WEAR – MATERIALS,MECHANISMS ANDPRACTICE

Edited by

Gwidon W. Stachowiak

WEAR – MATERIALS,MECHANISMS ANDPRACTICE

Editors:M.J. Neale, T.A. Polak and M. Priest

Guide to Wear Problems and Testing for IndustryM.J. Neale and M. Gee

Handbook of Surface Treatment and CoatingsM. Neale, T.A. Polak, and M. Priest (Eds)

Lubrication and Lubricant Selection – A Practical Guide, 3rd EditionA.R. Lansdown

Rolling ContactsT.A. Stolarski and S. Tobe

Total Tribology – Towards an integrated approachI. Sherrington, B. Rowe and R. Wood (Eds)

Tribology – Lubrication, Friction and WearI.V. Kragelsky, V.V. Alisin, N.K. Myshkin and M.I. Petrokovets

Wear – Materials, Mechanisms and PracticeG. Stachowiak (Ed.)

WEAR – MATERIALS,MECHANISMS ANDPRACTICE

Edited by

Gwidon W. Stachowiak

Copyright © 2005 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,West Sussex PO19 8SQ, England

Telephone (+44) 1243 779777

Chapter 1 Copyright © I.M. Hutchings

Email (for orders and customer service enquiries): cs-books@wiley.co.ukVisit our Home Page on www.wiley.com

Reprinted with corrections May 2006

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted inany form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except underthe terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the CopyrightLicensing Agency Ltd, 90 Tottenham Court Road, London W1T 4LP, UK, without the permission in writingof the Publisher. Requests to the Publisher should be addressed to the Permissions Department, John Wiley &Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England, or emailed topermreq@wiley.co.uk, or faxed to (+44) 1243 770620.

This publication is designed to provide accurate and authoritative information in regard to the subject mattercovered. It is sold on the understanding that the Publisher is not engaged in rendering professional services.If professional advice or other expert assistance is required, the services of a competent professional shouldbe sought.

Other Wiley Editorial Offices

John Wiley & Sons Inc., 111 River Street, Hoboken, NJ 07030, USA

Jossey-Bass, 989 Market Street, San Francisco, CA 94103-1741, USA

Wiley-VCH Verlag GmbH, Boschstr. 12, D-69469 Weinheim, Germany

John Wiley & Sons Australia Ltd, 42 McDougall Street, Milton, Queensland 4064, Australia

John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore 129809

John Wiley & Sons Canada Ltd, 22 Worcester Road, Etobicoke, Ontario, Canada M9W 1L1

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not beavailable in electronic books.

Library of Congress Cataloging in Publication Data

Wear – materials, mechanisms and practice / editor Gwidon W. Stachowiak.p. cm.

Includes bibliographical references and index.ISBN-13: 978-0-470-01628-2 (cloth : alk. paper)ISBN-10: 0-470-01628-0 (cloth : alk. paper)1. Mechanical wear. I. Stachowiak, G.W. (Gwidon W.)TA418.4.W415 2005620.1'1292—dc22 2005006833

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

ISBN-13: 978-0-470-01628-2 (HB)ISBN-10: 0-470-01628-0 (HB)

Typeset in 10/12pt Times by Integra Software Services Pvt. Ltd, Pondicherry, IndiaPrinted and bound in Great Britain by Antony Rowe Ltd, Chippenham, WiltshireThis book is printed on acid-free paper responsibly manufactured from sustainable forestryin which at least two trees are planted for each one used for paper production.

Contents

List of Contributors xiiiSeries Editors’ Foreword xviiPreface xix

1 The Challenge of Wear 1I.M. Hutchings

Abstract 11.1 Introduction 11.2 Definitions and Development of Wear Studies 11.3 Scope and Challenges 21.4 Conclusions 6References 6

2 Classification of Wear Mechanisms/Models 9K. Kato

Abstract 92.1 Introduction 92.2 Classification of Wear Mechanisms and Wear Modes 10

2.2.1 Mechanical, Chemical and Thermal Wear 102.2.2 Wear Modes: Abrasive, Adhesive, Flow and Fatigue Wear 112.2.3 Corrosive Wear 142.2.4 Melt and Diffusive Wear 15

2.3 General Discussion of Wear Mechanisms and Their Models 152.3.1 Material Dependence 152.3.2 Wear Maps 162.3.3 Wear Mode Transition 172.3.4 Erosion 17

2.4 Conclusion 18Acknowledgements 18References 18

3 Wear of Metals: A Material Approach 21S.K. Biswas

Abstract 213.1 Introduction 21

vi Contents

3.2 Mild Wear and Transition to Severe Wear 223.2.1 Mild Wear 223.2.2 Transition to Severe Wear 23

3.3 Strain Rate Estimates and Bulk Surface Temperature 273.3.1 Strain Rate Response Maps 283.3.2 Bulk Surface Temperature 303.3.3 The Phenomenological Argument 303.3.4 Micrographic Observations 31

3.4 Summary 343.4.1 Homogeneous Deformation – Severe Wear 343.4.2 Homogeneous Deformation – Mild Wear 353.4.3 Inhomogeneous Deformation – Severe Wear 35

Acknowledgements 35References 35

4 Boundary Lubricated Wear 37S.M. Hsu, R.G. Munro, M.C. Shen,and R.S. Gates

Abstract 374.1 Introduction 374.2 Lubricated Wear Classification 384.3 Lubricated Wear Versus “Dry” Wear 384.4 Wear Measurement in Well-Lubricated Systems 424.5 Measurement Procedures 44

4.5.1 Run-In Process 464.5.2 General Performance Wear Test (GPT) 494.5.3 Enhanced Oxidation Wear Test (EOT) 524.5.4 Boundary Film Persistence Test (BFPT) 534.5.5 Case Study with GPT and BFPT 554.5.6 Boundary Film Failure Test (BFFT) 57

4.6 Wear Mechanisms Under Lubricated Conditions 614.7 Modeling of Lubricated Wear 65

4.7.1 Wear 654.7.2 Contact Area 654.7.3 Rheology 664.7.4 Film Thickness 674.7.5 Contact Stress 674.7.6 Flash Temperatures 67

4.8 Summary 68Acknowledgments 69References 69

5 Wear and Chemistry of Lubricants 71A. Neville and A. Morina

5.1 Encountering Wear in Tribological Contacts 715.2 Lubricant Formulations – Drivers for Change 735.3 Tribochemistry and Wear 765.4 Antiwear Additive Technologies 77

5.4.1 Antiwear Technologies 775.4.2 ZDDP – Antiwear Mechanism 78

Contents vii

5.4.3 Interaction of ZDDP with Other Additives 835.4.4 New Antiwear Additive Technologies 87

5.5 Extreme Pressure Additives 885.6 Lubricating Non-Fe Materials 89References 90

6 Surface Chemistry in Tribology 95A.J. Gellman and N.D. Spencer

Abstract 956.1 Introduction 956.2 Boundary Lubrication and Oiliness Additives 95

6.2.1 Introduction 956.2.2 Monolayers, Multilayers and Soaps 966.2.3 Viscous Near-Surface Layers 1026.2.4 Boundary Lubrication in Natural Joints 1026.2.5 Summary 103

6.3 Zinc Dialkyldithiophosphate 1036.3.1 Background 1036.3.2 Analytical Approaches 1046.3.3 Summary of Film-Formation Mechanism 1046.3.4 Studies of Film Structure, Composition, and Thickness 105

6.4 Hard Disk Lubrication 1096.5 Vapor-Phase Lubrication 1126.6 Tribology of Quasicrystals 1156.7 Conclusions 118Acknowledgments 118References 118

7 Tribology of Engineered Surfaces 123K. Holmberg and A. Matthews

Abstract 1237.1 Introduction 1237.2 Definition of an Engineered Surface 1257.3 Tribomechanisms of Coated Surfaces 125

7.3.1 Scales of Tribology 1257.3.2 Macromechanical Friction and Wear 1267.3.3 Micromechanical Mechanisms 1317.3.4 Modelling Stresses and Strains in a Coated Microcontact 1327.3.5 Tribochemical Mechanisms 1337.3.6 Nanoscale Mechanisms 1357.3.7 Debris Generation and Transfer Layers 136

7.4 Contact Types 1397.4.1 Sliding 1397.4.2 Abrasion 1417.4.3 Impact 1417.4.4 Surface Fatigue 1417.4.5 Fretting 1427.4.6 Chemical Dissolution 1437.4.7 Lubricated 143

viii Contents

7.5 Advanced Coating Types 1447.5.1 Hard Binary Compound Coatings 1457.5.2 Multilayer Coatings 1467.5.3 Nanocomposite Coatings 1497.5.4 Hybrid and Duplex Coatings 151

7.6 Applications 1527.7 Conclusions 154References 155

8 Wear of Ceramics: Wear Transitions and Tribochemical Reactions 167S. Jahanmir

Abstract 1678.1 Introduction 1688.2 Structure and Properties of Ceramics 168

8.2.1 Alumina Ceramics 1688.2.2 Silicon Nitride Ceramics 1698.2.3 Silicon Carbide Ceramics 170

8.3 Wear Transitions 1708.3.1 Alumina 1718.3.2 Silicon Nitride 1748.3.3 Silicon Carbide 175

8.4 Damage Formation in Hertzian Contacts 1778.4.1 Brittle Behavior 1778.4.2 Quasi-Plastic Behavior 1778.4.3 Brittleness Index 180

8.5 Transition Loads in Sliding Contacts 1818.5.1 Quasi-Plastic Behavior 1818.5.2 Brittle Behavior 1838.5.3 Transition from Brittle Fracture to Quasi-Plasticity 184

8.6 Ceramics in Tribological Applications 185Acknowledgments 187References 187

9 Tribology of Diamond and Diamond-Like Carbon Films: An Overview 191A. Erdemir and Ch. Donnet

Abstract 1919.1 General Overview 1929.2 Diamond Films 194

9.2.1 Deposition and Film Microstructure 1949.2.2 Tribology of Diamond Films 1959.2.3 Practical Applications 204

9.3 Diamond-like Carbon Films 2079.3.1 Structure and Composition 2079.3.2 Tribology of DLC Films 2099.3.3 Synthesis of Carbon Films with Superlow-Friction and -Wear Properties 2159.3.4 Practical Applications 217

9.4 Summary and Future Direction 219Acknowledgments 219References 220

Contents ix

10 Tribology of Polymeric Solids and Their Composites 223B.J. Briscoe and S.K. Sinha

Abstract 22310.1 Introduction 22410.2 The Mechanisms of Polymer Friction 225

10.2.1 The Ploughing Term – Brief Summary 22510.2.2 The Adhesion Term – Brief Summary 227

10.3 Wear 22810.3.1 Semantics and Rationalizations 22810.3.2 Wear Classification Based on Generic Scaling Responses 23010.3.3 Phenomenological Classification of Wear Damages 23210.3.4 Wear Classification Based on Polymeric Responses 240

10.4 Tribology of Polymer Composites 24910.4.1 ‘Soft and Lubricating’ Phases in a Harder Matrix 24910.4.2 ‘Hard and Strong’ Phases in a ‘Soft’ Matrix 25010.4.3 Hybrid Polymer Composites 253

10.5 Environmental and Lubrication Effects 25410.6 A Case Study: Polymers in Hip and Knee Prosthetic Applications –

Ultrahigh-Molecular-Weight Poly(ethylene) (UHMWPE) 25610.7 Concluding Remarks 260Acknowledgements 261References 261

11 Wear of Polymer Composites 269K. Friedrich, Z. Zhang and P. Klein

Abstract 26911.1 Introduction 26911.2 Sliding Wear of Filler Reinforced Polymer Composites 270

11.2.1 Short Fibres and Internal Lubricants 27011.2.2 PTFE Matrix Composites 27211.2.3 Micro- and Nanoparticle Reinforcements 27511.2.4 Integration of Traditional Fillers with Inorganic Nanoparticles 27711.2.5 Functionally Graded Tribo-Materials 279

11.3 Artificial Neural Networks Approach for Wear Prediction 28011.4 Fibre Orientation, Wear Mechanisms and Stress Conditions in Continuous Fibre

Reinforced Composites 28211.5 Conclusions 286Acknowledgements 286References 287

12 Third-Body Reality – Consequences and Use of the Third-Body Concept to SolveFriction and Wear Problems 291Y. Berthier

Abstract 29112.1 Introduction 29212.2 Relationship Between the Third Body and Friction 292

12.2.1 Boundary Conditions 29212.2.2 Friction Analysis 292

12.3 Relationship Between the Third Body and Wear 293

x Contents

12.3.1 Wear Laws 29312.3.2 Material Hardness and Wear 294

12.4 What Methods Exist for Studying Friction and Wear? 29412.4.1 The Scientific Context Surrounding Tribology 29412.4.2 Physical Difficulties Related to Studying Contacts 29512.4.3 So Where to from Here? 297

12.5 The Third-Body Concept 29812.5.1 Artificial and Natural Third Bodies 29812.5.2 Contact Without the Third Body 29912.5.3 Types of “Solid” Third Body from the Mechanical Viewpoint 29912.5.4 “Action Heights” of Third Bodies 300

12.6 Functions and Behaviour of the Third Body 30012.6.1 Functions of the Third Body 30012.6.2 Operation of Solid Third Bodies 30112.6.3 Tribological Circuit of Third-Body Flows 30212.6.4 Rheology of the Third Body 30312.6.5 Scientific and Technological Consequences of the Tribological Circuit 303

12.7 Roles of the Materials in a Tribological Contact 30412.7.1 Indirect Role of the Materials – Scale of the Actual Mechanism or

Mechanical Device 30412.7.2 Direct Role of the Materials – Scale of First Bodies 30412.7.3 Optimal Direct Response of Material to the Tribological Contact 30512.7.4 Consequences on the Approach Used for Solving Technological Problems 306

12.8 Taking into Account the Effects of the Mechanism 30612.8.1 Choosing the Conditions to be Modelled 30612.8.2 Technological Consequences of the Effects of the Mechanism 307

12.9 Taking into Account the Effect of the First Bodies 30712.9.1 Local Contact Dynamics 30712.9.2 Technological Consequences of the Effects of the First Bodies 307

12.10 “Solid” Natural Third-Body Modelling 30812.10.1 Reconstruction of the Tribological Circuit 30812.10.2 Technological Consequences of the Third Body 309

12.11 Correspondence of the Strategy Proposed to Reality 31012.12 Control of Input Conditions 310

12.12.1 Objectives 31012.12.2 Procedure 31112.12.3 Precautions 311

12.13 Performing Experiments 31212.13.1 Initial Conditions 31212.13.2 Exterior of the Contact 31312.13.3 Interior of the Contact 313

12.14 Conclusions 314Acknowledgements 314References 315

13 Basic Principles of Fretting 317P. Kapsa, S. Fouvry and L. Vincent

Abstract 31713.1 Introduction 31713.2 Wear 319

Contents xi

13.3 Industrial Needs 32013.4 Fretting in Assemblies 32113.5 Fretting Processes 32213.6 Fretting Parameters 330

13.6.1 Nature of Loading 33013.6.2 Nature of the First Bodies 33113.6.3 Coatings 33213.6.4 Environment 33413.6.5 Frequency 33513.6.6 Temperature 335

13.7 Conclusions 336References 337

14 Characterization and Classification of Abrasive Particles and Surfaces 339G.W. Stachowiak, G.B. Stachowiak, D. De Pellegrin and P. Podsiadlo

Abstract 33914.1 Introduction 34014.2 General Descriptors of Particle Shape 34014.3 Particle Angularity Parameters 341

14.3.1 Angularity Parameters SP and SPQ and Their Relation to Abrasive andErosive Wear 342

14.3.2 Cone-Fit Analysis (CFA) 34414.3.3 Sharpness Analysis 349

14.4 Particle Size Effect in Abrasive Wear 35314.5 Sharpness of Surfaces 356

14.5.1 Characterization of Surface Sharpness by the Modified SPQ Method 35614.5.2 Characterization of Surface Sharpness by SA 358

14.6 Classification of Abrasive Surfaces 35914.7 Summary 364Acknowledgements 365References 365

15 Wear Mapping of Materials 369S.M. Hsu and M.C. Shen

15.1 Introduction 36915.1.1 Wear – A System Perspective 37015.1.2 Historical Material Selection Guide 370

15.2 Basic Definition of Wear 37215.2.1 Nature of Wear 37215.2.2 Wear Characterization 372

15.3 Wear as a System Function 37515.4 Wear Maps as a Classification Tool to Define the System 37615.5 Wear as an “Intrinsic” Material Property as Defined by Wear Maps 37715.6 Different Kinds of Wear Maps 37815.7 Application of Wear Maps 380

15.7.1 Material Comparison Based on Wear Maps 38115.7.2 Wear Transition Diagrams 38515.7.3 Material Selection Guided by Wear Maps 38915.7.4 Wear Mechanism Identification 391

xii Contents

15.7.5 Wear Modeling Guide Based on Wear Maps 39615.7.6 Wear Prediction Based on Wear Maps 405

15.8 Construction Techniques of Wear Maps 41115.8.1 Conducting Wear Experiments 41115.8.2 Wear Data 41215.8.3 Data Trend Analysis 41315.8.4 Wear Mapping 41415.8.5 Selection of Parameters for Mapping 41615.8.6 Assumptions in the Step-Loading Test Procedure 418

15.9 Application Map Concept and Examples 42015.10 Future Wear Map Research 421References 422

16 Machine Failure and Its Avoidance – Tribology’s Contribution to EffectiveMaintenance of Critical Machinery 425B.J. Roylance

Abstract 42516.1 Introduction 42516.2 Maintenance Practice and Tribological Principles 426

16.2.1 Maintenance Practice – A Brief Historical Overview 42616.2.2 Tribological Principles 42716.2.3 Tribology and Maintenance 431

16.3 Failure Diagnoses 43216.3.1 Failure Morphology and Analysis 43216.3.2 Dealing with Failure – Two Short Case Studies 43416.3.3 Comment 436

16.4 Condition-Based Maintenance 43616.5 Wear and Wear Debris Analysis 440

16.5.1 Wear Modes and Associated Debris Characteristics – Some ExperimentalResults and Their Application to RAF Early Failure Detection Centres 443

16.5.2 Summary of Laboratory Test Results 44516.5.3 Wear Particle Classification and Application 446

16.6 Predicting the Remaining Useful Life and Evaluating the Cost Benefits 44816.6.1 Remaining Useful Life Predictions 44816.6.2 Evaluating the Cost Benefits 449

16.7 Closure 450Acknowledgements 450References 451

Index 453

List of Contributors

Yves BerthierLaMCoS UMR5514, INSA LyonBat Jean D’Alembert, France

Sanjay K. BiswasDepartment of Mechanical EngineeringIndian Institute of ScienceBangalore, Karnataka, India

Brian J. BriscoeImperial College of Science and Technology and MedicineDepartment of Chemical Engineering and TechnologyLondon, UK

Christophe DonnetUniversité Jean Monnet de Saint-EtienneLaboratoire Traitement du Signal et Instrumentation, France

Ali ErdemirEnergy Technology DivisionArgonne National Laboratory, Argonne, USA

Siegfried FouvryLaboratory of Tribology and Systems DynamicsEcole Centrale de Lyon, France

Klaus FriedrichUniversitat KaiserslauternInstitut fur Verbundwerkstoffe GmbH, Germany

Richard S. GatesNational Institute of Standards and TechnologyGaithersburg, MD, USA

xiv List of Contributors

Andrew J. GellmanDepartment of Chemical EngineeringCarnegie Mellon UniversityPittsburg, PA, USA

Kenneth HolmbergVTT Manufacturing Technology, Finland

Stephen HsuNational Institute of Standards and TechnologyGaithersburg, MD, USA

Ian HutchingsUniversity of CambridgeInstitute for ManufacturingDepartment of EngineeringMill Lane, Cambridge, UK

Said JahanmirMiTi Heart CorporationGaithersburg, MD, USA

Philippe KapsaLaboratory of Tribology and Systems DynamicsEcole Centrale de Lyon, France

Koji KatoTribology LaboratorySchool of Mechanical EngineeringTohoku University, Sendai, Japan

Patrick KleinApplication Engineer Stationary HydraulicsBusak+Shamban Deutschland GmbH Handwerkstr. 5–770565 Shettgart, Germany

Allan MatthewsDepartment of Engineering MaterialsSheffield UniversitySir Robert Hadfield BuildingMappin Street, Sheffield, UK

Ardian MorinaSchool of Mechanical EngineeringUniversity of Leeds, UK

Ronald G. MunroNational Institute of Standards and TechnologyGaithersburg, MD, USA

List of Contributors xv

Anne NevilleSchool of Mechanical EngineeringUniversity of Leeds, UK

Dennis V. De PellegrinSchool of Mechanical EngineeringUniversity of Western AustraliaCrawley, Western Australia

Pawel PodsiadloSchool of Mechanical EngineeringUniversity of Western AustraliaCrawley, Western Australia

Brian J. RoylanceTribology and Condition Monitoring GroupDepartment of Mechanical EngineeringUniversity of Wales, Swansea, UK

Ming C. ShenZimmer, Austin, TX, USA

Sujeet K. SinhaDepartment of Mechanical EngineeringNational University of Singapore, Singapore

Nicholas D. SpencerLaboratory for Surface Science and TechnologySwiss Federal Institute of TechnologyZurich, Switzerland

Gwidon W. StachowiakSchool of Mechanical EngineeringUniversity of Western AustraliaCrawley, Western Australia

Grazyna B. StachowiakSchool of Mechanical EngineeringUniversity of Western AustraliaCrawley, Western Australia

Leo VincentLaboratory of Tribology and Systems DynamicsEcole Centrale de Lyon, France

Zhong ZhangNational Center for Nanoscience and Technology, ChinaNo.2 1st North Road, Zhong-Guan-Cum100080 Beijing, China

Series Editors’ Foreword

Tribology is concerned with understanding the behaviour and performance of the componentsof machines and equipment, with surfaces that are subject to relative motion, either fromother components or from loose materials. It therefore has a wide range of applicationsacross many industries, and also in medicine in understanding the mechanism of operationof the joints between bones. The Tribology in Practice Series of books aims to make anunderstanding of tribology readily accessible and relevant to industry, so that it can bebrought to bear on engineering problems.

This latest book in the series, Wear – Mechanisms, Materials and Practice, edited byGwidon Stachowiak provides a comprehensive review of the current understanding of thewear of all kinds of materials, and how it can be controlled and reduced. The authors ofthe individual sections of the book are world experts in the various subject areas. They aretherefore able to summarize the currently available knowledge and the ways in which it canbe used to solve practical problems. The book will therefore provide a valuable referencework for engineers in industry, as well as being useful for research workers in the field byproviding a summary of previous work.

As a Series, the Tribology in Practice Series is particularly concerned with design, failureinvestigation, and the application of tribological understanding to the products of variousindustries and to medicine. The scope of the series is as wide as the subject and applicationsof tribology. Wherever there is wear, rubbing, friction, or the need for lubrication, then thereis scope for the introduction of practical, interpretative material. The Series Editors and thepublishers would welcome suggestions and proposals for future titles in the Series.

M.J. NealeNeale Consulting Engineers, UK

T.A. PolakNeale Consulting Engineers, UK

M. PriestUniversity of Leeds, UK

Preface

Wear is the process occurring at the interfaces between interacting bodies and is usuallyhidden from investigators by the wearing components. However, this obstacle has beengradually overcome by scientists, revealing an intricate world of various wear modes andmechanisms. Since the early wear experiments our knowledge about wear has increasedconsiderably and a significant progress in the description of wear mechanisms has beenmade. Over the past decades our views and understanding of wear have changed, includingthe classification of wear mechanisms. Concepts such as abrasion, adhesion and fatigue,originally used in the classification of wear mechanisms, are, now, insufficient. New materialsand surface coatings wear in a specific manner. Complex reactions and transitions oftentake place on the wearing surfaces and our understanding of wear mechanisms occurring iscritical to the effective utilization of these materials. Furthermore, if we understand how amaterial resists wear and friction, then it should be much easier to improve that material.

It is now clear that all known forms of friction and wear are controlled by thin films ofmaterial present between the interacting surfaces. It has been recognized since ancient timesthat supplying liquid or grease to a contact offers a lower friction and wear. If such a filmis merely generated by wear of the bodies sliding in dry contact, the wear and friction areusually much higher. In general terms, this film formation controls wear to a large extentand is usually beneficial since it lowers friction and wear. However, there are also instanceswhen film formation raises wear and friction.

In May 2000 Chris Taylor, the editor of the Journal of Engineering Tribology, had askedme to guest-edit a special issue of the Journal of Engineering Tribology on the topic of‘Wear/Lubricated Wear’. I found this to be a very good idea and agreed. The special issuewas published almost two years later in 2002. The issue contained nine excellent paperscovering a broad range of topics representing our state of knowledge on recent developmentsin the area of wear/lubricated wear. World-leading researchers in the area of wear and wearcontrol, such as Koji Kato, Sanjay Biswas, Stephen Hsu, Ronald Munro, Ming Shen, RichardGates, Andrew Gellman, Nic Spencer, Said Jahanmir, Ali Erdemir, Christophe Donnet, BrianBriscoe, Sujeet Sinha, Klaus Friedrich, Zhong Zhang, Patrick Klein and Gwidon Stachowiak,contributed papers on various topics to this issue. On completion of this work it becameapparent that many researchers and engineers throughout the world would benefit from anexpanded version in a book form. So I had presented the idea of publishing this special issue

xx Preface

in a more expanded form to the Professional Engineering Publishers Ltd. The publisher wasvery supportive of the idea. In addition, the Editorial Board of the PEP book series mademany valuable suggestions regarding the book content. As a result several additional experts,such as Ian Hutchings, Anne Neville, Ardian Morina, Kenneth Holmberg, Alan Matthews,Yves Berthier, Philippe Kapsa, Siegfried Fouvry, Leo Vincent and Brian Roylance, wereinvited to contribute chapters in specific areas of wear and wear control. Altogether, sixadditional chapters were invited and, as a result, a unique piece of work has emerged.

The resulting book represents the current state of art in the area of wear, wear mechanismsand materials. The chapters discuss the latest concepts in wear mechanism classification,wear of metals, wear of polymer and polymer composites, fretting wear, wear mapping ofmaterials, friction and wear of diamond and diamond-like carbon films, wear of ceramics,concept of a third body in wear and friction problems and the tribology of engineeredsurfaces. Wear in boundary lubrication, effects of lubricant chemistry on wear, effects ofsurface chemistry in tribology, characterization and classification of particles and surfaces,and machine failure and its avoidance are also discussed. The strength of this book is in itscurrent knowledge of topic and its frequent references to engineering practice. However, thisbook is not limited to presenting what is already known about wear. It also attempts to presentthe myriad of new emerging problems and the possible ways of solving them. It shows usthat, although we already know a lot about wear, there are still some aspects of it to be yetuncovered and thoroughly investigated. It shows us that new ways and approaches to wearcontrol are still being discovered and implemented in practice. The book also demonstrateswhat type of new problems we are most likely to be dealing with in the future.

I am very grateful to the authors for sharing with us their knowledge and for their hardwork. In particular, I appreciate the time they dedicated to the meticulous preparation oftheir manuscripts. After all, it is not easy to put an extra task on top of the many other dutiesand commitments one already has. I am sure this book will provide a valuable reference forpeople with interest in wear and wear control.

Gwidon Stachowiak

1The Challenge of Wear

I.M. Hutchings

Abstract

While accurate predictive models for wear rate are still an elusive goal, it is clear thatsignificant recent progress has been made in our understanding of many aspects of wearmechanisms and that advances in materials, surface engineering and lubricants, as well as indesign methods and condition monitoring, have led to major improvements in the efficiency,lifetime cost and performance of many engineering systems. There is still much potential forfuture development, and challenges in tribology, especially in the vital field of wear, remain.

1.1 Introduction

To understand the degradation processes known as wear, to predict the rate of wear and to reduceit still form some of the most problematic challenges facing the engineer. The understandingof wear often involves a detailed knowledge of mechanics, physics, chemistry and materialscience, while its quantitative prediction, even to within an order of magnitude, remains inmany cases a distant goal. Although wear can often be reduced by lubrication, the extent of thatreduction can almost never be predicted accurately. The following chapters focus on particularaspects of wear and review the current state of our knowledge on this vitally important topic.

1.2 Definitions and Development of Wear Studies

The widest definition of wear, which has been recognized for at least 50 years, includes theloss of material from a surface, transfer of material from one surface to another or movement ofmaterial within a single surface [1]. Although a narrower definition of wear has been proposedas ‘progressive loss of substance from the operating surface of a body occurring as a result ofrelative motion at the surface’ [2], the wide range of engineering applications of concern to

Wear – Materials, Mechanisms and Practice I.M. Hutchings© 2005 John Wiley & Sons, Ltd

2 Wear – Materials, Mechanisms and Practice

the tribologist is served better by a broader definition. A simple and useful statement is thatwear is ‘damage to a solid surface, generally involving progressive loss of material, due torelative motion between that surface and a contacting substance or substances’ [3]. This includes(1) degradation by the displacement of material within the surface (leading to changes in surfacetopography without loss of material), as well as the more usual case of material removal; (2) thewear processes common in machines in which one surface slides or rolls against another, eitherwith or without the presence of a deliberately applied lubricant; and (3) the more specializedtypes of wear which occur when the surface is abraded by hard particles moving across it, oris eroded by solid particles or liquid drops striking it or by the collapse of cavitation bubblesin a liquid. This definition, quite deliberately, tells us nothing about the mechanisms by whichthe degradation takes place. These may be purely mechanical, for example involving plasticdeformation or brittle fracture, or they may involve significant chemical aspects, for exampleoxidation of a metal or hydration of a ceramic; in many practical cases, both chemical andmechanical processes play a role [4].

The study of tribology has a long history, extending for several centuries before the word itselfwas coined in 1965. Early studies of friction were performed by Leonardo da Vinci in the latesixteenth century, and the first quantitative understanding of fluid film lubrication originatedwith Beauchamp Tower in the late nineteenth century. Wear has entered the scientific arenarather more recently. The design and construction of early machines involved large clearancesand rather slow speeds of operation, with the result that, provided gross adhesion or excessivefriction could be avoided, changes in dimensions of sliding parts due to wear could often betolerated with little adverse effect on performance. It was the development of the high-speedinternal combustion engine in the early part of the twentieth century that provided the initialdriving force for the study of wear which has grown in importance to the present day. Ourunderstanding of wear mechanisms has developed most rapidly with the widespread use ofelectron microscopy and instrumental methods of microanalysis over the past 30 years. Thereare now many examples of advanced engineering products, some involving high-speed slidingor rolling and others small dimensions or hostile environments, whose development andsuccessful use are possible only through the understanding and successful limitation of wearprocesses. These include gas turbine engines, artificial human joints, automotive engines andtransmissions, tyres and brakes, hard disk drives for data storage and an increasing numberof electromechanical devices for domestic and industrial use. Wear is, however, not alwaysto be avoided: there are many manufacturing processes involving abrasive processing, forexample, in which wear is used productively to form and shape surfaces [5].

1.3 Scope and Challenges

In some applications such as bearings, wear (and friction) is of primary concern, while forothers the tribological performance of the system, although important, is not the main driverfor its design. Thus, in modern engineering, we find increasing use of materials with moreattractive combinations of density and mechanical properties than steel, or with benefitsin cost, performance or formability, such as polymers, ceramics and various compositematerials [6–8]. We also see a rapid increase in the use of surface engineering to providea cost-effective combination of near-surface performance with desirable bulk properties inengineering components [9, 10]. These developments all pose particular challenges for thetribologist.

The Challenge of Wear 3

The origins of these challenges are many. The conditions to which a surface is exposedduring wear are quite different from those involved in the measurement of conventionalmechanical properties such as tensile strength, indentation hardness or fracture toughness.The dimensions of oxide or lubricant films, of surface height variation or of wear debristypically lie in the range from 10 nm to 10 �m [9]. In the absence of a thick lubricantfilm, surfaces make contact with each other at local high spots (asperities) which interactand induce high stresses (up to the yield point in some cases) over distances of the orderof micrometres on a timescale of the order of microseconds. For typical speeds of relativemotion, the strain rates at these microscopic sites of mechanical interaction can therefore beof the order of 104–107s−1. Not only are the timescales very short and strain rates high, butall the energy of frictional work is dissipated through the interactions of these contacts, oftenleading to high but transient local temperatures [11]. Even in a lubricated contact, the powerdensity can be remarkably high: it has been estimated that in a thin elastohydrodynamic(EHL) oil film, the rate of viscous energy dissipation is of the order of 100 TW m−3,equivalent to dissipating the entire electrical power output of USA in a volume of 5 l.

The difficulties involved in fully describing, and then in formulating models for, thebehaviour of a wearing surface are not just associated with the extreme local conditions. Theproblem is much more complex than that, for at least three more reasons. First, the process ofwear itself changes the composition and properties of the surface and near-surface regions;the material which separates two sliding surfaces can be treated as a distinct ‘third-body’with its own evolutionary history and properties and these properties will often changeduring the lifetime of the system [12]. Second, the removal or displacement of materialduring wear leads to changes in surface topography. Third, the mechanisms by which wearoccurs are often complex and can involve a mixture of mechanical and chemical processes:for example, in the unlubricated sliding of two steel surfaces, material may be removed bymechanical means after oxidation, while under conditions of boundary lubrication the sourceof wear is often the mechanical removal of the products of chemical reaction between thesteel surface and the lubricant additives [13, 14]. Neither the mechanical nor the chemicalinteractions involved in sliding wear can yet be modelled accurately. The problem of frettingwear, in which contacting surfaces are exposed to small cyclic relative displacements, hassome similarities to, but also many differences from, that of continuous sliding [15].

The case of abrasive wear is in principle slightly more tractable since chemical effectsusually play a negligible role, but even here it would be necessary to model the deformationof the material to very large plastic strains, to incorporate realistic failure criteria (to accountfor ductile rupture or brittle fracture), to allow for changes in surface topography duringwear, to account for the inhomogeneity of the material (associated with its microstructure,as it is initially and as it becomes modified during wear) at the length scale relevant to theunit interaction with an abrasive particle and to sum the individual effects of the interactionswith perhaps many billions of abrasive particles.1 The properties of the abrasive particlesthemselves would also have to be incorporated into a full model: these will include theirbulk mechanical properties such as stiffness and strength, as well as a full description oftheir shape. The difficulties involved in describing the relevant aspects of particle shapealone have stimulated much research [16].

1 One gram of abrasive particles 10 �m in diameter represents about 109 particles.

4 Wear – Materials, Mechanisms and Practice

In view of the highly complex nature of wear processes and the difficulty of producingrealistic models for them, it is not surprising that many discussions of sliding wear start withthe simplest possible assumption of the relationship between wear rate and normal load:

Q = KW

H(1)

where Q is the volume removed from the surface per unit sliding distance, W is the normalload applied to the surface by its counterbody and H is the indentation hardness of thewearing surface. K is a dimensionless quantity which is usually called the wear coefficientand which provides a valuable means of comparing the severity of different wear processes.If K, W and H remain constant during wear, then it is implicit in equation (1) that the volumeof material lost from the surface is directly proportional to the relative sliding distance, orat constant sliding speed, to time. Equation (1) is usually called the Archard wear equation.Archard was perhaps the first to derive the relationship from a plausible physical model in1953 [17], although Archard himself acknowledged the earlier work of Holm in 1946, andthe empirical statement that wear is directly proportional to sliding distance and inverselyproportional to normal load was made as early as 1927 by Preston in a study of the polishingof plate glass [18]. An equation identical to equation (1) was also stated by Taylor in 1948without any indication that it was not already well known [19].2

For engineering applications, and especially for the wear of materials whose hardnesscannot readily be defined (such as elastomers), the wear rate is commonly stated ask = K/H = Q/W . k is often called the specific wear rate and quoted in units ofmm3 N−1 m−1. For a material with a hardness H of 1 GPa (a soft steel, or a hard aluminiumalloy, for example), the numerical value of k expressed in mm3 N−1 m−1 is exactly the sameas the value of K.

Figure 1.1 shows, very approximately, the range of values of K seen in various types ofwear. Under unlubricated sliding conditions (so-called dry sliding), K can be as high as 10−2,although it can also be as low as 10−6. Often two distinct regimes of wear are distinguished,termed ‘severe’ and ‘mild’. Not only do these correspond to quite different wear rates (withK often above and below 10−4, respectively), but they also involve significantly differentmechanisms of material loss. In metals, ‘severe’ sliding wear is associated with relativelylarge particles of metallic debris, while in ‘mild’ wear the debris is finer and formed of oxideparticles [13]. In the case of ceramics, the ‘severe’ wear regime is associated with brittlefracture, whereas ‘mild’ wear results from the removal of reacted (often hydrated) surfacematerial.

When hard particles are present and the wear process involves abrasion (by sliding orrolling particles) or erosion (by the impact of particles), then the highest values of K occur;3

the relatively high efficiency by which material is removed by abrasive or erosive wearexplains why these processes can also be usefully employed in manufacturing [5].

The values of K which occur for unlubricated sliding, or for wear by hard particles, aregenerally intolerably high for practical engineering applications, and in most tribologicaldesigns lubrication is used to reduce the wear rate; the effect of lubrication in reducing wear

2 Although the relevant paper was published in 1950, it had been presented at a conference in 1948.3 Equation (1) can be applied to abrasive wear as well as to sliding wear; an analogous equation can also be derivedfor erosion by solid particle impact, from which a value of K can be derived [20].

The Challenge of Wear 5

Boundary

EHL

Mild

Severe

Abrasion

Erosion

HL

10–14 10–12 10–10 10–8 10–6 10– 4 10– 2 1

Lubricatedsliding

Unlubricatedsliding

K

Wear by hardparticles

Figure 1.1 Schematic representation of the range of wear coefficient K exhibited under differentconditions of wear. HL = hydrodynamic lubrication; EHL = elastohydrodynamic lubrication

is far more potent than its effect on friction, and the increase in life which results from thereduction in wear is generally much more important than the increase in efficiency from thelower frictional losses. As Figure 1.1 shows, even the least effective lubrication can reducethe wear rate by several orders of magnitude, and as the thickness of the lubricant film isincreased in the progression from boundary to EHL and then to hydrodynamic lubrication,so the value of K falls rapidly. In the hydrodynamically lubricated components of a modernautomotive engine, values of K as low as 10−19 are achieved [21].

There is a great deal of current interest in improving lubricants so as to achieve lowwear rates with thinner films (associated with higher contact pressures). A good protectivelubricant film requires the right combination of adhesion to the substrate, film formation andreplenishment rate and shear strength [14]; allied with this is the need to find replacementsfor highly effective additives such as ZDDP (zinc dialkyl dithiophosphate) which containelements which are detrimental both to the environment and to the long-term operation ofautomotive exhaust catalysts. Boron compounds are receiving much attention in this context,and there are also attractions in lubricants which can be transported to the sliding surfacesin the vapour phase, especially for very small-scale devices (e.g. MEMS) and for systemsoperating at high temperatures [14, 21, 22]. There is also active research into lubricantsand lubricant additives which are effective for non-ferrous metals, ceramics and engineeredsurfaces [21].

6 Wear – Materials, Mechanisms and Practice

As our understanding of wear processes deepens, it becomes increasingly important tobe able to transfer that knowledge to engineers involved in both the design and operationof machines. Maps or wear regime diagrams provide powerful tools for the design process[4, 7, 23], while increasingly sophisticated methods have been developed to assess andmonitor the tribological health of operating machinery and determine the appropriate levelsof maintenance [24].

1.4 Conclusions

While accurate predictive models for wear rate are still an elusive goal, it is clear thatsignificant recent progress has been made in our understanding of many aspects of wearmechanisms and that advances in materials, surface engineering and lubricants, as well as indesign methods and condition monitoring, have led to major improvements in the efficiency,lifetime cost and performance of many engineering systems. There is still much potential forfuture development, and challenges in tribology, especially in the vital field of wear, remain.

References1. Almen, J.O., in Mechanical Wear (ed J.T. Burwell), American Society for Metals, 1950, pp. 229–288.2. Glossary of terms and definitions in the field of friction, wear and lubrication, Research Group on Wear of

Engineering Materials, Organisation for Economic Co-operation and Development, 1969, reprinted in WearControl Handbook (eds M.B. Peterson and W.O. Winer), American Society of Mechanical Engineers, 1980,pp. 1143–1303.

3. Standard terminology relating to wear and erosion, standard G-40-01, American Society for Testing andMaterials, 2001.

4. Kato, K., ‘Classification of Wear Mechanisms’, in Wear – Mechanisms, Materials and Practice(ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 9–20.

5. Hutchings, I.M., ‘Abrasion Processes in Wear and Manufacturing’, Proceedings of the Institution of MechanicalEngineers, Part J, Journal of Engineering Tribology, 216, 2002, 55–62.

6. Briscoe, B.J. and Sinha, S.K., ‘Tribology of Polymeric Solids and Their Composites’, in Wear – Mechanisms,Materials and Practice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 223–268.

7. Jahanmir, S., ‘Wear of Ceramics: Wear Transitions and Tribochemical Reactions’, in Wear – Mechanisms,Materials and Practice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 167–190.

8. Friedrich, K., Zhang, Z. and Klein, P., ‘Wear of Polymer Composites’, in Wear – Mechanisms, Materials andPractice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 269–290.

9. Holmberg, K. and Matthews, A., ‘Tribology of Engineered Surfaces’, in Wear – Mechanisms, Materials andPractice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 123–166.

10. Erdemir, A. and Donnet, C., ‘Tribology of Diamond and Diamond-Like Carbon Films’, in Wear – Mechanisms,Materials and Practice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 191–222.

11. Ashby, M.F., Abulawi, J. and Kong, H.S., ‘Temperature Maps for Frictional Heating in Dry Sliding’, TribologyTransactions, 34, 1991, 577–587.

12. Berthier, Y., ‘Third-Body Reality – Consequences and Use of the Third-Body Concept to Solve Friction andWear Problems’, in Wear – Mechanisms, Materials and Practice (ed G.W. Stachowiak), John Wiley & Sons,Ltd, Chichester, 2006, pp. 291–316.

13. Biswas, S.K., ‘Wear of Metals: A Material Approach’, in Wear – Mechanisms, Materials and Practice (edG.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 21–36.

14. Hsu, S.M., Munro, R.C., Shen, M.C. and Gates, R.S., ‘Boundary Lubricated Wear’, in Wear – Mechanisms,Materials and Practice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 37–70.

15. Kapsa, P., Fouvry, S. and Vincent, L., ‘Basic Principles of Fretting’, in Wear – Mechanisms, Materials andPractice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 317–338.

The Challenge of Wear 7

16. Stachowiak, G.W., Stachowiak, G.B., De Pellegrin, D.V. and Podsiadlo, P., ‘Characterization and Classificationof Abrasive Particles and Surfaces’, in Wear – Mechanisms, Materials and Practice (ed G.W. Stachowiak),John Wiley & Sons, Ltd, Chichester, 2006, pp. 339–368.

17. Archard, J.F., ‘Contact and Rubbing of Flat Surfaces’, Journal of Applied Physics, 24, 1953, 981–988.18. Preston, F.W., ‘The Theory and Design of Plate Glass Polishing Machines’, Journal of the Society of Glass

Technologists, 11, 1927, 214–256.19. Taylor, C.F., in Mechanical Wear (ed J.T. Burwell), American Society for Metals, 1950, pp. 1–7.20. Hutchings, I.M. ‘Tribology: Friction and Wear of Engineering Materials’, Butterworth Heinemann, Oxford,

UK, 1992.21. Neville, A. and Morina, A., ‘Wear and Chemistry of Lubricants’, in Wear – Mechanisms, Materials and

Practice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 71–94.22. Gellman, A.J. and Spencer, N.D., ‘Surface Chemistry in Tribology’, in Wear – Mechanisms, Materials and

Practice (ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 95–122.23. Hsu, S.M. and Shen, M.C., ‘Wear Mapping of Materials’, in Wear – Mechanisms, Materials and Practice

(ed G.W. Stachowiak), John Wiley & Sons, Ltd, Chichester, 2006, pp. 369–424.24. Roylance, B.J., ‘Machine Failure and Its Avoidance – Tribology’s Contribution to Effective Maintenance of

Critical Machinery’, in Wear – Mechanisms, Materials and Practice (ed G.W. Stachowiak), John Wiley &Sons, Ltd, Chichester, 2006, pp. 425–452.