Fundamentals of electrical contacts

He is a co-author of theTribology Handbook Russian edition 1979, English translation 1982, monographs Physics,Chemistry and Mechanics of Boundary Lubrication1979, Tribology of Electric C

The multidisciplinary study of the electrical contact in modern engineering is significant, but oftenneglected The scientist and engineers who have spent their professional lives studying andapplying electrical contacts know that these components are critical to the successful operation ofall products that use electricity In our civilization, all electricity transmission and distribution, mostcontrol, and most information exchange depends upon the passage of electricity through anelectrical contact at least once The failure of an electrical contact has resulted in severeconsequences, e.g., an energy collapse of a megapolis, a failure of the telephone system, and eventhe crash of an airplane.

Ragnar Holm, the prominent researcher, renowned engineer, and inventor, developed the validity

of “electrical contacts” as its own technical discipline with his book Electric Contacts (1958) The

50 years following its publication have given a firm confirmation of the accuracy of his predictionsand conclusions Since that time, however, there has been a huge increase in the application ofelectrical contacts For example, the era of the information highway and the development of theintegrated circuit have created new challenges in the use of electrical contacts The use of electricalcontacts on the microscopic scale presents numerous problems never considered by previousgenerations of researchers and engineers The future MEMS/NEMS technology is another areawhere the theory and practice of the electrical contact is of critical importance

The purpose of the authors has been to combine the progress in research and development in theareas of mechanical engineering and tribology, which Holm postulated to be key segments inelectrical contacts, with the new data on electrical current transfer, especially at themicro/nanoscale

This book complements the recent volume Electrical Contacts: Principles and Applications(published by Marcel Dekker, 1999) It takes a practical applications approach to the subject andpresents valuable design information for practicing mechanical and electrical engineers In fact, theinformation contained here will serve as an excellent source of information not only for anyonedeveloping equipment that uses electricity, but for postgraduate students who are concerned aboutthe passage of current from one conductor to another

The authors of this book have many years of research and practical experience One unusual andinteresting aspect of the book’s development is that it comes through the cooperation of thedifferent approaches to the subject from the West and the East They have succeeded in making thebulk of research and engineering data equally clear for all the segments of the internationalaudience

Paul G SladeIthaca, New York

Dr Milenko Braunovic´received his Dipl Ing degree in technical physicsfrom the University of Belgrade, Yugoslavia, in 1962 and the M.Met andPh.D degrees in physical metallurgy from the University of Sheffield,England in 1967 and 1969, respectively From 1971 until 1997, he wasworking at Hydro-Que´bec Research Institute (IREQ) as a senior member ofthe scientific staff He retired from IREQ in 1997 and established his ownscientific consulting company, MB Interface From 1997 until 2000 he wasconsulting for the Canadian Electricity Association as a technologyadvisor He is presently R&D manager with A.G.S Taron Technologies inBoucherville, QC, Canada.

During the last 30 years, Dr Braunovic´ has been responsible for the development andmanagement of a broad range of research projects for Hydro-Que´bec and the Canadian ElectricalAssociation in the areas of electrical power contacts, connector design and evaluation, acceleratedtest methodologies, and tribology of power connections He has also initiated and supervised theR&D activities in the field of shape-memory alloy applications in power systems Dr Braunovic´ isthe author of more than 100 papers and technical reports, including contributions to encyclopaediasand books, in his particular areas of scientific interests In addition, he frequently lectures atseminars world wide and has presented a large number of papers at various internationalconferences

For his contributions to the science and practice of electrical contacts, Dr Braunovic´ received theRagnar Holm Scientific Achievement Award in 1994, and for his long-term leadership and service

to the Holm Conference on Electrical Contacts he received, in 1999, the Ralph ArmingtonRecognition Award He is also a recipient of the 1994 IEEE CPMT Best Paper Award Hesuccessfully chaired the Fifteenth International Conference on Electrical Contacts held in Montreal

in 1990, and was a technical program chairman of the Eighteenth International Conference onElectrical Contacts held in Chicago in 1996 He is a senior member of the Institute of Electronicsand Electrical Engineers (IEEE), the American Society for Metals (ASM), the Materials ResearchSociety (MRS), the Planetary Society, the American Society for Testing of Materials (ASTM), andThe Minerals, Metals & Materials Society (TMS)

Dr Valery Konchitswas born on January 3, 1949 in the city of Gomel,Belarus He graduated from Gomel State University in 1972 He receivedhis Ph.D degree in tribology from the Kalinin Polytechnic Institute, Russia

in 1981

In 1972, he joined the Metal-Polymer Research Institute of the NationalAcademy of Sciences of Belarus in Gomel In 1993, he became the head ofthe laboratory in the Tribology Department Since 2001, Dr Konchits hasbeen Deputy Director of the Metal-Polymer Research Institute

The scientific interests of Dr Konchits lie mainly in electricalcontacts’ friction and wear, contact phenomena at their interfaces, andelectrophysical diagnostic methods of friction He is the author of morethan 80 papers and holds 10 patents He is also the co-author of a monograph in Russian, “Tribology

of electrical contacts” (authors: Konchits V.V., Meshkov V.V., Myshkin N.K., 1986, Minsk)

degree in electromechanics Has received his Ph.D from the Institute forProblems in Mechanics of the Russian Academy of Sciences in 1977 Thesame year, he joined the Metal-Polymer Research Institute in Gomel wheresince 1990 he has been Head of the Tribology Department He has alsobeen the director of MPRI since 2002 He earned his Dr.Sc degree intribology in 1985 and became a full professor of materials science in 1991.

He was elected as a correspondent member of the Belarus NationalAcademy of Sciences in 2004

He received the USSR National Award for Young Scientists in 1983, theAward for Best Research given by the Belarus National Academy of Sciences in 1993, and theAward of the Russian Government in Science and Technology in 2004

The scientific interests of Prof Myshkin lie mainly in the characterization at micro and nanoscalesurfaces, the contact mechanics of solids, wear monitoring, electric phenomena in friction,tribotesting equipment, and aerospace engineering

He has authored or co-authored more than 180 papers and 60 patents He is a co-author of theTribology Handbook (Russian edition 1979, English translation 1982), monographs Physics,Chemistry and Mechanics of Boundary Lubrication(1979), Tribology of Electric Contacts (1986),Acoustic and Electric Methods in Tribology (Russian edition 1987, English translation 1990),Structure and Wear Resistance of Surface Layers(1991), Textbook in Materials Science (1989),Magnetic Fluids in Machinery(1993), the English textbook Introduction to Tribology (1997), andTribology: Principles and Applications(2002)

Prof Myshkin is chairman of the Belarus Tribology Society and vice-president of theInternational Tribology Council He is also assistant editor-in-chief of the Journal of Friction andWear, and a member of editorial boards of Tribology International, Tribology Letters, IndustrialLubrication and Tribology, and the International Journal of Applied Mechanics and Engineering

In the preparation of the book, the authors have used a large number of published materials, either inthe form of papers in referenced journals, or from the websites of different companies andorganizations In both cases, proper permissions for using these materials have been obtained Inmany instances, the authors obtained the required information directly from the authors of thepapers or from the company authorities.

The authors are indebted to Dr Paul Slade for writing the preface of the book Special thanks go

to Dr Daniel Gagnon of Hydro-Que´bec Research Institute (IREQ) in Varennes, QC, Canada forproviding essential reference material and fruitful discussions concerning certain topics of powerconnections

The authors are grateful to Dr Mark I Petrokovets for fruitful discussion and his help inpreparation of Chapters 2, 3 and 5 We also thank Dr Denis Tkachuk for his valuable assistance inpreparation of the manuscript

Acknowledgement is made to the many individuals and company authorities for permission touse the original material and, in particular, to modify the original figures to maintain the uniformity

of graphic presentation throughout the book The following is a list of these individuals andcompany authorities

Prof George M Pharr, Department of Materials Science and Engineering, University ofTennessee, Knoxville, USA for providing the papers on nanoindenation testing methods andinstrumentation and allowing modification of some of the figures appearing in these papers.Prof Doris Kuhlmann-Wilsdorf, Department of Materials Science and Engineering, University

of Virginia, Charlottesville, USA for permission to use the information on fiber-brushes anddislocation nature of the processes occurring during friction

Dr Roland Timsit of Timron Scientific Consulting, Inc., Toronto, Canada for permission to usethe relevant material from his papers and publications

Dr Robert Malucci of Molex, Inc., Lisle, IL, USA, for permission to use the relevant materialfrom his papers and publications and modify some of the figures from his original publications cited

in this book

Dr Bill Abbott of Batelle, USA for helpful suggestions and discussions regarding the problems

of corrosion in electrical and electronic connections

Dr Sophie Noel, Laboratoire de Ge´nie Electrique, Supe´lec, Gif sur Yvette, France for helpfuldiscussions concerning the lubrication of electrical contacts and permission to use some of datafrom the publications cited in this book

Dr Magne Runde of the Norwegian University of Science and Technology, Norway, for helpfuldiscussions concerning the problem of electromigration in electrical contacts

Prof Zoran Djuric and Milos Frantlovic of the Center for Microelectronics Technologies andSingle Crystals, MTM, University of Belgrade, Serbia and Montenegro for providing theinformation on the wireless temperature monitoring system

Dr Bella Chudnovsky of Square D, USA, for helpful discussions concerning the whiskerformation in electrical contacts and for permission to use the information on the On-Line WirelessTemperature Monitoring System for LV, MV electrical equipment fond on the company web site(http://www.squared.com)

Prof L.K.J Vandamme of the Department of Electrical Engineering, Eindhoven University ofTechnology, The Netherlands for providing and allowing the use of reference materials concerningthe noise in electrical connections

Mr Larry Smith of USi, Armonk, NY, USA, for permitting the use of the images anddescriptions of the Power-donut unit found on the company web site (http://www.usi-power.com)

posted on the company web site (http://www.psiainc.com.).

Dr G Palumbo of Integran Technologies, Inc., Toronto, Canada for providing the information

on the grain size effects in nanocrystalline materials (http://www.integran.com)

Mr J Renowden of Transpower New Zealand, for providing the information concerning the fieldapplications of the microohmeter Ohmstik on power lines (http://www.transpower.co.nz)

Mr J Lebold of Boldstarinfrared, Canada for permission to use the infrared images from thecompany web site (http://www.boldstarinfrared.com)

R.N Wurzbach of Maintenance Reliability Group (MRG), York, PA, USA for permission to usedescription of the web-based cost benefit analysis method for predictive maintenance(http://www.mrgcorp.com)

ndb Technologie, Inc., Que´bec, Canada for permission to use the information about themicroohmeters found on their web site (http://www.ndb.qc.ca)

In addition, the authors would like to acknowledge the courtesy of the following companies forallowing the use of the information found on their respective websites: Omega Madge Tech., Inc.,(http://www.omega.com), FLIR Systems (http://www.flirthermography.com), Mikron Infrared,Inc (http://www.irimaging.com), Electrophysics Corp (http://www.electrophysics.com), InfraredSolution, Inc (http://www.infraredsolutions.com), Elwood Corp (http://www.elwoodcorp.com),Sensolink Corp., (http://www.sensorlink.com)

Lastly, it is a pleasure to acknowledge and express our gratitude to Mrs K Braunovic´ for hergenerous hospitality shown to the authors during the preparation of the book manuscript

This book provides detailed analytical models, state-of-the-art techniques, methodologies and toolsused to assess and maintain the reliability of a broad class of moving and permanent electricalcontacts in many technological devices, such as automotive and aerospace components, high- andlow-power contact joints, sliding and breaking contacts, electronic and control apparatus, andelectromechanical systems It provides a comprehensive outline of the tribological behavior ofelectrical contacts that is rarely discussed in the existing literature; these are problems ofconsiderable interest for researchers and engineers.

Focusing on the main mechanical and electrical problems in connections with the fieldapplications and the relationship between structure and properties, this volume provides a well-balanced treatment of the mechanics and the materials science of electrical contacts, while notneglecting the importance of their design, development, and manufacturing The book provides acomplete introduction to electric conduction across a contacting interface as a function of surfacetopography, load, and physical-mechanical properties of materials, and the interrelation ofelectrical performance with friction and wear; it takes into account material properties and lubricanteffects Consideration is given to the deleterious effects of different degradation mechanisms, such

as stress relaxation/creep, fretting, differential thermal expansion, and the formation ofintermetallics, as well as their impacts on operating costs, safety, network reliability, powerquality Various palliative measures to improve the reliability and serviceability of electricalcontact at macro-, micro-, and nano-levels are also discussed

This book diminishes a large gap between engineering practice widely utilizing empiricallyfound methods for designing and optimizing the contact characteristics and theory relating totribological and electromechanical characteristics of the contacts The main trends in the practicalsolutions of the tribological problems in electrical contacts are discussed in terms of contact design,research and development of contact materials, coatings and lubricants and the examples ofpractical applications in various fields are given throughout the book

Covering a wide range of references, tables of contact materials, coatings and lubricationproperties, as well as various testing procedures used to evaluate these properties, the book will be

an indispensable practical tool for professional, research, design and development engineers Thebook (or parts of it) can be used not only as a reference, but also as a textbook for advanced graduatestudents and undergraduates, as it develops the subject from its foundations and contains problemsand solutions for each chapter

Part I

Fundamentals of Electrical Contacts 1

Milenko Braunovic´, Valery V Konchits, and Nikolai K Myshkin Chapter 1 Introduction to Electrical Contacts 3

1.1 Introduction 3

1.2 Summary of Basic Features 6

Chapter 2 Contact Mechanics 9

2.1 Surface of Solids 9

2.2 Surface Topography 11

2.3 Modern Techniques of Measuring Surface Parameters 17

2.4 Contact of Smooth Surfaces 21

2.4.1 Plastic and Elastoplastic Contacts 23

2.5 Contact between Rough Surfaces 27

2.5.1 Greenwood–Williamson Model 27

2.5.2 Multilevel Model 29

2.5.3 Transition from Elastic to Plastic Contact 33

Chapter 3 Tribology 35

3.1 Friction 35

3.1.1 Laws of Friction 35

3.1.2 Real Contact Area 38

3.1.3 Interfacial Bonds (Adhesion Component of Friction) 38

3.1.4 Deformation at Friction 41

3.1.5 Friction as a Function of Operating Conditions 42

3.1.6 The Preliminary Displacement 44

3.1.7 Stick-Slip Motion 46

3.2 Wear 47

3.2.1 Stages of Wear 48

3.2.2 Simple Model of Wear 48

3.2.3 Basic Mechanisms of Wear 50

3.2.4 Abrasive Wear 52

3.2.5 Adhesive Wear 56

3.2.6 Prow Formation 57

3.2.7 Fatigue Wear 57

3.2.8 Corrosive Wear 59

3.2.9 Fretting Wear 59

3.2.10 Delamination 62

3.2.11 Erosion 64

3.2.12 Combined Wear Modes 64

3.3 Lubrication 65

3.4 Current Trends in Tribology 67

4.1 Metallic Contact Materials 71

4.1.1 Properties of Contact Materials 71

4.1.1.1 Copper 71

4.1.1.2 Aluminum 75

4.1.1.3 Silver 76

4.1.1.4 Platinum 78

4.1.1.5 Palladium 78

4.1.1.6 Gold 79

4.1.1.7 Rhodium 79

4.1.1.8 Tungsten 79

4.1.1.9 Nickel 80

4.1.2 Metals and Alloys for Heavy- and Medium-Duty Contacts 80

4.1.3 Metals and Alloys for Light-Duty Contacts 83

4.1.4 Materials for Liquid-Metal Contacts 85

4.1.5 Spring Contact Materials 87

4.1.6 Shape-Memory Alloys and Their Applications in Electrical Contacts 88

4.2 Coatings for Electrical Contacts 89

4.2.1 Basic Requirements 89

4.2.2 Surface Engineering Technologies 91

4.2.2.1 Surface Segregation 92

4.2.2.2 Ion Implantation 94

4.2.2.3 Electroplating 94

4.2.2.4 Electroless Plating 97

4.2.2.5 Cladding 97

4.2.2.6 Chemical Deposition 99

4.2.2.7 Plating by Swabbing 99

4.2.2.8 Physical Vapor Deposition Technology 99

4.2.2.9 Electro-Spark Deposition (ESD) 100

4.2.2.10 Intermediate Sublayers 101

4.2.2.11 Multilayered Contacts 101

4.2.3 Coating Materials 102

4.2.3.1 Coatings for Power Connectors (Copper and Aluminum Joints) 102

4.2.3.2 Coatings for Electronic/Electrical Applications 104

4.3 Composite Contact Materials 111

4.3.1 Composite Materials for Contacts of Commutating Apparatuses 111

4.3.2 Self-Lubricating Composites for Sliding Contacts 118

4.4 Nanostructured Materials 125

4.4.1 “Bulk” Properties Nanomaterials 127

4.4.2 Mechanical Properties 127

4.4.3 Electrical Properties 131

4.4.4 Magnetic Properties 136

4.4.4.1 Giant Magnetoresistance (GMR) 136

4.4.4.2 Ballistic Magnetoresistance (BMR) 138

4.4.5 Nanotubes 140

4.4.6 Thermal Stability 142

4.4.7 Characterization Techniques for Nanostructured Materials 143

4.4.7.1 Nanoindentation 143

4.4.7.2 Scanning Probe Microscopes 144

5.1 Contact Resistance 149

5.1.1 Circular and Noncircular a-Spots 149

5.1.2 Effect of Signal Frequency 154

5.1.3 Size Effects, Nanocontacts 157

5.1.4 Effect of Surface Films 160

5.1.5 Effect of Contact Geometry 166

5.1.6 Conductivity of Rough Contact 172

5.2 Interfacial Heating 180

5.2.1 Principles of Heat Conduction Theory 181

5.2.2 Simple Problems of Heat Conduction Theory 183

5.2.3 Contact Spots Heated by Electrical Current 188

5.2.3.1 Film-Free Metal Contact 188

5.2.3.2 Heating of Contact Spots Having Surface Films 190

5.2.3.3 Field Intensity in the Contact Clearance with Tunnel-Conductive Films 194

5.2.4 Formulation of Heat Problem with Friction 195

5.2.5 Flash Temperature of Electrical Contact 198

5.2.6 Thermal Instability of Friction Contact 200

5.2.6.1 Thermoelastic Instability 201

5.2.6.2 Instability Caused by Temperature-Dependent Coefficient of Friction 202

5.2.6.2 Instability Related to Friction Mode Variation 202

Chapter 6 Reliability Issues in Electrical Contacts 205

6.1 Significance of Electrical Contacts Reliability 205

6.2 Electrical Contact Requirements 206

6.3 Factors Affecting Reliability 206

6.4 Connection Degradation Mechanisms 208

6.4.1 Contact Area 209

6.4.2 Oxidation 211

6.4.3 Corrosion 212

6.4.4 Fretting 214

6.4.4.1 Mechanisms of Fretting 217

6.4.4.2 Factors Affecting Fretting 219

6.4.4.3 Fretting in Electrical Contacts 219

6.4.4.4 Contact Load 221

6.4.4.5 Frequency of Motion 223

6.4.4.6 Slip Amplitude 224

6.4.4.7 Relative Humidity 224

6.4.4.8 Temperature 226

6.4.4.9 Effect of Current 226

6.4.4.10 Surface Finish 228

6.4.4.11 Hardness 229

6.4.4.12 Metal Oxide 230

6.4.4.13 Coefficient of Friction 230

6.4.4.14 Electrochemical Factor 230

6.4.5 Intermetallic Compounds 230

6.4.7 Stress Relaxation and Creep 2406.4.7.1 Nature of the Effect of Electric Current 2416.4.7.2 Effect of Electric Current on Stress Relaxation 2426.4.8 Thermal Expansion 2476.5 Impact of Connection Degradation 2486.5.1 Prognostic Model for Contact Remaining Life 2506.5.2 Economical Consequences of Contact Deterioration 2566.5.3 Power Quality 258

8.1 Types of Electronic Connections 3098.2 Materials for Electronic Connections 3098.2.1 Solder Materials 3108.2.2 Lead-Free Solders 3128.2.2.1 Tin 3128.2.2.2 Tin–Silver 3128.2.2.3 Tin–Silver–Bismuth 3138.2.2.4 Tin–Silver–Copper 3138.2.2.5 Tin–Silver–Copper–Antimony 3148.2.2.6 Tin–Silver–Antimony 3148.2.2.7 Tin–Bismuth 3148.2.2.8 Tin–Copper 3158.2.2.9 Tin–Indium 3158.2.2.10 Tin–Indium–Silver 3168.2.2.11 Tin–Zinc 3168.2.2.12 Tin–Zinc–Silver 3168.2.2.13 Tin–Zinc–Silver–Aluminum–Gallium 3178.3 Degradation Mechanisms in Electronic Connections 3178.3.1 Porosity 3198.3.2 Corrosion/Contamination 3228.3.2.1 Pore Corrosion 3228.3.2.2 Creep Corrosion 3238.3.2.3 Tarnishing 3248.3.3 Fretting 3278.3.4 Frictional Polymerization 3348.3.5 Intermetallic Compounds 3368.3.6 Creep and Stress Relaxation 3488.3.7 Electromigration 3538.3.8 Whiskers 3578.4 Mitigating Measures 3618.4.1 Effect of Coating 3618.4.1.1 Gold Coatings 3618.4.1.2 Palladium and Palladium Alloys 3628.4.1.3 Tin Coatings 3648.4.1.4 Nickel and Nickel-Base Alloys 3648.4.2 Effect of Lubrication 364

Chapter 9

Sliding Contacts 369

9.1 Tribology of Electrical Contacts 3699.1.1 Interrelation of Friction and Electrical Processes 3709.1.2 Role of Boundary Films 3719.1.3 Main Means of Improving Reliability of Sliding Contacts 3719.1.4 Tribophysical Aspects in the Development of Sliding Contacts 3739.2 Dry Metal Contacts 3769.2.1 Low-Current Contacts 3769.2.1.1 Effects of Low Current and Electrical Field on Friction 3779.2.1.2 Effect of Interfacial Shear 378

9.2.2 High-Current Contacts 3869.2.2.1 Effects of Electrical Current on Tribological Behavior 3869.2.2.2 Influence of Electric Fields 3909.2.2.3 Effect of Velocity 3929.2.2.4 Effect of Material Combination of Contacting Members 3939.2.2.5 Electroplastic Effect in Sliding Contact 3949.2.2.6 Friction and Current Transfer in Metal Fiber Brush Contacts 3969.2.3 Stability of the Contact Resistance Electrical Noise 4009.2.3.1 Contact Noise in Closed Connections 4009.2.3.2 Electrical Noise in Sliding Contacts 4029.3 Lubricated Metal Contacts 4149.3.1 Introduction Lubrication Factors 4149.3.2 Electrical Properties of Lubricating Boundary Layers 4159.3.3 Conductivity of Lubricated Contacts 4199.3.3.1 Effect of Lubricant on Conductivity near the Contact Spots 4199.3.3.2 Effect of Lubricant on Conductivity of Contact Spots 4209.3.3.3 Experimental Studies of Electric Conductivity

of Lubricated Contacts 4279.3.3.4 Contact Resistance between Very Smooth Lubricated Surfaces 4309.3.3.5 Temperature Dependencies of Contact Conductivity 4319.3.4 Lubrication Factors in Sliding Contacts 4339.3.4.1 Effect of Lubricant Origin 4349.3.4.2 Lubricant Durability 4359.3.4.3 Tribochemical Aspects of Lubrication 4389.3.4.4 Effect of Velocity in Light-Current Contacts 4419.3.4.5 Effects of Lubricant Contact Properties 4429.3.4.6 Current Passage and Friction in High-Current

Lubricated Contacts 4449.3.5 Lubricants for Electrical Contacts 4499.3.5.1 Lubricants for Sliding Electric Switch Contacts 4509.3.5.2 Lubricants for Sliding Contacts of Sensors 4519.3.5.3 Selection of Contact Lubricants 4549.4 Composite Contacts 4549.4.1 Effect of Intermediate Layers on Electrical Characteristics 4559.4.1.1 Structure and Electrical Properties of Intermediate Films 4569.4.1.2 Mechanism of Current Passage through the Contact with

Intermediate Films 4609.4.1.3 Influence of Polarity on Conductivity in

Composite–Metal Contact 4679.4.2 The “Lubricating” Effect of Electrical Current 4719.4.2.1 Effect of Current on Friction Characteristics 4719.4.2.2 Mechanism of the “Lubricating” Action of the Electric Current 4739.4.2.3 Effect of Brush Material on Friction Behavior with

Electric Current 4779.4.3 Electrical Wear 4799.4.3.1 Wear of Currentless Contacts 4799.4.3.2 Effect of Current on Wear 4809.4.3.3 Factors Leading to Electrical Wear in the

Absence of Sparking 483

9.4.3.6 Some Ways to Reduce Electrical Wear 493

Lubrication 51510.3.3 Evaluation of Thermal Stability of Materials and Lubricants

by Electrical Methods 51710.3.4 Control of Surface Coatings and Films 51910.3.5 Novel Systems for Measuring and Analysis of Contact Characteristics 521

10.3.5.1 Method of “Triboscopy” 523Chapter 11

Monitoring Technologies 529

11.1 Thermal Measurements 53011.1.1 Infrared Thermography 53211.1.2 Basic Features of Infrared Thermography 53211.1.3 Types of Infrared Thermal Systems 53411.1.4 SME Temperature Indicators 53811.1.5 Temperature Stickers (Labels) 54011.1.6 Remote Temperature Sensors 54111.2 Resistance Measurements 54211.3 Monitoring Contact Load (Pressure) 54511.4 Ultrasonic Measurements 54611.5 Wireless Monitoring 54811.6 Cost Benefits of Monitoring and Diagnostic Techniques 552Appendix 1: Methods of Description of Rough Surface 555Appendix 2: Shape-Memory Materials 565Appendix 3: Electrical Contact Tables 585References 599

1 Contacts

1.1 INTRODUCTION

An electrical contact is defined as the interface between the current-carrying members of cal/electronic devices that assure the continuity of electric circuit, and the unit containing theinterface The current-carrying members in contact, often made of solids, are called contactmembersor contact parts The contact members connected to the positive and negative circuitclamps are called the anode and cathode, respectively

electri-Electrical contacts provide electrical connection and often perform other functions Theprimary purpose of an electrical connection is to allow the uninterrupted passage of electricalcurrent across the contact interface It is clear that this can only be achieved if a good metal-to-metal contact is established The processes occurring in the contact zone are complex and notfully explained within the limits of present knowledge Although the nature of these processes maydiffer, they are all governed by the same fundamental phenomena, the most important being thedegradation of the contacting interface and the associated changes in contact resistance, load,temperature, and other parameters of a multipoint contact

Electrical contacts can be classified according to their nature, surface geometry, kinematics,design and technology features, current load, application, and by others means.1–3 In general,electrical contacts can be divided into two basic categories: stationary and moving Figure 1.1represents the most general classification of electrical contacts according to contact kinematics,functionality, and design features

In stationary contacts, contact members are connected rigidly or elastically to the stationaryunit of a device to provide the permanent joint Stationary contacts are divided into nonseparable orall-metal (welded, soldered, and glued), and clamped (bolted, screwed, and wrapped) Nonsepar-able (permanent) joints have a high mechanical strength and provide the stable electrical contactwith a low transition resistance A nonseparable joint is often formed within one contact member.For example, in commutating devices, only materials with a complex composition and arc-resistantworking layers are used as the contact members They are made by contact welding, soldering,coating, deposition, electrospark alloying, and mechanical methods of joining

Clamped contacts are made by mechanically joining conductors directly with bolts or screws orusing intermediate parts, specifically, clamps These contacts may be assembled or disassembledwithout damaging the joint integrity The simplest case of a clamped contact is the joint of twomassive conductors with flat contact surfaces, such as busbars A more complex joint configuration

is a contact comprising several conductors, such as joints of a multistrand wire and clamp that areused for joining wire conductors in transmission lines

The nature of clamped and all-metal contacts is different This is because in the all-metalcontacts there is no physical interface between conductors, whereas in clamped contacts the inter-face is controlled by the contact pressure and the ability of the material to undergo plastic

3

deformation The lower the specific resistance and hardness of a material, the higher its corrosionresistance and, consequently, the lower the contact transition resistance For this reason, contactsurfaces are usually covered with soft, corrosion-resistant materials such as tin, silver, cadmium, orsimilar materials Different surface cleaning techniques are often used to improve thejoint connectability.

In moving contacts, at least one contact member is rigidly or elastically connected to themoving unit of a device Depending on their operating conditions, these contacts are dividedinto two categories: commutating and sliding Commutating contacts intermittently control theelectric circuit They fall into two categories: separable (various plug connectors, circuit breakers)and breaking The latter are used for a periodical closing and opening of an electrical circuit, such as

in different switches, contactors, relays, and similar devices Because of differences in breakingpower, current, and voltage, there is a great variety of breaking contacts The breaking contacts can

be classified as light-, medium- and heavy-duty:

† Light-duty contacts carry very low currents, operate at voltages up to 250 V, and display noappreciable arc-related electrical wear The successful operation of these devices dependsmainly on maintaining relatively low and stable contact resistance and also on the selec-tion of the contact materials The factors that must be taken into account are tendency tooxidize (tarnish); presence of dirt, dust or other contaminants on the contact surface; andcontact design (form, size, contact pressure, and finish) Light-duty contacts are intendedfor use in instrument controls, general automation, radio and data communication, andtelecommunication systems

† Medium-duty contacts carry appreciably higher currents (see 5A above) and operate atvoltages up to 1000 V For this group, electrical wear is of prime importance The factorsgoverning contact material selection to meet the very severe operating conditionsinclude tendency to welding, material transfer, and erosion (pitting) Applications ofmedium-duty contacts are control devices for industrial, domestic, and distributionnetwork applications

Current-Current pickoffs of electrical and welding machines

Rheostats, potentio- meters code senders

Current pickoffs of cranes and transport

Plug connectors and circuit breakers

Operate under conditions of friction and wear

FIGURE 1.1 Classification of electrical contacts

† Heavy-duty contacts carry very high currents (tens of kA) and operate at very high voltages(hundreds of kV) The most common types of these connectors are contactors, starters, andcircuit breakers.

In sliding contacts, the contacting parts of the conductors slide over each other without ation Current passage through the contact zone is accompanied by physical phenomena (electrical,electromechanical, and thermal) that produce changes in the state (characteristics) of surface layers

separ-of the contacting members that differ when operating without current (see Figure 1.2) The severity

of the processes occurring at contact interface depends on the magnitude and character of thecurrent passing through the contact, the applied voltage, operating conditions, and contactmaterials.4,5 The physical processes occurring in the contact zone of sliding contacts can beclassified as follows:

† Sliding contacts with a heavy electrical contact loadare contacts whereby currents orvoltages are commutated, inducing mechanical, thermal or electrical effects includingsparking and arcing These effects thus produce changes in the state (properties) of thecontact members The necessary condition of such an operating regime is that the voltageacross disclosed contacts exceeds the minimal electric arc voltage for the materials used;

† Sliding contacts with a moderate electrical contact loadare contacts where mechanical,thermal or electrical effects, excluding sparking and arcing, change the state of the matedsurfaces The voltage across opened contacts is between the softening voltage and theminimal electric arc voltage for the material used;

† Sliding contacts with a low electrical contact loadare contacts where no additionalphysical phenomena and changes are induced in the state of the mated surfaces Inthis case the voltage across open contacts is less than the softening voltage

The most important and widely used types of sliding contacts include contacts of electricalmachines, current pick-offs of transport and lifting machines, and of radio-electronic devices,and control and automatic systems As a rule, sliding contacts for electrical and transportationmachines are intended to commutate currents of a moderate and high intensity while those forradio-electronic devices and control and automatic systems are usually low-current level contacts

Physical effects

Thermal Electrical

FIGURE 1.2 Possible effects on the passage of electrical current through the interface

For electrical machines, there are two types of sliding electrical contacts: brush-collector andbrush-collector ring, in which the brushes with different polarities slide over one friction track(brush-collector) or different rings (brush-collector ring).1,6 Collectors are commonly made ofelectrolytic copper or copper with small additions of cadmium, silver, magnesium, zirconium, ortellurium Collector rings are made of copper alloys with zinc, lead, and aluminum and, in somecases (very high peripheral velocities), of ferrous metals and their alloys Brush materials are basedmainly on multicomponent composites of graphite, soot, copper, and coke powders.7

Sliding contacts are very important components in many devices used in automatic, chanical, and communication equipment, such as different potentiometers serving aselectromechanical sensors.8,9Their design is wide ranging and, despite their low material consump-tion, they are expensive parts of machines and devices due to an extensive use of noble metals intheir production From a mechanical viewpoint, the operating conditions of low-current slidingcontacts are quite favorable because sliding velocities are low and loads on the contact members arelight; thus, as a rule, these devices are protected against harmful environment factors

teleme-1.2 SUMMARY OF BASIC FEATURES

It has been established that real surfaces are not flat but comprise many asperities.1Therefore, whencontact is made between two metals, surface asperities of the contacting members will penetrate thenatural oxide and other surface contaminant films, establishing localized metallic contacts and,thus, conducting paths As the force increases, the number and the area of these small metal–metalcontact spots will increase as a result of the rupturing of the oxide film and extrusion of metalthrough the ruptures

These spots, termed a-spots, are small cold welds providing the only conducting paths for thetransfer of electrical current A direct consequence of this is a porous contact where infiltratingoxygen and other corrosive gases can enter to react with the exposed metal and reduce the metalliccontact areas This will eventually lead to disappearance of the electrical contact, although themechanical contact between the oxidized surfaces may still be preserved The real contact area Arisonly a fraction of the apparent contact area Aa, as illustrated in Figure 1.3

Constriction resistance Diameter of a-spot

The relationship between the applied normal load Fc, hardness of the metal, H, and the apparentcontact area, Aa, is given by the following expression:

It should be pointed out that the electrical interface of an a-spot is far different from the singlecircular contact spot Current passing across a contact interface is constricted to flow througha-spots Hence, the electrical resistance of the contact due to this constricted flow of current iscalled constriction resistance and is related to the basic properties of metals such as hardness andelectrical resistivity Holm1has shown that the constriction resistance for a single a-spot can beexpressed as

TABLE 1.1

Effect of Normal Load on Real Area of Contact for Clean Surfaces

DC potentiometers, whose resistive members are relatively thick and have a highspecific resistance.

There are many parameters that can be used to assess the operating efficiency of electricalcontacts Among these parameters, perhaps the most important are electric (the transition voltagedrop, commutation noise, erosion resistance), tribological (the wear resistance and friction coeffi-cient) and chemical (corrosion resistance) In the following chapters, detailed analyses of thefactors affecting the properties and performance of electrical contacts will be given

2.1 SURFACE OF SOLIDS

The features of a solid surface as a physical object are governed by its spatial arrangement as aboundary between two phases.10The atoms and molecules belonging to the surface have fewerneighbors than those in the bulk This simple fact has far-reaching consequences for geometry andphysics of a surface: the interactions between its atoms and their neighbors vary, distorting the forcefield that penetrates to the depth of several interatomic distances Given this fact, the excess ofenergy to surface energy appears; consequently, the surface interacts with the environment Thisprocess is termed adsorption There are physical and chemical types of adsorption

Physical adsorption is characterized by the van der Waals interactions between the adsorbateand the solid surface As a rule, its energy of the interaction is below 20 kJ/mol The polymolecularfilms adsorbed on the surface are removed relatively easily

The chemical adsorption energy is quite high (80–400 kJ/mol), usually producing a monolayer

on the surface that is hard to remove, even through the use of elevated temperatures In addition,chemical reactions between the surface and the active elements, such as oxidation in the environ-ment, should be remembered Unlike the case for chemisoption, these reactions result in a bulkphase on the surface

The environment exerts very different effects on a solid surface.10 In the 1920s, A Joffedemonstrated that halide crystals, e.g., NaCl, that are brittle in dry air, become ductile in a moistatmosphere and show an increase in strength Joffe ascribed this effect to a water film on the solidsurface, assuming that the water heals surface microcracks This circumstance holds significance intribological behavior of materials for which Joffe’s effect takes place For example, aluminumoxide is sensitive to water vapor, and high-strength steel exposed in pure hydrogen is sensitive to asmall concentration of oxygen An attack of some active environmental species on the solid surface

of metals or nonmetals may change the mechanical behavior of surface layers because of thewedging, or Rebinder, effect (Figure 2.1) This phenomenon was first observed by P Rebinder

As a rule, the species are of organic origin (fatty oxides, alcohols, soaps, etc.) and are present

in lubricants

The adsorbed film may have the opposite effect, producing surface hardening This hardeningoccurs in, for example, oxides on certain metals (the Roscoe effect) Hence, the surface adsorptivityproduces a fine boundary layer with a structure and behavior differing from those of the surfacelayer of the solid.Figure 2.2shows schematically that the structure of the boundary layer is quiteintricate The appearance of each sublayer depends upon the conditions of fabrication of a part Thelayers may mutually penetrate one another through the system of microcracks

The boundary layer may be in a diversity of physical states, ranging from nearly gaseous tosolid crystalline Both the basic parameters (temperature and pressure) and the pattern ofinteractions with the solid phase determine its state The mechanical behavior of boundarylayers demonstrates a wide spectrum of properties ranging from viscous and viscoelastic behavior

to perfect elasticity

9

When two solids approach each other, only their molecular fields interact and generate theattracting force responsible for their bonding or adhesion This latter state implies the appearance ofmolecular bonds between the mated surfaces The thermodynamic work of adhesion, ga, betweentwo bodies (1 and 2) is equal to the work of reversible adhesive detachment; this is frequentlydetermined by the Dupre equation:

gaZ g1Cg2Kg12;where g1and g2are the surface energies of surfaces 1 and 2 before contact (their free energies) and

g12is the interface energy

Metal oxides Adsorbed gas

Adsorbed moisture Polar molecules (Lubricant)

FIGURE 2.2 Structure of surface layer of metallic part

manufacturing or other considerations.11,12 A real surface is not ideal because of asperitiesappearing during machining and subsequent use The extent of deviation particularly dependsupon the structure of contacting materials The solid surface experiences the effects of variousfactors that can be classified into manufacturing, operational, and structural.

The height (amplitude) dimensions of asperities range extensively from decimal fractions of ananometer to several millimeters The spacing parameters of the asperities are still wider andsometimes extend the length of the part itself The lower limits of these ranges relate naturally

to the dimensions of atoms and molecules; the upper limits depend on the conditions of machiningand the structure of the materials in question There are apparently no physically imposed limits ofexistence of the asperities within these height and spacing ranges Nevertheless, the investigation ofrough surfaces and the development of suitable measuring equipment indicate that it is methodo-logically reasonable to divide the asperities into four dimensional levels: errors in form, waviness,roughness, and subroughness (Figure 2.3)

Errors in form are defined as shape deviations of a real surface or profile from the simplegeometry having a great spacing (S Z1–5,000 mm) and relatively small height (DZ1–50 mm) As

a rule, D/S%0.001 The errors in form are usually single, irregularly spaced surface departures Forcylindrical parts, they can be oval facets in the cross-section and taper, and barrel-shaped cambers

in the longitudinal section; lack of rectilinearity and flatness characterize shape deviations of aflat surface

Errors in form result from faulty machining, lack of tool precision, wear of tools, and elasticdeformations in the system of lathe-tool-workpiece produced by factors such as the variablecutting force

Sometimes shape deviations are determined quantitatively by the parameter; D being thegreatest distance between the real surface points and the surface enveloping the latter along thenormal (Figure 2.3) The enveloping surface is defined as the nominally shaped surface contactingwith the real surface and lying outside the material of the part, so that any deviation of the pointmost distant from the real surface within a specified area should have the minimum magnitude.Roughnessis usually excluded when errors in form are analyzed In reality, these errors and theroughness specified for the same surface are interrelated; therefore, a tolerance on the form erroralso imposes restrictions on the roughness For example, it is accepted that the roughness, Rz,should be at least 1.5–2 times less than the highest shape deviations

When studying a contact between real surfaces, it should be borne in mind that errors in formtend to redistribute the pressure within the apparent contact area; as a consequence, the stressconcentration occurs in the relevant contact areas, and rubbing surfaces undergo uneven wear.Wavinessis a combination of n quasiperiodic asperities with a relatively large spacing alongthe portion exceeding the specified sampling length, l, for measuring the surface roughness Thewaviness covers the following dimensional area: the spacing of asperities is 0.8–10 mm

subrough-(bigger parts may have a larger upper limit, up to 200–300 mm); the height is 0.01–500 mm Somecountries (Germany, Switzerland, and the United States, for example) have standards for waviness.There is no strict distinction between waviness and roughness Conventionally, for the con-venience of measurement and classification, either the waviness spacing (the lower spacing ofwaviness should exceed the assessment length used for roughness measurement) or the ratiobetween the spacing and the waviness height (assumed, as a rule, to be over 40) serves as such aseparating boundary.

Vibrations (forced or self-excited) in the lathe-tool-workpiece system are the main cause ofwaviness that may also result from friction and wear The cluster structure of the real contactbetween solids is mostly due to waviness, making the latter an important subject in studies ofenergy transfer across the contact zone

Roughness constitutes a surface microrelief, and it is defined as a population of asperities with arelatively small sampling; it is measured using the assessment length, l, shown in Figure 2.4.Usually, roughness is produced by tracks of machining tools (a cutter, a mill, an abrasive cutter,etc.), and the quality of roughness depends on the kinematical design, the method of machining, themechanical properties of a material, and vibrations in the lathe-tool-workpiece system The originalroughness of working surfaces undergoes significant modification during friction and wear, and itreaches the so-called “equilibrium roughness” that is apparently reproduced under normalfriction conditions

Subroughnesspresents the fine (nanometer scale) structure of a real surface, and it is closelyrelated to the so-called “physical relief.” It involves the accidental and imperfect location ofcrystallographic planes, chaotically distributed grains, and islet-type films including oxides andadsorbed ones For some partially crystalline polymers, the subroughness can be connected withalternating crystalline and amorphous regions of dozens of nanometers in size Its study can provide

a new insight into the theory of friction, wear, and lubrication

Hence, the real surface has a set of topographic elements that can be divided into two groupsand four levels of shape deviations: macrogeometry (errors in form and waviness) and microgeo-metry (roughness and subroughness) They possess different scales and patterns of distribution, andthey play different roles in the processes of friction and wear Conditionally, the regions of exist-ence of each can be represented by some subsets in the coordinate system with the axes being theheight of asperities, H, and the mean distance, S, between them (Figure 2.5) Even when the surfacehas the asperities of the same level, its description presents a nontrivial problem The descriptioncan be facilitated if its objectives are clearly understood At present, four approaches exist intribology to describing real surfaces: deterministic, parametric, probabilistic, and fractal

The deterministic approachpresents a surface description as some periodic, continuous, orpiecemeal continuous function It is educative to compare the sketch made by Coulomb in 1821(Figure 2.6a) with that borrowed from a contemporary monograph by Johnson in 1987(Figure 2.6b).13Although such imaging of the surface may seemingly be naı¨ve, it allows one of

FIGURE 2.4 Rough surface profile

the fundamental features of rough surfaces—their discrete (patchy) pattern—to be reflected,whereas stochastic behavior as another feature is ignored.

The parametric methodis based upon the description of a surface using a set of some ameters that are determined, as a rule, by analyzing surfaces in two or three dimensions Theroughness parameters are calculated in respect to some reference line (surface) that is plotted in

par-a definite mpar-anner The mepar-an line (surfpar-ace) is usupar-ally used par-as the reference line

The surface profile is generally characterized by a set of parameters There is a variety of suchsets; the parameters include height (amplitude), sampling, and those of a hybrid type A discussionfollows of the basic parameters used in engineering practice and research

The arithmetic average roughness, Ra, is defined as an arithmetic mean of the departures of theroughness profile from the mean line over one sampling length, l:

RaZ1l

ðl

where z(x) is the profile equation which is frequently set in a graphic or tabular form

In the last case, Rais calculated by the formula

RaZ1n

Xn iZ1

Usually, Rais averaged over several consecutive sampling lengths from 2 to 20 in accordancewith the national standard The parameter is identical to arithmetic average (AA) and center-lineaverage (CLA).

The root-mean-square (RMS) roughness, Rq, is defined as the RMS deviation of the profile fromthe mean line over sampling length:

RqZ 1l

ðl 0

Xn iZ1

z2i

!1=2:

The parameters Raand Rqdiffer insignificantly in magnitudes For the same surface, Rais lessthan Rqby 6–30% Thus, a regular sine-shaped profile with a single harmonics, the ratio Rq/Raisequal to p/23/2z1.11; for the Gaussian profile, RqZ 1.25Ra

The 10-point height (or zone roughness), Rz, is separation of the average of the five highestpeaks and the five lowest valleys within a single sampling length (peak [valley] is defined as localmaximum [minimum] above [below] profile mean line):

RzZ X5 iZ1

jzipj CX5 iZ1

The maximum peak-to-valley height, Rmax, is the largest single peak-to-valley height in fiveadjoining sampling lengths The parameter is a sensitive indicator of high peaks or deep scratchesbut has a large scatter due to random sampling The standards specify some spacing parameters.Spacing along the mean line, Sm, is the mean spacing between profile peaks at the mean line,measured over the assessment length Here, a profile peak is the highest point of the profile between

an upwards and downwards crossing of the mean line

Spacing of peaks, S, is the mean spacing of adjacent local peaks, measured over the assessmentlength A local peak is the highest part of the profile measured between two adjacent minima, and it isonly included if the distance between the peak and its preceding minima is at least 1% of the peak-to-valley of the profile Normally specified in a Russian standard, these parameters also exist inhybrid form.

Bearing ratio, tp, is the length of bearing surface (expressed as a percentage of the assessmentlength, L) at a given height, p, below or above the reference line (Figure 2.8) A plot of bearing ratioagainst surface height is named the bearing ratio curve (or Abbott–Firestone curve) The curveallows one to estimate the contact area of mating surfaces and to assess the expected rate of wear.The initial portion of the curve is conveniently approximated by the exponential function:

where 3 Za/Rmax, a is the distance between the highest line of profile and the specified level p band n are the parameters of the parabolic approximation of initial part of bearing ratio curve Theydepend on the type of machining

The probabilistic approachto describing the rough surface geometry is based on the theories ofprobability and random processes As a rule, it is more labor consuming, yet the effort is partlycompensated by more complete information about the surface topography This approach allowstreating two-dimensional or three-dimensional surfaces as some statistical sample of profile ordi-nates (or peaks of asperities) or as some realization of a random process

In the first case, the set of heights (the ordinates) of the profile z(x) is treated as a randomvariable that is described in the theory of probability by the cumulative probability function F(z).The function is the probability that random variable z(x), the profile height z, for example, takes avalue less than or equal to z:

f ðzÞ Z 1

s ffiffiffiffiffiffi2p

FIGURE 2.8 Schematic of bearing curve construction

where s is the standard deviation of the profile ordinates (identical to the roughness parameter, Rq,described above), which characterizes the scattering of the random variable, while a is its expectation(mean value of profile ordinates) If the ordinates are normalized by s and they are measured from themean line of profile, then a Z0 and the Gaussian (normal) probability density is written as:

f ðzÞ Z 1

s ffiffiffiffiffiffi2p

The probabilistic approach is generalized by the concept based on the theory of random fields(processes), which treats a profile as a random field This method was developed primarily forGaussian (isotropic and anisotropic) surfaces Its advantages become remarkable when analyzing athree-dimensional surface

This paper will not discuss the mathematical refinements of the method, which can be consulted

in publications Below are given the main results of applicable significance

The probability density of summit heights is described by the following equation:

pðx1Þ Z

ffiffiffi3p2p x

 1

C2pa3ðaK1Þ

par-The distribution p depends on the spectral moments only in some dimensionless combination

athat can be determined from the surface profilogram The bandwidth parameter a is associated withthe width of the surface spectral density The larger the parameter a, the broader the spectrum, i.e., theband of wavelengths making up the given surface is wider A narrow band (a/1.5) indicatesapproximate equal length of all the waves

The bandwidth parameter varies from 1.5 to infinity; the distribution of Nayak degenerates intothe known Rayleigh and Gaussian distributions at these limit values of the bandwidth parameter:

pRðxÞ Z

0; x!0;

2 ffiffiffi3pffiffiffiffiffiffi2p

(2.10)