tribologia_vol2

MODERN TRIBOLOGY HANDBOOKVolume OnePrinciples of Tribology

2001 by CRC Press LLC

MODERN TRIBOLOGY HANDBOOKMaterials Coatings, and Industrial Applications

Volume Two


The MECHANICS and MATERIALS SCIENCE SeriesSeries Editor

Bharat Bhushan

PUBLISHED TITLESHandbook of Micro/Nano Tribology, Bharat Bhushan Modern Tribology Handbook, Bharat Bhushan

FORTHCOMING TITLESRolling Mills Rolls and Bearing Maintenance, Richard C. Schrama Thermoelastic Instability in Machinery, Ralph A. Burton


MODERN TRIBOLOGY HANDBOOKVolume OnePrinciples of Tribology

Editor-in-Chief

Bharat Bhushan, Ph.D., D.Sc. (Hon.)Department of Mechanical Engineering The Ohio State University Columbus, Ohio

CRC Press Boca Raton London New York Washington, D.C.

MODERN TRIBOLOGY HANDBOOKVolume TwoMaterials Coatings, and Industrial Applications

Editor-in-Chief

Bharat Bhushan, Ph.D., D.Sc. (Hon.)Department of Mechanical Engineering The Ohio State University Columbus, Ohio

CRC Press Boca Raton London New York Washington, D.C.

Library of Congress Cataloging-in-Publication DataModern tribology handbook / edited by Bharat Bhushan. p. cm. (Mechanics and materials science series) Includes bibliographical references and index. ISBN 0-8493-8403-6 (alk. paper) 1. Tribology Handbooks, manuals, etc. I. Bhushan, Bharat, 1949- II. Series. TJ1075.M567 2000 621.89 dc21

00-046869

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microlming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of specic clients, may be granted by CRC Press LLC, provided that $.50 per page photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-8403-6/01/$0.00+$.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specic permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identication and explanation, without intent to infringe.

2001 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-8403-6 Library of Congress Card Number 00-046869 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

Foreword

The very size of this Modern Tribology Handbook reects the extent to which the subject has developed since the word tribology was introduced in 1966. While much progress has been recorded in recent decades and several research workers, some of whom are authors of chapters in these volumes, have revealed new facets of the subject and generated valuable data, it is as well to remember that the major users of tribological knowledge are the engineers who design, manufacture, and operate machinery. The general engineer who nds much value in handbooks will welcome the addition of this new compendium of tribological knowledge and data. It is important that the reader and user of this handbook be aware of the well-tried approaches to the measurement of friction and wear and the difculties sometimes encountered in the interpretation of the results. Throughout the long history of tribology, engineers have sought simple guidance on the magnitude of dominant quantities affecting the performance and life of machinery. Engineers in many elds frequently require estimates of the magnitudes of the friction and wear likely to be experienced by different combinations of materials sliding or rolling together in various environments. The presentation of practical information in the form of data banks for friction and wear based upon current knowledge and experience will thus be warmly welcomed. The frustration experienced by practicing engineers when seeking guidance from expert tribologists on representative values of such quantities is legendary! The basic concepts of contact, friction, wear, and lubrication have been embellished in impressive style by recent analytical and experimental approaches to these subjects, and the outcome is thoroughly reviewed in the initial and major section of the handbook dealing with macrotribology. Impressive studies have greatly enhanced our understanding of the physical and chemical nature of surfaces during the latter half of the 20th century, and the subject which underpins many aspects of tribology thus attracts special attention. Some of the topics, such as wear maps and elastohydrodynamic lubrication, are almost as new as the term tribology itself. Effective lubrication remains the ideal way of controlling friction and wear in most mechanical systems. The science and technology of generating uid-lm lubrication to protect tribological components is now rmly established. However, studies of macrotribology have been supplemented by remarkable investigations of micro-, nano-, and even molecular tribology in recent times. This is illustrated by studies of the physical and chemical properties of surfaces; the contact and adhesion between solids; the effects of surface modications and coatings upon friction and wear; lubricant rheology; very thin elastohydrodynamic lubricating lms; and the nature of boundary and mixed lubrication. This alone justies the substantial and welcome section of the handbook devoted to micro- and nanotribology. While most of the work is devoted to experimental studies, one chapter is devoted to the fascinating subject of molecular dynamics simulations in this eld.


Both the conventional and the newer tribological materials are considered in the third section of the handbook. This provides a timely opportunity for the reader to extend his or her knowledge of the advantages and limitations of ceramics, diamond, diamond-like carbon and related lms, and a wide range of coating composites. The last major section of the handbook is devoted to industrial components and systems. Familiar components which have typically enjoyed a century or more of development, such as slider bearings, rolling element bearings, gears, and seals are all considered, alongside components and systems encountered in road, rail, marine, and space vehicles. The special tribological problems faced in earth-moving and manufacturing equipment attract individual attention. It is refreshing to see newer applications of tribology included in the handbook. The term biotribology was introduced in 1973 to embrace the application of tribology to biological and particularly medical situations. While the success of joint replacement tends to dominate this eld, since it represents a remarkable and dominant feature of orthopedic surgery, there are also an increasing number of examples of the successful transfer of tribological knowledge to the biological eld. It is, however, the impact of information technology on society that has promoted major progress in tribology in recent times. The role of tribology has undoubtedly been central to the successful development of magnetic storage and retrieval systems. Spectacular achievements have been recorded in relation to computers, printers, cameras, and scanners, and the reader will welcome the chapters devoted to these developments. The Jost Report1 of 1966 emphasized that losses associated with the shutdown of machinery disabled by the failure of tribological components represented a troublesome economic millstone around the necks of machinery and manufacturing systems. Since that time, maintenance of machinery has changed considerably, with emphasis moving away, in many cases, from routine inspection and component replacement to more effective procedures. It is therefore tting that the closing chapter of the handbook should be devoted to machinery diagnosis and prognosis. It is now well recognized that the tribologist and maintenance engineer must work closely together in monitoring the health of machinery and the performance of tribological components that might so easily compromise the well-being of our industrial society. The Editor-in-Chief and his team are to be warmly congratulated in bringing together this extensive, timely, and useful Modern Tribology Handbook. Duncan Dowson, CBE, FRS, FREng, CEng, FIMechE FCGI Emeritus/Research Professor School of Mechanical Engineering The University of Leeds U.K.

Reference1. Department of Education and Science, 1966, Lubrication (Tribology) Education and Research, A Report on the Present Position and Industrys Needs, HMSO, London.


Preface

Tribology is the science and technology of interacting surfaces in relative motion and of related subjects and practices. The nature and consequences of the interactions that take place at the moving interface control its friction, wear, and lubrication behavior. Understanding the nature of these interactions and solving the technological problems associated with the interfacial phenomena constitute the essence of tribology. The eld of tribology incorporates a number of disciplines, including mechanical engineering, materials science, mechanics, surface chemistry, surface physics and a multitude of subjects, such as surface characterization, friction, wear, lubrication, bearing materials, lubricants, and the selection and design of lubrication systems, and it forms a vital element of engineering. The importance of friction and wear control cannot be overemphasized for economic reasons and long-term reliability. It is important that all designers of mechanical systems use appropriate means to reduce friction and wear, through the proper selection of bearings and the selection of appropriate lubricants and materials for all interacting surfaces. It is equally important that those involved with manufacturing understand the tribological origins of unwanted friction, excessive wear, and lubrication failure in their equipment. The lack of consideration of tribological fundamentals in design and manufacturing is responsible for vast economic losses, including shortened life, excessive equipment downtime, and large expenditures of energy. The recent emergence and proliferation of proximal probes (in particular tip-based microscopies and the surface force apparatus) and of computational techniques for simulating tip-surface interactions and interfacial properties has allowed systematic investigations of interfacial problems with high resolution as well as ways and means for modifying and manipulating nanostructures. These advances provide the impetus for research aimed at developing a fundamental understanding of the nature and consequences of the interactions between materials on the atomic scale, and they guide the rational design of material for technological applications. In short, they have led to the appearance of the new eld of micro/nanotribology. There are also new applications which require detailed understanding of the tribological processes on macro- and microscales. Since the early 1980s, tribology of magnetic storage systems has become one of the important parts of tribology. Microelectromechanical Systems (MEMS) have begun to appear in the marketplace which present new tribological challenges. Tribology of processing systems such as copiers, printers, scanners, and cameras is important, although it has not received much attention. Along with the new industrial applications, there has been development of new materials, coatings, and treatments, such as synthetic diamond, true diamond, diamond-like carbon lms, and chemically grafted lms, to name a few. It is clear that the general eld of tribology has grown rapidly during the past 50 years or so. Conventional tribology is well established, but micro/nanotribology is evolving and is expected to take center stage for the next decade. New materials are needed, and their development requires fundamental understanding of tribological processes. Furthermore, new industrial applications continue to evolve with their unique challenges. Much of the new tribological information has not made it into the hands


that need to use it. Very few tribology handbooks exist, and these are dated. They have focused on conventional tribology, traditional materials, and already-matured industrial applications. The objective of this handbook is to cover modern tribology with an emphasis on all industrial applications. A large number of leading tribologists from around the world have contributed chapters dealing with all aspects of the subject. The appeal of the subject is expected to be very broad, including researchers and practicing engineers and scientists. The handbook is divided into four sections. The rst section, on Macrotribology, covers the fundamentals of conventional tribology. It consists of 15 chapters on topics including surface physics, surface roughness, solid contact mechanics, adhesion, friction, contact temperatures, wear, lubrication and liquid lubricants, friction and wear measurement techniques, design of friction and wear tests, and friction and wear data bank. The second section on Micro/Nanotribology covers the fundamentals of the emerging eld of micro/nanotribology. It consists of studies using surface force apparatus, scanning probe microscopy, and molecular dynamic simulations. These studies complement our tribological understanding on the macroscale. The third section on Solid Tribological Materials and Coatings covers the materials; hard, wear-resistant, and solid lubricant coatings; and surface treatments used in tribological applications as well as coating evaluation techniques. The fourth and last section on Tribology of Industrial Components and Systems covers a large range of industrial applications. This section starts out with the most common tribological components followed by tribology of various industrial applications from the old and new economy. A Glossary of Terms in Tribology is added, which should be of general interest. We embarked on this project in October 1998, and we worked very hard to get all the chapters to the publisher in a record time of a little over 1 year. I wish to sincerely thank the authors for offering to write comprehensive chapters on a tight schedule. This is generally an added responsibility in the hectic work schedules of most researchers today. I also wish to thank the section editors who worked hard to solicit the most competent authors. They are listed in the handbook. I depended on a large number of reviewers who provided critical reviews, in many cases, of more than one chapter in a short time. They are listed in the handbook as well. I also would like to thank Mr. Sriram Sundararajan, a Ph.D. student in my lab, who patiently assisted in the handling of the chapters. I hope the readers of this handbook nd it useful. Bharat Bhushan Editor September 2000


The Editor

Dr. Bharat Bhushan received an M.S. in mechanical engineering from the Massachusetts Institute of Technology in 1971, an M.S. in mechanics and a Ph.D. in mechanical engineering from the University of Colorado at Boulder in 1973 and 1976, respectively, an M.B.A. from Rensselaer Polytechnic Institute at Troy, NY, in 1980, Doctor Technicae from the University of Trondheim at Trondheim, Norway, in 1990, a Doctor of Technical Sciences from the Warsaw University of Technology at Warsaw, Poland, in 1996, and Doctor Honouris Causa from the MetalPolymer Research Institute of the National Academy of Sciences at Gomel, Belarus. He is a registered professional engineer (mechanical). He is presently an Ohio Eminent Scholar and The Howard D. Winbigler Professor in the Department of Mechanical Engineering as well as the Director of the Computer Microtribology and Contamination Laboratory at the Ohio State University, Columbus. He is an internationally recognized expert in tribology on the macro- to nanoscales, and is one of the elds most prolic authors. He is considered by some a pioneer in the tribology and mechanics of magnetic storage devices and a leading researcher in the eld of micro/nanotribology using single probe microscopy. He has authored 5 technical books, 23 handbook chapters, more than 400 technical papers in reviewed journals, and more than 60 technical reports. He has edited more than 25 books, and holds 10 U.S. patents. He is founding editor-in-chief of the World Scientic Advances in Information Storage Systems Series, the CRC Press Mechanics and Materials Science Series, and the Journal of Information Storage and Processing Systems. He has given more than 200 invited presentations on ve continents and more than 50 keynote/plenary addresses at major international conferences. He organized the rst symposium on Tribology and Mechanics of Magnetic Storage Systems in 1984 and the rst international symposium on Advances in Information Storage Systems in 1990, both of which are now held annually. He is the founder of an ASME Information Storage and Processing Systems Division founded in 1993 and served as the founding chair from 1993 through 1998. His biography has been listed in over two dozen Whos Who books including Whos Who in the World, and he has received more than a dozen awards for his contributions to science and technology from professional societies, industry, and U.S. government agencies. Dr. Bhushan is also the recipient of various international fellowships including the Alexander von Humboldt Research Prize for Senior Scientists and the Fulbright Senior Scholar Award. He is a foreign member of the International Academy of Engineering (Russia), the Byelorussian Academy of Engineering and Technology, and the Academy of Triboengineering of the Ukraine, an honorary member of the Society of Tribologists of Belarus, a fellow of ASME and the New York Academy of Sciences, a senior member of IEEE, and a member of STLE, ASEE, Sigma Xi, and Tau Beta Pi. Dr. Bhushan has previously worked for Automotive Specialists, Denver, CO; the R & D Division of Mechanical Technology Inc., Latham, NY; the Technology Services Division of SKF Industries Inc., King of Prussia, PA; the General Products Division Laboratory of IBM Corporation, Tucson, AZ; and the Almaden Research Center of IBM Corporation, San Jose, CA.


Contributors

Dr. Phillip B. AbelNASA Glenn Research Center Cleveland, OH

Prof. Herbert S. ChengDepartment of Mechanical Engineering Northwestern University Evanston, IL

Dr. David I. FletcherDepartment of Mechanical Engineering The University of Shefeld Shefeld, U.K.

Dr. Koshi AdachiLaboratory of Tribology School of Mechanical Engineering Tohoku University Sendai, Japan

Richard S. CowanMultiUniversity Center for Integrated Diagnostics Georgia Institute of Technology Atlanta, GA

Dr. Richard S. GatesNational Institute of Standards and Technology Gaithersburg, MD

Dr. Xiaolan (Alan) AiThe Timken Company Canton, OH

William A. GlaeserBattelle Columbus, OH

Prof. Christophe Donnetcole Centrale de Lyon Dpartement de Sciences et Techniques des Matriaux et des Surfaces Laboratoire de Tribologie et Dynamique des Systmes cully, France

Dr. Niklas Axnngstrm Laboratory Uppsala University Uppsala, Sweden

Lois J. GschwenderWright Patterson Air Force Base Dayton, OH

Prof. Richard C. BensonDepartment of Mechanical and Nuclear Engineering The Pennsylvania State University University Park, PA

Dr. Jeffrey A. HawkU.S. Department of Energy Albany Research Center Albany, OR

Prof. Rob S. Dwyer-JoyceDepartment of Mechanical Engineering The University of Shefeld Shefeld, U.K.

Dr. Alan D. BermanSeagate Technology Costa Mesa, CA

Prof. Sture Hogmarkngstrm Laboratory Uppsala University Uppsala, Sweden

Dr. Ali ErdemirArgonne National Laboratory Energy Technology Division Argonne, IL

Bharat BhushanThe Ohio State University Columbus, OH

Dr. Kenneth HolmbergVTT Manufacturing Technology Espoo, Finland

Dr. Peter J. BlauTribomaterials Investigative Systems Oak Ridge, TN

Dr. John FerranteDepartment of Physics Cleveland State University Cleveland, OH

Dr. Hendrik HlscherInstitute of Applied Physics University of Hamburg Hamburg, Germany

David E. BreweU.S. Army Vehicle Propulsion Directorate NASA Glenn Research Center Cleveland, OH

Prof. John FisherSchool of Mechanical Engineering The University of Leeds Leeds, U.K.

Dr. Stephen M. HsuNational Institute of Standards and Technology Gaithersburg, MD


Dr. M. IshidaRailway Technical Research Institute Tokyo, Japan

Brent K. LokChevron Global Lubricants San Francisco, CA

Dr. A. William RuffConsultant Gaithersburg, MD

Prof. Jacob N. IsraelachviliDepartment of Chemical Engineering and Materials Department University of California at Santa Barbara Santa Barbara, CA

Prof. Kenneth C LudemaMechanical Engineering Department University of Michigan Ann Arbor, MI

Prof. Richard F. SalantDepartment of Mechanical Engineering Georgia Institute of Technology Atlanta, GA

Prof. Othmar MartiExperimentelle Physik Universitt Ulm Ulm, Germany

Dr. K. J. SawleyTransportation Technology Centre Pueblo, CO

Prof. Staffan Jacobsonngstrm Laboratory Uppsala University Uppsala, Sweden

Prof. Allan MatthewsResearch Centre in Surface Engineering The University of Hull Hull, U.K.

Dr. F. SchmidDepartment of Mechanical Engineering The University of Shefeld Shefeld, U.K.

Mark J. JansenAYT Corporation Brookpark, OH

Dr. William R. Jones, Jr.NASA Glenn Research Center Cleveland, OH

Dr. Daniel MaugisCNRS Laboratoire des Materiaux et Structures du Genie Civil Champ sur Marne, France

Dr. Karl J. SchmidJohn Deere Marine Engines Division Waterloo, IA

Dr. Ajay KapoorDepartment of Mechanical Engineering The University of Shefeld Shefeld, U.K.

Prof. Eric MockensturmDepartment of Mechanical and Nuclear Engineering The Pennsylvania State University University Park, PA

Prof. Steven R. SchmidDepartment of Aerospace and Mechanical Engineering University of Notre Dame Notre Dame, IN

Prof. Koji KatoLaboratory of Tribology School of Mechanical Engineering Tohoku University Sendai, Japan

Charles A. MoyerThe Timken Company (retired) Canton, OH

Dr. Shirley E. SchwartzGeneral Motors Powertrain Warren, MI

Dr. Martin H. MserInstitute fr Physik Johannes Gutenberg-Universitt Mainz, Germany

Prof. Francis E. KennedyThayer School of Engineering Dartmouth College Hanover, NH

Dr. Udo D. SchwarzInstitute of Applied Physics University of Hamburg Hamburg, Germany

Dr. Malcolm G. NaylorCummins Inc. Columbus, IN

Dr. Padma KodaliCummins Inc. Columbus, IN

Dr. Shashi K. SharmaWright Patterson Air Force Base Dayton, OH

Dr. Martin PriestSchool of Mechanical Engineering The University of Leeds Leeds, U.K.

David C. KramerChevron Global Lubricants Richmond, CA

Dr. Ming C. ShenSULZERMEDICA Austin, TX

Prof. Mark O. RobbinsDepartment of Physics and Astronomy The Johns Hopkins University Baltimore, MD

Dr. Mats LarssonBalzers Sandvik Coating AB Stockholm, Sweden

Carl E. Snyder, Jr.Wright Patterson Air Force Base Dayton, OH


Prof. Andras Z. SzeriDepartment of Mechanical Engineering University of Delaware Newark, DE

Dr. Jerry C. WangCummins Inc. Columbus, IN

Dr. Rick D. WilsonU.S. Department of Energy Albany Research Center Albany, OR

Dr. Urban Wiklundngstrm Laboratory Uppsala University Uppsala, Sweden

Prof. William R. D. WilsonDepartment of Mechanical Engineering University of Washington Seattle, WA

Mark L. SztenderowiczChevron Global Lubricants Richmond, CA

Dr. Simon C. TungGeneral Motors Research and Development Center Warren, MI

Dr. John A. WilliamsEngineering Department Cambridge University Cambridge, U.K.

Prof. Ward O. WinerWoodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, GA


Section EditorsSection 1: Macrotribology Bharat Bhushan (The Ohio State University, USA) Francis E. Kennedy (Dartmouth College, USA) Andras Z. Szeri (University of Delaware, USA) Section 2: Micro/Nanotribology Bharat Bhushan (The Ohio State University, USA) Othmar Marti (University of Ulm, Germany) Section 3: Solid Tribological Materials and Coatings Bharat Bhushan (The Ohio State University, USA) Ali Erdemir (Argonne National Laboratory, USA) Kenneth Holmberg (VTT Manufacturing Technology, Finland) Section 4: Tribology of Industrial Components and Systems Bharat Bhushan (The Ohio State University, USA) Stephen M. Hsu (National Institute of Standards and Technology, USA)


ReviewersProf. George Adams (Northeastern University, Boston, MA) Dr. Paul Bessette (Nye Lubricants Inc., New Bedford, MA) Prof. B. Bhushan (The Ohio State University, Columbus, OH) Prof. Thierry A Blanchett (Rensselaer Polytechnic Institute, Troy, NY) Dr. Peter J. Blau (Oak Ridge National Laboratory, Oak Ridge, TN) Dr. Ken Budinski (Eastman Kodak Co., Rochester, NY) Dr. Nancy Burnham (cole Polytechnique Federal de Lausanne, Switzerland) Dr. Jaime Colchero (Universidad Antonoma de Madrid, Spain) Dr. Christopher Dellacorte (NASA Glenn Research Center, Cleveland, OH) Dr. Urs. T. Duerig (IBM Research Division, Zurich, Switzerland) Dr. John Dumbleton (Biomaterials and Technology Assessment, Ridgewood, NJ) Dr. Norman S. Eiss Jr. (Retired) Dr. Ali Erdemir (Argonne National Laboratory, Argonne, IL) Prof. Traugott E. Fischer (Stevens Institute of Technology, Hoboken, NJ) Mr. William A. Glaeser (Battelle Memorial Institute, Columbus, OH) Prof. Steve Granick (University of Illinois, Urbana, IL) Prof. Judith A. Harrison (U.S. Naval Academy, Annapolis, MD) Dr. Jeffrey A. Hawk (U.S. Department of Energy, Albany, OR) Prof. Sture Hogmark (Uppsala University, Sweden) Dr. Kenneth Holmberg (VTT Manufacturing Technology, Finland) Dr. K. L. Johnson (Cambridge University, Cambridge, U.K.) Dr. William R. Jones (NASA Glenn Research Center, Cleveland, OH) Prof. Koji Kato (Tohoku University, Japan) Prof. Francis E. Kennedy (Dartmouth College, Hanover, NH) Dr. Jari Koskinen (VTT Manufacturing Technology, Finland) Dr. Minyoung Lee (G. E. Corp. R&D, Schenectady, NY) Prof. Frederick F. Ling (University of Texas, Austin, TX) Dr. Jean-Luc Loubet (cole Centrale de Lyon, France) Prof. Kenneth C Ludema (University of Michigan, Ann Arbor, MI) Dr. William D. Marscher (Mechanical Solutions Inc., Parsippany, NJ) Prof. Ernst Meyer (Institute fr Physik, University of Basel, Switzerland) Dr. Sinan Muftu (Massachusetts Institute of Technology, Bedford, MA) Dr. B. Nau (Fluid Sealing Consultant) Prof. Gerhard Poll (Universitt Hannover, Germany) Prof. David E. Rigney (The Ohio State University, Columbus, OH) Dr. A. William Ruff (Consultant, Gaithersburg, MD) Prof. Farshid Sadeghi (Purdue University, W. Lafayette, IN)


Prof. Steven R. Schmid (University of Notre Dame, Notre Dame, IN) Dr. Shashi K. Sharma (Wright Patterson Air Force Base, Dayton, OH) Dr. Simon Sheu (Alcoa, Pittsburgh, PA) Dr. William D. Sproul (Reactive Sputtering Inc., Santa Barbara, CA) Prof. Andras Z. Szeri (University of Delaware, Newark, DE) Dr. John Tichy (Rensselaer Polytechnic Institute, Troy, NY) Prof. Matthew Tirrell (University of California, Santa Barbara, CA) Dr. Andrey A. Voevodin (Wright Patterson Air Force Base, Dayton, OH) Prof. Mark E. Welland (Cambridge University, U. K.) Prof. J. A. Wickert (Carnegie Mellon University, Pittsburgh, PA) Dr. Pierre Willermet (Ford Motor Co., Dearborn, MI) Dr. John A. Williams (Cambridge University, U. K.) Mr. E. Zaretsky (NASA Glenn Research Center, Cleveland, OH) Dr. Ing. K.-H Zum Gahr (Forschungszentrum Karlsruhe, Germany)


Contents

Volume OneSECTION I MacrotribologyIntroduction Bharat Bhushan, Francis E. Kennedy, and Andras Z. SzeriPhillip B. Abel and John Ferrante

1

Surface Physics in Tribology1.1 1.2 1.3 1.4 1.5 1.6 1.7

Introduction Geometry of Surfaces Theoretical Considerations Experimental Determinations of Surface Structure Chemical Analysis of Surfaces Surface Effects in Tribology Concluding Remarks

2

Surface Roughness Analysis and Measurement Techniques Bharat Bhushan2.1 2.2 2.3 2.4 The Nature of Surfaces Analysis of Surface Roughness Measurement of Surface Roughness Closure

3

Contact Between Solid Surfaces3.1 3.2 3.3 3.4 3.5 3.6

John A.Williams and Rob S. Dwyer-Joyce

Introduction Hertzian Contacts Non-Hertzian Contacts Numerical Methods for Contact Mechanics Experimental Methods for Contact Mechanics Further Aspects


4

Adhesion of Solids: Mechanical Aspects4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9

Daniel Maugis

Introduction Adhesion Forces, Energy of Adhesion, Threshold Energy of Rupture Fracture Mechanics and Adhesion of Solids Example: Contact and Adherence of Spheres Liquid Bridges Adhesion of Rough Elastic Solids Application to Friction Kinetics of Crack Propagation Adhesion of Metals Conclusion

5

Friction5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11

Kenneth C Ludema

Introduction Qualitative Ranges of Friction Early Concepts on the Causes of Friction Adhesion, Welding, and Bonding of the Three Major Classes of Solids The Formation and Persistence of Friction Controlling Surface Films Experiments that Demonstrate the Influence of Films on Surfaces Mechanisms of Friction Measuring Friction Test Machine Design and Machine Dynamics Tapping and Jiggling to Reduce Friction Effects Equations and Models of Friction

6

Frictional Heating and Contact Temperatures6.1 6.2 6.3 Surface Temperatures and Their Significance Surface Temperature Analysis Surface Temperature Measurement

Francis E. Kennedy

7

Wear Mechanisms7.1 7.2 7.3 7.4 7.5 7.6

Koji Kato and Koshi Adachi

Introduction Change of Wear Volume and Wear Surface Roughness with Sliding Distance Ranges of Wear Rates and Varieties of Wear Surfaces Descriptive Key Terms Survey of Wear Mechanisms Concluding Remarks

8

Wear Debris Classication8.1 8.2 8.3 8.4 8.5

William A. Glaeser

Introduction How Wear Debris Is Generated Collection of Wear Debris Diagnostics with Wear Debris Conclusions


9

Wear Maps9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9

Stephen M. Hsu and Ming C. Shen

Introduction Fundamental Wear Mechanisms of Materials Wear Prediction Wear Mapping Wear Maps as a Classification System Wear Map Construction for Ceramics Comparison of Materials Modeling Wear by Using Wear Maps Advantages and Limitations of Current Wear Map Approach

10

Liquid Lubricants and Lubrication Lois J. Gschwender, David C. Kramer, Brent K. Lok, Shashi K. Sharma, Carl E. Snyder, Jr., and Mark L. Sztenderowicz10.1 10.2 10.3 10.4 Introduction Lubricant Selection Criteria Conventional Lubricants The Evolution of Base Oil Technology Synthetic Lubricants

11

Hydrodynamic and Elastohydrodynamic Lubrication11.1 11.2 11.3 11.4 11.5 Basic Equations Externally Pressurized Bearings Hydrodynamic Lubrication Dynamic Properties of Lubricant Films Elastohydrodynamic Lubrication

Andras Z. Szeri

12

Boundary Lubrication and Boundary Lubricating Films and Richard S. Gates12.1 12.2 12.3 12.4 12.5 12.6 Introduction The Nature of Surfaces Lubricants and Their Reactions Boundary Lubricating Films Boundary Lubrication Modeling Concluding Remarks

Stephen M. Hsu

13

Friction and Wear Measurement Techniques Sture Hogmark, and Staffan Jacobson13.1 13.2 13.3 13.4 13.5 The Importance of Testing in Tribology Wear or Surface Damage Classification of Tribotests Tribotest Planning Evaluation of Wear Processes

Niklas Axn,


13.6 13.7 13.8 13.9 13.10

Tribotests Selected Examples Abrasive Wear Erosive Wear Wear in Sliding and Rolling Contacts Very Mild Wear

14

Simulative Friction and Wear Testing14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8

Peter J. Blau

Introduction Defining the Problem Selecting a Scale of Simulation Defining Field-Compatible Metrics Selecting or Constructing the Test Apparatus Conducting Baseline Testing Using Established Metrics and Refining Metrics as Needed Case Studies Conclusions

15

Friction and Wear Data Bank15.1 15.2 15.3 15.4 Introduction Sources of Data Materials Found in Data Bank Data Bank Format

A. William Ruff

SECTION II Micro/NanotribologyIntroduction Bharat Bhushan and Othmar Marti

16

Microtribology and Microrheology of Molecularly Thin Liquid Films Alan D. Berman and J. N. Israelachvili16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 Introduction Solvation and Structural Forces: Forces Due to Liquid and Surface Structure Adhesion and Capillary Forces Nonequilibrium Interactions: Adhesion Hysteresis Rheology of Molecularly Thin Films: Nanorheology Interfacial and Boundary Friction: Molecular Tribology Theories of Interfacial Friction Friction and Lubrication of Thin Liquid Films Stick-Slip Friction


17

Measurement of Adhesion and Pull-Off Forces with the AFM Othmar Marti17.1 Introduction 17.2 Experimental Procedures to Measure Adhesion in AFM and Applications 17.3 Summary and Outlook

18

Atomic-Scale Friction Studies Using Scanning Force Microscopy Udo D. Schwarz and Hendrik Hlscher18.1 Introduction 18.2 The Scanning Force Microscope as a Tool for Nanotribology 18.3 The Mechanics of a Nanometer-Sized Contact 18.4 Amontons Laws at the Nanometer Scale 18.5 The Influence of the Surface Structure on Friction 18.6 Atomic Mechanism of Friction 18.7 The Velocity Dependence of Friction 18.8 Summary

19

Friction, Scratching/Wear, Indentation, and Lubrication Using Scanning Probe Microscopy Bharat Bhushan19.1 19.2 19.3 19.4 19.5 19.6 19.7 Introduction Description of AFM/FFM and Various Measurement Techniques Friction and Adhesion Scratching, Wear, and Fabrication/Machining Indentation Boundary Lubrication Closure

20

Computer Simulations of Friction, Lubrication, and Wear Mark O. Robbins and Martin H. Mser20.1 20.2 20.3 20.4 20.5 20.6 20.7 Introduction Atomistic Computer Simulations Wearless Friction in Low-Dimensional Systems Dry Sliding of Crystalline Surfaces Lubricated Surfaces Stick-Slip Dynamics Strongly Irreversible Tribological Processes


Volume TwoSECTION IIIIntroduction

Solid Tribological Materials and Coatings

Bharat Bhushan, Ali Erdemir, and Kenneth HolmbergKoji Kato and Koshi Adachi

21

Metals and Ceramics21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 21.9 21.10

Introduction Pure Metals Soft Metals and Soft Bearing Alloys Copper-based Alloys Cast Irons Steels Ceramics Special Alloys Comparisons Between Metals and Ceramics Concluding Remarks

22

Solid Lubricants and Self-Lubricating Films22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8 Introduction Classification of Solid Lubricants Lubrication Mechanisms of Layered Solids High-Temperature Solid Lubricants Self-Lubricating Composites Soft Metals Polymers Summary and Future Directions

Ali Erdemir

23

Tribological Properties of Metallic and Ceramic Coatings Kenneth Holmberg and Allan Matthews23.1 23.2 23.3 23.4 23.5 23.6 23.7 Introduction Tribology of Coated Surfaces Macromechanical Interactions: Hardness and Geometry Micromechanical Interactions: Material Response Material Removal and Change Interactions: Debris and Surface Layers Multicomponent Coatings Concluding Remarks

24

Tribology of Diamond, Diamond-like Carbon and Related Films Ali Erdemir and Christophe Donnet24.1 24.2 Introduction Diamond Films


24.3 24.4 24.5

Diamond-like Carbon (DLC) Films Other Related Films Summary and Future Direction

25

Self-Assembled Monolayers for Controlling Hydrophobicity and/or Friction and Wear Bharat Bhushan25.1 25.2 25.3 25.4 25.5 Introduction A Primer to Organic Chemistry Self-assembled Monolayers: Substrates, Organic Molecules, and End Groups in the Organic Chains Tribological Properties Conclusions

26

Mechanical and Tribological Requirements and Evaluation of Coating Composites Sture Hogmark, Staffan Jacobson, Mats Larsson, and Urban Wiklund26.1 26.2 26.3 26.4 26.5 Introduction Design of Tribological Coatings Design of Coated Components Evaluation of Coating Composites Visions and Conclusions

SECTION IV Tribology of Industrial Components and SystemsIntroduction Bharat Bhushan and Stephen M. HsuDavid E. Brewe

27

Slider Bearings27.1 27.2 27.3 27.4

Introduction Self-acting Finite Bearings Failure Modes Slider Bearing Materials

28

Rolling Element Bearings28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8 28.9 28.10

Xiaolan Ai and Charles A. Moyer

Introduction Rolling Element Bearing Types Bearing Materials Contact Mechanics Bearing Internal Load Distribution Bearing Lubrication Bearing Kinematics Bearing Load Ratings and Life Prediction Bearing Torque Calculation Bearing Temperature Analysis


28.11 Bearing Endurance Testing 28.12 Bearing Failure Analysis

29

Gears29.1 29.2 29.3 29.4 29.5 29.6 29.7 29.8

Herbert S. ChengIntroduction Gear Types Tribological Failure Modes Full-Film Lubrication Performance Mixed Lubrication Characteristics Modeling of Tribological Failures in Gears Failure Tests Conclusions

30

Rotary Dynamic Seals30.1 30.2 30.3 30.4 30.5 Introduction Mechanical Seals Rotary Lip Seal Nomenclature Defining Terms

Richard F. Salant

31

Space Tribology31.1 31.2 31.3 31.4 31.5 31.6 31.7

William R. Jones, Jr. and Mark J. Jansen

Introduction Lubrication Regimes Mechanism Components Liquid Lubricants and Solid Lubricants Liquid Lubricant Properties Accelerated Testing and Life Testing Summary

32

Automotive Tribology Ajay Kapoor, Simon C. Tung, Shirley E. Schwartz, Martin Priest, and Rob S. Dwyer-Joyce32.1 32.2 32.3 32.4 32.5 32.6 32.7 Introduction The Engine Transmission and Drive Line The Tire The Brakes Windshield Wipers Automotive Lubricants

33

Diesel Engine Tribology Jerry C. Wang33.1 33.2

Malcolm G. Naylor, Padma Kodali, and

Introduction Power Cylinder Components


33.3 33.4 33.5 33.6 33.7 33.8 33.9

Overhead Components Engine Valves Bearings and Bushings Turbomachinery Fuel System Fuels, Lubricants, and Filtration Future Trends

34

Tribology of Rail Transport Sawley, and M. Ishida

Ajay Kapoor, David I. Fletcher, F. Schmid, K. J.

34.1 Introduction 34.2 Wheel/Rail Contact 34.3 Diesel Power for Traction Purposes 34.4 Current Collection Interfaces of Trains 34.5 Axle Bearings, Dampers, and Traction Motor Bearings 34.6 New Developments and Recent Advances in the Study of Rolling Contact Fatigue 34.7 Conclusion

35

Tribology of Earthmoving, Mining, and Minerals Processing and R. D. Wilson35.1 35.2 35.3 35.4 35.5 35.6 35.7 35.8

Jeffrey A. Hawk

Introduction Wear Processes in Mining and Minerals Processing Equipment Used in Earthmoving Operations Equipment Used in Mining and Minerals Processing General Classification of Abrasive Wear Tribological Losses in the Mining of Metallic Ores, Coal, and Non-metallic Minerals Financial Cost of Wear in Earthmoving, Mining, and Minerals Processing Concluding Remarks

36

Marine Equipment Tribology36.1 36.2 36.3 36.4 36.5

Steven R. Schmid and Karl J. Schmid

Introduction Marine Oil Properties and Chemistry Diesel Engine Lubrication Steam and Gas Turbines Ancillary Equipment

37

Tribology in Manufacturing37.1 37.2 37.3 37.4 37.5

Steven R. Schmid and William R. D. Wilson

Introduction Unique Aspects of Manufacturing Tribology Metal Cutting Finishing Operations Bulk Forming Operations


38

Macro- and Microtribology of Magnetic Storage Devices Bharat Bhushan38.1 38.2 38.3 38.4 38.5 38.6 38.7 38.8 Introduction Magnetic Storage Devices and Components Friction and Adhesion Interface Temperatures Wear Lubrication Micro/Nanotribology and Micro/Nanomechanics Closure

39

Macro- and Microtribology of MEMS Materials39.1 39.2 39.3 39.4 Introduction Experimental Techniques Results and Discussion Closure

Bharat Bhushan

40

Mechanics and Tribology of Flexible Media in Information Processing Systems Richard C. Benson and Eric M. Mockensturm40.1 40.2 40.3 40.4 40.5 40.6 40.7 40.8 40.9 40.10 40.11 40.12 Introduction Introduction to Foil Bearings A Simple Foil Bearing Model Other Foil Bearing Models Air Reversers Introduction to Wound Rolls Air Entrainment in Wound Rolls Nip-Induced Tension and J-line Slip in Web Winding Web Tenting Caused by High Asperities Mechanisms that Cause a Sheet to Jam, Stall, or Roll Over in a Channel Micro-slip of Elastic Belts Transport of Sheets Through Roller/Roller and Roller/Platen Nips

41

Biomedical Applications41.1 41.2 41.3 41.4 41.5 41.6 41.7

John Fisher

Introduction Tribology in the Human Body Tribology of Artificial Organs and Medical Devices Natural Synovial Joint and Articular Cartilage Total Replacement Joints Wear and Wear Debris Induced Osteolysis Joint Replacement and Repair in the Next Millennium


42

Technologies for Machinery Diagnosis and Prognosis and Ward O. Winer42.1 42.2 42.3 42.4 42.5 Introduction Failure Prevention Strategies Condition Monitoring Approaches Tribo-Element Applications Equipment Asset Management

Richard S. Cowan

Glossary


IIISolid Tribological MaterialsBharat BhushanThe Ohio State University

Ali ErdemirArgonne National Laboratory

Kenneth HolmbergVTT Manufacturing Technology21 Metals and Ceramics Koji Kato and Koshi AdachiIntroduction Pure Metals Soft Metals and Soft Bearing Alloys Copper-based Alloys Cast Irons Steels Ceramics Special Alloys Comparisons Between Metals and Ceramics Concluding Remarks

22 Solid Lubricants and Self-Lubricating Films

Ali Erdemir

Introduction Classication of Solid Lubricants Lubrication Mechanisms of Layered Solids High-Temperature Solid Lubricants Self-Lubricating Composites Soft Metals Polymers Summary and Future Directions

23 Tribological Properties of Metallic and Ceramic Coatings and Allan Matthews

Kenneth Holmberg

Introduction Tribology of Coated Surfaces Macromechanical Interactions: Hardness and Geometry Micromechanical Interactions: Material Response Material Removal and Change Interactions: Debris and Surface Layers Multicomponent Coatings Concluding Remarks

24 Tribology of Diamond, Diamond-Like Carbon, and Related Films and Christophe Donnet

Ali Erdemir

Introduction Diamond Films Diamond-like Carbon (DLC) Films Other Related Films Summary and Future Direction

25 Self-assembled Monolayers for Controlling Hydrophobicity and/or Friction and Wear Bharat Bhushan .Introduction A Primer to Organic Chemistry Self-assembled Monolayers: Substrates, Organic Molecules, and End Groups in the Organic Chains Tribological Properties Conclusions


26 Mechanical and Tribological Requirements and Evaluation of Coating Composites Sture Hogmark, Staffan Jacobson, Mats Larsson, and Urban WiklundIntroduction Design of Tribological Coatings Design of Coated Components Evaluation of Coating Composites Visions and Conclusions

uring the industrial revolution, more importantly in the past 50 years, solid tribological materials and coatings have continued to play important roles in many engineering areas, mainly because mechanical systems rely on them for high performance, durability, and efciency. In particular, the development of advanced coatings with low friction and high wear resistance has become a leading research activity in tribology and is now called surface engineering. The increasingly multifunctional needs and more stringent operating conditions envisioned for future mechanical systems will certainly make solid tribological materials and advanced coatings far more important in the near future. To meet the increasing tribological needs of these advanced systems, researchers are constantly exploring new materials and developing novel coatings. As a result, great strides have been made in recent years in the fabrication and diverse utilization of new tribomaterials and coatings that are capable of satisfying the multifunctional needs of more advanced mechanical systems. Major developments in solid tribological materials include coatings with superlow friction and extreme hardness, providing very long wear life to sliding or rolling contact surfaces. Some of these novel coatings are now available for key industrial applications with high thermal/mechanical loadings and harsh tribological environments. Overall, the state-of-the-art in advanced material and coating technologies has now reached the point at which a tribocomponent can be fabricated from bulk ceramics or coated with a hard ceramic lm to provide improved tribological performance and durability. Progress in solid lubricants and self-lubricating lms (such as transition-metal dichalcogenides, diamond, diamond-like carbon, and composites) has led to signicant improvements in the wear lives of bearings, gears, seals, and cutting tools that typically operate under severe tribological conditions. With recent advances in fabrication methods, the cost of these new tribomaterials and coatings has become very affordable. This section focuses on the latest developments in solid tribological materials and coatings. Readers will nd a wealth of information, ranging from mechanistic modeling and understanding of the friction and wear behavior of various materials and coatings to how, where, and when these materials and coatings can be used to solve a challenging tribological problem. Chapters in this section cover all aspects of the metals, alloys, ceramics, composites, solid lubricants, and novel materials and coatings developed, tested, and used for tribological purposes. Each chapter has been written by leading experts in the eld. Tribological studies on traditional metals and alloys have continued at a steady pace during the last decade. The major research emphasis has been on further understanding the friction and wear mechanisms of these materials. During the same period, interest in ceramics and composites has increased tremendously, and these materials have become the major focus of tribological research, mainly because ceramics and composites offer perhaps the best prospect for realization of some new and advanced tribosystems (e.g., heat engines, high-speed bearings, high-temperature seals, and cutting tools). Obviously, ceramics and composites combine a wide range of attractive mechanical, thermal, and chemical properties that are not available in most metals and alloys. In-depth studies on ceramics have led to a better understanding of their friction and wear mechanisms, and this understanding has been used to design and develop a new generation of tribocomponents whose performance and durability far exceed that of traditional metal- and alloy-based tribocomponents. Koji Kato and Koshi Adachi provide an overview of the recent developments in the tribology of traditional metals and alloys, emphasizing the importance of advanced ceramic materials for demanding tribological applications (Chapter 21). Solid lubricants and self-lubricating lms have been around for a long time and are used largely to combat friction and wear under severe tribological conditions in which liquid or grease lubricants cannot function. In recent years, great strides have been made in the processing, fabrication, and diverse utilization of solid lubricants. Chapter 22 by Ali Erdemir is devoted to solid lubricants and self-lubricating lms. Recent progress in understanding the lubricating mechanisms of both traditional and new solid

D


lubricants is presented. The state-of-the-art in advanced solid lubrication methods and application practices is discussed, with particular emphasis on synthesis and applications of solid lubricant lms on tribological surfaces through advanced surface-engineering processes. Recent advances in surface engineering have led to the development of novel metallic and ceramic coatings that can meet the increasingly multifunctional needs of advanced mechanical systems. These advances were the result of dedicated research directed toward the modeling and mechanistic understanding of the tribological properties of these coatings. A chapter by Kenneth Holmberg and Allan Matthews is devoted to the recent progress made in tribological coatings and in understanding the friction and wear mechanisms of these coatings (Chapter 23). Special emphasis was placed on multilayered, compound, or gradient coatings used under both the dry and lubricated conditions. Diamond, diamond-like carbon, and other related coatings (such as carbon nitride and cubic boron nitride) are some of the hardest tribomaterials known and offer perhaps some of the lowest friction and wear coefcients under dry sliding conditions. A widespread application of diamond-like carbon coating is magnetic rigid disks and metal evaporated tapes used in magnetic storage devices. Chapter 24 by Ali Erdemir and Christophe Donnet provides an in-depth review of the recent progress made in the synthesis, tribology, and industrial uses of these coatings. Emphasis is on the state-of-the-art in understanding their friction and wear mechanisms, as well as on the uses of these coatings for diverse tribological applications. Referring to the structural and fundamental tribological knowledge gained during past decades, the authors stress the importance of surface physical and chemical effects on the friction and wear properties of these materials. Tribological issues associated with metal cutting and contact sliding are also addressed in detail. Self-assembled monolayers are organized, dense molecular-scale layers of long-chain organic molecules that are being developed for lubrication purposes. These lms are synthesized such that the functional groups of the organic molecules chemisorb onto a solid surface, which results in the spontaneous formation of robust, highly ordered and oriented, dense monolayers chemically attached to the surface. The lms with nonpolar end groups on the free end result in lms with hydrophobic properties. These lms have been successfully tried in the laboratory for microdevice applications. A chapter by Bharat Bhushan provides an in-depth review of the state-of-the-art of the science and technology of selfassembled monolayers (Chapter 25). Recent developments in deposition technologies have provided the exibility needed for design and development of multifunctional coatings that afford low friction and long wear life under demanding tribological conditions. These exotic coatings with nanocomposite structures or multilayer architectures are quite tough and are resistant to cracking during sliding contact. They also work extremely well in aggressive environments. Chapter 26 by Sture Hogmark, Staffan Jacobson, Mats Larsson, and Urban Wiklund focuses on tribological needs and design considerations for such multifunctional lms. Important tribological issues addressed in this chapter include premature coating delamination, coating deformation, brittle fracture and spalling, abrasive scratching, material pickup or transfer, and coating wear due to abrasive, erosive, and tribochemical interactions. New techniques used in the mechanical, structural, and tribological characterization of multifunctional coatings are also discussed. Several examples are provided to highlight the effectiveness of these coatings in metal-cutting and -forming operations and other tribological elds. The use of thin surface coatings (such as diamond, diamond-like carbon, molybdenum disulde, nitrides, carbides, and their composites and dopants) has substantially improved the tribological performance of rolling, rotating, or sliding mechanical parts and components in recent years. Specically, friction and wear of sliding contact interfaces has decreased by orders of magnitude. Compared to any material combinations used in the past, some of these new coatings were able to reduce friction coefcients to as low as 0.001, and wear rates to levels that in some cases are almost impossible to measure. A key reason for these remarkable developments is that researchers are now better equipped and have a deeper understanding of the fundamental mechanisms that control friction and wear. A second factor is that the thin coatings are now applied on solid surfaces that are in perfect compliance with the chemical,

mechanical, and thermal properties of the coating materials themselves, thus ensuring that premature failures due to thermal, mechanical, or chemical incompatibility are nonexistent. In short, while recent advances in new tribomaterials and coatings have been phenomenal, there remain several key challenges for future tribologists and surface engineers. In this eld, there are almost unlimited numbers of material combinations, surface parameters, and application conditions that one can manipulate or use to his/her advantage in a tribological application to achieve better performance and longer durability. Pioneers and dedicated researchers in the tribology eld have already made great strides in this respect, despite the very intricate and multifaceted nature of the eld. Today, as we embrace a new millennium with great hopes and expectations, we should look forward to opening up new possibilities for a highly industrialized and modern society. To move forward in this direction, we must develop new surface-engineered materials and coatings, together with novel design concepts in tribology. Specically, we need to devise new ways to build composite structures or systems that are based on multilayers or nanocomposites. We should also tailor or model the surface tribological properties of these structures and coating materials to achieve even higher performance and durability in future tribosystems. The tools (analytical, computational, and intellectual) for the successful execution of this task are now available. The greatest challenge for the future seems to be the formulation of new ideas and concepts and the integration of the vast knowledge base accumulated over the years in advanced tribological research and development. From the very beginning, mankind has been in search of new tribomaterials to achieve better and faster mobility. There should be no doubt that this trend will continue at a much accelerated pace in the new millennium, intensifying the need for new solid tribological materials and coatings.

21Metals and Ceramics21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 21.9 21.10 Introduction Pure Metals Soft Metals and Soft Bearing Alloys Copper-based Alloys Cast Irons Steels Ceramics Special Alloys Comparisons Between Metals and Ceramics Concluding Remarks

Koji KatoTohoku University

Koshi AdachiTohoku University

21.1 IntroductionFriction and wear can be kept low if the contact interface is well-lubricated. Even when the contact interface is not supplied with lubricants, friction and wear are changed by adsorbed gasses (Bowden et al., 1954) or by frictional repetition. This means that tribological properties are responses of a tribosystem that is lubricated on purpose or is under the effects of surroundings. Therefore, material properties of only one of two contacting bodies cannot be independently related to the tribological properties in a direct way. At the frictional contact surfaces, there exist frictional heating, high ash temperature, severe plastic shear deformation under contact pressure, and the agglomeration of wear particles to form the tribolayer (Rigney et al., 1977). These produce new surface properties that are different from the bulk material properties. Friction and wear take place at the contact interface between such unsteady surfaces. Nevertheless, metal and ceramic materials can be classied into groups for different applicational purposes, and the tribological usefulness of each group in practice can be qualitatively explained, to a certain extent, by the bulk material properties. These explanations are described in the following sections, which can be guides in the rst step of material selection for tribo-elements.

21.2 Pure MetalsPure metals are generally soft and ductile. Therefore, the contact junctions of asperities between them show large amounts of junction growth in sliding if the contact interface is not lubricated. Table 21.1 shows the friction coefcients observed with eight pure metals sliding on themselves in different atmospheres (Bowden et al., 1954). In air, oxygen, or water vapor, the friction coefcients of these eight frictional pairs vary from 0.8 to 3.0. Gold, nickel, platinum, and silver show relatively large values, which means that the adhesion is relatively strong at the contact interfaces of these metals and contact junctions grow sufciently to generate such large values. In hydrogen or nitrogen, copper, gold,


TABLE 21.1 Friction of Metals (Spectroscopically Pure) Outgassed in Vacuum (When clean, there is gross seizure.)Coefcient of Friction after Admitting Metals Aluminum on aluminum Copper on copper Gold on gold Iron on iron Molybdenum on molybdenum Nickel on nickel Platinum on platinum Silver on silver H2 or N2 4 4 5 Air or O2 1.9 1.6 2.8 1.2 0.8 3 3 1.5 Water Vapor 1.1 1.6 2.5 1.2 0.8 1.6 3 1.5

Data from Bowden, F.P. and Tabor, D. (1954), Friction and Lubrication of Solids, I, Clarendon Press, Oxford.

FIGURE 21.1 Effect of hardness on the relative wear resistance of pure metals. (From Khruschov, M.M. (1957), Resistance of metals to wear by abrasion as related to hardness, Proc. Conf. Lubrication and Wear, Inst. Mech. Engr., 655-659. With permission.)

and nickel show large values (between 4 and 5), which means adhesion is stronger in the inert gases than in air. The high friction of pure metals shown in Table 21.1 is applied in friction bonding of noble metals such as gold. Abrasive wear resistance of such pure metals linearly increases with hardness as shown in Figure 21.1 (Khruschov, 1957). On the other hand, adhesive wear does not show a clear relationship with hardness.

21.3 Soft Metals and Soft Bearing AlloysWhen hard metals such as steels slide on themselves without lubricants, high friction, gross seizure, and severe wear take place in air or vacuum. A soft-metal thin lm at the sliding interface between hard materials can reduce friction to the level of = 0.1 to 0.2. Gold, silver, lead, and indium are representative soft metals whose hardness values vary from about 0.3 GPa to about 0.5 GPa. In practical cases of soft metal-lubricated tribosystems, sliding velocities are relatively small and soft metals are not expected to work in the molten state.


FIGURE 21.2 Thin lm lubrication of soft pure metals in sliding of an Si3N4 pin on SUS440C stainless steel disk in high vacuum. (From Kato, K., Kim, H., Adachi, K., and Furuyama, H. (1996), Basic study of lubrication by tribocoating for space machines, Trans. Japan Soc. Mech. Eng., 62(600), 3237-3243. With permission.)

Figure 21.2 shows the lubricating properties of Ag, Au, Bi, In, Pb, Sb, and Sn observed with the friction pair of an Si3N4 pin against an SUS440C stainless steel disk in a vacuum of 106 Pa (Kato et al., 1996). It is recognized that the soft-lm thickness has its optimum value for the minimum friction coefcient (Bowden et al., 1954), but such optimum lm thickness can be held during running only when the soft metal is supplied continuously to repair the worn parts of the lm (Kato et al., 1990). When a soft-metal lm of a certain thickness is precoated on a hard material substrate, the life of the tribocomponent is determined by the wear life of the lm. Soft-metal lm lubrication is, therefore, convenient for relatively small and replaceable tribocomponents such as ball bearings. When a bearing system is expected to run in a state of hydrodynamic lubrication with oil, an unexpected solid contact is generated by the introduction of hard abrasive particles, misalignment, high load, or slow speed at the sliding contact interface. Soft alloys such as lead- or tin-based babbitts and aluminum-based alloys work well as bearing materials in such contact conditions. Lead-based babbitts contain a high percentage (>80 wt%) of lead with 1 to 10 wt% tin and 10 to 15 wt% antimony, and have a hardness value of about 0.2 GPa. An Sb-Sn phase is distributed as ne cubes throughout the structure. This material has the weakness of low fatigue strength because of segregation of the Sb-Sn phase during solidication. Tin-based babbitts contain a high percentage (>85 wt%) of tin with 5 to 8 wt% antimony and 4 to 8 wt% copper, and have hardness values of about 0.2 GPa. They have the phase of Sb-Sn or Cu6Sn5, and the presence of either or both of these intermetallic phases increases fatigue strength below 130C (Glaesure, 1992). These babbitts are soft enough to embed dirt or hard particles, but also provide good conforming under misalignment or high load. Even when the supply of oil is interrupted, babbitts ow or melt to protect the shaft from damage. The dry friction coefcient against steel remains at ~0.55 to 0.80 (Bowden et al., 1954). They are used below the contact pressure of 30 to 40 MPa and their fatigue strength is ~20 to 30 MPa. Aluminum-based alloys are used for bearings that require large fatigue strength and higher operating temperature than babbitt bearings. Aluminum-tin alloys show a fatigue strength three times larger than tin- or lead-based babbitts (Pratt, 1969) and provide better compatibility with steels. Aluminum-20 wt% lead alloy is less expensive and has fatigue strength almost equal to that of aluminum-20 wt% tin alloy and better wear resistance (Bierlein et al., 1969). Although these aluminum-based alloys exhibit better fatigue strength, corrosion resistance, wear resistance, and compatibility with steel than babbitts, their embeddability and seizure resistance are not as good as that of babbitts and a thin overlay of lead-tin becomes necessary. By considering all the tribological properties of babbitts and aluminum-based alloys, as well as the material costs, soft alloys are used for the oil-lubricated bearings. Because these alloys have relatively


TABLE 21.2Alloy

Copper-based Alloys and Hardness ValuesCu >60 >80 >80 >85 >78 99.5 Pb 2535 110 Sn 50 m) composites Thin-lm (0.25 m) lubricant lm. This is the area of hydrodynamic lubrication where friction is determined by the rheology of the lubricant. For nonconformal, concentrated contacts where loads are high enough to cause elastic deformation of the surfaces and pressure-viscosity effects on the lubricant, another regime elastohydrodynamic lubrication (EHL) occurs. Film thickness in this regime ranges from 0.025 to 1.250 m. As this parameter decreases, lm thickness decreases and surface interactions start taking place. This regime, in which both surface interactions and uid lm effects occur, is referred to as the mixed regime. Finally, at low values of ZN/P, the boundary lubrication regime is entered. The boundary lubrication regime is a highly complex arena involving metallurgy, surface topography, physical and chemical adsorption, corrosion, catalysis, and reaction kinetics (Godfrey, 1980; Jones, 1982). The most important aspect of this regime is the formation of protective surface lms to minimize wear and surface damage. For space mechanisms, AISI 440C stainless steel is the most common bearing material. The formation of lubricating lms is governed by the chemistry of both the lm former as well as the bearing surface and other environmental factors. The effectiveness of these lms in minimizing wear is


determined by their physical properties. These include shear strength, thickness, surface adhesion, lm cohesion, melting point or decomposition temperature, and solubility in the bulk lubricant. Typically, the EHL, mixed, and boundary lubrication regimes occur in lubricated space mechanisms, with the boundary lubrication regime being the most stringent. However, a subdivision of EHL starvation theory was described a number of years ago (Wedeven et al., 1971). It describes the situation occurring in ball bearings having a restricted oil supply, in which pressure buildup in the inlet region of the contact is inhibited, resulting in a lm thickness thinner than calculated by classical EHL theory (Dowson and Higginson, 1959; Hamrock and Dowson, 1981). However, starvation theory fails to adequately describe instrument bearing behavior because there is no oil meniscus. Another subdivision of EHL parched elastohydrodynamics describes a behavior where there is no free bulk oil in the system (Kingsbury, 1985; Schritz et al., 1994). The lubricant lms in this regime are so thin that they are immobile outside the Hertzian contact zone. This regime is of particular importance to space mechanisms because parched bearings require the least driving torque and have the most precisely dened spin axis. Finally, another area of EHL that is of importance to space mechanisms involves transient or nonsteady-state behavior. Unlike steady-state EHL behavior, non-steady-state behavior is not well-understood. However, many practical machine elements (e.g., rolling element bearings, gears, cams, and traction drives) operate under non-steady-state conditions. This is where load, speed, and contact geometry are not constant over time. In particular, stepper motors, which are commonly used in many space mechanisms, operate in this regime. This regime has been studied theoretically for line contacts (Wu and Yan, 1986; Ai and Yu, 1988; Hooke, 1994) and experimentally for point contacts (Sugimura et al., 1998).

31.3 Mechanism ComponentsSpacecraft contain a variety of instruments and mechanisms that require lubrication. Devices include solar array drives; momentum, reaction, and lter wheels; tracking antennas; scanning devices; and sensors. Each of these devices has unique hardware, and therefore lubrication requirements. Gyroscopes, which are used to measure changes in orientation, operate at high speeds, typically between 8000 and 20,000 rpm, with high accuracy. This makes the bearings the most important element of a gyroscope. Fluctuations in the bearing reaction torque, noise, and excess heat generation can cause a loss of null position in the gyroscope. The ideal lubricant for a gyroscope provides a high level of wear protection, produces minimal friction, and has a low evaporation rate (Kalogeras et al., 1993). Also, a xed, small (3 mg) amount of lubricant is used and must provide lubrication throughout the life of the gyroscope. Gyroscope gimbal supports are low-speed applications and the bearings operate in the boundary regime only. Momentum wheels, which typically operate between 3000 and 10,000 rpm, pose their own lubricant selection criteria. Currently, the majority of problems experienced by momentum wheels are related to the lubricant. Inadequate lubrication, loss of lubricant, and/or lubricant degradation are the reasons for the majority of wheel failures (Kalogeras et al., 1993). As higher speed wheels are designed, lubricants will be subjected to higher operating temperatures, which can increase creep or degradation rates. Current design practices to ease lubricant problems include use of improved synthetic lubricants, labyrinth seals and barrier coatings, lubricant-impregnated retainers, and a lubricant resupply system. Reaction wheels have similar design concepts as momentum wheels, but operate at lower speeds. The support bearings spend more time in the mixed lubrication regime. Therefore, lubricants chosen for reaction wheel use must also have good boundary lubrication characteristics. Control momentum gyroscopes (CMGs) combine the aspects of the gyroscope and the momentum wheel to provide attitude control of a spacecraft. Therefore, considerations of both groups must be weighed when selecting a lubricant for use in a CMG (Kalogeras et al., 1993). Devices that utilize scanning or rotating sensors represent another space mechanism that requires lubrication. An example would be a scanning horizon sensor. This device detects the Earths horizon,


which allows spacecraft to orient themselves. Moderate operational speeds (400 to 1600 rpm) and low loads in the bearings make lubricant selection easy. On the other hand, sensors that use oscillatory motion place a high demand on the lubricant. Typically, the angle of oscillation is slight and the bearing operates in the boundary lubrication regime only. With the small oscillatory angle, no new lubricant is brought in the contact zones (Postma, 1999). Slip rings are another example of a common mechanism used in space applications that requires lubricants. Low-speed operation and electrical conductivity are two important factors that affect lubricant selection. Excessive electrical noise is the most common failure mechanism in slip rings (Kalogeras et al., 1993). This is usually due to surface contamination, which can be reduced by proper lubricant selection. Many other mechanisms that require lubrication are used in space applications. Some examples include solar array drives (SADs), which rotate a spacecrafts solar arrays; ball, roller, and acme screws; and many types of gears and transmission assemblies (Saran and Larson, 1995).

31.4 Liquid Lubricants and Solid LubricantsBoth liquid and solid lubricants are used for space applications. The choice is often left to the designer. However, each class has merits and deciencies. The relative merits have been tabulated by Roberts and Todd (1990) and appear in Table 31.1.

31.4.1 Liquid LubricantsMany different chemical classes of liquid lubricants have been used for space applications in the last 3 decades. These include mineral oils, silicones, polyphenyl ethers, esters, and peruoropolyethers. Recently, a synthetic hydrocarbon (Pennzane) has replaced many of the older lubricant classes. Each of these types is discussed briey herein. However, because the great majority of current spacecraft use either a formulated Pennzane or one of the PFPE materials, these two classes are discussed in much greater detail. 34.4.1.1 Mineral Oils This class of lubricants consists of a complex mixture of naturally occurring hydrocarbons with a fairly wide range of molecular weights. Examples include V-78, BP 110, Apiezon C, Andok C (Coray 100) (Bertrand, 1991), and the SRG series of super-rened mineral oils, which includes KG-80 (Dromgold and Klaus, 1968). These latter uids have been highly rened, either by hydrogenation or percolation through bauxite to remove polar impurities. This makes them poorer neat lubricants, but greatly improves their response to additives. Apiezon C is still available commercially, but production of all others was discontinued many years ago. Nevertheless, the SRG oils have been stockpiled by some companies and are still used to lubricate bearings for momentum and reaction wheels. Their estimated shelf life is in excess of 20 years (Dromgold and Klaus, 1968).TABLE 31.1Dry Lubricants Negligible vapor pressure Wide operating temperature Negligible surface migration Valid accelerated testing Short life in moist air Debris causes frictional noise Friction speed independent Life determined by lubricant wear Poor thermal characteristics Electrically conductive

Relative Merits of Solid and Liquid Space LubricantsWet Lubricants Finite vapor pressure Viscosity, creep, and vapor pressure all temperature dependent Sealing required Invalid accelerated testing Insensitive to air or vacuum Low frictional noise Friction speed dependent Life determined by lubricant degradation High thermal conductance Electrically insulating

From Roberts, E.W. and Todd, M.J. (1990), Wear, 136, 157-167. With permission.


FIGURE 31.2 Screening test results (scanner and mechanism). (From Kalogeras, C., Hilton, M., Carr, D., Didziulis, S., and Fleischauer, P. (1993), The use of screening tests in spacecraft lubricant evaluation, Aerospace Corp. Report No. TR-93(3935)-6.)

31.4.1.2 Esters British Petroleum developed a triester-based lubricant in the 1970s designated BP 135. This material was laboratory tested but production was stopped and it never ew. Another ester used in the past is designated as NPT-4 (neopentylpolyol ester), but is no longer marketed. Nye Lubricants markets another series of low-volatility neopentylpolyol esters (UC4, UC7, and UC9). Esters are inherently good boundary lubricants and are available in a wide viscosity range. 31.4.1.3 Silicones This uid class was used early in the space program. They are poor boundary lubricants for steel on steel systems. Versilube F-50, a chloroarylalkylsiloxane, was an early example. Comparisons of this uid in boundary lubrication tests with a PFPE and a PAO have been reported (Kalogeras et al., 1993). Relative life is shown in Figure 31.2. The silicone performed poorly by degrading into an abrasive polymerized product. 31.4.1.4 Synthetic Hydrocarbons There are two groups of synthetic hydrocarbons available today: polyalphaolens (PAO) and multiply alkylated cyclopentanes (MACs). The rst class is made by the oligomerization of linear -olens having six or more carbon atoms (Shubkin, 1993). Nye Lubricants markets a number of PAOs for space applications. Properties for three commercial PAOs appear in Table 31.2. The other class of hydrocarbons is known as MACs. These materials are synthesized by reacting cyclopentadiene with various alcohols in the presence of a strong base (Venier and Casserly, 1993). The products are hydrogenated to produce the nal product, which is a mixture of di-, tri-, tetra-, or pentaalkylated cyclopentanes. Varying reaction conditions controls the distribution. For the last several years, only one product has been available for space use primarily the tri-2-octyldodecyl-substituted cyclopentane (Venier and Casserly, 1991). This product is known as Pennzane SHF-X2000, marketed as Nye Synthetic Oil 2001A. Various formulated versions are also available. A primarily disubstituted (lower viscosity, but higher volatility) version is also now available. Properties of the 2001A product appear in Table 31.3. Recent experience with this uid appears in Carr et al. (1995). A 6-year life test of a CERES elevation bearing assembly using a Pennzane/lead naphthenate formulation yielded excellent results (Brown et al., 1999).


TABLE 31.2 Typical Properties for Three Commercial PolyalphaolensProperty Viscosity at: 210F, SUS 210F, cs 100F, SUS 100F, cs 0F, cs Oil 132 39 3.9 92 18.7 350 440F 85F 2.2% 0.828 Oil 182 62.5 10.9 348 75.0 2700 465F 60F 2.0% 0.842 Oil 186 79.5 15.4 552 119 7600 480F 55F 1.9% 0.847

Flash point Pour point Evaporation 61/2 hours at 350F Specic gravity @ 25C

TABLE 31.3 Typical Properties of Nye Synthetic Oil 2001A (SHF X-2000)Viscosity at 100C Viscosity at 40C Viscosity at 40C Viscosity index Flash point Fire point Pour point Specic gravity at 25C Specic gravity at 100C Coefcient of thermal expansion Evaporation, 24 hr at 100C Refractive index at 25C Vapor pressure at 25C 14.6 cSt 108 cSt 80,500 cSt 137 300C 330C 55C 0.841 0.796 0.0008 cc/cc/C None 1.4671 10111010 Torr

TABLE 31.4

Physical Properties of Four Commercial PFPE Lubricants and Pennzane SHF X-2000Average Molecular Weight 9500 3700 6250 8400 1000 Viscosity at 200C (cSt) 255 230 800 500 330 Pour Point (C) 66 40 35 53 55 Vapor Pressure (Pa) At 20C 3.9 2.0 2.7 1.3 2.2 10 104 106 108 101110

Lubricant Fomblin Z-25 Krytox 143AB Krytox 143AC Demnum S-200 Pennzane SHF X-2000

Viscosity Index 355 113 134 210 137

At 100C 1.3 106 4.0 102 1.1 103 1.3 105 1.9 108

31.4.1.5 Peruoropolyethers These uids, designated as either PFPE or PFPAE, have been commercially available since the 1960s and 1970s in the form of a branched uid (Krytox) manufactured by DuPont (Gumprecht, 1966); a linear uid (Fomblin Z) (Sianesi et al., 1973); and a branched uid (Fomblin Y) (Sianesi et al., 1971), the latter two manufactured by Monteuous. Another linear uid (Demnum) was developed in Japan by Daikin (Ohsaka, 1985). The preparation and properties of these uids appear in Synthetic Lubricants and High-Performance Functional Fluids, (Shubkin, 1993). Some typical properties of these uids appear in Table 31.4. 31.4.1.6 Silahydrocarbons A new type of space lubricant has been developed by the Air Force Materials Laboratory (Snyder et al., 1992). These materials contain only silicon, carbon, and hydrogen, and therefore do not exhibit the poor


FIGURE 31.3 Kinematic viscosity as a function of temperature for a series of silahydrocarbons. (From Jones, W., Shogrin, B., and Jansen, M. (1998a), Research on liquid lubricants for space mechanisms, in Proc. 32nd Aerospace Mechanisms Symp., Cocoa Beach, FL, NASA/CP-1998-207191, 299-310.)

boundary lubricating ability observed with silicones. In addition, these unimolecular materials have exceptionally low volatility and are available in a wide range of viscosities. There are three types, based on the number of silicon atoms present in the molecule (i.e., tri, tetra, or penta) (Paciorek et al., 1990, 1991). A series of silahydrocarbons have been synthesized and their kinematic viscosities as a function of temperature have been measured (Figure 31.3) (Jones et al., 1998). For comparison, a Pennzane plot has been included. As can be seen, the viscosity properties of the silahydrocarbons bracket the Pennzane data. EHL properties of two members of this class have been measured (Spikes, 1996). A trisilahydrocarbon had an value of 16 GPa1 (0.3) at 21C, while a pentasilahydrocarbon had an value of 17 GPa1 (0.3). At 40C, the trisilahydrocarbon had an value of 11 GPa1 (1) and the penta, 13.5 GPa1 (1). For comparison, the value for Pennzane at 30C is 9.8 GPa1 (0.3), estimated by the same method. Therefore, these silahydrocarbons will generate thicker EHL lms than Pennzane under the same conditions.

31.5 Liquid Lubricant PropertiesNumerous reviews of liquid lubricants for space applications have been published (Fusaro and Khonsari, 1991; Stone and Bessette, 1998; Zaretsky, 1990). Liquid lubricant data also appear in some handbooks (Fusaro et al., 1999; Roberts, 1999; McMurtrey, 1985). Because most applications today use either PFPEs or Pennzane (MAC) formulations, these two classes will be presented in more detail.

31.5.1 Peruoropolyethers and MACsA liquid lubricant must possess certain physical and chemical properties to function properly in a lubricated contact. To be considered for space applications, these lubricants must have vacuum stability (i.e., low vapor pressure), low tendency to creep, high viscosity index (i.e., wide liquid range), good elastohydrodynamic and boundary lubrication properties, and resistance to radiation and atomic oxygen. Optical or infrared transparency is important in some applications.


FIGURE 31.4 Relative evaporation rates of aerospace lubricants. (From Conley, P. and Bohner, J.J. (1990), Experience with synthetic uorinated uid lubricants, in Proc. 24th Aerospace Mech. Symp., NASA CP-3062, 213-230. With permission.)

31.5.1.1 Volatility Although labyrinth seals are extensively used in space mechanisms, lubricant loss can still be a problem for long-term applications (Hilton and Fleischauer, 1990). For a xed temperature and outlet area, lubricant loss is directly proportional to vapor pressure. For a similar viscosity range, the PFPE uids are particularly good candidates compared to conventional lubricants, as shown in Figure 31.4 (Conley and Bohner, 1990). Vapor pressure data for four commercial PFPE uids and Pennzane appear in Table 31.4. 31.5.1.2 Creep The tendency of a liquid lubricant to creep or migrate over bearing surfaces is inversely related to its surface tension. Therefore, PFPE uids, which have unusually low surface tensions (LV , 17 to 25 dynes/cm at 20C), are more prone to creep than conventional uids such as hydrocarbons, esters, and silicones. However, these uids may be contained in bearing raceways by using low surface energy uorocarbon barrier lms on bearing lands (Kinzig and Ravner, 1978). However, there is a tendency for PFPE uids to dissolve these barrier lms with prolonged contact (Hilton and Fleischauer, 1993). Therefore, they are not effective in preventing the migration of PFPEs. Pennzane-based lubricants have higher surface tensions and are thus less prone to creep. 31.5.1.3 Viscosity-Temperature Properties Although liquid lubricated space applications do not involve wide temperature ranges, low temperatures (i.e., 10 to 20C) are sometimes encountered. Therefore, low pour point uids that retain low vapor pressure and reasonable viscosities at temperatures to 75C are desirable. The viscosity-temperature slope of PFPE unbranched uids is directly related to the carbon-to-oxygen ratio (C:O) in the polymer repeating unit, as shown in Figure 31.5 (Jones, 1995). Here, the ASTM slope is used for the correlation. High values of the ASTM slope indicate large changes of viscosity with temperature. In addition, branching (e.g., the triuoromethyl pendant group in the Krytox uids) causes deterioration in viscometric properties. A comparison of ASTM slopes for three commercial uids appears in Figure 31.6. Here, the low C:O ratio uid Fomblin Z has the best viscometric properties. The Demnum uid, with a C:O ratio of 3, has intermediate properties, while the branched Krytox uid has the highest slope. 31.5.1.4 Elastohydrodynamic Properties The operation of continuously rotating, medium- to high-speed bearings relies on the formation of an elastohydrodynamic (EHL) lm. This regime was briey discussed in the introduction. A more detailed


FIGURE 31.5 Viscosity-temperature slope as a function of carbon-to-oxygen ratio. (From Jones, W.R., Jr. (1995), Properties of peruoropolyethers for space applications, Trib. Trans., 38(3), 557-564. With permission.)

FIGURE 31.6 Viscosity-temperature slope (ASTM D 341-43) as a function of kinematic viscosity at 20C for Krytox (K), Demnum (D), and Fomblin (Z) uids. (From Jones, W.R., Jr. (1995), Properties of peruoropolyethers for space applications, Trib. Trans., 38(3), 557-564. With permission.)


TABLE 31.5 Pressure-Viscosity Coefcients * at Three Temperatures for Several LubricantsLubricant Ester Synthetic parafn Z uid (Z-25) Naphthenic mineral oil Traction uid K uid (143AB) 38C 1.3 1.8 1.8 2.5 3.1 4.2 99C 1.0 1.5 1.5 1.5 1.7 3.2 149C 0.85 1.1 1.3 (extrapolated) 1.3 0.94 3.0

Note: * given in units of Pa1 108. From Jones, W.R., Jr., Johnson, R.L., Winer, W.O., and Sanborn, D.M. (1975), ASLE Trans., 18(4), 249-262. With permission.

discussion appears in Wedeven (1975). The two physical properties of the lubricant that inuence EHL lm formation are absolute viscosity () and the pressure-viscosity coefcient () (Hamrock and Dowson, 1981). Both molecular weight and chemical structure inuence viscosity. Except for low-molecular-weight uids, values are only related to structure (Spikes et al., 1984). Pressure-viscosity coefcients can be measured directly with conventional high-pressure viscometers (Jones et al., 1975; Vergne and Reynaud, 1992) or indirectly from optical EHL experiments (Cantow et al., 1987; Aderin et al., 1992). Conventional viscometry normally uses the Barus equation (Barus, 1893) for correlations.

p = oe pwhere p = Absolute viscosity at pressure, p o = Absolute viscosity at atmospheric pressure = A temperature-dependent but pressure-independent constant

(31.1)

This implies that a plot of log p vs. p should yield a straight line of slope . Unfortunately, this simple relationship is seldom obeyed. The pressure-viscosity properties that are important in determining EHL lm thickness occur in the contact inlet where pressures are much lower than in the Hertzian region. Therefore, the slope of a secant drawn between atmospheric pressure and 0.07 GPa is typically used for lm thickness calculations. Some researchers (Jones et al., 1975) favor the use of another pressure-viscosity parameter, the reciprocal asymptotic isoviscous pressure (*) based on work by Roelands (1966). Pressure-viscosity coefcients (*) for several lubricants at three temperatures (38, 99, and 149C) are given in Table 31.5. Figure 31.7 contains values for the branched PFPE, Krytox 143AB. Data obtained by conventional (low shear) pressure-viscosity measurements are denoted with open symbols. Indirect measurements from EHL experiments (effective values) are shown with solid symbols. There is good agreement between the different sources as well as different measurement techniques. Figure 31.8 contains similar data for the unbranched PFPE Fomblin Z-25 as a function of temperature. Here, there is a denite grouping of the data, with effective values being substantially lower than values from conventional measurements. Two possibilities exist for this discrepancy. First, inlet heating can occur during the EHL measurements, thus leading to lower viscosities, lower lm thicknesses, and resulting in lower calculated values. The second possibility is a non-Newtonian shear thinning effect, which can occur with polymeric uids. Shear rates in EHL inlets can range from 105 to 107 sec1 (Foord et al., 1968). However, the EHL measurements do represent actual lm thicknesses that may be expected in practice. Effective values for several nonPFPE space lubricants, including Pennzane base uid and some Pennzane formulations, appear in Table 31.6 (Spikes, 1997).


FIGURE 31.7 Pressure-viscosity coefcients for PFPE Krytox 143 AB as a function of temperature. (From Jones, W.R., Jr. (1995), Properties of peruoropolyethers for space applications, Trib. Trans., 38(3), 557-564. With permission.)

FIGURE 31.8 Pressure-viscosity coefcients for PFPE Fomblin Z-25 as a function of temperature. (From Jones, W.R., Jr. (1995), Properties of peruoropolyethers for space applications, Trib. Trans., 38(3), 557-564. With permission.)


TABLE 31.6 Measured Viscosities and Calculated Pressure-Viscosity Coefcients ( Values) for Several Space LubricantsTemp (C) Pennzane 2001: Synth. Oil PAO-186: Synth. Oil NPE UC-7: Ester Pennzane 2001+5% Pb Naphthenate Pennzane 2001+3% Pb Naphthenate

Viscosity (cP) 40 80 100 120 88 19 12 8 90 21 13 8 37 10 6 4 (GPa1) 40 80 100 120 11.0 9.5 7.0 7.0 12.5 9.0 7.0 5.0 6.5 5.0 5.0 5.0 12.0 9.0 6.5 6.0 10.0 9.0 7.0 7.0 98 21 12 8 96 21 12 8

From Spikes, H.A. (1997), Film Formation and Friction Properties of Five Space Fluids, Imperial College (Tribology Section), London, U.K., Report TSO37/97.

FIGURE 31.9 Lubricant parameters for PFPE Y and Z uids. (From Spikes, H.A., Cann, P., and Caporiccio, G. (1984), Elastohydrodynamic lm thickness measurements of peruoropolyether uids, J. Syn. Lubr., 1(1), 73-86. With permission.)

From EHL theory, the greatest lm thickness at room temperature should be obtained with a lubricant having the largest value, assuming approximately equal inlet viscosities. However, for many applications, lubricants must perform over a wide temperature range. In this case, the EHL inlet viscosity can be the overriding factor if the temperature coefcient of viscosity is high. This can cause a crossover in lm thickness as a function of temperature for some PFPE uids, as shown by Spikes et al. (1984) in Figure 31.9. 31

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