Ceramic Cutting Tools

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CERAMIC CUTTING TOOLS Materials, Development, Performance

Edited by

E. Dow Whitney University of Florida Gainesville, Florida

I I nP

NOYES PUBLICATIONS Park Ridge, New Jersey, U.S.A.

and

Copyright 0 1994 by Noyes Publications No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission in writing from the Publisher. Library of Congress Catalog Card Number: 94-15234 ISBN: 0-8155-1355-O Printed in the United States Published in the United States of America by Noyes Publications Mill Road, Park Ridge, New Jersey 07656 10987654321

Library of Congress Cataloging-in-Publication

Ceramic cutting tools /edited p.

Data

by E. Dow Whitney.

cm.

Includes bibliographical references ISBN 0-8155-1355-O 1. Metal-cutting tools--Materials. 1. Whitney, E. Dow. TJ1186.C437 1994 666--dc20

and index. 2. Ceramic materials.

94-15234 CIP

To the memory of Professor George E. Kane of Lehigh University

vii

MATERIALS

SCIENCE

AND PROCESS

TECHNOLOGY

SERIES

Editors Rointan F. Bunshah, University of California, Los Angeles (Series Editor) Gary E. McGuire, Microelectronics Center of North Carolina (Series Editor) Stephen M. Rossnagel, IBM Thomas J. Watson Research Center (Consulting Editor)

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HANDBOOK OF DEPOSITION TECHNOLOGIES Edition: edited by Rointan F. Bunshah CHEMICAL

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edited by Robert F. Davis

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TO SEMICONDUCTORS:

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Ceramic

GRAPHITE,

DIAMONDS

and Other Materials-Processing

FIBER REINFORCED

FRICTION

COMPOSITES:

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FOR INDUSTRIAL

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CORROSION OF GLASS, CERAMICS David E. Clark and Bruce K. Zoitos HANDBOOK OF INDUSTRIAL and Gordon L. Barna

OF MATERIALS:

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FILMS AND COATINGS:

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CUTTING

TECHNOLOGY

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edited by G. K. Bhat

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Titles by Arthur H. Landrock edited by Sidney H. Goodman FOR ADHESIVE

PLASTICS AND ELASTOMERS

CARBON-CARBON D. Edie

Volume 1: edited by Jon G. P.

TOOLS: edited by E. Dow Whitney

ADHESIVES

FORMULATlNG

AND

by Peter J. Blau

AND CERAMIC

Related

SURFACE Wegman

ELECTRONICS

edited by Lawrence

TECHNOLOGIES:

REFRACTORIES

by Hugh 0.

edited by K. S. Mazdiyasni

APPLICATIONS:

AND PROCESSING

edited by

and Technology

AND TECHNOLOGY,

AND WEAR TRANSITIONS

SHOCK WAVES SPECIAL

CERAMIC

CERAMIC

CIRCUITS:

AND FULLERENES:

SOL-GEL TECHNOLOGY FOR THIN FILMS, FIBERS, PREFORMS, SPECIALTY SHAPES: edited by Lisa C. Klein

ADVANCED Binner

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BY COMPUTER:

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REPAIR:

by Raymond

F.

by Ralph D. Hermansen by Raymond

F. Wegman

edited by John D. Buckley and Dan

FACILITIES:

by William R. Acorn

Contributors

Donald E. Graham Carboloy, Inc. Warren, MI

J. Gary Baldoni Materials Technology Consultant Norfolk, MA

Walter W. GNSS Komet of America, Schaumburg, IL

Harold P. Bovenkerk Consultant HP Consulting Worthington, OH

Robert A. Hay Norton Diamond Northboro, MA

Sergei-Thomaslav Buljan Saint Gobain Norton Company Worcester, MA

Film

Choll K. Jun Greenleaf Corporation Saegertown, PA

John D. Christopher Machining Research, Inc. Florence, KY Kilian M. Friederich Cerasiv GmbH (formerly Plochingen, Germany

Inc.

Alan G. King Retired 1780A Rolling Hills Drive Twinsburg, OH

Feldmuehle)

R Krishnamurthy Department of Mechanical Engineering Indian Institute of Technology Madras, India

C.V. Gokularathnam Department of Mechanical Engineering Indian Institute of Technology Madras, India

xix

xx Pankaj

Contributors K. Mehrotra

Keith H. Smith

Kennametal, Inc. L&robe, PA Ernest

Greenleaf Corporation Saegertown, PA

Ratterman

E. Dow Whitney

General Electric Company Worthington, OH Shyam

K. Samanta

Mechanical Engineering University of Nevada Reno, NV

Department

Department of Materials Engineering University of Florida Gainesville, FL

Science and

Milton C. Shaw Department of Mechanical and Aerospace Engineering Arizona State University Tempe, AZ

NOTICE To the best of our knowledge the information in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arisingfrom,such information. This bookis intended for informational purposes only. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Publisher. Final determination of the suitability of any information or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. We recommend that anyone intending to rely on any recommendation of materials or procedures mentioned in this publication should satisfy himself as to such suitability, and that he can meet all applicable safety and health standards.

Foreword

Since ancient times, cutting tools have been used to remove excess material from forgings and castings. Today, metal cutting is one of the primary manufacturing processes for finishing operations. The cost of metal cutting operations and the productivity in manufacturing related to metal cutting depends on several factors. The most critical is the life of the cutting tool, followed by the amount ofdowntime. Low cutting speed and feed, and small depth of cut reduce productivity. However, catastrophic failure and random tool life cause extensive downtime and are the major causes for loss of productivity. There has been a considerable amount of basic research in the United States over the past three decades on high temperature structural ceramics for engine applications. As a result of this research a large database of information on structural ceramics was created. Most research has centered on the Al,O, and Sic families of materials because of their prospective use in high-temperature applications. Alumina retains its strength and hardness to a greater degree than do the less refractory cemented carbides. The commercial use of alumina as a cutting tool was pioneered by Ford Motor Co. and alumina cutting tools were selectively used for finish machining at high cutting speeds thus increasing productivity. During the early 1970’s Ford was eager to increase productivity in manufacturing. As a result of their research on structural ceramics, the

ix

x

Foreword

use of advanced ceramics as cutting tool materials was conceived. The advantage of advanced ceramic cutting tools over the more traditional tungsten carbide tools was that they could be operated at higher cutting speeds because of their chemical stability and greater hardness. However, several limitations were observed in mass production applications such as 1) the tools were limited to finish operations, and 2) catastrophic failure of the cutting tool inserts. Failure occurred randomly, leading to large variations in tool life. The associated damage to the cutters, machine set-up, etc., increased the cost of production and decreased productivity. Maximizing productivity in machining requires optimizing the trade-off between increases in cutting speed and the possibility of decreases in tool life and reliability. The properties required for good cutting tool materials can be broadly grouped into three categories: mechanical, physical and chemical. During the cutting operation, the tool tip is subjected to cutting forces the magnitude of which depends on the cutting conditions, work material properties, and cutting geometry. The tool/work interaction also results in a temperature field in the cutting zone. The temperature gradient in the tool material results in thermal stresses which depend of the thermophysical properties of the tool material such as thermal conductivity, coefficient of thermal expansion, elastic modulus, and thermal diffusivity. The sum of mechanical and thermal stresses is the total stress experienced by the cutting tool. Hence, a tool material which experiences low thermal stress for a given temperature field, could be subjected to higher mechanical stresses (i.e., severe cutting conditions) without exceeding its fracture strength. While the mechanical and thermal stresses are interrelated, the above approach is valid as a first order approximation. For a high performance cutting tool, the tool material must possess high strength at elevated temperatures, good oxidation resistance, a low coefficient of thermal expansion and high thermal conductivity. The cutting tool material must possess all these properties, individually and in combination, at the temperatures prevailing at the tool tip. These should be the basic design criteria for tailoring a high performance cutting tool material. Realizing that, I and a group of researchers at the Ford Scientific Research Laboratory first proposed the application of hot pressed silicon nitride (designated as SSS) as a

Foreword

xi

cutting tool material for high speed machining of grey cast iron. Ford subsequently conducted extensive and systematic research for developing a class of S&N, composites and characterized their performance and properties in metal cutting tests. The results of this research were presented in our 1980 patent. Further patents were awarded to Ford and publications by Ford scientists revealed that hot pressed Si,N, with Y,O, has a high speed and feed capability. Metal removal rates as high as 378 in3/min were achieved with this composition. It should be noted that for commercially cutting tools available at that time the maximum removal rate was only 15 in3/min. This quantum leap in increase in metal removal rate has accelerated the opportunities for improved productivity in machining operations of grey cast iron. Gradual wear and the absence of chipping were observed in production operations, where tool life improvements up to 20 times were demonstrated over commercial carbides, coated carbides and oxide ceramics. Advanced ceramic cutting tools composed of Si,N, andSi,N,-basedceramicsandcermetspromisesignificantproductivity improvement over traditional cutting tools. This potential is likely to grow as the technology for mass production of advanced ceramics becomes available to the cutting tool manufacturing industry. Most U.S. owned advanced ceramic cutting tool firms manufacture traditional cutting tools. These companies developed aluminabased cutting tools primarily to offer a full line of cutting tools. Much of the developmental work on newer ceramic cutting tool materials such as silicon nitride and sialon has been performed by advanced ceramic manufacturers and government and corporate research laboratories. In the mid 1980’s, GTE was the first of these manufacturers to enter the cutting tool industry as a producer. Since then, GTE has expanded its cutting tool operations through the purchase of Valeron Corporation. Other advanced ceramic producers such as Norton are presumably also developingsilicon nitride-basedcutting tools and may enter into commercial production. Greenleaf and Kennametal have been manufacturing sialon cutting tools based on a composition licensed from Lucas Aerospace (United Kingdom). Research and development activities related to advanced ceramic cutting tools are being developed around the world. If the experience in the United States is any indication, the work in the cutting tool manufacturer’s laboratories tends to be relatively applied and

xii

Foreword

developmental in nature. Such developmental work tends to be directed toward areas such as compositional changes, new additives, improvements in processing techniques, and changes in cutting tool configurations. Moreover, the magnitude of this R&D effort, at least in the United States, appears to be rather modest to date. However, advanced ceramic cutting tool technology is expected to benefit, perhaps substantially, from spillovers of technological information from the more basic research on structural ceramics currently underway in a number of government and academic programs. This book is a collection of several interesting papers. Some of the authors have discussed their own research and others have reviewed state-of-the-art ceramic cutting tools and their applications. I have not tried to review all the papers, however, I have selected a few to illustrate the diversity of the field. King’s chapter provides us with his reflections on the development of ceramic cutting tools. The evolution of powder processing techniques, sintering and hot pressing of alumina, silicon nitride and SiAlONs are discussed in his chapter. The chapter on “Aluminum Oxide Coatings for Cemented Carbide Cutting Tools” discusses the influence of different types of coatings for cutting tools. It has been established that the most successful coatings for machining ferrous materials are Tic, TiN and alumina. However, the coating that provides the greatest potential for productivity gain is alumina. Shaw’s chapter discusses the scientific reasoning behind the use of alumina as an effective anti-crater coating material for WC cutting tools. The use of alumina, however, has been limited due to its low resistance to fracture. Junand Smith discuss several significant developments in two major mechanisms of toughened alumina composites such as zirconia transformation toughening and whisker/fiber reinforced toughening. Baldoni and Buljan review their work on silicon nitride cutting toolsandpropose thatsiliconnitridecuttingtoolmaterials, becauseoftheir excellent mechanical and physical properties, may parallel the performance of cemented carbide for many applications. They came to the same conclusion as we did earlier at Ford of the potential to improve productivity using silicon nitride tools for machining grey cast iron. The work on the processes for making diamond at low pressures is reviewed by Hay. This section presents a brief explanation of the science and manufacturing techniques used to produce diamond

Foreword

Xl11 ***

cutting tools as well as their physical and mechanical properties. The paper also illustrates a number of field test results for these types of cutting tools. As stated earlier, the book presents the current understanding of ceramic cutting tool technology and we thank the authors for their contributions. April, 1994 Reno, Nevada

Shyam K. Samanta

Preface

It has been said that history has a way of returning to its origins, for very old ideas are often revived. Although the ceramic cutting tool is often considered to be a relatively “new” development in material removal technology, bow drills with flint tips were the first simple machines to use ceramic tools. Twenty-five centuries before Christ, Egyptian artisans used flint tool bits rotated with forked sticks to bore the insides of vases. Interest in ceramics as a high speed cutting tool material is based primarily on favorable material properties. As a class of materials, ceramics possess high melting points, excellent hardness and good wear resistance. Unlike most metals, hardness levels in ceramics generally remain high at elevated temperatures which means that cutting tip integrity is relatively unaffected at high cutting speeds. Ceramics are also chemically inert against most workmetals. This book describes the various classes of ceramic cutting tools and their applications. But more than that, this book is about manufacturing and productivity. In preparing the following pages a certain type of individual has been kept in mind; i.e., the person who experiences the thrill and joy of making things. And least we forget the principles upon which rest the prosperity of the United States, allow me to restate the following truism: “eficiency in manufactur-

xv

xvi

Preface

ing isfundamental to the growth ofAmerica’s economy.” To further emphasize this fundamental truth, the following passage is quoted which deals with this important problem [l]. “In our intensely competitive world, military wars come and go, but trade wars are never-ending. Every nation wants to take customers away from every other nation. In both wars, tools are a prime factor in determining the outcome. Armaments are the hardware of national security. Machine tools are the hardware of economic security. They are also the hardware of the war on poverty. ” Metal cutting, one of man’s oldest manufacturing processes (dating back to 1000 B.C. or earlier) assumes a significant role in today’s productivity scenario. This is due to significant advances which have been made in both machine tools and cutting tool materials. In terms of manufacturing efficiency the two are inseparable. Advances in technology in one area require that corresponding technical advances be made in the other. In terms of the cutting tool itself, development of more wear resistant tool materials for application in high speed machining has a profound impact o.n productivity. Figure 1 shows how cutting speeds have steadily increased since 1900. This increase in cutting speed has been made possible through the progressive evolution of tool materials. In his book, Man the Tool Maker, K.P. Oakley states the proposition, “Human progress has gone step by step with the discovery of better materials of which to make cutting tools, and the history of man is therefore broadly divisible into the Stone Age, the BronzeAge, the IronAge and theSteelAge. ” Certainly no other time in the history of America has the need to develop new and improved cutting tool materials been as important as it is today. Productivity, expressed in terms of how fast metal can be removed in machining operations, whether it be turning, milling, grooving, etc., is dependent on the availability of tool materials which can withstand the high temperatures and stresses generated in these operations without undergoingdegradationor change inshape. Cutting tools are thus the critical link between raw materials and the finished product. The rate determining factor in the chip making process is the cutting tool

Preface

xvii

material itself. Thus, the metal-working operation has always been dependent on the maximum capabilities of new tool materials. Machine tools and procedures have always been designed around the maximum capabilities of new tool materials. Obviously, to achieve high productivity a tool must be able to cut at high speed. As is seen in Figure 1, since about 1900, there has been an exponential increase in productivity capability as measured by cutting speeds available.

,, z&y

Tod

Sintered Carbide

Cast Nonferrous

High-Speed Steel 3

I g

5000 3000

I

Composite

Alf13-Tii

4

I

I

-

I

Ceramics /-

P % 1000 500 -

10 1800

I 1850

I 1900

YEAR OF INTRODUCTION

Figure 1. Improvements over time.

I 1950

I I 1970 1982 2000

TO PRACTICE

in cutting speeds for various cutting tools

The ceramic cutting tool represents a different class of cutting tool material with unique chemical and mechanical properties. Thus, there may be a tendency to avoid the use of ceramic tools where they may be applied advantageously. In order to realize the full potential of ceramics, it is essential to have a clear understanding of all the variables which affect the performance of these tools. In this regard it may be of interest to note that this publication is only the third book ever published devoted exclusively to the

xviii

Preface

science and technology of ceramic cutting tools. The classic is King and Wheildon’s Ceramics in Machining Processes, published in 1966 [2]. Two and one-half decades later, a Russian publication, Ceramic Tool Materials, was announced [3]. From this writer’s experience, this book published in the Ukraine, is not readily available, translation problems notwithstanding. Needless tosay, the authors feel a new book on ceramic tools is certainly justified. There is not now and probably never will be a “universal” cutting tool material. Many of the new ceramic tool materials today have very specific applications for which they are particularly suited. When properly applied, these new tools can provide the manufacturing engineer with a means of reducing machining costs and increasing productivity. It is the premise of this book that American manufacturers are not a dying breed, that there is glamour and glory in making things and that the neglect of production in favor of other manufacturing operationssuchas finance and marketingwhich has taken place in the United States since World War II is slowly being reversed. It is to the rebuilding of the manufacturing community that this book is dedicated. May it serve as a cornerstone rather than a tombstone. A very special acknowledgement is given to Rebecca Schulz for undertaking the extensive task of word processing in preparing the initial drafts and final manuscript. It is sincerely hoped that this book will be of service to those engaged in metalworking processes.

REFERENCES 1. L.A. Wilkie and R.S. Rimanoczy, The Principles of American Prosperity, The Fisher Institute, Dallas, TX (1981). 2. A.G. King and W.M. Wheildon, Ceramics in Machining Processes, Academic Press, New York, NY (1966). 3. G.G. Gnesin, 1.1. Osipova, G.D. Rontal, et al., Ceramic Tool Materials, Tekhnika, Kiev, Ukraine (1991). April, 1994 Gainesville, Florida

E. Dow Whitney

Contents

1

Ceramic Cutting Tools - Reflections on the Continual Process of Improvement.. ....................

1

Alan G. King

2

1

INTRODUCTION ....................................................................... SIGNIFICANT ADVANCES ...................................................... Alumina - Glass Bonded ..................................................... Sintered Alumina ............................................................... Hot-Pressed Alumina .......................................................... Early Advances in Science and Technology ........................ Recent Developments in Science and Technology (1980-1990). ............................................. CURRENT CERAMIC CUTTING TOOLS ................................ SUMMARY .............................................................................. REFERENCES ..........................................................................

.6 9 10 10

Tool Life ..............................................................

13

1

2 .2 3 .4

Milton C. Shaw 13 INTRODUCTION ..................................................................... 13 TEMPERATURE ...................................................................... .21 CHIPPING AND GROSS FRACTURE ................................... .24 PRESSURE.. ............................................................................ WORKPIECE COMPATIBILITY ............................................. 25

xxi

xxii

3

Contents STRUCTURAL INHOMOGENEITY ...................................... REFERENCES .........................................................................

.25 .26

Selection of Cutting Tool Materials ...................

28

John D. Christopher INTRODUCTION

....................................................................

WORK MATERIAL/ALLOY..

.................................................

.31

HIGH SPEED STEEL ..............................................................

.32

UNCOATED

.34

CARBIDES ........................................................

COATED CARBIDES

.............................................................

CERAMIC TOOLS .................................................................. Cold Pressed Alumina.. ....................................................

Hot Pressed Alumina/Tic ................................................ Whisker-Reinforced Alumina ........................................... Silicon Nitride ................................................................. Ceramic Summary ............................................................ TiC/TiN Cermets ............................................................. Polycrystalline Diamond and Cubic Boron Nitride ........... Polycrystalline Diamond, PCD.. ....................................... Polycrystalline Cubic Boron Nitride, CBN ....................... SUMMARY .............................................................................

4

.28

.38 .40

.40 .41 .42 .43 43 .44 .45 .45 .46 .47

Aluminum Oxide/Titanium Carbide Composite Cutting Tools . . . . . . . . . . . . . . . . . . . . .. ..*.............................. 48 Walter W. Grass and Kilian M. Friederich INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . 48 COMPOSITION, MICROSTRUCTURE AND PROPERTIES .48 GRADE APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 52 TOOL DESIGN . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 54 MACHINING RECOMMENDATIONS .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 REFERENCES . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

5

Cermet Cutting Tools .,.......................................

63

Walter W. Gruss and Kilian M. Friederich INTRODUCTION . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 COMPOSITION, MICROSTRUCTURE AND PROPERTIES .63 GRADE APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Contents TOOL DESIGN.. ...................................................................... TURNING AND BORING ....................................................... GROOVING AND THREADING ............................................ THREADING.. ......................................................................... MILLING ................................................................................. REFERENCES .........................................................................

6

xxiii .68 .68 .70 .80 .83 .85

Alumina-Silicon Carbide Whisker Composite Tools ........... ............. .......... ........ ....... ...... ..... ........ 86 Choll K. Jun and Keith H. Smith INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 TOUGHENING MECHANISMS AND MECHANICAL PROPERTIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Crack Deflection . . . . . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Whisker Pullout and Bridging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 APPLICATION OF CUTTING TOOLS . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 95 REFERENCES . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

7

Phase Transformation Toughened Materials for Cutting Tool Applications ............................ 112 R. Krishnamurthy

and C. V. Gokularathnam

INTRODUCTION .................................................................. DEVELOPMENT OF CERAMIC CUTTING TOOLS ............ TOUGHENED CERAMICS - CONCEPTS.. ........................... Toughening Mechanisms.. ............................................... Transformation Toughened Zirconia System ................... Y-TZP AND CeTZP SYSTEM APPLICATIONS ................... Y-TZP System.. .............................................................. Ce-TZP System .............................................................. ZTA MACHINING APPLICATIONS .................................... REFERENCES ........................................................................

8

Silicon Nitride Cutting

.112 113 123 12.5 126 133 135 159 .183

188

Tools .......... .. .. ..*........... 191

J. Gary Baldoni and Sergei-Thomaslav

Buljan

SILICON NITRIDE . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 193 SiAION . . . . . .. . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 Silicon Nitride-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

xxiv

Contents CUTI’ING TOOLS APPLICATIONS

....................................

Gray Cast Iron Machining ............................................. Steel Machining ............................................................. Superalloy Machining .................................................... SUMMARY ........................................................................... REFERENCES .......................................................................

9

Aluminum Oxide Coatings for Cemented Carbide Cutting Tools ......................................

.201

.202 .206 .210 .214 .215

221

Donald E. Graham

10

INTRODUCTION .................................................................. ADVANTAGES OF COATED TOOLS ................................. WEAR MECHANISMS ......................................................... Crater Wear.. ................................................................. Flank Wear.. .................................................................. Built-up Edge ................................................................ Notching.. ...................................................................... Multi-layer Coatings.. .................................................... SUMMARY ........................................................................... REFERENCES .......................................................................

.221 .222 .225 .226 .231 .234 .237 .237 .239 .240

Polycrystalline Diamond and Cubic Boron Nitride .........................................

241

Ernest Ratterman and Harold P. Bovenkerk EARLY HISTORY OF DIAMOND ....................................... .241 Recent History of Industrial Diamond ............................ .242 Other Super Hard Materials.. ......................................... .243 Properties of PCD and PCBN ........................................ .244 Guidelines for Machining with Polycrystalline Diamond Tools ........................................................ .249 Organization of PCD Machining Guidelines ................... .250 Select the Application . A Material/Industry Guide ........ .251 Guide to Selecting the Most Effective Grade of PCD.. .... .253 Description of PCD Tipped Tools .................................. .256 Selection of PCD Machining Parameters ........................ .257 .258 PCD Rake Angle Guidelines .... ...................................... PCD Lead Angle Guidelines .......................................... .261 PCD Nose Radius Guidelines.. ....................................... .261

Contents

xxv

PCD Coolant Use Guidelines ......................................... .262 PCD Tool Edge Preparation.. ......................................... .262 Summary of General PCD Application Guidelines ......... .263 CASE HISTORIES ................................................................ .264 GUIDELINE FOR MACHINING WITH POLYCRYSTALLINE CUBIC BORON NITRIDE (PCBN) ............ .269 Why Machine Instead of Grind?. .................................... .269 PCBN Machining Guideline Organization ...................... .271 Select the Application - A Material/Industry Guide ........ .272 Guide to Selecting the Most Effective Grade of PCBN ... .274 Description of PCBN Tools ........................................... .275 Select Parameters for Machining with PCBN Tools ....... .276 PCBN Depth of Cut Guidelines ..................................... .279 PCBN Rake Angle Guidelines.. ...................................... .281 PCBN Edge Preparation Guidelines ............................... .281 PCBN Lead Angle Guidelines ........................................ .283 PCBN Nose Radius Guidelines ...................................... .283 PCBN Coolant Application Guidelines.. ......................... .283 SUMMARY GUIDES FOR PCBN TOOLS ........................... .285 Speed ............................................................................ .286 Feeds ............................................................................. .286 Cutting-Tool Set-up ....................................................... .286 PCBN Tool Machining Case Histories ........................... .288 COST ANALYSIS OF MACHINING WITH SUPERABRASIVES .............................................................. .288 Costing Superabrasives.. ................................................ .297 Examples of Superabrasives Impact on Product Cost ..... .299

11

The New Diamond Technology and its Application in Cutting Tools ............................

305

Robert A. Hay INTRODUCTION .................................................................. BACKGROUND .................................................................... The CVD of Diamond.. .................................................. DIAMOND PROPERTIES ..................................................... Physical Properties of Diamond ..................................... Mechanical Properties of Diamond Film ........................ DIAMOND CUTTING TOOLS .............................................

.305 .305 .306 .309 .309 .309 .313

xxvi

12

Contents Single-Crystal Tools ...................................................... PCD .............................................................................. CVD Advantages ........................................................... Diamond Tool Use ......................................................... FIELD RESULTS .................................................................. CVD Thick Film Diamond ............................................. CVD Thin Film Diamond Tools ..................................... POTENTIAL.. ........................................................................ REFERENCES .......................................................................

.315 .315 .3 16 .317 .317 .317 .322 .324 .324

Machining Economics ......................................

328

Pankaj K. Mehrotra INTRODUCTION .................................................................. MATERIAL COST ................................................................ Direct Material Cost ...................................................... Cutting Edges Per Insert ................................................ Indirect Material Cost .................................................... RELIABILITY ........................................................................ LABOR COSTS ..................................................................... Cutting Time ................................................................. Tool Life ....................................................................... Non-Cutting Time.. ........................................................ Overhead/Labor Rates and Fixed Costs.. ........................ REFERENCES .......................................................................

13

.328 .330 .330 .334 .338 339 .339 .340 .343 .343 .344 .344

Summary and Prospectives on the Future of the Ceramic Tool in Manufacturing Operations . . . 346 E. Dow Whitney WHAT DOES THE FUTURE HOLD FOR CERAMIC CUTTING TOOLS? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . 348

Index

. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . . 349

1 Introduction to Ceramic Cutting Tools

Alan G. King

Twinsburg, Ohio

INTRODUCTION

Ceramic science in the first third of the 20th century was primitive. Ceramic engineering was largely by rote. Over time, the slide rule and log tables gave way to the calculator or computer. However, the contributions of the early ceramic engineer should not be discounted. Their skill, craftsmanship and attention to detail laid the foundation for many of today’s technical ceramics. Ceramic cutting tool development is paralleled by developments in ceramic processing science, materials selection and improved instrumentation. This chapter discusses the evolution of cutting tools in relation to the above criteria.

SIGNIFICANT

ADVANCES

Ceramic cutting tools have been in use for approximately 90 years. As new materials were developed during a specific era, the properties of the cutting tool improved. However, as new cutting tools were developed, new materials that demanded even more rigorous machining requirements were also developed. The following sections discuss in chronological order some advances in ceramic cutting tools.

2

Ceramic Cutting Tools

Alumina - Glass Bonded Ceramic tools have been in existence since the early 1900’s. These early tools consisted primarily of alumina and were bonded to metallic cutting tools with a glassy phase derived from additions of talc or clay. Because of the poor quality of the alumina powders available at the time, fracture toughness values of 3.0 MPa-m1’2 would have been typical (however, “fracture toughness” had not been conceptualized at the time). Strengths of even the best materials were low with values of = 340 MPa (50, 000 psi). The fundamental problem with glass-bonded alumina was the glassy phase softened at metal cutting temperatures. Therefore, these materials did not gain acceptance and their use was abandoned. A bridge to the next era of ceramic cutting tools could be attributed to Bridgman for his work in the field of high pressure physics for which he was awarded the Nobel Prize. His research required a material that could withstand both high stresses and temperatures. As no suitable material was available, Bridgman developed a device which now bears his name, the Bridgman anvil. Another result of his research was the investigation of diamond/graphite stability fields in the carbon phase diagram. Sintered Alumina During the early 1930’s Ryschkewitsch experimented with a relatively pure Al,O, cutting tool. The tool was marketed under the company name of Degussit. The addition of MgO as a sintering aid eliminated the glassy phase thereby improving the strength of the material. The tool was = 98% dense with a grain size of 3 pm. As is often the case today, application was found in metal cutting of cast iron where stresses are lower than those for machining steel. Another cutting tool, referred to as Microlite, was developed during the same time period in what was then known as the Soviet Union. Microlite consisted of pure alumina and magnesium oxide. This tool generated considerable interest among tool engineers even though its physical properties were comparable to the Degussit product. It is speculated that there may have been

Introduction to Ceramic Cutting Tools

3

a slight improvement in fracture toughness due to the 5 pm grain size. In the 1960’s, several different types of sintered alumina tools with a variety of additives were developed in the United States, Europe and Japan. Goliber at General Electric’s Carboloy Division developed a ceramic cutting tool based on alumina with a 10% addition of TiO. Prior to Goliber’s work it was known that TiO, could be used as a sintering aid, however, TiOz also caused discontinuous grain growth. The Al,O,/TiO tool was referred to as the O-30 grade. It had an equiax grain structure of approximately 2 pm, was sintered to nearly full density and had a transverse rupture strength of 586 MPa (85,000 psi). This was a remarkable material for the time, and received great acceptance. Hot-Pressed Alumina There were two principle hot pressed tools with significant market shares during the 1960’s. These were Carborundum’s CCT-707 and Norton’s VR-97. Hot pressing as a densification process is more forgiving than sintering in that full density is virtually assured. Powder properties are still important but not as critical as in sintering. For example, soft agglomerates can be devastating with sintering but are of little or no consequence when the ceramic is hot pressed. Given the full density and good microstructure of the CCT-707 and VR-97 both had excellent properties for ceramic tools at that time. CCT-707. The CCT-707 was developed under the trade Carborundum acquired the name Stupalox by VonMickwitz. technology and for a time marketed this single point turning tools along with its abrasive line. While Carborundum was a principle supplier of abrasive tools, it was not generally thought of as a This, along with internal management cutting tool supplier. difficulties, caused them to cease operations for both the abrasive and cutting tool industries. VR-97. This material was a pure alumina with MgO hotpressed to full density. Research on VR-97 was done by Norton Company where investigators observed that there was a generic connection between grinding wheels and single point

4

Ceramic Cutting Tools

machining. Unlike Carborundum, Norton management realized the difficulty in building a distribution network. To solve this problem, a Norton arranged a partnership with Vascoloy Ramet to Profits were split between the two distribute the inserts. companies and neither one realized a profit. Eventually Norton sold the VR line to Vascoloy who continued to market the tool for several years. The application of ceramic tools was beginning to mature to some degree. Machining costs leveled out at a low level as the surface speed of the workpiece was increased. While this was exciting, there were some provisos which limited the realization of One limit was, and still is, the this advantage in practice. capability of the machine tool to function well at high speeds without undo vibration. High speeds are acceptable if the cut is long and straight, but can be difficult if the part is intricate and/or delicate. As a result, ceramic tools found their only significant application on cast iron, where abrasion resistance was the overriding tool attribute. Early Advances in Science and Technology

In the early 1960’s, ceramic materials science was beginning to flourish. Kuczyunski, et al. developed a sintering theory bringing about a resurgence in materials research [ 11. Also, a great deal was being learned about dislocations in metals and this work was applied to the study of ceramics. Bridgman worked cooperatively with a consortium which included General Electric, Carborundum, and Norton in an attempt to synthesize diamond. They were not successful, but advanced technology for achieving high pressures and temperatures. Later, GE scientists developed the belt apparatus and the chemistry for practical diamond synthesis. Most cemented carbide tools were ground with synthetic diamond grinding wheels. This technology was undoubtedly a factor in the search for a process to make very fine polycrystalline diamond materials. GE developed a process to synthesize this type of diamond by discharging a large capacitor bank into the “belt” apparatus. DuPont scientists also working in this area, used explosives to obtain the phase change from hexagonal to cubic

Introduction to Ceramic Cutting Tools

5

carbon. In their process, graphite powder was floated onto a water bath and the shock wave from the explosion provided the particles with sufficient energy to cause a phase change. Coes working at Norton Co., developed a mechanochemical theory of grinding [2]. A portion of his research focused on the chemical reactions occurring at the metal-abrasive interface during metal cutting. Spine1 (Fe0*A1,03) was identified as a reaction species suggesting that oxygen had to be available for the ceramic to wear by this process. Wear research on alumina cutting tools followed Coes’ lead and it was found that oxygen was an important constituent in some wear processes. Several significant works were published during this time frame. Kingery, Bowen and Uhlmann authored the book Introduction to Ceramics [3]. This work provided a basic text for the scientific study of ceramic engineering and continues to be used as a teaching and reference source. Kingery also published his work on thermal shock crack initiation [4] and Hasselman published his theory on thermal crack propagation [5]. Another significant publication was Ceramics in Machining Processes [6]. This book combined science and experience into one source making research and development accessible to all interested parties. Instrumentation was advancing as transmission electron microscopy on surface replicas was providing detailed information on microstructure and wear phenomena. Surface area analyses were becoming more accessible. Optical microscopy had been available but its’ application was expanded - principally by German instruments. Mechanical testing equipment had become routine. Emission spectrograph was perhaps the central instrument for analyzing the relatively pure materials available at that time. Also during this period, serious attention was given to processing of high quality ceramic powders. Mazdiyasni and coworker conducted a sustained research effort on ceramic powders using organic precursors [7]. While a one-to-one relationship between this work and its direct application to tool materials was difficult to ascertain, the research stimulated thought about very pure ceramic powders with a controlled particle size distribution in the near sub-micron range. This was a significant advance in the technology we now call “advanced ceramics. ”

6

Ceramic Cutting Tools

Coble, then at G.E., developed the translucent alumina referred to as “Lucalox. ” Prior to this development, alumina A translucent alumina was quite ceramics were opaque. astonishing. The microstructure had to be fully dense. By controlling the sintering aid (MgO), ceramic powder properties, sintering atmosphere, and sintering cycle a translucent alumina was realized. Lucalox became an important key material in sodium vapor lamps. While this advance did not directly impact the tool material research, it did serve to focus attention on critical processing and sintering technology. Statistical experiment design was beginning to emerge as a valuable mathematical tool. These techniques had been around for about 30 years but were not extensively used until the 1960’s. Factorial experiment design was the child of Ronald A. Fisher in Great Britain in the 1930’s. Fisher was knighted for his valuable application of mathematics to experimental methodology. At Bell Laboratories, Shewhart adapted statistics to quality control systems. Deming was a staunch advocate for “statistical process control” (SPC). Although he was not successful in convincing U.S. industry of the merits of this program, he was effective in post-World War II Japan. While SPC was initially thought of as a manufacturing quality control tool, it gradually evolved into a process for continuous product improvement. Statistical methods apply to the ceramic cutting tool research and control just as they apply to other fields. Major customers, such as the automotive industry, require their tool suppliers to use SPC methods. Recent Developments

in Science and Technology

(198O-1990)

There has been a profound change in technical (advanced) ceramics since about 1980. A great deal of interdependent science and technology became available resulting in improved ceramics. Some of these ceramics are now being used as cutting tools.

Introduction to Ceramic Cutting Tools

7

A detailed discussion of advanced ceramics is beyond the scope of this chapter. However, a summary some of the salient advances which made advanced ceramics possible follows. Advanced ceramic powders. These ceramic powders were pure, finely divided and essentially free of contaminants. Powders were generally derived by chemical methods, with a major thrust coming from the Japanese. Superior powders were developed where each particle was spherical, had a very narrow submicron size distribution and were of high purity [8]. Morgan did some remarkable work with non-aqueous powder synthesis which may, in the future, see wider spread application. At Norton, extremely pure alumina was being made by distillation of aluminum isopropoxide which was hydrolyzed with water vapor and calcined. The emission spectrograph plates were devoid of any spectra other than Al. However, the Japanese were still the major source of high quality ceramic powders including: Al,O,, yttria stabilized zirconia, silicon nitride and silicon carbide. Advances in processing. A summary of some important advances in ceramic processing follows: + Prochazka sintered dense polycrystalline carbide [9].

beta silicon

+ The toughening mechanism of partially stabilized zirconia was first observed by Garvie, et al. [lo], and then explained by Evans and Heuer [ 111. + Claussen fabricated transformation toughened alumina

lxl. + A much better understanding of suspension chemistry was provided by several researchers including Askay, Sacks and Lange [ 13- 181. The work done by Lange focused attention on the importance of flaws in the ceramic structure which act as crack nuclei. By progressively removing crack nuclei populations by intelligent processing he was able to attain 2000 MPa (300,000 psi) transverse rupture strengths in yttria stabilized zirconia [19].

8

Ceramic Cutting Tools

+ Higher strength, hot pressed alumina (with zirconia additives) was developed in Japan. + The Soviet Union revealed that they had produced a polycrystalline diamond ceramic. +

Silicon nitride was developed principally for the ceramic heat engine. Jack and Wilson in England explored the chemistry of SiAlONs [20]. + Cutler made silicon carbide whiskers from calcining rice hulls in a reducing atmosphere. Advances

in processing

equipment

and

techniques.

During this period there were parallel advances in process equipment and techniques some of which are discussed in the following section. + It may appear inconsequential, but the ability to mill ceramic powders is crucial. Advanced ceramic milling media made in Japan are essentially free from producing mill chips. + Hot isostatic presses originally developed at Batelle are now widely used for densifying ceramics. + Much improved sintering furnaces are available that are cleaner and programmable. Graphite free furnaces and hot presses using refractory metals and vacuum purging provide the cleanest environment for sintering with the important option of neutral or reducing gas atmospheres. + Mensuration and instrumentation have been greatly improved. The scanning electron microscope (SEM-EDS) with energy or wave length dispersive capability is one of the most powerful problem solving tools available. Other instruments now available to the researcher include: particle size measuring equipment, the TEM, Fourier infrared transform spectroscopy (FTIR), electron

Introduction

microprobe spectrometry spectroscopy, secondary ion

to Ceramic

Cutting Tools

9

(EMP), inductively-coupled plasma (ICP), gas chromatography (GC), raman nuclear magnetic resonance (NMR) and mass spectrometry (SIMS).

+ The development of the transistor at Bell Labs resulted in an explosion of instruments, sensors, and most importantly the computer. Inexpensive and powerful, computers and the extensive array of software make many things possible which were prohibitively laborious not too long ago.

CURRENT

CERAMIC

CUTTING

TOOLS

Ceramic materials in the cutting tool market are becoming more diverse and differentiated. Major materials are: + Alumina-silicon carbide whisker composites. The addition of Sic, increases the fracture toughness to approximately 6MPa-m1’2. This composite must be hot pressed as the whisker tangle prevents sintering to a high density. + Silicon nitride has a toughness of 4-5MPa-m1’2. It is widely used for machining cast iron where the material’s abrasion resistance is excellent. S&N4 is shock resistant, with a high thermal conductivity and a moderate thermal expansion. + Titanium carbide/titanium nitride materials are identified as cermets having good abrasion resistance. + SiAlONs are solid solutions principally between silicon nitride and alumina. The presence of alumina provides improved resistance to oxidation.

10

Ceramic Cutting Tools

+ 70%A1,03-30%TiC is used for machining carbon alloy, tool steels, and stainless steel. + Polycrystalline diamond has excellent abrasion resistance and is used for cutting metals, glass and ceramics. It is also used in drill bits for oil and gas exploration. + Cubic boron nitride is second only to diamond in hardness. Whereas carbon is soluble in iron, cubic boron nitride is not. This makes its application on abrasive ferrous metals a good choice. + Alumina continues to be used as a cutting tool insert. + Cemented carbide is actually a cermet where the WC part is the ceramic constituent. Hardness and fracture toughness values can be manipulated to produce a family of cutting tool materials. It is incredibly strong, resistant to thermal shock, has a toughness up to 15MPa-ml’*, and at lower cutting speeds is very wear resistant.

SUMMARY

The advantages of ceramics over tool steel and cemented carbide are inherent as they result from the composition and crystal lattice. Ceramics are hard, inert and retain properties at When the tendency for brittle fracture is high temperatures. substantially reduced, ceramics have the potential for general application for machining steel and. displacing much of the cemented carbide inserts.

REFERENCES

1.

G.C. Kuczyuski, N.A. Hooton and C.F. Gibson, eds., Sintering and Related Phenomena, Gordon and Breach, NY (1967). 2. L. Goes, Jr., Abrasives, Springer-Verlag, NY (1971).

Introduction to Ceramic Cutting Tools

11

3. W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley & Sons, NY (1976).

4. W.D. Kingery, “Factors Affecting Thermal Stress Resistance of Ceramic Materials, ” J. Am. Cer. Sot. 38:3 (1955). 5. D. P. H. Hasselman, “Unified Theory of Thermal Shock Fracture Initiation and Crack Propagation in Brittle Ceramics, ” J. Am. Cer. Sot., 52:600 (1969). 6. A.G. King and W.M. Wheildon, Ceramics in Machining Processes, Academic Press, NY (1966). 7. K.S. Mazdiyasni, C.T. Lynch and J.S. Smith, “Preparation of Ultra-High Purity Submicron Refractory Oxides, ” J. Am. Cer. Sot., a:372 (1965). 8. Processing of Crystalline Ceramics, Materials Science Research, Vol. 11, (Palmour, Davis and Hare, eds.) Plenum Press, NY (1978). 9. S. Prochazka, “Sintering of Silicon Carbide,” in Ceramics for High Performance Applications, (Burke, Gorum and Katz, eds.) Brook Hill, MA (1974). 10. R.C. Garvie, R.H.J. Hammink and R.T. Pascoe, “Ceramic Steel,” Nature (London), 258:703 (1975). 11. A.G. Evans and A.H. Heuer, “Transformation Toughening in Ceramics: Martensitic Transformations in Crack Tip Stress Fields,” J. Am. Cer. Sot., a:241 (1981). 12. N. Claussen, “Fracture Toughness of Al,O, With an Unstabilized ZrO, Dispersed Phase, ” J. Am. Cer. Sot., s:49 (1976). 13. I.A. Askay and C.H. Schilling, “Colloidal Filtration Route to Uniform Microstructures, ” in Ultrastructure Processing of Ceramics, Glasses and Composites, (L.L. Hench and D.R. Ulrich, eds.), John Wiley & Sons, New York, pp. 439-447 (1984). 14. I.A. Askay, F.F. Lange and B.I. Davis, “Uniformity of A&O,ZrO, Composites by Colloidal Filtration, ” Comm. Am. Cer. Sot., C-190 - C-192, 66[10] (1983). 15. J. Cesarano III, I.A. Askay and A. Bleier, “Stability of Aqueous cr-A&O, Suspensions Stabilized with Polyelectrolytes, ” J. Am. Cer. Sot. ,7_l[4], pp. 250-255 (1988).

12

16.

17.

18.

19. 20.

Ceramic Cutting Tools M.D. Sacks, H-W Lee and O.E. Rojas, “Suspension Processing of Al,O,/SiC Whisker Composites, ” J. Am. Cer. Sm., 71[5], pp. 370-379 (1988). M.D. Sacks, “Properties of Silicon Suspensions and Cast Bodies,” Am. Cer. Sm. Bull., 63[12], pp. 1510-1515 (1984). M.D. Sacks, C.S. Khadlikar, G. W. Scheiffele, A.V Shenoy, J.H. Dow and R.S. Sheu, “Dispersion and Rheology in Ceramic Processing, ” in Ceramic Powder Science, Advances in Ceramics, Vol. 21, (G.L. Messing, K.S. Maxdiyasni, J.W. McCauley and R.A. Haber, eds.) American Ceramic Society, Inc., Westerville, OH, pp. 495515 (1987). F.F. Lange, “Processing Related Fracture Origins,” J. Am. Cer. Sot., f$:396 (1983). K.H. Jack and W.I. Wilson, “Ceramics Based on the Si-Al-ON and Related Systems,” Nature Physical Science, 238128 (1972).

2 Tool Life

Department

Milton C. Shaw Arizona State University of Mechanical and Aerospace Tempe, Arizona

Engineering

INTRODUCTION There are important similarities as well as important differences between ceramic cutting tools and those of tungsten carbide and high speed steel relative to endurance. Tools must be replaced for a number of reasons including loss of required geometry due to wear, plastic flow, chipping or gross fracture. Tool life may be terminated due to poor finish, excessive forces or horsepower dynamic instability (chatter) or excessive workpiece temperature leading to adverse surface integrity. Thus, tool life is an inprecise quantity as it depends on a number of performance characteristics as well as on design specifications for the part in question. In general, tool life is dependent on temperature, pressure compatibility between tool and work materials, and the structural inhomogeneity of the tool material. TEMPERATURE The temperature during cutting is the single most important tool life variable. In general, tool life varies as some very high Taylor [l] demonstrated that the tool power of tool temperature. life in minutes (T) for a high speed steel (HSS) tool operating at a constant feed varies with cutting speed (V) as follows:

13

14

Ceramic Cutting Tools VT” = Constant

(1)

Equation 1 is a good approximation for carbide and ceramic tools in the practical range of speed, feed and tool life. Representative values of the Taylor exponent (n) are as follows for different classes of cutting tool materials: Tool Material

n

HSS: Tungsten Ceramic

0.1 0.2 0.4

carbide (WC):

The Taylor exponent increases as the tool material becomes more refractory (greater resistance to thermal softening). This is because the dominant variable is the temperature at the tool tip. The temperature at the tip of a cutting tool [8] is found to vary approximately as follows [2]:

where:

u = specific cutting energy (energy per unit volume of chips produced) V = cutting speed t = undeformed chip thickness (feed rate in a turning operation) k = coefficient of thermal conductivity of the work material pC = volume specific heat of the work material.

The quantity (kpC)O.’ is the geometric mean of conductivity and specific heat is the thermal quantity of importance in all moving heat source situations. The thermal properties of the tool material do not appear in equation (2) since essentially all of the thermal energy ends up in the chips when machining at normal rates. Combining equations (1) and (2) for a given work material and feed rate:

Tool Life

Te

Substituting the foregoing classes of tool material:

15

(3)

j/c/‘/” &”

characteristic

values of (n) for different

HSS: T=d” WC: T = 0” C: T = O5 Thus, tool life varies with tool tip temperature to a high power even for a ceramic tool, but the tool temperature exponent decreases as the tool material becomes more refractory. The dominant wear mode is also found to shift with tool tip temperature. Opitz [3] showed empirically that the dominant wear mode for a given grade of tungsten carbide shifted as the quantity Vt0.6 increased as shown in Figure 2-1. For low values of Vt”.6, the dominant wear mode corresponds to blunting or rounding of the cutting edge. This is the dominant wear mode in broaching.

(a) Nose wear

Yp
(b) Tool face wear ll
(c) Wear-land

I 7< I’I’.~<

wear JO

(d) Cratering Vr o.6> JO

Figure 2-1. Types of predominant tool wear depending on product Vt0.6 where V = cutting speed in m/min and t = undeformed chip thickness in mm/rev (after [3]).

16

Ceramic Cutting Tools

However, as Vt”.6 increases, the dominant wear pattern shifts to a flank chamfer and then to development of a wear-land and finally crater formation. This is due to an increase in tool temperature as the quantity Vt”.6 increases. It is found experimentally that specific energy (u) appearing in equation (2) is essentially independent of cutting speed (V) but varies inversely with feed (t) approximately as follows: u--

Combining gives:

1

(4)

*O.Z

equations (2) and (3) for a given tool and work material Q _ p5t0.3

=

(~~090.5

(5)

The values of Figure 2-l have been changed to English units (Y=fpm, t=ipr) and (Vt”.6)o.5 substituted for Vt”.6 in Figure 2-2. Also, failure by plastic flow as suggested by Trent [4] has been added for very high values of (Vt”.6)o.5that are usually beyond the practical region. It is thus evident that Opitz’s empirical correlation between dominant wear mode and the product (Vto6) is associated with tool tip temperature. . I

Figure

p/t0.610.5

a) Nose wear

P

(3.3

b) Tool face wear

3.3 - 4. I

c) Wear

4.1 - 5.5

land wear

d) Crater wear

) 5.5

e) Plastic flow

>) 5.5

2-2. Types of predominant tool wear depending [Vt”.6]o.5where V = fpm and t = ipr (after [3,4]).

on

Tool Life

A type of groove formation frequently occurs soft steel, or other

tool wear not included at the chip edge. Deep when machining high materials with a strong

17

in Figures 2-l and 2-2 is groove wear (Figure 2-3) temperature alloys, very tendency to strain harden

Figure 2-3. Turning operation

showing wear grooves formed at free edges of chips A and B (after [5]). during chip formation. A large groove will generally form at the free edge of the chip at the main cutting edge while smaller grooves form at the free surface at the secondary cutting edge. When there is more than one groove on the end-cutting edge of the tool, there is a spacing corresponding to the feed per revolution. The work material that is not removed by the first groove on the end-cutting edge rubs without clearance on the tool thus generating the second groove, etc. The presence of grooves on the end-cutting edge is a source of surface roughness on the machined surface and frequently dictates tool life in finish machining. While groove formation is a complex problem, it is believed This was to be primarily a temperature-related phenomenon. suggested when a high temperature alloy (Waspalloy) was machined in total darkness and the radiation from the chip used to make a photograph on high speed infrared film. The photograph

18

Ceramic Cutting Tools

revealed at the point of chip generation two red-hot chip edges with a dark cooler central region (Figure 2-4). Since one should expect a greater rate of heat transfer from the edges of the chip than from the central region, the observed high temperature edges imply that more energy was expended in chip formation in the vicinity of the edges of the chip than elsewhere.

Dark lower temperature region Cutting

Figure

Glowing of chip

edge

edge

2-4. alloy Turning operation on high temperature photographed with infrared sensitive film to show glowing edge of chip (after [5]).

Shaw et al. [6] have explained the high temperatures observed at the free chip edges by noting that the edges of the chip deform in plane stress and the central region deforms in plane strain. Since a material subjected to plane stress flows at a lower stress than one subjected to plane strain, the material at the edges flow twice; once in the direction of the shear plane along with the rest of the material subjected to plane strain, and in a direction parallel to the main cutting edge. This double strain at the edges explains the observed higher temperature at the sides of the chip that gives rise to grooving for materials with high strain rate hardening.

Tool Life

19

Venkatesh [7] has published a study in which HSS, WC and cast alloy tools were compared with typical ceramic inserts with regard to wear modes pertaining. It was found that the primary grooves at A in Figure 2-3 were deepest for carbide and those at B were deepest for ceramic tools. It was also found that wear-land wear and crater wear had approximtely equal significance for ceramics and tungsten carbide tools. The wear that occurs on the wear-land on the tool flank is largely due to adhesive and abrasive wear of asperties since the temperature on this surface is normally well below the thermal softening temperature of the work until just before tool failure (when wear rate increases exponentially). The wear-land develops relatively slowly as long as the ratio of real to apparent area of contact (AR/A) on the wear land is a low value. As the wear-land lengthens, the temperature rises and the wear rate increases rapidly when the thermal softening temperature of the work is reached since at this point there is a rapid increase in AR/A. Because it is thermal softening of the work and not of the tool that is responsible for the sudden increase in wear-land wear rate, we should expect the same type of variation of wear-land (w) with time (T) for all types of tool material (Figure 2-5). The initial wear rate is high in Figure 2-5 since AR/A is large until the wearland area reaches a critical value. From A to B, the wear rate is relatively low since AR/A is low. When the wear-land temperature reaches the softening temperature of the work there is a rapid increase in AR/A and hence of wear rate. An interesting paradox associated with this wear-land behavior has been described by Vilenski and Shaw [8]. The temperature on the tool face is normally well above the thermal softening temperature of the chip and hence the ratio An/A for this surface is always high (~1). This accounts for the wear rate on the tool face being at least an order of magnitude greater than that on the tool flank and hence tools can perform successfully with a huge disparity of wear rate on the two surfaces. The type of wear on the tool face leading to a crater is different from that normally occurring on the tool flank. Crater wear is largely due to a combination of chemical degradation of the tool surface and chemical strengthening of the surface of the chip.

20

W

Ceramic Cutting Tools

Softening

Point of work

A

h

8

T Figure 2-5. Variation of wear land, w, with cutting time, T. This is a result of transfer of tool material to the chip during the development of a crater. For a carbide tool-steel workpiece combination, the austenitic surface of the chip has a greater affinity for carbon than the WC grains in the tool and carbon transfers from the tool face to the chip surface weakening the tool and strengthening the chip [9]. For an alumina tool, alumina from the tool combines with iron oxide on the surface of the chip to form a strong spine1 [lo]. The result is the same - the development of a crater. Different ceramics have different degrees of chemical stability and because of this coatings are effective in the control of crater development with tungsten carbide. The coating generally acts as a barrier against chemical and thermal transport across the tool-chip interface. While Al,O, is more chemically stable than WC in contact with hot iron, both WC and Al,O, tools will crater. This is due to the frequency of transport from tool to work being greater for WC while the amount of material per transfer is less. A&O, is a very brittle material and will fracture deeper beneath the tool surface even though the stress beneath the surface is lower. Therefore

Tool Life

21

large amounts of Al,O, are transferred to the chip. However, a WC tool with a thin Al,O, coating gives a lower rate of transport and the thinness of the coating limits the amount of material transferred per event. Thus, Al,O, is an effective anti-crater coating material for WC.

CHIPPING AND GROSS FRACTURE As a cutting tool becomes more refractory, it becomes more brittle and as a consequence brittle (tensile) fracture plays an increasing role in tool life. Ceramic tools are generally more inclined to chipping and fracture than WC or HSS tools and as a consequence there is a greater dispersion in tool life. Ceramic tools are very sensitive to adhesion particularly at low cutting speeds where the strength of the adhering chip material is frequently stronger than the tool material beneath the surface. This results in tool life frequently being greater at low cutting speeds for a tungsten carbide tool than for a ceramic tool even though the reverse is true at higher cutting speeds. Similarly, for the same reason, there is a critical temperature below which HSS gives a longer tool life than WC. Because of the relatively low ductility of ceramic tools, it is particularly important that the finished tool surfaces be completely free of sharp defects of thermal or mechanical origin. Since resistance to brittle fracture is so important to the performance of ceramic tools, there is need for some means of measuring this property. Fracture toughness (K,,) is a property frequently used to measure fracture resistance of a tool material [ 111. This is a quantity that gives the critical combination of crack size and crack opening tensile stress required for a sharp crack to While this property of a tool material propagate spontaneously. can be measured, it appears that a useful sound tool must be free of sharp cracks. Any defects present must correspond to a relatively low stress concentration and the critical event associated with fracture appears to be the initiation of a sharp crack. When this occurs there is sufficient stored energy in the system to cause the sharp crack to immediately propagate clear across the surface.

22

Ceramic Cutting Tools

The condition of slow subcritical crack growth appears to be absent in cutting tools in actual practice. A more meaningful measure of resistance to fracture of brittle tool materials appears to be transverse tensile stress performed on carefully prepared representative surfaces tested in four-point bending. Brittle materials tend to fail when the stress in the material reaches a critical value in tension. All engineering materials contain defects which tend to intensify the nominal mean stress and tensile failure occurs when the intensified tensile stress reaches a critical value [12,13]. The typical defect found in glass and sintered products such as tungsten carbide and ceramic tools is well approximated by a sphere [ 13,141. This results in the nominal unintensified uniaxial tensile stress at fracture being l/3 as great as the nominal unintensified uniaxial fracture stress in compression (Figure 2-6). From this it is evident that not only nominal tensile stress but also compressive stress contributes to brittle (tensile) tool fracture when spherical voids are present in a tool material. Just how the nominal tensile and compressive stresses on a tool combine to give the critical tensile stress for brittle fracture is illustrated in references 15 and 16.

Figure 2-6. Intensified tensile stress on surface of void; a) for uniaxial tension or; b) for uniaxial compression oc.

Tool Life

23

Brittle tool materials are not only sensitive to a built up edge or a thin built up layer attached to the tool but also to a suddenly applied load or an abrupt unloading of the tool at the end of a cut. For this reason, the probability of a tool failing at the end of a cut is far greater than at the beginning of a cut or under steady state cutting conditions [15-171. Three mechanisms that may result in fracture of a brittle tool material at the end of cut have been identified [17,18]: foot formation, differential contraction, stress reversal on sudden unloading. In foot formation, the direction of shear suddenly shifts from the steady state shear plane direction (AB in Figure 2-7) to AD as the end of the workpiece is approached. The reason for this is that the shear strength of the material increases with an increase in normal stress on the shear plane. Since the normal stress is less on AD than on AB, the strength along AD is less than along AB. A sudden shift in the direction of shear from AB to AD at the end of a cut causes a shift of the resultant force on the tool to a position closer to the tool tip with a corresponding increase in fracture probability.

Figure 2-7. Foot formation - AB = steady state shear plane, AD = fracture plane at end of cut.

24

Ceramic Cutting Tools

The second cause of fracture at the end of a cut is associated with a patchy layer of built-up chip material firmly bonded to the tool face. The coefficient of expansion of a ceramic tool is less for a steel work material chip. At the end of a cut, the layer will be hot and will tend to contract to a greater degree upon cooling than the ceramic tool surface, causing a tensile stress to develop across the gap (Figure 2-8).

Figure 2-8. Metal patches on tool face giving rise to tensile stress o, that can initiate tensile crack (dotted). The third cause of failure is stress at the tool tip changing from compressive to tensile due to overshoot of the tool when it is very suddenly unloaded. As a result of these actions, brittle tool materials fail more frequently when suddenly unloaded than when suddenly entering a cut or when cutting under steady-state conditions.

PRESSURE The mean pressure the chip exerts on the tool face varies approximately with the specific cutting energy (u). An increase in mean tool face pressure (or u) has two opposing effects on ceramic tool wear. On the one hand, wear is decreased with increased pressure since fracture is postponed by an increase in hydrostatic stress. On the other hand, an increase in u tends to increase the tendency for a built-up layer to form and this tends to increase the rate of wear for a brittle tool material. Of the two effects, material

Tool Life

25

transfer to the tool face is generally more significant and ceramic tool life is inclined to be most favorable for materials that machine with a low value of specific cutting energy. However, workpiece compatibility also influences the specific cutting energy and is discussed in the following section.

WORKPIECE COMPATIBILITY Some materials have a strong affinity for ceramic tool surfaces and form strong bonds. Two classes of work materials that fall in this category are titanium and aluminum alloys. These materials have a very strong affinity for oxygen or oxide surfaces and tend to form strong bonds with an Al,O, tool surface. In general, these materials give very poor tool life when machined with an Al,O, tool. This is the case for pure aluminum which has a very low specific cutting energy. Gray cast iron has a very low specific cutting energy and chips tend to fracture and hence the pressure on the tool face has a relatively low value due to the low strain associated with chip formation. The graphite flakes responsible for the low specific cutting energy also provide an atmosphere of graphite dust which in turn acts as an effective solid lubricant to decrease adhesion between chip and tool. Because of these two actions, Al,O, tools are often selected for machining gray cast iron.

STRUCTURAL INHOMOGENEITY Once a sharp crack is initiated in a relatively homogeneous brittle material such as glass or a fine grained aluminum oxide, it penetrates deep into the specimen without a change of direction as the energy stored in the system is released. Since the maximum tensile stress is generally near and parallel to the surface, the crack usually runs perpendicular to the surface leading to total destruction. A relatively recent development is to introduce second phase particles that will deflect a newly initiated sharp crack back to the surface. The result is the formation of relatively small wear

26

Ceramic Cutting Tools

particle instead of gross fracture. One method of achieving this is to incorporate 5 to 20 ~01% of a stronger, stiffer and more refractory (higher coefficient of expansion) material into a ceramic matrix. For example, Sic whiskers (=0.5l.tm diameter by =30pm length) are mixed and sintered with fine grained (5pm) AJO,. The whiskers are randomly distributed and are relatively close together. When a sharp crack initiates at a defect in the matrix, it does not go very far before it encounters a stronger SIC whisker and is deflected away from its original path. Another way of redirecting sharp cracks is to use a less refractory second phase particle that extracts energy from the crack tip making it easier to redirect. An example of this approach is to incorporate fine particles of partially-stabilized zirconium oxide (PSZ) into a fine grained Al,O, matrix. Zirconia undergoes a phase transformation on cooling that can be marginally prevented by adding a small amount of yttrium oxide. When such a particle is shocked it will transform instantaneously to its equilibrium state absorbing substantial energy from the surroundings. When a sharp crack initiates in an Al,O, matrix containing PSZ particles, energy is extracted from the crack as it strikes a PSZ particle and is redirected by the particle. The subject of crack deflection in brittle materials is A number of complex and is at present, largely empirical. tribology studies on materials containing a crack-deflecting second phase have been presented in recent years. Reference [ 191 is typical of these studies. In this investigation, it was found that the rate of wear (wear coefficient) was several orders of magnitude less for Al,O, sliding dry on Al,O, in a pin-on-disk test than when silicon carbide whiskers were present.

REFERENCES 1. 2. 3.

Taylor, F. W., Trans ASME Vol 28, pp.31-350 (1907). Shaw, M.C., Thermal Aspects in Manufacturing, pub. by ASME in PED, Vol 30, pp. 133-143 (1988). Opitz, H., Werkstottsch. u. Meschinenbsu, Vol 42, p.210 (1956).

Tool Life 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19.

27

Trent, E. M., J. Iron and Steel Institute, Vol 201 pp.847-855, 923-932 (1963). Shaw, M.C., Metal Cutting Principles, Oxford Clarendon Press (1984). Shaw, M. C., Thurmon, A.L. and Ahlgren, H.J., J. Eng. for Industry, Vol 88, pp.142-146 (1966). Venkotesh, V. C., Proc. of Intersociety Symposium on Machining of Advanced Ceramic Materials and Components, ASME, N.Y., pp.243-248 (1988). Vilenski, D. and Shaw, M.C., Annals of CZRP, Vol 18, pp.623-63 1 (1970). Opitz, H., Internst. Res. in Production Engineering, ASME, N.Y. pp.107-113 (1963). Eiss, N.S. and Fabinisk, R.C., J. Amer. Ceram. Sot., Vol 49, pp.221-226 (1966). Kalish, H.S., Manufacturing Engineering, Vol 89, pp.97-99 (1982) Griffith, A.A., Proc. First Internat. Congress of Applied Mech., Delft, pp.55-62 (1924). Takagi, J. and Shaw, M., J. Eng. for Industry, (Trans. ASME). Vol 105, pp.143-149 (1983). Shaw, M.C. and Avery, J. P., J. of Eng. for Industry (Trans. ASME), Vol 108, pp.222-229 (1986). Sampath, W.S., Lee, Y.M., and Shaw, M.C., J. Eng. for Industry, (Trans. ASME), Vol 106, pp.161-167 (1984). Lee, Y.M., Sampath, W.S. and Shaw, M.C, J. Eng. for Industry, (Trans. ASME), Vol 106, pp.168-170 (1984). Sampath, W.S., Ramaraj, T.C. and Shaw, M.C., Proc. of 12th NSF Manufacturing Systems Research Conference, pub. by ASME, Dearborn, MI, pp.371-373 (1985). Pekelharing, A. J., Annals of CIRP, Vol27-1, pp.5-10 (1978). Yust, C. S., Leitnaker, J.M. and DeVore, C.E., Proc. of the Intl. Conf. on the Wear of Matls, Houston, TX (1987).

3 Selection of Cutting Tool Materials

John D. Christopher Machining Research, Inc. Florence, Kentucky

There are many pathways taken to select a specific cutting tool for a machining process to manufacture a part. Unfortunately, many of these paths are those of least resistance, or more likely, least effort on the part of the decision maker. Without realizing the long range impact on the cost of the machined part or the rate at which parts are produced and delivered, purchasing agents, shop foremen, and manufacturing engineers often indiscriminately choose cutting tools. The choices are driven by many forces including eye-catching advertisements, friendly salesmen, and curiosity. Many companies purchase cutting tools based soley on low bids, with little regard for performance. Nationwide, poor productivity, excessive tool-changing downtime, and unacceptable part quality often result due to the performance of cutting tools purchased with cost as the primary determining factor.

INTRODUCTION Figure 3-l provides a longitudinal perspective (not a cutting speed recommendation) on the development of cutting tool materials. As metallurgical/material technology advanced, the improvement in the performance of cutting tools followed. Advancements in cutting tools depend entirely on improving the chemical composition and/or the manufacturing process of the tool material. Invariably, improvements in the metallurgical quality of

28

Selection of Cutting Tool Materials

29

1300 1200

'UC dXIDES

1100 1000 900 800 UJlTING SPEED FT./MN

LUNG&v1mixRl31

700 600

SIDEjLl500 400 300 X&HSS 200

n

100 0

-

i-m ; i

-a-

:&

SrEEL

b

/

1680 1710 1740 1770 1800 1830

i

1860 1890 1920 1950 1980 2010

YIBROF INlTCU3CTICti

Figure 3-1. Cutting tool development.

a cutting tool material results in longer tool life or more importantly, higher cutting speeds. Since the cutting speed is the dominant influence on the cutting temperature of the machining process, it naturally follows that tools that provide higher speeds have more tolerance for higher temperatures. Figure 3-2 illustrates this relationship. Tool materials with higher hot hardness will permit machining at higher productivity rates due to the higher allowable cutting speeds.

30

Ceramic Cutting Tools

RDCWELL IMRDNESS “C”

ROCKWELL

NllRDNESS "A" 95

65

60

55

50

45 0

400

OCARBON 'XCARBIDE

STEEL

800

1200

TEMPERATURE

(F)

+IiSS

1600

*COBALT

2000

HSS

'XCERAMIC

Figure 3-2. Hot hardness of various cutting tool materials.

Selecting the correct cutting tool material for a specific machining operation is the first step in creating the most effective process plan for manufacturing a part. The cutting tool material is dependent on the work material to be machined and the operation to be performed. Often, there are several possible choices of tool materials that will successfully (but not cost-effectively) produce parts. Additional factors then must be considered and these include:

Selection of Cutting Tool Materials + + + + +

31

machine tool horsepower, speed range, rigidity, productivity demands, tooling budget limitations, machine tool burden rate and labor and overhead rate.

Generally, the higher the combined hourly rate for the machine tool and operator, the greater the demand for higher productivity to reduce the cutting time per part. However, the size, performance capacity, and general condition of the machine tool may limit the productivity available from that particular machine. Whatever the limiting factor or factors may be, the wise process planner or programmer uses a valid cost analysis to determine his choice of either maximum productivity or minimum cost. Decisions made without an economic analysis will likely produce less than the maximum available output at a higher part cost than necessary.

WORK MATERIAL/ALLOY The most important consideration in selecting the correct cutting tool is the work material and its hardness. The material The may be metallic or nonmetallic, ferrous or nonferrous. majority of materials machined in the United States are ferrous materials, carbon, alloy, stainless steels, or cast irons. The cast irons may be gray, ductile, or malleable. There are usually two or three levels of tensile strength within many grades of alloy and stainless steels, as well as the three types of cast iron. Higher tensile strength levels invariably produce a higher hardness and a more difficult to machine material. A very thorough definition of the work material is a valuable aid in making an intelligent selection of the tool material. The purpose of this chapter is to survey the major categories of cutting tool materials with comments concerning unusual properties or limitations of each group, and the normal applications of each group. The various tool materials that will be discussed include:

32

Ceramic Cutting Tools + + + + + + +

high speed steel, uncoated carbides, coated carbides, ceramics, cermets, polycrystalline diamond and polycrystalline cubic boron nitride.

HIGH SPEED STEEL The earliest version of tool material used in machining was high carbon tool steel. This material was generally unalloyed steel and could be heat treated to a hard but shallow case. The addition of various alloying elements, particularly tungsten, chromium, and vanadium, added hardenability to the materials as well as much higher hot hardness. Table 3-l shows the chemical composition of selected high speed steels. These improvements in the chemical Table 3-1.

M-l M-2 M-7 M-4 M-42 T-15

HSS - Chemical Composition.

c

w

MO

Cl-

v

co

0.80 0.85 1.oo 1.30 1.10 1.50

1.50 6.00 1.75 5.50 1.50 12.0

8.00 5.00 8.75 4.50 9.50 --

4.0 4.0 4.0 4.0 3.7 4.0

1.0 2.0 2.0 4.0 1.2 5.0

----8.0 5.0

composition of the steel provided increased performance from higher cutting speeds than those available with the carbon steels. These increases in cutting speed introduced the tern1 “high speed” steel, usually abbreviated HSS. Today’s HSS tools are available in the normal ingot cast version and as the particle metallurgy (PM) (a patented process) version. The ingot cast materials, while capable of doing a satisfactory job in most applications, are limited by the permissible

Selection of Cutting Tool Materials

33

composition and/or heat treatment to achieve higher wear resistance (hardness) also produces a lower toughness and therefore a more brittle HSS. The PM steels, “mechanically” alloyed in the dry or particle condition, do not have the same limitations as those blended in the molten state. The result of this freedom in alloying is a series of high speed steel grades that can be heat treated to higher levels of hardness (68-70 Rc) without a severe reduction in toughness. These tools can provide longer tool life due to increased wear resistance without the risk of chipping or breaking from reduced toughness. As expected, this increased performance carries a higher cost. Generally, the effects of the various alloying elements are predictable. Table 3-2 illustrates the effects of various elements on the performance properties of high speed steels. Higher levels of carbon increase the hardness as they combine with other elements, particularly vanadium. The more popular grades, Ml, M2, M7, and Ml0 have l-2% vanadium. Vanadium carbides which form in

Table 3-2. Effects of Various Elements on the Performance HSS. MO: C: Cr: v: co:

of

Acts as a substitute for W Provides high hardness Increases hardenability Increases wear resistance Increases hot hardness

the microstructure of steels, are a major contributor to the wear resistance of the grade. Grade M4 (4% vanadium) is very wear resistant, compared to the previous four grades. The addition of cobalt (Co) to the chemical composition increases the hot hardness of the steel and elevates it to a “premium” category of higher The most wear resistant grade in the performance and price. premium category is T15, which has 1.5% carbon, 5% vanadium, and 5% cobalt. Figures 3-3 through 3-5 show the relative hot

34

Ceramic Cutting Tools

hardness, toughness, and wear resistance, respectively for nine grades of HSS. Hard coatings can be applied to the surfaces of finish ground HSS tools to improve their performance, particularly in machining ferrous alloys. The physical vapor deposition (PVD) process applies a single coating usually of titanium nitride (TiN). The coating is applied as the vapor solidifies on the tools within a vacuum chamber. Other coatings of titanium carbonitride (TiCN), zirconium nitride (ZrN), and chromium nitride (CrN) are also available. The PVD process operates at a temperature lower than the tempering temperature of the HSS and therefore does not degrade the hardness of the steel. While coating increases the cost of the tool, it has in many instances provided substantial improvements in performance. A typical example is found in tapping, where only a small amount of wear on the tap will produce an undersize and therefore unacceptable thread. The cost of the coating is offset by the increased tap life. However, as the coating thickness is only about 0.0002-0.0003 inches in thickness, it does not “armor coat” the tool, and will not always survive when machining abrasive materials or steels much harder than 40 Rc.

UNCOATED

CARBIDES

This tool material was first developed with tungsten carbide (WC) and cobalt as the binder. This material was satisfactory for machining gray cast iron, which was very common in the 1920’s and 1930’s. As the metallurgy of steel progressed, and its usage increased, the inability of these “straight” grades of carbide to successfully machine steels became obvious. The lack of alloying caused the tools to fail by cratering. The addition of titanium carbide and tantalum carbide to the composition of tungsten carbide and cobalt produced a series of grades that were crater resistant in turning and milling carbon and alloy steels. This improvement in carbide tool material increased the production rate of machining steels to that of cast iron in the 1940’s, 50’s, and early 60’s.

Selection of Cutting Tool Materials

35

Figure 3-3. Relative hot hardness for various HSS tool materials.

Figure 3-4.

Relative toughness

for various HSS tool materials.

Ceramic Cutting Tools

36

120

PERCENT

-

60

-

-

40

20

-

0 T15

H42

Ml0

T5

HZ

T4

HSS GRADE

Figure

Relative 3-5. materials.

wear

resistance

for various

-

-1

I

Ml

HSS

tool

A comparison between HSS and a C6 carbide turning an alloy steel is shown in Figure 3-6. The increase in speed from using carbide is approximately seven times.

Selection of Cutting Tool Materials

60

50

‘: .........i...... __i__

40

TOOL LIFE MINUTES

30

HSS

.........

20

... .; ..

.................

10

0

j.. ...

I0

50

100

150 CUTTING

200

250

300

350

400

450

SPEED-FEET/MINUTE

Figure 3-6. Cutting speed vs. tool life for HSS and carbide tools (300 BHN steel). The relative percentages of carbide to cobalt determine the wear resistance or toughness of the grades. In the United States the Cl to C8 designations are still commonly used by both producers and users of carbide tools. The Cl to C4 categories contain only the WC and Co, and are suitable for roughing applications or interrupted cutting. The higher numbers are harder with more carbide and less binder, providing higher wear resistance, lower toughness, and are more suitable for semifinishing and finishing cuts.

38

Ceramic Cutting Tools

The C5 to C8 grades contain the additional components of TiC and TaC for machining materials that generally produce a continuous (often work hardened) chip formation that causes tool cratering. Tool cratering can occur without severe flank and nose The result of wear that would produce parts out-of-tolerance. severe tool cratering is catastrophic tool failure and perhaps a damaged part. The relationship of hardness and toughness is the same for the grades, C5 to C8 as for Cl to C4. Lower numbered grades are tougher, while the higher numbered grades are harder and more wear resistant. The effect on tool life and cutting speed using various grades of carbide is shown in Figure 3-7. Uncoated carbides are still quite widely used in the machining industry. Virtually all of the carbide tipped tools, drills, reamers, milling cutters, saws, etc. use uncoated carbide grades (Cl-C4). Many machine tools, unable to fully utilize the higherperformance, more expensive coated carbides, due to lack of speed or horsepower, are able to cut with uncoated carbide tools. Materials that are very abrasive or high in hardness, are not ideal applications for coated carbides and are often machined with uncoated carbides. The differences in cast irons can be critical to the correct selection if the cutting tool material is uncoated carbide. Ductile (spheroidal graphite) and malleable (quenched and tempered) irons are machined with “steel cutting grades” of uncoated carbide (C5CS). Gray (flake graphite) cast iron, producing a discontinuous chip, is machined with the Cl-C4 grades of uncoated carbide.

COATED CARBIDES The most common tool wear on uncoated carbides is diffusion-related wear. The temperatures and pressures associated with the normal cutting parameters on ferrous alloys cause the cobalt binder on the surface of the carbide tools to diffuse out of the matrix with the hot chips produced during the cutting process. As the binder diffuses from both the top and side surfaces, the grains of carbide are displaced gradually, leaving wear scars on the flank and nose of the tool and the crater on the rake face.

Selection of Cutting Tool Materials

39

40

25

TOOL LIFE MINUTES

20

15

10

5

0

-...

i200

400

600 CUTTING

SO0

1000

1200

1400

SPEED-FEET/MINUTE

Figure 3-7. Cutting speed vs. tool life for various grades carbide tool materials (200 BHN malleable iron).

of

Attempts were made to reduce this diffusion process, all with limited success, until the development of the chemical vapor deposition (CVD) process. This process deposits various vaporized compounds on the surfaces of the carbide tools in a vacuum chamber. The first successful coating was titanium carbide (TIC). In addition to TIC, titanium nitride (TIN) and aluminum oxide (Al,O,) are now used in various combinations, with TIC serving as the base coat. The familiar gold colored exterior coating is usually TIN, although hafnium nitride (HfN), similar in appearance, is used by some manufacturers.

40

Ceramic Cutting Tools

It was noted by metallographic failure analysis studies on coated carbides that occasionally a damaged region (heat affected zone) at the interface of the carbide and the first coating was caused by the high temperatures associated with the CVD process. Machinability testing confirmed that the presence of this damaged region caused a weakening of the cutting edge manifested in heavy and/or interrupted cutting. The PVD coating process, commonly used on HSS tools, was tested on carbide and found to consistently Subsequent cutting tests with produce a damage free interface. PVD coated tools confirmed that an increase in tool life resulted This work has been publicly over the CVD coated tools. documented at SME cutting tool clinics by the Kennametal Company. A special group of PVD coated carbides are recommended for heavy or interrupted cutting applications. The substrates for the coated carbides are usually not cutting grades such as Cl or CS, but special compositions that are tailored to the use of the coated tool, having high toughness and deformation resistance. The wear resistance of the tool is usually dependent on the coating and not on the substrate. The presence of the coating will often increase the metal removal rate over an uncoated carbide 50% to 150%. Figure 3-8 shows the improved performance of the coated carbide over the uncoated tool when turning gray cast iron. These inserts are available in a wide variety of chip control geometries in all standard insert configurations. There is little doubt that the coated carbides are the closest product to an all-purpose cutting tool material for ferrous alloys. Their success is the reason that coated carbides account for approximately 60% of all sales of indexable inserts.

CERAMIC TOOLS Cold Pressed Alumina Early generations of ceramic tools mnaufactured in the late 1940’s and early 1950’s were primarily cold pressed aluminum oxide (Al,O,). While these tools were chemically inert and had good hot hardness compared to tungsten carbide, they were

Selection of Cutting Tool Materials

90

80

70

60

Tool Life minutes

5.

40

30

-i-

20

10

-i-

0 0

500

1000 Cutting

1500

2000

2500

3000

Speed-feet/minute

Figure 3-8. Performance improvement of coated carbide tools (class 35 gray cast iron).

vs. uncoated

notoriously low in toughness. This deficiency caused the tools to easily chip and break catastrophically, creating a poor image for early ceramic tools. The ceramic tools available today are of consistent high quality, and when correctly applied, are capable of delivering a cost-effective performance on finish cuts (light feed and depth of cut) of low hardness cast iron and medium hardness steels. Hot Pressed Alumina/Tic The development of the hot pressing process (without excessive grain growth) was a major step forward in producing

42

Ceramis Cutting Tools

high quality ceramic tools. This process allowed the addition of TIC to aluminum oxide, producing the HP Al,OJTiC, an excellent all purpose ceramic tool. This grade is available in a wide variety of standard insert configurations at an affordable price (20-25% higher than coated carbides). Although other ceramic materials may be better for specific applications, HP AI,O,/TiC is acceptable for most machining situations where ceramics are applicable. It is an excellent material for turning tool steels as hard as 60-63 Rc, capable of holding diameters to a tolerance off 0.00025” and producing surface finish values of less than 5 micro-inch. This versatile ceramic material also has excellent thermal stability and is capable of cutting dry or with a water base cutting fluid.

Whisker-Reinforced Alumina Silicon carbide (Sic) whiskers added to an A&O, matrix in random orientation, produces a ceramic tool material with very high toughness. This tool material is used in turning nickel-based alloys. These alloys which work harden when machined, cause a notching wear scar, usually at the depth of cut area on the side cutting edge of the tool. Notch wear can lead to the tool nose breaking off the insert, particularly when repeated passes are made at the same depth of cut. The high toughness of the whiskeralumina tool, along with the use of round inserts rather than nose radius style inserts, gives this ceramic material good success for high metal removal rate cuts on nickel base alloys. Several intelligent ramping techniques are recommended for these tools as an alternative to repeated passes at the same depth of cut. Varying the depth of cut minimizes the development of the severe notch at the same location on the cutting edge of the insert. Although the whisker-alumina tool is capable of providing a good performance in a variety of applications, other ceramic materials may be more cost effective. This ceramic tool material costs over twice the price of the hot pressed alumina/titanium carbide.

Selection of Cutting Tool Materials

43

Silicon Nitride Silicon nitride is produced by a variety of processes with different microstructures. Some versions have a binder material, others do not. At least one material is a complete matrix of silicon nitride whiskers. Variations in processing are the most likely explanation for the wide range in performance from one producer to another. While capable of several ceramic applications, S&N, is possibly the most ideal tool material for machining gray cast iron A “good” grade in turning, boring, and face milling operations. of silicon nitride will machine common grades (used in the automotive industry) of gray iron at cutting speeds of 4000-5000 feet per minute (fpm). Most of the silicon nitride tools are capable of continuous machining at 3000 fpm, which is considerably faster than the productivity obtainable with coated carbides. Laboratory tests in face milling class 30 gray cast iron have been performed at 7000 fpm with tool life values up to one hour of cutting time. Rotary tools, end mills and drills, are now manufactured by CNC grinding from solid blanks. These tools have enjoyed selective applications with great success over HSS, carbide, and coated carbide rotary tools of similar geometry. Ceramic Summary The major deficiency associated with the use of ceramic tools in production machining is low toughness. This results in chipping and breakage of the tools rather than wear. To alleviate this problem, several techniques have evolved to strengthen the cutting edge and produce wear rather than chipping or breaking. These include increasing the thickness or nose radii (round inserts have been produced) of the tool that can result in improved performance. However, the most recent and effective improvement in ceramic inserts is in the development of the edge preparation. There are three types of edge preparations that eliminate the perfectly sharp edge where the sides of the insert intersect with the rake face. The earliest technique was the hone, performed carefully by hand with a fine grain diamond hone. This operation

44

Ceramic Cutting Tools

is now automated. A radius is formed at the intersection of the face and side of the insert. The size of the radius can be varied to accommodate the application. The most common edge preparation is the T-land, which is a chamfer ground to a specific angle and width of the land. The angles vary from 10” to 35”, while the width of the land varies from 0.002-0.030 in, and occasionally higher. The most common combination is 20” x 0.004-0.006 in. The width of the land varies somewhat with the feed rate of the operation and is usually wider as the feed increases. The angle of the land is subjective, but generally decreases if the width of the land is very high. Although this technique is very successful in protecting the cutting edge, it is not an exact science. As optimization efforts proceed in the field and the data base increases, a better definition of the exact T-land for a specific material/operation will ultimately emerge. Inserts with ground T-lands can also have a subsequent honing operation which rounds the intersections of the land and the original faces of the insert. This added process eliminates any sharp intersections which may chip within the cutting zone.

TiUTiN

Cermets

These tool materials derive their name from the use of ceramic materials with a metallic binder. Today’s cermets are usually titanium carbide and titanium nitride with a binder material. They are an effective material for machining steels as they are both wear and crater resistant to the continuous chip formation of steels. Cermets are available in an assortment of insert shapes with chip This tool material is control grooves and edge preparations. capable of providing a performance equal to or greater than coated and uncoated carbides on steels in the soft to medium hardness range where other ceramics are usually ineffective. The popularity or acceptance of cermets is not as widespread in the United States as their performance deserves. In Japan, cermets represent about 30% of tool sales, as compared to about 5% in the U.S. There are grades of cermets available which have adequate toughness for milling and interrupted cutting on

Selection of Cutting Tool Materials

45

Inserts are also steels up to approximately 40 Rc hardness. available in positive rake geometry to minimize cutting forces and resultant part deflection. There are some practical guidelines to the application of cermets. These inserts are normally offered with T-lands, usually lighter than those on ceramic tools. Gray cast iron is generally not recommended as silicon nitride is more effective. However, ductile cast iron which cuts more like steel than gray cast is a good application. Since this TiC/TiN material does not have the excellent thermal stability of other materials, it is safer to cut without a fluid and risk intermittent flow which may cause thermal cracking of the insert. The cost benefits of cermets are usually higher productivity through higher cutting speeds and longer tool life. Since cermets are about 20% lower in cost than a coated carbide and exhibit better performance, there is a great untapped potential for TiC/TiN in the machining of steels. Polycrystalline

Diamond and Cubic Boron Nitride

The hardest substances known are 1) natural single crystal diamond, 2) polycrystalline diamond, and 3) cubic boron nitride. Polycrystalline tools are manufactured using extremely high temperatures and pressures. Polycrystalline

Diamond, PCD

The random orientation of the PCD tools corrects one of the major deficiencies of the natural diamond, the possible presence of a cleavage plane within the single crystal. This plane creates a natural failure site and can weaken the tool with a disastrous effect on performance. Therefore, single crystal diamonds used as cutting tools must be correctly oriented by a diamond expert. Single crystal diamond is an excellent special purpose tool for creating super fine finishes on items such as optical components. Usually the diamond is bonded to a standard carbide insert as a single tip on the insert. The insert is then ground (and perhaps polished) to provide a very smooth finish on the diamond.

46

Ceramic Cutting Tools

The PCD tool provides an excellent general purpose tool for machining nonferrous and nonmetallic, abrasive materials. The most common applications for PCD tools include copper and aluminum alloys machined at high cutting speeds. It is standard practice for aluminum automobile wheels to be turned at 8,000 to 10,000 feet per minute with PCD tools. A deficiency of these inserts is the lack of chip control, a problem on soft materials like aluminum. Another common application for PCD tools is machining nonmetallics such as hard fiber reinforced plastics and materials such as granite and marble. Because PCD is much more abrasion resistant than carbides, it provides higher cutting speeds and/or longer tool life than carbides in the same machining operation. PCD inserts cost lo-13 times more than carbide inserts and have only one cutting edge compared to the multiple edges on the carbides, this must be considered when deciding whether or not a PCD tool is cost effective. Often, there is no other choice for producing quality parts on a reasonable production schedule. PCD inserts are available in both positive and negative rake style. The diamond section can often be reground to extend the life of the insert and thus lower the cost per cutting edge. Polycrystalline

Cubic Boron Nitride, CBN

Cubic boron nitride tools are available in both tipped inserts (like PCD) and also in solid CBN inserts. The solid inserts cost about three times as much as the tipped insert, but offer multiple cutting edges and a much tougher cutting material. CBN inserts can be used successfully to turn nickel base alloys, but they have a difficult time competing with the cost of the whisker/alumina insert. A single tipped CBN insert can cost about 3 times as much as the whisker/alumina insert which can have 4-8 as many cutting edges. Therefore, CBN is generally not cost effective for production machining of nickel alloys. A second application for CBN is machining hard ferrous Parts with this hardness are usually alloys (65-68 Rc). manufactured on a grinder rather than machined. However, the metal removal rate in machining may be 10 times as great as the

Selection of Cutting Tool Materials

47

removal rate in grinding. Once again, CBN must compete with lower priced ceramics on steels in the 55-63 Rc range. The alumina/Tic ceramic is about 10% the price of a single tipped CBN insert and can have 4-8 cutting edges. It is wise to consider the entire economic picture of any machining operation in order to justify high performance/high price cutting tool materials such as CBN. A machining operation where solid CBN inserts can outperform other tool materials is face milling steels in the 2 60 Rc range. Solid CBN inserts have incredible toughness and when correctly applied can withstand the most severe interruptions without chipping and breaking.

SUMMARY

The choice of the cutting tool material should always be made with as complete an economic analysis as the situation permits. This analysis should be made after the built-in constraints of machine tool capability (hp and speed), production schedules, and most importantly, part quality are considered. Selecting a specific tool material on the basis of longer tool life when more than one type will provide a satisfactory product is a safe choice when the tool-change time is long and tool life is short. Fewer tool changes reduce non-productive down time. If tool-change time is very short, the selection should be a tool that will increase productivity, reduce cycle time, and ultimately lower direct labor and machine burden on the cost per part.

Aluminum Oxide/Titanium Carbide Composite Cutting Tools

Walter W. Gruss Komet of America, Inc. Schaumburg, IL

Kilian M. Friederich Cerasiv GmbH Plochingen, Germany

INTRODUCTION The first attempts to apply ceramic cutting tools for turning of gray cast irons were made in the early 1930’s. The high hot hardness, compressive strength, wear resistance and chemical inertness of ceramics promised success. However, the difficult manufacturing process of ceramics combined with unsuitable machine tools and lack of experience delayed implementation. Initially, only aluminum oxide ceramics (oxide ceramics) were used, but in the early 1970’s aluminum oxide/titanium carbide composites (carboxide ceramics) were introduced. They provided improved results in finish turning of ferrous metals and turning of hard ferrous metals. Optimization of the composition including the introduction of new sintering technologies resulted in further improvements in this group of cutting tool materials.

COMPOSITION,

MICROSTRUCTURE

AND PROPERTIES

Commercially available cutting tool materials belonging to the group of carboxide ceramics consist of aluminum oxide with additions of 30-40% titanium carbide and/or titanium nitride. The dispersion of these hard particles increases the hardness for

Aluminum OxideRitanium

Carbide Composite Tools

49

temperatures up to 800°C when compared to oxide ceramics (Table 4-1, Figures 4-l & 4-2). Simultaneously, the fracture toughness and bending strength is improved through crack impediment, crack deflection or crack branching caused by the dispersed hard particles. The higher hardness in combination with the higher toughness increases the resistance to abrasive and erosive wear considerably. The lower thermal expansion and higher thermal conductivity of the composite improve the thermal shock resistance and thermal shock cycling capabilities when compared to oxide ceramics. At temperatures exceeding 800°C, the titanium carbide and/or titanium nitride particles oxidize and begin to lose their reinforcing properties. The composite weakens and this phenomenon must be taken into consideration when selecting cutting conditions, such as cutting speed, depth of cut and feedrate. Table 4-1. Comparison of the Physical Properties of Oxide Ceramics with Aluminum Oxide/Titanium Carbide Composites.

Cutting Materials

Oxide Ceramic Al,O, + ZrO,

AI,OJTiC Composite

Hardness (Vickers)

2000

2200

Modulus of Elasticity (k-N/mm2)

390

400

Bending Strength (N/mm2)

350

600

Fracture Toughness (mN/mm2)

4.5

5.4

Coefficient of Thermal Expansion (IO-%‘)

7.5

7.0

Thermal Conductivity (W-m-‘-K-‘)

30

35

50

Ceramic Cutting Tools

Figure 4-1. Hardness vs. temperature of cemented carbide, oxide ceramic and aluminum oxide/titanium carbide composite.

Figure

4-2. Microstructure carbide composite.

of aluminum

oxide -30%

titanium

Aluminum Oxiderritanium Carbide Composite Tools

51

Powders with high purity and fine particle size (generally one micron or smaller) are selected for the manufacture of aluminum oxide/titanium carbide composites. Uniform blending of these components is achieved through dry or wet milling. Organic binders are added to provide sufficient strength for preforming processes. The heat treatment is very critical. The goal is to minimize porosity while maintaining a fine microstructure. The titanium carbide and nitride additives impede the densification through heat treatment. Various processeshave been developed to overcome this difficulty all basedon the simultaneous application of temperature and pressure. Originally, graphite dies at temperatures between 1600°C and 1750°C were used to mechanically densify the material at pressures between 200-350 bar. Graphite limits the maximum allowable pressure and temperature, and rest porosities of up to 1% may occur in the composite (see Figure 4-3). In recent years, hot isostatic pressing methods were introduced operating at pressuresof up to 200 Mpa with inert gas (N2, Ar) as the compacting media [3]. The process requires that the product be hermetically sealed or pre-sintered to 94% minimum density {closed pores only) to prevent penetration by the inert gas. Hot isostatic pressing increases density and reduces porosity, resulting in higher reliability of the composite.

Figure

4-3.

Defect

microsttucture

of A120J30%TiC

composite.

Ceramic Cutting Tools

52

GRADE APPLICATIONS The cutting tool industry classifies cemented carbide grades either by IS0 Standard R513 of the International Organization for Standardization or by the standards of the Joint Industrial Council of the United States. An abbreviated version of this classification method and how it applies to aluminum oxide/titanium carbide cutting tools is shown in Table 3-2. Table 4-2. Grade Classification Charts Showing Aluminum Oxide/Titanium Carbide Composite Cutting Ranges. Steel, Cast Steel, and Stainless 4--

Capability

Steel

Increased Wear Resistance Increased

Range IS0 Classification

PO1 PO5 PlO

I

I

Turning

Class

P30 C6

I

e

Metal, and Non-Metallic

Metal

Increased Wear Resistance Increased Toughness KOl

KO5

C4 I Aluminum oxide Titanium carbide

Turning

_I)

Aluminum oxide Titanium carbide Composites

Cast Iron , Non-Ferrous Capability Range IS0 Classification

P20

P15

c7

C8

Class

Toughness

KlO c3

K15

II) K20 c2

Aluminum Oxide/Titanium Carbide Composite Tools

53

As described, the aluminum oxide/titanium carbide composite grades are usually selected for machining of ferrous metals at high cutting speeds when a high accuracy of dimension and fine finish is required. Interrupted cuts are only recommended with very small chip sections and strong cutting edge designs as provided by round or square inserts with T-lands. In recent years, various industries have started to replace grinding of hardened steel components through turning with carboxide ceramic cutting tools. The economic justification of such a change includes: l

l

l

l

capital investment - CNC lathe versus grinder, perishable tooling - indexable insert tooling versus bonded abrasives, cycle time - turning versus grinding, OSHA/EPA - coolant eliminated with ceramic cutting tools.

The aggregate cost comparison often justifies the shift from grinding to turning. For example, the automotive industry now applies this process to turn hardened ring gears, side gears, axle shafts and similar components. Refer to the case study presented in Figure 4-4. Pinion: Hardened Steel HRc = 62 - 63 = 0.15-0.2 mm = 0.08 mm/rev. = 100-180 rn/min. Tool Designation Insert Designation

= CDJNR 2525 Ml 5 = DNGN 150812T Tool Life = 130 Pieces R. < 18crm

Machining Cost

Production

Tool Changing

Time

Figure 4-4. Case study - automotive

Co.9

pinion production.

54

Ceramic Cutting Took

TOOL

DESIGN

Aluminum oxide/titanium carbide composite cutting tools are exclusively used as indexable inserts. Solid ceramic tools or brazed tools made of this composite are rarely applied. The lower bending strength and toughness of this composite in comparison to cemented carbides has led to the design of toolholders with deeper pockets to accept thicker ceramic inserts. Insert holding systems applicable for ceramic composite cutting tools are similar to those designed for cemented carbides. Top clamping and/or hole clamping is common for regular insert styles, such as rounds, squares, triangles, 80” diamonds and 55” diamonds. However, in rough turning it is practical to use fixed or adjustable chipbreakers to obtain better distribution of the clamping force (Figure 4-5).

Clamping element with thrust plate (for short chip material)

Clamping element with adjustable chip breaker

Figure 4-5. Clamping

Notch Clamping

Center hole clamping

systems with various chipbreaker

types.

Aluminum Oxide/Titanium Carbide Composite Tools

55

Negative rake style inserts are preferable but in situations where low cutting pressure is required, positive rake style inserts are also used. Tool nose radii of 0.4 mm (0.016 inches) or smaller are rarely suitable for carboxide ceramic cutting tools. V-bottom style indexable tools with top clamp are recommended with single ended grooving inserts or round positive rake inserts, applicable for profiling or grooving (Figure 4-6). Edge preparations are another important factor in the performance of aluminum oxide titanium carbide composite cutting tools. The purpose of the edge preparation is to eliminate microchips from previous grinding operations and to strengthen and smooth the cutting edge. Table 4-3 provides information on typical edge preparations.

cut-off

Flat bottom grooving

Full nose radius grooving

Figure 4-6. Various v-bottom style grooving using inserts with top clamps.

Profiling

and profiling

tools

56

Ceramic Cutting Tools

Table 4-3. Typical Edge Preparations. The edge preparation for indexable inserts consists either of a chamfer or a hone or a combination of both. The chamfer is characterized by the width and the angle as shown below, while the hone is identified by the radius Y’. Edge preparation selection is especially critical for ceramic inserts and should be matched to the application for which the Insert IS to be applied. In general larger chamfers with or without hone are recommended for rough turning while smaller chamfers and/or hones are recommended for finish turning.

Process

Rake Angle

W(mm) x a

Finish Turning Finish Turning Roughing Roughing Heavy Roughing Roll Turning

Negative Positive Negative Positive Negative Neutral

0.08 x 25” 0.08 x 20” 0.2 x 20” 0.2 x 25“ 0.3 x 3o” 0.15 x 2V

l

l

l

Additional hone 0.03mm / 0.05mm recommended

MACHINING

RECOMMENDATIONS

The basic machining recommendations are cutting speed, feedrate and depth of cut. The shape and size of the work piece, the material and its hardness, surface finish and dimensional tolerances, machine tool capabilities and production quantities are factors determining machining recommendations. For carboxide ceramic cutting tools, the most critical parameter is the selection of the feedrate. The feedrate determines the surface finish but also may cause breakage of the insert when too high a feedrate is selected.

57

Aluminum Oxide/Titanium Carbide Composite Tools

Table 4-4 provides general guidelines for the selection of feedrate and tool nose radius to achieve a specific surface finish. For practical reasons, selection of the conditions that will achieve 50% of the maximum permissible roughness is recommended. Table 4-4. Peak to Valley Height Depending Corner Radius.

100

25

14.2

on Feedrate and

60

1

9.25-1

40

r=

5.5 -

25

2.9 - i

15

1 E 5 aI2

-210

B

2

5 1.33-q I)

7

5

5

0.93-B

% iti 5 It > 0.53-

3

ii g

0.34-

2

0.16]

1

0.09'

0.6

w

Pm

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

Feedrate (mm/rev.)

Figures 4-7 and 4-8 describe the relationship between feedrate and undeformed chip thickness for straight and round cutting edges. The maximum undeformed chip thickness is limited by the strength and toughness of the cutting edge in relation to the hardness and strength of the work piece materials.

58

Ceramic Cutting Tools

Figure 4-7. Feedrate (f) vs. undeformed cutting edges.

Figure 4-8. Feedrate (f) vs. undeformed cutting edges.

chip thickness for straight

chip thickness

for round

Aluminum Oxide/Titanium Carbide Composite Tools Tables 4-5 and 4-6 present feedrate permissible with various cutting tools when turning gray cast Additional guidelines are provided ferrous metals with aluminum oxide in Tables 4-7 and 4-8.

59

guidelines for the maximum styles of carboxide ceramic iron and medium carbon steel. for the turning of various titanium carbide cutting tools

Table 4-5. Guidelines for Maximum Feedrate vs. Nose Radius and Edge Preparation Permissible with AI,O,/TiC Composite Cutting Tools for Gray Cast Iron and Medium Carbon Steel.

Turning of medium carbon steel (200 BHN) Lead angle .080 8;:” 8d c

0 -3" -5"

0.015 0.022 0.010 0.014

Nose Radius .064 .048 .032

.016

0.020 0.014 0.009 0.013

0.006 0.005 0.004 0.003

0.014 0.011 0.007 0.010

0.010 0.007 0.006 0.005

Edge

Prep

w = 0.008" a=200

Turning of gray cast iron (180 BHN)

Lead angle

Nose .080

8

:z” 0 -5" -3"

.064

Radius .048 .032

0.018 0.016 0.025 0.022

0.012 0.016

0.016 0.011

0.011 0.008

0.014 0.010

0.011 0.008 0.007 0.006

Edge

Prep

.016 0.007 0.006 0.005 0.003

w = 0.008" a=200

60

Ceramic Cutting Tools

Table 4-6. Guidelines for Maximum Feedrate (mm/rev) Permissible with AI,O,/TiC Composite Cutting Tools for Gray Cast Iron and Medium Carbon Steel.

The feedrate “f”depends on the depth of cut and the diameter of the insert. After determining the maximum permissable chip thickness “chip,”from the first table below, select the factor “M” from the second table below to calculate the feedrate by following this formula:

f = chip, x M t Brinell 225 255 285 325 370 400 480 525 580 635 690

Han ess Vickers ShoreC 34 237 266 38 42 48 54 57 67 71 73 80 85

302 345 390 390 513 560 613 674 746

Recommended max. Rockwell

Chip

20

0.35

25 30 35 40 43 50 53 56 59 62

0.33 0.30 0.28 0.25 0.23 0.20 0.18 0.15 0.13 0.10 1

Depth of Cut (mm) 0.30 0.50 1.02 1.52 2.03 3.05 4.06

Fa or “M”for rounl insert: Bnlywith various IC 16 19 1 25 1 32 6 10 I 13 4.0 4.6 5.1 3.6 3.3 2.3 2.8 2.6 3.1 3.6 4.0 2.8 2.2 1.8 2.2 2.6 2.8 1.8 1.4 1.6 2.0 --1.7 1.8 2.1 2.3 1.5 1.4 --1.6 1.8 2.0 1.4 1.5 ___ ___ 1.4 1.5 1.7 --1.3 ___ ___----

I

Aluminum Oxide/Titanium Carbide Composite Tools Table 4-7. Machining Recommendations for Turning Steels with AI,O,/TiC Composite Cutting Tools.

Hardness Material

(BHN)

T I

CuttingSpeed (Si=M-)

61 Various

1

Feedrate

/ /

(IPR)

From

Mediin

To

From

Itiedian

To

Carbon Steels 1000Series

110 170 (5 Rc) 230(20 Rc) 290(31 Rc) 350(38 Rc) 400(43 Rc)

650 500 400 350 250 230

2300 2100 1800 1500 1300 1000

4000 3500 2600 2000 1500 1100

.003 .003 .003 .003 .003 .003

.012 .012 .012 .012 .012 .012

.030 .030 .030 .030 .030 .030

Alloy Steels

110 170 (5 Rc) 230(20 Rc) 290(31 Rc) 350(38 Rc) 400(43 Rc)

600 450 350 300 250 200

2100 1900 1600 1300 1200 800

3800 3200 2400 1800 1400 1100

.003 .003 .003 .003 .003 .003

.012 .012 .012 .012 .012 .012

.030 .030 .030 .030 .030 .030

336(36 Rc) 390(42 Rc) 450(48 Rc) 514(52 Rc) 578(56 Rc) 653(60Rc) 712(64 Rc)

300 250 230 200 175 150 100

800 850 750 660 550 450 400

1200 1100 980 800 650 550 450

---------------

.014 .Oll .008 .007 .006 .005 .004

max max max max max max max

Hardened Steels

-

62

Ceramic Cutting Tools

Table 4-8. Machining Recommendations for Turning Various Cast Irons and Other Materials with AI,O,/TiC Composite Cutting Tools.

l-

Cutting

Hardness Material

(BHN)

Speed

r

From

nhdiar

390 330 300 250 200 170 150 130 100

520 460 390 330 290 260 230 200 160

790 720 660 590 520 460 390 330 260

Malleable Cast Irons

150-200 200 - 250 250 - 300

650 650 350

2300 1650 1100

Nodular Cast Irons

140 190 (10 Rc) 240 (23 Rc)

1000 800 500

lnconel 600, Monel

115 200 (14 Rc) 360 (39 Rc)

lnconel700, Waspalloy Hastelloy, Rene

Chilled Cast Irons

Gray,

Feedrate T

(IPR) Ivlediin

To

___

.Oll .OlO .009 .008 .007 .006 .006 .005 .004

max max max max max max max max max

3906 230(1 150(3

003 003 003

.012 .012 .012

.030 .030 .020

1600 1400 1150

200(1 18oc) 13Oc

003 003 003

.012 .012 .012

.030 .030 .030

600 600 400

900 800 600

1200 1oOc 800

005 004 004

.006 .006 .006

.OlO .008 .008

360 (39 Rc) 450 (47 Rc)

600 400

900 800

1200 1oOc

005 004

.006 .006

.OlO .008

300 (32 Rc) 375 (40 Rc)

400 300

600 500

800 600

003 002

.005 .004

.008 .006

326 370 400 435 480 530 578 630 685

(35 (40 (43 (46 (50 (53 (56 (59 (62

Rc) Rc) Rc) Rc) Rc) Rc) Rc)

Rc) Rc)

-

I

-

To

=rom ___ ___ ___ ___ ___ _-_ ___ ___

-

REFERENCES 1. Evans, A.G., Philos. Mag., 2b p. 1327 (1972). 2. Wiederhom, S.M., J. Mat. Sci., 18 p. 766 (1983). 3. Burden, S.J. et al., Amer. Ceram. Sot. Bull., 67 p. 1003 (1988).

5

Cermet Cutting Tools

Kilian M. Friederich Cerasiv GmbH Plochingen, Germany

Walter W. Gruss Komet of America, Inc. Schaumburg, IL

INTRODUCTION Cermets, which consist of titanium carbonitride as the hard phase and nickel as binder, are finding increased acceptance as cutting tool materials in numerous applications. In Japan, they are more widely applied than uncoated cemented carbides. In North America and Europe, similar developments are expected. The success of cermets is based on their superior wear resistance when machining ferrous and non-ferrous metals over a wide range of cutting speeds. This chapter provides an overview of the development, composition and microstructure of cermets. Properties and grade selection are described and pertinent information necessary for the design and application of cermet cutting tools is presented.

COMPOSITION,

MICROSTRUCTURE

AND PROPERTIES

The term cermet is an acronym derived from the words ceramic and metal, the two major phases of this class of materials. The ceramic phase includes the carbides, nitrides and carbonitrides of titanium, molybdenum, tungsten, tantalum, niobium, vanadium, aluminum and their solid solutions with titanium nitride as the major constituent. The metallic binder phase consists of nickel alloyed with cobalt and constituents of the ceramic phase, 63

64

Ceramic Cutting Tools

depending on their solubility. The first “cemented carbides containing titanium nitride” were introduced by Kieffer et al. in 1969 [l] and 1971 [2]. The incorporation of titanium nitride as a hardness carrier was successfully demonstrated including the improved wettability of the nickel binder through additions of molybdenum. The widely known research work of Rudy [3-51 in the quaternary systems (Ti, MO), (C, N) and (Ti, W), (C, N) lead to a better understanding of the two phase nature of the hard particles and their typical core/rim microstructure resulting from spinodal decomposition. The advantages of these alloys in comparison to conventional cemented carbides were finer grain size, better wear resistance and higher thermodynamic stability, providing higher crater wear resistance and oxidation resistance. Attempts in Japan in the mid-70’s to increase the toughness of cermets were successful and resulted in a technological breakthrough when cermets were applied in milling operations. At the beginning of the 80’s cermets were introduced in Europe and also found increasing acceptance in the United States. Today a large variety of cermets are commercially available and are applied in turning, boring, grooving, threading, and milling of steels, cast irons, and non-ferrous metals. The manufacture of TIC/TIN cermets is carried out with powder metallurgy methods, including liquid phase sintering, similar to those applied for conventional carbides. The cermet microstructure shows (comparable to cemented carbides) hard, wear resistant particles imbedded in a ductile binder phase exhibiting high toughness and resistance to crack propagation. Figures 5-l and 5-2 show the microstructure of two typical cermet grades. When sintering cermets, the TiC/TiN particles react with the nickel, which is in liquid solution when reaching the eutectoid temperature, resulting in densification of the composition through particle rearrangement by capillary forces. Prior to densification, reduction of the oxide layer present on the TIC/TIN particles must be accomplished to assure good wettability by the liquid binder phase. This reduction is only possible with carbon which is not present in TiN. Therefore, carbon must be offered through other sources, such as Mo,C or WC, which are highly soluble in the

Cermet Cutting Tools

65

Figure 5-1. Microstructure

of a typical cermet grade for finishing.

Figure 5-2. Microstructure

of a typical cermet grade for roughing.

66

Ceramic Cutting Tools

liquid binder phase. Since molybdenum atoms are easily soluble in the TiC lattice, [5] the addition of molybdenum carbide is considered to improve wettability. Molybdenum is insoluble in the TIN lattice [6]. The affinity of molybdenum to nitrogen is lower than to carbon, resulting in Ti-N-rich and Mo-C-rich regions in the (Ti,Mo) (C,N) solid solution crystal and leads to the core rim microstructure typical for hard particle cermets [2]. According to Rudy [2,3,5], the core is rich in titanium and nitrogen while the rim is rich in molybdenum and carbon and is easily wetted by the liquid binder phase. In recent material developments, two different directions have been observed: + Hard particle composition can be controlled so that the core exhibits maximum hardness and wear resistance, while the rim facilitates good adhesion to the nickel matrix. Good adhesion forces crack propagation through the ductile binder phase and results in higher toughness. + Precise control of the chemical composition of the binder phase can be achieved with the goal to increase hot hardness and hot strength without causing brittleness. The application of aluminum-doped complex carbides and nitrides has lead to the addition of aluminum to the nickel binder phase resulting in improved high temperature properties. Simultaneously, the crystallization of titanium carbide at the rim of the hard parts provides better adherence to the binder phase [7]. The physical properties of cermets and conventional carbides are compared in Table 5- 1. These properties vary between manufacturers. Major differences between carbides and cermets occur in thermal conductivity, which proportionally affects the temperature shock resistance. The lower temperature shock resistance of cermets limits the application of coolants to finish turning, grooving, and threading.

67

Cermet Cutting Tools Table 5-1. Comparison Carbides with Cermets.

Cutting materials

Density in g/cm3 Hardness (Vickers) Modulus of Elasticity in kN/mm* Bending Strength in N/mm* Compressive Strength in N/mm* Coefficient of thermal expansion

in 10” K-l Thermal Conductivity in W.m-l.K_l

of Physical

Properties

Cemented Carbides

of Cemented

Cermets Roughing Grade

Finishing Grade

PlO

P20

10.8 1600

12.4 1500

6.1 1600

8.1 1600

540

550

500

500

1700

2100

1500

1700

4400

4400

4200

4400

7.9

6.9

7.6

7.9

27

38

12

10

GRADE APPLICATIONS Cutting tool manufacturers classify cermet cutting tool grades in a manner similar to IS0 Standard R5 13 of the International Organization of Standardization or by the standards of the Joint Industrial Council of the United States. Most cermet grades are equally suitable for machining of long and short chipping work piece materials, and ferrous and nonferrous metals, which greatly reduces the variety of grades required for the machining of ferrous metals, nonferrous metals and nonmetals. Cermets are capable of operating over a wide range of cutting speeds. The higher plastic deformation resistance and better chemical inertness of titanium carbonitrides in comparison to tungsten carbides permits higher cutting speeds and reduces edge build-up at lower cutting speeds.

68

Ceramic Cutting Tools

Cermets achieve excellent surface finishes and close size control in turning, grooving, threading, and milling applications. Frequently more expensive grinding operations are replaced through machining with cermet cutting tools. Coolant is applicable, except in rough machining operations, and generally results in longer tool life. Precision machining often requires various tool offset adjustments to maintain the required tolerances. The small, uniform flank wear of cermets minimizes the number of adjustments and reduces the related production down time.

TOOL DESIGN Cermet cutting tools are predominantly used as indexable inserts. Solid cermet cutting tools are now introduced commercially for some specific applications, such as small end mills and boring tools. Cermet indexable inserts are designed and manufactured to conform to IS0 standards, which assures full exchangeability with carbide indexable inserts and permits the use of toolholders and clamping systems originally designed for cemented carbides. Most manufacturers offer inserts for top clamping, hole clamping and screw clamping systems.

TURNING

AND BORING

Typical tool designs for turning and boring are shown in Figures 5-3 and 5-4. Hole clamping is very common for negative rake turning and boring toolholders, while screw clamping is preferable when small cross sections of the tool holding device are required. Cermet indexable inserts are generally applied in the range of small to medium feedrates which requires the design of chip control grooves suitable for such feedrates. Figure 5-5 shows a variety of cermet indexable inserts with chip control. Tool nose radii as small as 0.004 inches are applicable with cermet indexable

Cermet Cutting

Tools

69

70

Ceramic Cutting Tools

Figure 5-4. Typical boring tool for a cermet indexable insert.

inserts due to the high edge strength and plastic defonnation resistance of cennet materials.

GROOVING AND THREADING Figure 5-6 shows a grooving tool with a double ended cennet indexable insert. The double ended inserts are preferable for deep grooves, while triangular inserts "on edge" are more economical for shallow grooving, as required for snap rings and O-rings.

Cermet Cutting Tools

71

72

Ceramic Cutting Tools

Cermet Cutting Tools

73

Figure 5-7 shows threading with cermet indexable inserts in "lay-down!l position. The !llay down!l design is preferable, since it provides higher rigidity and allows selection of the side rake angle close to the helix angle of the thread by exchanging the shim seat.

Figure

5-7. insert.

Threading

with

a "lay-down"

style cennet

indexable

74

Ceramic Cutting Tools

Figure 5-8. Cermet indexable milling tools.

Cermet Cutting Tools

75

Table 5-2. Machining Recommendations for Finish Turning Various Steels with Cermet Cutting Tools.

Hardness

Cation Steels 1000 Series

100 300 (32 Rc) 400 (43 Rc)

Cutting S ed SFM From We&l 1 To 500 1000 I 1200 1000 400 800 200 600 800

Alloy Steels

150 250 (24 Rc) 350 (38 Rc) 400 (43 Rc)

500 450 400 200

950 800 600 450

Hardened Tool Steels

100 200 (14 Rc) 260 (27 Rc) 300 (32 Rc)

300 300 300 300

Stainless Steel 400 Series

150 200 (14 Rc) 350 (38 Rc)

stainless Steel I 200 & 300 Series stainless Steel I Precipitaion Hardened

Material

(BHN)

Nickel Based Alloys -

T

Feedrate

(IPR)

.004 .004 .004

MdkWI .008 .008 .007

.;;4 .012 .OlO

1100 1000 750 600

.004 .004 .004 .004

.008 .008 .007 .007

.014 .012 .OlO .OlO

800 600 550 550

900 800 750 750

.004 .004 .004 .004

.008 .007 .007 .006

.014 .012 .OlO .009

400 300 200

700 600 500

900 800 700

.005 .004 .004

.009 .009 .008

.012 .012 .012

200 (14 Rc) 250 (24 Rc) 350 (38 Rc)

300 250 200

600 550 500

750 700 600

.006 .004 .004

.009 .008 .007

.012 .Ol1 .OlO

175 (6 Rc) 200 (14 Rc) 350 (38 Rc)

300 250 200

600 400 350

650 600 500

.006 .006 .004

.012 .OlO .008

.014 .014 .012

120 200 (14 Rc) 360 (39 Rc)

300 250 150

500 400 300

650 550 400

.004 .004 .004

.006 .006 .006

.009 .009 .009

-

-

-

From

-

76

Ceramic Cutting Tools

Table 5-3. Machining Recommendations for Finish Turning Various Cast Irons and Other Non-Ferrous Materials with Cermet Cutting Tools.

Hardness Material

(BHN)

Gray, Malleable Cast Irons

110 180 (8 Rc) 240 (24 Rc) 320 (34 Rc)

Nodular Cast Irons

Cutting Speed _J‘SFM From

T

Feedrate (IPR) L.-.--L

1 .o’;;

From

L&caan

200 150

900 800 500 450

l&I 1000 700 600

.009 .008 .008 .006

.015 .013 .012 .OlO

.015 .014 .012

140 250 (24 Rc) 320 (34 Rc) 380 (41 Rc)

400 400 200 200

700 500 400 300

1000 800 600 500

.008 .007 .006 .005

.012 .OlO .008 .006

.015 .013 .OlO .008

Powdered Metals

20 32 50 76

1600 1200 1000 600

2000 1800 1500 1400

2200 2000 1800 1700

.003 .003 .002 .002

.006 .006 .006 .004

.008 .008 .008 .008

Non-Ferrous Alloys kee Machining

100 120

I800 I600

2000 1800

3000 .008 2800 .008

.018 .016

.020 .028

Non-Ferrous Alloys Non‘ree Machining

120 140

600 450

700 600

1000 .007 800 .006

.012 .OlO

.015 .015

500 500

IMedian

,

Cermet Cutting Tools

77

Table 5-4. Machining Recommendations for Rough Turning Various Steels with Cermet Cutting Tools.

Material

T

Hardness

W-W

1

Cutting Speed (m/min

T

1

Feedrate (nimlre I

From

Median

100 300 (32 Rc) 400 (43 Rc)

75 45 45

240 190 120

3’6b 0.20 240 0.15 210 0.15

0.35 0.30 0.25

Alloy Steels

150 250 (24 Rc) 350 (38 Rc) 400 (43 Rc)

150 140 120 60

285 240 180 135

335 300 230 180

0.18 0.18 0.15 0.13

0.30 0.30 0.25 0.20

0.35 0.35 0.30 0.30

Hardened Tool Steels

100 200 (14 Rc) 260 (27 Rc) 300 (32 Rc)

90 90 90 90

210 170 150 150

270 240 230 210

0.20 0.18 0.15 0.13

0.30 0.30 0.25 0.20

0.35 0.35 0.30 0.30

Stainless Steel 400 Series

150 200(14Rc) 350 (38 Rc)

105 75 55

170 150 120

230 190 170

0.20 0.15 0.10

0.30 0.28 0.25

0.35 0.33 0.30

200(14Rc) 250 (24 Rc) 350 (38 Rc)

75 75 55

150 135 120

180 170 150

0.15 0.10 0.10

0.25 0.23 0.20

0.30 0.28 0.28

Carbon Steels 1000 Series

Stainless Stee I 200 & 300 Series -

From

IWediin

-

020 0.35 0.20

78

Ceramic Cutting Tools

Tabie 5-5. Machining Recommendations for Rough Turning Various Cast Irons and Other Non-Ferrous Materials with Cermet Cutting Tools.

T Material

Gray,

I

Hardness

1

W-W

1

Cutting Speed (m/mir

From

Mediar

T

1

Feedrate nmkev)

To

From

Median

110 180(8Rc) 240 (24 Rc) 320 (34 Rc)

90 60 60 55

150 150 120 135

255 240 170 170

0.25 0.23 0.20 0.18

0.40 0.35 0.30 0.28

015 0.40 0.38 0.33

Nodular Cast Irons

140 250 (24 Rc) 320 (34 Rc) 380 (41 Rc)

75 60 55 55

120 105 90 90

210 170 150 135

0.28 0.20 0.18 0.15

0.33 0.28 0.25 0.20

0.38 0.33 0.30 0.25

Powdered Metals

20 32 50 76

380 285 240 140

490 490 360 360

535 535 490 490

0.08 0.08 0.05 0.05

0.18 0.18 0.15 0.13

0.23 0.23 0.23 0.23

Non-Ferrous Alloys %eeMachiniq

100 120

445 400

490 425

700 670

0.23 0.23

0.50 0.45

0.55 0.50

Non-Ferrous Alloys Non+ee Machiniq

120 140

145 110

140 145

240 0.20 185 0.18

0.33 0.28

0.43 0.43

Malleable Cast Irons

79

Cermet Cutting Tools Table 5-6. Machining Cermet Cutting Tools.

Recommendations

Hardness Material Carbon Steels 1OOOSenes

(BHN) 100 ZOO(14 250(24 300(32 350(38

Rc) Rc) Rc) Rc)

for Grooving

with

1

CuttingS Pet ed (rn/mil n) From I&diil ” To 120 230 335 105 210 335 90 190 270 80 180 230 75 150 190

0.05 0.05 0.05 0.05 0.05

0.10 0.10 0.10 0.10 0.10

0.13 0.13 0.13 0.13 0.13

Feedrate im/re From

Wdiar

To

Alloy Steels

150 250(24 Rc) 350(38 Rc) 400(43 Rc)

105 90 75 60

240 190 150 120

300 240 210 150

0.05 0.05 0.05 0.05

0.10 0.10 0.10 0.10

0.13 0.13 0.13 0.13

Stainless Steel 400Series

150 200(14 Rc) 350(38 Rc)

90 75 60

170 135 105

210 170 135

0.05 0.05 0.05

0.10 0.10 0.10

0.13 0.13 0.13

stainless Steel 200(14 Rc) 300Series 250(24 Rc) 350(38 Rc)

75 60 45

150 120 105

170 150 135

0.05 0.05 0.05

0.10 0.10 0.10

0.13 0.13 0.13

Tool Steels

150 200(14 Rc) 250(24 Rc) 300(32 Rc)

75 75 60 60

190 180 170 150

230 210 195 180

0.05 0.05 0.05 0.05

0.10 0.10 0.10 0.10

0.13 0.13 0.13 0.13

Gray Cast Irons

110 180 (8 Rc) 250(24 Rc) 320(34 Rc)

90 75 60 60

190 180 150 120

240 210 180 150

0.18 0.18 0.18 0.18

0.15 0.15 0.15 0.15

0.20 0.20 0.20 0.20

Nodular Cast Irons

140 250(24 Rc) 320(34 Rc) 380(41 Rc)

60 45 30 30

150 105 120 75

180 135 120 120

0.18 0.18 0.18 0.18

0.15 0.15 0.15 0.15

0.20 0.20 0.20 0.20

High Temperature Alloys

200(14 Rc) 250(24 Rc) 300(32Rc) 350(38 Rc)

30 15 15 8

40 30 25 15

45 40 30 25

0.05 0.05 0.05 0.05

0.10 0.10 0.10 0.10

0.15 0.15 0.15 0.15

-

-

80

Ceramic Cutting Tools

Cutting speeds are lower than in turning and boring due to the high cutting forces and heat associated with this machining process. Coolant is applicable and often reduces plastic deformation and edge build-up.

THREADING Single point thread turning is closely related to grooving, since a thread is similar to a helical groove. However, threading is more demanding due to the multiple passes required to form the profile and the rapid change in cutting forces at the entry and exit of each cut. Machining recommendations generally consist of cutting speed, infeed method, number of passes and most importantly, the depth of cut on each pass. Figure 5-9 shows two infeed methods recommended for thread turning with cermet cutting tools. The straight infeed with reducing depth of cut is recommended for short

infeed I nfeed

Straight or radial infeed method

Figure

Flank or modified compound infeed method

5-9. Infeed methods for threading.

81

Cermet Cutting Tools

chipping ferrous metals and high temperature alloys. The modified compound infeed is preferable for long chipping ferrous metals since this method results in better chip control. The number of passes and depth of cut per pass are given in Table 5-7. Rasically the depth of cut on the first pass should not exceed 0.25 mm (0.010 inches) and on the last pass should amount to at least 0.025 mm (0.001 inches). Recommendations for the median cutting speed are given in Table 5-8. Cutting speeds in threading are lower than in grooving and turning. Coolant is applicable.

Table 5-7. Threading Infeed Recommendations

Threads per Comer mm radius

Number Total of DOC Passes'

1

2

3

4

5

6

in Millimeters.

7

8

9

10

11

External Threads 1.102 0.944 0.787 0.709 0.630 0.551 0.472

0.1 0.13 0.15 0.18 0.2 0.23 0.25

0.64 0.71 0.84 0.91 1.01 1.17 1.37

5 6 7 8 9 10 11

0.2 0.18 0.15 0.08 0.03 0.2 0.18 0.15 0.1 0.05 0.03 0.2 0.18 0.15 0.15 0.1 0.05 0.03 0.2 0.18 0.15 0.15 0.1 0.08 0.05 0.03 0.2 0.18 0.15 0.13 0.13 0.1 0.08 0.05 0.03 0.23 0.2 0.15 0.15 0.13 0.1 0.1 0.08 0.05 0.03 0.23 0.2 0.18 0.15 0.15 0.13 0.1 0.1 0.08 0.05 0.03

0.78 0.89 0.97 1.18 1.27

7 8 9 10 11

0.18 0.15 0.13 0.13 0.1 0.08 0.03 0.18 0.15 0.15 0.13 0.1 0.1 0.05 0.03 0.18 0.15 0.15 0.13 0.1 0.1 0.08 0.05 0.03 0.2 0.18 0.15 0.13 0.13 0.1 0.08 0.08 0.05 0.03 0.2 0.18 0.18 0.15 0.13 0.13 0.1 0.08 0.08 0.05 0.03

Internal Threads 0.787 0.709 0.630 0.551 0.472

0.08 0.08 0.1 0.1 0.13

Note:Based on fvlarl4135' 80/220 BH with0.05 mm stockleftondiameterforcrest HarderworkpiecesorpoormachineconditionsmayrequireadecreaseinDOCperpass. l

cutting.

82

Ceramic Cutting Tools

Tabie 5-S. Cutting Speeds for Threading Tools.

Material

T

Hardness

with Cermet Cutting

1

Cutting Speed (m/min)

I

W-M From

Median

To

Rc) Rc) Rc) Rc)

60 60 50 45 45

135 120 120 105 90

230 210 185 170 150

Alloy Steels

150 250 (24 Rc) 350(38 Rc) 450 (48 Rc)

60 45 45 40

120 105 90 90

210 180 150 135

Stainless Steel 400 Series

1500 200 (14 Rc) 250 (24 Rc)

40 30 30

90 90 75

150 135 120

Stainless Steel 300 Series

200 (14 Rc) 250 (24 Rc) 350 (38 Rc)

40 30 30

90 90 75

150 135 120

Tool Steels

150 200 (14 Rc) 250 (24 Rc) 300 (32 Rc)

60 60 45 45

120 120 105 105

170 150 135 120

Gray Cast Irons

110 180 (8 Rc) 250 (24 Rc) 320 (34 Rc)

60 60 55 45

135 120 105 90

190 180 175 170

Nodular Cast Irons

140 250 (24 Rc) 320 (34 Rc) 380 (41 Rc)

60 60 45 30

120 90 75 60

150 135 120 105

200 250 300 350

15 8 8 8

25 15 15 15

40 30 25 25

Carbon Steels 1000 Series

High Temperature Alloys

200 250 300 350

150 (14 (24 (32 (38

(14 (24 (32 (38

Rc) Rc) Rc) Rc)

L

Cermet Cutting Tools

83

MILLING The application of cermet cutting tools in milling operations is growing rapidly. Typical applications are face milling, shoulder milling, and slot milling. Cutting speeds for milling are comparable to those recommended for turning. The selection of the feedrate, expressed in inches per tooth, is very critical because chipping, or breakage of the cutting edge may occur when too high a feedrate is chosen. Tables 5-9 and 5-10 contains guidelines for cutting speeds and feedrates for milling of metals. Maximum depth of cut depends on tool nose radius and inscribed circle of the insert. Please refer to manufacturers recommendations for the selection of grades. Table 5-9. Machining Recommendations with Cermet Cutting Tools.

Material

Hardness (BHN)

for Rough

T

Cutting Speed (m/mir

From

Mediir

To

T From

Milling

Feedrate ‘V) i bm/re dediir

1

I

Carbon Steels

100-200 200 - 250

105 90

230 180

270 230

.0.05 0.15 0.05 0.13

0:;8 0.15

Alloy Steels

200 - 250 250 - 350 350 - 400

90 75 55

180 145 105

230 180 150

0.05 0.05 0.05

0.15 0.13 0.1

0.18 0.15 0.13

750 90 200 - 300 60

150 180 130

180 230 150

0.05 0.05 0.05

0.15 0.1 0.1

0.18 0.13 0.13

Stainless Steels

Austenitic Ferritic

I daftensitic

Tool Steels

Cast Steels

150 - 130

130 - 130

175-225 225 - 275 275 - 375

75 70 60

150 135 105

180 170 150

0.05 0.05 0.05

0.15 0.13 0.1

0.18 0.15 0.13

loo- 150 150-200 200 - 250

105 90 60

210 180 120

270 240 180

0.05 0.05 0.05

0.15 0.13 0.13

0.18 0.18 0.15

54

Ceramic Cutting Tools

Table 5-10. Machining Recommendations for Finish Milling with Cermet Cutting Tools.

Material

Hardness @HN)

T

Cutting Speed fSFM )_ From

1

T

Feedrate (IPRI IMediar I From nAediir 1000 l& .002 .004 900 1100 .002 .004 .005 .

I

,

-I-

.ck

I

Carbon Steels

100-200 200 - 250

500 450

Alloy Steels

200 - 250 250 - 350 350 - 400

400 300 250

800 600 450

1000 .002 750 .002 500 .002

.004 .004 .003

.005 .005 .004

- 180 250

300 200

500 650 450

600 800 600

.002 .002 .002

.004 .004 .003

.005 .005 .004

Stainless Steels

Austenitic so

Ferritic 130- 180 I dartensitic

200 - 300

Tool Steels

175-225 225 - 275 275 - 375

500 400 250

loo0 850 500

1200 .002 1000 .002 600 .002

.004 .004 .004

.005 .005 .005

Cast Steels

100 - 150 150-200 200 - 250

500 400 250

1000 850 500

1200 .002 1000 .002 600 .002

.004 ,004 .004

.005 .005 .005

Gray Cast Irons

100 250

500 400

1000 800

1200 .002 900 .002

,004 ,004

.005 .005

Maleable Cast Irons

150 250

500 400

1000 800

1200 .002 1000 .002

,004 ,004

.005 .005

Nodular Cast Irons

150 250

200 150

400 300

500 400

004 004

.005 .005

-

.002 .002

Cermet Cutting Tools

85

REFERENCES 1. Kieffer, R., Ettmayer, P. and Freudhofmeier, M., Metal 25, 1335 (1971) 2. Rudy, E., J. Less-Common Met., 33, 43 (1973). 3. Rudy, E., Worchester, S. and Elkington, W., Plansee Seminar Beitrag, Nr. 30, Proceedings, Vol II. (1971). 4. U.S. Patent, # 397-165 (1976). 5. Rudy, E., Report, AFML-TR-65-2, Part V, Wright Patterson AFB, Ohio (1969). 6. Lengenauer, W. Ettmayer, P., Proc. 3rd Int. Conf. on the Science of Hard Materials, Nassau, Bahamas (1987). 7. Sridharan, S. Nowotny, H. and Wyne, F., Monatshefte Chemie, 114. 127 (1983).

Alumina-Silicon Carbide Whisker Composite Tools

Choll K. Jun and Keith H. Smith Greenleaf Corporation Saegertown, Pennsylvania

INTRODUCTION Aluminum oxide (alumina) has been used successfully as a cutting tool material for many years in many high speed machining applications, primarily due to its superior hardness and chemical However, the use of alumina was stability at high temperatures. limited because of its low resistance to fracture. As a result, many years of research effort have gone into investigating toughening mechanisms. Increasing world-wide interest in ceramics has produced several significant advances in two major fields of toughened alumina composites: + zirconia transformation + whisker/fiber

toughening,

reinforced

and

toughening.

Cutting tools incorporating fracture toughness improvement from zirconia transformation toughening are widely used in industry today [l-4]. Reinforcement of alumina with single crystal silicon carbide whiskers (Figure 6-1) is one of the most recent developments. These composites contain up to approximately 45 ~01% whiskers, depending on the composition of the matrix. Typically, the whiskers contain 13or a mixture of a and B phases of silicon carbide. Depending on the supplier, whisker dimensions

86

Alumina-Silicon Carbide Wh~ker Composite Tools

87

Figure 6.1. SEM micrograph of Silar-SC-9 whiskers (bar = 10~m) (adapted from ref. 9).

range from 0.05 to 1.0 ~m in diameter and 5 to 125~m in length [ 6] .In various investigations [ 5,7-11] the fracture toughness of AI2O3-SiC(w)composites was found to be at least double that of non-whisker reinforced alumina. However, the whiskers retard densification during sintering, requiring hot pressing of compositions with whisker loadings greater than 10-15 vol% [12]. As a result, most whisker-reinforced cutting tools have anisotropic properties due to the preferred orientation of the whiskers [7] . Superalloys are very difficult materials to machine because they workharden rapidly during metal cutting and maintain their strengths at high temperatures. As a result, when using cemented carbide inserts, it is impossible to achieve the same levels of productivity as expected during machining of other classes of metals. The introduction of aluminum oxide-titanium carbide ceramic inserts provided the first substantial gains in superalloy

88

Ceramic Cutting Tools

cutting speeds, up from 50-200 SFPM for uncoated cemented carbides to 400-800 SFPM [13]. This was possible due to the significantly greater hardness and strength of Al,O,-TIC at elevated The temperatures as opposed to cemented carbide [ 141. development of silicon nitride-based ceramics, with even greater hot hardness and improved fracture toughness, brought further improvements [14]. S&N, and SiAlON inserts can result in longer tool lives when compared with A&O,-TIC under the same metal cutting conditions, and are capable of somewhat greater speeds and feeds, particularly during roughing applications [ 15- 181. However, it was the commercialization of Al,O,-SIC,,, cutting tools (Table 61) in 1985 that resulted in an increase of superalloy machining speeds by an order of magnitude, up to 2500 SFPM [ 191.

Table 6-1.

Commercial

Whisker Reinforced

Cutting Tools.

Introduction

Tradename

Manufacturer

1985 [37]

WG-300

Greenleaf

1986 [37]

CerMax 490

Carboloy

1986 [37]

Kyon 2500

Kennamctal

1987 [38]

Reliant

High Velocity

1988 [39]

QlO

GTE Valenite

1988 [39]

KF2100

KyoceraFeldmuhle

1988 [40]

CC670

Sandvik Coromant

HP

Alumina-Silicon Carbide Whisker Composite Tools

89

Application of these whisker-reinforced cutting tools has focused primarily on the machining of nickel-based superalloys (ie., Inconel 7 18l, Incoloy 901 i, and Waspaloy’), in response to the significant market for aerospace parts produced from these alloys.

TOUGHENING PROPERTIES

MECHANISMS

AND MECHANICAL

Two toughening mechanisms for Sic whisker reinforced alumina composites have been theorized, crack deflection and whisker pullout [4,9,20-251. Crack Deflection A crack deflection model was proposed and well studied by Faber and Evans [20,21]. This model describes the effectiveness of second phase particles (in the form of rods) in increasing fracture toughness. The model is based on an analysis bf the tilt and twist of the crack front between particles, which determines the deflection-induced reduction in the crack driving force. Crack deflection can result in significant toughening in composites where there are strong crack/microstructure interactions. The degree of thermal expansion mismatch between the whisker and the matrix determines the extent of the interaction. In the case of the SIC whisker reinforced alumina composites, the thermal expansion coefficient of SIC (4.7 x 10-6/oC) is less than that of Al,O, (8.6 x 10m6PC) by a factor of approximately two. Because of the thermal expansion difference described above, hoop tension and radial compression exists in the matrix while the whisker is under radial compression. Whereas the whisker is in axial compression while the matrix is in axial tension. Since cracks propagate perpendicular to tensile stresses, cracks should be attracted to the SIC whiskers and propagate parallel or at right angles to them. This model has

’ “Inconel” and “Incoloy” are trade names of Into Alloys International, Inc. “Waspaloy” is a tradename of Precision Rings, Inc., of Indianapolis.

90

Ceramic Cutting Tools

been widely accepted to explain the toughening of ceramics and ceramic composites. Liu et al. [22] derived a crack deflection model for two dimensional randomly arranged rods by making an appropriate modification to the model of Faber and Evans. This model predicts more effective toughening in certain directions than that for three dimensionally arranged rods (whiskers). It suggests that toughening is not only a function of the length-to-diameter ratio and the volume fraction of the whiskers, but is also a function of the orientation between the whiskers and the cracks. When the crack plane and the direction of crack propagation were normal to the whisker plane, the toughness was increased approximately seven times and the K,, was increased over two times. However, most of the experimental toughness data available appear to be lower than the theoretical prediction. One reason may be that crack deflection is limited by strong whisker-matrix bonds and by defects in the whiskers. Optimal toughening of whisker reinforced alumina composites is expected to be achieved by flawless whiskers and optimal conditions in the whisker-alumina interface. Whisker Pullout and Bridging The whisker pullout mechanism requires whiskers with high transverse fracture toughness relative to the interfacial fracture toughness so that failure may occur along the whisker/matrix interface. Toughening results from the additional work required to pull the whisker out from the matrix. The stress transferred to the whisker must be less than its fracture strength, and the interfacial shear stress generated must be greater than the shear resistance of the whisker/matrix interface. The shear resistance is controlled by the degree of bonding. For Al,O,-SIC,,, composites, inter-facial compressive stresses increase the effective shear resistance of the whisker/matrix interface. Homeny et al. [23] estimated the critical whisker length for the pullout mechanism to be 2.6pm for the silicon carbide whisker reinforced alumina. The factors that result in whisker pullout may lead to whisker bridging. This related mechanism occurs when the whisker remains intact and bridges the crack surfaces in the wake region behind the propagating crack tip. Bridging requires modest interfacial strengths in order to transfer

Alumina-Silicon Carbide Whisker Composite Tools

91

the load to the whisker and high whisker tensile strengths to sustain the applied stress within the wake. Becher et al. 1241 calculated a bridging zone length on the order of 3 to 6pm for an alumina-20 volume% Sic whisker composite, which was found to be consistent with TEM observations. Al,O, - Sic,,, composites have been studied by many investigators. In general, it has been found that the fracture toughness increases as the whisker content increases, while the hardness only increases slightly. This behavior occurs until the whisker loading is so high that full densification is unobtainable. Some of the more significant papers on whisker composite processing and properties are reviewed below. Wei [5] claimed in his patent that the fracture toughness and fracture strength of polycrystalline alumina were significantly improved up to 9 MPaem/‘” and 899 MPa, respectively, by 20 ~01% Sic whisker reinforcement. The whiskers were claimed as having a monocrystalline structure, diameters of approximately 0.6pm, lengths from 10 to 80l_tm, and had essentially unidirectional orientation in the alumina matrix. Becher and Wei [7] and Wei and Becher [8] achieved fracture toughness values approaching 9 MPa*m”2 and fracture strengths almost to 800 MPa. It was reported that toughening was directionally dependent because of preferred whisker orientation. Vaughn et al. [9] found that the type of whisker used had a profound effect on the fracture toughness and work of fracture values (Table 6-2), and that small differences in whisker surface chemistry or morphology could be responsible for the effect. The enhanced properties were attributed to whisker bridging and pullout mechanisms. Jun and Exner [lo] and Exner et al. [ 1 l] investigated the effects of zirconia toughening on the mechanical properties of Sic whisker reinforced alumina composite cutting tools. They found that fracture toughness was improved by both increasing Sic whisker and zirconia additions (Figure 6-2). Hardness values increased with whisker additions but decreased with zirconia additions (Figure 6-3). The best combination of hardness and toughness was obtained with combined additions of whiskers and zirconia.

Table 6-2. Mechanical

Properties of Polycrystalline

A&O, and SC Whisker/A&O,

Matrix Composites.

Property

Alumina (1500”c)

Alumina (1650°C)

Alumina (1900°C)

Composite (Silar-SC- 1)

Composite (Tateho-SCW- 1-S)

Young’s Modulus

371

380

375

375

393

(MPa)

456 &40

385 +18

253 f8

641 f34

606 +146

Fracture Toughness

3.3

5.0

3.7

4.6

__

MPa-m L’2)

f0.2

H.2

+o. 1

+0.2

Work of Fracture

10

20

39

67

@Pa) Fracture Strength

(J/m21

21

93

Alumina-Silicon Carbide Whisker Composite Tools a.6 a.4 a.2 a 7.8 7.6 7.4 7.2

6.a 6.6 6.4 6.2 6 5.8 5.6 5.4

I

0 0%

0

20 +

7.5x

VOLUME 0 VOLUME

40 PCT Sic 15%

PCT

.?ROL

WHISKERS D IN

22.5%

x

30X

,!ATRIX

Figure 6-2. K,, vs. Sic whisker content (adapted from ref. 11j.

la

16

12 0 0

0%

40

20 +

7.57.

VOLUME 0 VOLUME

PCT 5iC 15% PCT

z”02

WHISKERS A IN

22.5%

x

307.

MATRIX

Figure 6-3. Hardness vs. SIC whisker content (adapted from ref. 11).

94

Ceramic Cutting Tools

Iio et al. 1261 found that the fracture toughness of alumina improved remarkably with increasing whisker content (Figure 6-4), up to 40 wt%, while the bend strength was maximized at 30 wt% (Figure 6-5). However, the composite density dropped with whisker contents greater than 20% (Figure 6-6). Toughness gains were strongly dependent on the composite’s microstructure, especially the distribution of Sic whiskers, rather than the grain size of the A&O, matrix.

8 6

A

v

”

22

4

0

0

v 8

*/ 0 I 10 Whisker

I 20 Content

I 30

I 40

(wt %)

Figure 6-4. Fracture toughness data (0) and (A) were measured using indentation microfracture methods or by the chevron-notch method, respectively (adapted from ref. 26).

0

10 Whisker

Figure 6-5.

20 Content

30

40

(wt %)

Bend strength vs. whisker content (adapted from ref. 26).

Alumina-Silicon Carbide Whisker Composite Tools

95

100 7 2 V 2

o-o-

98-

;3

0

0

0

0

185O’C H. P. 1900’C H.P.

10 Whisker

20 Content

30

40

(wt %)

Figure 6-6. Density vs. whisker content (adapted from ref. 26).

Homcny et al. [27] determined that the surface chemistry of the Sic whiskers had a major impact of the fracture toughness (Tables 6-3 and 6-4). The nature of the surface species appeared to affect the whisker/matrix inter-facial bonding and thus the extent of the crack/microstructure interactions. The presence of silicon oxycarbides and free carbon and/or hydrocarbons were associated with higher toughness values. Becher and Tiegs [28] reported the temperature dependence of fracture toughness. The toughness of alumina reinforced with 20 ~01% Sic whiskers was maintained at temperatures up to 1100°C in air (Figure 6-7. The fracture strength values decreased slowly with increasing temperature to 1100°C (Figure 6-Q while the weight gain rate increased with tcmpcraturc (Figure 6-9). The mechanical behavior temperature dependence was shown to be related to oxidation reactions in these composites, with creep phenomena contributing to the loss in strength and increased toughness above 1100°C. The purity and uniformity of the starting materials also had an affect on the properties.

APPLICATION

OF CUTTING

TOOLS

Successful application of whisker reinforced cutting tools begins with proper insert geometry selection, primarily the shape and the edge preparation of the insert. The strongest shape with the greatest lead angle

Table 6-3. Mechanical Properties of the 30 ~01% SIC Whisker/Al,O, Matrix Composite. (adapted from ref. 27).

Whisker/treatment

Theoretical density (%)

Fracture Strength (MPa)

Fracture Toughness (MPa*m”)

A1203*

98.7

510 k 14

4.1 IL 0.3

As-rec’d Silar-SC-9 Tateho-SCW- 1-S

100.0 99.7

641 + 34 606 k 146

8.7 f 0.2 4.6 k 0.3

Air Silar-SC-9 Tateho-SCW-1-S

99.4

513 * 30

4.9 z!I0.2

4% H, in Ar Silar-SC-9 Tateho-SCW-1-S

98.9 99.4

503 IL 31 466 I!Z33

7.4 k0.3 6.3 k 0.2

* Hot-pressing

parameters:

1500°C, 10 min., 33 MPa.

Table 6.4. Relationship Between Whisker Surface Chemistry and Fracture Toughness. (adapted from ref 27).

Whisker/treatment

Surface Species

Fracture Toughness (MPa*m”2)

Silar-SC-9 (as received)

Sic, SiO,C,, SiO,, C or (CH,),

8.7

Silar-SC-9 (10% H, in Ar)

Sic, C or (CH,),

7.4

Tateho-SCW-1-S (10% H, in Ar)

Sic, C or (CH,),

6.3

Silar-SC-9 (4% H, in NJ

Sic, C or (CH,),, SiO,N,

6.0

Sic, C or (CH,),, SiO,N,

5.3

Tateho-SCW- 1-S (air)

Sic, SiO,, C or (CHJ,

4.9

Tatcho-SCW-1-S (as rcceivcd)

Sic, C or (CH,),

4.6

Tateho-SCW-1-S (4% H, in N2)

1

9s

Ceramic Cutting Tools

, CREEP DAMAGE REGION

L 2 Y

7

ALUMINA

- 20 ~01% Sic,

AIR

t

0

150

300

450

600

750

900

TEMPERATURE

1050 1200

1350 1500

I’CI

Figure 6-7. Fracture toughness of 20 ~01% SIC whisker-alumina composite in air remains constant to at least 1000°C. Apparent toughness increases were obscrvcd at 1300°C and above, but these are associated with creep crack nucleation and growth (adapted from ref. 28).

ALUMINA

20 WI% SIC,

4 POINT

I

0 0

I 300

FLEXURE.

I

I 600

AIR

I

I

I

900

TEMPERATURE

I 1200

I 1500

1°C)

Figure 6-8. High fracture strengths are obtained for the 20 ~017~ Sic-whisker alumina composites to temperatures approaching 1100°C in air. Above 1 lOO”C, there is a significant loss in strength. The two plots are representative of the temperature dependent fracture strengths for composites fabricated by different processes. The increased strengths illustrated by the upper curve are obtained by improved techniques to clean, disperse and separate the whiskers from particulate matter present in the as-rccci\:cd whiskers (adapted from ref. 28).

Alumina-Silicon Carbide Whisker Composite Tools

99

q--l--A

/

10-7

’

500

I

I 700

I

I

I

900 TEMPERATURE

I

I 1100

I 1300

I

I 1500

(‘C)

Figure 6-9. Rate of weight gained for 20 ~015% SIC whiskeralumina composite in air increases with increasing temperature. Higher rates obtained with the alumina-based composite B (curve B) are attributed to the higher impurity content of the alumina starting powder (adapted from ref. 28).

and nose radius allowable must be chosen. This will result in the widest possible insert nose and the greatest length of contact between the insert and workpiece, maximizing the strength of the tool. Round inserts are popular for these reasons. Preparation of the insert edge is also critical. A sharp edge is prone to crack initiation. Therefore, a T-landed or honed edge is commonplace. A T-land approximately 0.003 in. by 20” is recommended by most suppliers as a good general purpose edge preparation. Edges with increases in the width and/or angle of the chamfer, or with a hone and T-land combination, are offered for more demanding applications.

100

Ceramic Cutting Tools

A Grooving

Figure

6-10.

Standard

edge

preparations.

The predominant failure modes for A12O3-SiC(w) tools are depth-of-cut notching (DOCN), flank wear, edge chippage, flaking, and fracture, with DOCN being the most predominant when the inserts are properly applied. Flaking occurs along the plane parallel to the rake of the insert since this is the weakest direction (resulting from the preferred orientation of the whiskers). Depth of cut notching is a phenomenon that results in extreme notching at the point where the tool contacts the outside surface of the workpiece. It is thought to be caused by seizure and pullout of tool material by the chip. This process is accelerated when surface

Alumina-Silicon Carbide Whisker Composite Tools

101

scale is present during roughing operations. Notching progresses to a point where the insert nose is weakened and fractures. Examples of used inserts with some of these failure modes are shown in Figure 6-l 1. Several steps can be taken to minimize the effect of DOCN, namely pre-chamfering the entry of the workpiece, taking ramped or varied depths of cuts to avoid concentrating the notching effect on a particular spot on the insert, and by avoiding situations where diamond or square shaped inserts are being notched on both sides of the nose. With the advent of whisker reinforced ceramic materials (of which Greenleaf WG-300 is a very good example) the high melting point (in excess of 2000°C) combined with the toughness and strength at high temperatures has led to the routine application of a cutting tool technology unknown in the past. The principle involved is one of maintaining a feed and speed combination which will generate a temperature ahead of the tool high enough to effectively decrease the forces associated with the shear formation, in effect to plasticize the material, to greatly facilitate its displacement. The most dramatic use of this technology has been in nickel base alloys which are among the most difficult materials to machine with standard tungsten carbide cutting tools. (Speeds are limited with carbide to the region of 150 surface feet per minute.) It has been shown that most of the heat generated in chip formation occurs in the shear zone immediately ahead of the tool (see Figure 6- 12). Some heat comes from the friction of the chip flowing over the top surface of the tool and a small amount from the flank contact with the surface of the part. The entire subject of heat generation in the metal removal process has been regarded as detrimental. Heat related failure of the cutting tool and excessive heat in the part causing distortion and poor size control have led to great emphasis on coolant application systems. It is clear, as a fundamental materials property question, hot metals are more readily displaced than cold metals. The concept of setting up a machining process to purposely generate a high degree of heat is alien to most and requires a careful educational and introductory program to be successful.

102

Ceramic Cutting Tools

Alumina-Silicon Carbide Wh~ker Comp~ite Tools

103

Approximate Heat Generation 1 75% 220% 35%

Approximate Heat Dissipation 4 5 6

Figure

6-12.

000/0 100/0 100/0

Heat dissipation

in ceramic

machining.

It must be remembered the feed rate as well as speedeffects the temperature. It is a common misunderstanding to think only in terms of speed versus heat. The increase of feed rate, for example, generatesa thicker chip which is then able to dissipate more of the heat being generated and, therefore, lowers the temperature in the shear zone. The opposite is also true. This means that there is a range of feeds and speeds at a given hardness value which will result in the desired temperature range. If the speed is reduced from some recommended value, then a similar percentage reduction in feed rate will return the operation to the temperature range (Figure 6-13). Extensive empirical data analysesshows that by regulating the feed and speed to produce a temperature in the area of 1200°C immediately ahead of the tool, and by utilizing a whisker reinforced ceramic insert, metal removal rates in nickel base alloys can be increased up lOX those possible with carbide tools. For example, on Inconel 718 at a hardness of 40 Rc, the nomogram speed is 1000 SFM and the

Ceramic Cutting Tools

104

Meters 1050

i. 8.0 "'c

875

Feet Starting point based on Insert RNGN-45 (RNGN-120800)

D.O.C.=.125"(3,175 mm)

700

8~ ..

Forging Scale Condition

~'-

525 350 175

c 4) .-o

MM 0,064

'tU:; a:o

0,127 "M~ 11... 4)

11.

Figure

6-13.

Example : 718 Inconel47

RC

700 SFM (213 m/min.) .00751PR

(0.191

mm/rev)

0,191

WG-300 machining recommendations.

feed rate is 0.00811per revolution. If for some reason we can only run at 500 SFM, then a reduction of feed to 0.004II per revolution will return the operation to the correct temperature range. The use of coolants will control the heat in the chips and stabilize the part piece without having any effect whatsoever on the heat in the shear zone. Plentiful coolant application is still appropriate. Selection of the strongest geometry, corner and edge condition are primary considerations. The use of the strongest shapewith the largest allowable radius and lead angle will give the most predictable results (Figure 6-14). Round inserts playa leading role in whisker reinforced ceramic application. This is not only becausea round insert is the strongest shape. A round insert has an advantage over all other shapesrelating to the depth-of-cut at which it is applied. A round insert has a built in ability to be

Alumina-Silicon Carbkle Wh~ker Composite Tools

105

100 90 80 ~

70

.c a c 4) ...

0)

40 35

10 VNGN VPGN

DNGN TNGN CNGN SNGN CNGN RNGN DPGN TPGN CPGN SPGN CPGN RCGN RPGN

Figure 6-14. Strength comparison of ceramic inserts. In declining order of comer strength, the strongests inserts are round, 100° diamond, sqaure, 80° diamond, triangle, 55° diamond and 35° diamond.

applied at various effective lead angles, dependent upon the depthof-cut. As depth increases,the lead angle decreaseand vice-versa. This feature is of paramount importance in the selection of tools for machining work hardened materials. This includes all nickel base alloys common in the jet engine industry. With work hardened materials, the hard surface layer results in rapid notch formation in the cutting tool. The speed of the notch formation is relative to the lead angle of the tool as well as the physical properties of the tool. As the lead angle increases,the force acting against the edge of the tool decreaseswhile the total load is spread over a longer area of the cutting edge. A round insert applied at light depths-of-cut provides the longest life (Figures 6-15, 6-16). In addition, nothing is wasted since a round insert only needs to be indexed by the amount it is consumed, unlike a straight edged tool which can only be turned to a new corner. Therefore, round inserts are the most economical.

106

Ceramic Cutting Tools

100% Chip

Figure

6-15.

Lead angle effect,

Thickness

round

vs. straight

edged inserts.

Alumina-Silicon Carbide Wh~ker Composite Tools

Figure 6-16. Illustration direction of cut.

107

showing direction of forces in relation to

The exception to this rule is in the machining of very thin sections where the resultant forces may cause part deflection. In this case, a square or diamond shape would be more appropriate. Nickel alloys cannot be machined in the finishing operation using honed edges. The use of hones permits the ductile nickel phase to be swept past the tool flank and pressure welded to the surface in small hair-like fragments. This is referred to as "smearing" (see Figure 6-17). For finishing operations a chamfer or T -land is recommended. A 'TI' edge which is approximately 0.002" wide X 20° will reinforce the edge and still permit finishing without smearing. Heavier lands such as T2 or T2A which is 0.006" X 20° in addition to a hone may be used for roughing (Figure 6-18). The techniques for ramping and pre-chamfering have come to the forefront with the expanded use of whisker reinforced materials. Pre-chamfering ensures minimum part run-out and a progressive entry and exit from the part to conserve the cutting edge from high shock loads. Ramping is by far the most significant method of extending tool life by eliminating notch formation. In the ramping technique, parallel cuts are avoided by

108

Ceramic Cutting Tools

Figure 6-17. Illustration of the "smearing" that results from using honed edges on nickel alloys.

of cut

Direction of cut

I /

/ Directior of cut

A.

B.lncorrect

Incorrect (causes

notching)

(causes

notching)

Figure 6-18. Chamfering techniques.

c. Correct (feed at angle to chamfer)

90°

Alumina-Silicon Carbide Wh~ker Composite Tools

109

first cutting a sloping or 'ramped' surface and then machining off the ramp with a straight cut (Figure 6-19). N umerous variations of this basic method are used. This includes sine waves, parabolic stock allowances in corners and saw tooth forms.

Figure

6-19.

Illustration

oframping-negative

inserts to extend tool

life.

REFERENCES I.

3.

4.

Evans, A.G and Faber, K. T., J. Amer. Ceram. Sac., 64 [7], 394-8 (1981). Faber, K.T., Toughening of Ceramic Materials by Crack Deflection Processes, Ph.D. Dissertation, University of California, Berkeley. Coyle, T.W., Transformation Toughening and Martensitic Transformation in ZrO2' Sc.D. Thesis, MIT, Cambridge. Rice,R.W., Ceram. Eng. Sci. Prad., 6 [7-8],589-607 (1985).

110 5.

6.

7. 8. 9. 10.

11. 12. 13. 14.

15. 16.

17. 18. 19.

Ceramic Cutting Tools Wei, G.C., Silicon Carbide Whisker Reinforced Ceramic Composites and Method for Making Same, U.S. Patent No. 4,543,345. Karasek, K.R., Bradley, S-A., Donner, J.T., Yeh, H.C., Schienle, J.L. rind Fang, H.T., J. Amer. Ceram. Sot., 72 [lo], 1907-13 (1989). Becher, P.F. and Wei, W.C., J. Amer. Ceram. Sot., 67 [12], C267-9 (1984). Wei, G.C. and Becher, P-F., Amer. Ceram. Sot. Bull., 64 [2], 298-304 (1985). Vaughn, W.L., Homeny, J. and Ferber, M.K., Ceram. Eng. Sci. Prod., 8 [7-81, 848-59 (1987). Jun, C.K. and Exner, E.L., Alumina-Zirconia Ceramics Reinforced with Silicon Carbide Whiskers and Methods of Making the Same, U.S. Patent No. 4,749,667. Exner, E-L., Jun, C.K. and Moravansky, L.I., Ceram. Eng. Sci. Prod., 9 [7-81, 597-602 (1988). Tiegs, T.N. and Becher, P.F., Amer. Ceram. Sot. Bull., 66 [2], 339-42 (1987). Baldoni, J.G. and Buljan, S.T., Amer. Ceram. Sot. Bull., 67 [2], 381-387 (1988) North, B., Ceramic Cutting Tools, 1986 Int’l Tool & Manufacturing Engineering Conference, May 19-22, 1986, Philadelphia, PA, Society of Manufacturing Engineers Paper #MR86-45 1 Whitney, E.D. and Vaidyanathan, P.N., Amer. Ceram. Sot. Bull., 67 [6], 1010-4 (1988). Baldoni, J.G. andBuljan, S-T., “Silicon Nitride Based Ceramic Cutting Tools”, Conference on Cutting Tool Materials and Their Application, March 11-13, 1986, Detroit, MI, Society of Manufacturing Engineers Paper -MR86-9 12 Funabashi, T., The Carbide and Tool Journal, May-June 1985, 22-27 (1985). Advanced Cutting Tool Materials, Kennametal Technical Brochure A85-144(50)J5, (1985). Smith, K.H., The Carbide and Tool Journal, Sept-Ott 1986, 8-l 1 (1985).

Alumina-Silicon Carbide Whisker Composite Tools

111

20. Faber, K.T. and Evans, A.G., Acta Metall., 31 (41, 565-76 (1983). 21. Faber, K.T. and Evans, A.G., Acta Metd., 31 141, 577-84 (1983). 22. Liu, H., Weisskopf, K.L. and Petzow, G., J. Amer. Gram. Sot., 72 141, 559-63 (1989). 23. Homeny, J., Vaughn, W.L. and Ferber, M.K., Amer. Ceram. Sot. Bull., 66 [2], 333-8 (1987). 24. Becher, P.F., Hsueh, C.H., Angelini, P. and Tiegs, T.N., J. Amer. Ceram. Sot., 71 [12], 1050-61 (1988). 25. Evans, A.G., .I. Amer. Ceram. Sot., 73 [2], 187-206 (1990). 26. Iio, S., Watanabe, M., Matsubara, M. and Matsuo, Y., J. Amer. Ceram. Sot., 72 [lo], 1880-84 (1989). 27. Homeny J. and Vaughn, W., J. Amer. Ceram. Sot., 73 [2J, 394-402 (1990). 28. Becher, P.F. and T.N. Tiegs, T.N., Ah. Ceram. Mat., 3 [2], 148-53 (1988). 29. “The Application Of WG-300 ‘Whisker’ Reinforced Ceramic/Ceramic Composites”, Greenleaf Corp. Technical Brochure G1015. 30. Smith, K.H., “The Application of Whisker Reinforced and Phase Transformation Toughened Materials In Machining of Hardened Steels and Nickel-Based Alloys,” in High Speed Machining Solutions for Productivity Proceedings of the Society of Carbide and Tool Engineers (G. Schneider, Jr.. ed) DD. 81-88. (1989).

Phase lkansformation Toughened Materials for Cutting Tool Applications

R. Krishnamurthy and C. V. Gokularathnam Department of Mechanical Engineering Indian Institute of Technology Madras, India INTRODUCTION Rapid advancements in aerospace, nuclear and other industries require enhanced in-service performance of engineering components. These requirements have resulted in the large scale development and use of difficult to machine materials that pose Though many new, nonconsiderable machining problems. conventional machining processes are being explored currently, traditional machining methods remain useful. A tool material must meet several stringent requirements that are dictated by the cutting processes. The deformation energy due to extensive plastic deformation ahead of the tool and the frictional energy due to interactions at the tool-chip and machined surface are converted into heat. Although most of the heat dissipates with the chip, some heat concentrates over an extremely small area near the tool tip, with the tool-tip temperature increasing to approximately 1000°C. Apart from the heating effect, the mechanical stresses at the tooltip are very high. To be effective, cutting tool materials must meet the following basic requirements: l

l

l

l

high high high high

hardness for efficient cutting action, mechanical resistance to cutting forces, resistance to wear, fracture and chipping, and hardness and strength at operating temperatures.

112

Phase Transformation Toughened Materials

113

The wear resistance of a cutting tool is dependent on hardness, physical and chemical stability of the tool, as well as its toughness during the actual metal cutting operation. These composite requirements have been met to a certain extent through concentrated research and development activities. This has resulted in the development and application of powder metallurgy HSS tools, coated and uncoated cemented carbides. These tools are quite useful at lower cutting temperatures. At higher cutting temperatures, the solution lies in the selection of more refractory and better wear resistance materials such as ceramics to meet the demands posed by modern machining.

DEVELOPMENT

OF CERAMIC

CUTTING

TOOLS

Degussa introduced the first commerical ceramic cutting tool in Germany in 1905 [ 11. Other early ceramic cutting tool patents were sought in England in 1912 and in the United States in 1942. In the former Soviet Union, attempts were made in 1943 to produce ceramic cutting tools at the Institute of Chemical Technology in Moscow. Ceramic cutting tools found commercial application in the U.S in 1954. Ford Motor Co. was the first to use these tools for a volume production application, however, the performance of these early ceramic tools was not an improvement over traditional cemented carbides. The main reasons for failure of these early ceramic cutting tools were given as l

l

l

inadequate strength and inconsistency in the performance, machine tools without the rigidity, power and higher spindle speeds required for optimum performance and, lack of urgency for higher speeds in cutting as the machining time was not included in manufacturing costs.

In essence, the technological and economic climate was not conducive for the development of ceramic tools. Traditionally, ceramics are materials are non-metallic, inorganic and possess higher melting points than many metals. Any development of ceramic cutting tool materials should not only ensure retention of

114

Ceramic Cutting Tools

these qualities,

but should offer increased performance. Ceramics for cutting applications must resist extreme strains. In order to obtain good chip formation and an economic cutting process, the ceramic material must possess high hardness at elevated the temperatures caused by friction of the cutting process. The tool edge must be resistant to pressure, possess good bending strength During metal cutting, the friction and sufficient toughness. between the cutting wedge and escaping chip causes wear on the cutting tool which progresses with cutting time. Cutting tools often exhibit different forms of wear, associated with many causes. Figure 7-l illustrates some of the causes of tool wear, most of which are thermally dependent. Chemical reactions at the contact surface between cutting material and work material (chip) lead to wear indicating that along with hardness and wear resistance, the ceramic should be chemically stable.

Torn off carbide particles ion mCutting

carbide

particles

LAlternating thermal strains in the cutting edge

Figure 7-1. Causes of tool wear.

tool

PhaseTransformation Toughened Materials

115

In addition to the above mentioned qualities, a ceramic cutting tool should also possessadequate thermal shock resistance in order to resist hazards due to abrupt changes in temperature in a series of cuts. Thus, an ideal ceramic cutting tool will be one which ensures higher hot hardness, high resistance to pressure, good bending strength coupled with adequate toughness, highest possible wear resistance, chemical stability and sufficient thermal shock resistance. The first ceramic machining tools were pure alumina (AI2O3) sintered at high temperatures. Such tools were suitable for finish turning of grey cast iron and certain soft steels. Material properties included excellent abrasion resistanceand comparatively higher hardness. However, there was a definite deficiency in fracture toughness and thermal shock resistance. Early alumina ceramics have been improved by controlled additions of magnesium oxide (MgO), zirconia (zrO2). Machining trials [2] conducted using newer, improved cold compacted AI2O3 ceramics have exhibited superior nose replication capabilities. However, these tools exhibited cutting edge chipping, leading to groove wear and subsequentcatastrophic failure of the cutting nose. Typical groove 'wear observed in ceramic tools is shown in Figure 7-2. Analysis of the grooved area using auger electron spectro-

Figure

7-2.

Typical

groove

wear in ceramic

tools.

116

Ceramic Cutting Tools

scopy (AES) indicated the formation of a spine1 (Fe,O,-Al,O,). Typical AES spectra indicate the presence of Fe in the tool (Figure During machining, the cutting tool-tip is subjected to 7-3). maximum pressure and temperature at distinct zones such as depth of cut line (DCL) zone and secondary cutting edge just behind the nose. The occurrence of higher temperatures or pressure under intimate contact with the sliding work surface may result in formation of groove wear in the DCL zone and secondary flank Literature [3-51 reports cutting temperatures of lOOOregion. 1200°C during machining of cast iron at a cutting speed of 200 m/min using a variety of carbide cutting tools. Because A&O, ceramics possess a lower thermal conductivity, higher cutting Mirao and Sata [6] recorded temperatures may be achieved. The higher cutting cutting temperatures as high as 1600°C. temperatures and the state of hydrostatic pressure at the cutting tip

Figure 7-3. AES - Pick up of Fe by the

Phase Transformation

Toughened

Materials

117

may promote chemical reactions at the groove. Figure 7-4 presents a phase diagram for FeO-Al,O,-Fe,O, illustrating the solid solubility of Fe,O, in A&O, and formation of Fe,O,-A&O, spine1 at approximately 1200- 1500°C [7]. Formation of such spinels weaken the matrix, resulting in chipping or spalling. Spalling of Al,O, has been attributed to the formation of a calcium oxide (CaO) - A&O, glass. Ham and Narutaki [8] reported a reaction between CaO (from the workpiece) and A&O, forming an unstable glass-like material. Such reactions can weaken the tool surface accelerating the wear process. Suh and associates [9] have studied the chemical stability of tool materials and have found good correlation between wear rate of various tool materials and the free energy of formation. They speculated that due to their lower negative free energy of formation, oxides and nitrides would be more stable than carbide cutting tools. However, there are exceptions (i.e., solubility and spine1 formation). King and Wheildon [ 101 suggested that the interface between the chip and ceramic tool possesses a number of solid chemical species that may be present under oxidizing conditions that promote wear. For Al,O, ceramic tools, chemical effects may often dominante. This realization served as the basis for development of improved tool materials. The susceptibility of A&O, ceramic tools to mechanical shock has limited their overall industrial application. The main reason for the poor fracture toughness of pure Al,O, tools is the absence of a ductile second phase for arresting cracking and spalling. In cemented carbide composites such as cobalt bonded tungsten carbide or nickel bonded titanium carbide, a ductile phase of cobalt or nickel is present. A similar approach has resulted in the development of an improved Al,O,-TIC composite tools with typical transverse rupture strength of 84-94 kgf/mm* compared to typical values of 70-84 In Al,O,-TIC kgf/mm* for traditional white alumina tools. composites, the Tic grains pin the cracks initiated at the tool surface. This is due to the additional expenditure of energy required for the cracks to propagate around the carbide particles. The particulate-dispersed matrix of A&O,-TIC may also exhibit better fracture toughness. Finish turning trials conducted on Mo-Cr

Ceramic Cutting Tools

118

: 2 0 L ::

E t’

1200

Hematite

k203

SS + Corundum

2o h203A12

Oj6’

SS

a0

Al203

A’2 03 AL203

SS

.&

1% 1350 1370

I I/

Fe2

03

!

Wiistite

Figure 7-4. Typical phase diagram of FeO-Al,O,-Fe,O,.

Fe 0

Phase Transformation

Toughened

Materials

119

alloyed cast iron liner materials in a high speed, precision VDF lathe using Al,O, and Al,O,-TiC tools have confirmed better performance of the Al,O,-TIC composites. Figure 7-5 illustrates a typical surface finish. Unlike the A&O, tools, the Al,O,-TiC tools did not exhibit chipping or spalling at the cutting edges. A certain amount of deformation of the cutting nose resulted in a better surface finish (Figure 7-6). Reinforced oxide ceramic matrices with pure crystals (whiskers) of silicon carbide (Sic) or silicon nitride (S&N,) exhibit enhanced fracture toughness [ 111. The sequential developments in ceramic cutting tools are illustrated in Figure 7-7. Ceramic cutting tools can be sub-divided into three main groups; oxides, mixed ceramics and non-oxide ceramics. Among the non-oxide ceramics, cubic boron nitride finds wide application while silicon nitride and diamond have specific applications. These tools were developed several years ago, and unlike oxide ceramics, they have a high resistance to thermal shock and a significantly greater bending strength. Figure 7-8 illustrates the typical hot hardness characteristic of S&N, and Al,O,-TIC tools. As shown in Figure 7-8, Si,N, tools have higher hot hardness at elevated temperatures.

vi -25

-125 0

NbC

I 500

I I I 1000 1500 2000 Temperature ,*C

Figure 7-5. Free energy of formation.

2%

120

Ceramic Cutting Tools

0.3 100

I 200

1 300

I 100

yo-yo

V=3OOm/min I I I 600 700 500

100

Time,

200

300

LOO

500

600

0

Block

0

White cemmic

700

set

cemmic

s=O.O8mm/rev. o+O.Smm Work : HO -

V=600m/m?1 0.1 0

I 100

I 200

I I 300 LOO Time,sec

I 500

Figure 7-6. Typical observations

6(

of surface finish.

Cr olloyed C.I.

r Oxide

Cemmics

Mixed

Catting

Ceramics

Non-oxide

Cemmics

Oxides +Non-oxidey

L

Non- oxides

I

I

I

mixed

Ceramics

Mixed Al&

Ceramics +TiC+Ti

Figure 7-7. The sequential development

cemmks

N

1 Sialon

in ceramics.

Silicon

Nitride

Si3N& I

5i3NL+ L

Sic

122

Ceramic Cutting Tools

l A1203+30%TiC 0 HIP AL203

Ooll

femperature

Figure 7-8. Typical hot hardness Al,O,-TIC tools.

750 ( *Cl

characteristics

900

of S&N, and

However, these tools do not exhibit superior performance in the machining of cast iron and steel work pieces. It was observed that Si,N, experienced higher order nose wear, decreasing the cutting performance. The higher order wear, despite the hot hardness of S&N, is attributed to the degradation of the cutting nose due to chemical reactions over the tool work interface. The smoother and broad triangular nose wear can be attributed to the occurrence of dissolution diffusion over the tool/work interface as well. Vigneau and Boulanger [ 121 reported similiar observations. Silicon nitride reacts chemically with iron and manganese in the presence of Tennonhouse and Runkle [13] oxygen at higher temperatures. suggested that manganese and oxygen can contribute to the wear of S&N, used to machine common iron based alloys.

Phase Transformation Toughened Materials

123

From the above illustrations, it can be inferred that if the potential of advanced ceramics and ceramic composites is to be fully realized, interface design must be perfected. This can be done by coating the substrate with a chemically stable, hard, wear resistant coating such as TiN and TIC on S&N, and other substrates. The inability of S&N, tools to machine ferrous parts has resulted in the development of oxide ceramics such as dualBased on the exceptional wear resistance of reinforced A&O,. A&O, and its brittleness, Al,O, was dual reinforced by ZrO, and Sic. The zirconia provides the transformation toughening, while Sic facilitates whisker reinforcement. Figure 7-9 shows the dual reinforced Al,O, has equivalent hot hardness, and it has improved chemical stability [ 141. The superiority of the dual reinforced A&O, tools is largely due to the addition of toughening and thermal conductivity attributes. The concepts of transformation toughening and its applications in the development of ceramic cutting tools are presented in the following sections.

TOUGHENED

CERAMICS

- CONCEPTS

Research and development on zirconia ceramics resulted in the development of two distinct classes of ceramics; fully stabilized This has paved the way for and partially stabilized zirconia. development of toughened ceramics. Swain [15] demonstrated that conventionally processed ceramic materials could be substantially This toughened as in the case of partially stabilized zirconia. breakthrough and its application to many other ceramic matrices, has led to the development of a new class of ceramics. At the same time, reinforcement of ceramic matrices by single crystals (whisker) was also being developed. The combination of these mechanisms, transformation toughening and whisker reinforcement, has led to the development of exceptionally tough and damage resistant ceramic materials. The basis of most of these toughening mechanisms is that there is a considerable reduction in the driving force at the propagating crack tip in the matrix of a ceramic material. Crack propagation in a matrix material is due to the

Ceramic Cutting Tools

124

IO

f

6 KIc rlPa’m 112 5

Jdl 4

I

white cemmic AL203

black ce romic A 1203 /Tic

-

L

5 i,N,

dualreinforced alumina

tungstenco rbide

300

30 I

150. k si

White ceramic A(203

Figure

black ceramic Al,O~/TiC

Sig N,,

duolreitorced alumina

7-9. Transverse rupture strength and stability reinforced Al,O, and other ceramics [ 141.

tungstefncot-bide

of dual

Phase Transformation Toughened Materials

125

crack driving force, which is opposed by the resistance of the The crack driving force is the stress intensity microstructure. factor. The resistance to this force may be increased by intrinsic toughening of the matrix or by means of crack tip shielding by extrinsic toughening. Since ceramics are generally brittle, it is the extrinsic toughening mechanism which can significantly contribute to the toughening of the ceramic matrix. This is illustrated in the later part of this chapter presenting developments in zirconia toughened alumina (ZTA). Toughening

Mechanisms

Usually a ceramic material is characterized by its strength Other properties such as creep behavior, thermal and toughness. expansion, conductivity and hardness are also useful parameters to identify. Ceramics are known for isolated pockets of high strength, and possess pre-existing or introduced flaws (< 50 pm). In a ceramic, the stress largely depends on the flaw size and shape. The relationship is stated as:

(1) where:

o = breaking strength K,, = fracture toughness c = flaw size Y = geometry factor

By controlling the size of the flaw, it is possible to enhance the strength, however, the ceramic has lower fracture toughness. This necessitates the addition of an agent for promoting toughness. Addition of certain materials promotes crack tip shielding which imparts enhanced toughness. The various mechanisms of crack tip shielding that impart toughness to ceramics are crack deflection and meandering, shielding, microcracking toughening, zone transformation toughening and contact shielding such as crackbridging by whisker and fiber reinforcements.

126

Ceramic Cutting Tools

Crack deflection and meandering: Cracks may be deflected from their planar path by grain boundaries in matrix dispersion and fracture resistant second phase particles. The reorientation of the crack path away from the favorable direction reduces the crack driving force. The toughness achieved is largely dependent on the nature of crack deflection and the phase that causes it. This type of toughening is usually independent of temperature. Microcrack toughening: Stress induced microcracks may take place in single and polyphase ceramics containing isolated stress pockets. These arise from thermal expansion anisotropy and thermal mismatch. This may be stress arising from the second phase inclusion undergoing a volume change due to phase transformation upon cooling. Such cracking will result in reduction of Young’s modulus and an increase in toughening. Crack deflection and Transformation toughening: introduction of microcracks in the matrix are a means for increasing toughness. Transformation toughening of ceramics is another mechanism facilitating matrix toughening. This mechanism is generally applied to the zirconia system. Transformation

Toughened

Zirconia System

Two decades of research on zirconia (ZrO,) has resulted in a variety of structural and functional applications. ZrO, belongs to the important class of materials generally known as fine ceramics, The utilization of engineering ceramics or advanced ceramics. zirconia as an engineering ceramic came about due to its well characterized allotropic transformation, ease of fabrication using powder metallurgy techniques and its chemical, thermal and physical stability during processing. The addition of magnesium oxide (MgO), calcium oxide (CaO), yttrium oxide (Y,O,), cerium oxide (CeO,) and other rare earth oxides in the form of mishmetal oxides in zirconia has resulted in the development of a class of innovative materials with a variety of engineering properties. ZrO, exhibits three well defined polymorphic phases; i.e., monoclinic (m), tetragonal (t) and cubic (c). The room temperature phase phase is monoclinic. Upon heating, the monoclinic

Phase Transformation Toughened Materials

127

transforms to the tetragonal phase at approximately 1000°C. The transformation to the cubic phase occurs at approximately 237OOC. Table 7-l provides some crystallographic data on zirconia [16]. Table 7-1. Zirconia Crystallographic

Data.

Crystal Structure Parameter

Monoclinic

Tetragonal

Cubic

Space Group

P2,K

P4.Jnmc

Fm3m

Density (kg/m3)

5560

6100 (talc) 5720 (exp)

6090 (talc)

5.094

5.124

Lattice Parameters (at 30°C)

a (4 b (A) c (A) I3

5.1415 5.2056 5.3128 99”18’

5.177

The room temperature monoclinic phase posseses a variety of interatomic distances and angles that suggests covalent bonding. Upon heating, there is a forward phase transformation of monoclinic to tetragonal with a corresponding decrease in the length of C-axis. The tetragonal zirconia is less ionic and has stronger covalency than the cubic phase [17]. The monoclinic to tetragonal transformation is complex. There is a severity in bonds due to the change in co-ordination of zirconium atoms and one of the two oxygen atoms. On further heating, the tetragonal phase transforms to the cubic phase. Thus in zirconia system, there exists at least three crystallographic modification which possess cubic (c), tetragonal (t) and monoclinic (m) symmetry and are stable at high, intermediate and low temperatures. Figure 7-10 presents a typical phase diagram of the ZrO, system. It is seen that we have:

128

Ceramic Cutting Tools 950°C 2370°C

2680°C Melt

4

Cubic

*

+

Tetragonal

e

Monoclinic

1 150°c On cooling the reverse transformation of t + m takes place. The t + m transformation is martensitic in nature, accompanied by an increase in volume; the tough zirconia alloy usually contains fine t-ZrO, particles. For such a material, the martensitic start temperature is suppressed below room temperature. The martensitic transformation can be induced by stress. This transformation is a source of toughness in the zirconia system. Usually the t + m transformation involves a set of transformation strains that increase the volume (volume dilation of around 4%) and change the shape of the grain as illustrated in Figure 7-l 1.

L : Liquid F : Fhorite

(cubic)

T : Tet ragonal TT : Transformable tetragonal M : Monoclinic T’: Non transformable tetragonal

L

I

I

I

0

5

IO

15

20

MOL% Yole5 Figure 7-10. Typical phase diagram of ZrO, system [ 161.

Phase Transformation Toughened Materials

129

@ @ I- ,: t

@ tetmgonal

monoclinic

monoclinic

Figure 7-11. Increase in volume and shape change for ZrO, [ 181.

Isolated grains usually transform. However, for grains embedded in the matrix, strain energy opposes the transformation [ 181. If the transformation is to progress, then it is necessary to supercool the system to create a sufficient driving force for transformation. The retention of t-ZrO, to room temperature can be controlled by several microstructural and chemical factors such as grain size and alloy content. For optimum properties, the metastability of Y,O, or CeO, - ZrO, should be such that the t + m transformation can be induced by application of an external stress/stress field, rather than from a thermal transformation which results from cooling the The material below the martensitic start temperature, M,. martensitic nature of the transformation has been studied by Hannink [19]. An important feature of the transformation is the existence of a unique lattice correspondence between the t- and mZrO, polymorphs. The essence of this martensitic transformation is that the retention of t-phase is the most important for utilization It has been of the transformation toughening phenomenon. reported that there exists a critical t-size or state attainable through thermal treatment/doping, below which the particle can be induced to transform with the aid of an external applied stress and above which the transformation will not occur.

130

Ceramic Cutting Tools

The total microstructural contribution to the critical stress intensity factor will largely depend upon on a number of factors such as size, morphology, dispersion and volume fraction of transforming zirconia particles, as well as temperature. To retain the tetragonal phase, dopants such as MgO, Y,O, or CeO, are added in specific proportions. The Y,O, doped ZrO, system is most common. The phase diagram in Figure 7- 12 illustrates the formation of yttria-partially stabilized zirconia (YPSZ) or yttria-tetragonal zirconia polycrystals (Y-TZP) [19]. The Y-TZP is the system used for transformation toughening. The YPSZ in the 4-6 mol% composition range is of little commercial interest as an engineering ceramic. The microstructure in this system is complicated by the presence of two tetragonal phases, t and t’. The t-phase is low in solute and may be stress induced to transform to the m-phase when suitably sized. The t’-phase is high in solute and must be decomposed to the stable cubic and metastable t-phase before stress induced transformation toughening is achieved.

I “‘xi

I

\I

-YzOJ

Figure 7-12.

c&tent (hOLE%j

Phase diagram of Y-PSZ [ 191.

Phase Transformation Toughened Materials

131

Commercial Y-TZP materials occur in the composition range of 1.75 - 3.25 mol% Y,O,. Materials of TZP can be made to exhibit higher strengths through proper sintering techniques and dopant contents. Typical strength properties of Y-TZP as influenced by the stabilizer content is shown in Figure 7-13 [ 191. Hot isostatic pressure sintering enhances the strength properties of Y-TZP. However, severe strength degradation of Y-TZP can occur when exposed to air or hot water in a controlled environment. Recently, better strength and toughness of Y-TZP has been achieved by the addition of 5-30 wt% A&O, to 2.5 mol% Y-TZP [191.

2000 0 0

HIP 14OO*C 150 MPa Normal sintering 1400 l C

g 1500 r

0

;

I 3

0

I 4

Y, 0s MOLE% Figure

7-13. Strength properties stabilizer content.

of Y-TZP

as influenced

by

The CeO, stabilized zirconia system has been developed for utilizing of transformation applications the phenomenon toughening. Tetragonal phase stabilization in the CeO,-ZrO, system can occur over a wide range of compositions 12-20 mol% CeO,. Ceria-tetragonal zirconia polycrystal (Ce-TZP) materials exhibit greater stability than the Y-TZP under similar

132

Ceramic Cutting Tools

environmental conditions [19). Typical strength properties of CeTZP as influenced by CeO, content are illustrated in Figure 7-14 [19]. While the fracture strength of Ce-TZP is not as high as that of Y-TZP, the toughness can be greater (maximum K’c for Y-TZP is 10 MPa*m”2). A detailed discussion on the performance of both Y-TZP and Ce-TZP is presented later in this chapter.

::

Figure

8

c3

7

I

6

7-14. Strength properties stabilizer content [ 191.

of Ce-TZP

as influenced

by

Phase Transformation Toughened Materials

133

In the zirconia toughened alumina (ZTA) system the retention of the tetragonal structure depends on the magnitude of the strain energy arising from the elastic constraints imposed by the surrounding A&O, material or on the shape and volume changes associated with t + m transformation of Y-TZP/Ce-TZP contained in the Al,O, matrix. The constraint is due to the different crystallographic orientation of neighboring grains or to a second phase matrix for two phase materials (i.e., Al,O, - Y-TZP). The strain energy arising from the constraints can be reduced by microcracking and/or plastic deformation which can accommodate some of the volume and shape changes associated with the transformation in a constrained situation. The Y-TZP is in a prestressed state due to fabrication stresses. The residual stresses on the Y-TZP can either increase or decrease the strain energy depending on its sense. If the transformation induces compressive stress and the residual stress is tensile, the net strain energy decreases. If both are in the same sense, the strain energy increases, resulting in lowering the transformation temperature. This phenomena may be advantageous for applications involving impact load and lower temperatures. It should be noted that with increasing temperatures, the fracture toughness decreases.

Y-TZP AND Ce-TZP SYSTEM

APPLICATIONS

As stated earlier, detailed research and development studies on both Y-TZP and Ce-TZP systems have been carried out with the objective of improved materials for machining of steel and cast irons. The research focused on fabricating inserts, grinding of fabricated inserts and application of the ground inserts as cutting tools. The design and utilization of transformation toughened ceramics such as Y-TZP is based on the stress induced tetragonal (t) to monoclinic (m) phase transformation of Y-TZP or Ce-TZP. Figure 7-15 illustrates the potential application range of the engineering ceramics. As shown in Figure 7-15, the covalent system SiC and S&N, have higher temperature applications, while the transformation toughened systems (i.e., TZP and ZTA, etc) are

134

Ceramic Cutting Tools

1600

z

l 51200 5 z z

Oevelopment

of high

performance

E 2 800 C ._0 z u ._ z. *

a

GO0

ZP

///A//A

0 500 Applied Figure 7-15. Potential application

1000 stress(MPa1 range of engineering

lC _

ceramics.

used in higher stress applications. The high stress application for ionic bonded, brittle oxide ceramics can be attributed to the stress induced transformation toughening of zirconia. Additional reasons for considering the zirconia toughened ceramics for low temperature application [20] are: + Creep rate of partially stabilized zirconia (PSZ) and TZP are high relative to those of Sic and S&N,. + Transformation toughening arises from stress induced phase transformation, which decreases towards the equilibrium temperature, T,:

Phase Transformation Toughened Materials

135

T, = 0.5 (M, + A,) where:

M, = martensitic A, = temperature

start temperature of reverse transformation

Figure 7-16 is a schematic representation of the expansion temperature curve for zirconia toughened ceramics. As seen in Figure 7-16, during heating there is a forward m -+ t, phase transformation with a decrease in volume. During cooling, (reverse transformation t -+ m) the matrix crosses the reverse transformation temperature, and a volume expansion occurs, resulting in cracking and crack branching. When the crack propagates, there is a release of constraint, facilitating toughening. Below the forward transformation temperature, RT, the stress induced across the crack tip causes a t + m transfornlation resulting in a volume increase and matrix toughening. This toughening mechanism is associated The types of with kinetic stabilization of phases in TZP. stabilization, thermodynamic and kinetic, are presented later in this chapter.

Y-TZP System Fabrication details: Fine powders of Y,O, mixed with ZrO, (prepared by chemical co-precipitation processes) were dispersed, mixed and milled in an agate planetary/transalatory ball mill. The Y,O, - ZrO, composition was mixed with 4 wt% polyvinyl acetate/polyvinyl alcohol binder and air dried. The particle size distribution of the Y,O, - ZrO, was determined by a laser particle analyzer. For example, the average size with different milling times for a 2 mol% Y,O, - ZrO, were as follows: Milling time (hours): 6 Particle size (pm): 0.83

12 0.82

24 0.7

36 0.65

48 0.59

Thus, the required powder size for sintering can be obtained regulating ball milling conditions.

by

136

Ceramic Cutting Tools

Phase Transformation Toughened Materials

137

Application of Y-TZP as cutting tool material: Machining trials on ferrous materials have been carried out using Y-TZP containing transformation toughened zirconia. The study involved sintering, grinding and machining applications. Sintering of Y-TZP Inserts: Fine powder Y-TZP containing varying mol% yttria were dispersed, mixed and milled in an agate planetary/transalatory ball mill in acetone for sufficient duration so that the average particle-size was approximately 0.83 p,rn microns. The powder was then pressed using a 4 wt% polyvinyl acetate/polyvinyl alcohol binder with a pressure of 200 MPa. The green compacts were then sintered in air in a high temperature, silicon carbide heat-treatment furnace. The samples were then tested. A block diagram illustration of the sintering schedule is presented in Figure 7-17. Sintering observations: Studies conducted on sintered composites reveal that the retention of high temperature tetragonal phase at room temperature depends on the density achieved during fabrication. This is illustrated in Figure 7-18 [21]. It is observed that as density increases, the tetragonal content also increases. With densification, the constraint imposed on neighboring grains increases, resulting in greater tetragonal phase retention. Critical grain size also influences the transformation process. Figure 7-19 illustrates typical influence of Y,O, content on critical grain size 1211. It is also observed that at approximately 2.5 mol% Y,O,, there is a sharp increase in grain size. Another parameter influencing grain size is sintering temperature. Figure 7-20 shows the influence of sintering temperature on average grain size [21]. Thus for achieving good results in transformation toughening, it is essential to retain as large a t phase as possible at room temperature, which in turn is determined by the Y,O, content and sintering temperature. Tetragonal phase formation during sintering: As stated earlier, the utilization of t --+ m transformation in Y-TZP (TTZ) is based on the retention and metastability of the tetragonal phase in the matrix. X-ray diffraction studies on composites of 2 mol% YTZP reveal that the amount of tetragonal phase was greater in the pressed and sintered state, than that of the loose powders. Figure 7-21 illustrates x-ray diffraction patterns of m(lll), t(ll1) and m(lli’)

r

Particle size Measurement (Laser - Granulomcter) -

-

Parrlcle size study on 2mol*/. Yttria mlred i’irconia

Raw

_

material

d I -

_

studies

Conventtonal

ortdc

Addikion

ot

technique

oxide

onrent~onal

Tcchniquc

Chcr”Icbt --

Conrolidotr

_

binder

with

, 220

MPo

coprcclpltatcd

t IO’/.

d

technique

Compaction

I An.lealmg IVarlous and

at

Check -I”

,,SO:C

,21,2

hou,s

_

Colctnotion

ot

760-C

‘. 10-c

? tot

temperatures

tlmesl

quenched

Sinter,“g

treatment

t

cm

t - Zr@l

Ouantl

t

tativc

Mothe.

-

~lordncrrt~iV~meosurancnl on vo,,ous processed samples with different loodr

and

fracture

Figure 7-17.

Block diagram of sintering study of Y-TZP inserts.

related toughness

tu

I2’12 hours __

‘1

Phase Transformation

60

Toughened

Materials

139

70 90 I( 80 Density/theoretical density(%)

Figure 7-18.

Retention of t-phase at room temperature on density [21].

depending

7 90 % tetr ogonal

0 0 0

0

I 1 V$l~Lonten+ in2

I $ol%,

I 4

ZrOZ Figure 7-19.

Influence of Y,O, content on critical grain size [21].

140

Ceramic Cutting Tools

1.2 1.0- Material=Zr02+2

mol%Y;!

03

z .5 O.BE ';i .E 0.6e o! g E 0.4z! aI 0.2-

I 1100 1200

0

I

I

I

1300 1400 1500 Sintering temperature ('C)

Figure 7-20. Influence of sintering temperature size [21].

I 1600

on average grain

Im(IIT)

mlllll

X,=0.834 Xt=0.166

twn

I!\

Figure

7-21. X-ray diffraction traces of m(ll l), t(lli) m( 111) reflection from sintered zirconia samples.

and

Phase Transformation

Toughened

141

Materials

reflection from the sintered zirconia samples. To understand the transformation of the metastable tetragonal phase to monoclinic, age hardening/precipitation hardening techniques were applied. The quenched samples were subjected to various low temperature annealing treatments. It was observed that quenching increased the amount of tetragonal phase. Table 7-2 presents the observations of the x-ray study. Table 7-2. Calculated Volume Fractions and Intensity Ratios for 2 mol% Yttria/Zirconia Compositions. (Powder, sintered, quenched (Q) and annealed (A) samples). Heat Treatment rowaer

xnl

“Ill

J4

“1

J,, (Q)

OS76 0.083

0.646 0.108

0.424 0.917

0.354 0.892

J zoo,24

0.897

0.921

0.103

0.079

J 2cQ/48

0.912

0.933

0.088

0.067

J 2co/120

0.900

0.924

0.100

0.076

J 95w24

0.061

0.080

0.940

0.921

(Q, 4

Annealing at 200°C for 24 hours transforms a reasonable quantity of tetragonal phase to the monoclinic phase. This indicates that the quenching process yields a large amount of transformable tetragonal particles. The effect of various heat treatments on the formation of tetragonal phase, density and bending strength are presented in Table 7-3. The hardness of TTZ is influenced by heat treatment. Figure 7-22 illustrates typical Vicker’s hardness values for different heat treatment conditions. The load sensitivity of the hardness values can be attributed to the transformation toughening of the TTZ. As stated earlier, TTZ can exhibit stress induced t + m transformation indicating a dip in the hardness value around 100150 pounds of indentation load. Annealing time also influences the

Ceramic Cutting Tools

142

Table 7-3. Effects of Heat Treatment on the Formation Phase, Density and Bend Strength of 2 mol% Y,O,-23-0,. Heat Treatment

760°C/2.5hrs, 1350°C/4hrs 76OTl2.5

Density (average)

Bend Strength (average)

(kgim3)

(MPa)

5970

429.5 1

61.9

374.17 380.79

36.0 71.8

292.01 445.09 392.79

75.2 40.5 48.8

% Tetragonal Phase

hrs, 1350”C/4hrs:

a) Furnace cooled b) Quenched Quenched

of Tetragonal

and Annealed

a) 130”C/24hrs b) 200”C/24hrs c) 352W24hrs

5738 5900 at: 5970 5981 6019

hardness of TTZ samples tested. Figure 7-23 illustrates a typical variation of hardness with annealing time. Increasing annealing time cause a decrease in hardness values. The observed rise and dip in hardness values around 4500 minutes of annealing can be attributed to the combined influence of stress induced transformation, microcracking and bulk particle interaction. Sintering time also influences the hardness as illustrated in Figure 7-24. At approximately 150 minutes of sintering, maximum hardness is obtained. Based on this observation, all the inserts of TTZ for machining application were subjected to the following heat treatment: 76O”C/150 min, 135O”C/150 min, annealing 2OO”C/24 hrs Grinding of Y-TTZ: For obtaining near net shape, the sintered and heat treated TTZ inserts were ground in a tool with cubic boron nitride (CBN) and diamond grinding wheels. During the grinding process, material removal is removed by microscopic deformation and fracture at the zones where the diamond/CBN

Phase Transformation Toughened Materials

143

Coptwipiiuted2mol% Y203 mixed I

loo A o* 0

Zr02sirtwed at135O*C for 150minuies Air-quenched (Fast cooling10 l C/lOsl ---Furnacecooled I I I I 1 I I SO 100 150 200 250 300 350 U Meowing force Ponds (GM-Force)

Figure 7-22a. Variation of Vickers hardness with indentation for different heat treatment conditions.

load

Ceramic Cutting Tools

144

1100 -

P

.

1000 -

‘N

.-___-.-*-

,I

900 -

800-

Coprecipitoted 2mol% Y2 03 mtxed ZrO2 Sinteredot1350DC for150minutes(Ml -.-

Air-quenched(Fast coolingld'cflos Quenched and annealed ----

OJ 0

I 900

I

1800

Measuring

at:

200-C/5760 M 200?/7200M

I I I I 2700 3600 4500 5400 force,Ponds (GM-Force)

I

6300 72

Figure 7-22b. Variation of Vickers hardness with indentation for different heat treatment conditions.

load

Phase Transformation Toughened Materials

145

900

800

Copmcipitateci Zmd% Y2O3mixedZr@

Quenched and annealed at: _---

ZOO? /1440 M

-.-

ZOO*C/2880M

. ..a---..

200*C/4320M

I 0

50

I

I

..1

ZOOb--7200M I

I

100 150 200 250 300 Measuring force Ponds (GM-Force

I

350

41

I

Figure 7-22~. Variation of Vickers hardness with indentation for different heat treatment conditions.

load

146

Ceramic Cutting Tools

1’ . \

200

d

.I .

.I

Coprecipitatd

Sintered crt.13SO°Cfor150minut~(M) Air-quenched(Fastcooling

b

lo3'c/1os) Quenchedand annealed&

-100

Zmol% Y2O3mixed fro2

-

-

--m

I

. . . . . . . . . . ..

ZOO*C/1440 M 13Ob1440

M

350*C/j440

M

Measuring force, Ponds (GM-Force)

Figure 7-22d. Variation of Vickers hardness with indentation for different heat treatment conditions.

load

147

Phase Transformation Toughened Materials

r

Coprecipitated 2mo1% Y;!03 mixed

llO(I-

100 O-

min.

Zr02 Sinteredat 1350tforl50 and air-quenched

f .

Measun'ng force

\

90(I-

IO GM- Force

.

---25GM-Force

\ . \

800

. . . . . . . . 50 GM-Fbrce

.

GM-Force

-.-IO0

G 70()E \ 2 60(I> I f

sot )-

G : 400Iz QI ; 300

200

100

&

O,

I

900

I

I

I

I

1800 2700 3600 4500 Annealingiime,minutes

I

I

5400

6300 72

Figure 23a. Typical variation of hardness with annealing

time.

Ceramic Cutting Tools

14s

1100

1000

9oc

800 “E 700 < a. 5600 L $ g 500 E ;

400

z -5 300 200

100

~precipitoted 2mOlo/ Y2O3 mixed Zr02, Sintered ot 1360 C fur 150 minutes(M) air- quenched

\ ‘1 \

0

Figure 23b.

I

900

1’ 1 \\/

Measuring force : 50 GM- Force ---100 GM-Force -.--200 GM -Force

..-----600

/

d

GM-Force I

I

I

I

1800 2700 3600 4500 Anneal ng time, minutes

I

5400

I

6300 7;

Typical variation of hardness with annealing time.

Phase Transformation

100 t

Toughened

Materials

149

Measuring fme P- 200 ponds(GM-Force1

SO

2nnl%

Y2O3 mlxed ZrO2

t

01

0

Figure

7-24.

Influence

I SirYering

I

tim%t)minu+es

1

I

180

21

of sintering time on hardness

abrasives cut the ceramics. The amount of material removed during grinding depends on the size and density of defects in the ceramic (i.e., flaws, cracks, and size of applied stress field). The size of the stress-field greatly influences the rate of material removal. When the stress field is smaller than the size of the defect, material is removed through plastic deformation and shearing. If the stress field is larger than the size of the defect, localized brittle microfracture and instantaneous rupture are the mechanisms of removal. Unlike metallic materials, grinding of ceramics is unique in that these materials are prone to defects. Also, brittle materials like ceramics have smaller E/H (Young’s modulus/hardness) values, resulting in small plastic zones, facilitating better surface quality during grinding. The grinding conditions are presented in Table 7-4.

Ceramic Cutting Tools

150

Table 7-4.

Surface Grinding

Work Material

‘%Q zro,

Density

of Ceramics.

(g/cc)

Hardness, GPa

3.98 5.9

18.5 12.5

H,

Young’s Modulus GPa 344 206

(2mol% yttria) Grmdmg

Wheel

Diamond CBN

BZ lAl-100-6-1-6 3K 12 V9-125-3-6

Wheel Speed Table Speed Depth of Cut

350,700,900, 1500, 1800m/min 21.5mmkc (Stepper motor driven) 1Opm

A typical surface profile recorded with diamond grinding is presented in Figure 7-25. It is seen that the ground surface texture was fairly uniform for most of the grinding speeds, with very close variation in R, and R, and RJR, values.

ll.67

1.n Il.691035 6.72

n.91

~62 ILLS a69 7.07

WJm-

Figure

7-25. Typical grinding.

surface profiles

recorded

with diamond

Phase Transformation Toughened Materials

151

Figure 7-26 represents a typical variation of surface finish (R,) with grinding speed for Y-TZP ceramics. As grinding speeds increase, the surface roughness increases, however at grinding speeds between 700-900 m/min, there is an improvement in surface quality. During, grinding, the Y-TZP ceramics experience higher temperatures and stresses (specific grinding pressure). While the grinding stresses facilitate stress induced reverse transformation of t + m, the grinding temperature favors m + t transformation. Thus during grinding, the Y-TZP insert will experience a cyclic t -+ m + t transformation; while the t + m transformation imparts toughness; the cyclic transformation t --+ m + t enhances the hardness. The improvement in surface finish in the range of 700-900m/min may be due to the t + m -+ t transformation. However, this trend can not be sustained due to the degradation of the grinding performance of the diamond wheel at the elevated temperatures associated with higher order grinding speeds. CA* 0 4.0 -

Zirconio

l Commercial Alumina

3b3.2-

0.6 0. 0

I 200

I I I I I COO 600 800 1000 12bO 14bO 1600 IE Grindingspeed,m/min

Figure 7-26. Typical variation of surface finish with grinding speed for Y-TZP ceramics.

152

Ceramic Cutting Took

As seen in Figure 7-26, diamond grinding of a commercial grade of cold compacted white A&O, results in a steady deterioration in surface quality with increasing grinding speeds. This is attributed to the inability of diamond to grind ceramics at the higher temperature values. Figure 7-27 illustrates the relative performance of CBN and diamond grinding of Y-TZP ceramics with respect to surface quality. The CBN produces a better surface quality due in part to its smaller grinding coefficient (= 0.3). (The CBN crystals are somewhat sharper than the diamond particles thus providing a better finish over a larger range of grinding speeds).

Zirconia 6.0 - . Ra -CBN grinding wheel -Diamondgrindingwheel 3.6 3.2-

01 0

I

I

400

I

I

Grin%g

I

I

1200 speed m/min.

I

IQ00

Figure 7-27. Relative performance of CBN and diamond grinding of Y-TZP ceramics with respect to surface quality.

Phase Transformation Toughened Materials

153

Grinding Speed and Specific Grinding Pressure: For assessing the grindability of TTZ, grinding force components F, (normal) and F, (tangential) were measured using an octagonal ring dynamometer. From these, the specific grinding pressure was calculated. Figure 7-28 illustrates the influence of grinding speed on specific grinding pressure for CBN and diamond grinding. CBN grinds TTZ with a higher specific grinding pressure than diamond. Cubic boron nitride resists oxidation up to 1300°C, while diamond is stable at somewhat lower temperatures (800°C). The CBN material is also softer than diamond, this usually results in grain flattening, unlike diamond which has better self-sharpening action. This results in a higher order specific grinding pressure. It is worth noting that higher specific grinding pressure could also be due to stress induced transformation of t --+ m phase, resulting in toughening of surface material. Grinding of toughened material is associated with deformation and shearing, resulting in higher order grinding stresses. This t -+ m transformation was observed by x-ray intensity profiles as stated earlier.

IXUY 1 Feed2 3

21.Smmhu. 1 DCpth of cut - 2Opm s -Hak-Zr@ - Wheel-0 CBN

Grinding

speed, mlmin

Figure 7-28. Influence of grinding speed on specific pressure for CBN and diamond wheel grinding.

grinding

Ceramic Cutting Tools

154

Figure 7-28, shows a steep increase in grinding pressure at an approximate grinding speed of 1000m/min. This may be attributed to the thermal decomposition of diamond. For comparative evaluations, grinding studies on commercial grade, cold compacted white Al,O, inserts were also conducted. Figure 7-29 illustrates the comparative evaluation of grinding of the two ceramics. With alumina, there is a small variation in the specific grinding pressure with grinding speed. This indicates that alumina is insensitive to thermal influences, while zirconia undergoes transformation toughening due to grinding temperatures and pressure. Feed- 2fSmm/sec Oepth of cut- 20pm Wheel- Diamond 8,C BN 1x10f2

Zr02

work-

and

A1203

z! A'203

1

xldO L

I

I

I

500

1000

1500

Grinding speed m/min

Figure 7-29. Comparative

evaluation of grinding of two ceramics.

Phase Transformation

Toughened

Materials

155

Figure 7-30 illustrates typical x-ray intensity profiles. It is observed when ground with CBN, TTZ experiences more stress induced transformation toughening. This results in possible folding of the surface asperities, and therefore an improved surface. From the grinding studies, it is observed that an overall improved grinding performance can be achieved by grinding YTZP with CBN wheels.

‘(111) jrinding Spctd - l8OOmlmin Feed - 21.Smhu Dtpth of cut- 2t)pm Whttl - Oiamond t CBN Work- ZrCq Unground ---Diamond grinding - - -CBN grinding

a0

20

I

111Ill

-60

-50

-40

Ii I’

” It = 3.6

II

II

-30

I

I

I

I

I 1

-20

I

’

-10 -huIIIl LO

Figure

7-30.

p

Typical x-ray intensity profiles of ground ceramics.

156

Ceramic Cutting Tools

Performance of Y-TZP: In metal cutting, one of the major drawbacks to widespread application of cold-compacted, white alumina is its unreliable performance due to sudden chipping of the nose portion (spalling of material near the cutting nose). This tendency is largely due to smaller fracture toughness of white ceramics, and can be eliminated/controlled by introduction of transformation toughened Y-TTZ ceramics for machining. Figure 7-31 illustrates the application of various cutting tool materials for different cutting conditions of speed and feed. From Figure 7-31, it can be seen that mixed ceramics (Al,O, + Tic), TTZ or ZTA occupy a position between nitride ceramics (silicon nitride) and pure ceramics (white alumina). Aoolicafion

l-l

Bomn

are0

nitride

Mixed

cemmic.

Coated

and

IT2 , ETA

carbide

Feed

Figure 7-31. Application of various cutting tool materials respect to cutting speed and feed.

with

157

Phase Transformation Toughened Materials

Surface Finish Control: Machining trials in free machining steel have been carried out in a high speed, precision VDF lathe. The machining conditions are presented in Table 7-5. Table 7-5. Machining Conditions.

Machine: VDF Lathe Spindle power: 18KW Cutting speed: SO, 100, 200, 320m/min Feed rate: O.lmm/rev Depth of cut: 0.25, 0.50, 0.75, l.OOmm ah Cutting tool geometry: v C-6) 5 (-5)

13 75

Or 90

1.8

The performance of a cutting tool can be assessed by factors such as cutting force, specific cutting pressure, cutting temperature and surface finish. Surface finish is largely dependent on cutting conditions and form-stability of the cutting tool wedge. For turning, an ideal tool is one that can replicate its nose without distortion. Nose replication is dependent on form-stability of the cutting wedge. The form-stability of the cutting tool depends on the hardness of the cutting tool at the machining temperature, the chemical stability of the tool material and its fracture toughness. Figure 7-32 presents typical variations in surface finish as influenced by machining conditions. As seen in Figure 7-32, it is possible to obtain a good finish with a cutting velocity of 200-300 m/min, feed rate of 0.1 mm/rev and a depth of cut approximately 0.5mm for TTZ tools. The improvement in surface finish after 0.7 mm depth of cut with a cutting velocity of 102 and 204 m/min and an almost saturated trend after 0.5 mm depth of cut for 328 m/min of cutting velocity has been observed to be associated with burnishing of the work surface by the deformed cutting nose. The deformation of the cutting nose can be attributed to either thermal softening or transformation toughening of the Y-TZP material under the condition of cutting pressure and temperature at the toolchip interface. Figure 7-33 illustrates a typical observation on the surface finish as influenced by the cutting speed.

Ceramic Cutting Tools

158 7-

Machiningspeed, @ !%.99m/min.Feed :O.l mm/rev. X 108.98m/minTTZ ceramic fool-insert 0 203.97m/min. A 327.82m/min.

Criticoldepth of cut

0

0.1

0.2

0.3

0.4 0.5 0.6 0.7 Depfhofcut,mm

0.8

0.9

1.0

I 1.1

Figure 7-32. Typical variation of surface finish as influenced machining conditions.

by

Depthof cut 0 l.Omm X 0.15mm 0 OSOmm A 0.25mm Feed:O.lmm/rev.

1.6-

0

0

I 30

I 60

I 90

I 120

1 I 1 I 150 180 210 240 Cuttingspeed m/min.

Figure 7-33. Typical observation by cutting conditions.

I 270

I 300

I 330

of surface finish as influenced

Phase Transformation Toughened Materials

159

Cutting Force: Figure 7-34 illustrates typical cutting force components measured using a Kistler piezo-electric lathe tool dynamometer during machining of free machining steel with YTZP (TTZ) inserts. At lower cutting speeds ( 200m/min) can be attributed to the deformation of the nose, possibly due to transformation toughening. During the machining trials, chipping of the cutting nose was not observed. Figure 7-35 is a typical variation of calculated specific cutting pressure as influenced by the cutting conditions. Figure 7-36 presents the comparative performance of TTZ inserts with commercial grade of white alumina inserts. It The TTZ inserts exhibited a comparable performance. It should be noted that the TTZ inserts used were cold compacted and sintered. If the inserts had been prepared as per industrial standards, (hot isostatic pressing (HIPing)) and perfect edge preparation, they would have exhibited an improved performance. Tool Wear: During machining, TTZ inserts experienced both crater and flank wear. Figure 7-37 illustrates a typical macrograph of crater and flank wear. The close boundary of the crater indicates that chipping of the cutting edge did not occur. The striation over the crater surface are the marks a sliding chip. From Figure 7-37, it can be observed that flank wear was relatively smooth with minimum abrasion marks. The presence of a fairly uniform flank-land boundary also indicates that the flank wear may be associated with small discrete plastic deformation of the asperities over the tool-flank, resulting in a smoothed flank land.

Ce-TZP System It is well known that zirconia exists in three different polymorphs: cubic (> 2370°C), tetragonal (1150-2370°C) and monoclinic (
200 180 -

0

Feedforce

X

bdiol

0

Cutting force

brce

Feed=O3mm/rev Oepthof cut.a.hm TTZ ceron~ctoolins9

160 : fl40B -1203 & cnloo5 5

80-

60 LO-

zoI

100

I I 200 300 Cutting speed mAin.

/

Fig.34Ial

FigILib,

cutting speed. m,m,n

Fig.34k

Figure 7-34. Cutting force components

during machining free machining steel.

j

161

Phase Transformation Toughened Materials

Cutting

Figure

spoed,m/nin

7-35. Typical variation of calculated specific pressure as influenced by cutting conditions.

cutting

bxhininqspeed ---TTZ Commit-tool insert(327.82m/minl Sandrik,Alum;na-~oI~sertl300.0m/minI 0 Ra *

lbax -.

Feed O.lmm/rev. .. %

Depth of cut,mm

Figure

7-36. Comparative performance of TTZ inserts commercial grade white alumina tools.

with

162

Figure

Ceramic Cutting Tools

7-37a.

Figure 7 -37b.

Micrograph

of crater

wear.

Micrograph

of flank wear .

Phase Transformation Toughened Materials

163

yttria and ceria can result in lowering the transformation temperature, thereby stabilizing the higher temperature phase at room temperature. Depending upon the alloy content and heattreatment, various forms of stabilized zirconia can be attained. If only a fraction of the high temperature phase is stabilized, and is referred to as partially-stabilized zirconia (PSZ). Yttria and ceria additions can lead to 100% tetragonal phase retention on quenching to room temperature. The room temperature t-phase is usually in a metastable state and can absorb energy, i.e., a propagating crack and that facilitates the t + m transformation. As stated earlier, the t + m transformation results in a volume expansion of about 4%. Thus, transformation absorbs energy and serves as a crack arrestor (absorber) and induces toughness. The more stable phase possesses a lower free energy, under given conditions of temperature, pressure and composition. The stabilization of a phase depends on thermodynamic or kinetic stabilization. Figure 7-38 illustrates these two types of stabilization ]221. With reduced dopant or temperature, the free energy decreases, resulting in phase transformation. The difference in free

/ \\ t

T (Al

\ -Iin, Tetmgonoi

/

J

J

;r

Monoclinic

Figure 7-38.

a) Thermodynamic

and b) kinetic stabilization

[22].

164

Ceramic Cutting Tools

energy between the two phases is referred to as AG. This is thermodynamic stability where both phases exist at an equilibrium temperature. However, with kinetic stabilization, an unstable phase can be retained in a metastable form, if sufficient activation energy is not provided at lower temperatures. In such an event, certain cubic/tetragonal phases can be retained at room temperature. This is illustrated in the phase diagram in Figure 7-39 [22]. The eutectoid reactions between t + m + c phases occur at 1050-150”C. During cooling, the metastable t-phase transforms to monoclinic (metastable phase m’). The concept of thermodynamic or kinetic stabilization is useful for understanding the behavior of TTZ ceramics, when subjected to grinding and other related processes, where both temperature and stress play significant parts in the transformation. 0

20

40

Wt(%) 60

100

80

2800 -3 Liquid

- ***.*

2400 -

‘*d,

\ 0 2000 -

‘;\\. \\_,.,

w

$600

+ 3

fet

i

i

\rs..-/.. -..

..--7

Cubic

-\

Cubic

0 z ‘02

20

A.*/

f I 40 (Mel Oh:”

Figure 7-39. Phase diagram of CeO,-ZrO,

i

I 80

[22].

10 Gel 2

Phase Transformation Toughened Materials

165

Sintering: Of the two systems, Y-TZP and Ce-TZP, CeTZP possess higher toughness and better resistance to lower temperature aging [19,23]. As with the Y-TZP system, several sintering schedules for the Ce-TZP system have been conducted. From the sintering studies, it was observed that 12 mol% ceriastabilized zirconia can be transformed to a single phase (100% t) by sintering at 135OOC for 2 hours. Subsequently, the sintered specimen were air quenched to achieve a higher density. The study also indicated that sintering at higher temperatures lead to a mixed m/t structure. This is not the most suitable structure for transformation toughening. With higher sintering temperatures, the TTZ ceramics experienced grain growth, exceeding the critical grain size for tetragonal retention. With spontaneous grain growth, transformation to the m-phase takes place upon cooling. Typical x-ray diffraction patterns taken on furnace cooled and air quenched Ce-TTZ samples are presented in Figure 7-40.

> ._ * :

(b)

c

Figure 7-40. X-ray diffraction pattern for a) furnace cooling and; b) air quenching of Ce-TTZ.

166

Ceramic Cutting Tools

Bending Strength and Weibull modulus: Ceria stabilized zirconia (12 mol%) powder was mixed with a suitable binder and cold pressed to rectangular bars in an uniaxial press at a pressure of 200 MPa. The cold compacted bars were then stabilized at 1350°C for 2 hours in air, followed by air quenching. Three-point bending tests were carried out in an universal Carl-Schenck testing machine. During the bend tests, the breaking load was estimated. Using the observed data, the bending strength was calculated. Weibull modulus was estimated following the method used for ceramics [24]. A representative Weibull plot of 12% Ce-TZPis presented in the Figure 7-41. To study the significance of annealing on bending strength, Ce-TTZ ceramic samples were sintered at 135OOC for 2 hours, air quenched and subsequently annealed at 1200°C for 2 hours, followed by air cooling. This resulted in improvement of the bending strength, but with a reduction in hardness (Table 7-6).

Flexural strength (MF’aI Figure 7-41. Weibull plot of 12 mol% Ce-TZP.

167

Phase Transformation Toughened Materials

Table 7-6. Bending Strength and Hardness Data for Ce-TTZ Ceramics. Bending Strength (MPa)

Vickers Hardness (H,)

%t

Sintering

390

882

100

Sir&ring & Annealing

440

762

83

Fracture Toughness: The sintered and air-quenched CeTTZ samples were tested for fracture toughness. The test samples were inserted with single notches of lmm widths and various precrack depths. The specimens were then subjected to three-point bending. From the breaking load, fracture toughness, IX,,, was evaluated. Figure 7-42 illustrates the influence of a/W ratio on fracture toughness. In these trials, the notch was inserted by a grinding wheel, which can induce strain in the material causing a t-m transformation. To avoid this, additional trials were conducted on Ce-TTZ specimens that were notched in the green state. These results are presented in Table 7-7. 12

61 0.2

12 mol % Ce-TZP SENB-3Point bend lmm diamond notch

I

I

I

I

I

0.3

0.L

0.5

0.6

0.7

0 3

Q/W

Figure 7-42.

Influence

of a/W on fracture toughness.

168

Ceramic Cutting Tools

Table 7-7.

Fracture Toughness

Data For Ce-TTZ Ceramics. K,, MPa*m’”

% t-transformed

Notch on Sintered Samples Notch on Green Compacts

9 5

45 52

Thermal shock resistance: Many of the structural ceramics are strong in mechanical/chemical environment both at low and/or high temperatures, however, they have poor resistance to thermal stresses due to thermal fluctuations in service. Hasselman [25] has made several contributions to the understanding of thermal shock based on analytical calculations. The basis of the approach was to analyze thermal stress failure in terms of the relationship between the usual thermo-mechanical properties of materials such as thermal expansion coefficient (a), Young’s modulus (E), heat transfer coefficient (h), thermal conductivity (k) and temperature difference (AT). This lead to the generation of a series of thermal shock parameters, i.e., R, R’, R” R”‘,R”” and AT,. The relevant thermo-mechanical properties used for the calculations are presented in Table 7-8. Table 7-8.

Relevant

Thermo-Mechanical

Properties.

Pronertv

Notation

Value

Young’s modulus Poisson’s ratio Thermal conductivity Thermal expansion coeff. Fracture toughness Bendinn strength

E

200 0.25 3.5 1o.9x1o-6 9 390

V

K a K1c (T

The fracture resistance parameters

R

=

GPa __ W.m-‘.K-’ K-’ h4Pa*mlD MPa

are:

a(1 - v) = 134.x Ea

Units

(2)

Phase Transformation

Toughened

Materials

169

R' = RX = 469.7 Wm-’ The damage resistance parameters E

Rffl =

are: = 1.75(Mpa)-’

(4)

Rflff = R”‘-v i = 36Ox10-6(m)

(5)

a2(1 -v)

Where

Vi

is the fracture surface energy.

The critical temperature difference, AT,, was evaluated by conducting a series of trials on Ce-TZP subjected to various heating cycles. Samples were soaked for half an hour at predetermined temperatures between 200 and 800°C and then quickly quenched in water maintained at 25°C. The quenched specimens were then subjected to three-point bend tests in a Schenck universal testing machine. Figure 7-43 presents the quenching temperature difference versus remaining strength. It was observed that the 12 mol% Ce-TZP exhibited a AT, at = 200°C. 12HOI Ce-TZP

I

I

200

Figure

7-43.

strength.

I

I

600 coo Initial temperature(‘C1

Quenching

temperature

I

800

difference

vs. remaining

170

Ceramic Cutting Tools

The thermal shock resistance parameters R and R’ apply to crack initiation problems and the thermal shock fracture resistance parameters, R”’ and R”” apply to crack propagation problems. It was observed that the thermal damage resistance of Ce-TZP is far better than most of the ceramic materials, including PSZ. Ceramic materials made of Ce-TZP are also more resistant to crack propagation. This is because Ce-TZP ceramics exhibit a higher order transformation toughening than any other ceramic material. Grinding Ce-TZP ceramics: Grinding ceramic materials takes on a greater importance with their increased use as structural engineering materials. The problems associated with grinding hard and brittle ceramics have been addressed by Inasaki [26]. When a sharp abrasive grain comes in contact with the hard brittle ceramic material, the ceramic experiences lateral and cone type of cracking due to cohesion failure as illustrated in Figure 7-44 [22]. These are penny like median cracks which form during loading (abrasive penetration) and grow up to the surface on unloading to form a radial crack array and lateral cracks. The zig-zag nature of lateral crack is typical of cracking in ceramics due to cohesion failure. The cracking of the material subjected to abrasion, will

racking

Median

cracking

Figure 7-44. Lateral and cone type cracking failure [22].

due to cohesion

Phase Transformation Toughened Materials pose a problem in that the depth of grinding, To facilitate controlled grinding force with in load is given as

171

the size of the chip becomes greater than posing difficulties in dimensional control. grinding, it is essential to keep the normal certain minimum load for cracking. This

p

_

AWIJ4AE!H>

(6)

H3

Where E is the Young’s modulus

and H is the hardness.

Diamond Wheel Grinding: Among the TTZ ceramics, CeTPZ has a greater K,, thus, it is easier to grind Ce-TZP than YTZP ceramics. Ce-TZP was ground using the conditions given in Table 7-9. Table 7-9. Grinding Conditions for Ce-TZP Ceramics.

Machine

Tool and cutter grinder

Operation

Surface grinding

Grinding

Wheel

Diamond: BZ 1Al - 100 - 6 - 1 - 6 (100mm diameter, 6mm width, grit size 91) B 120 RR 100 D CBN: (152mm diameter, 6.4mm width, grit size 120)

Wheel Speeds

1100, 2200, 3000 and 6000rpm

Depths of Cut

10, 20, 30 and 40pm

Work Feed Rate

27.5mm/min

Coolant

No coolant (dry grinding)

172

Ceramic Cutting Tools

The grinding set-up is similar to the one used for grinding of Y-TZP ceramics. Typical variations of grinding force with grinding speed are presented in Figure 7-45. Referring to Figure 7-45, it can be seen that while the tangential force is at a minimum for a grinding speed of 11.5 1 m/set, the normal force is at a maximum. It can also be seen that the normal grinding force increases with depth of grinding, while the tangential forces exhibit a different trend. During grinding, the Ce-TZP ceramic will experience thermodynamic and kinetic stabilization depending on grinding temperature and pressure. At low speeds, the grinding stress will be dominant, (compared to the grinding temperature) and depending upon the grinding stress, reverse transformation can also be promoted i.e., the surface will contain more m-phase. Typical variations of tetragonal phase with grinding speed is illustrated in Figure 7-46. At lower grinding speeds, the diamond particles will have a tendency to plow into the Ce-TZP ceramic, increasing the normal force, and promoting kinetic stabilization oft + m transformation. With higher grinding speeds, the grinding temperature is elevated, promoting the m + t transformation. The material becomes harder and increases the tangential grinding force. Figure 7-47 is a typical XRD pattern of sintered Ce-TZP compacts before and after grinding. The intensities of t(002) and Before grinding, the t(200) peaks are reversed after grinding. intensity of t(002) peak is smaller than that of t(200)-peak. As stated earlier, during grinding the Ce-TZP quenching stresses drive the t -+ m transformation, while the heat of grinding can induce forward transformation of m --+ t. The new t-phase formed due to grinding stresses, may have a different orientation than the t-phase. The new orientation is largely dependent on the energy of the tphase. The preferred orientation in such an environment, is usually found to be t(002). The phase content of the ground surface and grinding chips collected during the grinding trials are listed in Table 7-10.

Phase Transformation Toughened Materials

Depth of cut Qml

1100 5.76

6000

2200 3000 II.51 15.7

31.4

Rpm mkec

Grinding speed

0

1100 5.76

2200 3000 11.51 15.7 Grinding speed

6000 314

Rpm m/set

Figure 7-45. Variation of grinding force with grinding speed.

173

r

3-

l-

l-

lDepth of cut()rml 40

)-

I

1100 5.76

2200 11.51

I

I

3000 15.7

6000 31.4

Figure 7-46. Variation of t-phase with grinding speed.

RPm mkc

Phase Transformation

I

36

Figure

-28

Toughened

Materials

175

/

7-47. XRD traces of sintered Ce-TZP compacts grinding and; b) after grinding.

a) before

176

Ceramic Cutting Tools

Table 7-10.

Phase Content of the Ground Surface and Grinding

Chips.

(Depth of cut = 40pm, diamond grinding wheel)

Wheel Speed @pm)

1100 2200 3000 6000

Percentage Surface

78 0 100 64

Tetragonal Chip

56 100 100 100

Nature of Transformation

t+m t-+m t+m+t t+m+t+m

Figure 7-48, illustrates the influence of depth of grinding on the percent of t-phase during grinding. As the depth of grinding increases, the percent of t-phase increases, indicating the occurrence of t + m + t transformation for higher depths of grinding. This causes an increase in the radial grinding force as illustrated earlier.

Depth of cut, pm

Figure 7-48.

Influence of depth of grinding to % t-phase.

177

Phase Transformation Toughened Materials

The surface finish of ground Ce-TZP surface is also influenced by the grinding conditions. Figure 7-49 is a typical The variation of surface finish (R,) with grinding speed. observations indicated that it was possible to attain a good surface finish of R, = 0.17ym with a grinding speed of 15.7 m/set. The higher hardness of the t-phase could have yielded an improved surface finish. CBN Grinding: Studies on grinding of Ce-TZP ceramics with CBN wheels are illustrated in Figure 7-50. It was found that it was possible to grind Ce-TZP with lower forces, however, the surface was rougher than that ground by diamond wheels. Typical observations for CBN wheels are illustrated in Figure 7-5 1.

0.25

0.23'

‘iii a Go.21 E i In zo.19 : i 0.17

0.15

0

1

I

1100 5.76

I

I

2200 3000 II.51 15.7

6000 314

Rpm m/set

Grinding speed

Figure 7-49.

Typical variation of R, with grinding speed.

Ceramic Cutting Tools

178

CBN

Grinding

Depth 0

z 28 z

aC .-

of grinding 10ym

0

20ym

A

30ym

A

GOym

P ._ &

2._3 z isl = lg

0

!

I

2200

1100

Figure 7-50.

I 6000

I

3000 Grinding

speed,

rpm

Forces in CBN grinding wheels.

1.0 Depth of cut

: 20pm

E 0.6 a oc” 30.6G z 9 ;

04-

1;

2

2 0.2-

01 0

I 5

1 10

I 15

Figure 7-51. Comparative and CBN wheels.

1 20 Grinding

I I 25 30 velocity (m/s1

I 35

I co

! 15

surface roughness produced by diamond

Phase Transformation Toughened Materials

179

Machining Performance of Ce-TZP Cutting Tools: The sintered Ce-TZP compacts were ground to a standard cutting tool specification of SNUN 1218. The nose radius was placed in an optical profile grinding machine. Square inserts with a nose radius of 1.8 mm were used for machining spheroidal graphite cast iron. The machining conditions are presented in Table 7-l 1. Table 7-11.

Machining

Machine: Spindle Power: Cutting Speed:

Conditions

VDF Lathe 18KW 200-300m/min

for Ce-TZP Ceramics.

Feed Rate: Depth of Cut:

Cutting Tool (Ce-TZP): Phase structure: Hardness, &: Bending strength: Fracture toughness:

O.O63mm/rev 0.75mm

100% tetragonal 882 390MPa (3-point) 11MPamm”2(indentation) 9MPaamlP (SENB) 200°C

Thermal shock resistance: Geometry: V

a

h

I3

0

r

-6

5

-5

75

90

1.8

During machining of spheroidal Cutting Performance: graphite cast iron, short segmental chips are produced. Typical chips are shown in Figure 7-52. The brighter underside of the chip indicates a smoother flow over the tool rake face and reduced toolchip interface friction. Finish machining conditions were selected (i.e., V = 200300 m/min; a = 0.75mm; S = 0.063 mm/rev). The cutting force was measured with a Kistler dynamometer. The variation of force as influenced by cutting velocity is presented in Figure 7-53. As shown in Figure 7-53, between 225-275 m/min there was little At cutting velocities greater than change in cutting force. 275m/min, there was a pronounced increase in force, indicating the

180

Ceramic Cutting Tools

Figure

7 -52.

Types of chips produced.

30

z

25

N I.L. ,

--

-20 01 ~ ~ U

15

I 101

Ce- TZP tool insert

I 200

I 225 Cutting

Figure 7-53.

Variation

of cutting

J 250

Depth of cut; 0.75mm Feed: 0.063 mm/rev I f 275 300

speed I m/min force

with

speed.

Phase Transformation Toughened Materials

181

onset of nose deformation. Hot-stage microscopy on Y-TZP and Ce-TZP indicates that Y-TZP exhibits a thermodynamic stabilization m + t transformation at approximately 600°C while the Ce-TZP exhibits a m + t transformation at much lower temperatures. In metal cutting environments, higher temperatures at the tool chip interface have been reported. Also the cutting wedge experiences cutting pressures comparable to their hardness at the cutting temperature, thus the tool material in the cutting wedge portion, can undergo both thermodynamic and kinetic stabilization depending upon the cutting temperature and pressure. The steady cutting performance of Ce-TZP in the velocity range of 225-275m/min may be due to retention of lCO% tetragonal phase over the rake face, due to t -+ m + t transformations. With lower and higher cutting velocities, the surface may experience more t + m transformation, resulting in nose deformation and the observed force pattern for the cutting velocities in the range < 225 m/min and > 275 m/min. To assess the Transformation During Cutting: transformation during cutting, the tool tip was characterized by XRD (Torayo et al.), both before and after machining. The XRD showed 100% t-phase in both the cases. In order to assess whether any m + t phase transformation was possible, the conditions in optical profile grinding were altered to induce the m-phase (due to grinding) at the tool tip (during grinding of nose radius). It was found that this tool, having partly m-phase at its tip, showed a 100% t-phase, after machining at 225 m/min for about one minute. This shows that any m-phase present at the tool tip is converted to t-phase. Such a phenomenon is known to be indicative of cyclic transformation wherein the stress during cutting causes the t + m phase transformation and the frictional heat at the tool chip interface changes the m-phase back to the t-phase. From the sliding friction between the ceramic-steel interface, it has been observed that Ce-TZP exhibits cyclic transformation due to mechanical stress and frictional heat. Similarly, Ce-TZP could sustain its cutting ability due to cyclic This is transformation occurring over the cutting nose zone. possible because the temperature at the tool chip interface may be

182

Ceramic Cutting Tools

as high as 1000 to 1200°C. At these temperatures, only t-phase is stable and hence any m-phase produced due to the stress arising during cutting is transformed back to t-phase due to the frictional heat at the tool chip interface. Thus, it appears that Ce-TZP tool bits are capable of exhibiting complete cyclic (t-+m+t) transformations in a metal cutting environment, which facilitates transformation toughening, enhanced wedge retention and cutting performance. Similar transformations during cutting have been observed in yttria TZP (Y-TZP) tools as well. Tool Wear: Figure 7-54 shows the flank wear growth with machining time. It can be seen that a value of 0.3mm for flank wear land is considered to be the limit for tool wear assessment and is reached in about 4 minutes (for 225m/min cutting speed).

0.

Depth of cut -

0.7Smm - 0.063mmAw.

feed

Cuttingvelocity-225m/min 0.1 1

I 2

I 3

I 4

Machiningtime(min1

Figure 7-54.

Flank wear growth with time.

I 5

I 6

I 7t

Phase Transformation Toughened Materials

183

Figure 7-55 shows the typical wear pattern. It can be seen that crater formation and edge depression are the predominant forms of wear. The “ups” and “downs” marked by the striation like cracks on the vertical faces of the primary and secondary cutting edges are clear representations of the plastic deformation occurring in the CeTZP tool during machining. The striations are also a typical indication of transformation toughening of the material in the flank portion due to frictional heating and sliding contact pressure. It was observed that the tool could perform satisfactory cutting for up to 20 minutes, even with a flank wear land greater than 0.3 mm.

ZTA MACHINING APPLICATIONS The introduction of zirconia into alumina as a sintering and densification aid has been in practice since the early development of A&O, ceramics [27]. With developments in microstructure, the concept of dispersion strengthening gained momentum. This has lead to the development of A&O,-TiC and currently, toughening of In ZTA, the alumina by dispersion of ZrO, in alumina. microstructure contains different phases of alumina and zirconia. The toughening of ZTA is related to volume expansion and shear strain associated with the t + m transformation. Application of external stresses on ZTA causes the metastable tetragonal phase to transform to the monoclinic phase. This phase transformation, which is accompanied by a volume expansion of around 4% and a shear strain of about 6%, provides a compressive stress, which can reduce and eventually stop crack propagation. Studies on transformation toughening in ZTA [28] reveal the existence of a critical size of ZrO, particles, for the retention of metastable tetragonal phase. Further, it was shown that by altering the free energy associated with transformation, it was possible to toughen and even strengthen the ceramics. Control of microcracks generated due to volume expansion of t + m transformation during cooling from sintering temperature and due to the stress induced transformation during fracture process can also facilitate toughening and strengthening of ceramics.

184

Ceramic Cutting Tools a; bt) ~ G) bt) c: ::s u

u

~ cn

c: O

~

a; bt) ~ G) bt) c .-= ::s u

ti e .~ Q.. ..0 = 0 bt) ~

""""' cIS ~ 8 '+-0 0 - d) u

E ~ ~ d) cIS Q.

~ '-' a ~ t: ~ O bt) ~ L.,

~

~ L., .,.) u ~ I r-- -0 ~ $,. = ~ ~

Phase Transformation Toughened Materials

185

Studies by Coyle and Cannon [29] and Marshall and James [30] have illustrated the reversibility of the t + m transformation toughening and all of the transformed monoclinic phase can be fully reversed when the applied stress is removed. The application of surface compressive stress (depth of about 20ltrn) and consequent toughening of ZTA composites can be realized in applications such as grinding of ZTA and ZTA subjected to metal cutting environments. Studies on ZTA composites have also shown that microstructural alumina can be either dispersed with unstabilized zirconia, or PSZ. ZTA can also be alumina with dispersed PSZ or alumina zirconia duplex structures. Studies on alumina dispersed with unstabilized zirconia [31] have illustrated microcrack toughening and only stable microcracks exist in the composites containing a low volume percent of unstabilized zirconia can provide toughening. Similar trends were observed with thermal shock resistance. ZTA usually offers good thermal properties as a result of the presence of microcracks and zirconia particles, both having low thermal diffusivity. ZTA dispersed with unstabilized zirconia has not been used for metal cutting applications. ZTA with dispersed PSZ: The dispersion of PSZ in an alumina matrix is analogous to the ordinary PSZ ceramic in which tetragonal particles exist in a cubic matrix. The alloy oxides which are used to partially stabilize the toughening agent ZrO, is Y,O, or CeO,. Studies conducted by Lange [32] revealed that fracture toughness increases to 8 MPa*m at a volume of 50% of PSZ; beyond that, there was a reduction in fracture toughness. Elastic modulus and hardness decreased with the mol% of Y,O, in ZQ. For optimum toughening and strengthening of ZTA, the dispersed PSZ particles should be intercrystalline uniformly dispersed and present in high volume fractions. High fracture strength has been observed with 5pm particle size alumina and 0.6-1.5pm particle size PSZ. In their paper on engineering ceramics for high speed machining, Whitney and Vaidyanathan [33] illustrated the development of different ceramics for metal cutting applications. Toughening mechanisms for

186

Ceramic Cutting Tools

advanced ceramic materials and characteristics of dispersion strengthened S&N, tools were highlighted. Machining trials were conducted using alumina with dispersed PSZ particles by Mondal and others [34]. Characteristics of the cutting tools used are presented in Table 7-12. The tools were used to machine C-20 and C-50 materials. Machining trials indicated a small reduction in cutting force for ZTA tools compared to Al,O, tools. ZTA tools exhibited higher resistance to grooving and chip-notching wear due to their higher fracture toughness and chemical stability. Studies on the performance of alumina-zirconia tools in high speed face milling by Narutaki [35] have shown that ZTA tools exhibited superior crater wear resistance compared to pure A&O, tools. However, in turning the ZTA tools exhibited more Table 7-12. Characteristics of Cutting Tools.

Composition

Density (Theoretical)

a-A&O,

93.3

a-Al,O, + 14wt%Ca-PSZ

94.5

t + Z Phase in PSZ

Hardness H” 1390

7.0

45.3-46.1

1380

11.5

a-Al,O, + 14wt%Ca-PSZ + lwt% MgO 95.7

48.6-49.4

1525

14.7

a-Al,O, + 14wt% Y-PSZ

96.0

46.8-47.5

1400

13.3

a-Al,O, + 21wt% Y-PSZ

97.5

56.0-60.0

1425

14.4

a-Al,O, + 14wt% Y-PSZ + lwt% MgO 98.4

63.8-64.2

1544

15.2

Phase Transformation Toughened Materials

187

wear. The improved performance of ZTA tools in intermittent cutting like milling may be due to surface compressive stress induced by transformation toughening of the surface material. Studies conducted on dynamic fatigue and wear resistance of ZTA revealed stress induced transformation resulting in a reduction of crack propagation rate and consequently a rise in static fatigue performance. However, the wear resistance of ZTA was reduced when compared to plain alumina due to the formation of microcracks induced by friction stress-induced transformation on the surface. Phase transformation toughened zirconia has excellent properties such as higher order thermal shock resistance and moderate hardness. These qualities, provide better machining performance than cold compacted alumina tools. With a addition of 20% alumina, a ceramic referred to as Y-PSZ is created and are commerically known as Super-Z. These tools exhibit better cutting performance than TTZ tools. Super-Z has improved toughness and enhanced thermal shock resistance. Composites of Y-PSZ (3 mol% Y,O,) and alumina (20wt%) also exhibit a high fracture strength (2400 MPa) and fracture toughness of 17 MPa*m1’2 at room temperature. The Super-Z material is fabricated by hot isostatic pressing of sub-micron size powder particles. Machining trials have been carried out by Sornakumar and others [36] in a high speed VDF lathe. The tools have been used to cut spheroidal graphite cast iron. The surface roughness produced on the work piece and the tool life for machining at a cutting speed of 200 m/min, feed of 0.063 mm/rev and depth of cut 0.75 mm is presented in Table 7-13. It is seen that Y-PSZ tools exhibit performance comparable to ZTA in quality of surface production and performance in wear resistance.

Ceramic Cutting Tools

188

Table 7-13. Tool Life and Surface Roughness and ZTA.

Surface Roughness R,? CLm

Data for Super-Z

Tool Life* minutes

Super-Z

0.8

5**

ZTA

0.6

8.5

*Based on criteria recommended **If the inserts are prepared performance can be expected.

by IS0 of VB = 0.3mm. per industrial standards,

better

REFERENCES 1. Venkatesh,

2.

3. 4. 5. 6. 7.

8. 9.

V.C., Cutting Tools and Tool Materials, Third AIMTDR conference, IIT, Bombay, India, p.277 (1969). Sivasankaran, V., Performance of Ceramic Cutting Tools in Finish Machining of Alloyed Cast Iron, Ph.D. Thesis, IIT, Madras ( 1988). Deamley, P.A., Surface Engineering, l(l), p.43-58 (1985). Kan, S. J. Eng. for Industry, Trans ASME 98(2), pp. 607-613 (1976). King, A.G., Anzer.Ceram.Soc.BuZZ, 43(5), pp.395-401 (1964). Hirao, M. and Sata, T.,J., Japarz. Sot. Prec. Eng., 40(2), pp.156161 (1974). Levin, E.M., Robins, C.R., and McMurdri, H.F., Phase Diagram for Ceramists, American Ceramic Society, OH, pp.43-45 (1964). Ham, I. and Narutaki, N., J. of Eng. for Industry, Trans ASME, pp.95 l-959, Nov. (1973). Suh, N.P., Wear, 25, pp.11 1-124 (1973).

Phase Transformation Toughened Materials 10. 11. 12. 13. 14.

15. 16.

17. 18. 19. 20. 21. 22.

23. 24.

25. 26. 27. 28.

189

King, A.G. and Wheildon, W.M., Ceramics in Machining Process, Academic Press, New York and London (1966). Whitney, E.D., Vaidyanathan, P.W., Manufacturing Engineering, pp.36-37, May (1985). Vigneau, J. and Boulanger, J.J., Annals of CZRP, 3 1(l), pp.3539 (1982). Tennonhouse, G.J. and Runkle, F.D., Wear, 110, pp.75-81 (1986). Mehrotra, P.K., Ahuja, D.P. and Stephens, G.D., High Speed Machining Clinic, Paper# EM-91207, SME, (April 16-17, 1991). Swain, M., Materials Forum, 13, pp.237-253 (1989). Arunachalam, L.M., A Study of Transformation Toughened Zirconia and Its Application as a Cutting Tool, Ph.D. Thesis, IIT, Madras (1990). Morinaga, M., Adachi, H. and Tsukada, M., J. Phy. Chem. Solids, 44(4) p.301 (1983). Ruhle, M. and Evans, A.G., Progress in Materials Science, 33, pp.85-167 (1989). Hannink, R.H.J., Materials Forum, 11, pp.43-60 (1988). Claussen, N., Mat. Sci & Eng, 71, p.23 (1985). Lange, F.F., J. Mat. Sci., 17, pp.240-246 (1982). Annamalai, V.E., Transformation Behavior and Cutting of Ceria Tetragonal Zirconia Tool Application Polycrystals, Ph.D. Thesis, IIT, Madras (1992). Tsukuma, K., Amer. Ceram. Sot. Bull, 65( 10) pp. 1386-1389 (1986). Morrell, R., Handbook of Properties of Technical and Engineering Ceramics, Part I, An Introduction for the Engineer and Designer, Her Majesty’s Stationary Office, London, p. 105 (1985). Hasselman, D.P.H., Ceramurgia Zntl., 4, pp. 147-150 (1978). Insaki, I., Annals of CZRP, 36(2), pp.463-71 (1987). Wang, J. and Stevens, J. Mat. Sci., 24, pp.3421-3440 (1989). Garvie, R.C., Advances in Ceramics - Science and Technology of Zirconia III, Vol. 24, American Ceramic Society, OH, p.55-69 (1988).

190 29. 30. 31. 32. 33.

34. 35. 36.

Ceramic Cutting Took Coyle, T.W. and Cannon, R.M., Amer. Ceram. Sot. Bull, 60 p.377 (1981). Marshall, D.B. and James, R., J. Amer. Ceram. Sot, 69 p.215 (1986) Ruhle, M., Claussen, N. and Heuer, A.H., J. Amer. Ceram. Sot., 69(3), pp. 195- 197 (1986). Lange, F.F., J. Mat. Sci., 17, ~~-247-254 (1982). Whitney, E.D. and Vaidyanathan, P.N., Tool Materials for High Speed Machining, (J.A. Swartley Loush, ed.), pp.7782, ASM Intl, USA (1987). Mondal, B., Chattopadhyay Virkar, A. B. and Paul, A., Wear, 156, pp.365-383 (1992). Narutaki, N., Yamane, Y. and Hayashi, K., Annals of CIRP, 40(l), pp.49-52 (1991). Sornakumar, T., Ph.D. Thesis to be submitted, IIT, Madras.

8 Silicon Nitride Cutting Tools

J. Gary Baldoni Materials Technology Norfolk, MA

Sergei-Thomaslav Buljan Saint Gobain Norton Company Worcester, MA

Although present-day powder metallurgy technology has produced significant in near-net-shape forming advances techniques, considerable quantities of metal parts continue to be brought to finished form by turning, milling, boring, and other metal-cutting operations. The cost of metal part machining in the United States alone is estimated to be in excess of $100 billion annually; of this, about one billion is spent on the cutting tools used to fabricate these parts. Effective metal removal requires strong, hard, wear-resistant tools. The increasing demand for higher productivity and lower manufacturing costs is imposing a need for the development of improved cutting tools capable of operating at high machining speeds which increase the temperature at the tool-workpiece interface. This need has, since the turn of the century, resulted in the development of high-speed steel, cemented carbide, and coated carbide cutting tools and has become progressively oriented toward more refractory materials - ceramics (Figure 8-l). The first ceramic cutting tools introduced successfully into machining practice were based on aluminum oxide, A&O, [ 1,2]. Although available for 40 years, and proven successful in some

191

192

z

Ceramic Cutting Tools

Diamond and Cubic Boron Nitride

10,000 (3,000)

G 5,000 g (1,500)

A1203-Ti

$ s 5 8 J$

1,000 (300)

3z 2 U

100 (30)

A1203-SiC

A1203 I

Coated Carbid

500 (150)

Carbide

I

Cast Alloy I

High-Speed Steel (1::)

I II

Silicon Nitride

I I Carbon Tool Steel 1800

1900

2000

Year

Figure 8-1. Change in productivity due to the introduction cutting tool materials. Adapted from Ref. 23.

of new

machining operations, ceramic tools based on aluminum oxide have been unable to make a significant impact on the cutting tool market due to their relatively low fracture toughness and thermal shock These limitations have, at times, resulted in resistance. unpredictable failure of the tool during the cutting process and have underlined the need for developing tougher and more reliable ceramics which are able to address a considerably broader range of applications. This need brought about the development of tougher alumina composites and new ceramic materials [3-61.

Silicon Nitride Cutting Tools

193

The market potential for ceramic cutting tools has been greatly expanded with the recent commercial introduction of a new class of tool materials, silicon nitrides, which have been recognized as one of the toughest and most thermally-shock-resistant ceramics. Generic references to the material on which these cutting tools are based imply that silicon nitride tools are a single material. This is not the case; in fact, there are three distinct families of ceramic materials based on the compound S&N,. These are 1) silicon nitride containing glass-forming sintering aids; 2) silicon nitride-aluminum-oxygen solid solutions (SiAlONs); and 3) dispersoid-silicon nitride matrix composites. The former two families of materials were initially developed as candidates in vehicular engines, while the latter group was specifically designed for wear-resistant applications.

SILICON

NITRIDE

Single-phase S&N, is a highly covalent compound which exists in two hexagonal polymorphic crystalline forms, a and the more stable 0. Each of these structures is derived from basic S&N, tetrahedra joined in a three-dimensional network by sharing corners, with each nitrogen corner being common to the three tetrahedra. Either structure can be generated from the other by a 180” rotation of two basal planes 171. Actually, the a-S&N, to O-S&N, transition is achieved by a solution-precipitation reaction of S&N, and a molten glass. The strongly covalent bonds of S&N, produce a number of desirable engineering properties in this material: high strength, thermal stability up to approximately 1850°C, where it decomposes, good oxidation resistance, low coefficient of thermal expansion (good thermal shock resistance), and a modulus of elasticity greater than many metals. However, an adverse effect of this bonding, from the materials processing perspective, is a low self-diffusion coefficient which makes it virtually impossible to fabricate S&N, into a dense body by classical ceramic processing technology, viz. solid-state sintering,

194

Ceramic Cutting Tools

and as such requires sintering additives (densification aids) to achieve full density [8-lo]. The predominant impurity in S&N, powder, typically containing a large proportion of the a phase, is SiO,, which is present on the surface of the powder particles. The sintering aids, usually added as powders, are mixed by standard comminution procedures, ball or attritor milling, and the resultant mixture is shaped by a variety of techniques. The part is subsequently densified, usually in a nitrogen atmosphere, by firing at high temperatures. Hot pressing, overpressure sintering, or hot isostatic pressing at temperatures in excess of 1600°C are techniques which have been demonstrated to be amenable to the fabrication of dense silicon nitride parts. In the densification process, the sintering aid admixture reacts with the inherent SiO, to form a liquid, i.e., a glass, which facilitates S&N, particle rearrangement. This step contributes to part densification, and complete densification is achieved through a liquid-phase sintering mechanism. The a-S&N, particles dissolve in the liquid and precipitate as B-S&N, via a reconstructive phase transformation. As sintering continues, the B-S&N, nuclei grow as elongated grains and form an interlocked grain structure. Upon cooling, additional S&N, precipitates from the glass and contributes to grain growth. The sintering aids applied span a wide range of oxides and nitrides, producing a family of materials differing in composition and properties. Silicon nitride ceramics are represented by a two-phase material consisting of silicon nitride crystals and an intergranular bonding phase. The intergranular phase is a glass or partially devitrified glass based on SiO, and other sintering aids such as A&O,, Y,O,, MgO, etc. The mechanical properties of a particular S&N,-based ceramic are dependent upon the size distribution of the B-Si,N, grains [lo-151, assuming complete conversion of the a-phase to the B-phase, which is generally desired, and the quantity and species of the particular sintering aid(s) employed [8,10,16]. The grain size distribution of the S&N, grains strongly influence the fracture toughness (I$,) of the material, and hence the strength, since this property is directly proportional to K,,:

Silicon Nitride Cutting Tools

a =

195

KIC

IC

where

a = strength KIC = critical stress intensity factor C = inherent critical flaw size

This dependence is illustrated in the following example [ 13]. A Si3N4composition containing 6.0 w/o Y 203 and 1.5 w/o A12O3,referred to as A Y6, was hot-pressed for 90 min and 400 min with all other parameters kept constant. The densities of both ceramics were greater than 99% of theoretical. Examination of the resultant microstructures (Figure 8-2) showed that the extended time of liquid-phase sintering resulted in appreciable B-Si3N4grain growth. The results of quantitative stereology of these microstructures are given in Table 8-1. Particularly important to note is that while grain size has increased, the aspect ratio of the B-Si3N4 grains remained essentially constant. Under these circumstances, the development of larger grains of equivalent aspectratio through extended pressure-assistedsintering resulted in a 15% increase in fracture toughness, leading to the same incremental increase in strength (Table 8-1).

(a)

(b)

10~m Figure 8-2. Microstructures of A Y6-Si3N4 hot-pressed at constant temperature and pressure for (a) 90 min and (b) 400 min.

196

Ceramic Cutting Tools

Table 8-1. Microstructural Characterization and Mechanical Properties of AY6-Si,N, Hot-Pressed 90 min and 400 min at Constant Pressure and Temperature.

Densification Time (min)

Average Grain Size Equiv. Dia.

Average Grain Aspect Ratio

KIC (MPa*mln)

MOR (MPa)

(pm) 90

0.37

3.0

4.7kO.3

773f67

400

0.59

2.7

5.4f0.5

896f29

For sintered Y,O,-fluxed S&N, bodies containing 6 w/o Y,O, with or without A&O, additions, grain size has been reported to increase with sintering time or temperature, while the aspect ratio of the -S&N, grains showed little variation. The increase in grain size produced a linear increase in fracture toughness [ 171, and similar effects have been observed in other S&N, systems 118,191. A change of aspect ratio can strongly affect the fracture toughness [ 141, therefore, if grain growth is accompanied by an aspect ratio decrease, a net reduction in fracture toughness may be observed. A study of S&N, containing a large amount of sintering aids (15 w/o Y,O, + 3.4 w/o A&O,), where an increase in grain size was accompanied by a decrease in aspect ratio with extended sintering time, resulted in reduced fracture toughness [lo]. Based on these observations, it is hypothesized that crack deflection and grain de-bonding control the fracture toughness and strength of S&N, ceramics at lower temperatures (11OOO”C), where fracture has been observed to be predominantly intergranular [9,11,12,15]. Substantial published data have shown that the mechanical properties of silicon nitride ceramics at high temperatures are strongly influenced by the quantity and composition of the intergranular phase, although the size distribution of the Si,N, grains still has an effect. For example, the influence of A&O, addition content on the strengths of Si,N,6 w/o Y,O, ceramics

Silicon Nitride Cutting Tools

197

The two Al,O,-containing [16] is shown in Figure 8-3. compositions, both with densities > 99% of theoretical, had a room temperature strength of = 700 MPa. The material with 2.5 w/o A&O, maintained 90% of its 20°C strength to 970°C, then dropped to -150 MPa at 1400°C. The 1.5 w/o A&O, counterpart maintained 90% of its room-temperature strength to 1080°C before experiencing a decline in strength to = 200 MPa at 1400°C.

800-

0 w/o Al,O,

200 100I

A

I

20 " 800 Test

I

I

I

I

I

I

I

I

1000 1200 1400 1600 Temperature("C)

Figure 8-3. Strength of silicon nitride materials. (Open symbols are averages, closed symbols are individual data points). Adapted from Ref. 16.

198

Ceramic Cutting Tools

The behavior of the material with no alumina additions is significantly different. The absence of an Al,O, sintering aid made densification of this composition more difficult due to the reduction of and higher viscosity of this particular glass phase. This resulted in a reduced density (97-99% of theoretical) of the A&O,-free S&N, (6 w/o Y,O,) due to residual porosity, which reduced the room-temperature strength. However, the strength loss above 1000°C was appreciably reduced compared to the two A&O,-containing bodies. At 1400°C an average strength in excess of 400 MPa was measured. These examples serve to illustrate that, as industrially practiced, the vast majority of densified silicon nitride articles produced by the use of sintering aids can contain intergranular glass phases which may vary in both quantity and composition. Thus, silicon nitride cutting tools produced by this technology comprise a family of materials, and the properties of each individual S&N, tool material can differ, particularly in the temperature range encountered in high-speed metal cutting (>8OO”C), depending on composition, processing route, and means of densification.

SiAlON In the early 1970’s, ceramic research showed that aluminum and oxygen could be substituted for silicon and nitrogen, respectively, in the S&N, crystal structure to form what was termed an expanded lattice B’-SiAlON, a silicon-aluminum-oxygen-nitrogen solid solution [7]. The general composition of this material is

Si,,Al,O,N,,, where z denotes the number of oxygen atoms substituted for nitrogen and has a limiting value of 4.2 at 1700°C and 2.0 at 1400°C. It is claimed [7] that SiAlONs have physical and mechanical properties similar to silicon nitride due to the similar crystal structure and covalent bonding, and chemical properties approaching those of Al,O, due to solid solution effects.

Silicon Nitride Cutting Tools

199

To be fabricated into a dense body, sintering aids must be added to SiAlON compositions for the same reasons as discussed for silicon nitride. Rapid cooling from processing temperature produces a microstructure of IJ’-SiAlON grains with an intergranular glass phase. If Y,O, is the sintering aid, a portion of this glass can be devitrified to crystalline yttrium-aluminum-garnet (YAG) by heat treating and slow cooling. However, most sintered SiAlONs contain some residual glass phase, particularly at grain boundary triple-points. Like silicon nitride with sintering aid systems, the properties of SiAlONs are dependent upon the type and amount of sintering aid employed and the processing route followed during part fabrication. Additionally, SiAlONs are a more complex family of materials since the basic structural unit, unlike silicon Si,_,Al,O,N,_,~ can be of variable composition, nitride, S&N,, which is a compound of a specific stoichiometry. At present at least two grades of S&N,-based ceramic cutting tools are purported to be SiAlONs as opposed to monolithic silicon nitrides.

Silicon Nitride-Based

Composites

In addition to the two families of monolithic silicon nitride-based materials, S&N, with sintering aids and SiAlON; composites, in which a hard, refractory dispersoid is added to a (S&N, + sintering aids) matrix, have been explored as cutting tools [20-231. The addition of dispersed phases such as transition metal carbides or nitrides (TIC, TIN, HfC, etc.) to a S&N, matrix results in an increase in the hardness of the composite, which approximately follows rule of mixture behavior [24] (Figure 8-4a). An additional benefit from adding a dispersed second phase is the potential for increased fracture toughness of the composite via crack interactions with the dispersoid. If the dispersed phase has a higher resistance to fracture compared to the matrix, an advancing crack may bow between the second-phase particles [ 251. This increases the fracture toughness of the composite since the stresses required to propagate the bowed segments of the crack are higher than that needed to advance an unbowed crack.

200

Ceramic Cutting Tools

&-

$

6 -

Predicted by Crack Deflection Theory, A =_.I

Rule of Mixtures (Assuming TiCo.97)

I

I

0.2 0.3 (0.44 Volume Fraction TiC I+ (a) 0.1

Figure 8-4. a) Microhardness of Si,N, -TIC particulate

0.2 0.3 0.4 0.1 Volume Fraction TiC (b)

and, b) fracture

toughness of a series

composites.

A second crack-dispersoid interaction mechanism to produce increased fracture toughness, crack deflection, has been proposed 1261. Deflection toughening is predicted to occur when strain fields around a dispersoid particle, produced by thermal expansion coefficient and/or elastic moduli mismatch between the matrix and dispersoid, cause an advancing crack front to tilt and twist around the obstacle. Toughening occurs because the resulting non-planar a lower stress intensity than its planar crack experiences The increased energy expended can appreciably counterpart. increase the fracture toughness of the composite compared to the matrix material. An additional aspect of deflection theory predicts that the degree of toughening is dependent on dispersoid shape. Toughness increases up to four timesfor rod-shaped particles and up to two times for spherically-shaped dispersoids have been predicted, and crack deflection toughening has been experimentally observed in certain ceramic matrix composites [27]. In whisker

Silicon Nitride Cutting Took

201

(rod-shaped)-reinforced composites, pullout of and crack bridging by the acicular dispersoid could also contribute to fracture toughness improvements. The fracture toughness of S&N, (containing 6 w/o A&O, + 1.5 w/o A&O,)-based composites with TIC particulate additions at 10, 20, and 30 v/o has been observed to be statistically invariant with increasing TIC content (Figure 8-4b), and equivalent to that of the monolithic base material [24]. Microstructural analysis of the dispersoid-free and the 30 v/o Tic-containing Si,N, composite identically hot-pressed showed that the microstructure of the Tic-free material is characterized by larger Si,N, grain sizes and a S&N, grain size distribution substantially broader than that of the Tic-containing composite [ 131. The observed difference in Si,N, grain sizes could be attributed to the differences in the S&N, solution-precipitation and growth behavior, which are apparently influenced by the presence of titanium in the glass phase of the Si,N,-TIC composite [23]. Although it is clear that incorporation of a TIC dispersoid into a S&N, matrix could potentially provide an increase in the composite fracture toughness, in this instance the effect is offset by the reduction of S&N, grain size due to the S&N,-TiC reaction during densification under the processing conditions used. Such silicon nitride-based composites with increased hardness and fracture toughness equivalent to monolithic S&N, or SiAlON were found to be viable cutting tool materials, and a series of patents [28-301 based on this technology have been issued. Whisker-reinforced composites such as S&N, plus SIC (whisker) are also being evaluated as cutting tool materials, particularly for superalloy machining.

CUTTING TOOL APPLICATIONS The following discussion reflects on the status of development and the potential of silicon nitride cutting tools in three prominent areas of metal removal: gray cast iron, steel and superalloy machining.

202

Ceramic Cutting Tools

Gray Cast Iron Machining In machining operations, the cutting tool is subjected to stress and elevated temperatures resulting from friction and metal shear. While experiencing these conditions, the cutting edge undergoes continuous change due to wear processes(deformation, mechanical and chemical wear). The dominant wear mode of the tool vafies depending on conditions of use and the workpiece material [31]. Although chemical processes have been identified as potentially contributing to the wear of silicon nitride-based tools applied to the machining of gray cast iron [32,33], the dominant mechanism, based on extensive turning tests at cutting speeds as high as 25 m/s (5000 sfpm), is mechanical in nature, i.e., abrasion [20,21,23]. Diffusion experiments have shown minimal interaction between Si3N4-basedtools and gray cast iron (Figure 8-5a), as compared to similar tests with high alloy steel (Figure 8-5b).

(a)

(b)

Figure 8.5. Photomicrographs of workpiece-Si3N4-based tool material diffusion couples after exposure to 1100°C for 39 minutes (a) gray cast iron and (b) 4340 steel.

Silicon Nitride Cutting Tools

203

It is apparent then that since chemical processes contribute minimally to the dominant mechanism of wear, the mechanical properties of ceramic materials dictate tool performance for machining gray cast iron. Mechanical wear is controlled by the mechanical property interactions of two surfaces in sliding contact. The mechanisms which can contribute to this general classification of wear include plastic defomlation and abrasion. Gross plastic defomration of the cutting edge results in increased cutting forces and temperatures. This leads to increased wear rates and potential catastrophic failure and is a major contributing factor to the wear of tool steel and cemented carbide cutting tools in many metal-cutting applications [34,35]. This is not a major contributing factor to the wear of ceramic tools. However, on a microscopic scale, localized plastic deformation has been suggested as contributing to ceramic tool wear [36]. The primary mode of mechanical wear for ceramic cutting tools is abrasion, the removal of tool material by a scoring action of protruding asperities and hard phase inclusions in the workpiece and chip. The two-body abrasive wear resistance of metals which can accommodate large strains prior to fracture has been the subject of numerous investigations, and it has been shown that the wear resistance is determined by the metal’s hardness [37]. In this instance, two processes have been observed to occur as abrasive grains slide across metal surfaces: 1) formation of plastically deformed grooves which do not involve metal removal, and 2) removal of materials by the formation of microchips [38,39]. Considering less strain-tolerant materials such as ceramics, abrasive wear resistance is dictated by the material’s susceptibility to fracture and its hardness. The phenomenon has been modeled by the characteristic response of the respective tool materials to the penetration of an indenter, which emulates a protruding asperity of the workpiece material. With brittle materials, at a critical load a subsurface median vent crack is formed on the plane of symmetry of the applied stress field due to the tensile stress component generated by the indentation process. With additional loading, this subsurface crack grows to a critical size, at which point it becomes unstable and

204

Ceramic Cutting Tools

subsequently expands to the indented surface. This occurs during the remaining loading process to the maximum load and during unloading. radial Ultimately, a stable, fully propagated (half-penny) crack is obtained. The length of the radial crack in relation to the indentation size is determined by the fracture toughness of the material indented. At even higher loads, a third crack type, a lateral crack, can be generated. These originate below the indented surface and extend outward from the indentation in a plane parallel to the surface (40,411. For brittle ceramics, material removal by fracture that occurs in abrasion can be assumed to take place when lateral cracks of adjacent indentations, caused by penetration of sharp surface protrusions (or abrasive particles) of the opposing surface, intersect [40]. The removed volume is related to the indentation separation, depth of the indentation, and the sliding distance. Considering the dependence of the indentation size and the crack lengths emanating from such angular indentations on the material’s hardness and fracture toughness, the maximum volume removed by the system of indenters in a grinding operation has been found, for a given load and sliding distance, to be inversely related to the product K,,:3’4H”2, where K,, is the fracture toughness and H is the hardness [40]. From experimental wear data employing a pin-on-disk technique, the abrasive wear resistance (inverse of volume removed) of ceramic cutting tool materials was found to be directly proportional to K,, 3’4H1’2(Figure S-6), and this parameter provides, by a first approximation, a relative ranking of the abrasive wear resistance of brittle materials [20,42]. Cast-iron applications span most metal machining industries which are dominated by high-volume manufacture, such as automotive and earth-moving equipment components (brake drums, high-speed (high-productivity) engine blocks, etc.), where machining is critical for cost effectiveness. Using the K,c3’4H’n criteria for abrasive wear resistance, it is possible to predict the relative performance ranking of ceramic materials in cast-iron machining. Compared to alumina, the higher fracture toughness of Si,N,, combined with its superior thermal shock resistance and

Silicon Nitride Cutting Tools

1

5-

KIC = Fracture

Toughness

H = Hardness I ’

2468

I

I

I

I

I

/

10

12

14

KIc3’4H1’2 Figure 8-6. Abrasive wear resistance of ceramic cutting tool materials related to fracture toughness (K,,) and hardness (H). elevated temperature hardness, affords a ceramic material with improved reliability and dramatically increased tool life 1211. At this point, silicon nitride cutting tools have captured applications in cast-iron machining which have previously been exclusively served by cemented carbide and alumina-based tools. For example, with the use of silicon nitride-based tools in a brake drum machining operation, an improvement of >30% in productivity and as much as a ten-fold improvement in tool life over alumina-based ceramics was obtained 2221. In high-speed finishing and semiroughing applications, these tools, with their superior wear resistance, outperformed both aluminum oxide-based The tools and aluminum oxide-coated cemented carbides. capabilities of S&N, cutting tools allow their utilization on old as well as new machine tools. By applying silicon nitride tools, reported maximum metal removal rates in production by far exceed the productivity obtained with coated carbide or alumina tools [21,22].

206

Ceramic Cutting Tools

Based on present experience, it may be projected that further improvements in silicon nitride cutting tool materials, founded upon ceramic research for cutting tool applications and advanced S&N,-based composites with enhanced mechanical properties, i.e., whisker-reinforced S&N, [ 13,431, will bring about additional increases in performance and productivity. Steel Machining While silicon nitride-based cutting tools show outstanding wear resistance in cast-iron machining, the application of these materials to steel machining has for the most part been unsuccessful. The diffusion couples in Figure 8-5 show that the chemical reactivity of S&N, is higher in contact with steel than with gray cast iron. Crater formation on the tool is the predominant wear feature (Figure 8-7), providing evidence of the increased contribution of chemical wear in steel machining. Tool wear in gray cast-iron machining is observed only on the nose and flank of the tool. With high alloy 4340 steel, massive crater formation is produced at very short cutting times, which dramatically weakens the cutting edge, and leads to catastrophic tool failure. It is evident that the overwhelming contribution of the chemical wear components in steel machining completely obliterates the excellent abrasive wear resistance of typical cast-iron grade S&N, tool materials. While the mechanical properties of Si,N, ceramics can be enhanced through composite design, increases in their resistance to mechanical wear would not assure improved performance in this application. Based on considerations of estimated solubilities in a-iron, it has been demonstrated that the chemical wear-resistance of S&N,-based cutting tool materials can be enhanced by modifying For steel machining, their compositional character [44,45]. improvements in the chemical wear resistance of S&N,-Y,O,-TiC composite tools have been achieved through matrix modification by the addition of A&O,, a material which has been shown to be less With this compositional reactive with steel (Figure 8-8a). modification, the tool life of the higher alumina-containing

Silicon Nitride Cutting Tools

Gray Cast Iron (BHN 180) Cutting Speed: 1400 sfpm Cutting Time: 12 min Figure

8.7.

207

4340 Steel {BHN 300) Cutting Speed: 700 sfpm Cutting Time: 1 min

Worn silicon nitride-based cutting tools.

composites is considerably improved, to the extent that their use for the machining of steel is viable. Machining performance may also be further improved with composite design through the use of hard refractory compounds with increased thennodynamic stability. Figure 8-8b compares the perfonnance in steel machining of two composites (both utilizing a matrix phase of Si3N4 + 1.5 w/o A12O3+ 6 w/o y 203) containing 30 v/o transition metal carbide dispersed particulate phases. The HfC-bearing tool material exhibits considerably improved tool life compared to the TiC-containing composite, due to the fact that the solubility of HfC in iron is lower than that of TiC. It appears then that further development of silicon nitride cutting tools through the composite approach is an attractive and promising option [ 46]. Additional improvements in chemical wear resistance for Si3N4-based tools have also been obtained through the utilization of coatings [47,50]. The application of a coating whose solubility in ferrous alloys is several orders of magnitude lower than that of Si3N4, has been shown to considerably extend silicon nitride tool life in steel machining. Figure 8-9 illustrates the relative perfonnance of selected cutting tool materials in machining 4340 steel. The benefits

208

Ceramic Cutting Tools

2 w/o A&O, t

t

4340 Steel (BHN -300) 700 sfpm 0.010 ipr 0.050 in.

Cutting

Time (min)-

(a) 91

S&N, + Al,O, + TiC

82

7-

’

6-

2

5-

Workpiece: Speed: Feed: DOC:

4340 Steel 700 sfpm 0.010 ipr 0.050 in.

(BHN -300)

S&N, + Al,O, + HfC I

:,y

0

1

,

,

,

,

,

2

3

4

5

6

Cutting

Time (min) (b)

Figure S-S. Effect of (a) alumina content and (b) dispersed phase species on tool wear.

Silicon Nitride Cutting Tools

SNT (Uncoated)

209

SNT

=

Si,N,

+TiC

AT

=

Al,O,

+TiC

AT Composite TiC Coated Carbide

Cutting Time (min) Workpiece: Speed: Feed: DOC:

Figure 8-9. Machining materials.

performance

4340 Steel (BHN -300) 700 sfpm 0.010 ipr 0.050 in.

of a variety of cutting tool

realized through the application of either TIC or Ti(C,N) coatings on the S&N,-TiC composite are clearly evident. The characteristic plastic deformation of the tool nose, which is a major deterrent to the use of coated cemented carbides at high speeds, is not observed with the ceramic cutting tool materials. The wear rate of the TIC and Ti(C,N)-coated silicon nitride composite is significantly lower than that of the Tic-coated cemented carbide or the alumina particulate TIC composite.

210

Ceramic Cutting Tools

Superalloy

Machining

Superalloys are a class of metals developed for application at elevated temperatures. Turbine engine components, for example, must maintain their strength at temperatures above 650°C and maintain their resistance to hot corrosion and erosive wear. The high-temperature strength and stability of superalloys which meet these criteria severely limit their machinability and they are commonly machined with uncoated cemented carbides at low cutting speeds (0.25 to 1.0 m/s (50 to 200 sfpm)). Even at these speeds, tool life is very short, further reducing productivity. In order to increase productivity, high metal removal rates (higher cutting speeds) are desired; thus development (circa 1970) was primarily focused on alumina-titanium carbide composite tools since at that point they were the state-of-the-art ceramic cutting tool material. Although remarkable improvements in cutting speeds were attained (2-4 m/s (400-800 sfpm)), tool life was limited due to excessive depth-of-cut (DOC) notching. Tool change criterion had to be based on the DOC notch length to prevent catastrophic failure during machining [6]. Turning Inconel 718 with Al,O,-based tools in the speed range of 0.5-2.0 m/set, generates tool-chip interface temperatures between 800 and 1200°C [51]. The high temperatures and associated stresses generated during superalloy machining promote bonding across the tool/workpiece interface, causing an increase in friction, which can lead to seizure [52]. The transfer and bonding of workpiece material onto the rake face of cutting tools often produces a built-up edge (BUE) [53]. BUE formation is a consequence of plastic deformation and flow in the surface layer of the chip that contacts the tool, and it is strongly affected by cutting conditions (speed, feed, geometry) and thermal/chemical properties of the tool and the workpiece material couple. Once developed, the adhered BUE may promote further chemical interaction by elemental diffusion below the tool surface [54]. Chemical interactions between the tool material and workpiece have been studied, and a model based on thermodynamic properties has been proposed for estimating cutting tool wear [55,56]. The model assumes that the cutting tool material is removed by forming

Silicon Nitride Cutting Tools

211

a solid solution with the flowing workpiece chip. On this basis, A&O3 was considered to be a favorable tool material because of its high chemical stability with respect to iron and nickel. However, the limited performance of such materials in superalloy machining, resulting from low fracture toughness, underscores the necessity of a balanced approach in cutting tool design, one in which chemical and mechanical properties are considered simultaneously [46]. Depth-of-cut notching is a prominent feature of ceramic tool wear and often limits life of the tool. In a study using SiAlON tools for machining Incoloy 901, it was speculated that this mode of wear was a consequence of the chemical interaction between the tool and chip [57]. If DOC notching is a chemical wear process, then the expectation is that it would be temperature dependent and increase at higher cutting speeds. Similarly, it would be anticipated that more chemically stable ceramic tool materials, such as A&O,, would exhibit higher notch wear resistance than S&N, or SiAlON. The experimental evidence, however, is to the contrary. In addition to the fact that the notching of Si,N,decreases with cutting speed, it has been observed that A&O, cutting tools exhibit low resistance to notching, suggesting that the primary mode of wear at the depth of cut is not chemical in character [58]. The contact line at the depth of cut is a location of thermal and stress gradients, the severity of which varies depending on the properties of the workpiece, cutting conditions, and tool geometry. The machining characteristics of Inconel 718 lead to conditions of shear instability during deformation, producing a highly irregular chip morphology, characterized by tooth-like edges [59]. The tightness of the chip spiral decreases as the speed increases, which is attributable to the concomitant increase in separation of shear zones. Apparently, the shear rate sensitivity plays an important role in determining the fragmentation in the chip [58]. The mechanism of DOC notching in ceramic cutting tools has been attributed to the very irregular chip-workpiece separation process in the shear zone, which produced an interrupted seizure, pullout, and breakage on the cutting tool. Since there is no evidence of strong chemical effects in DOC notch regions, it is envisaged that the chip separation process and the combined action of the work-hardened workpiece surface

212

Ceramic Cutting Tools

and its hard constituents create abrasive wear conditions in the notch region. Similarly, DOC notch wear resistance has been reported to be primarily abrasive in character and, therefore, related to the mechanical properties parameter K,,3’4H”2 and to the ceramic’s resistance to thermal shock damage 1581. Because severe thermal gradients are at the DOC line, thermal shock resistance plays an important role in DOC notch formation. A study using A&O,- and S&N,-based cutting tools has shown that the degree of tool notching depends on the thermal shock resistance of a tool material. Thermal shock resistance of a cutting tool depends on the material’s thermal conductivity, diffusivity, thermal expansion coefficient, the strength (fracture toughness), and elastic modulus [60]. Table 8-2 lists the physical properties and the calculated thermal shock of figure of merit (R) of some ceramic cutting tool materials. Since ceramic cutting tool temperatures have been measured to be 1000°C when turning Inconel 718, the thermal shock parameter was calculated with available elevated temperature property data. The absolute value of R may vary depending on configuration, physical constraints, or imposed stresses. However, under equivalent use circumstances, this provides an acceptable method for relative ranking of a tool material’s thermal shock resistance. The parameter R is used to assess the material’s resistance to fracture initiation and is a measure of the maximum change in temperature for steady heat flow conditions. As seen in Table 8-2, the monolithic Al,O, tool has the lowest tolerable AT,(47”C), and additions of SIC whiskers increase this critical temperature range somewhat. The thermal shock resistance of the Al,OJSiC whisker cutting tool is increased due to higher thermal conductivity, reduction in thermal expansion, increased fracture toughness, and strength compared to the Al,O, cutting tool material. The S&N, materials possess higher R values, exceeding those of the A&O, tool materials, which are reflected in DOC notch wear resistance. This point has been demonstrated by comparing the notching of the Si,NJSiC whisker and Al,OJSiC whisker tools (Figure S-10). The notch wear resistance of both materials was

213

Silicon Nitride Cutting Tools Table 8-2. Comparison Tool Materials.

of Siiicon Nitride- and Alumina-Based

Whisker

*S&N,

0 -z

R

= 6.Owlo Y,O, + 1.5 w/o Al,O, + Bal. S&N, = MaxAT,; R = G( 1-v)/aE at 1000°C

g5 : 0 z

4-

g 2 O >

3

5

2-

Al;,2

3

;I,O,+SiC

(w)

t 200

I

SiIN,

: ;

lI

I 100

1

1 300

THERMAL SHOCK PARAMETER,

I

Si,N,+& I 400

(w) I 500

R (“C)

Figure S-10. Depth-of-cut notch wear related to the thermal shock resistance parameter “R” of selected ceramic cutting tool materials. (After Ref. 58).

214

Ceramic Cutting Tools

observed to deviate from purely abrasive wear, indicating that thermal shock damage contributes to DOC notching. The silicon nitride-based composite was observed to exhibit higher DOC notch resistance, compared to the whisker-reinforced alumina composite, demonstrating the advantages of considerably lower thermal expansion coefficient and higher strength (fracture toughness) in controlling this wear mode in spite of this material’s lower thermal conductivity [58]. Newer composite silicon nitride compositions appear to be extending the useful tool life in superalloy machining. The development of composites for this application has only begun, and the potential expansion of their application range and improvements in tool life have yet to be realized. An optimistic view of the outcome of this development is strongly supported by the realization that the primary factor limiting the perfomlance of silicon nitride is a chemical interaction between the tool and workpiece. Under these circumstances, it appears plausible that further chemical tailoring, in a manner similar to that applied in the case of tools for steel cutting, would bring about additional improvements in wear resistance and productivity.

SUMMARY In the past five years, silicon nitride cutting tools, due to improved fracture toughness and reliability, have gained wide acceptance in the metal removal industries. However, it should be recognized that cutting tool applications are very specific and demanding. In order to design a tool material for such use, the understanding of the multiplicity of factors influencing wear processes is a prerequisite. While materials developed for other purposes may provide a feasibility base for further refinement, their enlightened adaptation for machining use requires a thorough understanding of wear mechanisms under specific use conditions. This knowledge governs the adjustments required for compositional and/or microstructural tailoring to improve metal cutting performance.

Silicon Nitride Cutting Tools

215

The two dominant wear mechanisms influencing the performance of cutting tools are abrasion and chemical wear. Considerable research is being focused toward further improvement in the performance of silicon nitride-based tool materials through mechanical and chemical tailoring. Improvements in mechanical properties can be achieved through optimizing of the microstructure and controlling the grain size distribution of both monoliths and composites. Tailoring to obtain improved chemical wear resistance can utilize either matrix modifications or dispersoid additions. Further improvements in both abrasive and chemical wear resistance have been gained with the use of thin (2-10 pm) chemical and abrasive wear-resistant coatings on silicon nitride-based substrates. While in their infancy, silicon nitride-based cutting tool materials have demonstrated that they meet the present and future challenges of high-productivity machining. Furthermore, due to their excellent high-temperature stability, oxidation, and thermal shock resistance, and the possibility of tailoring the microstructure and properties, these silicon nitride cutting tool materials may parallel cemented carbides in many ranges of applications, with a definite advantage in the attainment of higher productivity. While the discussions and examples given have pertained mainly to turning, the use of silicon nitride-based cutting tools in other machining applications, such as milling, has indicated similar advantages and potential.

REFERENCES 1. King, A.G. and Wheldon, W.M., Ceramics in Machining Processes, Academic Press (1960). 2. Whitney, E.D., “Modem Ceramic Cutting Tools,” Powder Metal. Inter. 15[4]: 201-05 (1983). 3. Furakawa, M., et al., “Fracture Toughness in the System A&O,-TIC Ceramics,” Nippon Tungsten Review 18: 16-22 (1985).

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Ceramic Cutting Tools

4. Wahl, R.P. and Ilschner, B., “Fracture Behavior of Composites Based on A&O,-TIC,” J. Mater. Sci. 15: 875-85 (1980). 5. Whitney, E.D. and Vaidyanathan, P.N., “Microstructural

6. 7. 8.

9.

10.

11.

12.

13. 14.

15.

Engineering of Ceramic Cutting Tools,” Am. Ceram. Sot. Bull. 67(6): 1010-14 (1988). Baldoni, J.G. and S.-T. Buljan, “Ceramics for Machining,” Am. Co-am. Sot. Bull. 67(2): 381-87 (1988). Jack, K.H., “Review-SiAlONs and Related Nitrogen Ceramics,” J. Mat. Sci. 11: 1135-58 (1976). Lange, F.F., “Silicon Nitride Polyphase Systems: Fabrication, Microstructure, and Properties,” Inter. Metals Rev. 1: l-20 (1980). Lange, F.F., “Fabrication and Properties of Dense Polyphase Silicon Nitride,” Am. Ceram. Sot. Bull. 62(12): 1369-74 (1983). Ziegler, G., Heinrich, J., and Wotting, G., “Review Relationships Between Processing, Microstructure, and Properties of Dense and Reaction-Bonded Silicon Nitride,” J. Mater. Sci. 22: 3041-86 (1987). Lange, F.F., “Relation Between Strength, Fracture Energy, and Mmicrostructure of Hot-Pressed S&N,,” J. Am. Ceram. Sot. 56(10): 518-22 (1973). Knoch, H. and Gazza, G.E., “On the A to B Transformation and Grain Growth During Hotpressing of Si,N, Containing MgO,” Ceramurcia Inter. 6(2): 51-56 (1980). Buljan, S.-T., Baldoni, J.G., and Huckabee, M.L., “S&N,-SIC Composites,” Am. Ceram. Sot. Bull. 66(2): 347-52 (1987). Buljan, S.-T., et al., “Microstructure and Fracture Toughness of Silicon Nitride Composites,” Proc. Int. Cont. on Whisker and Fiber Toughened Ceram. ASM Inter.: 12631 (1988). Wotting, G., Kanka, B., and Ziegler, G., “Microstructural Development, Microstructural Characterization and Relation to Mechanical Properties of Dense Silicon Nitride,” in: Nonoxide Technical and Engineering Ceramics (S. Hampshire, ed.), pp. 83-96, Elsevier Applied Science (1986).

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16. Smith, J.T. and Quackenbush, C.L., “Phase Effects in Si,N, Containing Y,O, or CeO,: I, Strengthm,” Am. Gram. Sot. Bull. 59(5): 529-32 (1980). 17. Sarin, V.K., “On the A to B Transformation in Silicon Nitride,” in: Science of Hard Materials 3 (V.K. Sarin, ed.), pp. 151-60, Elsevier Applied Science (1988). 18. Tani, E., et al., “Effects of Size of Grains with Fiber-Like Structure of S&N, on Fracture Toughness,” J. Mater. Sci. Lett. 4: 1454-56 (1985). 19. Perjryd, L., “Microstructure and Mechanical Properties of CaO/MgO-Doped S&N, Sintered by Hot Isostatic Pressing,” Adv. Ceram. Mater. 3(4): 403-05 (1988). 20. Buljan, S.-T. and Sarin, V.K., “Machining Performance of Ceramic Tools,” in: Cutting Tool Materials (F.W. Gorsler, ed.), pp. 335-348, ASM (1981). 21. Sat-in, V.K. and Buljan, S.-T., “Advanced Silicon Nitride-Based Ceramic Cutting Tools,” SME Paper MR 83-189 (1983). 22. Buljan, S.-T. and Sarin, V.K., “Improved Productivity Through Application of Silicon Nitride Cutting Tools,” The Carbide and Tool Journal 14(3): 40-46 (1982). 23. Baldoni, J.G. and Buljan, S.-T., “Silicon Nitride-Based Ceramic Cutting Tools,” SME Paper MR 86-913 (1986). 24. Baldoni, J.G., Huckabee, M.L., and Buljan, S.-T., “Mechanical Properties, and Wear Resistance of Silicon Nitride Titanium Carbide Composites,” in: Tailoring of Multiphase and Composite Ceramics (R.E. Tressler, et al., ed.), pp. 329-345, Plenum Publishing (1986). 25. Lange, F.F., “The Interaction of a Crack Front with a Second-Phase Dispersion,” Phil. Mug. 22: 983-92 (1970). 26. Faber, K.T. and Evans, A.G., “Crack Deflection Processes - I. Theory,” Acta Metall. 3 l(4): 565-76 (1983). 27. Faber, K.T. and Evans, A.G., “Crack Deflection Processes - II. Theory,” Acta MetaEZ. 3 l(4): 574-84 (1983). 28. Sarin, V.K. and Buljan, S.-T., U.S. Patent 4,388,085; June 14, 1983; assigned to GTE Laboratories Incorporated.

218 29. 30.

31. 32.

33. 34. 35.

36. 37. 38. 39. 40.

41. 42.

43.

Ceramic Cutting Tools Sarin, V.K. and Buljan, S.-T.,U.S. Patent 4,425,141; January 10, 1984; assigned to GTE Laboratories Incorporated. Sarin, V.K., Penty, R.A. and Buljan, S.-T., U.S. Patent 4,497,228; February 5, 1985; assigned to GTE Laboratories Incorporated. Trent, E.M., Metal Cutting, 2nd ed., Butterworths (1984). Tennehouse, G.J., Ezis, A., and Runkle, F.D., “Interaction of Silicon Nitride and Metal Surfaces,” J. Am. Ceram. Sm. 68( 1): C30-C31 (1985). Babini, G.N., et al., “Role of Binder Phase in S&N, Cutting Tools,” Ah. Ceram. Mater. 2(2): 146-53 (1987). Baldoni, J.G. and Williams, W.S., “Deformation of Cemented Carbides,” Am. Co-am. Sot. Bull. 57(12): 1100-02 (1978). Baldoni, J.G., Buljan, S.-T., and Sarin, V.K., “Deformation and Wear of Cemented Carbide Cutting Tools,” 11th N. Am. Manufact. Res. Conf. Proc., pp. 342-347 SME (1983). King, A.G., “Ceramics for Cutting Metal,” Am. Ceram. Sm. Bull. 43(5): 395-401 (1964). Kruschov, M.M., “Principals of Abrasive Wear,” Wear 28: 69-88 (1974). Misra, A. and Finnie, I., “A Review of the Abrasive Wear of Metals,’ Trans. ASME 104: 94-101 (1982). Rabinowicz, E., Friction and Wear of Materials, John Wiley (1966). Evans, A.G. and Wilshaw, T.R., “Quasi-Static Solid Particle Damage in Brittle Solids - I. Observations, Analysis, and Implications,” Acta. Metall. 24: 939-56 (1976). Perrott, C.M., “Elastic - Plastic Indentation: Hardness and Fracture,” Wear 45: 293-309 (1977). Baldoni, J.G., Wayne, S.F., and Buljan, S.-T., “Cutting Tool Materials: Mechanical Properties - Wear Resistance Relationships,” ASLE Trans. 29(3): 347-52 (1986). Shalek, P.D., et al., “Hot-Pressed SIC Whisker/S&N, Matrix Composites,” Am. Ceram. Sm. Bull. 65(2): 35 l-56 (1986).

Silicon Nitride Cutting Tools

219

44. Buljan, S.-T. and Sarin, V.K., “Design and Wear Resistance of Silicon Nitride-Based Composites,” in: Inst. Phys. Cont. Ser. No. 75 (E.A. Almond, ed.), pp. 873-882, Adam Hilger, Ltd. 45. Buljan, S.-T. and Wayne, S.F., “Wear and Design of Ceramic Tool Materials,” Wear (in press). 46. Buljan, S.-T. and Wayne, S.F., “Silicon Nitride-Based Composite Cutting Tools; Materials Design Approach,” Ah. Ceram. Mater. 2(4): 813-16 (1987). 47. Sarin, V.K. and Buljan, S.-T., “Coated Ceramic Cutting Tools,” in: High Productivity Machining (V.K. Sarin, ed.), pp. 105-112, ASM (1985). 48. V.K. &u-in, S.-T. Buljan, and C. D' Angelo; U.S. Patent 4,406,667; September 27, 1983; assigned to GTE Laboratories Incorporated. 49. V.K. Sarin, S.-T. Buljan, and C. D’Angelo; U.S. Patent 4,416,670; November 22, 1983; assigned to GTE Laboratories Incorporated. 50. V.K. Sarin, S.-T. Buljan, and C. D’Angelo; U.S. Patent 4,421,525; December 20, 1983; assigned to GTE Laboratories Incorporated. 51. Huet, J.F. and Kramer, B.M., “The Wear of Ceramic Tools,” Proc. 10th N. Am. Manufact. Res. Conf. Proc., pp. 297-301, SME (1982). 52. Wright, P.K., Home, J.G., and Tabor, D., “Boundary Conditions at the Chip-Tool Interface in Machining: Comparisons between Seizure and Sliding Drictions,” Wear 54: 371-90 (1979). 53. Nakajima, K., Ohgo, K., and Awano, T., “Formation of a Built-Up Edge during Machining,” Wear 11: 369-79 ( 1968). 54. Ohgo, K., “The Adhesion Mechanism of the Built-Up Edge and the Layer on the Rake Face of a Cutting Tool,” Wear 51: 117-26 (1978). 55. Kramer, B.M. and Suh, N.P., “Tool Wear by Solution: a Quantitative Understanding,” J. Eng. Ind. 102: 303-09 (1980).

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56. Kramer, B.M. and Hartung, P.D., “Theoretical Considerations in the Machining of Nickel-Based Alloys,” in: Cutting Tool Materials. (F.W. Gorsler, ed.), pp. 57-74 (1981). 57. Bhattacharyya, SK., Jawaidand, A., and Wallbank, J., “Wear of SiAlON Tooling in the High Speed Machining of Aerospace Materials,” Muter. Technol. 10: 482-89 (1983). 58. Wayne, S.F. and Buljan, S.T., “Wear of Ceramic Cutting Tools in Ni-Based Superalloy Machining,” Trans. STLE (in print). 59. Lee, M., Horne, J.G., and Tabor, D., “The Mechanism of Notch Formation at the Depth-of-Cut Line of Ceramic Tools Machining Nickel Base Superalloys,” Proc. Inter. Conf. on Wear of Mater. ASME: 460-69 (1979). 60. Hasselman, D.P.H., “Figures-of-Merit for the Thermal Stress Resistance of High-Temperature Brittle Materials: A Review,” Ceramurgia Inter. 4(4): 147-50 (1978).

9 Aluminum Oxide Coatings for Cemented Carbide Cutting Tools

Donald E. Graham Carboloy, Inc Warren, MI

INTRODUCTION The history of cutting tools is marked by periodic inventions of materials that result in dramatic improvements in productivity. In the 1890’s it was high speed steel. Later came the cast non-ferrous materials, and later still in the 1920’s, cemented carbides became available. Since the development of carbide, many other cutting tool materials have been developed or improved including steel cutting carbides, cermets, ceramic cutting tools of various types, cubic boron nitride and diamond - but none have had the immediate and overwhelming commercial success as have overlay coatings. Titanium introduced carbide (Tic) coatings were commercially in 1969. Since then, a multitude of coatings have been developed and include titanium nitride (TIN), hafnium carbide and nitride, zirconium nitride, carbonitrides, boron containing coatings, and various oxides, including aluminum oxide (Al,O,). The most successful of the coatings for general (usually ferrous) machining applications are TIC, TiN and Al,O,. Each of these coatings is effective in its own particular arena. In a rather simplistic way, the TIN coating is preferred at low speeds because it is the most effective in preventing metal build-up. TIC, because of its hardness at low and intermediate temperatures, is most effective at “medium” speeds where mechanical abrasion is the 221

222

Ceramic Cutting Tools

predominate failure mechanism. The coating that provides the greatest potential for productivity improvement is A&O,. The advantage of A&O, lies in the fact that it is inert chemically and that it retains its hardness to higher temperatures than do the other coatings. Today, A&O, coatings are usually obtained in multiple layer products where all three of these coatings are combined in an attempt to realize the best properties of each. This will be described in more detail later. Two forms of A&O, coatings are available: the alpha form which is the stable version and was the first type used, and the kappa form. The alpha is the most common form and is probably the best form for cast iron machining. Recently K-Al,O, has been stabilized in thin layer form. Its advantages are a fine grain structure, layers that are usually smoother and more uniform, and it is generally freer from defects than the alpha form [l]. The deposition process for CVD A&O, coatings is well established. The formation of this oxide can be described by the following overall reaction:

zAlCl,(g)

+ 3CO,(g) + 3H,(g) -> Al,O,(s)

ADVANTAGES

+ 3CO(g) + 6HCl(g)

OF COATED TOOLS

Almost all of the carbides used in high productivity ferrous machining today are coated. The reasons for this popularity are many but the most important are increased productivity (and reduced tooling cost) and flexibility. With regard to productivity, the presence of a coating allows for a significant increase in cutting speed that can be obtained with no loss of life. And in fact, while a coated insert can replace an uncoated one under the same conditions and provide dramatically increased tool life, increasing cutting speed is the most cost effective way to utilize coatings. The productivity improvements possible with A&O, are shown in Figures 9-la and 9-lb where they are compared with TIC coatings and uncoated material. Figure 9-la shows tool life as a function

Aluminum Oxide Coatings for Cemented Carbide Tools

223

1000, AK3 1045 Steel 180 BHN 0.25 mm/rev. feed 2.5 mm DOC

a>

1001 1

10 Tool Life (min. to 0.25 mm flank wear)

1 IO

G4000 Cast Iron 210 BHN 2.5 mm DOC

101

Figure

100 10 Tool Life (min to 0.25 mm flank wear)

Productivity 9-1. coatings.

improvments

possible

with alumina

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Ceramic Cutting Tools

of speed for coated and uncoated inserts used to machine a steel [2]. The increase in cutting speed obtainable with Al,O, coatings can significantly improve the productivity of the machining process. The productivity advantage of A&O, is even more dramatic when machining cast iron (see Figure 9-lb) although the reasons for this are still not completely understood. Flexibility is also an important benefit. Increased knowledge and experience gained over the last two decades have led to the development of a wide variety of coated products, from very hard tools for high speed finishing operations to very tough tools that still have good tool life and speed capability. But because hardness, wear resistance and chemical stability can be obtained in the coating, and deformation resistance and toughness can be concentrated in the substrate, grades available today cover a very broad application range relative to uncoated inserts. For example, with uncoated carbide, a user is forced to accept a great reduction in breakage resistance in order to get high wear and deformation resistance. With the advent of coatings you can combine an extremely wear resistant surface layer with a tough substrate and end up with a tool that is both wear resistance and tough. The practical advantages of this are illustrated schematically in Figure 9-2 where wear resistance is plotted against breakage resistance [3]. In general, a certain minimum toughness is required for a tool to be generally useful. In Figure 9-2, that minimum toughness is defined as “A”. If one is restricted to the use of uncoated tools, he obtains “B” units of wear resistance which results in a certain number of minutes tool life. If, however, one can move to coated inserts, one gets “C” units of wear resistance greater tool life - at the same toughness. Conversely, if a shop required a specific tool life (10 minutes, or 4 hours, or a certain number of parts, etc.) arbitrarily defined as “B”, with an uncoated tool the user gets “A” units of toughness. If that happens to be the bare minimum, there will be no “slop” in the system; e.g. any little vibration, hard spot in the work material, etc. will result in chippage of the tool. If one moves to coated tools, “D” units of toughness can be used at “B” units of wear resistance. Thus the user has a tool that is much more forgiving.

Aluminum Oxide Coatings for Cemented Carbide Tools

225

C

Wear Resistance

Breakage Resistance

Figure 9-2. Wear and breakage resistance for coated and uncoated

tools.

The result of this is that any coated grade will cover a wider application range than uncoated tools which means there will be fewer application mistakes, a user can keep smaller, simpler inventories, and grade selection is easier.

WEAR MECHANISMS With today’s machine tools, high temperatures and heavy chip loads are common and the successful tool material must have the means to withstand them. Tools typically fail as a result of combinations of abrasion, chemical interaction with the work material, and/or chippage. Many of the commonly encountered

226

Ceramic Cutting Tools

failure mechanisms are dramatically affected by an Al,O, coating. Crater wear, flank wear, built-up edge, and, in some cases, notching, can all be reduced when a tool is coated with A&O,. Deformation resistance and toughness are primarily functions of the substrate and are not impacted by the presence or absence of a coating. Crater Wear The most important advantage that A&O, coatings provide is resistance to diffusional or crater wear. Cratering is a serious problem in machining ferrous materials and becomes more significant as the temperature (speed) is increased. This process can be visualized, in a simplistic way, as the dissolution of the tool material into the work material. Because this reaction depends primarily on the stability of the tool material in the presence of the work material, the most important coating parameter is the free energy of formation in the presence of the work material. To a first order of approximation those tool materials that have the lowest free energy of formation in the presence of the work material will be the most effective. Thus an assessment of the crater resistance of coatings could be made by looking at the thermodynamics of the tool/work material system and the diffusion rates of the elements of the coating in the work material. Unfortunately, these calculations are very complicated and have not been done. But while the exact calculations have not been done, experience has shown that the standard free energy of formation can be used to approximate the relative performance of various coatings. These values are shown in Figure 9-3. The practical result of this is shown in Figures 9-la and 9-lb where it can be seen that at high speeds where chemical factors are relatively more important than mechanical factors, the coating that provides the greatest tool life is the oxide coating. As shown in Figure 9-4, where crater resistance is plotted as a function of coating thickness, any of the popular coatings provide some crater resistance, but the lower the free energy of formation the greater the resistance to dissolution.

0

WC

-............. .. ..... -20 ,___.___.__...__._..............................................................................................................................................................................

-40

-60

-80

-100

-120

0

I

500

I

1000

I

1500

I

2000

2 00

Temperature (C) Figure 9-3. Standard free energy of formation

vs. temperature

for various tool materials.

16

8

AlSl 1045 Steel 180 BHN 0.36 mm/rev 260 m/min

4

8

12

Coat Thickness (microns) Figure 9-4. Crater resistance vs. coating thickness for various tool materials.

1

Aluminum Oxide Coatings for Cemented Carbide Tools

229

Hardness is another important variable. Even though the coating may stay intact chemically, the action of the chip against the chip contact zone can wear through the coating by abrasion. The hardness of A&O, at room temperature is lower than the other coatings but it retains its hardness more effectively as temperature is increased. This is shown in Table 9-l. Below temperatures of 500°C the Tic coating is harder and would be expected to provide the best wear resistance. Above that temperature - and in many Table 9-1. Hardness Properties

of Tool Coatings.

COATING

HARDNESS 25”C, kg/mm’

HARDNESS lOOO”C, kg/mm2

TIN

2000

190

TIC

2500

200

Al@,

2000

300

steel machining operations where the speed surpasses 200 m/min (650 sfpm) the temperature exceeds 1000°C - the Al,O, coating is harder and provides the best abrasion resistance, both on the flank and in the chip contact region. A third variable that is important is thickness. As would be expected, the thicker the coating the longer it will take to wear through, either chemically or mechanically. This is illustrated in Figure 9-5 where a series of curves describes crater wear versus time in cut for tools coated with different thicknesses of Al,O,. The coating thickness on each insert is shown in the figure. Three important conclusions can be drawn from Figures 9-4 and 9-5. + First, coating obtained then the

the crater wear rate increases significantly after the Relatively low wear rates were is penetrated. up to crater depths of about the coating thickness, rate increased.

230

Ceramic Cutting Tools

\

‘\

Aluminum Oxide Coatings for Cemented Carbide Tools

231

+ Second, even after the coating wore through and the wear rate increased due to contact between the chip and the substrate, the rate at which the crater depth increased was strongly influenced by the presence of coating at the edge of the crater. The slopes of the curves in Figure 9-5 decrease with increasing coating thickness. When the coating was 8.5 microns thick the rate of wear was onetenth that of the uncoated insert. + Third, crater resistance is directly proportional to coating thickness as shown in Figure 9-4. This linear relationship holds true regardless of how one chooses to define crater growth. The crater wear behavior of a coated insert has two major stages. During the first stage, which lasts until the chip first penetrates the coating, the very high chemical stability of the coating substantially retards crater growth. The duration of this stage of wear is directly proportional to coating thickness and is about twice as long per unit thickness for A&O, coating as for Tic and TiN coating as shown in Figure 9-6. Thus at high speeds, the oxide coating provides the greatest tool life. Once the crater penetrated the coating, the crater wear rate increases rapidly due to increasing contact between the chip and the substrate material, with its inherently higher wear rate. During this second stage, the coating at the edge of the crater impedes further growth, an effect that increases with coating thickness, at least up to the thicknesses studied. Flank Wear At lower speeds or when machining abrasive materials, abrasive wear is relatively more important than crater wear. Under those conditions the coating with the highest hardness will work best. This is shown in Figures 9-6 and 9-7 where identical inserts coated with 5 p,rn thick coatings of TiC and A&O, were used to machine iron and steel. Results of machining nodular iron (a very abrasive material) machined at a speed of 230 m/min (760 sfpm)

Oxide Coated 0.25z E+ iz 3 x

0.2-

0.15-

: ii O.l-

Nodular Iron 230 m/min 0.25 mm/rev

0.05-

0' 0

I 1

1 2

I 3

I 4

I 5

I 6

I 7

1 6

Time in Cut (min)

Figure 9-6. Flank wear vs. time for Al,O, and TIC coated tools, cutting speed = 230m/min.

0.4-

0.35iTiC Coating

/

0.3 z 0.25 .E. z $ Y Eg

OS2 0.15 0.1 0.05. 0.25 mm/rev 0.

I

I

I

0.5

1

1.5

i

I

2 2.5 Time (min)

I

I

3

3.5

Figure 9-7. Flank wear vs. time for A&O, and TiC coated tools, cutting speed = 300 m/min.

234

Ceramic Cutting Took

is shown in Figure 9-6. Under these conditions and cutting temperatures, the TiC coating is harder than the Al,O, coating and provides better tool life. As shown in Figure 9-7, the speed is increased while holding all other conditions constant. At 300 m/min (1000 sfpm) the temperatures are higher and the A&O, coating is harder and more wear resistant than the TiC coating. The effect of coating thickness is apparent in Figure 9-8 for both coatings on the same nodular iron. The flank wear resistance first increases with increasing coat thickness but then levels off as the thickness is increased beyond the 5-6 micron level. This is quite different from the crater wear situation where crater wear resistance showed no signs of leveling off up to thicknesses of almost 10 pm. The reason for this leveling off of performance has been described earlier [2] and is related to the fact that the critical region for flank wear appears to be a narrow zone at the bottom of the flank wear scar. The coating need cover only this zone to provide a contact bearing surface between the cutting edge and the workpiece to improve flank wear resistance. This bearing surface wears away slowly by a combination of mechanical abrasion and chemical reaction. As cutting speeds increase, chemical wear becomes more important and the Al,O, coating provides better tool life. Built-up Edge A third failure mechanism that is strongly influenced by the presence of an Al,O, coating is built-up edge. Built-up edge is a problem particularly at low speeds and is caused when particles of the work material become pressure welded to the tool edge. As speeds are increased, the chip passes the cutting edge too quickly for diffusional bonding to occur and consequently build-up decreases. With this in mind, a series of experiments was done wherein the volume of build-up on the edge of inserts coated with different coatings was measured as a function of speed. The substrates were equivalent in every way except for the composition of the coating. As shown in Figure 9-9, all coatings will prevent built-up edge to some extent but some are better than others.

Nodular Iron 130 m/min 0.25 mm/rev 20

0

I 1

I 2

I I I I I 4 5 6 7 3 Coating Thickness (microns)

1 8

I 9

Figure 9-8. Flank wear vs. time for A&O, and TIC coated tools, cutting speed = 130 m/min.

’ 3

236

Ceramic Cutting Tools

Aluminum Oxide Coatings for Cemented Carbide Tools While the nitride coatings are the most effective, effective and is preferred over uncoated inserts.

237

Al,O, is also

Notching Depth-of-cut line notching is a more complicated failure mechanism because so many factors can contribute to it. Notching is usually caused by mechanical means such as; chippage, abrasion from surface scale or a work hardened layer, etc., but on occasion it can be caused by chemical interaction between the work material and the tool in a manner similar to cratering. Figure 9-10 shows the effect on notching of the presence of coatings under conditions chosen to generate a notch by chemical factors. It can be seen that an A&O, coated tool provides better performance than the other The reasons for this behavior are the same as those tools. presented for the resistance of cratering. Multi-layer

Coatings

The trend in cutting tools today is to combine several coatings in one tool in order to obtain the advantages of each composition [4]. Assuming the individual layers are thick enough to be effective, it should be possible to obtain an insert that resists build-up because of the presence of TIN, resists abrasion and wear due to the presence of Tic, and resists cratering and high temperature flank wear because of the presence of A&O,. Conceivably, such a tool would be effective from very low to very high speeds. While this goal will probably never be fully achieved, improved bonding techniques and the development of superior substrates have resulted in a very broad range of products. Multilayer products are truly general purpose inserts. These multi-layer products usually have coating thicknesses that range from 5-15 pm (depending on the intended application) but are typically on the order of lo-12 pm. In the past, such thicknesses would severely compromise the insert strength, but because substrates are now finely tuned, e.g., better “marriage partners” for the coating, inserts with good toughness and wear resistance at the same time can be obtained.

1045 Steel 210 m/min 0.4 mm/rev 2.5 mm DOC 1 z+ E. c 0.8Ii. $ r 0.60 -E z 0.4Oxide Coat /

Time in Cut (min)

Figure 9-10. Notch depth vs. time for TiN, TiC and A&O, coated tools.

Aluminum Oxide Coatings for Cemented Carbide Tools

239

Usually inherent with the multiple layers is a smoother Continual recoating, particularly when the coating is thin. nucleation of the various layers keeps the grain size small which results in a smoother, more uniform coating. This is helpful in preventing built-up edge. As suggested earlier, even today’s broad range products are tailored for specific application areas. Coatings intended for low speed operations - threading, grooving, cut-off, or heavy duty cutting or milling - are usually thin (-5 pm) and are made up of titanium base coatings and seldom contain Al,O,. On the other hand, inserts that are intended for high speed operations or are targeted at the cast iron market usually have thicker A&O, layers. As one moves from the first application area to the second, more and more A&O, is added at the expense of the titanium base coatings.

SUMMARY Alumina overlay coatings provide significant advantages to tool life and productivity. Relative to other coatings, the oxide coating provides increased speed capability because of its good chemical stability, particularly in the presence of ferrous materials. This crater wear resistance is more than twice that of TIC and TiN coatings and is directly proportional to coating thickness. Alumina coatings are also effective at preventing flank wear, particularly at high speeds and when machining cast irons. Flank wear resistance also increases with coat thickness but reaches a point of diminishing returns where increases in thickness do not result in increases in wear resistance. The critical region for flank wear appears to be a narrow zone at the bottom of the flank wear scar. The coating need only cover this zone to be effective. Built-up edge can be eliminated or minimized relative to uncoated inserts by the presence of Al,O, coatings. On occasion, these coatings can also prevent depth of cut line notching.

240

Ceramic Cutting Tools

REFERENCES 1. J. Skogsmo and S. Vuorinen, Metallurgical Coatings 1990: Proceedings of the International Conference, Elsevier, (1990). 2. T.E. Hale and D.E. Graham, Cutting Tool Materials, ASM Conference Proceedings , ASM, p. 175 (198 1). 3. D.E. Graham and T.E. Hale, The Carbide and Tool Journal, Vol. 14, No. 3, p. 34 (1982). 4. A.T. Santhanam and P. Tierney, “Cemented Carbides,” Metals Handbook - Ninth Edition, Vol 16, ASM, p. 71.

10 Polycrystalline Diamond and Cubic Boron Nitride

Ernest Ratterman General Electric Company Worthington, OH

EARLY HISTORY

Harold P. Bovenkerk HP Consulting Worthington, OH

OF DIAMOND

Centuries ago, after man first discovered diamond crystals, it was realized that diamond was an unusually hard material. In the evolution of man, tools made of hard materials became increasingly important in the production of food, weapons and shelter and it did not take long for man to experiment with diamond as a tool. Thus, the evolution of tools followed a progression from shells and bones, to stone and native metals, to However, the rarity and fabricated metals and ceramics. availability of diamond limited the early applications to scribing and engraving tools. The popularity of diamond as a gemstone provided the motivation for serious prospecting. As a result, many diamonds of lesser quality became available for industrial uses such as, grinding, polishing and cutting. In the early part of the twentieth century, diamond tools made by mechanically hand setting or embedding in metal rods were used to a limited degree for drilling and abrading workpieces. Later, with the advent of processing bodies by sintering powdered metals, diamond powder was incorporated into metal matrices by this process. This form of diamond tool was first made in the 1920’s. In the 1930’s, diamond powder was incorporated into

243

242

Ceramic Cutting Tools

grinding wheels and other tools with a polymer or resin matrix. This was followed by tools made with a glass or vitreous matrix. The advent of increased availability of diamond not suited for jewelry coincided with the increased use of cemented carbides and hard ceramics. This began in the 1930’s but with the advent of World War II, the great demands on productivity in the fabrication of military goods soon turned industrial diamond from a material in surplus to a material in scarcity. Man has long been intrigued by diamond, what it was composed of and how nature made it. Alchemists in the Middle Ages tried to make diamond, however, such attempts were futile since the understanding of the chemical nature of diamond was not known. About 200 years ago, it was discovered that diamond was a form of the common element carbon. Seventy years ago, with the advent of x-ray crystallography, it was determined that diamond was composed of an array of carbon atoms with very strong chemical bonding of the atoms in a three dimensional cubic lattice. This information provided the scientific basis for diamond synthesis. Recent History of Industrial

Diamond

The shortage of diamond for industrial uses in the period surrounding World War II, provided a major impetus for diamond synthesis. One of the many attempts to make diamond was made by the General Electric Company. In 1955, researchers at GE announced that diamond had been reproducibly synthesized. This first diamond was a poorly crystallized powder not suitable for jewelry, and as such, the immediate focus was to exploit this discovery for production of industrial diamond powder. Following a major effort, in 1957 GE announced the commercial availability of synthesized diamond powder for use as an industrial abrasive for grinding and polishing applications. This first synthesis of diamond, as with the vast bulk of diamond made today, was accomplished by transforming carbon into the denser diamond form by utilizing extreme pressures and temperatures. It was realized soon after the original synthesis that

Polycrystalline Diamond and Cubic Boron Nitride

243

by varying conditions of pressure, temperature, time and the chemistry of the reaction that diamond crystals of widely different characteristics could be grown. These controlled characteristics include the size, shape, internal structure, surface characteristics and chemical composition of the grown crystals. This resulted in thousands of synthesized diamond products covering a full array of industrial applications. The advent of synthesized diamond remedied the problem of scarcity and stimulated the industrial use. The supply of differentiated diamond products (with the attendant performance advantages), the improvements in diamond tools and machines, and the increasing use of hard materials, has greatly stimulated diamond consumption. In 1940, it was estimated that the world consumption of industrial diamond was about one million grams, in 1993, use is estimated at more than 100 million grams. Since one gram is the exact equivalent of five carats, world consumption is in excess of 500 million carats. Other Super Hard Materials Following the reproducible synthesis of diamond, GE scientists theorized that boron and nitrogen could also be made into a close packed cubic structure like diamond. The compound, boron nitride, was known from the 1920’s but it had a soft, hexagonal After numerous structure like the graphite form of carbon. experiments, in 1957, GE announced the synthesis of cubic boron nitride, a very hard compound in the cubic structure with a density close to diamond. After further evaluation, it turned out that the cubic BN was almost the perfect complement to diamond as a material of industrial volume. Although not quite as hard as diamond, it had greater resistance to oxidation and a lower chemical reactivity. Small particles of diamond and cubic BN, although well suited for use in grinding wheels and as polishing agents, are not very useful as cutting tools. Large single crystals of diamond or cubic BN are very costly to grow and lack the toughness needed for most cutting tool applications. Therefore, from the beginning of synthesized diamond there was focus on making polycrystalline

Ceramic Cutting Tools bodies much like those used for steel, carbide, ceramic or cermet cutting tools. Success was finally achieved and by the early 1970’s, sintered diamond became commercially available, followed a few years later by well- sintered CBN. These materials were different than most cermets or ceramics in that the diamond and cubic BN grains are directly sintered together without the presence of a binder phase. The properties, therefore, are like diamond or cubic BN single crystals but with the advantage of being isotropic, hence of greater toughness and wear resistance. Since the working face of most cutting tools is two dimensional, sintered diamond (polycrystalline diamond or PCD) or sintered cubic BN (PCBN) are most commonly made as a thin layer (0.2 to 1 mm thick) on a cemented carbide substrate. This configuration has several advantages. The composite of diamond or cubic BN with cemented carbide offers greater toughness and is less expensive than solid PCD or PCBN. Furthermore, the carbide substrate can be readily brazed onto a standard carbide insert or a steel shank for a full range of machining applications. As technology improved, monolithic bodies of PCD and PCBN became available. In a further development, two-phase strongly bonded polycrystalline diamond and cubic BN became commercially available. PCD and PCBN are very important industrial products with cutting tool applications ranging from metals, glass and ceramic, to aggregates such as concrete and stone. Cutting tools of PCD are used in drill bits for oil and gas drilling and PCD is widely used in dies for drawing wire. Properties of PCD and PCBN In addition to the well known fact that diamond and cubic BN are the hardest materials ever found or made, there are other superior properties of these two materials. As shown in Tables lol-- 10-6, diamond is the stiffest material known and has very useful properties such as high thermal conductivity, low coefficient of friction and low thermal expansion. All of these properties are important to cutting tools.

Polycrystalline Diamond and Cubic Boron Nitride

245

In cutting tool applications, all of the properties listed in the Tables is important. In addition, since in many instances the cutting action takes place at high temperatures, hot hardness is very important. PCD and PCBN retain their hardness advantage over all cutting tool materials even at temperatures exceeding 1000°C. Table 10-l.

Comparative

Hardness

Material

Knoop Hardness Kg/mm2 (25°C)

diamond

6000- 11000

cubic BN

4000-5000

boron carbide

2200

tungsten carbide

2200

aluminum

2000

oxide

silicon carbide

1800-3900

hard steels

400- 800

Table 10-2. Modulus of Elasticity

Material

E-10 dynes/cm2

diamond

105

tungsten

34

steel

17-20

246

Ceramic Cutting Tools

Table 10-3. Thermal Conductivity

Material diamond

watts/cm’C

(typical)

20

cubic BN

13

silver

5

tungsten

1.7

iron

0.8

quartz

0.1

Table 10-4. Thermal Expansion

(25°C)

Coefficient

Material

lo-V”c (25OC)

diamond

0.8

silica

0.5

tungsten

4.5

alumina (sapphire)

5

Polycrystalline Diamond and Cubic Boron Nitride

Table 10-5. Friction Coefficient

Material

Coeffkient

diamond

0.05

teflon

0.05

tungsten carbide

0.20

alumina (sapphire)

0.20

iron

1.00

lubricated metal

0.10

Table 10-6. Transverse

Rupture Strength

Material

TRS lo3 lb/in2

PCD

120-200

PCBN

100

cemented

carbide

250-300

alumina

40-50

high speed steels

300-400

247

248

Ceramic Cutting Took

In addition to the above properties, the chemical reactivity between the cutting tool material and the workpiece must be taken into consideration. Diamond is totally inert to metals such as aluminum, copper, and zinc but reacts at high temperatures with metals having high carbon solubility such as iron, nickel and cobalt and carbide forming elements such as titanium, zirconium, tantalum and tungsten. In addition, diamond is inert to plastic, glass and ceramic. Machining aluminum, zinc or copper based alloy is ideal for diamond as is the cutting of plastic/glass composites. In addition, diamond is widely used in cutting wood and wood composites. To avoid chemical wear in metal cutting where a chemical reaction is possible, diamond should be used for cutting reactive metals only under conditions where cutting temperatures are below the reaction temperature. This can be done by controlling tool geometry, cutting speeds and coolants. Threshold temperatures for workpiece chemical reaction vary with the metal in question but can be typically at temperatures in the 400°C to 700°C range. If a carbon soluble or carbide forming metal is totally saturated with carbon, then machining with diamond is less of a problem. Diamond also can wear due to oxidation. Temperatures in excess of 700°C are needed before this becomes a factor. Despite limitations, diamond has become a widely used and very cost effective cutting tool material, especially in the automotive and aerospace industry. The silicon aluminum alloys, metal matrix composites and fiber reinforced plastics now used in these industries can only be effectively machined with PCD tools. PCBN is also inert to the same materials as diamond but because of its lower hardness does not generally compete with diamond in cutting these workpieces. However, for iron, nickel and cobalt based alloys, PCBN is less reactive than diamond hence chemical wear is minimized. PCBN also has more tolerance to oxidation than diamond. Taking these properties into account, the major cutting tool applications for PCBN are ferrous alloys and high temperature alloys based on nickel, cobalt and iron. In balance, diamond and cubic BN are very complimentary to each other in applications such as cutting tools and the growing

Polycrystalline Diamond and Cubic Boron Nitride uses of these cutting tool materials exceeds 100 million dollars. Guidelines

for Machining

are now an industry

with Polycrystalline

249 which

Diamond Tools

Polycrystalline diamond tools are successfully used in many industries for the machining of non-ferrous metals and many Machining applications for PCD tools non-metallic materials. continue to grow as newly developed and difficult to machine engineered materials are accepted by product manufacturing fundamental reasons why industries. There are many manufacturing management and process engineers must acquire basic knowledge about the effective use of PCD tools in modem When considering the potential for machining processes. effectively using PCD tools, manufacturing personnel may be faced with problems in any of the following areas: l

l

l

l

the need for improved and more consistent quality, having to machine “difficult-to-machine” materials, increased productivity, and improved tool life.

Of paramount importance in this scope of knowledge is a more than casual grasp of the economics of the machining process. PCD tipped tools may cost 10 to 15 times more than the Therefore, PCD tools must conventional tools they replace. provide technical performance capabilities well beyond A clear understanding of the relationship conventional tools. between technical performance and their purchase cost is of central importance. Some of the possible machining and performance capabilities of PCD tools are: An increase in process capability (Cp and Cpk) and throughput by improving part-to-part consistency in dimensions and finish during longer production runs of thousands to tens of thousands of parts. l

250

Ceramic Cutting Tools . Elimination

of excessive

tool changes.

Roughing and finishing of engineered difficult or impossible to machine. l

materials classed as

Reduction in costs brought about by lowering losses incurred from scrap, rework, shorting, reinspection, utilization of nonconfotming materials, field failures and warranties. l

Precise control of finish and dimension to enhance appearance, life and marketability of products. l

Organization of PCD Machining Guidelines This introduction to the uses of PCD tools is organized in the following sequence and is supported with tables, graphs and figures. Identify whether or not your application general scope of PCD tool applications. l

falls within the

Become familiar with the scope of commercially PCD tipped tools. l

l

available

Select the correct grade of PCD tool tip material.

Set up operating conditions of speed, feed and depth of cut based on established guidelines for the application. l

Develop a sound understanding of the factors which influence total machining costs relevant to the application. l

Polycrystalline Diamond and Cubic Boron Nitride Select the Application

- A Material/Industry

251

Guide

Current major applications for PCD tools are found in Table 10-7. This table indicates basic characteristics of workpiece, principle industries using such materials, and some typical end products. It should be emphasized that this listing contains a broad spectrum of non-ferrous metals and non-metallic materials. In general, PCD tools should not be applied in the machining of ferrous metals or high temperature alloys. The list of materials in Table 10-7 is not intended to exclude other non-ferrous metals or non-metallic materials. Since PCD tool materials are still evolving, so are their applications which are as yet beyond the scope of the hard experience base of these application guidelines. Supporting observations to this table are as follows: PCD tools are rapidly displacing conventional tools in the machining of silicon aluminum alloys principally in automotive but in other industries as well. For example, virtually all piston production worldwide is with PCD tools. Very high productivity and quality are the driving qualifying advantages. l

Metal matrix composites (MMC) are an example materials which cannot be effectively machined production applications with other than PCD tools. l

of in

High silicon aluminum engine blocks with cast iron cylinder liners are examples of dissimilar metal components which present a real challenge for machining operations. PCD tools may solve the tool life, finish and dimensional problems in such operations. l

. The ability of PCD tools to maintain an extremely sharp edge in very long production runs qualifies PCD in the manufacture of a range of bronze, copper, babbitt and copper/lead alloy components.

252

Ceramic Cutting Tools

Table 10-7. Workpieces

Machined

with PCD.

Workpiecc Materials

Producing Industries

Typical Products

Aluminum Alloys Lo, Mcd, Hi Silicon 319 336 380 384 390 413

Automotive Small Engine Electrical Equipment Home Appliances Computer/Electronic

Transmission Housings Engine Blocks Wheels Pistons Compressors Frames Pumps Discs

Metal Matrix Composites (MMC) 10% V to 30% v Silicon Carbide or Aluminum Oxide

Automotive Aerospace General Industry Sporting Goods

Brake Rotors Pulleys Jet Engine Components Bike Frames Tennis Rackets Golf Clubs

Aluminum/Cast Iron Dissimilar Metals

Automotive

Engine Blocks

Brass/Bronze Copper Copper/Lead

General Industry Electric Motors

Bearings Pumps Valves Liners Commutators

High Cobalt Cemented Carbides (>lO% Co)

Tubing Mold/Die

Molds Dies Punches

Wood Fiber Wood Laminates

Wood Products Construction

Furniture Wall/Floor Panels

Composites and Laminates of Resins, Glass, Metals, Asbestos, Ceramics

Aircraft Marine Automotive

Pulleys Structures Valves Rotors

Polycrystalline Diamond and Cubic Boron Nitride

253

PCD tools may be used in turning operations to replace tedious diamond grinding. This is a great productivity advantage in the manufacture and reconditioning of die, mold and punch components made of high cobalt cemented carbides. l

The high speed production of a wide range of wood and wood fiber products may make effective use of the very low wear rate of PCD in routing, milling, jointing and profiling these materials. l

Composites and laminates containing resins, glass, glass fibers, rubber, graphite, titanium, asbestos and ceramics These are may be readily machined with PCD tools. typically used in the aircraft, marine and automotive industries. l

Guide to Selecting The Most Effective Grade of PCD The properties and characteristics of PCD cutting tool surfaces are controlled in the manufacturing process. The key properties of abrasion resistance and impact resistance can be controlled through the size of the diamond abrasive particles used This flexibility allows for a in the manufacturing process. significant degree of optimization of tool life in balancing the varied application demands in the turning and milling of the materials cited in Table 10-7. There are three basic grades of PCD tool tip surfaces. These are defined as fine, medium, fine and coarse. The diamond particle size distribution which defines each grade, the important properties and characteristics and how these properties relate to application criteria are found in Table 10-8. A detailed application recommendation by major families of workpiece materials is shown in Table 10-9. In general, use of the coarse grade is recommended unless there are specific application demands with respect to finish which the coarse grade cannot meet. In addition, there are no tool

254

Ceramic Cutting Tools

Table 10-S. PCD Grade Descriptions.

PCD Grade

Avg. Diamond Particle Size twm)

Characteristics

Application Guide

Coarse

25

Most impact resistant

- All milling and interrupted cuts in silicon aluminum alloys

Most abrasion resistant

- Extremely abrasive materials - Cemented carbides

MedFine

5

Medium impact resistance Medium abrasion resistance

- General purpose turning of low and medium silicon aluminum alloys

- Non-ferrous metal Good finish - Composites/laminates

Fine

4

Best surface finishes

- Low abrasion, noninterrupted applications

EDM/EDG edge

-

Polycrystalline Diamond and Cubic Boron Nitride Table 10-9.

255

PCD Grade Selection.

Workpiece Materials

Grade of PCD

Aluminum Alloys Lo, Med, Hi Silicon 319 356 380 384 390 413

3 19 1 fine - surface finish 356 J coarse - tool life

Metal Matrix Composites (MMC) 10% v to 30% v Silicon Carbide or Aluminum Oxide

Coarse

Aluminum/Cast Iron Dissimilar Metals

Coarse

Brass/Bronze Copper Cooper/Lead

Fine - surface finish Coarse - tool life

High Cobalt Cemented Carbides (>lO% Co)

Coarse

Wood Fiber Wood Laminates

Fine

Composites and Laminates of Resins, Glass, Metals, Asbestos, Ceramics

Medium

380 1 medium or coarse 384 J 390 1 coarse 413 J

life or economic advantages in not using the strong coarse grade unless finish requirements demand otherwise. The exceptions to this generality are tools designed for the woodworking industries. The complex cutting edges required for woodworking can be readily produced in fine grade PCD with

256

Ceramic Cutting Tools

EDM or EDG processes. Such processesdo not require subsequent diamond grinding when used to make woodworking tools. Description of PCD Tipped Tools The most widely used PCD tools are tipped carbide inserts. These inserts are fabricated by creating a precision "packet" on a corner or comers of virtually any standard cemented carbide insert. (The specific grade of insert is important to the manufacturer, but is of no importance in its use as a PCD tool). The comer is then precision ground back to the radius and rake of the original. Examples of PCD tipped tools are shown in Figure 10-1.

Figure 10.1. Examples of PCD tipped tools (photograph courtesy of GE Superabrasives).

Polycrystalline Diamond and Cubic Boron Nitride

257

Virtually all machining of metals with PCD tools requires that the top rake surface be polished. This polishing operation provides much smoother chip flow, a sharper cutting edge, and reduces the risk of metal buildup on the cutting edge. Superabrasive tool manufacturers can supply PCD tipped tools for any application. All manufacturers can provide a detailed catalog clearly illustrating the range of tools available. In order to maximize the cost effectiveness of PCD tools, it is possible to have PCD inserts re-sized to original dimensions after normal maximum allowable flank wear. It is also possible to downsize PCD inserts to the next smaller inscribed circle (IC). The PCD tool supplier can provide details as to cost and logistics. Selection of PCD Machining

Parameters

The selection of machining parameters depends primarily on the specific nature of the properties and characteristics of the workpiece material. In addition, the basic nature of the process, turning, turning with interruptions and milling may also influence machining conditions. The capabilities and the limitations of the machine tool itself must also be considered. The general condition of the machine tool may well influence the life and other performance characteristics of any tool tip material. Hold PCD tools as rigidly as possible to eliminate extraneous vibrations and out-of-balance conditions in spindle and other rotating components. Avoid excessive overhang when setting up to run PCD tools. Set-up personnel and operators should be aware that the cutting edges of PCD tipped tools are as fragile as the edges of conventional carbide, ceramic and cermet tools. PCD tipped tools should always be handled with the same care and consideration given other tool materials. Effective economic use of PCD tools may depend on running the operation within the following recommended speed and feed limits. In general, PCD tools may be run at the same conditions as the carbide tools they replace. But maximum economic effectiveness may well depend on operating in the recommended guidelines.

258

Ceramic Cutting Tools

Table lo- 10 provides basic Speed/Feed Guidelines. starting guidelines for machining workpiece materials with PCD tools. These guides reflect the range of practical commercial operating experience. For those unfamiliar with the use of PCD tools, commence operations at or near the lower limits of speed and feed recommended. Always give prime consideration to producing parts or operations within specified tolerances on dimension and finish. As these conditions are satisfied, then begin the process of optimizing productivity through increases in both speed and feed. PCD Depth of Cut Guidelines. Most PCD tools are used in the form of tipped inserts. In this insert manufacturing process, a small PCD blank is brazed into a corner of the carbide insert. Thus the available length of PCD cutting edge is not the same length as the original carbide edge as it is limited by the size of PCD blank used in the process. It is therefore important that the depth of cut selected for any application not exceed 60% of the PCD cutting edge length. The purpose of this limitation is to insure that heat generated in the chip forming process does not soften the braze line between the PCD blank and the carbide insert. Note: This caution in no way limits the depth of cut capabilities of PCD tools in general. The specific guides and limitations are illustrated in Figure 10-2. PCD Rake Angle Guidelines PCD tools should be used with a positive back rake. However, it is important that the included angle between the rake face and the flank relief angle be as large as possible. This minimizes shear stress on the PCD cutting edge. These guides and cautions are illustrated in Figure 10-3. Milling cutters designed for operations with conventional inserts may not provide optimum performance when used with PCD inserts. A range of geometries have been successfully used with PCD inserts. In general, a positive axial/positive radial rake cutter can be used for many applications. But where machining conditions or workpiece properties are more demanding, positive axial/neutral radial and neutral axial/positive radial are effective In all cases, small positive rakes of 5” to 7” should be maintained.

Polycrystalline Diamond and Cubic Boron Nitride Table 10-10.

PCD Machining

259

Guidelines.

Workpiece Material

Speed (ft/min)

Feed (in/rev)

Aluminum Alloys Lo, Med, Hi Silicon 319 356 380 384 390 413

4% - 85 Si

3000-I 5000

0.004 - 0.025

9% - 14% Si

2000-8000

0.004 - 0.020

15% - 18% Si

1000-2500

0.004 - 0.015

Metal Matrix Composite (MMC) lOV% to 3OV% Silicon Carbide or Aluminum Oxide

Roughing

1000-1400

0.015 - 0.025

Finishing

1000-2000

0.005 - 0.015

Aluminum/Cast Iron Dissimilar Metals

Finishing

800- 1200

0.004 - 0.006

Brass/Bronze Copper Copper/Lead

Roughing

2000-2500

0.010 - 0.015

Finishing

2000-3500

0.004 - 0.008

High Cobalt Cemented Carbides (> 10% Co)

All

65-130

0.004 - 0.010

Wood Fiber Wood Laminates

All

300-1000

0.004 - 0.015

Composites and Laminates of Resins, Glass, Metals, Asbestos, Ceramics

All

1000-3500

0.004 - 0.012

260

Ceramic Cutting Tools

Note: Depth ofcut should nor cxced

Figure

60% oftoml FCD cutting cdgc available.

10-2. Guidelines for establishing tipped PCD inserts.

maximum

DOC with

Figure 10-3. Positive rake angles from 5-7” are recommended for machining with PCD tools. Relief angles should be a maximum of 15°C.

Polycrptalline Diamond and Cubic Boron Nitride

261

PCD Lead Angle Guidelines A conservative approach should always be used in setting up lead angles when machining with PCD tools. Leads of -15’ to -45’ should be used whenever possible. Lead angles from 0’ to slightly positive can be used where part geometry dictates. These guidelines are shown in Figure 10-4. PCD Nose Radius Guidelines Always select a nose radius as large as the work geometry will permit. In addition, use insert shapes which offer maximum strength and rigidity consistent with part geometry.

0” LEAD TOOL

LEAD ANGLE TOOL

to -45”

Figure 10-4. Illustration of recommended lead angles for PCD tools. Use negative leads of 15-45”. Zero to slightly positive leads may be used if necessary.

262

Ceramic Cutting Tools

PCD Coolant Use Guidelines Polycrystalline diamond blank tools can be used to machine parts either wet or dry. In most cases, tool performance is improved by the use of a properly applied cutting fluid. Soluble oil-water emulsions similar to those used when machining with cemented tungsten carbide tools are widely used with excellent results. Their lubricating qualities help to reduce frictional heating and the formation of built-up edges while providing good chip flow. To be effective, a cutting fluid must be supplied in a large steady flow to the rake surface of the PCD tools. Certain applications of PCD tools are used without any type of cutting fluid. The high thermal conductivity of PCD allows the tool to be used dry for many operations without reducing tool life. PCD Tool Edge Preparation PCD tools should always be run with an up-sharp edge. All PCD tipped inserts and tools are provided in this condition by suppliers. The only exceptions to this rule should be in milling or roughing applications where an approximate 0.0005 inch honed radius may be advisable. This guide is illustrated in Figure 10-5.

I

0.0005in.honc

Figure 10-S. PCD tools should always be run “up-sharp.” In milling and roughing a honed radius of 0.001” should be applied.

Polycrystalline Diamond and Cubic Boron Nitride

263

Summary of General PCD Application Guidelines To obtain the best tool performance and the most number of parts per cutting edge, the following guidelines should be closely followed: Use PCD cutting tools only to rough and finish nonferrous and nonmetallic materials. l

Select a rigid machine with enough horsepower to maintain the cutting speed where PCD tools perform best. l

PCD tools are very effective even when run at The maximum conventional carbide speeds. m: productivity of PCD can be obtained by operating at higher speeds. l

Establish speed and feed rates which will result in a cost-effective combination of high productivity and long cutting tool life. l

. Use rigid toolholders as possible.

and keep the tool overhang as short

Generally, the same tool geometry as that used for tungsten carbide tools is satisfactory but always reference specific guidelines for PCD tools. l

l

Use positive-rake

angles whenever possible.

Use the largest nose radius possible for better surface finishes and to spread the cutting force over a wider area. l

Use PCD tools with polished rake faces to reduce the friction of the chip and to produce better surface finishes. l

264

Ceramic Cutting Tools Establish the life of each cutting edge or tool (usually after a certain number of pieces are cut) and change tools regularly. l

Use coolant wherever possible to reduce heat, promote free cutting, and flush away the abrasive chips from the finished work surface. l

CASE HISTORIES Case histories 10-l through 10-5 describe the performance capabilities of PCD tools in several production operations in the automotive industry. These provide examples for operating conditions in which PCD tools have been very effectively used for productivity, quality and machining cost improvements.

Component: Material: Operation: Speed: Feed: DOC: Geometries:

Transmission Case 308 Aluminum Milling Flange Face 30OOft/min (910 m/min) 0.005 in/tooth (O.O12mm/tooth) 0.120in (3.0mm) maximum WC + 15’ combined axial-radial PCD + 8’ combined axial-radial

Insert:

WC SFC42E + Wiper

PCD SPC42E + wiper

Results:

3000 parts

40,000 parts

Case History 10-l. Case history of PCD tools rough milling aluminum transmission cases.

Polycrystalline Diamond and Cubic Boron Nitride

Material: Operation: Tool Geometry:

265

4227 Aluminum (5.5-7.0% silicon) Finish Milling Flange Face 6-7/8” Diameter Cutter 10-l/2” Square PCD Inserts +SORake, 1 lORelief (PCD) Previous (WC: +20”Rake, 25”Relief) l

l

l

l

Machining Parameters: Speed: 670m/min (2200 SFPM) Feed Rate: O.l8mm/Tooth (O.O07”/Tooth) DOC: 0.76mm (0.030”) Coolant: Yes Tool Cost: PCD - $80.00 WC - $6.00 l

l

l

l

l

l

Tool Life:

PCD - 250,000 pieces WC - 800 pieces

Burring Eliminated . 15% Productivity Increase Less Machine Downtime Tool Cost Per Piece Reduced 85% l

l

l

Case History 10-2. A case history milling manifolds with PCD tools.

aluminum

intake

266

Ceramic Cutting Tools

Material: Operation: Tool Geometry:

Low Silicon Aluminum Face Milling 10” Diameter Cutter 26 SPG-633 Inserts O’Radial, +5 Axial Rake l

l

l

Machining Parameters: Speed: 800m/min (2625 SFPM) Feed Rate: O.l25mm/Tooth (O.OOS”/Tooth) DOC: 0.50mm (0.020”) . Coolant: Yes Tool Cost: PCD - $65.00 WC - $5.25 l

l

l

l

l

l

l

l

Tool Life:

PCD - 150,000 pieces WC - 8,000 pieces

Improved Consistent Surface Finish Tool Cost/Piece Reduced Increased Productivity

Case History 10-3. A case history milling low silicon aluminum housings with PCD tools.

PolycrystallineDiamond and Cubic Boron Nitride

Material: Operation: Tool Geometry:

267

13- 16% Silicon Aluminum Finish Turning Diameter Tipped Tool +12’ Back Rake 1. lmm (0.040”) Nose Radius l

l

l

Machining l

l

l

Parameters: 200m/min

Speed: Feed Rate: DOC:

Coolant: . Tool Cost: l

l

l

l

l

Tool Life:

(655 SFPM)

0.127mmnooth (O.OOS”/Tooth) 0.127mm (0.005”) 5% Soluble Oil PCD - 10x Tungsten Carbide

PCD - 50,000 pieces WC - 900 pieces

Diameter Tolerance (k 0.0004”) Easily Maintained Improved Surface Finish Increased Productivity

Case History 10-4. A case history turning high silicon aluminum pistons with PCD tools.

268

Ceramic Cutting Tools

Machine: Material:

Diechesheim Tracer Lathe 57% Silicon Aluminum

Operation:

Contour Diameter

Facing

Tool Geometry:

l/2” Round

VPGA-432

Back Rake: Side Rake:

0” 0”

0” +5O

Tool Life (p&edge) PCD WC l

l

l

l

l

5000-7500 80- 100

1400 40

Carbide chipbreaker used to break chips Production increased from 700 to 1600 wheels/day Contouring cycle time reduced from 6 to 0.8 minutes 40 minutes/shift tool change downtime eliminated Scrap reduced significantly

Case History 10-S. A case history turning low silicon aluminum wheels with PCD tools.

Polycrystalline Diamond and Cubic Boron Nitride

269

GUIDELINE FOR MACHINING WITH POLYCRYSTALLINE CUBIC BORON NITRIDE (PCBN) PCBN tipped tools have been developed and introduced into the metalworking industry as a means of increasing productivity and product quality, while reducing overall machining costs. Such tools are effectively used to machine hardened steel, cast iron, hard facing materials, and high temperature alloys. There are many reasons why manufacturing management and process engineers must be aware of the machining performance capabilities of PCBN tools. Most important are: The relative ease of machining 45HRC to 70HRC and beyond. l

hardened steels of from

. The capability to machine gray cast irons, high alloy and chilled cast irons at much higher speeds and material removal rates than conventional tools. The ability to eliminate several tool changes, increase process capability (Cp and Cpk) and finish many “difficult-to-machine” materials. l

. The capability to eliminate costly grinding operations by turning steels in the hardened state. Almost 50% of all PCBN used worldwide has been to replace grinding operations. Therefore, this application area is worth special consideration. Why Machine Instead of Grind? The decision whether to machine or grind a specific component may be quite complicated. The decision hinges on the availability of machine tools, experience level of the manufacturing operation with machining and grinding processes and a multiplicity of other factors. Regardless of these complexities, there are a number of solid reasons to machine rather than grind. These are summarized in Figure 10-6.

270

Ceramic Cutting Tools

Machine I. Material Removal Rate

vs. Mach.

I 1 Grind

Grind Lathe

II. Machine Investment 16

vs. 0 1 Mach. \

Grinder Grind

III. Improved Part Accuracy

Optimize IV. Improved Grinding Efficiency

Grinding Operations

V. Environmental

Figure 10-6. Comparative

benefits of machining

versus grinding.

Polycrystalline Diamond and Cubic Boron Nitride

271

The advantages cited in Figure 10-6 are applicable to a wide The key element is metal range of metalworking industries. removal rate. Although grinding methods are becoming more aggressive, machining is faster than grinding by a factor of 3 in most cases. Second, the machine investment for grinding The equipment may be 3 times that of turning equipment. capability to perform many operations in a single chucking offers significant part accuracy over multiple setups for grinding operations. Even on components which may still have to be ground after hard machining, the amount of material and time Finally, there are involved can be significantly reduced. environmental expense considerations such as the cost of disposing of oil contaminated grinding swarf versus the cost of machining chip disposal or reclamation. Specific guidelines for machining hardened steels to replace grinding follow. Typical of all superabrasive tools, PCBN tipped tools cost 10 to 15 times the conventional tools they replace. The successful introduction of PCBN into the machining environment may well depend on a solid grasp of the many factors which influence total machining costs. These factors include the real dollar benefits of scrap and rework reduction or elimination and overall product quality improvement.

PCBN Machining

Guideline

Organization

This introduction to the uses of PCBN tools is organized in the following sequence with supporting tables and graphs: Identify that your application falls within the scope of materials and operations for which PCBN tools can be effectively applied. l

Select the correct grade of PCBN tool tip material for the application. l

l

Become familiar with commercially

available PCBN tools.

272

Ceramic Cutting Tools Set up operating conditions of speed, feed, depth of cut, coolant application and edge preparation best suited to the application. l

Develop a sound understanding influence total machining costs. determining the cost effectiveness case histories at the end of this understanding of effective PCBN l

Select the Application

of the factors which This will be critical to of PCBN tools. Refer to section for an improved tool uses.

- A Material/Industry

Guide

The current major applications for PCBN tools are found in Table lo- 11. This table indicates the basic characteristics of workpiece materials, principle industries using these materials and some examples of finished products. It should be noted that this list contains only hardened carbon steels, hardened alloy steels, cast iron, hard facing materials and high temperature alloys. PCBN tools are not appropriate nor applicable for machining aluminum alloys, other non ferrous metals, metal matrix composites, cast ironlaluminum bimetals, nor composites of resin, glass, graphite, wood, etc. Supporting

observations

are as follows:

PCBN tools solve a growing number of difficult machining productivity problems in the manufacture of hard steel gears, splines, shafts, and other components. Of major importance is the displacement of traditional grinding processes with hard turning on these materials. Components may be rough machined, fully heat treated and then turned or milled in the fully hardened state to final dimension and finish. l

PCBN tools are displacing conventional tools in milling, turning and boring a wide range of cast iron components in the automotive and truck industries. Significantly higher l

Polycrystalline Diamond and Cubic Boron Nitride

273

Table 10-11. Workpieces and Products Machined with PCBN Tools. Workpiece Materials

Industrial Use

Typical Products

Carbon Steels >HRc45 Alloy Steels >HRc45 Tool Die Steels >HRc45

Automotive Transportation Aircraft Power Drive Tools Appliances General Industry

Shafts Gears Bearings Dies Molds Tools

Gray Cast Iron ASTM Class 25-40 <2OOHB

Automotive Powertrain Diesel/Heavy Equip. HVAC Utility Engines

Clutch Plates Brake Rotors/Drums Cylinder Blocks Flywheels, Heads

Gray Cast Iron ASTM Class 50-60 210 HB to 390 HB

Heavy Machine Tools Paper Mill Drier Rolls Chemical Proc. Equip. Pressure Vessels

Camshafts Valve Bodies Dies, Gears

White/Alloy Cast Iron 400600 HB

Slurry Pumps

Impellers Casings, Valve Guides

Nickel or Cobalt Base Hi-Temp Alloys Inconel, Waspalloy, Stellite

Heat Exchangers Gas Turbine Engines Food Processing Medical Autoclaves Chemical/Paper Ind.

Turbine Blades Vanes Shrouds, Housings Hubs, Ducts Hip Joints

Iron Base Hi-Temp Alloys, A286, Incoloy

Oil and Gas Nuclear Plants

Pipe/Liners Fuel Rod

Sintered Iron

Automotive Diesel/Heavy Equip.

Valve Seats Hydraulic Pumps Gears, Cam Lobes

Hard Facing Alloy

Plastics, Rubber Glass Oil/Gas Gas Turbine Engines Motors, Pumps

Extrusion Screws Barrels Turbine Bearings/ Housings Compressor Hubs Cases, Shafts

274

Ceramic Cutting Tools machining dimension

speeds along with high levels of consistency and finish are the major advantages.

of

Hard facing metals, widely used to restore worn bearing and abraded surfaces, must be tediously ground to former dimensions. PCBN tools are used to turn or mill these surfaces to original dimensions and finishes in a fraction of the time required to grind them. l

PCBN tools are effectively used in the manufacture many aircraft engine and land turbine components. l

of

Guide to Selecting the Most Effective Grade of PCBN The CBN abrasive grains which are used in the manufacture of PCBN tool blanks have significantly superior hardness and strength properties in comparison to the materials used in the manufacture of other types of cutting tools. It follows then that the PCBN blank itself will possess hot hardness and abrasion resistance properties superior to conventional tools. The inherent superior resistance of PCBN to abrasive wear is not exactly matched by its resistance to chemical wear. The finish turning of hardened steels at typical depths of cut below 0.025 inch and feeds of less than 0.010 inch can generate high tool tip temperatures. These temperatures may be sufficient to create accelerated crater wear of the PCBN rake surface. This reaction generally disqualifies straight high content CBN tools from such application regimes. The solution to this problem lies in blending CBN with a ceramic component, such as titanium nitride, during the manufacturing process in approximately equal parts. Tool blanks produced in such a process are known as PCBN composite tool blanks. It is possible to produce a tool with high thermal and chemical stability which provides significant resistance to crater wear. This can be accomplished while still retaining sufficient hardness and strength for a wide range of conditions in finish A basic guide to differentiate the turning hardened steels.

Polycrystalline Diamond and Cubic Boron Nitride

275

applications for these two forms of PCBN tool material is found in Table 10-12. Description of PCBN Tools A widely used form of PCBN tools are tipped carbide inserts. These inserts are prepared by creating a precision “pocket” on a corner or corners of a standard cemented carbide insert. (The specific grade of insert is important to the manufacturer but is of no importance in its use as a PCBN tool). This corner is then precision ground back to the radius and rake of the original. Virtually any carbide insert size and geometry can be made into a PCBN insert. Table 10-12.

Selection Guidelines

for PCBN Tools.

Material Type Finishing

Roughing

PCBN

PCBN/Composite PCBN

PCBN

PCBN

PCBN

PCBN

Another popular form of PCBN insert is the “full face” or “full top” insert. In this form, the entire top rake surface of the insert is PCBN. Thus a square “full top” SNG-322 insert has four usable cutting edges versus only one edge for a tipped insert. It is of course possible for tool suppliers to provide a wide range of PCBN tool configurations in the brazed shank configuration as well. These three tool configurations are illustrated in Figure 10-7.

276

Ceramic Cutting Tools

Figure

10-7. PCBN tool configurations.

In order to maximize the cost effectiveness of PCBN tools, it is possible to have PCBN inserts re-sized to original dimensions after maximum normal flank wear. It is also possible to downsize PCBN inserts to the next smaller inscribed circle (IC). This is particularly advantageouswhen using "full top" inserts. The PCBN tool supplier can provide details as to cost and logistics.

Select Parameters for Machining with PCBN Tools The selection of machining parametersdependsprimarily on the specific surface condition, baseproperties and characteristics of the workpiece material. Secondary is a dependence on the nature of the machining operation itself. Finish turning, rough turning or milling requires specific attention to selection of the type of PCBN tool, its preparation and operation.

Polycrystalhe

Diamond and Cubic Boron Nitride

277

Hold PCBN tools as rigidly as possible and eliminate extraneous vibrations and out-of-balance conditions in spindles and other rotating components. Avoid excessive overhang when setting up PCBN tools. Set-up personnel and operators should be aware that the cutting edges of PCBN tools are as fragile as the edges of conventional carbide, ceramic and cermet tools. PCBN tipped tools should always be handled with the same care and consideration given other tool materials. Extreme care should be taken when measuring PCBN tools with caliper micrometers or depth gages. Effective economic use of PCBN tools depends operating within the following speed and feed limits. Running PCBN tools at speeds lower than the low guide limits shown below may lead to rapid and uneconomic flank wear or other forms of premature tool wear. The effectiveness of PCBN tools depends on taking full advantage of their higher hot hardness characteristics. This hot hardness characteristic allows chip formation to occur at higher temperatures and material to be removed at faster rates. Table 10-l 3 provides basic Speed/Feed Guidelines. guidelines for machining a range of workpiece materials with PCBN tools. These guides reflect the actual range of practical commercial operating experience. For those unfamiliar with PCBN tools, operations should be at or near the lower limits of speed and feed indicated for a given workpiece type and machining operation. Always give prime consideration to producing parts to within specified tolerances and finishes. As these conditions are satisfied, begin the process of optimization through considered increases in both speed and feed. Note that there are combinations of workpiece and nature of operation which indicate that either PCBN or a PCBN composite tool may be used. These situations require more guidance as follows: l Turning hardened steel: The basic tool wear mechanism in turning hardened steel is chemically induced crater wear. It is therefore recommended that PCBN composite tools with high resistance to

Table 10-13. Speed, Feed and DOC Guidelines for PCBN Tools.

Material

Operation

Grade

Surface Speed (ft/min)

Feed Rate (in/rev)

Depth of cut (in)

Gray Cast Iron (180-270 BHN)

Turning Milling

PCBN PCBN

2000-4000 2000-4000

0.006-0.025 0.006-0.012 in/tooth

0.005-O.100 0.010-0.100

Hard Cast Iron (>400 BHN)

Turning Milling

PCBN PCBN

250-500 400-800

0.006-0.025 0.006-0.012 in/tooth

0.005-O. 100 0.010-0.100

Hardened Stee1 (>45 Rc)

Rough Turning

PCBN and Composite PCBN*

220-350

0.006-0.025

0.030-O. 100

Hardened Stee1 , W5 Rc) Superalloys (> 35 Rc) Sintered Iron ti ee text, e ect

Finish Turning

PCBN

350-450

, Milling

, PCBN

, 400-800

Turning Milling

PCBN PCBN

Turning Milling arameters or

PCBN PCBN ac inmg wit

0.004-0.008 , 0.004-0.010 in/tooth

0.004-0.030 0.004-0.075

550-800 700-1000

0.004-0.012 0.004-0.008 in/tooth

0.004-O. 100 0.004-0.050

300-600 400-800

0.004-0.010 0.004-0.008 in/tooth

0.004-0.050 0.004-0.050

Polycrystalline Diamond and Cubic Boron Nitride

279

cratering always be tried first in these applications. In severe cases of turning grooves in splines and other interrupted cuts, wear may be dominated by fracture or spalling. In such cases, the tougher straight PCBN tools must be used. Turning gray cast irons (~270 HB): Machining these irons produces short segmented chips. It is wise to start such operations with straight PCBN tipped tools as abrasion Variation in the machining is the typical wear mode. characteristics of soft irons are frequently caused by significant variations in the chemical composition of the iron. This in turn may alter the wear mechanism of the PCBN tool. If rapid wear is observed that appears to be chemically induced, the operator may need to utilize a PCBN composite tool to overcome the problem. l

Milling gray cast irons (~270 HB): Straight PCBN tools should always be used, however, chemical wear and/or thermal cracking may justify trying PCBN composite tools. l

Turning hard cast irons (>400 HB): Machining tough hard irons produces segmented chips. They should be machined with straight PCBN as pure abrasion is the dominant wear mode. Due to variations in the composition of these irons, it may also be possible for chemical wear to In such cases, it is significantly influence tool life. recommended to try PCBN composite tools. l

PCBN Depth of Cut Guidelines. When turning or milling with tipped PCBN inserts, special attention must be paid to the depth of cut in relation to the actual In this insert length of the PCBN cutting edge available. manufacturing process, a small PCBN blank is brazed into a corner of the carbide insert. Thus the available length of PCBN edge is not the same length as the original carbide edge. It is limited by

Ceramic Cutting Tools

280

the size of the PCBN blank used in the process. It is therefore very important that the depth of cut selected for any operation not exceed 35% of the total PCBN edge available. The purpose of this limitation is to insure that heat generated in the chip forming process cannot reach and soften the braze line between the carbide insert and the PCBN blank. Note: This caution in no way limits the depth of cut capabilities of PCBN tools. The supplier can provide inserts to accommodate any depth of cut desired. The specific guides and limitations are illustrated and emphasized in Figure 10-8.

Note: Depth of cut should not exceed 35% of total PCBN cutting edge available.

Figure

1043. Guidelines for establishing tipped PCBN inserts.

maximum

DOC with

Polycrystalline Diamond and Cubic Boron Nitride

281

PCBN Rake Angle Guidelines PCBN tools are always used to best advantage with negative back rake. All the key applications for PCBN tools are in machining the highest strength engineered materials used in any industry. It is vital to maintain compressive stresses at the cutting edge of the PCBN tools. The guidelines for rake angle in general are shown in Figure 10-9. Guidelines for milling are shown in Figure 10-10. PCBN Edge Preparation

Guidelines

Special attention must be given to the preparation of the cutting edges of PCBN tools. This step is important in any machining operation but assumes critical importance when machining very difficult to “impossible” to machine metals at relatively high speeds.

Figure 10-9. Illustration of recommended (5-7’) for PCBN tools.

negative top rake angles

282

Ceramic Cutting Tools

Figure 10-10. Illustration

of double negative angles recommended for PCBN tools.

milling cutter rake

It is important to maintain the cutting edge of PCBN tools under a compressive stress. In order to achieve this, most applications for PCBN require a chamfered cutting edge. In some severe situations, a small honed radius must also be added. This process should be carefully done with a fine diamond hone to impart a 0.0005 inch to 0.0010 inch (10ym - 20pm) radius. The edge preparation guidelines shown in Table lo-14 provide recommendations to cover a range of PCBN tool applications. These are based on actual machining experience. User should also rely on PCBN edge preparation recommendations and information which your supplier may have with respect to any specific application. Note: PCBN inserts are not automatically supplied with a chamfered edge. It is important that the chamfer

Polycrystalline Diamond and Cubic Boron Nitride

283

needed for your application be specified when ordering PCBN tools. PCBN Lead Angle Guidelines A conservative approach should always be taken in setting up the lead angle when machining with PCBN tools. Negative leads of at least -15’ and preferably up to -45’ should be used whenever possible. Lead angles from 0” to slightly positive may be used where part geometry dictates. These guides are illustrated in Figure 10-11. PCBN Nose Radius Guidelines In order to minimize stress concentration on the ,PCBN cutting edge, always use as large a nose radius permitted by part geometry. In addition, use inserts that offer maximum strength and rigidity . PCBN Coolant Application

Guidelines

Light-duty, water-soluble oils of the type used for machining with carbide tools usually work well with PCBN tools and inserts. The main purpose of applying coolant is to retard flank wear which is critical to maintain size control of close tolerance parts. In some cases, it is desirable to machine with PCBN cutting tools without coolant. In the particular case of milling with PCBN, the application of any type of cutting fluid greatly increases the degree of hot and cold cycling (thermal cycling) of the PCBN cutting edges. Such extreme cycling can lead to rapid tool failure. There are other cases where a plentiful flow of cutting fluid should always be applied. These are: - When taking higher depths of cut with tipped inserts or brazed-shank tools, the high conductivity of the PCBN layer could conduct enough heat to cause a braze failure or other damage to the tool shank.

Cutting Forces

4

Straight PCBN Rough/Interrupte Turn

Material

Hardened Steel

15” x 0.008 in.

Cast Iron

20” x 0.008 in.

‘CBN Comoosite

Finish Turn

Milling’

Not Recommended

15” x 0.008 in.**

Rough/Interrupted Turn 15” x 0.008 in.*

Finish Turn

Milling

20” x 0.004 in.* Not Recommended 15” x 0.008 in.*

20” x 0.008 in. 15” x 0.008 in.

Not Recommended

20’ x 0.008 in.* 15” x 0.008 in.

Upsharp* Hard Facing

Upsharp*

20” x 0.008 in.** Not Recommended

Upsharp*

Not Recommended

20” x 0.008 in.** Not Recommended

Upsharp*

Not Recommended

Upsharp* High Temperature Alloy

Upsharp*

* Interrupted turning may also require a 0.0005 inch - 0.0010 inch hone. ** Milling requires a 0.001 inch - 0.002 inch hone.

’ Based on -5” axial and -5” radial rotary tools.

Table 10-14.

Guidelines

for Preparing

Edges of PCBN Tools.

Polycrystalline Diamond and Cubic Boron Nitride

285

0” LEAD TOOL

LEAD ANGLE TOOL

Figure 10-11. Recommended guidelines for setting lead angles for PCBN tools. Utilize negative leads of 15-45’. Positive or 0” lead tools may be used only if necessary.

- In grooving or other very deep cutting operations where coolants can assist in clearing the chips and carrying heat away from the cutting zone. Where the coolant system is not well maintained and/or the coolant itself is poorly applied to the cutting zone, it could be just as well to turn the coolant off and machine the operation dry.

SUMMARY

GUIDES FOR PCBN TOOLS

The selection, set-up and operating conditions for PCBN tools must be precise if the machining operation is to succeed.

286

Ceramic Cutting Tools

Speed 1. Always start within the recommended speed range for the type of material being machined and the grade of the PCBN tool or insert.

Feeds 2. Use the recommended feed rate wherever possible to obtain the best performance and longest life of the PCBN tool. 3. Feeds or depths of cuts less than 0.005 in. (0.12mm) are not recommended for straight PCBN tools for the following reasons: . A very light chip is produced the heat from the cut.

which cannot carry away

Excess heat at the cutting zone causes the work to expand, produces a tapered cut, and reduces the life of the tool. l

4. If the machining process requires the use of feeds or depths of cut less than 0.005 inch, the use of PCBN composite tools is recommended.

Cutting-Tool Set-up 5. Make sure that the pocket is clean and flat before installing an insert.

6. Do not clamp directly on the PCBN layer; use a chipbreaker

or a suitable alternative to distribute the clamping forces. Clamp the insert securely; do not use a small pipe or tube on the Allen wrench.

Polycrystalline Diamond and Cubic Boron Nitride

287

7. Regrind the PCBN tool at the first sign of dullness. Both brazed-shank and tipped-insert tools can reground. The amount of regrinding needed should determined by the PCBN toolmaker. l

be be

PCBN inserts can be reground to the smallest inscribed circle (IC) for which there may be a usable toolholder. This grinding must be done by a skilled PCBN toolmaker. l

8. Never allow a wear land to grow into the carbide substrate. Heat, chatter, surface finish and loss of workpiece accuracy will result. 9. Index and change tools on a regular basis. Dull tools increase machining forces causing chatter, which reduces the life of the PCBN tool. l

Impact damage can occur to dull tools more readily than to sharp tools during interrupted cuts. l

10. Set the tool on center. If shims are required to bring the tool to the correct height, use only one shim of the correct thickness instead of a series of small shims. 11. Keep the overhang of PCBN tools as short as possible prevent vibration and chatter. 12. Use negative-rake

to

tools wherever possible.

13. Set the side cutting edge angle (SCEA) as close to 45’ as possible. Avoid the use of a lead angle of less than 15’. 14. Use as large a nose radius on the PCBN tool as the job and machining operation will permit.

288

Ceramic Cutting Tools

PCBN Tool Machining

Case Histories

Several selected case histories are shown in Case Histories 10-6 through 10-13. These histories provide detailed information concerning the actual results of applying PCBN tools in several In each case, the nature of the industrial product situations. workpiece, tools used, operating conditions and a comparison of results with an alternate tool or process is indicated. The histories are instructive with respect to the practical uses of PCBN tools.

COST ANALYSIS OF MACHINING WITH SUPERABRASIVES In U.S. industry, lack of cost evaluation knowledge now outranks lack of technical know-how as the major obstacle to superabrasive implementation. The cost analysis issue has many facets. Characteristically, these may range from long established cost accounting cultures in U.S. industry to daily shop floor practices. Peter Drucker observed that large companies such as GM, GE, Western Electric and others grew rapidly in the 1920’s because of their cost accounting methods, not because of their blinding Some capabilities. manufacturing technology characteristics of this manufacturing cost accounting culture may be summarized as: + Radical changes in the actual labor cost of metalworking processes. The typical range of labor cost in today’s products are on the order of eight to twelve percent. Consequently, continual emphasis on labor cost reduction is of marginal value. + Concentration on a detailed compilation of “costs of production” while ignoring “costs of non-production”. Any business that has dedicated its costing resources to ferreting out non-production costs usually winds up in shock. Non-production costs may be found to be as much as fifty percent of total product cost.

Polycrystalline Diamond and Cubic Boron Nitride

289

Insert

Cutter

.324R

Component:

Ball Screw

Operation:

Thread Whirling

Material:

Hardened

Insert:

Radiused Tool

Speed:

1030 sfm (322m/min)

Feed:

2.8 in/min (7 lmm/min)

DOC:

0.500 in x 295 in long (12.7mm x 749 cm)

Coolant:

Compressed

Results:

Machining/hardening/grinding (168 hours) Hard turning with PCBN composite tool (105 minutes) 3 lead screws per set-up

Steel (60-62 HRC) 0.325in., 8.23mm,

15” x 0.003 in. 15’ x O.lmm

air

Case History 10-6. Case history of hard turning ball screw with PCBN composite tool.

290

Ceramic Cutting Tools

Insert

Cutter

.324R

Component:

Gear Pinion

Operation:

Finish Bore ID

Material:

5 120 Steel (62 HRC)

Insert:

0.236” Round (20 x 0.008”)

Speed:

430 ft/min

Feed:

O.O04”/rev

DOC:

0.004”

Results:

PCBN Composite

Benefits:

l

l l

Case History 10-7.

- 250 pee/comer

11 grinders replaced with 5 machining lathes PCBN inserts downsized Ability to hold _+0.004” ID tolerance

Detailed case illustrating grinding with turning by PCBN tools.

replacement

of

Polycrystaliine Diamond and Cubic Boron Nitride

291

Component

4.0 III

Material:

8620 Steel (62 HRC)

Operation:

Facing (Interrupted)

Insert:

CNGA-433 (20 x 0.004 in)

Speed:

365 ft/min

Feed:

0.003 in/rev

DOC:

0.005 in

Results:

PCBN composite - 300 parts

Benefits:

2X productivity increase over grinding

Case History 10-8. Case history detailing hard turning with PCBN composite tools replacing grinding.

292

Ceramic Cutting Tools

Component:

Pinion Gear

Material:

8620 Steel (62 HRC)

Operation:

Turn pilot diameter

Insert:

TNMA-332

Speed:

400 ft/min

Feed:

0.006 in/rev

DOC:

0.004 in

Coolant:

Yes

Results:

PCBN - 300 pcs/corner PCBN Composite - 900 pcskorner

Benefits:

3 machining lathes replaced 8 grinding machines

(20 x 0.004 in)

Case History 10-9. Case history hard turning pinion with PCBN composite

tools.

Polycrystalline Diamond and Cubic Boron Nitride

293

Component:

Engine Cylinder Head

Operation:

Milling Face Head

Material:

Gray Cast Iron (190-250 BHN)

Cutter:

Double

Insert:

SNG-632 15’ x 0.005 in (0.125 mm) chamfer

Speed:

3100 ft/min (950 m/min)

Feed:

0.0047 in/insert (0.12 mm/rev)

DOC:

0.020 in (0.5 mm)

Coolant:

Dry

Results:

PCBN - 17,000 pcskor SiN - 1900 pcs/cor

Negative diameter 32 inserts/cutter

10 in (250 mm)

Case History 10-10. Case history details of PCBN tools replacing SiN tools in machining gray iron.

294

Ceramic Cutting Tools

Component:

Automotive

Material:

Gray Cast Iron (200 BHN)

Operation:

Milling Front Face

Geometry:

Negative Axial - Negative Radial

Insert:

SNG-434 20 x 0.005 in (0.125 mm) chamfer SiN - 32 inserts/cutter PCBN - 18 inserts/cutter + wipers

Speed:

4700 ft/min (1430 m/min)

Feed:

0.010 in/insert (0.25 mm/insert)

- PCBN

0.0046 in/insert (0.11 mm/insert)

- SiN

Engine Block

DOC:

0.030 in (0.75 mm)

Coolant:

Dry

Results:

PCBN - 7200 pcs/cor SiN - 900 pcs/cor

Benefits:

Increased productivity Eliminated CL breakout

Case History 10-11. Case history details milling gray iron with PCBN tools.

Polycrystaliine Diamond and Cubic Boron Nitride

Component:

Engine Cylinder Head (4 cyl)

Operation:

Valve Seat Chamfering

Material:

Powdered

Insert:

TPE -732, 3 tools

Geometry:

Neutral Rake

Speed:

370 ft/min

Feed:

3.2 in/min (0.0035 in/rev)

DOC:

0.010 in plunge cut

Results:

WC - 300 PCs/corner avg. Inconsistent tool life/tool Marks visible on valve seat

Iron

3000 pcs/corner

Case History 10-12. Case history illustrating machining powdered metal.

295

Intake (28 HRC) Exhaust (42 HRC)

avg.

use of PCBN tools

296

Ceramic Cutting Tools

Component:

Transmission

Material:

Forged P.M. (58-62 HRC)

Operation:

ID Turn and Face

Insert:

TNG-223 (20 x 0.004 in)

Grade:

PCBN

Speed (SFM):

300

240

Feed (IPR):

0.004-0.008

0.005-0.010

DOC (In):

0.015

0.015

Coolant:

Dry

Dry

Results:

450 Gears

950 Gears

Gear

Case History 10-13. Case history details of PCBN tool machining powdered

metal components

at two surface speeds.

Polycrystalline

Diamond

and Cubic Boron Nitride

297

+ Most cost accounting methods isolate the factory floor from the rest of the business. As Drucker points out, “cost savings on the factory floor are real -- all other is speculation”. An examination of how these costing practices impede the implementation of superabrasives is necessary.

Costing Superabrasives Simply stated, the objective of introducing superabrasives in the manufacturing process is to improve productivity, reduce costs and increase quality. Given a manufacturing culture that has focused on control of expendable tool costs, introducing tools that cost lo-20 times more than conventional tools is cost accounting shock. A minor alteration of cost evaluations is not sufficient. A complete change in accounting strategy is necessary in order to properly evaluate superabrasives. Most accounting systems are not designed to take into account the cost influence of new process technology either “upstream” or “downstream” from the specific operation. Fully identifying and assessing the benefits is difficult and sometimes impossible in the traditional cost accounting system. Identifying so-called grinding costs is usually no more complicated than dividing the price paid for a grinding wheel by the total operations Similarly, present machining cost or the parts it produces. evaluations involve merely dividing the price paid for the tool tip by the number of parts it produces. are will recognize these Process engineers that oversimplified calculations of grinding and machining costs. Unfortunately, these rules-of- thumb persist even in many otherwise advanced and sophisticated manufacturing environments. There is some sound basis for using such simple rules. Conventional abrasives such as aluminum oxide and silicon carbide can be differentiated in the electric furnace processes and in the subsequent wheel manufacturing process. Yet the commercial price range and performance of these tools fall within a relatively narrow range. Where there is such marginal differentiation of price and

298

Ceramic Cutting Tools

performance of ail tools available, grinding cost analysis can be boiled down to its basic elements. If two percent more parts can be produced with wheels which cost five percent less, there are obvious marginal improvements in cost and productivity. The same principle applies when selecting among high speed steel tools, cemented carbide tools, and ceramic tool materials. Each product group is commercially available in a relatively narrow range of This simplistic cost analysis price and performance capabilities. system is severely lacking when faced with cost justifying Superabrasives may cost lo-20 times more than superabrasives. the conventional tools they replace but outperform them by factors of 10 to over 300 times. The simplistic formula of price divided by parts produced will generally miss the hidden cost benefits of using superabrasives entirely and at best, grossly understate them. Let’s take a few theoretical examples - suppose a PCBN tool is used to replace a carbide tool in a machining operation. The PCBN tool cost is 15X that of the carbide tool but produces Our simple cost analysis model 15X more parts per edge. dismisses PCBN as having any cost advantage. But use of PCBN reduced scrap from 12% to l%! Cutting speed was increased by 20% resulting in higher throughput. What about the 15 tool changes that didn’t cause an interruption in the process? How many more parts were made in lieu of 15 tool changes? Does the person responsible for controlling expendable tool costs know about the effects on tool change costs? Does this same person know about the cost implications of reducing scrap to nearly zero? In another case, PCD tools make it possible to double the number of aluminum wheels made in one tracer lathe per day. Scrap and rework are virtually eliminated. Additionally, the final finish on the outside of this wheel can be totally controlled in the final machining steps with PCD. This was impossible with conventional tools. The wheel designers had to accept whatever finish conventional tools could produce. With superabrasives, the customer has a range of desired surface finishes available! Thus, the use of superabrasives not only doubles productivity but also affects the appearance and marketability of the product. How can this be taken into account in the traditional cost analysis?

Polycrystalline Diamond and Cubic Boron Nitride

299

These examples make it clear that the cost and business impact of superabrasives may go well beyond the ability of any traditional cost/benefit analysis. The characteristics of Advanced Manufacturing Technology, and in particular superabrasives, necessitate computer integrated manufacturing (CIM) to simplify the task of cost justification.

Examples of Superabrasives Impact on Product Costs An engine cylinder block is being semi-finished and finish bored dry using a single-point tool boring head. After the semi-finishing pass is completed, a single tool is extended from the boring head by an actuator and the finishing pass is completed as the head is extracted from the cylinder bore. A total of twelve inserts are required to complete this operation on the gray cast iron V-6 engine. + + + + +

Insert - SNG-432 (15’ x 0.004 in chamfer) Speed - 2600 SFM Feed - 0.014 in/rev DOC - 0.015 in semifinish DOC - 0.005 in finish

The average bore cylindricity obtained with the silicon nitride tooling was 0.0006 in. When the change was made to PCBN inserts, average bore cylindricity was reduced to 0.0004 in. Since PCBN inserts conduct heat away from the workpiece, less heat shrinkage occurred in the bores, resulting in an improvement in cylinder honing.

Tool Cost

SiN

PCBN

1. Cost of new tool 2. Corners/tool 3. Cost of regrind 4. # of regrinds 5. Total corners/tool

$10.00 8 NA NA 8

$114 4 NA NA 4

300

Ceramic Cutting Tools 6. 7. 8. 9.

Cost/corner Total cylinders bored Total cost/cylinder Total cost/part (12 x #S)

$1.25 200 $0.00625 $0.075

$28.50 4700 !lxUHI6 $0.072

Tool Cost - Cost of tooling only. This is the cost often used as the major criterion for determining the economic justification for tool selection. Regrinding is also important, because it can bring the tool cost/part down significantly in some applications. The nature of this cylinder boring application, however, does not allow regrinding of inserts. As seen from the model, the price per part is essentially the same despite the significantly higher initial price of the PCBN tool. On-line Labor Cost

SIN

PCBN

1. Available prod. hr/shift 2. Labor rate - machine operator 3. Operator cost/shift 4. Parts products/shift 5. Labor cost/part

8 $30/hour $240 400 $0.60

8 $3O/hour $240 448 $0.5357

On-Line Labor Cost - Cost of operator to run machine. This cost in some cases will also include setup because it is done by the same person. On a per part basis the cost model shows a reduction in cost when PCBN is used due to the increase in productivity on this cylinder boring application. Tool Change Cost

SiN

PCBN

1. Hours req’d to change 1 cutter 2. Labor rate to change cutter 3. Cutter changes/shift 4. Cutter change cost/shift 5. Parts produced/shift 6. Cutter change cost/part

0.5 $30/hour 2 $60 400 $0.075

0.5 $3O/hour 0.08 $2.40 448 $0.0027

301

Polycrystalline Diamond and Cubic Boron Nitride

Tool Change Cost - Labor cost required to change tools. This may be the same as on-line labor cost depending on who is authorized to change tools. In the cylinder boring application PCBN requires a reduced number of tool changes, one every 12.5 shifts, compared to two per shift with SIN. Thus the tool change cost is significantly reduced. Scrap Cost 1. 2. 3. 4. 5.

# scrap parts/year Value added cost/part Parts produced/year Scrap cost/year Scrap cost/part

SIN

PCBN

1648 $112 377,000 $184,576 $0.4896

528 $112 312,000 $59,136 $0.1895

Scrap Cost - Cost of scrapped parts. PCBN produces a tighter part tolerance, resulting in a reduced scrap rate which is portrayed as a 61% scrap cost reduction shown in the model. Setup Cost

SiN

PCBN

1. Time required to index cutter 2. # cutters changed/shift 3. Labor rate to index cutter 4. Setup cost/shift 5. Parts produced/shift 6. Setup cost/part

NA NA NA NA NA NA

NA NA NA NA NA NA

Setup Cost - This is the cost for labor to index tooling or prepare the cutter for use, before it is actually delivered to the line. Since PCBN requires fewer tool changes, set-up cost can be reduced with respect to conventional tooling. This model is evaluating a cylinder boring application where no setup was required, however in some applications this cost is significant. Rework Cost

SiN

PCBN

1. # parts requiring rework/shift 2. Time required to rework part 3. Labor rate for reworking 4. Rework cost/shift

NA NA NA NA

NA NA NA NA

302

Ceramic Cutting

Tools

5. Parts produced/shift 6. Rework cost/part

NA NA

NA NA

Rework Cost - Cost of reworking

parts which do not meet specifications the first time they are machined. This cost will also be reduced due to the higher consistency of parts produced by the PCBN machining process. The cylinder boring application, however did not have statistics for this cost, but a reduction in scrap parts indicates a probable reduction in rework parts.

Inspection Cost

SIN

PCBN

1. Time req’d to inspect part 2.# parts inspected/shift 3. Inspection labor rate 4. Total inspection cost/shift 5. Parts produced/shift

NA NA NA NA

NA NA NA NA

6.Inspection cost/part

NA

NA

NA NA

Inspection Cost - Cost of labor for the inspection of parts to meet specifications. Once again with the tighter part tolerance that a PCBN tool yields, a higher confidence in product quality can be achieved, thus reducing the inspection time. The inspection procedure for the cylinder boring application did not change despite the significant improvement in process capability. Consequently, no inspection cost savings have been realized to date. Inventory Cost

SiN

PCBN

1. Raw/in-process (units req’d) 2. Inventory value/unit 3. Inventory carrying value 4. Rate of capital 5. Cost of carrying inventory 6. Parts produced/year 7. Inventory carrying cost/unit

NA NA NA NA NA NA NA

NA NA NA NA NA NA NA

Polycrystalline

Diamond

and Cubic Boron Nitride

303

Inventory Cost - Cost of carrying raw material and in-process parts before and/or after machining. This number is based on the scrap rate and predicted production rate. Since the scrap rate will be reduced using PCBN tools, the number of parts kept in inventory should be reduced accordingly. Increased productivity, however, may cause this cost to increase. No information on this cost was available for the cylinder boring application. Total Machining Cost/Part

SIN

PCBN

1. Tool cost/cylinder bore 2. On-line labor cost/part 3. Setup cost/part 4. Cutter change cost/part 5. Scrap cost/part 6. Rework cost/part 7. Inspection cost/part 8. Inventory cost/part 9. Total Cost/Part

$0.00625 $0.6000 $0.075 NA $0.4896 NA NA

so.oo6Q6 $0.5357 $0.0027 NA $0.1895 NA NA

Total Machining Cost - This yields total cost of machining and is the sum of the above costs on a per part basis for the cylinder boring application. As can be seen from the results above, the PCBN tool reduces cost $.44 per block, or 38%! Using a traditional machining cost analysis looking only at tooling cost per part, the silicon nitride and PCBN inserts appear equal leading the engineer to uninformed go/no-go decisions. In reality, they are very different. Certain costs were not attainable for this cost model, and this may be true for many applications. The purpose of the model is to include all relevant costs for any machining process and it is the responsibility of the engineer to determine which costs are pertinent to the particular application. The full effect which superabrasives have on this operation can be appreciated more fully by annualizing expendable tool costs and comparing this to annualized savings.

1. Tips in use/part 2. Tool life-parts

12 200

12 4700

304

Ceramic Cutting Tools 3. 4. 5. 6.

Parts/year Tips/year Cost/tip Tip cost/year

312000 18720 $1.25 $23400

312000 796 $28.50 $22700

The final bottom line on this application is that the actual total cost of expendable tools decreased very slightly and created a total production system cost reduction of 312000 x $.44 = $137,0OO/year!

11 The New Diamond Technology and its Application in Cutting Tools

Robert A. Hay

Norton Diamond Film Northboro, MA

INTRODUCTION

For centuries, chemists studied alchemy, wishing to turn common metals or chemicals into gold. Modem day alchemists have succeeded in turning common gases into diamonds, spawning a race to develop a product using this exciting new technology. Fascination with diamond has turned into excitement recently with the development of techniques for creating crystalline diamond films and coatings using low-pressure gases rather than the high pressure and temperatures previously considered essential. Processes for making diamond at low pressures are now commercially viable, with cutting tools as one of the first major applications. Within the last several years, several major companies have announced the successful development of chemically vapor deposited (CVD) diamond cutting tools for machining aluminum, composites and other non-ferrous metallic and non-metallic materials.

BACKGROUND

Research into diamond synthesis has been ongoing since the late 18th century when Smithson Tennant proved that diamond is a form of carbon [ 11. This triggered attempts by numbers of scientists and entrepreneurs to turn inexpensive carbon such as 305

306

Ceramic Cutting Tools

graphite into valuable diamond. There have been many reviews written on the progress of diamond synthesis [2-83. This section presents a brief explanation of the science and manufacturing techniques used to manufacture CVD diamond films for cutting tools. The CVD of Diamond The chemical vapor deposition (CVD) of diamond is based on two factors which both require high energy: 1) the carbon species must be activated since, at low pressure, graphite is thermodynamically stable and, without activation only graphite would be formed, 2) atomic hydrogen must be produced which activates and stabilizes the selectively removes graphite and diamond structure [9]. The deposition mechanism is complex and not fully understood at this time. The basic reaction involves the decomposition of a hydrocarbon such as methane as follows: CH, (g) + C (diamond)

+ 2 H, (g)

(1)

This simple process is in actuality a complex reaction where Atomic hydrogen is atomic hydrogen plays a crucial role. extremely reactive, it etches graphite at a rate which is 20 times as high as the rate at which it etches diamond. So when graphite and diamond are deposited together, graphite is preferentially removed while diamond remains. Hydrogen dissociates at very high temperatures (>2OOO”C) to produce atomic hydrogen. The rate of dissociation is a function of temperature, increasing rapidly above 2000°C. It also increases with decreasing pressure. In order to produce high quality diamond films, a large amount (>95%) of hydrogen is reacted with the hydrocarbon gas in a plasma so to not produce graphite in the growing diamond film.

New Diamond Technology and Application CVD Processes for Diamond

307

Cutting Tools

There are two major CVD diamond processes for cutting tool manufacturing. Microwave plasma: A microwave plasma has sufficient electron density and energy to dissociate hydrogen. A deposition schematic is shown in Figure 1 l- 1. Gases are introduced and flow past cutting tool inserts to be coated with diamond film. The inserts to be coated are generally located in the lower part of the plasma. The inserts/substrates are sometimes heated by the interaction with the plasma and microwave power but can also be heated (or cooled) separately with resistance heaters which allow control of the temperature. With this deposition system, diamond is produced with a morphology and properties that vary as a function of the substrate, temperature, gas ratio, and plasma intensity in the deposition zone. Deposition rates are low, typically l-10 km/hour. Microwave deposition has the advantage of being very stable and can run for long periods of time without interruption. This type of deposition system is used for the CVD diamond coating of cutting tools by several manufacturers [ lo- 121. DC arc plasma deposition: In DC arc plasma deposition a high intensity arc is generated between two electrodes by DC current. A DC arc plasma deposition schematic is shown in Figure 1 l-2. In the plasma discharge the temperature may reach 5000°C or higher. The very high temperature obtained in an arc discharge allow an almost complete dissociation of the hydrogen molecules. Since the availability of hydrogen atoms is a key element in the formation of CVD diamond, arc discharge systems have an advantage over other processes which produce far smaller amounts of hydrogen atoms. This abundant supply of atomic hydrogen and high gas velocity accounts for the extremely high deposition rate; over 100 l_tm/hour has been reported [ 131. This deposition rate is one to two orders of magnitude greater than the rate obtained by other deposition processes. Due to the high growth rates and large area growth potentials for the DC arc [14], this method is used to produce thick freestanding discs of CVD diamond that can be grown as thick as lmm and as large as 100 mm in diameter.

Ceramic Cutting Tools

308

4444444444444444

Microwave-Assisted

Plasma CVD DIAMOND

FILM

AVE GUIDE TUNER

Figure

Schematic 11-l. apparatus.

of microwave

assisted

plasma

4444444444444444

DC ARC Plasma CVD

Figure 11-2. Schematic

of DC arc plasma CVD apparatus.

CVD

New Diamond Technology and Application

309

Typically for thick film CVD diamond cutting tools, the diamond is 0.5 mm thick and the cutting tool tips are cut from the larger discs with a laser, lapped, polished, and brazed onto a carbide tool before grinding.

DIAMOND PROPERTIES Physical Properties of Diamond Graphite, soot and carbon black have the same chemical composition as diamond (all are forms of carbon). However, this trio and diamond have very different crystal structures. Graphite consists of layers of condensed, six-numbered aromatic rings of spzhybridized carbon atoms. These rings are strongly linked in a single plane and weakly held between the planes by van der Waals forces. These layers can slide over each other so that graphite is a soft material used as a lubricant. Soot and carbon black are microcrystalline forms of graphite. The crystallographic network of diamond consists completely of covalently bonded, aliphatic sp3-hybridized carbon atoms arranged tetrahedrally with a uniform distance between atoms. The tetrahedrons are connected at their tips to form the crystal lattice. Diamond’s structure accounts for many of its properties, such as extreme hardness, outstanding wear resistance, and low coefficient of friction, that make it useful as a freestanding piece that has been brazed to a tool or as a coating on cutting tool inserts, end mills and drills.

Mechanical Properties of Diamond Film The properties of chemical-vapor-deposited diamond film which make it important as a cutting-tool material include hardness, abrasion resistance, desirable friction characteristics, high thermal conductivity, low coefficient of thermal expansion, and chemical inertness.

310

Ceramic Cutting Tools

Single-crystal natural diamond exhibits an indentation hardness (Knoop scale) in the range 5700-10,400 kg/mm’. Chemical-vapor-deposited diamond that exhibits 100% sp3 bonding has measured hardness in the range SOOO-10,000 kg/mm2. A large number of these diamond films have measurable quantities of sp” bonding, particularly in the grain boundary area. The presence of these weaker graphitic bonds results in hardnesses of these diamond films that are below those of natural crystals [15]. The observed abrasion resistance of both freestanding chemical-vapor-deposited diamond and the diamond films applied as a coating exceeds that of tungsten carbide by one to greater than two orders of magnitude. Chemical-vapor-deposited diamond also has an abrasion resistance that is two to ten times greater than conventional high-pressure-high-temperature sintered polycrystalline diamond materials (PCD). When chemical-vapor-deposited diamond is polished, it exhibits friction properties equal to those of fluoropolymers at ambient temperatures. The friction properties of diamond are believed to be the result of the surface chemistry of the materials. Free dangling bonds on the surface of diamond have great affinity for hydrogen. The atomic surface layer of hydrocarbon thus formed creates an ultra low-friction, low adhesion surface when in contact with a wide variety of materials, including mating diamond surfaces. This low-friction, chemically inert surface results in a cutting-tool material that functions efficiently at a low cutting temperature without galling. The material is suitable for nonferrous and composite applications. The thermal conductivity of diamond is the highest of all materials. Chemical-vapor-deposited diamond has values of 8-20 W/(cm K), and type IIA single-crystal natural diamond has a value of 22 W/(cmK). The latter type is a variety that is effectively free of nitrogen as an impurity and has enhanced optical and thermal properties. The next-highest thermal conductivity material is silver with a value of 4.29 W/(cm K). The high thermal conductivity of chemical-vapor-deposited diamond results in a cutting tool that can conduct heat away from the cutting edge, so that tool life is increased and possible heat damage and distortion of the workpiece

New Diamond Technology and Application

311

Diamond also has a low specific being machined are reduced. heat; therefore, it conducts the heat generated in cutting to another heat sink, such as the substrate brazed or coated with diamond. Diamond has a comparatively low thermal expansion coefficient. Its percent of thermal expansion between ambient temperature and 750°C is about 0.2% The value is nonlinear, and the coefficient for diamond increases rapidly with temperature; therefore, the operating range of interest must be defined before the extent of thermal expansion mismatch can be determined. In order to take full advantage of the properties of chemical-vapor-deposited diamond either as a thick plate brazed to a substrate or as a coating, the matching of the thermal expansion rates of the substrate and diamond is critical for the diamond to perform well and not crack, fracture, or spa11 off. Thermal expansion rates are particularly important when the diamond film is deposited directly as a coating onto a substrate. The deposition process is typically carried out between 700 and 1000°C. This high temperature limits the materials onto which the film can be coated. The low thermal expansion of diamond further The engineering materials onto limits the available substrates. which the diamond can be nucleated and deposited for cutting tools are silicon nitride (S&N,) and silicon carbide (SIC) ceramics and tungsten carbide with a low content of cobalt (I 6%). Silicon nitride has the closest match of thermal expansion coefficient for these substrate materials, and it is a better chemical match for diamond nucleation and growth than tungsten carbide. Tungsten carbide presents greater difficulty because of cobalt in the grain boundaries introduced as a sintering aid. The cobalt at the tool surface causes graphite to form preferentially instead of diamond, and the result is a weakened adhesion of the coating as well as the diamond film itself. The mismatch in the thermal expansion coefficient is one of the key limitations in commercial development of chemicalvapor-deposited diamond films for cutting tools and other highforce applications, where the diamond film is subjected to very and shear forces as well as high high tensile, compressive, temperatures and large fluctuations in temperature.

312

Ceramic Cutting Tools

Another important factor in cutting tool performance is the chemical or thermal stability of the tool material either as a monolith or as a coating. Diamond is one of the most chemically inert substances. However, it cannot be used in machining ferrous, nickel, or titanium-based alloys without elaborate cooling methods (liquid carbon dioxide or nitrogen) because of its reactivity and chemical wear (carbon diffusion out of the diamond) at the high contact pressures and temperatures that are generated during the machining process. On the other hand, for machining aluminum and other ductile nonferrous alloys, plastics, and abrasive composite materials, diamond has no peer. The wear environment at the cutting tool tip involves extreme temperature. A temperature of up to 1000°C can be present locally during continuous turning, and the temperature may fluctuate between 700 and 200°C within microseconds in the interrupted cut of milling. Extreme force at the cutting tool tip results in contact pressures of 200-500 kg/mm2. Thus chemicalvapor-deposited diamond tool material must have sufficient bulk strength and fracture toughness to be utilized as a brazed or freestanding piece, and the diamond coating must have hightemperature hardness relative to the substrate and workpiece material, high-temperature chemical stability, good adhesion to substrate, and good microfracture toughness. The chemical stability of chemical vapor-deposited diamond film is superior to poly-crystalline diamond materials produced at high pressures. The latter begin to oxidize at 600°C, and at 700°C they degrade very rapidly in either air or vacuum because of the The cobalt grain boundary phase needed as a sintering aid. chemical-vapor-deposited diamond begins to degrade slowly in air at 700°C, but in vacuum or in an inert atmosphere it is stable at 12OOOC. This increased chemical and thermal stability over PCD material results in significant improvements in performance when machining is performed in very corrosive environments or at the high temperatures generated in processing very abrasive advanced composite materials. The characteristics of diamond film coating that provide good adhesion to substrates and good microfracture toughness are strong functions of film thickness, microstructure, and conditions

New Diamond Technology and Application

313

of deposition. The deposition conditions must be optimized to produce a diamond film with good adherence, which is a function of low intrinsic stress (stress in the film due to growth conditions), low extrinsic stress (stress due to mismatch of thermal expansion with substrate), and maximal mechanical and chemical bonding with the substrate surface [ 16,171. Coating thickness must also be optimized, if it is too thick, the diamond coating begins to exhibit inherently brittle bulk behavior due to increased stress, which can induce premature microfracture or debonding with the substrate. This residual stress in the coating also affects film microhardness, which is important because at the high cutting temperatures most materials lose their microhardness (this applies also to other hard coatings such as titanium carbide, titanium nitride, and aluminum oxide). High temperature microhardness is important in two modes of tool wear. Tool wear at the hot crater zone on the rake face (top surface) of the tool depends on the chemical inertness of the coating as well as the microhardness at high temperature. The abrasive wear resistance is directly related to the microhardness at the cutting temperature of the flank face of the tool. Inevitably the mechanical properties of the coating must be related in terms of its microstructure (grain size, crystallinity, defects, and so forth), which is a function of the parameters of the coating process.

DIAMOND

CUTTING

TOOLS

Diamond as a cutting tool material comes in three forms: single crystal, high-temperature/high-pressure polycrystalline blanks (PCD), and the newer CVD thick film blanks or thin film coatings. Each form has a different set of characteristics (Table 1 l-l) that determines its range of applications. These differences are inherent in the manufacturing processes used to make the diamond materials. Each manufacturing process emphasizes or accentuates certain characteristics at the expense of others. To use diamond effectively as a cutting tool material, its important to understand the trade offs and to pick the type of diamond that best suits the application.

Table 11-l. Properties of Diamond Tool Materials, C-2 Tungsten Carbide and Silicon Nitride. Property

Single Crystal Diamond

CVD Diamond

PCD

C-2 Cemented Carbide (WC)

Silicon Nitride (Si,NJ

Density (g/cc)

3.52

3.51

4.10

15.00

3.23

Young’s Modulus (GPa)

1050

1180

800

600

315

Compressive Strength (GPa)

9.0

16.0

7.4

5.0

3.0

Transverse Rupture Strength (GPa)

2.9

1.3

1.2

1.7

0.96

Fracture Toughness (MPa*m1’2)

3.4

5.5

9.0

11.0

6

Knoop Hardness (GPa)

50- 100

85-100

50-75

18

16.2

Thermal Conductivity (W/mK)

1000-2000

750-1500

500

100

38

Thermal Expansion (lo-%)

2.0-5.0

3.7

4.0

5.4

3.9

New Diamond Technology and Application Single-Crystal

315

Tools

Single-crystal diamond tools, which may be natural or synthetic, are valued for their extremely sharp edges and lowfriction surface, attributes that allow them to impart fine finishes. These characteristics are due to their crystalline structure and the fact that they are pure diamond. Ironically, the very structure that gives single-crystal diamond its attributes also contributes to its poor resistance to fracture. Since different crystallographic planes in the diamond lattice exhibit very different levels of wear resistance, tool life can vary significantly from piece to piece. Single-crystal diamond fractures relatively easily along certain cleavage planes, so its orientation in relation to the forces being applied is critical to its performance [ 181.

PCD While single-crystal diamond shines in high precision finishing applications, the introduction of PCD 20 years ago [19] enabled diamond-tool use in applications at the other end of the machining spectrum: PCD is ideal for rough-cutting highly abrasive materials that wear out most tools quickly. Compared to single-crystal and CVD diamond, PCD exhibits superior toughness (resistance to chipping and fracturing). PCD is relatively isotropic, meaning its properties are uniform in all directions, so that crystallographic orientation is not an issue. And since the crystallites are oriented in random directions, cracks rarely pass from one crystallite to another. PCD has greater fracture toughness than either single-crystal or CVD diamond and it also exhibits good tensile and compressive strength. These characteristics are a result of the diamond crystallites’ size, the amount of diamond bonds created during the sintering process, and the cobalt phase present in the PCD material. In both cemented tungsten carbide and PCD, the cobalt increases the fracture toughness of the sintered body.

316

Ceramic Cutting Tools

However, the cobalt binder also reduces PCD’s hardness, makes it susceptible to corrosion (especially when machining plastics), and causes oxidation at high temperatures as the cobalt catalyzes the conversion of diamond into graphite. Grain size, which helps determine wear resistance, is another critical difference between PCD and CVD diamond. In PCD, larger grain sizes are ideal for roughing, because they are more wear resistant. Smaller grains yield superior surface finishes. PCD typically is available in three different grain sizes for cutting tools - 5 pm, 10 km, and 25 pm. The smaller grains produce a good surface finish on a workpiece but wear quickly compared to larger grain-sized PCD [ 191. The CVD process also can be controlled to produce a variety of grain sizes, from smaller than 1 pm up to 50 p,m. But with CVD diamond, the small grains are as wear resistant as the larger grains. CVD Advantages CVD diamond is seen not only as an alternative to PCD in many machining situations, but it also can be used in applications that are off limits for PCD and single-crystal diamond tools. Compared to PCD, CVD diamond is harder and more rigid and exhibits a lower coefficient of friction, two to 10 times more abrasion resistance, higher thermal conductivity, and better chemical and thermal stability. It lags behind PCD only in terms of fracture toughness. Therefore, CVD diamond tools typically can cut at higher speeds than PCD tools: CVD diamond’s lower coefficient of friction and higher thermal conductivity allow the tools to run at faster speeds without generating harmful levels of heat. Running at higher speeds allows one to reduce the chip load on the tool while maintaining productivity. This helps compensate for CVD diamond’s lower fracture toughness. Also, CVD diamond, unlike PCD, can machine corrosive plastic-based materials with no dire consequences. Many plastics cause chemical corrosion, hence premature wear, of PCD tools.

New Diamond Technology and Application

317

Diamond Tool Use The three forms of diamond have some characteristics in common. Diamond tool applications are limited by their relatively low temperature capability and high reactivity with ferrous materials. Also, like other ceramic cutting tool materials, diamond is prone to chipping and breakage if used improperly. Diamond is best suited to the machining of aluminum and other ductile, non-ferrous alloys such as copper, brass and bronze, plus highly abrasive and advanced-composite materials such as graphite, carbon-carbon, carbon-filled phenolics, fiberglass, and honeycomb materials [20,21]. These materials quickly wear out tungsten carbide. In field applications, single-crystal diamond is used almost exclusively to impart high-precision finishes, taking shallow cuts to ensure maximum dimensional accuracy. PCD and CVD diamond can be used in many of the same applications, but PCD is more suited to roughing and to machining applications and materials that require high fracture toughness of the tool. CVD diamond excels at finishing, semi-finishing, and continuous-turning applications because of its superior wear resistance [20], and its hardness allows it to produce more precisely machined parts. CVD also is the best choice for applications that require complex tool geometries. PCD blanks, because of the way they are manufactured, are limited to simpler shapes. FIELD RESULTS Increased productivity in the field can be measured in several ways: the number of parts that can be cut by a single edge, the number of parts that can be cut in a given time, or the quality of the parts that are cut. In direct comparisons with PCD-tipped carbide tools, CVD-diamond-coated or thick film tools have demonstrated superiority in all three areas. CVD Thick Film Diamond Piston

turning

application:

In these

applications,

the

318

Ceramic Cutting Tools

cutting tools fail when they can no longer produce the tightly controlled piston’s surface finish. CVD diamond’s superior performance in these applications is a result of its high wear resistance coupled with a fine cutting edge that gives it the ability to produce very uniform and consistent groove profiles in very abrasive material. The lower fracture toughness of CVD diamond isn’t critical here, because there are few interruptions in the part and the depths of cut and feed rates are moderate due to semifinishing and finishing type cuts, which minimizes any impact. A piston O.D. turning/semi-finishing application on A-390 aluminum (16.5% to 19.0% silicon) was tested with CVD thick film diamond inserts. At a cutting speed of 2977 sfm, a feed rate of 0.021 ipr, and a 0.030” DOC, CVD diamond thick film tools produced 4800 parts, while a 25p.m grain-sized PCD insert produced only 2500 parts before the tool failed. A water and soluble oil coolant was applied for both tool materials. In a second such application, the operation was to finish the O.D. profile on an automotive piston of A-390 aluminum. The cutting conditions were as follows: the piston was O.D. turned in two passes, the cutting speed was 2290 sfm, then the feed rate was 0.020 ipr in the first pass and 0.012 ipr in the second pass. The depth of cut was 0.006” in both passes with a water/soluble oil coolant. The 25ym grain-sized PCD tool produced 10,000 parts on average before being changed due to failure to meet surface finish specification. The CVD thick film diamond tool produces 30,000 parts on average before it is changed. This represents a 33% reduction in the insert cost/piece alone, not taking into account other savings due to fewer tool changes, better consistency and lower tool inventories. Another example of CVD diamond’s superiority in piston machining is in finish cam turning. The machining conditions are a speed of 400 sfm, two passes are made each with a feed rate of 0.013 ipr and a depth of cut of 0.020”. Water soluble oil is used as the coolant. The work piece is an aluminum M-132 (12% silicon) with a Ni-Resist ring bonded into the OD of this diesel piston and the tool must cut through this as well on the aluminum alloy. The 1Opm grain size PCD averages 225 parts per insert before being changed. The CVD thick film diamond insert

New Diamond Technology and Application

319

produces 560 parts per insert on average. The corresponds to a 57% increase in tool life and in addition to this are the savings in labor due to decreased downtime. Finish-boring applications: In a finish-boring application on hard, anodized 6061-aluminum turbine cases, CVD thick film diamond tools increased productivity from 106 parts per tool (using a 5l_trn PCD tool) to 345. Machining conditions for this operation are a cutting speed of 1500 sfm, a feed rate of 0.002 ipr, and a DOC of 0.0005”, with a water soluble oil coolant. In this operation, surface finish is the highest priority and that is why a 5ym PCD tool was previously used in order to obtain the required excellent surface finish with good tool life. The CVD diamond thick film material not only increased tool life and productivity but also produced a better and more consistent surface finish on the part by providing finer finishes and maintaining its edge for more than 3 times as many parts per tool. A marine engine manufacturer was boring A-380 aluminum with lOl_trnPCD. The machining conditions were a cutting speed of 2291 sfm, a feed rate of 0.005 ipr, and a DOC of 0.020” with water and water soluble oil as coolant. The PCD tool produced over 1,800 parts per edge and the CVD thick film diamond tool produced over 5,000 parts per edge. Both tools were changed when the tools were unable to hold the dimensional tolerances. The CVD diamond tool outperformed the PCD tool again due to increased wear resistance combined with the ability to hold a fine edge and produce consistent fine finishes. Metal-matrix composites: In the machining of aluminum metal-matrix composites CVD thick film diamond has a wide range of performance. This depends on the cutting operation, turning vs milling, the particular alloy being machinined and the cutting speed and feed rate used. As mentioned previously, the CVD thick film diamond material does not have as high a fracture toughness as PCD. Hence the reason that CVD diamond has not performed as well as PCD generally in face milling type operations or turning operations which have significant interruptions at high speed, large depths of cut and/or high feed rates. This also applies to the machining of aluminum metal-matrix composites due to the type of forces that

320

Ceramic Cutting Tools

are generated by the impact of the hard particle reinforcement phase (A&O3 or Sic). The following two turning tests demonstrate the relationship between the effect of hard particle reinforcement size and cutting speed on the amount and type of wear on CVD diamond and PCD cutting tools. The first test was performed on A-359 aluminum with 10% Sic particles. The SIC particulates have an average particle size of 9pm. Parameters for this test were 3000 sfm speed, 0.008 ipr feed and 0.040” DOC. A flood coolant was applied. Tool life criterion was defined as 0.008” of flank wear. In Figure 1 l-3 it shows the CVD diamond tools cut 800 cubic inches of the material before 0.008” flank wear. In contrast, PCD tools with 25pm grains cut less than 400 cubic inches and PCD tools with 5pm grains less than 100 cubic inches before dulling. The second test was performed on A-359 aluminum with 20% Sic. The Sic particulates have an average particle size of 13p.m in this alloy. The cutting speed was 1000 sfm, the feed rate was 0.008 ipr, and the DOC was 0.040”. Again, flood coolant was used. Figure 1 l-4 shows that CVD thick film diamond tools cut almost 300 cubic inches of material before dulling, 25ym grain sized PCD tools cut slightly more than 200 cubic inches and 5ym grain PCD tools cut about 60 cubic inches. Tool Life (cu.in)

TURNING F3S.lOS-T6 Cutting speed = 3000sfm Feed rate = 0.008ipr Depth of cut = 0.040” TPG 432 Insert Flood Coolant Life criterion: 0.0080” (local wear)

Figure 11-3. Turning test results of diamond tools on A-359 aluminum alloy containing 10% SIC particles.

New Diamond Technology and Application

Tool Life (cu.in.) 300 [

321

I

TURNING F3S.20S-T6 Cutting speed = 13000sfm Feed rate = 0.008ipr Depth of cut = 0.040” TPG 432 Insert Flood Coolant Life criterion: 0.0080” (local wear)

Figure 11-4. Turning test results of diamond tools on A-359 aluminum alloy containing 20% SIC particles.

The poorer performance in the second test can be attributed to the increased size and volume of the Sic particles. Even though the cutting speed was reduced from 3000 to 1000 sfm, the larger mass and volume of the Sic particles more rapidly wore both diamond tool materials. The largest difference in performance from the first to second test was the CVD thick film diamond tools. The reason for this is due to the lower fracture toughness of the CVD thick film diamond. The impact of the larger particles and their larger number in the population in the 20% SIC alloy led to a microcracking and chipping on the cutting edge of the CVD material. The PCD materials were more worn by abrasion and less affected by the impact force of the hard particles due to their higher fracture toughness. when machining aluminum metal-matrix Therefore, composites with diamond tools, the PCD tools would be best used in the milling and roughing turning applications at low to moderate

322

Ceramic Cutting Tools

cutting speeds. CVD thick film diamond tools would be best used for semi-finishing and finish turning at low to moderate cutting This is particularly true in MMC’s with large hard speeds. particles at significant volume concentrations. In the 10% Sic alloy with its smaller particles and lower Sic volume concentration, the CVD thick film diamond has demonstrated significantly +2X) better tool life, then 25ym PCD. This is due to its increased hardness and abrasion resistance and this allows the ability to cut at higher speed with greater productivity.

CVD Thin Film Diamond Tools CVD diamond thin film cutting tools are becoming more readily available all the time. Several manufacturers worldwide are marketing several types of diamond thin film cutting tools. These manufacturers are coating tungsten carbide, silicon nitride and silicon carbide cutting tool substrates with 3-3Opm thicknesses of CVD diamond film [22-241. Thin film diamond coating is generally defined as being a layer of diamond crystals less than 50pm thick. Thin film diamond cutting tools are very attractive to potential users due to the fact it can be applied to complex tool geometries such as inserts with chipbreakers, endmills, routers, and drills. The coating like thick film CVD diamond exhibits high lubricity, generates low cutting forces and wears slowly. Thin film coatings offer all the machining advantages of CVD thick film or PCD as well as providing multiple cutting edges and potentially lower cost per cutting edge. Many papers and articles have been published regarding thin film diamond cutting tools. Craig [23] reported turning tests at Allison Division of General Motors where a diamond coated tungsten carbide was compared to uncoated carbide, TiN-coated carbide and PCD machining 6061 aluminum. Allison ran the test under the following parameters: 500 to 4500 sfm, feed of 0.005 ipr, and DQC of 0.025”. Force on the tool was used to measure the tools’ effectiveness. Forces ranged from 100 to 160 lbs. for the diamond coated WC insert. Forces on the uncoated carbide insert were 250 to 600 lbs; TiN-coated carbide, 600 lbs., and PCD, 150

New Diamond Technology and Application

323

to 200 bs. The test revealed that the diamond film made a large difference in the tool’s performance. The diamond coated insert had no built up edge until the coating was worn through. Kikuchi et al. [ 1 l] reported turning tests for 3-5pm thick diamond coated WC inserts. The diamond coated inserts had 3-18 times longer life than PCD and uncoated carbide tools in a variety of aluminum machining applications. Sen 1241 reported turning and milling tests for diamond coated silicon carbide inserts. The turning test on 18% Si aluminum alloy was done with coolant at 1200 m/min, O.lmm/rev, and 0.25mm DOC. The diamond coated SIC wore through on average in 70 minutes and a PCD lasted greater than 180 minutes. The milling test was done on the same alloy with one insert, with a cutting speed of 1400 m/min, DOC of 0.5mm, and a feed rate of A tool life of 110 minutes was O.lmm/tooth with coolant. achieved before the diamond coated (20-30 pm thick) SIC insert failed by wearing through the coating. Milling applications are considered to be extremely difficult for diamond coated inserts. Film adherence and edge strength are tested in this operation due to the impact involved in milling. Saijo et al. [25] reported good test results for diamond-coated tungsten carbide inserts when milling aluminum 18% Si alloy in the laboratory also. He reported mixed results in the field machining aluminum transmission cases. He also reported that the polishing of the diamond film is very effective to prolong the tool life of the diamond coated insert and improve the surface finish of the work material. Ito et al. [12] reported turning test results for diamond coated silicon nitride inserts. The diamond coated inserts had considerably longer life than uncoated carbide inserts in machining 12-17% silicon-aluminum alloys. The machined surface roughness was comparable to that for sintered diamond and far better than for the carbide inserts. CVD thin film diamond coated silicon nitride inserts have done well on the manufacturing floor as well. These inserts have demonstrated resistance to abrasive and corrosive wear in a finish facing job on a 30% carbon reinforced phenolic seal. The carbon makes this an abrasive material and as the plastic heats up, the

324

Ceramic Cutting Took

material becomes chemically corrosive. The phenolic is machined at a cutting speed of 400 sfm, feed rate of 0.003 ipr and a DOC of 0.025”. In this application prior to the diamond coated silicon nitride neither PCD or WC was the most cost effective tool material. Al,O, tools were being used since they had a good combination of abrasion and corrosion resistance necessary for the application. The Al,O, tools produced only 50 parts per corner and the CVD thin film S&N, inserts provided 650 parts per corner. Stephan [26] reports that CVD thin film coated Si,N, was able to achieve equal tool life as PCD per corner turning A-390 aluminum. The cutting conditions were 2231 sfm, 0.008 ipr, and the depth of cut was 0.040” with flood coolant. He also reports on the use of diamond coated S&N, routers for routing NomexTM (E.I. du Pont de Nemours, Inc.) honeycomb composites. The diamond coated S&N, routers run much quieter, and cooler with the freecutting diamond surface and produce a better quality edge with dramatically longer tool life. POTENTIAL The potential for CVD diamond cutting tools is incredibly large. The race to produce commercially viable tools is well underway and any delays to market to date are due to the fact that the manufacturers have needed to develop the coating as they simultaneously developed a manufacturing process. Commercial quantities of CVD thick and thin film diamond tools are being manufactured and applied successfully in the aerospace and automotive industries. As the coating and manufacturing technologies mature, the use of these tools will grow dramatically, coupled with the continuous substitution by lighter weight aluminum alloys, composites and nonmetallics for iron and steel in applications today and tomorrow. CVD diamond technology is poised to revolutionize the cutting tool industry.

REFERENCES 1. Tennant, S., Philos. Trans. R. Sot. London, 87, 97, 123 (1797).

New Diamond Technology and Application 2. Bundy,

325

F.P., Hall, H.T., Strong, H.M., and Wentorf, R.J., “Manmade Diamond,” Nature (London) 176: 51-54 (1955). 3. Bridgman, P.W., “Synthetic Diamonds,” Sci. Am. 193: 42-46 (1955). 4. Eversole, W.G., “Synthesis of Diamond,” U. S. Pat. No. 3,030,188, Apr. 17, 1962. 5. Angus, J.C., Will, H.A., and Stanko, W.S., “Growth of Diamond Seed Crystals by Vapor Deposition,” J. Appl. Phys. 39: 2915-2922 (1968). 6. Deryagin, B.V., and Fedoseev, D.B., “The Synthesis of Diamond at Low Pressure,” Sci.Am. 233 [5]: 102-109 (1975). 7. DeVries, R.C., “Synthesis of Diamond Under Metastable Conditions,” Annu. Rev. Mater. Sci. 17: 161-187 (1987). 8. Angus, J.C. and Hayman, C.C., “Low-Pressure Metastable Growth of Diamond and ‘Diamondlike’ Phases,” Science (Washington, D.C.) 241: 913-921 (1988). 9. Spitsyn, B.V., “The State of the Art in Studies of Diamond Synthesis From the Gaseous Phase and Some Unsolved Problems,” in: Applications of Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Murakawa, and A. Feldman, eds.) pp. 475-482, Elsevier Science Publishers B.V., Amsterdam (1991). 10. Soderberg, S., Westergren, K., Reineck, I., Ekholm, P.E., and Shahani, H., “Properties and Performance of Diamond Coated Ceramic Cutting Tools,” in: Applications of Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Murakawa, and A. Feldman, eds.), pp. 43-51, Elsevier Science Publishers B.V., Amsterdam (1991). 11. Kikuchi, N., Eto, H., Okamura, T., and Yoshimura, H., “Diamond Coated Inserts: Characteristics and Performance,” in: Applications in Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Murakawa, and A. Feldman, eds.), pp. 61-68, Elsevier Science Publishers B.V., Amsterdam (1991).

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Ceramic Cutting Tools

12. Ito, T. et al., “Diamond Coated Cutting tools Synthesized From CO,” in: Applications in Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Murakawa, and A. Feldman, eds.), pp. 77-83, Elsevier Science Publishers B.V., Amsterdam (1991). 13. Ohtake, N., Yoshikawa, M., Suzuki, K., and Takeuchi, S., “Growth Process of Diamond Film by Arc Discharge Plasma Jet CVD,” in: Applications in Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Murakawa, and A. Feldman, eds.), pp. 43 l-438, Elsevier Science Publishers B.V., Amsterdam (1991). 14. Woodin, R.L., Bigelow, L.K., Cann. G.L., “Synthesis of Large Area Diamond Films by a Low Pressure DC Plasma Jet,” in: Applications in Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Murakawa, and A. Feldman, eds.), pp. 439-444, Elsevier Science Publishers B.V., Amsterdam (1991). 15. Windischmann, H., Epps, G.F., and Ceasar, G.P., “Tensile Strength and Biaxial Young’s Modulus of Diamond Films,” in: New Diamond Science and Technology, (R. Messier, J.T. Glass, J.E. Butler and R. Roy, eds.), pp. 767-772, Materials Research Society, Pittsburgh (1991). 16. Reineck, I., Soderberg, S., Westergren, K., and Shahani, H., “Influence of Microstructure and Residual Stress on the Cutting Performance of Diamond Coated Tools,” in: New Diamond Science and Technology, (R. Messier, J.T. Glass, J.E. Butler and R. Roy, eds.), pp. 809-814, Materials Research Society, Pittsburgh (1991). 17. Schaefer, L., Jiang, X., and Klages, C.P., “In-situ Measuring of Stress Development in Diamond Thin Films,” in: Applications of Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Murakawa,

and A. Feldman, ed.s), pp. 121-128, Elsevier Science Publishers B.V., Amsterdam (1991).

New Diamond Technology and Application

327

18. Field, J.E., Strength and Fracture Properties of Diamond, in: The Properties of Diamond, (J.E. Field, ed.), pp. 281324, Academic Press, London (1979). 19. Krar, S.F. and Ratterman, E., Superabrasives: Grinding and Machining With CBN and Diamond, (S.M. Zollo, ed.) pp. 181-187, McGraw-Hill, Inc., New York (1990). 20. Hay, R.A. and Dean, C.D., “Cutting Tool Performance of CVD Thick Film Diamond,” in: Applications of Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Murakawa, and A. Feldman, eds.) pp. 53-60, Elsevier Science Publishers B.V., Amsterdam (1991). 21. Stephan, P.M., Hay, R.A., and Dean, C.D., “The New Diamond Technology and Its Application in Cutting Tools,” Diamond and Related Materials 1: 710-716 (1992). 22. Mason, F., “Diamond: The Material of the Future,” American Machinist, 134[2]: 23.

Craig, P.,

43-46 (1990). Derby, ” Cutting Tool 23-31 (1992).

“Thin-Film-Diamond

Engineering, 44[1]: 24. Sen, PK., “CVDITE - A New Type of Cutting Tool Insert,”

Industrial Diamond Review (England), 52[552]: 228230 (1992). 25. Saijo, K., Uno, K., Yagi, M., Shibuki, K., Takatsu, S., “The Tool Life of Diamond Coatings in Milling an Al-Si Alloy, ” in: Applications of Diamond Films and Related Materials, (Y. Tzeng, M. Yoshikawa, M. Kurakawa, and A. Feldman, eds.) pp. 69-76, Elsevier Science Publishers B.V., Amsterdam (1991). 26. Stephan, P.M., “Diamond Films Enhance Machining with Ceramics,” Am. Cer. Sec. Bull., 71[11]: 1623-1627 (1992).

12 Machining Economics

Pankaj K. Mehrotra Kennametal, Inc. Latrobe, PA INTRODUCTION The usefulness of advanced cutting tool materials lies in their ability to reduce the overall metal cutting manufacturing costs. A fairly wide range of factors affect machining economics, some directly, and others indirectly. Various cost models incorporating these factors can be devised depending on specific operations. As illustrated in Figure 12- 1 (depicting one such model), by increasing cutting speed, the cost per part decreases due to a reduction in labor or machining cost, reaches a minimum, and then begins to increase due to excessive tool wear and resulting high insert related cost [l]. Such models normally do not include the effect of changing tool material. Advanced cutting tool materials should be able to provide a lower cost at higher cutting speeds due to their enhanced performance, as conceptually illustrated in Figure 12-2. In this chapter, machining costs will be analyzed in terms of their basic elements without referring to any particular cost model. By doing so, an economic analysis of a variety of metal cutting operations can by conducted. The total cost to produce a part consists of material and labor costs (MC and LC, respectively): Total cost/part = MC/part + LC/part

(1)

In general, the cost of advanced tool materials on a per insert basis is higher than conventional carbide tools. However, the use of advanced tools significantly reduces both material and labor cost per part due to improved performance. This is accomplished 328

Machining Economics

329

cost

per

pad

machining cost __---________ fixed cost labor cost

------------------

t cutting speed

Figure 12-1. A model for estimating (Adapted from ref. [ 11).

the total machining

cost

cost

per

pati

toot material

cutting speed

Figure 12-2. Reduction of the total machining advanced cutting tool materials.

cost by applying

330

Ceramic Cutting Tools

by producing more parts per insert and/or removing metal at a higher rate. As a result, a lower overall manufacturing cost may be realized by the use of these tool materials. A typical exercise conducted to estimate the manufacturing cost per part produced is illustrated in Figure 12-3. The part, cutting speed, feed, etc. help to describe the operation. Insert cost, tool life, number of cutting corners available, etc. give an indication of material cost/part. Cutting time, part cycle time, machine cost, labor rates, etc. provide an estimation of the labor cost. The material and labor costs together (equation (1)) yield the total machining cost for a given part. Sometimes labor cost is split into machining cost and fixed cost, as shown in Figure 12-1. Alternatively, fixed costs may be included in a “labor rate” which is used to calculate the labor cost. As can be seen in Figure 12-3 and the following discussion, arriving at and optimizing Therefore, it is machining cost may be a complex process. recommended that any such cost analysis be performed with the help of a professional tooling engineer. MATERIAL

COST

Direct and indirect material costs are normally incurred in machining operations. Some, indirect material costs may be included in the labor rate or fixed cost. Direct Material Cost Direct material costs refer to the cost of the cutting tool insert. The cost of the actual component or part to be machined, although important, is usually not considered in analyzing machining economics. Nevertheless, the potential damage incurred during machining is a more expensive risk with a more valuable component. As a result, machining of expensive components is normally done under relatively conservative conditions. Part costs are based on the material of which the component is made, and its complexity of fabrication. For example, nickel or titanium based parts used in the aerospace industry are normally more expensive than some cast iron parts in the automotive industry.

Machining Economics

A

MACHINE COST/PIECE - CUTTINO

Figure

TWEIPIECE

331

x CVRRENT HOVRLY COST FOR THE MPARTMENT.

12-3. A typical tool performance report describing the information needed to calculate the total machining cost.

332

Ceramic Cutting Tools

In general, advanced cutting tools are more expensive than conventional carbide tools. There are several reasons for this, including higher raw material and manufacturing costs. The lower volume of these tool materials also does not allow the tool manufacturers to take advantage of the economy of scale. The cost of advanced cutting tool material inserts may vary from less than $5/insert for a cermet or white ceramic insert, to several hundred dollars for some large monolithic superhard tool materials. Although high insert cost may give “sticker shock” to some users, the insert cost per part produced should be considered for determining economic viability of a given tool material: IC InsertCostlpart = a XB Where: IC = a = 13 =

Insert Cost Number of parts produced per cutting edge Number of available cutting edges per insert

Tool life is a measure of the number of parts produced per cutting edge. It is affected by a combination of the cutting tool material and cutting conditions. Longer tool lives in an operation lead to lower insert cost per part produced. This aspect of machining will be discussed in detail later. Figure 12-4 shows an example where the cost of a ceramic tool is about 3 times higher than a conventional aluminum oxide coated carbide tool. However, the number of parts produced/edge is 2 times higher and cutting time/part is about 40% lower than that of the carbide tool. With the same number of cutting corners available for both tool materials, the total cost/part for the ceramic too is also about 40% lower than that of the carbide insert.

Machining Economics

ITOOL PERFORMANCE

333

REPORT1

Drum UACMWT

L

,wr

oPfIuTIo* Rough

Turn

OD

PART AND OPERATION

* A1203 + Assume

Coated a

labor

Carbide rate

of

$lOO/hr.

Figure 12-4. An illustration for the total cost for machining a component using an Al,O, coated carbide tool compared to a S&N, cutting tool.

334

Ceramic Cutting Took

Cutting Edges Per Insert As seen in equation (2), the number of useful edges available in an indexable insert also determines the insert cost/part produced, since cutting tools are normally purchased on a per insert basis. Varying shapes of inserts offer a range of cutting edges per insert, however, they also impose operational restrictions. These operational requirements normally have the largest influence on the choice of the insert shape or geometry. Although nominal insert costs vary widely depending on the manufacturer, sales volume and several other factors affect the final price. In general, the larger and more complex the insert shape, the more expensive it is. A round insert may be used for several cuts, depending on the insert size and depth-of-cut taken. A square insert gives 4 edges/side, trigon and triangular inserts 3 edges/side, and a diamond insert (80°, 55” and 35”) 2 edges/side. This is graphically represented in Figure 12-5. For negative rake inserts, the available number of corners are doubled by using both sides of the insert. For inserts with a positive rake clearance angle, corners of only one side can be used. Other details about the various insert shapes can be found in a tool supplier’s catalog [2]. These shapes impose operational restrictions on the metal cutting process. For example, a square insert (SNG-type) offers 8 possible corners to be used and a strong geometry, but it is normally used with a lead angle of +5’ or greater. On the other hand, an 80” diamond shaped insert (CNG-type) has a relatively weaker geometry and only half as many available corners. However, it can be used at negative lead angles. Inserts with positive rake clearance have only half as many corners available compared to their negative rake counterparts, but they can be used in a positive or neutral rake geometry to generate lower cutting In some forces, better surface finish and surface integrity. applications, the number of cutting corners available may be effectively doubled by using the same cutting corner in both left and right hand cutting modes as shown in Figure 12-6. Care should be taken that insert wear in the first cutting operation is minimized so that the other side is not damaged. Innovative users may get all 4 corners/side of a diamond shaped insert by using the

Machining Economics

335

increasing number of insert edges available for cutting

Figure 12-S. Various insert shapes showing different corners available for cutting.

number of

Figure 12-6. Illustration of the use of two cutting edges at the same insert corner.

336

Ceramic Cutting Tools

acute and the obtuse angle corners for suitable operations requiring different lead angles. Finally, an insert corner can be first used for a finishing cut with low depth-of-cut and little wear, followed by a roughing operation using the same corner. Again, care should be taken to avoid excessive wear in the first operation. These strategies can be used to maximize the number of parts produced per insert corner. For cutting tool materials with a high manufacturing cost, brazed tipped inserts are used, such as shown in Figure 12-7. For example, for superhard polycrystalline tool materials (diamond or CBN) manufactured using the super high pressure techniques, a small tip is cut and brazed to a less expensive carbide substrate. This gives only one useful cutting corner per insert but results in efficient tool material utilization. 'Mini-tips' use a smaller piece of the superhard material, and thus reduce cost. The depth-of-cut taken by these tipped inserts is usually restricted by the leg length (size) of the tip. Also, very heavy cuts and/or excessive speedsare

Figure

12-7. For a diamond tipped insert, an expensive diamond tip is brazed on an inexpensive carbide substrate.

Machining &onomics

337

to be avoided since they may generate enough heat to soften the braze, resulting in tip de-bonding. Recent advances in diamond coating technology could make all insert corners available for cutting, without restrictions on the depth-of-cut. This should reduce tool material cost per part produced. Some of the advanced cutting tool materials are very difficult to fabricate in certain complex shapes. As shown in Figure 12-8, a simple shape of a SiAION ceramic material can be brazed on to a more complex and less expensive carbide substrate. Such a tool can give the cutting performance of a ceramic, but only at a fraction of the cost. Users of expensive advanced cutting tool materials may further reduce their tooling costs in some operations by re-grinding used inserts to the next smaller size. Thus, a 1/2" I.C. insert may be ground to a 3/8" I.C. insert after its first use, at a fraction of the cost of a new insert of the same size. This practice takes one away from the cpncept of "throwaway inserts," but can be economical for some operations, such as those involving tipped superhard tool materials. Re-grinding operations necessitatethat tool wear in the first use not be excessive and that damage to the cutting edge is

--braze

,

carbide substrate

Figure

12-8. A simple Si3NJSiA1ON complex carbide substrate.

tool material brazed on a

338

Ceramic Cutting Tools

limited so that it can be removed by the regrinding operation. Furthermore, costs associated with handling used inserts, sorting and development of re-grinding technology should also be included to estimate the total cost of re-grinding. Indirect Material Cost There are some material costs directly associated with the cutting operation, but are usually pooled in the labor rate or fixed cost. Costs associated with coolants, lubricants, hand tools, machine tool, etc. could be included in this category. Lubricating coolants are used for a variety of reasons in metal cutting, such as for keeping the part cool and minimizing distortion in slender parts, dust control in cast iron machining, keeping tools and chips cool and to avoid overheating. Coolants help to maintain a lower operating temperature for carbide tools, thus minimizing tool wear. For ceramic and cermet tool materials, coolants accentuate thermal shock, which may shorten tool life or lead to tool fracture. For tipped superhard cutting tools, coolants minimize heating of the braze material, which otherwise will soften and cause the tip to de-bond. The cost of purchasing, using, reconditioning and disposing of coolants also falls in this cost category. Care should be taken in using and disposing coolants and lubricants due to their adverse impact on personnel and the environment 131. As hazardous materials are becoming more and more difficult and expensive to dispose off, disposal of used coolants is fast becoming a problem. As a result, many manufacturers are looking for metal cutting processes which do not use coolants, and thus reduce their costs. This trend favors ceramic and cermet tools, but creates difficulties for superhard tool materials with brazed tips. The advent of diamond or CBN coatings will allow cutting with superhard tool materials without coolants.

Machining Economics

339

RELIABILITY Reliability of advanced cutting tool materials is generally considered to be less than that of the conventional tools based on tungsten carbide and high speed steel [4]. The relatively low fracture resistance of advanced tool materials can result in a loss of one or more cutting edges either by accidental chipping due to careless handling, or by fracture due to unduly aggressive cutting This problem of conditions or by lack of machine rigidity. unpredictable fracture has prevented a widespread acceptance of advanced tool materials. Fracture during machining results not only in a loss of a cutting corner or the entire insert, but also possibly to a ruined surface finish, destroyed toolholder, wrecked Furthermore, this behavior machine tool or scrapped part. This necessitates continuous attention from an operator. unpredictability adds to the cost of machining process and makes reliable cost estimations difficult. One way to increase the reliability of these tool materials is to package them properly, and to handle them carefully. Sufficient testing should be conducted to optimize the cutting conditions to minimize tool failure by fracture. The use of coolants should be minimized or eliminated, if possible, when using ceramic or cermet tools. Development of tougher advanced tool materials and well thought out geometrical designs, such as a suitable edge preparation [5], will also help improve reliability and should lead to wider acceptance of advanced cutting tool materials.

LABOR COSTS In most metal cutting operations, labor costs are the major contributor to the overall cost of machining. In general, labor cost is based on the overhead or labor rate and the time required to machine a part or one full part cycle: Labor cost/part = Labor rate x Cycle time/part

(3)

340

Ceramic Cutting Tools

The cycle time/part or floor-to-floor time (CT) and non-cutting time (NCT):

time consists of cutting

Cycle time/part = CT/part + NCT/part

(4)

The cutting time refers to the actual time during which The non-cutting time may include time metal is being removed. spent on part changeover, tool change, tool off-set, part size or surface finish measurement, chip clean-up, etc. Cutting Time Probably the single most important factor in popularizing advanced cutting tool materials is the reduction of time needed to manufacture a given component. The use of advanced cutting tool materials has allowed increases in the cutting speed from 20-400% over conventional carbide tools. However, in some applications advanced tools may not be used in heavy roughing or severely interrupted cutting. These restrictions represent significant limitations in the use of these tools, and have prevented a widespread application of the advanced cutting tool materials. As shown in Table 12- 1, the metal removal rates by using silicon nitride tools in cast iron machining are typically about 3.5 In nickel alloy times that of Al,O, coated carbide tools. machining, metal removal rates can be usually increased by about 300% by applying silicon carbide whisker reinforced alumina or sialon cutting tool materials (Table 12-2) instead of PVD TiN coated carbide tools. Similarly, as Table 12-3 shows, about 20% increase in metal removal rates in machining steels can be achieved Table 12-4 illustrates about 40% by using A&O,-TIC tools. increase in metal removal rates in machining Al-alloy by using PCD tipped tools, and Table 12-5 shows A&O,-TIC and CBN tools to yield a metal removal rate in hard turning which is 4 to 8 times that of carbide tools 161. The increased metal removal rates demonstrated in Tables 12- 1 through 12-5 by using advanced cutting tool materials also necessitate higher horse power machine tools.

341

Machining Economics Table 12-1.

Machining

of Gray Cast Iron (175320

BHN).

Tool Material

Speed (sfm)

Feed (ipr)

Depth of Cut (in)

Metal Removal Rate (in3/min)

S&N,

2400

0.015

0.150

64.8

Al,O, coated carbide

650

0.015

0.150

17.6

Table 12-2. Machining (200-450 BHN).

of Ni-Based High Temperature

Alloys

Tool Material

Speed (sfm)

Feed (ipr)

Depth of Cut (in)

Metal Removal Rate (in3/min)

Al,O,-SIC,

800

0.006

0.100

5.8

SiAlON

700

0.006

0.100

5.0

PVD TIN Coated Carbide

175

0.006

0.100

1.3

Table 12-3. Machining (200-325 BHN).

of Medium Carbon Alloy Steels

Tool Material

Speed (sfm)

Feed (ipr)

Depth of Cut (in)

Metal Removal Rate (in3/min)

A&O,-TiC

1000

0.010

0.130

1 15.6

Al,O, Coated Carbide

700

0.012

0.130

13.1

342

Ceramic Cutting Tools

Table 12-4.

Machining

of Al-Alloys (50-150 BHN).

Tool Material

Speed (sfm)

Feed (ipr)

Depth of Cut (in)

Metal Removal Rate (ir?/min)

Tipped PCD

2500

0.018

0.130

70.2

PVD TIN Coated Carbide

1800

0.018

0.130

50.5

Table 12-5.

Machining

of Tool Steels (570-780 BHN).

Tool Material

Speed (sfm)

Feed (ipr)

Depth of Cut (in)

Metal Removal Rate (in3/min)

Tipped CBN

350

0.008

0.050

1.7

Al,O,-TIC

300

0.005

0.050

0.9

Carbide

50

0.005

0.050

0.2

Since most of the advanced tool materials are not as tough as carbides, these machine tools should also be stiffer. Thus, machine tools need to be upgraded to take advantage of these cutting tool materials. This may increase the fixed cost or labor rate component of the machining cost. As new metal forming methods produce near-net shaped parts, less and less metal needs to be removed (lower depth-of-cut) This trend favors the use of at higher metal removal rates. advanced cutting tool materials over the conventional WC-Co based tools. Ceramic and superhard tool materials can also replace grinding in some manufacturing applications. Grinding with an abrasive wheel is normally used for obtaining good surface finish and when working with hard workpieces. In several applications, grinding operations can be replaced by turning operations using alumina based or CBN based tool materials, at a significantly lower cost by reducing power consumption and cycle time [7].

Machining Economics

343

Tool Life Tool life is determined by time or by the number of parts a cutting edge can produce before the finished part goes out of specification. A longer tool life results in reduced frequency for tool off-setting, indexing or tool replacement, thus reducing labor cost and interruption to the work flow. Also, a longer tool life means a lower insert cost per part produced. In transfer lines or manufacturing cells, such as in some automotive operations, where an operator is not always available to attend to a machine, the desired tool life should correspond to a convenient time (such as a shift change) for the operator to index or change tools. Thus, any advanced tool material must show an advantage in terms of increments of this convenient time period. Shorter tool life increases, although significant, may not reduce the overall cost of production. On the other hand, in manufacturing operations where an operator is always in attendance and the cost of tool failure is high, such as in aerospace operations, an insert may be indexed after each pass, or only a fraction of the pass. In these operations a conservative approach to machining may leave some tool life unused, but may provide overall savings in manufacturing costs. Non-Cutting

Time

As advanced cutting tool materials allow higher and higher metal removal rates, non-cutting time of machine tools becomes a Although not more significant part of the floor-to-floor time. directly related to a given tool material, certain strategies can be employed to reduce labor time and cost related to non-cutting time on the machine tool. Some of these strategies are discussed below. Quick change tooling (QCT) refers to the hardware available to minimize time needed for tool changing [8]. Automation in part loading, unloading and process control measurements (such as part size, surface finish, etc.) also lead to reduction in labor cost and increase reproducibility of the operation. High metal removal rates with ceramic tools also increase the

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Ceramic Cutting Tools

Unmanned chip disposal amount of chips being produced. operations are frequently used to minimize labor costs for this type of operation.

Overhead/Labor Rates And Fixed Costs Labor rates or overhead rates refer to a method of assigning fixed costs of plant and personnel to the actual part producing activity. Real estate, plant, machinery, utility, staff employee, etc. are not directly involved in part production, but are essential to the manufacturing operation. As discussed earlier, some indirect material costs associated with actual part production may be included in the labor rate. New, advanced machine tools, automation, well trained staff employees, etc. needed for using advanced tool materials, increase the labor rates. For advanced tool materials to be economically viable, the reduction in cycle time/part caused by the use of these new tool materials should A recent trend in more than offset the increased labor rates. reducing labor rates is a cooperative effort between the tool user and tool supplier. The expertise of the highly trained technical staff of tool suppliers may be utilized to optimize the selection of tool and the manufacturing processes. Furthermore, a “full service supply” tool supplier can reduce the cost and confusion of having Finally, additional fixed cost to deal with many suppliers. reduction can be achieved by allowing tool vendors to run the in-plant tool crib. This may reduce inventory cost and develop a mechanism for paying only for the tools actually used or based on the productivity improvements actually realized on the shop floor.

REFERENCES 1. Fundamentals of Machining and Machine Tools, 2nd ed., (Boothroyd and Knight, eds.) Marcel Dekker, Inc., New York, NY, p. 182 (1989). 2. Kennametal Turning Products, Kennametal Inc., Latrobe, PA (1991).

Machining Economics

345

3. Metals Handbook, Ninth Edition, Vol. 16, Machining, ASM International, Metals Park, OH, p. 131 (1989). 4. B. North, “Ceramic Cutting Tools,” SME Technical Paper MR86-45 1 (1986). 5. Kennametal Turning Products, Kennametal Inc., Latrobe, PA, pp. 286-290 (1991). 6. Kennametal Turning Products, Kennametal Inc., Latrobe, PA, pp. 250-281 (1991). 7. Metals Handbook, Ninth Edition, Vol. 16, Machining, ASM International, Metals Park, OH, p. 429, pp. 708-735 (1989). 8. Kennametal KM Quick Change Tooling, Kennametal Inc., Latrobe, PA (1993).

13 Summary and Prospectives on the Future of the Ceramic Tool in Manufacturing Operations

E. Dow Whitney University of Florida Gainesville, Florida

Nearly three decades ago Professor Milton C. Shaw observed, “As the third ‘law’ of history indicates, new tool materials should be expected to appear in response to new requirements” [ 11. This ‘law’ has not been violated. Indeed, silicon nitride-based cutting tools were generally unknown to the manufacturing community when the latter words were written. Their introduction into the metalworking field since that time is further proof that the ‘third law’ holds. Today’s production engineer has available to him or her a wide variety of tool materials from which to choose. Even a casual perusal of this book will convince the reader that when concerned with productivity, the economic advantages of utilizing the proper tool materials in machining operations are considerable. This book is the result of bringing together a group of experts involved in a wide spectrum of ceramic cutting tool science and technology including research, development, testing and manufacturing. It was written primarily for production personnel involved in all aspects of metalworking operations such as plant managers, manufacturing engineers, production foreman and machine tool operators. At the same time, it would be particularly gratifying to the authors if this book is found to be useful in manufacturing engineering educational programs.

346

Summary and Prospectives

347

The timing for a modern book on ceramic cutting tools is In the past twenty years there has been a virtual excellent. explosion in the availability of new grades of ceramic cutting tools. These new tools are in effect spin-off products of extensive research being undertaken in this country and abroad on advanced structural ceramics for high temperature energy conversion systems. Indeed, a high speed metal cutting operation is often an excellent test of the thermal, mechanical and chemical attributes of a new Thus there is a synergism which structural ceramic composition. benefits both advanced structural ceramics technology as well as metalworking productivity. Are there problems still to be addressed which limit the further application of ceramic cutting tools? Certainly the problem of chemical reactivity-related wear limits the usefulness of advanced ceramic cutting tools in the machining of nickel-based and titanium alloys. The chemical reactivity problem associated with advanced ceramic cutting tools is related to a number of specific technical barriers. Critical leverage points for overcoming this chemical incompatibility problem go far beyond the conventional approach of relating tool wear to cutting parameters such as cutting speed, etc. It is the opinion of this writer that the chemical reactivity problem should be approached from the standpoint of molecular or chemical tribology, i.e., the study of tribology at atomic and molecular levels. This constitutes a new frontier of tribilogy research, and may prove useful if applied to specific wear problems involving ceramic tools [2]. Certainly any future research undertaken to study tool composition variations in order to minimize tool material/workmetal reactivity will benefit from molecular tribological considerations. In addition to these technical barriers there is an important non-technical barrier which must be overcome; i.e., new tool materials tend to be used under conventional machining conditions. Such an approach must be avoided in the evaluation of new tool materials for the machining of reactive metals such as titanium. Accommodations will also need to be made in other factors such as tool geometry, coolants, coolant delivery and other machining parameters so as to optimize tool performance.

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Ceramic Cutting Tools

WHAT DOES THE CUTTING TOOLS?

FUTURE

HOLD

FOR

CERAMIC

A new area of machining currently under development is This is a laser-assisted turning of ceramics with ceramics. rejuvenation of hot machining technology brought about through progress in laser techniques. Even as this final chapter is being prepared, reports are appearing in the scientific literature describing the possible synthesis of a new compound, cubic carbon nitride, a material predicted to have bulk moduli comparable to diamond. Only time will tell how this material will compare to diamond with regard to hardness, thermal stability and workmetal compatibility. Perhaps cubic carbon nitride be the first new ceramic cutting tool of the 21st century. There is not and probably never will be a “universal” cutting tool material. As already stated, many of the new ceramic tool materials available today have very specific applications. When properly applied, these tools provide the manufacturing engineer with a means of reducing machining costs and increasing productivity. It is somewhat ironic that the cutting tool insert, the lowest priced single unit in the tool machine system, offers the greatest opportunity for productivity improvement and cost reduction. Taking cutting speed as a measure of “degree of productivity,” we see that it is the performance of the tool itself which is often the limiting factor in the overall productivity scheme. Certainly improvement in productivity of manufacturing processes involving metal cutting still presents one of the most challenging problems of our times.

REFERENCES 1. M.C. Shaw, Cutting Tool Material Selection, (H.J. Swinehart, ed.) American Society of Tool and Manufacturing Engineers, Dearborn, MI (1968). 2. S. Granick, “Molecular Tribology,” Mat. Res. Sot. Bull., g[ lo], 33-35 (1991).

Index

a-A1203 - 222 a-SisN, - 193 Abrasion - 225 Advantages (of coatings) - 222 AI,03 - 2 Al,O,/TiC - 117,119 Al,OaRiO - 3,lO Alumina 115 glass bonded - 2 hot-pressed - 3 Aluminum oxide/titanium carbide composites - 48 capital investment - 53 composition - 48 cycle time - 53 grade applications - 52 hardness - 48,50 machining recommendations - 56 microstructure - 48, 50 oxide ceramics - 49 perishable tooling - 53 physical properties - 49 tool design - 54 Applications - 95, 251 cutting tools - 201 349

350

Ceramic Cutting Tools machininggr;ycastimn-U)2 steel - 206 superalloy - 210 finish-boring - 319 metal-matrix comp&es - 319 piston turning - 317

b-S&N, - 193 Breakage resistance - 224 Breaking strength - 125 Brittle/tensile fracture - 21 Broaching - 15 Built-up edge - 234 CCT-707 - 3 Cemented carbide - 10, 221 Ce02 stabilized zirconia - 131 CeO, tetragonal zirconia polycrystal (Ce-TZP) - 131 Ceramic cutting tools - 113 Ceramic summary - 43 Cermet boring - 68 composition - 63 cutting speeds - 80, 82 grade applications - 67 grooving - 70 lay-down - 73 machining recommendations - 75, 79 microstructure - 63, 65 milling finish - 84 rough - 83 physical properties - 66 properties - 63 threading - 70, 80 TiCYI’iN - 64 titanium carbonitride - 63 tool design - 68 turning - 68

Index Chemical degradation - 19 effects - 117 interaction - 225 stability - 20, 113, 117 strengthening - 19 Chip edge - 17 Chippage - 225 Chipping - 21 Coated carbides - 38 Coating thickness - 231 Coatings - 20 alumina - 221 TiC - 221 TiN - 221 Cold pressed alumina - 40 Commercial whisker reinforced tools - 88 Composites alumina-silicon carbide whisker - 86 Conductivity - 14 Costing superabrasives - 297 Crack deflection - 126 Crack deflection - 89 Crack meandering - 126 Crack propagation - 123 Crater wear - 19, 226 Critical flaw size - 195 Critical tensile stress - 22 Cubic boron nitride (CBN) - 10, 45, 241 Cutting edges - 334 Cutting speed - 13, 37 Cutting time - 340 Cutting tools development - 29 advances in processing - 7 development - 1 historical perspectives processing equipment - 8 selection - 28

351

352

Ceramic Cutting Tools

CVD coatings - 222 CVD thin film diamond tools - 322 Degussa - 113 Degussit - 2 Depth-of-cut (DOCN) - 100, 116, 237 Development - 113 Diamond properties - 309 chemical stability - 312 mechanical - 309 microfracture toughness - 312 microhardness - 313 physical - 309 thermal expansion - 311 Diamond technology - - 305 CVD diamond cutting tools - 305 DC are plasma deposition - 307 microwave plasma - 307 Differential contraction - 23 Direct/indirect material cost - 330, 338 Edge chippage - 100 Fixed costs - 344 Flaking - 100 Flank wear - 100, 231 Flaw size - 125 Foot formation - 23 Fracture - 100 Fracture toughness - 2, 21, 91, 117, 125, 194 Geometry factor - 125 Grain size distribution - 194 Groove wear - 17 Gross fracture - 21 Guidelines of machining - 249 Hardness - 91 Hardness properties - 229

Index High speed steel (HSS) - 32 Hot hardness - 30 Hot pressed alumina/riC - 41 Industrial diamond- 242 Insert cost - 332 k-AI,O, - 222 Labor costs - 328, 339 Lucalox - 6 Machining economics - 328 Magnesium oxide - 115 Martensitic transformation - 128, 129 Material cost - 328 Meandering - 126 Mechanical properties - 89, 194, 196 polycrystalline alumina - 92 Sic whisker/alumina composites - 92 Mechanical shock - 117 MgO-2 Microcrack toughening - 126 Microlite - 2 Multi-layer coatings - 237 Non-cutting time - 343 Notching - 100, 101, 237 Overhead/labor

rates - 344

PCBN case histories - 288 PCBN: machining guidelines application - 272 case history - 288 coolant application - 283 depth of cut - 279,286 DOC - 278 edge preparation - 281

353

354

Ceramic Cutting Tools lead angle - 283 machine/grind - 269 nose radius - 283 parameters - 276 products - 273 rake angle - 281 selecting grade - 274 speed/feed - 277,286 tools - 275 workpieces - 273

PCD case histories - 264 coolant use guidelines - 262 depth of cut guidelines - 258 lead angle guidelines - 261 machining parameters - 257 nose radius guidelines - 261 rake angle guidelines - 258 speed/feed guidelines - 258 tool edge preparation - 262 Phase transformation toughened materials - 112 Physical stability - 113 Plane strain - 18 Plane stress - 18 Polycrystalline diamond - 10, 45, 241 Polycrystalline diamond tools - 249 Pre-chamfering - 107 Pressure - 24 Productivity - 192 Properties (PCD, PCBN) chemical wear - 248 friction coefficient - 247 hardness - 245 modulus of elasticity - 245 thermal conductivity - 246 thermal expansion coefficient - 246 transverse rupture strength - 247 Properties of diamond tool materials polycrystalline diamond (PCD) - 315

Index single-crystal tools - 315 tool use - 317 316 Pure alumina - 115 Ramping - 107 Reliability - 339 Requirements hardness - 112 high hardness - 112, 113 mechanical resistance - 112 strength - 112 wear - 112,113,114 Selection corner - 104 edge condition - 104 geometry - 104 lead angles - 106 round inserts - 104 Si,N, - 88, 119 SiAION - 9, 88, 193,198 Silicon nitride - 9, 43, 191, 199 Spalling - 117 Specific energy - 16 Specific heat - 14 Strength - 195 Stress intensity - 195 Stress reversal - 23 Structural inhomogeneity - 25 Stupalox - 3 Super hard materials - 243 Super Z - 188 surface roughness - 188 tool life - 188 Superalloys - 87 t-land - 99 Taylor exponent - 14

355

356

Ceramic Cutting Tools

Temperature - 13 TiC/TiN cermets - 44 TiO, - 3 Titanium carbide/titanium nitride - 9 Tool coatings - 229 Tool life - 13, 332, 343 Tool tip temperature - 15 Total cost - 328 Toughening mechanisms - 89, 125 Toughening whisker/fiber reinforced - 86 zirconia transformation - 86 Toughness - 113 Transformation toughened zirconia system - 126 Transformation toughening - 126 TTZ and TZP applications - 137 bending strength - 166 CBN grinding - 152, 177 crater wear - 162 cutting force - 159 cutting performance - 179 diamond wheel grinding - 152,171 fabrication - 135 flank wear - 162 fracture toughness - 167 machining performance (Ce-TZP) - 179 performance - 152,161 sintering - 137, 165 surface finish - 157 tetragonal phase formation - 137 thermal shock resistance - 168 tool wear - 159, 182 transformation during cutting - 181 Weibull modulus - 166 Uncoated carbides - 34 VR-97 - 3

Index Wear land - 19 mechanisms - 225 mode - 15 rate - 19 resistance - 37, 224 Whisker bridging - 90 composites - 9 pullout - 90 reinforced alumina - 42 Work material / alloy - 31 Workpiece compatibility - 25

Y-T-I-Z grinding - 148 pressure - 153 speed - 153 Yttria-partially stabilized zirconia (Y-TZP) - 130 Yttria-tetragonal zirconia polycrystals (Y-TZP) - 130 Zirconia - 115 ceramics - 123 crystallographic data - 127 fully stabilized - 123 partially stabilized - 123 ZTA machining applications - 183 ZTA/PSZ - 185

357

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