y s t e m s when h i g h e r e n e r g y l o s s e s due t o f r i c t i o n c a n b e a c c e p t e d t h a n i s t h e c a s e w i t h o i l l u b r i c a t i o n , and t h e main p u r p o s e o f l u b r i c a t i o n i s a
55 d e c r e a s e i n t h e r a t e of wear. The problem o f t h e m i g r a t i o n o f t h e l u b r i c a n t from t h e s y s t e m i s n o t a s s e r i o u s a s i n t h e case o f o i l l u b r i c a t i o n . The g r e a s e c a n a l s o a c t a s a n
a n t i c o r r o s i v e , damper,
shock a b s o r b e r o r s e a l . The g r e a s e u s u a l l y c o n t a i n s 5-35% ( b y w e i g h t ) o f t h i c k e n e r . The b a s e o i l is m i n e r a l o r s y n t h e t i c : u s u a l l y ester o r p o l y s i l o x a n e f l u i d . Many o f t h e a d d i t i v e s u s e d i n t h e l u b r i c a t i n g o i l s a r e equ a l l y e f f e c t i v e i n g r e a s e s . I n o r g a n i c and o r g a n i c compounds a r e u s e d a s t h i c k e n e r s . Soaps u s e d a s t h i c k e n e r s a r e p r o d u c e d from c a r b o x y l i c a c i d s o r t h e i r g l y c e r i d e s ( f a t s and o i l s ) and a l k a l i o r a l k a l i n e - e a r t h h y d r o x i d e s and a l c o h o l s . They a r e p r e s e n t i n t h e g r e a s e i n t h e form o f c h a r a c t e r i s t i c f i b e r s t r u c t u r e s . S o a p s o f uns a t u r a t e d f a t t y a c i d s a r e more s o l u b l e i n m i n e r a l o i l s b u t t h e l o w o x i d a t i o n s t a b i l i t y of f a t t y a c i d s l i m i t s t h e i r a p p l i c a t i o n . The most i m p o r t a n t g r e a s e s w i t h s o a p t h i c k e n e r i n them a r e t h o s e b a s e d on c a l c i u m and l i t h i u m s o a p s . The o t h e r t h i c k e n e r s a r e t h e inorganic ones ( s i l i c a , o r g a n o p h i l i c b e n t o n i t e s )
,
p i g m e n t s ( a l i z a r i n , anthraq-
u i n o n e , i n d i g o , a z o i n d a t h r e n e , p h t a l o c y a n i n e d y e s t u f f s , Ultrarnarine B l u e ) , and p o l y m e r s ( P E , PP, PTFE, PA,
P I , c o n d e n s a t i o n p r o d u c t s of
a l k y l p h e n o l f a t t y a c i d and formaldehyde, p o l y u r e a s )
( r e f s . 59, 6 2 ) .
O r g a n o p h i l i c b e n t o n i t e s and p o l y u r e a s a r e u s e d i n g r e a s e s f o r u s e
a t m o d e r a t e t e m p e r a t u r e s where t h e y a r e more s u i t a b l e t h a n normal g r e a s e s . O r g a n i c t h i c k e n e r s a r e u s e d i n p o l y s i l o x a n e o i l s t o produce h i g h - t e m p e r a t u r e g r e a s e s . Some s p e c i a l g r e a s e s a l s o c o n t a i n s o l i d l u b r i c a n t s s u c h as MoS2 o r g r a p h i t e . The i n s t r u m e n t g r e a s e s m a n u f a c t u r e d w i t h m i n e r a l o i l a s a b a s e l i q u i d d e m o n s t r a t e r e l a t i v e l y low o x i d a t i o n r e s i s t a n c e , narrow range o f o p e r a t i n g t e m p e r a t u r e and a r e l a t i v e l y h i g h e v a p o r a t i o n r a t e ( r e f . 1 1 2 ) . The p r o p e r t i e s o f some i n s t r u m e n t g r e a s e s b a s e d o n min-
e r a l o i l are l i s t e d i n Table 3.10. The g r e a s e "BOX" c a n b e u s e d as a n a n t i - c o r r o s i v e w h i l e "BOZ" g r e a s e c o n t a i n i n g a b o u t 1%(by weight) MoS2 c a n be u s e d f o r l u b r i c a t i o n of h i g h l y l o a d e d m i n i a t u r e s y s t e m . The g r e a s e s b a s e d on s y n t h e t i c o i l a r e m o s t l y b a s e d o n e s t e r o r p o l y s i l o x a n e o i l s . Such g r e a s e s h a v e a wide r a n g e o f o p e r a t i n g t e m p e r a t u r e , l o w e v a p o r a t i o n r a t e and h i g h a g e i n g r e s i s t a n c e . The p r o p
e r t i e s o f some e s t e r - b a s e d i n s t r u m e n t g r e a s e s a r e l i s t e d i n T a b l e 3.11.
A s w e l l as b e i n g s u i t a b l e f o r u s e a t low t e m p e r a t u r e s , some
o f t h e s e g r e a s e s c a n be u s e d when t h e o p e r a t i n g t e m p e r a t u r e rises a b o v e 153OC. G r e a s e s 7 6 1 , 7 9 4 and 7 9 9 c a n b e u s e d f o r t h e l u b r i c a t i o n of m i n i a t u r e p o l y m e r i c s y s t e m s when t h e r u b b i n g e l e m e n t o r e l e m e n t s a r e made of PA, POM, PTFE, PBTP.
Ln
TABLE 3.10
cn
PROPERTIES OF SOME INSTRUMENT GREASES BASED ON MINERAL O I L
I
GREASE
FETT 852
MANUFACTURER
I PROPERTIES
1
I
ALVANIA RS (V3872)
D r .T I LLW I CH GmbH, HORB -AH LOORF (R.F.G.)
[
AEROSHELL (GREASE 5 )
INSTRUMENTENFETT BR2
BOX DOW CORN I NG
SHELL
I
BOZ
TECHNISCHE WACHSE, JENA (G.O.R.) I
Thickener Drop p o i n t , OC P e n e t r a t i o n a t 25OC A p p l i c a t i o n t e m p e r a t u r e range,
I A1 s t e a r a t e
Li stearate
107 41 2 OC
265-29 5
-25
- 25
80
120
-40
TABLE 3.11 PROPERTIES OF SOME .ESTER-BASED INSTRUMENT GREASES
ANDE ROL
I SOFLEX
GREASE PDP 38 C X 1000 MANUFACTURER PROPERTIES
SHELL
DOW CORNING
M i croge 1 26 0 26 0 296 280 a t t e m p e r a t u r e 39OC (22h) Li stearate
Thickener Drop p o i n t , OC P e n e t r a t i o n a t 25OC Evaporation rate, % A p l i c a t i o n t e m p e r a t u r e range,
NUOOEX I N C . , PISCATAWAY , NEW JERSEY
OC
1
'
1
-62 150
-54
20 5
KLUBER, MUNCHEN (F.R.G.)
I L i stearate
120
SUPER TEL
Li stearate
57
The g r e a s e s b a s e d on p o l y s i l o x a n e s d e m o n s t r a t e h i g h c h e m i c a l s t a b i l i t y , a g e i n g r e s i s t a n c e , a wide t e m p e r a t u r e r a n g e and a h i g h o p e r a t i n g t e m p e r a t u r e . The p r o p e r t i e s o f t h e s e g r e a s e s a r e l i s t e d i n T a b l e 3.12. Grease NP 51 c o n t a i n s a n a d d i t i o n o f 6 % (by w e i g h t ) MoS2, which makes i t a p p l i c a b l e i n h i g h l y l o a d e d t r i b o l o g i c a l s y s t e m s . Very s o f t g r e a s e K 7132 c a n a l s o be u s e d t o l u b r i c a t e p o l y -
meric systems ( e x c l u d i n g t h o s e w i t h e l e m e n t s made o f ABS). P r o p e r t i e s of t h e Xrytox 240 AC g r e a s e s (manufactured by Du Pont) based on p o l y e t h e r o i l s , a r e a s f o l l o w s : p e n e t r a t i o n ( a t 25OC) 283 and 282 r e s p e c t i v e l y : o p e r a t i n g t e m p e r a t u r e r a n g e - -4U to232OC and -34 t o 287OC r e s p e c t i v e l y . They a l s o h a v e a r e l a t i v e l y h i g h resistance t o radiation. The Bel-Ray Company o f F a r m i n g d a l e , N e w J e r s e y , h a s d e v e l o p e d a n a n t i - s e i z e and a n t i - s t i c k / s l i p h e l d i n a non-weeping,
p a s t e o f powdered PTFE s e c u r e l y
o r non-bleeding,inorganic g e l system t h a t is
h e a t s t a b l e (Bel-Ray 6 3 8 6 0 ) . I t w i l l n o t m e l t or r u n when exposed
t o h i g h t e m p e r a t u r e s , making i t s u i t a b l e f o r l o n g t e r m u s e a t up t o 1 2 O o C (from -7OC)
and i n t e r m i t t e n t e x p o s u r e t o 26OoC.
This p a s t e
c o n t a i n s 348 ( b y w e i g h t ) PTFE, h a s no d r o p p o i n t , and h a s a penetrat i o n v a l u e o f 284. S p e c i a l g r e a s e s which c a n b e u s e d a s l u b r i c a n t s u n d e r h i g h v a c uum c o n d i t i o n s a r e a p p l i e d f o r s e a l i n g s t a t i o n a r y and moving elements i n vacuum a p p a r a t u s and f o r l u b r i c a t i n g s p a c e i n s t r u m e n t a t i o n l o c a t e d o u t s i d e t h e h e r m e t i c a l l y s e a l e d s p a c e c r a f t . Such g r e a s e s must d e m o n s t r a t e good l u b r i c i t y , a v e r y l o w e v a p o r a t i o n r a t e ( m i n i mum p r e s s u r e of s a t u r a t e d v a p o u r s ) and s h o u l d resist l o w and h i g h t e m p e r a t u r e s . The s e a l i n g g r e a s e s a r e u s u a l l y b a s e d on m i n e r a l o i l s t h i c k e n e d w i t h s o l i d h y d r o c a r b o n s and r u b b e r . The p r o p e r t i e s of sane g r e a s e s o f t h i s k i n d are l i s t e d i n T a b l e 3.13.
The c h a r a c t e r i s t i c s
o f some g r e a s e s a p p l i e d t o t h e l u b r i c a t i o n o f s p a c e i n s t r u m e n t a t i o n
are g i v e n i n T a b l e 3.14. T h i c k e n e r s used i n g r e a s e s a p p l i c a b l e f o r t h e l u b r i c a t i o n o f vacuum o p e r a t i n g s y s t e m s a r e u s u a l l y n o t c h e a p s i n c e t h e y c o n s i s t of m a t e r i a l s l i k e f l u o r o p o l y m e r s , polyisobutylene, c e r e s i n and n a t u r a l r u b b e r . Very c h e a p material s u c h as a t a c t i c p o l y p r o p y l e n e (PP) which i s sometimes used as a t h i c k e n e r , i s n o t s u i t a b l e f o r g r e a s e s which must work i n h i g h vacuum i n s t a l l a t i o n s . An i n t e r e s t i n g t e s t o f s y n t h e t i c vacuum s e a l i n g g r e a s e s b a s e d on m i n e r a l o i l s and u s i n g a t a c t i c PP a s a t h i c k e n e r i s d e s c r i b e d i n r e f . 113. PP (20-30% by w e i g h t ) was mixed i n o i l heated to 160 k 10°C. A f t e r c o o l i n g , a l i g h t brown g r e a s e w i t h t h e c o n s i s t e n c y of v a s e l i n e w a s o b t a i n e d . Two s u c h g r e a s e s are compare w i t h t h e g r e a s e
TABLE 3.12 PROPERTIES
OF
SOME INSTRUMENT GREASES BASED ON POLYSl LOXANES 1
I
GREASE
MANUFACTURER PRDPERT I ES Thickener Drop p o i n t , OC P e n e t r a t i o n a t 25OC A p p l i c a t i o n t e m p e r a t u r e range,
PTFE FETT
K 7132
D r . TlLLWlCH GmbH, HO RB -AH L DO R F (F. R.G.)
DOW CORNING
GENERAL ELECTRI C
(F. R. G.) Na compound > 220
Powdered PTFE
230
355 OC
-30 200
KLUBER, MUNCHEN
- 73
-73
232
180
265-295 -60 230
CHEM IEWERK. NUNCHRITZ (G.D.R.) L i stearate 200
270-31 0
-30 150
TABLE 3.13 GREASES USED FOR SEALING I N VACUUM APPARATUS
GREASE
AP I EZON AP 100
MANU FACT URER
1
AP 101
I
SHELL
PROPE RT I ES Compos i t i on
Drop p o i n t , OC A p p l i c a t i o n temperature range,
M i n e r a l o i 1, sol i d h y d r o c a r bons. a d h e s i v e additive, polyi s o p r e n e rubber
10
5
. 10-11
BHMM HT[-300
KLUBE R, MUNCHEN (F. R . G. )
U.S.S.R.
Fluorinated polyalkylether, PT FE
48, 44, 44, 125
47 OC
-40
30
Vacuum t o , Pa Remarks
L,M,N,T
BARRIERTA L55/3 HV L55/5 HV
180 10-11
I
Glass v a l v e s o f P r e s s u r e o f s a t l a b o r a t o r y apu r a t e d vapours p a r a t u s , can 10-7, o p e r a t e i n cont a c t w i t h acids, Pa bases, o r g a n i c (Apiezons L,M,N so 1v e n t s respectively)
Mineral o i l , distillation residue of p e t r o l , rubber
62 -30
250
10-7 Stable i n water, vapour, aggress i v e media. Used i n b a l l anc s a f e t y valves
1'.3 * loM2 V i scosi t y 5000 Pa s ( a t OOC). Used i n moving e l e ments s e a l i n g
cn
TABLE 3.14
0
VACUUM GREASES USED IN SPACE
GREASE
SPACE 2110 (1)
ROPERT I E S
8135
FBA7
FM8,
FMlO
FPlO
F55M,F55MS
UNlSlLlCON HOCHVACUUMFETT TK HV
BRITISH PETROLEUM M i nera 1 o i l , oleophi 1 i c graphite w i t h lead, a n t i ox id a n t and a n t i -corr o s i o n addi tives
ompos i t i on
,ppIication
STRUCTOSCORAL
t e m p e r a t u r e range,
Synthetic oi I , oleoohi l i c graphite nrith lead
OC
F l u o r i natec M i n e r a l o i polyether, s o l i d hyPTFE d r o c a rbons
lineral >i 1, PTFE
-65
-65
200
2 50
I 0-9
10-IO
( FM8 ) ,o-lil
Low s l i d i n g speed (100 rpm) b e a r i ngs
Higher 5 1 i d i ng speed bear i ngs
(FMlO) S e r i e s o f space and vacuum s e a l i n g greases
'olyloxane, 'TFE ji
-65 175 10-5
P o l y s i loxanes
61 BHMl4 HIT-300
(see T a b l e 3.13)
i n T a b l e 3.15.
N e w c h e a p g r e a s e s con-
t a i n i n g a t a c t i c ?P a s a t h i c k e n e r h a v e h i g h e r d r o p p o i n t s and l o w e r e v a p o r a t i o n r a t e s t h a n BHMM
HIT-300 g r e a s e .
S p e c i a l g r e a s e which c a n b e u s e d a t h i g h p r e s s u r e s and t e m p e r a t u r e s may b e o b t a i n e d by a d d i n g PTFE ( 0 . 0 5 - 6 % by w e i g h t ) , c o p p e r p a l m i t a t e (0.1-5)
,
MoS2 ( 3 - l d ) , and g l y c e r o l ( 0 . 5 - d . 6 )
t o a typical
s o a p - c o n t a i n i n g g r e a s e ( r e f . 1 1 4 ) . T h i s g r e a s e c a n be u s e d f o r t h e l u b r i c a t i o n of e l e c t r i c a l c o n t a c t s , a s c a n m i c r o c r y s t a l l i n e wax, a v e r y u s e f u l c o n t a c t l u b r i c a n t when f r e t t i n g d o e s n o t o c c u r (ref.92), and s u c h g r e a s e s a s K o n t a s y n t h o r S y n t h e s i n ( K l i i b e r , Miinchen F.R.G.)
or S w i t c h C o n t a c t Compound ( C a s t r o l , G B ) . I n t h e case of l i t h i u m g r e a s e s based on m i n e r a l o i l s , t h e c r i t i c a l t e m p e r a t u r e o f g r e a s e d i s o r i e n t a t i o n c a n b e i n c r e a s e d by making them u n d e r optimum c o n d i t i o n s o f i s o t h e r m i c c r y s t a l l i z a t i o n (18OoC, 1 h )
( r e f . 1 1 5 ) . The
boundary l a y e r on s t e e l c a n b e h e a t - r e s i s t a n t up t o 2OO0C
(instead
o f 13OoC i n a t y p i c a l l i t h i u m g r e a s e ) when i n t e r m i t t e n t h e a t i n g i s applied. The thhOtrOpiC g r e a s e " T h i x o - g r e a s e
9 4 1 5 " , m a n u f a c t u r e d by
Moebius e t F i l s ( A l l s c h w i l , S w i t z e r l a n d ) , i s b a s e d o n d i e s t e r s w i t h e t h e r g r o u p s , h a s p u r e s y n t h e t i c aluminium t r i s t e a r a t e a s a thickener and c o n t a i n s a d d i t i v e s t o p r o v i d e e x c e l l e n t r e s i s t a n c e t o h i g h cont a c t p r e s s u r e s and a s i z a b l e r e d u c t i o n o f w e a r and f r i c t i o n i n l u b r i c a t e d m i n i a t u r e s y s t e m s . I t c a n b e u s e d i n s t e a d o f o i l when i t
i s i m p o r t a n t t h a t t h e l u b r i c a n t d o e s n o t m i g r a t e . The p e n e t r a t i o n of t h e g r e a s e i s ca.350,
e v a p o r a t i o n r a t e a t 100°C
less t h a n 0 . 6 %
( a f t e r 2 2 h o u r s ) , and t h e a p p l i c a t i o n t e m p e r a t u r e r a n g e -25 to 8OoC. The f r i c t i o n c o e f f i c i e n t ( n e a r l y t h e same i n d r y o r humid a i r ) of a journal steel-ruby
($3 0 . 1 1 mm) m i c r o b e a r i n g l u b r i c a t e d w i t h t h i s
g r e a s e i s p r a c t i c a l l y t h e same a s f o r a b a s e o i l l u b r i c a n t ( 0 . 1 1 ) . Greases t o be used mainly i n o p t i c a l i n s t r u m e n t s a r e u s u a l l y t h i c k , demonstrating high adherence t o t h e l u b r i c a t i n g s u r f a c e s . T h e i r u s e i n screw and c o r e l o c k i n g d e v i c e s a s s u r e s a s m o o t h e r mot i o n t o g i v e p r e c i s e a d j u s t m e n t c o n t r o l . They a r e u s e d on s l i d i n g and s p i n d l e mechanisms i n a wide a r r a y o f i n s t r u m e n t s s u c h as t e l e s c o p e s , b i n o c u l a r s , cameras, r a n g e f i n d e r s , v a r i a b l e c o n d e n s e r s , p o t e n t i o m e t e r s a n d c o i l s . The p r o p e r t i e s of some o f t h e s e g r e a s e s a r e l i s t e d i n T a b l e 3.16. Jena (G.D.R.)
The s p e c i a l g r e a s e s u s e d by C a r l Zeiss
( m a n f a c t u r e r o f o p t i c a l i n s t r u m e n t s ) , b a s e d on mineral
o i l s and t h i c k e n e d w i t h A 1 o r L i s t e a r a t e s , a r e l i s t e d i n Table 3.17 and 3.18. G r e a s e s of t h e FZ-A1 s e r i e s h a v e e i t h e r a s h o r t - f i b r o u s s t r u c t u r e (kz series) o r a long-fibrous s t r u c t u r e ( l g series).
m
TABLE 3 . 1 5
N
COMPARISON OF THE PROPERTIES O F VACUUM SEALING GREASES BASED ON MINERAL OtLS AND HAVING ATACTIC PP AS THICKENER WITH GREASE B H M M Hll - 300 ( s e e T a b l e 3.13) ( r e f . 113) I
GREASE PROPERT I ES D r o p p o i n t , OC E v a p o r a t i o n r a t e , % by w e i g h t a t 200 OC d u r i n g h 1
3 6 a t 100 1
OC
BM-1 VACUUM OIL (BY WEIGHT) PP
+ 25%
EM-4 VACUUM O I L
25% PP
BHMB
~n-
62
70
70
+
0.30 0.80 1.50
0.41
0.05
0.08
0.08 0.14
0.13 0.20 0.18
0.97 1.80
3..0
8.2 14.0
during h
3 6 Humidity absorption, Vacuum to, Pa
%
0.13 2.6
.
4
.
1.3
.
300
TABLE 3.16
GREASES USED FOR LUBRICATION OF OPTICAL INSTRUMENTS
GREASE
BEL-RAY VISCOLUBE GREASE I
MANUFACTURER
3EL-RAY COMPANY I N C . :ARMINGDALE, NEW JERSEY
1 Friz- I
SERXTELL2A
MV2
i e l s o f l a r g e molecular geight s y n t h e t i c f l u i d s
CARL Z E l S S JENA CHEMISCHE LABOR (G.D.R.)
KLUBER, MUNCHEN (F.R.G.)
I
I
Syn the t i c thickent r based on hydrocarbon o i 1
GEmrEFETT V 232
B
ROPERT 1 ES mpos i ti on
FETT SOVl sco
L i stear a t e as thickener
Organic thickener
qineral o i 1 , 41 stearate
Me1t i ng poi n t rop p o i n t , O C e n e t r a t i o n a t 25OC p p i~c a t i o n temperature range
216 328
-7 120
emarks, a d d i t i o n a l a p p l i c a t i o n !
204 99 312 334 -2 -18 93 105 Viscous, Gel o f lower sticky v i scos i t y fluid for eas i er movement o f component core-locking and s l i d i n g mechan i sms
250
Screws on binoculars
-60 150 Preci s i oi screws, gears, 9Y r o scopes
-60 180 White. Bearings o f spindles o f spi nni ng frames
90 220-250 - 20 90 Sticky
160 180-215 -20 140
m w
64
CT
0
a
v
s W 7 ln ln
-
W
N -I
CT
d >m 0 W ln
3 W
ln -
CT W ln
I
-I
a a
U - U h
* o
M W
0
0
M ?
I 0
N u )
I 0
b
0
0
M
0
7
-
I
-
m L n o
0
N I
m
0
7
0 4 -
0
n
L
q n
N
I n
4.
Ln
m LN n
L
N
-
\
D
o
00 I
Ln
f
\D I L n o
N F
m N
I
M
0
L n o
N c o 7
CT
0
a
a
v
W z
7 ln
-ln W
N -I
CT
a
V
m
> 0 W ln
3 lW n
-
N 7
-I
.-
0 7
-I
.-
N
-I
.-
0
co -I
.-
0
-f
-1
.N 0
.-
-1
W
CT
W ln
a a I
ln
W -
k
W R
R
CT 0
-
3 A
.-
W U
M l n
21 W
2:
w l n
I-u
a E
-14
m w a =
t a
65 They c a n b e u s e d between 15 and 50OC. Greases o f t h e FG-Li series c a n b e a p p l i e d when t h e a m b i e n t t e m p e r a t u r e is b e t w e e n -35 a n d 55OC ( o r -50 t o 1 3 0 O C when mechanisms o t h e r t h a n o p t i c a l i n s t r u m e n t s a r e l u b r i c a t e d ) . The d r o p p o i n t o f t h e s e g r e a s e s i s b e t w e e n 190 and
25OOC. The p e n e t r a t i o n i s g i v e n i n T a b l e 3.18.
The s t r u c t u r e o f t h e
L i 13 and L i 12 g r e a s e s i s l o n g - f i b r o u s a n d t h a t o f t h e other greases
short-fibrous. cussed i n r e f . 3.4,
The t r i b o l o g i c a l p r o p e r t i e s of t h e s e g r e a s e s aredis116; some of t h e s s p r o p e r t i e s c a n b e found i n F i g s .
3 . 5 and 3 . 6 .
1
-0.4
-0.2
tag v
I
0 0.2 ( v i n fD-*m/s)
0.4
.*
0.6
F i g . 3 . 4 . F r i c t i o n f o r c e F ( c i r c u m f e r e n t i a l f o r c e ) v s . c i r c u m f e r e n t i a l speed ( s l i d i n g speed) between r o t a t i n g e x t e r n a l c y l i n d e r ( r i n g ) h a v i n g f r i c t i o n s u r f a c e 1 000 mm2 and d i a m e t e r 55 mm, and i n t e r n a l c o a x i a l c y l i n d e r when grease of t h e FG-A1 s e r i e s i s i n t r o d u c e d between t h e f r i c t i o n s u r f a c e s ( T a b l e 3 . 1 7 ) . Thickness o f grease l a y e r was 3-20 gm. Ambient t e m p e r a t u r e 2OoC ( r e f . 1 1 6 ) .
66
c 0 00
FG 12 - ~ i
g 2.151 c
.a
LL
v
0
- Li 08
1 /FG
x h
LL 0 a
1.551
d
u
,
/
/
0.951
-0.4
-0.2
1
I
0
0.2
0.4
0.6 *
Fig. 3.5. F r i c t i o n f o r c e F ( c i r c u m f e r e n t i a l f o r c e ) v s . c i r c u m f e r e n t i a l speed v ( s l i d i n g speed) f o r the FG-Li s e r i e s of greases (see Table 3 . 1 8 ) . For d e t a i l s o f f r i c t i o n system used see c a p t i o n t o F i g . 3 . 4 ( r e f . 1 1 6 ) .
67
3.351.
F F6-A16 GA -1 *\9 \'\ 10 A
2.751N
'9 h
LL
2.151-
1.551.
0.951
K
I/T1
-20
,
-10
1
1
I
0
20
40
Temperature
I
I
50
*
OC
F i g . 3.6. F r i c t i o n a l f o r c e F ( c i r c u m f e r e n t i a l f o r c e ) v s . ambient temperature T f o r FG-Li s e r i e s o f greases. S l i d i n g speed 0 . 0 1 m / s . For d e t a i l s o f f r i c t i o n system used see c a p t i o n t o F i g . 3.4 ( r e f . 116).
68
3,4, SOLID
LUBRICANTS
S o l i d l u b r i c a n t s a r e more and more f r e q u e n t l y u s e d s i n c e t h e y
are v e r y good l u b r i c a n t s f o r t r i b o l o g i c a l s y s t e m s o p e r a t i n g u n d e r e x t r e m e c o n d i t i o n s : low and h i g h t e m p e r a t u r e s , vacuum, r a d i a t i o n , c o r r o s i v e environment, h i g h c o n t a c t p r e s s u r e s , h i g h frequencyoscill a t i n g motion, o r s t a r t - s t o p
( i n t e r m i t t e n t ) motion (e.g.
i n gas
b e a r i n g s ) . The most w i d e l y u s e d s o l i d l u b r i c a n t s a r e i n o r g a n i c s u b s t a n c e s : g r a p h i t e and molybdenum d i s u l p h i d e . The good l u b r i c a t i n g p r o p e r t i e s o f g r a p h i t e are due t o t h e l a y e r e d l a t t i c e s t r u c t u r e a n d a l s o depend o n a d s o r b e d f i l m s , i n p a r t i c u l a r of water v a p o u r , which p r o v i d e s u r f a c e s w i t h low a d h e s i o n . The u s e o f g r a p h i t e i s t h e r e f o r e e f f e c t i v e i n a humid a t m o s p h e r e ; i n a vacuum i t loses i t s anti- f r i c t i o n p r o p e r t i e s . A t t e m p e r a t u r e s o v e r 45OoC it f o r m s c a r b o n d i o x i d e . When s u i t a b l e o x i d e l a y e r s a r e p r e s e n t g r a p h i t e k e e p s i t s l u b r i c a t i n g p r o p e r t i e s u p t o 60OoC. M i x t u r e s o f g r a p h i t e w i t h m e t a l o x i d e s ( s u c h as PbO) o r m e t a l s a l t s a d h e r e t o m e t a l s u r f a c e s and r e d u c e f r i c t i o n c o n s i d e r a b l y ( r e f . 6 2 ) . The f r i c t i o n c o e f f i c i e n t o f g r a p h i t e i n a normal a t m o s p h e r e i s a b o u t 0 . 2
( r e f . 117). G r a p h i t e
i s a good l u b r i c a n t a t room t e m p e r a t u r e and a t 5OO0C b u t n o t a t i n t e r m e d i a t e t e m p e r a t u r e s ; a t room t e m p e r a t u r e , e n v i r o n m e n t a l cont a m i n a n t s s e p a r a t e g r a p h i t e l a m e l l a e and t h e y d e s o r b a t h i g h e r t e m p e r a t u r e s , and a t t e m p e r a t u r e s above 500°C,
as m e n t i o n e d b e f o r e ,
oxides a i d i n t h e l u b r i c a t i o n process ( r e f . 1 1 7 ) . Molybdenum d i s u l p h i d e (MoS2) h a s a lamellar s t r u c t u r e a n d c r y s t a l l i z e s i n t h e h e x a g o n a l s y s t e m w i t h t r i g o n a l symmetry. The s u l p h u r l a y e r s f o r m i n g t h e s u r f a c e o f t h e MoSZ c r y s t a l g i v e s t r o n g a d h e s i o n t o m e t a l s u r f a c e s . The f r i c t i o n c o e f f i c i e n t o f MoS2 c a n b e v e r y low ( l e s s t h a n 0 . 2 )
,
e s p e c i a l l y when v e r y h i g h c o n t a c t p r e s -
s u r e i s a p p l i e d , a l t h o u g h i n t h e p r e s e n c e o f water t h e f r i c t i o n c o e f f i c i e n t and
wear i n c r e a s e . U n l i k e g r a p h i t e , MoS2 i s a good
l u b r i c a n t i n a h i g h vacuum, d e m o n s t r a t i n g v e r y low f r i c t i o n c o e f f i c i e n t ( b e l o w 0 . 0 5 ) . A t e m p e r a t u r e i n c r e a s e from 2 0 t o 100°C reduces t h e f r i c t i o n c o e f f i c i e n t s i n c e environmental contaminants are d e t r i m e n t a l t o l u b r i c a t i n g e f f e c t i v e n e s s . Above 100°C, c o e f f i c i e n t remains r e l a t i v e l y s t a b l e MoS2 o c c u r s a b o v e 37OoC.
the friction
u n t i l s e v e r e o x i d a t i o n of
The s m a l l e r t h e p a r t i c l e s o f MoS2 a r e , t h e
higher t h e oxidation rate w i l l be. G r a p h i t e and MoS2 o r i e n t r a p i d l y when r u b b e d b e c a u s e o f t h e c r y s t a l s t r u c t u r e . The b a s a l p l a n e s become p a r a l l e l t o t h e f r i c t i o n s u r f a c e which f a c i l i t a t e s e a s y s h e a r . Powder b l e n d s o f h i g h - g r a d e
69 g r a p h i t e and MoS2 l e a d t o s y n e r g i s n or e q u i v a l e n c e , d e p e n d i n g o n o p e r a t i n g c o n d i t i o n s , w i t h r e g a r d t o f r i c t i o n a n d wear ( r e f s . 1 1 8 , 1 1 9 ) . T h i s phenomenon c a n b e e x p l a i n e d i n terms o f the physicochenical
p r o p e r t i e s o f t h e s o l i d l u b r i c a n t s . The f o r m a t i o n and t r a n s f e r o f l u b r i c a n t f i l m i s i n f l u e n c e d b y p a r t i c l e s i z e a n d by t h e r e l a t i v e volumes of t h e MoS2 and g r a p h i t e i n t h e b l e n d . The d i s t i n c t s y n e q i s n
w a s o b s e r v e d i n b l e n d s c o n t a i n i n g 1 5 t o 35% b y w e i g h t g r a p h i t e and i n b l e n d s w i t h c o n s i d e r a b l y h i g h e r amounts o f g r a p h i t e
-
from 6 0 t o
80%. The p r o c e s s of i n t e r c a l a t i o n , i . e . ,
t h e formation of chemical
compounds b y t h e i n s e r t i o n o f a t o m i c o r m o l e c u l a r s p e c i e s i n t h e v a n d e r Waals g a p b e t w e e n p l a n e s of l a m e l l a r s o l i d s c a n g r e a t l y improve t h e i n t r i n s i c l u b r i c a t i n g p r o p e r t i e s of such s o l i d s
( r e f . 120).
The a d d i t i o n o f 1 9 . 8 % ( b y w e i g h t ) of CoC12 t o g r a p h i t e w a s f o u n d t o i n c r e a s e l i f e of t h e l u b r i c a n t by more t h a n f i v e t i m e s , w h i l e g r a p h -
i t e + 1 9 . 3 % N i C 1 2 c o m p o s i t i o n more t h a n d o u b l e d t h e l o a d - c a r r y i n g c a p a c i t y compared w i t h g r a p h i t e and was e q u i v a l e n t t o EloS2. The amount by which t h e l i f e t i m e i s p r o l o n g e d d e p e n d s o n t h e c o n c e n t r a t i o n o f i n t e r c a l a n t i n g r a p h i t e and t h e r e s u l t i n g i n c r e a s e i n i n t e r l a y e r c a r b o n s p a c i n g due t o i n t e r c a l a t i o n . Another i n o r g a n i c l a m i n a r s o l i d l u b r i c a n t i s g r a p h i t e f l u o r i d e (CFxIn, where x c a n v a r y from a b o u t 0.3 t o 1.1. T h i s l u b r i c a n t c a n be roughly d e s c r i b e d a s a l a y e r l a t t i c e i n t e r c a l a t i o n
compound of
g r a p h i t e ( r e f . 1 2 1 ) . The g r a p h i t e f l u o r i d e d o e s n o t o x i d i z e i n a i r , b u t i t decomposes t h e r m a l l y above 54OoC t o f o r m c a r b o n t e t r a f l u o r o m e t h a n e , o t h e r l o w m o l e c u l a r w e i g h t f l u o r o c a r b o n s and c a r b o n . The f a i l u r e t e m p e r a t u r e f o r ( C F x ) n i s 49OoC, (4OOOC).
h i g h e r t h a n t h a t of MoS2
(CFxIn e x h i b i t s h i g h p l a s t i c i t y w i t h i n a l u b r i c a t e d con-
t a c t . The o r d e r o f p l a s t i c i t y i s ( C F x ) n > MoS2 > g r a p h i t e . (CFx)n i s p a r t i c u l a r l y u s e f u l i n polymide-bonded (CFx) c o a t i n g s (refs. 121, 1 2 2 ) . Graphite f l u o r i d e demonstrates b e t t e r l u b r i c a t i n g p r o p e r t i e s
t h a n g r a p h i t e i t s e l f b u t n o t q u i t e a s good a s XoS2. T e m p e r a t u r e , h u m i d i t y and t h e C:F r a t i o a f f e c t its p e r f o r m a n c e . D i c h a l c o n i d e s s u c h as WS2, s y n t h e t i c Nb1.158S2
have v e r y good
l u b r i c a t i n g p r o p e r t i e s ( r e f s . 1 2 1 , 1 2 3 ) . The p r o p e r t i e s o f W S 2 a r e s i m i l a r t o MoS2 b u t t h e o x i d a t i o n r e s i s t a n c e o f W S 2 a t t e m p e r a t u r e s h i g h e r t h a n 34OoC i s b e t t e r t h a n MoSZ. The loss of l u b r i c a t i q qual-
i t i e s of WS2 a n d MoS2 i n a i r c o i n c i d e s w i t h t h e t e m p e r a t u r e s a t which r a p i d c o n v e r s i o n t o t h e o x i d e o c c u r s ( r e f . 1 2 1 ) . I n a n i n e r t g a s o r vacuum t h e maximum o p e r a t i n g t e m p e r a t u r e i s a f u n c t i o n o f t h e t h e r m a l d i s s o c i a t i o n rates r a t h e r t h a n t h e o x i d a t i o n r a t e s o f
70
t h e l u b r i c a n t s . WS2 d e m o n s t r a t e s b e t t e r e l e c t r i c a l c o n d u c t i v i t y t h a n many
o t h e r d i c h a l c o n i d e s and i s u s e d i n e l e c t r i c a l c o n t a c t s .
The s y n t h e t i c Nb1.158S2
( s y n t h e s i z e d by niobium p e n t o x i d e r e a c t i n g
w i t h hydrogen s u l p h i d e a t 70OoC) when added t o l i t h i u m s o a p g r e a s e s d e m o n s t r a t e s a lower f r i c t i o n c o e f f i c i e n t a n d b e t t e r e x t r e m e p r e s s u r e p r o p e r t i e s t h a n MoS2 ( r e f . 1 2 3 ) . I n o r g a n i c , nonlaminar s o l i d s s u c h as PbO, CaF2 d e m o n s t r a t e good l u b r i c a t i n g p r o p e r t i e s t h a n k s t o t h e i r low s h e a r s t r e n g t h . S o f t oxi d e s ( e . g . PbO, B 0 ) h a v e r e l a t i v e l y low f r i c t i o n c o e f f i c i e n t s , 2 3 e s p e c i a l l y a t h i g h t e m p e r a t u r e s where t h e i r s h e a r s t r e n g t h i s reduced t o s u c h a d e g r e e t h a t d e f o r m a t i o n o c c u r s by p l a s t i c f l o w rat h e r t h a n by b r i t t l e f r a c t u r e . S n c r e a s i n g s u r f a c e t e m p e r a t u r e red u c e s b o t h c r y s t a l l i n e s h e a r s t r e n g t h and g l a s s v i s c o s i t y w i t h i n t h e s l i d i n g c o n t a c t . PbO l u b r i c a t e s e f f e c t i v e l y o v e r a narrow t e m p e r a t u r e r a n g e (500OC t o 650°C) and a t low s l i d i n g s p e e d s b u t i t i s even more e f f e c t i v e a t h i g h s l i d i n g s p e e d s and h i g h t e m p e r a t u r e s ( r e f . 1 2 1 ) . B o r i c o x i d e (B203) e x h i b i t s a h i g h f r i c t i o n c o e f f i c i e n t when t h e t e m p e r a t u r e i s lower t h a n 50OoC ( r e f . 1 1 7 ) . Near t h e m e l t i n g p o i n t , t h e f r i c t i o n c o e f f i c i e n t o f b o r i c o x i d e d e c r e a s e s to less t h a n 0 . 1 0 . T h i s i s b e c a u s e of a marked r e d u c t i o n i n t h e f o r c e n e c e s sary f o r viscous shear. Chemically s t a b l e f l u o r i d e s such a s CeF3, CaF2, L i F o r BaF2 c a n b e u s e d a s s o l i d l u b r i c a n t s . Cerium f l u o r i d e (CeF3) h a s a t y s o n i t e o r m o d i f i e d l a t t i c e l a y e r s t r u c t u r e . I t i s water i n s o l u b l e and res i s t a n t t o t h e r m a l d i s s o c i a t i o n and c h e m i c a l r e d u c t i o n . I t s m e l t i n g p o i n t i s 1438OC and i t s t h e r m a l e x p a n s i o n c o e f f i c i e n t m a t c h e s t h o s e of n i c k e l b a s e s u p e r a l l o y s and s t a i n l e s s s t e e l . CaF3 s i g n i f i c a n t l y reduces w e a r i n I n c o n e l a l l o y s , b o t h i n a i r and a r g o n atmospheres up t o 1 OOO°C; i t s powder i s a good f i l m - f o r m e r and t h e fusion-boded CeF3 c o a t i n g s have p o s s i b i l i t i e s a s s o l i d l u b r i c a n t s ( r e f . 1 2 4 ) . When u s e d a s a n a d d i t i v e i n g r e a s e s , CeF3 s u b s t a n t i a l l y i n c r e a s e s t h e l o a d - c a r r y i n g c a p a c i t y of a good g r e a s e , and r e d u c e s w e a r w i t h o u t i n f l u e n c i n g t h e f r i c t i o n c o e f f i c i e n t . The wear r e d u c t i o n i s s l i g h t l y b e t t e r t h a n MoS2 ( b o t h a t room t e m p e r a t u r e and a t 100°C)
( r e f . 1 2 4 ) . Compared w i t h MoSZ and CeF3
,
g r a p h i t e appears t o be
l e s s e f f e c t i v e as a g r e a s e a d d i t i v e when u s e d e i t h e r a l o n e o r a s a
m i x t u r e w i t h CeF3. CeF3 h a s s e v e r a l a d v a n t a g e s o v e r MoS2; i t is curr e n t l y 30% cheaper, has higher temperature s t a b i l i t y . Its off-white colour can be helpful i n c e r t a i n circumstances. CaF2, LiF and BaF2 l u b r i c a t e a t h i g h t e m p e r a t u r e s and o v e r a w i d e r r a n g e of t e m p e r a t u r e s t h a n PbO. C o a t i n g s w i t h c o m p o s i t i o n s
71
from t h e CaF2/BaF2 b i n a r y e u t e c t i c s y s t e m c a n o p e r a t e from a b o u t 500 t o 95OoC, d e m o n s t r a t i n g a f r i c t i o n c o e f f i c i e n t o f a b o u t 0 . 1 (ref. 121). O t h e r i n o r g a n i c s o l i d l u b r i c a n t s s u c h a s metal h a l i d e s (cadmium i o d i d e , cadmium c h l o r i d e , cadmium bromide, c o b a l t c h l o r i d e , l e a d i o d i d e and m e r c u r i c i o d i d e ) a r e h i g h l y c o r r o s i v e . Ammg t h e s o l i d l u b r i c a n t s w e c a n i n c l u d e t h i n f i l m s o f s o f t metals s u c h as l e a d , cadmium, t i n , indium, s i l v e r and g o l d u s e d t o c o a t h a r d s u b s t r a t e s . They a r e e s p e c i a l l y u s e f u l when b o t h l u b r i c a t i o n and c o r r o s i o n p r o t e c t i o n are r e q u i r e d , b u t t h e y g e n e r a l l y e x h i b i t higher f r i c t i o n c h a r a c t e r i s t i c s than l a y e r laminar s o l i d s such as MoS2 (see C h a p t e r 7 . 2 ) . O r g a n i c compounds
-
f a t s , s o a p s , waxes, polymers and t h e r m a l l y
s t a b l e s u b s t a n c e s such as p h t a l o c y a n i n e s - a r e a l s o u s e d a s s o l i d l u b r i c a n t s . They g i v e a low f r i c t i o n c o e f f i c i e n t b u t c a n n o t b e used a t t e m p e r a t u r e s h i g h e r t h a n t h e i r m e l t i n g p o i n t s . Metallic s o a p s
a r e u s e d , f o r example, a s t h i c k e n e r s i n g r e a s e s ( s e e C h a p t e r 3 . 3 ) . Polymers s u c h a s PTFE o r P I a r e w i d e l y u s e d a s s o l i d b o d i e s and a l s o o f t e n a s f i l m s . Using PTFE, a r e m a r k a b l y low f r i c t i o n c o e f f i c i e n t c a n b e e x p e c t e d - from 0 . 0 1 t o 0.04 from a t e m p e r a t u r e a s low
a s t h a t o f l i q u i d hydrogen t o t h e d e c o m p o s i t i o n t e m p e r a t u r e of t h e polymer ( r e f . 1 2 1 ) . PTFE u s e d i n powder form a s a n a d d i t i o n t o o i l s o r g r e a s e s ( t h i c k e n e r ) c a n r e d u c e s t i c k - s l i p e f f e c t s and demnstrates good a n t i - s e i z u r e p r o p e r t i e s ( s e e C h a p t e r s 3.2 and 3 . 3 ) . P h t a l o c y a n i n e s (metal f r e e o r c o p p e r ) d e m o n s t r a t e l u b r i c a t i n g p r o p e r t i e s i n f e r i o r t o MoS2 b u t under c e r t a i n c o n d i t i o n s a r e s u p e r i o r tograph-
i t e . The f o r m a t i o n of c h e l a t e s bonds them s t r o n g l y t o m e t a l s u r f a c e s . T h e i r most i m p o r t a n t u s e i s a s a t h i c k e n e r i n h i g h temperat u r e greases. S o l i d l u b r i c a n t s b a s e d on c a p r o l a c t a m , p r o d u c t s o f r e a c t i o n s of
metal ( s u c h as F e , Co, Cu, T i ) o x i d e s h y d r a t e s w i t h a c r y s t a l l i n e c a p r o l a c t a m , d e m o n s t r a t e good l u b r i c a t i n g p r o p e r t i e s ( r e f . 1 2 5 ) . Such s o l i d l u b r i c a n t s are p e r i o d i c c o l l o i d s y s t e m s w i t h t h e p r o p e r t i e s of t h i x o t r o p i c g e l s y s t e m s t a c t o i d s c h a r a c t e r i z e d by t h e i r
-
v e r y low s h e a r s t r e n g t h . The m e c h a n i c a l l u b r i c a t i n g a c t i v i t y o f t h e l u b r i c a n t i s b a s e d on t h e e n l a r g e m e n t s o f t h e c1earanc.e between t h e r u b b i n g s u r f a c e s when t h e r u b b i n g r e g i o n i s f i l l e d w i t h t h e v i s c o u s f l o w of c o l l o i d p a r t i c l e s , which makes a s i g n i f i c a n t r e d u c t i o n i n
wear and f r i c t i o n p o s s i b l e . The l u b r i c a t i n g a b i l i t y o f t h e s e l u b r i c a n t s i s a l s o d e p e n d e n t on t h e i r p h y s i c o c h e m i c a l and c h e m i c a l i n t e r a c t i o n s w i t h t h e r u b b i n g s u r f a c e s ( r e f s . 125, 1 2 6 ) .
12
S o l i d l u b r i c a n t s a r e used p r i n c i p a l l y a s d i s p e r s i o n s i n o i l s and g r e a s e s . They a r e a l s o a p p l i e d sometimes a s powders, b u t t h i s k i n d o f l u b r i c a t i o n i s i n e f f e c t i v e b e c a u s e t h e l o o s e powders c a n b e e a s i l y pushed o u t of t h e c o n t a c t r e g i o n . The s o l i d l u b r i c a n t s a r e t h e r e f o r e o f t e n mechanically burnished. Another p o s s i b i l i t y is t o u s e r e s i n s t o make compacts o f s o l i d l u b r i c a n t m a t e r i a l s and i f t h e compact i s b r o u g h t i n t o c o n t a c t w i t h t h e s u r f a c e o f a s o l i d r u b b i n g e l e m e n t , l u b r i c a n t f i l m w i l l b e c o n s t a n t l y r e g e n e r a t e d ( f o r example i n b a l l b e a r i n g s , see C h a p t e r 7 . 2 and 9 . 4 ) .
Chemical and e l e c t r o -
chemical methods f o r f o r m i n g m e t a l s u r f a c e s a r e a l s o u s e f u l f o r saw s o l i d l u b r i c a n t s . Dry s u r f a c e a n t i - f r i c t i o n c o a t i n g s formed by v a r i o u s s o l i d l u b r i c a n t s a r e o f most i n t e r e s t i n m i n i a t u r e s y s t e m s
(see C h a p t e r ' 7 . 2 ) . They c a n be a p p l i e d by b o n d i n g , s p u t t e r i n g , o r i o n p l a t i n g ( s o f t metals) ( r e f s . 1 1 7 , 1 2 1 , 1 2 7 , 1 2 8 ) . A n t i - f r i c t i o n dry coatings a r e discussed i n d e t a i l i n Chapter 7.2. Magnetic f i e l d s e n e r g y c a n a l s o b e used t o i n t r o d u c e t h e s o l i d lubricant particles into
t h e r u b b i n g r e g i o n and k e e p them t h e r e .
Such l u b r i c a n t s c o n t a i n a n a d d i t i o n of 1 0 - 3 0 % by w e i g h t of f e r r o m a g n e t i c m a t e r i a l s and t h e r u b b i n g e l e m e n t s s h o u l d b e made o f magn e t i c m a t e r i a l s ( r e f s . 1 2 9 , 1 3 0 ) . T h i s method c a n e x t e n d t h e l i f e -
t i m e of a t r i b o l o g i c a l s y s t e m c o n s i d e r a b l y , making it one o f t h e most e f f e c t i v e ways t o a p p l y s o l i d l u b r i c a n t s .
73
4 , UNLUBRICATED SYSTEMS
41 I
I
METALLIC SYSTEMS V a r i o u s m e t a l l i c t r i b o l o g i c a l s y s t e m s a r e u s e d i n s m a l l mecha-
n i s m s . A t y p i c a l example i s j o u r n a l b e a r i n g s where t h e j o u r n a l s
a r e made o f f r e e - c u t t i n g o r s t a i n l e s s s t e e l and t h e b e a r i n g p l a t e s which make u p t h e f r a m e o f t h e mechanism a r e made o f l e a d e d h i g h -
s t e e l journal
t e n s i l e b r a s s . The r o u g h n e s s o f t h e r o l l e r - b u r n i s h e d
i s u s u a l l y Ra
<
0.16 ,um and t h e b e a r i n g h o l e s a f t e r d r i l l i n g o r
p i e r c i n g o p e r a t i o n s a r e reamed, trimmed o r p u s h - b r o a c h e d so t h a t t h e n e c e s s a r y smoothness R a %
0.32 ,um i s o b t a i n e d . B e a r i n g s w i t h
s t e e l and b e a r i n g b u s h e s of b r a s s demon-
j o u r n a l s of f r e e - c u t t i n g
s t r a t e a r e l a t i v e l y h i g h f r i c t i o n c o e f f i c i e n t when o p e r a t e d without l u b r i c a t i o n . The r i s e i n t h e f r i c t i o n c o e f f i c i e n t d u r i n g t h e r u n ning-in
p r o c e s s w a s observed
diameter 3 H9/e9,
-
f o r bearings with
l e n g t h 1 mm, a b e a r i n g b u s h made of Cu58Pbl.5Fe0.02
b r a s s and j o u r n a l o f C S
( r e f . 131-133)
-
0.148, Mn
-
0.69,
Si
-
P
0.66,
-
0.102,
0.137 f r e e - c u t t i n g s t e e l ; t h e r o u g h n e s s o f t h e r u b b i n g s u r f a c e
( R a ) was 0 . 1 5 ,um and 0.3 ,um f o r t h e j o u r n a l a n d t h e b e a r i n g b u s h
r e s p e c t i v e l y . The v a l u e s of t h e f r i c t i o n c o e f f i c i e n t b e f o r e a n d a f t e r running-in are l i s t e d i n Table 4 . 1 . f r i c t i o n c o e f f i c i e n t w a s 0.1-0.3 and 0.6-0.8
a f t e r running-in.
The a v e r a g e v a l u e o f t h e
a t t h e beginning of t h e o p e r a t i o n
The s l i d i n g d i s t a n c e of t h e r u n n i n g -
i n was v e r y s h o r t .
S L I D I N G SPEED, mm/s C o n t a c t pressure,MPa
fb fa
0.0167
1 .67
0.167
8
4
3
2
3
2
1
2
1
0.5
0.26
0.21
0.12
0.42
0.15
0.35
0.28
0.27
0.24
0.27
0.69
0.73
0.62
0.71
0.65
0.52
0.60
0.71
0.73
0.76
The r a d i a l wear i n t h e s e b e a r i n g s d e p e n d s o n t h e s l i d i n g s p e e d and t h e c o n t a c t p r e s s u r e . The i n c r e a s e i n r a d i a l wear a s a f u n c t i o n
of c o n t a c t p r e s s u r e i s l i n e a r when t h e c o n t a c t p r e s s u r e i s below
74 1 MPa. Higher c o n t a c t p r e s s u r e s r e s u l t i n a v e r y r a p i d i n c r e a s e i n
t h e r a d i a l wear and t h i s i n c r e a s e i s e v e n more pronounced a t h i g h s l i d i n g s p e e d s . The s t e a d y s t a t e wear i n t e n s i t y a s a f u n c t i o n of s l i d i n g s p e e d and c o n t a c t p r e s s u r e i s shown i n F i g . 4.1. Only t h e wear of t h e b r a s s b e a r i n g bushes w a s o b s e r v e d .
L
0 Q)
3
I
/
Contact pressure ,
MPp
F i g . 4.1. Radial wear i n t e n s i t y as a f u n c t i o n o f c o n t a c t p r e s s u r e f o r m i n i a t u r e (0 3 mm) s t e e l - b r a s s j o u r n a l b e a r i n g s . 1 s l i d i n g speed 1.67 mm/s , 2 0.167 mm/s, 3 0.0167 mm/s. The d o t t e d l i n e s show t h e p o s i t l o n o f t h e b e a r i n g s when s e i z u r e i s p o s s i b l e . The l a s t p o i n t ( c i r c l e ) means t h a t t h e b e a r i n g s e i z e s when a h i g h e r s l i d i n g speed and c o n t a c t p r e s s u r e a r e a p p l i e d ( r e f . 131).
-
-
-
75 The w e a r d e b r i s i n t h e b e a r i n g s w a s of f o u r t y p e s : 1 flakes, 2
-
brass g r i t , 3
-
f i n e black pwder, 4
-
-
brass
f i n e yellow
powder. I t i s p o s s i b l e t o estimate t h e wear i n t e n s i t y o f t h e b e a r i n g by o b s e r v i n g t h e wear d e b r i s . When t h e r e i s wear d e b r i s o f t h e 1 s t and 2nd t y p e s , t h e b e a r i n g must have b e e n o p e r a t i n g u n d e r h i g h c o n t a c t p r e s s u r e and t h e wear i n t e n s i t y i s h i g h ; when t h e d e b r i s
i s o f t h e 3rd and 4 t h t y p e s , t h e c o n t a c t p r e s s u r e and c o n s e q u e n t l y t h e wear i n t e n s i t y a r e l o w . When t h e c r i t i c a l , p e r m i s s i b l e pv v a l u e ( p v
-
-
contact pressure,
s l i d i n g s p e e d ) i s e x c e e d e d , t h e b e a r i n g s may s e i z e u p . When
s e i z u r e o c c u r s , t h e j o u r n a l s u r f a c e i s c o v e r e d by welded b r a s s f l a k e s and t h e j o u r n a l becomes wedged i n t h e b e a r i n g b u s h by t h e s e f l a k e s . When t h e s l i d i n g s p e e d i s v e r y low ( l e s s t h a n 0 . 0 1 7 mm/s) s e i z u r e d o e s n o t t a k e p l a c e s i n c e t h e w e a r d e b r i s and t h e welded b r a s s f l a k e s o x i d i z e . The b e a r i n g c l e a r a n c e i s a n i m p o r t a n t f a c t o r i n preventing s e i z u r e ; w i t h a l o w clearance, s e i z u r e occurs a f t e r a s h o r t e r s l i d i n g d i s t a n c e . The p r o b a b i l i t y o f s e i z u r e i s l o w e r when t h e b e a r i n g c l e a r a n c e i s h i g h e r ( r e f s . 1 3 1 , 1 3 4 , 1 3 5 ) . S e i z u r e o f t h e m i n i a t u r e s t e e l - b r a s s j o u r n a l b e a r i n g s i s caused by t h e a d h e s i o n - d e c o h e s i o n
( s e p a r a t i o n ) phenomena on t h e i n t e r f a c e .
The b e s t c r i t e r i o n f o r t h e s e i z u r e of t h e b e a r i n g s i s t h e c r i t i c a l
time, which c a n b e d e f i n e d a s t h e time from t h e b e g i n n i n g o f t h e o p e r a t i o n of t h e bearing t o t h e r a p i d , s h a r p i n c r e a s e i n f r i c t i o n t o r q u e which marks s e i z u r e . The l e n g t h o f t h e b e a r i n g l i f e b e f o r e s e i z u r e d e p e n d s o n t h e n a t u r e of t h e l o a d i n g : u n d e r s t a t i c l o a d i n g t h e l i f e o f t h e b e a r i n g i s s h o r t e r t h a n u n d e r dynamic l o a d i n g ( r e f . 1 3 4 ) . F o r t h e b e a r i n g ( h a v i n g nominal d i a m e t e r 3 mm, l e n g t h 1 mm and c l e a r a n c e a d e q u a t e t o t h e f i t H 9 / e 9 ) o p e r a t i n g a t t h e s t a t i c l o a d i n g when p E < 6 , 1 0
=- MPa,
v E < 0.63,
1 . 5 7 5 m / s , and a t
t h e dynamic l o a d i n g p E < 1, 2 > MPa ( t h e a m p l i t u d e A)
,
the fre-
quency f o f t h e l o a d v a r i a t i o n s from 0 . 1 7 to 0 . 5 1 Hz, and when t h e c o n s t a n t a v e r a g e c o n t a c t p r e s s u r e was k e p t p = 8 MPa and s l i d i n g s p e e d v = 0.00157 m / s i n v e s t i g a t i o n s ( r e f . 1 3 4 ) h a v e shown t h a t t h e bearing l i f e of a dynamically loaded b ear in g i s double t h a t
t o a comparable s t a t i c a l l y l o a d e d b e a r i n g . When t h e b e a r i n g i s o n l y r e q u i r e d t o o p e r a t e f o r a s h o r t p e r i o d , t h e l o a d c a n be 3 t i m e s h i g h e r t h a n t h e maximum a d m i s s i b l e l o a d f o r t h e o p e r a t i o n o f t h e same b e a r i n g o v e r a l o n g p e r i o d . The r e l i a b i l i t y f u n c t i o n s f o r b e a r i n g s a n a l y s e d u n d e r s t a t i c and dynamic l o a d i n g , o b t a i n e d by t h e u s e o f t h e Gumbells t h e o r y , c a n be e x p r e s s e d a s f o l l o w s :
76
t + 150 7182 p-1.62 -1.42 V
+
45
- 3.811
(4.la)
where t i s i n s . The l i f e o f a b e a r i n g o p e r a t e d u n d e r s t a t i c l o a d i n g i s g i v e n by (refs. 134, 1 3 5 ) :
-t
= 23700 p
where
t
-1.62
V
-1.42
(4.2a)
is a v e r a g e life ( i n s ) , p i s i n MPa a n d v i s i n m / s .
T h i s f u n c t i o n i s shown g r a p h i c a l l y i n F i g . 4 . 2 .
\ W i n g speed
Fig. 4.2.
L i f e (:)
I
mm/s
o f s t a t i c a l l y loaded s t e e l - b r a s s m i n i a t u r e j o u r n a l b e a r i n g .
77
The l i f e of a d y n a m i c a l l y l o a d e d b e a r i n g i s
t
= 834
+
133 f
-
130 A
(4 .2b)
where t i s i n s , f i n Hz and A i n MPa. The c o r r e s p o n d i n g g r a p h i s g i v e n i n F i g . 4.3.
Amplitude , MPa
F i g . 4 . 3 . L i f e ( f ) o f d y n a r a i c a l l y loaded s t e e l - b r a s s m i n i a t u r e journal b e a r i n g .
N o c o r r e l a t i o n between t h e d i a m e t r a l c l e a r a n c e and t h e l i f e of
t h e b e a r i n g was found ( r e f . 134). The r e l a t i o n s h i p between b e a r i n g l i f e and b e a r i n g c l e a r a n c e i s p r e s e n t e d i n F i g . 4.4. When i n v e s t i g a t e d u s i n g a s t a n d a r d ASTM pendulum ( s e e C h a p t e r 8.2.2)
,
t h e f r i c t i o n c o e f f i c i e n t i n a s t e e l - b r a s s system w a s d i s -
t i n c t l y lower t h a n t h a t i n a s t e e l - s t e e l ( s p h e r e - p l a t e ) t r i b o l o g i c a l system ( r e f . 4 3 ) . S t e e l - s t e e l
s y s t e m s a r e less u s e f u l t h e r e -
f o r e , s i n c e t h e a d h e s i o n between t h e u n l u b r i c a t e d r u b b i n g e l e m e n t s
78
i s very high ( r e f . 1 3 6 ) .
A 50
0
4-
0
0
3-
o
o
0
0
0
0
0
0
o o
2-
0
0
0
0
0 0
0
1-
0
0
I
I
1
25
35
45
Diametrol clearonce
I
55
*
65
, pm
F i g . 4.4. R e l a t i o n s h i p between b e a r i n g d i a m e t r a l c l e a r a n c e and 1 i f e o f m i n i a t u r e (0 3 m m ) s t e e l - b r a s s j o u r n a l b e a r i n g . C o n t a c t p r e s s u r e 6 MPa, s l i d i n g speed 0.63 mm/s
(ref.
134)
.
Wear s t u d i e s o f medium c a r b o n s t e e l r u b b i n g o n i t s e l f
( r e f . 137)
show t h a t t h e h a r d n e s s a n d t h e s t a t e o f o x i d a t i o n of t h e r u b b i n g s u r f a c e s a r e t h e p r i n c i p l e f a c t o r s i n d e t e r m i n i n g wear r a t e s . When t h e h a r d n e s s i s i n c r e a s e d from 2 0 0 HV t o 6 0 0 HV, t h e l i n e a r wear
r a t e d e c r e a s e s 6 f o l d . When a t r i b o l o g i c a l s y s t e m ( p i n - o n - d i s k ) w a s o p e r a t e d u n d e r vacuum c o n d i t i o n s , t h e h i g h e s t wear r a t e was s t e e l s a m p l e s was 4 5 0 HV, w h i l e t h e l o w e s t was found when t h e i r h a r d n e s s w a s 600 HV. The w e a r r a t e found when t h e h a r d n e s s o f t h e
o f s a m p l e s w i t h a h a r d n e s s of 4 5 0 HV was 6-10
times h i g h e r t h a n
s a m p l e s w i t h a h a r d n e s s of 6 0 0 HV; t h e wear r a t e o f s a m p l e s w i t h a h a r d n e s s o f 2 0 0 HV w a s a p p r o x i m a t e l y t h e a r i t h m e t i c mean o f t h e
79
wear rates of the samples with 450 and 600 HV hardness. For a tribological system operating in a vacuum, higher wear rates were observed than in normal air conditions and severe material transfer occured. The dominating tribo-oxidative process in normal air conditions and the adhesive wear mechanism in a vacuum were observed. For systems operating in air, the friction coefficient was 0.25 at the begining of the test and had increased to about 1.0 by the end of the investigation Austenitic stainless steel rubbing on itself in air at room temperature (ref. 138) involves equal rates of wear in both elements (pin-on-flat plate system). The wear is the result of prow formation. Prows are formed by the adhesive transfer of material from one surface to the other and have a layered structure composed of thin platelets formed by asperity interaction. The prows formed under low loads demonstrate reduced ductility which is probably caused by oxygen pick-up and their high martensite content. Prows formed under high loads are more ductile and are flattened by repeated interaction. Studies of austenitic stainless steels sliding on tool steel (in argon) (ref. 139) show that their tribological behaviour depends on the ease of formation of strain induced martensite, the nature of the transfer process and the relative hardness of the stainless steel sample. When the hardness of the tool steel sample is higher the transfer of the stainless steel to the tool steel surface occurs: when the hardness of the stainless steel is higher the transfer of the tool steel to the stainless steel occurs. The oxidational wear of steel-steel systems operating at elevated temperatures is the main cause of damage to the rubbing elements (refs. 140, 144). Stainless steels with their generally good oxidation resistance might be expected to be more wear-resistant at elevated temperatures than other steels. Oxidational wear can also occur in other than steel-steel metallic systems; for example, it can occur in steel-copper (ref. 142), or copper-copper systems (ref. 1431, even when the sliding speeds and loads are small and the ambient temperature is not elevated. Oxidation processes play a major role in producing wear damage in the fretting of steels at room temperature (ref. 144) Adhesive wear in metallic systems is typically accompanied by the transfer of material. Transfer occurs when the shear strength of the adhesive bond between two asperities is greater than that of transferring material but the nature of the geometry is also
.
80
sometimes i m p o r t a n t ( e . 9 . f o r a s l i d e r on a f l a t s u r f a c e o f t h e same m a t e r i a l , t r a n s f e r g e n e r a l l y o c c u r s from t h e f l a t p a r t t o t h e s l i d e r ( r e f . 145) )
. The
o c c u r r e n c e o f t h e t r a n s f e r i s o b v i o u s from
t h e a p p e a r a n c e o f a c o a t i n g o f c o p p e r - c o l o u r e d m a t e r i a l on t h e
s t e e l . A c l e a r c o n n e c t i o n e x i s t s between t h e t r a n s f e r l a y e r and t h e g e n e r a t i o n o f l o o s e wear d e b r i s ( r e f . 1 4 6 , 1 4 7 , 1 4 8 ) . S t u d i e s o f copper-based a l l o y s s l i d i n g a g a i n s t h a r d s t e e l s show t h a t t r a n s f e r l a y e r s o r p a t c h e s d e v e l o p on t h e s l i d i n g s u r f a c e s d u r i n g p r o l o n g e d s l i d i n g . However, when t h e s t a i n l e s s s t e e l s l i d e s a g a i n s t samples made o f Cu-Nil Cu-A1,
Cu-A1203, o r Cu-Be
a l l o y s , t h e t r a n s f e r l a y e r s begin t o develop e a r l i e r , even b e f o r e l o o s e wear d e b r i s c a n b e d e t e c t e d ( r e f . 1 4 6 ) . S i m i l a r t o o l s t e e l s l i d i n g a g a i n s t Cu
-
t e s t s on
1.8 w t % B e and c o p p e r d i s p e r s i o n
hardened w i t h A 1 2 0 j o r C u z O p a r t i c l e s , a l s o i n d i c a t e d t h a t m a t e r i a l t r a n s f e r p l a y s a s i g n i f i c a n t r o l e i n b o t h f r i c t i o n and wear i n t h e s e s y s t e m s . The w e a r i s h i g h when t h e s m a l l t r a n s f e r p a r t i cles a r e not dispersed o r separated b e f o r e they can accumulate t o
form t h e t r a n s f e r l a y e r ; t h e u s e o f a f l u i d l u b r i c a n t i s o n e way t o p r e v e n t t h i s a c c u m u l a t i o n ( r e f . 1 4 6 ) . Wear e q u a t i o n s ( s u c h a s A r c h a r d ' s laws o f wear) a r e i n a d e q u a t e f o r r e a l s l i d i n g s y s t e m s b e c a u s e t h e y do n o t i n c o r p o r a t e t h e e f f e c t s o f material t r a n s f e r , a l t h o u g h t h e s t u d i e s d e s c r i b e d i n r e f . 1 4 9 , u s i n g pin-on-disk s y s t e m s i n which p i n s made o f b r a s s , c o p p e r and aluminium w e r e rubbed a g a i n s t d i s k s o f s t e e l , b r a s s , c o p p e r and aluminium, showed t h a t f o r d r y a d h e s i v e w e a r t h e f i r s t and t h i r d l a w s o f A r c h a r d ' s t h e o r y a r e v a l i d , i . e . t h a t t h e volume of worn m a t e r i a l i s proport i o n a l t o t h e s l i d i n g d i s t a n c e and i n v e r s e l y p r o p o r t i o n a l t o t h e y i e l d stress, o r t h e h a r d n e s s of t h e s o f t e r m a t e r i a l . R e a l i s t i c
wear e q u a t i o n s s h o u l d t a k e i n t o a c c o u n t t h e p r o p e r t i e s o f b o t h s l i d i n g p a r t n e r s (and t h e t r a n s f e r material which forms between them) , a s w e l l a s g e o m e t r i c a l and e n v i r o n m e n t a l e f f e c t s ( r e f s . 146, 1 4 7 ) . Geometrical f a c t o r s and a d h e s i o n a r e b o t h i m p o r t a n t b e c a u s e r e v e r s i n g m a t e r i a l s which h a v e a g i v e n g e o m e t r y , o r changing t h e i r geometry, c a n have profound e f f e c t s which a r e n o t y e t w e l l c h a r a c t e r i z e d o r u n d e r s t o o d ( r e f s . 1 4 7 , 1 4 8 ) . I n pin-on-disk s t u d i e s ( r e f . 1481, t h e p r e f e r r e d t r a n s f e r d i r e c t i o n w a s from t h e d i s k t o t h e p i n ( t h e w e a r l o s s from t h e d i s k i s l a r g e r ) ard a d h e s i o n studies have shown t h a t t h e p r e f e r r e d t r a n s f e r d i r e c t i o n is from t h e cohes i v e l y weaker material t o t h e c o h e s i v e l y s t r o n g e r o n e . When t h e l o a d s d u r i n g rubbing are lower t h a n t h e y i e l d s t r e n g t h o f t h e w e a k e r m a t e r i a l , t h e w e a r depends o n t h e p r o c e s s e s o f f o r m a t i o n
81 and r u p t u r e o f o x i d e s o f t h e r u b b i n g m a t e r i a l s ( r e f . 1 5 0 ) . The wear o f a l u m i n i u m - s i l i c o n a l l o y s d e p e n d s o n t h e s i l i c o n c o n t e n t ( r e f . 151)
.A
h i g h e r s i l i c o n c o n t e n t l e a d s t o improved
wear c h a r a c t e r i s t i c s . T h i s h a s b e e n shown f o r h y p e r e u t e c t i c a l u minium-silicon a l l o y s c o n t a i n i n g 17-26% (by weight) S i r rubbing ( p i n ) a g a i n s t a t o o l s t e e l d i s k . I n c r e a s i n g t h e volume f r a c t i o n o f p r i m a r y s i l i c o n r e s u l t s i n t h e t r a n s i t i o n from m i l d t o s e v e r e wear o c c u r r i n g a t a h i g h e r l o a d . The c o m p o s i t i o n a l v a r i a t i o n s which a f f e c t m a t r i x h a r d n e s s h a v e a much smaller i n f l u e n c e o n w e a r r a t e s t h a n v a r i a t i o n s i n s i l i c o n c o n t e n t . The wear b e h a v i o u r of p u r e aluminium (when aluminium p i n s r u b a g a i n s t a s o f t s t e e l d i s k ) exh i b i t s two d i s t i n c t r e g i o n s o f wear i n t h e low l o a d r a n g e s marked by a c l e a r t r a n s i t i o n p o i n t ( r e f . 1 5 2 ) . A t v e r y low l o a d s , wear i s a p p a r e n t l y c o n t r o l l e d by a p u r e l y o x i d a t i v e mechanism, w h i l e a t m o d e r a t e l y h i g h l o a d s it c h a n g e s t o a combined o x i d a t i v e - c u m m e t a l l i c t y p e o f wear. The t r a n s i t i o n p o i n t i s a f u n c t i o n o f t h e s l i d i n g s p e e d , t h e r e l a t i v e h a r d n e s s a n d r o u g h n e s s o f t h e d i s k surface, and a l s o t h e a m b i e n t c o n d i t i o n s o f t e m p e r a t u r e and h u m i d i t y . S e v e r e , p u r e l y m e t a l l i c wear o c c u r s o n l y a t v e r y h i g h l o a d s . The a d d i t i o n o f g r a p h i t e (maximum 2 % by volume) t o aluminium improves
i t s wear r e s i s t a n c e r e m a r k a b l y and d e c r e a s e s t h e f r i c t i o n c o e f f i cient (ref. 153). S i n t e r e d m e t a l s , s u c h a s a l l o y s b a s e d o n i r o n and c o n t a i n i n g phosphorus, are v e r y i n t e r e s t i n g as a n t i - f r i c t i o n materials s i n c e t h e y can o p e r a t e without t h e a d d i t i o n of l u b r i c a n t ( r e f s . 154, 1 5 5 , 1 5 6 ) . Such a l l o y s c o n t a i n 0 . 2
-
2 % P. I n Fe-P-S
alloys,
phosphorus makes t h e m a t e r i a l more h e t e r o g e n e o u s , f a c i l i t a t i n g t h e r u n n i n g - i n p r o c e s s , and s u l p h u r lowers t h e f r i c t i o n c o e f f i c i e n t .
- ( 0 . 5 - 11%P - ( 0 . 9 - 1 ) %S mater i a l o b t a i n e d by e x t r u s i o n i s 0.18-0.33 ( r e f . 1 5 5 ) . The a d d i t i o n o f 2.5 - 5 % M o o r 0 . 6 - 1%C improves t h e m a t e r i a l ' s p r o p e r t i e s . I n t h e nitriding-carburizing process of phosphatizing s i n t e r e d
The f r i c t i o n c o e f f i c i e n t o f Fe
i r o n m a t e r i a l , a w e a r - r e s i s t a n t s u r f a c e l a y e r c o n t a i n i n g Fe (CNP) m n alloy
t y p e compounds i s c r e a t e d ( r e f . 1 5 7 ) . For s i n t e r e d Fe-P-X
c o n t a i n i n g 1 - 4 % Cu '(where X i s 2 - 4 % N i l M o o r c a r b i d e a l l o y cont a i n i n g Mn, C r , M o and F e )
,
a t s l i d i n g a g a i n s t s t e e l , when t h e
c o n t e n t s of P and Cu a r e i n c r e a s e d t h e f r i c t i o n c o e f f i c i e n t and wear d e c r e a s e ( r e f . 1 5 6 ) . Such a l l o y s c a n b e u s e d a s a n t i - f r i c t i o n and w e a r - r e s i s t a n t materials. Phosphorus is a l s o i n t r o d u c e d i n t o s i n t e r e d c o p p e r o r b r o n z e m a t e r i a l s t o improve t h e i r m e c h a n i c a l and a n t i - f r i c t i o n p r o p e r t i e s
82 ( r e f s . 154, 1 5 5 ) . I n c o p p e r - b a s e d a l l o y w i t h 8-11% Sn, t h e a d d i t i o n of 0.2-1.7%
P l o w e r s t h e s i n t e r i n g t e m p e r a t u r e a t which m a t e r i a l
w i t h good t r i b o l o g i c a l p r o p e r t i e s i s o b t a i n e d . Amorphous m e t a l s and a l l o y s s u c h a s i r o n , c o b a l t a n d n i c k e l a l l o y s h a v e a f r i c t i o n c o e f f i c i e n t o f 0.2-0.5
and a r e l a t i v e l y low
wear r a t e ( r e f . 1 5 8 ) . The a d h e s i v e wear p r o c e s s i n s u c h m a t e r i a l s g e n e r a l l y i n v o l v e s t h r e e s t a g e s : t h e f i r s t s t a g e shows a smooth s u r f a c e c r e a t e d by t h e removal o f a s p e r i t i e s , t h e s e c o n d i n v o l v e s s e v e r e p l a s t i c deformation as w e l l a s c r a c k n u c l e a t i o n and propagat i o n , and t h e t h i r d s t a g e p a r t i a l l y c h a n g e s t h e wear mechanism from a d h e s i v e t o a b r a s i v e a s a c o n s e q u e n c e o f t h e embedment o f f r a c t u r e particles a t the interface. The f r i c t i o n c o e f f i c i e n t f o r p u r e metals w a s f o u n d t o b e r e l a t ed t o t h e t h e o r e t i c a l t e n s i l e s t r e n g t h , t h e o r e t i c a l s h e a r s t r e n g t h and a c t u a l s h e a r s t r e n g t h o f m e t a l s ( r e f . 1 5 9 ) . By r u b b i n g t h e m e t a l s a g a i n s t t h e m s e l v e s a t a v e r y low s l i d i n g s p e e d ( 0 . 0 1 2 m/s) , -8 a t l o w l o a d s a n d i n a h i g h vacuum o f 3 1 0 P a , it w a s f o u n d t h a t t h e h i g h e r t h e s t r e n g t h of t h e m e t a l , t h e lower t h e c o e f f i c i e n t of f r i c t i o n . The wear r e s i s t a n c e o f p u r e metals i n c r e a s e d w i t h increasi n g c o v a l e n t b i n d i n g e n e r g y ( r e f . 1 6 0 ) . Metals w i t h a p u r e m e t a l l i c bond, s u c h a s Ag, Al, Cu and Mg, a l l h a v e s i m i l a r , r e l a t i v e l y low wear r a t e s , b u t p u r e metals w i t h a m e t a l l i c , p a r t i a l l y c o v a l e n t bond, s u c h a s N i l Mo or W , a r e c h a r a c t e r i z e d by h i g h wear r a t e s which i n c r e a s e a s t h e c o n t r i b u t i o n o f t h e c o v a l e n t bond becomes more i m p o r t a n t . The r e l a t i o n s h i p between t h e
wear r a t e a n d b i n d i n g
e n e r g y c a n b e d e s c r i b e d w i t h p a r a b o l a which a p p r o a c h e s t h e b r i z o n t a l axis. T y p i c a l wear c o e f f i c i e n t v a l u e s f o r a d h e s i v e wear i n m e t a l - o n -metal sliding systems are10-3 while f o r abrasive wear they
-
-
are l o m 2 ( r e f . 1 6 1 ) . S e v e r e g a l l i n g wear o c c u r s when two c l e a n metals w i t h a h i g h d e g r e e o f m e t a l l u r g i c a l c o m p a t i b i l i t y s l i d e over each o t h e r ; t h e w e a r c o e f f i c i e n t s are u s u a l l y i n t h e a n d t h e w e a r p a r t i c l e s i z e s i n t h e r a n g e of range of 200-20 ,um ( r e f .161) With l e s s c o m p a t i b l e metals, m o d e r a t e wear occ u r s ; when t h e c o n t a c t p r e s s u r e i s low t h e t y p i c a l wear c o e f f i c i e n t s are around w h i l e wear p a r t i c l e s i z e s a r e i n t h e r a n g e of 20-2 ,urn. The w e a r c o e f f i c i e n t a p p e a r s i n t h e Holm-Archard r e l a t i o n s h i p f o r a d h e s i v e a n d a b r a s i v e wear:
.
wear c o e f f i c i e n t
*
load
*
sliding distance
(4.3)
wear volume = hardness
83
For metals i n c o n t a c t w i t h metals, t h e p r e s e n c e o f oxygen red u c e s t h e a d h e s i o n and c o r r e s p o n d i n g l y t h e f r i c t i o n c o e f f i c i e n t , a s compared w i t h metals i n t h e c l e a n s t a t e . The f r i c t i o n a t various loads i n t h e p r e s e n c e of oxygen i s i n t e r m e d i a t e between t h e c l e a n s t a t e and t h e l u b r i c a t e d s t a t e ( r e f . 1 6 2 ) . The w e a r r a t e o f metals
i s a l s o r e d u c e d by t h e p r e s e n c e of oxygen s i n c e t h e o x i d e s which form a c t e i t h e r a s a l u b r i c a n t o r a n a b r a s i v e m a t e r i a l , depending o n t h e i r h a r d n e s s ( r e f s . 1 1 7 , 1 6 3 ) . For a comparison o f t h e hardn e s s of some metals and o x i d e s , see T a b l e 4 . 2 ( b a s e d o n r e f . 1 6 3 )
.
TABLE 4.2
MOHS HARDNESS OF METALS AND METAL O X I D E S
METAL
HARDNESS
METAL 0x1 DE
Sn
1 .8
Al
2.2
cu20
Zn
2.6
Z nO
cu
3
M"304
3.5 ca. 5.5
Fe CuN iMn Hardened s t e e l
6-6.5
Nitrided steel
a
Sintered hard s t e e l
9
Mg (OH)
Fe203 Mg0 Mn203 Sn02
N i t r ides 2'3
HARDNESS 2.6
3.5-4 4-4.5 5-5.5 5.5-6
6 6-6.5
6.5-7 7.5-8.5 9
The p r e s e n c e o f h a r d o x i d e s ( s u c h a s A 1 2 0 3 ) between r e l a t i v e l y s o f t s u r f a c e s ( s u c h a s A l ) r e s u l t s i n a r e l a t i v e l y h i g h wear r a t e .
42 I
POLYMER I C SYSTEMS
I
4.2.1
. METAL-POLYMER
SYSTEMS
Metal-polymer t r i b o l o g i c a l s y s t e m s a r e u s e d more and more f r e q u e n t l y i n m i n i a t u r e mechanisms. A r e l a t i v e l y low f r i c t i o n c o e f f ic i e n t and s u f f i c i e n t l y h i g h w e a r - r e s i s t a n c e c a n b e a c h i e v e d i n t h e s e s y s t e m s by t h e p r o p e r s e l e c t i o n o f t h e polymer a n d t h e m e t a l t o be u s e d . S t e e l i s t h e most s u i t a b l e metal f o r u s e a s a c o u n t e r f a c e f o r polymeric elements ( r e f s . 1 6 4 , 1 6 5 ) . T y p i c a l s l i d i n g systems i n m i n i a t u r e mechanisms - j o u r n a l b e a r i n g s have a j o u r n a l made o f f r e e c u t t i n g s t e e l and a b e a r i n g bush i n i n j e c t i o n -moulde d , t h e r m o p l a s t i c polymer; t h e j o u r n a l s u r f a c e i s u s u a l l y r o l l e r
84 b u r n i s h e d t o Ra < 0 . 2 p m . The wear r a t e o f t h e j o u r n a l i s n e g l i g i b l e compared t o t h e w e a r r a t e o f t h e p o l y m e r i c b e a r i n g b u s h (when t h e p o l y m e r i c m a t e r i a l i s n o t r e i n f o r c e d ) . S t u d i e s have shown t h a t t h e b e s t t r i b o l o g i c a l p r o p e r t i e s o f s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s a r e o b t a i n e d when t h e p o l y m e r i c b e a r i n g b u s h e s a r e made from POM or PA m a t e r i a l s ( r e f s . 166-180). B e a r i n g s w i t h POM b e a r i n g b u s h e s d e m o n s t r a t e b e t t e r t r i b o l o g i c a l p r o p e r t i e s t h a n t h o s e w i t h PA b u s h e s . The f r i c t i o n c o e f f i c i e n t of s t e e l
-
POM b e a r i n g s i s less t h a n 0 . 3 w h e r e a s f o r s t e e l
PA b e a r i n g s it can b e a s much a s 0 . 4 5 .
The d e p e n d e n c e o f t h e f r i c -
t i o n c o e f f i c i e n t o n t h e s l i d i n g s p e e d f o r t h e s e b e a r i n g s i s shown i n F i n s . 4 . 5 a n d 4.6.
-1
-2
-3 -4
~
41
0.2
0.3
t
i-
Sliding speed, m/s
F i g . 4 . 5 . F r i c t i o n c o e f f i c i e n t v s . s l i d i n g speed f o r steel-POM h m i n i a t u r e j o u r n a l b e a r i n g s . Nominal diameter 2.15 mm, e x t e r n a l diameter o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 mm, d i a m e t r a l c l e a r a n c e 1 .5-2%. 1 - c o n t a c t pressure p = 0 . 5 MPa, 2 - 1 MPa, 3 - 2 MPa, 4 3 MPa.
-
85
/ 2.0 3.0
0.2
t I
0 . T
0.3
Sliding speed, rn/s
F i g . 4 . 6 . F r i c t i o n c o e f f i c i e n t vs. s l i d i n g speed f o r steel-PA 6 m i n i a t u r e j o u r n a l b e a r i n g s . Nominal diameter 2 . 1 5 mm, e x t e r n a l d i a m e t e r o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 rnm, d i a m e t r a l c l e a r a n c e 1 .5-2%. 1 - c o n t a c t p r e s s u r e p = 0 . 5 MPa, 2 - 1 MPa, 3 - 2 MPa, 4 - 3 MPa.
The r e l a t i o n s h i p b e t w e e n t h e f r i c t i o n c o e f f i c i e n t , t h e s l i d i n g speed and t h e c o n t a c t p r e s s u r e can be expressed with t h e following formula : f = (alv
+
-(a3v + a4) a2)p
(4.4)
w h e r e f o r b e a r i n g s u s i n g PA 6 , PA 6 6 , POM h o r PO14 c , t h e v a l u e s o f a l , a2, a3 a n d a4 r e s p e c t i v e l y , 0.12;
PA 66
-
0.3,
0.125,
0.5,
a r e : PA 6
0.12;
POM h
-
-
0.45,
0.2,
0.16,
0.1,
0.3,
0.5, 0.13;
POI4 c - 0 . 1 1 , 0 . 1 3 5 , 0 . 4 2 5 , 0 . 2 5 . T h e v a l u e s of p a n d v i n e q n . ( 4 . 4 ) s h o u l d be i n t r o d u c e d i n MPa a n d i n m / s r e s p e c t i v e l y .
86
The b e a r i n g s w i t h POM b e a r i n g b u s h e s d e m o n s t r a t e b e t t e r wear r e s i s t a n c e t h a n PA 6
wear r a t e s f o r POM
-
-
b a s e d b e a r i n g s . A c o m p a r i s o n of t h e r a d i a l
and PA
-
based bear in g s (based on t h e s t u d i e s
d e s c r i b e d i n r e f . 1 7 4 ) is presented i n F i g . 4 . 7 .
1
01
I
I
I
2
3
*
F i g . 4 . 7 . R a d i a l wear r a t e v s . c o n t a c t p r e s s u r e f o r s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s . Nominal d i a m e t e r 2.15 nun , e x t e r n a l d i a m e t e r o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 mm, d i a m e t r a l c l e a r a n c e 1 .5-2%, s l i d i n g speed 0.0167 m / s , s l i d i n g d i s t a n c e 3 0 km. 1 - P A 6, 2 - P A 66, 3 - POM c , 4 - POM h .
I n c r e a s i n g t h e c o n t a c t p r e s s u r e r e s u l t s i n a n i n c r e a s e i n t h e radial wear r a t e , p a r t i c u l a r l y f o r PA
-
b a s e d b e a r i n g s , b u t when t h e con-
t a c t p r e s s u r e i s h i g h enough ( a b o v e 2 MPa) t h e i n c r e a s e i n t h e r a d i a l wear r a t e of POM
-
based bearin g s i s a l s o s i g n i f i c a n t .
I n c r e a s i n g t h e c o n t a c t p r e s s u r e above 3 MPa r e s u l t s i n a dramatically h i g h e r wear r a t e ( r e f s . 1 6 8 , 1 7 0 ) . The e f f e c t o f t h e s l i d i n g s p e e d
87 o n wear i s smaller t h a n t h e e f f e c t of t h e c o n t a c t p r e s s u r e ( r e f s . 1 6 8 , 1 7 0 , 1 7 4 ) . The w e a r r a t e i n t e n s i t y c a n g e n e r a l l y be assumed
t o be a s t r a i g h t l i n e c o r r e l a t e d w i t h t h e pv ( p v
-
-
c o n t a c t pressurer
s l i d i n g speed) parameter of miniature steel-polymer j o u r n a l
bearings. O t h e r m a t e r i a l s a l s o a p p l i c a b l e f o r t h e b u s h e s i n t h i s k i n d of b e a r i n g a r e t h e p o l y t e r e p h t h a l a t e s PETP a n d PBTP ( r e f s . 1 6 8 , 1 7 0 , 1 8 1 ) . The f r i c t i o n c o e f f i c i e n t of t h e s e b e a r i n g s i s simil a r t o t h o s e w i t h PA b e a r i n g b u s h e s b u t t h e wear r a t e i s s l i g h t l y
175-177,
h i g h e r . O t h e r s u i t a b l e b e a r i n g m a t e r i a l s a r e PPO, PC a n d P I ( r e f s . 175-177). O p e r a t i n g c o n d i t i o n s s u c h as ambient t e m p e r a t u r e and h u m i d i t y play on important role i n t h e o p e r a t i n g behaviour of steel-polymer m i n i a t u r e j o u r n a l b e a r i n g s ( r e f s . 11, 1 8 2 - 1 8 4 ) . A h i g h e r t e m p e r a t u r e lowers t h e f r i c t i o n c o e f f i c i e n t f o r steel-POM m i c r o b e a r i n g s ( F i g . 4.8,
based on r e f . 1 8 4 ) .
t
-101
0
1
1
20
40
I
60
1
80
t
Temperature , OC F i g . 4.8. F r i c t i o n c o e f f i c i e n t v s . t e m p e r a t u r e i n steel-POM h m i c r o b e a r i n g s (nominal d i a m e t e r 0.14 mm). Load 16 mN, s l i d i n g speed 3.5 m / s ( r e f . 1 8 4 ) .
aa The e f f e c t of h u m i d i t y on t h e f r i c t i o n c o e f f i c i e n t o f s t e e l - P O M b e a r i n g s i s n e g l i g i b l e ( r e f . 1 8 4 ) b u t f o r PA-based b e a r i n g s a n i n -
crease i n h u m i d i t y c a n lower t h e f r i c t i o n c o e f f i c i e n t and wear rate ( r e f . 1 8 5 ) .
During t h e o p e r a t i o n o f s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s , t h e r e i s a marked t r i b o e l e c t r i f i c a t i o n e f f e c t ( r e f . 186). The p o t e n t i a l produced i n a b e a r i n g w i t h PA 6 b u s h e s r i s e s i n a r e l a t i v e l y s h o r t time t o s e v e r a l v o l t s ( + ) and s t a b i l i z e s a t t h i s l e v e l . The e f f e c t of t h e s l i d i n g s p e e d and t h e c o n t a c t p r e s s u r e o n t h e po’zential produced i s p r e s e n t e d i n F i g s . 4 . 9 and 4 . 1 0 .
j )
2 0.3 Sliding speed m/s
,
F i g . 4.9. P o t e n t i a l produced vs. s l i d i n g speed f o r s t e e l - P A 6 m i n i a t u r e j o u r n a l b e a r i n g s . Nominal d i a m e t e r 0 2 . 1 5 mm, e x t e r n a l d i a m e t e r o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 mm, d i a m e t r a l c l e a r a n c e 1 . 5 - 2 % . C o n t a c t p r e s s u r e p : 1 - 3 MPa, 0 . 5 MPa. 2 - 2 MPa, 3 - 1 MPa, 4
-
89
Contact pressure, MPa
F i g . 4 . 1 0 . P o t e n t i a l produced v s . c o n t a c t p r e s s u r e f o r steel-PA 6 m i n i a t u r e j o u r n a l b e a r i n g s . Nominal d i a m e t e r 0 2 . 1 5 mm, e x t e r n a l d i a m e t e r o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2 . 1 mm, d i a m e t r a l c l e a r a n c e 1 . 5 - 2 % . S l i d i n g speed v : 1 - 0 . 0 6 7 m / s , 2 - 0 . 0 1 6 7 m / s .
The c o r r e l a t i o n between t h e p o t e n t i a l produced and t h e t r i b o l o g i c a l p r o p e r t i e s of t h e b e a r i n g s h a s been a n a l y s e d ( r e f . 186). Other m i n i a t u r e t r i b o l o g i c a l systems such a s t h e s t e e l s p h e r e - p l a t e i n o s c i l l a t i o n motion i n t h e ASTM pendulum o r t h e s p h e r e -
see Chapter 8 . 2 ) , denonstrate s i m i l a r t r i b o ' l o g i c a l p r o p e r t i e s t o t h e aforementioned m i n i a t u r e j o u r n a l b e a r i n g s ( r e f s . 43, 44, 106, 166, 169, 170, 171).
-oscillating t a b l e ( p l a t e )(UTI set-up,
F i l l e d polymers used a s m a t e r i a l s f o r t h e b e a r i n g bushes o r o t h e r s l i d i n g e l e m e n t s i n m i n i a t u r e mechanisms have b e t t e r t r i b o l o g i c a l p r o p e r t i e s t h a n u n f i l l e d polymers used f o r t h e same purpose. T h i s i s e s p e c i a l l y t r u e of polymers f i l l e d w i t h s o l i d l u b r i c a n t s such a s PTFE ( o r P E ) , g r a p h i t e o r MoS2. The f r i c t i o n c o e f f i c i e n t and t h e wear r a t e g e n e r a l l y d e c r e a s e ( r e f s . 164, 1 6 6 , 1 6 8 , 169, 170, 174, 178, 179). When t h e polymer i s r e i n f o r c e d , e . g . , w i t h g l a s s f i b r e , t h e e f f e c t of t h e r e i n f o r c e m e n t on t h e t r i b o -
90
l o g i c a l p r o p e r t i e s d e p e n d s o n t h e o p e r a t i n g c o n d i t i o n s ( r e f s . 175187-1891. The n a t u r e of t h e r e i n f o r c i n g m a t e r i a l a l s o a f f e c t s t h e t r i b o l o g i c a l p r o p e r t i e s of t h e s y s t e m s . When r e i n f o r c e d p l y m e x
-177,
i s u s e d i n metal-polymer s y s t e m s , e . g . i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s , t h e r e i s w e a r of t h e m e t a l ( s t e e l ) e l e m e n t ( r e f s . 1 7 0 , 1 7 1 , 1 7 4 , 188, 1 8 9 ) . S t e e l - p o l y m e r j o u r n a l b e a r i n g s which h a v e a b e a r i n g b u s h made
from polymer r e i n f o r c e d w i t h g l a s s f i b r e (25-35% by w e i g h t ) demons t r a t e a r e l a t i v e l y h i g h f r i c t i o n c o e f f i c i e n t ( u p t o 0 . 8 ) . The f r i c t i o n c o e f f i c i e n t , a n d a l s o t h e wear r a t e o f t h e s t e e l j o u r n a l , c a n b e r e d u c e d by t h e s i m u l t a n e o u s a d d i t i o n o f a s o l i d l u b r i c a n t s u c h a s PTFE, g r a p h i t e o r MoSZ. F i l l i n g t h e polymer w i t h g l a s s f i b r e i s g e n e r a l l y more e f f e c t i v e t h a n f i l l i n g it w i t h glass b e a d s , a l t h o u g h t h e o p p o s i t e i s t r u e i n t h e c a s e o f PBTP w i t h a r e l a t i v e l y low s h e a r modulus o f , s a y , 750 MPa ( r e f s . 1 8 8 , 1 8 9 ) . G l a s s b e a d s r e l e a s e d by w e a r may a c t a s a b r a s i v e s . The wear r a t e o f r e i n f o r c e d polymer m a t e r i a l s d e p e n d s t o a g r e a t e x t e n t o n t h e o p e r a t i n g c o n d i t i o n s . The wear r a t e s o f some r e i n f o r c e d polymers u s e d a s m a t e r i a l s f o r b e a r i n g b u s h e s i n m i n i a t u r e steel-polymer j o u r n a l b e a r i n g s are p r e s e n t e d i n F i g s . 4 .ll and 4 . 1 2
( p l o t s a r e based on t h e d a t a found i n r e f s . 1 7 4 and 1 7 0 ,
1 7 1 , 1 7 2 r e s p e c t i v e l y ) . An i n c r e a s e i n c o n t a c t p r e s s u r e a n d / o r
s l i d i n g s p e e d r e s u l t s i n a h i g h e r wear r a t e . A t h i g h l o a d s , thermal e f f e c t s p l a y a n i m p o r t a n t r o l e i n t h e wear p r o c e s s . S i n c e g l a s s - f i l l e d polymers g i v e a h i g h e r c o e f f i c i e n t o f f r i c t i o n t h e h e a t i s g r e a t e r t h a n i n b e a r i n g s w i t h b e a r i n g b u s h e s made from unreinforced polymers. A t h i g h l o a d s ( h i g h p v
-
contact pressure x sliding
s p e e d ) , t h e t h e r m a l d e c o m p o s i t i o n o f t h e polymer may o c c u r i n t h e neighbourhood of t h e f i b r e s ( r e f . 1 9 0 ) , s i n c e f i b r e s exposed on t h e surface tend t o act as hot spots during s l i d i n g . This effect
i s p r o b a b l y t h e main r e a s o n why t h e w e a r r a t e o f POM b e a r i n g bushes i s lower t h a n t h e wear r a t e o f g l a s s f i b r e f i l l e d POM b e a r i n g b u s h e s ( r e f s . 175-177,
see T a b l e 4 . 3 ) .
The f r i c t i o n c o e f f i c i e n t s o f s e v e r a l f i l l e d p o l y m e r s when t e s t e d i n a p l a t e - s p h e r e s y s t e m ( U T I a p p a r a t u s , see C h a p t e r 8 . 2 ) a r e shown i n F i g . 4 . 1 3
( b a s e d o n r e f s . 1 7 1 , 1 7 2 ) . The f r i c t i o n
c o e f f i c i e n t s a r e g e n e r a l l y similar t o t h o s e o b s e r v e d i n m i n i a t u r e j o u r n a l b e a r i n g s ( r e f s . 1 6 7 , 1 7 0 , 1 7 1 , 1 7 2 , a n d p e r s o n a l communic a t i o n from t h e a u t h o r o f r e f s . 1 7 5 - 1 7 7 ) . The w e a r r a t e of t h e rubbing p l a t e s under t h e c o n d i t i o n s d e s c r i b e d i n t h e c a p t i o n t o F i g . 4.13 w a s n e g l i g i b l e .
91
A
1
100 .
L
Sliding distance , k m
F i g . 4 . 1 1 . R a d i a l wear r a t e s o f p o l y m e r i c b e a r i ng bush o f s t e e l -polymer m i n i a t u r e j o u r n a l b e a r i n g s w i t h bushes made o f PA 6 + 25% g l a s s f i b r e + 4% MoSg ( p l o t s 1 and 2) o r PA 6 + 25% g l a s s f i b r e + 4% g r a p h i t e ( p l o t s 3 and 4 ) v s . s l i d i n g d i s t a n c e . Nominal diameter 2.15 mm, e x t e r n a l diameter o f t h e p o l y m e r i c b e a r i n g bush 6 mm, b e a r i n g l e n g t h 2.1 mm, r e l a t i v e b e a r i n g c l e a r a n c e 1 .5-2%, sl i d i n g speed 0 . 0 6 7 m / s . 1,3 - cont a c t pressure p = 0 . 5 MPa, 2,4 - 0.2 MPa.
92
I
i
I
10
20
30
*
sliding distonce , km
F i g . 4.12. R a d i a l wear r a t e o f p o l y m e r i c b e a r i n g bush o f s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s w i t h bushes made f r o m u n r e i n f o r c e d a n d r e i n f o r c e d p o l y m e r s v s . s l i d i n g d i s t a n c e . Nominal b e a r i n g d i a m e t e r 1 mm, e x t e r n a l d i a m e t e r 2.7 mm, b e a r i n g l e n g t h ( l e n g t h o f c o n t a c t w i t h t h e j o u r n a l ) 1 mm, r e l a t i v e c l e a r a n c e 3-6%. 1 - PC ( s l i d i n g speed 314 mm/s, c o n t a c t p r e s s u r e 0 . 2 2 5 MPa), 2 - PA 6 6 (13.1 mm/s, 2.5 MPa), 3 - PBTP ( 1 3 . 1 m m / s , 1 .25 MPa), 4 - PBTP + 28% g l a s s beads 5 - 50 ,urn (13.1 mm/s, 1 . 2 5 MPa), 5 - PBTP + g l a s s f i b r e (13.1 m m / s , 1 , 2 5 MPa), 6 - PA 6 6 + 50% g l a s s f i b r e ( 1 3 . 1 m m / s , 1 . 2 5 MPa), 7 - PA 6 6 + g l a s s beads ( 1 3 . 1 mm/s, 1,25 MPa), 8 - PC + 3 0 % g l a s s f i b r e (314 mm/s, 0.225 MPa).
93 TABLE 4.3 SPECIFIC WEAR RATE OF POLYMERIC BEARING BUSH I N STEEL-POLYMER JOURNAL BEARINGS. JOURNAL MADE OF STAINLESS STEEL, GROUND SURFACE TO R = 0 . 1 - 0 . 2 g m , NOMINAL BEARING DIAMETER 5 mm, EXTERNAL DIAMETER 1 0 mm, B E A R ~ N GLENGTH 2.8 mm ( r e f s . 175-1 77)
.
SPECIFIC WEAR RATE,
km MPa
POLYMERIC MATERIAL v = 0.26 m/s p = 0.075MPa PA 66 PA 6 6 + 25% g l a s s f i b r e PA 12 PA 12 + 30% g l a s s f i b r e POM c POM c + 30% g l a s s f i b r e POM c + PTFE g l a s s f i b r e PC PC + 30% g l a s s f i b r e PPO ABS PETP PI P I + inorganic f i l l e r PTFE + i n o r g a n i c f i l l e r
v = 0.026m/s p = 0 . 7 5 MPa
v=0.0026 m / s p=7.5 MPa
8.2
9.1
7.0
6.6
10.6 2.5 1 .o
9.9 2.3 0.3
14.3 11.5 6.5 16 0.81 68 1 .8 2.9
44 1.3 13 1.5
13 200 0.7
3.3 0.20
0.13
51 0.41 7.3 2.4
15 28 1 .1 2.0 0.07 0.30
4.8 8.2 20
3.6 2.1 0.57 0.91
f
F i g . 4 . 1 3 . F r i c t i o n c o e f f i c i e n t f o r p l a t e - s p h e r e s y s t e m (UTI a p p a r a t u s , s e e C h a p t e r 8 . 2 ) . O s c i l l a t i n g p o l y m e r p l a t e 1 0 x 6 ~ 2mm, s t e e l s p h e r e ( b e a r i n g s t e e l ) 1/2". Load 6 . 1 N, s l i d i n g speed lOmm/s. 1 - PA 66 + 40% c a r b o n f i b r e , 2 PETP + 30% g l a s s f i b r e , 3 - PBTP + 30% g l a s s f i b r e , 4 - PBTP, 5 - PC, 6 - PC + 30% g l a s s f i b r e , 7 - PC + 30% g l a s s f i b r e + 15% PTFE, 8 - PA1 + 3% T i 0 2 + 0.5% PTFE, 9 - P A 1 + 3% T i 0 2 + 0.5% PTFE, 10 - POM h ( D e l r i n 500 NC 1 0 ) . 11 - POM h c h e m i c a l l y l u b r i c a t e d ( D e l r i n 500 CL) ( r e f . 1 7 1 ) .
-
94
The wear p r o c e s s i n m i n i a t u r e s t e e l - p o l y m e r s y s t e m s ( e . g . journ a l b e a r i n g s ) is of a n adhesive-cohesive n a t u r e s i n c e t h e s u r f a c e of t h e s t e e l e l e m e n t i s v e r y smooth, s l i d i n g s p e e d s a r e v e r y low and t h e c o n t a c t p r e s s u r e may b e h i g h . The t r a n s f e r o f t h e p o l y -
m e r i c m a t e r i a l t o t h e s t e e l s u r f a c e a n d s u c c e s s f u l s h e a r i n g o f new l a y e r s a r e t y p i c a l o f u n f i l l e d m a t e r i a l s s u c h a s PA
a n d POM (refs.
1 9 1 - 1 9 3 ) . The t h i c k n e s s o f t h e f i l m o f t r a n s f e r r e d m a t e r i a l s t a b i l i z e s b e f o r e s t e a d y - s t a t e wear o c c u r s d u r i n g c o n t i n u o u s s l i d i n g . The wear of t h e polymer o c c u r s by t h e p r o c e s s e s o f t r a n s f e r t o t h e c o u n t e r f a c e and t h e s u b s e q u e n t d e t a c h m e n t of t h e f r a g m e n t s f r o m t h e t r a c k ( r e f . 1 9 4 ) . The i n c r e a s e i n f i l m t h i c k n e s s is l i m i t e d b y t h e a d h e s i v e bond r e s i s t a n c e o f t h e polymer f i l m t o t h e m e t a l s u r f a c e . When t h e maximum t h i c k n e s s o f t h e f i l m i s r e a c h e d , t h e f i l m c a n l o s e i t s l o a d c a p a c i t y , i s d e s t r o y e d and e s c a p e s as w e a r debris when t h e e l a s t i c s t r a i n e n e r g y s t o r e d i n it becomes g r e a t e r t h a n o r e q u a l t o t h e a d h e s i o n a l energy a c t i n g o v e r t h e i n t e r f a c e ( r e f . 1 9 5 ) . The r e l a t i o n s h i p between t h e r a d i a l w e a r i n t e n s i t y o f t h e polymeric b e a r i n g bush i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s and t h e maximum t h i c k n e s s o f t h e polymer t r a n s f e r r e d f i l m i s p r e sented i n Fig. 4.14
( r e f . 1 9 6 ) . The i n c r e a s e i n f i l m t h i c k n e s s i s
accompanied by a n i n c r e a s e i n wear i n t e n s i t y ; t h i s e f f e c t may b e due t o t h e l o w e r h e a t t r a n s f e r from t h e f r i c t i o n r e g i o n by t h e
s t e e l s h a f t v i a t h e t h i c k e r polymer t r a n s f e r r e d f i l m . Nevertheless, t h e t r a n s f e r o f t h e polymer m a t e r i a l i n m e t a l - p o l y m e r
systems i s
g e n e r a l l y b e n e f i c i a l b e c a u s e o f i t s s e l f - l u b r i c a t i n g a c t i o n . The amorphous polymers ( e . g . P C , PPO) do n o t t r a n s f e r c o h e r e n t f i l m s t o t h e c o u n t e r f a c e d u r i n g s l i d i n g a n d t h e i r wear r a t e i s r e l a t i v e l y high (see Table 4 . 3 )
.
The wear d e b r i s formed d u r i n g s l i d i n g between a polymer and
s t e e l s u r f a c e a r e u s u a l l y e l l i p s o i d i n s h a p e , t h e t h i c k n e s s of t h e p a r t i c l e depending on t h e f o r m a t i o n p r o c e s s ( r e f s . 1 9 4 , 1 9 7 , 1 9 8 ) . I f t h e p a r t i c l e s e s c a p e from t h e i n t e r f a c e as s o o n a s t h e y are formed, t h e y a r e l e s s t h i c k t h a n i f t h e y a r e t r a p p e d between t h e s l i d i n g s u r f a c e s a n d bond a g a i n w i t h t h e polymer s u r f a c e . A h i g h e r
wear r a t e o f t h e polymer i s u s u a l l y accompanied by l a r g e r wear p a r t i c l e s ( r e f s . 1 9 4 , 1 9 7 , 1 9 8 ) . The r e l a t i o n s h i p between t h e r a d i a l wear i n t e n s i t y of t h e p o l y m e r i c b e a r i n g b u s h i n m i n i a t u r e steel-polymer j o u r n a l b e a r i n g s i s presented i n F i g . 4.15 196).
(ref.
95
E
Y
E 30 =L
*
I )
r, m c
Q, -c,
.s
20
L
a
Q,
3
A
0 -0
.d
&!
1c
6 "
4
C
-
I
I
-
,
2
4
3
Film thickness
7 I
c
5
*
, pm
F i g . 4 . 1 4 . Radial wear i n t e n s i t y o f p o l y m e r i c b e a r i n g bush i n a s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g v s . t h i c k n e s s o f t r a n s f e r r e d polymer f i Im. C i r c l e s : beari n g h o l e 1 mm, e x t e r n a l b e a r i n g bush d i a m e t e r 2.7 mm, b e a r i n g l e n g t h 1 mm. r e l a t i v e c l e a r a n c e 4-6%, s l i d i n g speed 1 3 . 1 mm/s, c o n t a c t p r e s s u r e 1.25 MPa. Squares: b e a r i n g h o l e 5 mm, e x t e r n a l b e a r i n g bush d i a m e t e r 10 mm, b e a r i n g l e n g t h 2.8 mm, r e l a t i v e c l e a r a n c e 1%, s l i d i n g speed 2.6 mm/s, c o n t a c t p r e s s u r e 0.75 MPa. 1 - POM h, 2 - POM C , 3 - PA 11, 4 - PA 66, 5 PBTP, 6 PA 12. 7 - PETP.
-
-
The a d h e s i v e wear o c c u r r i n g i n t h e m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g c o r r e l a t e s w i t h t h e a d h e s i v e bonding f o r c e between t h e r u b b i n g s u r f a c e s which may be d e t e r m i n e d o n t h e b a s i s of t h e t h e o r y of f l u c t u a t i o n s of Van d e r Waals d i s p e r s i o n f o r c e s ( r e f s . 1 9 9 , 2 0 0 ) . Using I s r a e l a s h v i l y ' s
formula (see refs. 2 0 1 , 2 0 2 ) ,
t h e a d h e s i v e f o r c e p e r u n i t c o n t a c t area Za f o r t h e f l a t c o n t a c t i n d i e l e c t r i c - d i e l e c t r i c s e p a r a t e d by t h e l a y e r o f t h e d i e l e c t r i c
96
l i q u i d can be calculated as follows:
where h r e p r e s e n t s h/2‘ii(h
-
Planck’s c o n s t a n t ) , H i s t h e distance
between s u r f a c e s ( t h i c k n e s s o f t h e l i q u i d l a y e r ) , u ,
is the fre-
quency of h i g h e s t a b s o r p t i o n , which may b e t a k e n as b e i n g t h e same f o r t h e m a j o r i t y o f d i e l e c t r i c s , a d e q u a t e t o t h e u l t r a v i o l e t wavel e n g t h ( A o ) o f 1 0 0 nm
,
a n d El0,
E20
and &30 a r e t h e d i e l e c t r i c
c o n s t a n t s of t h e two d i e l e c t r i c s a n d t h e s e p a r a t i n g l i q u i d r e s p e c tively.
Wear particle thickness p m
F i g . 4 . 1 5 . R a d i a l wear i n t e n s i t y o f p o l y m e r i c b e a r i n g bush i n a s t e e l -polymer m i n i a t u r e j o u r n a l b e a r i n g v s . wear p a r t i c l e t h i c k n e s s . S l i d i n g speed 26 mm/s, c o n t a c t p r e s s u r e p = 0.75 MPa. 1 - P I , 2 POM c , 3 - PC, 4 - P E T P , 5 - PA 6 6 , 6 - PA 1 2 , 7 PPO, 8 ABS.
-
-
-
97
F o r metal-polymer c o n t a c t w i t h o u t l i q u i d E 3 0 = 1, E 2 0 -m and w e o b t a i n
h oo (El0 -
I
1) (4.6)
The a d h e s i v e f o r c e p e r u n i t a r e a of c o n t a c t i n c r e a s e s parabol i c a l l y with t h e i n c r e a s e i n d i e l e c t r i c c o n s t a n t of t h e polymeric
m a t e r i a l . The c o r r e l a t i o n between t h e r a d i a l wear i n t e n s i t y o f t h e p o l y m e r i c b e a r i n g bush i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s and t h e r a t i o o f
rmp/ec( e c -
c o h e s i v e e n e r g y d e n s i t y ) i s discussed
elsewhere ( r e f . 1 9 6 ) . For steel-polymer m i n i a t u r e j o u r n a l b e a r i n g s w i t h a b e a r i n g bush made from u n f i l l e d polymer, t h e a d h e s i v e - c o h e s i v e p r o p e r t i e s of t h e polymers seem t o p l a y a dominant r o l e i n t h e wear mechanism
when t h e c o n t a c t p r e s s u r e i s r e l a t i v e l y h i g h a n d t h e s l i d i n g s p e e d v e r y s m a l l ( s a y below 0 . 0 2 m / s ) .
The f r i c t i o n a l h e a t i n g i n t h e
f r i c t i o n region i s then negligible (say a temperature rise i n t h e f r i c t i o n r e g i o n a T < 3 K)
. In
s u c h s i t u a t i o n s t h e wear o f t h e p o l y -
m e r m a t e r i a l o f t h e b e a r i n g bush depends o n b o t h t h e a d h e s i v e b n i i s a t t h e i n t e r f a c e between t h e r u b b i n g e l e m e n t s and o n t h e c o h e s i v e
p r o p e r t i e s of t h e m a t e r i a l b e i n g worn. The g e n e r a l f o r m u l a r e l a t i n g t o t h e volume of worn polymer m a t e r i a l ( V )
,
t a k i n g i n t o con-
s i d e r a t i o n t h e a d h e s i o n a l e n e r g y o n t h e i n t e r f a c e (Ea) a n d t h e c o h e s i o n a l e n e r g y of t h e polymer ( E c ) , c a n be w r i t t e n a s f o l l o w s : V = f
Ea
(4.7)
( 7 ) C
The a d h e s i o n a l e n e r g y on t h e i n t e r f a c e c a n be d e t e r m i n e d u s i n g t h e formula: I
Ea =
wps d 2
'Po
(4.8)
where W
is t h e s p e c i f i c energy of adhesion a t t h e polymer-steel PS i n t e r f a c e , d2 t h e bearing h o l e diameter, 1 t h e bearing l e n g t h , and
YO
t h e h a l f a n g l e o f c o n t a c t between j o u r n a l and b e a r i n g b u s h when
t h e r a d i a l wear w > 0 . The h a l f a n g l e o f c o n t a c t , 19, following formula:
I
,
can be c a l c u l a t e d using t h e
98
cos
, y o --
+
-d12
(s
+
2a
(s
2 dl
where dl
+
+
2wI2
+
(dl
+
s)
2 (4.9)
2a + 2w)
is t h e journal diameter, s t h e diametral clearance, a t h e
d e f o r m a t i o n o f t h e p o l y m e r i c b e a r i n g b u s h u n d e r l o a d , and w t h e r a d i a l w e a r o f t h e polymeric b e a r i n g bush. The d e f o r m a t i o n o f t h e p o l y m e r i c b e a r i n g bush u n d e r a p a r t i c u l a r load can be estimated using t h e formula: Pm a = g -
(4 .lo)
E
where g i s t h e w a l l t h i c k n e s s o f t h e b e a r i n g b u s h , pm t h e maximum v a l u e of t h e r e a l i s t i c c o n t a c t p r e s s u r e , a n d E t h e e l a s t i c i t y modulus o f t h e polymer u s e d , The maximum v a l u e o f t h e r e a l i s t i c c o n t a c t p r e s s u r e pm was d e t e r m i n e d ( r e f . 203) t o be pm -
3.97 Tsin
yo
p
,
<
p
when
( 4 .11)
20°
or 4.12) where t h e h a l f a n g l e of c o n t a c t
yo
(when t h e r a d i a l w e a r w = 0 ) i s
i n r a d i a n s . F o r s t e e l - P A b e a r i n g s A1 a n d A 2 c a n b e t a k e n a s 0.32 1 . 1 7 r e s p e c t i v e l y , a n d f o r steel-POM b e a r i n g s a s 0.33 a n d
and
1. 1 6 r e s p e c t i v e l y
.
The h a l f a n g l e of c o n t a c t
yo
c a n b e e s t i m a t e d u s i n g t h e follow-
Im
(4.13)
i n g f o r m u l a ( r e f . 203) :
yo where
p=
PdZ 1 Es mm
= k (-
P+
1
and t h e p a r a m e t e r s k and m f o r m i n i a t u r e steel-
-polymer b e a r i n g s w i t h PA 6 o r POM h b u s h e s are 0 . 7 7 a n d 0 . 3 5 ( P A 6 ) o r 0.78 a n d 0.39
(POM h ) .
The c o n t a c t p r e s s u r e d i s t r i b u t i o n p ( y ) i n t h e b e a r i n g s d i s c u s s e d c a n b e e x p r e s s e d by t h e f o r m u l a ( r e f . 2 0 3 ) :
p ( y ) = pm
[1 -
'Q ) 1 . 8 9 (-
YO
I
0.44
(4.14)
99 The c o h e s i o n a l e n e r g y Ec s h o u l d be t r e a t e d a s t h e c o h e s i v e e n e r g y o f t h e worn p o l y m e r i c m a t e r i a l , i . e . Ec = V ec
( 4 .15)
The c o h e s i v e e n e r g y d e n s i t y ec o f p o l y m e r s c a n be f o u n d i n r e f . 204 o r c a l c u l a t e d by t a k i n g i n t o c o n s i d e r a t i o n t h e r e l a t i o n s h i p ( r e f . 205)
A
2 -(
P2 where
ypl
and
(4.16)
ec2
yp2
are t h e s u r f a c e f r e e e n e r g i e s of t h e t w o poly-
mers u s e d . The s u r f a c e f r e e e n e r g i e s of PA 6 , POM h a n d POM c u s e d a s 4 5 . 5 a n d 37.6 mJ/m2
m a t e r i a l s €or b e a r i n g b u s h e s a r e 53.5,
respec-
t i v e l y (ref. 2 0 6 ) . The a n a l y s i s c a r r i e d o u t i n r e f s . 1 9 6 , 207 h a s shown t h a t t h e r e l a t i o n s h i p g i v e n by e q n .
( 4 . 7 ) f o r miniature steel-polymer jour-
n a l b e a r i n g s c a n be e x p r e s s e d a s f o l l o w s : (4.17)
where t h e p a r a m e t e r s a 5 a n d a 6 f o r b e a r i n g s w i t h a nominal diameter o f 2.15 mm c a n be t a k e n a s 2.34
a n d -1.11 r e s p e c t i v e l y .
The r e l a t i o n s h i p between t h e volume of t h e worn polymer mater i a l , V, a n d t h e r a d i a l w e a r w of t h e p o l y m e r i c b e a r i n g b u s h i s a+w V
f
Va = d l l
1
a r c cos
1
-
r ( r + d2) dl
S
(7 + r )
(4.l a )
0
where Va i s t h e volume of p o l y m e r i c b e a r i n g b u s h d e f o r m a t i o n q u i v a l e n t t o t h e r a d i a l d e f o r m a t i o n a , and r i s t h e r a d i u s ( i n t e g r a tion variable). Formula ( 4 . 1 7 ) a l l o w s u s t o p r e d i c t t h e a d h e s i v e w e a r i n unl u b r i c a t e d m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s when t h e s l i d i n g s p e e d i s v e r y low ( s a y 0.02 m / s a t maximum p r a c t i c a l c o n t a c t p r e s s u r e p < 3 MPa) and when t h e t e m p e r a t u r e r i s e i n t h e f r i c t i o n r e g i o n i s below 3 K . When t h e s l i d i n g s p e e d i s r e l a t i v e l y h i g h ( a n d t h e c o n t a c t p r e s s u r e is n o t t o o l o w ) , t h e t h e r m a l e f f e c t s i n t h e f r i c t i o n a r e a
100
s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n . The t h e r m a l e f f e c t o n t h e wear p r o c e s s o f polymer i s i m p o r t a n t ( r e f s . 1 9 1 , 1 9 3 , 208, 209-215). The d i s s i p a t i o n o f t h e f r i c t i o n a l h e a t i s s m a l l b e c a u s e o f t h e r e l a t i v e l y small d i m e n s i o n s o f t h e r u b b i n g e l e m e n t s a n d t h e low. t h e r m a l c o n d u c t i v i t y o f p o l y m e r s . The t e m p e r a t u r e i n t h e f r i c t i o n r e g i o n may r e a c h t h e m e l t i n g t e m p e r a t u r e o f t h e polymer i n some c o n t a c t s and l o c a l m e l t i n g w i l l o c c u r ( r e f s . 209, 2 1 4 ) . Some ext r u s i o n o f t h e m o l t e n polymer from t h e c o n t a c t r e g i o n o c c u r s i n non-composite polymer wear ( r e f . 2 1 1 ) . The d e s t r u c t i o n o f t h e c r y s t a l l i n e phase and a d e c r e a s e i n t h e molecular weight o f t h e polymer wear p r o d u c t s t y p i c a l l y accompany t h e wear p r o c e s s when t h e r e i s i n t e n s i v e h e a t g e n e r a t i o n i n s l i d i n g metal-polymer cont a c t s ( r e f s . 2 0 9 , 213, 2 1 4 , 2 1 6 ) . The t h e r m a l e n e r g y produced i n t h e f r i c t i o n a r e a , e s t i m a t e d by measuring t h e r i s e i n t e m p e r a t u r e , e f f e c t i v e l y d e t e r m i n e s t h e dynamics o f t h e wear p r o c e s s o f t h e polymer e l e m e n t i n metal-polymer t r i b o l o g i c a l s y s t e m s . T h e r e i s a c o r r e l a t i o n between t h e t e m p e r a t u r e rise i n t h e f r i c t i o n area o f s t e e l - p o l y m e r b e a r i n g s a n d t h e wear o f t h e p o l y -
m e r b e a r i n g b u s h ( r e f . 2 0 8 ) . The i n c r e a s e i n r a d i a l wear i n t e n s i t y which accompanies t h e t e m p e r a t u r e r i s e h a s a p a r a b o l i c c h a r a c t e r (Fig. 4.16, r e f . 1 9 6 ) . The f r i c t i o n a l ( t h e r m a l ) e n e r g y i s t r a n s f e r r e d from t h e f r i c t i o n r e g i o n o f t h e b e a r i n g s m a i n l y by t h e r o t a t i n g s t e e l j o u r n a l . The e n e r g e t i c l o a d i n g o f t h e p o l y m e r i c e l e m e n t i s c a u s e d p r i m i p n l l y by t h e s t o r i n g of t h e r m a l e n e r g y i n i t s i n c e t h e p o t e n t i a l e n e r g y i n t r o d u c e d a s a r e s u l t of d e f o r m a t i o n o f t h e b e a r i n g b u s h i s v e r y s m a l l ( r e f . 1 9 6 ) . When t h e e n e r g e t i c a p p r o a c h t o t h e wear o f m a t e r i a l s ( r e f s . 2 1 7 , 218) i s t a k e n i n t o c o n s i d e r a t i o n , t h e relations h i p between t h e f r i c t i o n e n e r g y and t h e wear r a t e of t h e polymeric b e a r i n g bush c a n be f o u n d . I n t h e s t e a d y - s t a t e wear p r o c e s s a t a d e t e r m i n e d s l i d i n g s p e e d and c o n t a c t p r e s s u r e , t h e w e a r i n t e n s i t y i s c o n s t a n t and one c a n assume t h a t it i s p r o p o r t i o n a l t o t h e r a t i o o f t h e e n e r g e t i c l o a d and t h e d u r a b i l i t y ( r e s i s t a n c e ) o f t h e mater i a l ( r e f . 2 1 9 ) . The c o h e s i v e e n e r g y d e n s i t y i s a n a p p r o p r i a t e i n d e x o f t h e d u r a b i l i t y o f a polymer. The maximum d e n s i t y o f t h e t h e r m a l e n e r g y s t o r e d i n polymer used a s b e a r i n g bush m a t e r i a l , e t r c a n be t a k e n a s t h e i n d e x o f i t s e n e r g e t i c l o a d i n g and c a l c u l a t e d a s f o l l o w s : (4.19)
where
pp
and c
P
et = YP cP A T a r e t h e d e n s i t y of t h e polymer and i t s s p e c i f i c
101
h e a t r e s p e c t i v e l y andAT is t h e temperature rise i n t h e f r i c t i o n
area.
E E aY
1)
2 (r,
c a l 4 c L
0
CU
z
I
8
10
Tempemture rise , K
I
12
-
-
F i g . 4.16. R e l a t i o n s h i p between r a d i a l wear i n t e n s i t y and temperature r i s e i n t h e f r i c t i o n r e g i o n for s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g o p e r a t i n g under v a r i o u s c o n t a c t p r e s s u r e s w i t h s l i d i n g speed v = 0.067 m/s. B e a r i n g h o l e diameter 2.15 mm, e x t e r n a l diameter 6 mm, b e a r i n g l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1 .5-2%.
The r a t i o of et t o t h e r a d i a l wear i n t e n s i t y Iw ,
*
et
-
et =W
c a l l e d t h e apparent s t o r e d f r i c t i o n a l ( t h e r m a l ) energy d e n s i t y
,
:e - Iw c o o r d i n a t e s y s t e m g i v e s t h e one curve c h a r a c t e r i s t i c describing t h e r e l a t i o n s h i p "ASFED" f o r s h o r t , when p l o t t e d i n a n
102 between
Iw a n d e:
by t h e f o l l o w i n g f o r m u l a :
Iw= a,
x a8 (et 1
For m i n i a t u r e s t e e l - p o l y m e r
(4.20) j o u r n a l b e a r i n g s i n which t h e bear-
i n g bush i s made of polymer ( e i t h e r r e i n f o r c e d o r u n r e i n f o r c e d ) , when t h e nominal d i a m e t e r o f t h e b e a r i n g h o l e is 2.15 mm t h e values o f t h e p a r a m e t e r s a , a n d a8 a r e 23.3 a n d - 1 . 1 7 1 8
respectively, (e, i n mJ/mm 3 ) ( r e f . 1 9 6 ) . The t e m p e r a t u r e rise AT o n t h e r u b b i n g s u r f a c e i n t h e f r i c t i o n area o f t h e s e b e a r i n g s c a n be e s t i m a t e d u s i n g t h e f o r m u l a ( r e f s .
et
when Iw i s i n ,um/km a n d
i n MJ/mm3
196, 220) A T = 1150
where
So
so pv
f d
1 A
In
1 k
(4.21)
i s t h e r e l a t i v e amount o f f r i c t i o n a l e n e r g y a b s o r b e d by
t h e polymeric m a t e r i a l ( e l e m e n t ) , p is t h e c o n t a c t p r e s s u r e ( i n MPa) , v t h e s l i d i n g s p e e d ( m / s ) b e a r i n g h o l e d i a m e t e r (nun) (W/m.K),
,
,
f the friction coefficient, d the
2 t h e t h e r r n a l c o n d u c t i v i t y o f polymer
k t h e r a t i o of t h e bearing h o l e diameter t o t h e e x t e r n a l
diameter o f t h e p o l y m e r i c b e a r i n g b u s h . The c o e f f i c i e n t
6, w a s
d e t e r m i n e d e x p e r i m e n t a l l y f o r m i n i a t u r e b e a r i n g s w i t h d = 2 . 1 5 mm, e x t e r n a l d i a m e t e r 6 nun, b e a r i n g b u s h l e n g t h 2 . 1 mm, a n d r e l a t i v e b e a r i n g c l e a r a n c e c a . 28, o p e r a t i n g u n d e r p < 3 MPa, v c 0 . 1 m / s -0.6 g i v i n g 6, = 0 . 0 8 p f o r t h e h e a r i n g s w i t h POM h , POM c , PA 6 , PA 66 b e a r i n g b u s h e s a n d
so
from PA 6 + 25% g l a s s f i b r e
= 0.14
p-OS5 f o r b e a r i n g b u s h e s made
+ 4 % g r a p h i t e (where p i s i n MPa). The
f r i c t i o n c o e f f i c i e n t f o r s u c h b e a r i n g s c a n be p r e d i c t e d u s i n g eqn. (4.4).
I t i s p o s s i b l e t o p r e d i c t t h e c u r v e c h a r a c t e r i s t i c o f t h e ra-
d i a l wear r a t e a s a f u n c t i o n o f t h e s l i d i n g d i s t a n c e by u s i n g e q n . ( 4 . 2 0 ) b u t o n l y when t h e r u n n i n g - i n p e r i o d of t h e b e a r i n g s c a n b e e s t i m a t e d . The e n e r g y n e e d e d f o r t h e t r a n s f o r m a t i o n o f t h e t r i b o l o g i c a l system under d i s c u s s i o n i n t o a system w i t h s t e a d y - s t a t e f r i c t i o n a n d wear p r o c e s s e s ( i . e . a f t e r r u n n i n g - i n )
n e e d s t h e same
amount o f e n e r g y E f r when t h e s l i d i n g s p e e d a n d c o n t a c t p r e s s u r e s are variables, i.e. Efr
=
F ( L ) Lr = Cr
(4.22)
where F(L) i s t h e a v e r a g e v a l u e o f t h e f r i c t i o n f o r c e d u r i n g t h e
103
r u n n i n g i n p e r i o d (assumed t o b e c o n s t a n t ) , L r i s t h e s l i d i n g d i s t a n c e needed f o r r u n n i n g - i n a n d C r i s a c o n s t a n t f o r t h e p a r t i c u l a r t r i b o l o g i c a l system. Efr
c a n be expressed as Efr-d
(4.23)
f 1 Lr p = C r
where 1 i s t h e b e a r i n g l e n g t h . The f r i c t i o n c o e f f i c i e n t c a n b e p r e d i c t e d u s i n g e q n .
(4.4).
C r c a n be assumed t o be 0 . 6 kJ f o r t h e m i n i a t u r e b e a r i n g s d e a l t
w i t h h e r e ( 0 2.15 mm) ( r e f . 1 9 6 ) . A s t h e r a t i o of t h e r a d i a l w e a r r a t e a f t e r running-in t o t h e
wear r a t e a f t e r e . g . a s l i d i n g d i s t a n c e o f 1 0 km f o r p a r t i c u l a r b e a r i n g s is c o n s t a n t , t h e f o l l o w i n g e q u a t i o n s c a n be u s e d f o r e s t i m a t i n g t h e s l i d i n g d i s t a n c e Lr
(needed f o r t h e running-in)
and
f o r d e t e r m i n i n g t h e r e s p e c t i v e r a d i a l wear r a t e wr: d
f 1 L r p = C r
(Ll0
-
Lr)
Iw -
Wl0
( 4 .24)
- wr
w l =~t l wr
where t l c a n be c a l c u l a t e d f o r t h e P A , POM a n d PA
+
glass fibre
based b e a r i n g s using t h e formula tl = m 2 p o ' 2 ( f o r t h e s e miniature, fi 2.15 nun, b e a r i n g s o p e r a t i n g u n d e r p E < 0 . 2 ,
v <0.1 m/s, MPa)
.
3 > MPa
and
m 2 i s 6 , 2 a n d 2 r e s p e c t i v e l y , p b e i n g measured i n
The r a d i a l wear r a t e a s a f u n c t i o n of t h e s l i d i n g d i s t a n c e during runniiig-in can be determined when t h e r e l a t i o n s h i p between t h e rad i a l wear r a t e w a n d t h e s l i d i n g d i s t a n c e i s t a k e n i n t o c o n s i d e r a tion (ref. 196) :
w = t LU
( 4 .25)
where t and u are p a r a m e t e r s . The a p p r o p r i a t e e q u a t i o n s are as follows.
w = t LU (4.26)
wr
= tL;
104
The wear ( a s a f u n c t i o n o f t h e s l i d i n g d i s t a n c e ) c h a r a c t e r i s t i c c u r v e s t a r t i n g from t h e p o i n t w i t h t h e c o o r d i n a t e s ( L r , wr) c a n be assumed t o be a s t r a i g h t l i n e t a n g e n t t o t h e l i n e w = t L U a t p o i n t (Lr,
wr).
E q u a t i o n s ( 4 . 1 7 ) a n d ( 4 . 2 0 ) c a n be u s e d t o p r e d i c t wear i n m i n i a t u r e s t e e l - po 1ymer j o u r na 1 b e a r i n g s The "a d h e s i v e - c o h e s i v e 'I
.
f o r m u l a ( 4 . 1 7 ) c a n be u s e d o n l y when t h e a n g l e of c o n t a c t 2
I
yo
between t h e j o u r n a l a n d t h e b e a r i n g b u s h i s less t h a n 'jY. The maxi-
mum a l l o w a b l e volume o f worn p o l y m e r i cf material i s , i n p r a c t i c e , y o = T / 2 . T h i s c r i t i c a l volume v e r y much less t h a n c r i t i c a l when c a n be c a l c u l a t e d u s i n g e q n .
( 4 . 9 ) where
4';
= y/2
should be i n t r o -
duced a n d t h e c r i t i c a l v a l u e o f t h e r a d i a l wear r a t e wc c a n be d e t e r m i n e d . When wc i s known, t h e c r i t i c a l , maximum volume of t h e worn p o l y m e r i c m a t e r i a l may be e s t i m a t e d u s i n g e q n .
( 4 . 1 8 ) . The
wear o n b e a r i n g s o p e r a t i n g a t low s l i d i n g s p e e d , when t h e t h e r m a l e f f e c t s a r e s m a l l , c a n b e p r e d i c t e d using eqn.
(4.17);
t h i s equa-
t i o n c a n b e t r e a t e d a s t h e master c u r v e r e l a t i n g t o t h e a d h e s i v e - c o h e s i v e p r o p e r t i e s a n d wear volume of p o l y m e r i c m a t e r i a l s . The e x p e r i m e n t a l d a t a d e s c r i b i n g t h e v a r i a t i o n i n wear r a t e ( u n d e r a g i v e n c o n t a c t p r e s s u r e ) w i t h t h e d i s t a n c e o f s l i d i n g a r e needed f o r wear p r e d i c t i o n . F o r t h e s t e e l - P A 6 m i n i a t u r e ( @ 2.15 mm) j o u r n a l b e a r i n g s , t h e v a l u e s of t h e t and u parameters i n eqn. ( 4 . 2 5 ) c a n b e assumed t o b e 1 8 . 5 a n d 0 . 6 9 r e s p e c t i v e l y (at p = 1 MPa) ( r e f . 1 6 9 ) . Having t h e s e v a l u e s t o d e s c r i b e t h e r e l a t i o n s h i p between t h e s l i d i n g d i s t a n c e L a n d t h e r a d i a l wear r a t e (eqn. (4.25)) and u s i n g eqn.
( 4 . 1 7 ) t h e r a d i a l wear r a t e o f t h e same m a t e r i a l
u n d e r d i f f e r e n t c o n t a c t p r e s s u r e s o r t h e wear r a t e f o r o t h e r m a t e r i a l s u n d e r t h e same i n i t i a l c o n t a c t p r e s s u r e s c a n be e s t i m a t e d . When t h e s l i d i n g s p e e d i s h i g h e r ( s a y v > 0 . 0 2 m / s )
and t h e
t e m p e r a t u r e r i s e i n t h e f r i c t i o n r e g i o n AT >>3 K , t h e " t h e r m a l " f o r m u l a ( 4 . 2 0 ) s h o u l d b e a p p l i e d t o p r e d i c t w e a r . The u p p e r p r a c t i c a l l i m i t a t which t h i s f o r m u l a c a n b e u s e d is a p p r o x i m a t e l y when A T < 2 0 K . The f o r m u l a e p r e s e n t e d c a n be u s e d t o p r e d i c t w e a r i n c y l i n d r i c a l bearings (Fig. 4 . 1 7 ) .
The wear p r o c e s s i n s u c h t r i b o l o g i c a l
s y s t e m s i s so complex t h a t c h a n g e s i n t h e d i m e n s i o n s ( t h e most imp o r t a n t being t h e bearing h o l e diameter) c a n a f f e c t t h e rate of r a d i a l wear, s i n c e t h e r u b b i n g c o n d i t i o n s c h a n g e . T h i s c o n c e r n s m a i n l y t h e e n e r g e t i c ( t h e r m a l ) s t a t e of t h e b e a r i n g t h e mechan i c a l i n t e r a c t i o n s between t h e j o u r n a l a n d t h e b e a r i n g b u s h ( h i g h curvature of contacting surfaces, r e l a t i v e l y high bearing clea-
105
r a n c e ) , t h e e f f e c t o f t h e p r e s e n c e of w e a r d e b r i s i n t h e f r i c t i o n a r e a and m a t e r i a l t r a n s f e r d u r i n g r u b b i n g . The f o r m u l a p r e s e n t e d c a n be used t o p r e d i c t t h e wear r a t e i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s which i n t h e i r geometry, s i z e and r u b b i n g c o n d i t i o n s , a r e t y p i c a l o f t h o s e u s e d i n m i n i a t u r e mechanisms (ref.178). For o t h e r b e a r i n g s t h e o p p o s i t e f o r m u l a e a f t e r s e v e r a l experimental
wear t e s t s c a n be e a s y found, i . e . t h e problem p r a c t i c a l l y r e d u c e s t o d e t e r m i n i n g e x p e r i m e n t a l l y p a r a m e t e r s a 5 , a 6 and a7, a8 i n e q n s . ( 4 . 1 7 ) and ( 4 . 2 0 ) r e s p e c t i v e l y .
1
F i g . 4.17. M i n i a t u r e c y l i n d r i c a l s t e e l - p o l y m e r j o u r n a l b e a r i n g . 1 - housing, 2 p o l y m e r i c b e a r i n g bush, 3 steel journal.
-
-
The a d h e s i v e - c o h e s i v e f o r m u l a ( 4 . 1 7 ) c a n n o t b e u s e d f o r b e a r i n g s w i t h b u s h e s made o f r e i n f o r c e d polymers s i n c e f o r s u c h p o l y mers t h e c o h e s i v e e n e r g y d e n s i t y ec is unknown. The wear o f r e i n -
106 f o r c e d polymers has p r o b a b l y a n a d h e s i v e - d e l a m i n a t i o n c h a r a c t e r ( r e f s . 211,
215, 2 2 1 ) . F a t i g u e i s marked by growing c r a c k s which
s e p a r a t e t h e l a y e r from t h e b u l k m a t e r i a l . The c r a c k s n u c l e a t e a t t h e m a t r i x / r e i n f o r c i n g material p a r t i c l e i n t e r f a c e , propagate p a r a l l e l t o t h e s u r f a c e ( t h e d e p t h depending o n t h e f r i c t i o n c o e f f i c i e n t ) and f i n a l l y s h e a r t o t h e s u r f a c e , producing w e a r s h e e t s . S i n c e t h e c r a c k p r o p a g a t i o n g e n e r a l l y c o n t r o l s t h e w e a r r a t e and t h e critical depths a r e quasi-linearly c o r r e l a t e d with t h e f r i c t i o n c o e f f i c i e n t ( r e f . 2 2 1 ) t h e r e s h o u l d b e a c o r r e l a t i o n between t h e wear i n t e n s i t y of t h e r e i n f o r c e d polymer m a t e r i a l a n d t h e f r i c t i o n c o e f f i c i e n t . Such a c o r r e l a t i o n i s shown i n Fig. 4.18 (ref.196).
Friction coefficient
F i g . 4.18. R e l a t i o n s h i p between r a d i a l wear i n t e n s i t y o f p o l y m e r i c b e a r i n g bush i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s and f r i c t i o n c o e f f i c i e n t . S l i d i n g speed v = 0 . 2 6 m/s, c o n t a c t p r e s s u r e p = 0.075 MPa ( d u r i n g wear t e s t s ) 1 - PC + 30% GF; 2 PA 12 + 3 0 % GF; 3 PA 66 + 25% GF; 4 POM c + 30% GF.
-
-
-
107 The f r i c t i o n c o e f f i c i e n t f i n s t e e l - r e i n f o r c e d polymer s y s t e m s c a n be e stima te d u s i n g t h e formula ( r e f . 222)
(4.27)
a r e volume f r a c t i o n s o f t h e m a t r i x polymer a n d P f i b r e r e s p e c t i v e l y , and f f and f a r e f r i c t i o n c o e f f i c i e n t s of P s t e e l - f i b r e and steel-polymer p a i r s r e s p e c t i v e l y .
where Vf a n d V
The o t h e r wear mechanism o f r e i n f o r c e d polymers i s especiall y n o t i c e a b l e when t h e s l i d i n g s p e e d i s r e l a t i v e l y low a n d c o n t a c t p r e s s u r e r e l a t i v e l y h i g h ( a s i n t h e b e a r i n g s d i s c u s s e d ) a n d depends on t h e t h i n n i n g of t h e f i b r e - r e i n f o r c e m e n t s , of t h e f i b r e s and t h e f i b r e s peeling-off 222,
s u b s e q u e n t breakdown
from t h e m a t r i x ( r e f s .
2 2 3 ) . The polymer wears r a p i d l y b e c a u s e o f t h e stress con-
c e n t r a t i o n and a s a r e s u l t of v o i d s being c r e a t e d i n t h e m a t r i x . The wear r e s i s t a n c e of t h e m a t e r i a l i s t h e r e f o r e d e t e r m i n e d by a
set o f t h e m e c h a n i c a l a n d a d h e s i v e - c o h e s i v e p r o p e r t i e s o f t h e f i b r e and polymer a n d t h e a d h e s i o n o n t h e f i b r e / m a t r i x i n t e r f a c e . The s e q u e n t i a l o c c u r r e n c e o f t h e a f o r e m e n t i o n e d t h i n n i n g , b r e a k down a n d p e e l i n g - o f f o f t h e f i b r e s from t h e m a t r i x g o v e r n s t h e wear of a f i b r e - r e i n f o r c e d p o l y m e r . I n t h e w e a r - t h i n n i n g o f t h e f i b r e s , l o a d N and t h e s l i d i n g d i s t a n c e L are t h e e s s e n t i a l f a c t o r s ; i n t h e breakdown o f t h e f i b r e s , t h e s t r a i n f p / E o f t h e m a t e r i a l c a u s e d by f r i c t i o n , t h e load N a n d t h e s l i d i n g d i s t a n c e a r e t h e m o s t i m p o r t a n t f a c t o r s ; and i n t h e p e e l i n g - o f f o f t h e strain f i b r e s from t h e m a t r i x , i n t e r l a m i n a r s h e a r s t r e n g t h f p / E of t h e m a t e r i a l l o a d and s l i d i n g d i s t a n c e a r e most i m p o r t a n t . So t h e wear volume V of t h e r e i n f o r c e d polymer c a n b e g i v e n by ( r e f . 222)
T,,
(4.28) As
f o r f i r s t - o r d e r a p p r o x i m a t i o n i t c a n b e assumed t h a t (4.29)
The s p e c i f i c wear r a t e ws c a n be w r i t t e n a s f o l l o w s : (4.30)
108 The r e l a t i o n s h i p b e t w e e n t h e s p e c i f i c wear r a t e ws a d fp/E f o r m i n i a t u r e steel-polymer
- xs
j o u r n a l bearings is shown i n Fig. 4.19.
% 8
-0
F i g . 4.19. S p e c i f i c wear r a t e w 5 o f p o l y m e r i c b e a r i n g bush i n miniature steel-polymer j o u r n a l bearings vs. f p /E * (see eqn. ( 4 . 3 0 ) ) . 1,2,3 - PA 66 + 25% GF; 4.5.6 -mPA 12 ? 3 0 % GF; 7,8,9 - POM c + 30% GF; 1 0 - PA 66 + 33% GF; 1 1 - PA 66 + 50% GF; 12 - PBTP + 3 0 % GF; 13,14,15 - PC + 30% GF. 1-9 and 13-15: b e a r i n g h o l e 5 mm, bush l e n g t h 2.8 mm, r e l a t i v e c l e a r a n c e 1%, s l i d i n g speed 0.26 m/s (1,4,7,13); 0.026 m/s (2,5,8,14); 0.0026 m / s (3,6,9,15) and c o n t a c t p r e s s u r e 0.075; 0 . 7 5 and 7 . 5 MPa c o r r e s p o n d i n g t o t h e s l i d i n g speeds. 10-12: b e a r i n g h o l e 1 mm, bush l e n g t h and o u t s i d e d i a m e t e r 1 and 2.7 mi, r e l a t i v e c l e a r a n c e 4-6%, s l i d i n g speed 0.0131 m / s , c o n t a c t p r e s s u r e 1.25 MPa.
7
The v a l u e s o f t h e s p e c i f i c w e a r r a t e were e s t i m a t e d u s i n g e q n . (4.18) and eqn.
( 4 . 1 0 ) where t h e r a d i a l w e a r r a t e w w a s t a k e n from
r e f s . 1 7 0 , 1 7 1 , 1 8 8 , 189 ( s e e a l s o F i g . 4 . 1 2 )
a n d r e f s . 175-177
109
(see a l s o T a b l e 4 . 3 ) . p,, c a l c u l a t e d from e q n . ( 4 . 1 1 ) o r ( 4 . 1 2 1 , w a s t a k e n a s p . The r e l a t i o n s h i p b e t w e e n ws and f p m / E . Ls, w i t h o u t , 2
t a k i n g i n t o c o n s i d e r a t i o n t h e v a l u e s f o r POM c
f
3 0 % GF, may b e
approximately described with t h e following formula: (4.31) w h e r e ws i s f o u n d t o be -7 2 1 0 mn /N
.
mm 2/N when f p m / E - z s i s i n t r o d u c e d a s
lo-’
The v a l u e s of ws f o r t h e b e a r i n g s w i t h a b e a r i n g b u s h made from POM c
+
30% GF a r e u s u a l l y h i g h ( s e e F i g . 4 . 1 9 )
and t h e y a r e
h i g h e r t h a n i n s i m i l a r b e a r i n g s w i t h a b e a r i n g b u s h made o f unf i l l e d POM c ( s e e T a b l e 4 . 3 )
. It was o b s e r v e d t h a t
t h e wear r a t e o f
g l a s s f i b r e r e i n f o r c e d POM c w a s sometimes l o w e r a n d sometimes h i g h e r t h a n t h e wear r a t e o f u n r e i n f o r c e d POM c , a s a l r e a d y ment i o n e d i n Tanaka’s s t u d i e s ( r e f . 1 9 0 )
.
A n a l y s i s o f t h e r e s u l t s o f t h e s e w e a r s t u d i e s shows t h e r e f o r e t h a t i n s t e e l - p o l y m e r b e a r i n g s w i t h a r e i n f o r c e d polymer b e a r i n g b u s h t h e mechanism o f s e q u e n t i a l w e a r
-
t h a t is, thinning of t h e
f i b r e s , f o l l o w e d by t h e i r breakdown a n d t h e n t h e p e e l i n g - o f f o f t h e f i b r e s from t h e m a t r i x
-
i s dominant i n t h e wear p r o c e s s ,
e v e n when s l i d i n g s p e e d s a r e low and c o n t a c t p r e s s u r e s a r e r e l a t i v e l y high. A t higher s l i d i n g speeds, thermal e f f e c t s a l s o
seem t o p l a y a n i m p o r t a n t r o l e a n d wear f o r m u l a ( 4 . 2 0 ) s h o u l d b e taken into consideration. The above c o n s i d e r a t i o n s l e a d t o t h e g e n e r a l c o n c l u s i o n t h a t t h e a d h e s i o n a t t h e i n t e r f a c e and t h e c o h e s i v e p r o p e r t i e s of t h e polymer p l a y a dominant r o l e i n t h e wear p r o c e s s of m i n i a t u r e t r i b o l o g i c a l s y s t e m s . When t h e p o l y m e r i c e l e m e n t i s made o f rei n f o r c e d polymer t h a t wear p r o c e s s i s a l s o g o v e r n e d by t h e p r o p -
e r t i e s o f t h e f i l l e r ( r e i n f o r c i n g m a t e r i a l ) a n d i t s bonding w i t h t h e m a t r i x . A t h i g h e r s l i d i n g s p e e d s a n d l o a d s , when t h e f r i c t i o m l h e a t i n g s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n , t h e wear p r o c e s s i s a l s o d e p e n d e n t o n t h e t h e r m a l e n e r g y p r o d u c e d and s t o r e d i n t h e polymeric element i n t h e f r i c t i o n area. 4.2.1
.
POLYMER-POLYMER SYSTEMS
Polymer-polymer
s y s t e m s a r e u s e d i n m i n i a t u r e mechanisms t o
r e d u c e c o s t s , e s p e c i a l l y f o r mass p r o d u c t i o n . The t r i b o l o g i c a l p r o p e r t i e s o f s u c h s y s t e m s c a n b e e v e n b e t t e r t h a n i n t h e case o f
110
metal-polymer systems ( r e f s . 43,
4 4 , 1 6 6 , 1 6 7 , 188, 1 8 9 , 224, 2 2 5 ) .
Both u n f i l l e d a n d f i l l e d p o l y m e r s a r e u s e d a s m a t e r i a l s f o r t h e rubbing elements. The m i n i a t u r e j o u r n a l b e a r i n g s ( b e a r i n g h o l e d i a m e t e r 1 mm) i n which a p o l y m e r i c j o u r n a l r u b s a g a i n s t a p o l y m e r i c b e a r i n g b u s h d e m o n s t r a t e r e l a t i v e l y good t r i b o l o g i c a l p r o p e r t i e s (refs. 170, 1 7 1 , 1 7 2 , 1 8 9 ) . A t a c o n t a c t p r e s s u r e o f 1 . 5 MPa a n d s l i d i n g s p e e d
o f 0.0131 m / s ,
when t h e j o u r n a l i s made o f POM h a n d t h e b e a r i n g
bush o f PBTP, t h e f r i c t i o n c o e f f i c i e n t i s a b o u t 0 . 4 , s i m i l a r b e a r i n g s w i t h a steel j o u r n a l it i s 0.5-0.6
while f o r
. The
wearrate
of t h e j o u r n a l and t h e b e a r i n g b u s h i n s u c h b e a r i n g s a r e compared i n Fig. 4.20
( b a s e d o n r e f . 170)
.
I
0
'a sliding distan re , Km
F i g . 4 . 2 0 . Wear r a t e s o f j o u r n a l and b e a r i n g s bush i n m i n i a t u r e j o u r n a l b e a r i n g v s . s l i d i n g d i s t a n c e . B e a r i n g h o l e d i a m e t e r 1 mm, e x t e r n a l b e a r i n g bush diameter 2.7 mm, b e a r i n g l e n g t h 1 mm, r e l a t i v e c l e a r a n c e 4-6%, s l i d i n g speed 0.0131 m/s.
1 - r a d i a l wear o f b e a r i n g bush i n b e a r i n g w i t h a r o l l e r - b u r n i s h e d j o u r n a l made of f r e e c u t t i n g s t e e l o p e r a t i n g a t c o n t a c t p r e s s u r e p = 1.25 MPa; 2 - r a d i a l wear r a t e o f b e a r i n g bush i n b e a r i n g w i t h a POM h moulded j o u r n a l o p e r a t i n g a t p = 1 .5 MPa, 3 - wear r a t e o f POM h j o u r n a l ; t h e wear r a t e o f t h e s t e e l j o u r n a l i s n e g l i g i b l e ( r e f . 170).
111
The u s e o f POM h i n s t e a d o f s t e e l a s j o u r n a l m a t e r i a l i n t h e s e b e a r i n g s e n s u r e s b e t t e r t r i b o l o g i c a l p r o p e r t i e s . From t h e s e studies it a p p e a r s t h a t l i m i t i n g t h e c o n t a c t p r e s s u r e i n t h e b e a r i n g
POM h-PBTP d o e s n o t depend o n t h e bonding s t r e n g t h ( o r c o m p r e s s i o n s t r e n g t h ) of t h e p o l y m e r i c m a t e r i a l o f t h e j o u r n a l b u t r a t h e r o n i t s t o r s i o n a l s t r e n g t h . The h i g h e r t h e r m a l e x p a n s i o n o f t h e p o l y -
meric m a t e r i a l u s e d o n t h e j o u r n a l a s compared t o steel means t h a t t h e b e a r i n g c l e a r a n c e i n polymer-polymer
b e a r i n g s h a s t o be l a r g e r
than i n steel-polymer bearings. The u s e o f f i l l e d polymer a s b e a r i n g bush m a t e r i a l i n polymer-polymer b e a r i n g s c a n improve t h e i r t r i b o l o g i c a l p r o p e r t i e s .
Sig-
n i f i c a n t wear r e d u c t i o n ( a s compared t o t h e wear i n POM h-PBTP b e a r i n g s ) was o b t a i n e d by making t h e b e a r i n g b u s h from t h e chemic a l l y l u b r i c a t e d POM h m a t e r i a l D e l r i n 5 0 0 C L , m a n u f a c t u r e d by Du Pont ( r e f . 1 7 0 ) . The wear r a t e s i n POM h-polymer
b e a r i n g s when
t h e b e a r i n g b u s h i s made o f v a r i o u s p o l y m e r s a r e compared i n F i g . 4.21
(based on r e f . 1 7 0 ) .
20
16
12
8
sliding distance, km F i g . 4.21. Radial wear o f bearing bush o f POM h-polymer journal b e a r i n g v s . s l i d i n g distance. Bearing hole diameter 1 mm, e x t e r n a l bearing bush diameter 2 . 7 mm bearing l e n g t h 1 mm, r e l a t i v e clearance 3-5%, s l i d i n g speed 0 . 0 1 3 1 m / s , contact pressure 0.25 MPa. 1 PA 1 1 + MS2, 2 - POM c , 3 POM h c h e m i c a l l y l u b r i c a t e d ( D e l r i n 500 CL manufactured by Du P o n t ) .
-
-
112
When t h e j o u r n a l i s made o f POM h , t h e wear of b e a r i n g b u s h e s made o f PA 6 6 o r PBTP r e i n f o r c e d w i t h g l a s s f i b r e o r g l a s s m i c r o ( r e f . 1 8 9 ) . The wear r a t e o f a b u s h made o f
beads i s d i f f e r e n t PA 6 6
+
3 0 % ( b y w e i g h t ) g l a s s f i b r e ( l e n g t h 0.3-0.8
mm, d i a m e t e r
0 . 1 5 mm) a f t e r a s l i d i n g d i s t a n c e o f 2 0 km i s s i g n i f i c a n t l y l o w e r
t h a n t h e wear o f a b e a r i n g b u s h made o f P A 6 6 b e a d s ( d i a m e t e r 5-50 ,um) ( F i g . 4 . 2 2 ,
+
28% g l a s s micro-
b a s e d on r e f . 1 8 9 ) .
Sliding distance, k m
F i g . 4.22. R a d i a l wear o f b e a r i n g bush o f POM h ( j o u r n a l ) r e i n f o r c e d polymer j o u r n a l b e a r i n g v s . s l i d i n g d i s t a n c e . B e a r i n g h o l e diameter 2.7 mm, b e a r i n g l e n g t h 1 mm, r e l a t i v e c l e a r a n c e 4-6%, s l i d i n g speed 0 . 0 1 3 1 m / s , c o n t a c t p r e s s u r e 1 . 5 MPa. 1 PBTP + 28% g l a s s microbeads, 2 - PBTP + 30% g l a s s f i b r e , 3 - PA 66 + 28% g l a s s microbeads, 4 - PA 66 + 30% g l a s s f i b r e .
-
The volume wear of t h e POM h j o u r n a l i n t h e s e b e a r i n g s a f t e r a s l i d i n g d i s t a n c e o f 1 0 , 2 0 a n d 30 km i n POM h-PBTP
+
30% g l a s s
113
+ 2 8 % g l a s s m i c r o b e a d s i s 30, 2 0 ; 6 0 , 50 a n d mm3 r e s p e c t i v e l y a n d i n POM h-PA 6 6 + 30% g l a s s f i b r e , POM h-PA 6 6 + 28% g l a s s b e a d s b e a r i n g s 5 , 2 ; 1 0 , 4 ; 1 5 , 1 2 mm 3 r e s p e c t i v e l y . The f r i c t i o n c o e f f i c i e n t o f s u c h b e a r i n g s i s l o w e r than t h e f r i c t i o n c o e f f i c i e n t of s i m i l a r bearings with a steel f i b r e , POM h-PBTP
90,
143
j o u r n a l . For POM h-PBTP f i b r e b e a r i n g s is 0 . 4 , g l a s s f i b r e , steel-PBTP
+
g l a s s m i c r o b e a d s , POM h-PBTP
+
glass
0.3 w h i l e f o r t h e s i m i l a r steel-PBTP
+
+
g l a s s m i c r o b e a d s b e a r i n g s i s 0 . 6 a n d 0.8
r e s p e c t i v e l y . F o r t h e POM h-PA 6 6
+
g l a s s f i b r e , POM h-PA 6 6
+
g l a s s microbeads b e a r i n g s t h e f r i c t i o n c o e f f i c i e n t is 0 . 2 2 and 0 . 2 and f o r t h e steel-PA 66
+
g l a s s f i b r e , s t e e l PA 6 6
+
g l a s s micro-
b e a d s b e a r i n g s 0 . 5 5 a n d 0.50 r e s p e c t i v e l y . POM h-PA 6 6
+
50% g l a s s
f i b r e bearings a l s o dernonstrate g o d tribolocjical properties (refs.17lr172).The r e l a t i v e l y h i g h d i f f e r e n c e between t h e t r i b o l o g i c a l p r o p e r t i e s o f b e a r i n g s w i t h b u s h e s made o f PBTP and PA 66 r e i n f o r c e d w i t h g l a s s f i b r e o r g l a s s m i c r o b e a d s may b e due t o t h e d i f f e r e n c e i n s h e a r m o d u l i o f t h e m a t r i x polymers ( f o r PBTP a b o u t 750 a n d f o r PA 6 6 1000 MPa ( r e f . 2 2 6 ) ) . A s a r e s u l t , d u r i n g f r i c t i o n t h e g l a s s microb e a d s c a n be more e a s i l y l i b e r a t e d from t h e PBTP m a t r i x m a t e r i a l b u t b e c a u s e t h e y remain i n t h e f r i c t i o n r e g i o n t h e y a c t a s a n a b r a s i v e , which r e s u l t s i n t h e h i g h e r w e a r r a t e o f t h e POM h j o u r nals. The f r i c t i o n a n d wear i n p o l y m e r i c s p h e r e - p l a t e s y s t e m s h a s b e e n i n v e s t i g a t e d ( r e f s . 43, 1 6 9 , 1 7 1 ) u s i n g t h e ASTM pendulum
(see Chapter 8.2)
. The
pendulum w a s hung o n t h e 1 / 2 "
diameter
polymer s p h e r e r u b b i n g a g a i n s t t w o polymer 1 0 x 6 x 3 mm p l a t e s ( s u p p o r t i n g t h e s p h e r e ) . The wear o f t h e r u b b i n g e l e m e n t s w a s e s t i m a t e d a f t e r t h e t r i b o l o g i c a l s y s t e m had b e e n d i s a s s e m b l e d . The f o l l o w i n g p o l y m e r s were u s e d a s materials f o r t h e r u b b i n g elements: PTFE, POM h , POM c , PC, PA a r o m a t i c ( P A a r ) , PA 11, PA 1 2 , PPO a n d ABS. The f r i c t i o n c o e f f i c i e n t s of s e v e r a l s y s t e m s are compared i n F i g . 4.23.
The f r i c t i o n c o e f f i c i e n t f o r s y s t e m s w i t h e l e m e n t s
m a n u f a c t u r e d from t h e same polymer i s h i g h . S t i c k - s l i p e f f e c t s a r e a l s o c h a r a c t e r i s t i c of s u c h c o m b i n a t i o n s . I n s y s t e m s where t h e e l e m e n t s a r e of d i f f e r e n t m a t e r i a l s , r e v e r s i n g t h e m a t e r i a l s o f t h e e l e m e n t s c h a n g e s t h e f r i c t i o n c o e f f i c i e n t . When, f o r example,
a s p h e r e made o f POM h r u b s a g a i n s t a PPO p l a t e t h e f r i c t i o n c o e f f i c i e n t i s 0.33,
b u t when a PPO s p h e r e r u b s a g a i n s t a POM h p l a t e
t h e f r i c t i o n c o e f f i c i e n t decreases t o 0.25;
o n t h e o t h e r hand it
i n c r e a s e s i f t h e PPO i s r e p l a c e d by PA 11. The wear i s e s p e c i a l l y h i g h i n s y s t e m s w i t h e l e m e n t s made of t h e same polymer. T h i s af-
114
f e c t s POM-POM c o m b i n a t i o n s i n p a r t i c u l a r . The wear i s a l s o h i g h i n s y s t e m s where o n e e l e m e n t i s made o f PPO a n d t h e o t h e r o f ABS. T h e r e i s r e l a t i v e l y low wear o f t h e r u b b i n g e l e m e n t s when t h e s p h e r e i s made o f POM h a n d t h e p l a t e of PA 1 2 .
0.5 1
0.4
0.3
0.2 0.1
F i g . 4.23. F r i c t i o n c o e f f i c i e n t s f o r some polymer-polymer systems ( s p h e r e - p l a t e i n ASTM pendulum). Load 7.07 N . 1 - POM h-POM h , 2 - POM C-POM C, 3 PC-PC, 4 - P A 1 1 - P A 1 1 , 5 - PPO-PPO, 6 POM h-PPO, 7 - PPO-POM h, 8 POM h-PA 1 1 , 9 - PA 11-POM h, 10 - POM h-PA a r , 1 1 - PA ar-POM h, 12 POM h-PC, 13 - PC-POM h .
-
-
-
F o r s p h e r e - p l a t e s y s t e m s i n v e s t i g a t e d u s i n g t h e ASTM pendulum and U T I a p p a r a t u s (see C h a p t e r 8 . 2 )
,
s t u d i e s have g i v e n r e s u l t s
s i m i l a r t o t h o s e d e s c r i b e d a b o v e ( r e f s . 4 4 , 1 6 9 , 171). The followi n g polymer-polymer s y s t e m s w e r e i n v e s t i g a t e d : POM h-POM h a n d polymer-POM h . A s w e l l a s t h e t y p i c a l POM h m a t e r i a l D e l r i n 500 NC 1 0 p r o d u c t o d Du P o n t , D e l r i n 8 0 2 0 ( d e m o n s t r a t i n g b e t t e r mouldi n g p r o p e r t i e s ) and D e l r i n 500 CL ( c h e m i c a l l y l u b r i c a t e d ) were u s e d . The o t h e r polymers t e s t e d were: PA 6 6 , PA 11, PA a r a n d PPO. These s t u d i e s r e v e a l e d t h a t i n t h e case o f POM h-POM h s y s t e m s t h e
worst t r i b o l o g i c a l p r o p e r t i e s o c c u r a t t h e r u b b i n g o f t h e same ( i d e n t i c a l ) materials. The h i g h e s t v a l u e o f t h e f r i c t i o n c o e f f i -
115 c i e n t , 0.48, o c c u r r e d when a p l a t e made o f D e l r i n 5 0 0 CL w a s rubbirq a g a i n s t a s p h e r e m a n u f a c t u r e d from
D e l r i n 8020; r e v e r s i n g t h e
m a t e r i a l s decreased t h e f r i c t i o n c o e f f i c i e n t t o 0.35.
This was t h e
l o w e s t f r i c t i o n c o e f f i c i e n t o f a n y of t h e n i n e POM h-POMh combinat i o n s i n v e s t i g a t e d . F o r t h e s y s t e m s w i t h a polymer s p h e r e a n d a POM h p l a t e ( D e l r i n 500 Nc 10, D e l r i n 500 CL) t h e PPO-Delrin
500 CL (0.15) and t h e PA 11- D e l r i n 500 NC 1 0 c o m b i n a t i o n had t h e h i g h e s t (0.33). The f r i c t i o n s y s t e m had t h e lowest f r i c t i o n c o e f f i c i e n t
c o e f f i c i e n t s and wear f o r polymer-POM h s y s t e m s were lower t h a n i n POM h-POM h s y s t e m s . The wear o f t h e s p h e r e s w a s h i g h e r t h a n t h e
wear o f t h e p l a t e s . When t h e o s c i l l a t i n g 10 x 6 x 3 mm PA 6 6 + 4 0 % c a r b o n f i b r e p l a t e i n U T I a p p a r a t u s r u b s a g a i n s t t h e 1/2" POM h s p h e r e , t h e f r i c t i o n c o e f f i c i e n t i s 0.25 ( r e f . 171). The f r i c t i o n c o e f f i c i e n t s f o r o t h e r POM h ( s p h e r e ) f i l l e d polymers c o m b i n a t i o n s i n s u c h t r i b o l o g i c a l s y s t e m are p r e s e n t e d i n F i g . 4.24. Some s y s t e m s i n F i g . 4.24 d e m o n s t r a t e f r i c t i o n c o e f f i c i e n t s l o w e r t h a n t h e f r i c t i o n c o e f f i c i e n t s f o r t h e s t e e l - f i l l e d polymer s y s t e m s . The PPS f i l l e d w i t h g l a s s f i b r e d e m o n s t r a t e d good wear r e s i s t a n c e when o p e r a t i n g a t e l e v a t e d t e m p e r a t u r e s ( a s h i g h a s 25OoC) w h i l e P A 1 c a n b e u s e d i n t r i b o l o g i c a l systems o p e r a t i n g w i t h i n t h e temperature range of - 1 9 0 t o 25OoC ( r e f . 5 6 ) . I n v e s t i g a t i o n s ( r e f s . 227, 228) i n t o polymer-polymer s y s t e m s ( t h r u s t washer t e s t s f o r two r i n g s r u b b i n g a t t h e c o n t a c t p r e s s u r e 0.28 MPa a n d s l i d i n g s p e e d 0.25 m / s ) c a r r i e d o u t f o r u n f i l l e d and f i l l e d polymers ( s u c h as PA 6 6 , PA 6 1 0 , POM c , PC, PPS, PBTP and UHMWPE) showed t h a t f o r u n f i l l e d polymers t h e r u b b i n g o f t h e same m a t e r i a l s r e s u l t s i n h i g h wear f o r b o t h t h e moving and s t a t i o n a r y e l e m e n t s , w i t h t h e g r e a t e s t wear o c c u r r i n g o n t h e moving s u r f a c e . When d i f f e r e n t polymers r u b t o g e t h e r , t h e moving e l e m e n t s t i l l h a s a higher w e a r r a t e than t h e s t a t i o n a r y one, a f t e r a p r o p o r t i o d i t y f a c t o r b a s e d o n p o l y m e r - o n - s t e e l wear h a s b e e n i n t r o d u c e d . The same e f f e c t o c c u r s i n t h e c a s e o f P T F E - f i l l e d p o l y m e r s . The a d d i t i o n o f r e i n f o r c i n g f i b r e g e n e r a l l y r e s u l t s i n i n c r e a s e d wear o f t h e s t a t i o n a r y e l e m e n t when s i m i l a r r e i n f o r c e d p o l y m e r s r u b a g a i n s t e a c h o t h e r ; however, t h e a d d i t i o n o f t h e r e i n f o r c i n g f i b r e s g e n e r a l l y r e s u l t s i n lower w e a r o f b o t h s u r f a c e s compared t o when r e i n f o r c e d polymer r u b s a g a i n s t s t e e l o r u n r e i n f o r c e d p o l y m e r s o r c o m p o s i t e s . Wear o f polymers c o n t a i n i n g g l a s s - f i b r e i s l o w e r t h a n t h a t o f t h e c a r b o n - f i b r e - r e i n f o r c e d p o l y m e r s . The a d d i t i o n o f PTFE t o t h e c o u n t e r f a c e m a t e r i a l reduces t h e d e t r i m e n t a l e f f e c t s of g l a s s fibre
116
( w i t h r e s p e c t t o wear) o n t h e o p p o s i n g s u r f a c e s . Good t r i b o l o g i c a l p r o p e r t i e s a r e o b t a i n e d when t h e s t a t i o n a r y e l e m e n t i s made o f PA 6 6 a n d t h e moving p a r t i s made o f POM h . B e t t e r wear r e s i s t a n c e c a n b e o b t a i n e d by making t h e s t a t i o n a r y e l e m e n t from PA 6 6 g l a s s f i b r e a n d t h e moving e l e m e n t from PA 6 6
+
t i o n c o e f f i c i e n t o f s u c h a s y s t e m i s a b o u t 0.09; t i o n c o e f f i c i e n t , 0.04, t i o n a r y surface)-POM c
w a s o b s e r v e d i n a PA 66
+
2 0 % PTFE s y s t e m
,
+
30% 20% PTFE. The f r i c -
a very low f r i c + 20% PTFE ( s t a -
which also d e m o n s t r a t e d
r e l a t i v e l y low v e a r . The a d d i t i o n o f PTFE e l i m i n a t e s s t i c k - s l i p e f f e c t s . When t h e t o t a l wear i n polymer-polymer s y s t e m s i s t a k e n i n t o c o n s i d e r a t i o n , c o m p o s i t e p a i r s w h i c h h a v e similar wear r a t e s when r u b b i n g a g a i n s t e a c h o t h e r are p r e f e r r e d t o p a i r s e x h i b i t i n g l a r g e d i f f e r e n c e s i n wear r a t e s .
0.5
0.4
0.3
0.2
0.1
F i g . 4.24. F r i c t i o n c o e f f i c i e n t s f o r POM h - f i lled polymer ( s p h e r e / o s c i l l a t i n g p l a t e i n U T I a p p a r a t u s ) systems. Load 6.1 N , maximum v a l u e o f s l i d i n g speed 0.01 m/s. 1 - PA 66 + 40% carbon f i b r e , 2 - PC + 30% g l a s s f i b r e + 15% PTFE, 3 PPS + 40% g l a s s f i b r e , 4 - PA1 + 3% Ti02+0.5% PTFE, 5 - PA1 + 12% g r a p h i t e + 3% PTFE.
-
117
Forty four polymer-polymer systems were investigated in refs. 185 and 2 2 9 (trust washer tests: two rubbing rings with external and internal diameters 4 3 and 3 7 mm respectively, thickness 8 mm, operating at contact pressure 0.1 MPa and slidinq speed 0.12 m/s). Their elements were made of the following polymers: PA 6, POM c, HDPE, PBTP, PTFE, PSI PVC and SAN. The best tribological properties were demonstrated by the POM c-HDPE, PA 6-HDPE, PBTP-HDPE,HDPE-PMMA and HDPE-PVC systems. The friction coefficients and the total wear were relatively 1ow.Where the same material was used for both elements, the total wear in the system increased in the following ascending order: POM c-POM c, PTFE-PTFE, PBTP-PBTP, HDPE-HDPE PMMA-PMMA. The wear in these systems is higher than in those with elements made from different polymers. Reversing the materials of the stationary and moving elements had little efect on the wear. When higher contact pressures (up to 0.6 MPa) were applied, the effect on the wear of intermolecular bonding energies or the polar surface energy component of these bonZing energies was more clearly recognizable (ref. 2 2 9 ) . When, for instance, the sliding combination POM c-HDPE was tested, although they both have a high wear rate, upon increasing the contact pressure a more rapid increase in w e a r was observed in the sliding polymer with the lowest intermlecular bonding energies, in this case HDPE. When rubbing against HDPE, POM c appears to be fairly wear-resistant but if, on the other hand, it rubs against PA 6, in which the high strength of the molecular hydrogen bridge bonds results in the polar component of the surface free energy being higher than it is for POM c, then POM c has a higher wear rate than PA 6. The investigations (ref. 2 3 0 ) carried out for polymer-polymer systems (pin-on-disk, pin diameter 2 nun, disk diameter 100 mm, load 10 N, sliding speed 2.4 m/s, sliding distance 4 5 0 ,um) where the pin materials were PTFE, POM, PA 66 and PP and the disk materials S A N , PS and PMMA showed that the friction coefficient is low when the pin is manufactured in PTFE and the friction coefficient is highest ( 0 . 2 9 ) for the PA 66-PMMA material combination. The friction was found to be decisively connected with adhesion (see below). Using a polymeric rider 1 2 mm thick (in the direction of sliding) and 1 2 mm (when measured perpendicular to the direction of sliding) rubbing against PVC strip approximately 2 5 nun wide and 380 mm long, the following material combinations were investigated: PVC-PVC, PTFE-PVC and PA 66-PVC. The tests were repeated using the
118
pin-on-disk (ring) system; the PVC ring was machined from a 150 m diameter pipe. The lowest wear occurred in the PA 66-PVC combination, while in the PTFE-PVC and PVC-PVC systems the wear was approximately 15 and 100 times higher respectively (ref. 231). When PA 66 slides on PVC, it transfers a thin film onto the PVCsurface. The PVC-PVC and PA 66-PVC systems have definite transition points where the wear regimes change from low to catastrophic wear rates. The transition point for a specific wear track length and ambient temperature can be defined by the relationship P = Po - kN, where P is the applied load, Po the value of load where the linear graph intersects the P-axis, N the number of passes per unit time and k the gradient of the load-speed graph in the linear range. When a rotating roller made of PTFE rubs against PTFE, PCA, PETP, PE and PP films (at contact pressure 100 Pa and sliding speed 0.075 m/s), the roller’s wear rate decreases in the above order of counterface materials, i.e., the wear of the PTFE roller is 2.5 times higher when rubbing against a PTFE film than when rubbing against PP film (refs. 232, 233). When the thickness of the polymer film is increased from say 5 0 to 250 ,um, the wear rate decreases hyperbolically. The transferred layer of PTFE material decreases simultaneously from about 0.4-0.5 ,urn to 0.08-0.1 w.Such effects are probably due to both the stress-strained state of the friction contact and the strength of the adhesive bond increasing with film thickness. Decrease in the adhesive bond strength is connected with a reduction in the area of contact caused by a loss in both the elasticity and flexibility of the film as a whole. The stronger bond between the contacting surfaces combined with a decrease in the film thickness also favours an increase in the friction coefficient. The minimum value of the friction coefficient can be plotted vs. polymer film thickness; it decreases to about 0.45 for a PTFE-PP system (ref. 232). The tribological properties of PE-PE systems are interesting (refs. 185, 234-2361. The effect of the density of PE on the friction coefficient is pronounced. The friction coefficient of LDPE-LDPE systems (low density PE: density below 0.93 mg/m3), operating at low contact pressure (and high sliding speed) can reach 1.2 but decreases rapidly to as low as 0.05 with increase in contact pressure and decrease in sliding speed (refs. 234, 235, 236). For HDPE-IIDPE systems (high density, over 0.94 mg/mm3) , the friction coefficient is below 0.4 (refs. 185, 234), falling (hyperbolically decreasing) to 0.1 when the density of the material is 0.965 n-g/m3
119 (ref. 234). The friction coefficient of UHMWPE-UHMWPE systems (density 0.94 mg/mm3) should be about 0.15 (ref. 234). The friction coefficient of the LDPE-LDPE combination decreases from 0.8 at 20OC to 0.2 at 80OC (ref. 235). The wear intensity in an HDPE-HDPE system operating under the aforementioned conditions (refs. 185, 229) was about 6 ,um/km and was similar for both rubbing elements. The wear in the UHMWPE-UHMWPE combination is controlled by the molecular orientation; orientation perpendicular to the wear surface can have disastrous consequences, whereas orientation parallel to the wear surface may have a slightly beneficial effect (ref. 238). At contact pressure 2 MPa, sliding speed 0.24 m/s (when tested using a tri - pin-on-disk machine), and surface roughness Ra of 0.37 ,um for the specimen with parallel molecular orientation, the wear rate was m3/N.m (1000 times more than the wear rate of the same UHMWPE rubbing against a steel disk with Ra = 0.01 ,um under the same conditions). Applied load is an important factor in causing wear and any reduction in it is beneficial. The running temperature in the friction area rapidly increases, becoming 112OC after 9 min; there was abundant evidence of adhesion , surface softening, distortion and melting of both polymer surfaces. The wear rate was very dependent on the length of the running periods, due to the high temperatures involved. Modifying the PE surface changes the tribological properties of PE-PE systems. For example, after the test surfaces had been fluorinated, the friction coefficient of the LDPE-LDPE system was reduced by about 25% (ref. 235). This is the result of the adhesive strength decreasing, since after fluorination the surface free energy of LDPE decreases from 31 mJ/m2 to 23 mJ/m2. The LDPE samples treated with fuming sulphuric acid demonstrated surface free energy of about 54 mJ/m2 and when rubbed against each other the friction coefficient was 10% higher than that of untreated LDPE samples. The static friction coefficient of halogenated polyethylenes depends on the surface free energy (ref. 237), increasing from 0.05-0.10 for a PTFE-PTFE combination (the surface free energy of PTFE is about 18 mJ/m2) to 0.9 for PVDC-PVDC systems (the surface free energy of PVDC is about 4 0 mJ/m2) , and is about 0.33 for LDPE-LDPE systems. The replacement of hydrogen atoms in the PE molecule (-CH2-) by fluorine atoms decreases the friction coefficient while their replacement by chlorine atoms increases it.
120
The glass/epoxy composite coated with a filled epoxy material containing 1-64 ,um particles of A1203 + Cu or stainless steel + A1203 when rubbing against the UHMWPE acetabular cup of a total surface hip replacement demonstrates very good tribological properties (ref. 239). A femoral shell containing particles of stainless
steel and A1203 in an epoxy matrix rubbing against an UHMWPE cup yielded the lowest friction force of all the shells tested; the addition of qraphite fibres to the UHMWPE cup articulated against the aforementioned composite caused an increase in the friction force but reduced surface damage to the cup. LDPE has lower friction and wear on a coated steel surface (a 50 nm thick film of poly (chloro-p-xylylene)) than on an uncoated surface (ref. 240). Surface mating makes no difference to wear or friction in the case of PVC and PCTFE. PTFE coatings (obtained for example by filling micropores in the anodized surface of aluminium rubbing components) demonstrate very good tribological properties (ref. 241). The thin (1-5 ,um) coatings on steel or bronze manufactured in composites with high-molecular surface active agents have a friction coefficient when rubbing in polymer-polymer systems of 0.04-0.05 (atcontact pressure 7.5 MPa and sliding speed 0.5 m/s) and can be used at temperatures up to 2OO0C (ref. 242)(see also Chapter 7.2). Tests for wear on small, precision gearing show that polymeric gears generally demonstrate tribological qualities similar to the polymer-polymer systems discussed previously (refs. 243-245). The wear of the teeth is low when the driving gear is made of PA (e.g. PA 6) and the driven gear of POM. The wear is higher for machined gears than for moulded gears. This is especially true of gears manufactwed in PA 6, PA 610 and POM (ref. 244). The wear in such gears is lower than in steel-polymer gears (Fig. 4.25, based on ref. 244). Material is transferred in all polymer-polymer systems, whatever the rubbing conditions, and invariably from a material of low cohesive energy density to one of higher cohesive energy density (ref. 246). In polymer-polymer systems (polymer disk rubbing against thin polymer film) in which the elements are made from LDPE, PTFE, PVC, PP, PMMF- or PETP, the thickness of the layer of material transferred increases with slidinq speed and time but decreases with load. The direction of the material transfer in polymer-polymer systems with elements made of the aforementioned polymers is shown in Fig. 4.26 (based on ref. 246).
121
50
1
2
3
4
-
Number of rotations, IO'
F i g . 4.25. Wear o f t e e t h o f d r i v e n gear o f p r e c i s i o n g e a r i n g . Module m = 0.8 mm, number o f t e e t h 43/22 ( d r i v e n gear), c i r c u m f e r e n t i a l l o a d p e r t o o t h w i d t h 1 0 N/mm, c i r c u m f e r e n t i a l speed 1 m/s. 1 machined d r i v i n g gear made o f f r e e - c u t t i n g s t e e l ( t h e wear o f t e e t h o f t h e s t e e l gear was n e g l i g i b l e ) and d r i v e n gear made o f POM; 2,3 - d r i v i n g gear (2) made o f PA 6 and d r i v e n gear (3) o f POM. P o l y m e r i c gears were moulded.
-
The t r a n s f e r of PTFE t o PE and PETP a n d PMMA t o p o l y c a p r o a m i d e (PCA) t a k e s p l a c e under s e v e r e s l i d i n g c o n d i t i o n s (refs. 1 6 5 , 2 4 7 ) .
A f t e r 3 min o f s l i d i n g ( r o l l e r - b e a r i n g pad s y s t e m ) a t c o n t a c t p r e s s u r e 0.05 MPa a n d s l i d i n g s p e e d 0.35 m/s, t h e t h i c k n e s s o f t h e l a y e r of m a t e r i a l t r a n s f e r r e d r e a c h e d a b o u t 0 . 4 p m and o s c i l l a t e d a b o u t t h i s v a l u e . The p r o c e s s o f t h e m e c h a n i c a l and t h e r m a l d e s t r u c t i o n o f t h e polymer macromolecules i s accompanied by t h e appearance of f r e e r a d i c a l s i n t h e f r i c t i o n area. The l o w s u r f a c e f r e e e n e r g y o f PTFE t r a n s f e r r e d t o PE w a s t h e main reason why t h e a d h e s i o n
122
bonds w i t h t h e PE s u r f a c e were weak and t h e t r a n s f e r r e d l a y e r w a s l o o s e . The PE s u r f a c e became amorphous and m e t h y l p e a r e d i n t h e s u r f a c e l a y e r . I n PTFE-PCA systems, a t h i c k
-
CH3 g r o u p s ap-
( h i g h l y p o l a r polymer)
( u p t o 2 ,um) and compact PTFE l a y e r w a s o b s e r v e d
o n t h e PCA s u r f a c e . A s a r e s u l t of t h e d e s t r u c t i o n o f t h e PCA, t h e -surface
l a y e r becomes amorphous a n d i n t h e f r i c t i o n a r e a t h e
f r e e r a d i c a l s -CH2-kH-NH-CO- a p p e a r a n d p l a y a n a c t i v e r o l e i n t h e t r a n s f e r p r o c e s s and i n t h e t r i b o c h e m i c a l r e a c t i o n s
P t
a a
a
(ref. 248).
a
21
IW
a
Br
' .
130
180
230
I ,
Cohesive energy density, mJ/rnm3
F i g . 4.26. D i r e c t i o n of m a t e r i a l t r a n s f e r i n some polymer-polymer systems.
When t h e two h i g h l y p o l a r p o l y m e r s PMMA and PCA r u b b e d t o g e t h e r , t h e e l e m e n t s q u i c k l y h e a t e d u p and
t h e materials b e g a n t o s t i c k
t o g e t h e r so much t h a t t h e p a r t i c l e s of PMMA w e r e p u l l e d o u t of t h e b u l k m a t e r i a l and a d h e r e d s t r o n g l y t o t h e PCA s u r f a c e . The f r e e
123
radicals CH 3 I C CH2
...- CH2 - -
CH 3
CH 3
-
I COOCH
I
*
C
...- CH2
-
I
-
I
COOCH
CH 3
I
-
+ CH2 = C C I I COOCH3 COOCH3
w e r e o b s e r v e d i n t h e f r i c t i o n a r e a . Even c o o l i n g t h e r u b b i n g e l e ments w i t h w a t e r d i d n o t i n t e r r u p t t h e t r a n s f e r o f PMMA t o t h e PCA s u r f a c e . This i s probably t h e e f f e c t of t h e s o - called "cool d ep ly m e r i z a t i o n " of PMMA (which i s p o s s i b l e e v e n a t
-
36OC ( r e f . 2 4 9 ) ) .
The g e n e r a l c o n c l u s i o n s which c a n b e drawn from r e f s . 1 6 5 a n d 2 4 7 a r e t h a t t h e d i r e c t i o n of m a t e r i a l t r a n s f e r i s from low p o l a r t o h i g h p o l a r polymer and t h a t t h e i n t e n s i t y o f t h e t r a n s f e r i n c r e a s -
es a s t h e s u r f a c e f r e e e n e r g y of t h e p o l a r polymer i n c r e a s e s . The e f f e c t s of v a r i o u s m a t e r i a l c o m b i n a t i o n s o n t h e trihlgical p r o p e r t i e s o f m i n i a t u r e polymer-polymer
systems have been observed
( r e f s . 43, 4 4 , 1 7 0 , 1 7 1 , 1 7 2 ) a n d a r e summarized i n F i g . 4 . 2 7 ( b a s e d on r e f , 1 7 1 1 .
Low friction
coefficient
High friction coefficient High w e f l r
Low wear
I
I
semirrystnlline and amorphous DoIymerj
POM
I
I
1
DA
semicrystalline
I
PO M
I
PC,PPU,ABS
I
Amorphous potymers
Fig. 4.27. Tribological properties o f some polymer-polymer combinations.
124
Systems with an element made of POM (semicrystalline polymer) rubbing against an element made from some other semicrystalline polymer (such as PA or PBTP) or amorphous polymer (e.g. PC or PPO), or systems where semicrystalline PA rubs against amorphous polymer or POM (or against another semicrystalline PA), demonstrate good tribological properties. The adhesion between rubbing surfaces in polymer-polymer systems seems to play an important role in the friction and wear processes. When the bonding force per unit area of contact in polymer-polymer systems was estimated using eqn. ( 4 . 5 ) , the correlation between the friction coefficient in POM h-polymer miniature systems was found. A useful practical correlation between the dielectric constant of the polymer used and the friction coefficient of the aforementioned systems is shown in Fig. 4 . 2 8 (ref. 2 0 0 ) .
0 POM h 0 PBTP
o PA11 0 PC 0 PETP
0 PPO
Dielect r tc constnnt
Fig. 4.28. Friction coefficient o f sphere-plate POM h-polymer miniature systems v s . dielectric constant of polymer used.
125
The data for the friction coefficients used in this plot were taken from refs. 4 3 and 4 4 . The physical properties of the polymers used may also vary during the friction process in the case of severe operating conditions (high local temperature, high normal and tangential stresses)(refs. 1 6 5 , 2 5 9 , 2 5 1 ) . In PA 6-PA 6 systems operating under contact pressure of between 0 . 2 6 5 and 0 . 5 3 MPa and sliding speed 0.8 m/s, after 1 4 4 0 m of sliding distance the decrease in the microhardness of the surface layer was around l o % , and in the volume of the crystalline phase about 8%; water absorption increased from 6 to 11%after 2 0 0 h and the dielectric constant of the polymer also decreased (ref. 2 5 1 ) . Erhard looked at the adhesional interactions in polymer-polymer systems (refs. 1 8 5 , 2 2 9 ) and discovered the relationship between the friction coefficient f and specific energy of adhesion at the polymer-polymer interface W (mJ/m2):
PP
f
=
0.12
+
' P
4.8
(4.32)
The specific energy of adhesion can be estimated using Duprk's formula (4.33)
where y1 and f 2 are the surface free energies of solids 1 and 2 , and y12 is the surface tension at Che interface. The value of can be estimated using Owens and Wendt's method or Wu's method (see Chapter 6 . 2 . 6 ) . An even more accurate way to estimate y12 can be found in ref. 2 5 . The relationship between the friction coefficient f of polymer-polymer (same material) systems and the physical properties of LDPE, PTFE, PCTFE and PMMA can be expressed by the fallowing formula (ref. 2 3 5 ) : (4.34)
rc
where 7 is the shear strength, 6 the microhardness and (in mN/m) the critical surface tension of wetting of the polymer used. The friction coefficient of PE sliding against itself can be expressed as (ref. 2 3 4 ) f=-
7 6c
(4.35)
126
where
zis
t h e s h e a r s t r e n g t h and
s t r e n g t h of t h e
rc
i s t h e compressive y i e l d
polymer u s e d .
A d e c i s i v e c o n n e c t i o n between f r i c t i o n a n d a d h e s i o n i n plymer-
-polymer
s y s t e m s was e s t a b l i s h e d by C z i c h o s ( r e f . 2 3 0 ) . The r e l a -
t i o n s h i p between t h e f r i c t i o n e n e r g y E f a n d t h e s p e c i f i c e n e r g y of a d h e s i o n Wpp
i s as f o l l o w s : E f = c1 e x p ( c 2 Wpp)
(4.36)
where c l and c 2 are c o e f f i c i e n t s . The r e l a t i o n s h i p i s shown by t h e graph i n F i g . 4 . 2 9 .
F i g . 4.29. R e l a t i o n s h i p between s p e c i f i c energy o f adhesion, Wpp, and f r i c t i o n energy, E f , f o r p o l y m e r -polymer systems. P i n - o n - d i s k system, p i n and d i s k d i a m e t e r 2 and 108 mm r e s p e c t i v e l y , l o a d 1 0 N, s l i d i n g speed 2.4 1 0 - 7 m/s, s l i d i n g d i s t a n c e 450 ,urn; 1,2,3 - p i n made o f PTFE, d i s k o f SAN, P S and PMMA r e s p e c t i v e l y ; 4,5,6 PP p i n , SAN, P S and PMMA d i s k ; 7,8,9 - POM p i n , SAN, PS and PMMA d i s k ; 10,11,12 PA 66 p i n , SAN, PS and PMMA d i s k ( r e f . 230).
-
127
When t h e bonding a d h e s i v e f o r c e p e r u n i t o f c o n t a c t a r e a , T a , w a s c a l c u l a t e d u s i n g eqn. ( 4 . 5 ) and t h e v a l u e s o f f r i c t i o n c o e f f i c i e n t t a k e n from r e f . 2 3 0 were p l o t t e d i n t h e c o o r d i n a t e s y s t e m f-Ta, t h e c o r r e l a t i o n between them became a p p a r e n t ( F i q . 4 . 3 0 ) .
4
__t
0
F i g . 4.30. R e l a t i o n s h i p between b o n d i n g adhesion f o r c e p e r u n i t c o n t a c t a r e a , Za, (eqn. (4.51, H = 1 nm, & j O = l ) and f r i c t i o n c o e f f i c i e n t f f o r polymer-polymer systems d e s c r i b e d i n t h e c a p t i o n t o F i g . 4.29.
The v a l u e s o f f and
2, f o r t h e PP-SAN and PP-PS s y s t e m s d i f f e r
w i d e l y from t h e r e s t o f t h e v a l u e s . T h i s c o u l d b e t h e r e s u l t o f t h e m u t u a l s o l u t i o n o f t h e polymers combined, s i n c e t h e s o l u b i l i t y p a r a m e t e r s o f , i n p a r t i c u l a r , PP and PS a r e similar ( r e f . 2 0 4 ) . S i n c e t h e f r i c t i o n f o r c e i s dependent s i m u l t a n e o u s l y on t h e s t r e n g t h of t h e a d h e s i v e bonds and t h e s t r e n g t h o f t h e weaker m a -
t e r i a l i n t h e polymer-polymer
s y s t e m , t h e r e l a t i o n s h i p between f
128
( e c - c o h e s i v e e n e r g y d e n s i t y , ec = d 2 , & - s o l u b i l i t y p a r a m e t e r ) c a n be f o u n d a n d i s p l o t t e d i n F i g . 4 . 3 1 . T h i s r e l a t i o n s h i p can be approximated w i t h t h e following formula : and
la e,
ag(Ta e c )
F =
where ag = 1 . 7 1 0
t o be 1 nm);
-4
za and
,
+ aI0
and b = 0 . 0 4
(4.37)
(when H I see e q n .
ec a r e e x p r e s s e d i n N / m 2
( 4 . 5 ) is t a k e n
and mJ/mm3
tively.
0
5
0.25
0.M
s” Y-
0.15
OJO
-
O.O!
no 250
500
750
1000
1250
F i g . 4.31. R e l a t i o n s h i p between Ta ec (7, bonding adhesion f o r c e p e r u n i t are a o f c o n t a c t , see eqn. (4.51, where H = 1 nrn, &30 = 1 and e i s t h e cohes i v e energy d e n s i t y of t h e weaker poyyrner systems d e s c r i b e d i n t h e c a p t i o n t o F i g . 4.29).
respec-
129 The tribological properties of polymer-polymer systems depend on the adhesional-cohesional properties of the rubbing materials. The combination of the same materials or materials which have similar solubility parameters is not advantageous. The highest practical strength of the adhesive bonds can be expected when the surface free energies of the polymers combined are the same or similar (refs. 2 5 3 , 2 5 4 ) . This means that the maximum strength of the adhesive bonds is wheny12in eqn. ( 4 . 3 3 ) is at its minimum. Low adhesion and the resulting low friction and wear can therefore be achieved in polymer-polymer systems when the surface free energies of the polymers combined are low and the surface tension at the polymer-polymer interface is high. The value of the product of the binding force per unit of contact area, (eqn. ( 4 . 5 ) ) , and the cohesive energy density of the weaker polymer, ec, should be small. When identical polymers must be applied, it is advantageous to minimize the ratio of shear strength to compressive yield strength (and simultaneously the surface free energy of the polymer too). For PE-PE systems, the friction coefficient decreases as the material’s density increases. In PCA-PA copolymer systems, the strength of the adhesive bonds increases as more free amide groups appear in the copolymer (Fig. 4 . 3 2 , ref. 2 5 5 ) . The tribological properties of polymer-polymer systems can be improved by filling one or both of the materials; for example, a glass-fibre reinforced polymer can be combined with a polymer filled with a solid lubricant such as PTFE. During rubbing in polymer-polymer systems, material is transferred from the polymer with the lower cohesive energy density (or which is less polar) to the polymer with the higher.cohesive energy density for which is more polar). The generation of thermal energy as a result of rubbing has a negative effect, leading to overloading (a too high pv value, where p is the contact pressure and v the sliding speed) and catastrophic wear of the lessthemd-resistant polymer. It is advisable to fill the polymer with a filler which has high thermal conductivity. Thin co’atings of composites of high-molecular surface active agents are recommended for metallic elements to give good tribological properties at temperatures up to 20OoC. When rubbing elements overheat, it may help to apply water for example (but not lubricant) as a coolant. The tribological problems of polymer-polymer systems need further study. In particular, we need to know more about the wear in such systems and how it is related to the physical properties
ra
130
of the rubbing materials and to the operating conditions. With a rational choice of materials, these systems can compete with metal-polymer or metallic triboloqical systems.
I
10
12
14
Contents of free amide groups
c
% mol
F i g . 4.32. Dependence o f a d h e s i v e bond i n PCA ( t h i n f i l m ) - P A copolymer systems on t h e number o f f r e e amide groups i n copolymer when t h e f i l m i s t o r n o f f . The copolymers used were a s f o l l o w s : P A 6 + P A 66 ( p r o p o r t i o n by w e i g h t 1 : 1 ) , P A 6 + P A 66 + P A 610 (1 :1 : 1 ) , P A 6 + P A 66 + P A 610 ( l : l : l ) , P A 6 + P A 66 t P A 1 2 ( l : l : l ) , PA 6 + P A 612 + P A 12 ( l : l : l ) , P A 6 + P A 66 + P A 610 + + P A 612 + P A 1 2 ( 1 : l : l : l : l ) .
131
4,3,
OTHER SYSTEMS
Many of the tribologicaL systems used in small mechanisms have elements made of non-metallic and non-polymeric materials or have polymeric elements rubbing against elements made of various other materials. Typical of these are ceramic materials such as sapphire or ruby (single-crystal aluminium oxide) , used for many years in the bearings of precision instruments (jewels). A steel, roller-burnished journal usually ruhs against such bearings. The friction and wear of elements in such a material combination is highly dependent on rubbing conditions. The friction coefficient of typical free-cutting steel (hardened) rubbina against sapphire is about 0 . 5 and of bearing stainless steel about 0.4 (ref. 9 5 ) . The effect of the hardness of materials rubbing against sapphire was found to be that when the metal or ceramic material is harder the friction coefficient is lower. The friction coefficient of monel alloy is 0 . 5 2 , while for WC 2 0 (WC + 2 0 % Co), WC 1 0 tungsten carbides and sapphire, with microhardness 1 1 0 0 0 , 16000 and 2 2 5 0 0 MPa respectively, it is 0 . 3 5 , 0 . 3 2 and 0 . 3 0 respectively. The friction (and wear) characteristics are anisotropic. The tribological properties of spherical microbearings, which have a steel element rubbing against corundum depend on the orientation on the optical axis of the crystal to the friction surface: the friction torque (as a function of the sliding distance) of the bearings is most stable when the optical axis of the corundum is perpendicular to the friction surface (ref. 9 5 ) . The wear resistance of microbearings with a bearing bush made of a ceramic material like sapphire is governed mainly by the wear resistance of the journal. When various materials (hardened free-cutting steel, bearing stainless steel, monel, WC 20, WC 1 0 , sapphire) were tested using a four-ball ( @ 3 mm) friction machine (the upper sphere of which was made of sapphire), the relative wear resistance of the above-mentioned materials, taking the wear resistance of free-cutting steel as 1.0, was as follows: 0.8, 1.1, 14.4, 13.8 and 2 3 . 7 respectively (ref. 9 5 ) . The wear is inversely proportional to the hardness. This is also generally true for the same ceramic materials rubbing against themselves: the diameter of the wear trace for sapphire, ruby, fused quartz, agate, diaspore and glass was negligible, 48, 2 4 0 , 1 7 0 , 1 5 0 and 2 2 0 ,un respectively (in the four-ball friction machine under the following rubbing conditions: operating time 5 0 min, sliding speed 0.0054 m/s). The
132 wear depends on t h e b r i t t l e n e s s a s w e l l a s t h e h a r d n e s s o f t h e m a t e r i a l u s e d . The m i c r o h a r d n e s s o f t h e a f o r e m e n t i o n e d materials
is 23600, 2 2 2 0 0 , 14700, 12780, 8840 a n d 4 4 0 0 MPa, w h i l e t h e microb r i t t l e n e s s ( d e f i n e d a s m i c r o h a r d n e s s a t 0 . 0 0 1 N when 50% o f i n d e n t a t i o n s t u r n i n t o c r a c k s ) i s 1 4 0 , 4 2 , 26, 182, n o t e s t i m a t e d and 4 8 MPa r e s p e c t i v e l y . The wear o f h a r d e n e d f r e e - c u t t i n g
steel
r u b b i n g a g a i n s t s a p p h i r e , r u b y , f u s e d q u a r t z , a g a t e , d i a s p o r e and g l a s s i s t a k i n g t h e wear when r u b b i n g a g a i n s t s a p p h i r e a s 1 . 0 , follows: 1.3,
1.1, 1.1, 1 . 6 ,
as
1 . 3 . I t c a n b e s e e n t h a t when a series
o f m i n e r a l s o f i n c r e a s i n g h a r d n e s s a r e t e s t e d , t h e wear o f t h e
s t e e l e l e m e n t f i r s t i n c r e a s e s and t h e n d e c r e a s e s . The i n c r e a s e i n wear i s c o n n e c t e d w i t h t h e h a r d n e s s o f t h e a b r a s i v e p a r t i c l e s (wear d e b r i s o f t h e m i n e r a l s ) i n t h e f r i c t i o n a r e a . The wear i n creases with increase of load. T h e o r e t i c a l and e x p e r i m e n t a l s t u d i e s u s i n g a f o u r - b a l l f r i c t i o n machine i n which t h e u p p e r b a l l w a s made o f s a p p h i r e a n d t h e l o w e r b a l l s o f h a r d e n e d f r e e - c u t t i n g s t e e l o r WC 1 0 t u n g s t e n c a r b i d e ( r e f . 9 5 ) , h a v e shown t h a t t h e r e l a t i o n s h i p between t h e wear i n t e n s i t y , I,,
o f s t e e l o r WC 1 0 m a t e r i a l s as a f u n c t i o n o f
t h e maximum r e a l c o n t a c t p r e s s u r e pm c a n b e a p p r o x i m a t e d w i t h t h e following formula: (4.38) where a l l a n d a12 c a n be t a k e n a s 3 . 1 3 1 0 - c u t t i n g s t e e l and 5 . 0 1 0 - l '
and
6.0
-9
and 0 . 4
f o r free-
f o r WC 1 0 r e s p e c t i v e l y .
The wear o f t h e c e r a m i c m a t e r i a l s d e p e n d s on t h e r u b b i n g cond i t i o n s . For i n s t a n c e , a t low s l i d i n g s p e e d s , a g a t e wears l i k e a p l a s t i c m a t e r i a l b u t a t h i g h s l i d i n g s p e e d s t h e wear i s s i m i l a r t o t h e wear of v e r y b r i t t l e m a t e r i a l s ( r e f . 9 5 ) . The wear i s p r o p o r t i o n a l t o t h e s l i d i n g d i s t a n c e . The r o u g h n e s s of t h e s u r f a c e s o f t h e r u b b i n g e l e m e n t s h a s a n i m p o r t a n t e f f e c t on t h e t r i b o l o g i c a l p r o p e r t i e s of t h e m a t e r i a l combinations being c o n s i d e r e d . F o r m i n i a t u r e t r i b o l o g i c a l s y s t e m s t h e optimum s u r f a c e roughn e s s of t h e r u b b i n g e l e m e n t s depends o n w h e t h e r t h e y a r e i n t e n d e d f o r a s h o r t o r l o n g s e r v i c e l i f e ( r e f . 2 5 6 ) . I n t h e f i r s t case t h e optimum s u r f a c e r o u g h n e s s w i l l be when t h e f r i c t i o n i s a t i t s min-
i m u m and i n t h e second when t h e f r i c t i o n i s s t a b l e and t h e wear i s minimum. The minimum f r i c t i o n i s a f u n c t i o n o f t h e i n i t i a l roughn e s s a n d s t a b l e f r i c t i o n i s a f u n c t i o n of t h e r o u g h n e s s a f t e r t h e r u n n i n g - i n p r o c e s s . I n m i c r o b e a r i n g s which h a v e r u b y j e w e l s w i t h a
133
working surface roughness, Ra of 0.02 ,um, the optimum initial surface roughness of the hardened steel journals should be 0.16 - 0.32 ,urn (ref. 9 5 ) . The real contact pressure in the range of 274-1010 MPa in such bearings has no effect on the optimum initial surface roughness of the journal surface. The effect of the initial surface roughness of a journal made of WC 10 in a microbearing with a sapphire bearing bush (Ra = 0.02 ,um) on the friction coefficient is presented in Fig. 4 . 3 3 (based on ref. 95).
1
0.5
I
1.0
Rz
1
1.5
1
2.0
,urn
F i g . 4.33. E f f e c t o f WC 1 0 j o u r n a l s u r f a c e roughness o n f r i c t i o n c o e f f i c i e n t for WC 10-sapphire m i c r o b e a r i n g . 1,2 - s t a t i c f r i c t i o n , 1 - b e a r i n g c l e a r a n c e , 2 - 5 ,urn, 2,3 - 20-30 ,urn, 3 k i n e t i c f r i c t i o n . Maximum c o n t a c t p r e s s u r e : 1 5 0 0 MPa, 2 - 1300 MPa, 3 - 350 MPa.
-
-
Increasing the sliding friction reduces the effect of the surface roughness on the friction coefficient. The surface roughness at which the friction coefficient is minimum depends on the operating
134 c o n d i t i o n s , however
,
optimum s u r f a c e r o u g h n e s s i s n o t t h e same as
minimum s u r f a c e r o u g h n e s s . F o r m i n i a t u r e s y s t e m s i n t e n d e d f o r l o n g s e r v i c e , t h e optimum i n i t i a l s u r f a c e r o u g h n e s s means t h e r o u g h n e s s a t which t h e running- i n wear i s minimum. T h i s r o u g h n e s s c o r r e s p o n d s n o t t o t h e lowest b u t t o t h e most s t a b l e f r i c t i o n . A l a b o r a t o r y model and t e s t s i n
a working s i t u a t i o n o f m i c r o b e a r i n g s w i t h a s t e e l j o u r n a l and r u b y j e w e l showed t h a t when t h e j o u r n a l ' s s u r f a c e roughness,.Ra ,um,
i s 0.02
t h e minimum r u n n i n g - i n w e a r o c c u r s when t h e s u r f a c e r o u g h n e s s
i s 0.1-0.3 ,um ( r e f . 9 5 ) . Whensteel j o u r n a l s of v a r i o u s s u r f a c e r o u g h n e s s e s w e r e rubbed a g a i n s t a r u b y
o f t h e working ruby s u r f a c e , R,
j e w e l ( R a = 0 . 0 2 ,um) t h e minimum r u n n i n g - i n wear o c c u r r e d when R, ( o f t h e s t e e l s u r f a c e ) w a s 0 . 2 ,urn. The optimum s u r f a c e r o u g h n e s s o f t h e hardened steel j o u r n a l i n such microbearings can be t a k e n
a s 0.02-0.04 ,um when t h e s u r f a c e r o u g h n e s s o f t h e r u b y j e w e l i s 0 . 0 2 ,urn. When t h e s l i d i n g s p e e d i s h i g h , a smaller v a l u e of Ra f o r t h e s t e e l j o u r n a l i s recommended. The e f f e c t of c o n t a c t p r e s s u r e on t h e optimum r o u g h n e s s i s s m a l l . The f r i c t i o n t o r q u e i n s t e e l - m i n e r a l m i c r o b e a r i n g s s u c h as t h o s e u s e d i n e l e c t r i c c o u n t e r s o r g y r o s c o p e s can b e e f f e c t i v e l y r e d u c e d by a p p l y i n g e x t e r n a l v i b r a t i o n s o f a f r e q u e n c y o f a b o u t 2 0 kHz ( r e f . 2 5 6 ) . The a x i s - o r i e n t e d v i b r a t i o n s are e f f e c t i v e when
t h e r o t a t i n g element i s i n a v e r t i c a l or h o r i z o n t a l p o s i t i o n . I n metal-ceramic s y s t e m s s u r f a c e c h e m i s t r y i s v e r y i m p o r t a n t t o f r i c t i o n and wear b e h a v i o u r ( r e f s . 1 6 2 , 2 5 7 ) . F o r o x i d e ceramics ( s u c h a s aluminium o x i d e , e . g . ,
s a p p h i r e , Mn-Zn and Ni-Zn
ferrites)
t h e f r e e e n e r g y o f o x i d e f o r m a t i o n f o r t h e l o w e s t metal o x i d e i s d i r e c t l y c o r r e l a t e d w i t h metal s h e a r p r o p e r t i e s which r e l a t e t o f r i c t i o n . F o r aluminium o x i d e i n c o n t a c t w i t h m e t a l s , oxygen i n c r e a s e s b o t h a d h e s i o n and f r i c t i o n , w h i l e c h l o r i n e , f o r example, d e c r e a s e s f r i c t i o n when used as a s u r f a c e c o n t a m i n a n t . All t h e transition metals (metals with
p a r t i a l l y f i l l e d d s h e l l s ) i n con-
t a c t w i t h ceramic t r a n s f e r r e d t o t h e ceramic c o u n t e r f a c e and t h e f r i c t i o n p r o p e r t i e s o f s u c h s y s t e m s c o r r e l a t e w i t h t h e t e n s i l e and s h e a r p r o p e r t i e s of t h e s e metals as w e l l ( r e f . 1 5 9 ) . The h i g h e r t h e s h e a r s t r e n g t h i s , t h e l o w e r t h e f r i c t i o n c o e f f i c i e n t w i l l be. As w e l l a s t h e adhesive-cohesive properties a t t h e i n t e r f a c e , t h e
p l o u g h i n g component o f f r i c t i o n i s a v e r y i m p o r t a n t f a c t o r i n t h e f r i c t i o n and wear b e h a v i o u r o f metal-ceramic
s y s t e m s . The f r i c t i o n
c o e f f i c i e n t o f t i t a n i u m i n c o n t a c t w i t h s i l i c o n c a r b i d e i n a vacum ( h e m i s p h e r i c a l s i l i c o n c a r b i d e r i d e r s l i d i n g on a metal p l a t e a t
135
a sliding speed of 0 . 0 5 m/s, load 0 . 0 5 - 0 . 5 N, and vacuum lo-’ Pa), is about 0 . 6 while for rhenium or rhodium it is about 0.4 (refs. 1 5 9 , 2 5 7 ) . The friction coefficient of copper in contact with sapphire is twice that of silver in contact with sapphire (ref. 1 6 2 ) . The wear resistance of carbon steels in contact with ceramic increases with hardness and the abrasive wear resistance of carbon steels is low for annealed steels and high for steels with a martensitic microstructure (ref. 1 3 7 ) . The friction in iron-sapphire or titanium-sapphire systems (Fe or Ti hemisphere ( @ 6 mm) slider-sapphire plate, amplitude of oscillations 5 mm, load 1 0 N, sliding speed 1 m/s) increases as a function of the number of passages; for Fe-sapphire systems from 0.4 to 1.1, stabilizing after about 2 5 passages, while for Ti-sapphire systems it increases from 0 . 2 to about 1 . 0 (oscillating largely about this value) (ref. 2 5 8 ) . The metal transfer to the sapphire surface was observed; the thickness of the film transferred was 0.1-0.2 ,um for Ti, while for Fe it was only 0 . 0 2 in the centre and 0.06 on the edge of the race. The most attractive ceramic material for tribological purposes seems to be magnesia-partially stabilized zirconia (magnesia-PSZ) (ref. 2 5 9 ) . Compared to other ceramics it is very tough and canpared to metals it is very hard. It is chemically inert (up to 8OO0C) and may be used in contact with metal (to provide better heat transfer from the friction area) in abrasive environments at moderately high temperatures. It can be used where good corrosion resistance or wear resistance is required; these include paper-wrapping dies and textile fibre guides. Friction coefficient values in the range of 0 . 2 - 0 . 3 were observed for PSZ sliding on hardmaterials such as tool steel, cast iron and carbides, while on soft metals (copper, aluminium, iron) they were between 0.4 and 0.5 and some metallic pick-up was seen on the PSZ. An increase in the friction coefficient was observed as the temperature was increased from room temperature to about 5OO0C, but above this temperature the friction decreased again, probably because water which had been absorbed was removed from the surfaces. The alumino-silicates ceramic (Gabro Clay) has a friction coefficient when rubbing against steel, gray cast iron, brass, aluminium and nickel of 0.3, 0 . 2 , 0.2, 0 . 2 5 and 0.15 respectively (ref. 2 6 0 ) . The friction coefficient increases with increase in load. The friction coefficient is lower at low loads and sliding speeds.
136 S i l i c o n n i t r i d e , Si3N4, h a s two t o t h r e e t i m e s t h e h a r d n e s s and o n e - t h i r d t h e f r i c t i o n c o e f f i c i e n t t o b e a r i n g s t e e l s ( r e f . 2 8 ) . I t m a i n t a i n s i t s s t r e n g t h and o x i d a t i o n r e s i s t a n c e up t o 12OOOC.
F a t i g u e l i f e f o r s i l i c o n n i t r i d e i s one t o two o r d e r s o f
magnitude
l o n g e r t h a n t h e r a t e d l i f e of b e a r i n g s t e e l s . S i l i c o n n i t r i d e h a s t h e l o w e s t f r i c t i o n and e l a s t i c modulus o f t h e c e r a m i c s p r e s e n t l y a v a i l a b l e . The s l i d i n g f r i c t i o n c o e f f i c i e n t v a l u e s f o r s t e e l on Si3N4, Si3N4-Si3N4, s t e e l - S i c ,
-steel systems (pin-on-disk, 0.15,
Sic-Sic,
steel-WC,
WC-WC
and steel-
s l i d i n g s p e e d 3 mm/s, l o a d 2 0 N ) are
0.29, 0.52, 0.19, 0.34 a n d 0.54 r e s p e c t i v e l y ( r e f . 2 8 ) .
0.17,
Four d i f f e r e n t ceramics (Si3N4, PSZ, A1203 and S i S i C ) u s e d f o r comparative f r e t t i n g tests i n c o n t a c t w i t h s t e e l i n a s t e e l b a l l -
-ceramic d i s k s y s t e m ( f r e q u e n c y 20 Hz, a m p l i t u d e 0 . 2 mm, l o a d 2 0 N, b a l l d i a m e t e r 1 0 m) g a v e t h e r e s u l t s shown i n F i g . 4.34
( b a s e d on
r e f . 2 6 1 ) . T h e s e tests show t h a t t h e s t e e l - P S Z s y s t e m d e m o n s t r a t e s the b e s t w e a r behaviour.
Steel
p 0.6
0.4 rn E 0.2 E *
L
0 W
5
Q
E"
3
d
0 0.2 >
0.4
Ceramic F i g . 4.34. Wear o f s t e e l and ceramic i n f r e t t i n g t e s t s . O s c i l l a t i o n o f ceramic d i s k r u b b i n g a g a i n s t A l S l 52100 s t e e l b a l l , 0 10 mm 800 HV. frequency 20 Hz, a m p l i t u d e 0 . 2 mm, l o a d 20 N , number o f o s c i l l a t i o n s
4.8 106.
137 Amorphous metals and a l l o y s u s e d i n s l i d i n g c o n t a c t w i t h ceramic demonstrate i n t e r e s t i n g t r i b o l o g i c a l p r o p e r t i e s
( r e f s . 30,
3 1 , 158, 2 6 2 ) . When t h e amorphous i r o n a l l o y s Fe40N38M04B18 Fe67CO18B14Sil and FeglB13.5Si3.5C2
,
( f o i l t h i c k n e s s 0.030-0.033mn)
a r e rubbed a g a i n s t a s a p p h i r e h e m i s p h e r e i n a vacuum a t r e l a t i v e l y low s p e e d s ( 0 . 0 5 - 1 . 5 mm/s)
a n d low l o a d s ( 0 . 2 - 2 . 5
p h i r e s p h e r e d i a m e t e r i s 3.2 mm)
,
N , when t h e s a p -
t h e i r f r i c t i o n c o e f f i c i e n t in-
creases from 1 . 0 - 1 . 5 a t room t e m p e r a t u r e t o 1.8-2.4 a t 35OoC ( r e f . 3 0 ) . This i s due t o a n i n c r e a s e in a d h e s i o n r e s u l t i n g f r o m s u r f a c e s e g r e g a t i o n of b o r i c o x i d e a n d / o r s i l i c o n o x i d e . The wear resistance o f t h e amorphous a l l o y Fe67Co18B4Sil w a s s u p e r i o r t o t h a t o f 304 s t a i n l e s s s t e e l . During t h e s l i d i n g p r o c e s s , c r y s t a l l i t e s 10-150
nm i n s i z e a p p e a r on t h e wear s u r f a c e . Between 500 and 75OoC t h e f r i c t i o n c o e f f i c i e n t i s low ( 0 . 2 - 0 . 3 ) and r e m a i n s c o n s t a n t because o f b o r o n n i t r i d e s e g r e g a t i o n on t h e f o i l s u r f a c e . The s e g r e g a t i o n o f c o n t a m i n a n t s coming from t h e b u l k o f t h e m a t e r i a l t o t h e s u r f a c e upon h e a t i n g i s r e s p o n s i b l e f o r t h e f r i c t i o n b e h a v i o u r o f t h e s e s y s t e m s i n a vacuum. I n a r g o n and a i r c o n d i t i o n s , t h e f r i c t i o n c o e f f i c i e n t o f Fe67C016B14Sil amorphous a l l o y r u b b i n g a g a i n s t
a 6 . 4 nun s a p p h i r e h e m i s p h e r e r i d e r i s 0 . 2 , t h e same as f o r 304 s t a i n l e s s s t e e l , a n d d o e s n o t depend on l o a d o r s l i d i n g s p e e d w i t h i n t h e a f o r e m e n t i o n e d r a n g e s . T h e r e i s no v i s i b l e wear o n t h e amorphous a l l o y . I f a 3.2 mm s a p p h i r e r i d e r i s u s e d t h e r e are marked d i f f e r e n c e s n o t o n l y i n f r i c t i o n , b u t a l s o i n w e a r . The f r i c t i o n c o e f f i c i e n t i n i t i a l l y low i n c r e a s e s w i t h s l i d i n g d i s t a n c e u n t i l eq'uilibrium c o n d i t i o n s are reached, 0.2
f o r t h e amorphous
a l l o y and a b o u t 0 . 5 f o r t h e s t e e l . Oxide w e a r d e b r i s p a r t i c l e s a r e g e n e r a t e d on t h e amorphous a l l o y s u r f a c e , w h i l e m e t a l wear d e b r i s p a r t i c l e s are p r i m a r i l y g e n e r a t e d o n t h e s t e e l s u r f a c e . I n t h e c a s e o f t h e amorphous a l l o y m i c r o s c o p i c b r i t t l e f r a c t u r e s a p p e a r a n d t h e o x i d e l a y e r s b r e a k u p , p r o d u c i n g f i n e o x i d e wear d e b r i s p a r t i c l e s . However, t h e s a p p h i r e a d h e r e s t o t h e s t e e l a c r o s s t h e i n t e r f a c e and w i t h t a n g e n t i a l m o t i o n , s e p a r a t i o n t a k e s p l a c e i n t h e
s t e e l and t h e bonds of t h e c o h e s i v e l y weaker s t e e l r u p t u r e ; c a v i t i e s t h e n form i n t h e m a t e r i a l . When t h e amorphous a l l o y Fe81B13.5Si3.5C2
(Ra = 2 ,um)
i s rub-
b e d a g a i n s t a s t e e l r o l l c o a t e d w i t h a 50 ,um t h i c k l a y e r (R, = 3 p n ) o f 80%WC-14%Ni-3.5%Cr-0.8%B-0.8%Fe-O.8%Si-O.l%C
it wears by plough-
i n g ; as t h e r o u g h n e s s i n c r e a s e s , t h e wear mode c h a n g e s t o microc u t t i n g and t h e n t o c r a c k n u c l e a t i o n and p r o p a g a t i o n ( r e f . 1 5 8 ) . A f t e r a n i n c u b a t i o n p e r i o d , t h e mass loss i n c r e a s e s l i n e a r l y w i t h
138
duration or sliding distance; the wear of this particular mrphous alloy increases with sliding speed, which is quite contrary to most crystalline metals. For wear resistance, the alloy is preferred in the crystallized state for sliding against smooth surfaces at low loads, but jn the amorphous state when the roughness and load are higher. The friction coefficient and wear intensity of boron nitride sliding on steel depend mainly on the sliding speed (ref. 18). When a certain sliding speed is reached (say over 10 m/s at the contact of two rolls @ 5 and 100 mm respectively, at load 20 N), the friction coefficient decreases to 0.5 (at low total wear in the tribological system) initially because of the formation of B2O3 oxides and finally by phase changes from sphalerite and wurtzite structures to graphitization of the ceramic surface. Similar graphitization of a silicon carbide surface (when heated to 15OO0C) with the graphite functioning as an abrasion and friction reducer was observed by Buckley and Miyoshi (ref. 257). The sliding of ceramic under load is accompanied by a plastic flow in ceramics such as magnesium oxide, aluminium oxide and silicon carbide under relatively modest conditions of rubbing contact (refs. 24, 257). The presence of surface films such as adsorbates, has a marked influence on the adhesion friction andwear, altering the amount of plastic deformation that will occur during rubbing. The wear encountered with ceramics is generally of an adhesive and abrasive nature. Fracture pits and multiangular wear debris, having crystallographically-oriented sharp edges,'have been observed with ferrite-ferrite and silicon carbide-siliconcarbide contacts (ref. 257). Relatively small tangential stresses can easily produce fracture at the surface of diamond-silicon carbide systems, for instance. Abrasive wear also occurs when a third particle harder than one or both of the rubbing surfaces becomes trapped at the interface. It can remove material from one or both surfaces. Such third-body abrasion can result in catastrophic wear in ceramic or other systems (ref. 263). Sic, Al2O3, SiAlON and PSZ demonstrate very good tribological properties when rubbing on themselves (ref. 24). The tests were carried out using a barrel roll-plate system where the diameter of the roll was 34 nun (width 10 nun, curvature radius 100 m m ) , the length of the plate was 10 mm and its thickness 5 nun. The system was operated at sliding speeds in the range of 0.1-4 m/s and applied load 2-40 N. The surface roughness, Ra, of the roll was 1 ,urn
139 and of t h e p l a t e 0.02-0.05 ,um. The f r i c t i o n c o e f f i c i e n t f o r Al2O3, PSZ, S i A l O N and S i c s y s t e m s was a b o u t 1 . 0 5 , 0 . 9 5 ,
0.95 a n d 0.75
r e s p e c t i v e l y a t a s l i d i n g s p e e d o f 0.25 m / s and a f t e r a s l i d i n g d i s t a n c e o f 2 km, and s l i g h t l y d e c r e a s e d w i t h i n c r e a s i n g l o a d . The f r i c t i o n c o e f f i c i e n t as a f u n c t i o n of t h e s l i d i n g s p e e d i n i t i a l l y i n c r e a s e d , r e a c h i n g a maximum o f 1.1 a n d 0.9 f o r A1203 a n d SiAlON s y s t e m s r e s p e c t i v e l y ( a t s l i d i n g s p e e d ca. 0 . 8 m / s ,
load 5 N , after
s l i d i n g d i s t a n c e 2 kn) w h i l e f o r PSZ a n d S i c s y s t e m s i t r e a c h e d a maximum v a l u e o f 0 . 9 and 0 . 8 r e s p e c t i v e l y ( s l i d i n g s p e e d c a . 0 . 3
m/s);
when t h e s l i d i n g s p e e d i n c r e a s e d t o 4 m/s, t h e f r i c t i o n co-
e f f i c i e n t decreased t o 0.65,
0.60,
0.55 a n d 0 . 5 5 f o r Al203, S W N ,
PSZ and S i c s y s t e m s r e s p e c t i v e l y . The w e a r o f t h e p l a t e i n c r e a s e s
p a r a b o l i c a l l y as t h e a p p l i e d l o a d i n c r e a s e s , w h i l e i n c r e a s i n g t h e s l i d i n g s p e e d i n i t i a l l y c a u s e s i t t o d e c r e a s e r a p i d l y ( h a v i n g min-
i m u m a t ca. 0 . 5 m / s ) from 55 ,um ( d e p t h o f t h e wear c r a t e r ) , 2 8 , 20 ,um ( a t 0 . 1 m / s , l o a d 5 N , a f t e r s l i d i n g d i s t a n c e 2 km) f o r SiAlON, A1203 and Sic s y s t e m s r e s p e c t i v e l y t o minimum v a l u e s 1 8 , 1 4 a n d 1 2 ,um, which t h e n i n c r e a s e t o 6 0 ,
55 and 1 6 ,um r e s p e c t i v e l y .
The wear
o f PSZ measured i n t h e r a n g e o f 0.3-1 m / s a l s o d e c r e a s e d from a b o u t 36 t o 28
m. A1203 wears by m i c r o c u t t i n g w h i l e w e a r by abra-
s i o n i s c h a r a c t e r i s t i c o f SiAlON m a t e r i a l i n p a r t i c u l a r . The r e l a t i o n s h i p between t h e s h e a r modulus K, o f t h e m a t e r i a l u s e d a n d t h e
w e a r c r a t e r d e p t h hc c a n b e e x p r e s s e d by t h e l i n e a r r e l a t i o n s h i p hc = -0.23 where hc i s i n +mi and K,
Ks + 53
(4.39)
i n GPa.
A t a low s l i d i n g s p e e d ( 1 m/s) t h e f r i c t i o n c o e f f i c i e n t f o r
Si3N4 s l i d i n g on i t s e l f i s 0 . 8 5 i n d r y a r g o n and n i t r o g e n a n d 0 . 8 i n l a b o r a t o r y a i r and oxygen ( r e f . 264) s y s t e m s : h e m i s p h e r e p i n @ 6 mm,
.Pin
wear ( i n p i n - o n - p l a t e i n d r y g a s e s ) as
l o a d s 1-30 N ,
compared t o wear of t h e p l a t e was n e g l i g i b l e . I n 98% humid a r g o n and a i r , t h e w e a r o f t h e p i n a n d t h e p l a t e w a s t h e same, v e r y s n a l l i n b o t h cases. I n d r y g a s e s , wear o c c u r s by f r a c t u r e and no e v i d e n c e of p l a s t i c d e f o r m a t i o n w a s o b t a i n e d . I n w e t g a s e s c o n t a i n i n g water v a p o u r , a t r i b o c h e m i c a l r e a c t i o n p r o d u c e s an amorphous s u b s t a n c e , p r o b a b l y a h y d r a t e d s i l i c o n o x i d e . T h i s i n c r e a s e s t h e adh e s i o n between wear p a r t i c l e s a n d p r o v i d e s a l a y e r on t h e s u r f a c e which p r o t e c t s t h e m a t e r i a l from f u r t h e r wear. The wear asymmetry o b s e r v e d between p i n and p l a t e c a n be e x p l a i n e d by t h e f o r m a t i o n o f a p r o t e c t i v e s h o e o f wear p a r t i c l e s a d h e r i n g t o t h e p i n b e c a u s e o f i t s s p h e r i c a l s h a p e . Aluminium s i l i c a t e s l i d i n g o n i t s e l f c a n
140
have a friction coefficient as low as 0.1 (ref. 2 6 5 ) and low-mass alumina ceramic ( 9 9 . 8 % A 1 2 0 3 1 sliding on itself G . 2 (ref. 2 6 6 ) Hot-pressed boron carbide B q C rubbing on itself at high temperatures in air or in a vacuum demonstrates very'good tribological properties (Fig. 4 . 3 5 , ref. 19). During sliding in air at very high temperatures, the dimensions of the samples increase because of the oxidation of the rubbing surfaces. At 5 0 - 6 0 0 ° C the B 2 O 3 grains and B O ( 0 H ) and B ( O H ) 3 fibres are present, at higher temperatures B(OH)3 with B 2 O 3 grains and r A l 2 O 3 , and at above 1000°C the surface layer consists of B ( O H l 3 only.
.
F i g . 4 . 3 5 . Wear and f r i c t i o n c o e f f i c i e n t o f h o t - p r e s s e d B 4 C s l i d i n g on i t s e l f as a f u n c t i o n o f t e m p e r a t u r e . C y l i n d r i c a l bushes r u b b i n g f r o n t a l l y ; i n t e r n a l d i a m e t e r 8 mrn, e x t e r n a l d i a m e t e r 16 mm, l e n g t h 1 5 mm, s l i d i n g speed 0.01 m/s, c o n t a c t p r e s s u r e 1 MPa. 1 1 Pa vacuum, 2 - a i r .
-
141
The ceramic-polymer s y s t e m s t e s t e d i n s l i d i n g and r o l l i n g i n a vacuum d e m o n s t r a t e i n t e r e s t i n g t r i b o l o g i c a l p r o p e r t i e s ( r e f . 2 0 ) . PC ( o r f i l l e d PTFE)
( d i s k ) s l i d i n g i n a pin-on-disk
system a g a i n s t
g l a s s o r d i a s p o r e g i v e s a f r i c t i o n c o e f f i c i e n t o f 0.03-0.04
(glass)
and 0.05 ( d i a s p o r e ) w i t h no wear o f t h e p i n and v e r y l i t t l e wear o f t h e d i s k (vacuum 1 . 3 3
Pa, s l i d i n g speed 0.5 m / s ,
l o a d 25 N ,
s l i d i n g d i s t a n c e 1 km). I n a s y s t e m s i m u l a t i n g a r o l l i n g b e a r i n g , t h e b e s t f r i c t i o n and w e a r b e h a v i o u r was f o u n d f o r d i a s p o r e - P C ( o r f i l l e d PTFE)
( t h e two r i n g s and s i x b a l l s r o l l i n g b e t w e e n them
were made o f d i a s p o r e a n d t h e c a g e f o r b a l l s was made o f PC ( o r f i l l e d pTFE).Of t h e o t h e r m i n e r a l s u s e d ( g l a s s , g a b r o a n d m a r b l e ) , t h e b e s t r e s u l t s w e r e o b t a i n e d when g l a s s was u s e d i n s t e a d o f t h e diaspore. Alumina c e r a m i c s c a n r e p l a c e t h e t r a d i t i o n a l metal i m p l a n t s i n a r t i f i c i a l h i p s ( r e f . 2 6 7 ) . The s i n g l e - c r y s t a l
alumina (sap-
p h i r e ) d e m o n s t r a t e s b e t t e r wear b e h a v i o u r t h a n a p o l y c r y s t a l a l u -
mina and i s b e t t e r t h a n s t e e l m a t e r i a l s i n r u b b i n g a g a i n s t UHMWPE. Glass i s a m a t e r i a l w i t h some d i s t i n c t i v e t r i b o l o g i c a l features ( r e f . 1 6 2 ) . When i t i s i n c o n t a c t w i t h metal i n m o i s t a i r , m e t a l i s t r a n s f e r r e d t o t h e g l a s s and t h e f r i c t i o n c o e f f i c i e n t i s t y p i c a l l y from 0 . 5 t o 0 . 7 ,
depending on t h e s h e a r s t r e n g t h of t h e m e t -
a l i n v o l v e d . I n a vacuum, g l a s s t r a n s f e r s t o t h e m e t a l , a n d t h e f r i c t i o n c o e f f i c i e n t is about 0.5.
I n o t h e r words, t h e f r i c t i o n
c o e f f i c i e n t s a r e s i m i l a r b u t t h e mechanisms of f r i c t i o n a r e comp l e t e l y d i f f e r e n t . This i s because t h e f r a c t u r e properties of g l a s s are s t r o n g l y a f f e c t e d by water; water v a p o u r a d s o r p t i o n d e -
creases t h e s t r e n g t h o f g l a s s . I n a vacuum, t h e f r i c t i o n c o e f f i c i e n t f o r m e t a l s s u c h as aluminium o r i r o n s l i d i n g on g l a s s i s a b o u t t h e same as t h e f r i c t i o n c o e f f i c i e n t o f g l a s s s l i d i n g on g l a s s . When s l i d i n g on metal i n a vacuum, g l a s s t r a n s f e r s o n t o t h e
metal s u r f a c e and t h e m e t a l s u r f a c e becomes c h a r g e d w i t h g l a s s ; a s a r e s u l t , t h e g l a s s i s e f f e c t i v e l y s l i d i n g o n g l a s s . The adh e s i v e wear p r o c e s s i s dominant when g l a s s i s r u b b i n g a g a i n s t glass. B o r o s i l i c a t e g l a s s 7 7 4 0 P y r e x of d e n s i t y 2 . 2 3 mg/mm3 , reinforced w i t h g r a p h i t e f i b r e s (modulus from 230 t o 6 9 0 G P a , s t r e n g t h 2.2-3.1 GPa, d i a m e t e r 6.5-11 ,um, volume c o n t e n t s i n t h e m a t r i x m a t e r i a l 3 1 . 5 - 7 1 . 8 % ) , d e m o n s t r a t e s good t r i b o l o g i c a l p r o p e r t i e s when s l i d i n g on p e a r l i t i c g r a y c a s t i r o n ( r e f . 3 2 ) . A g l a s s p i n ( @ 4 . 7 6 mm) was p r e s s e d a g a i n s t a d i s k u n d e r a l o a d o f 89 N and a t s l i d i n g speeds of 0 . 2 4 ,
0 . 6 0 and 0.96 m / s .
I t w a s found t h a t u n i d i r e c t i o n -
142
a1 fibre composites have lower wear rates and a lower coefficient of friction than chopped random fibre composites. High modulus fibre composites demonstrate a lower wear rate and coefficient of friction than high strength fibre composites. The friction coefficient and wear for both composite increase with increase in the sliding speed. At low sliding speeds there is only mild wear; a thin, smooth film appears on the composite surface and there is minimal plastic deformation of the counterface. At high speeds, wear is severe: extensive pitting of the thick surface film on the composite, plastic deformation and pitting of the counterface can be observed. The friction coefficients were from 0.09 ( 6 0 % high modulus fibres) to 0 . 3 9 ( 3 1 . 5 % discontinuous high strength fibres). The lowest wear (at friction coefficient 0.17) was observed when ultrahigh modulus fibres ( 6 7 % ) were used, and the highest when the glass was reinforced with high strength discontinuous fibres. Tests have been carried out to compare the wear resistance of brass, various steels, Fe-Cr-A1 alloy and titanium when sliding on glass fibre; they show that the wear resistance of porcelain, nitrided steel ( 0 . 4 5 % C ) , borated mild steels, nitrided Fe-Cr-A1 alloy and carburized titanium is respectively 5 5 , 7 8 , 68, 7 8 and 2 9 0 times more than the wear resistance of brass (ref. 2 6 8 ) . The industrial use of borated mild steel elements to guide glass fibres in textile machines supports the results of these laboratory studies. Glass-polymer systems can exhibit interesting tribological properties (refs. 1 9 0 , 194, 2 6 9 ) . The friction and wear in polymer (pin, 9 3 nun)-glass (disk, @ 60 nun) systems was found to depend on the sliding speed (ref. 2 6 9 ) . Polymer wear depends to some extent on the time taken to reach the critical speed and is around lpm/km or less in the lower speed range (for applied load 1 0 N, and sliding speeds 0.1-3 m/s). At the critical speed, the wear of the polymer rapidly increases. The values of the critical speed for LDPE, HDPE, PP, PA 6 and POM h are 0.5, 1.5, 1.1, 1.2 and 1 . 6 m/s respectively. The irrespective friction coefficients are 0.9, 0.4-0.8 (lower at lower sliding speed), 1 . 4 - 0 . 8 (higher at lower sliding speed, rapidly increasing at the critical speed) 2-1.3 (high at low speeds), and 0.5-0.6. The wear of the polymers on glass in the lower speed range is mainly due to the small local transfer of molten polymer to the frictional track; the wear in the higher speed range is due to the outflow of molten polymer through the rear edge of the polymer pin. LDPE wears by shearing
143
(at a distance from the frictional surface) and melting, and a substantial film is laid on the frictional track in the area of contact. The addition of carbon or glass fibre to POM c ( 2 0 % and 2 5 % by weight respectively) reduces the friction coefficient by 3 and 1 . 5 times respectively (for unfilled POM c it is 0.6-0.5) and clearly reduces the wear when the sliding speed (in the aforementioned pip -on-disk system, load 50 N) is higher than 0.6 m/s (ref. 1 9 0 ) . At lower speeds, the wear of glass-fibre-reinforced POM c is significantly higher than that of unfilled POM c when rubbing against qlass. The friction coefficient for PTFE similarly reinforced is 0.25 (practically the same for glass or carbon fibre reinforcement) but the wear of carbon-fibre-reinforced PIE5 is higher than glass-fibre-reinforced PTFE, especially at lower speeds. The wear of fibre-filled POM c is much greater than that of fibre-filled PTFE. Fibre-filled PQM c exhibits in general a much higher friction in sliding on steel than on glass. The high wear of glass-fibre-reinforced c is probably due to the marked thermal decomposition of polyacetal around the glass fibres on the frictional surface. The wear of PTFE (pin) rubbing against glass (disk)is connected with the transfer and return of the PTFE particles to and from the glass surface (ref. 1 9 4 ) . Graphite and carbon-graphite materials have good tribological qualities, their frictional properties being particularly good (refs. 3 4 - 3 7 , 1 5 4 , 162, 2 7 0 - 2 7 6 ) . The counterface for such materials are usually stainless steel, tool steels, hard chromeplated steels, hard coated aluminium, sintered metals, bronze, cast iron, ceramics, or the same graphite or carbon-graphite material. The surface roughness should be small. The friction coefficient against metals is about 0 . 2 The friction in air is controlled mainly by the presence of water molecules adsorbed on the frictional surface. During sliding, the carbon (graphite) material is transferred to the counterface. Radial journal bearings with a steel journal and a bearing bush made from carbon (graphite) materials demon’strate very good friction and wear behaviour (ref. 2 5 ) . The wear of such bearings (those with bushes made of electrographitized carbon) depends mainly on the load applied; for bearings which have bushes with a bearing hole diameter of 2 0 nun, external diameter 30 nun and length 2 0 nun, the maximum pv (p - contact pressure, v - sliding speed) is 0 . 3 MPa m/s at a sliding speed of 1 m / s , and 5 times higher at a sliding speed of 0.1 m/s (ref. 3 5 ) . When electrographitized carbon is impregnated with thermosetting polymer, the bearings can be used
.
144
at sliding speeds below 1 m/s and contact pressures below 0.1 MPa when ambient temperature is below 14OoC. Miniature bearing bushes manufactured from such materials and which have a bearing hole diameter of 0.5 mm, external diameter 3 mm and length 1 mm, operating at a sliding speed of 0.01 m/s and at very low contact pressure, have been successfully appled in flow-meters. Purebon hard carbon is used on guide bushing (bearing hole diameter 0.523 5 0.007, external bushing diameter 3.15 mm, bushing length 1.02 mm) for the rotor shaft in watt-hour meters designed to operate in the temperature range - 5 0 to 55Oc, at very low contact pressure, at a sliding speed below 0.01 m/s, and with a required life of 30 years at a constant friction coefficient (in a filtered air environment) (refs. 37, 273). The wear of hard carbon, hard carbon with resin impregnant and graphite with anti-oxidant impregnant varies with the ambient temperature. For example, the wear of hard carbon rubbing against hard chrome plate on steel increases rapidly with increase in temperature, while the wear of hard carbon with resin impregnant actually decreases in the range of 20-300°C. The wear rate of graphite with anti-oxidant impregnant decreases and reaches its minimum at about 12OoC and then rapidly increases as a function of temperature (ref. 37). The friction coefficients for bearing steel sliding on graphite materials or hard carbons are similar to the friction coefficients of such carbon-graphite materials rubbing against themselves (i.e. 0.2-0.3)(ref. 361, although the chrome stainless steel in steel-carbon-graphite systems has a slightly lower friction coefficient. Hard carbon material (super-hard coal sinter, microhardness 'up to 20 MPa) sliding against corundum (surface roughness Ra = 0 . 0 4 ,urn) has the lowest friction coefficient (ca. 0.16) and wear of any of these systems. In this sinter, the interphase material particles with low microhardness (ca. 1.2 MPa) act as a solid lubricant. Such materials can be used in gas bearings operating in air. Graphitized carbon materials impregnated with anti-friction metals (Ag, Cu, Cd, bronze, babbits, 95% Pb - 5% Sn or 70% Cu 30 % Pb alloys) have good tribological properties (refs. 154,272). Carbon-ceramic materials can be used in journal bearings or seals when the sliding speed is below 150 m/s, contact pressure 0.3-0.5 MPa, and temperature 600-650°C. Carbon-polymeric materials can be used in compressors or pumps at sliding speeds up to 15 m/s, contact pressure up to 20 MPa and at temperatures between-200 and 15OoC
145
(ref. 274). The carbon material manufactured by Nippon Carbon, containing 13-50% A1 or its alloy, 0.5-10% A1203 (concentrated mainly on the surface), demonstrates good frictional properties and extremely good wear behaviour (ref. 275). The Supragraf lamellae graphite materials manufactured by James Walker and Co. Ltd. have high chemical inertness, high thermal resistance and low density ( 1 . 4 mg/mm3), and can operate at temperatures of between -200 and 5OO0C in an oxidative atmosphere or at up to 250OoC in a reduced or inert atmosphere whilst having a friction coefficient of 0.05 (ref. 276). Tribological systems with an element or elements manufactured from graphite or carbon-graphite materials demonstrate better tribological properties in a vacuum than in air. The carbon transfer films are the key to effective operation (ref. 162). The lowest friction coefficients for vacuum-operated, 100% electrographitized carbon were obtained at sliding against electrolytic iron, copper or silver (0.15, 0.15, 0 . 2 0 ) and relatively low friction (0.2) and wear at sliding against aluminium oxide and stainless steel (friction coefficient ca. 0 . 4 ) (ref. 162). Pyrolitic graphite prepared by high temperature decomposition of hydrocarbons demonstrates the effect of orientation; during sliding on gold in a vacuum the friction coefficient was higher (up to 0.6) at prismatic orientation than at basal orientation (up to 0.4)(ref. 162). No gold was observed during sliding in basal orientation while during sliding in prismatic orientation small spheres of transferred gold could be seen. The prismatic orientation of graphite is from 500 to 1000 times more chemically active than the basal orientation. A single crystal diamond, a form of carbon, in sliding contact with metals demonstrates a decreasing friction coefficient with an increase in d-bond character of the transition metals (ref. 162). Titanium and zirconium, which are chemically very active, exhibit very strong interfercial bonding to diamond and a friction coefficient of about 0.7 (in a vacuum) while, by contrast, rhodium and rhenium have relatively low coefficients of friction ( 0 . 4 ) . The fact that oxygen increases the friction coefficient is related to the relative chemical thermodynamic properties and bonding of carbon to oxygen. All the aforementioned metals transfer to the surface of diamond during sliding. The advantage of using diamond is that no stick-slip effects occur in metal-diamond sliding systems (ref. 17). Some small mechanisms have tribological systems consisting of
146 elements made of materials such as paper (computer peripherals) or materials in a special form (perforated material, magnetic tapes). In the case of magnetic tapes they consist of finely dispersed r-Fe2O3, Cr02 or BaO.GFe203 particles bonded by used of polymeric binders onto a flexible substrate such as PETP. The magnetic tape is moved against a stationary (audio or computer) or rotating (video) read-write magnetic head which is generally made of Ni-Zn or Mn-Zn ferrite or Permalloy. The direct physical contact between the tape and the head takes place during the start-stop operations of the tape drive. Generally the friction in such systems is high at high humidity (above 60% relative humidity)(refs. 277, 278). Frictional damage to the tape increases the friction between the tape and the head. The damage to the tape is mainly due to a plastic flow of the binder in the magnetic layer (ref. 278). Ferrite heads causes less frictional damage to the tape than Permalloy heads and they have a much lower friction coefficient than Permalloy heads (0.20 . 3 at 35% relative humidity compared to 0.8-1.0 for Permalloy heads). The specific wear rate of Permalloy and hard Permalloy (containing niobium) heads is of the order of mm3/N.m and of Mn-Zn ferrite heads loq7 mm3/N.m (ref. 278). The wear of magnetic materials increases with relative humidity, the increase beinq more noticeable for ferrite heads than for Permalloy heads. The wear of the materials proceeds mainly by the abrasive action of hard magnetic powders contained in the magnetic layer of the tape, but the variation in the mechanical properties of the binder in the magnetic layer with relative humidity and possible tribochemical reactions in such conditions also influence the wear (refs. 277-279). Wear by paper occurs because of the abrasive action of small, hard particles contained in the paper (usually Si02, different minerals , Ti02) (refs. 280, 281). The abrasive wear by paper is proportional to the load and sliding distance and for Knoop hardness of the abraded material less than 7500 MPa, the wear is inversely proportional to hardness. Generally there are three categories of wear dependence on hardness (this was recognized by testing with paper a variety of materials, ranging from elastomers to diamond (ref. 281)) : the aforementioned inverse linear dependence, a transitional category of abrasive hardness and a third category where the hardness H, of the abraded materials is greater than the hardness of the abrasive. For all three categories the dependence of wear on hardness can be described in the form Hin; in the category
147 i n which t h e a b r a s i v e s a r e h a r d e r t h a n t h e a b r a d e d m a t e r i a l , n m l ; i n t h e c a t e g o r y i n which t h e h a r d n e s s e s are c o m p a r a b l e , n = l O ;
and
i n t h e c a t e g o r y i n which t h e a b r a s i v e s are s o f t e r , n x 5 . The app r o x i m a t e h a r d n e s s o f a b r a s i v e s i n p a p e r i s a b o u t 1 0 0 0 0 MPa. The m a t e r i a l s which are most r e s i s t a n t t o w e a r by p a p e r a r e diamond, s a p p h i r e , T i c and WC. I n mechanisms i n which m e t a l l i c e l e m e n t s are i n c o n t a c t w i t h
r a w c o t t o n , h i g h wear and s p a r k i n g c a n o c c u r b e c a u s e o f a t r i b o e l e c t r i c a l c h a r g e ( r e f s . 282, 2 8 3 ) . The a p p l i c a t i o n o f p o l y m e r s or p o l y m e r i c c o a t i n g s on t h e m e t a l l i c e l e m e n t s i s a d v a n t a g e o u s i n s u c h mechanisms. When r a w c o t t o n was s l i d i n g o n LDPE, PVB, PCA, PNP, epoxy (ED 1 6 ) and f u r a n - e p o x y o l i g o m e r s , a t s l i d i n g s p e e d s
0-8 m / s a n d c o n t a c t p r e s s u r e s 0.001-0.05
MPa, t h e h i g h e s t f r i c t i o n
c o e f f i c i e n t s were found f o r PVB a n d LDPE ( c a . 0 . 4 ) f o r PNP a n d ED 1 6 ( c a . 0 . 2 5 ) .
and t h e l o w e s t
The v a l u e s o f t h e t r i b o e l e c t r i c a l
c h a r g e d e n s i t y were h i g h e s t f o r LDPE and ED 1 6 a n d l o w e s t f o r PVB and PNP. The t r i b o e l e c t r i c a l f r i c t i o n component i s a s much as 50-60% of t h e t o t a l f r i c t i o n f o r c e i n s u c h s y s t e m s . The f r i c t i o n between a copper-MoS2 compound(32% by volume, s i n t e r e d a t 650°C i n a vacuum) and a s t e e l or s a p p h i r e s p h e r e (@ 1
mm) i n a vacuum 2 l o q 7 P a and a t s l i d i n g s p e e d 4 ,um/s and N i s accompanied by t h e t r a n s f e r o f t h e MoS2 f i l m
l o a d 0.2-0.5
( 1 . 2 n m ) ( r e f . 2 8 4 ) . The f r i c t i o n c o e f f i c i e n t i s 0 . 0 0 8 .
The p r o c e s s o f r o u g h n e s s f o r m a t i o n and w e a r o f human t e e t h under t h e f r i c t i o n of a t o o t h b r u s h
used w i t h a d e n t i f r i c e o c c u r s
according t o the s t r a i g h t l i n e l a w a s a function of t h e s l i d i n g
t i m e ( r e f . 2 8 5 ) . The t i m e n e e d e d f o r r e a c h i n g t h e u l t i m a t e s t e a d y s t a t e s u r f a c e r o u g h n e s s , ( R a = 0.3-0.5 ,urn), i s a b o u t 4 . 5 h . A f t e r t h e s u r f a c e r o u g h n e s s h a s s t a b i l i z e d , f u r t h e r wear o c c u r s w i t h o u t any c h a n g e i n t h e s u r f a c e r o u g h n e s s . The e f f e c t o f a c i d i t y i n t h e mouth i s q r e a t e r t h a n t h e s i m p l e a b r a s i v e e f f e c t o f t h e t o o t h b r u s h and d e n t i f r i c e . Summarizing t h e above c o n s i d e r a t i o n s , it c a n b e s t a t e d t h a t i n t h e case o f ceramics t h e f r i c t i o n a n d wear a r e a n i s o t r o p i c ; adhes i v e wear i s accompanied by a b r a s i o n , s u r f a c e c h e m i s t r y i s v e r y i m p o r t a n t t o f r i c t i o n and w e a r b e h a v i o u r , a n d c o n t a m i n a n t s o n t h e i r s u r f a c e a f f e c t t h e i r t r i b o l o g i c a l p r o p e r t i e s . These remarks a r e a l s o t r u e o f c a r b o n - g r a p h i t e m a t e r i a l s . A t h i g h t e m p e r a t u r e s graphi t i z a t i o n of t h e ceramic s u r f a c e may o c c u r and a r e d u c t i o n i n w e a r and f r i c t i o n c a n b e e x p e c t e d . Humidity h a s a r e m a r k a b l e e f f e c t on t h e b e h a v i o u r o f t r i b o l o -
148 gical systems such as the magnetic tape-head systems used in audio, computer or video applications. The wear of materials by paper depends on the hardness of the material used.
149
5 , LUBRICATED SYSTEMS 5,1, METALLIC SYSTEMS 5.1.1.
S O L I D METALS
Typical examples of boundary or mixed lubricated minuature systems are clock-type bearings with a steel journal (usually made of free cutting or stainless steel and roller-burnished to %<0.16 )m) and brass support (made directly in the plate which constitutes the frame of the mechanism) (Fig. 5.1).
F i g . 5 . 1 . C l o c k - t y p e b e a r i n g w i t h c o n i c a l (a) and s p h e r i c a l (b) recess p r o v i d e d t o r e t a i n l u b r i c a n t
Investigations (ref. 2 8 6 ) carried out on bearings like this with a nominal diameter of 3 mm and diametral clearances of 6 0 , 30 or 1 2 ,um show that their friction coefficient using classic clock oils (XU 1 2 0 or XU 4 3 0 , see Chapter 3.2) is 0.25 at low sliding speeds (below 0.003 m/s) and hicgh contact pressures (up to 5 MPa) , and
150
0.02 at high sliding speeds (over 0.067 m/s) and low contact pressures ( 0 . 0 5 MPa)(hydrodynamic lubrication, see Chapter 9.2). Increasing the ratio of the length 1 of the bearing to the nominal diameter d improves the operating conditions; the ratio (l/d)=1 appears to be optimal in the lubricated bearings mentioned above. The decrease in diametral clearance results in a decrease in the friction coefficient. The optimal relative clearance, taking into consideration the possibility of contact between the adges of the journal and support, is about 1%. The viscosity of the oil used has relatively little effect on the friction coefficient of clock-type bearings (ref. 286). Lubrication reduces the friction coefficient to a third of that of unlubricated bearings. The presence of a recess provided to retain lubricant (see Fig. 5.1) has little effect on the friction coefficient. The bearings operating at dynamical loadings (rapidly varied sliding speed or contact pressure) demonstrate lesser properties. The investigated (ref. 287) bearing with nominal diameter 2 mm, relative clearance ca. 1% lubricated using XU 430 oil when operates at a dynamic loading can be treated as the inertial system where the friction process is characteristic with a delay. Lubricated bearings have a time constant (the same at a defined sliding speed and varying contact pressure or at a defined contact pressure and varying sliding speed) 8-16 times smaller than the same bearings when unlubricated. The friction coefficient of a dynamically loaded bearing can be 2.5 times more than that of a statically loaded bearing (this is for bearings operating at a sliding speed below 0.001 m/s, and at a contact pressure of 0.5 MPa, and when the sliding speed varies with the acceleration from 1.7 to 100 mm/s2). Investigations (refs. 67, 68, 288, 289) in which steel-brass journal bearings (journal diameter 2 mm, bearing hole diameter 2.1 mm, and length 3 mm) were lubricated with various silicone oils showed that the use of anti-spread epilame coatings and carefully selected oil can reduce the friction coefficient to less than 0.1 at very low sliding speeds (lubrication with polysiloxane containing 1% addition of polar oxidized silicone or ( C F 2 ) 7 C F 3 ) (see Fig. 5.2, based on the data given in the aforementioned references). The friction coefficient at very low sliding speed when lubrication is by methylalkylpolysiloxanes can reach 0.1. However, with lubrication using dimethylpolysiloxanes, methylphenylpolysiloxanes or fluorinated polysiloxanes the friction coefficient is 0.2-0.3 (refs. 67, 66, 288, 289, 290-292). The fact that the max-
151
imum value of the friction coefficient on the friction coefficient
- sliding speed characteristics were observed in the case of lubrication with polysiloxanes containing 1% polar (oxidized) silicone o i l is probably caused by the higher speed at which the polar silicone film is adsorbed rather than by the speed of its fomation -
0.16
0.14 0*12
0.10 + c
.9 0.08 U *d
)c
e
0)
0.06
c
0
'3' 0,04 U
*J
t
0.02
-
1
0.01
0
I
0.02
0.03
0.04
&
b
0.05
sliding speed ,mls
Fig. 5.2. Friction coefficient v s . sliding speed for steel-brass miniature journal bearing .lubricated wi,th silicone oils. Journal diameter 2 mm, bearing hole diameter 2.1 mm, bearing length 3 mm, contact pressure 0.08 MPa. 1 - m e t h y l p h e n y l p o l y s i l o x a n e (viscosity 50 m d / s at 20OC)+1% polar (oxidized) dimethylpolysi loxane, 2 - dimethylpolysi loxane (100 rnm2/s) + 1 % polar dimethylpolysi loxane, 4 - dimethylpolysi loxane (500 rnm2/s)+0.5% (CF2)7CF3, 5 dimethylpolysiloxane (500 mm2/s), 3 - methylalkylpolysiloxane ( 5 0 0 m m 2 / s ) .
-
152 The wear of t h e b r a s s b e a r i n g e l e m e n t i s r e l a t i v e l y s m a l l . B e a r i n g s w i t h a j o u r n a l made o f s t a i n l e s s s t e e l q r o u n d t o Ra = 0 . 1 - 0 . 2
p m , d i a m e t e r 5 nun, b e a r i n g l e n q t h 2.8 mm, r e l a t i v e
c l e a r a n c e a b o u t 1%, and l u b r i c a t e d w i t h m i n e r a l i n s t r u m e n t o i l , d e m o n s t r a t e s p e c i f i c wear r a t e s of a b o u t 0 . 1 6 ,
0 . 1 9 and 0.50 ,um/km
MPa a t c o n t a c t p r e s s u r e s o f 0 . 0 7 5 MPa ( s l i d i n g s p e e d 0 . 2 6 m / s ) , 0.75 MPa ( s l i d i n g s p e e d 0.0026 m/s)
0.026 m / s )
and 7 . 5 MPa ( s l i d i n g s p e e d
r e s p e c t i v e l y . The e f f e c t o f h i g h c o n t a c t p r e s s u r e on
t h e wear r a t e c a n b e c l e a r l y s e e n . The u s e of b r o n z e s , i n c l u d i n g t i t a n i u m a n d b e r y l l i u m b r o n z e s , i n p l a c e of t h e u s u a l b r a s s can l e a d t o a d e c r e a s e i n t h e f r i c t i o n c o e f f i c i e n t a n d wear i n b e a r i n g e l e m e n t s ( r e f s . 293-295). plies in particular t o radial-axial
T h i s ap-
b e a r i n g s y s t e m s which have a
s t e e l h a r d j o u r n a l s u p p o r t e d o n two common c y l i n d r i c a l b e a r i n g s ( r a d i a l b e a r i n g s ) w i t h a s p h e r e rubbing a g a i n s t t h e p l a t e on t h e end o f t h e j o u r n a l t o s u p p o r t a x i a l l o a d s . I n s u c h b e a r i n g s t h e f r i c t i o n c o e f f i c i e n t decreases with i n c r e a s e i n t h e s p h e r e d i a m e t e r . The f r i c t i o n c o e f f i c i e n t d e c r e a s e s h y p e r b o l i c a l l y w i t h i n c r e a s e i n t h e a c t u a l c o n t a c t p r e s s u r e between t h e s p h e r e a n d t h e p l a t e ( r e f . 2 9 3 ) . A t c o n t a c t p r e s s u r e s of b e t w e e n 1000 a n d 1500 MPa, t h e f r i c t i o n c o e f f i c i e n t f o r a t o o l s t e e l - t o o l steel system ( l u b r i c a t e d w i t h c l a s s i c a l c l o c k o i l , s p h e r e d i a m e t e r 5.5-9.5
mm) i s between
0.5 and 0.25 b u t f o r c o n t a c t p r e s s u r e s o f 1500-3000 MPa it is between 0.25 and 0.15.
A t c o n t a c t p r e s s u r e s o v e r 2500 MPa t h e f r i c -
t i o n c o e f f i c i e n t decreases very l i t t l e . The t r i b o l o g i c a l b e h a v i o u r of m i n i a t u r e s p h e r e - p l a t e s y s t e m s t e s t e d u s i n g ASTM 2 - b a l l pendulum s y s t e m
(see C h a p t e r 8 . 2 ) i s
s i m i l a r t o t h a t of m i n i a t u r e j o u r n a l b e a r i n g s . The f r i c t i o n c o e f f i c i e n t s o f b a l l s made o f b e a r i n g s t e e l ( R a < 0 . 0 3 p) r u b b i n g a g a i n s t b r a s s o r s t e e l p l a t e s (Ra = 0.06-0.09 t i v e l y ) and l u b r i c a t e d
w i t h Synta-A-Lube
o r 0.03-0.04
respec-
(Moebius 9015) s y n t h e t i c
o i l w i t h a v i s c o s i t y o f 1 1 7 m 2 / s are a b o u t 0 . 2 and 0.08 r e s p e c t i v e l y , t h a t i s a b o u t 3-5 t i m e s smaller t h a n t h e f r i c t i o n c o e f f i c i e n t o f a n u n l u b r i c a t e d s l i d i n g system ( r e f . 4 3 ) . A comparison o f t h e f r i c t i o n c o e f f i c i e n t s o f s t e e l - b r a s s and s t e e l - s t e e l systems l u b r i c a t e d w i t h v a r i o u s o i l s a n d t e s t e d u s i n g a 2 - b a l l pendulum i s p r e s e n t e d i n F i g . 5 . 3 , b a s e d upon d a t a g i v e n i n r e f . 106 ( s e e a l s o Table 5.1)
. Lubrication
of steel-steel systems w i t h c h l o r i n a t e d
phenylmethylpolysiloxane r e s u l t s i n a m a r k e d l y h i g h f r i c t i o n c o e f f i c i e n t : t h e l u b r i c a t i o n of s t e e l - b r a s s o r s t e e l - s t e e l s y s t e m s u s i n g p o l y g l y c o l c o n t a i n i n g e t h e r and alcohol groups is also not advised.
TABLE
5.1
O I L S USE0 FOR LUBRICATION O F TRIBOLOGICAL SYSTEMS TESTED I N r e f . .
__ MINERAL OIL
PROPERT I ES
D e n s i t y , mg/mm 3
2 V i s c o s i t y , mm / s ( a t 20 and 5OoC) R e f r a c t i v e index Pour p o i n t , Flame p o i n t ,
OC
OC
Appl i c a t i o n temperature range, OC
SYNTHETIC O I L
CLOCK O I L
SILICONE OIL
I
1 Base c o m p o s i t i o n
106
1
2
M i nera 1 h i gh 1 y refined with additives
0.89 310 61 1.481 -18 245
0.90 598 99 1.482 -15 270
3
4
Mineral Polyw i t h 6G g l y c o l con t a i rr neat's -foot i ng oi1 ether and a 1coho 1 groups
0.91 138 37 lk74 -24 273 -10
80
1-05 117 26 1.51 1
-45 23 0 -3 0 80
5
6
F I UOr in a t e d PO'Yether
41 i phatic d ie s t e r
1.91 620
0.85 60 23 1.448
110
1.30 2 -30 I n f 1 ammable
-65 .230
-20
-50
180
90
7
8
TriChlor in a t e d f 1 u o r o p h e n y l - P'OPY 1 methylmethylPolYPolYs i loxant s i 1 oxane
-
1.08 121
48 1.436 -66 280 -50 130
1.23 375 110
1.382 -48
154
4
I
I , 0 9I I
I
1 ' I
I
I I I
I I
0.450
0.4 0
?
I
I I I
I I
I I I
I I
I
I
I
I I
I
I
I I
I
I
I- J2
3
I
?
I 0 I
I
I I I
I
I
P I
I
II
I
I I
I
I
+
I
I
1
I I I I I I
I _L i 4 Fresh oils
I
I
I I
I
0I
I I
I I 1
I
I
I I
I I
I
T
t
I
I I
+
I
I
I I I
9
I
I
I I I
I
I I
I I
I
I
Aged oils
I
I I
*
6
7
u
c a
.d
. U A
L L4-
a
0 0
t 0
.4
4
cl
.A
t
F i g . 5.3. F r i c t i o n c o e f f i c i e n t of s t e e l - b r a s s and s t e e l - s t e e l systems ( d o t t e d l i n e s ) l u b r i c a t e d w i t h o i l s l i s t e d i n T a b l e 5.1 and t e s t e d using a 2 - b a l l pendulum (see Chapter 8 . 2 ) . B a l l s made o f b e a r i n g s t e e l , p l a t e s o f b r a s s (CuZnbOPb) o r s t a i n l e s s s t e e l (X13CrNi18.8 a c c o r d i n g t o D I N ) . Pendulum mass 3 0 0 g. F i g . 5.3 a) g i v e s f r i c t i o n c o e f f i c i e n t s a t l u b r i c a t i o n w i t h f r e s h and aged o i l s , and i n F i g . 5.3 b) t h e e f f e c t s o f pendulum mass and f l u o r i n a t e d s i l i c o n e o i l v i s c o s i t y ( a t 20°C) on t h e f r i c t i o n c o e f f i c i e n t a r e shown. O i l s were aged u s i n g a b r a s s (CuZn37) vessel 50 mm i n d i a m e t e r , h e i g h t 25 mm and c o n t a i n i n g a 6-10 mm o i l l a y e r . The ageing time was 8 weeks a t 120°C ( o i l s 1,2 and 6, see T a b l e 5.1) and 180°C ( o i l s 5,7 and 8 ) .
155
Lubrication with fluorinated oil, on the other hand, results in relatively low friction coefficients. It can be advantageous to use aged oil, other than diester and chlorinated polysiloxaneoils. Decrease in contact pressure results in a decrease in the friction coefficient while decrease in fluorinated polysiloxane viscosity also results in a decrease in the friction coefficient. Relatively high friction coefficients in similar steel-brass and steel-steel systems lubricated with polyglycol oil containing ether and alcohol groups (Synta-A-Lube) were observed by Diirr (ref. 2 9 6 ) The surface roughness of the plate in a bearing steel-bearing steel system tested usinq an ASTM pendulum was found to affect the friction coefficient (ref. 2 9 7 ) . The surface roughness, Ra, of the ball surface was 0.03 ,urn or less and the plate roughness varied between 0 . 0 2 ,um and 1 . 3 8 ,um. Neat's-foot and polyglycol oil containing ether and alcohol groups were used for lubrication. The friction coefficient when neat's-foot oil was used increased from 0 . 0 9 to 0 . 1 2 with increase in surface roughness from 0 . 0 2 to about 0 . 3 ,um and stabilized at this level. A similar effect was found when the polyglycol oil was used for lubrication although only greater variation of the friction coefficient around the average value, 0 . 1 2 , was observed. Lubricated system tested using special four-ball or five-ball friction machines showed similar tribological properties to the aforementioned systems (refs. 2 9 6 - 3 0 0 ) . The wear of steel-steel systems depends markedly on the lubricant used. Four-ball tests (refs. 298, 2 9 9 ) showed that the use of classical clock oils from the XU series (see Chapter 3 . 2 ) results in friction increasing with viscosity while the wear simultaneously decreases. The silicone oils used for lubrication have little effect on the rate of wear. The mineral oils also have little effect. Greases based on mineral oils and containing aluminium or lithium stearate provide better tribological properties in steel-steel systems than mineral oils. Polyglycol oil containing ether and alcohol groups demonstrates good friction and wear reducing properties. Tests have shown that the friction is roughly the same when neat's-foot oil, trifluorpropylmethylpolysiloxane, classic clock oil or polyglycol with ether and alcohol groups are used, while the friction force is 3-4 times higher when phenylmethylpolysiloxane or dimethylpolysiloxane are used (ref. 2 9 6 , 2 9 7 , 300). At normal loads, the wear rate in systems lubricated with neat's-foot or polyglycol oil containing ether and alcohol groups is similar,
.
156
although at high contact pressures the wear is less when neat's-foot oil is used. The use of phenylmethylpolysiloxane instead of dimethylpolysiloxane results in a marked decrease in friction and wear. Increasing the viscosity of the phenylmethylpolysiloxane from 20 m2/s (at 25OC) to 2 0 0 m2/s reduces the friction coefficient from 0 . 1 5 to 0 . 1 2 but when the viscosity of oil is increased to 1 0 0 0 m2/s the friction coefficient remains unchanged. At higher viscosities, higher contact pressures can be applied. Increase the viscosity of the phenylmethylpolysiloxane also reduces wear. This effect is more noticeable at high contact pressures. The effect of the oil viscosity on the tribological properties of miniature systems is not clearly defined. It can be generally stated that the wear is lower at higher oil viscosities, decreasing in proportional to the increase in viscosity (refs. 2 9 9 , 3 0 0 ) . The friction coefficient may increase or decrease, although the variations in it are relatively small, at viscosities up to 1 0 0 0 mm2/s (refs. 6 7 , 6 8 , 1 0 6 , 2 9 9 , 3 0 0 , 3 0 1 ) . The steel precision elements used in diesel engine injection pumps operate in low-molecular hydrocarbon media. The greatest danger is if the precision surfaces of the plungers become to worn and seize up. The friction coefficient of such systems can be decreased from 0 . 2 5 (stable within the range of 3-9 MPa contactpressure)to 0 . 1 8 (stable within the range 1 - 1 9 MPa) by saturating the surfaces of the rubbing elements with nitride and oxygen (ref.302). The wear is also reduced by this action. The submicrorelief (roughness) of the surface is also important for a better wetting of the surfaces by a fuel and for improving the wear resistance. The lubrication of metallic systems markedly reduces friction and wear as compared to the friction and wear of unlubricated systems. One of the important roles of a good lubricant is to reduce metal-to-metal contact and therefore reduce the adhesive transfer. Fluid lubricants can reduce wear by dispersing and separatingsmall transfer particles before they can accumulate to form a transfer layer (ref. 1 4 6 ) . The wear coefficients (see eqn. ( 4 . 5 ) ) for the lubricated systems discussed are around 1 0 - 4 - 1 0 - 6 while wear particle sizes are in the range 2 0 - 2 ,urn (ref. 1 6 1 ) . The presence of a lubricant cannot prevent the sliding system from scuffing since at running-in or when the real contact pressures are very high, plastic fatigue (depending on asperity deformation and oxide formation influencing the strees states imposed on asperities) can initiate scu'ffing which is assisted by adhesion in the later stages of dam-
157 a g e ( r e f . 3 0 3 ) . The s e l e c t i v e t r a n s f e r e f f e c t o b s e r v e d ( e . g .
in
s t e e l - b r o n z e s y s t e m s l u b r i c a t e d w i t h g l y c e r o l when a 1 - 2 p n t h i c k c o p p e r l a y e r i s formed o n t h e f r i c t i o n s u r f a c e s d u r i n g r u b b i n g ) c a n r e d u c e s i g n i f i c a n t l y t h e wear and s c u f f i n g i n h i g h l y l o a d e d systems ( r e f . 304). I n v e s t i g a t i o n s c a r r i e d o u t on s t e e l - b r o n z e and s t e e l - s t e e l s y s t e m s l u b r i c a t e d w i t h m i n e r a l p o l y s i l o x a n e o i l (phenylplysiloxane, p u r e and c o n t a i n i n g 1%o l e i c a c i d ) and g l y c e r o l showed t h a t t h e p r e s e n c e o f oxygen h a s a n i m p o r t a n t e f f e c t on t h e f r i c t i o n and
wear o f s u c h s y s t e m s ( r e f . 3 0 5 ) . D e c r e a s i n g t h e oxygen c o n c e n t r a t i o n i n t h e f r i c t i o n r e g i o n , by r u b b i n g i n a vacuum o r by i n c r e a s i n g t h e c o n t a c t s u r f a c e o f t h e e l e m e n t s , r e s u l t e d i n a d e c r e a s e of t h e f r i c t i o n c o e f f i c i e n t and
wear r a t e . The l o w e s t wear w a s o b t a i n e d when g l y c e r o l w a s u s e d f o r l u b r i c a t i o n and t h e h i g h e s t when i n d u s t r i a l m i n e r a l o r p h e n y l p o l y s i l o x a n e o i l s were u s e d . The a d d i t i o n o f o l e i c a c i d r e s u l t s i n a marked r e d u c t i o n i n t h e w e a r r a t e and f r i c t i o n c o e f f i c i e n t ( f r o m t o 0 . 1 4 t o 0 . 0 8 ) a s compared t o t h e wear r a t e and f r i c t i o n c o e f f i c i e n t o b s e r v e d when t h e same s y s t e m i s l u b r i c a t e d w i t h p u r e phenylpolysiloxane. The r o u g h n e s s o f t h e o p e r a t i n g e l e m e n t s i n boundary l u b r i c a t e d s y s t e m s h a s a marked e f f e c t on t h e i r t r i b o l o g i c a l p r o p e r t i e s . The lowest f r i c t i o n c o e f f i c i e n t f o r a s t e e l - b r o n z e system l u b r i c a t e d w i t h m i n e r a l o i l i s when t h e s u r f a c e r o u g h n e s s , R a ,
of t h e steel
s h a f t ( a s h a f t - p a r t i a l b e a r i n g b u s h i n g s y s t e m w a s u s e d ) i s 0.32 p m o r l e s s ( R a o f t h e b r o n z e s a m p l e w a s 4-6
JX )I
and t h e h i g h e s t a t
Ra = 5 ,um ( r e f . 3 0 6 ) . The w e a r ( a n d a l s o t h e f r i c t i o n ) c a n b e re-
duced by a r t i f i c i a l l y c r e a t i n g m a c r o r o u g h n e s s on t h e b e a r i n g bushi n g s u r f a c e , b e c a u s e t h e a b r a s i v e wear d e b r i s c a n t h e n b e c o l l e c t e d i n t h e m i c r o p i t s . The l o w e s t wear i n s t e e l - s t e e l ( p i n - o n - d i s k ) s y s t e m s w a s found when a s o f t , rough s t a t i o n a r y p i n ( 3 0 0 HV
,
R,
= 4 ,urn) was s l i d i n g a g a i n s t a h a r d , smooth moving d i s k ( 6 0 0 HV,
R,
= 1 pm) (ref.
1 3 7 ) . Under boundary l u b r i c a t i o n c o n d i t i o n s , t h e
f r i c t i o n f o r c e i n i t i a l l y d e p e n d s upon t h e d i r e c t i o n a l r o u g h n e s s o f t h e c o n t a c t i n g s u r f a c e s i r r e s p e c t i v e o f w h e t h e r b a s e o i l o r dispers i o n s ( e . g . o f s o l i d l u b r i c a n t s s u c h a s g r a p h i t e , MoS2,PTFE) a r e used f o r l u b r i c a t i o n
( r e f . 3 0 7 ) . The f r i c t , i o n c o e f f i c i e n t i n l u -
b r i c a t e d s t e e l - s t e e l s y s t e m s i s l o w e r when t h e m a c h i n i n g t r a c e s on t h e h a r d e r s u r f a c e are p a r a l l e l t o t h e s l i d i n g d i r e c t i o n , b u t when t h e systems a r e u n l u b r i c a t e d o r b a r e l y l u b r i c a t e d , t h e d i r e c t i o n of t h e machining t r a c e s s h o u l d be p e r p e n d i c u l a r t o t h e s l i d i n g d i -
r e c t i o n ( r e f . 3 0 8 ) . I n i t i a l l y , when t h e s l i d i n g s u r f a c e s a r e mx~th and h a v e l i t t l e l u b r i c a t i o n , t h e t r a c e s c a u s e d b y r u b b i n g are p e r pendicular t o t h e s l i d i n g d i r e c t i o n . This self-organizing e f f e c t h a s b e e n o b s e r v e d f o r b r a s s , polymer and r u b b e r t r i b o l o g i c a l s y s -
tems (ref. 306). The p r o c e s s of m a t e r i a l t r a n s f e r i n l u b r i c a t e d s y s t e m s when d i r e c t c o n t a c t o c c u r s and s m a l l m e t a l p a r t i c l e s are g e n e r a t e d i s
s i m i l a r t o t h e t r a n s f e r i n u n l u b r i c a t e d s y s t e m s ( r e f . 1 4 6 ) . The s t r u c t u r e o f t h e t r a n s f e r l a y e r a n d o f t h e d e b r i s p a r t i c l e s are a l s o s i m i l a r . A l u b r i c a n t c a n r e t a r d s i g n i f i c a n t damage by dispersi n g t h e s m a l l metal p a r t i c l e s which seem t o b e p r e c u r s o r s o f t h e t r a n s f e r l a y e r . The t r a n s f e r i n l u b r i c a t e d c o p p e r - b a s e d a l l o y s s l i d i n g a g a i n s t s t e e l s y s t e m s i s a t y p i c a l example o f t h i s e f f e c t . The t r a n s f e r o f aluminium o n t o t h e c o u n t e r f a c e i n s t e e l - a l u m i n i u m a l l o y s y s t e m s i n c r e a s e s t h e f r i c t i o n c o e f f i c i e n t ( r e f . 3 0 9 ) . Under t r a n s f e r c o n d i t i o n s o f s l i d i n g t h e r e i s no c o r r e l a t i o n between t h e f r i c t i o n c o e f f i c i e n t ( w h i c h i s 0.10-0.20
a t lubrication with a
m i n e r a l o i l ) and t h e h a r d n e s s , u l t i m a t e t e n s i l e o r t e n s i l e y i e l d I
s t r e n g t h s o f t h e aluminium a l l o y . The p r e s e n c e o f a n o i l a d d i t i v e g e n e r a l l y l o w e r s t h e f r i c t i o n c o e f f i c i e n t . Under n o n t r a n s f e r cond i t i o n s t h e f r i c t i o n c o e f f i c i e n t i n c r e a s e s ( t o b e t w e e n 0 . 0 2 and 0.09)
a s h a r d n e s s and u l t i m a t e t e n s i l e s t r e n g t h o f t h e aluminium
a l l o y i n c r e a s e . Under t h e s e c o n d i t i o n s t h e o i l a d d i t i v e i n c r e a s e s t h e f r i c t i o n c o e f f i c i e n t o f t h e s y s t e m . The e f f e c t o n t h e t r a n s f e r and u l t i m a t e l y on t h e t r i b o l o g i c a l p r o p e r t i e s of t h e s y s t e m s i n v e s t i g a t e d a l s o depends o n t h e g e o m e t r y o f t h e r u b b i n g s u r f a c e s ; when t h e r e a l c o n t a c t p r e s s u r e s d e c r e a s e t h e t r a n s f e r i n t e n s i t y a l s o d e c r e a s e s ( r e f s . 146-148,
310).
When t h i n l a y e r s o f m e t a l ( u s u a l l y c o p p e r ) a r e formed on t h e r u b b i n g s u r f a c e s , t h e s e l e c t i v e t r a n s f e r e f f e c t l e a d s t o a cons i d e r a b l e r e d u c t i o n i n t h e wear o f a s l i d i n g s y s t e m ( r e f . 3 0 4 ) . A t y p i c a l s y s t e m i n which s e l e c t i v e t r a n s f e r o c c u r s i s a s t e e l -
-bronze
( o r brass) p a i r lubricated with glycerol. In such systems,
first t h e s u r f a c e l a y e r s o f t h e copper a l l o y a r e s e l e c t i v e l y d i s s o l v e d and t h e n atoms o f t h e a l l o y e l e m e n t s ( s u c h a s Zn, Pb, Fe o r All
e n t e r t h e g l y c e r o l a n d a s a r e s u l t t h e s u r f a c e l a y e r becomes
r i c h i n c o p p e r a t o m s . S i n c e t h e g l y c e r o l p r o t e c t s t h e c o p p e r from o x i d e s f o r m a t i o n , t h e s u r f a c e of t h e c o p p e r l a y e r i s a c t i v e and forms bonds w i t h t h e s t e e l s u r f a c e which i s p r o g r e s s i v e l y c o v e r e d by a 1 - 2 y m t h i c k c o p p e r l a y e r . The p r o c e s s o c c u r s i n a d i s c r e t e way. The s e l e c t i v e t r a n s f e r i s more i n t e n s i v e when t h e moving e l -
159
ement is made of copper alloy rather than steel. When contactpressures are high, the instrument grease CIATIM-201, based on mineral MWP oil (see Table 3.2) thickened by lithium stearate, or standard mineral oils can be used as lubricants instead of glycerol. In steel-steel systems, selective transfer can be brought about by small particles of lubricant (e.g. CIATIM-201) containing dispersed small metal particles (such as copper, bronze, brass, Pb or Ag powder) which dissolve in the lubricant during friction and form thin layers on the rubbing surfaces. The decrease in friction and wear as result of lubrication depends significantly on the type of lubricant used. The dependence of the friction coefficient on the chemical structure is expressed by the mobility of the molecules and molecular interactions (ref. 311). When long molecular chain lubricants are used the influence of the sliding speed on the friction coefficient is small (the viscosity increases with increasing length of the molecules). The anti-wear and anti-friction properties of mineral oil containing additives depend on the viscosity when the temperature in the friction area is lower than the temperature at which the chemical modification of the friction surface with active components of the additives occurs (ref. 312). The effect of the molecular weight distribution on the lubricating behaviour of mineral oils has been studied (ref. 313). The additives in lubricants are important for reducing friction and wear (see Chapter 3). An anti-wear additive such as zinc dialkyldithiophosphate (ZnDTP) is more effective in synthetic than in mineral oils (ref. 314). The optimum mass concentration of ZnDTP in petroleum oil used to lubricate a steel-aluminium bronze system is about 2.5% (ref. 312). ZnDTP owe their good anti-wear properties when used for a high-aluminium alloy under boundary lubrication to a good adsorption effect or the formation of a f r i c tion polymer (refs. 315, 316). The end result of the anti-wear and anti-seizure action of ZnDTP is to change the thermionic workfunction of the metal (ref. 317). The soluble Mo-S complexes are effective lubricant additives (ref. 313). The tribological action is preceded by their decomposition in the bulk oil. In this respect the molybdenum dithiophosphates resemble the zinc dithiophosphates. Nematic liquid crystals added to mineral oils used to lubricate steel-bronze and steel-steel systems demonstrate good qualities as additives (ref. 319). The friction coefficient was reduced from 0.10 to 0.05 (for a steel-bronze system) by the addition of 2% (by weight) of hydrohynonbis (n-heptyloxybenzoate).
160
P o l y e t h e r l u b r i c a n t s (see C h a p t e r 3 ) a l w a y s d e m o n s t r a t e b e t t e r f r i c t i o n behaviour than mineral o i l s ( r e f . 320). Polyethers can be u s e d as f r i c t i o n - r e d u c i n g components i n c o o l i n g l u b r i c a n t s c a p a b l e o f b e i n g d i l u t e d w i t h water ( t h e y c a n b e w a t e r - s o l u b l e o r comp l e t e l y o r p a r t l y h y d r o p h o b i c ) . The s u r f a c t a n t s ( a n i o n i c s , cat i o n i c s , n o n i o n i c s and a m p h o l y t i c s ) u s e d i n 1%a q u e o u s s o l u t i o n s
a s l u b r i c a n t s c a n g i v e a f r i c t i o n c o e f f i c i e n t below 0 . 1 ( f o r steel-steel systems) ( r e f . 321). F u e l s used a s l u b r i c a n t s i n b e a r i n g s t e e l - b r o n z e systems o p e r a t i n g i n t h e o x i d a t i o n w e a r r a n g e r e d u c e t h e wear. S t e e l - s t e e l s y s t e m s o p e r a t e d i n c o p p e r s a l t s o l u t i o n s ( 0 . 0 0 1 - 2 . 5 w t % ) i n g l y c e r o l d e m o n s t r a t e a v e r y low f r i c t i o n c o e f f i c i e n t ( a s l o w a s 0.015)
a n d low w e a r r a t e b e c a u s e o f t h e s e l e c t i v e
t r a n s f e r e f f e c t . S t e e l - s t e e l s y s t e m s o p e r a t i n g i n a g g r e s i v e media ( s u c h as a s o l u t i o n o f 1 0 % ( b y w e i g h t ) s o l a r o i l a n d 2 0 % N a C l i n w a t e r w i t h t h e a d d i t i o n o f b e n t o n i t e t o o b t a i n a d e n s i t y o f 1.18 mg/nun3) h a v e a f r i c t i o n c o e f f i c i e n t o f 0.3-0.4
a n d d e m o n s t r a t e low
wear when c o a t e d w i t h n i c k e l - p h o s p h a t e , which p r e v e n t s c o r r o s i o n ( r e f . 322). Boundary l u b r i c a t i o n i s a complex s u b j e c t . The l u b r i c a n t between t h e m i c r o r o u g h n e s s e s o f t h e s u r f a c e v i t r i f i e s a n d f o r m s a l a y e r b e t w e e n a few n a n o m e t e r s a n d 0 . 1 ,um t h i c k ( r e f s . 3 2 3 ) . When t h e t e m p e r a t u r e i n t h e c o n t a c t area i n c r e a s e s a t h i g h c o n t a c t p r e s s u r e , t h e s t r u c t u r e o f t h e l a y e r c h a n g e s i n t o l i q u i d and t h e l u b r i c a n t l a y e r d e c r e a s e s , which l e a d s t o a n i n c r e a s e i n f r i c t i o n . The a d s o r b e d boundary f i l m , p a r t i c u l a r l y when composed o f s u r face a c t i v e agent molecules, p l a s t i c i z e s t h e adsorbent surface, which leads t o t h e f r i c t i o n a n d w e a r d e c r e a s i n g b e c a u s e of t h e
stress r e d u c t i o n . The s h e a r s h o u l d b e l o c a l i z e d i n t h e plymolecular a d s o r b e d f i l m . The a p p l i e d l o a d i s l i m i t e d b e c a u s e t h e l o a d i n g c a p a c i t y o f p o l y m o l e c u l a r f i l m i s i n t h e r a n g e o f 5-10 MPa ( r e f . 9 7 ) . The r e s i s t a n c e a g a i n s t t h e b o u n d a r y f i l m t h i n n i n g s h o u l d b e
h i g h , t h e area o f c o n t a c t l a r g e a n d t h e r e a l a r e a of c o n t a c t n e a r t h e n o m i n a l ( c o n t o u r ) a r e a o f c o n t a c t . T h i s c a n b e a c h i e v e d by two-layer
l u b r i c a t i o n ( r e f s . 9 7 , 9 8 ) , where t h e b o u n d a r y film o f a
s u r f a c e a c t i v e a g e n t and t h e s u b l a y e r o f a s o f t metal o r p o l y m e r i c c o a t i n g form t h e c o n t a c t a r e a . T h i s t y p e o f l u b r i c a t i o n c a n r e d u c e t h e f r i c t i o n c o e f f i c i e n t markedly, e . g .
when f r e e c u t t i n g s t e e l
w i t h a cadmium c o a t i n g w a s s l i d i n g a g a i n s t i t s e l f i n thenaphthenic-paraffinic f r a c t i o n of a mineral o i l , t h e f r i c t i o n c o e f f i c i e n t was 0 . 2 9 , b u t when s u r f a c e a c t i v e a g e n t w a s added t o t h e l u b r i c a n t t h e f r i c t i o n c o e f f i c i e n t d e c r e a s e d t o 0.04 ( r e f . 9 7 ) .
161
The complex tribophysical and tribochdcal processes which occur during sliding lead to the production of reaction films (refs. 248, 324). This concerns, in particular, boundary lubrication with anti-wear additives. Tribochemical reaction films are formed by chemical reaction processes of the anti-wear additives involving ail active participation of both the friction surfaces, material and environmental factors. When additives acting by direct chemical reaction are used, (sulphur and chlorine compounds, fatty acids, fluorinated compounds), the metallic sulphides, chlorides or soaps are main constituents of the reaction films (ref. 3 2 4 ) . Additives acting through thermal and/or oxidative degradation processes (including metal dithiophosphates and more generally, phosphorus-containing organic compounds) form, after a very complex reaction has taken place products which have low molecular weights (oxides, mineral phosphates, sulphates etc.). The surface itself plays no direct part in the formation of polymeric and non-sacrificial reaction films, although it can act as a catalyst, and the high molecular weight compounds are formed through polymerization processes. The,films themselves are responsible for the reduction in the friction forces. Polymeric and non-sacrificial reaction films are formed when use is made of double-containing molecules such as benzene or centene (forming polymeric residues which giveanti-wear protection), zinc dithiophosphate (in some situations acting by polymerization processes), complex esters (acting by an in situ polymerization process), solid lubricants such as oil-soluble organic molybdenum compounds which form additives (MoS2 formed in situ in concentrated contacts) and borate additives (the reaction films are composed of two phases where iron is excluded and the non-sacrificial mechanism is involved). The materials used may lead to degradation of the lubricant since the metal surfaces play an important role in oxidation mechanisms. Copper and iron in particular appear to accelerate the oxidation reaction. Iron promotes oxidation much faster than copper (ref, 3 2 5 ) . Organometallic compounds are formed as a result of the lubricant-surface interactions. The boundary lubrication processes markedly accelerate the oxidation. A significant decrease can be seen in the activation energy of the oxidation when an oxide-free metal surface is used; in the case of cumene for example, from 3.4 to 2.6 kcal/mol (ref. 102). The metal surface may deteriorate as a result of interactions in the lubricant-surface system. The mechanochemical transformations of the oleic acid in the fric-
162
tion area of metal and abrasive lead to polymerization of theoleic acid, producing, as a result, the surface-active agents which increase wear of the metal (ref. 3 2 6 ) . Various models have been proposed for predicting the friction force, critical temperature of the lubricant, and wear rate, under conditions of boundary lubrication (refs. 3 2 7 - 3 3 0 ) . The recent qualitative model for friction forces given in ref. 3 2 7 takes into consideration the shear modulus, the area and the thickness of the sheared material (lubricant), but to allow for the thermofluctuational nature of the shear resistance expressed by relaxation the, intensive experimental studies need to be carried out on the energy values used in relaxation time formulae. An experimentally confirmed theoretical model allowing the estimation of the critical temperatures of oils and oils containing chemically active additives at various sliding speeds and realistic pressures is presented in ref. 3 2 8 . This model is based on the energy description of the failure of lubricant layers under conditions of boundary lubrication together with Kingsbury’s desorption model and Zhurkov’s formula. The following equations are used:
(5.1)
In TK
Tcr = T
In T
(5.2)
where T is the absolute value of the critical temperature, Tcr the actual value of the critical temperature, TK the ambient temperature, ET the solid breakdown activation energy due to the thermal effect, and 2 the structural-mechanical coefficient indicating the direction and efficiency of the stress field, i.e. thd rate at which the activation energy decreases; P, is effective (real)pressure, R the universal gas constant, v the sliding speed, to the period of thermal vibration of lubricant molecules, z the distance between the adsorption centres, Wmo the specific energy of adhesion interaction between the metal and lubricating film, and Wab the specific energy of adhesion interaction between the surfaces brought into contact. The value of to for mineral oil with the addition of fatty acids is 3 s, and z = 2 nun (on steels). The adhesive wear model for calculating the volume of adhesive wear is of the form (refs. 3 2 9 , 3 3 0 1 ,
163
(5.3) where Va is the adhesive wear volume, km the probability of forming a wear particle at a metal-metal junction, f the frictioncoefficient, the fractional film defect, Pm the contact pressure at which the material flows (under static load), and N is the normal load supported by the contacting asperities; for very slow sliding speeds (i.e. at boundary lubrication), N is equal to the total load of the contact; L is the sliding distance. The fractional film defect is defined as
Po
Po
Po = %/Ar
(Unlubricated area/Total area of contact)
(5.4)
and may be expressed in a modified form as
v ML where Tm is the melting point of the lubricant, M the molecular weight of the lubricant, Ec the heat of adsorption of the lubricant, and Ts the temperature at the interface. For a compound lubricant consisting of an additive a and abase fluid b, the fractional film defect d can be expressed as:
pa
a n d p b are the fractional additive and base fluid film where defect respectively, and B is the fractional surface coverage additive. A model has been developed for a pin-on disk system which takes the adhesive and fatigue wear into consideration (ref. 329). The so-called adhesive wear in lubricated contacts can be broken down into several elemental processes and the individual mechanisms analysed (ref. 331). Experiments on the lubricated wear of steel suggest that ridges in the surfaces along the sliding direction become fatigued and lead to the formation of flake-like fragments, resulting in wear. Fretting wear in lubricated contacts depends on the kind of lubricant used. Fretting wear in steel-steel systems lubricated with synthetic oil is higher than when mineral oil is used (ref. 104; see also Chapter 3.2). Fretting in aqueous solutions of non-
164
-corrosion-resistant steels results in the removal of material by chemical dissolution rather than by mechanical damage (ref. 144). 5.1.2. SINTERED METALS Sintered metals (bronze, iron) are used particularly for bushes in miniature journal bearings. Sintered, porous material is usually impregnated with a lubricating oil. The most frequently used material is bronze. Iron is stronger than bronze but less compatible with a mild-steel shaft and is susceptible to rust in a humid environment. The impregnating oil should have good oxidation resistance because it is constantly at the running temperature and is in intimate contact with the metal catalyst surfaces and the oxygen in the atmosphere. The oil should also demonstrate satisfactory boundary lubrication (oiliness) properties. The typical porosity of commonly used sintered bearing materials is 20-30%; standard porous bronze bearings of this sort are unsuitable for loads greater than 50 MPa (140 MPa for iron bearings) (ref. 332). The tribological properties of such bearings depend mainly on the properties of the oil and its circulation through the bearing. There should be a constant flow of oil from the loaded part to the unloaded part via the porous bearing. The bearing and the bearing gap should be completely filled with oil. The synthetic oils are better because they provide lower minimum friction coefficients on the Stribeck curve (ref. 333). They also have higher ageing resistance than natural oils but on the negative side they can creep from the bearing because of low surface tension. Polyether oils have been satisfactorily applied in miniature bronze bearings operating at 15OoC (ref. 65). The journals are usually made of hardened steel and have low surface roughness (Ra = 0.2 g m ) . They can be also coated by a polymer layer (ref. 872). The hydrodynamic effect in miniature porous bearings occurs at a smaller sliding speed than in a similar solid bearing (Fig. 5.4). The Stribeck curve changes during a life test. The comparison between the Stribeck curves for miniature bronze and iron bearings obtained during life tests can be seen in Fig. 5.5. The marked effect of the running-in on the friction can be observed; bronze bearings can be easily run in, but their friction coefficient and wear rate are higher than those of iron bearings. These observations about the running-in only apply when the shaft is statically loaded. The running-in creates a sufficient area of
165 flat surface to carry the load. This surface is parallel to mating shaft (refs. 335, 336). The temperature rise in the friction area as a function of sliding time during the running-in period is relatively rapid, going up to 50-60°C, but then it decreases toabout 4OoC when conditions for the hydrodynamic effect are reached (ref.337).
1
01
a
1
0.2
0.6 0.8 Sliding speed , mls 0.4
1.0
-
F i g . 5.4. F r i c t i o n c o e f f i c i e n t v s . s l i d i n g speed i n m i n i a t u r e porous (1,2) and s o l i d bronze b e a r i n g s i m pregnated w i t h m i n e r a l (1) and s y n t h e t i c o i l ( 2 ) , l u b r i c a t e d w i t h s y n t h e t i c o i l ( 3 ) . B e a r i n g h o l e diameter 3 mm, e x t e r n a l diameter 6 mm, l e n g t h 4 mm, d i a m e t r a l c l e a r a n c e 24 ,urn. Contact p r e s s u r e 0.48 MPa, v i s c o s i t y o f m i n e r a l o i l a t 2OOC 152.3 rnPa.s, of s y n t h e t i c o i l 110 rnPa.5 ( r e f . 3 3 3 ) .
The running-in period can also be detected by use of an electrical impulse counting method using a high frequency oscillator and the oil gap or the lubricant film between the shaft and the bearing bush as a capacitance/resistance element in an electrical circuit
166
( r e f s . 338, 3 3 9 ) .
----\ 0.075
0
/
----------0.15
0.225
0.30
-1650h 0.375
Edtding speed mls
F i g . 5 . 5 . Change i n S t r i b e c k c u r v e o f m i n i a t u r e s e l f - a d j u s t i n g porous bronze and i r o n ( d o t t e d l i n e s ) b e a r i n g s d u r i n g l i f e t e s t . B e a r i n g h o l e d i a m e t e r 3 mm, e x t e r n a l diameter 7 mm, l e n g t h 5.5 mm, d i a m e t r a l c l e a r ance 18 ,urn. C o n t a c t p r e s s u r e 0.2 MPa, o i l v i s c o s i t y 43 rnPa-s ( a t 2OoC) and 15 mPa.s ( a t 70OC). P o r o s i t y o f bronze and i r o n b e a r i n g 19 and 268 r e s p e c t i v e l y . S t a r t / s t o p c y c l e o f 25 m i n r o t a t i n g and 5 min s t a t i o n a r y . B e f o r e t h e t e s t was s t a r t e d , a f i l m b a r r i e r was a p p l i e d t o t h e p o s i t i o n s on t h e s h a f t n e x t t o t h e bearing being tested ( i n order t o prevent o i l from creeping o u t o f t h e b e a r i n g gap d u r i n g t e s t i n g ) ( d a t a f r o m r e f .
334). These methods i n d i c a t e t h e t r a n s i t i o n from mixed t o hydrodynamic l u b r i c a t i o n , i . e . t h e p o i n t a t which t h e f r i c t i o n c o e f f i c i e n t i s minimum. A s a r e s u l t o f t h e s t u d i e s d e s c r i b e d i n r e f . 3 4 0 , t h e
167
sliding speed at which this transition occurs in miniature porous bearings can be predicted using the followincj experimentally derived formula : vt
= 170
d
w2
a 1.01
v1
(5.7)
where vt is in m/s, and the bearing hole diameter d, the thickness of the wall of the bearing bush g, and the bearing length 1 are in mm; y is the relative clearance. This formula was determined for bearings with a nominal diameter of 1.5-3 mm, bearing wall thickness 1 - 3 mm, bearing length 3-6 mm, range of rotational speed 0.03-10000 rotations per minute and 0 . 0 2 5 - 0 . 5 MPa contact pressure. An increase in the applied load results in a reduction of the friction coefficient. In bronze miniature bearings (0 3 mm, external diameter 6 mm, bearing length 4 mm, and diametral clearance 2 4 ,um) impregnated with oil of viscosity 1 6 3 mPa-s (at 2OoC), the minimum value of the friction coefficient decreased from about 0 . 1 4 to 0.10 upon increasing the contact pressure from 0.05 to 0 . 1 5 MPa (Fig. 5.6, ref. 3 4 0 ) . The minimum value of the friction coefficient also decreases with increasing relative bearing clearance. The effects are greater when the viscosity of oildecreases. The friction coefficient at very low sliding speed ( 1 rotation per minute) decreases with increasing bearing clearance. Similar effects were also found in the investigations reported in refs. 3 4 1 345. The friction coefficient increases rapidly, especially when the relative bearing clearance is smaller than 0.005, and at high clearance it approaches a constant value. The friction torque can be expressed as a function of the relative clearance by the following formula : M f = a0 + a - 1
Yk
(5.8)
where a, and a are regression coefficients, and k is the exponent (k = 0.5, 1 or 2 ) . Prismatic porous bearings demonstrate better tribologicalprop erties than traditional cylindrical porous bearings (refs. 342-345). Various prismatic bearing shapes are presented in Fig. 5.7. These bearings demonstrate little dependence of friction torque on bearing clearance over a wide range of relative clearances (0.0015-0.015)1 high friction torque stability at small loadsl extremely accurate
168
journal alignment (within 1 ,urn), and longer life, since the wear debris are collected in the corners ("pockets") of the bearing hole. The more sides the polygon has, the greater the change in the friction coefficient as a function of the load direction (refs. 3 4 3 , 3 4 6 ) . The friction coefficient of prismatic bearings can be 3-5 times lower and their bearing life as much as 50 times longer than those of cylindrical bearings (refs. 3 4 2 , 3 4 5 ) .
0.60 + 0.40
c .-a u
.-J
LL LeQ)
.
0.20 -
0
u
c
.-a 0.10 -
Ti tk
.-I
0.08 -
63.3mm2/5
0.06 -
1 52.3mm2/5
0.04 -
50 mm2/s
0.02I
I
b.02
1
1
0.04 0.060.08D.(O
1
0.2
I
I
I
c
0.4 0.6 0.0 1.0
Contact pressure, MPa
F i g . 5 . 6 . Minimum v a l u e o f t h e f r i c t i o n c o e f f i c i e n t i n m i n i a t u r e porous b e a r i n g s as a f u n c t i o n o f c o n t a c t p r essu re a t v a r i o u s r e l a t i v e b e a r i n g clearances (p) and v i s c o s i t i e s o f o i l used (7) ( r e f . 3 4 0 ) .
169
F i g . 5 . 7 . V a r i o u s f o r m s o f m i n i a t u r e p r i s m a t i c porous b e a r i n g s .
The porosity of a bearing affects its tribological properties. The experimentally determined relationship between the specific density of the sintered niaterial used and the rotational speed nt at which the minimum value of the friction coefficient occurs, for miniature porous bearings lubricated with synthetic oil, is as follows (ref. 3 4 6 ) :
g
(5.9)
where p is the specific density of the sintered material (inmq/m3), g and 1 are the thickness and length respectively of the bearing bush wall (in mm) and y is the relative clearance and 17 the viscosity of the synthetic oil used (in mPa.s). The friction coefficient of such bearings decreases when the specific density of the sintered material is increased. The lubrication mechanism in miniature porous bearings depends on the temperature increase in the bearing (which activates physical effect such as the different thermal expansion of the oil and the porous matrix) and on the capillary flow because of the pressure balance in the capillary system formed by pores in the bearing bush and the gap between the surfaces of the journal and the bearing bush (refs. 3 4 7 , 3 4 8 ) . The volume of oil used for lubrication by such mechanisms is not higher than 4 % of the total volume of oil used f o r impregnating the bearing. A new system of lubrication has been elaborated to improve the efficiency of self
170 lubrication in miniature porous bearings (refs. 3 4 7 , 3 4 9 - 3 5 3 ) . Oil is extruded from pores by pressure of gas collected in a special chamber (Fig. 5 . 8 ) ; the thermal expansion of the gas in the chamber, brought about by the frictional thermal energy in the friction area, results in o i l extrusion. The maximuin oil extrusion from the bearing is 3-5 times higher than in traditional self-lubricating miniature porous bearings. The coefficient of oil extrusion uo can be expressed as follows: (5.10)
where Ve and Vgp are the respective volumes of extruded oil and gas in the bearing pores, wg = Vg/Vgp (Vg - gas volume in the chamber, Vgp - volume of gas in pores), Tg is the initial gas temperature, and are the coefficients of volume thermal expansion for sintered porous material and oil respectively, and AT is the temperature increase.
Po
Pm
F i g . 5.8. Porous b e a r i n g and body w i t h gas chamber. 1 - body, 2 gas chamber, 3 - porous b e a r i n g , 4 f r o n t e d thermal i s o l a t i o n impermeable f o r o i l and gas, 5 - j o u r n a l .
-
-
Experimental studies of miniature porous bearings with gas chambers have shown that the maximum o i l extrusion occurs when the parameter w4 in eqn. (5.10) is 0.1-0.2 (refs. 342, 3 4 9 , 3 5 0 ) .
171 The e n e r g y d i s s i p a t e d i n t h e f r i c t i o n p r o c e s s i s enough t o o b t a i n a p r e s s u r e i n t h e g a s chamber h i g h e r t h a n t h e c a p i l l a r y p r e s s u r e i n t h e p o r e s o f a s i n t e r e d b e a r i n g . The b e a r i n g l i f e is l i m i t e d n o t by a h i g h wear r a t e b u t by t h e r a p i d i n c r e a s e i n f r i c t i o n a l t o r q u e a s a r e s u l t o f t h e breakdown o f t h e s e l f - l u b r i c a t i o n s y s -
t e m . A c o m p a r i s o n between m i n i a t u r e b e a r i n g s w i t h a n d w i t h o u t g a s chambers is shown i n F i g . 5 . 9 .
operation t i m e , h
F i g . 5.9. P r o b a b i l i t y o f f a i l u r e - f r e e o p e r a t i o n o f two groups o f m i n i a t u r e s e l f - l u b r i c a t i n g porous b e a r i n g s . S l i d i n g speed 0.3 m / s , c o n t a c t p r e s s u r e 0.2 MPa, b e a r i n g h o l e d i a m e t e r 2 mm, e x t e r n a l d i a meter 6 mm, b e a r i n g l e n g t h 4 mm, d i a m e t r a l c l e a r ance 16-20 fim. 1 - b e a r i n g w i t h o u t gas chamber, 2 - b e a r i n g w i t h gas chamber ( r e f . 3 4 9 ) .
The p e r m e a b i l i t y o f t h e p o r o u s b e a r i n g b u s h h a s a n i m p o r t a n t i n f l u e n c e on t h e o p e r a t i o n o f s i n t e r e d b e a r i n g s . The h i g h e r permea b i l i t y i n t h e m i d d l e p a r t o f t h e b e a r i n g b u s h (when a n a l y s i n g
172
a l o n g t h e a x i a l d i r e c t i o n ) a s compared t o t h e f r o n t a l p a r t s r e s u l t s i n lower f r i c t i o n t o r q u e ( r e f . 2 5 4 ) . O b l i t e r a t i o n r e d u c e s t h e p e r m e a b i l i t y o f t h e p o r o u s b e a r i n g s i g n i f i c a n t l y ( r e f . 3 5 5 ) . The boundary l a y e r s c r e a t e d on t h e p o r e w a l l s c a n r e d u c e t h e p o r e d i -
ameter and d e c r e a s e t h e p e r m e a b i l i t y . The o i l s h o u l d c r e a t e a boundary l a y e r o f minimum t h i c k n e s s a n d maximum s t r e n g t h a n d s h o u l d a l s o h a v e good l u b r i c a t i o n p r o p e r t i e s . The c h e m i c a l i n e r t n e s s and t h e r m a l s t a b i l i t y o f t h e o i l a r e a l s o i m p o r t a n t s i n c e c o r r o s i o n wear c a n o c c u r a s t h e e f f e c t o f t r i b o c h e m i c a l r e a c t i o n s o f t h e p r o d u c t s o f t h e t h e r m a l o x i d a t i o n ( a t 16O-18O0C) o f s a y carbonic a c i d s w i t h t h e s i n t e r e d material
( r e f . 356).
The s e r v i c e l i f e o f p o r o u s b e a r i n g s i s l i m i t e d n o t by a n exc e s s i v e wear r a t e , b u t by t h e l o s s o f s e l f - l u b r i c a t i n g a b i l i t y , During t h e o p e r a t i o n o f t h e b e a r i n g , t h e o i l volume d e c r e a s e s and
i t s l u b r i c a t i n g p r o p e r t i e s d e t e r i o r a t e . T h e r e i s constant c r e e p o f o i l o u t o f t h e b e a r i n g o v e r t h e s h a f t , where i t e v a p o r a t e s ( r e f . 3 3 4 ) . Because o i l from t h e b e a r i n g c r e e p s o v e r t h e s h a f t and t h e s u r f a c e s o f t h e b e a r i n g h o u s i n g , t h e e v a p o r a t i n g s u r f a c e i s greatly i n c r e a s e d . T h e r e i s no s u b s t a n t i a l i n c r e a s e i n t h e f r i c t i o n c o e f f i c i e n t u n t i l a b o u t 50% o f t h e o i l c o n t e n t h a s b e e n l o s t (ref.357). I t is p o s s i b l e t o r e c h a r g e an o i l - s t a r v e d
p o r o u s b e a r i n g by a d d i n g
o i l t o t h e b e a r i n g s u r f a c e w h e r e v e r it i s a c c e s s i b l e ( a few d r o p s t o b e a r i n g s w i t h b e a r i n g h o l e d i a m e t e r 9 mm and l e n g t h 11 m m ) , a n d t h e r e b y i n c r e a s e t h e l i f e o f t h e b e a r i n g . The c o e f f i c i e n t o f f r i c t i o n c a n b e r e d u c e d by s e l e c t i n g a smaller w a l l t h i c k n e s s f o r t h e b e a r i n g b u s h b u t t h i s r e s u l t s i n l o w e r s t r e n g t h , l e s s volume of m e t a l , and t h e r e f o r e a s h o r t e r s e r v i c e l i f e f o r t h e b e a r i n g ( r e f . 3 5 8 ) . An i n c r e a s e i n l e n g t h r e s u l t s i n a l o w e r c o e f f i c i e n t o f f r i c t i o n and a l a r g e r metal volume and t h e r e b y a l o n g e r l i f e , a l t h o u g h a n undue i n c r e a s e i n l e n g t h may r e s u l t i n e d g e e f f e c t . P r i s m a t i c b e a r i n g s have a l i f e t i m e s e v e r a l t i m e s l o n g e r t h a n c y l i n d r i c a l bearings ( r e f . 342). B e a r i n g l i f e d e c r e a s e s w i t h i n c r e a s e o f t h e o p e r a t i n g temperat u r e of t h e t e m p e r a t u r e i n t h e f r i c t i o n a r e a . The t e m p e r a t u r e r i s e i n t h e f r i c t i o n a r e a c a n be a p p l i e d t o d e t e r m i n e t h e a d m i s s i b l e pv v a l u e o f t h e b e a r i n g s ( p
-
contact pressure, v
-
s l i d i n g speed).
The maximum s e r v i c e l i f e o b s e r v e d f o r m i n i a t u r e p o r o u s b e a r i n g s i s 2000-3000 h ( r e f . 3 5 9 ) .
The t y p i c a l o p e r a t i n g t e m p e r a t u r e r a n g e f o r p o r o u s b e a r i n g s i m p r e g n a t e d w i t h o i l i s from
-
4 0 t o 15OoC. B e a r i n g s i m p r e g n a t e d
w i t h s o l i d l u b r i c a n t ( s u c h a s g r a p h i t e , MoS2 o r PTFE) c a n b e u s e d
173
when t h e t e m p e r a t u r e v a r i e s i n t h e r a n g e from
-
200 t o 30OoC.
The
G l a c i e r DU m a t e r i a l b a s e d on p o r o u s b r o n z e i m p r e g n a t e d w i t h PTFE and Pb and w i t h a t h i n PTFE l a y e r on t h e s u r f a c e c a n b e u s e d i n t h e c h e m i c a l and food i n d u s t r y a s m a t e r i a l f o r b e a r i n g e l e m e n t s .
5,2,
P O L Y M E R I C SYSTEMS METAL-POLYMER SYSTEMS
5.2.1.
L u b r i c a t i n g m i n i a t u r e metal ( u s u a l l y s t e e l ) - p o l y m e r s y s t e m s i s v e r y e f f e c t i v e i n r e d u c i n g wear, s h o r t e n i n g t h e r u n n i n g - i n p e r i o d and d e c r e a s i n g s t i c k - s l i p e f f e c t s . However t h e s e l e c t i o n o f a l u b r i c a n t ( o i l ) s h o u l d b e made v e r y c a r e f u l l y , o t h e r w i s e it may have
l i t t l e e f f e c t o r e v e n a n e g a t i v e e f f e c t on t h e t r i b o l o g i c a l p r o p e r t i e s o f t h e s y s t e m . A s t u d y o f t h e e f f e c t s o f l u b r i c a t i o n gene r a l l y c o n f i r m s t h e c o n c l u s i o n drawn i n C h a p t e r 4 . 2 t h a t a d h e s i o n p l a y s an i m p 3 r t a n t r o l e i n t h e f r i c t i o n and w e a r mechanism o f p o l y -
.
meric m i n i a t u r e s y s t e m s (see a l s o ‘ r e f . 8 7 3 ) L u b r i c a t i o n , u s u a l l y w i t h one d r o p o f o i l , c a n l e a d t o a g r e a t r e d u c t i o n i n wear a n d , t o a lesser e x t e n t , a d e c r e a s e i n t h e f r i c t i o n c o e f f i c i e n t o f t h e p o l y m e r i c e l e m e n t i n s t e e l - p o l y m e r s y s t e m s i n which t h e p o l y m e r i c m a t e r i a l s have h i g h s u r f a c e f r e e energy ( r e f s . 5 , 6 , 4 4 , 106, 164, 169, 170, 171, 170, 189, 196, 199, 207, 208).
70,
77,
The wear
of t h e s t e e l element i n l u b r i c a t e d systems ( o f t e n a j o u r n a l i n miniature bearings) is negligible.
The e f f e c t o f l u b r i c a t i o n on
t h e f r i c t i o n c o e f f i c i e n t o f some p o l y m e r s u s e d i n t h e b e a r i n g bush o f m i n i a t u r e s t e e l - p o l y m e r s y s t e m s i s shown i n F i g . 5 . 1 0
( b a s e d on
d a t a t a k e n from r e f . 6 ) . The e f f e c t o f t h e e l a s t i c i t y o f t h e b e a r i n g bush c a n b e o b s e r v e d . F o r more e l a s t i c m a t e r i a l , s u c h a s PA 6 , t h e hydrodynamic l u b r i c a t i o n , o r more c o r r e c t l y e l a s t o h y d r o d y n a m i c lubrication since there i s r e l a t i v e l y g r e a t deformation of t h e b e a r i n g b u s h , c a n b e o b s e r v e d a t r e l a t i v e l y low s l i d i n g s p e e d s and t h e minimum v a l u e o f t h e f r i c t i o n c o e f f i c i e n t i s r e l a t i v e l y h i g h . S i m i l a r e f f e c t s h a v e b e e n o b s e r v e d by t h e a u t h o r (see below and refs.
1 9 6 , 3 6 0 ) . The f r i c t i o n c o e f f i c i e n t o f a s t e e l - P B T P
b e a r i n g (P, 1 mm) was r e d u c e d from between 0 . 4 and 0 . 7 bricated bearings) t o 0.08
miniature
( f o r unlu-
( a t boundary l u b r i c a t i o n ) by l u b r i c a t -
i n g i t w i t h f l u o r i n a t e d s y n t h e t i c o i l ( S i l b e r K7132 l v w i t h a v i s c o s i t y of 3 0 8 m2/s a t 2OoC;
see a l s o T a b l e 3 . 6 ) ( r e f . 1 7 0 ) .
The e f f e c t o f l u b r i c a t i o n on wear i n m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s i s more e v i d e n t t h a n i t s e f f e c t on f r i c t i o n ( r e f s . 44,
70,
164, 170, 196, 199, 207,
208,
361-364).
174
1
0.1
I
0.2
I
0.3
Sliding speed
1
0.4
I
0.5
,m/s
F i g . 5.10. E f f e c t o f l u b r i c a t i o n on f r i c t i o n c o e f f i c i e n t o f miniature steel-polymer j o u r n a l bearing. B e a r i n g h o l e d i a m e t e r 2 mm, b e a r i n g bush l e n g t h 1 0 mm, d i a m e t r a l c l e a r a n c e 0.03-0.08 mm, c o n t a c t p r e s s u r e 0.0625 MPa, l u b r i c a t i o n by d i r n e t h y l p o l y s i l o x a n e w i t h v i s c o s i t y 3 0 0 mm2/s ( a t 20OC). D o t t e d 1 i n e s g i v e char a c t e r i s t i c s o f l u b r i c a t e d b e a r i n g s . 1 - PA 6, 2 - PC, 3 - POM h .
The effect of lubrication on the wear of some steel-polymer bearings is presented in Fig. 5.11. It can be seen that the wear reduction in PA bearings is higher than in POM bearings. The decrease in wear is particularly high when the pv value increases (Fig.5.12). The wear in lubricated steel-PA bearings is higher than in lubricated steel-POM bearings. The increase in the wear rate of miniature lubricated steel-polymer journal bearings is generally proportional to the increase in pv value.
175
A 200
-
160
-
I
E c
a
Sliding d i s t ~ n c e km ,
F i g . 5 . 1 1 . E f f e c t o f l u b r i c a t i o n ( w i t h XU 430 t r a d i t i o n a l c l o c k o i l , see Table 3.1) on wear i n miniature steel-polymer j o u r n a l bearings. B e a r i n g h o l e d i a m e t e r 2 . 1 5 mm, e x t e r n a l d i a meter o f p o l y m e r i c b e a r i n g bush 6 mm, l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1.5%, s l i d i n g speed 0.067 m / s , c o n t a c t p r e s s u r e 3 MPa. D o t t e d l i n e s a r e f o r l u b r i c a t e d b e a r i n g s . Data on wear o f u n l u b r i c a t e d b e a r i n g s from r e f . 174. 1 PA 6, 2 - POM C , 3 - POM h.
-
The admissible pv value for miniature steel-polymer bearings can be increased several times as a result of lubrication (refs. 3 6 1 , 362, 3 6 5 ) . The temperature in the friction area was applied as the criterion for determining the admissible pv value ( 5 5 O C for unlubricated and 5OoC for lubricated bearings). The effect of lubrication on decreasing the admissible pv value is shown in Fig. 5.13.
176
I
1
0.2
p.v
0.4 I
1
0.6
MPasm/s
F i g . 5.12. E f f e c t o f l u b r i c a t i o n on wear v s . pv v a l u e (p - c o n t a c t p r e s s u r e , v s l i d i n g speed) f o r steel-PA 6 + 25% g l a s s f i b r e + 4% g r a p h i t e m i n i a t u r e j o u r n a l b e a r i n g . B e a r i n g h o l e diameter 2.15 mm, b e a r i n g bush e x t e r n a l diameter 6 mm , l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1.5%, s l i d i n g d i s t a n c e 1 0 km. 1 - u n l u b r i c a t e d b e a r i n g (data f r o m r e f . 174), 2 - m i n e r a l MWP o i l , 3 - s i l i c o n e - m i n e r a l OKB 122-16 o i l , 4 - XU 430 t r a d i t i o n a l c l o c k o i l .
177
8
6
4
2
0
I
0.2
1
0.4
1
I
0.6
sliding speed
0.8 J
-
m/s
F i g . 5.13. A d m i s s i b l e l o a d o f u n l u b r i c a t e d and l u b r i c a t e d m i n i a t u r e s t e e l - p o l y m e r j o u r n a l bearings ( l u b r i c a t e d w i t h t r a d i t i o n a l c l o c k o i l XU 430). B e a r i n g h o l e d i a m e t e r 2.15 mm, e x t e r n a l b e a r i n g bush diameter 6 mm, l e n g t h 2.1 rnm, r e l a t i v e c l e a r a n c e 1 . 5 % . Dotted l i n e s f o r l u b r i c a t e d bearings. 1 - PA 6 + 25% g l a s s f i b r e + 4% g r a p h i t e , 2 - PA 6, 3 - POM C, 4 - POM h.
The l u b r i c a t i o n o f o t h e r m i n i a t u r e s t e e l - p o l y m e r
systems h a s
a l s o a s t r o n g l y p o s i t i v e e f f e c t o n t h e i r f r i c t i o n a n d wear behavi o u r ( r e f s . 77, 1 0 6 and 3 6 6 ) . The f r i c t i o n b e h a v i o u r o f combinat i o n s o f l u b r i c a t e d s t e e l - P A 6 6 , s t e e l - P O M h , steel-POM h+PTFE, a n d steel-PTFE + 4 0 % b r o n z e + 1 5 % g r a p h i t e w a s t e s t e d u s i n g a 2 - b a l l pendulum ( b a l l s made of b e a r i n g s t e e l : see C h a p t e r 8 . 2 ) . The r e s u l t s , based o n d a t a from r e f . 1 0 6 , a r e shown i n F i g s . 5 . 1 4 , 5 . 1 5 , 5.16 a n d 5.17.
178
9 Q 0
0 0 0
0
I
Fresh
oi\S
F i g . 5.14. E f f e c t o f l u b r i c a t i o n on f r i c t i o n c o e f f i c i e n t o f steel-POM system t e s t e d u s i n g 2 - b a l l pendulum ( b a l l s made of b e a r i n g s t e e l : see Chapter 8 . 2 ) . 0 - u n l u b r i c a t e d , l,2,3,4,5,6,7,8 - numbers o f o i l s used, see T a b l e 5.1. Pendulum mass 300 g . For c o n d i t i o n s o f a g e i n g O f o i l s , see c a p t i o n t o F i g . 5.3
.
179
0
e 0.31
I I
?
I
I
Fig. 5.15. Effect of lubrication on friction coefficient of steel-PA 66 system. For additional information, see caption to Fig. 5.14
.
1. 8
Fresh oils
11 21 41 51 61 71
Aged
oils
Fig. 5.16. Effect of lubrication on friction coefficient of steel-PTFE + 40% bronze + 15% graphite system. For additional information, see caption to Fig.
5.14
.
180
s cu
a25
0
I
0 1
0.20
mO1
li
0 0 @4
4
P
c
a,
.a
u -2
0.15 0)
gl El
I
I
0,
O 1
c
I
I I
I
I‘
0
.3
“u 0.10
.-,I
&
I
I
I I I I
I I I
o_
7
I
I 0.05
Q*
5s
I
I
I
I FEt
0 POM h+PTFE
Fi?. 5.17. Effect of pendulum mass (circles) and viscosity (at 20OC) o f fluorinated silicone oil (squares) on friction coefficient on steel-polymer systems tested using 2-ball pendulum (see Chapter 8 . 2 ) . NB. 0 unlubricated system.
-
The increase in the friction coefficient as a result of lubrication can be observed for steel-POM and steel-PA 66 systems. For the steel-POM systems, the following oils were ineffective: polyglycol containing ether and alcohol groups, aliphatic diester and chlorinated phenylmethylpolysiloxane. The decrease in the friction
181
coefficient because of lubrication is relatively small as compared to steel-PA 6 6 and steel-PTFE + bronze + graphite systems. The lubrication of the steel-PA 6 6 system with the aforementioned oils (except chlorinated polysiloxane oil) has a deleterious effect on friction behaviour. The use of aged oils has relatively little influence on the friction behaviour of the steel-POM and steel-PTFE+ bronze + graphite systems, but in the steel-PA 6 6 system the friction coefficient was seen to increase (as compared to the friction coefficient of the system lubricated with fresh oil), particularly when polyglycol containing ether and alcohol groups and aliphatic diester were used. This is probably because of the detrimental interactions in a polymer-oil system between the polar products of ageing and polar polymer (PA 6 6 ) (see Chapter 6 . 6 ) . The efficiency of lubrication depends on the polymer-oil combination. Polymers which have relatively high surface free energy also demonstrate a high rate of friction and wear reduction when lubricated. Oils which have good lubricity and chemical inertness are the most suitable for lubricating polymeric systems. When the surface tension of the oil decreases (oil wets the polymer surface better) and the viscosity of the oil increases, the friction and wear behaviour of the lubricated discussed systems is improved (refs. 7 0 , 1 9 9 , 200, 3 0 1 , 3 6 7 - 3 7 1 ) . Lubrication with water is probably ineffective because the polymer surface is not properly wetted (refs. 5 3 , 3 7 2 , 3 7 3 ) . Using water to lubricate steel-polymer systems (LDPE, HDPE, PTFE, PA 6 , POM and PI polymers) led to a reduction in the friction coefficient and wear only in the steel-LDPE system (ref. 3 7 2 ) . The greatest increase in wear was observed in the systems with PI and PTFE. The increase in wear under water lubrication may be due to a modification of the surface structure of polymers by the water rather than to the modification of the counterface by polymer transfer. The use of water makes more sense when it is used as a cooling agent. Steel-polymer systems can be lubricated by an external lubricant or by lubricant dispersed in the polymer used. "External" lubrication is more efficient but "internal" lubrication is adventageous because the migration of the lubricant from the system is small. Dispersing 1-3% dimethylpolysiloxane with a viscosity of 5 0 - 1 0 0 m2/s (at 20°C) in PC has been shown to effectively lubricate steel-PC miniature bearings: the minimum value of the friction coefficient was 0 . 0 3 (at contact pressure 0 . 0 6 2 5 MPa) and wear observed at the sliding speed 0.5 m/s and contact pressure 0 . 3 MPa
182
after 50 h was negligible (ref. 6). Insoluble oils such as fluorinated polyether or polysiloxane, polyglycol, dispersed in a thin polymer coating (2-10 ,um thick) can efficiently lubricate a sliding steel-polymer coating system (ref. 3 7 4 ) . Tests on such systems using a sphere-on-disk arrangement (sphere @ 6 mm) have shown that epoxy coating containing about 1% (by weight) of a fluorinated m2/N at load insoluble oil gives the lowest wear rate (ca. 2 5 N and sliding speed 0.1 m / s ) and low friction coefficient (0.06-0.10). The lubricant (dimethylpolysiloxane) is dispersed in the polymer in the form of small drops (2-10 ,urn) and migrates onto the friction surface by diffusion and extension (ref. 70). This migration is more intensive when sliding takes place and the temperature on the friction surface rises. Materials with a thermoplastic polymer matrix and which contain 2% (by weight) silicone oil (and also glass fibre or PTFE for example) can be obtained from the LNP Corporation (ref. 375). LPN's Migralube composite provides low friction at start-up and initial running-in and can be used for high-speed operation. Rimplast composite is more resistant to wear (synergistic action of PTFE and silicone) and is especially useful when wear debris (e.g. from textile or paper processing) could be trapped in the friction area. Other examples of oil-containing polymers are PA and POM impregnated (up to 50wt% oil) with mineral oil. Some forms of these materials are Oilon (oil-filled PA) by Nylacast, PV 80 (oil-filled POM) by Railko and the Soviet SAMs (with a PA or POM matrix), which all demonstrate very good tribological properties (refs. 48, 57). The introduction of oil between metal and polymer surfaces reduces the bonding adhesive force. The decrease in the bonding force per unit of contact area, calculated using eqn. (4.6) at increasing dielectric constant of the liquid introduced (oil), is presented in Fig. 5.18 (ref. 196). The dielectric constant of the oil used for lubrication should be higher than the dielectric constant of the polymer. The author's studies (ref. 110) on the dielectric constant of instrument oils (the dielectric constant vs. oil temperature for the oils investigated is shown in Figs. 3.2 and 3.3) and studies on the wear of lubricated miniature steel-polymer journal bearings lead to the conclusion that there is a correlation between the radial wear intensity of such bearings and the bonding force per unit of contact area, calculated using eqn. (4.5) (Fig. 5.19, ref. 196).
183
F i g . 2.18. Bonding f o r c e p e r u n i t c o n t a c t on m e t a l ( Elr'see eqn.(4.5))area L,,J -polymer i n t e r f a c e v s . d i e l e c t r i c c o n s t a n t o f separating l i q u i d f i l m ( f i l m thickness H = 1 nm). 1 metal-POM (€20 = 4 ) , 2 - metal-PA 6 ( E 2 0 = 5 ) .
-
The presence of oil between the metal (steel)-polymer surface leads to a reduction in the specific energy of adhesion, which in turn reduces the wear (refs. 196, 368, 369, 376). The decrease in the specific energy of adhesion as a result of the presence of oil can be expressed with the formula (ref. 377): (5.11)
where Wps, Wpl and Wsl are the specific energies of adhesion at the polymer-steel, polymer-liquid and liquid-steel interfaces respectively: WPs,l is the specific energy of adhesion at the polymer-steel interface in the presence of liquid, and is the surface tension of the liquid.
184
s
I
I
I
5
6
Tnp,l
1
I
7 Nltnm'
1
8
c
F i g . 5.19. R a d i a l wear i n t e n s i t y o f p o l y m e r i c b e a r i n g bush o f m i n i a t u r e l u b r i c a t e d s t e e l - p o l y mer j o u r n a l b e a r i n g v s . bo nd i ng f o r c e p e r u n i t w, f i l m (eqn. (4 . 5 ) , c o n t a c t are a t h i c k n e s s H = 1 Jim). Be ari n g h o l e iameter 2.15 mm, e x t e r n a l bush di a met er 6 mm, l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1.5%, l u b r i c a t i o n w i t h mine r a l MWP, s i l i c o n e m i n e r a l OKB 122-16 and t r a d i t i o n a l c l o c k o i l XU 430 ( p o i n t s ) . S l i d i n g speed 0.067 m / s , c o n t a c t p r e s s u r e 7 MPa. 1 - POM h, 2 - PA 6.
zmp,l
The specific energy of adhesion at the interface is given by DuprB's
formula: (5.12)
where 21, 82 and are the surface free energies of bodies 1 and 2 and the specific surface tension at the interface respectively. Since from ref. (109)
185
-1
Y12
-1
(7l2 - 122l 2
(5.13)
W12 can be expressed as follows : 1
w12 "2(61
w2
(5.14)
Taking into consideration eqn. (5.14), eqn. (5.11) can then be rewritten in the form: (5.15)
The relationship between the decrease in the radial wear intensity and decrease in the specific energy of adhesion because of the introduction of oil between the rubbing surfaces in a steel-polymer miniature journal bearing is presented in Fig. 5.20 (ref. 196). W e n the surface tension of the oil approaches the value of the surface free energy of the polymer (for POM c used in the author's studies this was 37.6 mJ/m2 (ref. 206) ) the effect of the lubrication becomes less and less. This is probably caused by a decrease in the wettability of the polymer surface by the lubricant (ref. 369). The wear of the systems under discussion depends on the durability of the oil film adsorbed on the polymer surface, since the adhesive bond between the steel surface and the oil film is relatively strong. The wear rate of the polymer element is higher when there is direct contact between the steel and polymer surfaces. The portion of the surfaces cc on which actual steel-to-polymer contact takes place can be found using Bowden's well-known equation for mixed lubrication:
where f, fd and fl are the respective friction coefficients of non-hydrodynamically lubricated, unlubricated and hydrodynamically lubricated (i.e. the oil film separates the sliding surfaces) sliding contact. Once the values of f, fd and fl have been experimentally determined, OL can be computed from eqn. (5.16). The adhesional energy EaI1 acting over the interface in the lubricated journal steel-polymer bearing can be calculated (taking into consideration eqn.
186 (4.8))
using t h e formula: E
a,l
= d21
yi
+ (1
Taking i n t o c o n s i d e r a t i o n eqns. i n g t h e a d e q u a t e e x p r e s s i o n s f o r Wps
- a ) Wps,l]
a n d ( 5 . 1 5 ) a n d introduc-
(5.14)
a n d Wps,l i n t o e q n .
then g e t :
E
Y
1
E
11.0
-
lQ8-
10.6 10.4
-
10.2
-
10D
I
340
350
1
360
L
370
(5.17)
1
380
t
F i g . 5.20. R a d i a l wear i n t e n s i t y decrease vs. s p e c i f i c energy o f adhesion decrease a f t e r i n t r o d u c t i o n of o i l between r u b b i n g s u r f a c e s o f steel-POM c m i n i a t u r e j o u r n a l b e a r i n g . B e a r i n g h o l e d i a m e t e r 2.15 mm, e x t e r n a l b e a r i n g bush d i a m e t e r 6 mm, l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1.5%, s l i d i n g speed 0.067 m/s. c o n t a c t p r e s s u r e 3 MPa. 1 - m i n e r a l MWP o i l , 2 - s i l i c o n e - m i n e r a l OKB 122-16 o i l , 3 - t r a d i t i o n a l c l o c k o i l XU 430.
(5.17),
we
187
The relationship between the radial wear intensity and the adhesional energy acting over the interface for a lubricated miniature steel-polymer journal bearing is shown by Fig. 5.21 (ref.196).
E a w m c
0.6 -
.A
P, +-,
c
-d
01 'a5
I
0.6
I
-
0.7
F i g . 5.21. R a d i a l wear i n t e n s t y v s . adhesional energy a c t i n g o v e r i n t e r f a c e n l u b r i c a t e d m i n i a t u r e s t e e l - P A 6 j o u r n a l bear ng. B e a r i n g h o l e d i a m e t e r 2.15 rnm, e x t e r n a l bu h d i a m e t e r 6 mrn, l e n g t h 2 . 1 mm, r e l a t i v e c l e a r a n c e 1.5%, s l i d i n g speed 0.067 m / s , c o n t a c t p r e s s u r e s 3, 5, 7 MPa ( p o i n t s ) . L u b r i c a t i o n w i t h XU 430 t r a d i t i o n a l clock o i l .
It has been noted that the temperature in the friction area rises during the operation of lubricated miniature bearings (refs. 208, 361). The relationship between the radial wear intensity of the polymeric bearing bush and the temperature rise in the fric-
tion area in a lubricated miniature steel-polymer journal bearing
188
is presented in Fig. 5 . 2 2 (ref. 196). I t is worth pointing out the clear correlation between the wear and the enerqetical effects in the friction area.
Temperature rise K F i g . 5.22. R a d i a l wear i n t e n s i t y o f p o l y m e r i c b e a r i n g bush o f l u b r i c a t e d m i n i a t u r e steel-PA 6 j o u r n a l b e a r i n g v s . temperature r i s e i n f r i c t i o n a r e a . B e a r i n g h o l e d i a m e t e r 2.15 mm, ext e r n a l bush diameter 6 mm, l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1 . 5 % , s l i d i n g speed 0.067 m/s, c o n t a c t p r e s s u r e p € ( 3 , 7 >MPa, l u b r i c a t i o n w i t h MWP m i n e r a l o i 1 .
Polymers can only be effectively lubricated when the liquids (oils) used have different solubility parameters from the polymers. In the event that the solubility parameters are equal, the polymer is dissolved in the oil and the wear rate is high (refs. 5 3 , 1 0 7 ) . The solubility parameter & (in l o 3 q m)of instrument oils used to lubricate miniature systems can be estimated using the formula 6 ~ 6 . 7 &3 (where & is the dielectric constant of the oil) (ref. 2 0 5 ) . The solubility parameters of polymers can be found in Chapter 2.4. The polysiloxanes have a different solubility parameter ( 1 1 . 2 l o 3 from most polymers and are characterized by a relatively low surface tension that assures good wettability
49)
189 o f polymers. The problems o f t h e l u b r i c a t i o n o f p o l y m e r i c s y s t e m s w i l l b e d i s c u s s e d i n C h a p t e r 6 . 6 . S p e c i a l i n s t r u m e n t o i l s which c a n be used f o r t h e l u b r i c a t i o n o f m i n i a t u r e p o l y m e r i c s y s t e m s are described i n Chapter 3 . 2 . The wear o f l u b r i c a t e d m i n i a t u r e steel-polym.er j o u r n a l b e a r i n g s ( u s u a l l y l u b r i c a t e d w i t h one d r o p of o i l ) i s t h e e f f e c t of a d h e s i v e - c o h e s i v e i n t e r a c t i o n s on t h e i n t e r f a c e ( i f t h e t h e r m a l e f f e c t s can be n e g l e c t e d , i . e . t h e temperature rise i n t h e f r i c t i o n a r e a A T < 3 K ; see F i g . 5 . 2 2 ) . t e r i a l worn, V l ,
The volume o f t h e p o l y m e r i c ma-
c a n t h e r e f o r e be found u s i n g t h e f o l l o w i n g f o r (4.7) (ref. 196):
mula, s i m i l a r t o eqn.
(5.19) The l u b r i c a t e d wear c a n be d e t e r m i n e d f o r b e a r i n g s o p e r a t i n g under boundary o r mixed l u b r i c a t i o n . S i n c e , a s h a s a l r e a d y been mentioned, t h e h i g h e l a s t i c i t y o f t h e p o l y m e r i c b e a r i n g bush i s f a v o u r a b l e t o t h e hydrodynamic l u b r i c a t i o n e f f e c t a t r e l a t i v e l y low s l i d i n g s p e e d s , t h e s l i d i n g speed a t which t h e f r i c t i o n c o e f f i c i e n t i s l o w e s t , i . e . when t h e hydrodynamic e f f e c t b e g i n s t o occ u r , s h o u l d b e d e t e r m i n e d . The a u t h o r ' s s t u d i e s d e s c r i b e d elsewhere ( r e f . 360) have shown t h a t i n t h e c a s e o f m i n i a t u r e s t e e l - p o l y m e r j o u r n a l b e a r i n g s , t h e a n g u l a r s p e e d o f t h e j o u r n a l a t which t h e hydrodynamic l u b r i c a t i o n b e g i n s ( w ) c a n b e e x p r e s s e d by t h e f o r mula :
w
= CK
P
Y
hmin
(5.20)
fid where C and K a r e p a r a m e t e r s , p i s t h e c o n t a c t p r e s s u r e , y t h e rel a t i v e c l e a r a n c e , hmin t h e minimum t h i c k n e s s o f t h e o i l f i l m ,
,u t h e v i s c o s i t y ( d y n a m i c a l ) o f t h e o i l , and d t h e b e a r i n g h o l e d i ameter. Taking i n t o a c c o u n t t h e e l a s t i c i t y o f t h e s l i d i n g s u r f a c e , t h e p a r a m e t e r C c a n be d e t e r m i n e d by f i r s t o f a l l e s t i m a t i n g t h e c o e f f i c i e n t D i n t h e f o l l o w i n g way: D =
pd Erhmin
(5.21)
190
moduli and Poisson's ratios of contacting materials respectively). Knowing D , the parameter C can then be found:
c =
1
l + D
(5.22)
- m1
where ml equals 0 . 1 5 when 0.01GDDg 0 . 2 and 0 . 1 for D > 0 . 2 . Taking into consideration the outflow of oil from the bearing, the coefficient K was experimentally determined (for miniature steel-polymer journal bearings lubricated with instrument oil) as (5.23) K = b3 ~ p + 3 c where parameter b3 equals - 2 . 2 5 l o 5 and c2 can be expressed by the formula :
+
c3 = 2 7 p
7540
(5.24)
where p is contact pressure in MPa. The pressure drop a p (expressed in MPa in eqn. ( 5 . 2 3 ) ) between the gap where the thickness of the oil film is minimum and the gap where the distance between the journal and the bearing bush is maximum can be calculated using the following formula:
AP
=
rl(cos
el +
cos
e,)
-(
1
hmin
- s +l a )
(5.25)
where y1 is the surface tension of the oil, 0 1 and e2 are the contact angles of the oil on the materials used, s is the diametral clearance, and a the deformation of the polymeric bush under load; for hmin see eqn. (5.20). The minimum thicknes of the oil film hmin can be assumed to be (5.26)
where R,1 and R,2 are roughness height parameters (according to IS0 standard) of the steel and polymeric bearing bush surfaces respectively, The polymeric bearing bush deformation a can be calculated using eqn. (4.10). To determine the adhesional energy acting over the interface (EaI1 in eqn. ( 5 . 1 9 ) ) I the parameter CL in eqn. (5.19) needs to be known. oi can be determined by the analysis of the experimentally found Stribeck's curve and by use of eqn. ( 5 . 1 6 ) . Investigations into miniature steel-polymer journal bearings lubricated with typical instrument oils ( M W P mineral oil, OKB 1 2 2 - 1 6 silicone mineral oil and XU 4 3 0 traditional clock oil) have
191 shown that the friction coefficient f of the bearings in the range of non-hydrodynamic lubrication can be approximated using the following formula: f = fa(l
-
0.12 p
e
0*34
(5.27)
)
For p, p and d , see eqn. (5.20); p, ,u and d should be introduced in MPa, mPa.s and mm respectively; v is the sliding speed in m/s; for fd see eqns. (4.4) and (5.161. The fd should be calculated for every p and v value using eqn. (4.4). Since eqn. (4.4) was determined at vc0.5 m/s, f cannot be calculated from eqn. (5.27) when the sliding speed is higher than 0.5 m/s, because the fd value is unknown. The sliding speed is usually relatively low and eqn. (5.19) is valid at relatively low sliding speeds and contact pressures when thermal effects are negligible, so the parameter in eqn. (5.16) can be determined by use of eqn. (5.27). From eqn. (5.16) we have
- ff
-
1
fd 7 “( 1) + 1
(5.28)
1
The ratio of fd/fl was estimated experimentally and can be taken to be approximately al v + a2 -(a3 v + a,) + a14 P f, (1 - 0.12 p o . 3 P 4 Jd 5 a13 -U = (5.29) =1
where p , p and d are as in eqn. (5.20) and are in MPa, mPa-s and nun respectively, Vh is the sliding speed at which hydrodynamic lubrication begins (friction coefficient is minimum), v is the sliding speed, (vh and v are in m/s), and Xn is the outflow parameter for the oil, (other than XU 430); for K, see eqns. (5.20) and (5.23); for a l l a2, a3 and a4, see eqn. (4.4); parameters a13 and a14 for PA 6, PA 66, POM h and POM c are 0.07, 0.05, 0.03, 0.035 and 0.2, 0.35, 0 . 3 0 , 0.26 respectively. The ratio f/fl can be determined after estimating fd/fl because a s a result of eqn. (5.27) , - f-
- - fd
‘d
fl
(1
- 0.12 p
(5.30)
192 After determining fd/fl, the parameter &can be calculated from eqn. (5.28). The avalues determined were intoduced into eqn. (5.19) and the adhesional energy acting on the interface, Eall, I was determined (yo in eqn. (5.19) was calculated using eqn. (4.9)). After determining Ea,l the respective values of Ec,l (see eqns. (5.19) and (4.15)) were found on the basis of the experimentally determined radial wear rates and the calculated volumes of the worn polymeric material (estimated by use of eqn. (4.18)). The relationship between V1 and EcI1 is presented in Fig. 5.23 (ref.196). This relationship can be approximated by the following formula:
(5.31) for V1, EaI1 and EcI1 see eqn. (5.19); a15 and alG are parameters. The value of the parametera16= -1.25 in eqn. (5.31) is the same for all polymer+oil combinations, while the values of parameter a15 are as follows: POM c + XU 430 oil, 0.45 POM h + XU 430, 1.87 PA 6 + XU 430 , 2.34 POM c + MWP (or OKB 122-161, 1.11 POM h + MWP (or OKB 122-16), 3.0 and PA 6 + MWP (or OKB 122-16) , 3.95 The differences between the characteristic curves are probably due to the way in which the lubricant influences the wear process of each polymer. The nonpolar mineral MWP and silicone-mineral OKB instrument oils are fairly similar, while the traditional clock oil XU 430 contains polar additive (fatty oil); when the polymer is more polar and has higher surface free energy, the difference between lubrication with nonpolar oil is more pronounced. The resistance of the adsorbed oil film and also the interactions in the oil-polymer system (ref. 107, see also Chapter 6.5) also probably affect the differences in thecharacteristic curves V1 vs. EaI1/ECI1. It is clear from Fig. 5.22 that when the temperature rise A T in the friction area of lubricated steel-polymer bearings is high enough (nT>3 K), the thermal frictional energy produced should be taken into consideration in the wear analysis. Consideration of the relationship between thermal effects and wear in lubricated bearings gives similar to the tests on unlubricated steel-polymer miniature journal bearings (see Chapter 4.2.1); the relationship is plotted in Fig. 5.24 (ref. 196).
193
0
-0.08
0
0
a
0
0.06 b
0
0
0
A
0
D
8
0
B
m
0.04
0
L
'
>*
8 8
0
0
00
@Q
0.02
n A
¶
O
A rn
A 0
0 0
QO
.
D A
00 0
..
0
UP
1
0POMh +xu430 BPOMh 8POPlC DPOMC
0
I
1
1
1
20
40
60
80
1
100
1
I20
F i g . 5.23. Volume o f p o l y m e r i c m a t e r i a l worn (Va,]) vs. r a t i o of ad he si onal energy a c t i n g o ver t he i n t e r f a c e (E a , l ) t o cohesional energy ( E c , l ) f o r l u b r i c a t e d m i n i a t u r e steel-polymer j o u r n a l b e a r i n g . B e a r i n g h o l e diameter 2.15 mm, e x t e r n a l bush di a met er 6 m m , l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1.5%, s l i d i n g speed 0.067 m/s, c o n t a c t p r e s s u r e p = 3 MPa a t l u b r i c a t i o n w i t h MWP m i n e r a l o i l and OKB 122-16 s i l i c o n e -mi n era l oil and p E <3, 7 >MPa a t l u b r i c a t i o n w i t h XU 430 o i l .
1
140
*
194
(h (3.36,15) 3.0 0 PA6 A POMh A PDMc
7
D PA6
7
}tMWP
2.5
m POMh 2.0 E
x
A A DA 0
1.7
a
-
POMc PA6+25%GFJ
o PA6 7 PDMh / t X U 430 POMC a PA6+25% GFJ
A
A
m
A'%
d
3
-
m m
A A
1.0
0.5
0
1
I
10
20
1
30
I
40
F i g . 5.24. R a d i a l wear i n t e n s i t y I w 1 vs. a p p a r e n t s t o r e d f r i c t i o n a l energy d6nsi t y (ASFED, see Chapter 4.2.1) f o r l u b r i c a t e d m i n i a t u r e bearing. Bearing hole diameter 2 . 1 5 mm, e x t e r n a l bush d i a m e t e r 6 mm, l e n g t h 2.1 mm, r e l a t i v e c l e a r a n c e 1.5%, s l i d i n g speed v E < 0 . 0 6 7 , 0 . 1 8 2 > m/s , c o n t a c t p r e s s u r e p E < 3 , 7 > MPa.
I
50
c
195
This relationship can be approximated with the following formula: I = a17(et,l)a18 il w,l
(5.32)
*
where I W I 1 is the radial wear intensity in ,um/km, and etIl the apparent stored frictional energy density (ASFED, see Chapter 4.2.1) for lubricated bearings, measured in MJ/mm3, parameters a17 and a18 are 106.6 and - 1.4866 respectively. The relationship (5.32) can be used to predict wear in bearings similar to the lubricated miniature steel-polymer journal bearings analysed since eqn. (5.32) means that a20 a19 IW,l = et,l
(5.33)
where I w I l is the same as in eqn. (5.32), etll is the maximum density of the thermal energy stored in the polymeric material in mJ/mm3 (see eqn. (4.19)), and parameters a19 and a20 are 23.12 and 0.3273 respectively. The temperature rise in the friction area, which needs to be known in order to determine etll, can be estimated using a formula similar to that for unlubricated bearincjs (eqn. (4.21)). Since the friction coefficient for lubricated bearings can be determined from eqn. (5.27), by taking into consideration eqn. (4.4) the following formula can be used for estimating the temperature rise hT1 for lubricated bearings:
- (a3v+a4)
AT^ = 1150 601pv(alv+a2)p
(1-0.12 p
(5.34)
For p I vI p , d, A , k and 6olsee eqns. (4.4), (4.21) and (5.27); the values of the parameters a l l a2' a3 and a4 are given after eqn. (4.4). The coefficient J01 for lubricated bearings, as found in experiments on miniature steel-polymer journal bearings ((3 2.15 nun), can be taken for PA 6, PA 66, POM h and POM c bearings as 6,1 =0.06p'0'6 and for the bearings PA 6 + 25% glass fibre Sol = 0.08 p-0.5 (contact pressure p in MPa). By combining eqns. (4.19), (5.32) and (5.34), the radial wear intensity of lubricated miniature steel-polymer journal bearings similar to those investigated can be estimated. The radial wear rate of lubricated bearings as a function of the sliding distance can be predicted in a similar way as for unlubricated bearings (see Chapter 4.2.1). The value of Cr (eqns.
196 (4.22) and (4.23)) needed in eqn. (4.241, as estimated from experimental results, is 1.5 kJ for a bearing lubricated with MWP mineral oil or OKB 122-16 silicone-mineral oil and 2 kJ for bearings lubricated with X U 430 or XU 120 traditional clock oils. The tl parameter in eqn. (4.24) can be calculated on the basis that for lubricated bearings operating under contact pressure pE<3.7>MPa and sliding speed v < 0.2 m/s, tl = m2 v0.05 p-Om3 (v is in m/s , p in MPa) where m2 for bearings based on P A , POM and PA 6 + glass fibre and lubricated with MWP or OKB oils and XU 430 or XU 120 oils can be taken as 2.5, 2.4, 2.3 and 2.3, 2.2, 2.1 respectively. The values of t and u (eqn. (4.25)) for some lubricated steel-polymer miniature journal bearings are listed in Table 5.2 (ref. 196). TABLE 5.2 PARAMETERS t AND u (see eqn. (4.25) - w = t L U , where L AS DETERMINED E X P E R I M E N T I A L L Y FOR L U B R I C A T E D M I N I A T U R E B E A R I N G S . B E A R I N G HOLE D I A M E T E R 2 . 1 5 mm, EXTERNAL BUSH 2 . 1 mm, R E L A T I V E CLEARANCE 1.5-2%, S L I D I N G SPEED 0 . 0 6 7 3 MPa.
i s i n km and w i n ,urn) STEEL-POLYMER JOURNAL DIAMETER 6 mm, LENGTH m/s, CONTACT PRESSURE
The "adhesive-cohesive'' formula (5.31) can be applied to predicting the wear in lubricated steel-polymer miniature journal bearings (when bearing hole diameter is ca. 2 m m ) , when the thermal effects are small (temperature rise in the friction area bT1<3 K), and when the bearing bush is manufactured in unfilled polymer (with a determined cohesive energy density). For lubricated bearings operating at higher loads and sliding speeds (when the temperature rise AT1 in the friction area is 3 K < A T ~<20 K ) , and in which the bearing bush is made of unfilled or filled polymer, eqn. (5.321 should be used to predict the wear. Other comments concerning the practical use of the formulae presented here and in Chapter
197
4.2.1 for predicting wear in miniature steel-polymer journal bearings can be found in Chapter 4.2.1. 5.2.2.
POLYMER-POLYMER SYSTEMS
The lubrication of polymer-polymer systems, like the lubrication of metal-polymer systems, is effective despite the fact that the adsorption of the lubricant molecules on the polymeric surface is relatively poor. The solubilitv aarameter of the oil should be different from either polymer; the wetting ability of oil on the surfaces is also important. The lubrication of a miniature bearing made of POM (journal) and PBTP + glass fibre (or microbeads) with fluorinated synthetic instrument oil reduces the friction coefficient from 0.3-0.4 to 0.07-0.20 and greatly reduces wear (ref.189). The wear in lubricated polymer-polymer miniature systems has been investigated (ref. 108) using a pendulum system similar to the ASTM pendulum (see Chapter 8.2, Fig. 8.4) but with a rotating sphere to obtain pivoting friction at high load (30 N; rotational speed 550 rotations per minute for 10 days). Combinations of PA 66, PC and POM c lubricated with two special instrument oils were tested. These oils (products of Dr. Tillwich GmbH, Horb-Ahldorf, F.R.G.) were KunstoffBl K 7131, based on traditional clock oil but with a higher lubricity and chemical stability (viscosity 131mn2/s at 2OoC, surface tension 31.1 mN/m), and Silber K 7132 lv, a partially fluorinated synthetic oil on a polyether base with high chemical stability (viscosity 310 m 2 / s at 2OoC, surface tension 23.6 mN/m). The friction tests were carried out using the pendulum system at 0, 25 and 6OoC, and the wear tests using the pivoting friction system were carried out at ambient temperature. The results of the friction tests show a marked decrease in the friction coefficient because of lubrication. The frictioncoefficient in PA 66-PA 66 or PC-PC systems, which is 0.44 without lubrication, decreases after lubrication with K 7131 oil to 0.03 and 0.07 respectively, and to 0.02 and 0.024 after lubrication with K 7132 oil. The decrease of the friction coefficient in POM c - w c or POM c-polymer systems was smaller. This is probably because of the better adsorption of the oil molecules on the polar (PA 66) surface which has a higher surface free energy than the nonpolar POM c material. Increase in ambient temperature increases the friction coefficient. This is characteristic of POM c-POM c or PC-PC systems in particular, probably as a result of lower oil film re-
198
sistance on the POM c and PC surfaces. Increase in temperature from 0 to 6OoC increases the friction coefficient from 0.06 to 0 . 1 3 in POM c-POM c systems lubricated with K 7 1 3 2 o i l , or from 0.06 to 0.15 when they are lubricated with K 7 1 3 1 oil.The smallest friction variation as the effect of the temperature increasing was observed for POM c-PA 6 6 and POM c-PC systems. Exchanging the sphere and prism materials can sometimes cause a big change in the friction coefficient; for example, the friction coefficient for a lubricated PA 6 6 (sphere)-POM c system is 0 . 0 6 - 0 . 0 8 while for a POM c-PA 6 6 system it is only 0.03-0.04 (at 25OC). The smallest change as a result of reversinq the rubbing elements material was in the POP4 c-PC system lubricated with K 7 1 3 2 synthetic oil. The wear tests carried out using the pivoting friction system show that the least wear (negligible) occured in PA 66-PA 6 6 , PA 66 (sphere)-POM c, and PC-POM c systems lubricated with K 7131 oil and PA 66-PA 6 6 and PA 66-POM c systems lubricated with K 7 1 3 2 oil (ref. 108). The greatest wear was noted in POM c-POM c systems, the wear in the system lubricated with K 7 1 3 1 oil being about 3 times higher than the wear in the system lubricated with K 7 1 3 2 oil. There was also high wear in PA 66-PC and POM c-PC systems lubricated with K 7 1 3 1 oil and in PA 66-PC, PC-PC, PC-PA 6 6 and PC-POM c systems lubricated with K 7 1 3 2 synthetic oil. Reversing the materials in the PC (sphere)-POM c system lubricated with K 7 1 3 1 oil led to a very high increase in wear. Additional tests on the polymer and oil properties after the tribological tests showed some marked changes. There was a great decrease in the hardness of the polymers. The oils showed changes in their viscosity (e.g. an increase of about 20% after the pivoting friction test in the case of a POM c (sphere)-PA 6 6 system lubricated with K 7 1 3 1 oil), surface tension (decreasing) and refractive index (increasing for K 7 1 3 1 and decreasing for K 7 1 3 2 synthetic oil). These complex tribophysical and tribochemical effects occured in the polymer-oil systems during friction and wear (see also Chapter 6 . 6 ) . In the case of the polymer-polymer, sphere-plate systems investigated using the ASTM pendulum or UTI apparatus (see Chapter 8.2) lubrication generally improved their frictional behaviour and, in particular, their wear behaviour (refs. 4 3 , 44, 169, 1 7 1 ) . The friction coefficient reduction because of lubrication was noticeably higher in PA-PA, PETP-PETP and PPO-PPO systems than in POM-POM systems (ref. 1 6 9 ) which is probably because PA, PBTP and
199
PPO have better wettability than POM. The friction coefficient of polymer-POM h systems lubricated with polyglycol instrument oil containing alcohol and ether groups (Moebius Synta-A-Lube) was 0.15-0.20 for the POM h-POM h combination and 0.04-0.06 for other combinations (the polymers used were PC, PA aromatic, PETP and PPg) (ref. 169). The effect of lubrication with the aforementioned oil (Moebius 9 0 1 5 ) on the friction coefficient of various polymer-polymer systems tested with the ASTM pendulum is shown in Fig. 5 . 2 5 (based on data from refs. 43, 1 6 9 , 1 7 1 ) .
+J c
t
0.15
1 9
F i g . 5.25. F r i c t i o n c o e f f i c i e n t o f l u b r i c a t e d polymer-polymer systems t e s t e d u s i n g ASTM pendulum. L u b r i c a t i o n w i t h Moebius 9015 i n s t r u m e n t o i l . 1 - POM h-POMh, 2 - PC-POM h, 3 - P A aromatic-POM h, 4 PPO-POM h, 5 - PC-PC, 6 PA 11-PA 11, 7 PPO-PPO, 8 - PDM C-POMc. 9 - PETP-POM h.
-
-
-
The friction coefficient of POM h-POM h systems when the POM h used was Delrin 8 0 2 0 instead of the more usual Delrin 5 0 0 NC 1 0 (both Du Pont products), decreased from 0.16 when lubricated with Moebius 9 0 1 5 to 0.10 when lubricated with Moebius 9 0 2 4 oil, and then to 0 . 0 6 when lubricated with Silber K 7132 mv fluorinated oil
200
on a polyether base (ref. 44). The 9 0 1 5 and 9024 oils are similar in composition but differ in viscosity (117 and 2 6 6 m2/s respectively, at 2 0 O C ) . The higher the viscosity of the oil, the greater the decrease in the friction coefficient (and also wear)(refs.44, 169).
When the effect of various kinds of oils on the tribological behaviour of miniature polymer-polymer systems is compared, fluorinated polyether oils are found to be more effective than polyglycol oils containing alcohol and ether groups or mineral oils (refs. 44, 1 6 9 ) . The effect of various oils on the wear in a POM c-POM c system tested with the ASTM pendulum is presented in Fig. 5 . 2 6 (data drawn from ref. 1 7 1 ) .
3c
m
E E
2G
m I
s-
L (I
9
10
0
Fig. 5.26. Effect o f lubrication on wear in
POM c-POM c system (tested with ASTM pendu-
-
-
lum). Load 6.9 N . 1 unlubricated, 2,3 oils based on polyglycol containing alcohol a n d ether groups (Moebius 9015 (2) and 9024 (3)), 4 partially fluorinated polyether oil, Silber K 7132 mv.
-
The lubrication with an ester-based oil of polymer-polymer systems when one of the elements is manufactured in ABS is antiproductive
201
(ref. 58). A hemisphere made of LDPE with a radius of 5 mm rubbing against a LDPE plate 5 mm thick was lubricated with 1 5 different liquids whose surface tension ranged from 18.4 mN/m (for n-hexadecane) to 72.8 mN/m (water); the friction coefficient depended on the liquid used (ref. 375). The friction coefficient was reduced from 0.8 (unlubricated sliding) to 0.18 (lubricated with n-hexodecane). Generally, increasing the wettability of the surfaces leads to a reduction in the friction coefficient: this can be done either by using a liquid with a lower surface tension or by increasing the surface free energy of the polyethylene (activating the surface by treating it with fuming sulphuric acid for 5 minutes, which increases the polar component of the surface free energy). Oleylamine and some other long-chain boundary lubricants used give very low friction coefficients (as low as 0.06) on LDPE, forming a strongly adsorbed layer on the high energy polar surface. The frictionccefficient for unlubricated systems decreases from 0.8 at 2OoC to 0.2 at 80°C whereas in untreated LDPE systems lubricated with oleylamine the friction coefficient shows a marked increase at 40-50°C, presumably due to disorientation of the adsorbed lubricant layer. Oleylamine on treated LDPE produces only a slight increase in friction, at 60-70°C. The effect of better adsorption and the resulting decrease in friction between polymer-polymer surfaces has also been observed in two-catheter systems (ref. 379). The wettability of P S I LDPE, PVC, F E P , PS and SR surfaces was improved by radio frequency glow discharge in a helium environment (which increased of the surface free energy). The treated SR gives a lower friction coefficient than untreated SR when dragged across all untreated and treated polymer surfaces, whether the medium is distilled water or an isotonic saline solution. Friction between all the surfaces investigated is considerably lower in a blood plasma medium, probably because of the presence of adsorbed proteins at the polymer interfaces, The lubrication of PVC-PVC and PA 66-PVC sliding systems with water reduced the wear rate, preventing catastrophic wear even when the loads were much higher than those which would have caused catastrophic wear in dry conditions (ref. 231). This effect is the result of water cooling the rubbing surfaces. When the solubility parameters of water (6= 48.3 lo3'/=) and the polymers used are different, the use of water as a cooling medium can improve
202 the tribological behaviour of highly loaded polymer-polymer systems. Polymer-polymer systems can also be lubricated by oil migrating from the polymer matrix during friction (see Chapter 5.2.1). In such internally lubricated systems the oil creep from the system is smaller than in externally lubricated systems. The presence of silicone oil (dispersed in polymer material ca. 2wt%) reduces the friction coefficient and wear and eliminates stick-slip effects (refs. 227, 228). The wear in a PC (moving element)-PA 6 6 system was catastrophic when load was applied (thrust washer test, contact pressure 0.28 MPa, sliding speed 9 . 2 5 m/s) but very small (particularly for the PA 66 element) in a system made of PC-PA 66 + 13% PTFE (by weight) + 2% silicone oil, and the static friction coefficient decreased (as a result of the addition of PTFE + silicone oil) from 0.25 to 0.04 (kinetic friction coefficients were the same, 0.04). Reversing the materials in the system resulted in a considerable reduction in the wear but an increase in the static and kinetic friction coefficients to 0.06. When a PA 66 + 2% silicone oil-PC system was investigated, the wear of the PA 66 element was slightly higher than when the polymer was filled with both PTFE and silicone oil, and the wear of the PC element was considerably higher at friction coefficients of 0.03. The synergistic effect of PTFE and silicone oil was better observed in reinforced polymer systems (refs. 2 2 7 , 228). The wear of the moving element in a PA 66 + 2% silicone oil (moving element)-PC + 30% glass fibre (by weight) + 15% PTFE system was catastrophic at the static and kinetic friction coefficients 0.10 and 0.14 respectively. The addition of 13% PTPE to the polyamide element considerably reduced wear and also reduced both friction coefficients to 0.06. A lesser synergistic effect of PTFE and silicone oil was observed in the combination PA 66 + PTFE + silicone oil-PA 66 + 30% glass fibre. The presence of silicone oil in the PA 66 + PTFE material (or only silicone) as compared to the straightforward PA 66 + PTFE material led to a significant deterioration in the tribological behaviour of a PA 66 + PTFE-PA 66 + glass fibre system (this system demonstrated the best wear behaviour of the systems tested). When 1 5 % PTFE was also added to PA 6 6 + 30% glass fibre material, the wear behaviour of the systems formed by mating with the above polyamide materials containing silicone oil was markedly improved. For polymer containing carbon fibre-PA 66 + 30% carbon fibre + 1 5 % PTFE + 2 % silicone oil
203
systems, the wear in e.g. a PA 6 + 30% carbon fibre + 13% PTFE+ 2% silicone oil-PA 6 6 + 30% carbon fibre + 2 % silicone oil system was
2 times less than in a PA 6 6
+
30% carbon fibre-PA 6 6
+
30% carbon
fibre + 13% PTFE + 2 % silicone oil system, while the frictioncoefficients were about the same (ca. 9.11 and 0.15, static and kinetic respectively). The use of PC instead of PA 6 6 gave no advantages. From the above analysis, it is clear that the addition of silicone oil to thermoplastic material reduces wear when compared to the neat material, but the wear is still greater than PTFE-lubricated materials. The simultaneous addition of PTFE and silicone oil can have a positive synergistic effect, depending on the combination of materials. The problems of the lubrication of polymeric systems will be discussed in Chapter 6.6. For the lubrication of miniature polymer-polymer systems, special instrument oils can be used (see Chapter 3.2 and ref. 7 7 )
.
5,3,
OTHER SYSTEMS
Of the non-metallic and non-polymeric materials, minerals are often used as bearing materials to manufacture the bearing bushes (jewels) used in lubricated steel-mineral (ruby or sapphire) journal bearings. The journal in such bearings is usually made ofhardened free-cutting or stainless steel, and after roller-burnishing its surface roughness Ra <0.2 ,urn. The lubrication of steel-mineral systems reduces the friction coefficient and wear. Lubrication with traditional clock oil of a steel-mineral system, tested using the four-ball ( @ 3 mm) friction machine, reduced the wear of the steel over 100-fold as compared to wear in an unlubricated system. The effect of lubrication is especially noticeable when brittle materials are being used (ref. 95). The wear increases proportionally to the applied load. The ceramic materials investigated (ref. 95) were: sapphire, ruby, fused quartz, agate, diaspore and glass. The friction coefficient in lubricated steel-ruby or steel-sapphire miniature journal bearings operating under boundary lubrication conditions is usually 0.1-0.2 (refs. 1 , 360-3831. The friction coefficient decreases slightly as a function of applied load and demonstrates a minimum. The minimum appears when the realistic pressure is in the range of 340-540 MPa (ref. 9 5 ) . At very low sliding speeds (below 6 mm/s for a journal diameter of 0.11 in the ruby jewel bearinq of a balance the friction coefficient parabolically increases as the sliding speed increases (refs. 380-362).
204
Upon i n c r e a s i n g t h e b e a r i n g c l e a r a n c e i n t h e same j e w e l b e a r i n g , t h e f r i c t i o n c o e f f i c i e n t i n c r e a s e s from 0 . 1 0
y
= 4 . 5 % ) t o 0.13
(at
y
( a t r e l a t i v e clearance
= 2 5 . 5 % ) . The e f f e c t o f t h e c l e a r a n c e on
t h e f r i c t i o n c o e f f i c i e n t w a s b e s t o b s e r v e d however i n a m i c r o b e a r i n g w i t h a j o u r n a l made of WC 1 0 r u b b i n g a g a i n s t a s a p p h i r e j e w e l b e a r i n g ( r e f . 9 5 ) . The e f f e c t o f t h e s u r f a c e r o u g h n e s s on t h e f r i c t i o n c o e f f i c i e n t i s r e d u c e d when o i l w i t h good l u b r i c i t y i s used. The optimum r o u g h n e s s o f t h e j o u r n a l s u r f a c e ( a n d t h e j e w e l b e a r i n g s u r f a c e ) i s d i s c u s s e d and g i v e n i n C h a p t e r 4 . 3 . An example o f t h e S t r i b e c k c h a r a c t e r i s t i c curve for a m i n i a t u r e s t e e l - m i n e r a l (sapphire) journal bearing i s presented i n Fig. 5.27.
F i g . 5.27. S t r i b e c k c u r v e f o r m i n i a t u r e s t e e l - s a p p h i r e j o u r n a l b e a r i n g . B e a r i n g hole d i a m e t e r 0.9 mrn, l e n g t h 1.3 mrn, c l e a r a n c e 0.01 mm, cont a c t p r e s s u r e 0.0525 MPa, l u b r i c a t i o n w i t h c l o c k o i l o f v i s c o s i t y 45 m P a * s ( r e f . 383).
205 The o i l u s e d h a s a p r o m i n e n t e f f e c t on t h e t r i b o l o g i c a l b e h a v i o u r
of s t e e l - m i n e r a l b e a r i n g s . When f o u r o i l s
-
t r a d i t i o n a l clock o i l
(Moebius 8 0 0 0 ) , n o n p o l a r m i n e r a l o i l ( P a r a f f i n e PLS)
,
film-forming
s i l i c o n e + m i n e r a l o i l (Miracle-Plastic) , a n d n o n p o l a r f l u o r i n a t e d
-
were u s e d t o l u b r i c a t e a s t e e l - r u b y jewel b a l a n c e b e a r i n g , t h e h i g h e s t f r i c t i o n c o e f f i c i e n t ( 0 . 1 4 - 0 . 1 5 ) was f o u n d a t l u b r i c a t i o n w i t h p o l y e t h e r o i l and t h e l o w e s t when u s i n g t h e Moebius polyether
8000 and M i r a c l e - P l a s t i c
o i l s , which h a v e p o l a r m o l e c u l e s a d s o r b -
i n g s t r o n g l y on t h e r u b b i n g s u r f a c e s ( r e f . 3 8 0 ) . The f r i c t i o n c o e f f i c i e n t o f s t e e l - r u b y m i n i a t u r e j o u r n a l b e a r i n g s i s lower a t h i g h l o a d s when a n o i l s u c h a s p o l y g l y c o l c o n t a i n i n g a l c o h o l a n d e t h e r g r o u p s (Synta-A-Lube
Moebius) h a s MoS2 a d d e d ( r e f . 1 ) . An i n c r e a s e
i n t h e a m b i e n t t e m p e r a t u r e from 2 0 t o 3 7 O C r e s u l t e d i n a d e c r e a s e o f between 5 a n d 2 0 % i n t h e f r i c t i o n c o e f f i c i e n t o f a s t e e l - r u b y b a l a n c e j e w e l b e a r i n g when i t w a s l u b r i c a t e d w i t h F o m b l i n , E s s o
2OW50, Synta-A-Lube
and Synta-Visco-Lube
w i t h and w i t h o u t additives,
and Moebius 8000, and a n i n c r e a s e o f a b o u t 4 % i n t h e f r i c t i o n c c e f f i c i e n t when l u b r i c a t e d w i t h t h e n o n p o l a r m i n e r a l o i l PLS ( r e f . 3 8 1 ) . The f r i c t i o n c o e f f i c i e n t w a s l o w when Moebius 8000, Synta-A-Lube and Synta-Visco-Lube
w i t h a d d i t i v e s were u s e d f o r l u b r i c a -
t i o n ( 0 . 1 3 a t 2OoC and 0 . 1 1 a t 3 7 O C ) and r e l a t i v e l y h i g h when other o i l s were u s e d . The h i g h e s t f r i c t i o n c o e f f i c i e n t ( o v e r 0 . 2 0 ) was found w i t h l u b r i c a t i o n by n o n p o l a r m i n e r a l o i l a t an a m b i e n t t e m p e r a t u r e o f 37OC.
When t h e v i s c o s i t y o f t h e o i l i n c r e a s e s , t h e
f r i c t i o n c o e f f i c i e n t o f t h e boundary l u b r i c a t e d s t e e l - r u b y m i n i a t u r e j o u r n a l b e a r i n g s d e c r e a s e s ( r e f . 3 8 2 ) . When t h e s l i d i n g s p e e d i s h i g h , hydrodynamic e f f e c t s c a n c a u s e a d i s t i n c t i n c r e a s e i n t h e
f r i c t i o n c o e f f i c i e n t ( t o over 0.4) but t h e f r i c t i o n c o e f f i c i e n t c a n b e e a s i l y d e c r e a s e d by s i m p l y h e a t i n g t h e o i l t o d e c r e a s e i t s v i s c o s i t y . The f r i c t i o n c o e f f i c i e n t i s a l s o a f u n c t i o n o f t h e humidity of the a i r . A t l o w s l i d i n g speeds it increased s i g n i f i c a n t l y a s t h e s l i d i n g s p e e d d e c r e a s e d , when t h e s t e e l - r u b y m i n i a t u r e j o u r n a l b e a r i n g s w e r e l u b r i c a t e d w i t h Synta-A-Lube
o i l containing
1%p a l m i t i c a c i d a n d o p e r a t i n g i n a i r a t 1 0 0 % r e l a t i v e h u m i d i t y .
I n t h e same o i l b u t c o n t a i n i n g o t h e r a d d i t i v e s , t h e e f f e c t o f hum i d i t y on t h e f r i c t i o n b e h a v i o u r o f l u b r i c a t e d b e a r i n g s w a s much
l e s s (see F i g . 6 . 1 8 ) . The l i f e o f s t e e l - m i n e r a l m i n i a t u r e b e a r i n g s d e p e n d s m a i n l y on t h e a g e i n g r e s i s t a n c e o f t h e o i l u s e d i n them; b e c a u s e o f t h e g r e a t o p e r a t i o n a l p r e s s u r e , t h e o i l a g e i n g c a n b e r a p i d and t h e p r e s e n c e i n t h e f r i c t i o n a r e a of t h i s t h i c k , h i g h l y v i s c o u s s u b s t a n c e (aged
206 oil and wear products) can lead to the journal wedging. The possibility of this happening can be reduced by for instance coating the journal surface with a polymer, which reduces the pressure on the contact surfaces underneath (ref. 323). The friction torque in lubricated steel-mineral microbearings, e.g. those used in electric counters or gyroscopes, is less when external vibrations are more intense than those in the same unlubricated bearings (see Chapter 4 . 3 ) . Pendulum friction tests carried out on steel-ruby (refs. 296, 300) and steel-sapphire (ref. 106) lubricated systems have shown that their frictional behaviour depends significantly on the lubricant used. The friction coefficients in a steel-sapphire (sphere-plate) system, lubricated with the oils listed in Table 5.1 and tested with a 2-ball pendulum, are shown in Fig. 5.28 (data are based on information from ref. 106). Lubrication with fluorinated propylmethylpolysiloxane is the most effective; lubrication with diester oil gives the highest friction coefficient (over 0.2). Decreasing the load raises the friction coefficient when lubrication is supplied by, for example, polyglycol containing alcohol and ether groups with chlorinated phenylmethylpolysiloxane, and reduces it when lubrication is supplied by a mineral oil. In mineral oil, ageing leads to the formation of polar compounds, which reduces the friction coefficient. In the case of fluorinated polyether oil, the friction coefficient was found to increase markedly. Increase in the viscosity of fluorinated polysiloxane reduces the friction coefficient. The friction coefficients in a steel-ruby system tested using an ASTM pendulum were: 0.4 when unlubricated; 0 . 3 6 when lubricated with dimethylpolysiloxane; 0.15-0.25 when lubricated with phenylmethylpolysiloxane; and between 0.08 and 0.12 when lubricated with fluorinated propylmethylpolysiloxane, traditional clock oil (which gave the lowest friction coefficient), polyglycol containing alcohol and ether groups, paraffinum liquidum iester, and polyether oils (ref. 296). Dimethylpolysiloxane was used at viscosities of between 20 and 100000 mm2/s (at 25OC) and phenylmethylpolysiloxane at viscosities of between 20 and 1000 mm2/s (at 25OC) , but there was no noticeable effect of the viscosity on the friction coefficient (ref. 300).
20 7
0.10
0.05
I 2
1 I I 1
1 l I 1
1 1 l 1 I I 1 1 1 1 1 1 I l l 1 1 1 1 1 1 1 1 1 I I 1 1 1 I I 1 1 1 I I
1P 1 1 1 1
In
\ (\r
E E 00
00 t 3m
52
~ I I I I I
I1
1
I I
1
1
1
1
1
F i g . 5.28. F r i c t i o n c o e f f i c i e n t s i n l u b r i c a t e d s t e e l - s a p p h i r e system t e s t e d w i t h a 2 - b a l l pendulum. Numbers o f o i l s used a r e l i s t e d i n T a b l e 5 . 1 . C i r c l e s a r e f o r l o a d 3 N, squares 2 N, d o t t e d l i n e s f o r l u b r i c a t i o n w i t h aged o i l s ( f o r c o n d i t i o n s o f a g e i n g see c a p t i o n t o F i g . 5.3). F r i c t i o n c o e f f i c i e n t s a r e a l s o given f o r t h e system l u b r i c a t e d w i t h f l u o r i n a t e d s i l i c o n e o i l o f v a r i o u s v i s c o s i t i e s ( a t 20OC).
The lubrication of ceramic materials reduces adhesion and lessens the possibility of brittle fracture during sliding (ref.257). In an iron-sapphire system, dodecane reduces the friction coefficient from 1 to 0.2,dodecane + 0.5% (by weight) stearic acid reduces it to 0.15 and ester (isopalmitate of trimethylol propane) to 0.22 (ref. 258). For a titanium-sapphire system, the friction coefficients for the same lubricants were 0.9, 0.16-0.7 and 0.25-0.4 respectively. Details of this system can be found in Chapter 4.3. The friction coefficient in an iron-sapphire system
208 lubricated with dodecane + stearic acid and with ester was constant as a function of the sliding distance but when lubricated with dodecane alone, the friction coefficient increased slightly. In the case of a titanium-sapphire system, where the adhesion on the interface is higher, the lubricant does not reduce effectively the adhesion and transfer of the material to the sapphire surface, but in the presence of the molecules of stearic acid, the strong adhesion on the titanium-transferred titanium interface is hindered and the friction coefficient decreases to as low as 3.16. A s h ilar effect can be noted when ester is used. Lubrication with SAE 40 oil of a metal-alumino-silicates ceramic (Gabbro Clay) system resulted in a large friction coefficient reduction (ref. 260). For lubricated sliding against steel, gray cast iron, brass, aluminium and nickel the friction coefficients were 0.05, 0.04, 0.04, 0.03 and 0.05 respectively (reduced from 0.3 when unlubricated). When the applied load was increased the friction coefficients also increased. When silicon nitride (Si3N4), lubricated with mineral or ester base oil, slides against steel or against itself (see Chapter 4.3 for sliding conditions), the friction coefficients are 0.11 and 0.13-0.12 respectively (0.36 and 0.24 respectively when lubricated with silicone oil) (ref. 28). The reduction of the friction coefficients obtained was very small (see Chapter 4.3). As a result of lubrication with silicone oil, the friction coefficient increased from 0.15 and 0.17 respectively (at unlubricated sliding) to the above given values. The friction coefficients upon lubrication with mineral or ester oil are similar to those of steel-steel systems. With an increase in temperature from 25 to 250°C, the friction coefficient increased from 0.11 and 0.13-0.12 to 0.13 and 0.14 for steel-Si3N4 and Si3N4-Si2N4 systems respectively. The friction coefficients of steel-Sic, steel-WC, Sic-Sic, WC-WC and Al203-Al203 systems lubricated with mineral or ester oil were found to be 0.13, 0.13-0.14, 0.14-0.18, 0.18 and 0.17, whereas with silicone oil lubrication they were 0.42, 0.39, 0.23, 0.43 and not estimated, respectively. Lubrication of silicon nitride rolling-spinning contacts with graphite can give a wear coefficient an order of magnitude lower than oil boundary-lubricated steel (ref. 28) When Armco iron, mild steel, copper and titanium were slid against bonded silicon carbide abrasive paper in air at 100% relative humidity, the friction coefficient and wear were lower than when iron and mild steel were used in dry air (ref. 384). The wear
.
209 of the copper was slightly higher but its friction coefficient lower: the reverse was true of titanium. When iron or mild steel are used, the friction coefficient decreases with increasing humidity, while copper and titanium exhibit a maximum and a minimum respectively in the middle humidity range. The wear of hot-pressed Si3N4 sliding on itself in gases saturated with water vapour is minimum at 98% relative humidity (the friction coefficient isabout 0.7)(ref. 264). In such conditions a tribochemical reaction produces an amorphous substance, probably a hydrated silicon oxide, that increases the adhesion between wear particles and forms a layer on the surface of the material. It protects the material from further wear. The increase in wear of A1203 caused by water vapour (ref. 335) is probably due to the absence of a protective layer, since A1203 is stable with respect to oxidation; A1203 is subject to stress corrosion cracking, which is increased by water, so the presence of water vapour facilitates the fracture of the material (ref. 264), Alumina ceramics, particularly single crystal materials like sapphire, have demonstrated very good wear behaviour in saline lubricated hip-joints in which the head is made of alumina and the socket of UHMWPE (ref. 267). When the cartilage rubs against glass, with water (or water + 3% oleic acid) lubrication, increasing the contact pressure from 0.1 MPa to 3 MPa (at a sliding speed of 0.1 m/s) reduces the friction coefficient from 0.4 to 0.2, or in the case of lubrication with mineral paraffin oil (or paraffin oil + 3% oleic acid) from 0.05 to 0.02 (with a minimum at a contactpressure of around 2.5 MPa) (ref. 71). Lubrication with synovial fluid gives a friction coefficient in the range 0,006-0.015 (the minimum being at about 2 MPa contact pressure). This effect is caused by some mechanism other than the formation of the boundary layer on the rubbing surfaces. Graphite and carbon-graphite materials exhibit very good tribological behaviour in lubricated systems. Graphite or hard carbon elements can operate in corrosive liquids and non-corrosive aqueous solutions, and hard carbon elements can also operate in non-corrosive organic solvents (ref. 37). The friction coefficient in graphite or carbon-graphite-steel bearings (with a steel journal) is in the range 0.05-0.10 with mixed lubrication and 0.01-0.05 with hydrodynamic lubrication (ref. 35). The highest wear occurred when hard carbon EK 10 (Ringsdorf-Werke, F.R.G.) was used and the lowest when the polymeric resin bonded carbon materials V 1017 or
210 V 1024 were used. Hard carbon materials impregnated with such alloys as Sn-Pb or Sn-Pb-Sb and rubbing against steel with boundary lubrication by Freol oil (contaxing Freon (Du Pont) - used in some coolinq systems) demonstrate very good frictional and corrosive behaviour (frictiogi coefficient below 0.10) (ref. 272) Stainless steel-graphite systems operating in electrolytes can be effectively protected against high corrosive wear by passivation, if acid, non-oxidative media are used (ref. 3 6 6 ) . In graphite-graphite systems, the mere presence of lubricant vapours greatly irniiroves their tribological behaviour (ref. 271). Effective lubrication of steel-diamond s y s tems operating in a vacuum (2.6 Pa) has been obtained using the traditional clock oil XU 430 containing dispersed PTPE particles (reT. 17) The lubrication of ceramics with oils which have good lubricity and ageing resistance and which can be applied at high contact pressures has a very positive effect on their tribological behaviour. Because of the high surface free energy of ceramics,the use of anti-migration coatings (epilames) is essential to prevent oil creep (see Chapter 6.2). Graphite or carbon-graphite materials are particularly suitable when the rubbing elements will be coming into contact with water or aggressive media.
.
211
6, LUBR I CAT I ON PROBLEMS INTRODUCTION The l u b r i c a t i o n o f f i n e mechanisms d i € f e r s from t h e l u b r i c a t i o n o f machines b e c a u s e t h e y a r e u s u a l l y l u b r i c a t e d f o r l i f e d u r i n g t h e assembly p r o c e s s . A s m a l l amount o f o i l (one d r o p ) must e n s u r e e f f i c i e n t l u b r i c a t i o n o v e r a l o n g p e r i o d of t i n e . The amount o f o i l i s u s u a l l y 0.5-100
mg. Watch b e a r i n g s need o n l y 0 . 0 1 mg of o i l f o r sev-
e r a l years of operation. The r e q u i r e m e n t s f o r i n s t r u m e n t o i l s h a v e b e e n d i s c u s s e d i n C h a p t e r 3 . 1 . Here w e w i l l a n a l y s e some o f t h e most i m p o r t a n t probl e m s c o n c e r n i n g t h e i n t e r a c t i o n s betweeii o i l s and r u b b i n g e l e m e n t s , t a k i n g i n t o a c c o u n t t h e e f f e c t o f t h e e n v i r o n m e n t . The f i r s t prob-
l e m i s how t o k e e p a s m a l l amount of o i l i n t h e b e a r i n g s y s t e m . The o i l l i e s between two e l e m e n t s , u s u a l l y w i t h h i g h s u r f a c e f r e e e n e r g i e s ( e . g . metals, m i n e r a l s ) and b e c a u s e t h e t e n s i o n o f o i l i s 30 o r more t i m e s l e s s t h a n t h e s u r f a c e f r e e e n e r g y o f m e t a l o r m i n e r a l , t h e o i l c r e e p s from t h e b e a r i n g ( r e f s . 3 8 7 - 3 9 0 ) . The s u r f a c e tension o f t h e o i l a l s o d e c r e a s e s w i t h time b e c a u s e o f t h e a g e i n g p r o c e s s ( r e f s . 3 8 9 , 3 9 1 ) . The s p e c i f i c l o a d s c a n r e a c h 1 0 0 0 MPa which, comb i n e d w i t h t h e dynamic c o n d i t i o n s o € o p e r a t i o n and h i g h tempratures, a r e c o n d u c i v e t o m i g r a t i o n o f t h e o i l from t h e b e a r i n g . T h i s migrat i o n f r o m t h e rubbing region can b e prevented by s p e c i a l coatiny
.
t e c h n i q u e s ( e p i l a m i z a t i o n ) These t e c h n i q u e s are v e r y important i n p r e c i s i o n e n g i n e e r i n g , and w i l l b e d i s c u s s e d below. The second i m p o r t a n t problem i s how t o e s t i m a t e t h e optimum volume o f o i l f o r e f f i c i e n t l u b r i c a t i o n . The amount o f o i l must b e s u f f i c i e n t f o r t h e prescribied p e r i o d o f o p e r a t i o n b u t it s h o u l d n o t exceed t h e c r i t i c a l volume b e c a u s e o f t h e r i s k o f s p r e a d i n g . I f t h e volume o f oil i s t o o l a r g e , c a p i l l a r y f o r c e s t e n d t o s p r e a d t h e l u b r i c a n t around t h e f r i c t i o n r e g i o n . As a r e s u l t , t h e amount of oil i n t h e b e a r i n g d e c r e a s e s and a l s o o t h e r p a r t s o f t h e f i n e mechanism become c o n t a m i n a t e d w i t h u n d e s i r e d o i l . The r e s i s t a n c e o f t h e l u b r i c a n t t o a g e i n g i s v e r y i m p o r t a n t as t h e o p e r a t i n g c o n d i t i o n s a r e t o u g h - h i g h s p e c i f i c l o a d s , dynamic ( i m p a c t ) l o a d s , s m a l l h e a t t r a n s f e r from t h e f r i c t i o n r e g i o n
,
con-
taminants ( d u s t , i n d u s t r i a l g a s e s ) , i r r a d i a t i o n , g r e a t temperature v a r i a t i o n s ( a n d h i g h t e m p e r a t u r e s ) . These a r e t h e main r e a s o n s f o r
212
t h e l u b r i c a n t ageing. S p e c i a l l u b r i c a t i o n problems a r i s e when p l a s t i c b e a r i n g s are l u b r i c a t e d . To s e l e c t t h e c o r r e c t o i l it i s n e c e s s a r y t o c o n s i d e r t h e p h y s i c a l and chemical i n t e r a c t i o n s between t h e o i l and t h e polymeric material, e s p e c i a l l y u n d e r normal r u b b i n g c o n d i t i o n s . The i n c r e a s i n g u s e o f p l a s t i c s i n p r e c i s i o n e n g i n e e r i n g makes t h e problem of p l a s t i c s l u b r i c a t i o n e s p e c i a l l y s i g n i f i c a n t f o r t h e tribology of m iniature systems.
6,2, P R E V E N T I N G O I L FROM SPREADING O R C R E E P I N G 6.2.1.
INTRODUCTION
T h e r e a r e t h r e e ways t o p r e v e n t o i l from s p r e a d i n g away from t h e b e a r i n g : t h e o i l c a n b e inherently n o n s p r e a d i n g , it c a n b e made n o n s p r e a d i n g b y t h e c a r e f u l u s e o f a d d i t i v e s ,or t h e s o l i d s u r f a c e c a n b e s p e c i a l l y t r e a t e d . The f i r s t s o l u t i o n i s t h e s i m p l e s t b u t t h e c h e m i c a l s t a b i l i t y o f n o n s p r e a d i n g o i l s , o b t a i n e d m a i n l y by v a r i o u s r e f i n i n g p r o c e d u r e s from p o r p o i s e jaw, b l a c k f i s h neat's-foot
,
olive,
a n d bone o i l s , i s v e r y p o o r . T h e s e o i l s a g e and t h e i r
l u b r i c a t i o n p r o p e r t i e s change w i t h t i m e . The s e c o n d and t h i r d s o l u t i o n s w i l l b e d i s c u s s e d below. The l a t t e r , t h e most common solution, w i l l be described f i r s t 6.2.2.
.
FUNDAMENTALS
The dynamics of l i q u i d s p r e a d i n g o n a s o l i d s u r f a c e c a n b e d e s c r i b e d by t h e change i n t h e c o n t a c t a n g l e 0 . F u l l s p r e a d i n g o c c u r s when 8-
0 . The v a r i a t i o n i n t h e c o n t a c t a n g l e of a l i q u i d l a i d o n
a s o l i d s u r f a c e is c o n d i t i o n e d by t h e f o l l o w i n g forces (reEs.387-390): 1) Cohesive f o r c e s o f t h e l i q u i d ,
2 ) I n t e r a c t i o n f o r c e s between m o l e c u l e s of s o l i d and l i q u i d ( a d h e -
sive forces)
,
3) A t t r a c t i v e ( m o l e c u l e ) f o r c e s of s o l i d a t t h e p e r i m e t e r o f t h e drop I 4 ) Gravity forces.
The f i r s t two k i n d s o f f o r c e s r e s i s t t h e l i q u i d s p r e a d i n g and t h e l a s t two e n c o u r a g e i t . The e q u i l i b r i u m ( w i t h o u t t a k i n g g r a v i t y f o r c e s i n t o c o n s i d e r a t i o n ) i s d e s c r i b e d by Young's e q u a t i o n rlcos 0 =
s
-
rs1
213
-
where 0
contact angle,
rl -
f a c e f r e e e n e r g y of s o l i d ,
surface tension of l i q u i d ,
'dSl -
rs -
sur-
interfacial tension.
The s p r e a d i n g o f t h e o i l d r o p i s t h e r e f o r e t h e r e s u l t o f t h e a c t i n g force A T r e l a t e d t o t h e perimeter u n i t of t h e liquid-solid interphase circle
*T=
!'s
-
where 0,
-
-
a/sl
Y1
'OS
(6.2)
'm
momentary v a l u e o f t h e c o n t a c t a n g l e .
Because t h e v a l u e o f
rsl i s
r e l a t i v e l y small, t h e s p r e a d i n g
dynamics depend on t h e d i f f e r e n c e between
ys
and
rl.
I t i s p o s s i b l e t o a c h i e v e a r e l a t i v e l y small d i f f e r e n c e between
yS
and
y1
when t h e o i l d r o p ( w i t h s u r f a c e t e n s i o n 12-36 mN/m)
is
l a i d on t h e polymer s u r f a c e ( t h e s u r f a c e f r e e e n e r g y o f polymers i s lower t h a n 1 0 0 m J / m 2 ) ( r e f s . 3 9 2 , 3 9 3 ) . The s u r f a c e f r e e e n e r g y o f polymers c a n b e d e s c r i b e d w i t h t h e v a l u e o f t h e c r i t i c a l s u r f a c e t e n s i o n of w e t t i n g
rc i n t r o d u c e d
t h a t t h e p l o t of cos 0 vs.
by Zisman ( r e f . 387)
rl f o r a homologous
. He
found
s e r i e s o f l i q u i d s on
a g i v e n low e n e r g y s o l i d i s g e n e r a l l y a s t r a i g h t l i n e . The e m p i r i c a l
rc i s d e f i n e d a s t h e v a l u e o f yl a t t h e rl w i t h t h e h o r i z o n t a l l i n e , c o s
value of
p l o t cos 0 v s .
yl
inercept of the 0 = 1. L i q u i d s w i t h
less t h a n f c would b e e x p e c t e d t o s p r e a d o n t h e s o l i d s u r f a c e . The i m p o r t a n t r e q u i r e m e n t f o r t h e o i l i s s t r o n g a d h e s i o n t o t h e
s o l i d s u r f a c e . The a d h e s i o n between s u r f a c e i s d e s c r i b e d by t h e c l a s s i c Duprb e q u a t i o n
wa
y1
=
(1
+
cos 0)
(6.3)
where Wa i s t h e s p e c i f i c work ( e n e r g y ) o f a d h e s i o n . The work of adh e s i o n d e c r e a s e s as value of
rs
yl
( a t a given
d e c r e a s e s and 0 i n c r e a s e s , i . e . , when t h e
rl)
decreases. The u s e o f low s u r f a c e f r e e
e n e r g y s o l i d s t o p r e v e n t t h e s p r e a d i n g of t h e o i l d r o p , i n t h e o i l - o n - s o l i d c o m b i n a t i o n , i s n o t a s a t i s f a c t o r y s o l u t i o n t o t h i s prob-
l e m . Because t h e work of c o h e s i o n W k of t h e o i l is W k = 2
r1 , l t h e
non-wetting o f t h e s o l i d s u r f a c e by t h e o i l w i l l o c c u r when Wa<3Wk; 1 l i m i t e d w e t t i n g ( O o < 0 (180°) a t W a ) 2 W k , and f u l l y w e t t i n g a t
wa)
W k . I t i s p o s s i b l e t o d e s c r i b e t h e r e f o r e t h e optimum l u b r i c a t i o n of t h e b e a r i n g i n t e r m s o f n o n s p r e a d i n g and maximum a d h e s i o n
as
1
'a>
wa
'z k' = MAX (W,)
(6.4)
214 P r a c t i c a l e x p e r i e n c e w i t h i n s t r u m e n t o i l s shows t h a t the optimum r e l a t i o n s h i p between t h e a d h e s i o n o f t h e o i l t o a s o l i d s u r f a c e and t h e t e n d e n c y t o s p r e a d o c c u r s a t t h e c o n t a c t a n g l e 0 = 25-40°. 6.2.3.
METHODS
The p r e v e n t i o n o f o i l m i g r a t i o n c a n b e a c h i e v e d by the g m t r i c a l o r c h e m i c a l m o d i f i c a t i o n o f t h e s o l i d s u r f a c e . An example o f geo-
metrical m o d i f i c a t i o n o f t h e s u r f a c e o f t h e m i n e r a l c o v e r b e a r i n g o f a b a l a n c e i s shown i n F i g . 6 . 1 , where t h e o i l c a n o n l y r e a c h and f i l l the barrier circle.
F i g . 6.1. Ge o m e t ri ca l m o d i f i c a t i o n o f t h e s u r f a c e o f a cover balance b e a r i n g t o prevent o i 1 m i g r a t i o n .
The o t h e r g e o m e t r i c a l a p p r o a c h t o n o n s p r e a d i n g i s b a s e d o n the e d g e e f f e c t ( F i g . 6 . 2 ) . For t h e d i s p l a c e m e n t o f t h e d r o p o n t h e v e r t i c a l p l a n e , t h e s u r f a c e o f t h e d r o p must t u r n o n t h e a n g l e 90°.
For t h i s
t h e d r o p h e i g h t must b e enough t o change t h e c o n t a c t a n g l e of 90'. A d d i t i o n a l e n e r g y i s needed f o r t h i s e f f e c t and t h e o i l c a n n o t c r o s s t h e edge b a r r i e r . The e f f i c i e n c y of t h i s method h a s b e e n shown ( r e f . 9 5 ) i n t h e c a s e o f t h e d r o p s of c l a s s i c c l o c k o i l s l a i d o n ruby
p l a t e s . The c o n t a c t a n g l e s were a b o u t 20° a n d t h e o v e r l o a d s required t o d i s p l a c e t h e d r o p w e r e a b o u t 30 g i n t h e c e n t r e of t h e p l a t e but
215
a t t h e e d g e were 1 0 0 - 1 2 0 q . I t is a l s o known ( r e f s . 3 9 4 - 3 9 8 )
that
t h e t e n d e n c y t o s p r e a d i n c r e a s e s w i t h t h e i n c r e a s e i n surface r o u g h n e s s from Ra = 0 . 6 3 p m t o Ra = 0 . 0 2 p m r e s u l t s i n a n i n c r e a s e i n t h e c o n t a c t a n g l e from 13O t o 2 4 O and i n t h e i r r e m i s s i b l e o v e r l o a d s from 17.5 q t o 31.1 g ( r e f . 9 5 ) .
F i g . 6.2.
The edge e f f e c t d u r i n g o i l d r o p s p r e a d i n g .
The f o l l o w i n g c h e m i c a l methods o f s o l i d s u r f a c e m o d i f i c a t i o n
are u s e d : 1) T h e s u r f a c e i s c o m p l e t e l y c o a t e d b y m o n o l a y e r s o f low s u r f a c e
e n e r g y compounds on which o i l d r o p s w i t h h i g h e r s u r f a c e t e n s i o n
w i l l not spread, 2 ) The o i l d r o p i s s u r r o u n d e d w i t h a narrow r i n g c o a t i n g ( e p i l a m e )
o f a compound w i t h low s u r f a c e e n e r g y , or t h e e n t i r e s u r f a c e o f t h e e l e m e n t is c o a t e d and t h e c o a t i n g i s t h e n removed f r o m t h e
area where t h e o i l d r o p w i l l b e l a i d ( F i g . 6 . 3 ) . T h i s i s t h e s 0- c a 11ed
I'
s t o p - o i 1" method ,
3 ) The s o l i d s u r f a c e i s c h e m i c a l l y t r e a t e d t o l o w e r t h e s u r f a c e
f r e e energy.
Barrier film I
F i g . 6.3.
S t o p - O i l method f o r p r e v e n t i n g o i l m i g r a t i o n .
216 The f i r s t method w a s i n v e n t e d b y Woog i n F r a n c e i n t h e n i n e t e e n t w e n t i e s , t h e s e c o n d method w a s d e v e l o p e d i n U S Navy l a b o r a t o r i e s a f t e r t h e l a s t w a r , and t h e c h e m i c a l t r e a t m e n t method w a s i n v e n t e d by Osowiecki a n d a p p l i e d i n i n d u s t r y i n S w i t z e r l a n d i n 1 9 6 4 . The c o a t i n g ( e p i l a m e ) i s l a i d o n t h e s o l i d s u r f a c e from l i q u i d o r g a s s o l u t i o n ( o r d i s p e r s i o n ) . The t h i c k n e s s o f t h e l a y e r u s u a l l y d o e s n o t e x c e e d 0 . 5 ,urn. The a c t i v e p o l a r g r o u p s o f t h e e p i l a m e macromolecules a r e p h y s i c a l l y a d s o r b e d o r c h e m i s o r b e d o n t h e s o l i d s u r f a c e . O s o w i e c k i ' s method is v e r y e f f e c t i v e i n t h e case o f m i n e r a l s o l i d s u r f a c e s . The e f f e c t o f t h e c h e m i c a l t r e a t m e n t of t h e s u r f a c e
i s v e r y s t a b l e and f o r example, u l t r a s o n i c c l e a n i n g o r c h e m i c a l i n t e r a c t i o n s b e t w e e n t h e o i l a n d t h e m a t e r i a l o f t h e s u r f a c e l a y e r do n o t c h a n g e t h e e n e r g e t i c a l p r o p e r t i e s o f t h e s u r f a c e . Only s t r o n g a l k a l i s o r a c i d s can s p o i l t h e t r e a t e d s u r f a c e l a y e r o f ruby o r s a p p h i r e j e w e l s . O s o w i e c k i ' s method h a s b e e n a p p l i e d by Reno S.A. and by KIF P a r e c h o c i n S w i t z e r l a n d t o p r e v e n t o i l m i g r a t i n g from t h e r u b b i n g r e g i o n of t h e c o v e r b e a r i n g i n t h e watch b a l a n c e , e s p e c i a l l y i n t h e I n c a b l o c system. 6.2.4.
COATINGS (EPILAMES)
The r e q u i r e m e n t s f o r t h e s p e c i a l c o a t i n g s ( e p i l a m e s ) t o p r e v e n t o i l from s p r e a d i n g or c r e e p i n g are a s follows ( r e f . 3 9 0 ) : 1) Low s u r f a c e f r e e e n e r g y , 2 ) S t r o n g adhesion t o t h e s o l i d s u r f a c e , 3 ) High c h e m i c a l s t a b i l i t y , 4 ) The c o a t i n g compound must b e i n e r t t o t h e o i l a n d t h e c o a t e d ma-
terials ' 5 ) Broad t e m p e r a t u r e r a n g e o f u s e , 6 ) S u i t a b l e h a r d n e s s and s h e a r s t r e n g t h ,
7) Homogeneity o f t h e c o a t i n g l a y e r on t h e whole s o l i d s u r f a c e ( t h i c k n e s s , m e c h a n i c a l and p h y s i c o c h e m i c a l p r o p e r t i e s )
,
8 ) The s m a l l e s t p o s s i b l e t h i c k n e s s , 9 ) The compound f o r t h e e p i l a m e s h o u l d be s o l u b l e i n common s o l -
vents,
10) The t e c h n i q u e s f o r d e p o s i t i n g c o a t i n g s must n o t b e t o o s o p h i s t i cated. The s u r f a c e f r e e e n e r g y o f t h e e p i l a m e d e p e n d s o n t h e c h e m i c a l s t r u c t u r e of t h e m a c r o m o l e c u l e s . The s u r f a c e free e n e r g y o f t h e epilame d e c r e a s e s f o r t h e macromolecules w i t h s i d e c h a i n s o f t h e f o l l o w i n g s t r u c t u r e s ( r e f . 387) :
217 -CH3
>
-CF2-
>
-CF2H
) -CF3
The e p i l a m e s w i t h
-CF3 g r o u p s a r e c h a r a c t e r i z e d by t h e i r low s u r face f r e e energy ( c a 6 m J / m 2 , Table 6 . 1 )
.
TABLE 6.1. CRITICAL SURFACE TENSION OF WETTING, TURES OF SURFACE GROUPS ( r e f . 387)
rc
OF LAYERS WITH DIFFERENT CHEMICAL STRUC-
SURFACE CONSTITUTION
A. Fluorocarbon s u r f a c e s -C F3 -CF2H -CF
8. Hydroc rbon
3
6 15
and -CF2-
17
-CF2
18
-CH2- CF
20
3
-CF2-CFH-
22
-C F2-CH2-
25
- CFH- CH 2-
28
u r f aces -CH3 ( c r y s t a l ) -CH
3
(monolayer)
20-22 22-24
-CH2-
31
=CH- (phenyl r i n g edge)
35
C . Chlorocarbon s u r f a c e s -CCIH-CH2-
39
- C C I 2-CH2-
40
=cc 1
43
0. N i t r a t e d hydrocarbon s u r f a c e s -CH20N02 ( c r y s t a l )
40
(monolayer) -C(NO ) 2 3 -CH2NHN02 ( c r y s t a l )
42
44
The compound f o r t h e c o a t i n g i s u s u a l l y d e p o s i t e d o n t h e s o l i d s u r f a c e from a l i q u i d of g a s s o l u t i o n ( d i s p e r s i o n ) . The PTFE c o a t i n g s a r e normally a p p l i e d from d i s p e r s i o n i n w a t e r b u t it i s d i f f i c u l t t o m e e t t h e r e q u i r e m e n t of a b r i e f h e a t i n g p e r i o d a t h i g h t e m p e r a t u r e s , which i s n e c e s s a r y t o s i n t e r t h e PTFE p a r t i c l e s , obt a i n good f i l m a d h e s i o n and e l i m i n a t e t h e water and a d d i t i v e s . This t r e a t m e n t is o f t e n t o o i m p r a c t i c a l f o r t h e m a n u f a c t u r e o f i n s t r u ments. A more g e n e r a l l y a p p l i c a b l e and s i m p l e r method, which d o e s
218
i s t h e u s e of s o l u b l e compounds. The
n o t r e q u i r e any p r e - t r e a t m e n t ,
low s u r f a c e - e n e r g y f i l m s c a n b e r e a d i l y l a i d down from a s o l u t i o n i n a n a p p r o p r i a t e s o l v e n t s u c h as x y l e n e h e x a f l u o r i d e o r b e n z o t r i f l u o r i d e w h i c h , a f t e r slow e v a p o r a t i o n , l e a v e a s u i t a b l e c o a t i n g . The compounds a c t u a l l y u s e d on t h e s o l i d s u r f a c e h a v e t h e i m p o r t a n t a d v a n t a g e o f n o t b e i n g r e a d i l y d i s s o l v e d by any o f t h e c l e a n i n g s o l v e n t s commonly u s e d i n t h e i n s t r u m e n t f i e l d . T h r e e g r o u p s of compounds a r e commonly used f o r t h e e p i l a m e s : o r g a n i c materials such as o l e i c o r s t e a r i c a c i d s ; p o l y s i l o x a n e s ; and f l u o r i n a t e d p o l ymers. Woog i n F r a n c e f i r s t d e v e l o p e d and p a t e n t e d a p r o c e s s i n which a d i l u t e s o l u t i o n o f t h e f a t t y a c i d s i n a v o l a t i l e s o l v e n t
w a s used t o c o a t t h e b e a r i n g s u r f a c e s w i t h a t h i n f i l m (epilame) o f f a t t y a c i d . An example o f t h i s t y p e o f epilarne is A r e t o l , manufact u r e d by Moebius e t F i l s i n A l l s c h w i l ( S w i t z e r l a n d ) . The n e x t s t e p was t h e d e p o s i t i n g e p i l a m e s from t h e g a s p h a s e . An i n e r t g a s , e . g . a r g o n , f l o w i n g t h r o u g h t h e chamber w i t h f a t t y a c i d , t a k e s i t s evapo r a t e d a t ( 1 2 0 O C ) m o l e c u l e s and b r i n g s them t o t h e c o n t a i n e r w i t h t h e e l e m e n t s t o be c o a t e d . The e s s e n t i a l d i s a d v a n t a g e s o f t h e f a t t y -acid epilames are: 1) C r i t i c a l s u r f a c e t e n s i o n o f w e t t i n g
T~
i s v e r y h i g h (ca. 30 nN/m),
2 ) Chemical r e a c t i o n o f t h e e p i l a m e w i t h t h e c l a s s i c o r g a n i c o r min-
e r a l instrument o i l s a c c e l e r a t e s t h e i r ageing, 3) Great c h a n g e s i n t h e e p i l a m e p r o p e r t i e s o c c u r w i t h t i m e and tern perature, 4 ) T y p i c a l s o l v e n t s u s e d i n t h e i n s t r u m e n t f i e l d c a n remove t h e epilame
.
E p i l a m e s b a s e d on p o l y s i l o x a n e s show some i n t e r e s t i n g properties. A series o f t h e s e e p i l a m e s h a s b e e n e l a b o r a t e d by Cetehor i n Besanpn ( F r a n c e ) ( r e f s . 64,68) -401)
,
. As
c o n c l u d e d by Tabor and W i l l i s
(refs. 399-
t h e a d s o r p t i o n o f d i m e t h y l p o l y s i l o x a n e (DMPS) molecules o n t o
a c o p p e r s u r f a c e is p o s s i b l e when DMPS i s d e p o s i t e d from t h e s o l u t i o n ( e . g . i n Freon 113) and t h e n h e a t e d t o 100°C. The t h i c k n e s s ( s ) o f t h e a d s o r b e d DMPS l a y e r i s a p a r a b o l i c f u n c t i o n v s . t i m e ( t )( s = fi).A f t e r a b o u t 2 0 0 h o u r s o f h e a t i n g t h e c o a t i n g t h i c k n e s s i n c r e a s e s t o a b o u t 0 . 1 1 ,UIII. According t o Tabor and W i l l i s , t h e good a d s o r p t i o n of DMPS o n t o a m e t a l l i c s u r f a c e i s due t o t h e p l a r groups -COOH
o b t a i n e d as a r e s u l t of t h e o x i d a t i o n of m e t h y l g r o u p s -CH3
i n t h e macromolecules o f DMPS
.
The f i r s t e x p e r i m e n t s c a r r i e d o u t by C e t e h o r showed t h a t t h i s method c a n b e a p p l i e d t o t h e p r o d u c t i o n o f e p i l a m e s o n m e t a l l i c and m i n e r a l s u r f a c e s . The e p i l a m e s f i l m w a s o b t a i n e d u s i n g DMPS ( w i t h
219
v i s c o s i t y 30000 mm2/s a t 20OC) d i s s o l v e d i n F r i g e n 1 1 3 . The e l e m e n t w a s immersed i n t h e s o l u t i o n a n d t h e n h e a t e d a t 100°C f o r 2 h o u r s . The r e m a i n s o f t h e n o n a d s o r b e d compound were c l e a n e d by u l t r a s o n i c s i n c h l o r i n a t e d s o l v e n t . The c r i t i c a l s u r f a c e t e n s i o n o f t h i s s o r t o f e p i l a m e i s 2 4 mN/m. On a s u r f a c e l i k e t h i s , o i l s w i t h a s u r f a c e t e n s i o n o f 25-30 mN/m c a n b e l a i d down. The c o n t a c t a n g l e s are 20-30'. The maximum v a l u e o f t h e work of a d h e s i o n , W a r f a c e t e n s i o n o f t h e o i l i s 27 mN/m.
o c c u r s when t h e s u r -
The DMPS e p i l a m e s o b t a i n e d by
t h i s i n s i t u method may o n l y b e removed m e c h a n i c a l l y . The d e v e l o p m e n t of t h e p o l y s i l o x a n e e p i l a m e s h a s b e e n d i r e c t e d t o t h e o x i d i z e d p o l y s i l o x a n e compounds d i s s o l v e d i n s o l v e n t s . F o r t h i s , the DMPS w i t h p o l a r g r o u p s -COOH h a s b e e n p r e v i o u s l y formed by t h e o x i d a t i o n o f DMPS i n a n o z o n e a t m o s p h e r e a t a t e m p e r a t u r e o f
100°C. About 0 . 5 % o f DMPS i s t h e n d i s s o l v e d i n F r i g e n 113.The microelements are inmersed i n t h i s solution f o r 2-4 minutes. A f t e r d r y i n g , t h e
elements are washed w i t h c h l o r i n a t e d s o l v e n t t o remove t h e unadsorbed m o l e c u l e s o f t h e compound. The p r o p e r t i e s o f t h i s e p i l a m e a r e s i m i -
l a r t o t h e o b t a i n e d by t h e i n s i t u method. T h i s t y p e of e p i l a m e h a s b e e n p a t e n t e d , a n d was i n d u s t r i a l l y m a n u f a c t u r e d by Rhone-Poulenc (Rhodorsil) i n France. The c r i t i c a l s u r f a c e t e n s i o n of w e t t i n g i s r e l a t i v e l y h i g h . To l o w e r t h e
tc,
rc
f o r these epilames
f l u o r i n a t e d polysiloxanes a r e
a p p l i e d . The o x i d a t i o n i n t h e o z o n e a t m o s p h e r e o f f l u o r i n a t e d p o l y s i l o x a n e FS 1265 (Dow C o r n i n g ) c o n t a i n i n g t h e g r o u p s CF2-CF2 made
it p o s s i b l e t o manufacture a n epilaine w i t h
rC
= 17-19 mN/m.
i c a l s u r f a c e t e n s i o n o f w e t t i n g , a b o u t 1 7 mN/m,
The c r i t
w a s achieved i n
e p i l a m e s b a s e d o n t h e f l u o r o p o l y s i l o x a n e s w i t h (CF2)n-CF3
groups
o x i d i z e d i n t h e o z o n e a t m o s p h e r e . The c o n t a c t a n g l e f o r o i l s w i t h a s u r f a c e t e n s i o n o f 26-27 mN/m on s u c h e p i l a m e s i s 30-35O. E f f e c t i v e e p i l a r n e s h a v e b e e n d e v e l o p e d i n t h e Naval R e s e a r c h L a b o r a t o r y i n Washington D . C .
( r e f s . 4 0 2 , 4 0 3 ) . They a r e b a s e d o n
c o m m e r c i a l l y a v a i l a b l e f l u o r i n a t e d polymers whose low
;yc
values
r e n d e r them e s p e c i a l l y a t t r a c t i v e f o r s u c h a p p l i c a t i o n . The b e s t compounds a r e f l u o r i n a t e d esters of p o l y ( a c r y l i c a c i d ) or poly(mtha c r y l i c acid) with the values of
lowest v a l u e o f
;yc
( 6 mN/m)
rc
11.1 and 1 0 . 6 r e s p e c t i v e l y .
The
is found o n a s u r f a c e w i t h a n a d s o r b e d
f l u o r i n a t e d f a t t y a c i d monolayer. The f i r s t f l u o r i n a t e d c o a t i n g s b a s e d o n PTFE o r c o p o l y m e r s o f t e t r a f l u o r o e t h y l e n e ( T P E ) a n d h e x a f l u o r o p r o p y l e n e (HFP) w i t h a criti c a l surface tension 404)
,
rc
1 8 . 5 a n d 1 6 . 2 mN/m r e s p e c t i v e l y ( r e f s . 4 0 2 , were v e r y l i m i t e d i n t h e i r a p p l i c a t i o n b e c a u s e of t h e rigorous
220
s u r f a c e t r e a t m e n t r e q u i r e d t o d e p o s i t t h i n , c o n t i n o u s , adherent p l y m e r c o a t i n g s , s i n c e i n o r d e r t o a s s u r e good f i l m a d h e s i o n , t h e objects t o b e c o a t e d need a b r i e f b a k i n g i n a n over o r t o t a l immersion i n a f l u i d i z e d b e d o f t h e h o t polymer powder a t h i g h t e m p e r a t u r e s . The f l u o r o p o l y e s t e r s , however, c a n b e d e p o s i t e d a s a n a d h e r e n t f i l m from a s o l u t i o n i n a v o l a t i l e f l u o r o c h e m i c a l s o l v e n t a t room temperature. The two f l u o r o p o l y e s t e r s a r e o n l y u s e f u l w i t h t h e S t o p - O i l method b e c a u s e t h e a d h e s i o n o f t h e o i l t o t h e f l u o r o p o l y e s t e r c o a t i n g i s v e r y poor due t o t h e i r low s u r f a c e e n e r g y . A s l i g h t j a r o f t h e i n s t r u m e n t might d e t a c h t h e o i l d r o p . The o i l d r o p i s t h e r e f o r e p l a c e d on t h e h i g h e n e r g y s u r f a c e and s u r r o u n d e d by t h e low e n e r g y anti-migration coating. R e s e a r c h ( r e f s . 402-404) h a s b e e n c a r r i e d o u t on f l u o r i n a t e d polymers ( p r o d u c t s o f t h e Minnesota ?lining and K a n u f a c t u r i n g Co.)
- pentadecafluoro-octyl m e t h a c r y l a t e ( PFOMA) and C a F 7SO 2MC 3 H 7C 2H 4OCOCH=CH 2 , 2-N- propy l p e r f l u o r o o c t a n e s u l f o n a m i d o / e t h y l a c r y l a t e (PFOSEA) and iyc 10.6 and 11.1 mN/m r e s p e c t i v e l y . The polymers w e r e s u p p l i e d i n t h e form o f a 20% s o l u t i o n i n p u r e x y l e n e h e x a f l u o r i d e . A measured q u a n t i t y o f
w i t h t h e f o r m u l a s C7F15CH2OCOC(CH3)=CH2, 1 H
t h e s o l u t i o n w a s poured o n t o t h e smooth s u r f a c e s o f t h e t e s t p a n e l . The s o l v e n t w a s t h e n e v a p o r a t e d s l o w l y u n d e r c o n t r o l l e d c o n d i t i o n s a t room t e m p e r a t u r e i n a d u s t - f r e e environment f o l l o w e d by e x p o s u r e f o r 24 h o u r s i n a c l e a n vacuum oven a t 5OoC and 1 0 kPa. The r e s u l t i n g smooth and s p e c u l a r f i l m s were between 0.15 and 0.23 mm t h i c k , f i r m and t r a n s p a r e n t , and were s o a d h e r e n t t h a t o n l y s m a l l fragments c o u l d b e removed from t h e p a n e l by c h o p p i n g t h e c o a t i n g . The p a n e l s used w e r e s t a i n l e s s s t e e l , P y r e x , g l a s s epoxy, p a p e r b a s e epoxy, p a p e r b a s e p h e n o l i c , f i b e r - r e i n f o r c e d and p a p e r - r e i n f o r c e d Bakelite. The p a n e l s w e r e immersed a t room t e m p e r a t u r e f o r p e r i o d s o f one and f o u r weeks i n o n e of t h e f o l l o w i n g o r g a n i c l i q u i d s : hexadecane,para f f i n o i l , tricresylphosphate, dicyclohexyl , propylene carbonate, and b i s ( 2 - e t h y l h e x y l ) s e b a c a t e . C o a t i n g s on m e t a l s u r f a c e s ( s t a i n less s t e e l , b r a s s and aluminium) w e r e a l s o s t u d i e d . E q u i l i b r i u m
c o n t a c t a n g l e s o f hexadecane and w a t e r on t h e d r i e d s u r f a c e s w e r e measured b e f o r e and a f t e r e x p o s u r e i n t h e o r g a n i c l i q u i d s . These i n v e s t i g a t i o n s ( r e f s . 402,403) r e v e a l e d t h a t t h e r e was no change i n t h e p h y s i c a l a p p e a r a n c e o f t h e c o a t i n g s a f t e r immersion i n n o n p o l a r l i q b i d s such a s hexadecane o r p a r a f f i n o i l , and o n l y o c c a s i o n a l p i t t i n g o r o p a q u e n e s s were n o t e d a f t e r p r o l o n g e d exp o s u r e i n some p o l a r l i q u i d s . W e t t a b i l i t y w a s o n l y s l i g h t l y a f f e c t -
221 e d . The polymer c o a t i n g s on m e t a l s u b s t r a t e s were u n a f f e c t e d by e v e n t h o s e p o l a r o r g a n i c o i l s which had some a d v e r s e e f f e c t s o n t h e
same c o a t i n g s on r e s i n o u s s u r f a c e s , e m p h a s i z i n g t h e i n f l u e n c e o f t h e u n d e r l y i n g s u r f a c e . The a d h e s i v e q u a l i t i e s o f t h e f i l m s f o r any o f t h e p a n e l s o r m e t a l l i c s u r f a c e s t e s t e d were n o t a f f e c t e d by exp o s u r e t o a n e n v i r o n m e n t o f 1 0 0 % R.H.
and 4 9 O C f o r one week.
These i n v e s t i g a t i o n s ( r e f s . 402,403) i n t o t h e p r e v e n t i o n o f o i l s p r e a d i n g ( S t o p - O i l method) showed t h a t f o r m e t a l and m i n e r a l s u r f a c e s t h e two p o l y m e r s i n v e s t i g a t e d were v e r y good. I f d u r i n g n o r m a l u s e t h e i n s t r u m e n t w i l l b e s u b j e c t e d t o a b r u p t t e m p e r a t u r e c h a n g e s from subzero t o high temperature i n c o n d i t i o n s of high humidity condit i o n s , t h e PFOSEA c o a t i n g s h o u l d b e p r e f e r r e d b e c a u s e t h i s p l y e s t e r w i t h s t a n d s s u c h c o n d i t i o n s ; o t h e r w i s e PFOMA s h o u l d b e u s e d . The PFOMA c o a t i n g ( 2 % PFOMA s o l u t i o n i n h e x a f l u o r o x y l e n e ) h a s b e e n t e s t e d and u s e d f o r t h e p r e v e n t i o n o f l u b r i c a n t m i g r a t i o n from t h e b a l l b e a r i n g s of servo-motors
( r e f . 4 0 5 ) . A v a r i e t y of l u b r i -
cant types w e r e considered: s i l i c o n e s , d i e s t e r s , s i l i c o n e - d i e s t e r b l e n d s , g r e a s e s , soap-thickened d i e s t e r s , soap-thickened
silicone-
- d i e s t e r blends ( a l l d i e s t e r l u b r i c a n t s contained 0.5% antioxidant and 2 . 0 % r u s t i n h i b i t o r ) . The l i q u i d l u b r i c a n t s r a n g e d i n v i s c o s i t y from 1 0 0 0 t o 4 2 0 0 0 nun2/,
a t -53.9OC.
B a r r i e r f i l m s o f PFOMA l e s s
t h a n 1,um t h i c k were d e p o s t e d on t h e f o r e h e a d s u r f a c e s o f t h e b a l l - b e a r i n g r i n g s . T h e s e i n v e s t i g a t i o n s showed t h a t PFOMA c o a t i n g s a r e v e r y e f f e c t i v e i n p r e v e n t i n g t h e l u b r i c a n t from s p r e a d i n g . B e a r i n g s t r e a t e d w i t h such a b a r r i e r have i n c r e a s e d t h e l i v e s o f serw-motors from 300 t o n e a r l y 4000 h o u r s . A PFOMA c o a t i n g c a n b e a c c i d e n t a l l y formed o n t h e r u b b i n g s u r -
f a c e s o f t h e b a l l b e a r i n g s . When t h e y become worn, t h e t r a c e s of PFOMA may r e s u l t i n d r y r u b b i n g o f m e t a l - t o - m e t a l
surfaces. I f t h e
l u b r i c a n t d o e s n o t q u i c k l y come between t h e r u b b i n g s u r f a c e s , s e i z u r e is p o s s i b l e . I n h i s i n v e s t i g a t i o n s i n t o gyroscope b e a r i n g s , Ahlborn ( r e f . 4 0 6 ) showed t h a t when t h e s l i d i n g s p e e d o f t h e b a l l i s less t h a n 1 . 0 7 m / s
and t h e r e a l i s t i c s p e c i f i c l o a d s
( 8 0 0 MPa,
p r a c t i c a l l y no s e i z u r e s o c c u r . S e i z u r e i s p o s s i b l e o n l y when t h e v i s c o s i t y o f t h e l u b r i c a n t i s t o o h i g h and t h e o i l c a n n o t q u i c k l y come between t h e m e t a l l i c s u r f a c e s a f t e r t h e d e s t r u c t i o n o f t h e PFOMA l a y e r and when t h e h e a t i n g i n t h e r u b b i n g r e g i o n i s very high. S e l e c t i n g t h e optimum s o l v e n t f o r t h e p r e p a r a t i o n o f b a r r i e r e p i l a m e f i l m s i s v e r y i m p o r t a n t . B e r n e t t ( r e f . 407) h a s i n v e s t i g a t e d t h e e f f e c t o n PFObIA f i l m o f a l t e r n a t i v e s o l v e n t s f o r t y p i c a l h e x a f l u o r o x y l e n e (HFX) ( w i t h b o i l i n g p o i n t 116OC)
. These w e r e
:
222 1 ) a more v o l a t i l e
and
more
easily
available
CC12F-CC1F2
( F r e o n TF, b o i l i n g p o i n t 48OC) and 2 ) a s l i g h t l y more v o l a t i l e s o l v e n t a n a l y z e d a s i s o m e r s o f perfluorobutyltetrahydrofuran (PBTF) ( b p = 92-102OC) . I n t h e s e i n v e s t i g a t i o n s f l u o r o p o l y m e r f i l m s p r e p a r e d from o n e F r e o n TF and two d i f f e r e n t PBTF s o l u t i o n s were compared w i t h f i l m s p r e p a r e d from HFX s o l u t i o n f o r w e t t a b i l i t y and s t a b i l i t y a f t e r exposure t o e l e v a t e d t e m p e r a t u r e s i n a i r o r i n c o n t a c t w i t h o r g a n i c l i q u i d s : d i m e t h y l s i l o x a n e DC 200 ( 5 0 m2/s a t 2OoC1 ( 2-ethylhexyl)
s e b a c a t e , V e r s i l u b e F-50
from G e n e r a l E l e c t r i c Co.) t o NRL MB-20B) additives)
and NRL-20B
, Nye
, bis
(chlorinated polysiloxane
s y n t h e t i c instrument o i l
(formulated
( m i x t u r e of H e r c o l u b e A , d i e s t e r , and
( r e f . 4 0 8 ) . The t e s t s c a r r i e d o u t showed t h a t a t a m b i e n t
t e m p e r a t u r e s , F r e o n TF i s a s u i t a b l e s u b s t i t u t e f o r HFX i n many
cases. F o r c e r t a i n r e q u i r e m e n t s , however, s u c h a s p r e v e n t i o n o r g a n i c l i q u i d s p r e a d i n g a t e l e v a t e d t e m p e r a t u r e s , polymer s o l v e d i n HFX o r polymers s o l v e d i n PBTF ( t w o d i f f e r e n t ) a r e s u p e r i o r t o polymer d i s s o l v e d i n F r e o n TF, p r o v i d e d t h e r e were no t r a c e i m p u r i t i e s p r e s e n t i n e i t h e r s o l v e n t . The r e s u l t s i n d i c a t e t h a t whenever F r e o n TF
i s u s e d as a s o l v e n t f o r t h e s e f l u o r o p o l y m e r s , t h e p e r f o r m a n c e o f t h e f i l m s is a d v e r s e l y a f f e c t e d . W e t t a b i l i t y and a d h e s i o n o f t h e f i l m s p r e p a r e d from PFOMA d i s s o l v e d ( 2 0 % ) i n HFX, 2 0 % s o l u t i o n o f PFOMA i n BFX d i l u t e d t o 2 % w i t h HFX, s o l u t i o n o f 2 % PFOMA i n F r e o n TF, s o l u t i o n o f 2 % PFOMA i n PBTF-a a n d s o l u t i o n of 2 % PFOMA i n PBTF-b, w e r e a l m o s t u n a l t e r e d by e x p o s u r e s t o 21OoC i n a i r f o r 11 h o u r s and t o 100°C i n o i l s f o r 18 h o u r s . F i l m s of polymer d i s s o l v e d i n F r e o n TF and i n PBTP-a had t h e same
rc a s
f i l m s of polymer d i s s o l v e d i n HFX ( 2 0 % s o l u t i o n ) ; how-
e v e r , e x p o s u r e t o e l e v a t e d t e m p e r a t u r e s i n a i r , and e s p e c i a l l y i n c o n t a c t w i t h o i l s , p r o d u c e d g e n e r a l l y b a d and o c c a s i o n a l l y d i s a s t r o u s e f f e c t s o n w e t t a b i l i t y i n polymer d i s s o l v e d i n F r e o n TF films. F u r t h e r i n v e s t i g a t i o n s h a v e b e e n c a r r i e d o u t t o d e m o n s t r a t e hav t h e polymer c o n c e n t r a t i o n ( w h i c h g o v e r n s t h e f i l m t h i c k n e s s ) ard the s o l v e n t t y p e a f f e c t t h e q u a l i t y o f c u r e d b a r r i e r f i l m s o f PFOMA. T h e s e i n v e s t i g a t i o n s ( r e f . 4 0 9 ) r e p o r t t h e r e s u l t s of a n experimen-
t a l s t u d y o f t h e e f f e c t s o f t h e s e v a r i a b l e s on b a r r i e r f i l m w e t t a b i l i t y , s u r f a c e , and polymer p r o p e r t i e s , u t i l i z i n g c o n t a c t a n g l e and m i c r o s c o p y s t u d i e s , and r e l a t e them t o t h e i r o p t i m i z a t i o n i n b a r r i e r f i l m o i l r e p e l l e n c y a p p l i c a t i o n s . The r e s u l t s show t h a t o f t h e s e v e n s o l v e n t s u s e d : I-IFX, 1,1,2-trifluoro-1,2,2-trichloroethane (CClF) , p e r f l u o r i n a t e d c y c l i c e t h e r C8F160 (PCE) and t h e i r m i x t u r e s : HFC/CClF 7 5 / 2 5 , PCE/CClF 7 5 / 2 5 , PCE/CClF 90/10 and PCE/CClF/HFX
223
80/10/10, t h e b e s t is t h e P C E / C C l F 9 0 / 1 0 m i x t u r e . The m o s t e f f e c t i v e b a r r i e r f i l m f o r m u l a t i o n of t h e s e s o l v e n t s was i n a 0 . 2 % w/w s o l u t i o n o f PFOMA. T h i s f o r m u l a t i o n c o n s i s t e n t l y g a v e smooth (0.5-0.15 ,um t h i c k ) , uniform, h i g h l y o i l - r e p e l l e n t
f i l m s . Barrier f i l m s on
p o l i s h e d metal s u b s t r a t e s w e r e more r e s i s t a n t t o l u b r i c a n t s t h a n t h o s e on g l a s s s u b s t r a t e s . The c h o i c e o f s o l v e n t s i n t h e c a s t i n g s o l u t i o n a p p e a r e d t o b e t h e most i n f l u e n t i a l f a c t o r . I t is p s t u l a t e d t h a t c h a n g e s i n s u r f a c e t e n s i o n g r a d i e n t s and
s o l u b i l i t i e s during
s e l e c t i v e e v a p o r a t i o n from mixed s o l v e n t s a f f e c t t h e s u r f a c e p r o p e r t i e s of t h e d r i e d f i l m s . S c a n n i n g e l e c t r o n m i c r o s c o p y showed d i f f e r e n c e s i n u n i f o r m i t y and also i n d i c a t e d s u r f a c e c h a n g e s induced by e x p o s u r e o f t h e f i l m s t o l u b r i c a n t s a t e l e v a t e d (lOO°C) temperatures. The f o l l o w i n g t h r e e o i l s , r e p r e s e n t a t i v e of l u b r i c a n t s u s e d f o r m i n i a t u r e b e a r i n g s , were u s e d f o r c o m p a t a b i l i t y s t u d i e s ; M i l i t a r y
a formulated e s t e r - d i e s t e r instrument
S p e c i f i c a t i o n MIL-L-81846, o i l with
rl
= 25.5 mN/m,
u s e d i n b e a r i n g s i n normal o p e r a t i n g con-
d i t i o n s ; a n unformulated chlorophenyl p o l y s i l o x a n e ,
;yl
= 2 1 mN/m
,
u s e d f o r h i g h t e m p e r a t u r e b e a r i n g o p e r a t i o n ; and b i s ( 2 - e t h y l h e x y l ) sebacate, a d i e s t e r base stock, unformulated, with
rl
= 31.1 mN/m
( t h e s e b a c a t e was p e r c o l a t e d by a l u m i n a b e f o r e u s e ) . T r i p l y d i s t i l l e d water hexadecane
(rl (rl
= 72.0 = 27.6
mN/m), mN/m)
m e t h y l e n e i o d i d e (fl = 50.8 mN/m)
and
w e r e s t a n d a r d r e f e r e n c e l i q u i d s used
f o r c o n t a c t a n g l e measurement. M e t h y l e n e i o d i d e and h e x a d e c a n e had b e e n p u r i f i e d t h r o u g h v a r i o u s a d s o r b e n t columns p r i o r t o u s e . The w e t t a b i l i t y t e s t s show t h a t a f t e r immersion i n t h e o i l s t h e s u r f a c e o f t h e f i l m s became p i t t e d a n d rough and t h e c o n t a c t angles
were l o w e r t h a n b e f o r e immersion. These c h a n g e s , however, w e r e s m a l l , e s p e c i a l l y f o r f i l m s d e p o s i t e d o n m e t a l s ( p a s s i v a t e d a n d unp a s s i v a t e d b e a r i n g s t e e l s ) , The p o s s i b i l i t y o f o i l e x p o s u r e hcreasi n g t h e s u r f a c e p o l a r i t y w a s examined and t h e d e t e r m i n a t i o n ( u s i n g t h e g r a p h i c a l method o f Fowkes ( r e f . 4 1 0 ) ) of t h e d i s p e r s i o n comp o n e n t o f t h e s u r f a c e f r e e e n e r g y o f t h e PFOMA f i l m showed t h a t t h e d had d e c r e a s e d s l i g h t l y a f t e r o i l immersion i n d i c a t i n g a pssible JJs c h a n g e i n s u r f a c e p o l a r i t y ( f o r t h e c o n t r o l PFOMA f i l m s , d w a s very close to the
rc
rs
v a l u e , i n d i c a t i n g no p o l a r c o n t r i b u t i o n ) . P h y s i c a l
a l t e r a t i o n s by t h e l u b r i c a n t s u c h as s w e l l i n g , c r a z i n g a n d t h e l i k e
w e r e n o t s e e n , b u t should b e c o n s i d e r e d i n any d i s c u s s i o n of polym e r - o i l i n t e r a c t i o n s . I t would a p p e a r t h a t s u r f a c e r o u g h n e s s i s n o t a m a j o r f a c t o r , and t h a t a s l i g h t change i n t h e s u r f a c e p o l a r i t y o f PFOMA may o c c u r on o i l e x p o s u r e .
224
F l u o r i n a t e d m a t e r i a l s ( s u c h a s PFOMA) u s e d f o r b a r r i e r f i l m s have t h e i m p o r t a n t a d v a n t a g e o f n o t b e i n g r e a d i l y d i s s o l v e d by a n y of t h e c l e a n i n g s o l v e n t s commonly u s e d i n t h e i n s t r u m e n t f i e l d ( r e f . 4 0 2 ) . O r g a n i c s o l v e n t s s u c h a s x y l e n e o r p e t r o l e u m e t h e r had no
a d v e r s e e f f e c t o n PFOMA c o a t i n g s o n s t a i n l e s s s t e e l , b r a s s and aluminium s u r f a c e when t o t a l l y immersed f o r 3 weeks a t a m b i e n t t e m p e r a t u r e . Ammonia-containing aqueous w a t c h c l e a n i n g s o l u t i o n s a r e c a p a b l e o f d e t a c h i n g t h e c o a t i n g a f t e r p r o l o n g e d immersion w i t h o u t d i s s o l v i n g i t . F l u o r i n a t e d s o l v e n t s s u c h a s F r e o n TF u n d e r s t a n d a b l y a c t a s good s o l v e n t s o n t h e polymer. T h r e e i n d u s t r i a l l y m a n u f a c t u r e d a n t i - m i g r a t i o n coatings (epilams) a r e l i s t e d i n Table 6 . 2 .
These t h r e e c o a t i n g s are manufactured on
t h e b a s i s o f f l u o r o p o l y m e r m a t e r i a l and t h e c r i t i c a l s u r f a c e tension of w e t t i n g of t h e s e epilarnes i s t h e r e f o r e v e r y low. The a d m i s s i b l e t e m p e r a t u r e r a n g e f o r t h e d r i e d c o a t i n g s i s v e r y broad. The techniques f o r d e p o s i t i n g c o a t i n g s a r e v e r y s i m p l e a n d c h e a p . The d i s a d v a n t a g e of t h e s e anti-migration c o a t i n g s l i e s i n t h e handling o f them;there m u s t b e adequate v e n t i l a t i o n t o avoid vapour build-up and t h e s u r f a c e s need t o b e c a r e f u l l y c l e a n e d b e f o r e d e p o s i t i n g c o a t i n g s (see Chapter 6 . 2 . 6 )
.
I n v e s t i g a t i o n s c a r r i e d o u t i n r e c e n t y e a r s o n metal-on-polymer and polymer-on-polymer m i n i a t u r e s y s t e m s h a v e shown t h a t l u b r i c a t i n g such c o m b i n a t i o n s of m a t e r i a l s i s a v e r y e f f e c t i v e way t o s i g n i f i c a n t l y r e d u c e t h e i r w e a r r a t e s (see C h a p t e r 5 . 2 ) . T h e r e i s s t i l l a p r o b l e m , however, w i t h t h e o i l d r o p m i g r a t i o n from t h e rub-
b i n g r e g i o n . The c a u s e o f o i l m i g r a t i o n i n p o l y m e r i c s y s t e m s i s n o t q u i t e c l e a r ; p u r e p o l y m e r s have r e l a t i v e l y v e r y low (
< 60
rc
mN/m) ( r e f . 1 8 5 ) . Because o f t h e low work of a d h e s i o n W a be-
tween polymer and o i l , t h e d r o p c a n b e e a s i l y d i s p l a c e d o n t h e p o l ymeric s u r f a c e . Fox and Zisman ( r e f . 4 1 1 ) h a v e shown t h a t t h e s p r e a d i n g o f l i q u i d s on low-energy s o l i d s u r f a c e s d o e s n o t depend on t h e p o l a r and n o n p o l a r c o m b i n a t i o n s o f p o l y m e r s and l i q u i d s . The a d h e s i o n o f h y d r o c a r b o n s u r f a c e s t o l i q u i d s i n a l i p h a t i c h y d r o c a r b o n s wasbetter t h a n i n l i q u i d s c o n t a i n i n g o x y g e n ' o r f l u o r i n e b u t n o t as good a s i n a r o m a t i c h y d r o c a r b o n s . The a d s o r p t i o n e n e r g y of h y d r o c a r b o n l i q u i d s on low-energy s u r f a c e s w a s l a r g e l y d u e t o d i s p e r s i o n i n t e r a c t i o n s ; t h e o b s e r v e d h e a t s o f a d s o r p t i o n s e t a maximum v a l u e for t h e dispersion p o t e n t i a l i n t h e s e systems ( r e f . 4 1 2 ) . The i r r e g u l a r i t i e s and r o u g h n e s s o f r e a l i s t i c polymer e l e m e n t s a f f e c t t h e s p r e a d i n g o f o i l s . The f o r m a t i o n o f a n a d h e s i v e bond be-
TABLE 6 . 2 INDUSTRIALLY MANUFACTURED ANT I - M I GRAT ION COATINGS (EPI LAMES) *
DESCRIPTION AND PRODUCER PROPERT I ES
FC-721 3 M COMPANY MINNEAPOLIS, MINNESOTA (U.S.A.)
F1 uoropo 1 ymer Freon TF (2% s o l u t i o n )
Ma t e r ia 1 Solvent C r i t i c a l surface tension o f w e t t i n g ;Yc, o f d r i e d f i l m
11-12
mN/m
Density, mg mn-3 B o i i i n g p o i n t , OC F i l m t h i c k n e s s , ,urn
1.5 48 1.3 ( d i p c o a t i n g ) and 0.05 (FC-721 d i l u t e d t w e n t y f o l d )
ANTI SPREAD M2/200/50 D r . TlLLWlCH GmbH. HOKB-AHLDORF ( F .R .G .) F1 uoropo 1 yme r F r i g e n 113 (0.5 or 2% s o l u t i on) T e s t s i l i c o n e (50 mm2 s - ' a t 20°C) o i 1 forms a d r o p w i t h c o n t a c t a n g l e 5-45O 1.54
FIXDDROP BS, K MOEBIUS e t Fils,ALLSCHWIL, BASEL (SWITZERLAND) F I u o r o p o l ymer F r i gen 113 TR-T (Hoechs t ) Freon (Du Pont) S i l i c o n e o i I forms a d r o p w i t h c o n t a c t a n g l e a b o u t 45
1.54
47.6 0.5 (0.5% s o l u t i o n ) 2 (2% s o l u t i o n )
47.6
0.01 -0.05
Thermal s t a b i 1 i t y of d r i e d
film, Flash p o i n t Toxi c i t y Appearance Chemical s t a b i Ii t y
Product iv i t y Storage
1
OC
S t a b l e u p t o 205 Nonf lammable C l e a r and c o l o r l e s s Non-solubi l i z i n g s o l v e n t s such as heptane, t o l u e n e , acetone and w a t e r D i p p i ng, b r u s h i n g , s p r a y i ng . A i r d r y i n g : 15-20 s When i n c o n t a c t w i t h d i e s t e r o i l s , s h o u l d be baked f o r 15 min a t 100°C Should be s t o r e d a t o r near room t e m p e r a t u r e
-75 t o +200 Nonf 1ammab 1e A t 250°C f l u o r i n e i s s e c r e t e c C l e a r and c o l o r l e s s Non-sol u b i 1 i z i ng s o l v e n t s . Can be used f o r c o a t i ngs dep o s i t e d on polymers Dipping, b r u s h i n g , s p r a y i n g . D i p p i n g : 5-10 s a t 20°C. D r i e s a t between room temper a t u r e and 8OoC i n 1-15 m i n ca. T O O g p e r m2 12 months i n c l o s e d c o n t a i n e r
' - 7 5 t o +200
, Nonf lamnable A t 250OC f l u o r i n e i s s e c r e t e C l e a r and c o l o r l e s s Non-solubilizing solvents. Can be used for c o a t i n g s dep o s i t e d o n polymers (K) Dipping, b r u s h i n g , s p r a y i n g . D r i e s a t 5O-8O0C i n 2-5 m i n
12 months i n c l o s e d c o n t a i n e
N
N Ln
226
tween a l i q u i d and a s o l i d s u r f a c e depends on t h e development of a maximum a r e a o f m o l e c u l a r c o n t a c t and t h e d i s p l a c e m e n t o f a i r from the micro-irregularities
on t h e s u r f a c e . The r a t e of p e n e t r a t i o n o f
l i q u i d s i n t o c a p i l l a r i e s and s l i t s on t h e s u r f a c e a f f e c t s t h e dyna-
mics of f l o w . I n g e n e r a l t h e dynamics of s p r e a d i n g a r e c o n t r o l l e d by t h e c o e f f i c i e n t c o s E l m / ( g q ) ; where g i s a f a c t o r which depends on t h e geometry of t h e system ( 7 i s t h e v i s o s i t y of a l i q u i d )
r1
( r e f . 4 1 3 ) . The dynamics of DMPS s p r e a d i n g on PTFE depend on t h e volume of d r o p s and t h e r o u g h n e s s o f t h e s u b s t r a t e t o g e t h e r w i t h t e m p e r a t u r e and v i s c o s i t y ( r e f . 4 1 4 ) . The s p r e a d i n g v e l o c i t y w a s shown t o grow w i t h d e c r e a s i n g d r o p volume, t h e e f f e c t b e i n g more pronounced a t h i g h v i s c o s i t i e s of DMPS. The s p r e a d i n g on t h e rough s u r f a c e w a s s l o w e r t h a n on t h e smooth one owing t o t h e e n e r g y bar-
r i e r c r e a t e d by s u r f a c e i n h o m o g e n e i t i e s . An i n c r e a s e i n t e m p e r a t u r e increases t h e spreading velocity. The m i g r a t i o n of t h e o i l from p o l y m e r i c m i n i a t u r e b e a r i n g s i s p r o b a b l y a l s o a f f e c t e d by t h e a p p l i c a t i o n o f f i l l e r s , p l a s t i c i z e r s , s o l i d l u b r i c a n t s e t c . F i l l e r s such as g l a s s or c a r b o n r i s e t h e s u r f a c e f r e e e n e r g y of polymers ( r e f . 2 0 6 ) . The inhomogeneity of t h e s u r f a c e s c a n b e v e r y h i g h . The c o a t i n g ( e p i l a m e ) c a n b e u s e d t o make t h e polymer s u r f a c e uniform o r a s a b a r r i e r f i l m . The d e p o s i t i n g o f c o a t i n g s on polymers i s a q u i t e n o v e l and d i f f i c u l t problem. The f o l l o w i n g two ways a r e p o s s i b l e : 1) m o d i f i c a t i o n o f t h e s u r f a c e p r o p e r t i e s o f p o l y m e r i c s o l i d s by t h e a d s o r p t i o n of a p p r o p r i a t e e . g .
p a r t i a l l y f l u o r i n a t e d compounds a t p o l y -
mer-air i n t e r f a c e s d u r i n g t h e f o r m a t i o n o f t h e polymer s u r f a c e s , and 2 ) d e p o s i t i n g a t y p i c a l c o a t i n g ( e . g . f l u o r o p o l y m e r ) as on m e t a l l i c and m i n e r a l s u r f a c e . The f i r s t s o l u t i o n h a s been s t u d i e d i n t h e l a b o r a t o r y ( r e f . 4 1 5 ) . I t w a s found t h a t f o r t h e polymers b e i n g looked a t ( P S I PMMA, PAM, PVDC copolymer c o n t a i n i n g 2 0 % p o l y a c r y n i t r i l e ) t h e p a r t i a l l y f l u o r i n a t e d compounds u s e d ( a t concentrat i o n & 1%by w e i g h t ) w e r e a b l e t o d e c r e a s e t h e c r i t i c a l s u r f a c e
rcl
tension o f PMMA, PAM and PVDC from 39, 35-40 and 38-44 mN/m t o a b o u t 1 9 , 11 and 20 mN/m r e s p e c t i v e l y . The f l u o r i n a t e d compounds used w e r e so i n s o l u b l e i n t h e PS t h a t t h e y e i t h e r s e p a r a t e d o u t a s a n o t h e r p h a s e w h i l e t h e s o l v e n t e v a p o r a t e d o r else t h e y formed c l o u d y , opaque f i l m s . T h e s e l a r g e d e c r e a s e s i n c r i t i c a l s u r f a c e t e n s i o n r e f l e c t e d t h e change i n s u r f a c e c o m p o s i t i o n which had t a k e n p l a c e , t h e polymer m o l e c u l e s b e i n g r e p l a c e d i n t h e i n t e r f a c e by c l o s e l y packed CF2 and CF3 g r o u p s . The f l u o r i n e - c o n t a i n i n g s u r f a c e a c t i v e a g e n t s were e q u a l l y e f f e c t i v e when added t o t h e monomer prior
227 t o p o l y m e r i z a t i o n o r t o s o l u t i o n s o f t h e polymer i n a v o l a t i l e solv e n t . The less v i s c o u s t h e polymer i s , t h e more r a p i d l y a d s o r p t i o n e q u i l i b r i u m w i l l b e a t t a i n e d . One f u r t h e r p r o p e r t y of a f i l m formed by t h i s t e c h n i q u e i s t h a t it s h o u l d b e s e l f - h e a t i n g
-
t h a t i s , any
s u r f a c e a c t i v e m o l e c u l e s l o s t from t h e f i l m w i l l b e r e p l a c e d by t h e d i f f u s i o n o f a d d i t i o n a l m a t e r i a l i n t o t h e i n t e r f a c e . The r a t e o f s e l f - h e a t i n g w i l l be d e p e n d e n t upon t h e r a t e of d i f f u s i o n o f t h e f l u o r o c a r b o n d e r i v a t i v e s i n t h e b u l k polymer, and may b e accelerated by h e a t i n g t h e s o l i d polymer or o t h e r w i s e l o w e r i n g i t s v i s c o s i t y .
a normal polymer c o a t i n g s ( e p i 1 a m e s ) from t h e s o l u t i o n of base material (e.g. fluoropolymer) r e q u i r e s consideraThe d e p o s i t i o n
t i o n of t h e i n t e r a c t i o n s i n b a s e m a t e r i a l - p o l y m e r and ( e s p e c i a l l y ) s o l v e n t - p o l y m e r s y s t e m s . The r e s u l t s o f T i l l w i c h and S t e h r ' s studies ( r e f s . 4 1 6 , 4 1 7 ) showed t h a t f l u o r i n a t e d p o l y m e r s , p a r t i c u l a r l y fluorinated polyacrylates
,
c a n be used t o p r e p a r e c o a t i n g s (epilams)
f o r p o l y m e r i c s u r f a c e s . Of t h e c h l o r i n a t e d , a l c o h o l , e s t e r , and k e t o n e s o l v e n t s , t h e b e s t i s 1,1,2-trifluoro-1,2,2-trichloroethane
(CClF). I t i s i n e r t t o t h e polymers commonly u s e d i n p r e c i s i o n eng i n e e r i n g , e v a p o r a t e s r a p i d l y and i s r e s i s t a n t t o a g e i n g ( t h e r e
w e r e no p o l y m e r i z a t i o n e f f e c t s i n t h e a n t i - m i g r a t i o n c o a t i n g which w a s i n a c l o s e d c o n t a i n e r f o r 24 months and i n c o n t a c t w i t h a i r f o r 3 months).
The problem o f f i n d i n g a n optimum c o a t i n g f o r t h e s o l i d s u r f a c e o f materials u s e d i n p r e c i s i o n e n g i n e e r i n g n e e d s f u r t h e r s t u d y . E s p e c i a l l y important are t h e t r i b o l o g i c a l p r o p e r t i e s of epilames i f t h e y are a c c i d e n t a l l y p l a c e d i n t h e rubbing r e g i o n o r i f o i l g e t s o n t o t h e e p i l a m e s u r f a c e . I n t h e case o f hydrodynamic l u b r i c a t i o n o f a m i n i a t u r e b e a r i n g , t h e e p i l a m e b a r r i e r f i l m p l a y s an i m p o r t a n t r o l e i n p r e v e n t i n g , t h e o i l from s p r e a d i n g from t h e h i g h o i l p r e s s u r e r e g i o n . The v a l u a b l e e f f e c t o f l u b r i c a t i o n on t h e t r i b o l o g i c a l p r o p e r t i e s o f p o l y m e r i c m i c r o c o u p l e s and t h e problems w i t h o i l m i g r a t i o n from t h e r u b b i n g r e g i o n i n s u c h b e a r i n g s g i v e added importance t o t h e technique
o f e p i l a m i z i n g polymers. The e f f e c t s o f t h e
t r o b o e l e c t r i f i c a t i o n o f polymers must be c o n s i d e r e d i n p a r t i c u l a r . 6.2.5.
SELF-COATING (AUTOEPILAMIZING)
The i d e a o f s e l f - c o a t i n g i s v e r y s i m p l e . The o i l ( s p e c i a l l y p r e p a r e d o r w i t h s p e c i a l a d d i t i o n s ) does n o t s p r e a d a f t e r b e i n g l a i d on t h e s o l i d s u r f a c e b u t a l a y e r w i t h low s u r f a c e e n e r g y i s formed. T h i s l a y e r i s formed a s t h e r e s u l t o f c h e m i c a l a c t i o n o r
the selective adsorption of molecules e.g. the very active molecules of the special additives in the lubricant. Attempts have been made to synthesize a self-coating oil at the Naval Research Laboratory in Washington D.C. (refs. 367, 402, 410). Three classes of non-spreading liquids have been distinguished: 1) Autophobic liquids examplified by molten stearic acid, octyl alcohol, tricresyl phosphate and trichlorodiphenyl, 2 ) Numerous esters able to spread completely on metal surfaces b u t unable to spread on glass, silica or sapphire. The ester hydrolyzes immediately upon adsorbing onto these hydrated solid surfaces; of the two products of the hydrolytic reaction, the one with the greater average lifetime of adsorption remains to coat the surface with a close-packed monolayer thereby blocking further progress of the hydrolysis reaction. If this protective monolayer has a critical surface tension of wetting which is less than the surface tension of the liquid ester, nonspreading results, 3 ) Liquids whose surface tension is so high and adhesional energy so low that the energy of adhesion is lower than that of cohesion and spreading is thus thermodynamically impossible. If such liquids existed they would differ from auto-phobic liquids in not leaving a film behind them when rolled over a horizontal polished solid surface. Pure non-spreading instrument oils with good lubrication properties and chemical stability have unfortunately never been found. The best results have been obtained with the esters. The oils can be prevented from spreading by the addition of selected solutes which act in one of three ways. The first way is based on the ability of the solute to adsorb onto a high-energy surface and form a monolayer with a critical surface tension of wetting less than the surface tension of the original oil. The second way is based on the addition of a more volatile solute which creates a surface tension gradient at the edge of an oil drop that opposes the spontaneous spreading of the oil. The third way is based on the ability of the solute to react chemically with the solid surface, forming a low energy surface. The autoepilamizing effect of the DMPS oils is obtained (refs. 6 1 , 68, 288) by adding < l % of oxidized fluorinated polysiloxane (with groups CF2-CF2) to the original DMPS. The DMPS oil with the addition of 1% of polar fluorinated polysiloxane formed, on metal surfaces, drops with a contact angle of 12-15'. However the contact
-
229
a n g l e f o r a 0.5% s o l u t i o n ( o f t h e o x i d i z e d f l u o r o p o l y s i l o x a n e i n DMPS) on metal s u r f a c e s w a s a b o u t 35'.
I t h a s been i m p o s s i b l e t o
achieve t h i s autoepolamizing e f f e c t f o r t h e o t h e r polysiloxanes w i t h t h i s method b e c a u s e o f problems o f s o l u b i l i t y . The r e s u l t s of some i n t e r e s t i n g i n v e s t i g a t i o n s a r e r e p o r t e d i n r e f . 4 1 9 . 90 of t h e v a r i o u s f l u o r i n a t e d compounds ( w i t h g r o u p s -CF3 and -CF2) w e r e t e s t e d . The b a s e o i l s were: met hyl al kyl pol ys i l oxanes , chlorinated methylpolysiloxanes, f l u o r i n a t e d polysiloxanes, mineral o i l s , s y n t h e t i c esters and some multicomponent i n s t r u m e n t o i l s (MBC-30-9, NS-6n, MBL-12, Synt-A-Lube). T h e s e i n v e s t i g a t i o n s proved t h a t t h e a d d i t i o n o f f l u o r i n a t e d c a r b o n a c i d s C6F13COOH with 0.2-0.3% by w e i g h t i s e f f e c t i v e i n p r e v e n t i n g a l l o f t h e t e s t e d o i l s from s p r e a d i n g . The c o n t a c t a n g l e s of t h e a u t o e p i l a m i z i n g o i l s r a n g e d from 15'
t o 60°.
The d i f f e r e n c e s i n t h e c o n t a c t a n g l e f o r s t e e l ,
b r a s s and r u b y s u r f a c e s were v e r y small. The c o n t a c t a n g l e s o f MBL-12
and Synt-A-Lube o i l s c o n t a i n i n g 0 . 3 % C6F13COOH were 2 0 and
12O respectively on steel surfaces. A t t h e drop diameters 1 . 0 0
MBL-12)
and 0.95 mm (Synt-A-Lube)
the e f f e c t i v e drop "shearing"
c e n t r i g u g a l a c c e l e r a t i o n was o v e r 4 0 0 g ( f o r p u r e o i l s a b o u t 15 g and 4 g f o r MBL-12 and Synt-A-Lube
oils respectively).
The a u t o e p i l a m i z i n g e f f e c t on t h e s t e e l and s a p p h i r e s u r f a c e s o f t h e b i s ( 2 - e t h y l e x y l ) s e b a c a t e (EHS) , a l i p h a t i c esters, squalane, n-hexadecane, m i n e r a l o i l and p o l y s i l o x a n e s (DC 200, DC 510, DC 550, DC710) was a c h i e v e d ( r e f . 4 2 0 ) by i n c l u d i n g a f l u o r i n a t e d h y d r o c a r -
bon o r p o l y s i l o x a n e compound i n t h e t e s t l i q u i d . The n o n s p r e a d i n g e f f e c t w a s a c h i e v e d by l o w e r i n g ( w i t h t h e a d d i t i v e ) t h e s u r f a c e t e n s i o n o f o r i g i n a l l i q u i d by l e s s t h a n 5-6 mN/m.
The a d d i t i o n o f
f l u o r i n a t e d hydrocarbon r e s u l t e d i n t h e f o r m a t i o n o f w e l l - a d s o r b e d -CF3 o r -CF2 monolayers. The p o l y s i l o x a n e formed v e r y t h i c k adsorbed f i l m s on s t e e l , b r a s s , s a p p h i r e , q u a r t z and g l a s s ; t h e s e f i l m s had low
yc
and o i l d i d n o t s p r e a d on them. The c o n c e n t r a t i o n of p o l y -
siloxane i n t h e l i q u i d s t e s t e d (except i n squalane) w a s
< 1 % (by
w e i g h t ) and maximum 5% i n t h e case o f f l u o r o e s t e r s . The l o n g t e r m n o n s p r e a d i n g o f t h e t e s t e d l i q u i d s w a s a c h i e v e d when t h e a d d i t i o n o n l y s l i g h t l y d e c r e a s e d t h e s u r f a c e t e n s i o n o f t h e l i q u i d and had good s e l e c t i v e a d s o r p t i o n p r o p e r t i e s ( e . 9 . a d d i t i o n o f t h e p o l y s i l o x a n e t o a l i p h a t i c e s t e r s o r f l u o r i n a t e d e t h e r s t o EHS) forming l a y e r s with
rc <(
;yl.
The a d d i t i o n o f a n o n v o l a t i l e compound, l o w e r -
i n g s l i g h t l y t h e s u r f a c e t e n s i o n o f t h e l i q u i d and forming t h e s u r face layer with
y c = yl,
a f f e c t t h e base l i q u i d i n s u c h a manner
t h a t i n t h e b e g i n n i n g t h e d r o p s p r e a d s , b u t t h e n l a t e r decomposes
230 i n t o s e v e r a l s m a l l , nonspreading d r o p s . The e a s y s l i d i n g of t h e nonspreading d r o p s o v e r t h e s o l i d s u r f a c e was c h a r a c t e r i s t i c of t h e l i q u i d s c o n t a i n i n g t h e amine s a l t s o f f l u o r i n a t e d carbon a c i d s . T o t a l s p r e a d i n g o c c u r r e d i n t h e c a s e o f d r o p s o f l i q u i d s w i t h v e r y a c t i v e ' p o l a r a d d i t i o n s lowering t h e s u r f a c e t e n s i o n more t h a n 6 mN/m and a d s o r b i n g s t r o n g l y o n t o t h e s o l i d surf aces. Research ( r e f . 418) h a s shown t h a t t h e b e s t a d d i t i o n compounds a r e t h o s e which have a s i m i l a r chemical s t r u c t u r e t o t h e b a s e l i q u i d , a r e more v o l a t i l e and have g r e a t e r s u r f a c e t e n s i o n . S p r e a d i n g of t h e d i s t i l l e d s q u a l a n e was e f f e c t i v e l y p r e v e n t e d by adding a 5% s o l u t i o n of i s o p r o p y l b i p h e n y l , which h a s a s u r f a c e t e n s i o n g r e a t e r t h a n 7 mN/m and a b o i l i n g p o i n t 50'
lower t h a n s q u a l a n e .
I t i s c l e a r from t h i s a n a l y s i s o f t h e r e s e a r c h c a r r i e d o u t so
f a r t h a t t h e problem o f a u t o e p i l a m i z i n g i n s t r u m e n t o i l s i s v e r y complex and t h e s u g g e s t e d s o l u t i o n s a r e f a r from s a t i s f a c t o r y . The chemical s t r u c t u r e and c o n c e n t r a t i o n of t h e a d d i t i o n must be v e r y c a r e f u l l y s e l e c t e d . The compound must be e a s i l y s o l u b l e i n t h e o i l a t a p p l i c a t i o n t e m p e r a t u r e , a d s o r b q u i c k l y from t h e s p r e a d i n g edge of t h e d r o p and form a s u r f a c e l a y e r w i t h
rc<
The s u r f a c e ten-
s i o n of t h e b a s e o i l must n o t b e lowered by more t h a n 5-6 mN/m. N o n - v o l a t i l i t y and chemical s t a b i l i t y on t h e s o l i d s u r f a c e a r e a l s o v e r y i m p o r t a n t r e q u i r e m e n t s which a r e d i f f i c u l t t o s a t i s f y . 6.2.6.
COATING (EPILAME) TECHNOLOGY
The s e l e c t i o n o f t h e c o r r e c t technology f o r e p i l a m i z i n g i s extremely i m p o r t a n t . Here by " t e c h n o l o g y " w e u n d e r s t a n d t h e p r o c e d u r e f o r preparing the s o l i d surface t o receive t h e coating (epilame), t h e p r o c e s s o f d e p o s i t i n g , and checking t h e q u a l i t y o f t h e c o a t i n g . The s o l i d s u r f a c e s h o u l d be v e r y c l e a n b e f o r e t h e d e p o s i t i o n o f a c o a t i n g , to e n s u r e a s t r o n g a d h e s i v e bond between t h e c o a t i n g and t h e s o l i d s u r f a c e . There a r e many c l e a n i n g t e c h n i q u e s t o choose from ( r e f s . 421-423). Elements which a r e assembled on a p r o d u c t i o n l i n e t o keep c o s t s low a r e u s u a l l y c l e a n e d by chemical methods. Cleaning t e c h n i q u e s u s i n g a s o l v e n t a r e e f f e c t i v e enough f o r prepari n g a s o l i d s u r f a c e f o r t h e d e p o s i t i o n o f an e p i l a m e ( r e f s . 95, 424). The e l e m e n t s are u s u a l l y c l e a n e d by u s i n g s e v e r a l s o l v e n t s s u c c e s s i v e l y : f o r example non-polar ( e . g . b e n z i n e ) followed by p o l a r (such as water o r a l c o h o l ) . I f water i s used, t h e l a s t s t e p i n t h e c l e a n i n g p r o c e d u r e i s t h e removal of i t s r e s i d u e , f o r example w i t h
231 a c e t o n e , or by u s i n g a c e n t r i f u g e o r d r y i n g w i t h h o t a i r . Microelements such a s watch components a r e u s u a l l y c l e a n e d once i n b e n z i n e ,
twice i n s o a p s o l u t i o n s ( a t a t e m p e r a t u r e o f 60-70°C) , t h r e e times i n d i s t i l l e d water ( a t 60-70°C) and t w i c e i n a c e t o n e . B e f o r e e a c h washing p r o c e d u r e t h e e l e m e n t s a r e d r i e d u s i n g a c e n t r i f u g e ( r e f . 95). U l t r a s o n i c s i s normally used t o c i r c u l a t e t h e c l e a n i n g l i q u i d s i n t h e b a t h s . The t y p i c a l f r e q u e n c y o f t h e u l t r a s o n i c h e a d s u s e d t o m i c r o e l e m e n t s i s a b o u t 2 0 kHz. The t i m e r e q u i r e d t o c l e a n a n e l e ment depends on power o f t h e h e a d and t h e s i z e o f t h e s u r f a c e t o be c l e a n e d ( r e f . 4 2 5 ) . The optimum power l o s s d u r i n g u l t r a s o n i c c l e a n i n g i n water i s 1.5-2 l o 2 W / m m 2 and i n o r g a n i c s o l v e n t s 1-1.5 l o 2 W/mm2.
O f t e n t h e u l t r a s o n i c c l e a n i n g i s completed w i t h a n o r d i n a r y
washing p r o c e s s . The e v a l u a t i o n of t h e c l e a n l i n e s s of t h e s o l i d s u r f a c e i s d i f f i c u l t . T h e r e a r e many methods b u t t h e i r e f f i c i e n c y depends on t h e c l e a n l i n e s s r e q u i r e m e n t s . For i n d u s t r i a l a p p l i c a t i o n s , t h e o p t i c a l methods a r e v e r y handy and c h e a p ( r e f . 4 2 4 ) . W e t t a b i l i t y i s a l s o commonly employed t o m o n i t o r t h e c l e a n l i n e s s of a s u r f a c e . The other, more s o p h i s t i c a t e d methods a r e e l l i p s o m e t r y , m i c r o f l u o r e s c e n c e , evap o r a t i v e r a t e a n a l y s i s (ERA) and s p e c t r o s c o p i c methods ( A E S , ESCA, I S S , SIMS)
( r e f . 421, C h a p t e r 8 . 6 ) .
A c o a t i n g ( e p i l a m e ) c a n b e d e p o s i t e d on t h e s o l i d s u r f a c e by
d i p p i n g , b r u s h i n g o r s p r a y i n g . Dip c o a t i n g i s u s u a l l y t h e most conv e n i e n t and p r o d u c e s a u n i f o r m f i l m . Dipping c a n l a s t anywhere between a few s e c o n d s and o v e r h a l f a m i n u t e f o r t h e modern PFOMA e p i l a m e s . A f i n e c a m e l - h a i r b r u s h c a n b e u s e d t o p a i n t b a n d s o f any d e s i r e d w i d t h (between 1 and 3 mm f o r e x a m p l e ) . T h i s method i s u s u a l l y a p p l i e d w i t h t h e Stop-Oil t e c h n i q u e . Spraying i s a l s o v e r y e f f i c i e n t b u t d o e s n o t p r o d u c e a v e r y u n i f o r m f i l m and h a n d l i n g problems a r i s e (e.9. t h e need f o r a d e q u a t e v e n t i l a t i o n ) . The f r e s h c o a t i n g ( e p i l a m e ) i s u s u a l l y d r i e d i n a i r a t room t e m p e r a t u r e o r h o t t e r (see T a b l e 6 . 2 ) . The f i l m c a n a l s o b e c u r e d a t 5OoC i n vacuo f o r 3 t o 4 h o u r s ( r e f . 4 0 9 ) . The t y p e o f s o l v e n t and t h e mode o f f i l m d r y i n g a f f e c t t h e p r o f i l e s o f t h e f i l m . The c o m p o s i t i o n and f i l m p r o p e r t i e s o f b a r r i e r f i l m s o l u t i o n s ( a s i n t h e case o f PFOMA compounds) a n d t h e p r o f i l e s o f a d e q u a t e b a r r i e r f i l m s o b t a i n e d d u r i n g d r y i n g and c u r i n g i n vacuo are p r e s e n t e d ( b a s e d on r e f . 409) i n T a b l e 6 . 3 and F i g . 6.4. An example of t h e i n d u s t r i a l p r o c e d u r e f o r d e p o s i t i n g F i x o d r o p BS e p i l a m e f i l m s f o l -
l o w s ( r e f s . 6 1 , 4 2 6 ) . The c l e a n e d a n d d r i e d e l e m e n t s , p l a c e d i n a
232
b a s k e t , are dipped i n t o t h e epilame b a t h f o r 30 t o 60 seconds w i t h s l o w r o t a t i o n . A f t e r t h e t r e a t m e n t , t h e b a s k e t i s l i f t e d o u t o f the l i q u i d and t h e e x c e s s e p i l a m e i s e x p e l l e d b y h i g h s p e e d r o t a t i o n f o r a few s e c o n d s . The e l e m e n t s t h e n have t o b e d r i e d f o r 2 t o 5 m i n u t e s a t 50 t o 8OoC.
I t i s e s s e n t i a l t o d r y t h e e l e m e n t s immedi-
a t e l y a f t e r d i p p i n g because a s a consequence of t h e h i g h v o l a t i l i t y of f r i g e n i n high atmospheric humidity, condensation can p r e v e n t the proper formation of t h e epilame film. TABLE 6.3. COMPOSITION AN0 FILM PROPERTIES OF B A R R I E R FILM SOLUTIONS ( r e f . 409)
CURE FILM PROPERTI ES
SOLUTION COMPOSITION
DRY FILM TYPE (See Fig.6.4)
SOLVENT^
POLYMER PERCENTAGE
2.0
H FX
2.0 2.0
CCI F PCE HFX HFX/CC PCE/CC PCE/CC PCE/CC PCE/CC
0.5
0.5 0.5
0.5 0.2 0.2
F 75/25 F 75/25 F 90/10 F 90/10 F/HFX 80/10/10
L
A
APPEARANCE
I
I
Retracted, raised centre Wavy, raised-edge: Smooth Retracted Retracted Wavy s u r f a c e Smooth Smooth Retracted, raised centre
I
a: HFX PCE
-
h e x a f l u o r o x y l e n e , C C l F - 1,1,2-trifluoro-1,2,2-trichloroethane, p e r f l u o r o c y c l i c e t h e r , C8F,60. I
Solution A
c
m
F i g . 6.4.
-
Drying
n
-
Cured
.~arrierfilm
substrate
0
B a r r i e r f i l m p r o f i l e s d u r i n g d r y i n g (see T a b l e
6.3)
233
The q u a l i t y o f t h e e p i l a m e f i l m depends on i t s s u r f a c e f r e e e n e r g y , m e c h a n i c a l p r o p e r t i e s , a d h e s i o n t o t h e s o l i d s u r f a c e , topography ( o n t h e s o l i d s u r f a c e ) , t h i c k n e s s and o i l r e s i s t a n c e . The s u r f a c e f r e e e n e r g y c a n be a p p r o x i m a t e d by t h e v a l u e o f t h e c r i t -
i c a l s u r f a c e t e n s i o n of w e t t i n g yc i n t r o d u c e d by Zisman ( r e f . 3 8 7 ) . Measuring t h e c o n t a c t a n g l e 0 u s i n g a homologous s e r i e s o f l i q u i d s i s a s i m p l e way t o f i n d t h e v a l u e o f
7,.
R o t a t i n g t h e d r o p allows
more p r e c i s e measurements t o be made o f t h e s u r f a c e and a l s o of the i n t e r f a c i a l t e n s i o n o f t h e e p i l a m e - l i q u i d s y s t e m s ( r e f . 4 2 7 ) . The method proposed by Owens and Wendt ( r e f . 3 9 2 ) g i v e s t h e v a l u e s o f bonding component ponent
2:
rs
=
r;
7;
( p o l a r component) and d i s p e r s i o n f o r c e com-
o f t h e s u r f a c e f r e e e n e r g y r s f o l l o w i n g Fowkes ( r e f . 4 2 8 ) : h
1,
d
+
Ys
Using two l i q u i d s ( e . g . water and m e t h y l e n e i o d i d e ) w i t h known and
rf
and measuring t h e c o n t a c t a n g l e s f3 t h e v a l u e s o f
ft
and
y $ o f t h e e p i l a m e f i l m c a n b e e s t i m a t e d from t h e s y s t e m o f two e q u a t i o n s b a s e d on t h e f o l l o w i n g m o d i f i c a t i o n o f Young's e q u a t i o n :
More p r e c i s e v a l u e s of
r:
andf:
of t h e e p i l a m e f i l m c a n b e e v a l -
u a t e d u s i n g t h e formula f o r c a l c u l a t i o n o f i n t e r f a c i a l t e n s i o n ( f o r p o l y m e r - l i q u i d or polymer-polymer s y s t e m s ) ( r e f . 393)
rsl
ysl
p r o p o s e d by Wu
When i s known, by u s i n g Young's e q u a t i o n and d e t e r m i n i n g t h e v a l u e s o f 0 f o r two l i q u i d s w i t h known a n d e , i t i s p o s s i b l e , as
rt
w i t h t h e Owens and Wendt method, t o e v a l u a t e t h e epilame f i l m .
yz
and
J I ;o f
the
O t h e r p o s s i b l e methods f o r t h e d e t e r m i n a t i o n o f f s o f t h e e p i l a m e f i l m are t h e methods b a s e d on t h e known s o l u b i l i t y prmter
or p a r a c h o r o r t h e method w i t h t h e d e t e r m i n a t i o n o f t h e i n t e r m o l e c u l a r interacations ( r e f s . 429-431,
5 9 6 ) . The s i m p l e method f o r ap-
p r o x i m a t e e v a l u a t i o n o f t h e s u r f a c e e n e r g e t i c s o f an e p i l a m e f i l m
yl.
i s t o t e s t i t u s i n g t e s t l i q u i d s w i t h a known The s e r i e s o f t e s t l i q u i d s may c o n s i s t o f , f o r example, p o l y s i l o x a n e s ( e . g . w i t h
234
r1 of
19.7
,
21.9 and 23.1 mN/m
( r e f . 432)). For t h e t e s t i n g o f t h e
s u r f a c e e n e r g e t i c s o f t h e A n t i s p r e a d e p i l a m e s manufactured by Dr. T i l l w i c h GmbH i n Horb-Ahldorf
(F.R.G.)
test s i l i c o n e o i l (with a
v i s c o s i t y of 50 mm2/s a t 2OoC) i s u s e d ( r e f . 433). A d r o p o f t h i s
t e s t o i l w i t h a d i a m e t e r of a b o u t 1 mm c a n n o t s p r e a d on t h e Antis p r e a d e p i l a m e f i l m and t h e c o n t a c t a n g l e s h o u l d be 5-45O. To d e t e r m i n e t h e s u r f a c e e n e r g e t i c s o f s t r o n g l y curved s o l i d s u r f a c e s i s r a t h e r d i f f i c u l t . I n t h i s case t h e a n a l y s i s of t h e s p r e a d i n g dynamics o f t h e t e s t l i q u i d can be proposed a s a method. The a u t h o r h a s shown ( r e f . 206) t h a t t h i s method can be s a t i s f a c t o r i l y used t o e s t i m a t e t h e s u r f a c e f r e e e n e r g y o f polymer s u r f a c e s w i t h a c u r v a t u r e r a d i u s o f a b o u t 1 mm.
The methods used t o t e s t t h e mechanical p r o p e r t i e s and t h e adh e s i o n o f p o l y m e r i c c o a t i n g s t o s o l i d s u r f a c e s can be a p p l i e d t o t h e i n v e s t i g a t i o n of epilame f i l m . . An example o f t h e s e methods i s t h e s i m p l e p e e l t e s t , where t h e f o r c e r e q u i r e d t o p e e l t h e s u r f a c e a p a r t o r t h e a n g l e a t which t h e s u r f a c e s s p o n t a n e o u s l y r e - a d h e r e can be determined ( r e f s . 430, 431, 434). R o l l i n g methods, where a r i g i d c y l i n d e r o r s p h e r e r o l l s down an i n c l i n e d p o l y m e r i c p l a n e , o r t h e method o f Schallamach waves or c u r v e s
o f detachment moving
through t h e polymeric i n t e r f a c e can a l s o be a p p l i e d . The t h i n epilame f i l m s a r e t r a n s p a r e n t and a r e d i f f i c u l t t o see w i t h t h e naked e y e . T h i s d i f f i c u l t y s e r i o u s l y hampers i n s p e c t i o n o f
t h e f i l m s f o r l o c a t i o n and c o n t i n u i t y . F i t z Simmons from t h e Naval Research L a b o r a t o r y h a s proposed ( r e f . 435) t h e i n c o r p o r a t i o n i n t h e fluoropolymer s o l u t i o n of a f l u o r o s c e n t i n d i c a t o r so t h a t t h e r e s u l t i n g s o l i d i f i e d c o a t i n g might f l u o r e s c e s u f f i c i e n t l y under u l t r a v i o l e t r a d i a t i o n (UV) t o be r e a d i l y d e t e c t a b l e . T e s t s were c a r r i e d o u t f o r 2 s o l u t i o n s by weight i n x y l e n e h e x a f l u o r i d e of a
perfluoroalkyl-substituted m e t h a c r y l a t e polymer. The e x p e r i m e n t a l f o r m u l a t i o n c o n t a i n e d , i n a d d i t i o n , a f l u o r e s c e i n dye a c o n c e n t r a t i o n of 0.2% based on t h e weight of t h e f l u o r o p o l y m e r . The f i n a l s o l u t i o n was f i l t e r e d t o remove any i n s o l u b l e haze. The dye was a h i g h p u r i t y , s o l v e n t s o l u b l e , h e a t s t a b l e ( t o 315OC) m a t e r i a l which a b s o r b s W above 300 nm and d t s t h i s as a v i s i b l e b l u i s h l i g h t . The f l u o r e s c e n t b a r r i e r f i l m s i n v e s t i g a t e d were n o t a d v e r s e l y a f f e c t e d by t h e h e a t c u r e i n t h e d e p o s i t i o n of t h e c o a t i n g . Fluor e s c e n t b e h a v i o u r under l o n g wave UV was shown t o p e r s i s t , a l b e i t d i m i n i s h e d , i n s o l i d i f i e d f i l m s a f t e r s t o r a g e a s l o n g a s 44 months. I t was a l s o v e r i f i e d t h a t n e i t h e r a f i v e - y e a r o l d f l u o r e s c e n t s o l u -
t i o n nor f i l m s f r e s h l y p r e p a r e d from it a t t h a t l a t e d a t e w i l l re-
235 spond t o s h o r t wave W , e v e n t h o u g h a s o l i d i f i e d c o a t i n g s t o r e d d r y f o r t h a t l e n g t h of t i m e remains f l u o r e s c e n t . T h e r e w a s , however, no i m p a i r m e n t of t h e b a r r i e r e f f e c t i v e n e s s i n c o a t i n g s d e p o s i t e d from a s o l u t i o n which h a d s u f f e r e d a l o s s o f f l u o r e s c e n c e t h r o u g h ageing. The b a r r i e r p e r f o r m a n c e a l s o remainded u n a f f e c t e d e v e n f o r a s o l i d i f i e d f i l m which had b e e n e x p o s e d t o water ( v a p o u r o r b u l k ) s u f f i c i e n t l y f o r its fluorescence t o be a s a r e s u l t of t h e fluorescent compound d i s s o l v i n g . C o n t a c t a n g l e measurements f o r a v a r i e t y o f o r g a n i c l i q u i d s and o i l - r e t e n t i o n experiments with a chlorinated s i l i c o n e demonstrated t h a t t h e r e a r e no e s s e n t i a l d i f f e r e n c e s b e t w e e n t h e e x p e r i m e n t a l f l u o r e s c e n t f o r m u l a t i o n and c u r r e n t commercial f o r m u l a t i o n s w i t h r e s p e c t t o b a r r i e r performance, e i t h e r f o r f r e s h l y p r e p a r e d b a r r i e r f i l m s o r f o r a g e d f i l m s . The f i l m s r e m a i n e d t i g h l y a d h e r e n t t o t h e m e t a l s u b s t r a t e , e v e n a f t e r p r o l o n g e d s t o r a g e (34 months) i n a water-saturated
atmosphere w h i l e i n s i m u l t a n e o u s d i r e c t c o n t a c t w i t h
a chlorinated lubricating o i l . The m a j o r drawback o f f l u o r e s c e n t b a r r i e r f i l m s i s , however, t h e l o c a l v a r i a t i o n i n f l u o r e s c e n t i n t e n s i t y which r e s t r i c t s i t s u s e f o r m o n i t o r i n g t h e c o n t i n u i t y o f t h e c o a t i n g s . The l o s s o f f l u o r e s c e n c e due t o h u m i d i t y i s a l s o a c h a r a c t e r i c t i c o f f l u o r e s c e n t b a r -
r i e r f i l m s . When i n s t r u m e n t b a l l b e a r i n g s f o r example o p e r a t e u n d e r c o n d i t i o n s w h i c h i n v o l v e r o t a t i o n a l s p e e d s a s h i g h a s 2 4 0 0 0 rpm and t e m p e r a t u r e s i n e x c e s s o f 175OC i n c o n d i t i o n s o f h i g h h u m i d i t y , t h e a p p l i c a t i o n o f f l u o r e s c e n t b a r r i e r f i l m s i s n o t recommended ( r e f s . 435, 4 3 7 ) . A s t u d y t o o b t a i n a f l u o r e s c e n t dye s u i t a b l e t o r e p l a c e t h e p r e s e n t l y u s e d o r g a n i c dye was p r e s e n t e d r e c e n t l y by M e s s i n a ( r e f . 436, see a l s o C h a p t e r 9 . 8 ) . The method f o r t h e i n s p e c t i o n o f a b a r r i e r f i l m p r o p o s e d by B o r j a ( r e f . 437) seems a more e f f e c t i v e way t o v e r i f y t h e p r o p e r a p p l i c a t i o n o f b a r r i e r f i l m s . The b a r r i e s f i l m c a n b e o b s e r v e d and i t s t h i c k n e s s measured by means o f a p o l a r i z a t i o n i n t e r f e r o m e t e r . I n t e r f e r e n c e f r i n g e s a r e superimposed on t h e b a r r i e r f i l m s u r f a c e s . The i n t e r f e r e n c e f r i n g e s , t h e l i n e s of c o n s t a n t p h a s e d i f f e r e n c e ,
a r e l i k e c o n t o u r l i n e s of t h e b a r r i e r f i l m s u r f a c e which f o l l o w each o t h e r a t a l e v e l s p a c i n g e q u a l t o h a l f a wavelength (A) o f t h e t r a n s m i t t e d l i g h t . The u s e o f a sodium v a p o u r lamp
(A=
589 nm)with
monochromatic l i g h t of e x c e l l e n t q u a l i t y e n a b l e s a c c u r a t e m e a s u r e m e n t s t o b e made b e c a u s e t h e i n t e r f e r e n c e b a n d s a p p e a r i n p a r t i c u l a r l y good c o n t r a s t . S m a l l l o c a l i r r e g u l a r i t i e s on t h e s u b s t r a t e , s u c h a s s c r a t c h e s , a r e c l e a r l y shown up by d i s p l a c e m e n t s o f t h e i n -
236
terference bands. A single scratch across the observed area of the barrier film surface without scoring enables the metal substrate to be observed by fringe displacement. The nth part of the fringe spacing represents a depth (h) of the surface variation or scratch i.e.,
The displacement is measured in terms of n. If, for example, the fringes are displaced at a step by three quarters of a fringe spacing, then n = 4 / 3 . A qualitative scale utilizing light optics microscopy (with crossed polars and coaxial lighting) has been established relating thickness to colour (i.e. the thickness can be directly related to the colour of the film). A film of optimum thickness (less than 2 5 0 nm) is invisible (transparent) but has a slight brownish tinge. Thicker, less desirable films (on the steel surface of the ball bearing) have Newtonian fringes (rainbow colours). Even thicker , unacceptable films are grey and are very susceptible to mechanical abrasion and flaking. The optimum thickness, of the film less than 2 5 0 nm, ensures maximum resistance to abrasion while maintaining non-wettabiiity and selective uniformity (as shown by experiments). From the experiments reported earlier by Kinzig and Ravner (ref.409), it appears that the PFOMA films were usually thin enough to exhibit interference colour zones; thicknesses of between 5 0 nm and 1 5 0 0 nm were estimated with an interference colour gauge calibrated in25 nm steps, Summarizing, we can conclude that by utilizing the aforementioned interference technique, particularly the thickness versus colour scale, production plant personnel can effectively and economically verify the proper application of invisible barrier film. Testing the oil resistance of any epilame film is usually reduced to checking the initial wetting of the film and the reaction to it. The film samples are immersed in the oil for certain time intervals at controlled temperatures, usually elevated. Then they are washed with detergent and water to make them free of oil. The measurement of contact angles with the test fluids has been shown to be a sensitive detector of surface changes (refs. 4 0 2 , 4 0 3 , 4 0 5 , 409, 435). The advancing contact angle with these liquids should be measured before and after oil immersion. SEM analysis of the film surface is also an efficacious tool to reveal the surface changes after oil immersion (ref. 409). It could be clearly seen (at 1 0 0 0 magnification) that the surface of a con-
237 t r o l PFOMA f i l m b e f o r e immersion was so smooth t h a t a s c r a t c h w a s e s s e n t i a l l y t h e o n l y v i s i b l e f e a t u r e . The s u r f a c e became p i t t e d and rough a f t e r immersion f o r 1 9 h o u r s a t 100°C
i n t h e o i l s used f o r
t h e l u b r i c a t i o n of m i n i a t u r e b e a r i n g s : e s t e r - d i e s t e r w i t h mN/m,
unformulated chlorophenyl p o l y s i l o x a n e
b i s (2-ethylhexyl) sebacate with
r1
= 31.1
xl =
r1
2 1 . 0 mN/m
= 25.5
and
mN/m w i t h uneven f i l m s ,
p i t t i n g o c c u r r e d m o s t l y i n t h e t h i c k e r r e g i o n s , and t h e t h i n n e r
areas w e r e less a f f e c t e d . Small p a t c h e s and b l i s t e r s a p p e a r e d where t h e f i l m d i d n o t a d h e r e f i r m l y t o t h e s u b s t r a t e ( g l a s s and m e t a l ) ; where t h e f i l m w a s s c r a t c h e d it was s e e n t o p u l l away from t h e sub-
s t r a t e . S p e c t r o s c o p i c methods c a n be u s e d t o i n s p e c t t h e e f f e c t s o f c h e m i c a l r e a c t i o n s i n t h e o i l - e p i l a m e f i l m s y s t e m . The ESCA i s a s p e c t r o s c o p i c t o o l p a r e x c e l l e n c e f o r s t u d y i n g i n c o n s i d e r a b l e det a i l a s p e c t s o f s t r u c t u r e and bonding i n t h e s u r f a c e r e g i o n s of p l ymers. The t y p i c a l sampling d e p t h i s < 1 0 nm and s i n g l e ESCA e x p e r iment p r o v i d e s i n f o r m a t i o n which makes it p o s s i b l e t o s t u d y t h e f i n e s t d e t a i l s o f t h e s u r f a c e r e g i o n s o f inhomogenous s a m p l e s .
6 , 3 , E S T I M A T I O N O F OPTIMUM V O L U M E
O F O I L DEPOSIT
The p r o p e r d o s e of o i l i n a m i n i a t u r e b e a r i n g i s v e r y h p r t a n t . T h e r e s h o u l d b e enough o i l t o e n s u r e e f f e c t i v e l u b r i c a t i o n over a l o n g p e r i o d of o p e r a t i o n b u t i t s volume s h o u l d n o t b e h i g h e r t h a n t h e c r i t i c a l v a l u e above which s p o n t a n e o u s s p r e a d i n g o c c u r s . The t h i c k n e s s of t h e o i l f i l m
(dose) should b e h i g h because t h e speed
of t h e o x i d a t i o n process (ageing) of t h e o i l d e c r e a s e s w i t h i n c r e a s i n g t h i c k n e s s . The r e q u i r e m e n t s f o r t h e optimum volume o f t h e o i l d e p o s i t a r e t h a t t h e o i l dose should p r o v i d e e f f e c t i v e l u b r i c a t i o n d u r i n g t h e r e q u i r e d p e r i o d o f o p e r a t i o n and t h a t t h e o i l s h o u l d n o t m i g r a t e from t h e r u b b i n g r e g i o n . The e s t i m a t i o n o f t h e minimum q u a n t i t y o f o i l r e q u i r e d f o r t h e determined p e r i o d o f o p e r a t i o n o f t h e b e a r i n g i s very d i f f i c u l t and t o do so i n f o r m a t i o n o n t h e b e h a v i o u r a t y p i c a l b e a r i n g l u b r i c a t e d i s n e c e s s a r y . The q u a n t i t a t i v e comparison o f t h e l u b r i c a t i o n c o n d i t i o n s of v a r i o u s b e a r i n g s c a n be
c a r r i e d out using t h e u n i t
o i l consumption p a r a m e t e r Z ( r e f . 9 5 )
where V i s t h e volume of a n o i l d o s e and S1 t h e l u b r i c a t e d s u r f a c e o f t h e moving e l e m e n t . Because o f t h e m e c h a n i c a l mixing o f t h e o i l
238
and t h e t e m p e r a t u r e i n c r e a s e i n t h e r u b b i n g r e g i o n t h e o x i d a t i o n p r o c e s s of t h e o i l is a c c e l e r a t e d . The e s t i m a t i o n o f S1 s h o u l d i n clude t h e e n t i r e surface of t h e rubbing elements i n contact with t h e o i l . The Z f o r a m i c r o b e a r i n g o p e r a t i n g under t h e same e n v i r o n m e n t a l c o n d i t i o n s , l o a d e d w i t h t h e same power f l u x M u ( M moment,
W -
-
rotative
a n g u l a r s p e e d ) and w i t h s i m i l a r r o t a t i o n characteristics
( e . g . s t a r t - s t o p o p e r a t i o n , unchanged d i r e c t i o n o f r o t a t i o n ) h a s a p p r o x i m a t e l y t h e same v a l u e s . F o r example i n t h e case o f themicrob e a r i n g s o f a watch l u b r i c a t e d w i t h t h e o i l MBP-12 when t h e o i l h a s a l i f e t i m e o f 1.5 y e a r s , Z i s a p p r o x i m a t e l y 0 . 0 1 5 mm ( r e f . 95). I n t h e same watch, t h e b a l a n c e m i c r o b e a r i n g is more h e a v i l y l o a d e d t a k i n g i n t o a c c o u n t t h e movement, s l i d i n g s p e e d and s l i d i n g distance, and n e e d s more o i l . The u n i t o i l consumption p a r a m e t e r Z becomes 0 . 2 mm.
I n d e s i g n p r a c t i c e two s i t u a t i o n s o c c u r : 1) t h e t y p e o f o i l i s d e t e r m i n e d and t h e minimum a d m i s s i b l e v a l u e o f Z i s known o r 2 ) Z
i s n o t known a s t h e o i l i s b e i n g a p p l i e d f o r t h e f i r s t t i m e .
In the
f i r s t c a s e , t h e optimum v a l u e o f t h e volume o f o i l t o b e d e p o s i t e d c a n be d e t e r m i n e d b u t i n t h e second case o n l y t h e maximum admiss i b l e volume ( l o o k i n g a t t h e s p r e a d i n g c o n s t r a i n t s ) o f t h e o i l c a n be c a l c u l a t e d . The maximum a d m i s s i b l e v a l u e of o i l - d o s e c a n be e s t i m a t e d f o r t h e p a r t i c u l a r s h a p e and g e o m e t r i c a l f e a t u r e s o f t h e r u b b i n g e l e ments, when t h e t y p e o f o i l h a s been d e t e r m i n e d and t h e v a l u e and
(rl
d i r e c t i o n o f a p o s s i b l e i n e r t i a o v e r l o a d are a l s o known is a t t h e maximum t e m p e r a t u r e i n t h e r u b b i n g r e g i o n when t h e maximum o p e r a t i n g e n v i r o n m e n t t e m p e r a t u r e i s known). L e t u s c o n s i d e r a t y p i c a l journal microbearing (Fig. 6 . 5 ) .
The o i l i n t r o d u c e d i n t o t h e
b e a r i n g t e n d s t o f i l l t h e c a p i l l a r y c l e a r a n c e between t h e j o u r n a l and t h e b e a r i n g bush. Because t h e g r a v i t y and boundary f i l m f o r c e s a c t i n g on t h e o i l d r o p c a n be n e g l e c t e d i n r e l a t i o n t o t h e L a p l a c e c a p i l l a r y f o r c e s ( r e f . 9 5 ) , t h e d i s p l a c e m e n t o f t h e o i l s t o p s when t h e p r e s s u r e ApA a t t h e A meniscus and t h e p r e s s u r e A p B a t t h e B meniscus a r e e q u a l i . e . sing
and
yB
Ap,
= ApB
pA
or
i n terms o f n , m , o i ,
=
PB ( F i g .
6.5).
Expres-
a n d c o n t a c t a n g l e s O 1 and
e2, w e o b t a i n PA =
[cos
PB = cos O1
o1 +
+
n cos (02
c o s ( 6 2 + 01)
]m
=
n
+
-
o(
) - cos
[ cos
e2 +
m
O2
cos
+
cos (01
(el+
'p ) ]
(6.11)
239
F i g . 6.5.
O i l i n the e q u i l i b r i u m p o s i t i o n i n a t y p i c a l j o u r n a l microbearing.
The c a l c u l a t i o n o f m and n i s u s u a l l y c a r r i e d o u t a t themininnun a x i a l c l e a r a n c e (assumed t o be h a l f of t h e r a d i a l c l e a r a n c e ) . The c a l c u l a t e d v a l u e of t h e a d m i s s i b l e n s h o u l d be less t h a n o r e q u a l t o t h e h e i g h t o f t h e j o u r n a l b e v e l . The v a l u e o f n should b e c a l c u l a t e d a t t h e assumed v a l u e o f t h e maximum i n e r t i a o v e r l o a d . The o i l may s p r e a d o u t o f t h e m i c r o b e a r i n g when t h e o i l d i s placement under i n e r t i a o v e r l o a d s c r o s s e s a c r i t i c a l l i m i t ( C L ) . The a n t i - s p r e a d i n g f o r c e s which a r e a f u n c t i o n o f t h e s u r f a c e t e n s i o n , t h e w e t t i n g p e r i m e t e r and t h e v i s c o s i t y of t h e o i l , a c t i n t h e case of both q u a s i - s t a t i c
(monotonic low speed o v e r l o a d s ) and
dynamic o v e r l o a d i n g . However, t h e i m p o r t a n t a n t i - s p r e a d v i s c o s i t y f o r c e s can p r o v i d e a n t i - s p r e a d i n g a c t i o n o n l y when t h e r e i s a dynamic o v e r l o a d , so t h e q u a s i - s t a t , i c o v e r l o a d s a r e more dangerous. The scheme f o r t h e c a l c u l a t i o n o f t h e r e s i s t a n c e a t r a d i a l overload f o r t h e o v e r CL d i s p l a c e m e n t o f t h e o i l i n a t y p i c a l j o u r n a l microbearing is presented i n Fig. 6.6.
A n a l y s i s w i l l be p r o v i d e d f o r t h e
s i t u a t i o n where t h e d i s p l a c e m e n t o f t h e o i l d r o p h a s a n e g l i g i b l e i n f l u e n c e on t h e boundary f i l m f o r c e s so t h e meniscuses of t h e d i s p l a c e d d r o p change i n s i g n i f i c a n t l y . The r e s i s t a n c e f o r c e R which depends on t h e d e s i g n f e a t u r e s o f t h e m i c r o b e a r i n g , and t h e p r o p e r -
t i e s o f t h e o i l i s determined by t h e i n e r t i a f o r c e d i s p l a c i n g t h e o i l d r o p below t h e CL. R a t t h e r a d i a l o v e r l o a d f o r t h e a n a l y s e d
240 bearing can be calculated from the following formula (ref. 95) Rr -
2 Ylda [cos 0
+ cos
(@
+
OL
da P o s 0 + cos ( 6 2 fogh
+
n2 g d cos ( 8
1’
+ a)
o( sin 2
(6.12)
)
At the axial overloading
-
Ra -
a/,
01 )
l2
(6.13)
where da - admissible displacement of the oil drop (when the overload is ended, the oil drop returns to the base position), 0 - assuming the same contact angle for the materials of the journal and the bearing bush, po - oil density, g - gravity acceleration, sin k =
sin 2
cos ( 0
+
+ S ) 2
3m
cos ( e
+
(d
+ m)
)
d n
(the other parameters are shown in Figures 6.5 and 6 . 6 ) .
F i g . 6.6.
O i l p o s i t i o n i n a t y p i c a l j o u r n a l microbearing a t t h e r a d i a l i n e r t i a o v e r 1 oad .
It is well known from practice that the oil s h o u l d preferably fill the clearance space as shown in Fig. 6.7. Taking this into
241
c o n s i d e r a t i o n , maximum a d m i s s i b l e volume o f o i l , a s f a r a s nons p r e a d i n g i s c o n c e r n e d , c a n b e e s t i m a t e d u s i n g t h e f o l l o w i n g formula
S
(2d
-
s)hs
+
2(-
3+-d- s dl 1+-
ylsl
+
d-s 3+d2 d-s y2ss)]
+-
1
dl
(6*14)
d2
where s i s t h e d i a m e t r a l c l e a r a n c e ( i . e . t h e d i f f e r e n c e between t h e d i a m e t e r s of t h e b e a r i n g bush a n d t h e j o u r n a l ) , y i s t h e d i s t a n c e from t h e c e n t r e o f g r a v i t y of t h e crosssection o f t h e round s o l i d t o t h e a x i s o f r o t a t i o n , and S i s t h e a r e a of t h e c r o s s s e c t i o n .
/
F i g . 6.7.
The optimum f i l l i n g w i t h o i l o f t h e b e a r i n g c l e a r a n c e space.
I n drawing up t h e above g i v e n f o r m u l a , Gulden's second t h e o r e m f o r t h e c a l c u l a t i o n of t h e volume o f round s o l i d s h a s b e e n a p p l i e d . The a c t u a l g e o m e t r i c a l s h a p e of t h e o i l s p a c e i s r e p r e s e n t e d by round s o l i d s i n F i g . 6 . 8 . I n t h i s f i g u r e t h e d i a m e t e r s d l a n d d 2 u s e d i n formula ( 6 . 1 4 ) a r e g i v e n . T h i s a p p r o x i m a t i o n , i n d i c a t e d i n
242
Fig. 6.8 with dotted lines gives an oil volume estimated error of about 15% (ref. 95)(about as accurate as an actual dose).
F i g . 6.8. 1 i nes)
.
The a p p r o x i m a t i o n of a r e a l i s t i c o i l space w i t h round s o l i d s ( d o t t e d
After the evaluation of the volume of the oil deposit V and the lubricated surface S1, the unit oil consumption parameter 2 can be calculated using formula (6.9). If the calculated value of Z is equal to or greater than the minimum admissible Z, the estimated oil volume will be the one required. If 2 is less than the minimum admissible 2, it must be increased. Z can be increased either by bearing modifications, such as increasing the journal bevel or the external diameter of the bearing bush, or by major changes in the bearing design, such as the use of a covered bearing instead of the typical "open" bearing. The method for estimating the optimumvolume of an oil deposit given above for the typical "open" journal microbearing is of course valid for other types of miniature bearings.
243
I n m i n i a t u r e b e a r i n g s such a s watch j e w e l b e a r i n g s , which a r e r e s i s t a n t t o dangerous r a d i a l i n e r t i a o v e r l o a d s of more t h a n l o 4 g , t h e t y p i c a l volume o f t h e o i l d e p o s i t is 1 0 - 2 - 1 0 - 3
mm3. For s u c h
b e a r i n g s t h e a n a l y s i s f o r nonspreading o f a n o i l i s t h e r e f o r e superf luous
.
6,4, LUBRICANT DURABILITY The o p e r a t i n g c o n d i t i o n s o f i n s t r u m e n t l u b r i c a n t s a r e v e r y sev e r e . They must p r o v i d e e f f e c t i v e l u b r i c a t i o n o f m i c r o b e a r i n g s over l o n g p e r i o d s , o f t e n under v e r y h i g h s p e c i f i c l o a d s ( u p t o 2-3 GPa) and impact l o a d s . Other a d v e r s e f a c t o r s a r e h i g h t e m p e r a t u r e v a r i a t i o n s , a c t i v i t y of t h e wear d e b r i s and r u b b i n g e l e m e n t m a t e r i a l s , i n f l u e n c e of t h e environment (oxygen , i n d u s t r i a l g a s e s and impur i t i e s ) , h u m i d i t y , and v a r i o u s t y p e s of r a d i a t i o n ( W , r a d i o a c t i v i t y , c o s m i c ) . The l u b r i c a n t q u a n i t i e s a p p l i e d f o r l i f e l u b r i c a t i o n a r e v e r y s m a l l . The l u b r i c a n t s h o u l d t h e r e f o r e b e n o n s p r e a d i n g and s h o u l d d e m o n s t r a t e e x c e l l e n t l u b r i c i t y and h i g h d u r a b i l i t y . By "dura b i l i t y " w e mean t h e chemical and p h y s i c a l s t a b i l i t y of a lubricant. Generally speaking, f o r instrument l u b r i c a n t s , with an i n c r e a s e i n t h e l u b r i c i t y and nonspreading p r o p e r t i e s , t h e d u r a b i l i t y decreases. The chemical s t a b i l i t y c a n be e s t i m a t e d v i a t h e d e t e r m i n a t i o n
of t h e dynamics of o x i d a t i o n and p o l y m e r i z a t i o n p r o c e s s e s , which r e f l e c t t h e a g e i n g of t h e l u b r i c a n t . Ageing i s s t i m u l a t e d by t r i b o l o g i c a l and e n v i r o n m e n t a l e f f e c t s . An extreme i n s t a n c e of a g e i n g i s t h e t o t a l coking o f a l u b r i c a n t . The sum of v a r i o u s e n e r g e t i c a l e f f e c t s ( t r i b o l o g i c a l and e n v i r o n m e n t a l ) f o r t o t a l c o k i n g i s a cons t a n t v a l u e ( r e f . 4 3 9 ) . The o x i d a t i o n and p o l y m e r i z a t i o n p r o c e s s e s a r e n o t t h e o n l y chemical changes i n i n s t r u m e n t o i l s , High humidity and t e m p e r a t u r e can provoke a h y d r o l y t i c r e a c t i o n i n s e v e r a l o i l s ( e . g . s y n t h e t i c o i l based on t r i c r e s y l p h o s p h a t e ) and t h e s e c r e t i o n of c o r r o s i v e a g e n t s . A
marked i n c r e a s e i n t h e v i s c o s i t y and d e n s i t y of oil o c c u r s a s
a r e s u l t of t h e o x i d a t i o n and p o l y m e r i z a t i o n p r o c e s s e s . The s u r f a c e t e n s i o n of o i l d e c r e a s e s non-monotonically, t h e a c i d i t y and t h e a b s o l u t e r e f r a c t i v e i n d e x e s i n c r e a s e , t h e c o l o u r of t h e o i l changes and t h e o i l l o s e s i t s l u b r i c a t i n g p r o p e r t i e s . The speed of t h e agei n g p r o c e s s depends s i g n i f i c a n t l y on t h e r e l a t i o n of t h e a r e a of c o n t a c t w i t h oxygen t o t h e t o t a l volume of t h e o i l d e p o s i t . The a g e i n g dynamics of o i l a r e l i m i t e d by t h e p r o p e r t l e s of t h e o i l and t h e o p e r a t i n g c o n d i t i o n s . O i l s o f n a t u r a l o r i g i n , whether
244 animal , vegetable or mineral, are generally non-resistant to ageing (see Chapters 3.2 and 8.5.4). The marked improvement in the ageing resistance of instrument oils has been achieved using synthetic oils. Operating conditions can vary significantly for the same oil. The ageing process is a function of the operating time, and it is very difficult to determine the ageing-resistance of the oil used in actual microbearings. Before an oil is used to lubricate newly designed bearings, especially for series manufacturelitshould be subjected to a searching ageing test. The methods for the accelerated testing of the ageing-resistame of instrument oil can be divided into three groups (ref. 440): 1) static, 2) dynamic and 3) mechanical-dynamic. In the first case the oil is subjected to thermal orradiation energy fluxes and the effect of oxygen is tested together with the catalytic influence of materials which will be used in the manufacture of bearing elements (ref. 9, 441). Baader's test can serve as an example of the dynamic method. The copper (or other material) wires or plates are immersed in an oil bath at a temperature of 95 or 8OoC with a frequency of 20 or 25 times per minute for 12 days in a typical test. Mechanical dynamic ageing tests have been developed in recent years by Dilrr (ref. 442) and Gumz (ref. 443). In these tests the oil is subjected to mechanical and impact loads and to wear debris as in actualbearings, to assess their effect on the ageing process of the oil. The technical details of the aforementioned ageing methods are presented in Chapter 8.5.4. The estimate of the ageing-resistance of an oil depends on the testing method used. The classic clock oil with a viscosity of 96 mm 2 / s at 2OoC, and adicity number 0.31 mg KOH/g was aged (ref. 442) by static heating in an open brass vessel, the dynamic (Baader's test) and mechanical-dynamic methods (UTI apparatus see Chapter 8.5.4) changed its viscosity, measured at 20°C, to 110 mm 2/ s after 2 2 0 days, 1000 mm2/s after 16 days and 1000 mm / s after 6 days respectively. The acidity number was changed to 6.2 , 17.8 and 18.2 mg KOH/g respectively. The testing temperature was 95OC. The area of contact of the oil with air was about 400 and 5200 mm2 for the static and dynamic methods respectively. After mineral oil with a viscosity of 1500 mm 2/ s at 2OoC and acidity number 0 was aged in a open vessel and in Gumz apparatus (see Chapter 8.5.4)its properties changed to 1520 and 16000 mm 2/ s and 0 . 5 and 42.7 mg KOH/g respectively (ref. 443). The tests were carried out at 95OC for a period of 1200 h.
-
245 The s y n t h e t i c o i l s aged i n t h e s e t e s t s showed
significantly
b e t t e r a g e i n g p r o p e r i e s t h a n n a t u r a l o i l s . The r e s p e c t i v e p r o p e r 2 t i e s of e s t e r - b a s e d i n s t r u m e n t o i l ( v i s c o s i t y 1 9 mm / s a t 2OoC, a c i d i t y number 0 . 3 mg KOH/g) a f t e r a g e i n g i n UTI a p p a r a t u s a t 95OC f o r 1 2 d a y s and w i t h 25 l o 3 mm 3/ s a i r f l o w i n g across t h e o i l , change d t o 2 1 mm2/s and 0 . 4 mg KOH/g. The Moebius o i l b a s e d on p l y a l k y l 2 g l y c o l w i t h e t h e r and a l c o h o l g r o u p s ( v i s c o s i t y 200 mm / s a t 2OoC, a c i d i t y number 1 . 5 mg KOH/g) w a s a g e d w i t h t h e s t a t i c method and t h e Gumz mechanical-dynamic method ( r e f . 4 4 3 ) . A f t e r 1 2 0 0 h a t 95OC 2 t h e v i s c o s i t y and a c i d i t y number i n c r e a s e d t o r e s p e c t i v e l y 2 0 5 m / s 2 and 12.8 mg KOH/g ( s t a t i c method) and t o 5 0 0 nun / s and 1 9 . 9 rrg KOH/g (Cumz mechanical-dynamic m e t h o d ) . The c o l o u r o f t h e o i l changed
from c l e a r - g r e e n t o c l e a r - b r o w n and g r e e n r e s p e c t i v e l y , The absolute r e f r a c t i v e i n d e x i n c r e a s e d from 1.4888 t o 1.4905
( s t a t i c method)
and t o 1.4933 (Gumz m e t h o d ) . These i n v e s t i g a t i o n s i n d i c a t e t h a t t h e a g e i n g dynamics o f a n o i l depend on t h e method of t e s t i n g . Mechanical-dynamic methods, which s i m u l a t e most r e a l i s t i c a l l y t h e o p e r a t i n g c o n d i t i o n s of o i l i n a f i n e mechanism b e a r i n g , a r e t h e most v a l u a b l e f o r e s t i m a t i n g t h e a g e i n g - r e s i s t a n c e o f a n i n s t r u m e n t o i l . U n f o r t u n a t e l y , t h e y do n o t provide c r i t e r i a f o r d e t e r m i n i n g t h e a p p l i c a b i l i t y of a n o i l as regards its r e s i s t a n c e t o ageing. The c l a s s i c t e s t i n g methods p r o v i d e q u a n t i t a t i v e e s t i m a t e s o f t h e a g e i n g r e s i s t a n c e of a n o i l . An o i l c a n b e r e c o g n i z e d a s res i s t a n t t o a g e i n g and c a n b e s u b j e c t e d t o p r a c t i c a l t e s t s i f a f t e r 6 months of s t a t i c a g e i n g a t 4OoC o f 30 g o i l i n a g l a s s l a b o r a t o r y
wash-vessel c o n t a i n i n g an a d d i t i o n o f c o p p e r and i r o n powder,t.he following c o n d i t i o n s are m e t ( r e f . 4 4 1 ) : 1 ) I n c r e a s e i n v i s c o s i t y i s n o t more t h a n 2 5 % , 2 ) A c i d i t y number i n c r e a s e s by maximum 2-3
units,
3) S u r f a c e t e n s i o n d r o p s by maximum 3-4 mN/m, 4) Change i n c o l o u r i s n e g l i g i b l e . To t e s t t h e a g e i n g r e s i s t a n c e of o i l i n r e a l b e a r i n g s i s n o t e a s y b e c a u s e t h e t e s t i n g t i m e h a s t o be f a i r l y l o n g (more t h a n one y e a r ) , and t h e o i l volume i s s o s m a l l t h a t t h e r e a r e d i f f i c u l t i e s i n c o l l e c t i n g a n o i l sample from t h e o p e r a t i n g b e a r i n g . The
estima-
t i o n of t h e a g e i n g g r a d e o f s u c h a s m a l l amount o f o i l ( o f t e n nun 3 ) is d i f f i c u l t and as y e t no s a t i s f a c t o r y s o l u t i o n h a s been
< 0.5
.
found The a g e i n g g r a d e o f t h e o i l t a k e n from a real b e a r i n g c a n be e s t i m a t e d u s i n g a n o p t i c a l microscope, b u t t h i s o n l y e n a b l e s t h e
246 overall coking state of the oil to be recognized. Eicke (ref. 391) has investigated the ageing of small amounts (about 0.2 mg)of instrument oils by determination of the changes in surface tension. This method is quite interesting but unfortunately, in the ageing process, the characteristics (surface tension as a function of ageing time) of many instrument oils pass through a minimum (Fig.6.9).
1
I
100
I
300’ 1
200
Ageing time ,days
F i g . 6.9. S u r f a c e t e n s i o n of an i n s t r u m e n t m i n e r a l o i l as a f u n c t i o n o f ageing t i m e ( r e f . 389).
In the oxidation phase of the ageing process the surface tension of oil drops, and in the polymerization phase it increases. This nonmonotonicity militates against accurate ageing grade determination by estimation of surface tension of an oil at any time duringoperation. Huber (refs. 369, 444) has proposed two solutions to this prob-
247
lem. The ageing dynamics can be characterized directly using the important property of viscosity. The viscosity of an oil sample of about 0.5 mm3 can be evaluated using the special micro-viscometer (see Chapter 8 . 5 . 3 ) . This method is effective but the amount of oil samples needed is relatively large. Huber’s second proposition is quite interesting. The ageing effects can be clearly seen in the IR oil spectrogram (Fig. 6 . 1 0 ) . The necessary volume of a sample for IR analysis is > 0 . 5 mg, i.e., it is comparable with oil volumes introduced into miniature bearings. The oil ageing grade can be described with an ageing number ( A Z )
AZ =
E ( ~ 1 7 7 0cm-l)
+
(E-1720
cm-l)
+
E (-1610
cm-’)
(6.19
E ( 1 4 6 0 cm-’)
where E (....I in the numerator is the absorptivities at the wave numbers of intensive formation of ageing chemical products in mineral instrument oils (Fig. 6.10) i.e. 1780 1 7 6 0 cm-l C = 0 in anhydrides, lactones; 1740 1 7 0 0 cm-l C = 0 in esters, aldehydes, ketones, fatty acids; 1620 1 5 9 5 cm-l C = 0 in metal salts.
... ... ...
Wave length , pm --c
-
Nave number ,cm”
F i g . 6.10.
I R spectrogram o f a n m i n e r a l i n s t r u m e n t o i l ( r e f . 4 4 4 )
248
The wave number 1460 cm-l has been chosen as the standard because after ageing the absorptivity in this band does not change (and it is also at its highest). The absorptivity E is defined as JO
E = log -
(6.16)
J
where Jo and J are light intensities before and after the test respectively. The results of Huber's investigations into the relationships between A 2 and the increase in the viscosity and acidity number of the special instrument oil MS2 (paraffin oil with antioxidants) are presented in Fig. 6.11 and Fig, 6.12 respectively.
A 200
-
q-150 L
-
I\r
Q)
n E 2
c 0
100 -
c
.-1
Iu
cn -x
50
-
I 01
I
I
200
400
1
I
600
800
c
viscosity increase, % Fig. 6.11.
Ageing number
AZ
and i n c r e a s e i n v i s c o s i t y o f a p a r a f f i n o i l ( r e f .
4441.
The oil had been aged using a modified Baader's test (DIN 51554). The ageing characteristics of the oil MS2 and pure paraffin oil are presented in Fig, 6.13. It is possible to see that the A 2 values for MS2 and pure paraffin oil differ significantly. Good correlation exists between AZ and the increase in viscosity and acidity
249
number,
250
20C ru Q
n E
15C
=3
c
m 100
c . A
QI 0
q
5c
0
I
M
I
80 Increase of acidity number, % 40
60
c
F i g . 6 . 1 2 . Ageing number (AZ) and i n c r e a s e i n a c i d i t y number o f a p a r a f f i n o i l ( r e f . 444).
Huber’s IR method can also be applied to the analysis of the ageing resistance of instrument oils based on polyalkylglycol with ether and alcohol groups which are often applied for the lubrication of miniature systems. Mineral oils or greases with a lot of additives can also be studied using this method. In this case, neglecting the soap bands in the spectrogram is necessary, or other spectroscopic techniques, e.g. differential IR spectroscopy, should be used. The IR spectroscopy method for the estimation of the ageing of a small amount of grease used as lubricant in a rolling bearing has been applied by Bilobrov et al. (ref. 4 4 5 ) . The grease Ciatim-203, applied for the lubrication of aircraft instrumentation, is based on paraffin (transformer) oil condensed with a Li-soap of stearic
250 a c i d and c o n t a i n i n g a n t i o x i d a n t s . A c o r r e l a t i o n w a s found between
E1720/E1450 (where E - i s IR a b s o r p t i v i t y a t 1720 c m - l d 1450 cm-' r e s p e c t i v e l y ) and t h e o x i d a t i o n g r a d e o f t h e g r e a s e . The E1720/E1450 e q u a l t o 0.9 h a s b e e n t a k e n a s t h e c r i t e r i o n v a l u e of t h e maximum a d m i s s i b l e o x i d a t i o n g r a d e of t h e g r e a s e ( t h e 8.5% ( i n w e i g h t ) o f t h e g r e a s e o x i d i z e d a t 0 . 9 E1720/E1450).
a, t
Instrument oil MS2 0
I
1
I
20
40
60
c.
F i g . 6 . 1 3 . Ageing number (AZ) as a f u n c t i o n o f ageing t i m e ( r e f .
444).
The o i l a g e i n g dynamics i n a c t u a l w o r k i n g s y s t e m s have b e e n s t u d i e d by o i l p r o d u c e r s over l o n g p e r i o d s of o p e r a t i o n . A s a n example, t h e i n c r e a s e i n v i s c o s i t y o f v a r i o u s i n s t r u m e n t o i l s a f t e r n a t u r a l a g e i n g i s p r e s e n t e d i n F i g . 6.14 ( r e f . 446). The e f f e c t of a d d i n g a n t i o x i d a n t t o c l a s s i c c l o c k o i l and t h e a p p l i c a t i o n of s y n t h e t i c o i l ( p o l y a l k y l g l y c o l w i t h e t h e r and a l c o h o l g r o u p s ) c a n be c l e a r l y o b s e r v e d . The comparison o f t h e e f f e c t of a n a t u r a l a g e i n g o f c l a s s i c c l o c k o i l and s i l i c o n e o i l o n t h e a m p l i t u d e of a b a l a n c e i n a watch i s shown i n F i g . 6 . 1 5
( r e f . 64). A f t e r 18 months
of o p e r a t i o n , t h e a m p l i t u d e of t h e b a l a n c e l u b r i c a t e d w i t h s i l i c o n e
o i l d e c r e a s e d by 3.5% w h e r e a s t h e a m p l i t u d e of t h e b a l a n c e lubricate d w i t h c l a s s i c c l o c k o i l d e c r e a s e d by a s much as 17.2% a f t e r o n l y 12 months o f o p e r a t i o n ,
251
I
I
I
c
I
3 Ageing (operat h g ) time, years 2
1
01
F i g . 6.14. I n c r e a s e i n v i s c o s i t y o f i n st rument o i l s d u r i n g n a t u r a l ageing. 1 - pure n e a t ’ s - f o o t o i l ; 2 - p u r e c l a s s i c c l o c k o i l ; 3 classic clock o i l ( w i t h a n t i o x i d a n t s ) ; 4 - s y n t h e t i c o i l - p o l y a l k y l g l y c o l (Synt-A-Lube) (ref.450)
-
.. --
Ho ri za ntal
- \
250
I
O -
-\ \
f
A
I
I
Vertical
I
‘0
4
I
I
12 16 Ageing(operat1ng) time, months 8
I
*
20
r i g . 6.15. A m p litud e o f a watch b al a nce as a f u n c t i o n o f n a t u r a l ageing ( o p e r a t i ng ) t i m e a t h o r i z o n t a l and v e r t i c a l watch p o s i t i o n s and l u b r i c a t e d w i t h s y n t k t i c and n a t u r a l o i l s ( r e f . 6 4 ) .
252
The problems involved in the estimation of natural lubricant ageing need further investigation. IR spectroscopy methods can only be used to estimate ageing in a few kinds of natural lubricants. The determination of the ageing number (AZ) does not give direct information about the tribological effect of ageing. The relationships between viscosity, acidity number etc., and AZ should be determined for the practical significance of the estimated AZ values. It is necessary to develop a standard method for the objective evaluation of the ageing resistance of instrument lubricants (refs. 440,
447).
The system should be made up of the following elements: 1 ) A device for mechanical-dynamic simulation of lubricant egeing, 2 ) Microviscometer
with a wide measuring range suitable for small 3 0.5 mm ) amounts of lubricant, 3 ) A device for a quick estimation of the acidity number of small ( <
amounts of lubricant, 4 ) A device for determining the surface tension of small volumes of
oil, 5 ) A device for the evaluation of the refraction number of oils.
The viscosity, acidity number and surface tension can be used as the criterion parameters for the estimation of the ageing grade of a lubricant. These parameters have an important effect on the tribological behaviour of a bearing. Because it is easy to determine the absolute refractive index and because af the significance of changes in it as a result of ageing, this parameter seems to be a specially important element of the criterion parameters set. Turning back to the viscosity parameter, it should be remembered that the increase in viscosity due to ageing is significantly higher in natural oils than it is in synthetic oils (refs. 4 4 2 , 4 4 3 , 4 4 8 ) . The physical stability of a lubricant is characterized by a low rate of evaporation. The evaporation resistance of oils depends on their chemical structure and purity. Increases in the temperature and in the area of contact with air accelerate the evaporation process. During the first stage of evaporation, the viscosity of the oil increases; afterwards, the liquid phase completely disappears. These effects are clearly very dangerous when miniature bearings are in operation, as the amount of oil used is very small. Practical evaporation tests are carried out by exposing a determined amount of oil (approximately 50 mg) in 1 mg drops instainless cups or bowls (ref. 4 4 1 ) . The test temperature is 5Oi2OC for natural oils and 7 O t 2 O C for synthetic oils (refs. 441, 4 4 9 ) . o i l s
253
suitable for practical application do not evaporate significantly. A maximum loss in weight of 1-2% after 3 months is typical. Among the natural oils, neat’s-foot and bone oils have a low evaporation rate. Classic clock oil are very evaporative. Synthetic oils are generally very evaporation resistant (see Chapter 3.2). Oils with a high percentage of low-molecular fractions demonstrate a low evaporation resistance. A comparison of evaporation rates of classic clock oils with various types of oils based on polyalkylglycols with ether and alcohol groups is presented in Fig. 6.16 (ref.450). The evaporation rate of a mineral product of narrow fractions composition is lower than that of a product of wide fractions composition with the same viscosity (ref. 451).
0 -10
y t4 5 a -4
.d
u
U
*d
c2 +4 $ -4 U
x +4 -4 0
30
90 0 30 60 Evaporating time, days 60
90
F i g . 6 . 1 6 . E v a p o r a t i o n r a t e s of c l a s s i c c l o c k o i l and o i l s based on p o l y a l k y l g l y c o l w i t h e t h e r and a l c o h o l groups a t 50°C ( r e f . 4 5 0 ) .
6 , 5 , L U B R I C A T I O NUNDER EXTREME C O N D I T I O N S Lubricated miniature systems operating under extreme enviromnental conditions need special analysis. Extreme conditions such as low or
254
h i g h t e m p e r a t u r e s , r a p i d and wide r a n g i n g t e m p e r a t u r e v a r i a t i o n s , a t r o p i c a l o r c o r r o s i v e environment, h i g h d u s t contamination o r p r e s e n c e of i n d u s t r i a l g a s e s , vacuum, s t r o n g r a d i a t i o n (UV, r a d i o a c t i vity)
-
a l l make p r o p e r l u b r i c a t i o n v e r y d i f f i c u l t . V a r i a t i o n s i n
v i s c o s i t y a s a f u n c t i o n o f t e m p e r a t u r e , e v a p o r a t i o n and a g e i n g are t h e p r i n c i p a l lubricant c r i t e r i a parameters a f f e c t i n g t h e t r i b o l o g i c a l p r o p e r t i e s of l u b r i c a t e d s y s t e m s . T h e s e p a r a m e t e r s s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n when s e l e c t i n g a l u b r i c a n t f o r a p p l i c a t i o n i n s y s t e m s r e q u i r e d t o o p e r a t e under e x t r e m e c o n d i t i o n s . N a t u r a l o i l s a r e g e n e r a l l y n o t a p p l i c a b l e a t low o r exceptionally high temperatures. S p e c i a l h i g h l y - r e f i n e d p a r a f f i n low-temperature i n s t r u m e n t o i l ( e . 9 . MWP o i l ) c a n be u s e d a t t e m p e r a t u r e s as low a s -4OOC.
The a d d i t i o n t o a m i n e r a l o i l o f 0 . 5 % p o u r p o i n t d e p r e s s a n t
a f f e c t s not the freezing point but also the viscosity ( r e f . 9 7 ) . S y n t h e t i c o i l s a r e more s u i t a b l e . O i l s b a s e d on m o n o e t h e r s o r p o l y a l k y l g l y c o l s w i t h e t h e r and a l c o h o l g r o u p s a r e u s e f u l a t e n v i r o n m e n t a l t e m p e r a t u r e s o f a b o u t -5OOC b u t t h e v i s c o s i t y o f t h e o i l c a n 2 r e a c h o v e r 10000 mm / s . S o v i e t l o w - t e m p e r a t u r e c l o c k o i l s s u c h a s o r MN-45 d e m o n s t r a t e , a t t e m p e r a t u r e s of -6OOC and -45OC re2 s p e c t i v e l y , a v i s c o s i t y of a b o u t 50000 and 11500 mm / s . The v i s c o s -
MN-60
i t y o f p o l y a l k y l g l y c o l w i t h e t h e r and a l c o h o l g r o u p s o i l S y n t a -Frigo-Lube
(Moebius 9 0 3 0 ) a t -4OOC i s 9 0 0 0 mm 2 / s . S i l i c o n e o i l s
a r e q u i t e u s e f u l a t low t e m p e r a t u r e s . Changes i n t h e v i s c o s i t y o f d i m e t h y l p o l y s i l o x a n e s a s a f u n c t i o n of t e m p e r a t u r e a r e q u i t e s m a l l , 2 e . 9 . t h e DMPS w i t h a v i s c o s i t y of 10 mm / s a t 2OoC h a s a v i s c o s i t y 2
of a b o u t 2 0 0 mm / s a t -6OOC.
The c h l o r i n a t e d phenylmethylplysiloxanes
a r e v e r y u s e f u l a s l u b r i c a n t s a t low t e m p e r a t u r e . They c a n b e u s e d a t a minimum t e m p e r a t u r e of a b o u t -65OC. Adequate e p i l a m i z i n g i s needed when p o l y s i l o x a n e s a r e u s e d . S u i t a b l e g r e a s e s b a s e d on s y n t h e t i c o i l c a n a l s o b e u s e d . I n some c a s e s t h e a p p l i c a t i o n of powder s o l i d l u b r i c a n t s s u c h a s MoS2 o r WS2 i s p o s s i b l e . A t extremely high temperatures, t y p i c a l l u b r i c a n t s w i l l age
q u i c k l y , e v a p o r a t e and m i g r a t e from t h e b e a r i n g . H i g h l y r e f i n e d p a r a f f i n i c o i l s c a n n o t b e a p p l i e d when t h e o p e r a t i n g t e m p e r a t u r e i s h i g h e r t h a n 150-2OO0C
( r e f . 6 0 ) . The e v a p o r a t i o n r a t e i s r a t h e r
h i g h a t t h e s e t e m p e r a t u r e s . Only i n s t r u m e n t o i l s b a s e d on p o l y siloxanes o r polyether can be considered f o r use a t temperatures of 200-250°C. H e r e t h e c h l o r i n a t e d o r f l u o r i n a t e d p o l y s i l o x a n e s ( e s p e c i a l l y phenylmethylpolysiloxanes) o r f l u o r i n a t e d p o l y e t h e r s a r e t h e
most u s e f u l ( r e f s . 65, 81, 405, 409). O i l s b a s e d on plyphenylethws
255
are a l s o u s e f u l a t such t e m p e r a t u r e s , and e s p e c i a l l y i n t h e presence of r a d i a t i o n , a s i n n u c l e a r r e a c t o r s . The e v a p o r a t i o n r a t e of a l u b r i c a n t o p e r a t i n g under e l e v a t e d t e m p e r a t u r e s c a n be d e c r e a s e d by s p e c i a l b e a r i n g d e s i g n . The s o l u t i o n s a r e b a s e d on i n c r e a s i n g volume of o i l u s i n g s p e c i a l o i l r e s e r v o i r s , and on i s o l a t i n g t h e o i l from t h e environment i n s p e c i a l cells. F i g . 6.17 p r e s e n t s a n example ( r e f . 9 5 ) .
F i g . 6.17. Bearings w i t h a d d i t i o n a l o i l d e p o s i t chamber i s o l a t e d from t h e environment.
A p a r t i c u l a r l y d i f f i c u l t problem i s posed by t h e l u b r i c a t i o n o f
s y s t e m s when t h e o p e r a t i n g t e m p e r a t u r e i s e x p e c t e d t o v a r y over a wide r a n g e . T h i s o c c u r s e s p e c i a l l y i n t h e c a s e o f a i r - ard spacecraft i n s t r u m e n t a t i o n . I n t h i s s i t d a t i o n o n l y s y n t h e t i c oils w i t h v e r y low v a r i a t i o n s i n v i s c o s i t y as a f u n c t i o n o f t e m p e r a t u r e , w i t h a low pour p o i n t and h i g h t e m p e r a t u r e d u r a b i l i t y c a n b e u s e d . T y p i c a l n a t u r a l and s y n t h e t i c o i l s s u c h a s t h o s e b a s e d on e t h e r a l c o h o l s , c a n be a p p l i e d i n a r e l a t i v e l y narrow t e m p e r a t u r e r a n g e ( s a y f r o m -20 t o +80°C). The p r o p e r l u b r i c a n t s f o r wide r a n g e t e m p e r a t u r e v a r i a t i o n s a r e t h e p o l y s i l o x a n e o i l s . They c a n b e u s e d when t h e t e m p e r a t u r e r a n g e s from s a y -50 t o +2OO0C. The v i s c o s i t y i n d e x o f p o l y s i l o x a n e o i l s i s a b o u t 200 and f o r more s p e c i a l i z e d c h l o r i n a t e d
phenylmethylpolysiloxanes r e a c h e s a b o u t 1 9 5 a t t e m p e r a t u r e s of -65 t o +23OoC. An e f f e c t i v e s o l u t i o n t o t h e a f o r e m e n t i o n e d l u b r i c a t i o n problens
i s t h e a p p l i c a t i o n of s o l i d l u b r i c a n t s ( s e e C h a p t e r s 3.4 and 7.2). Here molybdenum d i s u l p h i d e (MoS2) i s w i d e l y u s e d . The rubbing micro-
e l e m e n t s are c o a t e d w i t h MoS2 u s i n g v a r i o u s t e c h n i q u e s . The MoS2 c o a t e d e l e m e n t c a n r u b a t v e r y l o w o r v e r y h i g h t e m p e r a t u r e s (300-
256 -4OOOC) b u t t h e o x i d a t i o n p r o c e s s and a i r h u m i d i t y may a f f e c t t h e t r i b o l o g i c a l p r o p e r t i e s of MoS2 c o a t e d m i c r o e l e m e n t s ( r e f s . 37,117,
1 2 7 , 452, 453, see a l s o C h a p t e r 7 . 2 ) .
The u s e of W S 2 i s a l s o pos-
s i b l e . The maximum u s e f u l t e m p e r a t u r e s f o r t h e MoS2 ( o r WS2) l a y e r l a t t i c e a l s o depend on l u b r i c a n t p a r t i c l e s i z e and t h e i n f l u e n c e o f a d j u v a n t s and b i n d e r s . F o r a p p l i c a t i o n i n a h i g h t e m p e r a t u r e ( a b o u t 4OO0C)
a i r environment e s p e c i a l l y i n humid c o n d i t i o n s , g r a p h i t e
s o l i d l u b r i c a t i o n i s e f f e c t i v e . The o t h e r p o s s i b l e s o l i d l u b r i c a n t c o a t i n g s f o r u s e i n a high temperature c o n d i t i o n s are g r a p h i t e f l u o r i d e s (4OOOC)
,
p o l y i m i d e s ( P I ) ( 4 O O O C ) , polyimide-bonded g r a p h i t e
f l u o r i d e s ( 35OoC) r e m a r k a b l y d u r a b l e i n s t a r t / s t o p e n d u r a n c e t e s t i n g . P o l y i m i d e s c o m p o s i t e s ( p o l y i m i d e s w i t h powdered s o l i d lubricant a d d i t i v e s , s u c h a s g r a p h i t e and MoS2, P I - bonded MoS2) e x h i b i t a v e r y low ~ 0 . 0 5f r i c t i o n c o e f f i c i e n t when r u b b i n g a g a i n s t s t e e l ( r e f s . 1 2 1 , 4 5 3 ) . O t h e r u n c o n v e n t i o n a l s o l i d l u b r i c a n t s such a s s o f t o x i d e s and f l u o r i d e s p r o v i d e l u b r i c a t i o n t o a s h i g h a s 900°C ( r e f . 1 2 1 ) . They c a n be used a s f u s e d c o a t i n g s ( 0 . 0 1 - 0 . 0 2
mm t h i c k )
on m e t a l s u b s t r a t e s o r a s t h e l u b r i c a t i n g component o f m e t a l m a t r i x c o m p o s i t e s . F o r wear c o n t r o l , t h e s p u t t e r e d h a r d c o a t i n g s o f some s e l e c t e d c a r b i d e s , n i t r i d e s ( e . g . s i l i c o n e n i t r i d e ( r e f . 2 8 ) ) and o x i d e s a r e s e r v i c e a b l e t o 1000°C i f a d e q u a t e a d h e s i o n t o t h e subs t r a t e i s m a i n t a i n e d a t a l l t e m p e r a t u r e s r e q u i r e d by t h e application rref. 1 2 1 ) . The e f f e c t o f climate on t h e t r i b o l o g i c a l p r o p e r t i e s o f l u b r i c a t e d m i n i a t u r e systems is v e r y s i g n i f i c a n t . I n p a r t i c u l a r , t h e l u b r i c a n t s u s e d i n f i n e mechanisms o p e r a t i n g u n d e r t r o p i c a l climate c o n d i t i o n s should be r e s i s t a n t t o t h e e f f e c t s of humidity, high t e m p e r a t u r e and t h e a c t i v i t y o f m i c r o o r g a n i s m s . L u b r i c a n t s w i t h a h i g h l y r e f i n e d p a r a f f i n o i l b a s e w i t h a n t i o x i d a n t s c a n b e used i n t h e s e c o n d i t i o n s b u t s y n t h e t i c o i l s a r e t h e most s u i t a b l e . E s t e r -based o i l s w i t h good l u b r i c i t y , low e v a p o r a t i o n r a t e and mn-spreadi n g p r o p e r t i e s c a n b e proposed a s good t r o p i c a l i n s t r u m e n t o i l s . I n
a series of s u c h s p e c i a l o i l s (MCT-3, MCT-20, MCT-30 and MPT-25) h a s been d e v e l o p e d ( r e f . 8 7 ) . The o i l s a r e c o r r o s i o n - s t a b l e when u s e d t o l u b r i c a t e m e t a l l i c ( s t e e l and b r a s s ) s u r f a c e s a t 1 0 0 % r e l a t i v e a i r h u m i d i t y and a t elevated t e m p e r a t u r e s . The oils c o n t a i n a n t i o x i d a n t s and a c t i v e f u n g i c i d e s which r e s p e c t i v e l y make t h e U.S.S.R.
them c h e m i c a l l y r e s i s t a n t and r e s i s t a n t t o m i c r o o r g a n i s m s . P o l y a l k y g l y c o l ( w i t h e t h e r and a l c o h o l g r o u p s ) o i l s s u c h a s Synt-A-Lube 9 0 1 0 (new f o r m u l a ) a r e h u m i d i t y s t a b l e , i . e . t h e f r i c t i o n c o e f f i c i e n t of l u b r i c a t e d m i c r o b e a r i n g s ( e . g . s t e e l - r u b y ) d o e s
257
n o t i n c r e a s e w i t h h u m i d i t y v a r i a t i o n s when t h e r e i s a low s l i d i n g speed a r e a ( F i g . 6.18), ( r e f . 6 1 ) . The new Moebius o i l s b a s e d o n p o l y a l k y l g l y c o l s c o n t a i n h i g h - p r e s s u r e l u b r i c i t y a d d i t i v e s and dea c t i v a t o r of c o p p e r . The l u b r i c i t y a d d i t i v e i s i m p o r t a n t f o r improvi n g t h e h u m i d i t y r e s i s t a n c e of s y n t h e t i c o i l b u t t h e a d d i t i v e s h o u l d n o t h y d r o l i z e i n t h e p r e s e n c e o f water ( r e f . 8 8 ) .
A
former formula (!OD%) new formula (!OD%)
----*--*----* - _ _ _ _ 4
rmer formula (1 'Yo\ former
OD9
10
1
10
Sliding
I j
new formula (1%
1
20
speed ,mrn . s-'
l b
30
F i g . 6.18. Comparison of f r i c t i o n c o e f f i c i e n t v a r i a t i o n s (as a f u n c t i o n o f s l i d i n g speed) u s i n g a s t e e l - r u b y p i v o t m i c r o b e a r i n g w i t h a diameter o f 0.11 mm, l u b r i c a t e d w i t h o l d and new f o r m u l a Synt-A-Lube Moebius o i l , a t v a r i o u s h u m i d i t i e s of a i r ( r e f . 6 1 ) .
I f it i s p o s s i b l e (because of a d m i s s i b l e h i g h e r f r i c t i o n ) , t h e a p p l i c a t i o n o f c o r r o s i o n - r e s i s t a n t g r e a s e s c a n be a d v i s e d when mini a t u r e s y s t e m s are r e q u i r e d t o o p e r a t e under t r o p i c a l climate cond i t i o n s . The u s e o f wax g r e a s e s f o r t h e l u b r i c a t i o n of electromec h a n i c a l i n s t r u m e n t s o p e r a t i n g i n China and I n d o c h i n a h a s beenprove d t o be f e a s i b l e ( r e f . 9 ) . I n e x t r e m e s i t u a t i o n s , t h e a p p l i c a t i o n o f s o l i d l u b r i c a n t s such a s PTFE, g r a p h i t e o r PE
can be proposed.
The o p e r a t i n g of l u b r i c a t e d m i n i a t u r e s y s t e m s i n t h e p r e s e n c e o f i n d u s t r i a l g a s e s , SO2,
C02,
NH3,
H2S,
e x h a l a t i o n from p o l y m e r i c
m a t e r i a l s o r h i g h d u s t contamination ( b u i l d i n g machines, chemical i n d u s t r y i n s t r u m e n t a t i o n ) i s v e r y d a n g e r o u s f o r t h e o i l s used.Under C02,
SO2 o r NH3
d r y and w e t a t m o s p h e r e , t h e a c c e l e r a t e d a g e i n g ( e x -
p r e s s e d i n a d e c r e a s e i n s u r f a c e t e n s i o n ) o f b o t h n a t u r a l o i l s (especially neat's-foot
o i l ) and s y n t h e t i c o i l s h a s been o b s e r v e d (ref.
3 9 1 ) . M i n e r a l o i l s s e e m t o b e t h e most r e s i s t a n t . E x h a l a t i o n s o f p o l y m e r i c materials a f f e c t t h e p r o p e r t i e s of a n o i l , e s p e c i a l l y a t e l e v a t e d t e m p e r a t u r e s . The p o l y a l k y l g l y c o l s w i t h e t h e r and a l c o h o l groups o r e s t e r - b a s e d o i l s , normally v e r y d u r a b l e i n a g e i n g , a r e e s p e c i a l l y n o n - r e s i s t a n t t o polymeric m a t e r i a l e x h a l a t i o n s ( r e f . 4 5 4 ) . More d u r a b l e i n a n e x h a l a t i o n a t m o s p h e r e a r e t h e m i n e r a l o i l s ,
e s p e c i a l l y t h o s e w i t h a n t i o x i d a n t s , UV a d d i t i v e s e t c . S i l i c o n e o i l s
a r e v e r y r e s i s t a n t , p r o b a b l y b e c a u s e t h e y a r e poor s o l v e n t s f o r t h e p o l y m e r i c m a t e r i a l e x h a l a t i o n s and b e c a u s e t h e y a r e c h e m i c a l l y inert. Such o i l s a r e a l s o s u i t a b l e i n d u s t y a t m o s p h e r e s . High vacuum i n s t r u m e n t a t i o n o r d e v i c e s i n s t a l l e d " i n s p a c e " need s p e c i a l l u b r i c a t i o n . The l u b r i c a n t used s h o u l d e x h i b i t a l o w e v a p o r a t i o n rate. H e r e , e t h e r ( e s p e c i a l l y polypheny1ether)lubricants a r e of most i n t e r e s t ( r e f . 9 ) a t room o r l o w e r t e m p e r a t u r e s . Mineral
(2
2
380 mm / s ) a r e recommended when t h e o i l s with high v i s c o s i t y t e m p e r a t u r e i s a b o u t 100°C. Because of t h e a b s e n c e o f o x i d i z e d l a y e r s on r u b b i n g s u r f a c e s t h e a p p l i c a t i o n o f o i l s w i t h EP additives
i s n o t e f f e c t i v e . S i l i c o n e o i l s o r g r e a s e s c a n b e u s e d as lubricants f o r s p a c e i n s t r u m e n t a t i o n . An e f f e c t i v e s e a l i n g s y s t e m i s needed when o i l s o r g r e a s e s a r e u s e d t o r e d u c e e v a p o r a t i o n l o s s e s and t o p r o t e c t o p t i c a l e l e m e n t s from c o n d e n s a t i o n o f e v a p o r a t e d l u b r i c a n t molecules. S o l i d l u b r i c a n t s a r e w i d e l y used f o r t h e l u b r i c a t i o n o f m i n i a t u r e s y s t e m s o p e r a t i n g i n a h i g h vacuum e s p e c i a l l y f o r a r e l a t i v e l y s h o r t t i m e . Dry l u b r i c a n t s a r e g e n e r a l l y l i m i t e d t o areas where t e m p e r a t u r e e x t r e m e s o r o u t g a s s i n g r e q u i r e m e n t s p r e c l u d e t h e u s e of o i l s o r g r e a s e s . An example o f t h e p r e s e n t u s e of d r y l u b r i c a n t s i n s p a c e c r a f t i s t h e a c t u a t o r o r p o s t i o n i n g d e v i c e s . They are lubricated w i t h t r a n s f e r l u b r i c a t i o n from a c o m p o s i t e r e t a i n e r c o n t a i n i n g PTFE and MoS2 o r w i t h a bonded MoS2 c o a t i n g ( r e f . 4 5 5 ) . The l u b r i c a n t m a t e r i a l s now u s e d on s p a c e c r a f t a r e p r i m a r i l y : MoS2 e i t h e r bonded,
composites:
b u r n i s h e d , s p u t t e r e d (see Chapter 7 . 2 )
or in
s o f t m e t a l s s u c h a s Pb and A u ion vapour d e p o s i t e d a n d
PTFE i n c o m p o s i t e s o r b u r n i s h e d . Care i n h a n d l i n g c o a t i n g s once
259 t h e y a r e a p p l i e d i s a m a j o r c o n s i d e r a t i o n i n p r e s e r v i n g t h e coatirgs p r i o r t o launch. I n many cases, s u c h a s w i t h MoS2, ground t e s t i n g o r s t o r a g e i n humid a i r c a n l i m i t c o a t i n g l i f e , and i n e r t g a s p u r i f i c a t i o n i s r e q u i r e d . T h e r e i s a need f o r d r y l u b r i c a n t s w i t h better ground t e s t i n g t o l e r a n c e ( r e f . 455)
.
The t h r e e c u r r e n t l y most-used s o l i d l u b r i c a n t s f o r b a l l b e a r i n g s i n a vacuum have been compared u n d e r t h e same c o n d i t i o n s and f o r s p e c i f i e d d u t i e s ( r e f . 1 2 8 ) . MoS2-sputtered f i l m , though e x h i b i t i n g p a r t i c u l a r l y low and smooth t o r q u e , h a s a l i m i t e d l i f e . The b e a r i n g e v e n t u a l l y " f a i l s " b e c a u s e of r i s i n g f r i c t i o n and s t e e l
wear o c c u r r i n g a f t e r f i l m wear-out. The PTFE-composite c a g e material shows g e n e r a l l y good performance, though t h e a v e r a g e b e a r i n g t o r q u e i s r a t h e r u n s t e a d y a t a l l l o a d s . I o n - p l a t e d l e a d f i l m on t h e raceways, i n c o n j u n c t i o n w i t h a c a s t l e a d e d b r o n z e c a g e , a l s o gives good performance and no t o r q u e " f a i l u r e s " . Cage d e b r i s c o n t r i b u t e s subs t a n t i a l l y t o torque noise but t h e average torque i s noticeably s t e a d i e r t h a n from t h e PTFE-caged b e a r i n g . T h e r e w a s no s t e e l wear d e t e c t a b l e from t r a c k p r o f i l o m e t r y . Many e x c e l l e n t vacuum-deposited h a r d c o a t i n g s have been d e v e l oped r e c e n t l y ( s e e Chapter 7 . 3 ) . Some of t h e s e h a r d c o a t i n g s , s u c h
a s T i N coupled w i t h s o f t c o a t i n g s such a s s p u t t e r e d MoS2, c o u l d possibly provide long l i f e dry l u b r i c a t i o n using only t h e i n i t i a l c o a t i n g , w i t h o u t r e l y i n g upon a t r a n s f e r mechanism from a r e s e r v e o f l u b r i c a n t . With t h e s e c o a t i n g s , h i g h e r s t r e s s l e v e l s are p s s i b l e , t h u s r e d u c i n g t h e s i z e and w e i g h t o f s p a c e c r a f t mechanisms. Compar i s o n of t h e r e s u l t s of a v e r a g e f r i c t i o n t o r q u e i n p i v o t b e a r i n g s w i t h d i a m e t e r 3 mm w i t h T i N - T i c , sented i n Fig. 6.19
( r e f . 456)
.
and TiN+MoS2-Tic c o a t i n g s a r e p r e -
Extreme l u b r i c a t i o n c o n d i t i o n s a r e e n c o u n t e r e d when m i n i a t u r e s y s t e m s a r e exposed t o s t r o n g r a d i a t i o n ( s p a c e , n u c l e a r r e a c t o r s
e t c . ) . I n n u c l e a r r e a c t o r s t h e l u b r i c a n t i s a d d i t i o n a l l y exposed t o t h e a c t i v i t y o f e l e v a t e d t e m p e r a t u r e s a s h i g h a s 2OO0C (sometimes 4OO0C) and c o o l i n g media s u c h a s C 0 2 . The e f f e c t o f r a d i o a c t i v i t y ( a , and iy - r a d i a t i o n ) o n o i l s i s t o b r e a k down m o l e c u l e s and f r e e r a d i c a l s r e a c t i n g w i t h a d j a c e n t m o l e c u l e s . High v i s c o s i t y and
p
c o k i n g p r o d u c t s a p p e a r i n t h e o i l a s a r e s u l t . G r e a s e s exposed t o r a d i o a c t i v e r a d i a t i o n f i r s t s o f t e n and t h e n become p o r o u s and l i q u i d . The d e s t r u c t i o n of s o a p c e l l s t r u c t u r e s f p o l y m e r i z a t i o n and o i l s e c r e t i o n occur. P a r a f f i n o i l s are less r a d i a t i o n r e s i s t a n t t h a n o i l s with aromatic f r a c t i o n s (ref. 9 ) . Aliphatic diester-based o i l s
a r e more s t a b l e t h a n m i n e r a l o i l s . The c o m p a r i s o n between t h e be-
260
h a v i o u r of s i l i c o n e and m i n e r a l o i l s exposed t o r a d i o a c t i v e r a d i a t i o n i s shown i n F i g . 6 . 2 0
( r e f . 6 2 ) . Halogened p o l y s i l o x a n e s are
less s t a b l e t h a n dimethyl- o r p h e n y l p o l y s i l o x a n e s . O f t h e s y n t h e t i c o i l s , p o l y p h e n y l e t h e r o i l s a r e most r e s i s t a n t t o gamma r a d i a t i o n e s p e c i a l l y a t high t e m p e r a t u r e s .
Number of oscillations
F i g . 6.19. Comparison o f f r i c t i o n t orq ue s o f p i v o t b e a r i n g s (diameter 3 mm) w i t h Ti c- c o a te d s t e e l bushes and j o u r n a l s coated w i t h TIN or TiN+MoS2; t e s t e d i n a 'vacuum. The a m p l i t u d e of o s c i l l a t i o n was 30°. The r a d i a l and a x i a l loads a r e g i v e n on t h e p l o t ( r e f . 4 5 6 ) .
A p p l i c a t i o n of MoS2 f o r t h e l u b r i c a t i o n of b e a r i n g s exposed t o r a d i o a c t i v e r a d i a t i o n i s recommended ( i f t h e t r i b o l o g i c a l p r o p t i e s a r e s u i t a b l e ) . Other s o l i d l u b r i c a n t s f o r u s e i n n u c l e a r r e a c t o r s i n s t r u m e n t a t i o n exposed t o e l e v a t e d t e m p e r a t u r e s a r e such m i x t u r e s a s PbS+MoS2+B2O3o r MoS2+graphite+natrium s i l i c a t e ( r e f . 9 ) .
261
Radiation dose, 10'6neutrons.mm-2
F i g . 6.20. Comparison o f behaviour o f s i l i c o n e and m i n e r a l o i l s when undergoing r a d i o a c t i v e r a d i a t i o n .
6 , 6 , L U B R I C A T I O N OF POLYMERIC SYSTEMS The l u b r i c a t i o n o f m i n i a t u r e polymeric b e a r i n g s i s a v e r y e f f e c t i v e way t o improve t h e i r t r i b o l o g i c a l p r o p e r t i e s (see Chapter 5 . 2 ) . T h e t r i b o l o g i c a l b e h a v i o u r o f l u b r i c a t e d metal-polymer o r polymer-polymer systems depends t o a g r e a t e x t e n t on t h e physicochemical e f f e c t s o c c u r r i n g i n a p o l y m e r - l u b r i c a n t system ( r e f s . 1 0 7 , 1 0 8 ) . The a b s o r b i n g o f a l u b r i c a n t by polymeric m a t e r i a l i s more
i n t e n s i v e i n t r i b o l o g i c a l dynamic c o n d i t i o n s t h a n i n c o n d i t i o n s of s t a t i c c o n t a c t . The a d s o r p t i o n o f l u b r i c a n t m o l e c u l e s o n t o a poly-
m e r s u r f a c e i s a l s o a f f e c t e d by t r i b o l o g i c a l c o n d i t i o n s b e c a u s e o f t h e t r i b o e l e c t r i f i c a t i o n o f polymeric m a t e r i a l . The i n f l u e n c e o f t h i s "momentary" p o l a r i t y of a polymer s u r f a c e on t h e d u r a b i l i t y of t h e boundary f i l m of a l u b r i c a n t can be v e r y i m p o r t a n t and must be t a k e n i n t o c o n s i d e r a t i o n ( r e f . 457) The e f f e c t p l a s t i c i z i n g t h e s u r f a c e l a y e r o f a polymeric mate-
.
262
r i a l i s g r e a t e r when t h e material i s t r i b o l o g i c a l l y l o a d e d . Polymer i s f r e q u e n t l y p l a s t i c i z e d f o r t h e a p p l i c a t i o n o f low-molecular l u b r i c a n t s ( e . g . m i n e r a l o i l s ) . The e n t r y of l u b r i c a n t m o l e c u l e s d i sperses t h e s u r f a c e l a y e r of polymers b u t a t t h e same time f a c i l i t a t e s t h e o r i e n t a t i o n o f t h e secondary s t r u c t u r e s correspond w i t h t h e f r i c t i o n f o r c e s ( r e f . 4 5 8 ) . The p l a s t i c i z a t i o n o f a polymer s u r f a c e l a y e r f o l l o w s t h e wear o f m a t e r i a l , so t h a t d u r i n g t h e r u b b i n g e a c h newly-formed l a y e r i s p l a s t i c i z e d and c a n b e t r e a t e d as “ s e l f - l u b r i c a t e d ’ ‘ ( r e f . 459) A c r i t i c a l p e r i o d o f s l i d i n g i s re-
.
q u i r e d t o produce a m o d i f i e d s u r f a c e o n t h e polymer
(PPO r u b b i n g
a g a i n s t s t e e l w i t h DMPS l u b r i c a t i o n ) and t h e d u r a t i o n o f t h i s p e r i o d i n c r e a s e s s i g n i f i c a n t l y when t h e r e i s no boundary l u b r i c a t i o n . The t h i c k n e s s of t h e m o d i f i e d s u r f a c e l a y e r on t h e polymer i s of t h e o r d e r o f 8 p m . Estimates o f i t s g l a s s t r a n s i t i o n t e m p e r a t u r e i n d i -
c a t e t h a t wear-induced p l a s t i c i z a t i o n o c c u r s r a t h e r t h a n s i m p l e p h y s i c a l m i x i n g . The m o d i f i e d s u r f a c e l a y e r d e v e l o p e d on t h e polym e r e n s u r e s t h a t f r i c t i o n and wear r e m a i n low e v e n a f t e r t h e r g n w a l o f a l l e x c e s s f l u i d from t h e system. The w e t t a b i l i t y of p o l y m e r i c m a t e r i a l by a l u b r i c a n t i s v e r y i m p o r t a n t . The r e d u c t i o n o f wear i n s t e e l - p o l y m e r m i n i a t u r e j o u r n a l b e a r i n g s i s a f u n c t i o n o f t h e w e t t a b i l i t y o f polymer by t h e o i l app l i e d ; t h e b e t t e r t h e w e t t a b i l i t y , t h e l e s s t h e wear i s ( r e f s . 363, 367, see a l s o C h a p t e r 5 . 2 ) . The f r i c t i o n c o e f f i c i e n t o f a l u b r i c a t ed p o l y e t h y l e n e - p o l y e t h y l e n e system d e c r e a s e s w i t h a r e d u c t i o n i n t h e s u r f a c e t e n s i o n o f t h e l u b r i c a n t ( r e f . 3 7 8 ) . The wear o f t h e polymer i n a s t e e l - p o l y m e r system depends o n t h e r e l a t i o n between t h e s o l u b i l i t y p a r a m e t e r s o f t h e l u b r i c a n t and t h e polymer (ref. 53). For amorphic polymers ( s u c h a s PMMA o r P P O ) , when t h e v a l u e s of t h e s o l u b i l i t y parameters a r e s i m i l a r , t h e w e a r r a t e is very high (and c a n exceed t h a t f o r a n u n l u b r i c a t e d s y s t e m ) . The g r e a t e r t h e d i f f e r e n c e between t h e s o l u b i l i t y p a r a m e t e r s o f t h e l u b r i c a n t and t h e polymer, t h e l o w e r t h e wear r a t e o f t h e polymer a s a r e s u l t o f l u b r i c a t i o n w i l l b e . F o r t h e c r y s t a l l i n e polymers, whose wear r a t e i s due i n t h e f i r s t p l a c e t o m a t e r i a l t r a n s f e r e f f e c t , t h e wear r a t e o f t h e polymer e l e m e n t i n l u b r i c a t e d s t e e l - p o l y m e r s y s t e m s increases w i t h a n i n c r e a s e i n t h e s o l u b i l i t y p a r a m e t e r o f t h e l u b r i c a n t . The e f f e c t o f t h e s o l u b i l i t y p a r a m e t e r o f a l u b r i c a n t on t h e f r i c t i o n c o e f f i c i e n t o f s t e e l - p o l y m e r s y s t e m s i s n e g l i g i b l e . When amorphic polymers r u b a g a i n s t s t e e l , t h e i n f l u e n c e o f l u b r i c a n t on wear i s associated with strong p l a s t i c i z a t i o n of t h e surface l a y e r but i n t h e case o f c r y s t a l l i n e polymers t h e l u b r i c a n t m o d i f i e s thematerial-
263
- t r a n s f e r t o t h e steel surface. I f t h e m a t e r i a l - t r a n s f e r process p l a y s a r e l a t i v e l y s m a l l r o l e i n t h e wear mechanism ( e . g . P O M ) , t h e e f f e c t of l u b r i c a n t on t h e wear r a t e o f a p o l y m e r i c m a t e r i a l i s small. Complex p h y s i c a l and chemical p r o c e s s e s o c c u r d u r i n g t h e rubb i n g o f e l e m e n t s i n metal-polymer of polymer-polymer s y s t e m s . A s a r e s u l t new f u n c t i o n a l groups and f r e e r a d i c a l s come i n t o e x i s t e n c e i n t h e rubbing r e g i o n ( r e f s . 103, 1 6 4 , 165, 2 4 9 ) . The c o m p l e x i t y of t h e p r o c e s s e s makes them v e r y d i f f i c u l t t o u n d e r s t a n d b u t t h e r e s u l t
i s t h a t t h e t r i b o l o g i c a l system m e t a l - l u b r i c a n t - p o l y m e r -lubricant-polymer
o r polymer-
i s v e r y " e l a s t i c " i . e . it i s p o s s i b l e t o c o n t r o l
the processes which o c c u r d u r i n g r u b b i n g , l e a d i n g t o s y s t e m s w i t h optimum t r i b o l o g i c a l p r o p e r t i e s . Tribochemical p r o c e s s e s embodying a c t i v e f r e e r a d i c a l s can be a p p l i e d t o t h e g e n e r a t i o n v e r y t h i n l a y e r s on t h e r u b b i n g s u r f a c e s from l i q u i d o r gaseous o r g a n i c compounds g e n e r a t e d w i t h i n t h e r u b b i n g r e g i o n . A s t h e r e s u l t of "sel e c t i v e t r a n s f e r " i n t h e p r e s e n c e of a l u b r i c a n t which c a n r e a c t w i t h mechano-chemical d e g r a d a t i o n p r o d u c t s o f polymer, t h e product i o n o f f r i c t i o n polymers o r c o l l o i d m e t a l l a y e r s i s p o s s i b l e . The a c t i v e f r e e r a d i c a l s formed by i n t e r a c t i o n of s t e e l - p o l y caproamide s u r f a c e s i n t h e p r e s e n c e of m i n e r a l o i l w i t h t h e a d d i t i o n of 2-20% (by w e i g h t ) o f & - c a p r o l a c t a m e monomer, c a n b e blocke d by monomer. A d e c r e a s e i n a d h e s i o n i n t e r a c t i o n s between r u b b i n g s u r f a c e s o c c u r s a s t h e r e s u l t of t h e s o - c a l l e d c o l d p o l y m e r i z a t i o n o f t h e monomer i n t h e o i l . T h i s h a s been s e e n t o r e s u l t i n a dec r e a s e i n t h e wear, f r i c t i o n c o e f f i c i e n t and t e m p e r a t u r e i n t h e rubbing r e g i o n a s compared t o l u b r i c a t i o n w i t h p u r e m i n e r a l o i l ( r e f s . 4 6 1 , 4 6 2 ) . The f r i c t i o n polymer was formed d u r i n g t h e rubb i n g o f s t e e l - P A 6 6 elements l u b r i c a t e d w i t h p o l y s i l o x a n e s (ref -463). This product w a s t h e r e s u l t of t h e o x i d a t i o n of polysiloxane a t t h e e l e v a t e d t e m p e r a t u r e r e s u l t i n g from f r i c t i o n a l h e a t , and t h e r e a c t i o n o f methyl groups of p o l y s i l o x a n e and f r e e amide r a d i c a l s res u l t i n g from t h e mechanical d e s t r u c t i o n o f polyamide. An i m p o r t a n t improvement i n t h e t r i b o l o g i c a l p r o p e r t i e s o f t h e m a t e r i a l combin a t i o n s i n v e s t i g a t e d was a c h i e v e d i n t h i s way. Selective t r a n s f e r o f f e r s another p o s s i b i l i t y f o r controllinq t h e t r i b o l o g i c a l p r o p e r t i e s of polymeric s y s t e m s . T h i s e f f e c t occ u r s f o r example d u r i n g t h e r u b b i n g o f p o l y m e r i c m a t e r i a l c o n t a i n i n g an a d d i t i o n o f a s o f t m e t a l chemical compound ( e . g . c o p p e r ) . The e f f e c t of s e l e c t i v e t r a n s f e r h a s been observed d u r i n g t h e rubb i n g of polycaproamide (PCA) o r PTFE, w i t h t h e a d d i t i o n of 4 0 % (by
264
w e i g h t ) CuzO, a g a i n s t s t e e l , l u b r i c a t e d w i t h g l y c e r i n e ( a l u b r i c a n t which p r e v e n t s o x i d a t i o n o f t h e r u b b i n g s u r f a c e ) . The p r o d u c t s o f t h e o x i d a t i o n of g l y c e r i n e react w i t h a steel s u r f a c e i n t h e pres e n c e o f Fe and Cu i o n s . The f r i c t i o n a l h e a t i n i t i a t e s t h e c r y s t a l l i z a t i o n p r o c e s s of c o p p e r and t h e c o p p e r i o n s d i f f u s e i n t o a n o x i d e f r e e steel surface. A s a r e s u l t t h e steel s u r f a c e i s continuously covered w i t h a t h i n , s o f t c o p p e r l a y e r . The m e t a l l i z a t i o n of t h e polymer s u r f a c e w i t h c o p p e r i s t h e r e s u l t o f a c t i v a t i o n o f t h e s u r f a c e l a y e r by h y d r o x y l g r o u p s d e r i v e d from g l y c e r i n e and by c a r b x y l g r o u p s r e s u l t i n g f r o m o x i d a t i o n o f t h e g l y c e r i n e p r o d u c t s and a b l e t o a d s o r b m e t a l i o n s . The s e l e c t i v e t r a n s f e r e f f e c t i n t h e a f o r e mentioned s y s t e m s c a u s e d a d e c r e a s e o n t h e e f f e c t i v e wear r a t e of p o l y m e r i c m a t e r i a l . When no l u b r i c a t i o n w a s used i n t h e case o f PCA-based c o m p o s i t i o n s c o n t a i n i n g c u p r o u s o x i d e , t h e wear r a t e w a s 6 times lower a s compared t o t h a t o f p u r e PCA s a m p l e s . When l u b r i -
c a t e d w i t h g l y c e r i n e under s i m i l a r t e s t i n g c o n d i t i o n s , t h e w e a r int e n s i t y d e c r e a s e d almost 1 0 0 times and w a s o n l y
F i l l i n g PIFE
w i t h c u p r o u s o x i d e l e d t o a t h r e e f o l d d e c r e a s e i n t h e wear intensity when o p e r a t i n g w i t h o u t a n y l u b r i c a t i o n . When g l y c e r i n e w a s u s e d , t h e d e c r e a s e w a s t e n f o l d a s compared t o t h e c o m p o s i t i o n c o n t a i n i n g Cu20 and amounted t o a b o u t 5 The e f f e c t of p r e v i o u s c o n t a c t ( e . g . by immersion) between a polymer sample and a l u b r i c a n t on t h e t r i b o l o g i c a l p r o p e r t i e s of a l u b r i c a t e d p o l y m e r i c s y s t e m s h o u l d b e t a k e n i n t o c o n s i d e r a t i o n . An i n c r e a s e i n t h e time o f e x p o s u r e of t h e polymer s p e c i m e n s i n t h e o i l c a u s e s a f u r t h e r i n c r e a s e i n t h e f r i c t i o n c o e f f i c i e n t o f steel polymer s y s t e m s . T h i s e f f e c t h a s been o b s e r v e d f o r t h e p o l y c a p r o amide r u b b i n g a g a i n s t s t e e l when t h e h i g h l y r e f i n e d m i n e r a l i n s t r u ment o i l MWP w a s used a s a l u b r i c a n t ( r e f . 4 6 3 ) . The f r i c t i o n c o e f f i c i e n t of t h e samples p r e v i o u s l y immersed i n MWP o i l w a s h i g h e r t h a n f o r t h o s e r u b b i n g a g a i n s t s t e e l w i t h o u t l u b r i c a t i o n . The longe r t h e e x p o s u r e time, t h e h i g h e r t h e f r i c t i o n c o e f f i c i e n t w a s . This e f f e c t w a s more pronounced when t h e t e s t t e m p e r a t u r e w a s h i g h e r . T h i s c a n b e e x p l a i n e d by a n i n c r e a s e i n t h e a c t u a l a r e a of c o n t a c t due t o t h e a d s o r p t i o n p l a s t i c i z a t i o n o f t h e s u r f a c e l a y e r o f p o l y meric m a t e r i a l . S i m u l t a n e o u s l y w i t h t h e i n c r e a s e i n t h e a c t u a l area of c o n t a c t , t h e r e w a s a d e c r e a s e i n t h e e f f e c t i v e s h e a r r e s i s t a n c e o f t h e m a t e r i a l ; under t e s t c o n d i t i o n s however, t h i s phenomenon c o u l d n o t compensate t h e i n c r e a s e i n t h e f r i c t i o n c o e f f i c i e n t caused by s o f t e n i n g
of t h e material.
The m e c h a n i c a l p r o p e r t i e s o f t h e polymeric samples d e c r e a s e d
265
a f t e r c o n t a c t w i t h a l u b r i c a n t . T h i s a p p l i e s mainly t o t h e hardness. The h a r d n e s s o f PA 6 6 and PA 1 2 samples d e c r e a s e d s i g n i f i c a n t l y a f t e r c o n t a c t d u r i n g 28 d a y s w i t h such o i l s as v a s e l i n e o i l and e s t e r - b a s e d o i l (2-ethylhexylphtalate)(refs. 108, 4 6 5 ) . The e f f e c t
of o i l e x p o s u r e , was observed t o be s l i g h t f o r PA 11, P C , PETP and PA 1 2 immersed i n i n s t r u m e n t o i l s such a s methylphenylpolysiloxane, f l u o r i n a t e d m e t h y l a l k y l p o l y s i l o x a n e and p o l y a l k y l g l y c o l w i t h e t h e r and a l c o h o l g r o u p s . The a u t h o r s o f t h e s e i n v e s t i g a t i o n s of i n t e r a c t i o n s i n t h e oil-polymer
system i n s t a t i c c o n d i t i o n s a s s e r t s t h a t
such m a t e r i a l s a s R i l s a n ( P A l l ) , C r a s t i n ( P E T P ) , Hostaform (POM c ) , Noryl ( P P O ) , D e l r i n (POM h ) and Makrolon ( P C ) a r e t h e b e s t materials i n v e s t i g a t e d f o r elements of l u b r i c a t e d m i n i a t u r e s y s t e m s . The exposure i n o i l s o f samples made of t h e r m o p l a s t i c s decreases t h e i r t e n s i l e and bending s t r e n g t h . The m i n e r a l and e s t e r - b a s e d o i l s a r e e s p e c i a l l y harmful ( r e f . 1 6 9 ) . The lower t h e c o n t a c t a n g l e o f l i q u i d s such a s w a t e r s o l u t i o n s of N a O H ,
CH3COOH, H2S04 on polym e r s u r f a c e s such a s PA o r P P , t h e g r e a t e r t h e c r e e p of t h e polymer sample w i l l be ( r e f . 4 6 6 ) . I n t h e case o f t h e l u b r i c a t i o n of m i n i a t u r e polymeric s y s t e m s it i s a l s o v e r y i m p o r t a n t t o d e t e r m i n e t h e e f f e c t of polymeric mat e r i a l on t h e p r o p e r t i e s o f t h e l u b r i c a n t u s e d . Polymeric m a t e r i a l c a n a f f e c t t h e l u b r i c a n t by premature coking a s a r e s u l t of polym e r i z a t i o n , change i n t h e p r o p e r t i e s of s t a b i l i z i n g and i n h i b i t i n g a d d i t i v e s , g e l l i n g , unhomogenizing, change i n c o l o u r , a c i d i t y numb e r , v i s c o s i t y , s u r f a c e t e n s i o n and r e f r a c t i v e i n d e x . T i l l w i c h ( r e f s . 108, 465) h a s i n v e s t i g a t e d t h e i n f l u e n c e o f p o l y m e r i c mat e r i a l on t h e v i s c o s i t y , a c i d i t y number and r e f r a c t i v e i n d e x o f o i l s . Ten t h e r m o p l a s t i c s and 1 2 o i l s were t e s t e d a t t e m p e r a t u r e s of 6 0 , 8 5 and 95OC f o r a p e r i o d o f 28 d a y s . The e f f e c t o f e x h a l a t i o n s from polymer g r a n u l e s was t e s t e d i n c l o s e d , g l a s s v e s s e l s w i t h d i ameters of up t o 1 0 0 m . O i l samples of 1 0 m l volume were p l a c e d n e a r t o t h e c o v e r . The c l a s s i c Baader's t e s t was t h e n c a r r i e d o u t w i t h 30 g of polymeric g r a n u l e s i n s t e a d copper b a r s . The i n f l u e n c e o f d i f f e r e n t polymeric m a t e r i a l s o n t h e o i l s tested v a r i e d g r e a t l y . The o i l s t e s t e d can b e a r r a n g e d i n t h e f o l l o w i n g o r d e r based on d e c r e a s i n g r e s i s t a n c e t o t h e a c t i v i t y of polymers
( P A 6 6 , PA 11, PA 1 2 , POM h , POM c , PC, PA (amorphic-Trogamid T I , p o l y a l k y l t e r e p h t h a l a t e PETP ( C r a s t i n S 6 0 0 ) , p o l y t e t r a m e t h y l t e r e p h -
t h a l a t e PETP (Dynamit N o b e l ) , PPO : DMPS, E t s y n t h a K 2363 ( s p e c i a l synthetic instrument o i l (polysiloxanealcohol) f o r t h e l u b r i c a t i o n o f m i n i a t u r e polymeric s y s t e m s ) , p a r a f f i n u m subliquidum, E t s y n t h a
266 K 7132 l v ( f l u o r i n a t e d p o l y e t h e r s y n t h e t i c o i l ) , f l u o r i n a t e d n e t h y l -
p h e n y l p o l y s i l o x a n e , E t s y n t h a Gold, E t s y n t h a K 7 1 3 1 ( c l a s s i c c l o c k o i l ) , v a s e l i n e o i l , e s t e r - b a s e d o i l ( 2 - e t h y l h e x y p h t h a l a t e ) ,aliphatic a l c o h o l , f r e s h n e a t ' s - f o o t o i l . F i v e of t h e aforementioned o i l s have been acknowledged a s b e i n g v e r y good l u b r i c a n t s f o r m i n i a t u r e , polymeric s y s t e m s , i . e . DMPS, E t s y n t h a K 2363, K 7132, K 7131 and f l u o r i n a t e d methylphenylpolysiloxane o i l s . A s p e c i a l i n s t r u m e n t o i l f o r l u b r i c a t i o n o f polymeric m i n i a t u r e
systems
-
Etsyntha K 7132 l v
-
changed i t s v i s c o s i t y n e g l i g i b l y
when s u b j e c t e d t o Baader's t e s t ( r e f . 4 6 7 ) . The a g e i n g o f t h e same o i l w i t h B a a d e r ' s t e s t m o d i f i e d by s u b s t i t u t i n g polymer g r a n u l e s 2 f o r copper b a r s , changed t h e v i s c o s i t y from 308 mm / s a t 2OoC t o 2
3 2 0 mm / s a f t e r 8 0 d a y s . The same t e s t c a r r i e d o u t on c l a s s i c c l o c k 2
o i l w i t h a v i s c o s i t y of 123 mm / s a t 20°C
showed t h a t a f t e r 2 0 d a y s
of a g e i n g ( w i t h t h e modified B a a d e r ' s t e s t , u s i n g polymer g r a n u l e s ) t h e i n c r e a s e i n v i s c o s i t y o f t h e o i l was n e g l i g i b l e , b u t f u r t h e r 2 a g e i n g c a u s e d a r a p i d i n c r e a s e i n v i s c o s i t y t o a b o u t 520 mm / s a f t e r 6 0 d a y s . The c l a s s i c B a a d e r ' s a g e i n g t e s t l e d t o t h e same v i s c o s i t y i n c r e a s e a f t e r 20 d a y s b u t f u r t h e r t e s t i n g saw a s i g n i f i 2 c a n t l y lower i n c r e a s e i n v i s c o s i t y : a f t e r 6 0 d a y s t o a b o u t 215mn/s 2
and a f t e r 80 days t o a b o u t 300 mm / s . V a r i a t i o n s i n l u b r i c a n t p r o p e r t i e s a s t h e r e s u l t o f polymer- l u b r i c a n t i n t e r a c t i o n s can be s i g n i f i c a n t . It i s p o s s i b l e a t high s p e c i f i c l o a d s and a v e r y low s l i d i n g speed t o c a u s e t o t a l c o k i n g of t h e o i l ( r e f . 5 8 ) . Because of t h i s t h e r e a r e s e v e r a l i n s t r u m e n t o i l s on t h e market which a r e s p e c i a l l y manufactured f o r u s e i n t h e l u b r i c a t i o n of polymeric m i n i a t u r e systems (see C h a p t e r 3.21. These o i l s are r a t h e r f u l l y s y n t h e t i c and based on e t h e r a l c o h o l , f l u o r i n a t e d p o l y s i l o x a n e s , o r esters. One of t h e s e o i l s i s a s p e c i a l l y f o r m u l a t e d c l a s s i c c l o c k o i l (KunstoffB1 K 7 1 3 1 , manufactured by D r . Tillwich GmbH,
( E t s y n t h a Chemie), i n W e s t Germany).
The m i g r a t i o n of o i l from t h e r u b b i n g r e g i o n , e s p e c i a l l y when polymeric m i n i a t u r e s y s t e m s a r e l u b r i c a t e d f o r l i f e , i s a s e r i o u s problem. The problem i s n o t a s i m p o r t a n t f o r metal-polymer combinat i o n s because of t h e f o r m a t i o n of a r e l a t i v e l y s t r o n g a d s o r b e d o i l boundary f i l m on t h e m e t a l s u r f a c e . The m i g r a t i o n o f o i l is, howe v e r , troublesome i n t h e c a s e o f t h e polymer-polymer systems o f t e n used i n f i n e mechanisms. The r e a s o n f o r t h e o i l m i g r a t i o n i s n o t f u l l y c l e a r . The s u r f a c e f r e e energy o f used plymers is not high (30-50 mJ/m 2 ) and it is n o t s i g n i f i c a n t l y h i g h e r t h a n t h e s u r f a c e t e n s i o n of t h e o i l s u s e d .
267
Because of many various additives to the polymer, mode-of .manufacturing the polymeric elements, adsorption and reaction with the environmental atmosphere, the polymer surface is very inhomogenous and as a result, the molecular force field depends on other factors than the chemical structure of the molecules (ref. 468). Oil drops will probably spread on thermoplastic polymer surfaces, depending on the shape of the magnifying drop (refs. 416, 4171, and this can happen very quickly (Fig. 6.21 of ref. 169). The migration of the oil drop from the rubbing region of polymeric systems especially when there is impact (acceleration) overloading or element vibration, can be the result of relatively low work of adhesion between low surface free energies of the contacting oil-polymer surfaces.
Soreodtng t i m e , min
F i g . 6 . 2 1 . Comparison o f s p r e a d i n g dynamics o f a d r o p of n e a t ’ s - f o o t o i l s p r e a d i n g on polymer p l a t e s w i t h o u t c o a t i n g ( e p i lame) (full 1 i n e s ) and w i t h a c o a t i n g o f epilame ( F i x d r o p B S , sol u t i o n l : l o o ) ( d o t t e d l i n e s ) ( r e f .169).
Coating (epilamizing), to regulate the polymer element surface or to create a barrier film, solves this problem. The modern epilames based on fluoropolymers are suitable for epilamizing plastic
268
elements s u r f a c e (see Chapter 6.2.4).
The e f f e c t of e p i l a m i z i n g of
a polymer s u r f a c e of t h e s p r e a d i n g of o i l d r o p c a n b e c l e a r l y s e e n when t h e d r o p s p r e a d i n g dynamics shown i n F i g . 6 . 2 1 a r e compared. The problem o f p r o p e r l u b r i c a t i o n o f polymeric m i n i a t u r e systens
is f a i r l y r e c e n t and needs f u r t h e r r e s e a r c h . C o n t r o l l i n g t h e e f f e c t s i n a l u b r i c a n t - p o l y m e r i c m a t e r i a l system i s a p r e c o n d i t i o n f o r p r o g r e s s i n t h e t r i b o e n g i n e e r i n g o f t h e m i n i a t u r e s y s t e m s . The p r e d i c t i o n of t h e t r i b o l o g i c a l p r o p e r t i e s o f m e t a l - l u b r i c a n t - p o l y meric m a t e r i a l o r polymeric m a t e r i a l - l u b r i c a n t - p o l y m e r i c m a t e r i a l systems i s v e r y d i f f i c u l t ; however, t h e r e s u l t s o f p r e v i o u s i n v e s t i g a t i o n s e n a b l e some u n d e s i r a b l e c o m b i n a t i o n s t o be i d e n t i f i e d and t h e i r u s e avoided.
269
7 , SPECIAL TRIBOLOGICAL COATINGS 7 , 1 , INTRODUCTION Special tribological coatings are usually applied for their anti-friction or anti-wear qualities. Anti-friction coatings are used when lubrication by oils or greases is precluded, for example, when the operating temperature is high, store time very long or when the environment (oxygen atmosphere, vacuum, radioactivity, etc.) is incompatible with the presence of a lubricant. Dry lubrication is an effective way to resolve many very difficult prablems of lubrication in miniature systems (see Chapter 6). P7ear-resistant coatings are applied on cheap materials to obtain high wear resistance of the rubbing surfaces, particularly when abrasive, impact or corrosive wear is expected. Various techniques can be applied to produce dry lubricant films or wear-resistant layers on a surface. For anti-friction coatings, the following techniques are typical: burnishing, bonding,sputtering (physical vapour deposition (PVD) techniques, used for solid lubricants such as M O S ~ ) ,ion plating and electroplating (soft metals), and fluidization (polymeric coatings). Anti-wear coatings are produced by electrodeposition, thermomechanical diffusion processes, or by chemical or physical vapour deposition (CVD and PVD); the wear resistance of the surface can also be modified by ion implantation or laser beam treatment. Details on modern techniques for depositing anti-friction or anti-wear coatings can be found in refs. 117, 4 6 9 , 8 7 4 .
7 2 A N T I- F R I CTIO N I
I
COATINGS
Anti-friction coatings can be made of inorganic or organic substance. The most frequently used inorganic coatings are those formed using molybdenum disulphide (MoS2) or soft metal (silver, lead, gallium, gold, tin, indium, bismuth, cadmium). The FeMo2S4 coating proposed recently (ref. 4 7 0 ) can give a friction coefficient (when sliding against steel at low contact pressure) below 0.05. Good tribological behaviour is also demonstrated by combined multi-layer coatings such as gallium + MoS2 (ref. 471).
270 The MoS2 coatings are deposited using various methods (burnishing, bonding, sputtering). The best properties are demonstrated by thin ( < 1 ,um) coating obtained by sputtering, which ensures good adhesion of the coating to the substrate material. The optium sputtered film thickness for tribological applications, for example in rolling bearings, is 0.2-0.3 ,um (ref. 117). MoS2 coatings can operate both at very low temperatures (as low as -15OOC) and at high temperatures (up to 4OO0C in air and 600-700°C in a vacuum) as well as in the presence of radioactivity. They are often used in mechanisms operating in space. MoS2 coatings are usually deposited directly onto a metal surface (often steel) or onto a thin Cr3Si.2 sublayer on the steel surface. This method somehow increases the lifetime of the coating (ref. 117) although the reasons for this are not clear. The previous degassing of MoS2 in a vacuum improves the quality of the coating. The substrate temperature should be kept between 20 and 32OoC to achieve the lowest possible friction coefficient ( 0 . 0 4 ) (ref. 117) The crystallqraphic orientation and structural properties of the lamellae are important for the tribological behaviour (refs. 472, 473). The crystallites should be arranged with their basal planes parallel to the substrate surface. Such films are less degraded by exposure to high humidity environments, less prone to oxidation (which would form MoOj), have a low friction coefficient and longer endurance lives than films with randomly oriented crystallites. The surface free energy of such films is low (a few dozen mJ/m2 (ref. 126)1 . Films formed of MoS2Dx (Dx - additional substance not reacting with MoS2) operating in air give very low friction coefficients as compared to hydrodynamic or qas lubrication (refs. 126, 474). This effect is the result of the large decrease in interlamellar forces when the atoms of the additional substance are placed on the basal planes ( O O O l ) , decreasing the adhesion and enabling easy relative sliding of the (0001) planes. Also (0001) planes oriented parallel to the friction surface in graphite or graphite + MoS2 coatings give very low friction coefficients in a vacuum (below 0.02) ;the highest friction coefficient was for graphite films, the lowest (0.01) for MoS2 (ref. 475). The very low friction coefficient of the MoS2 films in a vacuum may be obtained by low energy ion or electron bombardment during rubbing (ref. 126). MoS2 coatings have no anti-corrosion properties and can initiate corrosion, particularly when operating in a humid atmosphere.
.
271
The quality of the tribological behaviour of these coatings is greatly reduced when they are operated in humid air. The friction coefficient of steel + MoS2 film-ruby journal jewel microbearings (bearing hole diameter 0.11 mm) was over 0 . 6 in humid air (100% relative humidity) while in dry air (1% relative humidity) it was only about 0.2, being constant as a function of the sliding speed and increasing to 0.22 after l o 6 revolutions (ref. 4 5 2 ) . The great advantage of MoS2 coatings is their low friction, particularly in a vacuum. The friction coefficient decreases with increase in contact pressure. For such films, the contact pressure during operation may be as high as 3000 MPa. Previous running-in improves the tribological behaviour of the coating in a vacuum (refs. 4 7 6 , 4 7 7 ) . Running-in should be carried out at as high a pressure as possible (up to 0.8 of the seizure pressure in vacuum). Increasing the contact pressure step by step during running-in raises the seizure load and increases the lifetime of the bearing during normal operation in a vacuum. Running-in in a vacuum is more effective than in air. However, if the coating is stored in air after running-in, the wear of the coating during operation in a vacuum will again be greater. Running-in increases the microhardness of MoS2 (ref. 4 7 8 ) . The thinner the coating is, the higer its microhardness. When operating in air, burnished MoS2 films demonstrate better friction behaviour when the substrate is very rough, but when the films are operating in dry argonla smooth substrate gives better results (ref. 4 7 9 ) . The wear of MoS2 films in dry argon is about two orders of magnitude greater than in moist air because of the difference in failure mechanisms. In dry argon, failure is caused by the gradual depletion of MoS2 by lateral flow from the contact region and the production of a very fine, powdery debris, mostly ci-iron, while in moist air failure is due to the transformation of coelesced MoS2 films into a powdery material consisting mainly of a-iron and MoO3. If the substrate surface roughness i s increased, a lower wear rate can be expected both in moist air and in dry argon by providing reservoirs for the MoS2. The tribological behaviour of MoS2 burnished films is better when they are deposited on other,previously formed films, or when they are modified by various additional substances. For example, copper films were applied to a steel-brass pair (lubricated with glycerol + hydrochloric acid) using the selective transfer method (see Chapter 5.1.1) ; the films were 2 ,um thick, porous and chemical-
272 ly active (ref. 471). The gallium was easily burnished into this film (the melting temperature of gallium is 29.7OC) , filling the pores in the copper layer. The MoS2 was then burnished into this prepared substrate film to form a solid solution of MoS2 in gallium, and also probably of gallium sulphides, with good anti-friction properties. The other method for realizing combined MoS2 m t ings was to burnish the MoS2 into a 2-3 ,um thick substrate layer obtained by sulphurizing and carburizing the steel surface. The porous structure of the layer containing hard carbide particles leads to a relatively low friction coefficient (0.1-0.15) and low wear , too. In tests on burnished MoS2 films deposited on bearing races (6026 and 6028 ball bearings with a shaft diameter of 6 and 8 nun, operating in a vacuum of Pa, at oscillatory motion with amplitude 1.5-4O , frequency 5-20 Hz and loaded with radial and axial forces of 75-2000 and 25-700 N respectively), the films had an lifetime 2 107-5 lo6 cycles (ref. 471). The lifetime of similar bearings without coatings and lubricated with CIATIM-221 grease (based on ethylpolysiloxane and thickened with Ca stearate) was only 2 106-1 105 cycles respectively. The wear resistance of Mo%films can be improved by the addition of graphite (up to 10% by weight)(ref. 4801, probably as a result of the graphite and molybdenum disulphide synergism (ref. 118). The modification of MoS2 films by the addition of phthalocyanines is also effective. The greatest decrease in wear, however, was achieved by adding 10% (by weight) of metal-free phthalccyanine (ref. 480). This was probably the result of strong bonding of the phthalocyanines with the metal substrate surface because of the formation of chelates, along with the influence of the phthalccyanine on the formation of oriented crystallites on the film surface. Thin, ternary alloy films of Fe-Mo-S (FeMozSq), applied to a surface by sputtering, show very good tribological properties because of the formation of an interfacial film at the contact point (ref. 470). During the sliding of a hemisphere pin of radius 6 nun and made of chromium-plated, steel against Fe-Mo-S film 5 ,urn thick on bearing steel, at a sliding speed of 1 nun/s and load lo-30 N, the friction coefficient was less than 0.01 (smaller than with a MoS2 coating). The wear in steel-(Fe-Mo-S2) systems is very mil. The high temperature MoS2 diffusion films can be obtained by the thermochemical processing of an element made of molybdenum or chromium steel, Such films (5-120 ,um thick when formed on mlybdenm
273 or 10-250 ,um thick on chromium steel) are formed with or without the addition of Pb, MoS2-ZnS solid solution or as MoS~+iron (or chromium) sulphides (when the film is formed on chromium steel) (ref. 481). The admissible temperatures and contact pressures for these films at operation in a vacuum or in inert gases are 850, 850 (with Pb release), 400 and 7OO0C, and 400-500, 800-1000 , 200-300 and 800-1000 MPa respectively. These coatings have been investigated in various combinations in the search for low friction and wear-resistant journal bearings to operate in the pure helium atmosphere of nuclear reactor cooling systems at a temperature of 300-350°C, sliding speed 0.01-0.8 m/s and contact pressure 2-0.5 MPa (ref. 481). The best tribological behaviour was found for the combination of a 40 pm thick coating on the moving element sliding against a 60 ,um thick coating of the same material on the stationary element. The best coating was the one on chromium steel (0.2% c, 13% Cr) containing MoS~+iron (or chromium) sulphides. A journal bearing with a combined journal and bearing bush coating thickness of 90-100 ,urn, nominal diameter 40 mm, external bearing bush diameter 55 mm, length 40 mm, radially and axially loaded with 100 and 400 N and operating at the sliding speed 1.2 m/s in pure helium (at a pressure of 0.14 MPa and a temperature of 3OO0C) demonstrated a low friction coefficient (0.03-0.04) and had a lifetime of 25000 h (until total wear of the coatings). Other frequently used coatings are those made of inorganic substances deposited using soft metals. Their friction coefficient is usually greater than that of MoS2 films but they also fulfil an anti-corrosion function. The best properties are shown by soft metal films formed by ion-plating, which provides good adhesion of the film to the substrate surface. The thickness of the coating has a strong influence on the friction coefficient. If the film is too thin, shear of the film by counterface surface occurs and if the film is too thick, the deformation component of the friction force increases. In the case of a hard steel slider on a hardsteel plate coated with indium, the most effective lubrication was when the film was 0.1 to 1.0 ,um thick (ref. 117). Lead films demonstrate very good tribological properties (refs. 128, 482). Sputtered, ion-plated or electro-deposited single lead coatings all have about the same sliding distance (endurance life). Their friction coefficient is about 0.06 in a vacuum. To extend the sliding distance, double layer coatings are used. The underlying layer should be made of a material which can neither dissolve
274
lead nor be dissolved by it and which has a lubricating quality. The introduction of copper, platinum or molybdenum substantially extends the sliding distance (by improving wear behaviour) (ref. 4 8 2 ) . The lead coating breakdown process during sliding has an adhesive character. Other soft metals used for films are indium, gallium, gold, silver, tin and cadmium. Alloys of these and other metals are also used. A film of copper-tin alloy on bearing races ensures a long lifetime for miniature ball bearings in a vacuum or in air; a silver-lead alloy can be used to form a dry lubricating film on bearings operating in a vacuum or air at temperatures up to 3OO0C (ref. 4 8 3 ) . A composite material based on silver and containing e.g. rhenium oxide particles can be used as a coating on bearings operating at temperatures up to 700-9OO0C (refs. 483, 8 7 9 ) . Fusible metal films containing polymer additives demonstrate interesting properties (ref. 4 8 4 ) . Films based on Ga-In eutectic alloy and containing up to 5% (by weight) of powdered (less than 1 0 ,um) PE, PP, PS or PMMA were obtained by rotary printing on a brass plate of microhardness 1 5 0 0 MPa and surface roughness (Ra) 0.22-0.25 ,um. In the tribological studies, a pin with a spherical working surface ($3 0.75 mm, Ra = 0 . 1 2 ,urn) made of chromium steel ( 0 . 2 % C, 1 3 % Cr) was used a s the counterface. The sliding speed was very low ( 1 . 2 mm/s). The friction coefficient and wear intensity as a function of applied load for various coatings are presented in Fig. 7.1. There are significant differences in the tribological properties of the coatings without polymer additive and those with additive (in particular PE), especially at high loads. The polymeric additives facilitate the formation of a plasticized layer on the metal surface, thereby improving the tribological behaviour of the system. Anti-friction coatings of organic materials such as polymers, high-molecular surface active agents, or phthalocyanines also fulfil an anti-corrosion function. Apart from the phthalocyanines coating they cannot be used at elevated temperatures. This kind of coating is formed by using such polymers as PA, PE, POM, PPS, PTFE or PI. Polymeric coatings such as PA or PE coatings are usually deposited on metallic elements by fluidization and their optimum thickness is about 0.3 mm (ref. 4 8 5 ) . The use of a thin polymeric coating instead of a thick polymeric solid element leads to better heat transport from the friction area and as a result to better wear resistance; e.g. the use of PA coatings can reduce wear by
275 3-7 times as compared to when a thick polymeric element is used (ref. 485). When a PA coating rubs against steel, the friction coefficient is lower (below 0.15) than for thick PA elements. The
PA coating material is often filled with MoS2, graphite, talc, PTFE or PE. PA 11 (Rilsan) has very good tribological properties when used to form polymeric coatings. POM coatings also demonstrate good tribological behaviour (ref. 486)
.
0.3
I
% I a,
*t
.d
U
I
5 0.2 0
0
2 Load ,N
60
r i g . 7 . 1 . F r i c t i o n c o e f f i c i e n t and wear i n t e n s i t y ( d o t t e d l i n e s ) as a f u n c t i o n o f a p p l i e d l o a d f o r a t r i b o l o g i c a l system made o f s t e e l - e u t e c t i c Ga-In a l l o y f i l m , w i t h no a d d i t i v e ( 1 ) and w i t h added PMMA ( 2 ) , PS ( 3 ) , PP (4) o r PE (5) ( r e f . 484)
.
Polymeric coatings deposited in the presence of oxygen have poorer tribological properties than those made under vacuum or
276 anti-oxidative conditions (refs. 407, 4 8 8 ) . 1 mm thick PE coatings, formed on aluminium in a vacuum (10-l Pa) at a temperature of 588 K and sliding against steel, have a wear rate (more than one order of magnitude lower) and a lower friction coefficient than similar coatings formed in air (ref. 407). The PE coatings formed on catalytically active metals such as copper or steel demonstrate better frictional behaviour than those deposited on a catalytically inert surface such as aluminium (ref. 487). The sharp increase with rising temperature in the friction coefficient of oxidized PE formed on a catalytically inert surface is caused by rendering amorphous and cross-linking of PE macromolecules and the increase in its polarity. The tribological qualities of PE coatings can be further improved by the addition of graphite or titanium dioxideI2-4wt% alkanon (ref. 485). Coating with chlorinated polyether (Penton) gives even better properties than PA - 2 to 3 times less wear can be expected (ref. 485). A PPS-lined aluminium sleeve (GLAMAT 33, Glacier Metal Co.) designed to be oil lubricated and to operate at up to 15OoC has shown very good tribological behaviour in the bearings of small electric motors (ref. 489). PTFE and PI coatings can operate at high temperatures. Bonded, sprayed or fused PTFE or PTFE/FEP coatings can operate at up to 2 5 0 - 3 0 3 O C . Sometimes PTFE is forced into the pores on the surface of metals or other materials, as in the case of DU material(G1acier Metal C o . ) where the surface layer of the PTFE+Pb composite is 1 0 - 3 0 ,um thick. When the DU material slides slowly against steel, its friction coefficient is higher than that of PTFE (O.las against 0.05) but its wear is dramatically lower, particularly at high loads (ref. 45). Aluminium and magnesium alloys which have been anodized to give a hard, porous surface layer are impregnated with PTFE and show a marked reduction in friction and wear (refs. 2 4 1 , 4 9 0 ) . Iron, copper,nickel, cobalt, aluminium, magnesium and zinc alloys as well as non-metals can be coated with nickel+PTFE film to get a very low wear rate and greatly improve friction behaviour (ref. 490). PTFE coatings can operate in a vacuum and at very low temperatures. PI coatings are especially resistant to elevated temperatures (up to 3 5 0 O C ) . When PI coatings of varnish were tested in argon, dry air and air containing water vapour, differences were observed in their friction and wear behaviour: low friction and a short lifetime were observed at temperatures above 100°C, which is at-
277 tributed to second order relaxation in the molecular bonds of the polymer between 25 and 100°C; in this temperature range the friction coefficient is relatively high (0.1-0.3) while in the temperature range 10O-40O0C it is very low (0.02) (ref. 121). The addition of graphite fluoride (CF,), or MoS2 to the PI varnish clearly improves the wear and friction properties at room temperature. 20-25 ,um thick PI films varnished onto stainless steel can be divided into two tribological groups: those with high friction, and those with low friction but a film wear rate an order of magnitude higher than those of the high friction group (ref. 491). The wear mechanism of such films is predominantly adhesive. The size of the wear particles is larger for the group of PI coatings with low friction. When PI was used as bonding material for a (CFx)n coating on start/stop foil gas bearings operating at temperatures up to 350°C, the coating showed very good tribological properties, particularly in its wear resistance (ref.121). Coatings of high-molecular surface active agents demonstrate very good tribological behaviour. The thickness of a varnished coating should be 1-5 ,um. A very low friction coefficient (0.03-0.04) was found for films deposited on stainless steel using a composite of silicon organic varnish with polybenzimidazol during sliding against phosphorus bronze at a sliding speed of 0.5 m/s, contact pressure up to 10 MPa and temperature up to 2OO0C (ref. 242). When the second element, made of phosphorus bronze, was also coated with the same varnish, the friction coefficient was found to be a parabolic function of the contact pressure, with its minimum (0.05) at about 7.5 MPa and with values of 0.2 at 1.5 MPa and 0.07 at 10 MPa. These coatings are cheap and can be applied in the mass production of miniature journal bearings and also in the manufacture of precision pairs for diesel injection pumps. Phthalocyanine coatings have very good thermal resistance they can operate at 70OoC - and have a very low friction coefficient (0.1). However , they adhere strongly to the metal surface (by the formation of chelates) but their wear resistance is low, and because of this they are rarely used. A special polymeric coating 50 nm thick, deposited using ply(chloro-p-xylylene) on a steel surface rubbing against LDPE, decreased the friction coefficient and wear of the LDPE as compared to an uncoated steel-LDPE system (ref. 240). The tribological behaviour of PTFE-PTFE systems is greatly improved when PTFE is used
278 as a coating on various alloys, in which the porous surface layers are impregnated with PTFE (refs. 241, 490). Epoxy composite materials demonstrate some interesting tribological properties when lubrication is applied. The friction coefficient in a lubricated epoxy-steel system can be as low as 0.01-0.03 (without lubrication, it would be at best 0.25-0.30)(ref. 492). This low friction coefficient is achieved by impregnating a very thin epoxy coating (2-10 ,um) with about 1% (by volume) of fluorinated synthetic oil, non soluble in the epoxy material used, which is dispersed in microdrops in the coating (ref. 374). The lifetime of such a coating, when tested in a ball-on-disk system (using asteel ball @ 6 mm, sliding speed 0.1 m/s and applied load 5 N), was found to be 105-107 cycles, giving a friction coefficient of about 0.06. The effect of air humidity on the tribological behaviour of the system was very small. Internally lubricated polymeric coatings of this sort seem to offer an effective solution to some of the most difficult problems concerning the lubrication of miniature systems. One particularly difficult problem is how best to lubricate a system which will be operating at very high temperatures. The wear resistance of the anti-friction coatings is another important problem. The search is still going on for new anti-friction coatings which are wear-resistant and can operate at high temperatures. One very promising and relatively new inorganic material, which will probably be soon competing with MoS2 as the most efficient anti-friction coating, is (CF,),; it can operate in dry air at temperatures of up to 5OO0C while demonstrating a friction coefficient below 0.1 (during sliding against steel - ref. 121). Solid lubricants such as synthetic niobium disulphide (Nbl.15sSz), cerium trifluoride (CeF3), and lanthanum trifluoride (LaF3) can be considered for coatings on metals or ceramics. Their maximum operational temperature is higher than that of MoS2, being 7OO0C for Nbl,15SS2 (ref. 123) and 1000°C for CeF3 and LaF3 (refs. 121,124) in an air atmosphere. Other fluorides, such as LiF, CaF2 and BaF2, also demonstrate good tribological properties, especially the temperature range 200-8OO0C. A fused 30 ,um thick fluoride coating consisting of 62% BaF2 and 38% CaF2 gives a friction coefficient below 0.2 in the temperature range 100-8OO0C and very little wear when the temperature is below 50OoC (ref. 121)). Soft oxides such as lead monoxide (PbO) have relatively low friction coefficients, particularly at high temperatures. A 30 ,um thick Pb0-4PbO-Si02 coating sliding against stainless steel was found to lubricate ef-
279 fectively in the temperature range from 500-6OO0C, giving a friction coefficient below 0.2 (at low sliding speeds; but higher than 1 m/s) (ref. 121). The temperature range for effective lubrication can be extended by increasing the sliding speed, up to as high as 5 0 m/s, which can also reduce the friction coefficient to as low as 0.05. Oxide and fluoride coatings can be deposited by the known procedures for applying glass or porcelain enamel glazes. Fluoride coatings should be fused in an inert or reducing atmosphere to avoid the coatings being contaminated with oxides of the substrate metal. Fluoride metal composite coatings (used to improve on the wear resistance of fluoride coatings) can be deposited by plasma arc spraying on a wrought metal substrate. Such coatings are surface ground to the desired thickness (0.1-0.2 mm) and a smooth surface finish. A 0.25 mm thick coating of 30% Nichrome, 30% silver, 25% calcium fluoride, and 15% special sodium free glass (composition 5P,% Si02, 21% BaO, 8 % CaO, and 13% K20) has a friction coefficient of the order of 0.2 and high wear resistance at all temperatures from room temperature to 87OoC (ref. 121). When the coating was tested in a moderate vacuum (7 Pa), cold nitrogen gas (-107OC) and in air (room temperature), in an oscillating journal bearing with a coated bearing bush, the friction coefficients were 0.15, 0.22 and 0.20 respectively. The wear was still very low when the temperature during sliding in air was raised to 87OoC. The temperature limit imposed by oxidation of the alloy is about 900°C for composite with a nickel super alloy matrix and about 6OO0C for cobalt matrix coatings (ref. 121). Completely non-metallic fluoride-oxide composite coatings obtained by plasma spraying have the advantage over metal matrix fluoride coatings of being free of the limitations imposed by oxidation. However, the friction coefficient at room temperature for a coatings of 80% Zr02 and 20% CaF2 is 0.4 , though this drops to about 0.2 at 65OoC (the wear is several times less at 65OoC than at room temperature) (ref. 121). A special kind of coating used for the dry lubrication of rolling bearings in spececraft mechanisms is known as a transferred coating (refs. 128, 455, 493-495, 875, 876). This coating (maximum thickness 1.5 ,um) is formed in situ and is replenished by a rubbing transfer from a reserve of material during use. Three groups of transferred coatings can be distinguished (ref. 455): those formed by transfer but then used without further transfer; those formed and replenished during operation; and those transferred
280 over any of the above discussed coatings. Transferred films can be formed for example by rubbing on a ball bearing retainer made of a composite containing a dry lubricant. Typical transferred films are formed of MoS2 or PTFE. Polymeric composites such as PTFEiqlass fibre+MoS2 or PI+MoS2, which can be used at temperatures above 100°C, are often used. The selective transfer effect (see Chapters 5.1.1 and6.6) can also be applied to form thin films with good tribological properties. In polymeric composite-metal sliding systems these films are usually formed of copper; when the polymeric material is filled with FeS or Cu2S the following interactions between metal and FeS or Cu2S occur during rubbing (ref. 47)
-
Fe (in metal)
+
FeS (in polymer)
Fe
+
cu2s
FeS + Fe (in friction area)
-
or
FeS
i2
Cu
The polymeric composite containing Cu2S is more suitable for forming a lubricating film by selective transfer. The friction coefficient of dry lubricant anti-friction coatings can be predicted when the molecular (adhesion) component of the friction coefficient is known (ref. 469). The friction coefficient f is expressed as (ref. 497)
where fa and fd are the adhesion and deformation components of the friction coefficient, Tn is the shear stress due to adhesion in the interface, pr the normal contact pressure, O. the coefficient of hysteresis losses, k the coefficient dependent on the type of contact deformation, h the depth of penetration, and R the asperity radius. To determine the adhesion component fa of the friction coefficient, 2, and pr need to be known; they can be estimated by use of a simple model consisting of a ball stressed between two parallel, coated plates and rotating very slowly arround the axis perpendicular to the plates (ref. 496). The dimensions of the plates should satisfy Brinell's test of hardness. The normal load N at which plastic deformations occur in the contact area can be selected by use of 2G N the condition 0.126 HB Rb 1.26 HB (where HB is the Brine11 hardness of the plates, and Rb is the radius of the ball). As the ball rotates, the force F, determined by the value of the shear
==
RZ
281 stress Zn and normal contact pressure pr (because of the applied normal load N), can be found. When the value of F has been determined, it can be inserted into the formula = 3FRh/4.T-r3 (where Rh and r are the radii of the ball's holder and the intentation of the ball on the plate respectively). The normal contact pressure is defined by the formula pr = N/x-r2 The value of the indentation radius r is measured after loading. The adhesion component of the friction coefficient for the coating-steel systems investigated in ref. 496 was found to generally decrease as a function of temperature, decreasing rapidly in the range from room temperature to about 2OO0C, and increase at high temperatures when the bonding substance weakened or the coating lost its mechanical resistance These anti-friction coatings are usually applied under extreme operating conditions. Spacecraft mechanisms operating in a vacuum are often lubricated with dry lubricants such as MoS2, PTFE or soft metal coatings (refs. 128, 453, 455, 493-495, 498). Quality control is vital if the films are to provide reliable service in the mechanism. For sputtered MoS2 films, a wear rate of 0.1 nm per cycle was found to be a reasonable minimum wear criterion (when tested in an argon atmosphere on properly treated LFW-1 rings following the procedure described in ref. 499). The use of MoS2 films in air is not recommended: in a C02 atmosphere their frictionccefficient is two times lower than in air and their wear rate islower too (ref. 500). The additional lubrication of MoS2 and inorganic coatings in general dramatically reduces their endurance life (ref. 501). The lubrication of organic coatings, however, is very effective in reducing the friction coefficient and especially the wear rate (ref. 457). The lubricant should be carefully selected after taking into consideration the general rules of polymer lubrication (see Chapters 5.2 and 6.6). The aforementioned use of an MoS2 sputtered coating on steel for the lubrication of a steel-ruby jewel bearing gives a friction coefficient of about 0.2 but only in dry air. Sputtered, 0.5 ,um thick MoS2 films on a steel journal (previously coated with a TIC or TiN 2-6 ,um thick CVD hard coating) rubbing in a vacuum against a steel bearing bush coated with TIC in an oscillatory bearing, have demonstrated very good tribological behaviour (ref. 456: see also Fig. 6.19, Chapter 6.5). The MoS2 film sputtered on TiN coating was more effective, but only to lo5 oscillations. Lubricating oscillatory journal bearings operating in a high vacuum in this
zn
.
.
282
way made it possible to apply radial and axial loads of 10 N to the bearings under investigation (pl 3 mm, diametral clearance below 16 ,urn). The sliding bearing tested gave 10 times greater torque but the torque was more constant with time over the complete oscillation angle than for the ball bearing (shaft diameter 4 mm). A burnished, 3-15 ,um thick MoS2 coating on the teeth of a steel pinion mated with a precision brass gear gave better friction and wear behaviour than similar gearing with polished teeth on the steel pinion (ref. 5 0 2 ) . Anti-friction coatings are often used in miniature ball bearings installed in spacecraft mechanisms because of their exceptional ability to reduce torque and extend the lifetime of the bearings (ref. 877). Their use is generally limited to areas where temperature extremes or outgassing requirements preclude the use of oils or greases. Dry lubricants are used in many spacecraft actuators and positioning devices where total cycle life is relatively low. The most commonly used coatings are: MoS2, either bonded, burnished, sputtered or in a composite; soft metals such as Pb or Au, vapour deposited; and PTFE, in a composite or burnished. The differences in the lifetime of an R4 miniature ball bearing on raceways coated with various films are presented in Fig. When the three solid lubricants currently most used for ball bearings in a vacuum (i.e. sputtered MoS2, ion-plated lead films and PTFE composite as cage material) were tested for bearing torque and wear, sputtered MoS2 films gave the lowest and smoothesttorque, but they have a limited life (ref. 128). The failure of such bearings is due to increasing friction and steel wear after the film has worn out. Ion-plated lead films on raceways in conjunction with a cast leaded bronze cage give steady enough average torque but the cage debris contributes substantially to torque noise. The average torque is noticeably steadier than for a bearing which makes use of a PTFE cage. Hard coatings such as Tic or TiN coupled with a soft,sputtered MoS2 coating can provide long life dry lubrication using only the initial coating (without relying on a transfer mechanism from a reserve of lubricant). Such a combination can withstand higher stress levels and makes it possible to reduce the size and weight of the mechanism. These coatings have been tested in oscillating motion in a miniature ball bearing (bore 4 mm, external diameter 13 mm, bearing thickness 5 mm) as well as in the aforementioned
283
j o u r n a l b e a r i n g ( r e f . 4 5 6 ) . The i n s i d e r i n g o f t h e b e a r i n g w a s coated w i t h T i c (2-6 ,um)+MoS2 (0.5 ,urn), t h e o u t s i d e r i n g had a T i c c o a t i n g , and t h e steel b a l l s and s t e e l c a g e were c o a t e d w i t h MoS2. The MoS2 c o a t i n g on t h e T I C r e d u c e s t h e b e a r i n g t o r q u e , a s compare d t o t h e same b e a r i n g w i t h no MoS2 c o a t i n g , when t h e b e a r i n g o p e r a t e s i n a vacuum. The b e a r i n g t o r q u e i n d r y a i r ( 1 % r e l a t i v e h u m i d i t y ) w a s h i g h e r t h a n i n a vacuum: 1 . 5 t i m e s h i g h e r f o r t h e b e a r i n g w i t h o u t MoS2 f i l m and o v e r 2 t i m e s h i g h e r f o r t h e b e a r i n g w i t h MoS2 f i l m . The p r e s e n c e o f MoS2 had no e f f e c t on b e a r i n g t o r q u e i n a vacuum. The t o r q u e s i n a i r w e r e t w i c e a s h i g h when s t a n d a r d o i l l u b r i c a t i o n was a p p l i e d .
1000h 1650 1240 2200 800
600
4 00
20c
0
F i g . 7.2. Endurance l i f e o f R 4 b a l l b e a r i n g i n a vacuum, 1 . 3 3 10-6 Pa, as a f u n c t i o n of t h e k i n d o f c o a t i n g d e p o s i t e d on raceways. T h r u s t l o a d 35 N, r o t a t i o n a l Speed 300 r/min. 1 - gold p l a t e , 2 - s i l v e r p l a t e , 3 - s i l v e r p l a t e + MoS2 f i l m , 4 - s i l v e r p l a t e + s u l phur f i l m , 5 - l e a d f i l m , 6 - b i s m u t h l e a d f i l m , 7 - bismuth f i l m , 8 - f u s e d PTFE ( r e f .
455). D r y l u b r i c a n t c o a t i n g s a r e v e r y u s e f u l n o t o n l y t o r e d u c e fric-
t i o n and wear b u t also when c o r r o s i o n r e s i s t a n c e i s i m p o r t a n t ; f o r
284
example, organic coatings such as those based on fluoropolymers are used on small components such as fasteners. Very thin PTFE/FEP bonded coatings (5-25 ,um thick) are applied in bulk by dip spin or thimble spray techniques and are designed to give the fasteners different coefficients of friction to meet the individual torque/tension requirements of tightening, and to reduce the torque values required to achieve a given clamp load. For instance, the friction coefficient of Xylan grades (Whitford Plastics) can be varied between 0.105+6.5% and 0.22210% (ref. 504). A coating such as Xylan 1010 deposited on a moving element demonstrates a low friction coefficient (ca. 0.12) and high corrosion resistance during fretting (in the experiments, an uncoated element made of tool steel or titanium was oscillating in relation to a Xylan-coated element made of tool steel) (ref. 505). The elimination of stick-slip by using a coating like this is also of great importance. Dry lubricant anti-friction coatings provide effective s o l u tions to many difficult problems of lubrication of miniature systems, especialy when they operate under extreme conditions (vacuum, low and elevated temperatures, radioactivity, corrosive media). For the most used coatings, MoSZ, soft metals or PTFE, the friction coefficient can be reduced to below 0.1 at sliding against metal surfaces. MoS2 coatings are most effective in systems operating in a vacuum. One general disadvantage of the anti-friction coatings is their relatively low wear resistance. If one tries to increase the wear resistance, for example by including hard particles, then a higher friction coefficient can be expected (above 0 . 2 ) . Some special inorganic coatings can be applied in air up to ~ 0 0 - 9 0 0 ~ Corrosion-resistant, ~ . organic, polymeric coatings can operate at temperatures up to 3 5 O o C . Thin films of varnish manufactured from a high-molecular surface active agent can bevery effective; they have been found to have a very low friction coefficient (0.05) and low wear during coating-coating sliding at temperatures up to 2 0 0 ~ ~ . The above analysis of anti-friction coatings can be used as a guide to select dry lubricants for new applications, but tests should be performed on the actual system, especially to check the lifetime of the coatings. The running-in and storage conditions can also have an important effect on the ultimate tribological behaviour of a system, particularly in the case of MoS2 coatings. However, if the MoS2 coatings are intended for operation in space, ground testing or storage in humid air can severely limit coating
life after launch. Dry lubricant coatings with better ground-testinq tolerance are needed.
7 , 3 , ANTI-WEAR COATINGS Hard anti-wear coatings are used when rubbing elements are highly loaded, abrasive material is present, impact or corrosion wear is likely to occur, or elevated temperatures can be expected. The surface of the rubbing element is then protected by a special metallic or inorganic non-metallic hard coating to reduce wear. The surface layer of the material may also be modified by ion implantation or laser treatment. The metal coatings of pure metals or their alloys are deposited by various methods. Coatinq deposited by padding are thick (minimum 0.5 mm) and made by depositing metals such as Fe , Co, Ni or Cu or their alloys usually on steel. This technique is used for the recovery of worn-out elements, as is the technique which consists of spraying on coatings at least 50 ,um thick composed of wear-resistant steels such as bearing steel or hard alloys such as 1OMnSi6 (steel) or NiCrBSi. Flame-sprayed and fused NiCrBSi coatings with or without additional hard particles of carbides and oxides are resistant to both adhesive and abrasive wear. The size of any hard particles introduced should be greater than the size of abrasive particles which enter the rubbing region (refs. 506, 507). Electrodeposited coatings of Cr, Ni, Fe demonstrate relatively good wear behaviour (especially Cr coatings) but the cost of their production (in particular hard Cr coatings) is relatively high and the coatings do not adhere very strongly to the substrate, especially when small elements of complex geometrical shapes are coated. A cheaper method is to deposit Ni coatings on metallic or non-metallic elements using a chemical method. The chemical method for depositing Ni coatings 20-40 ,um thick is especially suitable for small elements. Chemically-nickeled polymeric elements demonstrate good wear resistance and are used in the typing elements of typewriters. The most suitable polymer for coating by the Ni chemical method is ABS (ref. 508). Ni coatings can be hardened so as to produce a 100% increase in their hardness (refs. 509, 510). Ni composite coatings have higher resistance when they are deposited from electrolyte containing dispersed small particles (3 to 6 ,um) of diamond, Sic, Tic or A1203, Co+Cr3C2 (refs. 510-512). Self-lubricated coatings with
286 high wear resistance, such as Ni+graphite, Ni+MoS2 or BC, or Ni+PTFE, can also be formed in this way. A similar technique is used for depositing wear-resistant Cu+A1203 (or Fe+A1203) and self-lubricating wear-resistant Cu+BaSo4, Cu+MoS2 coatings (refs. 512, 513). Composite electroless plated Ni+P+diamond coating also exhibits excellent wear resistance (ref. 514). The relatively low hardness of the above mentioned electrolytic composite coatings is due to the low amount of small hard particles (less than 1% by weight) which can be actually deposited. The disadvantage of chemically deposited nickel coatings is the slowness of the process of producing them and their susceptibility to seizure during unlubricated sliding. The Ni+P chemically obtained coatings demonstrate high abrasive wear resistance after thermal treatment at 45OoC and when the phosphorus content is around 7% (ref. 515). Nickel coatings are more useful than chromium coatings for boundary lubrication when adhesive wear is expected. When the contact pressure is very high, the friction coefficients of the boundary-lubricated chromium coating-high-alloy steel or chromium coating-Al203 systems are high: 0.6 and 0.4-0.7 respectively? while when nickel hardened coatings or nickel non-hardened coatings were used the friction coefficients were 0.09, 0.10 and 0.23-0.30, 0.10-0.12 respectively (ref. 516). The adhesive wear resistance (at boundary lubrication) of the aforementioned chemically-deposited hardened and unhardened coatings is better than the adhesive wear resistance of Ni electrodeposited coatings. chemically deposited Ni coatings are also useful in resisting impact wear, being used for instance on the printing elements of computer printers. Chemically-nickeled elements are not very resistant to abrasive wear, unlike hard chromium coatings. Titanium and chromium coatings (0.1-0.5 mm thick) deposited on steels by thermochemical (diffusion) processes exhibit good corrosion and wear resistance at sliding and rolling friction. Thin coatings of Ti, Cr, W and Mo, less than 5 ,urn thick, are useful at low contact pressures and can also be deposited on metals and non-metals by CVD or PVD processes. Cr and Mo coatings applied by PVD demonstrate excellent abrasive wear and corrosion resistance and no hydrogen embrittlement (ref. 517). A flame-sprayed Mo coating (ca. 0.9 mm thick) is more resistant to adhesive wear than plasma-sprayed WC/Co coating (when sliding against bearing steel). Its abrasive wear resistance is lower than ordinary mild 0.37% C steel (ref. 518). Metallic coatings such as
287 stainless steel, Ni-Cr or Ti-Ni deposited by plasma or detonation spraying (the particles of material to be deposited are accelerated by the force of a detonation) demonstrate very good tribological behaviour. Coatings (0.18-0.20 mm thick after grinding, deposited by detonation)of powdered stainless steel or Nichrome (20% Cr-80% Nil, further modified by diffusion with boron and aluminium, were deposited on various steels, giving the following results (ref. 519). The type of steel used as a substrate had no effect on the tribological properties of the coating. The adhesive wear of the coatings formed on stainless steel and Nichrome was respectively 4 and 9 times less than on 0.45% C steel. The wear of Nichrome coatings was slightly lower than the wear of WC + 15% Co hard coatings. In a vacuum (1.33 Pa) , the wear of the stainless steel and Nichrome coating was roughly 2 and 3 times less than the wear of 0.45% C steel. The wear of Nichrome and WC+15%Co coatings was about the same. At temperatures up to 800°C (at sliding in a vacuum) the wear of the Nichrome coating was less than that of A1203 and A1203-Ti02 coatings , and only slightly more than the wear of a WC + 15% Co coating (ref. 520). Detonation-sprayed Nichrome coatings demonstrated wear behaviour better than or similar to that of WC + 15% Co detonation-sprayed coatings during unlubricated sliding in an air atmosphere or in a vacuum (also at elevated temperatures). The friction coefficients at contact pressure 1 MPa and sliding speed 0.4 m/s for stainless steel, Nichrome and WC + 15% Co coatings were 0.33, 0.28 and 0.24 at sliding in air and 0.53, 0.43 and 0.38 in a vacuum. As a result of increasing the temperature to 8OO0C (at sliding in a vacuum) the friction coefficient of stainless steel coatings increased to 0.65 but the friction coefficients of Nichrome and WC + 15% Co coatings only slightly increased. Self-lubricating wear-resistant coatings can be obtained by sintering metal powders and impregnating this porous coating with solid lubricant. Industrial experience has shown that the electrocontact sintering of the surface layer of sintered porous bearing bushes can extend the lifetime of bearings by 1.3-2.6 times (ref. 521). Coatings of inorganic non-metallic substances are deposited by material overlaying, surface conversion (e.g. phosphatizing, anodizing), or material diffusion into a substrate. The sprayed oxide coatings (Al2O3, Zr02, ZrSiO4, Si02, Cr203) can operate in an oxidative and radioactive atmosphere, demonstrating high wear
288 resistance. These coatings are relatively thick (minimum 0.2 mm) and brittle. To give better adherence, the substrate sublayer must be specially prepared. Sprayed WC coatings have better thermal and wear resistance than sprayed oxides. The detonation-sprayed A1203 (or A1203-Ti02) coatings in A1203-WC + 1O%Co (coating), A1203-A1203 systems at unlubricated sliding have shown relatively good wear behaviour, with a friction coefficient of 0.4-0.5 (refs. 520, 522). Detonation-sprayed A1203 and WC + 1 0 % Co coatings are used on the working surfaces of gas bearings in gyroscopes for example. Such coatings also demonstrate wear resistance when used on the cutting elements of tape and card punches. WC-Co plasma-sprayed coatings demonstrate good abrasive wear resistance when rubbing against flint or Sic abrasives (ref. 518). The disadvantage of detonation-sprayed WC+Co coatings is their relatively high friction coefficient during unlubricated sliding (up to 0.7). The friction coefficient can be reduced by adding graphite (C)+Ni to the base material: a content of 5% (by weight) of C+Ni can reduce the friction coefficient to 0.45 (during fretting of the coating against itself at contact pressure 19.6 MPa, amplitude 125 ,um, and frequency 25 Hz) with only a slight decrease in wear resistance (ref. 523). The use of other solid lubricants to modify the rough surface of a WC coating can reduce the friction coefficient but the lifetime of the coating is short. The plasma-sprayed Tic-Ni (70%-30%) on XC 35 steel and detonation-sprayed CrjCr2-NiCr (80%-20%; NiCr = 80% Ni - 20% Cr) on Inconel coatings demonstrated good tribological behaviour after grinding (Tic-Nil to R, = 0.58 pm and lapping (CrjC2-NiCr) to R, = 0.1 am, when sliding against themselves at elevated temperatures (ref. 524). At sliding in air at room temperature (contact pressure 1 MPa, sliding speed 0,083 m/s), the friction coefficient for Tic-Ni coating was low (below 0.3) and the wear was small, since adsorbed vapours were lubricating the system. A similar effect (at a higher coefficient,ca. 0.5) was observed for Cr3C2-NiCr coatings. Within the temperature'range from 100 to 4OO0C, the friction coefficient is relatively high (ca. 0.8 for Tic-Ni and 0.65 for Cr3C2-NiCr coatings) and the wear is pronounced. At temperatures from 4OO0C to the maximum operating temperature (800°C for Tic-Ni and 1000°C for Cr3C2-NiCr) the friction coefficient is constant for TIC-Ni (below 0.5) and decreases for CrjC2-NiCr (to 0.3 at 1000°C). This is the temperature range where oxidation of
289 the surfaces occurs and the lubricating effect of the oxides reduces the friction coefficient and gives practically negligible wear. At the limiting temperature, where oxidation is very strong and the friction coefficient relatively low, the oxides spa11 and the wear becomes high. During sliding in a vacuum (1.5 Pa) the above effects do not occur; the friction coefficients increase as a function of the temperature (to 0.9 for Tic-Ni at 7OO0C and above 1.0 for Cr3C2-NiCr at 900OC). Because of strong adhesion between Ni bonding elements, Cr3C2-NiCr coatings can be applied in radioactive environments. WC and WC+Co coatings (15-55 ,um thick) deposited by electrophoresis and sintering on steels demonstrate high wear and thermal resistance and have high density. They are used to coat the surfaces of measuring instruments, which are subject to wear. Cemented carbide coatings a few micrometers thick, deposited by metallic surface fusion (MSF) on elements of computer card machines, demonstrate high abrasive wear resistance when in contact with paper (ref. 525). The so-called MSF process allows a molten overlay of electrically conductive material to be applied to a metallic base. Apart from cemented carbide, chromium,gold and other substance can also be deposited by this method. Ferrous bases are usually clad, but stainless steel and chromium substrates, as well as chromium-plated tungsten and molybdenum elements, may be treated. In practice, the best results are obtained when elements are dabbed with a light oil before being treated. A good surface finish on the substrate (Ra g 0.4 ,um) is essential. Tungsten carbide can be deposited on nickel substrates by carbide electrodeposition (ref. 526); tantalum has also been successfully plated and other carbides (e.g. chromium carbide) can also be deposited. This method can be used for plating complex shapes. Enameled oxide glazes (A1203 and other metal oxides) on steels provide useful protection against the action of flowing agressive media at elevated temperatures. Since such coatings are very thick (minimum 0.1 nun) they are sensitive to impact and can split, especially at temperatures above 800-900°C. Conversion coatings are the effect to conversion of the metal (All Ti, Tat W, Mot Hf, Zr) surface to metal oxide due to an anodic process (anodizing) or the formation of insoluble phosphate crystals on Fe, A l l Ti, Cu or galvanized steel surfaces (phosphatizing). Anodized coatings are extremely hard and have good abrasive
290
wear but their impact wear resistance is low. When these porous anodized coatings (100-300 ,um thick) are deposited on aluminium or magnesium alloys and impregnated with a solid lubricant such as PTFE, they give a low friction coefficient and low adhesive wear (refs. 244, 409, 527). HAE anodizing process of magnesium alloys leading to thin and thick ( 3 ,um to 80 ,um) coatings results in high abrasive wear resistance, especially of thick coatings (ref. 529). Anodized coatings are not useful for elements when pivoting friction occurs. Zinc or manganese phosphatizing by immersing (or spraying) steel or non-precious metal alloys in the salts of phosphoric acid is cheap, manganese phosphatized coatings in particular demonstrating quite good wear behaviour. Phosphatizing is usually applied with thermal diffusion hardening, or sometimes with nitriding. The soft phosphatized coatings (5-30,m thick) have good adherence to the substrate and a short running-in period. The disadvantage of using them is their short lifetime and low thermal and corrosion resistance because of their porous structure. Their quality can be improved by repeated phosphatizing to form new thin surface layers (1-2 ,um thick). Diffusion processes (carburizing, nitriding, boriding,siliciding , sulphidizing, chromizing, vanadizing) give hard, wear-resistant coatings. After dispersion hardening (in the case of mild steels) or carburizing, the surface layer (minimum 0.2 mm thick) of steel takes on a martensitic structure with a hardness of 700-900 HV. Hardened or carburized elements are useful at dynamic loads and they demonstrate good fatigue wear resistance. Nitrided layers 5-330 ,um thick exhibit even higher wear and thermal resistance. The adhesive wear of both unlubricated and lubricated nitrided surfaces is low when the sliding distance is not too long (ref. 530). However, the best tribological properties are shown by the coatings obtained by carbonitriding and sulphonitriding. The carbnitrided layers (5-300 ,urn thick), especially after hardening and annealing, are very wear- and thermal-resistant. The surface is rich in iron nitrides E-Fe,N and also ;yt-Fe4N.Such coatings are very seizure-resistant at boundary lubrication when sliding against themselves (ref. 516). They also demonstrate good adhesive wear resistance during lubricated sliding against A1203. They exhibit very good resistance at dynamic loading too. Sulphonitriding produces a surface layer 5-300,um thick, which has a shorter running-in period than nitrided layers, together with high wear and seizure resistance. The disadvantage of such layers is often their
291 brittleness. When the sulphonitriding is carried out in an oxidative atmosphere, the surface layer can be enriched in oxides to improve the wear resistance of the coating. Borided coatings of steels, metal alloys and sinters (10-500 ,um thick) exhibit interesting tribological behaviour. Although the coatings is relatively cheap, the wear resistance is very high and it also has good anti-friction properties. Alloyed steel can also be borided; as this results in some alloing elements dissolving in the boride layer while others are concentrated under it, it affects the wear behaviour of the layer (ref. 531). The wear of boride layers is relatively high when fatigue, abrasion and tribo-oxidation occur. Surface layers rich in FeB and Fe2B borides are wear-resistant when sliding against themselves. In tests, a journal bearing ( @ 20 nun), with a steel journal with a 0.45% C borided and thermally processed surface, and a bearing bush of the same steel chromized by diffusion and thermally processed, and lubricated with instrument grease based on MWP mineral oil thickened with Li stearate, exhibited excellent wear behaviour at oscillating motion and high load (4 MPa at the diametral clearance 150 ,um) (ref. 532). The wear was 10 and 6 times less than in similar bearings in which the journal was made of 0.45% C steel and the bearing bushes were made of bronze and a PTFE+metal composite respectively. The adhesive wear behaviour of the borided layer can be improved with chromium and molybdenum from the alloyed steel which also, along with vanadium, improve the abrasive wear resistance (ref. 531). The usually high fatigue wear of borided layers can also be decreased with molybdenum or vanadium. The borided elements in computer matrix printheads have demonstrated long lifetime (ref. 533; for details see below). The corrosion-resistant steels and alloys used in the chemical industry can be borided in salt compositions based on Na2B407--4-B and Na~B407-NaBFq-NaC1-B compounds (ref. 534). Bonded corrosion-resistant steels and titanium alloy have a 90,um thick FeBiFe2B layer exhibiting microhardness of ca. 20000 MPa. Boriding can double or triple the abrasive wear resistance of an element. The tribological behaviour of borided stainless steel during lubricated sliding (lubricated with demineralized water) can be greatly improved by adding a 2 pm thick, diffusion-chromized surface layer) (ref. 535); at the same time, its corrosion resistance in aqueous media demonstrating some aggresivity is improved. When a borided coating on chromium-molybdenum alloyed steel was tested in a
292 pin-on-disk system, the wear was found to increase tenfold as the relative humidity of air was raised from 10 to 95% (ref. 531). In a nitrogen or argon atmosphere the wear was 2-3 times higher than in dry air. The diffusion siliciding of mild steels improves their tribological behaviour significantly. The porous layer of Si solution in a-Fe after impregnation with lubricant is wear-resistant and also exhibits a low friction coefficient. The maximum realistic contact pressure is 150 MPa. The unlubricated sliding of silicided steel against mild steel gives a friction coefficient of up to 0.3 when the porosity of the surface layer is near the optimum value of 40% (ref. 536). Vanadizing gives a 20,um thick, very hard surface layer (ca. 2500 HV) which greatly improves the wear resistance of steels. The adhesive wear resistance of vanadized bearing steel in pin-on-disk testing is constant as a function of air humidity and severaltimes smaller than borided alloyed steel (ref. 530). Vanadized surfaces are superior to borided, nitrided or carburized surfaces in tribo-oxidative and abrasive wear situations. Chemical vapour deposition (CVD) coatings demonstrate excellent adherence to a substrate, they are thin (3-12 ,urn, usually 4-7 ,urn), and they are wear- and corrosion-resistant. The temperature of the CVD process is usually relatively high (750-1100°C),although medium and low temperature processes (400-700°C, and room temperature to 35OoC respectively) are also applied when depositing certain coating materials. The high temperature process is used for depositing metals with a high melting point and hard materials such as carbides, nitrides, carbonitrides, borides and oxides. Both metallic and non-metallic materials are used as substrates. Tic and TiN CVD coatings are frequently used; they have good wear resistance as well as a relatively low friction coefficient. The latter was below 0.3 in a steel+TiC-ruby jewel microbearing system, below 0 . 2 in a Tic-Tic ball-on-disk sliding system in a helium atmosphere, and around 0.3 in Tic-steel systems in air (refs. 537, 5 3 8 ) . The friction coefficient in a vacuum is veryhigh however (over 1.0). The friction coefficient in dry air was also relatively high (ca. 0.5). The friction coefficient can be qreatly reduced by applying lubrication. The application of a Tic, TiN or (CrIFe)7C3 carbide coating in concentrated boundary lubricated contacts appreciably increases the load capacity of a system (ref. 539). TIC coating on
293 cemented carbide (WC + 6 % C o ) balls used in miniature oil-grease lubricated bearings greatly extends their endurance life as ccinpared to similar bearings with steel balls (ref. 740). 2 ,um thick Tic coatings without interlayers on cemented carbide and steel demonstrate good resistance to fatigue and permanent deformation while when thick Tic coatings (5-7 pm) are applied on a steel substrate (when abrasive wear is expected) the chromium carbide interlayer is desirable (ref. 541). Other CVD coating such as Sic also demonstrate very good friction and wear behaviour. Tests have found the friction coefficients of steel-Sic (coating), Tic-Sic, and Sic-Sic systems at unlubricated sliding in a ball-on-disk system.in humid air (90-95% relative humidity) to be 0.15, 0.20 and 0.27 and in dry air 0.30, 0.25 and 0.47 respectively (ref. 542). The greatest wear was observed in the Sic-Sic system. CVD coatings of Tic (5-20 ,um) or Cr7C3 (3-11 ,um) demonstrate good seizure resistance when sliding against themselves at boundary lubrication (ref. 516). The pins in Tic-Tic or TiC-A1203 pin-on-coated-disk sliding systems showed less wear than the pins in Cr7C3-Cr7C3 or Cr7C3-Al203 systems. Tic and Cr7C3 coatings are resistant to abrasive wear by flint and corundum, but only the Tic coating is resistant to abrasive wear by Sic. Neither coating is useful when metal fatigue occurs. They demonstrate good corrosion resistance in the presence of dew and Tic is also corrosion resistant in salt water vapour. A W2C-Ni coating 7-10 pm thick with a 3-8 ,urn Ni interlayer was tested in the same conditions (ref. 5161, and although it was found to be unsuitable under boundary lubrication conditions, it did demonstrate high abrasive wear resistance to flint, corundum and Sic, it can be applied at fatigue loading and it is corrosion resistant in the presence of dew. Tic and TiN coatings can also be deposited by physical vapour deposition (PVD) processes (sputtering, ion-plating) The principal advantage of PVD techniques over the CVD process is the lower temperature of the substrate. The coatings are very thin (2-5 ,urn) and have a uniform surface. Comparisons between the tribological behaviour of TIC and TiN coatings deposited on 0.35% C hardness and high-speed steels using CVD and PVD techniques can be found in ref. 543. For the tests, the coatings rubbed against high-speed steel in the presence of abrasive A1203 80 ,um particles. In the case of TiN the wear of the PVD coating is about twice as high as that of the CVD coating. For PVD coatings the wear of TiN is about 10 times higher than the wear of Tic, but for CVD coatings the
.
294 wear of TIC is 3 times higher than the wear of TIN. The friction coefficients are similar for TiN coatings obtained by either method (ca. 0 . 3 ) but the friction coefficient for the Tic PVD coating is lower than that for the Tic CVD coating (0.24 as against 0.38). additional tests on a TiC+TiN coating obtained by the CVD process revealed wear rates and friction coefficients which were average for Tic and TiN CVD coatings. The lower wear resistance of the Tic CVD coating was brought about by the evident spalling of the coating. The plasticity of the coating seemed to play an important role in its wear. Sputtered Tic and TiN coatings demonstrate better resistance to oxide conversion at high temperatures (i.e. up to 54OoC) than bulk Tic and TiN materials (ref. 121). The rate of oxidation of the coatings is probably lower because of high coating density and the passivating nature of the initially formed oxide films. This protects the coating from catastrophic oxidation. A sputtered Cr2O3 coating deposited on the working surface of nickel-chromium foil bearings and rubbing against a journal coated with chromium carbide in start-stop tests demonstrated long lifetime (ref. 544). The coating had not worn out after 9000 start/stop operations (the temperature range was room temperature to 65OOC). Smooth, ion-plated TiN coatings (0.06-0.8 ,um thick), when rubbing against a smooth tool steel surface a pin-on-disk system, had a high friction coefficient which increased as a function of sliding distance (ref. 545). The transfer of steel onto the coating was observed (the contact pressure was 14 MPa). The wear in the steel-TiN coating system was G O times less than in a steel-steel system. The best tribological behaviour, however, was found for the TiN-TiN system. The friction coefficient was barely 0.1 and the wear was around 400 times less than in a steel-steel system. A modified ion-plating method known as the ICL process originating in the G . D . R . enables very hard, wear resistant coatings to be formed from a metastable amorphic allotropic form of carbon called i-C layers (ref. 546). The source of the carbon in the ICL process is the stream of ions of particles containing carbon. The i-C coatings can be deposited (at a substrate temperature of 25OoC) on steels, carbides, non-ferrous and light metals, and ceramics. The speed of coating formation is 6 ,um/h. 15 ,um thick coatings demonstrate wear resistance 2-4 times higher than untreated surfaces, their friction coefficient is 0.1-0.5 and they have high corrosion resistance. The thread guides used in textile machines
295
have a very long endurance life when coated with i-C. The modification of the chemical composition and microstructure of a thin layer of material by ion-implantation produces remarkable decreases in both friction and wear as well as improvements incorrosion resistance and fatigue life. The most frequently used are nitrogen ions. The wear rate of pure iron implanted with N+ ions ( 5 1 0 1 5 ions/mm2) and sliding against M-50 steel in a pin-on-disk system lubricated with n-hexadecane and mineral oil, was 4 0 % (during running-in) and then 20% (steady-state wear) lower than the wear rate of untreated iron (but the friction coefficient was unchanged)(ref. 5 4 7 ) . Implanting nitrogen ions ( 5 1 0 1 5 ions/mm2) in AISI 1 0 1 8 mild steel and 3 0 4 stainless steel at lubricated sliding did not significantly improve the wear properties of 1 0 1 8 steel but for 3 0 4 steel the wear decrease as a result of ion implantation, was as large as a factor of 5 0 when the contact pressure was low (the material yield stress was not exceeded) (ref. 5 4 8 ) . The wear rate of mild steel was halved by nitrogen ion implantation ions/mm2) for unlubricated sliding against steel (ref. 5 4 9 ) Lubricated sliding tests on untreated, thermally nitrided, N+ implanted and nitrided plus N+ - implanted 17-4 PH stainless steel specimens have shown that thermally nitrided plus implanted steel has the best wear resistance (ref. 5 5 0 ) . The wear was reduced by up to two orders of magnitude as compared with the wear of the untreated specimen. Different ions (titanium, argon, nitrogen, iron) implanted in ferritic AISI E52100 steel have been shown to have different effects on the tribological behaviour of the steel (ref. 5 5 1 ) . The investigations were carried out in a ball-on-cylinder tester (implanted cylinder surface, unimplanted martensitic 5 2 1 0 0 steel ball) in air, in a fully formulated lubricant and in highly purified hexadecane. Ti implantation decreased the friction coefficient from 0 . 5 5 to 0 . 3 2 during unlubricated sliding and to 0.22 in hexadecane, but iron and nitrogen implantation increased the friction coefficient in hexadecane to 0.7. In the fully formulated lubricant only the break-in pattern was modified. The wear was decreased by all implants. The formation of Tic in the surface region was observed in material implanted with titanium ions. The implants strengthen the materials sufficiently to reduce wear and this strengthening is probably the result of the enhanced carbon content that accompanies implantation and the probable formation of fine carbide precipitates.
.
296 Copper alloys such as lead bronzes implanted with phosphorus ions ions/mm2 as optimum) exhibit higher abrasive wear resistance than unimplanted material (ref. 549). The titanium alloy + TA6V implanted with nitrogen (N2) ions (140 keV) with doses (2.8-5.6) 1015 ions/mm2 demonstrated a friction coefficient of 0.2-0.4 (as compared to 0.7-0.0 for untreated alloy) and low wear during unlubricated sliding against steel in a pin-on-disk configuration at a very low sliding speed (ref. 553). Implanting WC+Co ions/mm2) doubles its wear resistance too with nitrogen ions (ref. 549). Laser treatment can be used on ferrous and non-ferrous alloys and brinqs about important changes in the surface composition. Laser beam treatment causes elements with different melting points to melt simultaneously, producing homogenous surface layers (ref. 554). The structure of the laser-treated surface is fortified with small, dispersed carbide inclusions, which considerably increases the abrasive wear resistance. As compared to untreated steels laser-treated mild steels demonstrate significantly higher wear resistance during both lubricated and unlubricated sliding. Commonly used 0.45% C steel treated with a C02 laser beam (2.5 kW) exhibits high adhesive wear resistance at boundary lubrication when sliding against normal 0.45% C steel (ref. 555). The differences in the wear intensity of this steel before any treatment, after laser treatment, and after being hardened by RE’ current are shown in Fig. 7 . 3 . The improvement in wear resistance brought about by laser treatment is particularly significant at very high contact pressures. The friction coefficient for material treated by laser is 0.045 as compared with 0.06 for hardened steel. This effect is caused by a significant increase in the microhardness of the surface layer after laser beam treatment (it is up to 3 times harder). The velocity of the laser beam displacement during treatment affects the final wear resistance. The wear resistance of laser-treated mild steel was reduced by 150% by doubling the velocity of the laser beam displacement (in the range of 5-30mn/s) (ref. 557). Shallow surface melting laser treatment (down to 2 0 ,urn) isbetter when the treated surface will be operating under boundary lubrication with no abrasive material particles in the lubricant, while deep-melting treatment (down to 1000-1500 ,urn) is better when high wear resistance in the presence of abrasive material is required (ref. 557). The laser treatment of chromium steel
297 (0.4% C -1% Cr) reduces the wear twofold (at boundary lubrication) as compared to the wear of nitrided and carburized steel (ref. 558). The laser treatment of non-ferrous alloys is a very effective way to improve their tribological behaviour. The abrasive wear resistance of aluminium bronzes was found to be twice as high after laser treatment (560 W, optimum surface power density 800-1000 w/mm2) (ref. 559).
Contact pressure, MPu
Fig. 7.3. Linear wear intensity v s . contact pressure for 0.45% C steel at boundary lubrication (sliding speed 0.33 m/s). 1 unhardened by RF current, 3 - treattreated, 2 ed by laser beam (ref. 555).
-
-
The laser beam can also be used to improve the process ofcoating or to deposit special anti-wear coatings. Both the brittleness (when there is a high content of FeB in the surface layer) and the boriding time can be reduced by the use of laser beam heating. The 0.35% C and 0.40 C-1% Cr steels borided in the presence of a C02 laser beam (800 W, velocity of laser beam displacement 2 0 m/s)
29 8 were found to have an abrasive wear resistance 1.5-2 times higher than carburized steel. A high power C02 laser beam (15 kW, surface density of the power 1 . 5 l o 4 W / m m 2 ) has been used for prcducing wear-resistant coatings of the mixture of Cr302+Cr502 (18.8% by weight), Mo (70%), Ni (5%), Si ( 5 % ) , and Cr ( 1 . 2 % ) (ref. 5611, giving excellent bonding of the coating to substrates of, for example, cast iron and superalloys, uniform distribution of the elements throughout the layer and no undesired porosity. The hardness obtained was about 1 0 0 0 HV. The wear resistance obtained was excellent (under boundary lubrication with mineral oil and contact pressure up to 250 MPa). When adhesive and abrasive wear occur simultaneously, the linear wear intensity (Iw) of a coating can be predicted using the following formula (ref. 5 6 4 ) : 'Ua
Iw
P
= -(
n
1
(7.2)
wed where ,ua is the adhesion number, p the contact pressure, Wed the abrasive plough energy density, and n = 2 to 3 . The adhesion number which characterizes the adhesion between the rubbing surfaces, can be determined experimentally as follows: (7.3) where Ftad and Fnad are the tangential and normal forces at the passage from static to kinetic friction. The abrasion plough energy density can also be estimated experimentally, since Wed
-- a' Fa La = va
(7.4)
a'
where Wa is the abrasion energy, Va the volume of material worn by abrasion, Fa the shear force on the sliding distance, and L a the abrasion sliding distance. When the IUad and Wed values are known, they can be introduced into eqn. (7.2) and the linear wear intensity simply estimated. Experimental studies have shown that the predicted and the experimentally determined values of the linear wear intensity for various coatings are in close agreement (ref. 5 6 4 ) .
299 The quantitative models for the relative abrasive and chemical dissolution wear rates of potential tool coating materials are presented in ref. 878. Hard anti-wear coatings offer a likely solution to the problem of providing high wear resistance for highly loaded miniature elements operating under extreme conditions in the presence of abrasives (e.g. paper) or under impact wear conditions (refs. 562 , 563). Chemical nickeling is cheap and is used, for example, to coat the polymeric typing elements of typewriters (and also the printing elements of some computer printers). The formation of cemented carbide hardfacings on the elements of card machines extends their endurance life significantly (FicJ. 7.4 based on data from ref. 533) shows the effects of various coatings on wear in the guiding matrix and needle system of a computer mosaic printer. The excellent wear resistance of the bonded coating can be observed. A CVD Tic coating used in a micrometer to protect the contacting element from wear, reduced the measurement error after 120000 measurements from about 50 ,um (when hardened steel was used) to below 1,um (ref. 538). The freewheel blocking body action needs wear-resistant working elements with a high, but not too high, friction coefficient. A CVD CrxCy coating on the bearing steel working element increased the friction coefficient (from 0.7 to 9.7-0.9), wear resistance (increasing the lifetime from around 200 to 4000 hours) and corrosion resistance as compared to uncoated bearing steel (refs. 537, 538). A journal microbearing with a ruby jewel bearing and a steel journal CVD-coated with Tic, demonstrated a friction coefficient of 0.2-0.3 together with high wear and corrosion resistance during unlubricated sliding (ref. 537). The same bearing with an uncoated journal, lubricated with ESSO 20 W 50 oil, had a friction coefficient of 9.16. Journal bearings ( 0 3 mm) with a TiN or Tic CVD-coated steel bearing bush and a TiN+MoS2 (sputtered) or TiC+MoS2 coated journal demonstrate very good tribological behaviour and load capacity during oscillating motion in a high vacuum (ref. 456). The endurance life of such bearings can reach l o 5 o s cillations. Miniature ball bearings with TiN or Tic CVD-coated raceways, having steel balls and cages (coated with sputtered MoS2 film), demonstrate better tribological behaviour in a vacuum than in air (during oscillating motion: ref. 456). Very good tribological be-
300 haviour was also found for similar ball bearings having balls made of cemented carbide CVD-coated with Tic uncoated raceways (or a raceway coated with MoS2 sputtered film) and a bronze cage coated with MoS2.
I
100
200
300
I
400
1
500
600
700
t
Printing t i m e h
F i g . 7.4. G u i d i n g h o l e d i a m e t e r v s . p r i n t i n g t i m e i n mosaic computer p r i n t e r f o r v a r i o u s coatings. Substrate m a t r i x m a t e r i a l tool s t e e l C 15; n e e d l e m a t e r i a l - hardened and annealed high-speed s t e e l X82WMo6.5 h a v i n g hardness 850 t 100 H V l . Needle s t r o k e 0.5 mm, frequency 100 Hz, t h i c k n e s s o f g u i d i n g m a t r i x 0 . 3 mm. 1 - t i t a n i d i n g , 2 - d i s p e r s i o n hardening, 3 - c a r b u r i z i n g a t low temperatures, 4 - chemical n i c k e l i n g and gas n i t r i d i n g , 5 - boriding.
-
Miniature gyroscope ball bearings (inner diameter 4 . 7 nun) with steel balls and lubricated with oil-grease (cage impregnated with MIL-L-00831768 o i l (phenol 258 BB), and 4-5 mg Andok C grease) had an endurance life (at the end of which the lubricant became opaque
301 and lost its lubricating qualities, and there were wear tracks on the races) of only 3000 h at 24000 rpm (ref. 540). The same bearings with WC + 6 % Co balls coated with TIC (CVD) were still running smoothly and the lubricant was still clear after 20000 hours at 24000 rpm. Hard anti-wear coatings are a very useful means of reducing wear. At adhesive wear under boundary lubrication conditions and when the loads are not very high, the cheap, chemically deposited and hardened nickel coatings can be satisfactorily applied and will give good anti-seizure and anti-corrosion properties. Nitrided coatings are also useful., demonstrating high wear. resistance against steel and A1203, and also abrasive wear resistance against flint; they are especially useful at dynamic loads (fatigue wear). Borided coatings are very resistant to adhesive wear, especially when sliding against themselves (they have low wear resistance against A1203) and demonstrate high abrasive wear resistance against flint and alumina. They can be used to reduce fatiguewear, and are also corrosion-resistant. TIC, Cr7C3 and W2C-Ni (with a nickel interlayer) CVD-deposited coatings have very good abrasive wear resistance against flint, alumina and Sic (except for Cr7C3) but little fatigue wear resistance (except for W2C-Ni). Under boundary lubrication conditions, Tic coatings demonstrate very good tribological behaviour when sliding against themselves or against Al2O3, and average behaviour when sliding against steel.
302
8 , EXPERIMENTAL TECHNIQUES 8,1, I NTRODUCT I ON The i n v e s t i g a t i o n o f t h e t r i b o l o q i c a l p r o p e r t i e s of m i n i a t u r e s y s t e m s p o s e s s p e c i a l problems. The s m a l l d i m e n s i o n s o f r u b b i n g e l e m e n t s and t h e s m a l l v a l u e s o f v a r i o u s measurements r e q u i r e t h e a p p l i c a t i o n o f v e r y s e n s i t i v e and a c c u r a t e i n v e s t i g a t i o n methods. The i d e n t i f i c a t i o n of t h e f r i c t i o n , wear and t h e r m a l p r o p e r t i e s o f rubbing microelements w i l l be d i s c u s s e d i n d e t a i l , t a k i n g i n t o c o n s i d e r a t i o n t h e most i n t e r e s t i n g methods and d e v i c e s i n v e n t e d i n recent years. The d e t e r m i n a t i o n o f t h e p r o p e r t i e s o f l u b r i c a n t s u s e d i n t h e l u b r i c a t i o n o f m i n i a t u r e mechanisms i s a r a t h e r d i f f i c u l t b u t i n t e r e s t i n g f i e l d i n t h e t r i b o l o g y o f m i n i a t u r e s y s t e m s . The a n a l y s i s o f t h e methods w i l l b e mainly concerned w i t h t h e problem o f d e t e r mining t h e p h y s i c o c h e m i c a l p r o p e r t i e s o f microvolumes o f o i l s . T h i s
i s v e r y i m p o r t a n t f o r t h e s t u d y o f t h e e f f e c t s of i n t e r a c t i o n s i n t h e l u b r i c a n t - r u b b i n g elements-ambient system. Aspects such a s t h e t e s t i n g of c o a t i n g s (epilames), c l e a n i n g and s p e c i a l i n v e s t i q a t i o n t e c h n i q u e s w i l l a l s o be d i s c u s s e d and many p r a c t i c a l s u g g e s t i o n s w i l l be made.
8,2, FRICTION 8.2.1.
INTRODUCTION
There a r e many t e c h n i q u e s f o r m e a s u r i n g C r i c t i o n ( f o r c e o r t o r q u e ) i n m i n i a t u r e t r i b o l o g i c a l s y s t e m s . Here w e w i l l d i s c u s s t h e methods and a p p a r a t u s f o r e s t i m a t i n g t h e f r i c t i o n i n microb e a r i n g s r a t h e r t h a n t h e methods and t e c h n i q u e s f o r m e a s u r i n g s d l forces o r torques. The f r i c t i o n f o r c e o r f r i c t i o n t o r q u e i s g e n e r a l l y e s t i m a t e d by measuring t h e d e f o r m a t i o n o r d i s p l a c e m e n t o f an e l a s t i c s y s t e m which b a l a n c e s t h e a c t i o n of f r i c t i o n l o a d s or by m e a s u r i n g t h e change i n c u r r e n t i n t e n s i t y o r v o l t a g e as t h e r e s u l t of a f r i c t i o n l o a d a f f e c t i n g a magnetic o r e l e c t r i c f i e l d . Methods f o r measuring t h e f r i c t i o n i n m i n i a t u r e s y s t e m s have been i n v e n t e d f o r b o t h o s c i l l a t i n g ( r e v e r s e ) motion o f t h e r u b b i n g
30 3 e l e m e n t and f o r u n i d i r e c t i o n a l motion. They w i l l be d i s c u s s e d h e r e i n t h i s order. 8.2.2.
O S C I L L A T I N G MOTION O F THE RUBBING ELEMENT
The c l a s s i c method h e r e i s t h e ASTM pendulum ( r e f . 5 6 5 ) which i s w i d e l y u s e d . A 1/2" s p h e r e s l i d e s on two p l a t e s f i x e d t o g e t h e r
a t gn a n g l e o f 90° ( F i g . 8 . 1 ) . The pendulum i s d e f l e c t e d by 25O and t h e n allowed. t o o s c i l l a t e f r e e l y . The a m p l i t u d e o f t h e pendulum (a)as a f u n c t i o n o f t h e number o f o s c i l l a t i o n s g i v e s t h e Acl damping c u r v e l and AIX ( a m p l i t u d e d e c r e a s e ) as a f u n c t i o n of cx g i v e s t h e AR c u r v e ( F i g . 8 . 1 ) .
30 4
The s t e e p e r t h e damping c u r v e , t h e g r e a t e r t h e f r i c t i o n between t h e r u b b i n g e l e m e n t s ( s p h e r e and p l a t e s ) . The ACY c u r v e (when
aq
where c i s t h e pendulum c o n s t a n t . G e n e r a l l y , t h e e q u a t i o n d e s c r i b i n g t h e pendulum o s c i l l a t i o n i s
as f o l l o w s ( r e f . 1 0 6 ) :
(where I i s t h e moment o f i n t e r t i a , and c ' , p ' and k' areconstants). Because of s m a l l v a l u e s o f c x , s i n c i N a . The t e r m k ' s i n d i s o f t e n 2 n e g l i g i b l e ( r e l a t i v e t o t h e term p ' a ) . The t e r m p'R2 d e s c r i b e s t h e a i r damping. When t h e pendulum i s s o formed t h a t t h e c o n s t a n t p ' i s v e r y s m a l l , i . e . t h e a i r dampinq i s n e g l i g i b l e , e q u a t i o n ( 8 . 2 ) c a n be r e w r i t t e n as
12
+
S +
COL+
+
C'M
k'sicjn 6 = 0
or k signd = 0
where
+
k = mlLs
mgr f c o s y ( I +~
mls)
m - t o t a l m a s s o f pendulum 9 - g r a v it y a c c e l e r a t i o n 1s-
see F i g . 8.2
IS - moment o f i n e r t i a ( s
-
g r a v i t y c e n t r e : see F i g . 8 . 2 )
r - radius of sphere f - c o e f f i c i e n t of f r i c t i o n ' 9 - see F i g . 8.2. The g e n e r a l s o l u t i o n of e q u a t i o n ( 8 . 4 ) i s
an = cxoCosOt
+
ck
(1
-
%
-
ck
(1
- c o s w t ) , oi >
= O(ocosWt
COSWt),
dc <
0,
0
oi
=
o-,
"h = aP
305
& MASS
F i g . 8 . 2 . Schew of the ASTMpendulum.
The d i f f e r e n c e between two s u c c e s s i v e a m p l i t u d e s i s 4
ck
and t h e r e -
fore A.
-
An = 4
k C
n = 4 n
rf lscos
9
(8.51
The f r i c t i o n c o e f f i c i e n t c a n b e e s t i m a t e d as f o l l o w s : A.
f = where Ao,
-
An
lscOslq
4r
n
= AA
lscoslq 4r
(8.6)
An a r e t h e a m p l i t u d e s a t t h e b e g i n n i n g and a f t e r
n-oscillations
r e s p e c t i v e l y , a n d n i s t h e number o f o s c i l l a t i o n s .
Because t h e d i m e n s i o n l e s s v a l u e A A is t h e r e l a t i o n of t h e dj.m?nsion a m p l i t u d e d i f f e r e n c e Aa t o t h e pendulum r a d i u s R (see F i g . 8 . 2 )
we
get: lscoslp f =
4rR
A a = K A a
(8.7)
where K i s t h e pendulum ( a p p a r a t u s ) c o n s t a n t . I n F i g . 8.3 t h e exp e r i m e n t a l l y d e t e r m i n e d r e l a t i o n s h i p between A a and f i s g i v e n (ref. 106). Good r e s u l t s have b e e n o b t a i n e d when t h e pendulum o s c i l l a t i o n s w e r e recorded u s i n g a s p e c i a l magnetic i n s t a l l a t i o n . A " H a l l " e f f e c t which i n t e r a c t s w i t h a m a q n e t i c t a p e i s f i x e d a t t h e l o w e r
30 6 end o f t h e pendulum. The d i s t a n c e between t h e g e n e r a t o r and t h e m a g n e t i c t a p e i s a b o u t 1 m. The m a g n e t i c t a p e is m a g n e t i z e d i n s u c h a manner t h a t a f t e r e v e r y 1 mm o f pendulum d i s p l a c e m e n t , t h e g e n e r a t o r g i v e s one i m p u l s e ( b e c a u s e o f t h e change i n p o l a r i t y o f t h e m a g n e t i z e d t a p e ) . The number o f i m p u l s e s i s t h e r e f o r e p r o p o r t i o n a l t o t h e a m p l i t u d e o f t h e pendulum. A s p e c i a l e l e c t r o n i c s y s -
t e m w i t h a c o u n t e r enables t h e d i r e c t e s t i m a t i o n of t h e f r i c t i o n c o e f f i c i e n t of t h e s p h e r e - p l a t e s y s t e m .
,
0
10
20 30 40
50
t
60 ba ,m m
F i g . 8.3. R e l a t i o n s h i p between f r i c t i o n c o e f f i c i e n t and a m p l i t u d e d i f f e r e n c e .
The pendulum s y s t e m shown i n F i g . 8 . 4 f o r d e t e r m i n i n g t h e f r i c t i o n c o e f f i c i e n t o f polymer-polymer s y s t e m s w a s b u i l t by D r . T i l l w i c h GmbH (F.R.G) ( r e f . 1 0 8 ) . The pendulum h a s a s p h e r e - s h a p e d r u b b i n g e l e m e n t and i s hunq on a p r i s m w i t h two p e r p e n d i c u l a r p l a n e s . The pendulum i s d r i v e n by a s p e c i a l c a r which i s moved p n e u m a t i c a l l y and a t t a c h e d e l e c t r o m a g n e t i c a l l y t o t h e pendulum arm. The pendulum o s c i l l a t i o n s a r e c o u n t e d by an i n f r a - r e d s y s t e m which g i v e s a d i g i t a l o u t p u t s i g n a l . The t r i b o l o g i c a l s y s t e m ( r u b b i n g e l e m e n t s w i t h any o i l f i l l i n g t h e p r i s m and l u b r i c a t i n g t h e r u b b i n g e l e m e n t s ) i s t h e r m o s t a t e d u s i n g a P e l t i e r e l e m e n t ( 3 0 W ) . Convection c o o l i n g i s p r o v i d e d by a c i r c u l a t i n g e t h y l e n e g l y c o l - w a t e r b l e n d . The f r i c t i o n v i b r a t i o n s o f t h e p r i s m are d e t e c t e d by a p i e z o m e t e r . A thermoelement w i t h a d i a m e t e r o f 0 . 1 mm measures t h e t e m p e r a t u r e i n t h e f r i c t i o n region. A laser s y s t e m i s u s e d t o o b s e r v e t h e
wear of t h e s p h e r e s u r f a c e
and t h e s p a r i n g l u b r i c a t i o n o f t h e rubbing elements.
30 7
F i g . 8.4. (a) D r . T i l l w i c h CmbH pendulum system f o r t r i b o l o g i c a l i n ves t i g a t i on o f m i n i a t u r e p o l ymer-pol ymer sys terns, (b) i nvest i g a t e d r u b b i n g elements, ( c ) thermoelenent f o r measurement o f temperature i n f r i c t i o n region. 1 - frame, 2 sphere, 3 p r i s m , 4 - w i r e c o n n e c t o r , 5 - piezometer, 6 - measurement head, 7 - P e l t i e r element, 8 - pendulum arm, 9 - w e i g h t , 10 - I R element system, 1 1 - g r a n i t e p l a t e , 12 - c a r guide, 13 - c a r .
-
-
30 8
The s y s t e m i s c o n t r o l l e d by a m i n i c o m p u t e r w i t h t a p e r e c o r d e r , d i s p l a y , p r i n t e r and a n a l o g - d i g i t a l t r a n s f o r m e r a n d s u i t a b l e i n t e r f a c e s . The measured and c o n t r o l l e d p a r a m e t e r s a r e a u t o m a t i c a l l y d i s p l a y e d o r t y p e d on r e q u e s t . Such a s y s t e m e n a b l e s l o n g - t e r m t e s t s t o b e c a r r i e d o u t which may l a s t up t o 4 weeks ( a b o u t 4 km of s l i d i n g d i s t a n c e ) . The minimum p r a c t i c a l number o f o s c i l l a t i o n s n e c e s s a r y f o r t h e d e t e r m i n a t i o n of t h e f r i c t i o n c o e f f i c i e n t i n t h e s p h e r e - p l a t e s pendulum s y s t e m i s a b o u t 1 2 0 . To r e d u c e t h e o s c i l l a t i o n s o f t h e pendulum p e r p e n d i c u l a r t o t h e p l a n e o f t h e pendulum movement t h e two-sphere s y s t e m h a s b e e n p r o p o s e d ( r e f . 1 0 6 ) . The f r i c t i o n i n b a l a n c e jewel b e a r i n g s c a n be i n v e s t i g a t e d by o b s e r v i n g t h e f r i c t i o n damping o f t h e o s c i l l a t i o n s o f a b a l a n c e . The f r i c t i o n t o r q u e M (u)c a n b e e s t i m a t e d u s i n g t h e f o l l o w i n g relationship :
where I
w n
-
-
moment o f i n e r t i a o f t h e b a l a n c e angular velocity number o f o s c i l l a t i o n s . A s p e c i a l system c a l l e d t h e "BalisomGtre" has been b u i l t f o r
t h e a n a l y s i s of t h e f r i c t i o n b e h a v i o u r of b a l a n c e p i v o t b e a r i n g s by t h e L a b o r a t o i r e S u i s s e de Recherches H o r l o g S r e s ( a t p r e s e n t C e n t r e S u i s s e d ' E l e c t r o n i q u e e t d e M i c r o t e c h n i q u e S.A. ) i n NeucMtel ( S w i t z e r l a n d ) ( r e f . 3 8 0 ) . The B a l i s o m S t r e s y s t e m i s p r e s e n t e d i n F i g . 8 . 5 . The o s c i l l a t i o n s o f a b a l a n c e are i n v e s t i g a t e d u s i n g a p h o t o e l e c t r o n i c s y s t e m . The c o n t r o l s y s t e m i s t h e HP 2 1 0 0 computer w i t h a 1 6 K memory. The d a t a c a n b e r e a d from a s y s t e m of counters. The p e r i p h e r a l s of t h e s y s t e m c o n s i s t of an x-y p l o t t e r a r e a d e r , a p e r f o r a t o r and a t e l e t y p e w i t h keyboard.
Investigations i n t o t h e f r i c t i o n a l p r o p e r t i e s of miniature s y s t e m s b a s e d on t h e o s c i l l a t i o n o f t h e r u b b i n g e l e m e n t i n a pendulum s y s t e m g i v e i n f o r m a t i o n a b o u t t h e f r i c t i o n o v e r a r e l a t i v e l y short s l i d i n g distance o r operation t i m e , e.g.
d u r i n g t h e running-
- i n p e r i o d . For o b s e r v a t i o n s over a l o n g e r p e r i o d of t h e f r i c t i o n v a r i a t i o n s d u r i n g o s c i l l a t i o n of a r u b b i n q e l e m e n t , a s p e c i a l p i e m of a p p a r a t u s , UTI-Refbuncjs-
built a t the Institut
und V e r s c h l e i s s p r i i f g e r z t , h a s been
f i i r U h r e n t e c h n i k und Feinmechanik o f t h e
U n i v e r s i t y o f S t u t t g a r t (F.R.G.) ( r e f . 4 4 ) . A d i a g r a m o f t h e ap-
309 p a r a t u s is g i v e n i n F i g . 8 . 6 . A p l a t e , which i s o n e of t h e r u b b i n g e l e m e n t s , i s mounted on a t a b l e . The t a b l e o s c i l l a t e s a n d t h e plate
r u b s a g a i n s t t h e immobile e l e m e n t
-
a sphere, cylinder o r
p i n . The f r i c t i o n f o r c e i s measured u s i n g t h e s p r i n g s h o l d i n g t h e sample and t h e i n d u c t i v e gauge f o r e s t i m a t i n g t h e d i s p l a c e m e n t o f t h e f l a g (which i s f i x e d t o t h e r u b b i n g e l e m e n t ) as a r e s u l t o f friction.
F i g . 8.5.
Scheme o f t h e
LSRH
BalisomStre.
310
SAM PLE
CYUNDR ,PIN)
I
I
-
PLATE
I
TABLE
I /
FLAT SPQlN6
FLAG INDUCTIVE GAUGE
Fig.
8.6. Principle
of t h e UTI apparatus.
The s l i d i n c j v e l o c i t y of t h e t a b l e i s c o n s t a n t o v e r t h e whole d i s t a n c e of t h e s h i f t ( F i g , 8.1 b)
is 0.002
-
.
The r a n u e of s l i d i n g v e l o c i t y
5 0 m/s. The d i s t a n c e of t h e s h i f t of t h e t a b l e can
r a n g e from 0 . 5 t o 15 m. The p r i n c i p l e o f p l o t t i n g t h e f r i c t i o n f o r c e when u s i n g t h e UTI s y s t e m i s shown i n F i g .
8.8.
31 1
F i g . 8.7. S l i d i n g speed as a f u n c t i o n o f t i m e i n an ASTM pendulum (a) and UTI apparatus ( b ) .
FRICTION FOPCE CHAPACTERISTIC
F i g . 8.8. P r i n c i p l e o f t h e f r i c t i o n f o r c e p l o t u s i n g UTI apparatus.
312 8.2.3.
UNIDIRECTIONAL MOTION O F THE RUBBING ELEMENT
The methods f o r measurinq t h e f r i c t i o n moment i n m i n i a t u r e b e a r i n g s when t h e motion of t h e r u b b i n g e l e m e n t i s u n d i r e c t i o n a l a r e based on b a l a n c i n g t h e f r i c t i o n t o r q u e w i t h t h e e x t e r n a l m t . The b a l a n c i n g moment can b e p r o v i d e d m e c h a n i c a l l y o r e l e c t r i c a l l y . The most common method i s t h e pendulum system ( F i g . 6 . 9 ) ( r e f s . 44,
182, 1 8 3 ) .
a)
I
/
SHAFT
F i g . 8.9. The measurement o f f r i c t i o n t o r q u e w i t h t h e pendulum system, f o r d r y (a) and l u b r i c a t e d (b) b e a r i n g s .
The b a l a n c i n g moment i s p r o v i d e d by t h e pendulum d e v i a t i o n . The method i s i l l u s t r a t e d i n F i g . 8.9 a ) and b ) , f o r d r y and l u b r i c a t e d m i n i a t u r e b e a r i n g s r e s p e c t i v e l y . The d e v i a t i o n o f t h e pendulum's c e n t r e of g r a v i t y i s n o t measured a g a i n s t t h e movable bush b u t a g a i n s t t h e f i x e d s h a f t . The moment Ms
i n t h e bush under d r y run-
n i n g c o n d i t i o n s i s ( r e f . 182) : Ms
= P
-
b = P(a
+
e) = Mf
+
Md
(8.9)
31 3 and for l u b r i c a t e d b e a r i n g s :
Ms = P
-
b = P(a
-
-
e ) = Mf
Md
(8.10)
where Mf
-
bush moment f r i c t i o n moment
Md
-
d i s l o c a t i o n moment
Ms
a , b - pendulum d e v i a t i o n (see F i g . 8 . 9 )
e
= s/2
(s
-
radial clearance).
I n b o t h c a s e s o n l y t h e d e v i a t i o n o f t h e c e n t r e o f g r a v i t y from t h e f i x e d s h a f t a x i s y i s measured. The d e v i a t i o n may b e measured d i r e c t l y by means of n o n - c o n t a c t
i n d u c t i v e gauges.
The a d v a n t a g e s o f t h e pendulum method are e a s y and f a s t meas u r e m e n t s and t h e p o s s i b i l i t y o f a u t o m a t i n g t h e m e a s u r i n g p r o c e s s . A damping s y s t e m i s n e c e s s a r y however, b e c a u s e o f v a r i a t i o n s i n
t h e f r i c t i o n moment; t h e pendulum c a n o s c i l l a t e a n d t h e a v e r a g e v a l u e o f t h e f r i c t i o n moment c a n b e d e t e r m i n e d . The b a l a n c i n g moment c a n b e o b t a i n e d by d i p p i n g t h e e l e m e n t i n a l i q u i d , a s i n t h e c l a s s i c Langue m e a s u r i n g s y s t e m ( P i g . 8 . 1 0 ) ( r e f s . 163, 566-568). The h y d r o s t a t i c method i s v e r y s i m p l e a n d v e r y a c c u r a t e . The damping i s n a t u r a l . The f r i c t i o n t o r q u e of t h e b e a r i n g b e i n g t e s t e d is b a l a n c e d w i t h t h e d i f f e r e n c e i n t h e p r e s s u r e o f t h e l i q u i d on e a c h p i n immersed i n t h e l i q u i d and f i x e d t o a t h r e a d by which i t hangs on t h e b e a r i n g h o l d e r . The r a d i a l l o a d p o f t h e b e a r i n g
is : P = 2(Gw
+
Q
-
S a
pL) +
(8.11)
Gh
where
c./J -
w e i g h t on one p i n
-
w e i g h t of p i n
S
-
w e i g h t of h o l d e r w i t h b e a r i n g bush ( b u s h e s ) area of pin section
a
-
f i r s t depth o f p i n d i p p i n g density of l i q u i d .
Q Gh
pL -
The a n g l e o f f r i c t i o n
'4
a f t e r displacement o f t h e p i n s can b e
c a l c u l a t e d u s i n g t h e f o l l o w i n g f o r m u l a ( r e f s . 567, 568) :
siny
= 2
?L P d
2e
cosoL
+
(AT + A t ) D P d
(8.12)
314
where
d
-
bearing diameter
e
-
h o l d e r e c c e n t r i c i t y ( d i s t a n c e between c e n t r e of t h e b e a r -
X
D
p i n displacement e x t e r n a l d i a m e t e r of h o l d e r
i n g bush and e x t e r n a l c y l i n d e r of t h e b e a r i n g b u s h h o l d e r ) o(
-
a n g l e of l o c a t i o n of h o l d e r e c c e n t r i c i t y
nT,M - a v e r a g e a d h e s i o n components of t h e s t r e n g t h i n t h e t h r e a d branches.
F i g . 8.10. H y d r o s t a t i c method f o r measuring f r i c t i o n torque o f bearings. 1 - s h a f t , 2 b e a r i n g t o be i n v e s t i g a t e d , 3 - h o l d e r , 4 - prism, 5 - thread, 6 - pin, 7 - weight, 8 - spring.
-
I f t h e j o u r n a l diameter i s d t h e f r i c t i o n moment Mf c a n be j r e x p r e s s e d as :
315 1
Mi = 7 P d 7. s i n ?
(8.13)
The minus sign i n formula (8.12) r e l a t e s t o t h e case when t h e j o u r n a l r o t a t e s c l o c k w i s e . The a d h e s i o n components A T and A t are s o small
as t o b e n e g l i g i b l e when t h e p i n s a r e m a n u f a c t u r e d i n b r a s s a n d a l c o h o l is used ( r e f . 5 6 7 ) . The b a l a n c i n g moment i n t h e f r i c t i o n a p p a r a t u s c a n b e p r o v i d e d by a s p i r a l o f f l a t s p r i n g . The microtribometer d e s i g n e d i n LSRH ( N e u c h h t e l , S w i t z e r l a n d ) ( F i g . 8.11) f o r t h e e s t i m a t i o n o f t h e frict i o n t o r q u e i n m i n i a t u r e j o u r n a l b e a r i n g s ( @ ca. 0 . 1 mm) c a n m e a s u r e t h e f r i c t i o n moment t o 1 . 8
-
1 0 - l ,uN m a t t h e s e n s i v i t y
JAN m ( r e f s . 184, 382, 5 6 9 ) . The r a n g e o f s l i d i n g s p e e d s
i s from ca.
0 . 0 5 t o 5 0 mm s - l . The j o u r n a l 2 i s p i v o t e d on t h e b e a r i n g s 5 a n d 6 ( F i g . 8 . 1 1 ) . The f r i c t i o n t o r q u e i n t h e t e s t e d b e a r i n g s 3 and 4
t u r n s t h e h o l d e r 1 and l o a d s t h e s p i r a l s p r i n g 8 which a t t h e o t h e r e n d i s f i x e d t o t h e h i g h l y s e n s i t i v e f o r c e gauge 9 . The o s c i l l a t i o n s of t h e h o l d e r are damped w i t h t h e F o u c a u l t b r a k e 10 s o t h e a v e r a g e v a l u e o f t h e f r i c t i o n moment i s d e t e r m i n e d . The f r i c t i o n t o r q u e i n t h e v e r t i c a l c e n t r e o f t o r s i o n of t o r s i o n a l s u s p e n s i o n - f r e e s p r i n g b e a r i n g s h a s a l s o b e e n d e t e r m i n e d by m e a s u r i n g t h e angle ( r e f . 570).
1
F i g . 8.11. Diagram o f t h e LSRH m i c r o t r i b o m e t e r . 1 - h o l d e r , 2 - j o u r n a l , 3 and 4 bearings t o be t e s t e d , 5 and 6 - s u p p o r t b e a r i n g s , 7 - d r i v e ,
-
8 - s p i r a l s p r i n g , 9 - f o r c e gauge, brake.
10
-
Foucault
316
The f l a t s p r i n g i s o f t e n u s e d as t h e e l e m e n t which g i v e s t h e b a l a n c i n g f o r c e (moment) and t h e d e f o r m a t i o n of t h e s p r i n g c a n b e d e t e r m i n e d by v a r i o u s methods. The e x t e n s o m e t e r s y s t e m i s a s i m p l e way t o measure t h e f o r c e a c t i n g a g a i n s t t h e s p r i n g as a r e s u l t o f f r i c t i o n i n a b e a r i n g . A diagram of t h i s k i n d of system, used t o i n v e s t i g a t e miniature bearings ( r e f . 286)
,
i s presented i n Fig.
8.12.
F i g . 8.12. Exp eri me nt al a pparatus f o r t h e e s t i m a t i o n o f f r i c t i o n torque w i t h a f l a t extensometer s p r i n g . 1 motor, 2 reducer, 3 i rn pu l ser, 4 - c l u t c h , 5 - b a l l b e a r i n g s , 6 - base p l a t e s , 7 - j o u r n a l , 8 - b e a r i n g bushes t o be t e s t e d , 9 - h o l d e r , 10 - w e i g h t s , 11 - b a l l , 12 - f l a t s p r i n g w i t h extensometers, 1 3 - extensometer b r i d g e .
-
-
-
The motor 1, v i a t h e g e a r i n g a n d b e l t t r a n s m i s s i o n 2 , i m p u l s e r 3 ( f o r t h e e s t i m a t i o n o f r o t a t i o n a l s p e e d ) , and c l u t c h 4, d r i v e s t h e j o u r n a l 7. The h o l d e r 9 w i t h t h e two b e a r i n g b u s h e s t o b e t e s t e d 8 i s hung on t h e r o t a t i n g j o u r n a l 7 . The b e a r i n g s u n d e r i n v e s t i g a -
t i o n a r e l o a d e d w i t h t h e w e i g h t s 1 0 . The f r i c t i o n t o r q u e i s measured using t h e f l a t s p r i n g 1 2 with extensometers glued onto it which a r e c o n n e c t e d t o t h e e x t e n s o m e t e r b r i d g e 1 3 . The t e s t r i n g d e s c r i b e d h a s b e e n u s e d t o measure f r i c t i o n t o r q u e i n j o u r n a l b e a r i n g s w i t h a d i a m e t e r o f up t o 3 nun i n t h e r a n g e o f s l i d i n g s p e e d s from 0 . 0 0 0 0 3 3 t o 0 . 1 m s - l a n d s p e c i f i c
317 ( c a l c u l a t e d ) b e a r i n g l o a d s from 0 . 2 t o 5 MPa. Based on t h i s i d e a of t h e h o l d e r o f t h e b e a r i n g b u s h e s t o b e t e s t e d h a n g i n g on t h e r o t a t i n g s h a f t , two o t h e r e x p e r i m e n t a l s e t -ups w i t h e x t e n s o m e t e r s g l u e d t o t h e f l a t s p r i n g h a v e b e e n b u i l t , t e s t e d , and u s e d i n e x p e r i m e n t s w i t h m i n i a t u r e s e l f - l u b r i c a t i n g p o r o u s b u s h e s and a l s o p o l y m e r i c b e a r i n g b u s h e s . The l o a d i n t h e s e s y s t e m s i s a p p l i e d by t h e e x t e n s i o n s p r i n g . T h i s method h a s t h e a d v a n t a g e o f o m i t t i n g t h e s t a t i c b a l a n s i n g of t h e l o a d i n g s y s t e m which i s n e c e s s a r y i n w e i g h t l o a d i n g s y s t e m s . A l s o , t h e l o a d c a n b e c o n t i n o u s l y v a r i e d by s i m p l y c h a n g i n g t h e s p r i n g l e n g t h . An o r d i n a r y dynamometer h a s b e e n s a t i s f a c t o r i l y u s e d as. t h e l o a d i n g spring. The d e f o r m a t i o n o f t h e f l a t s p r i n g i n t h e t r i b o m e t e r can b e e s t i m a t e d w i t h t h e c a p a c i t y method ( F i g . 8 . 1 3 ,
ref.
187) o r t h e
o p t i c a l method ( F i g . 8 . 1 4 , r e f . 5 7 1 ) .
4
\
1 3
F i g . 8.13. T r i b o m e t e r b u i l t i n ETH ZOrich. 1 - j o u r n a l , 2 - b e a r i n g bush, 3 - guides, 4 - arms, 5 - l e v e r s , 6 - crossed s p r i n g , 7 - meas u r i n g head, 8 i n f r a r e d thermonxtry head.
-
31 8
F i g . 8.14. T r i b o m e t e r f o r o p t i c a l measurement o f f r i c t i o n torque. 1 - j o u r n a l , 2 - bearing bush t o be t e s t e d w i t h h o l d e r , 3 - base, 4 f l a t s p r i n g , 5 - m i r r o r , 6 - l i g h t source, 7 - recorder.
The l i g h t beam r e f l e c t s on t h e m i r r o r f i x e d t o t h e s p r i n g . The deformation o f t h e s p r i n g because of t h e f r i c t i o n f o r c e i n a b e a r i n g c a n be o b s e r v e d by m e a s u r i n g t h e beam d e v i a t i o n , u s i n g a s c a l e , r e c o r d i n g c h a r t o r p h o t o e l e c t r i c system. I t i s v e r y i m p o r t a n t t o estimate t h e f r i c t i o n t o r q u e i n t h e
j o u r n a l b e a r i n g s a t varying s p e c i f i c l o a d s and s l i d i n g speeds b e c a u s e i n a c t u a l u s e t h e s e two p a r a m e t e r s w i l l o f t e n b e c h a n g i n g c o n t i n o u s l y o r r a p i d l y . For t h i s purpose, t h e e x p e r i m e n t a l s e t - u p shown i n F i g .
8.15 w a s d e v e l o p e d ( r e f . 2 8 7 ) .
The j o u r n a l 11 rubs a g a i n s t t h e b e a r i n g b u s h e s 2 p l a c e d i n t h e h o l d e r 1. The f r i c t i o n t o r q u e i s e s t i m a t e d by t h e b r i d g e e x t e n s o -
meter s y s t e m measuring t h e d e f o r m a t i o n ( c a . 0 . 1 - 0 . 2 mm) o f t h e s p r i n g s 5 . The l o a d i s a p p l i e d t o t h e b e a r i n g s by t h e a c t i o n o f t h e e l e c t r o m a g n e t a t t r a c t i n g t h e h o l d e r 2 . The j o u r n a l i s r o t a t e d by t h e motor v i a t h e e l e c t r o m a g n e t i c c l u t c h 8 . The run-down method € o r t h e measurement o f t h e f r i c t i o n t o r q u e
i s v e r y s i m p l e and i s u s e d f o r t h e o r i e n t a t i o n e v a l u a t i o n o f t h e f r i c t i o n t o r q u e r e f s . 9 , 5 7 1 ) . The heavy drum ( F i g . 8 . 1 6 ) i s s e t i n motion by t h e e n e r g y i m p u l s e which i s d i s s i p a t e d by overcoming the friction i n
he b e a r i n g . I f t h e t i m e t taken f o r t h e system t o
run down and t h e number o f r o t a t i o n s are d e t e r m i n e d , t h e f r i c t i o n t o r q u e Mf can b e e s t i m a t e d from:
319 Mf
471 n t2
(8.14)
= ___
where I i s t h e moment of i n e r t i a of t h e drum.
F i g . 8.15. Experimental set-up f o r t h e e s t i m a t i o n o f f r i c t i o n t o r q u e i n m i n i a t u r e j o u r n a l b e a r i n g s d u r i n g r a p i d changes i n l o a d or s l i d i n g speed. 1 - h o l d er, 2 b e a r i n g bushes t o be t e s t e d , 3 - thermoelement, 4 electromagnet, 5 - f l a t s p r i n g s w i t h extensometers, 6 base frame, 7 - b e l t t r a n s m i s s i o n , 8 - e l e c t r o m a g n e t i c c l u t c h , 9 - photodiode w i t h l i g h t i n g , 10 impulse d i s c .
-
-
-
-
320
F i g . 8.16. Run-down method f o r d e t e r m i n i n g f r i c t i o n t o r q u e . 1 - drum w i t h p i v o t , 2 b e a r i n g t o be t e s t e d .
The f r i c t i o n moment can be b a l a n c e d u s i n g an e l e c t r i c a l s y s t e m . The " e l e c t r i c a l s p r i n g " method i s p r e s e n t e d i n F i g . 8 . 1 7 .
F i g . 8.17. " E l e c t r i c a l s p r i n g " method f o r estimating the torque i n miniature bearings. 1 b e a r i n g t o be t e s t e d , 2 holder, 3 l e v e r , 4 - displacement gauge, 5 - t r a n s ducer g i v i n g t h e b a l a n c i n g moment.
-
-
-
Under t h e f r i c t i o n t o r q u e , t h e h o l d e r 2 w i t h t h e l e v e r 3 t r i e s t o r o t a t e . The o p e r a t i n g gap i n t h e d i s p l a c e m e n t gauge 4 v a r i e s and t h e e l e c t r i c a l s i g n a l from it is a m p l i f i e d and t h e n s e n t t o t h e t r a n s d u c e r 5 g i v i n g t h e b a l a n c i n g moment. The f r i c t i o n moment Mf
32 1
is proportional t o the current intensity i i n the transducer’s c o i l (Mf = C i, C
-
c o n s t a n t ) . The a c c u r a c y o f t h e Mf measurement
i s n o t h i g h e r t h a n 3-5% ( r e f . 5 7 1 ) .
The i n d u c t i v e method a p p a r a t u s i s p r e s e n t e d i n d i a g r a m m a t i c form i n F i g . 8.18 ( r e f . 5 7 1 ) . When t h e s h a f t 1 r o t a t e s a t c o n s t a n t s p e e d on t h e removable e l e m e n t 3 of t h e a p p a r a t u s , t h e f r i c t i o n moment Mf and t h e compensating moment Mc a r e b o t h e x e r t i n g f o r c e . The compensating moment i s c r e a t e d by t h e i n d u c t i o n s y s t e m 5. The c u r r e n t i n t e n s i t y i n t h e c o i l i s a u t o m a t i c a l l y d e t e r m i n e d by t h e p h o t o e l e c t r i c s y s t e m 6 and 7 a n d t h e a m p l i f i e r 8. When M f = Mc, i t i s p o s s i b l e , b a s e d on t h e c u r r e n t i n t e n s i t y i , t o estimate t h e
-
f r i c t i o n moment from t h e s i n g l e e q u a t i o n Mf = C
i2
(C
-
constant).
The a c c u r a c y of t h e measurements i s n o t h i g h e r t h a n 1%.
I
‘ I
1
F i g . 8.18. I n d u c t i v e method f o r e s t i m a t i n g f r i c t i o n torque, 1 - j o u r n a l , 2 - bearing t o be t e s t e d , 3 - h o l d e r , 4 - d i s c , S ind u c t i o n system, 6 l i g h t source, 7 - p h o t o di o de , 8 amplifier.
-
-
-
The f r i c t i o n t o r q u e i n m i n i a t u r e j o u r n a l b e a r i n g s can a l s o b e measured u s i n g t h e s y s t e m i l l u s t r a t e d i n F i g . 8.19.
The t o r q u e meter used i n t h i s s y s t e m ( r e f . 572) c o n s i s t s o f t h e c o r e 1 ( w i r e made from n i c k e l o r n i c k e l a l . l o y ) , t h e bobbin 2 and t h e w i n d i n g 3. The winding r e c e i v e s e l e c t r i c c u r r e n t from t h e g e n e r a t o r 4 . The t o r s i o n of t h e n i c k e l w i r e causes a v a r i a t i o n i n the inductance of t h e w i n d i n g and b a l a n c e s t h e b r i d g e 5 . The p o t e n t i a l U1-U2
(proportional t o t h e applied torque Mf) attached
i s r e g i s t e r e d by t h e
pen r e c o r d e r 6 . The optimum p a r a m e t e r s o f t h e t o r q u e
32 2
meter when u s e d t o d e t e r m i n e t h e f r i c t i o n t o r q u e i n s t e e l - b r a s s m i n i a t u r e b e a r i n g s w e r e as f o l l o w s 1.5 V and 1 kHz r e s p e c t i v e l y :
:
s u p p l y , v o l t a g e and f r e q u e n c y ,
number of c o i l s of w i n d i n g , 3000-5000
( c o p p e r w i r e , @ 0 . 1 2 nun); and n i c k e l c o r e d i a m e t e r , 2 nun. The s e n s i v i t y o f t h e s y s t e m was a p p r o x i m a t e l y 700 mV/N.m
a t t h e maxi-
.
mum t o r q u e measured ( a b o u t 0 . 1 2 N - m ) The a c c u r a c y o f t h e s y s t e m i n d e t e r m i n i n g t h e f r i c t i o n t o r q u e was a b o u t 4 % .
*fr it t ion torque
AND RECTIFIER
-
F i g . 8.19. System f o r measuring f r i c t i o n t o r q u e o f m i n i a t u r e j o u r n a l b e a r i n g s . 1 - n i c k e l core, 2 bobbin, 3 winding, 4 - generator, 5 - bridge, 6 recorder.
-
-
An e l e c t r o d y n a m i c s y s t e m h a s b e e n d e v i s e d t o measure t h e f r i c -
t i o n t o r q u e a t t h e i n i t i a t i o n o f movement i n b e a r i n g s w i t h a u n i d i r e c t i o n a l or o s c i l l a t i n g j o u r n a l motion ( r e f . 5 7 3 ) . The s y s t e m
i s i l l u s t r a t e d i n F i g . 8.20. The l e v e r 2 i s p i v o t e d on t h e b e a r i n g 1. The e l e m e n t c o o p e r a t i n g w i t h t h e l e v e r d i s p l a c e m e n t gauge 3 i s
f i x e d a t one e n d o f t h e l e v e r , and t h e magnet 5 a t t h e o t h e r . The
e l e c t r i c w i r e 6 a t t a c h e d t o t h e c u r r e n t s o u r c e 7 i s l o c a t e d between
32 3 t h e p o l e p i e c e s o f t h e magnet. The c u r r e n t meter a t t a c h e d t o t h e
master resistor 9 is c o n n e c t e d t o t h e d i s p l a c e m e n t meter 4 a n d t h e wire 6 . The c u r r e n t f l o w i n g t h r o u g h t h e w i r e 6 r e p e l s t h e magnet 5.. Once t h e c u r r e n t h a s b e e n i n c r e a s e d t o a v a l u e a t which t h e r e p e l l i n g f o r c e i s g r e a t e r t h a n t h e f o r c e n eces s ar y t o exceed t h e f r i c t i o n t o r q u e i n the b e a r i n g 1, t h e lever moves and t h e movement i s r e g i s t e r e d by t h e d i s p l a c e m e n t gauge 3. The d i s p l a c e m e n t m e t e r 4 t r a n s m i t s t h e s i g n a l t o t h e c u r r e n t meter 8 and t h e c u r r e n t
a c t u a l l y f l o w i n g t h r o u g h t h e w i r e 6 i s r e g i s t e r e d . The s y s t e m should be stan d a r d i z e d b e f o r e t h e experiments.
F i g . 8.20. E l e c t r o d y n a m i c system f o r measuring f r i c t i o n t o r q u e . 1 - b e a r i n g t o be t e s t e d , 2 l e v e r , 3 - displacement gauge, 4 - displacement meter, 5 - magnet, 6 w i r e , 7 - c u r r e n t source, 8 - c u r r e n t meter, 9 - master r e s i s t o r .
-
Another s y s t e m f o r m e a s u r i n g f r i c t i o n t o r q u e i s shown i n F i g . 8.21.
I n t h i s system, t h e b e a r i n g t o be t e s t e d i s l o c a t e d i n t h e
h o l d e r 3 ( r e f s . 9 , 5 7 1 ) . On r o t a t i o n o f t h e j o u r n a l , t h e f r i c t i o n moment r o t a t e s t h e h o l d e r a n d t h e c o i l a t t a c h e d t o i t i n t h e magn e t i c f i e l d . The l i q h t beam from t h e l i g h t 5 r e f l e c t s o n t h e mirmr 2 a n d f a l l s on t h e p h o t o e l e m e n t 6 . The t r i o d e a l l o w s t h e c u r r e n t
32 4 t o p a s s a n d b e r e g i s t e r e d on t h e meter 8. T h i s c u r r e n t a f f e c t s t h e m a g n e t i c f i e l d which, a c t i n g w i t h t h e magnet, b a l a n c e s t h e f r i c t i o n t o r q u e . The f r i c t i o n t o r q u e i s p r o p o r t i o n a l t o t h e c u r r e n t i n t e n s i t y . The i m p o r t a n t a d v a n t a g e o f t h i s method i s t h e p o s s i b i l i t y of m e a s u r i n g r a p i d l y c h a n g i n g f r i c t i o n t o r q u e .
F i g . 8.21. " P h o t o e l e c t r i c balance" system f o r measuring f r i c t i o n t o r q u e . 1 - c o i l , 2 - m i r r o r , 3 - h o l d e r , 4 - b e a r i n g t o be t e s t e d , 5 - l i g h t source, 6 - photoelement, 7 t r i o d e , 8 - c u r r e n t meter.
-
F o r a p r e l i m i n a r y estimate o f t h e t r i b o l o g i c a l p r o p e r t i e s of t h e materials f o r t h e e l e m e n t s o f m i n i a t u r e s y s t e m s t h e well-known pin-on-disc
a p p a r a t u s i s a l s o sometimes u s e d ( r e f s . 1, 163, 183,
456, 542). The d i a m e t e r s of t h e p i n s o r s p h e r e s are m o s t l y between 0.3 and 6 mm. The m e c h a n i c a l methods p r e s e n t e d h e r e f o r e s t i m a t i n g t h e f r i c t i o n i n m i n i a t u r e b e a r i n g s are v e r y s i m p l e a n d u s e f u l i n l a b o r a t o r y i n v e s t i g a t i o n s c a r r i e d o u t on small s e r i e s of b e a r i n g s . Using s u c h methods, t h e a v e r a g e f r i c t i o n t o r q u e c a n b e e s t i m a t e d o v e r a limited r a n g e o f s l i d i n g s p e e d s . The s y s t e m s w i t h a s p i r a l o r f l a t s p r i n g
are p r a c t i c a l l y n o n - i n e r t i a l and t h e i n s t a n t a n e o u s f r i c t i o n t o r q u e can b e e s t i m a t e d . The e l e c t r i c a l methods a r e v e r y u s e f u l f o r t h e i n v e s t i g a t i o n o f l a r g e s e r i e s o f b e a r i n g s ; t h e y g i v e a c c u r a t e meas u r e m e n t s o f t h e f r i c t i o n t o r q u e a n d t h e measurements c a n b e f u l l y a u t o m a t e d o v e r a wide r a n g e o f l o a d s and s l i d i n g s p e e d s .
325
8,3,
WEAR
The e s t i m a t i o n o f t h e wear of t h e r u b b i n g e l e m e n t s i n m i n i a t u r e systems i s very important f o r t h e p r e d i c t i o n of t h e i r durab i l i t y : however, o n l y a few measurement s y s t e m s e x i s t f o r t h i s p u r p o s e . These can b e d i v i d e d i n t o two g r o u p s : t h o s e which a l l o w t h e wear t o b e measured w i t h o u t d i s a s s e m b l i n g t h e s y s t e m s a n d t h o s e which r e q u i r e t h e r u b b i n g e l e m e n t s t o b e d i s a s s e m b l e d . The f o r m e r g r o u p i n c l u d e s t h e w e a r a t t a c h m e n t p r e s e n t e d i n F i g . 8.22 (ref. 131).
F i g . 8.22. Wear attachment (a) and nomogram (b) f o r e s t i m a t i o n of r a d i a l wear of m i n i a t u r e j o u r n a l bear i ngs
.
The r o t a t i n a j o u r n a l rubs a g a i n s t two b e a r i n g b u s h e s which are t o b e i n v e s t i g a t e d . The l o a d i s a p p l i e d t h r o u g h t h e b a l l b e a r i n g from two e x t e n s i o n s p r i n g s . The e l e m e n t s a r e p l a c e d i n t h e s p e c i a l wear a t t a c h m e n t ( F i g . 8.22 a ) . The r a d i a l w e a r of t h e b e a r i n g ( j o u r n a l + b e a r i n g b u s h ) c a n be estimated w i t h o u t d i s m a n t l i n g t h e t r i b o l o g i c a l s y s t e m . The a x i s of t h e j o u r n a l i s d i s p l a c e d b e c a u s e o f t h e r a d i a l w e a r . The d i f f e r e n e between t h e d i s t a n c e Aa and % i s measured w i t h a s p e c i a l l y a d a p t e d Abb4 l e n g t h meter. The r a d i a l wear o f t h e b e a r i n g b u s h c a n t h e n
326 b e e s t i m a t e d u s i n g t h e nomogram ( F i g . 8.22 b ) . The t e s t s e t - u p i s p r o v i d e d w i t h 1 0 wear a t t a c h m e n t s . The j o w n a l s a r e p l a c e d i n 5 p a r a l l e l rows a n d d r i v e n i n series by t h e motor v i a an o v e r l o a d c l u t c h . T h i s s y s t e m h a s been u s e d t o e s t h t e t h e wear of metal a n d p o l y m e r i c m i n i a t u r e b e a r i n g s o p e r a t i n g a t c o n t a c t p r e s s u r e s o f up t o 7 MPa a n d s l i d i n g s p e e d s from 0 . 0 0 0 0 1 6 t o 0.2 m/s. I n v e s t i g a t i o n s i n t o t h e wear of m i n i a t u r e b e a r i n g s h a v e b e e n c a r r i e d o u t i n T a l l i n T e c h n i c a l U n i v e r s i t y (U.S.S. R . ) t h r e e e x p e r i m e n t a l s e t - u p s shown i n F i g .
8.23
using the
( r e f . 5 7 4 ) . The s y s -
t e m i n which t h e j o u r n a l r o t a t e s on t h e b u s h e s b e i n g i n v e s t i g a t e d ( F i g . 8.23 a ) a s s u r e s less s c a t t e r o f r e s u l t s t h a n the s y s t e m p r e s e n t e d i n F i g . 8 . 2 3 b . The s y s t e m shown i n F i g . 8 . 2 3 c i s v e r y s i m p l e a n d u s e f u l i n l a b o r a t o r y t e s t i n g b u t t h e h i g h a c c u r a c y of t h e h o l d e r 4 is i m p o r t a n t .
a)
n
F i g . 8.23. Schemes o f t h r e e wear i n v e s t i g a t i o n rigs. 1 - e l a s t i c j o i n t , 2 - d r i v e shaft, 3 j o u r n a l , 4 - b e a r i n g bushes t o be t e s t e d , 5 h o l d e r , 6 - wear r a t e meter, 7 - h o l d e r of t u r n i n g lathe, 8 - weight.
I n t h e t e s t r i g b u i l t i n ETH Z i i r i c h , S w i t z e r l a n d , t h e wear i s e s t i m a t e d a l o n g x a n d y a x e s ( r e f s . 178-180). The r o t a t i n g s h a f t 1 (Fig. 8.13) rubs a g a i n s t t h e bush 2 , which s l i d e s on t h e h o r i z o n t a l g u i d e s 3. The d i s p l a c e m e n t x of t h e bush a s t h e e f f e c t o f wear i s
32 7
r e g i s t e r e d by t h e c a p a c i t y t r a n s d u c e r s . The y d i s p l a c e m e n t r e s u l t i n g from wear o f t h e measurement h e a d i s a l s o c o n t r o l l e d u s i n g t h e c a p a c i t y gauge. The a c c u r a c y o f t h e wear measurements a l o n g t h e x and y a x e s i s no h i g h e r t h a n
3 ,um.
The t e s t s e t - u p used i n t h e LDZ L a n d i s and Gyr AG i n Zug, S w i t z e r l a n d , ( r e f s . 1 7 5 , 1 7 6 ) , i s p r e s e n t e d i n F i g s . 8 . 2 4 and 8.25.
F i g . 8.24. E xp eri me nt al set-up f o r e s t i m a t i n g r a d i a l wear i n m i n i a t u r e s t e e l - p o l y m e r b e a r i n g s w i t h o u t disassembling the bearings. a top view ( g e n e r a l ) , b - measuring system ( s e c t i o n A - A ) . 1 - base, 2 - b a l l b e a r i n g s o f d r i v e s h a f t , 3 j o u r n a l , 4 bush t o be t e s t e d , 5 - w eights, 6 - measuring edge, 7 l i g h t source, 8 mirror ( r e f s . 175, 176).
-
-
-
-
-
32 8
F i g . 8.25. D e t a i l s o f t h e wear measuring system i n t h e apparatus shown i n F i g . 8.24.
The wear of t h e b e a r i n g s i s o p t i c a l l y d e t e r m i n e d w i t h a microscope by measuring t h e change i n d i s t a n c e A between t h e m e a s u r i n g e d g e 6 and t h e j o u r n a l 3 ( F i g . 8 . 2 6 ) .
F i g . 8.26. Measuring t h e r a d i a l wear i n t h e system shown i n F i g . 8.24 and 8.25.
The wear i n p o l y m e r i c b e a r i n g s w a s s u c c e s s f u l l y e s t i m a t e d u s i n g a r a d i o t r a c e t e c h n i q u e ( r e f . 880)
.
329 The wear r a t e i n a m i n i a t u r e s y s t e m c a n a l s o be e s t i m a t e d a f t e r d i s m a n t l i n g t h e system ( r e f s . 44,
168, 5 7 5 ) . The wear i s d e t e r m i n -
e d u s i n g a microscope t o measure t h e d i a m e t e r o f s m a l l b o r e s o r u s i n g a s t y l u s i n s t r u m e n t t o measure t h e r o u g h n e s s . The worn s u r f a c e can be v i s u a l i z e d u s i n g t h e f u l l y a u t o m a t e d profilometer-comp u t e r s y s t e m ( r e f s . 576, 5 7 7 ) . I f p r o f i l e t r a c i n g w i t h a s t y l u s i s i m p r a c t i c a l , as i n t h e c a s e o f b e a r i n g s o r b o r e s w i t h s m a l l d i a -
meters ( l e s s t h a n 3 nun), a s u r f a c e r e p l i c a t e c h n i q u e can b e u s e d which r e p r o d u c e s t h e o r i g i n a l s u r f a c e w i t h r e a s o n a b l e a c c u r a c y ( r e f . 575). The wear o f t h e b e a r i n g a f t e r o p e r a t i o n c a n b e measured u s i n g a microscope t o e v a l u a t e t h e d i a m e t e r of s m a l l b o r e s . The practical a c c u r a c y o f such measurements was a b o u t 0 . 5 ,um when u s e d f o r t h e e x a m i n a t i o n o f wear i n p l a s t i c b e a r i n g s of 1 mm d i a m e t e r ( r e f s . 44,
168).
The wear o f t h e s p h e r e s o r p l a t e s i n a pendulum o r U T I - app a r a t u s (see C h a p t e r 8 . 2 ) can b e e s t i m a t e d w i t h a s t a n d a r d profilc-
meter s u c h a s t h e T a l y s u r f 1 0 0 . The wear o f t h e b e a r i n g b u s h e s can b e measured w i t h a r o t a r y p r o f i l o m e t e r s u c h a s t h e T a l y s u r f Rotary ( r e f s . 4 4 ,
168) ( F i g . 8 . 2 7 ) .
F i g . 8.27. The use o f a r o t a r y p r o f i l o m e t e r t o e s t i m a t e the r a d i a l wear r a t e o f a b e a r i n g bush.
330 I t i s v e r y s i m p l e t o estimate t h e wear r a t e o f s m a l l b o r e s u s i n g
t h e r e p l i c a method ( r e f . 5 7 5 ) . R e p l i c a s o f a b e a r i n g s u r f a c e can e a s i l y b e e v a l u a t e d w i t h c o n v e n t i o n a l i n s t r u m e n t a t i o n . Many replica
materials i n t h e form of powders and f i l m s a r e a v a i l a b l e . T h e p m d e r t y p e o f r e p l i c a m a t e r i a l i s mixed w i t h a s u i t a b l e f l u i d t o form a p a s t e . I n p r e v i o u s i n v e s t i g a t i o n s ( r e f . 575) t h e C e l l o n t y p e s o f r e p l i c a m a t e r i a l , 0 . 0 7 mm i n t h i c k n e s s , were found t o b e s u i t a b l e f o r t h e e x a m i n a t i o n o f s m a l l b o r e s . The r e p l i c a f i l m s w e r e c u t t o s i z e and a s m a l l amount o f s o l v e n t ( a c e t o n e o r m e t h y l a c e t a t e ) w a s smeared on t h e s u r f a c e o f t h e c l e a n s p e c i m e n . The f i l m w a s l a i d o v e r t h e specimen and g r a d u a l p r e s s u r e was a p p l i e d from o n e end u n t i l a l l t r a p p e d a i r w a s removed. A f t e r a few m i n u t e s t h e r e p l i c a w a s p e e l e d o f f . S i n c e t h e f i l m was t h i n a n d t e n d e d t o c u r l , i t w a s k e p t f l a t between t h i n g l a s s p l a t e s and a l l o w e d t o c u r e f o r a b o u t 30 m i n u t e s i n a h o t a i r b a t h a t 8OoC. T h i s g a v e r e p l i c a s w i t h few wrinkles. The t r a n s p a r e n t r e p l i c a s w e r e examined by i n t e r f e r e n c e micros c o p y . From t h e Rt v a l u e s o f t h e r e p l i c a s measured by i n t e r f e r o metry it was o b s e r v e d t h a t t h e i n i t i a l r o u g h n e s s o f t h e b o r e f i r s t d e t e r i o r a t e d and t h e n g r a d u a l l y improved. T h i s was m a i n l y due t o s c o r i n g o f t h e s t e e l s h a f t , which h a d b e e n r u b b i n g a g a i n s t a bronze bush w i t h a d i a m e t e r o f 3 mm a n d l e n g t h 4 mm a t s l i d i n g s p e e d 0.078 m / s a t 0 . 0 0 9 MPa p r e s s u r e . The f i n a l r e p l i c a s were a l s o studi e d w i t h t h e s t y l u s i n s t r u m e n t b o t h b e f o r e assembly and a t t h e e n d of t h e experiment. To e l u c i d a t e s u r f a c e v a r i a t i o n s and modificat i o n due t o w e a r , t h e d i g i t i z e d s u r f a c e p r o f i l e s w e r e a n a l y s e d on a computer. P l o t s o f t h e s p e c t r u m a n d t h e a u t o c o r r e l a t i o n f u n c t i o n of t h e r e p l i c a p r o f i l e a t t h e e n d of t h e e x p e r i n e n t c l e a r l y showed t h e d i s p l a c e m e n t o f t h e s p e c t r u m toward l o n g e r w a v e l e n g t h s and t h e smoothing o r r e d u c t i o n o f f i n e i r r e g u l a r i t i e s . Thus t h e s u r f a c e o f t h e b o r e w a s m o d i f i e d by wear s o a s t o h a v e l o n g waves i n s t e a d o f s h o r t o n e s . These l o n g waves w e r e m o s t l y due t o t h e f i n e projections produced on t h e s u r f a c e by t h e w e a r . I n v e s t i g a t i o n s i n t o t h e wear o f m i n i a t u r e s y s t e m s a r e v e r y i m p o r t a n t b u t a t t h e moment t h e r e i s l i t t l e c h o i c e o f method. The n e e d t o d e v e l o p m o r e a c c u r a t e a n d p r o d u c t i v e methods i s e v i d e n t . Dismantling a system f o r c l o s e examination is u n d es ir ab le because t h e removal o f wear d e b r i s from t h e r u b b i n g r e g i o n s e r i o u s l y a f f e c t s t h e wear p r o c e s s .
331
8,4,
THERMAL EFFECTS T o measure t h e t e m p e r a t u r e i n t h e f r i c t i o n r e g i o n of m i n i a t u r e
s y s t e m s i s q u i t e d i f f i c u l t . The e l e m e n t s are s m a l l and t h e temper a t u r e i n c r e a s e s are n o t u s u a l l y hirjh. The m e a s u r i n g e l e m e n t s s h o u l d be l o c a t e d a s n e a r a s p o s s i b l e t o t h e r u b b i n g r e g i o n , s o t h e y need t o b e veq7 s m a l l . The p r e s e n t m e a s u r i n g s y s t e m s , o f which t h e r e a r e n o t t o o m y , can be d i v i d e d i n t o c o n t a c t ( o r q u a s i - c o n t a c t ) a n d n o n - c o n t a c t s y s t e m s ; an example o f t h e f o r m e r i s t h e t h e r m o c o u p l e s y s t e m , a n d o f t h e l a t t e r , t h e i n f r a r e d - t h e r m o m e t r y s y s t e m . The t h e r m o c o u p l e s y s t e m h a s b e e n u s e d t o d e t e r m i n e t h e t e m p e r a t u r e rise i n s t e e l polymer j o u r n a l b e a r i n g s (refs. 174, 3 6 1 ) . The t h e r m o c o u p l e w a s p l a c e d i n a 2 nun d i a m e t e r h o l e d r i l l e d a l o n g t h e a x i s o f t h e s t e e l j o u r n a l ( F i g . 8 . 2 8 ) . A t h e r m i s t o r h a s a l s o b e e n s a t i s f a c t o r i l y app l i e d i n s t e a d of t h e Cu-constantan thermocouple. I n f r a r e d - t h e r m o m e t r y c a n b e used f o r d e t e r m i n i n g t h e s u r f a c e t e m p e r a t u r e of e x t r e m e l y s m a l l r e g i o n s o f r u b b i n g c o n t a c t ( r e f s . 578, 5 7 9 ) . I t h a s been s u c c e s s f u l l y u s e d t o measure t h e s u r f a c e t e m p e r a t u r e of a s t e e l j o u r n a l i n i n v e s t i g a t i o n s c a r r i e d o u t on s t e e l - p o l y m e r b e a r i n g s i n ETH Ziirich ( s e e F i g . 8 . 1 3 and r e f .
178).
The Williamson i n f r a r e d - t h e r m o m e t e r h a s b e e n u s e d f o r measurements i n t h e r a n g e of 25 t o 1 3 O o C w i t h an a c c u r a c y of 2 3OC. Temperatures have been measured on a r e a s o f d i a m e r e r
Q
4 mm.
The B a r n e s I n f r a r e d R a d i o m e t r i c Microscope (Model RP4 a l i q u i d nitrogen-cooled
-
2 A ) with
indium antimonide (InSb) d e t e c t o r has
been used t o s t u d y t h e s u r f a c e t e m p e r a t u r e d i s t r i b u t i o n i n t h e c o n t a c t a r e a of t h e sphere d i s c t r i b o l o g i c a l system p r e s e n t e d i n F i g . 8.29 ( r e f . 5 7 8 ) . The d e v i c e c o n s i s t s o f a f i x e d specimen ( e . 9. s p h e r e , c y l i n d e r ) l o a d e d a g a i n s t a t h i n , o p t i c a l l y - f l a t ,
sap-
phire (A1203) rotating disc, transparent t o infrared radiation. S i n c e t h e t a r g e t s p o t s i z e ( 1 3 t o 3 6 ,um) i s much smaller t h a n t h e a c t u a l e l a s t i c o r p l a s t i c area o f c o n t a c t , i n f o r m a t i o n o n s u r f a c e t e m p e r a t u r e d i s t r i b u t i o n can be o b t a i n e d . Loads o f between 0 . 1 and 1 0 N and s l i d i n g s p e e d s r a n g i n g from
t o 1 0 m/s can be a p p l i e d
i n t h i s a p p a r a t u s . R a d i a t i o n , e m i s s i v i t y , f r i c t i o n , and a r e a o f c o n t a c t can be measured a n d s u r f a c e damage i n f o r m a t i o n o b t a i n e d from s c a n n i n g e l e c t r o n m i c r o g r a p h s o f t h e worn s u r f a c e s .
332
F i g . 8.28. System used f o r t h e e s t i m a t i o n o f t h e temperature i n c r e a s e i n r u b b i n g r e g i o n o f m i n i a t u r e s t e e 1 -polymer j o u r n a 1 b e a r i ngs
.
Infrared microscope
Reflectire objective lens
F i g . 8.29. Contact geometry i n r o t a t i n g s a p p h i r e d i s c - i n f r a r e d microscopy system ( r e f . 578).
333 The a c c u r a t e d e t e r m i n a t i o n o f s u r f a c e t e m p e r a t u r e s i n t r i b o l o g i c a l p r o c e s s e s from r a d i a n c e measurements i s a d i f f i c u l t t a s k w i t h many p i t f a l l s and s o u r c e s o f errors. T h e r e are t h e e r r o r s ass o c i a t e d w i t h i n f r a r e d measurements i n g e n e r a l , w i t h t h e p r e s e n c e o f t h e s a p p h i r e i n t h e a f o r e m e n t i o n e d c a s e , and w i t h t h e s l i d i n g c o n t a c t p r o c e s s i t s e l f . A l i s t o f p o t e n t i a l problems and f a c t o r s t o c o n s i d e r , t o g e t h e r w i t h a p p r o p r i a t e comments o r recommendations,
i s g i v e n i n r e f . 578. A c c u r a t e measurements o f i n c r e a s e s i n s u r f a c e t e m p e r a t u r e a r e v i t a l t o o u r understanding o f m i n i a t u r e systems because t h e s e i n creases a r e l i k e l y t o p l a y an i m p o r t a n t r o l e n o t o n l y . i n t h e mechanisms o f f r i c t i o n and wear b u t a l s o i n t h e f a i l u r e o f l u b r i c a n t f i l m s and i n t h e f o r m a t i o n i n s i t u o f p r o t e c t i v e f i l m s on s o l i d s u r f a c e s (see C h a p t e r s 4 and 5 ) .
8,5, QUALITY OF LUBRICANTS a . 5 . 1 . INTRODUCTION
A N D COATINGS
EP I LAMES)
The e v a l u a t i o n o f t h e l u b r i c a n t s and c o a t i n g s ( e p i l a m e s ) before, d u r i n g and a f t e r o p e r a t i o n i n m i n i a t u r e s y s t e m s i s v e r y i m p o r t a n t . The e v a l u a t i o n o f t h e l u b r i c a n t p r o p e r t i e s b e f o r e u s e i s q u i t e s i m p l e b e c a u s e o f t h e r e l a t i v e l y l a r g e volume o f l u b r i c a n t available f o r s t u d y . The d i f f i c u l t y a r i s e s i n t h e d e s i g n o f s i m u l a t o r s f o r t h e i n v e s t i g a t i o n and p r e d i c t i o n o f l u b r i c a n t b e h a v i o u r i n r e a l t r i b o l o g i c a l systems. To d e t e r m i n e t h e q u a l i t y o f a l u b r i c a n t d u r i n g o r a f t e r u s e i n
a m i n i a t u r e s y s t e m i s v e r y d i f f i c u l t due t o t h e e x t r e m e l y s m a l l amounts i n v o l v e d - a s l i t t l e as 10-7g f o r t h e l u b r i c a t i o n o f a m i n i a t u r e watch b e a r i n g . The i d e n t i f i c a t i o n o f t h e p h y s i c a l and chemical p r o p e r t i e s o f used (aged) l u b r i c a n t i n m i n i a t u r e systems i s a problem which h a s n o t y e t been s a t i s f a c t o r i l y s o l v e d .
The a n a l y s i s of t h e q u a l i t y o f t h e c o a t i n g ( e p i l a m e s ) on a m i c r o e l e m e n t , e s p e c i a l l y i n i n d u s t r i a l c o n d i t i o n s ( m a s s production) ,
i s a l s o v e r y d i f f i c u l t . The q u a l i t y o f t h e e p i l a m e s i g n i f i c a n t l y a f f e c t s t h e d u r a b i l i t y of r u b b i n g e l e m e n t s and t h e e f f i c i e n c y o f lubricants.
334 8.5.2.
LUBRICITY OF LUBRICANTS
The l u b r i c i t y o f a l u b r i c a n t i s i t s main t r i b o l o g i c a l p r o p e r t y and y e t t h i s p r o p e r t y h a s b e e n n e v e r p r o p e r l y d e f i n e d . The l u b r i c i t y i s i m p o r t a n t of c o u r s e when boundary l u b r i c a t i o n c o n d i t i o n s o b t a i n i n a t r i b o l o g i c a l s y s t e m . From t h e many p r o p o s e d d e f i n i t i o n s o f l u b r i c i t y , t h e one d e f i n i n g i t as t h e r e s i s t a n c e of t h e boundary f i l m t o r u p t u r e ( r e f s . 5 8 0 , 581) seems t h e m o s t a d e q u a t e . The b u i l d i n g o f t h e boundary f i l m as t h e s o r p t i o n p r o c e s s can be e x p r e s s e d a s t h e v a r i a t i o n o f t h e thermodynamic p o t e n t i a l o f t h e s y s t e m ( r e f s . 582, 533, 8 3 1 ) . A G = AH where
G
-
T AS
(8.15)
is t h e thermodynamic p o t e n t i a l . , H t h e e n t h a l p y , T t h e t e m -
p e r a t u r e and S i s t h e e n t r o p y . The s o r p t i o n r e l a t e d t o t h e mass (m) and t i m e ( t ) u n i t s is - -AG (8.16) mt and s i m i l a r l y t h e d e s o r p t i o n i s + &? ( 8 . 1 6 a ) . The boundary f i l m ' s mt r u p t u r e loads are t h e e x t e r n a l s p e c i f i c l o a d (p) a n d s l i d i n g s p e e d ( v ) . The p r o d u c t p - v d e f i n e s t h e e x t e r n a l e n e r g y ( r e l a t e d t o t h e s u r f a c e and t i m e u n i t s ) i n t r o d u c e d i n t o t h e s y s t e m . The boundary f i l m w i l l b e d u r a b l e o n l y when a t t h e l e a s t p.v =
G m t
(8.17)
and p and t are c o n s t a n t . I t i s p o s s i b l e t o conclude t h a t t h e l u b r i c i t y a t t h e f o r m i n g o f
t h e boundary f i l m can be e x p r e s s e d a s t h e t h e r m a l e f f e c t r e l a t e d t o t h e s o r b a t e mass and t i m e u n i t (J.mol-'.s-') and a t t h e r u p t u r e p r o c e s s a t t h e u n i t power r e l a t e d t o t h e s u r f a c e u n i t (W/m 2 ) . The a f o r e m e n t i o n e d d e f i n i t i o n s can b e u s e d f o r t h e q u a n t i t a t i v e e v a l u a t i o n of t h e l u b r i c i t y o f l u b r i c a n t s . F i s c h e r ' s p r o p o s i t i o n ( r e f s . 584, 535, 882) f o r t h e l u b r i c i t y L is:
(8.18) where f i s t h e f r i c t i o n c o e f f i c i e n t of u n l u b r i c a t e d s y s t e m a n d f
i
t h e f r i c t i o n c o e f f i c i e n t o f l u b r i c a t e d system. The l u b r i c i t y can be t h e r e f o r e q u a n t i t a t i v e l y e v a l u a t e d on t h e b a s i s of t h i s f o r m u l a . The d i s a d v a n t a g e o f t h e above l u b r i c i t y def i n i t i o n i s t h a t the reduction i n t h e f r i c t i o n c o e f f i c i e n t brought a b o u t by t h e l u b r i c a n t d o e s n o t d e s c r i b e c o m p l e t e l y i t s l u b r i c a -
335 tion properties. V a r i o u s e x p e r i m e n t a l methods have b e e n u s e d f o r t h e e v a l u a t i o n o f t h e l u b r i c i t y o f a l u b r i c a n t , a l l o f which c a n be r e d u c e d gene r a l l y t o t h e i n v e s t i g a t i o n o f t h e f r i c t i o n a n d / o r wear o f a l u b r i c a t e d b e a r i n g . The pendulum method a l r e a d y d e s c r i b e d i n C h a p t e r 8.2.2 i s o f t e n used f o r t h e i n v e s t i g a t i o n of t h e l u b r i c i t y o f i n s t r u m e n t o i l s . The 4 - s p h e r e and 5 - s p h e r e a p p a r a t u s w i t h s p h e r e s o f 3 mm, 5 mm ( u p p e r s p h e r e ) a n d 6 mm i n d i a m e t e r r e s p e c t i v e l y h a s
a l s o been s a t i s f a c t o r i l y a p p l i e d , q i v i n g r e s u l t s which are compar a b l e w i t h t h e r e s u l t s o f t h e pendulum method ( r e f s . 296-300,
882).
F i s c h e r d e v i s e d t h e two-sphere a p p a r a t u s shown i n F i g . 8.30 ( r e f s . 584, 5 8 5 ) .
SCOIR,
\
,indicator
I'
SPl.ing
F i g . 8.30. Two-sphere apparatus f o r t h e determination o f the l u b r i c i t y of l u b r i c a n t s .
The two s t e e l s p h e r e s 1 ( 5 mm i n d i a m e t e r ) a r e p r e s s e d a g a i n s t t h e
s t e e l p l a t e 2 . The t o r q u e ( f r i c t i o n c o e f f i c i e n t ) n e e d e d f o r t h e l u b r i c i t y from f o r m u l a ( 8 . 1 8 ) i s measured by t h e t o r s i o n s p r i n g 4 a n d i n d i c a t e d on t h e scale 5. The r o t a t i n g e l e m e n t s are t h e two s p h e r e s . The d i r e c t i o n o f r o t a t i o n o f t h e two s p h e r e s i s t h e same, as i s t h e i r r o t a t i o n a l s p e e d ( 6 0 r . p . m ) . The f r i c t i o n t o r q u e i s t r a n s m i t t e d t o t h e i n d i c a t i n g s y s t e m by t h e arm 6 . The l u b r i c i t y of i n s t r u m e n t o i l h a s b e e n e f f e c t i v e l y t e s t e d w i t h t h e U T I o r LSRH a p p a r a t u s d e s c r i b e d i n d e t a i l i n s e c t i o n s 8 . 2 . 2 and 8 . 2 . 3
( r e f s . 43, 44, 381, 3 8 2 ) . A s t a n d a r d b u t m i n i a -
t u r i z e d p i n ( o r s p h e r e ) -on d i s c o r s p h e r e - o n c y l i n d e r tribometers can a l s o b e u s e d t o e v a l u a t e t h e l u b r i c i t y o f i n s t r u m e n t o i l s (refs. 374 , 8 8 3 ) .
336 The e s t i m a t i o n o f t h e t h i c k n e s s o f a boundary f i l m o f l i q u i d o r t h e d e t e r m i n a t i o n o f i t s m e c h a n i c a l p r o p e r t i e s can b e a l s o applied t o t h e preliminary evaluation of t h e l u b r i c i t y of instrument oils (refs. 8.5.3.
582, 8 8 4 , 8 8 5 ) .
PHYSICOCHEMICAL PROPERTIES O F LUBRICANTS
I d e n t i f y i n g t h e p h y s i c a l and c h e m i c a l p r o p e r t i e s o f i n s t r u m e n t o i l s b e f o r e u s e i s q u i t e s i m p l e and t h e u s u a l methods d e s c r i b e d e l s e w h e r e (e.9. r e f . 9 ) c a n b e a p p l i e d . The p h y s i c o c h e m i c a l p r o p e r t i e s u s u a l l y d e t e r m i n e d f o r i n s t r u m e n t o i l s a r e ( r e f s . 5 8 , 59) : v i s c o s i t y a n d i t s v a r i a t i o n s a s a f u n c t i o n of t e m p e r a t u r e , s u r f a c e t e n s i o n ( s p r e a d i n u a b i l i t y ) , v o l a t i l i t y , d e n s i t y , r e f r a c t i v e index, c o l o u r , pour p o i n t , flammability ( f l a s h p o i n t ) , s a p o n i f i c a t i o n and n e u t r a l i z a t i o n numbers, and a s h c o n t e n t . Complex and i m p o r t a n t p r o p e r t i e s s u c h as l u b r i c i t y ,
ageinq, s t a b i l i t y a g a i n s t polymers,
e t c . , are d i s c u s s e d e l s e w h e r e ( s e e S e c t i o n s 8 . 5 . 2 ,
8.5.4
and 6 . 6 ) .
I n t h i s s e c t i o n , some methods w i l l b e d e s c r i b e d which c a n i d e n t i f y t h e p h y s i c a l a n d c h e m i c a l p r o p e r t i e s o f a l u b r i c a n t when i t s v o l w i s so s m a l l t h a t t h e a p p l i c a t i o n o f t h e u s u a l methods i s n o t pos-
s i b l e . I n a d d i t i o n t o t h e a f o r e m e n t i o n e d methods
,
some methods f o r
determining t h e s o l u b i l i t y parameters and d i e l e c t r i c p r o p e r t i e s of an o i l , so i m p o r t a n t f o r t h e s t u d y o f t h e t r i b o l o g i c a l p r o p e r t i e s of l u b r i c a t e d polymeric systems, w i l l be d i s c u s s e d . The d e t e r m i n a t i o n o f t h e v i s c o s i t y o f s m a l l amounts o f o i l ( e .
3
g. 0 . 5 mm ) i s d i f f i c u l t . A t y p i c a l v i s c o m e t e r can b e u s e d o n l y
when t h e volume of t h e o i l i s n o t smaller t h a n 1 0 0 mm3 ( r e f s . 586, 5 8 7 ) . The V e r s i n o m i c r o v i s c o m e t e r ( r e f . 5 8 8 ) , where t h e o i l l a y e r i s s h e a r e d between two s p h e r i c a l s u r f a c e s , c a n be u s e d f o r t h e e s t i m a t i o n o f t h e v i s c o s i t y of 20-70 mm3 of o i l w i t h v i s c o s i t y i n 2 6 10 P a - s . The r a d i u s o f t h e s p h e r e i s from t h e range o f 5 . 1 1 t o 4 . 7 6 mm and t h e h e m i s p h e r e 5.16 mm. The c a p i l l a r y methods f o r m e a s u r i n g v i s c o s i t y are g e n e r a l l y s u i t e d t o l a r g e volumes o f o i l , t h e y are u n s u i t a b l e f o r rheologically
-
n o n s t a b l e l i q u i d s , and t h e i r e f f e c t i v e r a n g e i s l i m i t e d . C a p i l l a r y micromethods which measure t h e flow o f an o i l d r o p i n a c a p i l l a r y t u b e which can b e i n c l i n e d from a v e r t i c a l p o s i t i o n c a n b e u s e d t o d e t e r m i n e t h e v i s c o s i t y o f smaller amounts o f o i l . The a c c u r a c y o f t h e v i s c o s i t y measurement u s i n g t h i s method i s a b o u t 3 % . The d i a -
mm a n d t h e l e n g t h o f t h e o i l d r o p a b o u t 40 mm. T h i s c a p i l l a r y micromethod, w i t h t h e capillary
m e t e r of t h e c a p i l l a r y tube i s 0.01-0.5
3 37
t u b e p l a c e d i n a t h e r m o s t a t e d m e t a l l i c c o v e r , h a s been used f o r t h e e s t i m a t i o n of t h e v i s c o s i t y change i f o i l i r r a d i a t e d w i t h r a y s ( F i g . 8.31; r e f . 6 1 4 ) .
Fig. 8.31. Capillary microviscometer.
The k i n e m a t i c v i s c o s i t y o f a s m a l l amount o f o i l ( 1 mm 3
h a s been
c a l c u l a t e d u s i n g t h e formula 'J=
where
'r,
2 d sinv 3 2 v d ( l + a')
(8.19)
i s t h e k i n e m a t i c v i s c o s i t y , vd t h e s p e e d of o i l f l o w , g
t h e g r a v i t y a c c e l e r a t i o n , d t h e d i a m e t e r o f c a p i l l a r y tube, F i g . 8.31, a' o i l drop, m;
-
p-
Couette c o r r e c t i o n (a'=
p/1
sin?
,
1
-
y-
see
length of
constant =
The o t h e r c a p i l l a r y method u s e f u l when t h e volume o f o i l i s v e r y s m a l l ( < 0 . 5 mm 3 ) i s a l s o b a s e d on P o i s e l l e ' s l a w f o r a Liquid column of l e n g t h 1, moving a t a v e l o c i t y v i n a c y l i n d r i c a l c a p i l l a r y of d i a m e t e r d: d2A (8.20) 32 v p l i s t h e dynamic v i s c o s i t y o f o i l and a p t h e p r e s s u r e d i f ??=
where
7
338 f e r e n t i a l . T h i s formula can be used o n l y when l / d i s h i g h enough, e.g.
l/d
30 ( r e f s . 589-592).
Another d e v i c e f o r e s t i m a t i n g t h e v i s c o s i t y o f very s m a l l volumes o f o i l i s p r e s e n t e d i n s c h e m a t i c form i n F i g . 8.32 a ( r e f s . 590-592).
F i g . 8.32. Scheme o f t h e d e v i c e f o r d e t e r m i n i n g v i s c o s i t y o f microvolumes of o i l (a) and t h e s t r e a m l i n e s of o i l flow i n the c a p i l l a r y tube ( b ) .
The g l a s s t u b e 1 i s f i x e d t o t h e moving t a b l e of a microscope. The c a p i l l a r y tube 2 w i t h t h e oil t o be t e s t e d i s p l a c e d i n t o t h e p a r t i t i o n 3. The p r e s s u r e d i f f e r e n t i a l between t h e two s i d e s of t h e p a r t i t i o n i s measured w i t h t h e manometer 4 . The d i r e c t i o n o f t h e moving a i r streams i s c o n t r o l l e d w i t h t h e v a l v e s 6 . The v e l o c i t y v o f t h e o i l column i n t h e c a p i l l a r y t u b e i s e s t i m a t e d by measuring t h e t i m e needed f o r t h e d i s p l a c e m e n t o f t h e o i l between two dashes on t h e s c a l e i n t h e viewing area o f t h e microscope. The s t r e a m l i n e s o f t h e o i l moving i n s i d e t h e c a p i l l a r y t u b e a r e shown i n F i g . 8.32 b. The maximum s p e e d i s r e a c h e d i n the a x i s o f t h e t u b e . Owing t o t h e a c t i o n o f s u r f a c e f o r c e s t h e s t r u c t u r e o f l i q u i d s i n f i n e c a p i l l a r i e s may d i f f e r from t h e i r s t r u c t u r e i n b u l k . I n v e s t i g a t i o n s have shown t h a t t h e v i s c o s i t y i n t h e c a p i l l a r y o f l i q u i d s such a s l u b r i c a t i n g o i l s i s e q u a l t o t h e i r b u l k v i s c o s i t y when t h e d i a m e t e r of t h e q u a r t z c a p i l l a r y t u b e i s 2 ,um o r l e s s ( r e f s . 590, 5 9 1 ) . The s m a l l e s t volume o f o i l needed f o r t h e e s t i mation o f t h e v i s c o s i t y by t h e c a p i l l a r y method can be r e d u c e d
3 39
t h e r e f o r e t o a b o u t 2. lo-’
mm
3
.
F i r s t task is t h e accurate determination of t h e diameter of t h e m i c r o c a p i l l a r y used. T h i s d i a m e t e r can be d e t e r m i n e d i n a s i m p l e manner from t h e c a p i l l a r y p r e s s u r e a s t h e p r e s s u r e o f a i r o r i n e r t g a s compressed by a l i q u i d , o r by measuring t h e gas p r e s s u r e p needed t o form and d e t a c h a bubble from t h e c a p i l l a r y end immersed i n a l i q u i d ; n i t r o g e n may be used f o r t h i s p u r p o s e ( r e f . 5 9 1 ) . Then t h e d i a m e t e r d can be worked o u t i n t h e f o l l o w i n g way: (8.21) where 6,,
is t h e s u r f a c e t e n s i o n o f o i l .
The c a p i l l a r y r a d i u s measured by t h i s method d i f f e r s from t h e act u a l d/2 by t h e t h i c k n e s s h o f t h e f i l m c o a t i n g t h e c a p i l l a r y s u r f a c e : d = do
+
2h.
The d i a m e t e r o f a c a p i l l a r y t u b e a t d > 5 ,um can h e measured by t h e use o f m i c r o s c o p i c methods. The measuring system p r e s e n t e d i n F i g . 8.33 i s a m o d i f i c a t i o n of
the
above-described d e v i c e f o r t h e d e t e r m i n a t i o n o f o i l visco-
s i t y by t h e c a p i l l a r y method ( r e f s . 5 9 1 , 5 9 2 ) .
I
A
F i g . 8.33. Diagram o f t h e d e v i c e for d e t e r m i n i n g v i s c o s i t y of small volume o f o i l . 1 - quartz c a p i l l a r y , 2 pressure chamber, 3 - o i l ampoule, 4 microscope.
-
An empty, s e a l e d q u a r t z c a p i l l a r y 1 i s cemented w i t h epoxy r e s i n
o r a s i m i l a r a d h e s i v e i n t o t h e s t o p p e r o f a h i g h - p r e s s u r e chanter 2.
340 A f t e r cementing, t h e c a p i l l a r y ends a r e trimmed, one o f them b e i n g t h e n i n s e r t e d i n t o t h e ampoule 3 c o n t a i n i n g t h e o i l t o b e t e s t e d , and t h e o t h e r i n t o t h e h i g h - p r e c s u r e chamber. The h i g h - p r e s s u r e chamber communicates w i t h a pneumatic s y s t e m by means o f which t h e gas p r e s s u r e i n t h e chamber can be chanqed r a p i d l y . The p r e s s u r e
i s measured by a s e t of s t a n d a r d gauges which are i n t e r c h a n g e a b l e f o r d i f f e r e n t ranges. The movements o f t h e meniscus o f t h e o i l f i l l i n g t h e c a p i l l a r y a r e o b s e r v e d w i t h a long-focus microscope by t h e d a r k background method. The t i m e o f t r a v e l can be measured w i t h a s t o p w a t c h . When t h e f r e e end o f t h e c a p i l l a r y comes i n t o c o n t a c t w i t h t h e o i l i n t h e ampoule, t h e o i l i s sucked i n t o t h e c a p i l l a r y i f t h e g a s p r e s s u r e i n t h e c a p i l l a r y po < pc ( p c b e i n g t h e c a p i l l a r y p r e s s u r e ) o r
i s f o r c e d o u t i f p > p , ; when p = p, t h e meniscus s t o p s . The c a p i l l a r y p r e s s u r e po i s r e g i s t e r e d on approach t o e q u i l i b r i u m from b o t h d i r e c t i o n s ( a t p > p c and p < p c )
, the
advancing meniscus always
moving a l o n g a w e t t i n g f i l m l e f t b e h i n d by t h e r e c e d i n g meniscus. The e q u a l i t y o f t h e measured v a l u e s o f pc shows t h a t t h e r e i s no wetting h y s t e r e s i s . A t a pressure s l i g h t l y exceeding t h e c a p i l l a r y p r e s s u r e , t h e l i q u i d i s f o r c e d o u t o f t h e c a p i l l a r y s l o w l y so t h a t t h e f o r m a t i o n and detachment of a g a s bubble from t h e c a p i l l a r y end can be observed and t h e minimum p r e s s u r e po needed f o r t h i s can be measured. The v i s c o s i t y o f o i l can be c a l c u l a t e d u s i n g formula ( 8 . 2 0 ) . The p r e s s u r e d i f f e r e n t i a l Ap = p,
-
p l . The p r e s s u r e p1 i s n o t
e q u a l t o t h e p r e s s u r e r e c o r d e d by t h e gauge b e c a u s e t h e r e i s a p r e s u r e drop due t o
movement o f t h e gas column i n t h e c a p i l l a r y .
To d e t e r m i n e p1 t h e e q u a t i o n o f gas movement i n a c a p i l l a r y f o r Knudsen numbers K = 0 . 0 0 1
-
0 . 1 may be u s e d :
(8.22)
9 g the viscosity of t h e g a s , 1 t h e l e n g t h o f t h e gas column, and F; = 1.38 A i s t h e co9 e f f i c i e n t of s l i p ( A being t h e f r e e path o f t h e gas molecules).
where p i s t h e gas p r e s s u r e i n t h e chamber 2 ,
For t h e c a p i l l a r i e s from 2 0 t o 0 . 0 2 ,um i n d i a m e t e r , K = 0.003-0.03 ( r e f . 591). Simultaneous solutAon o f e q u a t i o n s ( 8 . 2 0 ) and ( 8 . 2 2 ) g i v e s dL(pc - p) (8.23) l1V = 32 7
34 1
where lI i s t h e e f f e c t i v e l e n g t h of t h e l i q u i d column a n d
(8.24)
Usually 1 > 1 /3. 9
Then t h e d i f f e r e n c e between l 1 and 1 i s n o t
more t h a n a f e w p e r p e r c e n t . S i n c e t h e t e r m of t h e e q u a t i o n (8.24)
i s s m a l l compared t o 1, and as t h e v i s c o s i t y of a i r depends l i t t l e on t h e p r e s s u r e , t h e dependence o f v l l on p when pc i s c o n s t a n t should be p r a c t i c a l l y l i n e a r . A microviscometer (rheometer) f o r i n v e s t i g a t i n g q u a n t i t i e s of
around 0 . 5 mm 3 of o i l i s shown i n F i g . 8 . 3 4 . I t i s b u i l t on t h e b a s i s o f an e l e c t r i c i t y c o u n t e r . A l a y e r o f t h e o i l t o b e i n v e s t i g a t e d i s s h e a r e d between two p l a n e s , i n t h i s case a r o t a t i n g c y l i n d e r and a s a p p h i r e p l a t e . The r a n g e of v i s c o s i t i e s which can b e measured i s from 1 0 0 t o 1 0 0 0 0 m P a s s . The r e p r o d u c i b i l i t y o f t h e measurements w i t h t h i s t y p e of a p p a r a t u s i s a b o u t 1 0 % .
Fixing cone Steel ball d, 1 mm
--
Sapphire bearing
Body
F i g . 8.34. Diagram o f microviscometer (rheometer for dealing with volumes of oil o f around 0.5 mi
(ref.389)
342
The c o n f i g u r a t i o n o f t h e s u r f a c e s w i t h t h e s h e a r e d o i l l a y e r between them i s v e r y i m p o r t a n t . S t u d i e s o f p l a n e - p l a n e ,
sphere-in-
- s p h e r e and c o n e - p l a t e s u r f ace c o n f i g u r a t i o n s h a v e shown t h a t when a small amount of non-newtonian
l i q u i d ( e . g . a n a g e d o i l ) i s shear-
ed between them, t h e c o n e - p l a t e c o n f i g u r a t i o n i s o p t i m a l . The
s h e a r i n g speed i n a rheometer o f t h i s k i n d is c o n s t a n t and t h e t o r q u e s u f f i c i e n t l y l a r g e t o b e measured. A d i a g r a m of a cone-plate m i c r o v i s c o m e t e r f o r d e a l i n g w i t h o i l volumes d 1 mm3 i s p r e s e n t e d i n F i g . 8.35. The c o n e i s t h e r o t a t i n g e l e m e n t . I t makes p o s s i b l e s i m p l e a r a n g e i n g t h e p l a t e i n t h e r e l a t i o n t o t h e cone. The a d d i t i o n a l a d v a n t a g e of t h i s d e s i g n i s t h e f a c i l i t y w i t h which t h e p l a t e w i t h t h e o i l d r o p c a n be t h e r m o s t a t e d .
sdlk Plate
4
-
....................
Fig. 8.35. Cone-and-plate microviscometer (microrheometer)
.
The a n g l e
a
( F i g . 8 . 3 5 ) s h o u l d b e small (0(,<5O)
t o maintain a
, s h e a r t o r q u e Ms and can b e e s t i m a t e d u s i n g t h e f o l l o w i n g f o r m u l a s :
c o n s t a n t s h e a r i n g s p e e d . The s h e a r stress shear rate
fS
L=
N
Ms
=
3Ms 2 7 r3
2T w r3 2 3 ci c o s a! ( 1 -
(8.25)
- . 2) 2 0:
10-6
(8.26)
(8.27)
34 3
7 is
where
-
r
i n MPa, M i n m N - m ,
r a d i u s o f cone ( m )
w - a n g l e s p e e d o f cone ( r a d / s )
-
CC
'7 -
a n g l e ( F i g . 8.35) i n r a d v i s c o s i t y of l i q u i d ( P a - s ) . The s h e a r t o r q u e Ms c a l c u l a t e d f o r a cone o f r a d i u s 3 mm, 0
, and w = 8 7 . 3 r a d / s , was about 14.3 mN.m when t h e s p a c e between cone and p l a t e was f i l l e d t o t a l l y w i t h 1 mm3 of oil w i t h a=
1
v i s c o s i t y 50 P a - s . An i n t e r e s t i n g method f o r e s t i m a t i n g t h e v i s c o s i t y of s m a l l amounts o f o i l which is s u i t a b l e f o r d e t e r m i n i n g v i s c o s i t y a s a f u n c t i o n o f t h e d i s t a n c e from t h e w a l l i s t h e s o - c a l l e d blow-off method ( r e f s . 593, 594)
. The
method is a s f o l l o w s . One o f t h e walls
o f a channel formed by two p l a n e - p a r a l l e l p l a t e s i s c o a t e d w i t h a l a y e r o f l i q u i d . I f a c u r r e n t of a i r o r i n e r t g a s i s t h e n p a s s e d through t h e c h a n n e l , flow i s g e n e r a t e d i n t h e f i l m due t o a t a n g e n t i a l f o r c e i n d u c e d by t h e f l o w i n g g a s . The f i l m p r o f i l e becomes wedge-shaped, t h e s l o p e of t h i s wedge b e i n g dependent on t h e v i s c o s i t y o f t h e l i q u i d . I f t h e l i q u i d v i s c o s i t y o v e r t h e whole d i s t a n c e t o t h e w a l l i s i n v a r i a b l e , t h e s l o p e remains c o n s t a n t u n t i l t h e w e t t i n g boundary is r e a c h e d . The l i m i t a t i o n of t h i s method l i e s i n t h e e s t i m a t i o n o f t h e v i s c o s i t y of n o n - v o l a t i l e l i q u i d s a t t h e w e t t i n g boundary. I n a new m o d i f i c a t i o n of t h e blow-off method t h e l i q u i d f i l m c o a t s t h e moving p l a t e , s o t h e v i s c o s i t y can be measured a t t h e w e t t i n g
J
boundary. The d i s p l a c e m e n t o f t h e p l a t e w i t h l i q u i d f i l m a t v a r i ous speeds i n t h e o p p o s i t e d i r e c t i o n t o t h e blow-off
s t r e a m makes
it p o s s i b l e t o l o c a t e t h e s t a t i o n a r y p o s i t i o n o f t h e wedge-shaped
f i l m a t t h e some d i s t a n c e from t h e w e t t i n g boundary. The t h i c k n e s s o f t h e f i l m remains c o n s t a n t and may t h e r e f o r e b e p r e c i s e l y measured. A d e v i c e f o r d e t e r m i n i n g v i s c o s i t y by t h e blow-off
p r e s e n t e d i n F i g . 8.36
method i s
( r e f . 5 9 4 ) . The body 1 i s made of b r a s s and
covered w i t h a b r a s s l i d 2. The p l a t e 3 moves i n t h e r e c t a n g u l a r c u t o u t made i n t h e body. The speed may b e v a r i e d i n t h e r a n g e o f
mm/s. The l i d h a s two windows f o r a l a s e r beam ( t h e a n g l e between t h e l a s e r beam and h o r i z o n t a l p l a t e i s 38O) , and i s t h e r m o s t a t e d . The t h i c k n e s s o f t h e c h a n n e l i s 0 . 2 2 0 . 0 1 mm. A c u r r e n t o f g a s ( a i r o r n i t r o g e n ) i s p a s s e d t h r o u g h t h e chann e l t o blow-off t h e l i q u i d b e i n g t e s t e d . For measuring t h e p r e s s u r e g r a d i e n t , two narrow openings 9 i n t o t h e c h a n n e l a r e p r o v i d e d 5
0
t o 125.
344 w i t h c o n n e c t i o n s t o c o n n e c t a p r e s s u r e gauge ( t h e c o n n e c t i o n s a r e o m i t t e d from t h e f i g u r e ) . The blow-off
flow i s p r o v i d e d by a vac-
uum-cleaner o r vacuum pump. The p r e s s u r e g r a d i e n t i s c a . 40 mm o f w a t e r column p e r t h e 1 0 mm. The s h e a r stress i s c a . 4 P a .
F / -5
7 I
5
r
2
17
Fig. 8.36. Device f o r determining v i s c o s i t y by b l o w - o f f method. 1 - body, 2 lid, 3 moving p l a t e , 4 - d r i v i n g rod, 5 laser beam window, 8 - glass windows, 6 - connectors, 7 p l a t e w i t h prism, 9 - openings t o pressure g r a d i e n t measuring system.
-
-
-
The experiment i s c a r r i e d o u t a s f o l l o w s . A s u b s t r a t e , o p t i ‘ c a l l y p o l i s h e d t o g i v e a s u r f a c e f i n i s h q u a l i t y Ra< 0 . 0 1
am, is
coated with a f i l m of t h e l i q u i d t o be t e s t e d ( u s i n g a f r e s h l y drawn g l a s s r o d ) . Then t h e p l a t e 3 w i t h t h e c o a t e d f i l m i s q u i c k l y p l a c e d i n t h e d e v i c e . The l a s e r beam i s f i x e d a t a d i s t a n c e (xo) of 2 - 3 nun from t h e f i l m (see F i g . 8 . 3 7 ) . Then t h e p l a t e is moved a t a c o n s t a n t v e l o c i t y and t h e blow-off
c u r r e n t i s s e t up. The t i m e o f s i m u l t a n e o u s moving and blowing-off i s r e g i s t e r e d . While t h e p l a t e i s moving t h e t i m e f o r t h e l a s e r beam t o r e a c h t h e boundary of t h e f i l m i s measured and a f t e r t h i s t h e f i l m t h i c k n e s s from 0 t o h (see F i g . 8.37) i s c o n t i n u o u s l y measured, u s i n g t h e e l l i p s o m e t r i c method. F o r e v e r y chosen v e l o c i t y v o f t h e p l a t e , t h e t h i c k n e s s h o f t h e s t a t i o n a r y f i l m l a y e r i s d e t e r m i n e d . The v a l u e s o f v and h a r e p l o t t e d and t h e v i s c o s i t y
o f t h e l i q u i d as
a f u n c t i o n o f t h e d i s t a n c e from t h e w a l l ( p l a t e ) may b e c a l c u l a t e d as f o l l o w s ( r e f . 594) : r h
9=7
(8.28)
34 5
7
i s t h e s h e a r stress. The accuracy o f t h i s method o f d e t e r m i n i n g v i s c o s i t y i s a b o u t
where 15%.
F i g . 8.37. B l o w - o f f method w i t h moving p l a t e c o a t e d by l i q u i d f i l m (a) and t h e o r e t i c a l dependence o f measured f i l m t h i c k n e s s as a f u n c t i o n o f time t (b).
The t h r e e main t y p e s o f v-h c h a r a c t e r i s t i c c u r v e o f l i q u i d flow a r e shown i n F i g . 8.38 ( r e f . 5 9 5 ) . Curve 1 i s c h a r a c t e r i s t i c f o r t h e flow o f n o n p o l a r l i q u i d s w i t h poor i n t e r m o l e c u l a r i n t e r a c t i o n and r e l a t i v e l y r i g i d m o l e c u l a r c h a i n s . The v i s c o s i t y and s t r u c t u r e o f t h e l i q u i d a r e c o n s t a n t a l o n g t h e whole d i s t a n c e from t h e w a l l . T h i s flow p r o f i l e is c h a r a c t e r i s t i c of l i q u i d s such a s v a s e l i n e o i l . Curve 2 i n F i g . 8.38 is c h a r a c t e r i s t i c f o r l i q u i d s w i t h s m a l l i n t e r m o l e c u l a r i n t e r a c t i o n s and w i t h enough e l a s t i c m o l e c u l a r c h a i n s a d s o r b i n g on t h e s o l i d s u r f a c e . The v i s c o s i t y a s
a f u n c t i o n o f d i s t a n c e from t h e w a l l r e a c h e s a maAimum and a t a d i s t a n c e of 15-20 nm t h e v i s c o s i t y d e c r e a s e s t o t h e b u l k viscosity. T h i s i s c h a r a c t e r i s t i c of d i n e t h y l p o l y s i l o x a n e s . The t h i r d c u r v e i s c h a r a c t e r i s t i c f o r l i q u i d s w i t h h i g h i n t e r m o l e c u l a r interactions and w i t h r i g i d m o l e c u l a r c h a i n s . The i n t e r a c t i o n s between t h e l i q u i d molecules a r e o f t h e d i p o l e - d i p o l e t y p e . The i n f l u e n c e o f t h e f o r c e f i e l d o f t h e s o l i d phase r e a c h e s t o a d i s t a n c e o f a b o u t 40 nm
.
The above-described blow o f f method i s u s e f u l f o r d e t e r m i n i n g 3 t h e v i s c o s i t y o f s m a l l amounts o f o i l ( c 0 . 5 mm ) and a t t h e same
t i m e c h a r a c t e r i z e s t h e v i s c o s i t y i n t h e t h i n boundary f i l m , which
346
i s v e r y i m p o r t a n t f o r a n a l y s i s of t h e e f f e c t s of l u b r i c a t i o n on t h e t r i b o l o g i c a l p r o p e r t i e s o f m i n i a t u r e systems.
h
E
c
v
Fig. 8.38. P r o f i l e s o f f l o w v e l o c i t i e s o f various l i q u i d s . 1 non-polar l i q u i d , 2 - poor p o l a r l i q u i d , 3 - p o l a r l i q u i d .
-
There a r e many methods f o r i d e n t i f y i n g t h e s u r f a c e t e n s i o n o f o i l . U n f o r t u n a t e l y , t h e m a j o r i t y of t h e s e methods cannot be app l i e d when t h e volume of l i q u i d ( o i l ) i s 1 mm3 o r less ( r e f . 5 9 6 ) . The e x i s t i n g methods can be d i v i d e d i n t o s t a t i c methods ( c a p i l l a r y method, l a y i n g ( s e s s i l e ) d r o p o r gas-bubble,
hanging d r o p o r gas
bubble , immersed p l a t e methods) o r q u a s i - s t a t i c methods ( r i n g method, drop weight o r volume method, maximum p r e s s u r e i n g a s bubb l e o r i n drop method, r o t a t i n g d r o p method, e t c . )
. Dynamical mth-
ods ( o s c i l l a t i n g s t r e a m s , o s c i l l a t i n g d r o p ) a r e n o t p r a c t i c a l . The most s u i t a b l e methods f o r e s t i m a t i n g t h e s u r f a c e t e n s i o n o f a s m l l amount o f o i l ( e . g . one d r o p ) a r e t h e l a y i n g ( s e s s i l e ) d r o p and hanging o r d r o p w e i g h t methods. The t h e o r e t i c a l p r i n c i p l e o f t h e s u r f a c e t e n s i o n measurements
i s r e p r e s e n t e d by t h e L a p l a c e e q u a t i o n :
34 7
(8.29) where Ap i s t h e d i f f e r e n c e i n p r e s s u r e s from e x t e r n a l a n d i n t e r n a l space,
y1
and
y2
t h e d e n s i t y of t h e f i r s t and s e c o n d p h a s e , g t h e
g r a v i t y a c c e l e r a t i o n , z t h e d i s t a n c e ( i n v e r t i c a l d i r e c t i o n ) between p o i n t on t h e l i q u i d s u r f a c e and some f i x e d r e f e r e n c e p o i n t ,
E12
the
s u r f a c e t e n s i o n o f l i q u i d on t h e i n t e r f a c e a n d R1 and R2 a r e t h e main r a d i i o f c u r v a t u r e a t f i x e d p o i n t s on t h e s u r f a c e . The l a y i n g (sessile) d r o p method i s based on i d e n t i f y i n g t h e s h a p e of t h e d r o p l a i d on t h e h o r i z o n t a l p l a t e ( F i g . 8 . 3 9 ) .
F i g . 8.39. L i q u i d d r o p ( 1 ) on h o r i z o n t a l p l a t e ( 2 ) .
laying (sessile)
The s u r f a c e t e n s i o n o f t h e l i q u i d can b e c a l c u l a t e d u s i n g t h e formula ( r e f . 5 9 6 ) :
(8.30) where h i s t h e d i s t a n c e between t h e d r o p t o p and e q u a t o r i a l p l a n e
(see Fig. 8 . 3 9 ) . Formula ( 8 . 3 0 ) i s a p p l i c a b l e o n l y f o r r e l a t i v e l y l a r g e d r o p s and t h e margin o f measurement e r r o r may b e o v e r 3%. Many r e s e a r c h p a p e r s have been d e v o t e d t o p e r f e c t i n g t h e above formula t o g i v e more a c c u r a t e s u r f a c e t e n s i o n measurements. D o r s e y ' s p r o p o s i t i o n
i s p a r t i c u l a r l y i n t e r e s t i n g ( r e f . 5 9 6 ) . I n s t e a d of t h e d i s t a n c e h , Dorsey p r o p o s e s measuring t h e d i s t a n c e H ( s e e F i g . 8 . 4 0 ) : H = x0 t g a 1
- yo
(8.31)
348
F i g . 8.40. Diagram of l i q u i d d r o p and c o o r d i n a t e s f o r a p p l i c a t i o n o f Dorsey (x,y) and lvaschenko ( x ’ , y ’ ) method o f determining surface tension.
I v a s c h e n k o a n d Eremyenko ( r e f . 597) u s e d a m e a s u r i n g microsaape t o estimate t h e c o o r d i n a t e s x3, x4, z 4 a n d z 5 , and t h e a n g l e (
see F i g .
X1
8.40) a n d c a l c u l a t e d t h e d i s t a n c e H from t h e f o l l o w i n g
relationship: (8.32) U s i n g t h e e s t i m a t e d d i s t a n c e H a n d t h e measured r a d i u s o f t h e d r o p i n t h e e q u a t o r i a l p l a n e , r , t h e s u r f a c e t e n s i o n of t h e l i q u i d c a n b e c a l c u l a t e d by a p p l y i n g t h e Dorsey f o r m u l a :
2
a= r where
0’05200 0.41421
-r -
a = (P2
-
12 Yl)
-
0.12268
+
0.0481(:
-
0.4121)
- c a p i l l a r i t y constant.
(8.33)
34 9 A t good w e t t a b i l i t y of t h e s o l i d s u r f a c e by t h e
o i l under in-
v e s t i g a t i o n , t h e d r o p formed o n t h e s o l i d s u r f a c e may h a v e n o eq u a t o r i a l p l a n e ( F i g . 8 . 4 1 ) . I v a s c h e n k o a n d Eremyenko ( r e f . 597) h a v e e s t i m a t e d t h e c o o r d i n a t e s o f t h e t a n g e n t s which form a n g l e s r e l a t i v e t o t h e v e r t i c a l a x i s y i n a case l i k e t h i s .
o f 45O and 60'
F i g . 8.41. Diagram o f l i q u i d d r o p f o r d e t e r m i n i n g s u r f a c e t e n s i o n by lvaschenko method.
The s u r f a c e t e n s i o n can b e e v a l u a t e d u s i n g T a b l e 8 . 1 (from r e f . 597). T h i s method can b e u s e d o n l y when 0.3900
where
and HsOO are t h e d i s t a n c e s from d r o p t o p t o t h e c o r n e r s o f
t h e 45O and 60'
a n g l e s (see Fig.
8 . 3 7 ) . Deryabin e t a l .
( r e f . 598)
h a v e a p p r o x i m a t e d t h e s h a p e of t h e d r o p w i t h a n e l l i p s o i d a n d c a l c u l a t e d t h e e q u a t o r i a l r a d i u s r a n d H (see a b o v e ) :
H =
where x
yl,
2 2 x2 y 1
-
2
2
x1 y2
(8.34)
x 2 and y 2 a r e t h e c o o r d i n a t e s o f two p o i n t s ( i n a
p l a n e of symmetry) on t h e w e t t i n g l i n e o f t h e d r o p ( F i g . 8 . 4 0 ) . Once r and H h a v e b e e n e s t i m a t e d t h e s u r f a c e t e n s i o n o f t h e l i q u i d forming t h e d r o p can be c a l c u l a t e d u s i n g t h e a p p r o p r i a t e r e l a t i o n s h i p s f o r a d r o p w i t h i t s e q u a t o r l a y i n g on a s o l i d s u r f a c e .
W
TABLE 8.1 VALUES OF THE FUNCTION
=
H450
H60°’H450 0.390 39 1 392 393
394 395 396 39 7 398 399 400 40 1 40 2 40 3 40 4 405 406 407 40 8 409 41 0 41 1 412 41 3 41 4 415 416 41 7
cn
2
0
1
H60°
f(r) (see Fig. 8.41) 450
2
(a
3
2.4428 2.2842 2.1395 2.0106 1 .8953 1.7886 1.6946 1.6094 1.5279 1.4537 1.3826 1.3200 1.2610 1 .2075 1.1570 1.1079 1.0636 1.0219 9825 9442 9094 8757 8442 8149
2.4265 2.2689 2.1261 1.9993 1.8841 1.7768 1.6856 1.6010 1.5193 1.4463 1.3762 1.3138 1.2559 1 .2027 1.1519 1.1031 1.0594 1.0175 9786 9405 9059 8727 8416 8120
7838
7363
7865 7594 7338
71 16
7090
2.4758 2.3147 2.1672 2.0333 1.9179 1 .8089 1.7141 1 .6266 1.5436 1.4681 1.3956 1.3326 1.2710 1.2175 1 .1665 1.1174 1.0719 1.0303 0.9904 9517 9165 8823 8512 8207 7919 7644
2.4592 2.2994 2.1531 2.0220 1 .go67 1.7986 1.7043 1.61 79 1.5353 1.4610 1.3891 1.3263 1 .2660 1.2123 1.1619 1.1127 1.0678 1.0261 3864 9470 91 30 8788 8478 8177 7891 761 7
7387 7139
0
FOR DETERMINING SURFACE TENSION O F LAYING OIL DROP
7568 7311 7068
4 2.4102 2.2539 2.1127 1.9874 1 .8728 1.7691 1.6768 1.5928 1.51 18 1.4389 1-3699 1.3075 1.2500 1 .2980 1.1468 1.0983 1 .0548 1.0130 9746 9368 9026 8695 8390 8086 7809 7539 7285 70 47
-
see formula (8.33))
5 2.3939 2.2393 1. O m 1.9756 1.8616 1.7593 1.6680 1.5846 1 .SO43 1.4314 1.3637 1.3012 1 .2440 1.1924 1.1418 1.0936 1.0501 1.0090 9706 9332 899 1 8663
8358 8058 7789 7515 7258 7026
6 2.3779 2.2246 2.0847 1.9637 1.8505 1.7503 1.6594 1.5764 1.4969 1A240 1.3574 1.2949 1 .2383 1.1867 1.1368 1.0889 1.0461 1.0052 9668 9300 8958 86 28 8327 8031 7761
7489 7232 7004
7
8
2.3461 2.2099 2.0696 1.9518 1.8405 1.7415 1.6512 1.5682 1.4897 1.4166 1.3513 1.2885 1.2331 1.1815 1.1319 1.0845 1.0423 1.0016 9631 9270 8925
2.3621 2.1956 2.0597 1.9406 1.8294 1.7327 1.6431 1.5601 1.4825 1.4095 1.3450 1.2822 1.2280 1.1763 1.1270 1.0801 1.0384 0.9980 9593 9235
8598
8571 8266
8296 8004 7730 746 1 7210 6979
8893 7977 7700
7436 7188 6956
9 2.3301 2.1814 2.0447 1.9293 1.8191 1.7238 1.6349 1.5520 1.4753 1.4027 1.3388 1.2760 1.2228 1.1712 1.1222 1.0760 1.0344 0.9944
9555 9199 8860 8542 8237 7947 7672 741 1 7162 6932
TABLE 8.1 (cont.)
H60dH450 0.418 41 9 420 42 1 42 2 42 3 42 4 425 426 427 428 429 43 0 43 1 432 433 434 435 436 43 7 438 439 440 44 1 442 443 444 445 446 447 448
1
0
2
3
4
5
~~
0.6908 66 85 6467 6256 6061 5880 5695 5525 5349 5195 5039 4900 4759 4624 4493 4362 4240 4124 4006 3897 379 1 3689 3593 3495 340 1 3314 3223 31 37 3059 298 1 2904
6
7
8
9
6772 6556 63 40 61 44 5952 5766 5594 5419 5258 5103 495 4 4817 4678 45 42 441 4 4288 4172 4051 3937 3834 3731 363 1 3535 3437 3348 326 1 31 70 3091 3013 2934 2857
6750 6533 6320 6122 5934 5749 5576 5401 5243 5084 4940 480 1 4663 4529 4400 4277 41 59 4040 3926 3820 3721 3621 3526 3430 3340 3252 3161 3082 3006 29 26 2849
6728 651 2 6298 61 00 5917 5728 5558 5333 5224 5063 4927 4787 4652 4518 4388 4265 41 48 4027 3916 3812 371 1 361 3 3516 3420 3332 3242 3151 3074 2998 2919 2841
6706 6489 6277 6081 1 goo 571 1 5542 5366 5209 5050 4915 4773 4639 4505 43 74 4252 41 37 401 7 3907 3802 3700 3603 3505 3412 3323 3233 31 45 3067 2989 291 2 2834
~
6884 6662 6443 6237 6042 586 1 5677 5506 5335 5178 5026 4884 4747 46 08 448 1 4350 4229 4113 3996 3887 3782 3680 3582 3484 3393 3304 3214 31 30 3051 2973 2896
6860 6638 6420 6219 6024 5842 566 1 5489 5321 5166 5013 4872 4736 4597 446 7 4337 4219 41 0 0 3984 3876 3771 3670 3574 347’1 3386 3296 3205 31 22 3043 2966 2887
6836 661 7 6399 6198 6006 5822 56 44 5469 5307 5151 4996 4857 4720 4579 4455 4325 4206 4087 3972 3866 3762 3660 3565 346 4 3378 3287 3196 31 14 3035 2958 2879
681 4 6596 6378 6180 5988 5802 5627 5452 5292 5135 4985 4845 4704 4567 4440 431 3 41 93 4075 3960 3855 3750 3650 3556 3455 3367 3278 3188 3106 3028 2950 2871
6794 6576 6358 6162 5970 5784 5612 5436 5 276 51 19 4970 4833 4693 4554 4428 4300 41 82 4062 3948 3845 3741 3641 3545 34 46 3356 3269 31 79 3098 3022 29 42 2864
TABLE
8.1
(cont.)
H60dH450
0
0.449 450 45 1 45 2 453 454 455 456 45 7
0.2827 275 1 2682 2612 2546 2479 241 8 2356 2298
3
2674 2605 2540 2472 241 2 2350 2293
2534 2466 2406 2344 2287
6
2806 2730 2660 2591 2528 2460 2400 2338
-
2798 2723 2655 2584 2521 2454 2394 2332
2789 2716 26 49 2577 2514 2448 2388 2327
278 1 271 1 2640 257 0 2507 2442 2382 2321
1
7 2774 2704 2631 2564 2499 2436 2375 2315
1
8 2766 2697 2624 25 58 2493 2430 2369 2309
1
9 2759 2690 2618 2552 2486 2424 2362 2303
35 3
The main advantage o f t h e l a y i n g d r o p method i s t h e independence o f t h e e s t i m a t e d s u r f a c e t e n s i o n o f t h e c o n t a c t a n g l e . The b e s t equipment used t o e v a l u a t e t h e n e c e s s a r y q u a n t i t i e s a s s u r e s a c curacy of t h e s u r f a c e t e n s i o n e s t i m a t i o n t o w i t h i n 0 . 5 % ( r e f . 597). The d r o p weight method i s one o f t h e most common f o r d e t e r m i n i n g t h e s u r f a c e t e n s i o n even though is t h e o r e t i c a l p r i n c i p l e s a r e n o t c l e a r . The method i s b a s e d on t h e f a c t t h a t t h e volume AV o f a d r o p of l i q u i d t h a t i s formed e x t r e m e l y s l o w l y and t h a t d e t a c h e s from t h e t i p of a t u b e o f d i a m e t e r d (see F i g . 8 . 4 2 ) i s g i v e n by (8.36) where f c i s a c o r r e c t i o n f a c t o r .
A F i g . 8.42. Drop f o r m a t i o n as a f u n c t i o n o f wettabi 1 i t y o f tube material.
The v a l u e o f t h e c o r r e c t i o n f a c t o r i s a much-discussed subj e c t : its approximate v a l u e 0 . 6 1
( r e f . 5 9 6 ) . Another problem is t h e
shape of t h e d r o p (see F i g . 8 . 4 2 ) . The s h a p e o f t h e d r o p i s d e t e r mined by t h e w e t t a b i l i t y o f t h e t u b e by t h e l i q u i d . A t good w e t t a b i l i t y t h e d r o p s a r e l a r g e ( F i g . 8.42 a ) and a t p c o r w e t t a b i l i t y t h e y a r e s m a l l ( F i g . 8.42 d ) . I n c a l c u l a t i n g t h e s u r f a c e t e n s i o n w i t h formula ( 8 . 3 6 ) , t h e c h o i c e o f t h e v a l u e of d , t h e d i a m e t e r o f t h e t u b e , p o s e s a problem. The i n v e s t i g a t o r s who developed t h i s
354 method a d v i s e a p p l y i n g e i t h e r t h e i n t e r n a l o r t h e e x t e r n a l r a d i u s o f t h e t u b e depending on t h e good o r poor w e t t a b i l i t y o f t h e t u b e s u r f a c e by t h e l i q u i d . I n s h o r t , i t may be concluded t h a t t h e d r o p weight method f o r e s t i m a t i n g s u r f a c e t e n s i o n i s f a r i n f e r i o r t o t h e l a y i n g d r o p method d i s c u s s e d above. The s u r f a c e t e n s i o n o f v e r y s m a l l volumes o f o i l may b e e s t i mated by u s i n g t h e same s y s t e m used f o r d e t e r m i n i n g v i s c o s i t y and p r e s e n t e d i n F i g . 8.33.
I f t h e c a p i l l a r y d i a m e t e r i s measured ( e .
g . by m i c r o s c o p i c method) a t known p r e s s u r e p , t h e s u r f a c e t e n s i o n o f t h e o i l may be e s t i m a t e d u s i n g e q u a t i o n ( 8 . 2 1 ) . The i n t e r a c t i o n s between t h e o i l and t h e s o l i d s u r f a c e a r e simply determined by t h e c o n t a c t a n g l e . The s t a t i c v a l u e o f t h e c o n t a c t a n g l e d e t e r m i n e s t h e u n i t work o f a d h e s i o n Wa between t h e o i l and t h e l u b r i c a t e d s u r f a c e :
wa where
= 612(1
+ cos e )
6,, i s t h e s u r f a c e t e n s i o n o f o i l and
(8.37) 0 is t h e c o n t a c t angle.
of t h e o i l t o s p r e a d from t h e a r e a o f r u b b i n g c o n t a c t b u t a l s o r e s u l t s i n a decrease i n t h e work o f a d h e s i o n s o t h e o i l d r o p can " s l i d e " on t h e s o l i d s u r f a c e . As mentioned i n Chapter 6 . 2 . 2 , t h e optimum c o n t a c t a n g l e i n t h e l u b r i c a t i o n o f m i n i a t u r e e l e m e n t s i s around 25-40°. Measuring t h e c o n t a c t a n g l e i s v e r y s t r a i g h t f o r w a r d . I t can be determined simply u s i n g an o p t i c a l microscope. I f t h e d i a m e t e r do and h e i g h t ho o f t h e o i l d r o p ( F i g . 8.43) a r e measured, t h e cont a c t a n g l e 0 i s g i v e n by t h e formula A n i n c r e a s e d c o n t a c t a n g l e r e d u c e s t h e tendency
13 = 2 a r c t g
2hO -
(8.38)
d0 The c o n t a c t a n g l e can a l s o b e d i r e c t l y measured u s i n g an i n s t r u m e n t microscope ( r e f . 5 9 9 ) . The h e a t g e n e r a t e d d u r i n g t h e f r i c t i o n p r o c e s s a f f e c t s t h e t e m p e r a t u r e o f t h e r u b b i n g e l e m e n t s . The c o n t a c t a n g l e s h o u l d a l s o b e known as a f u n c t i o n o f ambient t e m p e r a t u r e . T h i s can b e d e t e r mined u s i n g t h e d e v i c e p r e s e n t e d i n F i g . 8.44
( r e f . 6 0 0 ) . The o i l
d r o p i s p l a c e d on t h e m e t a l l i c p l a t e 3 which i s f i x e d t o t h e copp e r p i n 7 w i t h t h e screw 6 . The copper p i n i s h e a t e d by t h e elec-
t r i c h e a t e r 8. The t e m p e r a t u r e o f t h e o i l d r o p i s measured w i t h a thermometer p l a c e d i n a p o c k e t s o l d e r e d t o t h e p l a t e 3 and f i l l e d w i t h Wood's a l l o y .
355
Fig. 8.43. Parameters needed f o r d e t e r m i n i n g the co nta ct angle.
F i g . 8.44. Diagram o f t h e device f o r d e t e r m i n i n g t h e c o n t a c t a n g l e as a f u n c t i o n o f t e mpe rat ure . 1 - a n g l e measuring e yep i ece micrometer, 2 microscope, 3 p l a t e , 4 - o i l drop, 5 l i g h t i n g , 6 - screw, 7 - copper e l e c t r i c heater. pin, 8
-
-
-
356 A s i m p l e t e c h n i q u e and t h e n e c e s s a r y a p p a r a t u s f o r measuring
c o n t a c t a n g l e s a r e d e s c r i b e d i n r e f . 6 0 1 . The p r o c e d u r e i s b a s e d on o b s e r v a t i o n o f t h e a n g l e a t which l i g h t from a p o i n t s o u r c e i s r e f l e c t e d from t h e s u r f a c e o f a l i q u i d d r o p a t i t s c o n t a c t p o i n t w i t h a s o l i d . The p r i n c i p l e o f t h e r e f l e c t i o n method i s shown i n
F i g . 8.45.
'
AC
Contact Angle
F i g . 8.45. P r i n c i p l e o f t h e r e f l e c t i o n method f o r determining the contact angle (ref. 601).
I f l i g h t from a p o i n t s o u r c e A s t r i k e s t h e d r o p B a t t o o low an a n g l e , no r e f l e c t i o n
i s s e e n a t t h e f o c a l p o i n t C ( F i g . 8.45 a ) .
A s t h e a n g l e i s i n c r e a s e d , a b r i g h t s t a r o f l i g h t suddenly a p p e a r s
a t t h e f o c a l p o i n t . T h i s l i g h t ( r e p r e s e n t e d by s o l i d a r r o w s ) i s r e f l e c t e d from t h e extreme edge of t h e d r o p . L i g h t s t r i k i n g t h e drop a t o t h e r p l a c e s (shown by dashed a r r o w s ) i s n o t v i s i b l e a t t h e f o c a l p o i n t ( F i g . 8.45 b ) . I f t h e a n g l e i s t o o h i g h (Fig. 8.45 c), l i g h t i s r e f l e c t e d t o t h e f o c a l p o i n t ( s o l i d a r r o w s ) b u t n o t from t h e edge o f t h e drop. A t t h e a n g l e where t h e r e f l e c t e d l i g h t j u s t appears, o r disappears, t h e angle of displacement o f t h e p o i n t
357
s o u r c e and f o c a l p o i n t (A-C)
from t h e v e r t i c a l e q u a l s t h e c o n t a c t
a n g l e ( F i g . 8.45 d ) . The a p p a r a t u s i t s e l f i s shown i n F i g 8 . 4 6 .
The g o n i o m e t e r h a s
a f l a t , a d j u s t a b l e h e i g h t t u r n t a b l e , on which t h e t e s t sample i s p l a c e d , and a l e v e l i n g screw t o keep t h e t a b l e s u r f a c e h o r i z o n t a l . L i g h t from a 6 - v o l t f l a s h l i g h t b u l b mounted i n a b l o c k a t t h e e n d of a l o n g , h i n g e d a r m i s r e f l e c t e d back from t h e d r o p s u r f a c e t o
a peep h o l e a d j a c e n t t o t h e l i g h t s o u r c e . The a r m i s r a i s e d by t u r n i n g a screw s e t i n t h e arm mounting b l o c k a t a 4 5 O a n g l e . The a n g l e o f e l e v a t i o n i s measured w i t h a p r o t r a c t o r . A small T e f l o n p l u g i n t h e screw t i p smooths t h e a c t i o n o f t h e screw a g a i n s t t h e
a r m . All machined p a r t s are made o f aluminium.
\leveling screw
F i g . 8.46. Apparatus f o r d e t e r m i n i n g t h e c o n t a c t a n g l e by t h e r e f l e c t i o n method ( r e f . 6 0 1 ) .
35 8 The t e c h n i q u e g i v e s r e s u l t s which a g r e e w i t h t h o s e o b t a i n e d by t h e u s u a l d r o p p r o f i l e method, w i t h i n e x p e r i m e n t a l e r r o r . The app a r a t u s d e s c r i b e d h a s t h e advantage of e a s e o f c o n s t r u c t i o n and o p e r a t i o n , ruggedness and low c o s t a l o n g w i t h p r e c i s i o n . Such t e c h n i q u e was s a t i s f a c t o r i l y used f o r t h e c o n t a c t a n g l e measuremm 3 ) ( r e f .
ments o f a f i v e - r i n g polyphenyl e t h e r ( d r o p s i z e 0.5-1.5
886).The s t a t i c c o n t a c t a n g l e does n o t c h a r a c t e r i z e c o m p l e t e l y t h e i n t e r a c t i o n s between s o l i d s u r f a c e and o i l . The tendency o f an o i l t o m i g r a t e from l u b r i c a t e d m i c r o b e a r i n g s can be b e t t e r i n v e s t i g a t e d by s t u d y i n g t h e s p r e a d i n g dynamics o f t h e o i l on t h e s u r f a c e o f t h e r u b b i n g element. For t h i s p u r p o s e , t h e c o n t a c t a n g l e as a funct i o n o f t i m e can be simply measured o r p h o t o g r a p h i c a l l y r e c o r d e d u s i n g t h e above d e s c r i b e d methods. The s p r e a d i n g o f o i l can be r e c o r d e d w i t h a cinecamera. The a u t h o r h a s s a t i s f a c t o r i l y a p p l i e d a 1 6 mm cinecamera t o t h e i n v e s t i g a t i o n o f t h e s p r e a d i n g v e l o c i t y o f i n s t r u m e n t o i l d r o p s on t h e s u r f a c e o f p l a s t i c b e a r i n g microelements ( r e f . 2 0 6 ) . The f i l m was a n a l y s e d s t e p by s t e p a f t e r a 1 5 - f o l d e n l a r g e m e n t . The accuracy o f t h e d e t e r m i n a t i o n o f t h e p o l y m e r - o i l p h a s e boundary was a b o u t 0 . 0 1 mm. The s o l i d - o i l boundary can be p r e c i s e l y determined by t h e u s e of i n t e r f e r e n c e microscopy o r e l l i p s o m e t r y ( r e f s . 6 0 2 , 603)
.
In the l a t t e r c a s e , t h e movement o f t h e edge o f o i l f i l m s less t h a n 1 0 0 nm t h i c k can be s t u d i e d ( r e f . 6 0 3 ) .
The s u r f a c e c r e e p o f o i l on e . g .
t h e r i n g of a r o l l i n g bearing
( r e f s . 887, 888) can be o b s e r v e d and photographed u s i n g u l t r a v i o l e t s t i m u l a t e d luminescence o f t h e o i l ( r e f s . 888, 8 8 9 ) . The u n i v e r s a l d e v i c e f o r d e t e r m i n i n g t h e s u r f a c e p r o p e r t i e s o f o i l by s t u d y i n g t h e s h a p e o f p r e s e n t e d i n F i g . 8.47
suspended
or
sessile
drops i s
( r e f . 5 9 6 ) . The p h o t o g r a p h i c a p p a r a t u s c a n
o f c o u r s e be r e p l a c e d by a cinecamera. The t e m p e r a t u r e o f t h e d r o p i s c o n t r o l l e d by flowing w a t e r . L o s s o f o i l by e v a p o r a t i o n i s an i m p o r t a n t p r o p e r t y . The e s t i -
mation of t h e e v a p o r a t i v i t y o f o i l i s b a s e d on t h e mass l o s s which occurs i n prescribed conditions : N = m1
-
m 1m2 1 0 0 %
where N i s t h e e v a p o r a t i o n loss i n %, o i l b e f o r e and a f t e r t e s t .
(8.39) and ml and m2 t h e m a s s o f
359
Fig. 8.47. Scheme o f the set-up f o r d e t e r m i n i n g s u r f a c e t e n s i o n by t h e suspended o r s e s s i l e drop method. 1 - photothermostat, -apparatus, 2 - drop, 3 4 - long-focus o b j e c t i v e , 5 - t u b u l e f o r drop forming, 6 - s y r i n g e , 7 teleobjecl i g h t f i l t e r , 9 - short-focus tive, 8 o b j e c t i v e , 10 - diaphragm, 1 1 cinelamp.
-
-
-
-
For t h e m a j o r i t y o f i n s t r u m e n t o i l s , t h e t y p i c a l methods f o r studying evaporation described elsewhere (e.g.
r e f . 9 ) can b e ap-
p l i e d . S p e c i a l t e c h n i q u e s however have a l s o been d e v i s e d f o r t h e e s t i m a t i o n o f e v a p o r a t i o n loss f o r i n s t r u m e n t o i l s . I n t h e ASTM method ( r e f . 5 6 5 ) 1 -I 0 . 2 g o f i n s t r u m e n t o i l i s p l a c e d i n a cleane d and weighed t e s t v e s s e l ( F i g . 8 . 4 8 ) . The t e s t v e s s e l i s t h e n weighed a g a i n w i t h t h e o i l i n i t ( w i t h an a c c u r a c y of 0 . 2 mg) and placed i n t h e test apparatus (Fig. 8.49).
The t e st t e m p e r a t u r e is
7OoC. The n i t r o g e n must flow through t h e t e s t v e s s e l a t a r a t e o f
3
14 mm /h t o p r e v e n t o i l o x i d a t i o n . The e x p e r i m e n t l a s t s f o r 1 0
d a y s , t h e t e s t v e s s e l b e i n g weighed f i r s t a f t e r 18 h o u r s and t h e n daily. The GOST t e s t ( r e f . 4 4 9 ) f o r d e t e r m i n i n g t h e e v a p o r a t i v i t y of i n s t r u m e n t o i l i s c a r r i e d o u t a s f o l l o w s . The o i l o r m e l t e d g r e a s e sample ( 7 d r o p s , i . e .
c a . 0.10-0.16
g) is placed i n t h e s p e c i a l
s t a i n l e s s s t e e l v e s s e l ( F i g . 8.50) and weighed w i t h an a c c u r a c y o f 0 . 2 mg. The v e s s e l i s i n c l i n e d u n t i l t h e bottom o f i t i s c o v e r e d
w i t h a t h i n l a y e r o f o i l . The v e s s e l i s t h e n weighed a g a i n . The v e s s e l is p l a c e d on t h e g l a s s p l a t e and p u t i n t o a l a b o r a t o r y dryi n g c a b i n e t . Vessels c o n t a i n i n g f a t t y o r m i n e r a l o i l s a r e k e p t i n t h e d r y i n g c a b i n e t f o r 4 h o u r s a t a t e m p e r a t u r e o f 5Ok2OC.
When s y n t h e t i c o i l s a r e t e s t e d , t h e d r y i n g t i m e is 2 4 h o u r s a t a temper a t u r e o f 75f2OC.
360
F-
I-
F
I
Fig. 8.48. Test vessel f o r determining evaposphere, r a t i o n r a t e o f instrument o i l s . A B - s p i r a l ( w i t h rod a t the end) t o ensure b e t t e r mixing o f n i t r o g e n w i t h o i l , C , D , E elements f o r prevention o f o i l foaming, F outflow.
-
-
36 1
F i g . 8.49. Apparatus for t e s t i n g e v a p o r a t i v i t y o f instrument o i l s .
F i g . 8.50. T e s t v e s s e l f o r d e t e r m i n ng e v a p o r a t i v i t y o f i n s t r u m e n t oils. 1 handle. v e s s e l made o f s t a i n l e s s s t e e l , 2
-
-
362 A f t e r t h e v e s s e l has been taken o u t t h e d r y i n g c a b i n e t , i t is
c o o l e d i n a d e s i c c a t o r t o a m b i e n t t e m p e r a t u r e and weighed w i t h a n a c c u r a c y o f 0 . 2 mg. A p r a c t i c a l test f o r determining e v a p o r a t i o n loss of i n s t r u -
ment o i l c a n b e carried o u t by l a y i n g 50 mg o f o i l , i n d r o p s o f a b o u t 1 mg, o n t o t h e b o t t o m o f a s t a i n l e s s s t e e l c u p ( r e f . 9 6 ) . The t e s t t e m p e r a t u r e s h o u l d b e k e p t a t 5OoC.
O i l s w i t h good eva-
p o r a t i o n r e s i s t a n c e lose less t h a n 1-2% of t h e i r i n i t i a l m a s s a f t e r 3 months of t e s t i n g . The d e n s i t y of t h e o i l must be known i f t h e s u r f a c e t e n s i o n i s t o be d e t e r m i n e d . I f it i s n e c e s s a r y t o d e t e r m i n e t h e s u r f a c e t e n s i o n w i t h a n a c c u r a c y o f 0 . 2 % , t h e d e n s i t y s h o u l d b e known w i t h a n a c c u r a c y of a b o u t 0 . 0 5 % . The p y c n o m e t r i c o r p y c n o m e t r i c - d i l a t o -
metric methods are t h e most u s e d and m o s t a c c u r a t e ( r e f s . 5 9 6 , 604, 6 0 5 ) . The m o s t a c c u r a t e d e t e r m i n a t i o n o f t h e d e n s i t y o f t h e o i l as a f u n c t i o n o f t e m p e r a t u r e i s g i v e n by t h e t w o - c a p i l l a r y pycnoneter- d i l a t o m e t e r s y s t e m i l l u s t r a t e d i n F i g . 8.51 ( r e f . 5 9 6 ) .
F i g . 8.51. Two-capi 1 l a r y pycnometer-di latometer f o r determining d e n s i t y o f o i l as a f u n c t i o n o f temperature. 1,g tubules, 2,4.6,8 control marks, 3 , 7 - c a p i l l a r i e s , 5 - o i l r e s e r v o i r .
-
-
The a p p a r a t u s i s s t a n d a r d i z e d u s i n g a c o n t r o l l i q u i d of known dens i t y . The c o n t r o l l i q u i d i s i n t r o d u c e d i n t o t h e v e s s e l 5 u n t i l t h e meniscus r e a c h e s t h e marks 4 and 6 . The e n d s 1 and 9 are h e r m e t i c a l l y c l o s e d and t h e pycnometer i n t h e n p l a c e d i n a t h e r m o s t a t f o r
36 3
2-3 h o u r s . The d i s t a n c e s
h l and h2 o f t h e meniscus from t h e con-
t r o l marks 2 and 8 are measured. The a v e r a g e v a l u e o f h l and h2 i s t h e n determined a t o t h e r t h e r m o s t a t i c a l l y - c o n t r o l l e d t e m p e r a t u r e s . A f t e r t h i s p r o c e d u r e t h e pycnometer i s p r e c i s e l y weighed, t h e cont r o l l i q u i d i s poured o u t and t h e pycnometer i s a g a i n weighed. Having t h e m a s s o f t h e master l i q u i d , t h e pycnometer’s e q u a t i o n can b e f o r m u l a t e d :
+
V1 = a
bhT
+
L
chT
+
...
(8.40)
where V 1 i s t h e volume of m a s t e r l i q u i d , h
T
i s t h e average value
of h l and h2 a t t h e d e t e r m i n e d t e m p e r a t u r e , and a , b and c a r e c o n s t a n t s d e t e r m i n e d f o r example by t h e
least
s q u a r e s method.
The d e t e r m i n a t i o n o f t h e d e n s i t y o f an o i l i s c a r r i e d o u t i n t h e same manner a s f o r t h e c o n t r o l l i q u i d . I f t h e v a l u e s o f h f o r a number o f t e m p e r a t u r e s a r e measured and t h e mass o f t h e o i l de-
p
termined, t h e d e n s i t y
’
o f o i l can be e s t i m a t e d u s i n g t h e formula
m = a
+
bhT
+
2 chT
where m i s t h e mass o f t h e o i l , hT t h e a v e r a g e v a l u e o f t h e d i s t a n c e s hl and h2 d e t e r m i n e d a t t h e t e m p e r a t u r e T I and a , b and c a r e t h e same c o n s t a n t s a s i n t h e pycnometer’s e q u a t i o n ( 8 . 4 0 ) . The a c c u r a c y o f d e n s i t y d e t e r m i n a t i o n by t h e above d e s c r i b e d
pycnometer-dilatometer
method i s about 0 . 0 4 % .
The d e t e r m i n a t i o n o f t h e s o l u b i l i t y p a r a m e t e r o f i n s t r u m e n t oils, p a r t i c u l a r l y o f t h o s e used f o r t h e l u b r i c a t i o n o f m e t a l -polymer o r polymer-polymer s y s t e m s , i s v e r y i m p o r t a n t ( s e e Chap-
ters 5 . 2 and 6 . 6 ) . The s o l u b i l i t y p a r a m e t e r & i n t r o d u c e d by Hildebrand i s d e f i n e d a s f o l l o w s : (8.42) E i s t h e energy o f v a p o r i z a t i o n t o a g a s a t z e r o p r e s VaP s u r e , i . e . , a t i n f i n i t e s e p a r a t i o n o f t h e m o l e c u l e s , and V i s t h e
where
molar volume. The s o l u b i l i t y p a r a m e t e r i s t h e s q u a r e r o o t o f t h e c o h e s i v e energy d e n s i t y and i d e n t i f i e s t h e s o l v e n c y b e h a v i o u r o f an o i l o r polymer. The s o l u b i l i t y p a r a m e t e r can be found when t h e e n e r g y of v a p o r i z a t i o n r e l a t e d t o t h e volume of e v a p o r a t e d o i l i s determined.
364 To d e t e r m i n e t h e energy o f e v a p o r i t a t i o n o f o i l , common methods used f o r e n t h a l p y e s t i m a t i o n can be a p p l i e d ( r e f s . 6 0 4 , 6 0 6 ) . The s o l u b i l i t y p a r a m e t e r can a l s o b e a p p r o x i m a t e l y e v a l u a t e d on t h e b a s i s o f t h e r e l a t i o n s h i p between i s o t h e r m a l compressibility,
8 , and s u r f a c e t e n s i o n , 6,,,
a s g i v e n by McGowan ( r e f . 6 0 7 ) :
3
X . 6T2
= 1.33
-
-9 10
(8.43)
where
(Vs
-
s p e c i f i c volume; Vs =
perature-constant)
.
71 ; 9 -
density; p
-
The c o m p r e s s i b i l i t y $ , s h o u l d b e i n t r o d u c e d i n Pa-’ t e n s i o n 6,
in
p r e s s u r e , T -tern and s u r f a c e
N/m.
The c o m p r e s s i b i i t y is i n v e r s e l y p r o p o r t i o n a l t o t h e c o h e s i v e 1 0 9 ) , and t h e r e f o r e t h e r e l a t i o n s h i p between
energy d e n s i t y ( r e f
t h e s u r f a c e t e n s i o n o f an o i l and t h e s o l u b i l i t y p a r a m e t e r
6
can
be e x p r e s s e d a s
d-zc
3
a
b12
(8.44)
where c can be t a k e n as a b o u t 0 . 6 5 . I t i s n e c e s s a r y t o e s t i m a t e t h e d i e l e c t r i c c o n s t a n t of an o i l
t o be a b l e t o p r e d i c t
i t s i n f l u e n c e a s a l u b r i c a n t on f r i c t i o n .
T h i s i s p a r t i c u l a r l y t r u e i n t h e c a s e o f l u b r i c a t e d metal-polymer o r polymer-polymer systems ( s e e Chapters 5.2 and 6 . 6 ) . The d i e l e c -
t r i c c o n s t a n t & o f an o i l can be determined from t h e r e l a t i o n between t h e c a p a c i t y C o f a condenser f i l l e d w i t h a sample o f t h e o i l and t h e c a p a c i t y Co o f t h e same condenser w i t h o u t o i l ( i n a vacuum o r a i r ) , i n t h e f o l l o w i n g way:
&=C
(8.45)
CO
Taking t h e r e l a t i v e v a l u e s o f t h e d i e l e c t r i c c o n s t a n t i n a vacuum t o be 1 and i n a i r 1 . 0 0 0 6 , t h e i n t r o d u c t i o n o f o i l i n t o t h e condenser i n c r e a s e s t h e c a p a c i t y by E - t i m e s . There a r e s e v e r a l methods f o r e s t i m a t i n g t h e d i e l e c t r i c con-
stant o f l i q u i d s but t h e volume o f l i q u i d needed i s u s u a l l y very
365
l a r g e , o v e r 1 0 0 0 mm3. The most common method u s e s a c y l i n d r i c a l c o n d e n s e r which i s d e s i g n e d t o make i t p o s s i b l e t o t h e r m o s t a t t h e l i q u i d sample ( r e f s . 6 0 8 , 6 0 9 ) . On t h e b a s i s o f t h e c y l i n d r i c a l c o n d e n s e r , t h e a u t h o r h a s cons t r u c t e d a p i e c e of a p p a r a t u s f o r d e t e r m i n i n g t h e d i e l e c t r i c cons t a n t of i n s t r u m e n t o i l s . The p r i n c i p l e o f t h e a p p a r a t u s i s ill u s t r a t e d i n F i g . 8.52
( r e f . 1 1 0 ) . The c y l i n d r i c a l c o n d e n s e r i s
formed by t h e s t e e l c y l i n d e r 1 and s t e e l t u b u l e 2 . Both t h e s e e l e ments are n i c k e l - p l a t e d .
The s p a c e between t h e b a s e 3 a n d t h e
t u b u l e 2 i s f i l l e d w i t h epoxy r e s i n . The i s o l a t i n g e l e m e n t s 4 and 5 are made o f T e f l o n . The c o n d e n s e r c a n b e h e r m e t i c a l l y c l o s e d w i t h screws 6 and 1 0 a n d p l a c e d i n a t h e r m o s t a t . The volume o f o i l r e q u i r e d i s a b o u t 1 7 0 mm 3
.
F i g . 8.52. Condenser f o r d e t e r m i n i n g d i e l e c t r i c p r o p e r t i e s o f i n s t r u m e n t o i l s . 1,2 - condenser f o r m i n g r o d ( 1 ) and t u b u l e ( 2 ) , 3 base, 4 epoxy r e s i n , 5 T e f l o n washers, 6 f i x i n g nut, 7 w i r e s , 8 screw, 9 T e f l o n s e a l , 10 - o i 1 samp 1 e
-
.
-
-
-
-
-
-
366
The c a p a c i t y C o f a c y l i n d r i c a l c o n d e n s e r can be e s t i m a t e d using t h e formula (8.46)
where :
-
EO
a b s o l u t e v a l u e o f d i e l e c t r i c c o n s t a n t ( d i e l e c t r i c permea b i l i t y o f vacuum):
-
E 1 Re,Ri
d i e l e c t r i c c o n s t a n t o f medium f i l l i n g t h e c o n d e n s e r l e n g t h of t h e condenser e x t e r n a l and i n t e r n a l r a d i i o f t h e c y l i n d r i c a l s u r f a c e of t h e condenser
I f t h e d i e l e c t r i c c o n s t a n t i s determined a t high f r eq u en cies ,
t h e i n d u c t a n c e a n d c a p a c i t y o f t h e measurement c i r c u i t s h o u l d be taken i n t o c o n s i d e r a t i o n . A diagram of t h e s u b s t i t u t i o n a l c i r c u i t f o r t h e measurement s y s t e m i s p r e s e n t e d i n F i g . 8.53.
The induc-
t a n c e can b e r e d u c e d by u s i n g v e r y s h o r t w i r e s between c o n d e n s e r and c a p a c i t y meter. I f i n d u c t a n c e L
‘5
0 , t h e reduced conductance
Y o f t h e measurement c i r c u i t can be worked o u t from t h e f o r m u l a
Y = - +1i w ( C W + C )
(8.47)
RO
where Ro
w
-
-
o i l resistance frequency
Cw
-
capacity of w i r e s
C
-
c a p a c i t y of c o n d e n s e r f i l l e d w i t h o i l .
Because t h e o i l resistance Ro i s u s u a l l y v e r y h i g h , t h e cond u c t a n c e Y i s g i v e n by Y
=iw
(Cw
+
I f two w i r e s of d i a m e t e r
a
W
C)
%
(8.48) and l e n g t h lww i t h t h e d i s t a n c e
betweem them a r e u s e d t o c o n n e c t t h e measurement c o n d e n s e r and
c a p a c i t y meter, t h e c a p a c i t y of t h e w i r e s c a n be c a l c u l a t e d u s i n g t h e formula
367
(8.49)
The d i e l e c t r i c c o n s t a n t
E
of t h e o i l s h o u l d b e e s t i m a t e d i n
t h i s way:
E=-
c
- cw
co-
(8.50)
cw
F i g . 8.53. S u b s t i t u t i o n a l c i r c u i t o f t h e system used f o r d e t e r m i n i n g d i e l e c t r i c p r o p e r t i e s o f i n s t r u m e n t o i l s . Ro o i l r e s i s t a n c e , Cw - capac i t y o f w i re s, C - c a p a c i t y o f condenser f i I l e d w i t h o i l , L - i n du ct a nce o f w i r e s .
-
I f t h e c o n d e n s e r p r e s e n t e d i n F i g . 8.52 i s u s e d , t h e d i e l e c t r i c
loss factor (loss t a n g e n t ) t a n $of a n i n s t r u m e n t o i l c a n also be determined. The most i m p o r t a n t t h e r m a l ( p o u r and f i r e p o i n t s ) , o p t i c a l ( r e f r a c t i o n index, c o l o u r ) , and chemical (chemical composition, n e u t r a l i z a t i o n and s a p o n i f i c a t i o n numbers) p r o p e r t i e s , t h e a s h c o n t e n t and o t h e r p r o p e r t i e s o f i n s t r u m e n t o i l s c a n be d e t e r m i n e d by t h e a p p l i c a t i o n of t h e s t a n d a r d methods and a n a l y t i c a l t e c f i n i w s u s e d i n a n a l y t i c a l c h e m i s t r y (see e . g .
r e f s . 9, 604).
T u r n i n g now t o t h e t e s t i n g of i n s t r u m e n t g r e a s e s , t h e i r main p h y s i c o c h e m i c a l p r o p e r t i e s c a n be e s t i m a t e d u s i n g s i m i l a r methods t o t h o s e u s e d f o r o i l s . However, t h e s p e c i a l p r o p e r t i e s o f g r e a s e s r e q u i r e t h e a p p l i c a t i o n of s p e c i a l methods. T h e s e p r o p e r t i e s are t h e i r d r o p p i n g p o i n t and t h e i r c o n s i s t e n c y ( p e n e t r a t i o n i n d e x ) .
36 8 The procedure g e n e r a l l y a p p l i e d f o r d e t e r m i n i n g t h e s e p r o p e r t i e s i n g r e a s e s can a l s o be used t o t e s t i n s t r u m e n t g r e a s e s . The g r e a s e ' s tendency t o b l e e d o r s e p a r a t e o i l i s i m p o r t a n t t o know because i n many a p p l i c a t i o n s , a s i n raceway o f a b a l l b e a r i n g , a d e g r e e o f o i l s e p a r a t i o n is n e c e s s a r y f o r s u c c e s s f u l operat i o n , and e . g .
i n some e l e c t r o n i c s a p p l i c a t i o n s z e r o o i l s e p a r a -
t i o n may be r e q u i r e d , e s p e c i a l l y w i t h s i l i c o n e l u b r i c a n t s ( r e f . 8 9 0 ) . A s p e c i a l t e s t h a s been d e v i s e d by Deckel GmbH, F.R.G.)
(Miinich,
f o r estimating t h e c a p i l l a r y e f f e c t s of instrument greases
which m i g r a t e from l u b r i c a t e d m i n i a t u r e s y s t e m s a s a r e s u l t o f t h i s phenomenon ( r e f . 9 ) . For t h i s t e s t two p o l i s h e d and c l e a n e d t h i c k f l a t s p r i n g s a r e used. The s p r i n g s a r e l a i d one on t o p o f t h e o t h e r and t h e n t h e upper one i s moved lengthways f o r a b o u t 10 mm and p r e s s e d o n t o t h e lower one w i t h a l o a d o f 4 . 3 N . Narrow g r e a s e s t r i p s a r e l a i d a l o n g t h e edges El and E2 ( F i g . 8 . 5 4 ) of t h e f l a t s p r i n g p l a t e s . A f t e r 2 4 h o u r s a t a t e m p e r a t u r e o f 2OoC t h e upper p l a t e i s c a r e f u l l y t u r n e d t h r o u g h 180' w i t h a n e e d l e and t h e o i l which has m i g r a t e d from t h e g r e a s e by c a p i l l a r y a c t i o n can be e s t i m a t e d i n p e r c e n t o f
t h e t o t a l surface of p l a t e contact.
El E2
Fig. 8.54. Determining capi 1 l a r y e f f e c t o f instrument greases ( r e f . 9 ) .
The micromethods a p p l i e d f o r t e s t i n g r e l a t i v e l y s m a l l amounts of grease
-
10-1 g ) have been r e p o r t e d i n r e f . 6 1 0 . The microsample o f g r e a s e i s a n a l y s e d i n f o u r s t a g e s , employing t h e
f o l l o w i n g a n a l y t i c a l methods: h i g h t e m p e r a t u r e vacuum f r a c t i o n a t i o n , t h e r m o a n a l y s i s , t h i n l a y e r chromatography, infrared s p e c t -
369 roscopy and mass s p e c t r o s c o p y . I n t h e f i r s t s t a g e o f t h e a n a l y s i s g e n e r a l i n f o r m a t i o n on t h i c k e n e r and d i s p e r s a l l i q u i d i s o b t a i n e d by means o f i n f r a r e d s p e c t r o s c o p y . I n t h e second s t a g e , t h i n l a y e r chromatography i s used t o e s t i m a t e t h e g e n e r a l q u a n t i t y o f g r e a s e components c h a r a c t e r i z e d by t h e i r chromatographic m o b i l i t y . For t h e chromatographic a n a l y s i s a b l e n d o f h e p t a n e and s u l phide e t h e r i n t h e proportion 6 : l
(by volume) i s a p p l i e d . The
m i x t u r e e n a b l e s t h e g r e a s e sample t o be d i v i d e d i n t o 8 o r 9 comp o n e n t s . The i n d i v i d u a l s o l v e n t s may be used t o g e t h e r w i t h b l e n d eluents. I n t h e t h i r d s t a g e , t h e components o b t a i n e d i n t h e second s t a g e a r e a n a l y s e d . S p e c i a l chromatographic methods ( d e t e r m i n a t i o n of f i x i n g indexes R f ,
choice of masters, a r t i f i c i a l blends, quali-
t a t i v e r e a c t i o n s w i t h v a r i o u s r e a g e n t s ) and a l s o methods o f molec u l a r a n a l y s i s ( i n f r a r e d and mass s p e c t r o s c o p y ) a r e a p p l i e d a t t h i s stage. I n t h e f o u r t h and f i n a l s t a g e , t h e r m o a n a l y s i s and h i g h - t e m p e r a t u r e vacuum f r a c t i o n a t i o n a r e a p p l i e d where t h e mass o f -7 g r e s p e c t i v e l y . s u b s t a n c e needed i s lo-* g and 10
-
A system f o r t h e e v a l u a t i o n of l u b r i c a n t s h a s been proposed i n
r e f . 6 1 1 . The same system may be adapted f o r d e t e r m i n i n g t h e q u a l i t y o f s u b s t a n c e s used f o r t h e l u b r i c a t i o n o f m i n i a t u r e s y s t e m s . 8 . 5 . 4 . EFFECTS O F INTERACTIONS I N LUBRICANT-RUBBING ELEMENTS-AMBIENT SYSTEM
The e f f e c t o f i n t e r a c t i o n s i n t h e l u b r i c a n t - r u b b i n g e l e m e n t s -ambient system p l a y a v e r y i m p o r t a n t r o l e i n t h e t r i b o l o g i c a l p r o c e s s e s which o c c u r i n t h e f r i c t i o n r e g i o n . These i n t e r a c t i o n s a f f e c t t h e q u a l i t y o f l u b r i c a n t ( a g e i n g ) and l e a d t o physico-chemi c a l m o d i f i c a t i o n s of t h e s u r f a c e o f t h e r u b b i n g e l e m e n t s . I t is r a t h e r d i f f i c u l t t o observe t h e ageing process o f a lu-
b r i c a n t during t h e a c t u a l o p e r a t i o n o f a microbearing because only a v e r y s m a l l amount o f t h e aged l u b r i c a n t can b e t a k e n from t h e m i c r o b e a r i n g f o r a n a l y s i s . The c r i t e r i a f o r t h e d e t e r m i n a t i o n o f t h e dynamics o f t h e a g e i n g p r o c e s s i n working m i c r o b e a r i n g s u s u a l l y include variations i n v i s c o s i t y , surface tension, r e f r a c t i o n i n d e x , and s a p o n i f i c a t i o n
( o r a c i d i t y ) number; t h e s o - c a l l e d age-
i n g number ( A Z ) may a l s o be d e t e r m i n e d (see Chapter 6 . 4 ) .
A micro-
q u a n t i t y o f aged o i l t a k e n d i r e c t l y from an o p e r a t i n g m i c r o b e a r i n g can be a n a l y s e d f o r p r a c t i c a l purposes by d e t e r m i n i n g t h e v i s c o s i t y and s u r f a c e t e n s i o n (by t h e methods d e s c r i b e d i n Chapter 8.5.3 )
3 70 and by d e t e r m i n i n g t h e a g e i n g number ( A Z ) u s i n g t h e method p m p s e d by Huber ( r e f . 4 4 4 ) . The a g e i n g number ( A Z ) of o i l t a k e n from an o p e r a t i o n a l microb e a r i n g i s based on an a n a l y s i s o f t h e v a r i a t i o n s i n I R s p e c t o grams. F o r s p e c t o g r a p h i c a n a l y s i s o n l y 0 . 5 mg o f o i l i s needed. T h i s m i c r o q u a n t i t y of o i l i s p l a c e d i n t h e s p e c i a l AgCl c u v e t t e shown i n F i g . 8.55 (manufactured by R I I C o f London, among o t h e r s ) .
Fig.8.55.
AgCl c u v e t t e
The m i c r o b e a r i n g c o n t a i n i n g t h e o i l i s i n t r o d u c e d i n t o t h e t o p p a r t o f t h e c u v e t t e and t h e o i l is e x t r a c t e d by c e n t r i f u g i n g . T h i s method i s i n e x p e n s i v e and a v o i d s t h e r i s k of l o s s e s and i m p u r i t y d u r i n g s o l v e n t removal. I t s d i s a d v a n t a g e i s t h a t t h e c u v e t t e deforms d u r i n g t h e c e n t r i f u g i n g and i t s dimensions change. Because t h e t h i c k n e s s o f t h e o i l l a y e r i s v e r y s m a l l ( e . g . 0.025 mm) a s m a l l change i n t h e c u v e t t e dimensions a f f e c t s t h e o i l volume needed. Data p r o c e s s i n g t e c h n i q u e s developed f o r t h e m a n i p u l a t i o n and enhancement o f I R s p e c t r o s c o p i c d a t a o f t h e s t u d y o f l u b r i c a n t o x i d a t i o n p r o d u c t s a r e d e s c r i b e d by Coates and S e t t i i n r e f . 891. O t h e r methods such as chemiluminescence measurement o f l u b r i c a n t s ( r e f . 892) and e l e c t r o n i c d e t e c t i o n of l u b r i c a n t o x i d a t i v e breakdown ( r e f s . 639, 893) have g r e a t p o t e n t i a l f o r t h e s t u d y o f ageing of instrument l u b r i c a n t s . O i l r e s i s t a n c e t o a g e i n g is u s u a l l y d e t e r m i n e d by
carryingout
some q u i c k tests u s i n g s p e c i a l methods and equipment. Three mthods
371 a r e a p p l i e d : s t a t i c , dynamic and mechanical-dynamic 6.4)
. These
(see Chapter
a c c e l e r a t e d tests e n a b l e t h e d u r a b i l i t y o f i n s t r u m e n t
o i l s t o b e e s t i m a t e d b e f o r e use when t h e o p e r a t i n g c o n d i t i o n s o f t h e system t h e y w i l l be l u b r i c a t i n g a r e known b e f o r e h a n d . I n t h e s t a t i c t e s t s , a g l a s s o r , more o f t e n , a b r a s s o r s t e e l cup i s used. Enough o i l i s p l a c e d i n t h e cup t o c o v e r t h e bottom w i t h a t h i n l a y e r ( u s u a l l y a b o u t 1 mm t h i c k ) . The d i a m e t e r o f t h e cup can b e up t o 1 0 0 mm ( r e f . 9 6 ) . The o i l i s t h e n t e s t e d i n a i r a t t h e r e q u i r e d t e m p e r a t u r e (40-5OoC f o r 2-3 months ( r e f . 9 6 ) 100°C
,
f o r 4 h o u r s ( r e f . 6 1 2 1 , o r 120-180°C f o r 8 weeks ( r e f . 1 0 6 ) ) .
Because o f t h e c a t a l y t i c e f f e c t s , t h i s t e s t i s v e r y s e v e r e , e s p e c i a l l y when a b r a s s cup i s used. The o t h e r s t a t i c method i s b a s e d on f i l l i n g a l a b o r a t o r y v e s s e l w i t h about 30 g o f o i l c o n t a i n i n g an a d d i t i o n o f copper o r i r o n powder. The t e s t l a s t s f o r 6 months at a t e m p e r a t u r e o f 4OoC i n f r e e flowing a i r . T h i s t e s t i s e s p e c i a l l y a p p l i e d when d e t e r m i n i n g t h e a c t i o n of a n t i o x i d a n t s t a b i lizers in o i l (ref. 96). A s i m i l a r t e s t , i n which t h e 1 0 m l o f o i l p l a c e d i n t h e g l a s s
l a b o r a t o r y v e s s e l c o n t a i n s 1 0 g o f polymer g r a n u l a t e , h a s been proposed by T i l l w i c h ( r e f s . 108, 4 6 5 ) . She s t u d i e d t h e e f f e c t o f polymer on t h e a g e i n g p r o c e s s o f v a r i o u s i n s t r u m e n t o i l s k e p t a t 6OoC o r ( b e t t e r ) 8OoC f o r 28 d a y s .
The o t h e r method f o r d e t e r m i n i n g t h e i n f l u e n c e of t h e polymer m a t e r i a l on t h e a g e i n g p r o c e s s o f i n s t r u m e n t o i l was d e s c r i b e d e a r l i e r by Huber ( r e f . 4 5 4 ) . The o i l samples ( a b o u t 20-30 mg) a r e placed i n a
covered g l a s s ( F i g . 8.56) c o n t a i n i n g polymer granulate
o r microelements. The glass v e s s e l i s t h e n k e p t a t 60 o r 1 0 0 ° C f o r 30 o r 60 days.
-
Polymer
, port icies Fig. 8 . 5 6 . Vessel f o r t e s t i n g o f e f f e c t o f (ref. 4 5 4 ) . polymer vapours on instrument o i l s
372 The dynamic method t o i n d u c e r a p i d a g e i n g i n i n s t r u m e n t o i l s was d e v i s e d by Baader. The method c o n s i s t s of immersing a copper p l a t e i n a b o u t 60 m l o f o i l a t an e l e v a t e d t e m p e r a t u r e i n t h e p r e s e n c e o f a i r . The Baader a p p a r a t u s i s p r e s e n t e d i n F i g . 8.57 ( r e f . 6 1 4 ) . The frequency of t h e copper p l a t e immersion i s 25 t i n ~ s p e r minute, i . e .
36000 ? 1 0 0 p e r day. The t e s t i s c a r r i e d o u t a t
80, 9 5 o r l l O ° C usually f o r a period of 1 2 days.
S W DIN 12 215-08
-Cone
NS 45127 see (DIN 12 242)
F i g . 8.57. Baader apparatus ( D I N 5 1 5 5 4 ) . 1 t e s t instrument, 2 - c o o l e r , 3 - p l a t e ’ s holder .
-
The most r e c e n t mechanical-dynamic methods f o r a c c e l e r a t e d t e s t i n g o f t h e a g e i n g r e s i s t a q c e o f i n s t r u m e n t o i l , d e v i s e d by D U r r and Gumz, a r e most i n t e r e s t i n g .
The o i l i s t e s t e d i n a s p e c i a l a p p a r a t u s where a mechanical l o a d i s a p p l i e d and t h e p r e s e n c e of wear d e b r i s i s e n s u r e d . The p r i n c i p l e of t h e UTI-apparatus dev i s e d by D i i r r a t S t u t t g a r t U n i v e r s i t y (F.R.G.) i s i l l u s t r a t e d i n
373 Fig. 8.58
b a s e d on r e f . 4 4 2
. The
e l e m e n t s 1 0 and 11 a r e made o f
s e l e c t e d m a t e r i a l s which rub and produce c a t a l y t i c a l l y a c t i v e wear d e b r i s . A i r o r o t h e r gas can b e p a s s e d t h r o u g h t h e o i l sample by means of t h e pump 2 . The d r i v e i s t r a n s f e r r e d t o t h e r u b b i n g element 1 0 v i a t h e t r a n s m i s s i o n box 3 , c l u t c h 4 and t h e s h a f t 7 placed i n t h e ceramic ( A 1 2 0 3 1 b e a r i n g s 6 and 8. Load i s a p p l i e d t o t h e rubbing e l e m e n t s by means of t h e w e i g h t s 5 . The o i l e v a p o r a t e s i n t h e v e s s e l 13, l i q u e f i e s i n t h e condenser 1 and flows back t o t h e v e s s e l through t h e c u t - o u t a l o n g t h e s h a f t . The t e m p e r a t u r e of t h e equipment i s c o n t r o l l e d by f l o w i n g w a t e r . The a p p a r a t u s h a s been used t o d e t e r m i n e t h e a g e i n g r e s i s t a n c e of v a r i o u s i n s t r u m e n t o i l s f o r p e r i o d s o f more t h a n 4 0 days a t 95OC.
F i g . 8.58. UTI-apparatus f o r r a p i d a g e i n g o f i n s t r u m e n t o i Is. 1 condenser, 2 pump, 3 - g e a r i n g box, 4 - c l u t c h , 5 w e i g h t s , 6.8 A1203 b e a r i n g s , 7 s h a f t , 9 - thermoelements o f rubbing p a i r , s t a t , 10,11 12 t e s t e d o i l sample, 13 - o i l v e s s e l .
-
-
-
-
-
-
-
374 The a p p a r a t u s d e v i s e d by Gumz ( r e f . 443) e n a b l e s t h e a g e i n g r e s i s t a n c e o f i n s t r u m e n t o i l s t o b e t e s t e d under c o n d i t i o n s o f impact l o a d i n g ( F i g . 8 . 5 9 ) . The d i s c 1 w i t h p r e s s e d - i n p i n s 2 r o t a t e s and t h e hammer 4 loaded w i t h t h e f l a t s p r i n g 3 s l i d e s inpact-
- w i s e on t h e p i n s . The d i s c w i t h t h e p i n s i s immersed i n t h e o i l . The p i n s and t h e hammer may b e manufactured from any m a t e r i a l w h i h s u i t s t h e purpose o f t h e t e s t . The o i l i s aged f o r 1 2 0 0 h o u r s a t 95OC. The r a t i o o f t h e o i l volume t o t h e a r e a of c o n t a c t withmetal 2 2 i s about 0.0018 m l / m m compared w i t h a b o u t 0 . 0 5 m l / m i n a worki n g c l o c k j o u r n a l b e a r i n g of 1 . 2 mm d i a m e t e r . I n Baader's t e s t t h i s r a t i o r e a c h e s a b o u t 0.032 ml/mm
2
.
1
1
I
F i g . 8.59. Gum-apparatus f o r a g e i n g o f instrument o i l s . 1 - disc, 2 pins, 3 - f l a t s p r i n g , 4 - harmer.
-
I n s t r u m e n t o i l s a r e used t o l u b r i c a t e i n s t r u m e n t s o p e r a t i n g under t h e i n f l u e n c e o f s t r o n g r a d i a t i o n ( e . g . i n n u c l e a r r e a c t o r s o r i s o t o p e h a n d l i n g ) . The a g e i n g r e s i s t a n c e o f o i l s under s u c h c o n d i t i o n s can b e i n v e s t i g a t e d by means o f t h e a c c e l e r a t e d tests described i n r e f . 614.
1 0 0 o i l samples w e r e t e s t e d i n aluminium
c a s e t t e s p l a c e d i n a r a d i a t i o n chamber ( 1 l i t r e ) where t h e r a d i a t i o n dose was 3 kGy/h and t h e t e m p e r a t u r e 3OoC, u s i n g t h e i s o t o p e 6 o C o a s a s o u r c e . The c r i t e r i a f o r d e t e r m i n i n g t h e e f f e c t o f r a d i -
a t i o n on o i l p r o p e r t i e s w e r e t a k e n t o be gas s e c r e t i o n , v i s c o s i t y and r e f r a c t i v e i n d e x changes. The v i s c o s i t y measurement method h a s been d e s c r i b e d i n Chapter 8.5.3. r a c y of
For t h e e s t i m a t i o n of t h e r e f r a c t i o n
i n d e x w i t h an accu-
an i n t e r f e r o m e t e r was used. The gas s e c r e t i o n was
3 75
determined u s i n g t h e s p e c i a l a p p a r a t u s shown i n F i g . 8 . 6 0 .
F i g . 8.60. Apparatus f o r s t u d y i n g gas secretion. 1 reservoir, 2 - capillary, 3 test-tube.
-
-
The g l a s s v e s s e l c o n s i s t i n g o f a r e s e r v o i r 1 and c a p i l l a r y 2 i s introduced i n t o the test-tube in a desiccator
3 and t h e whole a p p a r a t u s i s p l a c e d
o r vacuum chamber. A f t e r opening t h e chamber, t h e
o i l from t h e r e s t - t u b e goes up t h r o u g h t h e c a p i l l a r y and f i l l s t h e r e s e r v o i r . The gas s e c r e t i o n d u r i n g r a d i o l y s i s e x p e l s the equivalent volume of o i l . I f t h e q u a n t i t y o f s e c r e t e d gas is s m a l l , t h e f o l lowing e q u a t i o n i s v a l i d ( r e f . 6 1 4 ) : dV dt
dV dt
Vo
dt
(8.51)
where V
9
Vo t
;yr
-
volume of s e c r e t e d g a s
-
volume o f o i l p u t i n t o t h e a p p a r a t u s radiation t i m e
-
i n s t a n t a n e o u s gas s e c r e t i o n o r v e l o c i t y of g a s s e c r e t i o n from t h e u n i t of o i l volume.
A f t e r i n t e g r a t i n g formula ( 8 . 5 1 ) t h e t o t a l g a s s e c r e t i o n d u r i n g t h e p e r i o d of r a d i a t i o n i s g i v e n by t h e formula (8.52)
3 76
where Vor,
mor,
Vo
-
o i l volume b e f o r e and a f t e r r a d i a t i o n ,
mo
-
o i l mass b e f o r e and a f t e r r a d i a t i o n .
The o i l d e n s i t y v a r i a t i o n i s i n s i g n i f i c a n t . The mass o f o i l i n t h e a p p a r a t u s i s determined by weighing b e f o r e and a f t e r r a d i a t i o n . I f t h e g a s s e c r e t i o n i s known, t h e t o t a l r a d i a t i o n - c h e m i c a l gas s e c r e t i o n G may be d e t e r m i n e d u s i n g t h e f o l l o w i n g formula: (8.53) where
-
o i l density
T
-
absolute temperature
D
-
p g R
atmospheric pressure u n i v e r s a l gas c o n s t a n t r a d i a t i o n dose. A f t e r t h e o i l sample h a s been weighed, t h e r a d i a t i o n t i m e may
b e prolonged and i n t h i s way t h e r e l a t i o n s h i p between g a s secret i o n and r a d i a t i o n dose may be determined. The e f f e c t o f o i l o n t h e s u r f a c e o f m e t a l l i c e l e m e n t s depends mainly on i t s c o r r o s i v e n e s s . The i n v e s t i g a t i o n s i n t o t h e c o m s i v e n e s s o f i n s t r u m e n t o i l s a r e t h e same a s f o r o t h e r machine o i l s
(see e . g . r e f . 9 1 , t h e s p e c i a l GOST 7934.5 s u i t a b l e standards.
-
74 s t a n d a r d o r o t h e r
S p e c i a l a t t e n t i o n needs t h e t o x i c i t y t e s t i n g o f i n s t r u m e n t l u b r i c a n t s . However, even when new o i l s a r e f r e e from t o x i c i t y , haza r d s could d e v e l o p a s a r e s u l t o f a g e i n g ( d e g r a d a t i o n ) and/or cont a m i n a t i o n d u r i n g use ( r e f . 8 9 4 ) . The s t u d y o f t o x i c i t y o f a d d i t i v e s (and t h e i r l u b r i c a t i o n a b i l i t y ( r e f . 895)) i s a l s o i m p o r t a n t . Types o f t o x i c i t y t e s t a r e d e s c r i b e d i n r e f . 894. I t i s v e r y i m p o r t a n t t o i n v e s t i g a t e t h e i n f l u e n c e o f instrum?nt
o i l s on t h e p h y s i c a l p r o p e r t i e s o f p o l y m e r i c e l e m e n t s used i n mini a t u r e mechanisms. The a c t i o n o f o i l on p o l y m e r i c m a t e r i a l s may b e determined by examining t h e changes i n p a r t i c u l a r p r o p e r t i e s of polymeric samples. The m o s t i m p o r t a n t method f o r j u d g i n g t h e e f f e c t of o i l on t h e p r o p e r t i e s of p o l y m e r i c e l e m e n t s a p p e a r s t o be m i c r o h a r d n e s s t e s t i n g b e f o r e and a f t e r c o n t a c t w i t h o i l . The t e s t i s q u i c k l y c a r r i e d o u t and needs o n l y one e l e m e n t . The microhardness can b e measured w i t h a microhardness m e t e r w i t h a diamond pyramid. The o b j e c t may be i l l u m i n a t e d w i t h a l u m i n e s c e n t lamp ( r e f . 6 1 5 ) . I n p r e v i o u s
371 t e s t s , t h e e l e m e n t has been i n t h e form o f a 15x10~3mn parallelepiped. An i n t e r e s t i n g method f o r s t u d y i n g t h e a c t i o n of an i n s t r u m e n t l u b r i c a n t ( o i l ) on polymeric microelements h a s been proposed by T i l lwich (refs. 208, 465). The p o l y m e r i c p l a t e i s t e s t e d a f t e r contact with t h e h e l p o f t h e s p e c i a l a p p a r a t u s shown i n F i g . 8.61. The steel cone
i s p r e s s e d i n t o t h e p o l y m e r i c m a t e r i a l w i t h a f o r c e o f 20 N for 10, 30 o r 6 0 s. The c r a t e r formed i s measured by o p t i c a l method w i t h a ref l e x - c o n t r a s t d e v i c e ( F i g . 8 . 6 2 ) . The e f f e c t o f t h e c o n t a c t w i t h o i l i s a l s o e v a l u a t e d by measuring t h e v a r i a t i o n s i n t h e d i s t a n c e between two marks made o n t h e polymer p l a t e ( F i g . 8 . 6 1 ) .
F i g . 8.61. Apparatus f o r t e s t i n g a polymer s u r f a c e a f t e r c o n t a c t w i t h polymer p l a t e ( r e f . 465). o i l . 1 - weight, 2 - guide p l a t e , 3
-
The i n v e s t i g a t i o n of t h e tendency o f a polymeric m a t e r i a l t o c r a c k a s a r e s u l t o f c o n t a c t w i t h o i l under l o a d i s also a reasonably s e n s i t i v e method f o r d e t e r m i n i n g polymer r e s i s t a n c e t o t h e influence of l u b r i c a n t . The polymer samples, u s u a l l y i n t h e shape o f a p a r a l l e l e p i p e d , may be t e s t e d a f t e r c o n t a c t w i t h o i l t o d e t e r m i n e changes i n t h e i r bending and t e n s i l e s t r e n g t h o r impact value (refs. 169, 616). The e f f e c t o f o i l on p o l y m e r i c m a t e r i a l i s a l s o d e t e r m i n e d by t h e a n a l y s i s o f a b s o r p t i o n o f o i l by t h e polymer. The r a t e o f abs o r p t i o n can be e s t i m a t e d by w e i g h t i n g o r measuring t h e volume o f a sample b e f o r e and a f t e r t e s t i n g . The m i g r a t i o n o f t h e o i l molecules i n t o t h e polymeric m a t e r i a l can b e observed u s i n g t h e Electron Probe M i c r o a n a l y s i s (EPMA) o r m a e r f o r d Back-Scattering (I1Bs) mthods ( r e f . 459)
.
318
F i g . 8.62. O p t i c a l d e v i c e ( a ) f o r measuring c r a t e r b r e a d t h (b) ( r e f .
465).
These two methods c l e a r l y d e m o n s t r a t e t h e e x t e n t t o which t h e o i l molecules e n t e r i n t o t h e s u r f a c e l a y e r o f a polymer d u r i n g amtact. They do n o t , however, p r o v i d e any i n d i c a t i o n of t h e way inwhich the o i l is i n c o r p o r a t e d . If t h e polymer m a t e r i a l p l a s t i c i z e s , as i s usually t h e case, t h e p l a s t i c i z a t i o n reduces t h e g l a s s - t r a n s i t i o n t e m p e r a t u r e by weakening t h e i n t e r c h a i n bonding f o r c e s i n the polym e r . The g l a s s - t r a n s i t i o n t e m p e r a t u r e can b e d e t e r m i n e d u s i n g t h e e q u i l i b r i u m s t a t e dynamic method. There a r e many p o s s i b l e ways o f d e t e r m i n i n g t h e g l a s s - t r a n s i t i o n t e m p e r a t u r e of p o l y m e r i c materials, e.g. 8.5.5.
d i l a t o m e t r y , DST o r DTA analysis, EPR and many others (see ref. 6 0 4 ) . PROPERTIES OF COATINGS AND EPILAMES Methods f o r s t u d y i n g t h e q u a l i t y of c o a t i n g s and e p i l a m e s have
been d e s c r i b e d i n Chapter 6 . 2 . 6 .
The main p r o p e r t i e s of an epilame
f i l m t o b e examined a r e i t s s u r f a c e f r e e e n e r g y ( w e t t a b i l i t y ) , i t s mechanical p r o p e r t i e s and a d h e s i o n t o s o l i d s u r f a c e s , its topogra@y on a s o l i d s u r f a c e , i t s t h i c k n e s s and i t s o i l r e s i s t a n c e . The s u r f a c e f r e e e n e r g y can b e d e t e r m i n e d by measuring the wnt a c t a n g l e . For t h i s t h e t e c h n i c a l means d e s c r i b e d i n S e c t i o n 5.3
3 79 o f t h i s C h a p t e r may b e s a t i s f a c t o r i l y a p p l i e d . Other t e c h n i q u e s have been s p e c i a l l y developed f o r t e s t i n g t h e s u r f a c e p r o p e r t i e s o f ep-
ilame f i l m s ( r e f s . 4 3 2 , 4 3 3 ) . The LSRH t e s t s s e t - u p ( N e u c h 2 t e l
,
S w i t z e r l a n d ) c o n s i s t s o f t h e series of 6 s p e c i a l l i q u i d s l i s t e d i n Table 8.2
( r e f . 4 3 2 ) . O i l I i s a f l u o r i n a t e d compound, o i l s I1 and
I11 a r e p o l y s i l o x a n e s and o i l s I V , V and V I a r e n o n p o l a r l i q u i d s
b a s e d on c l o c k o i l s . F l u o r i n a t e d epilame c o a t i n g s are identified using o i l No. I . T h i s o i l w i l l s p r e a d q u i c k l y on f l u o r i n a t e d epilane f i l m w h i l e oils I1 and I11 w i l l n o t s p r e a d . On t h e e p i l a m e f i l m s b a s e d on p o l y s i l o x a n e s , o i l I does n o t s p r e a d b u t o i l s I1 and I11 do spread. The w e t t a b i l i t y o f t h e e p i l a m e f i l m s by o i l I V depends on the quality o f t h e c o a t i n g : when o i l s I V does n o t s p r e a d t h e e p i l a m e f i l m i s uniform and well-done.
I r r e g u l a r i t y of t h e oil d r o p s s h m i n s u f f i -
c i e n t p o l a r i t y o r adherence o f t h e e p i l a m e f i l m . A s i m i l a r e f f e c t o b s e r v e d a t t h e w e t t i n g o f t h e epilame f i l m by o i l I11 shows t h a t t h e epilame f i l m h a s been b a d l y made. T A B L E 8.2 O I L S USED I N LSRH ( N e u c h s t e l , S w i t z e r l a n d ) FOR T E S T I N G CLEAN SURFACES AND SURFACES COATED B Y E P I L A M E F I L M S O I L No.
I II III IV V VI
COLOUR C o l ou r 1 ess B r ig h t - r e d Dark- r e d Yellow Green
Blue
VISCOSITY
AT 2ooc, m m 2 / s
35 70 54 36 47 97
SURFACE TENSION,
d / m
19.7
21 .g
23.1 29.1 34.6 41.6
The c l a s s i c a l t e s t o f epilame f i l m s u s i n g s t e a r i c a c i d s h o u l d show t h a t none o f t h e s i x o i l s s p r e a d on t h e checked s u r f a c e . E l e ments covered by epilame f i l m must b e c l e a n e d w i t h c h l o r i n a t e d solv e n t o r b e n z i n e , and t h e t e s t r e p e a t e d . I n t h i s way s t e a r i c a c i d epilame f i l m s a r e c l e a n e d and t h e i r v i r g i n s u r f a c e r e s t o r e d . The e p i l a m e f i l m s b a s e d on amines, used f o r c o a t i n g n i c k e l - p l a t e d e l e ments, can be c o n s i d e r e d t o be w e l l made when none of the o i l s spread. 2 -1 The a p p l i c a t i o n of p o l y s i l o x a n e ( w i t h v i s c o s i t y 50 mm s at 2OoC)
has been proposed f o r t e s t i n g f l u o r i n a t e d e p i l a m e f i l m s
(ref.
4 3 3 ) . The c o n t r o l l i q u i d d r o p s ( a b o u t 1 mm i n d i a m e t e r ) a r e formed
on a h o r i z o n t a l l y p l a c e d e l e m e n t and s h o u l d g i v e a c o n t a c t angle of between 5 and 45OC a f t e r 4 h o u r s . The d r o p s must n o t be v i s i b l y deformed o r i r r e g u l a r . The element t o be t e s t e d s h o u l d n o t have been a l r e a d y used. The t e s t l i q u i d i s s u p p l i e d w i t h t h e o r i g i n a l epilame.
3 80 The f l u o r i n a t e d e p i l a m e f i l m s made by t h e 3 M Company (Minnesota, U.S.A.)
c a n b e t e s t e d u s i n g h e p t a n e o r s i l i c o n e o i l s . The d r y f i l m
s h o u l d n o t b e w e t t a b l e by t h e s e l i q u i d s . The m e c h a n i c a l p r o p e r t i e s o f e p i l a m e f i l m s and t h e i r adherence
t o a s o l i d s u r f a c e c a n b e i n v e s t i g a t e d u s i n g s i m i l a r methods t o t h o s e u s e d f o r t e s t i n g p o l y m e r i c c o a t i n g s (see r e f . 4 3 1 ) . The method r e c e n t l y d e v e l o p e d by T i n g e t a l . ( r e f . 896) t o d e t e r m i n e t h e i n t e r f a c i a l bond s t r e n g t h between a t i n s u r f a c e c o a t i n g a n d i t s sub-
s t r a t e c a n b e a l s o t a k e n i n t o c o n s i d e r a t i o n . Epilame f i l m a d h e s i o n can be t e s t e d by t h e u s u a l s h e a r i n g o r t e a r i n g - o f f m e t h o d s , o r by s p e c i a l methods s u c h as t h e p i n , u l t r a c e n t r i f u g e , u l t r a s o n i c o r o p t i c a l methods.
I n t h e p i n method, p i n s made o f a m a t e r i a l used f o r
t h e a c t u a l p r o d u c t i o n o f m i c r o e l e m e n t s are p l a c e d i n h o l e s b o r e d i n t h e metal p l a t e (Fig. 8 . 6 3 ) .
Pin Plate
F i g . 8.63. P i n method f o r t e s t i n g polymer f i l m adhesion.
The e n d s o f t h e p i n s are ma&ine-levelled on t h e s i d e o f t h e p l a t e and t h e e p i l a m e f i l m i s d e p o s i t e d t h e r e . The adherene or rre&anical resistance o f t h e f i l m can b e d e t e r m i n e d by r e c o r d i n g t h e f o r c e o r m m n t needed t o e i t h e r draw o u t o r r o t a t e t h e p i n . The u l t r a c e n t r i f u g e method i n v o l v e s removing t h e c o a t i n g from a c y l i n d r i c a l o r spherical sample by c e n t r i f u g a l f o r c e . I n the u l t r a s o n i c method t h e coating i s removed as a r e s u l t o f i n e r t i a l f o r c e s o f an a x i a l l y o s c i l l a t i n g m a g n e t o s t r i c t i o n head of a c y l i n d r i c a l o r c o n i c a l shape; t h e coati n g i s d e p o s i t e d on t h e f r e e e n d o f t h e h e a d . The o p t i c a l method
i s b a s e d o n t h e e f f e c t o f v a r i a t i o n s i n the r e f r a c t i v e i n d e x as a f u n c t i o n o f t h e stress when a c o a t i n g i s t o r n o f f from a s u r -
3 81 f a c e : t h i s method can be used when t h e microelements and d e p o s i t e d epilame f i l m a r e t r a n s p a r e n t , f o r i n s t a n c e when t h e e l e m e n t s a r e made o f g l a s s . The topography o f an epilame b a r r i e r f i l m d e p o s i t e d on a microelement can be s t u d i e d u s i n g o p t i c a l methods (see C h a p t e r 6 . 2 . 6 ) . The t h i c k n e s s o f t h e epilame f i l m may be d e t e r m i n e d by i n t e r f e r o metry and t h e f i l m ' s r e s i s t a n c e t o o i l s , u s i n g t h e f l u i d s t o det e r m i n e w e t t a b i l i t y v a r i a t i o n s , by SEM a n a l y s i s o r ESCA methods
(see a l s o Chapter 6 . 2 . 6 ) . viewed i n r e f .
These methods a r e a l s o c r i t i c a l l y re-
8 9 7 . The methods used f o r s t u d y i n g t r a n s f e r r e d f i l m
( r e f . 898) can be a l s o t a k e n i n t o c o n s i d e r a t i o n .
8,6, C L E A N I N G The c l e a n i n g o f microelements i s an i m p o r t a n t p r o c e d u r e d u r i n g e x p e r i m e n t a l i n v e s t i g a t i o n s on them and d u r i n g t h e i r m a n u f a c t u r e . The c l e a n i n g method depends on t h e d e g r e e o f c l e a n l i n e s s r e q u i r e d . Clean s u r f a c e s can be b r o a d l y d i v i d e d i n t o two c a t e g o r i e s : atomi c a l l y c l e a n s u r f a c e s , and t e c h n o l o g i c a l l y o r p r a c t i c a l l y c l e a n s u r f a c e s . Atomically c l e a n s u r f a c e s a r e r e q u i r e d f o r s p e c i a l purposes and can o n l y be r e a l i z e d i n a h i g h vacuum (see r e f s . 4 2 1 , 422). The most commonly used method f o r t e c h n o l o g i c a l c l e a n i n g i s
s o l v e n t o r chemical c l e a n i n g . The c l e a n i n g l i q u i d i s u s u a l l y a wa-
t e r s o l u t i o n o r o r q a n i c s o l v e n t . Cleaning w i t h w a t e r s o l u t i o n i s based on t h e chemical r e a c t i o n s between t h e c o n t a m i n a n t s and a c i d o r a l k a l i n e s u b s t a n c e s d i l u t e d i n t h e w a t e r . The chemical r e a c t i o n s a r e g e n e r a l l y s u p p o r t e d by a d i s p e r s a l p r o c e s s . The mechan i c a l removal o f contaminants from t h e s o l i d s u r f a c e , by u l t r a s o n i c s for example , a c c e l e r a t e s and improves t h e c l e a n i n g p r o c e s s . The optimum t e m p e r a t u r e range f o r u l t r a s o n i c c l e a n i n g combined w i t h t h e use o f a w a t e r s o l u t i o n i s 4O-5O0C
( r e f . 425). U l t r a s o n i c
c a v i t a t i o n i s g r e a t e r i n water s o l u t i o n s than i n organic solvents. The s e l e c t i o n o f t h e b e s t s u b s t a n c e s and c l e a n i n g t e c h n i q u e can be made e x p e r i m e n t a l l y . Some examples o f c l e a n i n g l i q u i d s used f o r c l e a n i n g e l e m e n t s o f m i n i a t u r e mechanisms a r e l i s t e d i n r e f . 899. G e n e r a l l y , t h e a c i d s o l u t i o n s can b e used when t h e s u r f a c e s
a r e contaminated w i t h r u s t , s c a l e , o x i d e s o r s a l t s o f n o n - f e r r o u s m e t a l s . The a c i d s widely used f o r p r e p a r a t i o n o f w a t e r s o l u t i o n s a r e : s u l p h u r i c (H2S04)I p h o s p h o r i c ( H 3 P 0 4 )
,
t a r t a r i c (Hoot
*
CHOH *
382 *
CHOH
-
COOH)
,
-
c i t r i c (HOC(CH2COOH)2COOH H20) , chromic (H2Cr04), , h y d r o f l u o r i c (HE'), and nitric (HNo3) a c i d . Special
hydrochloric (HC1)
s u b s t a n c e s knm a s c o r r o s i o n i n h i b i t o r s a r e u s u a l l y i n t r o d u c e d t o the a c i d water s o l u t i o n s t o coat t h e cleaned surface with a t h i n a n t i - c o r r o s i o n l a y e r . The c o r r o s i o n i n h i b i t o r s a r e s p e c i f i c f o r each m e t a l . Some r e a d y - f o r - u s e
a c i d s o l u t i o n s used f o r u l t r a s o n i c
c l e a n i n g a r e now commercially a v a i l a b l e . To g i v e a few examples o f current U.S.
p r o d u c t s : O a k i t e ' s 34 M i s used f o r c l e a n i n g o x i d e s
from copper , aluminium and copper a l l o y s ; Phosphate Degreaser (Magnus) i s used f o r t h e removal o f o x i d e s and o i l s from copper and s t e e l s u r f a c e s ; and T r u l i t S ( T r u Chemic) can b e used t o remove o x i d e s and o i l s from t h e s u r f a c e s o f v a r i o u s m e t a l s . A l k a l i n e w a t e r s o l u t i o n s a r e more o f t e n a p p l i e d f o r c l e a n i n g t h a n a c i d s o l u t i o n s . These s o l u t i o n s a r e used mainly t o remove f a t t y c o n t a m i n a n t s , which a r e a l s o o f t e n removed by o r g a n i c s o l v e n t s ; however, a c i d s o l u t i o n s a r e more e f f i c i e n t when t h e f a t t y contaminants a r e b l e n d e d w i t h i n o r g a n i c compounds. The fundamental chemical r e a c t i o n d u r i n g c l e a n i n g w i t h a l k a l i n e s o l u t i o n i s f a t s a p o n i f i c a t i o n . The r e a c t i o n p r o d u c t s a r e g l y c e r i n e and sodium o r p o t a s s i u m s o a p s . A n e c e s s a r y c o n d i t i o n i s an a d e q u a t e pH v a l u e i n t h e s o l u t i o n and f o r t h i s sodium o r p o t a s s i u m h y d r o x i d e s (NaOH o r KOH) a r e r e q u i r e d . The most commonly used s u b s t a n c e s f o r c l e a n i n g
w i t h w a t e r s o l u t i o n s a r e phosphates
,
s i l i c a t e s , sodium c a r b o n a t e ,
sodium hydroxide and sodium b o r a t e . A s i n a c i d s o l u t i o n s , t h e c o r r o s i o n i n h i b i t o r s are s p e c i f i c for e a c h m e t a l . There are o t h e r a l k a l i n e w a t e r s o l u t i o n s made of o r d i n a r y d e t e r g e n t s . The a d d i t i o n o f s u r f a c e a c t i v e d e t e r g e n t s t o e i t h e r a c i d o r a l k a l i n e w a t e r sol u t i o n s d e c r e a s e s t h e i n t e r f a c i a l t e n s i o n between t h e s o l i d b e i n g c l e a n e d and t h e washing l i q u i d , which f a c i l i t a t e s t h e e n t r y o f t h e l i q u i d i n t o t h e s m a l l p o r e s and c r e v i c e s . The fundamental c r i t e r i o n f o r s e l e c t i n g t h e o r g a n i c s o l v e n t f o r c l e a n i n g i s t h e s o l u b i l i t y p a r a m e t e r (see Chapter 8 . 5 . 3 ) . The s o l u b i l i t y p a r a m e t e r s o f t h e contaminant and s o l v e n t s h o u l d be s i m i l a r . The s u r f a c e t e n s i o n of t h e s o l v e n t must a l s o be v e r y low. From t h i s p o i n t o f view, t h e h a l o g e n a t e d s o l v e n t s a r e most suitable. The c h l o r i n a t e d s o l v e n t s most s u i t a b l e f o r c l e a n i n g i n u l t r a s o n i c app a r a t u s a r e m e t h y l e n e c h l o r i d e , t e t r ach l o r o e t h y l e n e , t r i ch l o r o e t h y l e n e and 1, 1,1 - t r i c h l o r o e t h a n e . The b i g d i s a d v a n t a g e o f t h e s e s o l v e n t s , however, i s t h e i r t o x i c i t y . Only 1 , 1 , l - t r i c h l o r o e t h a n e has a r e l a t i v e l y low t o x i c i t y ; t h i s i s a v a i l a b l e i n t h e form o f Genklene ( I C I L t d . )
,
Chloroethane (Dow Chemical Co.)
,
Baltane
383 (Rhone P r o g i l )
,
and Wacker 3x1 (Wacker-Chemie GmbH)
. The
disadvan-
tage of t h e s e p r o d u c t s i s , however, t h e i r water h y d r o l y s i s , meani n g t h e y have r e l a t i v e l y low s t a b i l i t y . Among t h e b e s t a n d most w i d e l y u s e d s o l v e n t s are t h e F r e o n s , f i r s t i n t r o d u c e d by Du P o n t . These a r e m a i n l y a l i p h a t i c hydroc a r b o n s i n which one o r s e v e r a l hydrogen atoms are changed by f l u o r i n e , f l u o r i n e a n d c h l o r i n e , o r f l u o r i n e and bromine, o r by a l l t h e a f o r e m e n t i o n e d e l e m e n t s . The series o f f r e o n s are e n u m e r a t e d i n t h e f o l l o w i n g way: t h e l a s t d i g i t i s t h e number o f f l u o r i n e atoms i n t h e m o l e c u l e , t h e m i d d l e d i g i t i n d i c a t e s t h e number o f hydrogen a t o m s , a n d t h e f i r s t one t h e number o f c a r b o n atoms ( l e s s 1) i n t h e f r e o n m o l e c u l e . The c h l o r i n e atoms i n t h e m o l e c u l e are
d e t e r m i n e d by t h e a n a l y s i s o f t h e g e n e r a l f o r m u l a C n ( H , C 1 , F ) 2 n + 2 . The symmetric i s o m e r i s d e s c r i b e d o n l y w i t h t h e t h r e e - d i g i t number b u t t h e nonsymmetric isomer i s i d e n t i f i e d w i t h t h e l e t t e r A . The bromine f r e o n s a r e marked w i t h t h e l e t t e r B and t h e d i g i t w i t h den o t e s t h e number o f atoms. I f t h e f r e o n i s a c y c l i c compound, t h e
l e t t e r C i s w r i t t e n b e f o r e t h e number. The p r i n c i p l e a d v a n t a g e s o f f r e o n s a r e : n o n - f l a m m a b i l i t y ,
low
s u r f a c e t e n s i o n a n d t o x i c i t y . They are a l s o i n e r t a g a i n s t p o l p r i c m a t e r i a l s . Freon 113 ( e . g . F r e o n TF, from Du P o n t ) i s w i d e l y u s e d f o r cleaning microelements, with t h e h e l p o f u l t r a s o n i c d ev ices f o r example. The h a l o g e n a t e d s o l v e n t s are n o t c o r r o s i v e a g a i n s t metals i f no water i s p r e s e n t . Freons are a l s o u s e f u l s o l v e n t s f o r c l e a n i n g polymer s u r f a c e s . The e f f e c t s of s h o r t - p e r i o d and l o n g - p e r i o d
con-
t a c t between v a r i o u s polymers and h a l o g e n a t e d s o l v e n t s are shown i n T a b l e s 8 . 3 and 8.4, b a s e d on d a t a from I C I a n d Du P o n t tests r e s p e c t i v e l y , a n d drawn from r e f . 425.
The p o l y a c e t a l s , PTFE and
polyamides can o n l y b e a p p l i e d when t h e r e i s a l a s t i n g c o n t a c t w i t h c h l o r i n a t e d s o l v e n t s . Freon 1 1 3 i s o n l y a c t i v e a g a i n s t p o l y s t y r e n e a t e l e v a t e d t e m p e r a t u r e s . The comparison between t h e act i v i t y o f f r e o n s 1 1 3 a n d 1 1 4 B2 a g a i n s t p o l y m e r i c m a t e r i a l s i s p r e s e n t e d i n T a b l e 8 . 5 ( t a k e n from r e f . 6 1 7 ) . The c l e a n i n g p r o c e d u r e s v a r y , d e p e n d i n g o n t h e f i n a l e f f e c t r e q u i r e d . Generally, microelements a r e c l e a n e d using s u c c e s s i v e l y n o n - p o l a r and p o l a r s o l v e n t s (see C h a p t e r 6 . 2 . 6 ) . The method o f c l e a n i n g i t s e l f is n o t i m p o r t a n t , t h e f i n a l s t a t e o f t h e c l e a n e d s u r f a c e o f a microelement b e i n g t h e c r i t e r i o n o f t h e q u a l i t y o f a cle a ning procedure.
3 84 TABLE 8.3 EFFECT OF HALOGENATED SOLVENTS ON POLYMERS DURING THE PROCESS OF CLEANING
-
SHORT P E R I O D S OF CONTACT
METHYLENE CHLORI OE
TRICHLOROETHYLENE
TETRACHLOROETHYLENE
POLYMER
1.1,l-TRICHLOROETHANE
TRICHLOROTRI FLUOROETHANE
TEMPERATURE 20°C
PA PE PC PS PTFE P MMA PVC ABS CA UF
Boili ng
Boi 1 i ng
2OoC
1 2
0
0
1
2
3 3
2
3
3 0
0
03
0 1 2
2OoC
Boili ng
0 1
2OoC
1 2
0
3
2
Boi 1 i ng
2OoC
Boiling
0 2
0 0
0 0
0
0
3
3
3 3
0
3
0
0
1
0
0
1
2
2
0
3
1 0
2 2
1 0 0 0
2 2 0
1 0 0
3
0 0 0 0 0 0
1
3
3
1 0 3 3 3
2
2
1
3 3 0
3
3
3
3
1
2
0 1
0
0
0
0
1 0
0
0 0
TABLE 8.4 EFFECT OF HALOGENATED SOLVENTS ON POLYMERS AFTER CONTACT FOR 100 h AT TEMPERATURE 54OC OR
4 h AT BOILING TEMPERATURE
TRICHLOROETHYLENE
TETRACHLOROETHYLENE
1,l,l-TRI-
TRICHLOROTRI FLUOROETHANE
CHLOROETHANE
POLYMER
Boi 1 i n g
54OC
PA POM PE PC PS PT FE PMMA PP PS PPO PVC ( h a r d ) A6 S
0
O1
I
54OC
0
1
0
0 1 2
1
1 1
6
I
-
I
0
6 4 6
O1 1
1
4 6 6 6 4 4
4
I
54OC
Boi 1 i n g
54OC
1 0
4 2
I
Boi 1 i n g
0
6
-
TEMPERATURE
0 0 0 0
:I
0
3
0
0 1 0
0 1
0 0
0 0
1
2
4 6
4 6
0
0
6
4 4 6 4 4 5
2
0 I
0 '
6 5 4
3
I
Boi 1 i n g
I
6
I
Designations: 0 unaffected, 1 small p l a s t i c i z a t i o n , no s w e l l , 2 - s m a l l s w e l l and p l a s t i c i z a t i o n , 3 - c r a c k i n g and f r a g i l i t y , 4 - s w e l l , w a r p i n g and plasticization, 5 - p a r t i a l dissolving o r destruction, 6 total dissolving or d e s t r u c t i o n .
-
3 85 TABLE 8.5 EFFECT OF PROLONGED CONTACT OF FREONS 113 OR 114 82 WITH SOME POLYMERS
SWELLING, POLYMER
FREON 113
,
1920 h
-
25-30°C
PE PVC ( h a r d 1 PVC ( s o f t )
0.96
PT FE EP UF+asbestos Rubber
0.75 0.5
0.1
4.3
% FREON 1 1 4 8 2
1125 h
,
20-25OC
3.38 0 14 1.4
5
1.7 -5
20.3
2.1
C o n t r o l l i n g t h e q u a l i t y o f h a l o g e n a t e d s o l v e n t s b e f o r e use i s v e r y i m p o r t a n t f o r t h e cleaning to be e f f e c t i v e . The t o x i c i t y and c o r r o s i v e e f f e c t of the solvent are p r n p e r t i e s which have an i m p o r t a n t i n f l u e n c e b o t h on s o l v e n t h a n d l i n g and t h e f i n a l s t a t e o f t h e c l e a n e d s u r f a c e . The t o x i c i t y r i s k s can be e l i m i n a t e d by u s i n g a c l o s e d a p p a r a t u s equipped w i t h e f f i c i e n t v e n t i l a t i o n .
Corrosion i s
more d i f f i c u l t t o p r e v e n t . I n i n d u s t r i a l p r a c t i c e t h e c o r r o s i v e e f f e c t of s o l v e n t s can be t e s t e d b e f o r e use by u s i n g t h e method and d e v i c e developed i n LSRH ( N e u c h s t e l , S w i t z e r l a n d ) ( r e f . 6 1 8 ) A diagram o f t h e t e s t v e s s e l i s shown i n F i g . 8 . 6 4 .
.
Test strips,
8OxlOx0.5 m, of s o f t s t e e l b l a d e , f i n e l y s a n d - b l a s t e d and reh e a t e d i n a vacuum a t 8OO0C a r e p l a c e d i n c o n t a c t w i t h t h e l i q u i d t o be t e s t e d . The s t a t e o f t h e s t r i p i s checked a t l e a s t once a day. The s t r i p s a r e hung on a g l a s s o r p o l y m e r i c s t e m 4 mm i n d i -
ameter. The i n s p e c t i o n can be made more v i g o r o u s by e x p o s i n g t h e t e s t s t r i p s removed from t h e s o l v e n t t o a humid atmosphere. The v e s s e l f o r e x p o s u r e t o s a t u r a t i n g humidity i s a g l a s s , r e c t a n g u l a r v e s s e l w i t h a d i a g o n a l s u p p o r t i n s i d e t o which t h e t e s t s t r i p s a r e a t t a c h e d . The c o v e r i s made of o r g a n i c g l a s s . The i n s i d e w a l l s are covered w i t h t h i c k b l o t t i n g p a p e r h u m i d i f i e d w i t h d e m i n e r a l i z e d w a t e r . At t h e bottom o f t h e v e s s e l , d e m i n e r a l i z e d w a t e r i s maint a i n e d a t a c o n s t a n t l e v e l o f ' 2 0 nun. Before u s e , t h e o r i g i n a l p o l y e t h y l e n e p r o t e c t i o n s h o u l d b e removed from t h e LSRH t e s t s t r i p w i t h a c l e a n p a i r o f t w e e z e r s . One s t r i p i s s u f f i c i e n t f o r one v e s s e l o f s o l v e n t . The s t r i p is hung a t t h e b e g i n n i n g o f t h e day i n such a manner t h a t i t i s c o m p l e t e l y immersed i n t h e h o t o r c o l d l i q u i d . The s t r i p is examined a f t e r 24
3 86
h o u r s , p r e f e r a b l y w i t h a magnifying g l a s s . I f t h e s o l v e n t i s s a t i s f a c t o r y , no r u s t s h o u l d be r e v e a l e d . I f t h e s u r f a c e i s c o v e r e d w i t h s m a l l s p o t s , t h e s o l v e n t must be changed immediately. A f t e r t h e f i r s t e x a m i n a t i o n , i f t h e s t r i p i s n o t a l r e a d y c o r r o d e d , it i s hung i n t h e v e s s e l i n a s a t u r a t e d atmosphere f o r a b o u t 2 4 h o u r s . I f t h e s o l v e n t i s e x a c t l y r i g h t , t h e r e w i l l b e n o r u s t a t a l l on t h e s t r i p . A few i s o l a t e d d o t s of r u s t a r e t o l e r a b l e ; however, i f t h e i r number i n c r e a s e s , t h e s o l v e n t i s s u s p e c t . I n s t e a d o f s t e e l s t r i p s , copper s t r i p s can be t e s t e d t o i n v e s t i g a t e t h e c o r r o s i v e a c t i o n of s o l v e n t s on copper a l l o y s .
I \I
-1
Fig. 8.64. T e s t vessel f o r i n v e s t i g a t i o n o f corrosive e f f e c t s o f solvents ( r e f . 618).
A n a l y s i s o f t h e c l e a n l i n e s s o f t h e c l e a n e d s u r f a c e i s a very i m p o r t a n t t a s k . We can b r o a d l y speak o f i n d u s t r i a l l y a p p l i c a b l e and l a b o r a t i r y t e s t i n g . The method u s e d f o r t e s t i n g t h e c l e a n l i n e s s o f m i n i a t u r e e l e m e n t s f o r v a r i o u s meters d u r i n g t h e p r o d u c t i o n p r o c e s s i s d e s c r i b e d i n r e f . 4 2 4 . The s t a t e of t h e s u r f a c e s of c l e a n e d e l e m e n t s is d e s c r i b e d by c l a s s i f y i n g them i n t h e approp r i a t e c l e a n l i n e s s c a t e g o r y ( s t a g e ) . The c l e a n l i n e s s t a g e s a r e def i n e d i n Table 8 . 6 ;
t h e y a r e used by d e s i g n e n g i n e e r s
t o get a
d e s c r i p t i o n o f t h e f i n a l s t a t e o f t h e s u r f a c e s of a microelement.
387 TABLE 8.6 CLEANLINESS STAGES USED I N INDUSTRIAL PRACTICE ( r e f . 424)
I I
CLEANLINESS STAGE
I
I
REQUl REMENTS OF COMPONENTS ( f o r 5 s c o n t r o l times)
A P P L I C A T I O N EXAMPLES
I
I
N o contamination should be i s i b l e a t 10 x m a g n i f i c a t i o n
A
N o contamination should be v i s i b l e a t 3 x magnification
B
N o contamination should be v i s i b l e t o the naked eye
Bearings f o r e l e c t r i c i t y tnaters
- sensitive contacts - p r i n t e d c i r c u i t boards w i t h s l i d i n g contacts - expansion systems e . g . Thermogyr - synchronous motors - escapement balance bearings -
normal contacts and r e l a y s normal p r e c i s i o n mechanical equipment normal p r i n t e d c i r c u i t boards r o t a r y magnets
-
normal assembly i n machine c o n s t r u c t ion switchboards
-
D
~
Standard workshop cleanliness
I
The l a b o r a t o r y methods u s e d f o r d e t e r m i n i n g t h e c l e a n l i n e s s o f c l e a n e d s u r f a c e s r a n g e from t h e v e r y s i m p l e t o t h e v e r y s o p h i s t i cated ( r e f s . 421-423, 425, 6 1 9 ) . A v e r y s e n s i t i v e and y e t s i m p l e
method is t o measure t h e c o n t a c t a n g l e o f a t e s t l i q u i d , e . g . wa-
t e r . On c l e a n s u r f a c e s , t h e d r o p o f water s p r e a d s q u i c k l y and t h e c o n t a c t a n g l e i s v e r y small ( 0 - 0 ) . T h i s method can d e t e c t t h e d i f f e r e n c e i n c l e a n l i n e s s between a
c l e a n s u r f a c e and o n e contam-
i n a t e d w i t h a s o l u t i o n c o n t a i n i n g 0 . 0 1 % f a t . However, it can o n l y be a p p l i e d t o w e t t a b l e s u r f a c e s (see Table 8 . 7 ) .
The w e t t a b i l i t y
t e s t u s i n g w a t e r can be q u i t e m i s l e a d i n g i f t h e s u r f a c e i s contaminated with hydrophilic impurities; for instance, i f t h e surface
i s c o v e r e d w i t h a l a y e r o f d e t e r g e n t o r an i n o r g a n i c s a l t , t h e w a t e r w i l l s p r e a d e r r o n e o u s l y i n d i c a t i n g t h a t t h e s u r f a c e is c l e a n . A l s o w e t t a b i l i t y s t u d i e s on rough s u r f a c e s are v e r y d i f f i c u l t , as r o u g h n e s s i n t e r f e r e s w i t h t h e c o n t a c t a n g l e measurements and can r e s u l t i n s p u r i o u s r e s u l t s . The t e s t c a n n o t b e a p p l i e d t o check t h e c l e a n l i n e s s o f low e n e r g y s u r f a c e s , i . e . s u r f a c e s w i t h a s u r face f r e e e n e r g y of less t h a n 72 m J / m L , which i n c l u d e s all p o l y m e r s . The LSRH t e s t s e t c o n s i s t i n g o f a series o f 6 o i l s m e n t i o n e d
388 i n Chapter8.5.5, i s very s u i t a b l e f o r t e s t i n g t h e c l e a n l i n e s s o f microelement s u r f a c e s . TABLE 8 . 7 WETTABILITY
OF SURFACES BY WATER
( r e f . 421)
W ETTAB LE
Metal plus f o r e i g n oxides (alumina, s i l i c a t e s ) Metal p l u s adsorbed chemical f i Ims ( w e t t i n g agents) Hydrated s i 1 i ca (Si -OH surface bonds)
NON-WETTAB LE ~~
-
Go1 d Metal f r e e o f oxides Organic polymers o r surfaces w i t h o r g a n i c f i l m s Surfaces w i t h organics i n the surface Metals p l u s c e r t a i n adsorbed ions (F-) HF etched s i 1 icon ( S i - F bonds) Strongly heated s i l i c a (Si-0-Si surface bonds)
The e v a p o r a t i v e r a t e a n a l y s i s (ERA) t e c h n i q u e h a s been shown to be a v e r y e f f e c t i v e way o f e s t i m a t i n g t h e d e g r e e o f s u r f a c e c l e a n l i n e s s ( r e f . 6 2 0 ) . The p r i n c i p l e o f t h i s t e c h n i q u e i s a s f o l lows: a r a d i a t i v e compound ( d i s s o l v e d i n a s u i t a b l e s o l v e n t ) i s d e p o s i t e d on t h e s u r f a c e t o be a n a l y s e d . The r a t e of e v a p o r a t i o n o f a v o l a t i l e m a t e r i a l from a s u r f a c e i s an i n v e r s e f u n c t i o n of t h e amount o f p r e - e x i s t i n g c o n t a m i n a t i o n . I t i s p o s s i b l e , o f murse, t o monitor t h e r a d i o a c t i v i t y remaining on t h e s u r f a c e ; a contamin a t e d s u r f a c e e x h i b i t s a g r e a t e r amount o f r a d i o a c t i v i t y t h a n a c o r r e s p o n d i n g c l e a n s u r f a c e . The c o n t a m i n a t i o n i s measured i n MESERAN numbers, which s i g n i f y t h e amount o f r a d i o a c t i v i t y remain-
i n g on t h e s u r f a c e ; t h e lower t h e MESERAN number, t h e c l e a n e r t h e surface. The m i c r o f l u o r e s c e n c e method e n a b l e s t h e r a p i d and n o n d e s t r u c t i v e detection, i d e n t i f i c a t i o n of organic p a r t i c u l a t e s without t h e n e c e s s i t y of removing them from o r a l t e r i n g t h e s u r f a c e i n question ( r e f . 6 2 1 ) . I t a l s o makes it p o s s i b l e to i d e n t i f y p a r t i c l e s embed-
ded i n t r a n s p a r e n t l a y e r s and t h e p r e s e n c e o f t h i n o r g a n i c f i l m s . The d e t e c t i o n o f f i l m s i s p a r t i c u l a r l y s e n s i t i v e on e i t h e r t e x t u r e d s u r f a c e s o r t h o s e covered w i t h some form o f p a t t e r n . Using t h i s method, f i l m s have been d e t e c t e d t h a t were as t h i n a s 0 . 3 nm, a s determined by e l l i p s o m e t r y ( r e f . 6 2 2 ) .
3 89
The s u r f a c e p o t e n t i a l d i f f e r e n c e (SPD) t e c h n i q u e c a n b e used f o r m o n i t o r i n g t h e c l e a n l i n e s s o f m e t a l l i c s u r f a c e s ( r e f . 623). The e x p e r i m e n t a l s e t - u p i s shown i n F i g . 8.65. SPD r e p r e s e n t s t h e d i f f e r e n c e i n p o t e n t i a l between t h e test s u r f a c e and a r e f e r e n c e e l e c t r o d e ( p r o b e ) , and any change i n t h e t e s t s u r f a c e c o n d i t i o n
i s r e f l e c t e d i n changes i n SPD. SPD i s u s e f u l f o r d e t e c t i n g b o t h hydrophobic and h y d r o p h i l i c contaminants
b u t t h e s u r f a c e must b e
d r i e d b e f o r e measurements a r e t a k e n .
Electrometer
Ground
1
7
,Test
surface
F i g . 8.65. Measurement o f surface p o t e n t i a l d i f f e r e n c e (SPD) by i o n i z a t i o n method ( r e f .
623).
The indium adhesion t e s t ( r e f . 624) i s a p p l i c a b l e t o b o t h rouqh and p o l i s h e d s u r f a c e s . Both hydrophobic and h y d r o p h i l i c contamin a n t s can be d e t e c t e d . I n t h i s t e s t t h e s u r f a c e t o be t e s t e d i s brought into c o n t a c t w i t h indium and f o r c e i s a p p l i e d t o make a bond; s u b s e q u e n t l y t e n s i l e f o r c e i s a p p l i e d t o d i s r u p t t h e bond. The c o e f f i c i e n t of adhesion i s d e f i n e d a s t h e r a t i o o f t h e t e n s i l e force f o r adhesive f a i l u r e t o t h e j o i n i n g f o r c e ; t h e higher t h e c o e f f i c i e n t of adhesion, t h e c l e a n e r t h e s u r f a c e . The s p e c t r o s c o p i c methods a r e very e f f e c t i v e f o r m o n i t o r i n g s u r f a c e c l e a n l i n e s s . The most commonly used t e c h n i q u e s a r e : Auger E l e c t r o n Spectroscopy ( A E S ) I E l e c t r o n S p e c t r o s c o p y f o r Chemical A n a l y s i s (ESCA) I o n S c a t t e r i n g Spectranehy ( I S S ) and Secondary Ion Mass Spectroscopy (SIMS). Table 8 . 8 p r e s e n t s a comparative summary o f t h e s e t e c h n i q u e s . A r e l a t i v e l y new t e c h n i q u e known a s plasma chromatography h a s been e f f e c t i v e l y u t i l i z e d as an u l t r a - s e n s i t i v e a n a l y t i c a l t e c h n i q u e which p e r m i t s t h e c h a r a c t e r i z a t i o n o f t r a c e contaminants o f t h e o r d e r of p a r t s p e r b i l l i o n o r less (ref. 626).
390 TABLE 8.8
COMPARISON
O F ESCA, AES, ISS/SIMS TECHNIQUES
CHARACTE R I ST I C
ESCA
Minimum Oetectabi 1 i t y L i m i t (one p a r t i n ) elements
...
I SS/S I MS
AE S
1 o4
1o6
103
Examines e n t i r e surface
Topography
( r e f . 625)
I
Can sweep sample w i t h spots a few hundred microns wide. Can u l t i mately produce e l e c t r o n spot s i z e o f 5 pin
Examine e n t i r e surface
In-depth p r o f i l i n g
Possible but slow S p u t t e r i n g r a t e t o order a few hundred Q m i n
Natural consequence o f the technique (0.5 nm/min)
Destructive ? (What m a t e r i a l s can be examined 1 )
Most
Solids only
Depth t o which i n formation i s produced Chemistry
S o l i d s organics o n l y very specia1 conditions
5 nm
2 nm
S e n s i t i v e t o oxidation state and c r y s t a l structure
Some s e n s i t i v i t y t o oxidation state
1 monolayer
S l M S gives m l e c u l a r composition
'
Some i n t e r e s t i n g c o m p a r a t i v e s t u d i e s o f t h e c l e a n l i n e s s o f bearing steel surfaces a f t e r solvent treatments using various t e c h n i q u e s are d e s c r i b e d by B a r n e t t a n d Ravner i n ref. 438. The s u r f a c e - a n a l y t i c a l t e c h n i q u e s o f AES
I
X-ray p h o t o e l e c t r o n s p e c -
t r o s c o p y (XPS) a n d w e t t a b i l i t y c o r r e l a t e d w e l l . W e t t a b i l i t y w a s d e t e r m i n e d by c o n t a c t a n g l e measurements u s i n g t h e sessile d r o p method w i t h d i s t i l l e d water I m e t h y l e n e i o d i d e ( C H 2 1 z ) decane (C16H34).
a n d hexa-
The k i n e t i c s of r e c o n t a m i n a t i o n of c l e a n e d s u r f a c e s depend n o t o n l y on t h e a m b i e n t s t o r a g e b u t a l s o on t h e k i n d o f material a n d t h e method u s e d t o c l e a n t h e s u r f a c e ( r e f . 421). The k i n e t i c s o f r e c o n t a m i n a t i o n of d i f f e r e n t s u r f a c e s when e x p o s e d t o l a b o r a t o r y
a i r are shown i n F i g . 8.66. One p o s s i b l e e x p l a n a t i o n f o r t h e rec o n t a m i n a t i o n p r o c e s s t a k i n g h o u r s o r d a y s i s t h a t t h e contaminat i o n i s a d s o r b e d i n p a t c h e s due t o t h e h e t e r o g e n o u s n a t u r e o f t h e
39 1
s u r f a c e and i t t a k e s a l o n g t i m e f o r t h e f o r m a t i o n o f a c o n t i n u o u s f i l m of c o n t a m i n a t i o n .
1
s
4 5 6
Time
Time (h)
(h)
F i g . 8.66. Recontamination o f cleaned s u r f a c e s i n a d r i e d (a) and wet (b) atmosphere. 1 - Ti02, 2 - S r T i O j , 3 - CrzO3, 4 - N i O , 5 AlzO3, 6 S i O 2 ( r e f . 627).
-
-
The i s s u e o f s t o r i n g c l e a n s u r f a c e s i s v e r y i m p o r t a n t b e c a u s e i f t h e y can be k e p t c l e a n t h e y need n o t b e used immediately a f t e r c l e a n i n g . To s t o r e a c l e a n s u r f a c e i n a p l a s t i c bag i s a c a r d i n a l e r r o r a s most p l a s t i c s g i v e o f f p l a s t i c i z e r s and o t h e r l o w mlecular weight m a t e r i a l ] w i t h t h e r e s u l t t h a t t h e s e m a t e r i a l s a d s o r b and form a f i l m on t h e c l e a n s u r f a c e ( r e f . 6 2 8 ) . Another approach i s t o s t o r e them i n c o n t a i n e r s c o n t a i n i n g c l e a n and o x i d i z e d nichrome o r aluminium p e l l e t s . The r a t i o n a l e b e h i n d t h i s approach i s t h a t t h e hydrocarbons p r e s e n t i n s i d e t h e c o n t a i n e r w i l l s e l e c t i v e l y a d s o r b on t h e aluminium p e l l e t s and t h e t e s t s u r f a c e w i l l s t a y c l e a n . The aluminium p e l l e t s f u n c t i o n a s " g e t t e r s " f o r t h e o r g a n i c contaminants. White h a s c a r r i e d o u t a s t u d y o f t h e e f f e c t i v e n e s s o f v a r i o u s c o n t a i n e r s f o r s t o r i n g c l e a n nichrome, which i s v e r y s e n s i t i v e t o contamination ( r e f . 629)
. The
nichrome s t a y e d cleanest
when c l e a n e d and o x i d i z e d aluminium p e l l e t s w e r e used i n t h e cont a i n e r ( F i g . 8 . 6 7 ) . For t h e aluminium p e l l e t s t o s t a y e f f e c t i v e ] t h e y must be p e r i o d i c a l l y c l e a n e d of adsorbed m a t e r i a l ] which can be done by d e g r e a s i n g and r e f i r i n g i n a i r a t 50OoC. T a m a i e t a l .
39 2
found t h a t t h e c o n t a m i n a t i o n o f o x i d e s u r f a c e s by hydrocarbons can be reduced i f t h e s e a r e s t o r e d i n a w e t a t n o s p h e r e ( r e f . 6 2 7 ) .
Stored for 16h
in
1
C ntact an Le on nichrome after sto& lo' 20'
a'
Laboratory oir Gloss stoppered bottle . empty
oluminium desiccptor
Dosiccatar contatnuq activoted
charcool
Activated alumina
ALurntnium shot
Fig. 8.67. E f f e c t o f storage c o n d i t i o n s on recontamination o f nichrome surface (ref. 629).
8,7,
SPECIAL
INVESTIGATIONS
Various methods have been i n v e n t e d f o r i n v e s t i g a t i n g t h e ,spec i a l p r o p e r t i e s o f r u b b i n g m i n i a t u r e e l e m e n t s . These p r o p e r t i e s include s u r f a c e roughness, w e t t a b i l i t y , various e f f e c t s during journal bearing operation, etc. The s u r f a c e roughness i s t r a d i t i o n a l l y s t u d i e d w i t h t h e h e l p v a r i o u s t y p e s o f p r o f i l o m e t e r s b u t t h r e e - d i m e n s i o n a l computer analy s i s h a s a l s o been s a t i s f a c t o r i l y a p p l i e d for p l o t t i n g t h e s u r f a c e roughness o f s t e e l j o u r n a l s w i t h a d i a m e t e r o f 5 nun ( r e f s . 178, 1 7 9 ) , g i v i n g p e r s p e c t i v e and t o p o g r a p h i c views o f t h e j o u r n a l s u r -
f a c e roughness. The w e t t a b i l i t y o f t h e s m a l l s p h e r e s i n l u b r i c a t e d m i n i a t u r e r o l l i n g b e a r i n g s i s a v e r y i m p o r t a n t f a c t o r i n t h e b e h a v i o u r of such b e a r i n g s (see C h a p t e r 9.4). A diagram of t h e d e v i c e for e s t i mating t h e w e t t a b i l i t y o f s m a l l s p h e r e s i s p r e s e n t e d i n F i g . 8.68.
393
Grop plotter
1
Li
F i g . 8.68. Device f o r e s t i m a t i n g wettabi l i t y o f smal 1 spheres ( r e f . 6 3 0 ) .
The sample s p h e r e i s immersed i n t h e l i q u i d ( o i l ) . The immersion f o r c e r e q u i r e d t o do t h i s i s r e g i s t e r e d v i a t h e l a b o r a t o r y b a l a n c e and t h e c h a r a c t e r i s t i c curve o f t h e immersion f o r c e i s p l o t t e d a s a f u n c t i o n o f t h e immersed d e p t h of t h e s p h e r e . The a n a l y s i s o f t h e p l o t e n a b l e s t h e advancing and r e c e d i n g c o n t a c t a n g l e s t o be determined f a i r l y a c c u r a t e l y . An e x p e r i m e n t a l curve i s p r e s e n t e d i n F i g . 8 . 6 9 . A s t e e l sphere w i t h d i a m e t e r 2.38 mm was immersed i n w a t e r . The immersion f o r c e F can be c a l c u l a t e d u s i n g t h e f o r m u l a (8.54) The mass weighed by t h e b a l a n c e i s (8.55)
I n the above formulas h i s t h e immersion depth (see F i g . 8.701, and ?A a r e t h e d e n s i t i e s o f l i q u i d and
R is t h e s p h e r e r a d i u s ,
pL
39 4
a i r r e s p e c t i v e l y , and g i s a c c e l e r a t i o n due t o g r a v i t y .
Emmersion
L
J
h2
Fig. 8.69. Experimental p l o t recorded a t immersion and emmersion o f s t e e l sphere (diameter 2.38 mm) i n w a t e r ( r e f . 630).
F i g . 8.70.
Sphere immersed i n l i q u i d .
The advancing c o n t a c t a n g l e CX (see F i g . 8 . 7 0 )
can be c a l c u l a t e d
i n t h e f o l l o w i n g way:
a = a r c cos
hl
-
R
R
(8.56)
where h l i s g i v e n i n t h e p l o t p r e s e n t e d i n F i g . 8 . 6 9 . The r u b b i n g s u r f a c e of t h e b e a r i n g bush s h o u l d be s t u d i e d i n o r d e r t o u n d e r s t a n d t h e i n f l u e n c e o f s u r f a c e i n t e r a c t i o n between
39 5 j o u r n a l and b u s h e s on t h e t r i b o l o g i c a l p r o p e r t i e s o f m i n i a t u r e b e a r i n g s . The r u b b i n g s u r f a c e o f s i n t e r e d b r o n z e b e a r i n g b u s h e s ( @ 6 nun) w a s a n a l y s e d before a n d a f t e r e x p e r i m e n t s , u s i n g t h e opt i c a l s y s t e m p r e s e n t e d i n F i g . 8.71 ( r e f . 6 3 1 ) .
+
Bearin bush
F i g . 8.71. D e v i c e f o r a n a l y s i s o f r u b b i n g s u r f a c e of s i n t e r e d b e a r i n g bush.
The b e a r i n g gap o f a p o r o u s b e a r i n g
(0 8 mm) w a s viewed d u r i n g
o p e r a t i o n through a hollow g l a s s s h a f t ( F i g . 8.72, r e f . 334). I n t h i s set-up a mirror-smooth,
a t 5 0 0 r.p.m.
p o l i s h e d h o l l o w g l a s s s h a f t w a s rotated
With t h e a i d o f a s m a l l m i r r o r i n t h e h o l l o w g l a s s
s h a f t and a m i c r o s c o p e w i t h a l a r g e f o c a l l e n g t h , e v e r y p o i n t i n t h e b e a r i n g gap c o u l d be o b s e r v e d . The r u n n i n g - i n p r o c e s s o f p o r o u s b e a r i n g s can be o b s e r v e d usi n g h o l o g r a p h i c i n t e r f e r o m e t r y ( r e f s . 336, 6 3 2 ) . S u r f a c e r e p l i c a s , which can b e m a d e from s i l i c o n e M2000 P o l a s t o s i l r u b b e r by p o u r i n g t h e m i x t u r e o f r u b b e r and h a r d e n i n g a g e n t d i r e c t l y o n t o t h e p a r t o f t h e s u r f a c e i n v o l v e d , are p l a c e d i n t h e immersion c e l l of t h e set-up f o r contouring r e p l i c a s i n t r a n s m i t t e d l i g h t ( F i g . 8.73 a and b )
. A beam
of l i g h t o f wavelength
A
= 6 3 2 . 8 nm from an H e - N e
l a s e r 1 ( F i g . 8.73) i s d i v i d e d by t h e beam s p l i t t e r 2 i n t o an obj e c t i v e beam and a r e f e r e n c e b e a m . T h e o b j e c t i v e beam is r e f l e c t e d by a 1 0 0 % m i r r o r 4 and p a s s e s t h r o u g h t h e immersion c e l l 6 . The t r a n s p a r e n t r e p l i c a 5 i s imaged by t h e h o l o g r a p h i c m i c r o s c o p e 7
39 6 o n t o t h e h o l o g r a p h i c p l a t e 9 . The r e f e r e n c e beam, on r e f l e c t i o n from a n o t h e r 1 0 0 % mirror 3 , i s c o n v e r t e d by t h e o b j e c t i v e 8 i n t o a s p h e r i c a l wave i n c i d e n t d i r e c t l y o n t o t h e h o l o g r a p h i c p l a t e 9 . A hologram i s r e c o r d e d and t h e h o l o g r a p h i c image i s r e c o n s t r u c t e d ,
i n t h i s c a s e by a s p h e r i c a l beam c r e a t e d from t h e l a s e r beam by means of microscope o b j e c t i v e s .
tF i g . 8.72. Device f o r l o o k i n g i n t o b e a r i n g gap d u r i n g rubbing o f elements ( r e f . 3 3 4 ) .
Holographic c o n t o u r i n g of t h e s u r f a c e u t i l i z e s t h e phenomenon of i n t e r f e r e n c e , which o c c u r s d u r i n g r e c o n s t r u c t i o n o f a doubly exposed hologram. Each e x p o s u r e i s performed w i t h a d i f f e r e n t i m mersion f l u i d . The r e l a t i v e d i f f e r e n c e i n d e p t h between two cons e c u t i v e contour l i n e s is c a l c u l a t e d as follows :
A z = zm+l
where
A
-
'ni
-
n1
A -
n2
(8.57)
i s t h e wavelength o f t h e l i g h t u s e d , and n1 and n2 a r e t h e
r e f r a c t i v e i n d i c e s o f t h e immersion f l u i d s . Owing t o t h e i n c l i n a t i o n o f t h e r e f e r e n c e p l a n e t h e c o n t o u r l i n e s can be o b t a i n e d i n t h e form o f a series o f p r o f i l o g r a m s d e s c r i b i n g t h e e n t i r e i n v e s t i g a t e d a r e a . With t h e i r h e l p t r i b o l o g i c a l p a r a m e t e r s such a s t h e r a d i i o f t h e peaks and v a l l e y s , t h e s l o p e s and s t r a i g h t n e s s o f t h e r i d g e s o f t h e peaks and t h e permea b i l i t y of t h e d u c t s i n t h e v a l l e y s can be d e t e r m i n e d d i r e c t l y . The h o l o g r a p h i c method has been s a t i s f a c t o r i l y used to d e t e r -
m i n e stresses i n j e w e l p i v o t b e a r i n g s ( r e f . 6 3 3 ) . These i n v e s t i -
397
g a t i o n s were c a r r i e d o u t i n s t a t i c c o n d i t i o n s u s i n g an o r d i n a r y He-Ne
l a s e r . The s t a r t i n g p o i n t was an i d e a l b e a r i n g , and holograns
c o r r e s p o n d i n g t o e v e r y l o a d w e r e c b t a i n e d e i t h e r from a l a b o r a t o r y model (which was very d i f f i c u l t t o o b t a i n ) o r by u s e o f a computer.
%3"
I
Reference plane
Distance elements
F i g . 8.73. (a) Experimental s e t - u p f o r c o n t o u r i n g r e p l i c a s i n transmi t t e d l i g h t : 1 He-Ne l a s e r , 2 - beam s p l i t t e r , 3.4 100% r e f l e c t i o n m i r r o r s , 5 - t r a n s p a r e n t replica, 6 immersion c e l l , 7 - holog r a p h i c microscope, 8 objective, 9 - holog r a p h i c p l a t e ; (b) Immersion c e l l ( r e f . 6 3 2 ) .
-
-
-
-
T h e breakdown o f l u b r i c a n t f i l m between j o u r n a l and b e a r i n g
e l e m e n t h a s been observed u s i n g a g l a s s p r i s m b e a r i n g ( F i g . 8 . 7 4 , r e f . 6 3 4 ) . The b r a s s j o u r n a l 1 i s r o t a t e d i n t h e b e l t 9 and bmu&t i n t o c o n t a c t w i t h one s i d e o f t h e glass p r i s m 2 . The n a t u r a l l i g h t p a s s i n g through b o t h a d e f l e c t i n g p l a t e 3 and quarter-wave p l a t e 4 i s t r a n s f o r m e d i n t o e l l i p t i c p o l a r i z a t i o n which i s p r o j e c t e d o n t o the area of contact. After r e f l e c t i o n a t t h e metallic surface, t h i s l i g h t i s s h i e l d e d by t h e d e f l e c t i o n p l a t e 5 . The phase o f t h e r e f l e c t i o n on t h e m e t a l l i c s u r f a c e , and t h e t o t a l r e f l e c t i o n on the g l a s s s u r f a c e a r e d i f f e r e n t from each o t h e r , and c o n t a c t o r non- c o n t a c t between t h e s l i d i n g s u r f a c e s can be d i s t i n g u i s h e d w i t h
398 g r e a t a c c u r a c y ( 0 . 0 2 pm) by means o f s h i e l d i n g t h e r e f l e c t i o n .
The
a r e a o f c o n t a c t i s k e p t p e r f e c t l y dark and photographed through t h e microscope 6 . The s h u t t e r of t h e camera i s snapped by m e a n s o f t h e r e l a y and p h o t o t r a n s i s t o r 11. I n t h i s way, even i f t h e number
of r e v o l u t i o n s i s changed, t h e same c o n t a c t l i n e o f t h e j o u r n a l i s recorded. The o i l i s s t o r e d i n t h e l u b r i c a t i n g t a n k 8.
F i g . 8.74. Apparatus f o r s t u d y i n g breakdown o f l u b r i c a n t f i l m between j o u r n a l and beari n g . 1 - a x i s made o f brass, 2 - prism, 3 a quarter-wave p l a t e , 4,s - p o l a r i z i n g p l a t e , 6 - microscope, 7 c e n t r e s , 8 - o i l tank, 9 b e l t , 10 - lamp, 11 - p h o t o - t r a n s i s t o r ( r e f . 634).
-
-
S t u d i e s o f m i n i a t u r e b e a r i n g s under s p e c i a l c o n d i t i o n s (low o r high t e m p e r a t u r e s I o r i n a c o n t r o l l e d environment)
,
r e q u i r e special-
l y designed apparatus ( r e f . 635). A device f o r t e s t i n g t h e t r i b o l o g i c a l p r o p e r t i e s o f m i n i a t u r e j o u r n a l b e a r i n g s a t low temperat u r e s i s d e p i c t e d i n F i g . 8.75. The j o u r n a l 1 r o t a t e s on t h e r o l l i n g b e a r i n g s . The b e a r i n g bush t o be t e s t e d 2 i s p l a c e d i n t h e h o l d e r 3 and loaded v i a t h e c y l i n d e r 4 by t h e w e i g h t s Q, and Q2. The j o u r n a l is d r i v e n by t h e e l e c t r i c motor 7. The r o t a t i o n s p e e d
i s measured u s i n g t h e t a c h o g e n e r a t o r 1 0 on t h e s c a l e 11. The extensometric system c o n s i s t s of t h e l e v e l 5 with t h e extensometer 1 2 , a m p l i f i e r 13 and o s c i l l o s c o p e 1 4 .
The c o o l i n g system e n a b l e s t h e t e m p e r a t u r e t o be lowered below O°C,
t o -12OOC.
L i q u i d n i t r o g e n vapour i s used a s t h e c o o l i n g me-
399 dium. The Dewar v e s s e l 1 5 , a u t o t r a n s f o r m e r 1 6 , s p i r a l 1 7 , tube 2 3 and low t e m p e r a t u r e chamber 18 (made o f b r a s s ) make up t h e c o o l i n g system.
F i g . 8.75. Device for t e s t i n g t r i b o l o g i c a l p r o p e r t i e s o f m i n i a t u r e j o u r n a l bearings a t low temperatures. 1 - j o u r n a l , 2 - b e a r i n g bush, 3 holder, 4 - cylinder, 5 level, 6 f i x i n g n u t , 7 - DC motor, 8 rectifier, 9 - v o l t a g e r e g u l a t o r , 10 tachogenerator, 1 1 - scal e , 12 - extensometers. 13 - a m p l i f i e r , 14 - o s c i l l o s c o p e , 15 Dewar vessel, 16 a u t o t r a n s f o r m e r , 17 - h e a t i n g s p i r a l , 18 - low t emp era t u re chamber, 19 discs, 20 - i s o l a t i n g co ver made o f PTFE, 21 thermoelement, 22 e l e c t r o n i c potentiometer, 23 t u be ( r e f . 6 3 5 ) .
-
-
-
-
-
-
-
-
-
-
-
M i n i a t u r e b e a r i n g s can be t e s t e d i n a c o n t r o l l e d e n v i r o n m e n t using t h e apparatus i l l u s t r a t e d i n Fig.
8.76.
The r o l l i n g b e a r i n g s
t o b e t e s t e d are s u p p o r t e d on t h e p r i s m s 4 a n d t h e i r motion i s mas u r e d by t h e pendulum s y s t e m 1. The p r i s m s 4 a r e p l a c e d on t h e p l a t e 3 s u p p o r t e d on t h e k n i f e b e a r i n g 2. The a p p a r a t u s i s contained i n t h e PMMA box 1 4 . The f r e e e n d o f t h e p l a t e 3 is s u p p o r t e d by t h e f l a t s p r i n g w i t h g l u e d e x t e n s o m e t e r 15. The f l a t s p r i n g c a n b e a d j u s t e d by t h e e l e m e n t 1 7 of t h e l e v e l - s p h e r e s t r a n s m i s s i o n 5. A t t h e v a r i a t i o n
400
o f t h e i n c l i n e a n g l e o f t h e p l a t e 3 t h e pendulum k e p t by t h e f r i c t i o n t o r q u e d e v i a t e s f o r t h e a n g l e 9 a t t h i c h t h e pendulum w e i g h t moment i s h i g h e r t h a n t h e f r i c t i o n t o r q u e . The v a l u e o f t h e a n g l e 'f
can be r e a d on t h e s c a l e 7 on which t h e l i g h t beam r e f l e c t e d by
t h e mirror 1 2 i s focussed.
1
F i g . 8.76. Device f o r s t u d y i n g t r i b o l o g i c a l p r o p e r t i e s o f m i n i a t u r e b e a r i n g s i n a cont r o l l e d environment. 1 pendulum system, 2 - k n i f e bearing, 3 - p l a t e , 4 - prisms, 5 l e v e l mechanism, 6 - knob, 7 , 9 scales, 8.11 t e l e s c o p e s , 10 - t e l e s c o p e i l l u m i n a t o r , m i r r o r s , 1 4 - c o v e r , 15 - extenso12,13 b e a r i n g s b e i n g t e s t e d , 17 pushmeters, 16 e r , 18 - spheres ( r e f . 6351.
-
-
-
-
-
-
-
The i n c l i n a t i o n a n g l e of t h e p l a t e 3 v a r i e s s l o w l y , e . g .
0.5
rad/min, and t h e pendulum a c c e l e r a t i o n i s p r a c t i c a l l y 0 . The equat i o n o f reduced f r i c t i o n t o r q u e f o r t h e b e a r i n g s b e i n g t e s t e d i s M f = G L sin
(8.58)
The a n g l e o f d e v i a t i o n o f t h e pendulum i s g i v e n by
s
'P'E
(8.59)
where: S i s t h e d i s p l a c e m e n t o f t h e l i g h t beam r e f l e c t e d by t h e m i r r o r 1 3 and r e a d on t h e s c a l e 9 , D i s t h e d i s t a n c e between t h e m i r r o r and t h e s c a l e and G and L a r e t h e pendulum w e i g h t and length respectively.
40 1 The f r i c t i o n t o r q u e can be determined u s i n g t h e formula Mf % G L
S 2 D
(8.60)
The l i f e and t o r q u e o f b a l l b e a r i n g s depends on t h e s t a t e o f t h e l u b r i c a n t i n t h e b e a r i n g . I m p u r i t i e s , i n p a r t i c u l a r , have a s t r o n g e f f e c t on t h e b e h a v i o u r o f l u b r i c a t e d b e a r i n g s . To p r e d i c t t h e l i f e of s e a l e d b e a r i c g s it is important to d i s c o v e r how d u s t p r o o f t h e y a r e . The amount o f powder o f known g r a n u l a r i t y which would g e t through a b e a r i n g seal can be e s t i m a t e d on t h e b a s i s o f t h e d a r k i n g g r a d e of t h e l u b r i c a n t used ( r e f . 6 3 6 ) . The d a r k i n g grade o f t h e l u b r i c a n t , D, can be determined a p p r o x i m a t e l y w i t h t h e formula
D = lOg-=k
bcd3 d4
P
(8.61)
+ A 40
where Po and P a r e t h e i n t e n s i t i e s o f i n c i d e n t i n g and t r a n s m i t t e d l i g h t r e s p e c t i v e l y , k i s t h e p r o p o r t i o n a l i t y c o n s t a n t , b t h e thickn e s s o f t h e sample l a y e r , c t h e c o n c e n t r a t i o n of t h e powder i n t h e l u b r i c a n t , d t h e average d i a m e t e r o f t h e powder g r a i n and J o t h e l i g h t wavelength. A t determined b , d and
Jo v a l u e s t h e d a r k i n g g r a d e o f t h e l u -
b r i c a n t i s p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n o f powder i n t h e l u b r i c a n t . The d a r k i n g g r a d e o f t h e l u b r i c a n t can be e s t i m a t e d u s i n g t h e d e v i c e i l l u s t r a t e d i n F i g . 8.77 ( b a s e d on r e f . 6 3 6 ) . A v e r y s m a l l amount o f l u b r i c a n t (below 0 . 1 9) is p l a c e d i n t h e measuring probe. The l i n e a r i n d i c a t o r 12 measures t h e d a r k i n g g r a d e o f t h e l u b r i c a n t on a s c a l e o f 0 - l o o o
. The
o t h e r probe can measure t h e
ageing r e s i s t a n c e o f t h e l u b r i c a n t by d e t e r m i n i n g t h e a b s o r p t i o n o f t h e i n f r a r e d r a d i a t i o n i n t h e range o f 1700-1600
cm-l.
This
measuring probe can a l s o be used t o d e t e r m i n e t h e a g e i n g resistance o f g r e a s e s , a p p l y i n g t h e t h i n f i l m test f o r e s t i m a t i n g t h e v o l a t i l i t y of l u b r i c a t i n g greases ( r e f . 637). The e l e c t r i c a l methods a r e v e r y e f f e c t i v e f o r d e t e r m i n i n g vari o u s l u b r i c a n t p r o p e r t i e s , e s p e c i a l l y t h e r m a l and a g e i n g (oxidation)
r e s i s t a n c e , s i n c e e l e c t r i c a l s i g n a l s can b e e a s i l y t r a n s m i t t e d , handled and monitored. The t h e r m a l r e s i s t a n c e o f a l u b r i c a n t ( t h e t e m p e r a t u r e of d e s o r p t i o n ) can b e e s t i m a t e d by measuring t h e varia t i o n i n t h e e l e c t r i c a l r e s i s t a n c e o f a v e r y t h i n and s m a l l p i e c e o f w i r e o r f o i l p l a c e d on t h e f r i c t i o n s u r f a c e ( r e f . 6 3 8 ) . A t it w a s mentioned a l r e a d y ( S e c t i o n 5 . 5 o f t h i s c h a p t e r ) it is p o s s i b l e
402
t o d e t e r m i n e t h e o x i d a t i v e s t a b i l i t y of l u b r i c a n t s by u s e of a s i m p l e e l e c t r o n i c ( s e m i c o n d u c t o r ) g a s s e n s o r which r e s p o n d s t o gaseous o x i d a t i o n p r o d u c t s . A s e n s o r s u c h a s t h e FIGARO model 2 0 2 , s i m i l a r t o t h o s e used t o d e t e c t a c o m b u s t i b l e atmosphere , r e s p o n d s e q u a l l y w e l l t o d e u t e r a t e d and n o n - d e u t e r a t e d l u b r i c a n t s and h a s been s u c c e s s f u l l y used t o m o n i t o r l u b r i c a t i o n systems i n c l u d i n g s y n t h e t i c hydrocarbons
,
a z e l a t e e s t e r s , and p e n t a e r y t h r i t o l t e t r a -
esters ( r e f s . 6 3 9 , 8 9 3 ) .
12
Fig. 8.77. Device f o r determining darking or ageing grade o f l u b r i c a n t . 1 light source, 2 - l u b r i c a n t sample, 3 - conphotoelement, 6 denser, 4 - f i l t e r , 5
-
-
-
system f o r compensation o f non-balancing voltage, 7 logarithmic a m p l i f i e r , 8 system compensating p o l a r i z i n g c u r r e n t o f logarithmic amplifier, 9 differential a m p l i f i e r , 10 system of c o n t r o l l e d variable, 11 amplification corrector, 12 linear indicator.
-
-
-
-
-
The t r i b o l o g i c a l p r o c e s s e s such as m a t e r i a l t r a n s f e r , k i n e t i c s o f t h e f r i c t i o n and wear, etc. i n u n l u b r i c a t e d and lubricated s y s -
t e m s can b e s u c c e s s f u l l y s t u d i e d by u s e o f t h e a f o r e m e n t i o n e d t e h , 8.5.4 , 8.5.5 , 8 . 6 ) The t e c h n i q u e s for
niques (Chapters 6.2.6
.
40 3
t h e s e purposes a r e a l s o d e s c r i b e d i n d e t a i l i n r e f s . 2 , 1 1 7 , 1 6 5 , 304, 8 9 8 , 901-903.
404
9, TRIBOLOGICAL ASPECTS OF FINE MECHANISM ASSEMBLIES 9 , 1 INTRODUCTION I
In the many assembliesused in fine mechanisms the tribological phenomena are important factors in determining the energy dissipation (and loss) and life of the whole mechanism. Typicalassemblies, such as bearings, gears, transmissions, couplings etc., are W e discussed mainly from the tribological point of view, since other design problems are analysed in detail in, for example, refs. 3, 3 8 3 , 6 4 0 and 9 5 5 . The fundamental formulae for determining tribological behaviour (the friction coefficient or torque, wear rate or life) and also other parameters which have a direct effect on the tribological behaviour of an assembly are also considered here.
9.2, TYPICAL P L A I N B E A R I N G S Cylindrical journal bearings with elements made of various materials - metals , polymers , ceramics, graphite and carbon-graphites are often used. The clock-type journal bearing has already beenpresented in Fig. 5.1. The bearing hole is cylindrical and drilled in the mechanism plate, which is usually of brass. The journal is made of a free-cutting or stainless steel and is roller-burnished to Ra<0.2 pm. Clock-type bearings often have a mineral bearing bush (of ruby or sapphire) and a steel journal. Other ceramic materials such as cemnted carbide (WC), agate or glass are also used in the manufacture of bearing bushes and journals (WC is used for rubbing against ceramic bearing bush materials Some typical ceramic bearing bushes (bearing jewels) are illustrated in Fig. 9.1. The forms and dimensions of bearing jewels are given by D I N 8 2 5 6 , 8 2 5 7 , 8 2 6 2 or GOST 7 1 3 7 - 7 8 , 8 8 9 6 - 7 6 standards. Bearing bushes made of sintered metals (bronze, iron) and impregnated with lubricant are also useful since their self-lubrication can last for 2 0 0 0 - 3 0 0 0 h (see Chapter 5 . 1 . 2 ) . Bushes or various forms (see Fig. 9.2 and ref. 1 9 6 ) manufactured in plyrrreric materials (mainly thermoplastic polymers) can also be an effective answer to the bearing problems of miniature mechanisms, particularly in mass production (refs. 1 9 6 , 9 0 4 - 9 0 8 ) . The bushes are glued or forced into a metallic body (refs. 6 4 1 , 6 4 2 ) . Sometimes the bearing holes are
.
4 05
formed directly in the body when it is made of a polymeric material. Polymeric bearing bushes are also manufactured by Outsert technique on a metallic plate (ref. 6 4 3 ) . Metallic bushes lined with a polymeric material or impregnated like the sintered porous bronze in W Glacier Metal Co. bearings are also applied (refs. 37, 48, 4 8 9 ) . The surface modification of metal elements by ion implantation improves the tribological behaviour of metal-polymer bearings (ref. 9 1 1 ) . Miniature bearings with both journal and bearing bush made from polymeric materials are also used. Miniature bearings with bushes made of graphite or carbon-graphite demonstrate very good tribological behaviour (refs. 35, 3 7 , 9 0 9 , 9 1 0 ) .
F i g . 9.1.
T y p i c a l ceramic b e a r i n g bushes ( b e a r i n g j e w e l s ) .
Metal and ceramic bearings are lubricated to ensure a low wear rate and relatively small friction coefficient (about 0.2 under boundary lubrication conditions). The rubbing elements are often coated (epilamized) to prevent oil migration from the bearing. Lubrication can also be applied to polymeric and other bearings which improves their tribological behaviour, reducing wear in particular (refs. 873, 9 1 2 - 9 1 4 ) . Hydrodynamic lubrication is rarely found in the bearings used in fine mechanisms because of lubrication with only one drop of oil is usually applied and because of the typical operating conditions (low sliding speed and high contact pressure). However, hydrodynamic lubrication is possible at very low sliding speeds in poly meric bearings because of the high elasticity of the polymeric el+ ment (ref. 360, see also Chapter 5 . 2 . 1 ) . Self-lubricating bearings with bushes made of sintered porous metal or polymer impregnated with oil also can operate under hydrodynamic lubrication conditions (refs. 9 1 5 , 9 1 6 ) .
406
il
Fig. 9.2. T y p i c a l p o l y m e r i c bushes o f m i n i a t u r e bearings.
The admissible operating conditions for lubricated bearings depend mainly on the lubricant used. Typical thermoplastic polymeric materials are unsuitable at elevated temperatures. A comparison of the behaviour characteristics of some self-lubricating and unlubricated bearings is given in Table 9.1. The admissible pv values are given only for comparison and as a guide since they also depend on the geometry and design of the bearing, as well as on the environmental conditions, More exact pv values for miniature steel-polymer journal bearings can be found in Chapter 5.2.1 (Fig. 5 . 1 3 ) . Bearings based on sintered porous metals filled with a solid lubricant (PTFE, MoS2) and PTFE or PI bearings are applicable for
4 07
use in a vacuum. T A B L E 9.1 COMPARISON OF BEHAVIOUR C H A R A C T E R I S T I C S O F S E L F - L U B R I C A T I N G AN0 UNLUBRICATEO JOURNAL BEARINGS (based on r e f s . 644 and 645).
i S INTEREO S I NTEREO POROUS POROUS OIL-IMPREGNATED METAL I M PREGNATED WITH GRAPH I T E OR MoS2
BEAR ING
o r r o s ionresistance
0.04-0.12
I I
I
hemical ntertness dmi s s i b l e ealistic ontact Nressure,
POM PREGREASEO IN BRONZE
1
good
I
poor
1
no
I 1
0.10-0.15
0.07-
0.080.25
0.20
-200 280
-40 80
-4-k 1 I 1.2-0.4 0.1-0.2
-130 80
I good
v e r y good
(sea w a t e r
v e r y good
I
I
2-4
1 I
80-180
1
-200 350
iz:
70
110
J dmi s s ib l e PV, 1Pa m/s p - contact Nressure, - sliding peed)
ITIZED CARBON
I
1
'r i c t i o n oefficient
PTFE/ LEA0 MPREGNATEO POROUS METAL
0.5-2.5
1.5-2.5
0.05
0.2
1:;-
good
100200
The design of bearings depends on the materials applied. For metal and ceramic miniature bearings, a typical fit used for d>l mm (d is the nominal diameter) is the IS0 free-running fit d9-H9 and for d < l mm a cd9-H9 fit. The fit for bearings with bearing bushes (and sometimes also journals) made of materials with a relatively high thermal expansion coefficient should be chosen very carefully. During the operation of bearings made of polymeric materials the bearing clearance required depends on the humidity of the ambient air. This is because PA 6 and PA 66 materials absorb water. The minimum value of the diametral clearance (s,) of the
408
bearing when the bearing bush is forced into the metal body of the mechanism, can be determined using the following formula (ref. 220): sm> kt +
% + kp
(9.1)
where kt and kh are the reductions in bearing hole diameter resulting from increases in temperature and humidity respectively, and k is the most probable decrease in bearing hole diameter required P by the forcing of the bearing bush into the metal body. Eqn. (9.1) is valid for bearings with a steel journal (or any journal made of a material with a low thermal expansion coefficient). The value of kt is given by: (1+J) (1-k2) 1 (1-23)+k
kh
=
(3"
-(-
d
1 1-k
-
AT (9.2) 21nk where at is the coefficient of thermal expansion, d ist the naninal bearing hole diameter, J is Poisson's ratio, k is the ratio of the bearing hole diameter to the external diameter of the bearing bush, and AT is the predicted temperature rise. The value of kh can be predicted using the formula kt = cXtd
T)] 1
(9.3)
where p is a coefficient which for PA can be taken as 0.005-0.001, has a higher value for PA 6, a lower one for PA 66, and for other polymers is equal to 0; W is the water content in the polymer, in %; and d is the nominal diameter of the bearing hole. The value of kD is given by the formula (9.4) where 3 and k are as in eqn. (9.2); el and e, are the minimum and maximum bush interferences, respectively; and 6, and 6, are mean square deviations from nominal values of the external diameter of the bearing bush and the diameter,of the body hole (6, = T1/6 and 6, = T2/6, where T1 and T, are the working tolerances for the external bearing bush and the body hole diameters respectively). In order to determine the maximum value of the realistic contact pressure in a bearing it is important to be able to predict the elastic deformation on rubbing elements in journal bearings. The solution of the complex elastorheological problem of journal
409
and bearing bush contact in a miniature bearing, after taking friction into account, has made it possible to determine the contact pressure distribution p(Y) in the following way (ref. 2 0 3 ; and see Fig. 4 . 1 7 ) : (9.5)
yo
where pm is the maximum value of contact pressure, 2 is the angle of contact, and a and b are parameters. The maximum value of the contact pressure pm can be foundusing either the formula when Ip0<200 or the formula A2
pm = (A1 + -1
when
'po > 20'
(9.7)
'PO
where A1, A2 and c are parameters; y o is as in eqn. ( 9 . 5 ) (in radians) ; and p is the bearing contact pressure (p = N/(dl)), where N is the applied normal load, d the bearing hole diameter, and 1 the length of contact between the journal and the bearing bush). can be determined using the folThe half angle of contact, lowing formula:
yo,
where
do
= Es
.N
mm
,
N being the applied normal load, E the elas-
ticity modulus of the bearing bush material, and s the diametral clearance; kl, m, c, A l l A 2 , a and b for various miniature journal bearings are listed in Table 9 . 2 (based on ref. 2 0 3 ) . Having these values and using eqns. ( 9 . 6 ) or ( 9 . 7 ) the value of pmcan be found and compared with the admissible value of the yield stress for softer material. When pm is known, the radial deformation of the element under load can be determined by use of eqn. (4.10) (based on Hookers law). For polymeric bearings, the important thing to determine is the so-called seizure number (ref. 646), which should not be exceeded for safe operation. The seizure number, L,, is defined in
TABLE 9.2 COMPUTED VALUES OF PARAMETERS kl,
* * 0
m, c , A l , A * , a and b PARAMETERS
kl
BEAR I N G
rn
C
Al
A2
a
b
Unl ubr i cated
0.77
0.32
3.97
0.49
1.12
1.60
0.40
Lubricated
0.70
0.27
3.98
0.31
1.17
1.83
0.44
Unlubricated
1.40
0.44
3.99
0.49
1.12
1.56
0.40
Lubricated
1.24
0.40
3.98
0.28
1.19
1.85
0.45
Uniubricated
0.77
0.35
3.97
0.32
1.16
1.87
0.44
Lubricated
0.71
0.34
3.97
0.28
1.17
1.89
0.44
Unl ubr i cated
0.78
0.39
3.97
0.37
1.15
1.87
0.44
Lubricated
0.72
0.38
3.97
0.33
1.16
1.89
0.44
Unl ubr i c a t e d
0.64
0.21
3.98
0.31
1.17
1.83
0.44
Lubricated
0.67
0.24
3.98
0.27
1.18
1.83
0.44
Un’ubricated
2.18
0.49
3.98
0.25
1.20
1.88
0.45
Lubricated
2.17
0.49
3.98
0.24
1.20
1.89
0.45
Steel-brass
S t e e 1 -ruby
PA 6 S t e e 1 -po 1 ymer POM h
Polymer (POM h)-polymer (PA 6)
polymer(PA 66+20% PTFE)-polymer(PA 66+30% glass f i b r e s )
1
411
the following way: Ls -
PV -
(9.9)
Y
where p and v are respectively the bearing contact pressure and sliding speed at which the temperature rise in the friction area is so high that necking (tightening) of the bearing bush on the journal occurs and thermal ratchetting occurs in the bearing; L+J is the relative radial clearance. Taking into consideration eqn. ( 9 . 2 ) , putting kt = s ( s being the diametral clearance) and introducing A T from eqn. (4.21), the following formula can be used for the estimation of Ls for unlubricated miniature steel-polymer journal bearings: Ls =
[(1-25) 1150
J fdd In TF1 CC
+ k2]h 1
(1+J) (1-k
1
(9.10)
where the nomenclature can be found after eqns. ( 4 . 2 1 ) and ( 9 . 2 ) . The value of the friction coefficient fd can be predictedusing eqn. (4.5) or determined experimentally. The tribological properties of the bearings are frictional losses (defined by the friction coefficient or torque) and mass losses (defined by the wear rate). The life of the bearing can be defined by the length of time up to seizure (the rapid and high increase of the friction coefficient or torque) or by the time needed for the assumed wear rate limit to be exceeded. The steady-state friction torque in miniature bearings during prolonged use is often a very important factor in their design. The estimation of the aforementioned losses in a tribological system is especially meaningful for miniature journal bearingswhen radial wear can reach significant values, as in the case of polymeric bearing. The tribological behaviour of the commonly used steel-polymer bearings can be defined by determining their so-called wear and frictional torque numbers (refs. 6 4 7 , 6 4 8 ) . The frictional torque Mf is expressed by the formula
(9.11)
J ,
- Ip,
412 where d is the nominal diameter of the bearing hole, 1 is the length of contact of the journal and bearing bush, p(q) is the contact pressure distribution, and qA the half angle of contact when radial wear is taken into consideration (see eqn. (4.9) Since from Archard's formula it results that (ref. 649)
.
(9.12) (where w(g) is the radial wear of the bearing bush measured perpendicularly to friction surface (Fig. 9.3); dw = dw(V)cos(P , L is the sliding distance, p(9) is as in eqn. (9.11) , and Wo is the wear modulus), the frictional torque, Mf, after the introduction of p((p) from eqn. (9.12) to eqn. (9.11) and integration, can be expressed as follows: I
=
2
d 2 If Wo
sinqi
(9.13)
After taking into consideration the static balance equation of the journal (see Fig. 9.31, p cos 4 dl = dl
f
p(y) (cos
+ f sinq )ydq
(9.14)
0
and remembering that tanfi = f, from eqn. (9.12) we obtain w
=
2P
(9.15)
If we take into consideration the suggestion from ref. 647, the wear number Lw can be expressed as follows: (9.16)
when f = 0 we obtain Lw the same as in ref. 647. The frictional torque Mf can therefore be determined from the formula (9.17)
413
Fig.
9.3. Contact
o f journal ( 1 ) w i t h b e a r i n g bush ( 2 ) . I
For determining To eqn. (4.9) can be applied. If we accept the suggestion from ref. 648 the frictional torque number LM will be given by
The variation in the frictional torque Mf depends on the variation in the value of the frictional torque number during the wearing out process: 1 2 Mf = 7 d lfpLM
(9.19)
when the coefficient of friction is assumed to be constant. The frictional torque increases to a maximum of 1.273 times the value of frictional torque at zero wear. Wear prediction in unlubricated and lubricated miniature steel-polymer journal bearings is discussed in detail in Chapters 4.2.1 and 5.2.1 respectively.
414
Steel-brass miniature journal bearings operating under a very high load (bearing contact pressure over 7 MPa and sliding speed below 2 m/s) will not seize up when the operating time is very short. The reliability functions of such unlubricated bearings are unlubricated bearings are given in Chapter 4.1. The reliability model of the seizure of such bearings, based on the adhesion-decohesion hypothesis of seizure, results in the following general reliability formulae (ref. 6 5 0 ) : - the distribution function of time period to the bearing's seizure F(t) = 1
-
-
exp
[A1q-
+ ,u 0 )t - - e *1
1
(9.20)
the frequency function -*lt)]
expLq(l-e A, -slt)-(A,+ ,uO)t]
(9.21)
- the failure (seizure) rate (9.22)
where p 0 and,u1 are parameters of distribution and frequency functions respectively, and is constant. In miniature metal or mineral bearings (generally when the bearing bush is made of relatively hard material), which usually operate under boundary lubrication conditions, the size and shape of surface irregularities are major factors in bearing performance (ref. 6 5 2 ) . The misalignment of the journal and two bearing surfaces and the ordinary surface roughness and waviness normally found on 'miniature journal bearings can prevent line contact. The contact is then made at two points on the bearing (bush). The journal, being misaligned and having surface irregularities of its own, starts by rolling over a series of surface irregularities on the bearing, resulting finally in its rolling up the bearing wall to a height at which neither rolling nor interlocking can be sustained, and in two-point contact and relatively steady-state slip (Fig. 9 . 4 ) . depends on the fricThe efficiency of the bearing system, tion and surface irregularities, and can be expressed by the formula (ref. 6 5 2 )
A,
Tb,
415
1-12b =
U
-
U -
( l + - ) -U l2
(9.23)
P
where l1 and l 2 are input and output thrust and load moment arms, P is the input thrust, and G the shaft assembly weight (see also Fig. 9 . 4 ) .
Input thrust P
F i g . 9.4. C o n t a c t o f J o u r n a l ( 1 ) w i t h b e a r i n g s u r f a c e (2) h a v i n g i r r e g u l a r i t i e s ( 3 )
.
When the journal slips on one surface irregularity (Fig. 9 . 4 ) factor u is calculated from the formula u = R - sine cos 0
(9.24)
where R is the journal radius, and @the angle of friction: f = tan@(f is the friction coefficient) and contact angle 6 can be determined as follows:
416
S
( l + - 2R -
r -12R
(9.25)
where s is the diametral clearance, and r the height of the surface irregularity. If, however, surface irregularities interlock, a condition that applies at breakaway, then u
= R
sin ( 4 + 2 e )
(9.26)
If the journal slips on two surface irregularities, then u = R
sin 4 EiEZi-3
(9.27)
and if the surface irregularities are removed (the condition corresponding to the condition where u is minimum) eqns. ( 9 . 2 4 ) , ( 9 . 2 6 ) and ( 9 . 2 7 ) all reduce to u
=
R sin#
(9.28)
The maximum efficiency qbmax is calculated as follows:
bmax
--
1 - 1 + -
U
I1 U
(9.29)
12
The efficiency is zero when
-GP <' l -l
u
-
u
(9.30)
The effect of the thrust-to-weight ratio (P/G) on the efficiency is presented in Fig. 9.5 a (ref. 6 5 2 ) . The efficiency decreases drastically as surface irregularities become larger. The maximum efficiency differs substantially for the following three operating conditions: interlock, sliding on one point, and sliding on two points (Fig. 9 . 5 b, ref. 6 5 2 ) . The maximum efficiency also varies with the bearing clearance, journal size, and the ratio of the input and output load moment arms. The effects of these parameters are presented in Fig. 9 . 5 c, d and e respectively (ref. 6 5 2 ) . When there are no irregularities on the bearing surface the clearance has no effect on the efficiency (Fig. 9 . 5 c).
417
0.0005 0.001 0.002
Thrust-to-weight
ratio P/G
0605 0.01 0.02
0.05 01
surface irregulority si?e r/R
c)
Bearing clearance s/2R
Journal size ~ / i 1
e)
Moment-orm
ratio
142
Fig. 9.5. Efficiency o f miniature journal bearing v s . thrust-to-weight ratio (a), surface irregularity size (b), clearance (c), journal size (d) and Mment-arm ratio (e). Graphs prepared for bearing with friction coefficient f = 0.2, input thrust moment arm (see Fig. 9.4) 1 1 = 3 R ( R is the journal radius), output load moment arm 12 = 4 1 1 , s/2R = 0.04 ( s is the diametral clearance), and maximum moment arms 1 1 and 12 = 3 R (Fig. 9.5 e) (ref. 652).
418
A very smooth surface finish reduces the influence of journal size on maximum efficiency (Fig. 9 . 5 ) . Let us consider the effect of the moment arm ratio on the maximum efficiency for a typical s t e p -up gear train supported by journal bearings. At the start of engagement, bearing efficiency is at a peak and tooth efficiency is at a low point. They are almost equal at this point. The tooth efficiency increases rapidly as the teeth begin to mesh and the bearing efficiency decreases slowly; as a result, the bearing efficiency dominates the overall system efficiency. Depending on the type of application, step-up or step-down, changes in the moment-arm ratio can have differing effects on the maximum efficiency (Fig. 9.5e). For lubricated miniature journal bearings, the sliding speed at which the friction coefficient is minimum and hydrodynamic lubrication begins to occur can be determined by use of the method for lubricated steel-polymer bearings described in Chapter 5.2.1. For other bearings (with a bearing bush made of a hard material) the same procedure can be applied, only the K parameter should be determined experimentally or in some other way. The author's own analysis (ref. 6 5 1 ) has shown that the K parameter circumscribing the outflow of oil from miniature steel-brass or steel-ceramics clock-type journal bearings can be approximately determined using Vogelpohl's formula
19.31)
where 1 and d are as in eqn. (9.11). For miniature journal bearings whose bearing bush is made of sintered porous bronze, the sliding speed at which the friction coefficient is minimum and hydrodynamic lubrication begins to occur can be predicted using eqns. ( 5 . 7 ) and ( 5 . 9 ) . The hydrodynamic lubrication in bearings where the pivoting friction occurs is discussed in ref. 917. The above considerations apply to miniature cylindrical journal bearings. As was already mentioned in Chapter 5.1.2, miniature journal bearings with a prismatic bearing bush (Fig. 5.7) demonstrate better performance than cylindrical bearings, by assuring the precise positioning and high stability of the journal axis during operation. The frictional torque, M can be calculated fP' using the following formula (refs. 3 4 3 , 6 5 3 ) :
419
where the parameters used are shown in.Fig. 9.6; fc is the computational friction coefficient] f = tan& is the friction coefficient] and #'is the angle of friction.
F i g . 9.6.
Load and geometrical form o f p r i s m a t i c b e a r i n g .
The fractional element in eqn. (9.32) is the computational friction coefficient f As may be seen in eqn. (9.321, the frictional C' torque in prismatic bearings depends on the bearing's geometrical form (circumscribed by angleCI) and on the load direction (circumscribed by angle q ) . The variations in the frictional torque of these bearings can therefore be high at variations in the load direction. The dependence of the computational friction coefficient on the load direction for bearings with a triangular ( c C = 120°), square ( d = 90') , or hexagonal ( C I = 60') bearing bush at the angles of friction = 8'30' (f = 0.15) and ff= 16'50' (f = 0.3) , are presented in Figs. 9.1 a and 9.1 b respectively. The greatest range of variation in load direction is possible in bearings with a triangular bush. The admissible range of variation in load direction is a function only of 4 and for triangular, square and hexagonal bushes is 1 2 0 ° 1 90' and 60' respectively. The index of thevariation of the computational friction coefficient, Kc, can be defined as: -
Kc -
a fC
-q-
(9.33)
420
I
1.0 160"
I
75'
I
I
goa
1050
120"
-
o(
Fig. 9.7. Computational friction coefficient fc vs. load direction (q, see Fig. 9.6) (a,b), and ratio of maximum to minimum value o f computational friction coefficient v s . angle CC (see Fig. 9.6) (c). Triangular ( a = 1 2 0 ° ) , square ( a = go"), hexagonal ( a = 60") f o r m o f bearing bush, angle of friction ?Y= 8"30' (f = tanfl= 0.15, f friction coefficient) ( a ) , d = 16'15' (f = 0.3) (b).
-
421
For bearings with triangular, square and hexagonal bearing and fl= 16"50', Kc is on average (at the linear bushes at o = 8'30' approximation) 0,004, 0 , 0 0 2 and 0.001 per degree and 0.006, 0.003 and 0.015 per degree respectively. It follows from eqn. (9.32) that the minimum value of the computational friction coefficient fc occurs when Cp+s=90" or d + g = 90" + ct and equals sin$. When y+flis ca. 90°+ the var2 iations in the computational friction coefficient as a result of variations in load direction are at their lowest, but the computational friction coefficient itself is relatively high. The selection of the load direction has to be a compromise since at
the computational friction coefficient is smaller, but the variations in it are greater. If the spread of the values of the computational friction coefficient, Sc, is defined in the following way, where the spreads of the values of v , o l andeare notated as SIP' Scl and Sr:
(9.35)
then at the assumed cC and Sq = S,y values the effect of the variations of the angle of friction e o n the variations of the computational friction coefficient is higher than the variations of the angle q . The ratio of the value
(9.36)
It can be seen that the effect of variations in the angle of frictionfon the computational friction coefficient is at its greatest in bearings with a triangular bush, and at its smallest in those with a hexagonal bush. The smaller the angle of friction,
422 the greater its effect on the computational friction coefficient. The ratio of the maximum to the minimum value of the computational friction coefficient as a function o f K i s shown by Pig. 9.7 c and d. The bearing clearance also has an effect on the frictional torque of the bearing. The experimentally determined frictional torque regression formula for cylindrical and prismatic miniature journal bearings with a sintered porous bronze bush is expressed by eqn. (5.8)(refs. 341, 343). The spread of the frictional torque SM at the spread of the bearing clearance S y can be estimated using the following formula: (9.37) where Mf is the frictional torque, and u/ the relative clearance. After the differentiation of eqn. (5.8) uponyand assuming that k = 2, we obtain from eqn. (9.37) that
(9.38) Taking into consideration that for bearings with cylindrical sintered porous bronze bushes the value of parameter a is up to 16 times greater than for similar prismatic bearings (refs.341-343) and assuming that the values of . a (see eqn. (5.8)) are aoc and a for cylindrical and prismatic bearings respectively (the value OP of aoc is slightly greater), the following expressions canbe found: for cylindrical bearings Mfc
=
aoc +
16ac
-
(9.39)
Y2 (9.40)
and for prismatic bearings (9.41)
-
2a
(9.42)
423
The characteristics Mf = f(y) and SM/Sw = f,(Cy), based on the above eqns. ( 9 . 3 9 ) , ( 9 . 4 0 ) , ( 9 . 4 1 ) and ( 9 . 4 2 ) , are plotted in Fig. 9.8. These plots indicate that there is a rapid increase in the frictional torque and the spread of its values at the relative clearance~(0.005 in cylindrical bearings and aty<0.002 in prismatic bearings.
Y
F i g . 9.8. F r i c t i o n a l t o r q u e Mf and spread o f i t s v a l u e s S M r a t i o t o spread o f r e l a t i v e clearance SM/S,+,I vs. r e l a t i v e b e a r i n g c l e a r ance. 1,3 c y l i n d r i c a l b e a r i n g , 2,4 - prismat i c (square) b e a r i n g ; 1,2 f r i c t i o n a l torque,
-
3,4
-
I
IS~/Sy)l.
.
-
The experimentally determined frictional torque characteristics (with the spreads of the values) for bearings with a bearing hole diameter (or length of a side of the square in the case of a prismatic bearing) of 2 nun and a bush length of 4 mm are presented in Fig. 9.9 (refs 341, 3 4 2 ) . The bearing bushes were made of sintad porous bronze 9O%Cu, 10%Sn, porosity 2 2 - 2 4 % ) and impregnated with the instrument oil NIICP-MN-60 y. The journal was made of free-cutting steel and roller-burnished to RZ = 0.2-0.4 am. The sliding speed was very low and the load was 0.15 N.
424
A
F i g . 9.9. F r i c t i o n a l t o r q u e Mf and spread o f i t s measured v a l u e s vs. d i a m e t r a l c l e a r a n c e s f o r c y l i n d r i c a l ( 1 ) and p r i s m a t i c (square) (2) m i n i a t u r e (0 2 mm) s t e e l - s i n t e r e d porous bronze b ea ri ng s
.
As compared to the classic cylindrical miniature journal bearings, prismatic bearings ensure more precise and stable positioning of the journal during operation. The spread of the values of the frictional torque in prismatic bearings is also smaller when the relative bearing clearances are smaller than 0.005. The wear debris in prismatic bearings can collect in the corners ("pockets"), which also has a positive effect on the journal's position, steady-state frictional torque and the life of the bearings. The disadvantage of prismatic bearings is the dependence of the frictional torque upon the load direction. Journal bearings with a conically-shaped bearing bush are sometimes used in fine mechanisms to support elements performing small and slow oscillatory movements. The bending strength of conical journals is considerably greater than that of cylindrical journals. Conical bearings are therefore shock-resistant. The bearing bushes may take the form of screws. Due to easy clearance adjustment the dead movement in such bearings can be kept very small. TWO typical forms of conical bearings used in fine mechanisms are presented in Fig. 9.10. The friction torque Mfc in edge type bearings (Fig. 9.10 a) and in bearings with a conical bearing surface (Fig. 9.10 b) m y be calculated using the following formula:
425
-
Mfc - 2
N COSE
(9.43)
c1
where N is the load perpendicular to the journal's axis, f is the friction coefficient and C 1 for edge-type bearings is d and for conical surface bearings 0.635 (dl+d ) ; for a, d, dl and d2, see 2 Fig. 9 . 1 0 a and b.
Fig. 9.10. Miniature conical bearings. a b - w i t h a c o n i c a l b e a r i n g su rf a ce.
-
edge-type,
The rubbing surfaces of conical bearings should be very smooth and the elements should be made of a corrosion-resistant material which produces soft wear debris particles. Tapered bearings may be used in precision (e.g. optical) instruments as they offer the very important advantage that the bearing clearance in them can be easily eliminated. Spherical bearings have ball-shaped journals. The bearing bush may be spherical or conical. A conical bush is easy to manufacture and permits the eliminLtion of the clearance as the journal wears, which is impossible in the case of a spherical bush. The real contact pressures are significantly smaller in bearings with spherical bushes than in bearings with conical bushes. The wear in the first group of bearings is therefore dramatically smaller. Bearings with a conical bush are useful in the lever systems of measuring instruments where any effect of the bearing clearance on the functioning of the mechanism is to be avoided. Spherical bearings with a spherical bush are applied in hip joint prostheses (see Chapter 10).
426
The tribological behaviour of various spherical bearings with a hard ball head and a relatively soft polymeric spherical cup (e. g. made of UHMWPE) applied in hip joint prostheses can be compared by determining the wear modulus and frictional torque numbers (refs. 647-649). The wear modulus Wo already defined by eqn. (9.12 is the normal stress on the sliding surface when the depth equals the sliding distance. For the bearings discussed here it may be found using the formula (ref. 649)
-
O N
I2 4 -dWZ R(0.92Y)os dt
(9.44)
13
-
0.424yos 1
dWZ where w is the angular speed (rad/s) dt the wear intensity (nm/s), __ wz the maximum cup wear depth in the direction of the ball axis along the direction of the applied load N (in N), the angle Of contact between the load direction and line ball centre-point on the edge of the contact area (rad)I and R is the ball radius ( m ) . I The angle of contact may be calculated using the formula
vis
yo,
I
cos y o s =
R
2
+ (AR + wZ) - (R 2 R (AR
+
+ AR)2
(9.45
wz)
where A R is the radial clearance. I Once the angle of contact yOshas been determined, the volume of material worn, Vs, can be estimated from (ref. 647) (9.46)
and the wear area Aws can be estimated from the formula Aws = 2 7 R
2
(1
-
cos q i s )
(9.47)
The wear number Lws is (refs.647, 649) 1
L,s
=
I 2 2(0.92IqOs
-
(9.48) I
0.424
qos
)
and the frictional torque number LMs is (ref. 648) (9.49)
427
The frictional torque M f s may therefore be calculated using the formula Mfs
= fNRLMs
(9.50)
where f is the friction coefficient. As in cylindrical journal bearings, the frictional torque increases to a maximum of 1.273 times the frictional torque at zero wear (when the friction coefficient is assumed to be constant), The frictional torque in a cylindrical journal bearing and in a spherical (ball cup) bearing, both with a bearing bush made of relatively soft material such as polymer is shown in Fig. 9.11 (ref. 648).
1.' I.27
>
30 60 Half angle of contact , O
90
F i g . 9 . 1 1 . F r i c t i o n a l t o r q u e number f o r c y l i n d r i c a l j o u r n a l b e a r i n g ( 1 ) and s p h e r i c a l ( b a l l cup) b e a r i n g (2), b o t h w i t h a r e l a t i v e l y s o f t b e a r i n g bush such a s polymer.
428
In centre bearings, a conical journal with a spherical end is placed in a spherical recess or in a conical recess with a concave spherical centre portion. In both cases the recess radius is greater than the radius of the spherical end of the journal. Two cases of a journal supported on centre bearings should be noted: the one where the force (load) acts along the shaft and the one where the force acts transversely upon the shaft. In the first case the frictional torque Mfcl may be e s t d t d using the formula (refs. 3, 383)
(9.51)
-1 - - 1
where P is the axial load, f is the friction coefficient, q 1 , q 2 and El, E2 are Poisson's ratios and elasticity moduli for journal and bearing materials respectively, and R and Rb are the radii j of the spherical end of the journal and the spherical recess of the bearing respectively. When the force (load) acts normally to the journal's axis, one component of the frictional torque, the effect of the pivoting friction, can be neglected and therefore the frictional torque Mfc2 can be predicted using the following formula (ref. 3): Mfc2 = NR.f coS@ 3
(9.52)
where N is the normal load, R . and f are as in eqn. ( 9 . 5 1 ) , and Q 3 is the so-called "rolling upwards" angle (the friction angle at which the journal begins sliding on the block). The rolling upwards angle in the case of contact of sphere-endd journal with a spherical bearing may be predicted as follows: tan4
<
f
(9.53)
2 ( Rb-R
where sc is axial clearance. The friction coefficient for transversally loaded centre bearings (polished steel pivots and blocks) may be assumed to be 0.3-0.4 (ref. 3). Transversally loaded centre bearings should have blocks made of material with a low elasticity modulus to reduce
429
the frictional torque, while the bearing block material in axially loaded centre bearings should have a high elasticity modulus to achieve the same effect. The bearing block in tranversally loaded centre bearings may therefore be made of agate or glass and in axially loaded bearings of sapphire or cemented carbide. The journals are made of hardened steel or special alloys such as Permometall Paraloy or Permium. Journal materials should demonstrate a low friction coefficient when rubbing against the block material (ceramic, tool steel, beryllium bronze) and should have high resistance to wear and corrosion, Low frequency, high amplitude vibrations may dramatically increase the wear in centre bearings such as those in instrcments in aircraft or vehicles. The use of CVD coatings on a s t - f 1 journal (Cr7C 3-Tic coating) or on a Nivarox journal (hardenable ?.L-CO alloy, boron diffusion coating) greatly reduces the wear of the journals in centre bearings operating under vibration (ref. 5 3 7 ) . The use of block materials with a smaller elasticity modulus than that of the journal material is also advantageous for decreasing journal wear under vibration. Some examples of suitable block materials are agate, glass and beryllium bronze. Since the journals are usually made of steel and the frame of the mechanism or instrument in which the bearing blocks are placed is usually made of brass, a light alloy or a polymer, and since these materials have different thermal expansion coefficients, there is a danger of jamming or an inadmissible increase in axial clearance when the ambient temperature undergoes considerable variations. In the presence of vibrations, a too-large increase in axial clearance results in the appearance of heavy dynamic loads and in highly accelerated wear. Thrust jewel bearings of the cover plate type are used when both transverse and axial forces act. The transverse loads are taken on a jewel and the axial loads on a thrust block shaped like an end-stone, on which the conical or spherical end of the pivot is supported. Since the journal in such bearings is very thin (e. g. 0.1 mm) the transition from the journal (pivot) diameter to the shaft diameter is arc-shaped to avoid stress concentration. The jewel end-stones are either mounted in the plate or pressed in the cover plate (Fig. 9 . 1 2 ) . The 0 . 0 2 - 0 . 0 3 mm clearance between the end-stone and the jewel, increasing towards the end-stone periphery, means that due to the capillary forces the drop of oil placed on the pivot stays in the wedge-like space instead of spreading, so that the spherical end of the pivot is permanently lubricated.
430
F i g . 9.12. T r u s t b e a r i n g (cover p l a t e t y p e ) w i t h p r e s s e d - i n j e w e l . 1 - end-stone, 2 cover p l a t e , 3 - d r o p o f o i l , 4 - j e w e l , 5 - p l a t e , 6 - trumpet-shaped p i v o t .
The frictional torque Mft in thrust bearings may be calculated using the formula (ref. 3 )
Mft - 1 Nfld
+
%
6
1- 31
2
P
+%I 2
1
R
j
l
(9.54)
Rb
where N and P are the transverse and axial loads respectively; f l and f2 are the friction coefficients between the journal and the jewel and between the journal and the end-stone respectively; dl, d2 and El, E 2 are Poisson's ratios and elasticity moduli of the journal and end-stone materials respectively: and R . and Rb are 7 the journal spherical and end-stone curvature radii respectively: usually the end-stone is flat, i.e. Rb--+m. The first term in eqn. ( 9 . 5 4 ) denotes the frictional torque in the journal-jewel system, and the second the pivoting frictional torque in the sphere-shaped end of the journal/end-stone system. If the journal is loaded by its own weight only (as happens in watch balances) the frictional torque will be different in the
431
vertical and horizontal position of the journal. The frictional torque for the vertical position will be considerably smaller than for the horizontal position (as can be seen from a comparison of the first and second terms in eqn. ( 9 . 5 4 ) ) . TO obtain a constant frictional torque regardless of the journal position, the pivot end is provided with a concave spherical cap instead of a convex one. Since trumpet-shaped pivots are usually very thin in order to reduce frictional torque (the first term of eqn. ( 9 . 5 4 ) ) sudden shocks suffered by the mechanism may damage them. To prevent this, shock-resistant thrust bearings are employed (see e.g. DIN 8 2 8 3 standard). The Swiss Incabloc system (a product of Portescap) is often applied where an elastic lyre-shaped flat spring enables small radial and axial displacements of the jewel and end-stone respectively under shock. The other Swiss system, Antichoc 2000 (Antichoc S.A.), is very simple since only one injection-moulded polymeric element (POM) fulfils the functions of the jewel, the end-stone and the elastic anti-shock element (Fig. 9 . 1 3 ) .
-a 0.5 mm
Fig. 9.13. Shock-resistant thrust bearing Antichoc 2000 system. 1 polymeric (POM) bearing element, 2 metallic block, 3 steel pivot.
-
-
-
-
432
The frictional behaviour of the polymeric element in this system, when a steel journal was used as the mating element, was determined by use of the LSRH microtribometer (see Chapter 0 2). The results of these studies are shown in Fig. 9.14 (based on ref. 184).
11 0.10
o.’m
d15
OI30
0:40
c
Friction coefficient
99.41 8060-
-
W 40-
E
M
L
3 L)
u
10-
b
86-
L)
t 0,
4-
dL
2-
P
JI
I1
2
4
b
$
rb
1’4 2’0 *
Specific frictional torque, prn N/N F i g . 9 . 1 4 . F r i c t i o n a l behaviour o f shock-resistant t h r u s t bearing presented i n F i g . 9 . 1 3 (Weibull’s diagrams). a radial friction, b - axial friction. 1,2,3 p i v o t end diameter 0 . 1 2 , 0 . 1 8 and 0 . 2 6 nnn r e s p e c t i v e l y , r a d i a l load 0.016 N , angular speed between 1 0 and 100 rad/s, ambient temperature 20°C, r e l a t i v e humi d i t y 50%. 5 , 6 a x i a l load 3 . 9 lom2 and 1.6 N r e s p e c t i v e l y , p i v o t end d i a meter 0.14 mm, angular speed between 10 and 1 0 0 rad/s ( r e f . 1 8 4 ) .
-
-
-
433
0.4-
operating time, h
F i g. 9.15. F r i c t i o n c o e f f i c i e n t v s . oper a t i n g time f o r f r e e - c u t t i n g s t e e l - t h e r m o p l a s t i c polymer (POM h) b e a r i n g w i t h b e a r i n g h o l e di a met er 4.6 mm and bush l e n g t h 4.1 mm. S l i d i n g speed 1.73 m/s, c o n t a c t p r e s s u r e 521 MPa. 1 - polymer f i l l e d with lubricant, 2 polymer f i l l e d w i t h l u b r i c a n t and a d d i t i o n a l l u b r i c a t i o n w i t h special s y n t h e t i c oil f o r polymeric systems, 3 polymer f i l l e d w i t h l u b r i c a n t and a d d i t i o n a l l u b r i c a t i o n w i t h comp l e x ba ri um soap grease, 4 polymer f i l l e d w i t h l u b r i c a n t and a d d i t i o n a l lub r i c a t i o n w i t h l i t h i u m g rease c o n t a i n i n g PTFE p a r t i c l e s and based on s p e c i a l e s t e r o i l ( r e f . 3 39 ).
-
-
-
434
For the effect of the temperature on the friction behaviour of this bearing system see also Fig. 4 . 8 . The effect of air humidity on the tribological behaviour of the system is negligible. The frictional torque in the system is independent of the journal (assembly) axis position. The advantages of plain bearings are their simple design and low cost, particularly when polymeric injection-moulded elements are used. The friction coefficient of such bearings is relatively high,although hydrodynamically lubricated miniature bearings (e.g. sintered porous bearings) give a very low friction coefficient at a relatively low sliding speed and contact pressure. The important requirement that the frictional torque should remain stable during a long period of operation is very difficult to fulfil since in polymeric bearings for instance, the wear of the bearing also affects the frictional torque (Fig. 9.11). Lubrication can decrease the frictional coefficient but its value can vary remarkably in various modes during the operation of a bearing (Fig. 9.15). Plain miniature bearings usually operate at very low sliding speeds and if higher speeds are expected (e.g. over 0.2 m/s), then hydrodynamically lubricated bearings such as sintered porous bearings or polymeric bearings (from which no oil can escape) can be applied. The stick-slip effects in miniature bearings, particularly in the case of polymeric bearings, can be considerably reduced by the proper choice of lubricant. The wear is reduced by lubrication and by use of anti-wear coatings or wear-resistant materials such as ceramics (see Chapter 7 . 3 ) . The use of solid lubricants, usually in the form of an anti-friction coating, enables miniature plain bearings to operate under extremely arduous conditions (see Chapters 6.5 and 7 . 2 ) .
.
435
9 ~ 3 ,S P E C I A L BEARINGS Special bearings are used when special requirements must be met (typically very low frictional torque or very high sliding speeds). There are many designs for special bearings used in fine mechanisms or precision instruments. Gas bearings enable extremely low frictional torque to be obtained and are used in gyroscopes and other precision instruments. They can be used over a very wide temperature range, from cryogenic to pyrogenic applications and in the presence of radioactivity. They can operate under start-stop conditions inhelium environment and at cryogenic temperature (ref. 918). The application of gas bearings is advantageous in the absence of dynamic loads however, compact and high-precision bearings are applied e.g. to the rotary head assemblies for portable VHS-type VTRs (refs. 9 1 9 , 9 2 0 ) . The calculation procedure which applies to gas bearings can be found in refs, 3 , 3 8 3 , 6 5 4 and 655. The wear in gasostatic and gasodynamic bearings is negligible, although corrosion wear may occur because of the condensation of water vapour from the decompressing air. The materials used should therefore be corrosion-resistant; hardened stainless steels are applied on shafts. The bearing bushes should be manufactured from anti-friction material such as bronze, brass, or carbon-graphite, which are particularly useful when the sliding speed is higher than 1 0 m/s. When the mass of the bearing elements needs to be minimized, aluminium alloys with deeply anodized working surfaces are applied to the bearing elements. When a radial or conical gasostatic bearing is operating at high sliding speed it becomes gasodynamic. At the moment when the pressure of the gas sucked into the smallest gap between the rotating shaft and bearing bush balances the external shaft load, the external gas supply (as in a gasostatic bearing) may become disengaged. This disengagement is not desirable since during s t a r t -stop operation the initial frictional torque and wear of the rubbing elements is very high, and an external gas supply would raise the load capacity of the bearing. When gasodynamic bearings are designed without external gas pressure, anti-friction materials, such as the carbon-graphites used in air bearings, or anti-friction coatings (see Chapter 7 . 2 ) are applied to prevent high frictional torque and seizure during start-stop operation, when the shaft and bearing bush working surfaces are in direct contact.
436
Gas bearings may operate at very high rotational speeds (500,000 r.p.m. in dental drills). The frictional torque and the thermal effects may be great in high-speed gas bearings but the autocooling effect of the flowing gas stream is sufficient to keep the temperature in the bearing within reasonable limits. When the shaft is driven by a gas turbine the changes of the bearing seizing up are small, probably because there is no drive moment when the gaspressure supply becomes disengaged. High-speed gas bearings are susceptible to self-excited vibrations of the rotating element because of the elasticity of the gas film and the possibility of an inbalance in the rotating system, when the axis of the shaft in a radial bearing is eccentrically displaced and rotates around the bearing bush axis at a rotational speed of approximately half the rotational speed of the shaft (the "half frequency whirl effect", refs. 6 5 4 - 6 5 6 ) . This effect may occur when the bearing is slightly loaded (the bearing is used only to guide the shaft). The bearing may be stabilized by increasing the gas pressure or by machining shallow grooves or notches on the working surface of the bearing bush. The gasodynamic effect is applied to obtain the flying effect of the head in magnetic disk files. The air bearing flying head situation of submicron spacing thickness can be achieved at speeds above several meters per second (refs. 250, 657). Wear occurstherefore only during start-stop operations (see Chapter 1 0 ) . Foil bearings (Fig. 9 . 1 6 ) , used in magnetic recording devices, can operate by rotation of the shaft or transport of the foil. The bearings can be lubricated by a fluid placed between the shaft and foil or by a fluid supplied under pressure. Foil bearings demonstrate a relatively small friction coefficient (even below 0.001) and constant friction torque when the sliding speed is increased to high values also offering the possibility of frequently reversing the direction of sliding when there is no great friction torque or variation i n friction torque. The calculation procedure for such W r i n g s is based on the solution of Reynold's equation and can be found in refs. 6 5 7 - 6 5 9 , for example. For foil bearings without externalfluid pressure supply, the thickness of the fluid film ho may be estimated using the formula: ho = 0 . 6 4 3 Rs(
)3
62,v
where Rs is the shaft radius (see Fig. 9 . 1 6 ) ,
(9.55)
the fluid viscosity,
437 v the relative sliding speed, and F' = F/a; F the force of foil tension, a - width of foil (see Fig. 9.16).
F i g . 9.16. P r i n c i p l e o f the f o i l b e a r i n g .
The friction coefficient ff in foil bearings can be predicted by use of the following formula: L
1
ff = 0.0279(%)'
+ 0.0466(--) 30 3 P
(9.56)
where w is the angular relative speed and p the average bearing contact pressure. In high density digital recording with a rotary head, the tape has no contact with the head because of the form of the foil bearing. The head contour for rotary heads usually has two slots on a sphere or pseudosphere to maintain uniform sub-micrometer spacing over the head surface (refs. 661, 662). Rotary head recording devices have spherical heads with a diameter of between 2 0 a n d 30 mn. Magnetic miniature bearings show little frictional torque and practically no wear. Since it is impossible for a ferromagnetic body to reach a state of stable equilibrium in a constant field of a permanent magnet or electromagnet (Earnshaw's theorem) , when magnets are used, a magnetic relief or suspension of the supported element can be obtained in the direction of one coordinate. The positioning of this element in the directions of the remaining coordinates must be ensured by other means, e.g. mechanical (contact)
430
bearings. Suspension in the field of a permanent magnet is very easy to achieve and is used to support the shaft in watt-hour meters where the magnets can be made in the form of concentric rings and inwhich the self-lubricated bearing, made for example of carbon-graphite, ensures that the shaft (rotor) is coaxial with the magnet. In a clock balance bearing, the repelling force of like magnetic poles can be used for carrying axial loads. In this type of bearing the transverse forces are taken by a jewel bearing. The magnetic materials used must demonstrate a very great coercive force and great magnetic energy. Magnets made of such materials as Nd-Fe-B, Sm2(Co,Fe)l, alloys are small and light-weight (refs. 7 4 0 , 921, 9 2 2 ) . The design parameters for bearing magnets are described in refs. 3, 383, 6 4 0 and 6 6 3 . Some newest designs can be found in ref. 9 2 3 . The complete compensation of gravity by means of a magnetic field requires the application of some automatic field control or some other means for instance, the utilization of the superconductivity effect. The magnetic levitation of a ferro-magnetic shaft can be maintained by controlling the current in the winding of the electromagnet to keep magnetic attraction equal to the force of gravity on the supported shaft assembly. A shaft supported in this way is rotated by means of an electric motor or gas turbine. In a vacuum an inductive drive is often used. Levitation of an element made of a superconductive material is possible, This relatively simple system can be used satisfactorily in a vacuum, e.g. for supporting the rotor of a gyroscope (ref. 664). Also a diamagnetic element (made of pyrolitic graphite for example) may be used in a magnetic field; although the load capacity of such bearings is very small they can be applied in some measurement instruments. The calculation procedures for such bearings are described in detail in refs. 6 6 4 - 6 6 6 . Magnetic bearings are very useful in applications in measurement instruments used in dynamometry, accelerometry, precision balances and so on, and in high rotational speed devices - ultracentrifuges, gyroscopes for space navigation, vacuum pumps and textile machines. They have many advantages over other bearings: there is no mechanical contact between the rotor and stator: the rigidity of the bearing can be modified: and they are suitable for operation in a vacuum, in a corrosive atmosphere, in and over a wide range of temperature variations (-150 to + 4 5 0 ° C ) .
439
Mercury bearings are applied in delicate measurement instruments, as they demonstrate a very low starting frictional torque, lower than kinetic frictional torque. The operating principle of a mercury bearing is shown in Fig. 9.1la. The load capacity of such bearings is not high ( 0 . 0 1 N at 2.5 mm drop diameter; refs. 3, 383, 924). Self-alignment is a serious problem in such bearings. An increase in the load capacity and a degree of self-alignment can be achieved by using the bearings presented in Fig. 9.17b.
F i g . 9.17. Mercury b e a r i n g s ; s i m p l e ( a ) and combined s i n k e r b e a r i n g w i t h s e l f - a l i g n m e n t (b). 1 - rotor, 2 mercury, 3 - s t a t o r , 4 alignment rings (borosil i c a t e g l a s s ) , 5 - sinker ( b o r o s i l i c a t e g l a s s ) ( r e f . 6 6 7 ) .
-
-
The frictional torque of mercury bearings can reach mN-m or less, i.e. several times smaller than the frictional torque of a centre bearing with the same load capacity. The mercury drop should be covered with a liquid with as low a viscosity as posdible in order to protect the mercury from oxidation. Mercurybearings can operate in the vertical position. Their load capacity is low in relation to their size. It is possible to use lubrication by mercury drops in the case of a journal bearing (refs. 924, 925). Capillary bearings demonstrate astatic, stiction-free, and passive (i.e. not requiring a power input) properties (refs. 668, 669). The configuration of a simple capillary bearing is shown in
440
Fig. 9.18. The rod is free to rotate about and to translate along its long axis. Such capillary bearings have been used to support the axis in a compass and the bearing was satisfactorily operated when the loops were filled with water, isopropanol or silicone (ref. 668). Capillary bearings are stable provided that the area of the liquid free surface can only increase under decentralizing loads. Other configurations of capillary bearings such as the circular ring form are possible. A detailed analysis of such bearings can be found in refs. 668, 669, and 926).
Fig. 9.18. Simple capillary bearing.
Spring suspensions are also used in fine mechanisms. The spring supports can be applied instead of plain or rolling bearings in the case of oscillating - and particularly vibratory motions of small amplitude. Since the spring supports have no clearances they are accurate and stiction free but not astatic because of random residual elastic forces; however, elastic retardation can be made very small by proper fixing and choice of materials. The spring supports do not wear out and.they are particularly well suited for operations in conditions of external vibrations. If such bearings are properly designed they are much less sensitive to the effects of stray loads occurring in .transport than other types of bearing. The deflection of spring supports produces a restoring moment or force, which is advantageous in the design of measurement instruments, and in electrical instruments the use of spring bearings offers a safe and direct way of supplying current to the moving part. Typical types of spring supports are the flat leaf spring (loaded only by transverse force or by moment applied to its end,
44 1 e.g. in the suspension of a pendulum), crossed springs, the torsion spring (leaf or circular cross-section spring used for supporting swinging motion), free suspension springs (used in electrical measuring instruments), and stressed suspension springs. A detailed discussion of the different kinds of spring suspension as well as design procedures for them can be found in refs. 3, 383 and 640. In mass production, moulded polymeric flexible couplings (joints) are also very useful (refs. 927-930). In summarizing the above considerations, it should be pointed out that conventional bearings suffer "stiction" unless they are "dithered". Gas and magnetic bearings are usually not passive and require a power input, which often .results in stray forces. Spring suspensions can be made stiction free, but not astatic. Astatic stiction-free, passive bearings can be made by using capillary forces but they demonstrate a relatively low load capacity.
9,4,
ROLLING BEARINGS
The rolling bearings applied in fine mechanisms or precision instruments are either knife-edge bearings or rolling bearings with intermediate rolling elements. The knife-edge bearing consists of a knife-edge and flat, concave cylindrical or convex spherically-shaped block. Under load, the edge acquires a more or less regular cylindrical shape of small radius. A knife-edge bearing is a rolling contact-type bearing without intermediate rolling elements. A knife-edge bearing is of the open type and the pivot can carry out only slight rocking movements. To obtain a small friction coefficient in knife-edge bearings the initial knife-edge radius should be small (below 10 ,urn). The friction coefficient fr of such bearings can be predicted by use the following f I
fr = 2klj
1
-
1 j$
E,L (9.57)
where k = 0.4-0.5, and N is the load: El, E2 and $,, $2 are elasticity moduli and Poisson's ratios of the knife and block materials respectively, 1 is the length of the knife-edge, and r and R are the knife-edge and block radii respectively. Knife-edges are made of steel as a rule. Agate knife-edges have not provided to be advantageous from a metrological point of
442
view when used in balance bearings, though they can be useful in these cases when the balance is operating in a corrosive environment. The maximum admissible angle of deflection of knife-edge bearings cannot exceed the value at which the knife begins sliding on the block (usually 8-12"). Rolling bearings with intermediate rolling elements are often applied. In fine mechanisms as a rule miniature ball rolling bearings (shaft diameter below 10 mm) are used. Typical standard bearings of a normal grade of tolerance are used except when an extremely high degree of accuracy is required or when special design requirements make the use of a standard bearing unsuitable: in special bearings the bearing dimensions are smaller than those of standard bearings. The quality of a bearing in given conditions is determined by rotation accuracy, the bearing clearance and the frictional torque. Various types of miniature rolling bearings are applied. Standard ball bearings are commonly used but when the diameter of the shaft is very small (e.g. below 1 nun) sub-miniature bearings of the cup-and-cone type or bearings without a cage or inner ring are preferred (refs. 3, 383, 670 and 671). Special high-precision bearings used at high rotational speeds often have no cage (ref. 672). Because instrument bearings require the lowest possible operating torque,most of them have lightweight cages and open race curvatures to give a minimum contact area. Precision instrument bearings are available in three tolerances levels - ABEC 5P and 5T, 7P and 7T, and 9P (a semi-precision grade which meets ABEC 3 tolerances is also available). For mounting instrument bearings, press fits should be avoided, the accuracy of the mounting surfaces should be equal to the accuracy of the mating bearing surface, misalignment should not exceed 0.25" to ensure low torque and running accuracy, and loading across the bearing during assembly should be avoided (ref. 673). Examples of mounting miniature bearings can be found in refs, 674-679, 931, 932, 941 and 942. Vibrations in bearings are caused by geometrical.inaccuracy of the bearing elements. The active elements of a bearing have the greatest influence (i.e. when the internal ring rotates, the accuracy of its geometry and that of the rolling elements has a decisive influence)(ref. 680). Geometrical inaccuracy has more effect on the frictional torque level for lubricated bearings than for unlubricated cleaned bearings. In the case of relatively high frequencies a lubricant acts as a vibration damper.
443
The frictional torque in miniature ball bearings depends on the bearing clearance and operating conditions. The frictional torque decreases with increasing clearance. However, increasing the bearing clearance over 30 pm results in an increase in frictional torque (ref. 681). Frictional torque increases linearly with the rotational speed and applied load. The dependence of the frictional torque on the rotational speed and applied load can therefore be expressed with the following formula (ref. 681): Mfr = a0 n + b o C N
(9.58)
where n and EN are the rotational speed and the sum of acting (loading) forces respectively: and . a and bo are constant parmters for the bearing being used. The frictional torque in miniature bearings can be reduced by preliminary running-in, improving accuracy in the manufacture of the bearing elements (and improving the accuracy of mounting), additional motion of the ring of the bearing relative to the shaft, special bearing design, or by lubrication. The frictional torque and the spread of its averages can be more than halved by running-in. A similar effect can be obtained by the additional motion of the bearing ring. The special gyroscope gimbal bearing shown in Fig. 9.19 was designed for minimum starting frictional torque. Also the use of bobbin-type cages reduces the frictional torque of a bearing (ref. 682). The lubrication of miniature bearings is an effective way to decrease their frictional torque and increase their life. Oils, greases and solid lubricants are used. Oil lubrication is used for the high rotational speed bearings 3pplied in gyroscopes or dental drills (in the form of oil-mist lubrication). Lubrication from the vapour phase appears to be effective at high temperature e.g. at a bulk temperature 37OoC by lsing a lubricant comprising a homogenous gas phase mixture of iitrogen and phosphate ester vapours (ref. 933). Grease lubricaZion is usually applied in sealed bearings. The usefulness of various types of instrument oils and greases is determined by the )peratin9 conditions (see Fig. 9.20). Lubrication by oil impregna:ion (of the polymeric cage) and grease pack is an excellent nethod, ihile oil impregnation alone is very good, grease pack good and iinimum oil fair. The use of microporous polymer lubricant instead if the usual grease pack is also effective (ref. 6 8 3 ) . Typical ap-
444
250
fiuori noted
synthetic oils dica microgel
MO
-_---
Mineral oils t
DoLyurea thickener-
-- _- _-- --
torque
- ,- -
speeds
0porat i ng co ndi Iio n s
F i g . 9 . 2 0 . C r i t e r i a f o r s e l e c t i n g o i l s and greases f o r l u b r i c a t i o n o f m i n i a t u r e r o l l i n g bearings ( r e f s 674 and 6 7 6 ) .
445
proximate volumes of grease to be introduced into a bearing are equal to the volume of one or two balls for low torque bearings and to the volume of three or four balls for high rotational speed bearings (ref. 671).
F i g . 9.19. F r i c t i o n r e d u c i n g gimbal b e a r i n g . I n t e r m e d i a t e r i n g i s d r i v e n (by d r i v e n f l a n g e o r gear) a l t e r n a t e l y c l o c k w i s e and c o u n t e r c l o c k w i s e by servo motor o r o t h e r means.
Miniature bearings used in mechanisms operating under extreme conditions (e.g. in space applications) are lubricated with solid lubricants (see Chapters 3 . 4 , 6 . 5 and 7 . 2 ) . The solid lubricants are applied as anti-friction coatings or as fillers of polymeric cage (retainer) materials. The type of lubrication is a critical factor in torque, Oil-lubricated bearings demonstrate generally lower low-speed torque than the same bearings lubricated with grease. At high speeds, grease lubricated bearings may have lower torque, especially if a channeling grease is used. A unique process for applying a thin film of grease to all bearing surfaces is grease plating. A mixture of grease and solvent (later removed by heating) is used in order to obtain a thin film of grease. Grease plating gives lower torque than a grease pack and retains the lubricant on the bearing surfaces better than oil lubrication. At oil lubrication, in particular with low surface tension oils (e.g.
446
silicones polyethers), the application of anti-migration coatings (epilamizing) is necessary (the Stop-Oil method is very useful see Chapter 6.2.3). Lubricant distribution on the inner ring of an inner-ring rotating bearing may depend on the migration of oilfran the bearing raceway up onto the land and subsequent transfer to retainer surfaces (ref. 888). The performance of miniature instrument bearings in aircraft and marine guidance systems is adversely affected by the presence of contaminants on the contacting surfaces. Effective cleaning of such bearings prior to assembly is very important (ref, 438). A l s o atmosphere-borne particulate contamination of precision miniature bearings has an extremely serious effect on their performance. To minimize such effects during transport or storage, the bearings or complete components (e.g. electromechanical devices or gyroscopes) are often packaged in ultraclean polymeric (PE or PA) containers. However, the currently used antistatic agents incorporated into PE or P A films have shown adverse effects on contact with precision bearings and their lubricants (ref. 111). This problem has not yet been satisfactorily resolved. The choice of oil or grease depends on the type of bearing and device, and the operating conditions. Some lubricants in general use are: MIL-G-23827 A or MIL-G-81322 A , for office machinery (typewriters, computer peripherals); MIL-G-23827 A, for electromechanical devices (VTR, telecommunications devices); MIL-G-23827 (or 3545 C), for small motors (servomotors); MIL-L-6085 A or MIL-L-81376 for control devices (gyroscopes); oil-mist lubrication (spindle or turbine oil) and MIL-G-23827 A , for high-rotational devices (dental drills); MIL-G-23827 A (or 3545 C ) or MIL-L-15719 A, for domestic equipment (mixers, juice extractors); and NLGI No.0-1 grease, for other devices such as fishermen’s spinning reels (ref. 684). More precise information about the use of instrument lubricants in miniature rolling bearings can be found fn refs. 685 and 686, or obtained from bearing manufacturers. The lifetime of miniature bearings cannot be predicted using the standard methods used for large machinery bearings since miniature bearings usually operate at very low loads. The most important factors in determining the lifetime of miniature bearings are the lubrication, sealing, cage design and performance, bearing mounting and of course the operating conditions. In applications involving electrical potentials across bearings, electrical wear is one of the greatest maintenance obstacles because it damages the bear-
447 ing itself, and it deteriorates the lubricant due to wear particles (ref. 934). Under the low oxygen environment, formation of FeF3 and subsequent FeF3 catalyzed degradation of perfluoropolyalkylether (PFPE) oil constitutes an important PFPE degradation pathway under boundary lubricating condition in steel bearings (ref. 935). The ageing and evaporation of a lubricant and its migration from the bearing are the most important factors in the evaluation of the effect of lubricant on the life of the bearing. The factors affecting grease life in sealed ball bearings were found to be grease-leakage rate, residual oil content, oxidation, and amount of wear particles (ref. 936). The use of anti-migration coatings (see Chapter 6.2) and the diagnosisof the ageing grade of the lubricant used [e.g. by applying IR methods - see Chapter 8.5.4) are important. The lubricant film behaviour can be evaluated by measuring electrical resistance between the bearing rings (ref. 937). Transfer film lubrication is often applied in bearings operating under extreme conditions. The lubricant is usually incorporated within a self-lubricating composite and the lubrication is provided by the continuous transfer of solid lubricant to sliding/rolling elements (refs. 875, 938). When bearings with a cage (retainer) made of self-lubricating material are used, the wear of the cage is usually the dominant factor in determining the bearing life. If the admissible wear is known, the life t (in hours) of a self-lubricating bearing can be predicted using the following formula: 0.02
wa
t = (d+D)nb
dw
(9.59)
dw is the wear intensity where wa is the admissible wear, in mm; dL (L is the sliding distance for a ball rubbing against the cage at N rotations of the bearing ring), and nb the rotational speed (in rpm); and d and D are the inner and outer diameters of the bearing respectively (in mm) A quasiempirical method for assessing life and reliability of solid-lubricated bearings that considers fatique, raceway wear, ball separator wear, and lubricant transfer rate control from sacrificial self-lubricating ball separators can be found in ref.939. The lifetime of miniature bearings may be evaluated experimentally by accelerated tests (ref. 940). In some miniature bearings, the bearing elements are made of special materials such as polymers or ceramics, and these bearings
.
448
are applicable in a corrosive environment, when chemical inertness, no lubrication, no noise, good electrical insulation and small weight are required. In the bearings manufactured by STAR Kugelhalter GmbH (F.R.G.), the rings are made of POM, the cage of PA 66, and the balls of glass: in the bearings manufactured by Gebr. Schmeing Kunststoffwerk (F.R.G.) the rings are made of POM, the cages of PA 66 or PP, and the balls of glass or steel; and inother bearings, the rings are made of UHMWPE, the cage of PP or PA 66, and the balls of glass or steel. The balls used in special bearings are often manufactured from other materials such as polymers (PA, POM, PP, PTFE composites, fluoropolymers or PUR composites) or ceramics. Such bearings have a friction coefficient of 0.004-0.009' and run smoothly with no stick-slip effects (refs. 587, 688). The load capacity and maximum rotational speed of bearings with elenwts made of the aforementioned materials are of course smaller than those of traditional steel bearings. Predicting the load-speed boundaries of bearings with polymeric rings can be found in refs. 943, 944. The high hardness and excellent high-temperature performance of ceramics are attractive for critical rolling-bearing applications. Silicon nitride having the lowest friction and elastic modulus of the available ceramic seems to be a most interesting material in such applications (refs. 28, 945-948, see also Chapter 4.3). The lubrication of ceramic bearings improves their tribological characteristics (Chapter 5.3, refs. 949, 950). Bearing materials and bearings for high temperatures and extreme conditions are reviewed in refs. 951-953. Polymeric and ceramic bearings or bearings with the insulation layers are applicable in electromechanisms. Such special electrically insulated rolling bearings are discussed in ref. 954.
9,5,
GUIDES
Sliding or rolling guides are often used and are especially important in manipulators and positioning devices. Detailed procedures for their design can be found in refs. 3, 383, 640 and 955. The frictional forces in guides should be small. This depends not only on the friction coefficient but also on the guide design. Miniature guides designed with the help of handbooks for machine design demonstrate in practice unexpected frictional resistance and non-uniform movement during sliding, especially at small loads.
449
The miniature guides used in displacement transducers, measuring heads, cam mechanisms, change-over switches, locking devices and other instrument assemblies were investigated to find their tribological properties (ref. 6 8 9 ) . The model used in the investigations was a simple cylindrical guide consisting of a shaft of diameter 2 nun made of unhardened free-cutting steel. Two standard agate bearing bushes pressed into a metallic holder were used (the distance between the bushes was about 20 m m ) . The investigations showed that the static friction coefficient of the unlubricated guide varies from 0.15 to 0.25 when the applied load is small (below 0.1 N). The friction coefficient is relatively small (0.13-0.15) and steady-state when the applied load is greater than 0.3 N (contact pressure 0.021 MPa). Smaller clearances of between 10 pm and 85 pm resulted in higher friction coefficients and a higher spread of their measured values. If the guide is lubricated with an instrument oil (Soviet B-1) and then cleaned with a mixture of acetone and gasoline to remove the multimolecular layer of lubricant and leave only the boundary-oriented layer of lubricant, its static friction coefficient (0.13-0.15) will be less dependent on the clearance and load applied. The friction coefficient in the latter lubricated guide is equal to the friction coefficient in an unlubricated guide when the applied load is over 0.3 N. When greater amounts of oil are used, the friction coefficient increases and its spread is very high (up to 1.4). This was observed for oil amounts of 0.002-0.060 mg/nun; the oil layer was between 0.35 and 10pnthick on the 100 mm long steel shaft. The use of this amount of lubricant for the lubrication of miniature guides should therefore be avoided. A very smooth surface on the cylindrical steel shaft (Ra = 0.08 pm) results in a relatively high friction coefficient; the optimum roughness of the shaft surface is when R a = 0.32 pm
.
The use of rolling guides, as opposed to sliding guides,drastically reduces the friction resistance, improves the uniformity of the movement and the accuracy of the guides (wear also has a significant effect on their accuracy). Linear ball bearings (ball bushings) used with a shaft as linear motion guides characterize with the low friction coefficient and small its variations if the waviness and roughness of the rolling surfaces are low (ref. 956). The wear in rolling guides is mostly due to fatigue. Materials used in the rubbing elements of sliding guides are usually unhardened mild steels (0.3 - 0.4% C ) for the guiding ele-
450
ment and 0.5% C or tool steel for the sliding (guided) element. When the guide is more highly loaded, the combination of materials is similar to that in plain bearings. In rolling guides elements are made of unhardened 0.5% C or tool steel and the rolling elements of 0.4% C steel. In more highly loaded guides, the guiding elements are made of hardened tool steel and the rolling elements of 0.4-0.5% C steel. The elements of highly loaded guides operating at high speed are made of the same materials as those used in the manufacture of rolling bearings. An effective way to lengthen the life of guides is the use of anti-wear coatings (see Chapter 7.3). When the weight of a guide needs to be kept to a minimum, its elements can be made of a light alloy, such as aluminium, magnesium or titanium, and the surface anodized to make it hard. The following material (coating) combinations demonstrate low friction and low wear properties in unlubricated systems (from the best to worst wear behaviour respectively): A1 0 -MoS2, Tic-Sic, Fe-Sic, Tic-TiN, Tic-Tic, Tic-Fe, Tic-WC, 2 3 Fe-MoS2, Tic-Cr3C2 (Fe = steel, ref. 6 9 0 ) . The application of PTFE composites is particularly effective for guides used in mechanisms operating under extreme conditions and to avoid stick-slip effects. Such and other composites for guides are reviewed in ref. 957. The lubrication of guides can decrease the friction coefficient and significantly decrease wear when contamination by abrasive particles is avoided by special guide design. The use of a lubricant with an anti-stick-slip additive is especially effective. The guide for a Fourier’s interferometer used in space was effectively lubricated with XU 430 clock oil (see Table 3.1) containing dispersed PTFE particles (ref. 17). The guiding element was made of bearing steel (HRC 60-64), was cylindrical in shape (diameter 12 m m ) , and had a surface roughness of Ra<0.04 pm; it was in sliding contact with two spherically-ended diamond pins (the guided part). The steel-diamond combination was applied since it does not suffer from the stick-slip effect during sliding. The guided element was positioned in a vacuum with an accuracy of 5 0.01pm over a sliding distance of 2 mm at a sliding speed of 0.3 mm/s. The greases thickened with PTFE particles (see Tables 3.12-3.14) are veryuseful for minimizing the stick-slip effect in guides. Difficult wear problems appear when a guided element is made of material containing hard abrasive particles (e.g. paper, magnetic tape or card). Paper can wear very hard-resistant materials, but can itself be worn or damaged by relatively s o f t materials.
451 The wear by paper shows an inverse linear dependence on the material hardness (see Chapter 4.3). Such materials as diamond, sapphire, Tic and WC are therefore wear-resistant when rubbing against paper. The use of wear-resistant coatings (see Chapter 7.3) is a very effective way to reduce the wear of elements that are in contact w i t h paper, in computer peripheral equipment for instance (refs. 280, 525). The application of cemented carbide hard facings by the metallic surface fusion process to elements of card machinesgreatly reduces their failure rate (ref. 525). The abrasiveness of paper is also reduced by as much as one to two orders of magnitude by the individual and combined effects of paper damage, paper debris and moisture (ref. 691) The finely dispersed hard particles (e.g.of T-Fe203 or Cr02) that form the magnetic medium in magnetic tapes cause abrasive wear during rubbing against guiding elements. The wear of the head capstan is accompanied therefore by wear of the guides and other path components (see Chapter 4.3). A1203 is used for the tape guide flanges, and nonmagnetic stainless steel for the guide barrels (ref. 2 8 0 ) . The abrasiveness of papers, ribbons and tapes is greatly increased by an external abrasive becoming attached to their surfaces. Under contaminated conditions the external abrasive (also produced by themselves, as in the case of magnetic media such as oxidetapes) may predominate and mask the inherent abrasiveness of these materials themselves. The guiding elements of textile machines are exposed to rapid wear, especially when glass threads are used. The use of low-cost borided mild steels instead of stainless steels can reduce wear (ref. 268). The application of Fe-Cr alloy and titanium, nitrided, borided or cemented, ensures high wear resistance in guiding elements. The processing of PETP fibres filled with pigments causes wear of thread guides. To reduce this wear, the guides can be provided with special coatings e.g. plasma-sprayed ceramics (Cr203, 8 7 % A1203 - 13% Ti02) or hard plated chromium (ref. 958). The friction resistance of the aforementioned guides (especially sliding guides) is relatively high. Extremely low friction resistance, the possibility of very smooth and accurate movements (no stick-slip effect), and no backlash are characteristic of gasstatic guides (refs. 383, 654). Aerostatically lubricated table systems for measuring instruments, in which all relative sliding elements including the lead screw to drive the table are aerostat-
.
452
ically lubricated, make it possible to keep the pasitioniy error to below 70 MI, with stiffness in the feed direction of 8 N/,um and in the vertical direction of 1 6 0 N/,um (ref. 6 9 2 ) . The advantages and disadvantages of gasostatic guides are the same as those of gasostatic thrust bearings. The use of elastic elements (flat springs, membranes or tapes) to act as guides is also an effective way to keep the friction resistance low. A selection of design parameters for such guides is given in refs. 3, 383, 6 4 0 and 9 5 5 . Flat spring guides are rigid, demonstrate small friction resistance, have a natural restoring force (which is desirable in some cases) and facilitate a reliable current supply to the moving part of an instrument. Membrane guides are applied in various instruments in physics when an exactlyrectilinear motion and very low friction losses are required. They do not wear and are particularly suitable in cases of high frequency vibrating movements. The disadvantage of the flat spring and membrane guides is their small working range. This can be improved by use of a tape guide of rolamite (ref. 6 9 3 ) or by use of a special U-shaped eight-flat-springs system (Fig. 9 . 2 1 ) .
A-A -
2
1
Al I
\
-%
I /
/
--.-I-'--&--
Fig. 9.21. U-shaped flat springs linear guide. 1 - body, 2 U-shaped flat spring, 3 guided stiffening ribs, 5 foil (made of element, 4 beryl 1 ium bronze) (ref. 694).
-
-
-
-
453 The U-shaped flat springs are made of beryllium bronze 0.05 mm thick foil and are stiffened by plated copper ribs (0.5 mm wide, 2 nun high,0.5 mm apart). A transverse load of 20 N results in a 0.3 mm transverse displacement of the guided element. The guide can operate without back-force at a distance of 2 50 nun in the temperature range 300 K to 2 K. Prediction of the relaxation oscillation (stick-slip effect) in guides is extremely important. Stick-slip motion of an elastic system may occur if the frictional force loads an elastic system, which carries one friction surface and presses it against a moving surface. It occurs if the kinetic friction increases with decreasing sliding speed at low speeds; no stick-slip occurs when the kinetic friction decreases with decreasing sliding speed, at very low speeds (ref. 6 9 5 ) . The presence or absence of stick-slip depnds on the sliding speed, the dynamics of the system and the nature of the dependence of kinetic friction on sliding speed. The motion during slip is approximately sinusoidal. If the velocity during "slip" is sufficiently high a noticeable temperature rise ("flash") may be produced. The experimentally checked simple theory of the stick-slip motion for the pin-on-disk configuration of a sliding system can be found in ref. 696. An extensive bibliography concerning stick-slip motion is given in ref. 697. The relaxation oscillations (usually periodic and primarily tangent to the contact plane) are accompanied by normal motions (during the transition between stick and slip phases of tangential oscillations) The normal vibrations are , however also characteristic of unlubricated smooth sliding, for example between steel surfaces (refs. 698, 9 7 7 ) .
.
9,6, GEARS
AND TRANSMISSIONS
Various gear transmissions are applied in fine mechanisms. The tribological problems are important since the friction and wear losses in such systems should be as small as possible. Fine-pitch gears are more often used to transmit motion rather than power, so fatigue is not a common problem with them; surface durability and wear characteristics are more important. The most important factors in gear wear are the materials, lubricant and surface finish. When gears operate at high loads and sliding speed the local temperature rise can melt polymeric gears, destroy the lubricant film andcause accelerated ageing of the lubricant.
454
The theory of good fine-pitch design (teeth geometry) is now well known (refs. 3, 6 4 0 , 6 9 9 and 9 5 9 ) . The optimization of the aforementioned parameters and profile precision is vital for the good tribological behaviour of fine-pitch gears in various applications. Friction losses depend on the type of profile used (teeth geometry) and are heavily dependent on the friction coefficient between the rubbing surfaces of the teeth. Friction is however not constant during the process of teeth contact and varies from the teeth meshing to their disengagement(ref. 7 0 0 ) . In the case of unlubricated involute free cutting steel-brass teeth contact thefriction coefficient was observed to vary from 0.05 to 0.3, and from 0.02 to 0.2 for the same gear lubricated with XU 430 clock oil(see Table 3.1). The average friction coefficient decreases with increase in the tooth contact ratio. Variations in instantaneous torque are smaller for a Swiss clockwork gear than for an involute gear, but the involute gear ensures a constant gear ratio within the limits of manufacturing accuracy. It is very difficult to maximize gear efficiency while keeping the gear volume - the space needed at a defined gear ratio within acceptable limits, since the gear volume can be simplydecreased by increasing the number of stages, but this results in higher friction losses (also in the bearings) and is not economical.Reducing the pitch (module) of the gears is costly. The optimum geometrical spacing of the gears is discussed in ref. 701. Special gears with a very small number of teeth (less than eight) on a pinion, complementary profile gears, Wolfram's planetary gears, harmonic drive or cyclo gears can be applied to give small volume gearing systems (ref. 7 0 2 ) . The efficiency of the gear train depends mainly on the efficiency of the journal bearings used (ref. 652; see also Chapter 9.21, since an assembly can never be more efficient than its least efficient part. When ordinary journal bearings are used the gear train efficiency is nearly independent of the tooth profile and in practice, though not in theory, involute gear teeth are just as efficient ascycloidal gear teeth. The frictional torque in reduction gears reduced to the driving shaft depends mainly on the first, and, to a lesser degree, on the second shaft bearing torque, counting from the driving shaft (ref. 3 ) . To reduce the necessary driving torque it is essential to employ bearings offering low friction resistance such as conical, centre or ball bearings to support the first shaft and to reduce its assembly
-
455
weight. The quality of support of the succeeding shafts does not exert a noticeable influence upon the driving torque and they may be supported by ordinary journal bearings. It is important to minimize gear wear. Gears manufactured from a combination of polished pinion teeth made of hardened free-cutting or stainless steel with lead brass machined gears show relatively good wear and friction behaviour. However the weight of the gear, the inertia moment and the costs are high. Stainless steel gears are also often applied. Resistance to pitting is important because pinion (or small gear) teeth can quickly reach a state of destructive pitting. Anodized aluminium alloy gears are also relatively wear-resistant but are lighter weight and have a smaller inertia moment. Gears made from powder metal are less expensive than machined metal gears and demonstrate reasonably good behaviour, particularly gears formed of Fe-based powder containing NiiMoiMn sintered at 1250°C, hardened in oil at 860°C and tempered at 18OoC, which have a long life (ref. 703). Such gears run-in easily. They have a 0 . 0 3 - 0 . 3 thick surface austenic layer on a martensite base phase and demonstrate as a result high fatigue and pitting resistance and high admissible contact pressure. Moulded polymeric gearing (PA, POM, PC, PP, PETP, PBTP, HDPE, PUR, ABS, phenolic) is in wide use because of its relatively low cost, corrosion resistance, light weight,quietness, and the fact that it needs no lubrication. It is applied in hardened steel-polymer or polymer-polymer material combinations. When steel-polymer systems are used the heat transport from the friction region of teeth is better than in polymer-polymer gears, and in highly loaded gears operating at high rotational speeds, tooth wear is less than in similar, fully polymeric gears. The wear in a metal driving gear-polymeric driven gear system is half that a polymeric driving gear-metal driven gear system (excluding aluminium or brass gears) (ref. 2 4 3 ) . The wear is mostly in the polymeric gear since the wear in hardened steel gears is relatively low (in the most common steel-polymer gears). The best tribological behaviour is demonstrated by steel-POM and steel-PA gears. However, at small loads the wear in polymer-polymer gears may be lower than in steel-polymer gears (see Fig. 4 . 2 5 ) . The best tribological behaviour in the case of polymer-polymer gears was found when the driving gear was made of PA (e.g. PA 6 ) and the driven gear of POM (ref. 2 4 4 ) . The wear in machined gears is greater than that in moulded gears. Wear increases parabolically with increasing load. The admissible
456
load does not exceed 40 N/mm (the circumferential force divided by the tooth's width)(refs. 244, 245, 704-710, 908). The wear is proportional to the circumferential speed, but increases with the duration teeth contact (refs. 243, 706, 960). In steel-POM gears the wear was largely influenced by the applied load and less influenced by the number of revolutions (ref. 961). In steel-polymer complementary gears, the wear of the polymeric gear may be less than in ordinary gears (ref. 711). Wear also increases with the module. The roughness of the working surface of the steel teeth should be small (Ra below 0.63 pm). The manufacturing accuracy should be kept in the 9-12 class of tolerances of TGL RGW, 642 (DIN 3963) and IS0 standards (refs. 710, 712, 713). When the load is higher than 15 N/mm, the thermal effect on wear should be taken into consideration. A relative humidity of 40-60% has no effect on gear behaviour, but when it reaches 90% the wear rate of PA gears is considerably higher. The presence of dust or other particles such as paper or magnetic particles greatly increases the wear in fine-pitch gears. This is particularly true of polymeric gears. The use of filled polymers, reinforced with glass fibres and/or filled with solid lubricants, gives lower wear rates although the wear of the counterface may increase, as in other tribological polymeric systems (see Chapter 4.2). Material combinations such as PA + glass fibres-PA + PE or PBTP + glass fibres-PA + PE give low gear wear (ref. 705). A comparison of the wear in fine-pitch steel-polymer and polymer-polymer gears is presented in Fig. 9.22. Silent gears are more expensive to make since tighter manufacturing tolerances are required. Tighter tolerances will provide greater noise reduction for low quality gearing than they will for high quality gearing, i.e. improving the accuracy of lower quality gears usually proves to be a cost effective way of reducing drive noise (ref. 774). The use of a sandwich gear is also effective (ref. 962). The pitch used should be as fine as possible for the load conditions. Improving the surface finish has only a minor effect on noise reduction although Ra should be below 0.5 ,urn. However, if the gear is made of softer material, an integral ratio allows the gear to cold-work and conform to the pinion, thereby promoting quiet operation. In a start-stop operation of a polymeric gear train, in a quartz clock for example, the noise can be reduced by the use of a high viscosity damping and lubricating oil or grease (see Chapter 3.2 and below).
457
2
3
300
I/
200
4
7
E
-
=L L
0
s 1oc
1 L
I
Number of rotations, '01 Fig. 9.22. Wear o f polymeric gearing as a f u n c t i o n o f number o f r o t a t i o n s . For wear d e f i n i t i o n , d e s c r i p t i o n o f gear used, and o p e r a t i n g parameters see c a p t i o n t o F i g . 4.25. (1-6) d r i v i n g gear made o f f r e e c u t t i n g s t e e l (wear was n e g l i g i b l e ) , d r i v e n gear manufactured i n : '1 PA 11, 2 PA 12, 3 PA 12 + 30% glass f i b r e s , 4 POM, 5 PA 610, 6 PA 6; (7-12) polymer-polymer gearing: (7 and 9) d r i v i n g gear (7) o f PA 12, d r i v e n gear (9) of POM; (8 and 12) d r i v i n g gear (12) of PA 6, d r i v e n gear (8) o f PA 12; (10 and 11) d r i v i n g gear (11) o f POM, d r i v e n gear (10) o f PA 12; grease l u b r i c a t e d steel-polymer gearing w i t h d r i v i n g gear made of (13-15) POM, 14 PA 6, f r e e c u t t i n g s t e e l , d r i v e n gear manufactured i n : 12 15 PA 12 + 30% glass f i b r e s (based on r e f s . 244, 707).
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
458
Lubrication is an effective way to improve the tribological behaviour (especially the wear behaviour) and quietness of gears. The lubrication of fine-pitch gears is usually by a single "for-life" application. Fine-pitch gears are often "open", i.e. not enclosed in a sealed gear box. Boundary lubrication films are therefore more prevalent than hydrodynamic fluid films. Wear reduction as the result of boundary lubrication of fine-pitch gears is considerable (see Fig. 9 . 2 2 ) . It is important to use anti-migration coatings (epilames - see Chapter 6 . 2 ) in boundary-lubricated fine-pitch gears to keep the oil or grease in the friction region of the teeth. When higher frictional losses can be permitted, the use of greases is more advantageous. The wear of PBTP gears can be greatly reduced by dipping the element in a non-agressive grease-dispersion (ref. 7 1 6 ) . Once, for-life lubricated gears of POM c (Hostaform C 1 3 0 2 0 ) and PA 6 (Miramid VE 3 0 ) operating at a load of 20 N / m in computer peripheral devices in the presence of paper dust have a life of 4 . 2 l o 7 - 4 . 2 1 0 8 rotations (ref. 7 1 0 ) . Solid lubrication, with burnished MoS2 films for example (see Chapter 7 . 2 ) , improves the tribological behaviour of metal fine-pitch gears, especially those operating under extreme conditions. Self-lubricating polymeric gears incorporating solid lubricant (MoS2, graphite, PTFE, PE) have very good wear resistance. Lubrication with special oils (when frictional losses must be kept very small) or suitable greases (see Chapters 3 . 2 and 3 . 3 ) is important to reduce wear in polymeric gears and to reduce noise during start-stop operations. The use of anti-migration coatings (epilames) is then necessary, Oil and grease can be also effective carriers of contaminants. Harmonic drives are very useful since their reduction ratio in a single stage is high (from 6 0 to as high as 3 2 0 ) , they have zero backlash (although they demonstrate a "soft" torsional spring rate around null torque), high efficiency and torque capacity, no "stiction" (stick-slip motion) upon start-up (consequently starting torque variations are minimal) and a low wear rate (refs. 7 1 7 , 7 1 8 and 9 6 3 ) . They are therefore highly suitable for use in robots and servomechanisms, especially in space. The life of harmonic drives is governed primarily by bearing ratings and gear tooth wear is very low. MoS2 lubrication of the teeth of harmonic drives used in space extends their life (ref. 7 1 9 ) . Fine-pitch worm gears are very useful in drive systems; a worm on a motor shaft enables motion to be transmitted to the perpen-
459
dicularly placed axis of a mating worm-wheel with a relatively high reduction-ratio and low noise. Various designs of miniature worm gears can be found in ref. 7 2 0 . The design of worm gears is discussed in refs. 7 2 1 and 7 2 2 . Metal (for instance the best hardened steel)-polymer and polymer-polymer material combinations are often applied in worm-wheel systems, for example in tachometers or timers (refs. 7 2 1 , 7 2 3 ) . The use of POM (e.g. Hostaform C 1 3 0 2 1 ) on the worm and PA 6 on the worm-wheel gives a long-life worm gear. POM is the best polymer for use on the worm-wheel in steel-polymer systems. Fully polymeric worm gears have a long life, Inaccuracies in the worm profile (especially when the worm is made of steel) and in the mounting of the worm gear, and a very rough working surface result in greater wear. The wear in POM-PA 6 6 and PA 6 6 + PI-PA 6 6 systems, both unlubricated and lubricated with mineral oil and ethylene glycol, is higher on the worm's teeth (ref. 7 2 4 ) . Polymeric fine-pitch worm gears can operate without lubrication, but oil, or better still grease, lubrication with a suitable lubricant (see Chapters 3 . 2 and 3 . 3 ) dramatically improves their tribological behaviour, in particular, extending their life. Screw transmissions are useful in positioning systems (ref. 725). The design of screw transmissions is discussed in refs. 3, 6 4 0 and 9 5 5 . A transmission ratio of between 1 : 5 0 0 and 1 : 2 0 0 0 can beobtained in differential screw transmission. The driving element (a nut or screw) can produce a rotary on translatory motion. The efficiency of the transmission in driving systems should be high. Screw transmissions with a low frictional torque and a stick-slip free, extended life can be obtained by careful selection of materials and lubricant. A steel screw (even of unhardened steel) mated with a soft metal nut (e.g. lead brass or bronze) demonstrates good tribological behaviour in lubricated conditions (by using PTFEparticles containing oil or grease, or lubricant which gives a selective transfer effect - see Chapter 5 . 1 . 1 ) . The lead screw transmission used in ultra-high precision table systems can be aermtatically lubricated (ref. 6 9 2 ) . The use in driving systems of a steel-polymer (e.g. PA composite) combination gives good tribological behaviour, especially with lubrication. The friction and wear behaviour of screw transmissions, especially those used in positioning or measuring systems, depends greatly on the geometrical design and accuracy of the rubbing elements. Elimination of the screw backlash by use of an elastic element results in higher wear. Better tribological behaviour (in particular low friction, i.e. high efficiency,
460 and stick-slip free) can be found in rolling screw transmissions (ref. 640). The frictional torque comes mainly from the wedging action and friction between adjacent balls. Rolling screw tramanissions however are not self-locked. The use of microporous polymer lubricant plugs (hung on the travelling nut) to lubricate thescrew surface ensures the continuous presence of lubricant in the rubbing region (ref. 683). Cam mechanisms (for a design procedure see ref. 726) withsteel or polymeric elements are usually lubricated when highly loaded. Very smooth rubbing surfaces, especially that of the follower, are important for good tribological behaviour in cam mechanisms. The general rules governing the selection of polymeric materials, as laid out in Chapters 4.2 and 5 . 2 , are applicable to cam systems too. Ratchet, Geneva (or star wheels) mechanisms for stepwise (incremental) forward or intermittent motion respectively should be made of hardened stainless steels or have anti-wear impact-resistant coatings (see Chapter 7.3) or be manufactured from polymeric materials with high impact strength and good tribologicalbehaviour in sliding (e.g. Hostaform S VP27073; see ref. 727). In friction transmissions (for design procedures see refs. 3, 640, 705 and 9 5 5 ) , forces or torques are transmitted from thedriving element to the drive element by means of friction generated by their interacting pressure. A two-step transmission of this kind is used for example in the Abbe comparator, enabling the driven element to be set without any backlash and with an accuracy of approximately 0.5 y m (ref. 3). The efficiency of friction transmissions is not high (usually no higher than 0.7). When the rubbing elements are metal, they are made of alloyed steel (usually bearing steel) hardened to HRC 60, and the working surfaces are polished. When the rubbing surfaces are coated by corrosion-resistant, anti-wear coatings (see Chapter 7.3) they can operate without lubrication; lubricant is mainly used to protect the rubbing elements from corrosion. Polymeric elements are sometimes used instead of metal ones in friction transmissions because the accuracy required in their manufacture is much lower than for metal ones, which lowers the cost for making them, especially when they are mass produced, Usually the driving element is made of polymer and the driven one of steel. This is important for reducing the possibility of local wear-out of the material of the driven element in the event of its locking.
461
Typical material for the polymeric element is PA 6 but using POM gives less wear (ref. 705). The use of PA 66, PBTP or HDPE is also reasonable. Polymeric elements in friction transmissions fail because of overheating and flattening of the polymeric roller, which results in rapid wear. The coefficients of rolling friction in a PA-steel friction transmission reduced to roller radius are given in Fig, 9.23. The friction coefficient depends not only on the friction material combination and ambient temperature but also on the roller's diameter, interacting pressure, circumferential Speed and the topography of the contacting surfaces.
20
610
4'a
60
i*O
Id0
-
Tempmture , O C
-
Fig. 9.23. Rolling friction number f / R (f friction coefficient, - roller radius) for P A - s t e e l friction transmission. Roller radius R = 50 mm, circumferential speed 3 m/s. 1 PA 6 containing cast PA 6 (hard)(ref. PA 6, 3 cast PA 6 (soft), 4 monomers, 2
R
-
705).
-
-
-
462
Metal or polymeric rollers with elastomer facing are useful since they give a higher friction coefficient and simplify the design of the transmission, by sometimes making an external interacting pressure unnecessary. They are especially useful in friction mechanisms for feeding strip materials in various office and computer devices. Both rollers are made of metal or one of them is coated with an elastomeric material which has a high coefficient of friction when in contact with, for example, paper. The metal roller is sometimes knurled to increase friction, Flexible connector transmissions (drives) are often used in fine mechanisms for transmitting motion by means of various types of connectors (cables, belts, chains, etc. 1 Resilient m m d o r s can be made of organic materials (natural or polymers) or of metals. Organic connectors demonstrate relatively high elastic elongation and relaxation and are usually in the form of flat bands, round section strings or cords. Metal connectors are manufactured in the shape of high-carbon steel bands and sometimes, in special circmstances (corrosive medium, small pulley diameters), phosphorus bronze or beryllium copper bands. Also patent steel wire or bronze wire are employed as connectors. Pulleys can be made of metal (steel, aluminium alloys, brass), pclymer ( P A 6, PA 66, POM, PBTP, HDPE, PVC, PUR, phenolic resin), or ceramics. The design of flexible connector transmissions is described in refs. 3, 6 4 0 and 9 5 5 . Various types of belts or chains, usually made of PUR and cooperating with metal (stainless steel, anodized aluminium alloy) or polymeric sprockets or pulleys (made of the above-mentioned polymers), are used in low-powered systems as precise zero backlash control mechanisms with an average of 50% of the teeth in contact, which helps to minimize noise; they are non-corrosive, self-lubricating, smooth operating lightweight and have non-magnetic qualities; however, such drive belts or chains are not as strong as steel cables and fail as a result of fatigue (mostly at improperly applied field splices). The wear and shorter life is caused mainly by inaccuracies in meshing contacts; added lubrication by spraying with a light mist of silicone oil will extend their life. Many different types of PUR precision drive belts and chains are manufactured by Winfred M. Berg Inc. Bead-belt chains, consisting of polymeric beads moulded onto a flexible cord, are also useful for obtaining a slip-free flexible connector transmission (ref. 728). Synchronous (timing) belts are thin and flexible have a toothed profile (of trapezoidal shape as standard) that mates with corres-
.
4 63
ponding grooves in the pulleys, thereby providing the same positive engagement as a chain or gears. A stable length of the belt (made of PUR or Neoprene) is obtained by reinforcing with steel, glass or aramid cord. Synchronous belts operate well on miniature drives and in applications involving high speeds or small pulleys; however they require fairly accurate alignment of the pulleys. The rubbing of teeth in meshing and demeshing results in wear, mainly of the belt teeth. Accuracy in the manufacture and assembly of the transmission is the most important factor in determining the lifetime of a synchronous belt (refs. 7 2 9 , 7 3 0 , 9 6 4 , 9 6 5 ) . Neoprene belts with a PA coating are made wear-resistant without lubrication (with chemically inert lubricant as in the case of highly loaded PURbelts operating at high speeds) and when the coating is worn out the change in colour of the working surface indicates the need to change the belt. The circular form of the tooth profile minimizes stress concentrations, providing more uniform load distribution, increased capacity, and a smoother, quieter action. Closer tooth spacing increases torque capacity and extends life (ref. 7 1 4 ) .
9 7 COUPLINGS : I
CLUTCHES AND BRAKES
The couplings used to join two shafts end-to-end may be rigid or flexible. Rigid couplings demand almost perfect alignment of the mating shafts, and if they are misaligned, either initially or as the result of wear, accelerated wear of the shafts, the coupling and the shaft bearings can be expected. Some designs f o r rigid and elastic miniature couplings can be found in refs. 3, 6 4 0 and 9 5 5 . Elastic couplings such as disk slip or Oldham's coupling are used to offset misalignment of the shafts, but these can be very inefficient. At higher rotational speeds frictional losses and wear in such couplings can be high even with lubrication. The centre block of Oldham's coupling may be made of bronze, PA or PUR instead of steel. Elastic metal bellows couplings, or polymeric Ones made for instance of POM, can cope with angular misalignments of up to 9 ' and parallel to about 0.04 of the shaft's diameter, and can operate at rotational speeds in excess of 10,000 rpm (ref. 7 0 9 ) . Misalignments of up to 4' (angular) and about 0.04 of the shaft's diameter (parallel) are possible in spring couplings. Bellows and spring couplings are practically wear free. Other elastic couplings such as membrane, elastomeric (PUR) vibration-damping couplings are also useful and extremely efficient. The flexible shaft couplings have
464
an efficiency of about 0.9, they are suitable for connecting shafts of various angles and they may be bent while in operation, although bending requires more power to overcome the friction between the shaft and the sheathing, and increases the wear of the flexible shaft. Universal Cardan joints are used to join shafts that mustintersect at angles greater than those that can be accommodated by flexible couplings. The Cardan joint includes a cross assembly and two yokes. Such joints are available in single or double configurations and with or without slip provisions. Elements are made of carbon steel or heat-treated alloy steel, bronze or stainless steel. The use of POM yokes and a PA cross instead of metal elements is advantageous since no oils or grease lubrication is needed at low torque and rotational speed (ref. 7 0 5 ) . At higher loads and speeds, lubrication with special oils or greases (see Chapter 3.2 and 3.3) can greatly extend the life of polymeric systems. The design of universal Cardan joints is discussed in refs. 3, 6 4 0 , 7 3 1 and 9 5 5 . Clutches (for design see refs, 3 and 6 4 0 ) permit easy connection or disconnection of shafts, at rest or in motion. Claw clutches consist of two disks provided with teeth in their crowns and pressed together by a spring. Lubrication with oils or greases with good lubricity (e.g. traditional clock oils) results in a dramatic decrease in wear of such clutches. One-way ratchet clutches are used to transmit torque from the driving shaft to the driven shaft by means of a pawl ratchet wheel. These elements are made usually of hardened stainless steel. In friction clutches, pressure between friction surfaces determines the torque transmitted from the driving element to the driven element. Clutches with flat surfaces (metal or polymeric elements) are typical. The pressure is produced by a spring. Two simple miniature polymeric clutches applicable in the mass production of fine mechanisms are illustrated in Fig. 9.24 (ref. 7 3 2 ) . A stable friction coefficient can be obtained by accurate cleaning of the mounted elements. The use of metal and polymeric materials such assteel, aluminium alloys, brass, POM, PA and PA + glass fibres in friction clutches ensures reasonably high wear resistance. One-way friction clutches with intermediate blocking elements, made of a bearing stainless steel, have a friction coefficient of 0.7 between rubbing elements but corrosion and wear occur as a result of the lack of lubrication (ref. 5 3 7 ) . Improving the surfaces of the blocking elements with the anti-wear CVD coating Cr C has been found to in-
X Y
465
crease the friction coefficient to 0.7-0.9, leading to better gripping as a result. No corrosion was observed and the endurance life of the clutch appeared to be extended from about 200 to 4 0 0 0 hours (refs. 537, 538).
Y
F i g . 9 . 2 4 . M i n i a t u r e polymeric clutches w i t h small diameter (a) and small thickness ( b ) . 1 - polymeric gear, 2 shaft, 3 polymeric coupling elements.
-
-
Magnetic clutches are of the electromagnetic type or have permanent magnets. The most widely used is basically a conventional disk-type mechanical friction clutch with electrical actuation. The mating surfaces of the electromagnet and armature are covered with friction materials. In a miniature electromagnetic clutch of this type (but with only one friction surface, the second being the Armco of the electromagnet) friction sintered metallic(and non-metal1ic)materials based on copper instead of on synthetic resin and asbestDs can be used (ref. 7 3 3 ) . The following formula can be used
for defining the quality of the friction material for such clutches: k = -100 f wt
(9.60)
where k is the duty paramter, f the friction coefficient, w the linear wear (inp n ) after 1 km of sliding distance, and t is the time needed for running-in
.
(h)
Tests have been carried out to determine the wear of the soft magnetic materials in systems of Armco-Armco and Armco-sintered iron powder with 3 % A1203 operating in the aforemmtioned electromagnetic clutches (ref. 734); the Arrnco-sintered iron powder f 3% A1203 system showed very good wear behaviour. The running-in process should be carried out in the presence of a magnetic field. The wear process in the systems discussed can be defined using the following criteria (parameters): wear intensity (linear wear per sliding distance), level of the change of the friction surface z = wRa (w linear wear, Ra - surface roughness parameter), and effectiveness of the friction, zf = f/w (f - friction coefficient)(refs. 734, 735). Miniature magnetic particle clutches, such as those used in computer devices, operate best when no magnetic particles adhere or coagulate (agglutinate) during inoperative periods (refs.736-738). MoS2 colloidal dispersion can be used to blend alkyl resin with the magnetic particles so that after the resin has hardened, the MoS2 lamellae are bonded with the resin to the magnetic particles (ref. 738). A coating of alkyl resin + MoS2 prevents the occurrence of eddy currents in the magnetic powder and decreases the friction between magnetic particles, thus extending their life thanks to less fretting corrosion. The size of the magnetic particles used should be varied so that the empty spaces between large particles are filled by small particles. The use of other materials such as talc, graphite, magnesium or zinc oxide is also effective against agglutination of the magnetic particles. In liquid magnetic-particle clutches the magnetic particles are dispersed in silicone oi1,which demonstrates only small viscosity variations as a result of heating and is ageing-resistant. Clutches and brakes have similar operating principles in so far as a brake is basically a clutch with one stationary member. Small, essentially mechanical brakes actuated electrically are used in micromotors, especially for start/stop operations. The design of brakes in which a spring causes engagement when current is removed
4 61
can be found in ref. 739. The life of these miniature brakes is about 5 l o 6 operations at a frequency of engagements of 3 0 0 0 h-’. Non-friction clutches or brakes can be of the hysteresis or eddy current type, which offer the advantages of flexibility, smooth starting and intensive vibration damping; however they lose a certain amount of motion, depending on the loading torque. When the lost motion is small the efficiency of the clutch can be 0 . 9 2 - 0 . 9 6 , but this decreases to as low as 0 . 7 0 - 0 . 7 5 as the amount of lost motion increases. Magnetic clutches with permanent magnets are used in measuring instruments working under pressure to transmit m v m t from the pressure chamber to the outside without the use of stuffing-boxes. They consist of two magnets fixed on shafts and placed on each side of a wall. There are two main types of magnetic clutch in use: the face type (where the separation wall may be flat) and the radial type. The face clutch is stronger and of simpler design but has the disadvantage that there is considerable axial thrust force acting upon the clutch magnets. This force must be transmitted by correspondingly strong bearings and thus additional frictional torque is created. Magnets formed from the rare-earth neodymium alloyed with iron and boron, e.g. Nd15B8FeT7 (ref. 7 4 0 1 , are stronger than the best samarium-cobalt magnets and make it possible to make magnetic clutches of small dimensions but high torque capacity. A design procedure for magnetic clutches can be found in refs. 3 and 663.
9 8, I
CONTACTS, BRUSHES
Electrical contacts or brushes are widely used in connectors, relays, switches, potentiometers, micromotors etc. The most important and complex problem in the design of electrical contacts is to simultaneously ensure the smallest wear and the lowest stable electrical contact resistance. The wear in electrical contacts occurs as a result of friction and impact erosion, and also as a result of the action of the electric current on the contact mterials. The process of frictional transfer plays an important role in the wear and electrical behaviour of contacts (refs. 89-94, 7 4 1 - 7 4 3 ) . Because of the need for low and stable electrical contact resistance,contact materials are chosen for their good electrical properties and nobility i.e. the ability to resist the formation of insulating films such as oxides and tribopolymers (see ref. 1 0 3 ) . Thus copper and its alloys are the most frequently used contact
468
substrates, while noble and seminoble metals such as gold or silver are often applied as thin coatings to make the surface relatively inert. Since gold-plated electrical contacts have become so expensive in recent years, tin or Sn-Pb solder plating is being used in electrical contacts because they acquire self-limiting oxides in many environments (refs. 7 4 2 , 7 4 3 ) . A serious problem for electrical contacts, especiallythe cheaper ones, is their wear by fretting - a small amplitude oscillatory relative motion. Fretting apparatus for testing electrical contacts is described in refs. 7 4 2 and 7 4 3 . Many contacts have little rigidity, which makes them susceptible to forces which can cause micromotion ranging from a fraction of a micrometer to tens of micrometers; such forces include external vibrations (for example, the edge-card connectors widely used in computers and electronic instruments are subject to vibrations during transportation and may also be caused to vibrate by such mechanical devices as disk drives or blowers) and differential thermal expansion and contraction of the components to which the contacts are mounted. As a result of fretting, the protective surface layers may expose the base substrates and if a significant thickness of non-conductive film is formed, the electrical integrity of the contact may be destroyed. However, in noble metal or base metal contacts operating in an inert atmosphere the only fretting wear occurs when the base metal contacts are subjected to vibrations in reactive atmospheres such as oxygen, chlorine, sulphur dioxide etc., where fretting corrosion may occur. The following phenomena may cause high contact resistance in base metal electrical contacts subjected to fretting motion (refs. 7 4 2 , 7 4 3 ) : the generation, accumulation and oxidation of metal wear debris at the contact interface, separating the surfaces electrically; and, more rarely, the formation of an oxide film on the surface which is not penetrated by asperities of the counterface under load and fretting motion. When the contact surface is modulated, it has cavities for trapping the oxide wear debris so the contact resistance under fretting motion can be low, enabling the use of non-noble metals with modulated surfaces as substitutes for noble metal (ref. 7 4 3 ) . This solution also reduces friction (the friction coefficient can be reduced from 0 . 7 to 0 . 2 for Sn - 34 Pb, copper, electroless nickel contacts). With fretting, especially in the absence of lubrication, the thin top layer of contact metal may wear out and the contactresist-
469
ance will change to that characteristic of the underlying metal. Hard underplatings (e.g. nickel) and hard substrates can increase the durability even of gold platings under fretting conditions (ref. 7 4 2 ) . Gold is often preferred as the contact material since it forms little frictional polymer (as the effect of tribopolymerization); instead of gold, Pd - Ag and Pd - Ag - Au alloys (e.g. 6 0 Pd - 4 0 Ag) can also be used under fretting conditions. Palladium, rhodium, ruthenium and several other noble contact metals and their alloys catalyze the formation of insulating frictional polymers originating in adsorbed organic air pollutants, but unlike during fretting corrosion, the contact surfaces suffer little or no wear. During the fretting of dissimilar metals, the interface composition changes because of transfer, wear and film formation (refs. 7 4 1 - 7 4 3 ) . The direction of net metal transfer is from the soft to the hard surface. When gold is used as one surface metal and a harder material such as palladium in used as the other, the whole system becomes gold and contact resistance tends to remain low. Alloys such as 7 5 Au - 2 5 Cu which are harder than palladium experience transfer of palladium (ref. 7 4 2 ) . When gold is mated to tin or Sn-Pb solder combinations, the systems become all base metal and contact resistance may degrade even more rapidly than for tin-tin or solder-solder contacts because of the greater difficulty which asperities of these dissimilar metals have in fracturingoxide to re-establish metallic contact, Wear by fretting is lower when the number of cycles for base metals to attain a given increase in contact resistance is low and, with catalytic metals, when the vapour concentration of organics which can produce significant amounts of frictional polymers is low (ref. 7 4 2 ) . If the wipe is long, the contact resistance will be more unstable, but by increasing the normal load of the electrical contacts the contact resistance can be made more stable, especially in friction polymerizing systems. Contact resistance is less variable at high open-circuit voltage because of fritting of the insulating films. The contact reliability of connectors also depends on the shape of the contacts, topography, ductility, porosity, layer thickness, diffusion barrier, insulating compounds, contaminants and ambient conditions (temperature and atmosphere) (refs. 7 4 4 , 7 4 5 , 966-9661. The folded cantilever used for printed circuit boards, the pin-socket contact and the butting contact (tulip form) are typical contact-spring configurations in separable electronic connectors.
470
The surface roughness of the contact working surface, Rt, is about 0 . 3 pn. The thickness of the layer depends on the operating conditions (atmosphere). In a normal atmosphere the thickness of an Au - Co layer should be 1.5-2 pm (usually it is about 1.8 @I), deposited on a 2-3 pn thick N i underlayer. Sustained temperatures over 1 5 0 ’ can result in the formation of a nickel oxide layer, which leads to a higher contact resistance. The use of Au - Ag and Au - Co alloys and Pd is recommended for the contacts of connectors operating at elevated temperatures (ref. 7 4 6 ) . The connector’s industrial suitability can be adequately determined by testing the contacts in H S+S02+N02 to simulate an industrial atmosphere (refs. 2
744,
747).
Gaseous organic compounds in the atmosphere can adsorb on the contact surface and subsequently be converted to adherent insulating solids due to catalytic activity of the metal. The effect of rubbing is to accelerate these reactions, probably due to the continuing renewal of the surface on which the reactions take place. The vapours of polymers used as insulating materials in connectors have a great effect on the contact resistance of electrical contacts. The greatest effect on the contact resistance of Au, 92 Au - 8 Ag, and 7 0 Au - 3 0 Ag was observed when ABS was used,and the smallest when PTFE, in the case of the alloys, or PC, in the case of Au, was used; PC and PS were also tested (ref. 7 4 4 ) . Glass-fibre reinforced polymers are unsuitable for this purpose. Similar effects have been noted €or Au, Ag and Pd contacts when tested in the atmosphere of vapours of PTFE, PC, PA 6, glass-fibre reinforced epoxy resin, phenolic resin, ABS, PC and PE (ref. 7 4 8 ) . A rise in the ambient temperature accelerates the process of the formation of insulating solids on the contact surfaces. Palladium and palladium alloys, as possible low cost replacements for gold in contacts containing electrical components, are especially “sensitive” to adsorbing gaseous organic compounds from the atmosphere. Contacts used in electrical connectors, printed circuit boards with edge contacts, switches, instrument slip rings , ptenticxmters, and micromotors demonstrate sliding wear. In sliding contacts, contact behaviour is controlled by metal transfer, metal remova1,friction and electrical noise. Wear phenomena determine the product life, reliability and especially the cost of noble metal contacts. Wear occurs in metal contacts by adhesion, abrasion, brittle fracture, fretting (see above) and delamination (ref. 7 4 9 ) . Running-in dominates the wear process in many cases. Adhesive wear can occur
471
in mild or severe regimes. Adhesive wear by prow formation occurs in many contacts, including gold, palladium, silver, tin and solder, and entails material transfer from the member with the largestslidling surface to the smaller one, from which it may subsequently emerge as loose debris, Repeated sliding in the same track leads to a process called rider wear in which, in the severe regime, only the smaller member loses material by transfer. Adhesion between many different contact metals, such as gold and rhodium, is often very strong. The ductibility of the metal is the key to the occurrence of high wear by prow formation. When ductile wrought metals (such as inlays) are used, adhesive wear may also occur but this tendency can be reduced if the materials are hard and if contact lubrication is applied (see below). Abrasive wear and, especially, brittle fracture wear can be reduced by ductile electrodeposits and wrought inlays. Hard ductile substrates and underplates help reduce sliding wear. Multilayer finishes of thin gold with a smooth, finely porous surface on less noble, hard underlying materials are interesting low friction and wear-resistant contact materials (refs. 91,
749-756).
In an empirical wear equation developed for a connector system of the spring-pin type (ref. 2 8 0 ) , the wear width w is proportional to the l/a - power of (N/No) and to the l/b - power of n, where N and No are respectively the contact force and minimum force for appreciable wear, and n is the number of cycles of wiping or insertion. The constants a, b, No and wo (initial wear width) are dependent on the contact materials and geometry. The friction coefficient (influencing the value of "insertion forces") increases with N, and the friction force depends on N unpredictably. The friction coefficient of electroplated gold contacts is 0.15-0.30 and palladium contacts 0.4-0.7 (ref. 7 5 0 ) . A s a criterion for the definition of the life of connector contacts the number of slides to the wear-out of the contact surface layer can be applied (ref. 91). The life is longer when the contact material is hard. Palladium contacts demonstrate good wear resistance during sliding. In relays, as the switch contacts start to separate, the number of contacting asperities decreases and as the last remaining contacting asperity decreases in contact area, contact resistance increases until contact voltage reaches the melting potential (about 0.5 V for most metals). At low voltage, rupture of the moltenbridge separating the surfaces opens the circuit but when source voltage exceeds 1 4 V an arc may form which will pit the contacts and shorten
472
their life (refs. 91, 757-759). Since the dielectric strength of air decreases with barometric pressure, at high altitudes contact arcing occurs at lower voltages and lasts longer as the contacts separate. Commercial switches used in computers, laboratory equipment, office machines, communication equipment, aircraft and military field and shipboard equipment designed for an office-type environment, usually not sealed to resist entry of liquids and actuated by hand, have a life of 20 - 100 million operations, depending on the applications (refs. 91, 760, 761). The contact forces are in the range 0.08-0.10 N. Apart from metal contact elements, cheap switches with contacts made of conductive elastomers are also used in keyboards; they have a life of about lo6 operations (at the frequency 3 s-l) (ref. 91). The sliding contacts used in instrument slip rings micromotors (brush-collector systems) should demonstrate especially good tribological, as well as electrical, properties. The following material combinations used in metal sliding contacts (brush-slip ring system) demonstrate very good wear behaviour and electrical properties: AuCu14Pt9Ag4-AuAg2OCul0, PdAg30Cul4PtlOAulOZnl-AuCu14.5Pt9A~4, AuCu14Pt9Ag4-5 ,um Ni+0.5-1 ,um Rh+0.2 ,um Au (ref. 91). The sliprings are made of the alloy (when an extended life is required) or are electroplated with a noble metal. The sliding contacts in potentiometers with special requirements (e.g. in x-y recorders) are made of noble metals. In encoders, when very small voltage and current electrical signals are being used, the contacting springs (made e. g . of CuSn6, CuNil8Zn2O alloys) and contact element (made of CuZn alloys) are gold plated. It is important that the sliding contacts be lubricated. The tribological properties of the brush-aollector system play an essential role in the behaviour and length of life of DC electric micromotors with brush commutation. Metal brush-metal collector and metal-based composite brush-metal collector systems are used. In micromotors which use a very small current, such as those used in tape recorders, dictaphones and cine cameras, the brush is mostly made of flexible sliding wires or strips. Collectors aremanufactured also as an electro-deposited slip ring. Extended life and good electrical properties characterize brush-collector systems using AuAg20Cu10-AgCu10, AuAg20Cu10 (or AuCu14Pt9Pg41-AgPd30, and AuCu14Pt9Ag4-AuAg20CulO material combinations (ref. 91). The PdCul4PtlOAulOZnl-Cu+5 pm Ni + 3-5 ,um hard gold brush-collector system has an extremely long life but only moderate electricalprop
473 erties can be expected. A cheap solution is to use the CuSn6+5 pm Ag-Cu + 5 pm Ni + 3-5 ,um hard gold material combination. The applied contact load is 0.03-0.08 N for a single brush. Lubrication mostly reduces running-in wear. The life of micromotors with the aforementioned brushes extends to 5 0 0 0 h. At higher currents, brushes made from silver-graphite (3-10% graphite by weight) or copper-graphite composites, where polymeric bonding is applied and fusible additives such as Sn, Pb, In, Gal Zn, T1, Cd are introduced, are used.%q usually rub against a copper collector. The friction and wear in such systems depends greatly on the operating conditions. The transfer of conductive solid lubricant material onto the counterface plays a particularly important role (refs. 741, 762-765). Good tribological behaviour has been found for such compositions as Cu70C26.7In3.3 and an electroconductive metal-polymer composite consisting of a pack of highly conductive foil layers bonded with an adhesive based on an electroconductive binder filled with functional additives (refs. 762, 766, 767). When copper-graphite brushes are used the transferred film does not sufficiently protect the surface of the copper collector from oxidation, whereas when a metal-polymer composite is used this film does protect the collector surface from corrosion, enabling proper operation of a micromotor at elevated humidity and temperature; a transferred film of silver-based composites is highly conductive and appears to prevent the formation of dense copper oxide film on the collector surface (refs. 762, 7 6 8 , 769). During operation of a brush-collector (or a slip ring) system the magnitudes of the contact voltage drop may differ for anodic (AU+) and cathodic (AU-) brushes by a factor of up to 2 or 3 . As a rule, the relationship is AU+>AU-for metal-containing brushes. Polar effects most clearly occur when brushes contain metals in proportions of 40-60% (by weight) (ref. 7 7 0 ) . Metal-containing brushes sliding on a copper collector in typical micromotors form collector films with "gaps" within the protoxide layers beneath the transfarhg layer , which explains the observed inequality AU> AU-. Thin metal wire (strips) brushes or composite metal-graphite (with fusible additives) brushes differ in their tribological behaviour. The frictional losses in a micromotor in which the first type of brush is used are relatively small since they are less than in bearings (sintered porous bearing bushes) (ref. 7 7 1 ) . The use of jewel bearings decreases the bearing losseswith 66% as compared with the losses in sintered porous bearings (ref. 969). The relative
4 74
frictional losses of the brush-collector systems of typical Dc micromotors are shown in Fig. 9 . 2 5 . It can be remarked that the frictional losses are of one order of magnitude hiqher in the mtal-graphite brushes with surface commutation system than in those with quasi multi-point or linear commutation with flexible metal strips.
16’)
5-
21o-~-
5-
21
5-
‘1 lo-6 0
2doO
4000
6000
Rotat tonal speed,
8600
10000
c
r pm
-
Fig. 9.25. R e l a t i v e brush f r i c t i o n a l losses (Mf - f r i c t i o n a l torque, J rot o r ’ s moment o f i n e r t i a ) vs. r o t a t i o n a l speed i n DC micromotors. 1.2 micromotor w i t h three s l o t s core r o t o r and half-opened housing, 3.5 and 5 W res p e c t i v e l y , w i t h composite metal-graphite brush pressed t o c o l l e c t o r ( d i a meter 4.6 mm) w i t h f l a t (wire) s p r i n g ( 1 ) and h e l i c a l s p r i n g ( 2 ) ; 3 , 4 micromotors w i t h core-less drag-cup r o t o r and closed housing, 2 and 1.5 W respect i v e l y , w i t h brush ( m e t a l l i c , w i r e type) s u r f a c e 0.2 mm2, c o l l e c t o r diameter 1 . 1 mm ( 3 ) and s i m i l a r brush w i t h surface 0.12 mm2, c o l l e c t o r diameter 1 mm (4) ( r e f . 771).
-
-
475 The wear in brush-collector systems depends on applied load and sliding speed. The optimum load is when the electrical and mechanical wear is minimum (ref. 91). In brush-slip ring systems the contact pressure should be less than 0.1 MPa. For micromotors the effect of the rotational speed on wear is more significant than the effect of the sliding speed. When the critical rotational speed is exceeded the wear increases rapidly (ref. 91). This is because of the critical effect of the collector’s run-out on the wear. The admissible run-out should be kept below 5 ,um. The lubrication of electrical contacts has a great effect on their behaviour. The simplest form of lubrication is self-lubrication, by a transferred film of solid lubricant for example. This is especially effective in the aforementioned metal-polymer composite (brush material)-copper (collector material) sliding systems (refs. 7 6 2 , 766, 7 6 9 , 9 7 0 ) . The use of a solid lubricant, e.g. microcrystalline wax for gold connector contacts, is effective as long as it is not displaced (e.g. by fretting). Other solid lubricants such as graphite and octadecylamine hydrochloride (ODA-HC1) have been tested successfully in some connector applications (ref. 2 8 0 ) . Immobile solid lubricants are more easily displaced on smooth rather than on rough surfaces. As already mentioned, contact surfaces should be especially smooth when thin noble metal finishes are used. Boundary lubrication of electrical contacts with thin films of fluids is more effective than with solid because thin films of fluids are more persistent on surfaces. They stabilize the contact resistance by slowing down the rate of wear of thin noble metals on film-forming substrates, and in the case of base metals by shielding the surfaces and wear debris from the environment, thereby reducing the oxidation rate. Lubricants can also disperse frictional polymers, although they also contribute to polymer build-up. Boundary lubrication can greatly reduce friction and wear in contacts. Gold or gold electroplated contacts lubricated with a microcrystalline wax or polyphenyl ether (say about 50 nm thick) demonstrate extended life. The use of lubricants for palladium and other lower cost contacts minimizes wear and stabilizes contact resistance, significantly reducing the effects of fretting (ref. 9 2 ) . Fretting of unlubricated palladium contacts in ordinary room air increases contact resistance to unacceptable levels. Contact resistance degradations in Pd-Pd contacts can be minimized by immers-
476 ing the contacts in fluids (or by placing a drop of fluid on the mated contacts where it remains as a "thick" layer during the run) or by coating the contact by immersing it in a mixture of lubricant and solvent (e.g. l,l,l-trichloroethane, widely used in manufacturing processes for cleaning electronic assemblies and contact materials); after withdrawal the solvent quickly evaporates, leaving behind a thin coating. The thicker this coating is, the greater the improvement in contact behaviour. The following fluid lubricants were used in fretting tests on Pd-Pd contacts (ref. 92): a mixture of isomeric five-ring polyphenyl ethers, synthetic paraffinic hydrocarbon (hydrogenated alkene) , United States Pharmacopeia (U.S.P.) mineral oil, silicone oil, polychlorotrifluoroethylene, and oleyl sarcosine acid. Lubrication with a microcrystalline wax layer of finely divided particles was also applied as a comparison. The fluids tested kept the resistance at a low level. The contact resistances from runs lasting up to lo5 cycles fell within the "flooded" band (below 15 mR ) whereas during unlubricated operation, at just after l o 3 cycles the contact resistance increased rapidly because of the formation of frictional polymers. Contact resistance during the tests with silicone oil was at the upper boundary of the flooded band, polyphenyl ether gave intermediate behaviour and the results when using polychlorotrifluoroethylene (having viscosity 2 3 4 0 mm / s at 25OC) were at the lower boundary. The Pd-Pd contacts used in electronic components such as switches, slip rings,encoders, potentiometers and other devices with wiping contacts, and in separable connectors with sponge reservoirs over or through which the contacts pass during engagement bathed in fluid, are able to maintain low contact resistance. Investigations into fretting (ref. 92) within Pd-Pd contacts lubricated with a coating of polyphenyl ether or synthetic hydrocarbon fluid obtained by retraction from 0 . 5 - 5 % by weight solutions in l,l,l-trichloroethane showed that thin coatings are less effective than thick coatings (over 5% solutions by weight) in maintaining stable contact resistance. The cycles necessary to obtain 100 mR of contact resistance increase particularly rapidly as the percentage of lubricant in the solvent is increased up to 2%. In the range of 5-100% solutions the cycles necessary to obtain 100 m R o f contact resistance are similar. Coatings at levels over 2% arehighly effective. Lubrication with thin coatings of microcrystalline wax (for comparison, experiments were also conducted with microcrystalline wax dispersed in the mentioned carrier at levels of 0.5
477
and 0.1% by weight) is not very effective, probably because of the tendency for such solid coatings to be easily displaced during fretting, thereby producing effectively clean contacts. The number of cycles required to produce a given increase in contact resistance during fretting decreases with increasing track length (refs. 92, 742, 7 7 2 ) . This is true both for the unlubricated and lubricated Pd-Pd contacts studied with tracks of from 10 to 160 ,um length. For unlubricated contacts with long wipes (in excess of, say, 2 0 ,um ) the decrease of the number of cycles to attain 100 mR of contact resistance is very small, while for lubricated contacts with longer wipes (in excess of, say, 60 ,um) the improvement as a result of adding lubricant is small. Coatings formed with small (below 2 % by weight) amounts of fluids can be highly effective at small to intermediate amplitudes, while coatings from 2 % or higher concentrations of lubricant are highly effective with 20 ,um long wipes. Stabilization of the contact resistance by the use of lubricants is also related to the amount of frictional polymer formed (refs. 92, 7 4 2 ) . The frictional polymer can be dispersed by extra lubricant and polymers may also be less tough if oil is incorporated in their matrix to produce a gelatinous solid (ref. 7 7 3 ) . The final effectiveness of fluids in stabilizing contact resistance is attributable therefore to the nonadherence of the polymer that is formed. The reduced effectiveness of a thin fluid coating of lubricant with increasing wipe distance is explained by the fact that the effectiveness of fluids is related both to the volume of polymer formed and to the amount of fluid on the surface. Since the quantity of polymer produced with each wipe increases with the length of travel, the ability of the lubricant to accommodate it may be exceeded, pushing polymer to the ends of the track. This accounts for contact resistance being at a maximum at these locations (ref. 92). The organic lubricants (e.g. mineral oils, synthetic hydrocarbon) can also form frictional polymers. The quantity of frictional polymer from fretting Pd-Pd contacts without lubrication was much smaller than at lubrication with a thin film of synthetic hydrocarbon obtained by immersion and withdrawal from 0.5% (by weight) solution in l,l,l-trichloroethane (ref. 9 2 ) . The use of a lubricant coating can be deleterious if it evaporates or creeps from the surface, since the polymer remaining behind would contribute to high contact resistance. Lubrication as a means of inhibiting increases in contact re-
478 sistance (by shielding the surface from the air and thereby retarding the rate of oxide formation), as well as reducing wear, is especially effective for solder-solder contacts. The use of thick coatings (up to 2000 nm) obtained e.g. from a 20% U.S.P. mineral oil solution in a volatile solvent, stabilizes the contact resistance for at least l o 5 cycles (ref. 742). A surface treated with 20% polyphenyl ether solution is free of oxides (i.e. is still bright) after lo5 cycles of fretting (60Sn40Pb contacts, 20 pm wipe) and has a contact resistance of 0.9 mR, while the surface of the same contact lubricated with a 5 % coating is covered with an insulating black oxide and has a contact resistance of 37 R . The effectiveness of lubricants appears to be only slightly dependent on their composition and both fluids and greases are of value (refs. 741, 742, 774-776). However, those lubricants which are better adsorbed, either physically or, better still by chemisorption, and which have good electrical conductivity are more suitable (refs. 91, 777-781). If an oil is used, it should have medium viscosity to avoid the hydrodynamic lubrication effect (which leads to a very thick insulating film of oil) and also to avoid creeping from the contacts. High surface tension oils are therefore of interest. The oil or grease used should also have a low evaporation rate. The mixture of isomeric five-ring polyphenyl ethers (having a viscosity of 1200 mm 2/ s at 25OC) is a particularly good contact lubricant (as compared to other good contact lubricants such as polychlorotrifluoroethylenes ) because of its nonwetting character (the polyphenyl ether coating consists of tiny droplets) and low volatility, which minimize creep and tend tokeep it on the surface. If lubricant disappears from the surface, the frictional polymer which remains behind may then contribute to high contact resistance (ref. 92). The creep can be studied by examining the lubricant on the contact surface and seeing whether it has reached areas where it should not be; ellipsometry or, more simply, fluorescence detection (refs. 436, 603, 897) are suitable methods for this kind of study. However, a green fluorescent organic phosphor dye used in polyphenyl ether appeared to degrade rapidly with time and exposure so that after only a short time the fluoroscence of the lubricant was no longer readily detectable using an ultraviolet light source. A suitable replacement dye was found to be BBOT [2,5-bis (5-tert-butyl-2-benzoxazolyl)thiophene], a scintillation-grade blue fluorescent dye (ref. 436). The fluorescence intensity from a fresh lubricant film applied from a solution con-
taining 0.01% (by weight) BBOT ( 2 % in the lubricant: the BBOT dye was added directly to the l,l,l-trichloroethane solvent prior to mixing with polyphenyl ether (PPE): 0.5wt% PPE solution in the solvent) is about six times greater than the intensity from a film applied from a solution containing 0.02% ( 4 % in the PPE film) organic phosphor dye prepared in the same manner, and changes very little with time. The aforementioned BBOT concentration has no detrimental effect on the contact resistance (a contact system consisting of gold-plated sample brass panels and a gold wire was used for the measurements which showed this). Fluorescence detection using BBOT dye can also be used to measure and monitor the thickness and distribution of the contact lubricant film. The lubricant used should demonstrate good ageing resistance, also at elevated temperatures (usually up to 125OC in electronic devices) and should be chemically inert to polymers used as insulating materials, Synthetic or highly purified mineral oils (or greases based on such oils) are very useful. The characteristics of some special, commercially available oils for the lubrication of electrical contacts are listed in Table 3 . 7 . Solid state switches containing semiconductors that have controllable resistivities allow electrical circuits to be controlled without the use of moving parts and can replace the conventional type of switch in many applications. Brushless DC micromotors have their coils switched by transistors rather than mechanical brushes. They have a longer life and lower RFI generation than other DC micromotors. Research is, however, still going on into contact alloys and lubricating films for conventional electrical contacts. The tribological study of electrical contacts would benefit from a more general synthesis of the various case studies (refs.94, 741). Further study is needed on the contact of rough surfaces in electrical contacts, the mechanism of the formation of the real area of mechanical and electrical contact, and the effect of boundary films on contact resistance. The calculations of the contact resistance are based on too many quantitative empirical parameters. The friction and wear problems of electrical contacts and especially the effect of material transfer in unlubricated and lubricated contacts and the identification of the relationships between the properties of the transferred films and their tribological and electrical characteristics need further investigation. The basis for selecting the lubricant for contacts has not yet been clearly defined. Taking advantage of the selective transfer effect (see Chap-
480 t e r 5 . 1 . 1 ) seems t o be a n i n t e r e s t i n g and i m p o r t a n t way t o improve t h e t r i b o l o g i c a l and e l e c t r i c a l b e h a v i o u r o f l u b r i c a t e d e l e c t r i c a l c o n t a c t s and t o e x t e n d t h e i r l i f e , e s p e c i a l l y u n d e r e x t r e m e i n d u s t r i a l operating conditions.
481
10, SPECIAL TRIBOLOGICAL PROBLEMS Special tribological problems arise in many mechanical devices used in various a r e , for example, computer devices and medical implants (prostheses, assists and artificial organs). Mechanical elements play an important role in computer input-output devices, peripheral equipment, magnetic recording technology etc. There are many problems associated with high density recording devices where the recording head should be kept in direct contact with the magnetic medium because during start/stop operations sliding between head and medium occurs. This situation is undesirable from the viewpoint of system reliability since there is wear of both the magnetic medium and the magnetic head, which leads to a poor signal and eventually system failure. In disk file memories the recording head is stationary while the magnetic medium rotates at high speed. The magnetic medium consists of 7-Fe2O3 and A1203 particles held ridgidly in a polymeric binder, of continuous thin metal films such as Co-Ni, Co-Pt, Co-Cr, or of the new Co-Ni-Cr/Cr sputtered thin film which is useful for Winchester type small hard disk drives (ref. 7 8 2 ) . The friction and wear in head-magnetic medium systems can be reduced by adding a lubricating agent to the magnetic medium, by depositing solid or thin fluid lubricating film on the magnetic medium (refs. 7 8 3 - 7 8 7 , 971-974,
985-987).
Lubricated disk composites ($-Fe203 film/substrate 4% wt. Mg-A1 alloy surface anodized in a chromic acid) can show good endurance of up 2 0 , 0 0 0 cycles for the contact-start-stop (CSS) motion of a floating head-slider (ref. 7 8 8 ) . The wear endurance life can be extended by increasing the hardness of the substrate layer. Heat-resistant, defect-free Cr03 anodization substrates with a Knoop hardness of 4 2 0 0 MPa have been successfuly used to minimize wear; hematite (with additive elements'Ti+Co, Ti+Cu+Co, Ru+Co and 0 s ) 1 7 0 nm thick films were deposited onto the substrate by reactive magnetron-sputtering. The T-Fe2O3 thin film disk exhibits high wear-resistance, as compared to the wear-resistance of sputtering-deposited or plated Co-alloy disks where the hardness of the substrates is nearly equal. An additional SiOz top-coating and lubrication of a-Fe203 disks can further extend their endurance life. A r-Fe203
482
sputtered or plated disk lubricated with PTFE can perform 2 0 , 0 0 0 CSS operations (10 times the number of CSS which might be expected over 5 years of operation) and has a stiction force (after being saturated for three or four days at 2S0C and 50% relative humidity of less than 60 mN, which is 1/5 of the suspension spring buckling load (ref. 7 8 9 ) . The friction coefficient of a polymer film is low when its thickness is kept over 0.1 pm (ref. 9 7 2 ) . Disks with a chemically plated Co-Ni-P medium 0.08 pm thick, and a S i 0 2 overcoat 0 . 0 8 gm thick, spin-coatedand lubricated with a polar solid lubricant, demonstrated sufficient wear durability for 2 0 , 0 0 0 CSS cycle tests at 0.2 ,urn flying height with Mn-Zn ferrite heads (ref. 7 8 5 ) . The degree of oxidation in polysilicate films made by the spin-coating method using a tetrahydroxysilane alcohol solution can be improved by irradiation with a laser beam: after laser annealing, the polysilicate film becomes similar to the sputtered Si02 film. Laser annealed polysilicate films have sufficient wear durability to withstand head-disk sliding motion, but because of high surface activeness in the polysilicate film there are strong friction and stiction effects between the head and disk surfaces. The friction coefficient of the annealed film (measured for a Mn-Zn ferrite head at 6 0 mN loading force) is initially very high (even over l . O ) , but decreases to 0.4 after 2 0 h of exposure to air and stabilizes at 0.3 after 1 5 0 h. After the film has been cleaned to remove adsorbates (from the air) from the polysilicate film surface the friction coefficient returns to about 0.7. The stiction force at the head-disk contact after one week is about 100 mN even when films which have been exposed a long time are used. Liquid lubricant film behaves in a similar way to the laser annealed polysilicate films and has a similar friction coefficient, as it is easily removed from the head disk-contact area, after 100 h storage in air, but the stiction force is twice as high. NOn-pOlar solid lubricant film demonstrates a low, stable friction coefficient of around 0.11 and a low stiction force of 2 0 mN as a function of the exposure time in air, but the wear resistance during start-stop operation of the head-disk system is low. Since the adhesion of such lubricants to the polysilicate film is poor, the headgathers lubricant from the disk surface and as a result its flying becomes unstable. Polar solid lubricant film is strongly adsorbed to the polysilicate film, resulting in good wear durability of the head-disk system and a small friction coefficient and stiction force. The tribological behaviour of the head-disk systems tested
483
is compared in Table 10.1. Optimum lubrication is achieved with polar solid lubricant, giving the good wear durability mentioned above, a friction coefficient of below 0.3 and a stiction force of less than 5 0 mN per head at 25OC and 5 0 % relative humidity for a week using heads loaded with a force of 60 mN. The wet durability was excellent when tested at 4OoC and 80% relative humidity for 7 months. TABLE 10.1
COMPARISON OF THE TRIBOLOGICAL BEHAVIOUR OF Mn-Zn HEAD-DISK (0.2 p m t h i c k Co-Ni-P p l a t e d medium, 0.02 p m S i 0 2 o v e r c o a t ) SYSTEM U S I N G VARIOUS METHODS OF LUBRICATION (based on r e f . 785)
FRICTION COEFFICIENT
STI CT I ON AFTER LONG HEAD-0 I SK CONTACT
WEAR
None
Med i urn
High
Low
Liquid
Med iurn
High
Low
Nonpolar, s o l i d
Low
Low
High
Polar, s o l i d
Low
Low
Low
LUBR I CANT
Liquid lubricants, especially fluorocarbon oils, are used to minimize friction and wear, especially for Winchester-type media. The thickness of the lubricant film must be carefully controlled (see below) since if the film is too thin, poor lubrication and head failure can be expected and if the film is too thick, stiction of the head often occurs, which can result in head-disk damage during start-up. The thickness of the oil film should be kept between 0.01-0.1 ,um. The film is formed from the oil solution. Fluorocarbon oils such as the fluorinated polyethers Fomblin Y CF 3 I
CF3
- [ - (0-CF - CF2)m-
and Fomblin
(0
- CF ) n- ]
-
0
- CF3
2
are highly efficient lubricants both for magnetic tape and disks (refs. 7 8 6 , 7 8 7 , 9 7 5 , 9 7 6 ) .
484
Investigations into the effect of various fluorinated Fomblin polyether oils on the tribological behaviour of the Co-Cr (sputtered on PETP and PI substrates) floppy disk-dummy or standard floppy disk-head systans have shown that these lubricants reduce friction and wear to a fair extent (ref. 786). The applied loads were 120-150 mN. Thin films of oils were obtained from diluted solution in 1,1,2-trichlorotrifluoroethane by dip coating in a dust-free environment. The thickness of the lubricant is a complex function of the drawing speed, solution concentration (0.5-2% by weight of Fomblin neutral Z 25 oil having a molecular weight of 10,000 and bulk viscosity of 200 mm 2/ s at 25OC) and the evaporation rate of the solvent. At very low drawing speeds (below 2 m/s), the thickfor ness of the lubricant films was about 0.005, 0.03 and 0.05 0.5, 1.0 and 2.0% concentrations respectively. When the drawing speed was 15 mm/s the thickness of the lubricant films was about 0.03, 0.06 and 0.10 ,um respectively. The friction coefficients determined at a sliding speed of 1 mm/s for a 0.04 ,um thick oil film using Co-Cr coated disk and A1203, BaTi03 or amorphous carbon head test materials were: 0.47/0.40, 0.38/0.36 and 0.18/0.16 (static/kinetic) respectively during unlubricated sliding: 0.22/0.10, 0.24/0.12 and 0.16/0.12 at lubrication with Fomblin 2 25 oil: and 0.17/0.09, 0.16/0.10 and 0.14/0.09 at lubrication with polar 2 DOL 4000 FCpnb1-h oil (molecular weight 4000, viscosity 290 mm 2/ s ) . The presence of oil greatly reduces the effect of the head material on the friction coefficient. Wear tests have shown that the improved lubricant adhesion as a result of polar functional groups increases the wear resistance in disk-standard head systems, The relative disk life in lo6 passes ( = 50 h of operation) when the thickness of the lubricant film was 0.04 ,um, was 0.01, 1.4, 1.1, 1.7, 1.9 and 2.8 for unlubricated and lubricated sliding with Fomblin neutral 2 15 (molecular weight 8300, viscosity 150 mm 2/ s ) , 2 25, mixture 2 25 + polar 2 DOL, and polar 2 DOL 2000 (molecular weight 2 0 0 0 , viscosity 2 75 mm / s ) and 2 DOL 4000 oils respectively. When 0.01, 0 . 0 2 and 0.04 ,um thick Z DOL 4000 oil films were tested the average life was determined as 1.5, 3.1 and 3.5 respectively (in 106 passes). The experiments were carried out with a spherical head profile (50.8 mm in radius). The shorter life found when a flat profile and soft pressure pad were used was probably caused by the uneven distribution of stresses in the area of contact. The studies were carried out using the disk on a single side. No significant effect of the media stiffness on the wear was observed.
485 A surface treatment can also improve wear resistance of thin film magnetic recording media (ref. 9 7 1 ) . In this method a thin film of higher fatty acids, their metal salts, amides, esters,etc. is deposited on the magnetic film surface by the vacuum method, called "lubricant deposition method". The problem of stiction is an important one. Stiction usually causes lubricant film disruption and deformation of the flexible slider, which can result in head failure. Stiction is an adhesion problem, depending on the texture of the contacting surfaces and the applied head load in the real area of contact. The adsorption and condensation of water vapour on the disk surface also needs to be considered in the context of stiction. The interaction between the water and the lubricant film plays an important role the initiation of stiction. Stiction tests on the Winchester head-disk interface (ref. 7 9 0 ) have shown that the disks with higher water adsorption susceptibility and a rougher surface exhibit higher stiction forces. The adsorbed water film thickness was 0.028-0.035 @m (increasing with increase in vapour pressure) on the relatively rough disk surface. The stiction force increases rapidly when the 3 water vapour density is greater than 2 4 g/m Loaded disk-head systems show much higher stiction than those without load for both the unlubricated and lubricated disks. Unlubricated disks in contact with unloaded sliders exhibit higher stiction than lubricated unloaded disk-head systems. The magnitude of the stiction in unlubricated and lubricated loaded systems is similar. If the surface of the disk is smoother? the stiction force will be smaller. The stiction process begins when water vapour condenses at the disk-head interface due to a change of temperature and the process becomes fully developed when the water film is dried up by purging air. Water drops diffuse through the lubricant film and displace theslubricant from the disk surface by lateral advance. The surface tension of the thin water film during the drying process pulls the two solid surfaces together, producing deformation at the area of contact. The adhesion is greater when the film is thin. The stiction force is thus determined by the amount of water condensation? the degree of water diffusion through the lubricating oil film and the amount of lubricant replaced by water molecules at the contactarea. The most severe stiction occurs at solid-solid junctions. The effect of temperature and humidity on the head-disk friction coefficient is also clear (ref. 7 9 0 ) . The friction coefficient at room temperature and a relative humidity of 50% is controlled
.
486 by the amount of lubricant present on the disk surface and increases with decreasing lubricant film thickness. If the temperature or humidity is increased, the friction coefficient stays about the same; it is not dependent on the lubricant film thickness. This was found for disks with a rough surface. Such disks exhibit high stiction forces. Smooth disks have a higher friction coefficient and are less dependent on the thickness of the lubricant film (an unlubricated disk and one lubricated with a 0.004 pm thick film were tested) under all test conditions. The smooth disks demonstrated very low water adsorption. Micropores found in their media layer improve their durability by providing lubricant replenishment during operation. Pin-on-disk studies carried out with a spherical profile ferrite pin and coated magnetic disk show that under mixed lubrication the transition from mild to severe wear takes place at a decreasing sliding speed and increasing load (refs. 791, 974). The critical values of Nv (N - load, v - sliding speed) can be placed on ahyperbola plotted in the v - N coordinate system. It was confirmed that the wear in the real magnetic disk-flying head system during start-stop operation is small when the effective contact load is in the mild wear region as determined from the pin-on-disk wear studies. As a result of the wear tests carried out on typical pin-on-disk triboloqical svstems, reliable disk-flyinq head systems can be desiqned. Friction and wear problems in maqnetic tape head systems have already been discussed in Chapter 4.3. Since the surface roughness of tape is substantially higher than that of disks, interniittent contacts in magnetic tape-head systems are quite frequent and wear has a predominantly abrasive or abrasive-adhesive character (refs. 278, 280, 792-797, 978-979). Metal pigment videotape containing graphite fluoride (obtained by fluorination of carbon black) demonstrates very good tribological behaviour (ref. 784). The lubrication effect appears and the friction coefficient decreases sharply from 0 . 4 5 to 0.3 when the degree of lubrication is more than 30%. It was found that tape containing 3-5% graphite fluoride had even better wear resistance when 3-10% by weight of various other solid lubricants (carbon black, PTFE powder, MoS2, natural graphite) were also added - much better than tapes containing natural graphite or other lubricants (similarly demonstrating the very small positive effect of the wear resistance of the tape). Tape containing graphite fluoride has a stable still-frame performance over
487
long wear periods when exposed to elevated temperatures (in the range 2O-6O0C) and high humidity ( 8 0 % relative humidity). Graphite fluoride reduces tape-head friction and wear so efficiently because of its low surface free energy (as compared to other solid lubricants), which reduces the adhesion between the rubbing surfaces. The presence of the graphite fluoride also results in high packing density and high tape orientation. The lubrication of magnetic tapes with liquid fluorocarbon lubricants (Fomblin Y and Z used in 0.5-1.5% by weight solutions in 1,1,2-trichlorotrifluoroethane) , where the surface film accounts for 3 0 - 4 0 % of the total amount of lubricant oil and the other 60-70% of the oil fills the surface pores of the media, greatly improved the frictional properties of the tapes investigated (cobalt modified iron oxide of the Audio Type-II)(ref. 7 8 7 ) . Fluorocarbon oils are not removed from the surface when it is scraped by the head. This was not the case for the other lubricants tested (200 and 60,000 mPa s (at 20°C) silicone oils 1% solutions in n-heptane and internal lubricant 1% isocetyl stearate +0.6% of 200 mPa s viscosity silicone oil over the oxide). Fluorocarbon oils are chemically inert and do not react with the binder to plasticize the surface. An increase in friction during tape wear was observed (ref. 9 8 0 ) . Changes in surface smoothness were not significant enough to account for the increase in friction, but a measured noticeable increase in the real area of contact and mechanical and/or chemical changes observed in the tape were found to be correlated with it. The wear of the head in a magnetic tape-head system depends on the type of tape used: Cr02 (magnetic medium) tapes are abrasive while Fe tapes are rather adhesive (ref. 7 9 7 ) . Metal andpolycrystalline ceramic head materials show greater wear than crystals of ferrite or garnet. The transfer of tape material dominates the wear of the head in the case of Fe tapes, the amount of material transferred being determined by the quantity of abrasive particles in the coating. The transfer of tape material in the presence of humidity is responsible for the corrosion of metal heads (ref. 7 9 5 ) . The transfer changes in wear mechanism and increases the friction coefficient since the tape rubs on tape debris. The transfer of wear debris Of airradiated nickel-zinc ferrite head to a magnetic tape can be measured by autoradiography of the worn magnetic tape (ref. 9 8 1 ) . The use of materials based on Ni-Fe-Nb permalloy with the addition of 1.1% A1 or 2% Mo - 0.5% A1 gives wear resistant heads (ref. 7 9 8 ) . The application of the extremely hard A1203-TiC as guard
488
substrate greatly improves the wear rate (by 0 . 2 ,um/1000 h at ambient temperature and at a tape speed of 4.75 cm/s) (ref. 7 9 9 ) . When high coercivity metal tape is used, tilted sendust sputtered ferrite heads demonstrate high wear resistance due to the structure being occupied by a large amount of ferrite (ref. 8 0 0 ) . Head wear in devices using smooth magnetic tapes in association with air-film lubrication is generally low, although isolated asperities on the nominally smooth tapes can have a great effect on the head wear (ref. 7 9 2 ) . The characterization of magnetic tapes in terms of the number od isolated asperities (as well as average roughness),either directly by using an optical method such as a grazing incidence laser asperity counter (ref. 7 9 2 ) or indirectly by measuring electrical dropout, is thus essential. A method for determining a linear wear comparable to a surface roughness height can be found in ref. 9 8 2 . Methods for measuring tape-debris generation are described in ref. 9 8 0 . The temperature of microscopic areas in a head-tape interface can be measured using infrared radiometric technique (ref. 983)
The flying head sliders used in high density magnetic disk recording machines operate under submicron spacing conditions. Spacing below 0.5 ,um at a sliding speed of several m/s is now being used in practice. A modified form of Reynold's equation on mlecular slip-flow effects may be applied to the design of flying heads (refs. 657, 7 9 1 , 801, 8 0 2 ) . Studies of the dynamics of slider systems should take into consideration not only ultra-thin film air bearings but also suspension gimbal springs and actuator arm mechanisms (refs. 804, 8 0 5 ) . Whitney mechanisms have better dynamic characteristics than Winchester mechanisms. A warp in the slider modifies the pitch and dynamic response of the slider but chip and pimple size on the rails influences the steady state fly- heights on the rails (ref. 905) .At low frequency disk waviness the warped slider flies lower than a flat slider. For stretched surface re03rd.q disk the flybg height of the slider is n w l y indep=ndent of radidl track position over a wide range, which is in contrast to conventional disks for which the flying height increases with radius at increased sliding speeds (ref. 8 0 6 ) . The flying height increases when the edges of the head approach the rims of the disk. The heads used for stretched surface disks enable a continuous run on a single track in excess of 8,000 hours and f o r over 30,000 contact start-stop cycles (ref. 8 0 6 ) . Thin film media exhibiting excellent recording characteristics are mechanically delicate. High performance air bearing sliders are
negative pressure sliders (refs, 8 0 7 , 8 0 8 ) which require only a small loading force and the wear resulting from the contact start/ stop motion is small. A negative pressure slider can fly at a lower sliding speed. The flying height fluctuates much less than a conventional slider. When SiOz cover protective film was used on the transducer parts of the negative pressure sliders after 20,000 contact start/stop cycles (using r-Fe203 sputtered media) no output level degradation or microscopic wear were observed on either the head or the medium (refs. 8 0 8 , 8 0 9 ) . In digital tape recording devices, rotary heads are used. The transverse scan device which features non-contact recording as well as non-contact, reel-to-reel tape transport using cartridge tapes and which uses both a pressurized air bearing female guide and a spherical monolithic head, allows more than l o 7 head scans t o be carried out without tape wear (ref. 810). The back surface of the tape is supported flexibly by a pressurized air bearing female guide with two rows of feeding holes, and the recording surface of the tape is supported by a hydrodynamic air film produced by the rotating head rotor. 0 . 1 5 ,um spacing can be obtained at the head gap. Although the rotary head device does not feature start/stop contact, intermittent contacts do occur between head and tape. This leads to a combination of abrasive and adhesive wear as in longitudinal tapes. The performance of a magnetic disk storage system depends very much on the dynamic properties of the electromechanical actuator used to move the magnetic recording heads. Except for the need to reduce the mass and dimensions of the actuators, the most important problem is to minimize the friction associated with the hearings (guides) which support the recording head and actuator coil. Friction noise producing an error in position is only partially compensated by servo electronics and limits the ultimate track density. The magnitude of the frictional forces divided by the moving mass should be very small (the equivalent acceleration (1 m/s 2 (ref. 8 1 1 ) ) . The tribological aspects of guide systems have already been discussed in Chapter 9 . 5 , A cutaway view of a hydrostatic air bearing guide in the planar actuator of a magnetic recording head disk file is presented in Fig. 10.1. The air bearing surfaces are 4 . 8 mm in diameter, 6 . 4 nun long, 4 4 . 5 mm apart, and raised by 0 . 3 nun fmnthe pressurized (0.3 MPa) stationary rod. The diameter of the 6 orfices in each bearing is 0.2 nun. The radial clearance of the moving sleeve is 8 pm, its length 9 0 mm and its outside diarneter
490
6.35 nun. A graphite filled PI was chosen as the sleeve material to match the coefficient to thermal expansion of the aluminium rod.
F i g . 1 0 . 1 . H y d r o s t a t i c a i r b e a r i n g guide used i n l i n e a r a c t u a t o r used t o move magnetic r e c o r d i n g head, 1 sleeve, 2 r a i s e d a i r b e a r i n g su rf a ce, 3 relief vent, 4 o r i f i c e , 5 - pressurized stat i o n a r y rod ( r e f . 811).
-
-
-
Disk media must be corrosion-resistant. Generally Co-Cr media are the most corrosion-resistant followed by Co-Ni and Co-P, in that order (ref. 812). This is true in both accelerated,business environment (BE) tests and in the case of electromechanical corrosion. During BE tests, corrosion products are formed on top of the disk surface rather than by pitting, showing the presence of sulphur and oxygen. The corrosion behaviour of Ni-Fe media is excellent in elevated humidity C12, SO2 and O2 atmospheres (ref. 813). Co-Cr films show some resistance to magnetic and chemical degradation. Surface changes occur in samples exposed to high relative humidity test atmospheres but attack does not readily occur for specimens exposed to dry test atmospheres containing high concentrations of C 1 2 , SO2 corrosives. The nature of the substrate has a critical influence on the behaviour of a sputtered film (ref. 813). Aluminium substrates are prone to degradation in high relative humidity C 1 2 , and S02/02 containing atmospheres, while Si02 glass substrates are not. In films deposited on dissimilar metal substrates, degradation by galvanic attack does not seem to be a factor.
491
Corrosion is strongly affected by the deposition of electrically conducting overlayers through the mechanism of galvanic corrosion at breaks in these layers (ref. 8 1 4 ) . Determination of such parameters of the magnetic media surface as roughness (defects), lubricant film thickness and the spacing between head and medium surface is important when characterizing the behaviour of the head-medium system, Computer Analyzed Microscopic Interferometry (CAMI) is a non-destructive method of obtaining three dimensional and quantitative interferometric images of very high depth resolution at a given size of viewing field, which give very precise surface roughness and volume measurements (ref. 7 9 4 ) . This method is also very useful for wear studies in head-medium tribological systems. Electron Spectroscopy for Chemical Analysis (ESCA) or Fourier Transform Infrared Spectroscopy (FTIR) may be used to estimate lubricant thickness (refs. 8 1 5 , 8 1 6 ) . Thethickness of fluorocarbon oil film may be measured with an accuracy of 50% with the ESCA method, which is suitable for film thicknesses of below 0 . 0 1 pm. The FTIR method is a cost-effective technique which enables thicker films to be studied. The principle of the technique is to measure the IR signal of the lubricant, as represented by the area under the absorption peak of the fluorocarbon, or simply its peak height, which increases with increasing film thickness. The head flying performance my be studied by simple electrical resistance, Acoustic Emission (AE) methods for detecting slider-to-disk contact, interferometric methods allowing accurate spacing determination (refs. 2 8 0 , 8 0 2 , 8 1 7 , 9 8 6 , 9 8 7 ) . The A E m t h d can detect contact between sliders and disks made .of any material as well as measuring the projection height distribution over an entire disk surface (refs. 8 1 7 , 9 8 6 ) . The computerized Hitachi interferometric measuring system can be used to study spacing in the range 0.1-2 gm (ref. 8 0 2 ) . The system was used to study the correlation between spacing determined theoretically (by solving Reynold’s equation, taking into account the gas or air compressibility effect and molecular slip-flow effects) and spacing determined experimentally; it was found that the mean theoretically determined spacing of over 0.15 pm agreed with the measured one and that at smaller spacings the experimental values were lower than the theoretical ones. Wear of the head and medium can be studied by the microscopic or radiographic techniques (refs. 8 1 8 , 9 8 1 ) . As has already been mentioned, CAMI is highly suitable for studying wear. Interfero-
492
metric images of wear scars are invaluable in providing insight into wear mechanisms and the volume of material lost at a given location on the specimen can be calculated. Magnetic sensors are useful for estimating wear in head-magnetic tape systems (refs. 7 9 6 , 8 1 9 ) . Since wear tests can be so time consuming, accelerated wear tests are also of interest. They have been used to estimate the abrasiveness of magnetic tape (refs, 8 2 0 , 8 2 1 ) and to study head materials at low tape speeds and low contact pressures (0.01-0.1 MPa) (refs. 8 1 9 , 8 2 2 ) . An accelerated simple pin-on-tape wear test for head-video tape systems gives wear data within a few minutes and the wear coefficients determined give a similar ranking of materials or tapes to that found in actual use, although the values of the wear coefficient obtained in the accelerated tests are several orders of magnitude higher (ref. 7 9 7 ) . One of the tribological problems in computer/data processing office equipment is the wear produced by paper and ribbon. The wear scars produced by these two types of materials are typical of mild wear behaviour, frequently having a bright polished appearance. During wear testing, paper and ribbons have been found to have similar characteristics and present similar tribological problems (refs. 2 8 0 , 281, 6 9 1 , 8 2 3 ) . Wear by paper is fundamentally an abrasion process, caused by hard particles in the paper, and depends on the hardness of the material used. For a variety of materials, from elastomers to diamond, the dependence of the wear on the material's hardness can be estimated from the following formula (refs. 281, 691, 823) :
v = -K
Nm L H"
(10.1)
where V is the volume of wear, K the abrasive wear constant for the paper, N the load, L the sliding distance, and H the hardness of the abraded material, m and n are phenomenological constants. Three regions of hardness dependence have been identified (ref. 2 8 1 , see also Chapters 4.3 and 9 . 5 ) : in the region where the hardness of the abrasive is greater than that of the abraded material, n =: 1; in the region where the hardnesses are similar, n '* 10; and in the region where the abraded material is harder n 5. Paper can be worn or damaged by relatively soft materials. Damage to the paper decreases its abrasivity, whilst the drier it is, the more abrasive it becomes (ref. 6 9 1 ) . The above factors, individually or combined, may reduce or increase the abrasivity of paper by as much
493
as one to two orders of magnitude, so that it is difficult to assign a specific value to the abrasive constant K for a given paper. The similarities between paper and ribbon wear suggest that the nature of the wear process is also similar, i.e. the wear is caused by abrasion. This was confirmed for woven printer ribbons impregnated with liquid ink (ref. 823). The wear of un-inked fabrics or ribbons was found to be typically two to three orders of magnitude smaller than the wear of inked fabrics. Since the major constituents of ribbon inks are frequently materials known to lubricate and reduce wear, such as mineral oils, oleates and oleic acid, ribbon wear does not have an adhesive character, Abrasive wear by paper or ribbon results from small amounts of sub-micron size abrasive particles in them. An increase in humidity results in the wear of the ribbon increasing with almost a square root relation. Abrasive wear by ribbons obeys the elementary relation for this kind of wear. The abrasiveness of commercially available papers and ribbons can vary by over two orders of magnitude (refs. 280, 823). The major difference in the relative abrasiveness of different papers and ribbons is to do with difference in the size rather than the amount of their abrasive particles. Their abrasiveness is also strongly dependent on prior use and humidity. In the relative humidity range of 10-60% the abrasiveness can change by as much as a factor of ten for paper and a factor of two for ribbons. After miltiple use, the abrasiveness of both materials can dramatically decrease, by up to one to two orders of magnitude after ten or so passes. To investigate the ability of paper and ribbon to generate wear and other materials to resist such wear, a piece of apparatus is needed which provides a large amount of unused surface area of paper or ribbon. The apparatus developed by Roshon for this purpose (ref. 8 2 4 ) basically consists of a large circular drum with the paper or ribbon specimen mounted on the drum's periphery. The drum rotates, while a wear specimen slowly moves across the surface of the drum in the direction of the drum axis, which effectivelygives a helical wear path. The abrasiveness of both paper and ribbon increases when external abrasive media become attached to their surfaces. This is also true of magnetic media which are themselves very abrasive and provide a similar wear situation to that of paper and ribbon. The aspect of contamination is very significant since computer systems are more and more frequently used in stores and factories (ref. 796). In such conditions the external abrasive may predominate and
494 mask the inherent abrasiveness of the paper, ribbon or tape (refs. 280, 796). Furthermore, computer input-output devices frequently produce large amounts of paper and ribbon debris, which can also produce wear by abrasion. All these contamination problems should be taken into consideration when designing for sensitive components and surfaces to further reduce wear, on top of the use of hard materials or anti-wear coatings of course (see Chapter 7.3). The problems of wear associated with paper and ribbon may be divided into two categories (ref. 280). One of them concerns guiding surfaces, for which loads or contact pressures are generally small, but sliding speeds may be high e.g. 10 m/s or more. The second category consists of problems associated with elements such as punches or printer type, in which high sliding speeds and also high loads under impact conditions may be encountered, The basic concept of abrasiveness applies to both these two categories although the wear predictions may differ in their degree of complexity. When impact operating conditions occur the hard materials or hard anti-wear coatings used should have greater ductility and toughness. The same techniques for wear testing can be used to classify both abrasiveness and abrasion resistance for both categories of wear problem associated with paper and ribbon. Printer mechanisms often involve impact. High print forces are desirable in order to get sharp print quality. This induces flexural fatigue in the type element and results in an increased wear rate. A greatly worn type character gives poor print quality (refs. 280, 984),and may also upset the intended operation characteristics since many printing mechanisms are sensitive to the distance travelled by the impacting hammer. To achieve a wear life in the lo8 109 cycles when printing on paper or ribbon, hard materials or anti-wear coatings (see Chapter 7.3) must be used to decrease the abrasive wear. The wear is marked by the smoothing off of the contact surfaces of type fonts. In matrix printers, print wire tips tend to be worn on the sides, exhibiting a "pencil sharpener effect" as the wire penetrates into the ribbon-paper medium while rubbing occurs on the slideways (refs. 533, 825). The use of ruby (ref. 826) or borided tool steel (ref. 533; see also Chapter 7.3) slideways is very effective in extending the life of wire-slideway systems. A cam actuator or a hammer hitting the back of a type element (and causing the latter to hit the platen-supported paper) in a printer mechanism involves impact between metals, Fretting and fa-
495
tigue wear (e.g. pitting) occurs on such metallic components (ref, 2 8 0 ) . The amount of wear depends not only on the normal impact speed but also on the tangential approach speed component and the relative stiffness of the impacting elements (promoting slip on the interface). The resulting wear characteristics are typically in the form of a gently declining slope, due to the progressively conforming wear geometry. Rebounding print hammers are stopped or de-energized by the use of a "back-stop" for which elastomer impact-damper covers are provided. Since elastomer impact dampers operate at higher than ambient temperature (due to mechanical energy dissipation) they are geometrically designed to avoid excessive heat buildup. Impact wear of a protective elastomer layer may be due to thermal, fatigue or abrasive mechanisms (refs. 164, 280). Print disks and typeballs are usually nonmetallic elements, requiring the use of impact resistant materials. Wear occurs by degradation of the material, or the matrix in a composite, by contact fatigue and abrasion (ref. 2 8 0 ) . A reasonably long life can be obtained by applying thick hard nickel plating to polymer print disks or typeballs (ref. 8 2 7 ; see also Chapter 7 . 3 ) . A wear life in the range of 105-106 cycles can be reacned for nickel-plated polymer disks. In printing methods other than solid impact printing, such as the optical, thermal, magnetic or ink-jet methods where material particles are applied, erosion wear problems occur. In ink-jet printing internal components such as nozzles and pumps are subjected to the erosive action of ink. There is very little wear when the ink contains no small abrasive particles (in electrostatic printing) but when the ink contains magnetic particles of 15 nm in size (in magnetostatic printing) wear of polymer pump components can be observed (ref. 2 8 0 ) . Medical implants such as prostheses should demonstrate very good tribological properties as well as biological inertness. Their life depends significantly on the wear of the rubbing elements and must be extended as much as possible. The development of total replacement hip, knee, ankle, elbow, shoulder and hand joints (and the appropriate surgical techniques) has been the major success of orthopaedic surgery this century and would not have been possible without extensive in vitro and in vivo studies of the tribological problems, especially wear, associated with such artificial joints. Other implants such as prosthetic heart valves and artificial organs are also the subject of tribological evaluation, to ensure
496
low frictional losses. Hip joint prostheses are the most frequently applied total replacement joints. In vitro (using a special total joint replacement simulator or a typical laboratory wear machine; see ref, 11) and in vivo wear studies of metal-UHMWPE artificial hip joints give different results (refs. 11, 828, 830, 988-990). In Charnley's first low friction arthroplasty of the hip based on a stainless steel femoral component and an unfilled PTFE acetabular cup the femoral head very quickly penetrated into the acetabular cup, so lates UHMWPE was adopted as the socket material, and is now used almost exclusively in the total replacement joints. The average rate of penetration of the femoral stainless steel head into the UHMWPE acetabular cup of Charnley's prostheses examined in vivo was found to be 0.19 mm/year (ref. 828). A large spread of individual penetration rates (from 0.005 to 0.623 mrn/year) can be expected. The average clinical wear factor is 2.9 l o m 6 to 7.18 10-6m3N-1 m-l. mm3N-l m-', The corresponding laboratory wear factor was 1.2 found using a pin-on plate reciprocating wear machine for a stainless steel-UHMWPE system lubricated with distilled water, in which the surface roughnes of the steel, Ra = 0.054 ,um, was similar to that of the original prosthesis head. The entrapment of bone-acrylic cement particles between articulating surfaces has a very strong negative effect on the true wear rate of UHMWPE in the total hip prostheses (refs. 1 1 , 828-831). The major factors responsible for the ramaining discrepancies between the average in vivo and in vitro wear factors and the wide spread of the study results are (ref. 828): the level of patient activity, the effective roughness of the femoral head (the surface may be affected by acrylic cement particles) and inherent variations in wear factors depending on the polymeric materials used. Various counterface materials are used for manufacturing the femoral heads. Typical materials are stainless steel, Co-Cr-Mo, Ti-6A1-4V alloys, and alumina (A1203). The presence of toxic vanadium in Ti-6A1-4V was successfully avoided by use of the new Ti-5A1-2.5Fe alloy (ref. 832). The tribological behaviour of a head coated with an oxide layer about 1-3 ,um thick produced by induction heating of the Ti-5A1-2.5Fe alloy surface is the same as that of a head manufactured from alumina ceramic. Single crystal alumina ceramics have better physical properties and show better tribological behaviour than polycrystal alumina ceramics (ref. 267; see also Chapter 4.3). Coating steel with alumina would extend the
497
life of UHMWPE cups from approximately 15 years to 15,000 years (ref. 833). Continuous fibre woven E-glass/epoxy composite femoral shells have the same elastic properties as bone and demonstrate relatively good tribological behaviour (ref. 239). Epoxy/E-glass shells coated with (2:l) stainless steel shell/epoxy + (3:lO) A1203/epoxy (steel and A1203 particles 1-64 ,um in size, when rubbing against a UHMWPE acetabular cup, exhibit only 15 times more wear than the traditional Vitallium ball-UHMWPE system but the friction coefficient is 10% smaller. The addition of graphite fibre to UHMWPE (1:30, graphite fibres: UHMWPE) increases the friction coefficient but reduces the surface damage to the UHMWPE. The investigations cited were carried out using the total hip simulator. The wear of UHMWPE in a prosthesis depends on its molecular weight. High wear is associated with low molecular weights of the polymer at the articular surface (ref. 834). High wear is accompanied by the production of large particles of debris (up to 0.1-1 mm when using the hip joint simulator. The wear of UHMWPE whensliding against a steel counterface decreases hyperbolically with increasing average molecular weight (ref. 835). The molecular orientation to the wearing surface in UHMWPE is important; orientation perpendicular to the wearing surface is undesirable (ref. 238). $-irradiation seems to have an adverse effect on the wear behaviour of UHMWPE (ref. 11). The use of reinforced (e.g. by carbon fibres) UHMWPE results in 88% higher stiffness and 1 7 % greater withstanding of compressive loads as compared with plain UHMWPE (ref. 837). Time-dependent deformation over a 2 4 h period was reduced significantly in the UHMWPE reinforced with carbon fibres. The wear of carbon-filled UHMWPE is however slightly higher than that of plain UHMWPE (ref. 11). Since the dimensional changes in a prosthesis are largely due to creep or flow of the polymer (actual wear of the polyethylene component accounts for only 3-30% of the total change (ref. 834)) it is essential to minimize the creep, for instance by reinforcement or by use of silane cross-linked PE (reducing creep by over 60%; see ref. 833). Worn UHMWPE acetabular cups show three distinct regions (ref. 828): a highly worn, smooth area in the superior half of the socket, a much rougher low wear area, and a ridge separating these two areas. The surface shows pitting in both the high and low wear areas, particularly in the low one, Adhesive wear is evident in the high wear region together with some scrathes resulting from cement entrapment. Adhesion appears to be the predominant wear mechanism
498
in UHMWPE when the counterface surface is very smooth (Ra< 0.1pm) in steel-UHMWPE systems (refs. 835, 8 3 7 ) . The wear process in a hip joint prosthesis (femoral metal head-polymer acetabular cup) can be identified using wear modulus and wear number perameters (refs. 6 4 7 , 6 4 9 ; see also Chapter 9 . 2 ) . During the wear process the increase in the wear modulus is about 1 0 0 2 N/mm per 1 nun sliding distance in the Charnley and the CharnleyMuller hip joint prosthesis (ref. 6 4 9 ) . The Charnley prosthesis is superior to the Charnley-Muller prosthesis if running-in is carried out before implantation (refs. 6 4 7 , 6 4 8 ) . The frictional torque needed to rotate the ball cup bearing increases to a maximum 1.273 times in frictional torque at zero wear (ref. 648; see also Chapter 9.2 and Fig. 9 . 1 1 ) . Knee joint prostheses sometimes show severe wear of the UHMWPE tibial component. The true wear of UHMWPE components in knee prostheses in an order of magnitude larger than in total hip prostheses (ref. 8 3 8 ) . Dimensional changes of the tibial components as the result of wear may be as great as or greater than the dimensional changes due to creep. The wear is dominated by the high contact pressures. The design and alignment factors are more important than material variability (refs. 8 3 8 - 8 4 0 1 , The wear is also, as in hip joint prosthesis, greatly affected by the presence of entrapped cement particles. Prostheses which demonstrate high wear exhibit regular periodic cracking of the friction surface, probably as a result of fatigue (ref. 8 3 8 ) . Around three hip implants are carried to every knee implant (ref. 1 1 ) ; the other types of joint prostheses, mostly of the upper limb (shoulder, elbow, wrist joints) are used relatively rarely. The materials used are similar to those applied in hip or knee joint replacements (refs. 11, 8 4 1 - 8 4 7 ) . The wear problems in such prostheses are not as important as in hip or knee prostheses, which may be due to their relatively short lifetime or removal f o r other reasons (ref. 11). To improve loncj term performance and to eliminate the adverse effects of wear debris, the concept of encapsulation of the joint articulating parts and their artificial lubrication has been proposed (refs. 11, 8 4 8 - 8 5 0 1 . A low friction coefficient ( 0 . 0 2 ) can be expected when UHMWPE is used in blood plasma (ref. 1 1 ) . A very mall friction coefficient ( 0 . 0 0 6 - 0 . 0 1 5 ) was obtained for a cartilageglass sliding system lubricated with synovial fluid (ref. 71; see also Chapter 5 . 3 ) Biocompatible polyethylene glycol OH-(CH20Q12)x-H
.
4 99
with a viscosity of 8 2 mPa s at 37OC gives a remarkable reduction in the wear rate of all-metal prostheses and in friction of metalUHMWPE joints (a shoulder joint prosthesis was used for the experiments; ref. 850) Elastohydrodynamic lubrication of artificial hip joints is very effective; fluid lubricating film can be achieved even with the low viscosity synovial fluid (refs. 9 9 1 , 9 9 2 ) . The coefficient of friction in prototype artificial hip joints which have compliant surface lining ( 2 mm thick, a hardness of about 4 MPa) lubricated with the low viscosity synovial fluid present in diseased joints after surgery is remarkably low and comparable with normal healthy human joints (ref. 9 9 2 ) . Marked variations in the coefficient of friction for the sliding of bovine articular cartilage and UHMWPE against surgical grade steel (EN 58J), glass or cartilage in the presence of synovial fluid, saline solution or distilled water when interfacial electrical potentials were applied were observed (ref. 9 9 3 ) . Such frictional variations were dependent upon the lubricant present, the rubbing materials and the test conditions. The failure of joint prostheses is mainly due to the effects of loosening and fracture, but in order to increase the lifetime of the prosthesis attention must be paid to all possible causes of failure (e.g. wear) (ref. 11). The new improved encapsulated total joint replacements, or the new designs of cups with large rims, and improvements in surgical techniques play an important part in efforts to prevent acrylic cement particles from getting into the friction area and in efforts to reduce wear by artificial lubrication (for a review of papers on the mechanical properties of acrylic cement see ref. 851). The use of cement-less prostheses such as the alumina-alumina hip joint prosthesis is also an effective solution to these problems of prosthesis failure. Prosthetic heart valves are immersed in the blood. Presently about 4 0 types of artificial valves are used. Ball and disk valves are most often applied. The valve must be durable since a life expectancy of a least 25-30 years is desirable (heart valves cycle at approximately 4 2 million times a year). Materials used in the valve must be compatible with tissue and blood i.e. must not be corroded or initiate clots, they must have a density close to that of blood, and they must be able to withstand common hospitalsterilization techniques. Materials currently used for valve cages include titanium (having a relatively small density), Co-Cr-Mo alloy
.
500
(Stellite 21), and stainless steel (ref. 8 5 2 ) . For ball valves, the best available ball materials is SR; other materials used include pyrolitic carbon, stellite, titanium and fluorosilicone. The application of SR gives good friction and wear behaviour since blood demonstrates excellent lubricating properties. The ball rotates randomly when moving, which prevents wear at specific locations. However SR is no good for disk-type valves as it is too soft to be used as an occluder. The tendency of a disk to tilt when flow is not perpendicular to its surface results in its rubbing against the struts at the same point on the disk, which would cause high wear in a relatively soft material like SR. Various materials have been tested for eventual use in the occluders of disk valves. In the Bj6rk-Shiley tilting disk valve prosthesis (having the highest possible orifice area-to-tissue diameter ratio), consisting of a free-floating disk suspended in a Stellite cage (encircled with a PTFE rim), carbon, PTFE, POM h (Delrin), PP, and pyrolitic carbon were studied to evaluate their usefulness as disk materials (refs. 8 5 3 - 8 5 6 1 , The best wear resistance results (in vitro testing) were obtained for POM h and pyrolitic carbon. POM h (Delrin) is acceptable for a heart valve occluder, with a lifetime in excess of 3 0 years. The disadvantage of POM h is its propensity to adsorb moisture during the sterilization process (steam autoclaving). The lifetime (in vitro wear tests) of pyrolitic carbon was found to be about 400 years. Other properties of the pyrolitic carbon are also excellent as regards its possible use as occluder material. A description of simulators for in vitro testing of artificial heart valves may be found in refs. 8 5 2 , 8 5 3 and 8 5 7 . A review of papers on heart valve replacement materials is presented in ref. 8 5 8 . The heart valve prostheses can also be very useful in artificial heart systems, particularly the diaphragm or sac artificial heart systems (refs. 8 5 9 , 8 6 0 ) . In such systems the typical ball or disk valve protheses used must demonstrate a lifetime of over 8 10 cycles (a five year duration). In the design of artificial hearts only materials compatible with blood, other body fluids and tissue can be taken into consideration. This severely restricts the list of materials to those aforementioned and in fact is the major limiting factor in artificial heart design. Catheter systems are used frequently for the delivery of intravenous fluids and the removal of urine: they are now being used more and more frequently in such complicated procedures as the com-
pression of plaques in coronary arteries and the obstruction of blood flow to specific areas of the body. The more complicated catheter procedures often require the use of double catheter systems consisting of a stiff outer catheter and a flexible, buoyant, flow-directed, inner catheter, which is often balloon-tipped. The use of such catheter systems is however restricted because of excessive friction generated between the two catheters. The friction between PE, FEP, PVC, SR and PS polymers commonly used as catheter materials can be considerably reduced by oxidizing their working surfaces by exposing them to radio frequency glow discharge (RFGD) in a helium environment (ref. 379; see also Chapter5.2.2). The exposure of the polymers to RFGD produces an oxidized hydrophilic surface with unaltered roughness (under relatively mild plasma conditions). There was a considerable reduction in the friction between RFGD-treated SR (as compared to untreated SR) when dragged across all untreated polymer surfaces, in particular in the presence of a blood plasma medium (distilled water and isotonic saline solution were also used). Treating the outside surface of SR is sufficient to improve the frictional characteristics of two-catheter systems employing SR as an inner catheter material. The composition of the outer catheter, whether of PE, FEP, PVC or even PSI should not affect the improved frictional performance of the treated SR catheter. Preliminary platelet retention tests indicate also that the treated catheter might be less thrombogenic. Special tribological problems sometimes arise in areas such as the design of micromotor bearings, special microseals, microrobots, optical instruments and textile machines, or when the presence of magnetic, electric or ultrasonic fields, external vibrations, high radiation, cryogenic temperatures, hydrogen presence etc. can be expected. The typical bearings used in electric micromotors are miniature ball or self-lubricating sintered porous bearings. Very good results were obtained (ref. 4 8 9 ) applying Glamat 3 3 (Glacier Metal Co.) for the bearings in fractional horsepower induction motors. The Glamat 3 3 bearings are in the form of a rolled aluminium sleeve lined with PPS, a high temperature thermoplastic polymer which can operate at up to 2OO0C and which cross-links lightly during the curing process. They are designed to be oil lubricated, having a window or other convenient arrangement for oil entry. Such bearings, with their excellent wear behaviour, could be a contender for motors using ball bearings which have to withstand relatively heavy radial loads. The u s e o f special miniature rolling bearings
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shielded and for-life lubricated with a high quality lubricant is often the solution to many difficult design problems in micromotors (refs. 679, 861, 8 6 2 ) . Brushless micromotors (see refs. 863, 8 6 4 ) are of interest since there are no tribological problems withbrushes and the endurance life can reach 20,000 h. The ideal material for a microseal combines the following properties: good memory, toughness and wear resistance, low friction, chemical resistance, wide temperature range of use, and low cost. Selection of a seal material is no easy matter and usually must be done in close consulation with a seal manufacture. Although many different types of materials can be used for seals, elastomers are most commonly used, however, they often give relatively high friction, especially after a period at rest or at low sliding speeds. Seals made of PTFE (virgin or more often as a composite containing a filler such as bronze, glass or carbon fibre or graphite to improve wear resistance and mechanical properties) can be used to overcome some of the problems associated with elastomeric materials because of the low sliding friction coefficient of PTFE. Friction data and the indications for comparising elastomeric seal materials based upon extensive testing are available in refs. 9 9 4 - 9 9 7 . Elastomers filled with dry lubricants exhibit relatively low friction coefficients (ref. 9 9 8 ) . The friction and wear of PTFE-based reciprocating seals are discussed in refs. 9 9 9 , 1 0 0 0 . Magnetic fluid seals are useful particularly at high sliding speeds (refs. 1 0 0 1 , 1002).
Microrobots are widely used (in particular in assembly processes): ref. 865 and are usually built of assemblies (components) the tribological aspects of which have been discussed in Chapter 9. The lubrication problems are not difficult to solve when the environmental operating conditions are not extreme (see Chapters 3, 6 , 7.2 and ref. 1 0 0 3 ) . Optical instruments require the use of the special lubricants already discussed in Chapter 3.3. The oils and greases used should be chemically inert, demonstrate very low evaporation rates and should not spread, since the optical elements are too sensitive to be covered with microdrops or vapours of lubricant. The use of anti-friction coatings (Chapter 7. 2) is therefore of great importance especially when for-life lubrication problems need to be solved. In textile machines the guiding elements for fibres (in particular glass, synthetic fibres) are required to demonstrate high wear resistance. The use of special coatings is of most interest for solving such tribological problems, which have already
503
been discussed in Chapters 4 . 3 , 7 . 3 and 9 . 5 . The use of carburized, nitrided or borided titanium (or Fe-Cr alloys) can solve wear problems in guides for glass fibres (ref. 268). The presence of a magnetic field affects the friction and wear in tribological systems. Ferromagnetic materials rubbing in a magnetic field demonstrate oxidative wear since the realistic contact pressures in the microcontact areas of the surfaces are higher than when there is no magnetic field, and magnetostrictive microoscillations occur (refs. 7 3 4 , 7 3 5 ) . The magnetic field, being favourable for the formation of oxides, results in the anti-seizure properties demonstrated by the rubbing surfaces, which also gives a constant frictional torque. The running-in process in tribological systems having elements manufactured in soft ferromagnetic materials can therefore be accelerated and made smoother by having it carried out in the presence of a magnetic field. The friction in a ferromagnetic metal-metal system in the presence of a magnetic attraction force is distinctly different from the usual friction (ref. 866). The static friction coefficient increases parabolically upon increasing the force factor Kf (where Kf is the ratio of the magnetic attractive force to the total. normal force acting on the sliding element). When Kf is 0 the friction coefficient is 0.55 and increases to 1.05 when Kf is increased to 1.0. In the investigations cited (ref. 8661, when the Kf factor was 0, small powder-like wear particles and small scratches on frictional surfaces were observed. When the magnetic attractive force was applied the powder had the appearance of carbon and adhered to the surface along the larger scratches. The stick-slip phenomenon was observed. When the combined force was applied (normal load and magnetic attraction) the powder was silvery and crowded along the scratches. The phenomena observed are complex and need further study. The possibility of controlling the tribological behaviour of systems by varying the attractive magnetic force is however evident. The wear in Ni-Ni or Fe-Fe sliding systems depends on the direction of applied magnetic field (ref. 1004). A magnetic field vertical to the sliding surface has the effect of accelerating the severe-mild wear transition in air, so it gives a remarkable reduction in specific wear. A magnetic field horizontal to the surface enhances specific wear. From the fact that severe wear continues in both atmospheres of nitrogen and argon under the vertical magnetic field, it can be concluded that this effect is attributable to the chemisorption activity of ferromagnetic metals to oxygen. The chem-
504 isorption of oxygen onto the ferromagnetic metals is activated by the application of vertical magnetic field to the observed reduction in specific wear. The effect of a magnetic field on the friction in steel-polymer systems has also been studied (ref. 8 6 7 ) . Increasing the magnetic field strength in the range 0- 6.8 A/m (the highest strength used) increased the static friction coefficient, although a local minimum was observed at 5.2 A/m. The friction coefficient increased more rapidly at increasing intensity of the magnetic field after the local minimum had been reached. A magnetic field can be applied to prevent the migration of lubricant drops from the friction area. Drops of polysiloxane or fluorinated polyether oils containing 10% (by weight) of 15-30 nm Fe304 particles (with the 1-5% (by weight) addition of stabilizing surface active media) did not spread on a bearing steel surface when a perpendicular magnetic field of 300 mT was applied (ref, 99; see also Chapter 3.2). Fluorinated polyether oil containing Fe304 particles was tested using a standard five-ball friction machine in the presence of a magnetic field of 5 0 0 mT and at a temperature of 2OO0C; it exhibited an endurance life twice as long as when tested without the presence of a magnetic field. Such oils are suitable for systems operating at elevated temperatures (up to 200OC). An electric field (or magnetic field) affects the behaviour of the lubricant molecules of a boundary layer. An external electric field changes the direction of their polarization. The orientation of the hydrocarbon molecule dipoles may continue after the electric field has been turned off (ref. 8 6 7 ) . The triboelectric charge in lubricated polymer systems result in the orientation of the lubricant molecules perpendicular to the rubbing surfaces and in the occurrence of the phenomenon called "momentary polarity" in such nonpolar polymers as PTFE or PE (refs. 164, 457), which improves their tribological behaviour. Ultrasonics propagation in the area of friction reduces the friction force but increases the wear (ref. 8 6 8 ) . The increase in wear is a result of the increase in frictional work (energy) introduced to (or stored in) the system. The frictional work is a function of the friction force sliding speed and sliding time. An external ultrasonic field results in a high increase in the momentary sliding speed. During sliding in the presence of the ultrasonic field the magnitude and direction of the friction force vec-
505
tor change. The friction force vector is rotated and the angle of the rotation at which disruption of the frictional bonds occurs and the friction force becomes zero is determined by the ratio of the shear and also depends on the ratio of the sliding (v) and vibrational (vv) speeds. With increase in the vv/v ratio the frictional work rapidly increases (Fig. 10.2; ref. 868).
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ib
Load,N
Frequency, kHZ Amplitude, pm
F i g . 10.2. F r i c t i o n a l work a t i n t r o d u c t i o n o f u l t r a s o n i c v i b r a t i o n s , perpendicular t o f r i c t i o n p l a n e : WfQ f r i . c t l o n a l work i n u l t r a s o n i c s f i e l d , Wf f r i c t i o n a l work. 1 vv/v (vv vib r a t i o n a l speed, v s l i d i n g speed), 2 frequency, 3 amplitude, 4 s t a t i c normal l o a d .
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External vibrations perpendicular to the friction plane reduce the friction force. The reduction is at a minimum when the vibrational overload is arround 1. Small vibrational overloads with low
506
frequencies result in only a small friction reduction but in a great wear increase. The most effective friction reduction is obtained at the resonant frequency of elements. Torsional low frequency vibrations are used to reduce friction in miniature rolling bearings, although the parasitic torques in a constant direction limit the practical applications of this method. An extensive discussion of the use of vibrations to reduce the friction in instrument bearings can be found in ref. 2 5 6 (see also Chapters 4 . 3 and 5.3).
High levels of radiation, e.g. in nuclear reactor installations, make it necessary to use predominantly solid lubricants (applied in anti-friction coatings) in tribological systems. Lubricants based on polyethers are of most interest when gamma radiation, and also high temperature, can be expected (see Chapter 3 . 2 ) . When cryogenic temperatures occur, a solid lubricant and application of polymers (PTFE, PE composites) is effective. The friction coefficient and wear rate of bonded MoS2 (or/and graphite) coatings increase at a decrease of the temperature to cryogenic values (e.g. 4 K) (ref. 1005). The coefficients of thermal expansion of substrate and coatings materials should be similar. A special tribological problem appears when hydrogen is present in the friction area. This can occur when there is hydrogen in an atmosphere,when it migrates from the bulk material or when it appears as the product of the decomposition of water vapours, fuels or lubricants, or as the product of the destruction of polymers (when one of the rubbing elements is manufactured in a polymer containing hydrogen atoms in the macromolecule). The hydrogen easily diffuses into the metal surface and initiates cracks which accumulate and eventually destroy the metal surface layer. The process of hydrogen wear occurs in heavily loaded tribological systems when one of the rubbibg elements is made of steel, cast iron or titanium. Hydrogen wear can be effectively reduced by ensuring that the selective transfer effect occurs in the friction area (ref. 3 0 4 ; see also Chapter 5.1.1). The metal elements used should be free of hydrogen atoms. Cr and Mo anti-wear coatings applied by PVD demonstrate no hydrogen embrittlement (see Chapter 7 . 3 ) . In steel-polymer systems the positive potential of the steel surface, arising from triboelectrification, should be compensated by introducing into the polymer component polymer particles which exhibit negative potential during rubbing (e.g. PTFE, PE). The negative hydrogen ions will not then migrate intensively to the steel sur-
507
face (ref. 304). When striving to reduce friction as much as possible f u l l advantage should be taken of the so-called ultra-low friction phenomenon. This phenomenon was found (refs. 165, 869) in metal-PE and metal-PP sliding systems operating in a vacuum (10- 3 - 10-1 Pa). A friction coefficient of about 0.001 can be obtained in such systems when the polymer surface is bombed with accelerated atoms of (flux 1011 atoms/mm2 s, energy 2 keV). helium or other elements The experiments were carried out at ambient temperature, contact pressure 0.025-0.076 MPa and sliding speed 0.2 m / s . The wear of the polymer in such conditions is twice as low as it would be without surface bombardment. This is an example of the possibility ofordering the structure of a material and increasing its stability by the action of a highly concentrated energy stream (ref. 869). Further ordering of the material structure and improvements in its tribological behaviour can also be obtained by a simple running-in process but because of the relatively low density of the frictional energy stream the reduction in the friction coefficient and wear intensity is small.
508
11, CLOS I NG COMMENTS The tribology of miniature systems is a special area of the science of tribology. The small dimensions of the rubbing elements, often extreme operating conditions, the need to keep the friction and wear losses as low as possible, and the need to ensure a long endurance life, for example at for-life lubrication, make the tribological problems of design and investigation of such systems especially difficult. Special lubrication techniques the use of the anti-migration coatings known as epilames, and the use of special instrument lubricants are all characteristic of miniature systems. This work has attempted to cover the fundamental tribological problems of miniature systems. However, the tribological problems of miniature systems need further, more intensive study (see refs. 870 and 1006). The effect of special conditions on the tribological behaviour of miniature systems, particularly those used in space installations,will be one of the main subjects of this study. As an example of the lubrication studies.needed, the problem of the interactions between material and lubricant under space conditions needs to be tackled (ref, 871). The fundamentals of the triboengineering of miniature systems should formulated to give designers the possibility of predicting tribological behaviour and so make it possible to design miniature systems with satisfactory frictional and mass losses and with a determined endurance life. At the same time , tribological studies should keep pace with the enormous technological advances being made in precision engineering (mechatronics) practice. The tribology of miniature systems should therefore be included among the future research priorities in the field of tribological research.
509
REFERENCES CHAPTER 1
1
Simon-Vermot A . , Miiller C.: Reibungsmessungen an kleinen Teilen und unter geringen Krzften, in: B e r i c h t e d e r I X . Kongress
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Internationale der Chronometrie i n S t u t t g a r t
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Deutsche Gesellschaft fiir Chronornetrie, Stuttgart 1 9 7 4 , Berichte D 3.5. Czichos H.: T r i b o l o g y . Elsevier, Amsterdam - Oxford - New York 1 9 7 8 . Trylidski W.: F i n e M e c h a n i s m s a n d P r e c i s i o n I n s t r u m e n t s . Pergamon Press - Wydawnictwa Naukowo-Techniczne, Oxford Warsaw 1 9 7 1 . Rymuza 2.: Micropair as a tribological system. P o m i a r y A u t o m a t y k a K o n t r o l a ( 1 9 8 3 ) 5 , 1 7 3 , in Polish, Massin M.: Introduction a la tribologie en microm6canique; physico-chimie des surfaces. M k c a n i q u e - M a t e r i a u x E l e c t r i c i t k ( 1 9 8 1 ) 378-379, 261. Massin M.: Le frottement des matiBres plastiques en microm6canique. M C c a n i q u e - M a t e r i a u x - E l e c t r i c i t 6 ( 1 9 7 7 ) 3 3 0 - 3 3 1 , 12*
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Davidson A. (ed.): H a n d b o o k o f P r e c i s i o n E n g i n e e r i n g . Vol. 2. Macmillan, London 1 9 7 0 . 8 Pigors 0.: V e r s c h l e i s s v e r h a l t e n v o n W e r k s t o f f e n . VEB Deutscher Verlag fiir Grundstoffindustrie. Leipzig 1 9 8 5 . 9 Spengler G., Wunsch F.: S c h m i e r u n g u n d L a g e r u n g i n d e r F e i n w e r k t e c h n i k . VDI-Verlag, Dcsseldorf 1 9 7 0 . 10 Molotilova B.V. (ed.): P r e c i s i o n A l l o y s - H a n d b o o k . Metallurgya, Moscow 1 9 8 3 , in Russian. 11 Dumbleton J.H.: T r i b o l o g y o f N a t u r a l a n d A r t i f i c i a l J o i n t s . Elsevier, Amsterdam - Oxford - New York 1 9 8 1 . 1 2 Fedorchenko I.M., Pugina L.I.: C o m p o s i t e S i n t e r e d A n t i - F r i c t i o n M a t e r i a l s . Naukova Dumka, Kiev 1 9 8 0 , in Russian. 1 3 Schumacher M.: S e a w a t e r C o r r o s i o n H a n d b o o k . Noyes Data Corporation, Park Ridge, N.J. 1 9 7 9 . 1 4 Misami K., Tosyo K.: J a p a n e s e P a t e n t 4 9 1 3 6 8 6 , 16.02.76. 1 5 B r i t i s h P a t e n t 1 2 9 5 5 0 8 : Method of fabrication high strength self-lubritacing materials - 0 8 . 1 1 . 7 2 . 16 Fedorchenko I.I., et al.: Investigations of some properties of materials based on iron with natrium fluoride additions for friction couples of reactor set-ups. P o r o s h k o v a y a M e t a l l u r g y a ( 1 9 7 7 ) 7, 64, in Russian. 1 7 Dreischer H., Hirsch H.: PTFE-modifizierte Schmierstoffe in der Feingerxtetechnik. F e i n g e r a t e t e c h n i k 1 5 ( 1 9 8 5 ) 7, 1 9 9 . 1 8 Karyuk G.G., et al.: Frictional properties of materials based on compact modifications of boron nitride. P o r o s h k o v a y a M e t a l l u r g y a ( 1 9 8 4 ) 9, 8 2 , in Russian. 1 9 Tkachenko Y.F., et al.: High temperature friction and some properties of hot-pressed boron carbide. P o r o s h k o v a y a M e t a l l u r g y a ( 1 9 8 4 ) 1 2 , 4 1 , in Russian. 7
5 10 20 21 22
Rosenzweig I.I., et al.: About problem of friction mineral-polymer pairs in vacuum. T r e n i e i l z n o s 6 ( 1 9 8 5 ) 4, 7 3 6 , in Russian. Zum Gahr K.H.: M i c r o s t r u c t u r e a n d Wear o f M a t e r i a l s . Elsevier, Amsterdam and New York 1 9 8 7 . Habeeb J.J., et al.: Wear and lubrication of ceramics, in: Proceedings o f the I n s t i t u t i o n o f Mechanical Engineers I n t e r n a t i o n a l Conference T r i b o l o g y F r i c t i o n , L u b r i c a t i o n and Wear, F i f t y Y e a r s On. Mechanical Engineering Publications, Bury St. Edmunds 1 9 8 7 , Vol. 1, p. 5 5 5 .
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Naga S., et al.: An experimental study of tribological behaviour of ceramic materials, in: E u r o t r i b 8 5 P r o c e e d i n g s o f t h e 4 t h I n t e r n a t i o n a l T r i b o l o g y C o n g r e s s . Ecully, France 9-12 September 1 9 8 5 , edited by La Societd Franpaise de Tribologie. Elsevier, Amsterdam 1 9 8 5 , Vol. 3,1.2. Denape J., Lamon J.: Le comportement en frottement sec de ceramiques a haute temperature, in: E u r o t r i b 85 P r o c e e d i n g s o f t h e 4 t h I n t e r n a t i o n a l T r i b o l o g y C o n g r e s s . Ecully, France 9 - 1 2 September 1 9 8 5 , edited by La Societ6 Franqaise de Tribologie. Elsevier, Amsterdam 1 9 8 5 , V o l . 3,1.2. O C Z O ~K., Barowicz J.: Constructional ceramic materials. P r z e g l g d M e c h a n i c z n y ( 1 9 8 4 ) 4, 1 7 , in Polish. Smith W.F.: P r i n c i p l e s o f M a t e r i a l s S c i e n c e a n d E n g i n e e r i n g . McGraw-Hill Book Company, New York 1 9 8 5 . Hornbogen E.: W e r k s t o f f e , A u f b a u und E i g e n s c h a f t e n v o n K e r a m i k , M e t a l l e n , P o l y m e r - u n d V e r b u n d w e r k s t o f f e n . 4 Aufl. Springer-Verlag, Berlin - Heidelberg - New York - Tokyo 1 9 8 7 . Bhushan B., Sisley L.B.: Silicon nitride rolling bearings for extreme operating conditions. A S L E T r a n s a c t i o n s 2 5 ( 1 9 8 2 ) 4, 417.
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Sydenham P.H.: Mechanical design of instruments. 2: Materials working with imperfect resources. M e a s u r e m e n t a n d
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1 3 ( 1 9 8 0 ) 11, 4 1 9 .
34
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40
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T i l l w i c h M.,
e t a l . : S c h m i e r s t o f f e i n d e r Feinwerktechnik.
S c h m i e r t e c h n l k und T r i b o l o g i e 29 (1982) 5, 200.
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512 Problem o f c h e m i c a l s t a b i l i t y o f c l o c k o i l s
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Hajek J.:
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Weinheim - D e e r f i e l d Beach B a s e l 1984. S c h u l z e D.K.W.: Polyphenylethersynthesen, S t a n d und Aussichten zur Herstellung flirssiger Polyphenylether. S c h m i e r u n g s t e c h n i k 16 (1985) 7, 216. h o c h w e r t i g e Schmierstoffe 64 P l a g g e A. : P e r f l u o r i e r t e P o l y e t h e r und I n t e r f l i i s s i g k e i t e n f u r Extrembedingungen. S c h m i e r u n g s t e c h n i k 15 (1984) 11, 338. 65 Biischel E.: Erhdhung d e r Lebensdauer von K l e i n s t m o t o r e n . S c h m i e r t e c h n i k u n d T r i b o l o g i e 24 (1977) 6, 150. 66 Nick1 G. , C h r i s t a k u d i s D. : Untersuchungen iiber d i e Schmierstoffeigenschaften d e r E t h e r a l k o h o l e . S c h m i e r u n g s t e c h n i k 15 (1984) 9, 273. 6 7 Massin M.: La l u b r i f i c a t i o n d e s micromecanismes p a r l e s polymeres s i l i c o n e s , i n : B e r i c h t e d e r I X . K o n g r e s s I n t e r n a t i o n a l e d e r C h r o n o m e t r i e , S t u t t g a r t 1974. Deutsche G e s e l l s c h a f t f a r Chronometrie , S t u t t g a r t 1974 , B e r i c h t e D 3.7. 68 Massin M.: L e s f l u i d e s s i l i c o n e s a p p l i q u e s a l a l u b r i f i c a t i o n des micromecanismes. C e t e h o r , B e s a n ~ o n ( F r a n c e ) i n t e r n a l r e p o r t 1974. 69 Tokarzewski L., Zakrzewski J.: S i l i c o n e o i l s h a v i n g b e t t e r p r o p e r t i e s . P r z e m y s t C h e m i c z n y 55 (1976) 10-11, 536, i n Polish. 70 Rymuza Z . : L u b r i c a t i o n o f polymers w i t h s i l i c o n e o i l s . Z a g a d n i e n i a E k s p l o a t a c j i M a s z y n (1983) l(53), 37, i n P o l i s h . 71 B e l y i V.A. , e t a l . : I n v e s t i g a t i o n o f l u b r i c i t y o f s y n o v i a l f l u i d . T r e n i e i l z n o s 5 (1984) 6, 984, i n R u s s i a n . 72 Vinogradov E.J., Z l o b i n V.S.: L u b r i c a t i n g Material. U . S . R . R . p a t e n t 950754, (1982), i n R u s s i a n . 73 Basset D . , e t a l . : O i l - s o l u b l e f l u o r i n a t e d compound as a n t i wear and a n t i f r i c t i o n a d d i t i v e s . A S L E T r a n s a c t i o n s 27 (1984) 4, 380. 74 Papay A.G.: O i l - s o l u b l e f r i c t i o n r e d u c e r s t h e o r y and a p p l i c a t i o n s . A S L E P r e p r i n t N o . 82-AM-3F-1. 75 Zaynovskaya T . A . , e t a l . : Complex molybdenum compounds as add i t i v e s i n t o l u b r i c a t i n g o i l s . Khimya i T e k h n o l o g i a T o p l i v i M a s e l (1984) 4, 38, i n R u s s i a n . 76 J o n e s M.H., S c o t t D. ( e d s . ) : I n d u s t r i a l T r i b o l o g y . E l s e v i e r , Amsterdam and N e w York 1983. 77 Rymuza 2 . : O i l s f o r l u b r i c a t i o n o f p o l y m e r i c m i c r o p a i r s . P o m i a r y A u t o m a t y k a K o n t r o l a (1985) 1, 14, i n P o l i s h . 78 Mamiediarov M.A., e t a l . : E f f e c t o f c h e m i c a l s t r u c t u r e on viscosity t e m p e r a t u r e p r o p e r t i e s of c y c l i c complex e t h e r s . K h i m y a i T e k h n o l o g i a T o p l i v i M a s e l (1984) 8, 33, i n R u s s i a n . 79 D a l i n M.A. ( e d . ) : H i g h e r O l e f i n s . Khimya, L e n i n g r a d 1984, i n Russian. et;al.: Instrument o i l s , i n : N e f t , 80 V o l c h i n s k a y a N . I . , Prociessy i Produkty j e j Uglubl i e n n o j P e r a r a b o t k i , Sbornik, p a r t 111. CNIITENEFTEKHIM, Moscow 1983, p . 70, i n R u s s i a n . 81 Sharma S . K . , e t a l . : High t e m p e r a t u r e l u b r i c a n t s o i l s and g r e a s e s . T r i b o l o g y I n t e r n a t i o n a l 16 (1983) 4, 213. 8 2 B e l o v P.S., e t a l . : A l k y l n a p h t a l e n e s as components of h i g h t e m p e r a t u r e s l u b r i c a n t s . Khimya i T e k h n o l o g i a T o p l i v i Masel (1984) 4, 31, i n R u s s i a n . 83 Rymuza Z.: Low- and h i g h t e m p e r a t u r e s i n s t r u m e n t o i l s . P o m l a r y A u t o m a t y k a K o n t r o l a (1985) 4, 113, i n P o l i s h .
63
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513 84 85 86 87 88 89
Rymuza 2 . : L u b r i c a t i o n o f m i c r o p a i r s o p e r a t i n g u n d e r e x t r e m e c o n d i t i o n s . Z a g a d n i e n i a E k s p l o a t a c j i M a s z y n ( 1 9 8 5 ) 1, 4 1 , i n Polish. Rymuza 2.: L u b r i c a t i o n o f small mechanisms o p e r a t i n g u n d e r vacuum c o n d i t i o n s . P o m i a r y A u t o m a t y k a K o n t r o l a ( 1 9 8 5 ) 9 , 220, i n Polish. D r i e s c h e r H . , H i r s c h H . : Interferometerspiegelantrieb f i i r Satelliten-IR-Fourier-Spektrometer. F e i n g e r g t e t e c h n i k 1 4 (1984) 8 , 347. G a l t s o v a N.E.: T r o p i c a l c l o c k and i n s t r u m e n t o i l s . P r i b o r o s t r o y e n i e (1961) 5 , 2 4 , i n Russian. Etchin A.I., e t a l . : E f f e c t of water on l u b r i c a t i n g p r o p e r t i e s o f s y n t h e t i c o i l s . Khimya i T e k h n o l o g i a T o p l i v i Masel (1983) 2 , 2 2 , i n R u s s i a n . BesanGon a n d o t h e r F r e n c h f i r m s : L u b r i c a t i o n des Cetehor c o n n e c t e u r s a c o n t a c t s s e m i p e r m a n e n t s , i n : E u r o t r i b 85
-
Proceedings o f
90 91 92
93 94 95 96
98
International
t h e JSLE
I n t e r n a t i o n a l T r i b o l o g y Conference,
Tokyo, J a p a n , J u l y 8-10, 1985 ( e d i t e d b y Y. T a m a i ) . E l s e v i e r , Amsterdam 1985, vol. 3, p . 981. Fuks G . I . : A d s o r p t i o n a n d l u b r i c i t y o f o i l s . T r e n i e i I z n o s 4 ( 1 9 8 3 ) 3 , 398, i n R u s s i a n . e t a l . : About t w o - l a y e r s l u b r i c a t i o n , i n : Fuks G . I . , I n v e s t i g a t i o n s on Physicochemistry o f
99 100 101
102 103 104
105 106
T r i b o l o g y Congress,
.
Proceedings o f
97
the 4 t h
E c u l l y , F r a n c e 9-12 S e p t e m b e r 1985, e d i t e d b y L a S o c i e t e F r a n G a i s e de T r i b o l o g i e . E l s e v i e r , Amsterdam 1985, V o l . 4 , 5.2. Huck M . : E i n s a t z von S c h m i e r m i t t e l n a u f G l e i t - und S t e c k k o n t a k t e n . M e t a l l o b e r f l S c h e 36 ( 1 9 8 2 ) 9 , 429. K e i l A., e t a l . : E l e k t r i s c h e K o n t a k t e und i h r e W e r k s t o f f e . S p r i n g e r - V e r l a g , B e r l i n - H e i d e l b e r g - New York - Tokyo 1984. A n t l e r M. : E f f e c t o f l u b r i c a n t s o n f r i c t i o n a l p o l y m e r i z a t i o n o f p a l l a d i u m e l e c t r i c a l c o n t a c t s . A S L E T r a n s a c t i o n s 26 ( 1 9 8 3 ) 3 , 376. K o n t a k t - 6 1 D O D U C O N T A . DODUCO K6 D r . E . D i j r r w k h t e r , P f o r z h e i m (F.R.G.) Myshkin N . K . : T r i b o l o g i c a l a s p e c t s of u s e of e l e c t r i c a l cont a c t s . T r e n i e i l z n o s 5 ( 1 9 8 4 ) 1, 34, i n R u s s i a n . J e w e l B e a r i n g s . M a s h i n o s t r o y e n i e , Moscow Handelsman Y.M.: 1973, i n R u s s i a n . Mock G.B.: The l u b r i c a t i o n o f d e l i c a t e m a c h i n e r y , i n :
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B a s h i z d a t , Ufa 1971, p . 79, i n R u s s i a n . K u z n e t s o v A.A., e t a l . : P r e v e n t i n g m i g r a t i o n of o i l s i n r u b b i n g systems. Khimya i T e k h n o l o g i a T o p l i v i Masel (1983) 9 , 22, i n Russian. Rymuza 2.: N o n - m i g r a t i n g i n s t r u m e n t o i l s . P o m i a r y A u t o m a t y k a K o n t r o l a ( 1 9 8 4 ) 6, 1 7 9 , i n P o l i s h . S a l e m A.E.M.: E f f e c t of worn metals o n t h e Abou E l Naga H . H . , o x i d a t i o n o f l u b r i c a t i n g o i l s . Wear 96 ( 1 9 8 4 ) 3 , 267. F e k l i s o v a T . G . , e t a l . : Some p e c u l i a r i t i e s of t r i b o c h e m i c a l o x i d a t i o n o f h y d r o c a r b o n s . T r e n i e i l z n o s 6 ( 1 9 8 5 ) 2 , 339, i n Russian. H e i n i c k e G.: T r i b o c h e m i s t r y . Akademie-Verlag, B e r l i n 1984. Wunsch F.: Bedeutung des c h e m i s c h e n Aufbau v o n S c h m i e r s t o f f e n f i i r d i e B i l d u n g von T r i b o k o r r o s i o n . S c h m i e r u n g s t e c h n i k 12 ( 1 9 8 1 ) 1, 1 2 . Lockwood F . , Klaus E.E.: E s t e r o x i d a t i o n - t h e e f f e c t of a n i r o n s u r f a c e . A S L E T r a n s a c t i o n s 25 ( 1 9 8 2 ) 2 , 236. P e t r a k H . , Diwyak W.: Priifung der S c h m i e r f x h i g k e i t von d l e n und L a g e r w e r k s t o f f e n m i t e i n e m 2-Kugel-Reibungspendel. F e i n w e r k t e c h n i k + M e s s t e c h n i k 84 (1976) 3, 109.
5 14 107 108
109 110 111
112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130
Rymuza 2. : Physicochemical e f f e c t s d u r i n g l u b r i c a t i o n o f p o l ymers. T e c h n i k a S m a r o w n i c z a T r y b o l o g i a 19 (1980) 4-5, 16, i n Polish. T i l l w i c h M., e t a l . : PrUfgerZt f a r L a n g z e i t v e r s u c h e z u r Bestimmung t r i b o l o g i s c h e r W e r k s t o f f - S c h m i e r s t o f f Kenndaten i n K u n s t s t o f f l a g e r n sowie uberpriifung a u s g e s u c h t e r Reibsysteme auf t r i b o l o g i s c h e Reaktionen, i n : T r i b o l o g i e Band 5 (W. Bunk, e t a l . , e d s . ) . S p r i n g e r - V e r l a g , B e r l i n 1983, p. 125. Van Krevelen D.W.: P r o p e r t i e s o f Polymers. Elsevier, London - New York 1976. Amsterdam Rymuza Z . : Untersuchung d e r Dielektrizitltskonstante f r i s c h e r und g e a l t e r t e r f e i n w e r k t e c h n i s c h e r o l e . T r i b o l o g i e und S c h m i e r u n g s t e c h n i k 34 (1987) 4 , 229. B e r n e t t M.L., Ravner K.: A n t i s t a t i c a g e n t s , l u b r i c a n t s and p r e c i s i o n b e a r i n g s . L u b r i c a t i o n E n g i n e e r i n g 38 (1982) 8, 481. Rymuza 2.: I n s t r u m e n t g r e a s e s . P o m i a r y A u t o m a t y k a K o n t r o l a (1985) 7, 174, i n P o l i s h . S h i f r i s G.S., e t a l . : Vacuum g r e a s e b a s e d on a t a c t i c PP. P l a s t i c h e s k y e Massy (1984) 11, 45, i n Russian. Evtushenko G.S., e t a l . : A n t i - f r i c t i o n l u b r i c a t i n g composit i o n . U . S . R . R . p a t e n t 1060670, 1982, i n R u s s i a n . Matveyevski R.M. , e t a 1 : E f f e c t o f polymorphic t r a n s f o r m a t i o n s o f l i t h i u m g r e a s e s . T r e n i e i l z n o s 5 (1984) 6, 972, i n Russian. K a l l e r A.: Rheologische Gebrauchskennwerte von S c h m i e r f e t t e n f u r den E i n s a t z i n o p t i s c h e n P r X z i s i o n g e r 3 t e b a u . S c h m i e r u n g s t e c h n i k 13 (1982) 7, 206. Buckley D.H.: S u r f a c e E f f e c t s i n A d h e s i o n , F r i c t i o n , Wear a n d L u b r i c a t i o n , E l s e v i e r , Amsterdam and New York 1981. F i s c h e r F.G., e t a l . : G r a p h i t e and molybdenum d i s u l p h i d e -synergisms. NLGl Spokesman 46 (1982) 9, 190. Bartz W . J . , e t a l . : T r i b o l o g i c a l b e h a v i o u r o f two- and t h r e e -component bonded s o l i d l u b r i c a n t s . Wear 115 (1987) 1-2, 167. Conte A.A.: G r a p h i t e i n t e r c a l a t i o n compounds as s o l i d l u b r i c a n t s . ASLE p r e p r i n t No. 82-AM-2A-2 (1982). S l i n e y H.E. : S o l i d l u b r i c a n t m a t e r i a l s f o r h i g h t e m p e r a t u r e s a review. T r i b o i o g y I n t e r n a t i o n a l 15 (1982) 5, 303. F u s a r o R.L.: G e o m e t r i c a l a s p e c t s of t h e t r i b o l o g i c a l p r o p e r ties o f g r a p h i t e f i b e r r e i n f o r c e d p o l y i m i d e c o m p o s i t e s . ASLE T r a n s a c t i o n s 26 (1983) 2, 209. Hironaka S., e t a l . : S y n t h e t i c niobium s u l f i d e a s a s o l i d l u b r i c a n t . J o u r n a l o f t h e Japan S o c i e t y o f P e t r o l e u m I n s t i t u t e 26 (1983) 1, 82. Dumdum J . M . , e t a l . : L u b r i c a n t g r a d e cerium f l u o r i d e a new s o l i d l u b r i c a n t a d d i t i v e f o r g r e a s e s , p a s t e s and s u s p e n s i o n s . NLGl Spokesman 4 7 (1984) 4, 111. Krachun A.T., Morar V.E.: Solid Lubricants Based on Caprolactam. S h t i i n t s a , K i s h i n e v 1988, i n R u s s i a n .
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515 (1984) 4,
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516 150 151
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1978, p. 25. Fitzsimmons V.G. , S h a f r i n E.G. : The d e t e c t i o n , w e t t a b i l i t y and d u r a b i l i t y of fluoroscent b a r r i e r films. ASLE T r a n s a c t i o n s 17 (1974), 135. Messina F.D.: F l u o r o s c e n c e d e t e c t i o n o f t h i n - f i l m e l e c t r i c a l c o n t a c t l u b r i c a n t s . ASLE T r a n s a c t i o n s 2 7 ( 1 9 8 4 ) 4 , 2 9 5 . B o r j a P.C.: I n v i s i b l e b a r r i e r f i l m . L u b r i c a t i o n E n g i n e e r i n g 3 7 (19811, 4 4 6 . B e r n e t t M.K., Ravner H.: S u r f a c e a n a l y s i s of b e a r i n g s t e e l s a f t e r s o l v e n t t r e a t m e n t s . 11. L u b r i c a n t Coated B e a r i n g Surf a c e s . ASLE T r a n s a c t i o n s 25 ( 1 9 8 2 ) 1 , 5 5 . Sobadska K.: E n e r g e t i c a l model of t r i b o l o g i c a l a g e i n g o f o i l . Technika Smarownicza T r y b o l o g i a 8 (1977) 3, 61, i n Polish. Rymuza Z.: Ageing o f o i l s u s e d f o r l u b r i c a t i o n of f i n e mechanisms. P o m i a r y AutomatyKta K o n t r o l a ( 1 9 8 0 ) 2 , 6 2 , i n P o l i s h . Refner A . : D i e S p e z i e l l e n Aufgaben d e r Schmierung i n d e r Feint e c h n i k . F e i n w e r k t e c h n i k 7 5 (1%71) 1 1 , 4 4 7 . D i i r r F.: E i n B e i t r a g z u r k u n s t l i c h e n A l t e r u n g von Uhrenslen. Ac t e s d e Co'll oq u i u m r t n t e r ' m t i o n a I e n M i c r omecan i q u e , CETEHOR, Besanpon 1 9 7 3 , p . l k - 2 6 k . Gumz S . : Alterungspriifung von Olen d i e i n d e r F e i n w e r k t e c h n i k Vervendung f i n d e n . F e i n w e r k t e c h n i k + M i c r o n i c 7 8 ( 1 9 7 4 ) 4 , 111.
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531 Huber A , , Verhar M.: B e s t i m u n g e i n e r A l t e r u n g s z a h l an k l e i n s t e n Schmierolmengen m i t IR-Spektroskopie. F e i n w e r k t e c h n i k + M e s s t e c h n i k 87 (1979) 5, 238. e t a l . : A p p l i c a t i o n o f IR-Spectroscopy f o r 445 B i l o b r o v V.M., determination of o x i d a t i o n grade of a l u b r i c a n t during operat i o n o f r o l l i n g b e a r i n g s . E 1 e c t r o t e khn i che s k a ya P romys h 1 e n nos t - S e r y a E l e k t r i c h e s k i y e M a s h i n y (1981) Vypusk 6 (1241, p. 22, i n Russian. 446 Hajek J.: Clock l u b r i c a t i o n t e c h n o l o g y . Jemnd M e c h a n i k a a O p t i k a (1982) 5, 123, i n Czech. 44 7 Rymuza 2 . : Ageing o f i n s t r u m e n t o i l s . T e c h n i k a S m a r o w n i c z a T r y b o l o g i a (1980) 2, 9, i n P o l i s h . 448 Klimov A.K. , e t a l . : V a r i a t i o n o f v i s c o s i t y of s y n t h e t i c l u b r i c a n t s a t oxygen a b s o r p t i o n . Khimya i T e k h n o l o g y a T o p l i v i M a s e l (1979) 2, 4 8 , i n R u s s i a n . 449 Watch o i l s and g r e a s e s . Method f o r t h e d e t e r m i n a t i o n o f evap o r a t i o n . GOST S t a n d a r d , N o . 7934. 1-74, i n R u s s i a n . 450 Oschner F.: Remarks on s y n t h e t i c c l o c k o i l s and some s p e c i a l l u b r i c a t i o n problems. Jemna’ M e c h a n i k a a O p t i k a (1970) 6, 177, i n Czech. 451 L i t v i n o v a N.M. , e t a l . : Dependance of e v a p o r a t i o n o f o i l b a s e f o r low-loaded mechanisms on i t s f r a c t i o n c o m p o s i t i o n . Khimya i T e k h n o l o g y a T o p l i v i M a s e l (1979) 6, 28, i n R u s s i a n . 452 Rochat G . , e t a l . : L u b r i f i c a t i o n s o l i d e d e s m o b i l e s micromGcaniques, i n : A c t e s de 10e C o n g r e s I n t e r n a t i o n a l de C h r o n o metric, GenSve 1979, S o c i e t d S u i s s e de Chronometrie, GenSve (19791, Vol. 3, p. 413. S e l f l u b r i c a t i n g composites f o r extreme e n v i r o n 453 Gardos M.N.: ment a p p l i c a t i o n s . T r i b o l o g y I n t e r n a t i o n a l 15 (1982) 5, 273. 454 Huber A. : Wirkung von Kunstof fasdiingstungen auf Feinmechaniksschmiermittel. B e r i c h t e d e r I X . Kongress l n t e r n a t i o n a l e d e r C h r o n o m e t r i e , S t u t t g a r t 1974, Deutsche G e s e l l s c h a f t f i i r Chronometrie, S t u t t g a r t 1974, D 3.9, p. 739. 455 C h r i s t y R . I . : Dry l u b r i c a t i o n f o r r o l l i n g element s p a c e c r a f t parts. T r i b o l o g y I n t e r n a t i o n a l 15 (1982) 5, 265. 456 M a i l l a t M . , e t a l . : S l i d i n g and r o l l i n g b e a r i n g s f o r p i v o t systems. P r o c e e d i n g s o f Second Space T r i b o l o g y Workshop ESTL, Risley U.K., October 1980 (ESA SP - 158 December 1980) , p. 3. A n t i - W e a r a n d A n t i - F r i c t i o n C o a t i n g s . Mashino457 Koutkov A.A.: s t r o y e n y e , Moscow 1976, i n Russian. Surface Dispersing o f Dynamically Contact458 Gorokhovskyi G.A.: i n g P o l y m e r s and M e t a l s . Naukova Dumka, Kiev 1972, i n Russian. 459 S k e l c h e r W.L., e t a l . : The i n f l u e n c e o f p o l y d i m e t h y l s i l o x a n e on t h e f r i c t i o n and wear of polyphenylene o x i d e u n d e r boundary l u b r i c a t i o n c o n d i t i o n s . A S L E T r a n s a c t i o n s 25 (1982) 3, 391. 4 60 Rymuza 2 . : F r i c t i o n polymers and e f f e c t o f s e l e c t i v e t r a n s f e r i n metal-polymer p a i r s . T e c h n i k a S m a r o w n i c z a - T r y b o l o g i a (1978) 5-6, 130, i n P o l i s h . 461 B e l y i V.A., e t a l . : Adhesive wear of polymers. T r a n s a c t i o n s o f t h e A S M E - J o u r n a l o f L u b r i c a t i o n T e c h n o l o g y 99 (1977) 4, 396. 462 B e l y i V.A., e t a l . : Methods o f d e c r e a s i n g w e a r i n metal-polym e r c o n t a c t s . T r a n s a c t i o n s o f t h e ASME - J o u r n a l o f L u b r i c a t i o n T e c h n o l o g y 100 (1978) 2, 185. 463 Koutkov A . A . , Tabor D . : L u b r i c a t i o n of n y l o n by p o l y s i l o x a n e f l u i d s . T r i b o l o g y (1970) August, 163. 4 64 B e l y i V.A., e t a l . : L u b r i c a t i o n of metal-polymer f r i c t i o n u n i t s , i n : P r o c e e d i n g s o f t h e JSLE - ASLE I n t e r n a t i o n a l Lub r i c a t i o n C o n f e r e n c e , Tokyo 1975, E l s e v i e r , Amsterdam 1976, p. 410. 444
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558
SUBJECT INDEX Abrasion-resistant coatings, 285, 451, 495 Abrasiveness of paper and ribbons, 451, 493 ABS, 23 Accelerated wear test Acetabular cup, 496 Acoustic emission (AE) method, 491 Additives, 43, 159 Adhesion, 97, 124, 354, 380 Adhesive wear, 79, 124 Adhesive-cohesive wear formula, 97 Agate, 15 Ageing, 243 - dynamics, 243 - methods, 244, 370, 401 - number, 244, 370 Agressive media, 381 Air-film lubrication, 451, 488 Alumina, 141 Aluminium - based alloys, 11 - silicon alloys, 81 Alumino-silicate ceramics, 135, 139 Amorphous metals and alloys, 82 Angle of contact, 354 Anodizing, 289 Antichoc system, 431 Anti-friction coatings, 44, 259 , 269 Anti-migration coatings, 44, 216, 230, 333, 378 Anti-seize and anti-stick/slip paste, 57 Anti wear coatings, 285, 451, 495 Apparent stored frictional energy density (ASFED), 101 Archard's laws of wear, 80 Armco, 65 Artificial - heart, 500 - valves, 499 - joints, 496 ASTM pendulum, 303 Auger electron spectroscopy (AES), 389 Autoepilamizing, 227 - oils, 50, 227 Autophobic liquids, 228
Autoradiography, 487 Baader's test, 51, 243, 372 Babbits, 7 Ball bearings, 282, 300, 399, 441 Barrier film, 215, 378, 446 Bearing materials, 6, 404 Bearing calculation, 97, 404 - lubricated steel-polymer, 191 - unlubricated steel-polymer, 97 Bearings - plain, 404, 435 - rolling, 282, 300, 441 Belt drives, 462 Bjark-Shiley heart valve prosthesis , 500 Blends of graphite and MoS2, 68 Blow-off method, 343 Borja's method, 235 Boriding, 290 Boron - carbide, 16, 140 - nitride, 16, 138 Boundary lubrication, 149, 160 - ceramic systems, 203 - metallic systems, 149 - polymeric systems, 203 Brakes, 463 Brasses, 6, 73, 152 Breakdown of lubricant film, 397 Bronzes, 6 Brushes, 467 Brushless micromotors, 479 Cable drives, 462 Cadmium-based alloys, 7 Calculation - of friction coefficient and wear rate, 97, 132, 163, 191, 290, 446 - of plain bearings, 97, 191, 408 Cam mechanisms. 460
559 Capillary b e a r i n g s , 439 e f f e c t , 368 method, 336 Carbon-graphites, 18, 143, 2 0 9 , 438 c o u n t e r f a c e s f o r 143 C a r b u r i z i n g , 290 Card m a c h i n e s , 288, 289, 450 C a t a s t r o p h i c wear of ceramics, 138 C a t h e t e r s y s t e m s , 500 Centre bearings, 428 C e r a m i c s , 1 6 , 1 3 4 , 208, 448 Chain d r i v e s , 4 6 2 C h a r n l e y p r o s t h e s i s , 496 Chem ic a1 n i c k e l i n g , 285 s t a b i l i t y of o i l , 243 v a p o u r d e p o s i t i o n (CVD) , 292 Chromizinq, 285, 290 C l e a n i n g , 230, 381 C l e a n l i n e s s e v a l u a t i o n , 231, 386 C l e a r a n c e , 407 Clock o i l s , 34 t y p e b e a r i n g s , 73, 1 4 9 , 4 0 4 lubricated, 149 u n l u b r i c a t e d , 73, 4 0 4 C l u t c h e s , 463 Coatings, 4 4 , 269 a n t i - f r i c t i o n , 269 anti-migration, 216 a n t i - w e a r , 285 - on f a s t e n e r s , 284 l u b r i c a t i o n , 281 t r a n s f e r r e d , 279, 447 C o b a l t - b a s e d a l l o y s , 11 C o h e s i v e e n e r g y d e n s i t y , 97 Cold p o l y m e r i z a t i o n , 263 Collector , 4 7 2 Compatibility of o i l s and materials, 51, 198, 2 6 1 , 371, 376 - o f s o l v e n t s and m a t e r i a l s , 383 Composites, 1 6 Computer analyzed microscopic i n t e r f e r o m e t r y (CAMI) , 4 9 1 d e v i c e s , 481 C o n i c a l b e a r i n g s , 424 Connector t r a n s m i s s i o n s , 4 6 2 C o n t a c t a n g l e , 354 C o n t a c t s , 467 C o n t a m i n a n t s , 386, 401, 4 4 6 Conversion c o a t i n g s , 289 Copper-based a l l o y s , 1 0 , 80 , 1 4 4 , 296
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C o r r o s i o n , 1 3 , 385, 490 i n h i b i t o r s , 43 of m a g n e t i c m e d i a , 490 Corrosiveness o f o i l s , 5 3 , 376 of s o l v e n t s , 385 Corundum, 1 5 C o u p l i n g s , 463 Cover p l a t e t h r u s t b e a r i n g s , 429 C r a c k i n g a c i l i t y of p o l y m e r s , 377, 383 C r e e p i n g of o i l s , 2 1 2 Cryogenic temperature materials, 8 C y l i n d r i c a l b e a r i n g s , 404, 4 2 7
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Damping o f o s c i l l a t i o n s c u r v e , 303 o i l s , 4 7 , 456 D e g r a d a t i o n of l u b r i c a n t , 1 6 1 D e n s i t y e s t i m a t i o n m e t h o d s , 362 Depositing anti-migration coati n q s , 2 2 6 , 230 Detecting c o n t a m i n a n t s , 231, 386, 4 0 1 epilame and l u b r i c a n t f i l m s , 234, 381, 390, 478 r u n n i n g - i n p e r i o d , 1 6 5 , 395 D e t o n a t i o n - s p r a y e d c o a t i n g s , 287 Diamond, 1 6 , 145 Diaspore, 1 4 1 D i c h a l c o n i d e s , 69 Dielectric c o n s t a n t o f o i l s , 52 d e t e r m i n a t i o n , 364 o f p o l y m e r s , 20 Diesel e n g i n e i n j e c t i o n pumps, 256 Disk f i l e memories, 481 Drives, 453 Drop w e i g h t method, 353 DuprB's e q u a t i o n , 213 D i i r r ' s a p p a r a t u s , 372 Dustproof t e s t i n g , 4 0 1 Dynamics o f l i q u i d s p r e a d i n g , 212
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E a r n s h a w l s t h e o r e m , 437 Edge e f f e c t , 2 1 4 Effect of climate on l u b r i c a t i o n , 46, 208, 256 of e l e c t r i c f i e l d on f r i c t i o n a n d wear, 504 of l u b r i c a n t o n p o l y m e r , 2 6 1 of m a g n e t i c f i e l d on f r i c t i o n a n d wear, 503 of polymer an lubricant, 265, 371
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560
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of r a d i o a c t i v i t y on l u b r i c a n t s , Fine-pitch 4 6 , 259, 374 g e a r s , 453 of r u n n i n g - i n on f r i c t i o n c o e f worm g e a r s , 458 F i t z Simmons method, 234 ficient, 164 - of t e m p e r a t u r e on f r i c t i o n and Flame-sprayed c o a t i n g s , 286 wear, 87, 100, 255 F l e x i b l e connector transmis- of t r i b o e l e c t r i f i c a t i o n on wear, s i o n s , 462 F l u o r i d e c o a t i n g s , 278 88 - of u l t r a s o n i c s on f r i c t i o n and F l u o r i d e s , 70 F l u o r o c a r b o n l u b r i c a n t s , 482 wear, 504 Efficiency Fluoropolymer c o a t i n g s , 2 1 6 , of b e a r i n g , 4 1 4 284 of g e a r s , 454 Fluoropolymers, 2 4 of l u b r i c a t i o n , 181 F l y i n g e f f e c t o f head, 488 E l e c t r i c f i e l d e f f e c t on f r i c t i o n measurements, 4 9 1 and wear, 504 F o i l b e a r i n g s , 436 E l e c t r i c a l c o n t a c t s , 467 F o r - l i f e l u b r i c a t i o n , 2 , 211, f r i c t i o n , 473 443, 482 l u b r i c a t i o n , 47, 6 1 , 475 Fourier transformed i n f r a r e d r e l i a b i l i t y , 469 s p e c t r o s c o p y (FTIR) , 4 9 1 wear, 460 Freewheel, 2 9 9 E l e c t r o d e p o s i t e d c o a t i n g s , 285 Fretting E l e c t r o n s c a t t e r i n g f o r chemical b e h a v i o u r of ceramics, 136 a n a l y s i s (ESCA), 2 3 7 , 381, 3891 e l e c t r i c a l c o n t a c t s , 468 491 b e h a v i o u r of m e t a l s , 163 E l e c t r o p h o r e s i s c o a t i n g s , 289 behaviour of l u b r i c a n t s , Elements of t r i b o l o g i c a l s y s t e m s , 1 51 Enameled o x i d e g l a z e s , 289 Friction coefficient E n c a p s u l a t i o n of a r t i f i c i a l j o i n t - of carbon-graphites, 1 4 3 , elements , 49 8 209 Energy of a d h e s i o n , 9 7 , 183, 354 o f ceramics, 131, 203 Epilame, 4 4 , 2 1 6 , 333, 378 o f magnetic t a p e s , 1 4 6 , t e c h n o l o g y , 230, 333, 378 437, 486 E p i l a m i z a t i o n , 2 1 4 , 378 measuring t e c h n i q u e s , 302, of polymers, 267 312, 399 E r o s i o n , 495 o f polymers, 83, 173 Ester t e s t r i g s , 302, 312, 399 based g r e a s e s , 55 Friction o i l s , 36 - m o d i f i e r s , 43 Estimation of s u r f a c e f r e e energy, t r a n s m i s s i o n s , 460 233, 378 F r i c t i o n a l torque E t h e r s , p o l y e t h e r s , 38, 475, 482 i n r o l l i n g bearings, 443 Evaporation measurements, 302, 312, methods, 358 399 of o i l , 252 number, 413, 4 2 6 r a t e s f o r g r e a s e s , 55 spread, 4 2 1 r a t e s f o r o i l s , 33 F u e l s used a s l u b r i c a n t s , 1 6 0 E v a p o r a t i v e r a t e a n a l y s i s (ERA), F u s i b l e metal f i l m s , 2 7 4 388 Extreme l u b r i c a t i o n c o n d i t i o n s , 259 Gabbro, 1 6 p r e s s u r e a d d i t i v e s , 43, 159 Gas E x t r u s i o n of o i l , 1 7 0 b e a r i n g s , 288, 435 chamber, 170 l u b r i c a t i o n , 54 F a i l u r e of j o i n t p r o s t h e s i s , 4 9 9 Gears, 453 Femoral h e a d s , 4 9 6 Geneva mechanisms, 4 6 0 Fe-Mo-S c o a t i n g s , 2 7 2 Glass, 1 6 , 1 4 1 Ferrites, 16, 146 G r a p h i t e , 68 F i l l e d polymers, 28, 1 0 7 f l u o r i d e , 6 9 , 486 F i n e mechanisms, 4 0 4 G r a p h i t e s , 1 8 , 143, 209, 438
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561 G r a p h i t i z a t i o n of ceramic s u r f a c e , 138 G r e a s e s , 54 f o r h i g h p r e s s u r e s and temperatures, 61 o p t i c a l instruments, 61 - - s e a l i n g , 57 - - s p a c e c r a f t mechanisms, 258 - t e s t i n g , 336, 401 thixotropic, 61 Guides, 448 G u m z ' s a p p a r a t u s , 374 Gyroscope b e a r i n g s , 438, 4 4 6
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Harmonic d r i v e s , 458 Hard c o a t i n g s , 285 s e l f - l u b r i c a t i n g , 285 Hardness - of metals, 83 of metal o x i d e s , 83 Heads, 1 4 6 , 437, 451, 487 Heart v a l v e p r o s t h e s i s , 499 H e a t - r e s i s t a n t p o l y m e r s , 26, 32 High - molecular surface a c t i v e a g e n t s , 277 pressure o i l s , 44 temperature o i l s , 44 - vacuum l u b r i c a t i o n , 4 6 , 57, 258 Hip j o i n t p r o s t h e s e s , 4 9 6 H o l o g r a p h i c method, 395 Huber's method, 2 4 9 , 370 Human body, 8 - j o i n t s , 496 teeth, 147 Hydrodynamic l u b r i c a t i o n , 1 6 7 , 1 8 9 , 418 Hydrogen wear, 506
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I - c o a t i n g s , 295 Impact wear, 4 9 4 I m p l a n t s , 495 I n c a b l o c k s y s t e m , 431 Indium a d h e s i o n t e s t , 389 I n d u s t r i a l g a s e s , 258 Infrared s p e c t r o s c o p y ( I R ) o f aged o i l s , 249 - thermometry, 331, 488 Inorganic - non-metallic substance c o a t i n g s , 287 s o l i d l u b r i c a n t s , 7 0 , 259 I n s p e c t i o n of epilame o r l u b r i c a n t f i l m s , 234, 381, 390 I n s t r u m e n t l u b r i c a n t s , 33
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Interactions i n o i l - p o l y m e r s y s t e m s , 198, 2 6 1 , 371, 376 i n solvent-polymer systems, 383 I n t e r c a l a t i o n , 69 I n t e r n a l l u b r i c a t i o n , 1 9 , 32, 181, 202 I n t e r n a l l y l u b r i c a t e d polymers, 19, 32, 181 Ion - - i m p l a n t a t i o n c o a t i n g s , 295 - - p l a t i n g c o a t i n g s , 273, 295 s c a t t e r i n g spectrometry (ISS), 3 89 Iron-based s i n t e r e d materials, 9 , 164 Israe l a s h v i l y s f o r m u l a , 9 5
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Jasper, 16 J e w e l b e a r i n g s , 205, 2 9 9 , 405 Journal bearings polymer-polymer, 1 0 9 , 1 9 7 , 410 - p r i s m a t i c , 1 6 7 , 418 s t e e l - b r a s s , 73, 152, 4 1 0 , 414 s t e e l - m i n e r a l , 1 3 1 , 203, 405, 414 s t e e l - p o l y m e r , 83, 1 7 3 , 406
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K i n e t i c s of r e c o n t a m i n a t i o n , 390 K i n g s b u r y ' s d e s o r p t i o n model, 162 Knee j o i n t p r o s t h e s e s , 498 Knife-edge b e a r i n g s , 4 4 1 Laser t r e a t m e n t c o a t i n g s , 2 9 6 Laying d r o p method, 346 Lead films, 273 L e v i t a t i o n , 438 L i q u i d c r y s t a l s , 159 Low t e m p e r a t u r e o i l s , 4 4 , 255 L u b r i c a n t s , 33 d u r a b i l i t y , 243 - g r e a s e s , 54 oils, 33 s o l i d , 68 Lubrication o f a r t i f i c i a l j o i n t s , 498 boundary, 1 4 9 o f c o a t i n q s , 281 o f e l e c t r i c a l c o n t a c t s , 47, 61, 475 u n d e r extreme c o n d i t i o n s , 253, 443 f o r - l i f e , 2 , 211, 443, 482 of g e a r s , 458
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562 of g u i d e s , 450 hydrodynamic, 167, 1 8 9 , 418 of m a g n e t i c media, 482 mechanism i n p o r o u s b e a r i n g s , 169 mixed, 1 4 9 , 1 9 1 of p o l y m e r i c s y s t e m s , 43, 173, 2 6 1 problems , 2 11 of r o l l i n g b e a r i n g s , 443 w i t h s i l i c o n e o i l s , 150 s p e c i a l p r o c e d u r e s , 5 0 , 443, 482 by t r a n s f e r , 279, 447 two-layer, 1 6 0 a t v a r y i n g a m b i e n t temperat u r e , 255 L u b r i c i t y , 334
Material t r a n s f e r , 7 9 , 9 4 , 158 Metallic g l a s s e s , 1 7 , 82, 137 Metal-based c o m p o s i t e s , 1 9 Metals, 6 , 73, 1 4 9 , 4 9 6 , 503 Magnetic - b e a r i n g s , 437 - c l u t c h e s , 465 f i e l d e f f e c t on f r i c t i o n and wear, 503 on m i g r a t i o n of l u b r i c a n t , 50, 7 2 h e a d s , 437, 451, 481 - m a t e r i a l s , 8 , 438, 467 media, 437, 451, 481 l u b r i c a t i o n , 482 - s e n s o r s , 492 - t a p e s , 1 4 6 , 437, 486 Marble, 1 4 1 Maximum a d m i s s i b l e v a l u e o f o i l d o s e , 238 Medical i m p l a n t s , 495 Mercury b e a r i n g s , 439 MESERAN number, 388 Metal h a l i d e s , 71 Metallic s u r f a c e f u s i o n p r o c e s s , 289 Metals, 6 , 73, 82, 131, 149, 164,206, 273 Methods of a c c e l e r a t e d t e s t i n g o f a g e i n g r e s i s t a n c e , 2 4 4 , 369 of e p i l a m i z a t i o n , 2 1 4 M i c r o c r y s t a l l i n e wax, 475 M i c r o f l u o r o s c e n c e method, 388 Micrometers, 299 Micromotors, 472, 501 M i c r o r o b o t s , 501 Microseals, 501 Microstructure of t r i b o l o g i c a l system, 2 M i c r o t r i b o m e t e r , 315
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M i g r a t i o n of o i l , 2 1 2 , 2 6 6 , 4 4 6 M i n e r a l o i l s , 34, 4 1 , 55 M i n e r a l s , 1 5 , 131, 203 Miniature t r i b o l o g i c a l system, 1 Mixed l u b r i c a t i o n , 149, 1 9 1 Modified i o n - p l a t i n g , 2 9 4 Molybdenum d i s u l p h i d e (MoS2), 68, 270, 282 c o a t i n g s , 270, 282 - compound, 1 4 7 - compounds ( b l e n d s ) , 43 Momentary p o l a r i t y of p o l y m e r s , 261 Mo-S complexes, 152 Motion c h a r a c t e r i s t i c , 4 Mounting of r o l l i n g b e a r i n g s , 4 4 2 N a t u r a l o i l s , 34 Natural substances used f o r lubrication, 4 1 N e a t ' s - f o o t o i l , 33 N e g a t i v e p r e s s u r e s l i d e r s , 488 N e m a t i c l i q u i d c r y s t a l s , 159 Nickel, 8 - b a s e d a l l o y s , 11 N i c k e l i z i n g , 285 N i t r i d i n g , 290 Noise r e d u c t i o n i n g e a r s , 456 Non-spreading o i l s , 50, 2 2 7
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O i l - i m p r e g n a t e d c o a t i n g s , 278 O i l s , 33 r e q u i r e m e n t s , 33, 2 1 1 s e l e c t i o n , 48, 180 t e s t i n g , 336, 4 0 1 u n i t consumption p a r a m e t e r , 237 Oldham's c o u p l i n g , 463 Optical instruments l u b r i c a t i o n ,
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61
Optimum volume of o i l d e p o s i t , 237 Organic compounds, 71 O s c i l l a t i n g m o t i o n , 303 O s o w i e c k i ' s method, 2 1 6 O v e r l o a d i n g o f l u b r i c a t e d microb e a r i n g , 240 Owens and Wendt's method, 233 O x i d a t i o n a l wear, 79 O x i d a t i v e s t a b i l i t y o f lubricants, 243, 369 Oxide c o a t i n g s h a r d , 287 s o f t , 278
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Padding, 285 P a p e r , 1 4 6 , 450, 4 9 2 Partially stabilized zirconia ( P S Z ) , 1 3 5 , 138 P e l t i e r e l e m e n t , 306
563 Pendulum s y s t e m s , 303, 312, 399 P e r m e a b i l i t y of porous b e a r i n g , 171 P h o s p h a t i z i n g , 290 P h t h a l o c y a n i n e c o a t i n g s , 277 P h y s i c a l vapour d e p o s i t i o n ( P V D ) , 293 P i n method, 380 P l a i n b e a r i n g s , 4 0 4 , 501 Plasma-sprayed c o a t i n g s , 288 P l a s t i c i z i n g of polymers, 198, 261 P o i s e i l l e ' s f l o w , 337 P o l y a c e t a l (POM) , 21 Polyamides (PA) , 2 0 P o l y e t h e r s , 38, 475, 482 P o l y e t h y l e n e s (PE) r 2 2 , 30, 4 9 6 P o l y c a r b o n a t e (PC) , 2 1 P o l y g l y c o l s , 38 P o l y i m i d e ( P I ) , 2 6 , 276 P o l y m e r i c c o a t i n g s , 2 7 4 , 501 Polymers, 20, 83, 107, 1 4 1 , 1 7 3 , 1 9 7 , 2 6 1 , 383, 448, 455, 460, 4 6 4 , 470, 4 9 6 , 501, 504 e f f e c t o n l u b r i c a n t s , 265, 37 1 e p i l a m i z a t i o n , 267 - f i l l e d , 28, 107 - f l u o r i n a t e d ( f luoropolymers) , 24 h e a t - r e s i s t a n t , 26 l u b r i c a t i o n , 43, 173, 261 - p l a s t i c i z i n g , 1 9 8 , 2 6 1 , 383 - p o l a r i t y , 261 w e t t a b i l i t y , 262 Polyoxymethylene (POM) , 2 1 P o l y ( p h e n y 1 e n e o x i d e ) (PPO) , 22 P o l y p r o p y l e n e ( P P ) , 22 P o l y s i l o x a n e s , 40 P o l y s t y r e n e (PS) r 2 4 Polyterephthalates , 21 P o l y t e t r a f l u o r o e t h y l e n e (PTFE) , 24, 30, 4 1 , 2 7 6 Porous b e a r i n g s , 1 6 4 , 287, 404, 422 Porous metals, 9 , 1 9 , 81, 1 6 4 Predicting tribological behaviour o f boundary l u b r i c a t e d metal systems, 1 6 2 o f c o a t i n g s , 280, 2 9 0 - of p l a i n b e a r i n g s , 404 o f polymer-polymer s y s t e m s , 123 of porous b e a r i n g s , 167, 404, 422 - of r o l l i n g b e a r i n g s , 446 - of steel-polymer journal b e a r i n g s , 85, 9 7 , 1 9 1
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P r e v e n t i n g o i l from s p r e a d i n g o r c r e e p i n g , 212, 2 6 6 , 443 P r i n t e r mechanisms, 285, 2 9 1 , 299 , 4 9 4 P r i s m a t i c b e a r i n g s , 1 6 7 , 418 P r o f i l o m e t e r t e c h n i q u e f o r wear r a t e e s t i m a t i o n , 329 Prostheses , 496 P r o s t h e t i c h e a r t v a l v e s , 499 P y r o l i t i c g r a p h i t e , 438 Q u a r t z c l o c k , 456 R a d i o a c t i v i t y , 4 6 , 259, 287, 289, 374, 506 Radio f r e q u e n c y glow d i s c h a r g e (RFGD), 500 R a t c h e t mechanisms, 4 6 0 Raw c o t t o n , 1 4 7 Reaction f i l m s , 1 6 1 Recommendations f o r s e l e c t i n g oil, 48 R e c o n t a m i n a t i o n , 390 R e l a x a t i o n o s c i l l a t i o n s , 453 R e l i a b i l i t y f u n c t i o n s , 7 6 , 410 Requirements f o r i n s t r u m e n t l u b r i c a n t s , 33 , 211 R e i n f o r c e d p o l y m e r s , 28, 107 R e p l i c a method, 330, 396 Rheometers, 341 Ribbons, 4 9 2 R o b o t s , 501 Rolling - b e a r i n g s , 399, 4 4 1 - - l i f e t i m e , 446 l u b r i c a t i o n , 443 upwards a n g l e , 428 R o t a r y h e a d s , 437, 489 Rouqhnes s e f f e c t on t r i b o l o g i c a l b e h a v i o u r , 2 , 1 3 2 , 157 - f o r m a t i o n , 1 4 7 , 392 Ruby, 1 5 , 1 3 1 , 205, 299 Run-down method, 319 Running-in - d e t e c t i n g 1 6 5 , 395 - effect on f r i c t i o n coefficient, 164 e n e r g y , 102 R u s t i n h i b i t o r s , 43
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23 S a p p h i r e , 1 5 , 1 3 5 , 207 Scanning electron microscopy (SEMI , 236, 381 Screw t r a n s m i s s i o n s , 459 S e a l i n g i n vacuum a p p a r a t u s , 57 S e a l s , 501 SAN,
564 Secondary i o n mass s p e c t r o scopy (SIMS), 389 S e i z u r e , 57, 75 number, 409 S e l e c t i v e t r a n s f e r , 157, 263 S e l f c o a t i n g , 227 Self-lubricating coatings,
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278, 285, 287 Self-lubrication,
19, 32, 181,
202, 405, 443 S e s s i l e d r o p method, 346 S h o c k - r e s i s t a n t b e a r i n g s , 431 Shoulder j o i n t p r o s t h e s e s ,
498 SiAloN, 16, 138 S i l e n t g e a r s , 456 S i l i c i d i n g , 292 Silicon c a r b i d e , 16, 138 n i t r i d e , 16, 136, 139 S i t a l l , 15 Silver-based a l l o y s , 8 S i n t e r e d m e t a l s , 9 , 81, 164,
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Synergism o f MoS2 and g r a p h i t e a c t i o n , 69 S y n o v i a l f l u i d , 41 S y n t h e t i c o i l s , 36 System, 1 T a c t o i d s , 71 Tape memories, 481 r e c o r d e r h e a d s , 146, 437, 451,
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487 Tapered b e a r i n g s , 425 Temperature e f f e c t on t r i b o l o g i c a l b e h a v i o u r , 100, 331 - measuring t e c h n i q u e s , 331 - rises due t o f r i c t i o n , 102,
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331 Testing o f l u b r i c a n t s , 243, 333, 401 - of t r i b o l o g i c a l b e h a v i o u r of m i n i a t u r e s y s t e m s , 302, 394,
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289 Sliding e l e c t r i c a l contacts,
470 S o f t m e t a l c o a t i n g s , 71, 273 S o f t m e t a l s , 71, 273 S o l i d l u b r i c a n t s , 68, 269 S o l u b i l i t y p a r a m e t e r , 52, 188,
402 T e x t i l e machines, 294, 451, 502 Thermal e f f e c t s due t o f r i c t i o n ,
102, 331
T h e r m i s t o r s , 331 Thermodynamical p o t e n t i a l , 334 Thermoelements, 331 363, 382 Thennochemical d i f f u s i o n p r o c e s S o l v e n t s , 382 ses, 286, 290 S p a c e c r a f t mechanisms, 258, T h i c k e n e r s , 55 280, 445 Thin f i l m m a g n e t i c media, 481 S p e c i a l b e a r i n g s , 255, 435 T h i x o t r o p i c g r e a s e , 61 S p h a r e l i t e s t r u c t u r e , 138 T h r u s t j e w e l b e a r i n g s , 429 S p h e r i c a l b e a r i n g s , 425 T i l l w i c h ' s method, 377 S p i n e l , 16 Timing b e l t d r i v e s , 462 Spring s u p p o r t s , 440, 452 T i t a n i u m , 13, 135, 207, 286 S p u t t e r i n g , 293 Topography o f e p i l a m e o r S t a r w h e e l s , 460 l u b r i c a n t f i l m s , 234, 381, 390 S t e e l s , 6, 73, 83, 131, 152, Total 203, 289, 292 coking o f l u b r i c a n t , 243 S t i c k - s l i p e f f e c t , 57, 451, r e p l a c e m e n t j o i n t s , 496 453 T o x i c i t y of o i l s , 53, 376 S t i c t i o n , 482 T r a d i t i o n a l c l o c k o i l s , 34 Stop-Oil method, 215, 378, Transfer 446 l u b r i c a t i o n , 279, 447 S t o r a g e of c l e a n s u r f a c e s , 391 o f m a t e r i a l , 158, 263 S t r i b e c k ' s curve, 164, 204 T r a n s f e r r e d c o a t i n g s , 279, 447 S u l p h o n i t r i d i n g , 290 T r a n s i t i o n from mixed t o hydroSurf a c e dynamic l u b r i c a t i o n , 167, 418 p o t e n t i a l d i f f e r e n c e (SPD) T r a n s m i s s i o n s , 453 t e c h n i q u e , 389 T r i b o c o r r o s i o n , 51 - t e n s i o n , 346, 379 T r i b o e l e c t r i f i c a t i o n , 88, 261 - - of aged o i l s , 247 Tribology, 1 t e n s i o n measurement, 346, 378 - o f m i n i a t u r e s y s t e m s , 1, 508 Surgical materials, 9 Tribological Switches, 471, 479 a s p e c t s of f i n e mechanism a s s e m b l i e s , 404 Synchronous b e l t d r i v e s , 462 - c o a t i n g s , 269
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565 experimental techniques , 302, 394, 403 system, 1 - - c o n c e p t u a l p l a n e s of analysis, 3 T r o p i c a l climate l u b r i c a n t s , 4 6 , 256 - e f f e c t o n l u b r i c a t i o n , 256 Tungsten c a r b i d e , 18, 131, 289, 300 Two-layer l u b r i c a t i o n , 1 6 0 T y p e w r i t e r u n i t s . 285, 494
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Ultrasonics e f f e c t on f r i c t i o n and wear, 504 U l t r a - l o w f r i c t i o n phenomenon, 507 U n i d i r e c t i o n a l m o t i o n , 312 U n i t o i l consumption p a r a m e t e r , 237 U n i v e r s a l Cardan j o i n t s , 4 6 4 Upper limb j o i n t p r o s t h e s e s , 498 UTI apparatus - f o r f r i c t i o n t e s t s , 308 - f o r r a p i d a g e i n g of o i l s , 373 Vanadizing, 292 V e r s i n o ’ s v i s c o m e t e r , 336 Vibrations - e f f e c t on t r i b o l o g i c a l b e h a v i o u r , 1 3 4 , 505 - on r o l l i n g b e a r i n g s , 4 4 2 V i s c o m e t e r s , 336 Viscosity d e t e r m i n a t i o n , 336 - i n d e x i m p r o v e r s , 43 - of o i l e f f e c t on f r i c t i o n , 20 6 V o g e l p o h l ‘ s f o r m u l a , 418
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Wear i n artificial joints, 141, 496 of c a r b o n - g r a p h i t e s , 1 4 3 , 209 of c e r a m i c s , 131, 1 4 1 , 203 debris, 94 of e l e c t r i c a l c o n t a c t s , 470 of human t e e t h , 1 4 7 of m a g n e t i c media, 481 of m e t a l s , 73, 1 4 9 , 1 6 4 modulus, 4 1 2 number, 4 1 2 , 4 2 6 by p a p e r and r i b b o n s , 1 4 6 , 450, 4 9 2
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of p o l y m e r s , 83, 109, 1 4 1 , 173, 1 9 7 - p r e d i c t i o n , 9 7 , 132, 162, 191, 290, 446 - r e s i s t a n t c o a t i n g s , 285, 451, 49 5 t e s t m e t h o d s , 325 t e s t r i g s , 325 W e t t a b ilit y - of p o l y m e r s , 2 6 2 - s t u d i e s , 213, 378, 390, 392 W e t t i n g , 213, 387, 390, 392 Woog’s method, 215 W o r m g e a r s , 458 W u r t z i t e s t r u c t u r e , 138 Wu’s method, 233
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X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (XPS), 390 Zhurkov’s f o r m u l a , 1 6 2 Zinc dioctyl-dithio-phosphate (ZnDTP) , 43, 159
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