electrical contacts: fundamentals, applications and...

Part I

Fundamentals of Electrical Contacts

Milenko Braunovic, Valery V. Konchits, andNikolai K. Myshkin

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q 2006 by Taylor & Francis Group, LLC

Preface

The multidisciplinary study of the electrical contact in modern engineering is significant, but often

neglected. The scientist and engineers who have spent their professional lives studying and

applying electrical contacts know that these components are critical to the successful operation of

all products that use electricity. In our civilization, all electricity transmission and distribution, most

control, and most information exchange depends upon the passage of electricity through an

electrical contact at least once. The failure of an electrical contact has resulted in severe

consequences, e.g., an energy collapse of a megapolis, a failure of the telephone system, and even

the 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 predictions

and conclusions. Since that time, however, there has been a huge increase in the application of

electrical contacts. For example, the era of the information highway and the development of the

integrated circuit have created new challenges in the use of electrical contacts. The use of electrical

contacts on the microscopic scale presents numerous problems never considered by previous

generations of researchers and engineers. The future MEMS/NEMS technology is another area

where 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 the

areas of mechanical engineering and tribology, which Holm postulated to be key segments in

electrical contacts, with the new data on electrical current transfer, especially at the

micro/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 and

presents valuable design information for practicing mechanical and electrical engineers. In fact, the

information contained here will serve as an excellent source of information not only for anyone

developing equipment that uses electricity, but for postgraduate students who are concerned about

the passage of current from one conductor to another.

The authors of this book have many years of research and practical experience. One unusual and

interesting aspect of the book’s development is that it comes through the cooperation of the

different approaches to the subject from the West and the East. They have succeeded in making the

bulk of research and engineering data equally clear for all the segments of the international

audience.

Paul G. Slade

Ithaca, New York


The Authors

q 2006 by Taylor & Francis Group, L

Dr. Milenko Braunovic received his Dipl. Ing degree in technical physics

from the University of Belgrade, Yugoslavia, in 1962 and the M.Met. and

Ph.D. degrees in physical metallurgy from the University of Sheffield,

England in 1967 and 1969, respectively. From 1971 until 1997, he was

working at Hydro-Quebec Research Institute (IREQ) as a senior member of

the scientific staff. He retired from IREQ in 1997 and established his own

scientific consulting company, MB Interface. From 1997 until 2000 he was

consulting for the Canadian Electricity Association as a technology

advisor. He is presently R&D manager with A.G.S. Taron Technologies in

Boucherville, QC, Canada.

During the last 30 years, Dr. Braunovic has been responsible for the development and

management of a broad range of research projects for Hydro-Quebec and the Canadian Electrical

Association in the areas of electrical power contacts, connector design and evaluation, accelerated

test methodologies, and tribology of power connections. He has also initiated and supervised the

R&D activities in the field of shape-memory alloy applications in power systems. Dr. Braunovic is

the author of more than 100 papers and technical reports, including contributions to encyclopaedias

and books, in his particular areas of scientific interests. In addition, he frequently lectures at

seminars world wide and has presented a large number of papers at various international

conferences.

For his contributions to the science and practice of electrical contacts, Dr. Braunovic received the

Ragnar 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 Armington

Recognition Award. He is also a recipient of the 1994 IEEE CPMT Best Paper Award. He

successfully chaired the Fifteenth International Conference on Electrical Contacts held in Montreal

in 1990, and was a technical program chairman of the Eighteenth International Conference on

Electrical Contacts held in Chicago in 1996. He is a senior member of the Institute of Electronics

and Electrical Engineers (IEEE), the American Society for Metals (ASM), the Materials Research

Society (MRS), the Planetary Society, the American Society for Testing of Materials (ASTM), and

The Minerals, Metals & Materials Society (TMS).

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Dr. Valery Konchits was born on January 3, 1949 in the city of Gomel,

Belarus. He graduated from Gomel State University in 1972. He received

his Ph.D. degree in tribology from the Kalinin Polytechnic Institute, Russia

in 1981.

In 1972, he joined the Metal-Polymer Research Institute of the National

Academy of Sciences of Belarus in Gomel. In 1993, he became the head of

the laboratory in the Tribology Department. Since 2001, Dr. Konchits has

been Deputy Director of the Metal-Polymer Research Institute.

The scientific interests of Dr. Konchits lie mainly in electrical

contacts’ friction and wear, contact phenomena at their interfaces, and

electrophysical diagnostic methods of friction. He is the author of more

than 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).

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q 2006 by Taylor & Francis Group, L

Prof. Nikolai Myshkin was born on May 12, 1948 in the city of Ivanovo,

Russia. He graduated from the Power Engineering Institute in 1971 with a

degree in electromechanics. Has received his Ph.D. from the Institute for

Problems in Mechanics of the Russian Academy of Sciences in 1977. The

same year, he joined the Metal-Polymer Research Institute in Gomel where

since 1990 he has been Head of the Tribology Department. He has also

been the director of MPRI since 2002. He earned his Dr.Sc. degree in

tribology in 1985 and became a full professor of materials science in 1991.

He was elected as a correspondent member of the Belarus National

Academy of Sciences in 2004.

He received the USSR National Award for Young Scientists in 1983, the

Award for Best Research given by the Belarus National Academy of Sciences in 1993, and the

Award of the Russian Government in Science and Technology in 2004.

The scientific interests of Prof. Myshkin lie mainly in the characterization at micro and nanoscale

surfaces, 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 the

Tribology 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), and

Tribology: Principles and Applications (2002).

Prof. Myshkin is chairman of the Belarus Tribology Society and vice-president of the

International Tribology Council. He is also assistant editor-in-chief of the Journal of Friction and

Wear, and a member of editorial boards of Tribology International, Tribology Letters, Industrial

Lubrication and Tribology, and the International Journal of Applied Mechanics and Engineering.

LC

Acknowledgments

In the preparation of the book, the authors have used a large number of published materials, either in

the form of papers in referenced journals, or from the websites of different companies and

organizations. In both cases, proper permissions for using these materials have been obtained. In

many instances, the authors obtained the required information directly from the authors of the

papers 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-Quebec Research Institute (IREQ) in Varennes, QC, Canada for

providing essential reference material and fruitful discussions concerning certain topics of power

connections.

The authors are grateful to Dr. Mark. I. Petrokovets for fruitful discussion and his help in

preparation of Chapters 2, 3 and 5. We also thank Dr. Denis Tkachuk for his valuable assistance in

preparation of the manuscript.

Acknowledgement is made to the many individuals and company authorities for permission to

use 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 and

company authorities.

Prof. George M. Pharr, Department of Materials Science and Engineering, University of

Tennessee, Knoxville, USA for providing the papers on nanoindenation testing methods and

instrumentation 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 and

dislocation nature of the processes occurring during friction.

Dr. Roland Timsit of Timron Scientific Consulting, Inc., Toronto, Canada for permission to use

the relevant material from his papers and publications.

Dr. Robert Malucci of Molex, Inc., Lisle, IL, USA, for permission to use the relevant material

from 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 Genie Electrique, Supelec, Gif sur Yvette, France for helpful

discussions concerning the lubrication of electrical contacts and permission to use some of data

from the publications cited in this book..

Dr. Magne Runde of the Norwegian University of Science and Technology, Norway, for helpful

discussions concerning the problem of electromigration in electrical contacts.

Prof. Zoran Djuric and Milos Frantlovic of the Center for Microelectronics Technologies and

Single Crystals, MTM, University of Belgrade, Serbia and Montenegro for providing the

information on the wireless temperature monitoring system.

Dr. Bella Chudnovsky of Square D, USA, for helpful discussions concerning the whisker

formation in electrical contacts and for permission to use the information on the On-Line Wireless

Temperature 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 of

Technology, The Netherlands for providing and allowing the use of reference materials concerning

the noise in electrical connections.

Mr. Larry Smith of USi, Armonk, NY, USA, for permitting the use of the images and

descriptions of the Power-donut unit found on the company web site (http://www.usi-power.com).


Dr. Young-kook (Ryan) Yoo, Director of Global Sales and Marketing of PSIA Corp. Sungnam

462-120, Korea, for permission to use descriptions of different surface analytical equipment as

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 field

applications 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 the

company web site (http://www.boldstarinfrared.com).

R.N. Wurzbach of Maintenance Reliability Group (MRG), York, PA, USA for permission to use

description of the web-based cost benefit analysis method for predictive maintenance

(http://www.mrgcorp.com).

ndb Technologie, Inc., Quebec, Canada for permission to use the information about the

microohmeters found on their web site (http://www.ndb.qc.ca).

In addition, the authors would like to acknowledge the courtesy of the following companies for

allowing 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), Infrared

Solution, 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 her

generous hospitality shown to the authors during the preparation of the book manuscript.


Introduction

This book provides detailed analytical models, state-of-the-art techniques, methodologies and tools

used to assess and maintain the reliability of a broad class of moving and permanent electrical

contacts in many technological devices, such as automotive and aerospace components, high- and

low-power contact joints, sliding and breaking contacts, electronic and control apparatus, and

electromechanical systems. It provides a comprehensive outline of the tribological behavior of

electrical contacts that is rarely discussed in the existing literature; these are problems of

considerable interest for researchers and engineers.

Focusing on the main mechanical and electrical problems in connections with the field

applications 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 not

neglecting the importance of their design, development, and manufacturing. The book provides a

complete introduction to electric conduction across a contacting interface as a function of surface

topography, load, and physical-mechanical properties of materials, and the interrelation of

electrical performance with friction and wear; it takes into account material properties and lubricant

effects. Consideration is given to the deleterious effects of different degradation mechanisms, such

as stress relaxation/creep, fretting, differential thermal expansion, and the formation of

intermetallics, as well as their impacts on operating costs, safety, network reliability, power

quality. Various palliative measures to improve the reliability and serviceability of electrical

contact at macro-, micro-, and nano-levels are also discussed.

This book diminishes a large gap between engineering practice widely utilizing empirically

found methods for designing and optimizing the contact characteristics and theory relating to

tribological and electromechanical characteristics of the contacts. The main trends in the practical

solutions 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 of

practical applications in various fields are given throughout the book.

Covering a wide range of references, tables of contact materials, coatings and lubrication

properties, 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. The

book (or parts of it) can be used not only as a reference, but also as a textbook for advanced graduate

students and undergraduates, as it develops the subject from its foundations and contains problems

and solutions for each chapter.


Table of Contents

Part IFundamentals 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

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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

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Chapter 4

Contact Materials ....................................................................................................71

4.1 Metallic Contact Materials ....................................................................................................71

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4.1.1 Properties of Contact Materials ................................................................................ 71

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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

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Chapter 5

Current and Heat Transfer across the Contact Interface ......................................149

5.1 Contact Resistance................................................................................................................149

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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

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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

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6.4.5.1 Effect of Electrical Current ...................................................................... 232

6.4.6 Electromigration ...................................................................................................... 237

6.4.7 Stress Relaxation and Creep.................................................................................... 240

6.4.7.1 Nature of the Effect of Electric Current .................................................. 241

6.4.7.2 Effect of Electric Current on Stress Relaxation ...................................... 242

6.4.8 Thermal Expansion.................................................................................................. 247

6.5 Impact of Connection Degradation ......................................................................................248

6.5.1 Prognostic Model for Contact Remaining Life ...................................................... 250

6.5.2 Economical Consequences of Contact Deterioration ............................................ 256

6.5.3 Power Quality .......................................................................................................... 258

Part II

Applications of Electrical Contacts ..............................................................................................261


Chapter 7

Power Connections ................................................................................................263

7.1 Types of Power Connectors ................................................................................................263

7.2 Design Features and Degradation Mechanisms ..................................................................263

7.2.1 Bolted Connectors .................................................................................................. 263

7.2.1.1 Fretting in Bolted Connectors .................................................................. 269

7.2.1.2 Fretting in Aluminum Connections.......................................................... 271

7.2.1.3 Intermetallics ............................................................................................ 272

7.2.1.4 Creep and Stress Relaxation .................................................................... 275

7.2.2 Bus-Stab Contacts.................................................................................................... 276

7.2.3 Compression Connectors ........................................................................................ 279

7.2.3.1 Degradation Mechanisms in Compression Connectors .......................... 281

7.2.3.2 Corrosion .................................................................................................. 282

7.2.3.3 Fretting in Compression Connectors........................................................ 283

7.2.4 Mechanical Connectors .......................................................................................... 284

7.2.4.1 Binding-Head Screw Connectors ............................................................ 285

7.2.4.2 Insulation Piercing Connectors ................................................................ 289

7.2.4.3 Wedge Connectors.................................................................................... 289

7.2.5 Welded Connectors.................................................................................................. 290

7.3 Mitigating Measures ............................................................................................................292

7.3.1 Contact Area–Connector Design ............................................................................ 292

7.3.2 Contact Pressure ...................................................................................................... 294

7.3.3 Surface Preparation.................................................................................................. 296

7.3.4 Mechanical Contact Devices .................................................................................. 297

7.3.4.1 Retightening.............................................................................................. 300

7.3.4.2 Bimetallic Inserts...................................................................................... 301

7.3.4.3 Transition Washers .................................................................................. 301

7.3.4.4 Multilam Contact Elements...................................................................... 302

7.3.4.5 Shape-Memory Alloy Mechanical Devices ............................................ 302

7.3.4.6 Self-Repairing Joints ................................................................................ 303

7.3.5 Lubrication: Contact Aid Compounds .................................................................... 304

7.4 Installation Procedures ........................................................................................................306

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Chapter 8

Electronic Connections..........................................................................................309

8.1 Types of Electronic Connections ........................................................................................309

8.2 Materials for Electronic Connections ..................................................................................309

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8.2.1 Solder Materials ...................................................................................................... 310

8.2.2 Lead-Free Solders.................................................................................................... 312

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8.2.2.1 Tin ............................................................................................................ 312

8.2.2.2 Tin–Silver ................................................................................................ 312

8.2.2.3 Tin–Silver–Bismuth.................................................................................. 313

8.2.2.4 Tin–Silver–Copper.................................................................................... 313

8.2.2.5 Tin–Silver–Copper–Antimony ................................................................ 314

8.2.2.6 Tin–Silver–Antimony .............................................................................. 314

8.2.2.7 Tin–Bismuth ............................................................................................ 314

8.2.2.8 Tin–Copper .............................................................................................. 315

8.2.2.9 Tin–Indium .............................................................................................. 315

8.2.2.10 Tin–Indium–Silver.................................................................................... 316

8.2.2.11 Tin–Zinc.................................................................................................... 316

8.2.2.12 Tin–Zinc–Silver ........................................................................................ 316

8.2.2.13 Tin–Zinc–Silver–Aluminum–Gallium .................................................... 317

8.3 Degradation Mechanisms in Electronic Connections ..........................................................317

8.3.1 Porosity .................................................................................................................... 319

8.3.2 Corrosion/Contamination ........................................................................................ 322

8.3.2.1 Pore Corrosion .......................................................................................... 322

8.3.2.2 Creep Corrosion........................................................................................ 323

8.3.2.3 Tarnishing ................................................................................................ 324

8.3.3 Fretting .................................................................................................................... 327

8.3.4 Frictional Polymerization ........................................................................................ 334

8.3.5 Intermetallic Compounds ........................................................................................ 336

8.3.6 Creep and Stress Relaxation.................................................................................... 348

8.3.7 Electromigration ...................................................................................................... 353

8.3.8 Whiskers .................................................................................................................. 357

8.4 Mitigating Measures ............................................................................................................361

8.4.1 Effect of Coating .................................................................................................... 361

8.4.1.1 Gold Coatings .......................................................................................... 361

8.4.1.2 Palladium and Palladium Alloys.............................................................. 362

8.4.1.3 Tin Coatings ............................................................................................ 364

8.4.1.4 Nickel and Nickel-Base Alloys ................................................................ 364

8.4.2 Effect of Lubrication .............................................................................................. 364

Chapter 9

Sliding Contacts ....................................................................................................369

9.1 Tribology of Electrical Contacts ..........................................................................................369

9.1.1 Interrelation of Friction and Electrical Processes .................................................. 370

9.1.2 Role of Boundary Films .......................................................................................... 371

9.1.3 Main Means of Improving Reliability of Sliding Contacts.................................... 371

9.1.4 Tribophysical Aspects in the Development of Sliding Contacts............................ 373

9.2 Dry Metal Contacts ..............................................................................................................376

9.2.1 Low-Current Contacts ............................................................................................ 376

9.2.1.1 Effects of Low Current and Electrical Field on Friction ........................ 377

9.2.1.2 Effect of Interfacial Shear ........................................................................ 378

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9.2.1.3 Adhesion, Transfer, Wear Debris Formation, and

Surface Transformation ............................................................................ 380

9.2.2 High-Current Contacts ............................................................................................ 386

9.2.2.1 Effects of Electrical Current on Tribological Behavior .......................... 386

9.2.2.2 Influence of Electric Fields ...................................................................... 390

9.2.2.3 Effect of Velocity .................................................................................... 392

9.2.2.4 Effect of Material Combination of Contacting Members........................ 393

9.2.2.5 Electroplastic Effect in Sliding Contact .................................................. 394

9.2.2.6 Friction and Current Transfer in Metal Fiber Brush Contacts ................ 396

9.2.3 Stability of the Contact Resistance. Electrical Noise ............................................ 400

9.2.3.1 Contact Noise in Closed Connections...................................................... 400

9.2.3.2 Electrical Noise in Sliding Contacts ........................................................ 402

9.3 Lubricated Metal Contacts ..................................................................................................414

9.3.1 Introduction. Lubrication Factors............................................................................ 414

9.3.2 Electrical Properties of Lubricating Boundary Layers .......................................... 415

9.3.3 Conductivity of Lubricated Contacts ...................................................................... 419

9.3.3.1 Effect of Lubricant on Conductivity near the Contact Spots .................. 419

9.3.3.2 Effect of Lubricant on Conductivity of Contact Spots ............................ 420

9.3.3.3 Experimental Studies of Electric Conductivity

of Lubricated Contacts ............................................................................ 427

9.3.3.4 Contact Resistance between Very Smooth Lubricated Surfaces ............ 430

9.3.3.5 Temperature Dependencies of Contact Conductivity .............................. 431

9.3.4 Lubrication Factors in Sliding Contacts ................................................................ 433

9.3.4.1 Effect of Lubricant Origin........................................................................ 434

9.3.4.2 Lubricant Durability ................................................................................ 435

9.3.4.3 Tribochemical Aspects of Lubrication .................................................... 438

9.3.4.4 Effect of Velocity in Light-Current Contacts .......................................... 441

9.3.4.5 Effects of Lubricant Contact Properties .................................................. 442

9.3.4.6 Current Passage and Friction in High-Current

Lubricated Contacts .................................................................................. 444

9.3.5 Lubricants for Electrical Contacts .......................................................................... 449

9.3.5.1 Lubricants for Sliding Electric Switch Contacts .................................... 450

9.3.5.2 Lubricants for Sliding Contacts of Sensors ............................................ 451

9.3.5.3 Selection of Contact Lubricants .............................................................. 454

9.4 Composite Contacts ..............................................................................................................454

9.4.1 Effect of Intermediate Layers on Electrical Characteristics .................................. 455

9.4.1.1 Structure and Electrical Properties of Intermediate Films ...................... 456

9.4.1.2 Mechanism of Current Passage through the Contact with

Intermediate Films.................................................................................... 460

9.4.1.3 Influence of Polarity on Conductivity in

Composite–Metal Contact ........................................................................ 467

9.4.2 The “Lubricating” Effect of Electrical Current ...................................................... 471

9.4.2.1 Effect of Current on Friction Characteristics .......................................... 471

9.4.2.2 Mechanism of the “Lubricating” Action of the Electric Current............ 473

9.4.2.3 Effect of Brush Material on Friction Behavior with

Electric Current ........................................................................................ 477

9.4.3 Electrical Wear ........................................................................................................ 479

9.4.3.1 Wear of Currentless Contacts .................................................................. 479

9.4.3.2 Effect of Current on Wear........................................................................ 480

9.4.3.3 Factors Leading to Electrical Wear in the

Absence of Sparking ................................................................................ 483

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9.4.3.4 Influence of the Electric Field in the Clearance ...................................... 489

9.4.3.5 Wear with Sparking and Arcing .............................................................. 491

9.4.3.6 Some Ways to Reduce Electrical Wear .................................................. 493

Part III

Diagnostic and Monitoring Technologies ....................................................................................495


Chapter 10

Electrical Methods in Tribology ..........................................................................497

10.1 Surface Characterization ....................................................................................................497

10.2 Diagnosis of Contact Area and Friction Regimes ............................................................503

10.2.1 Formation of Contact Area.................................................................................. 503

10.2.2 Control of Sliding Contact with the Presence of Oxide Films .......................... 508

10.2.3 Experimental Study of Metallic Contact Spots Formation ................................ 509

10.3 Evaluation of Tribological Performance of Materials and Lubricants..............................511

10.3.1 Evaluation of Load-Bearing Capacity and Lubricity of Surface Films ............ 511

10.3.2 Estimation of Lubricant Interlayer Shear Strength under Imperfect

Lubrication .......................................................................................................... 515

10.3.3 Evaluation of Thermal Stability of Materials and Lubricants

by Electrical Methods.......................................................................................... 517

10.3.4 Control of Surface Coatings and Films .............................................................. 519

10.3.5 Novel Systems for Measuring and Analysis of Contact Characteristics............ 521

10.3.5.1 Method of “Triboscopy” .................................................................... 523

Chapter 11

Monitoring Technologies ......................................................................................529

11.1 Thermal Measurements ......................................................................................................530

11.1.1 Infrared Thermography........................................................................................ 532

11.1.2 Basic Features of Infrared Thermography .......................................................... 532

11.1.3 Types of Infrared Thermal Systems.................................................................... 534

11.1.4 SME Temperature Indicators .............................................................................. 538

11.1.5 Temperature Stickers (Labels) ............................................................................ 540

11.1.6 Remote Temperature Sensors.............................................................................. 541

11.2 Resistance Measurements ..................................................................................................542

11.3 Monitoring Contact Load (Pressure)..................................................................................545

11.4 Ultrasonic Measurements ..................................................................................................546

11.5 Wireless Monitoring ..........................................................................................................548

11.6 Cost Benefits of Monitoring and Diagnostic Techniques..................................................552

Appendix 1: Methods of Description of Rough Surface ........................................................555

Appendix 2: Shape-Memory Materials ....................................................................................565

Appendix 3: Electrical Contact Tables ....................................................................................585

References ....................................................................................................................................599

Francis Group, LLC

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Introduction to Electrical

Contacts

1.1 INTRODUCTION

An electrical contact is defined as the interface between the current-carrying members of electri-

cal/electronic devices that assure the continuity of electric circuit, and the unit containing the

interface. The current-carrying members in contact, often made of solids, are called contact

members or contact parts. The contact members connected to the positive and negative circuit

clamps are called the anode and cathode, respectively.

Electrical contacts provide electrical connection and often perform other functions. The

primary purpose of an electrical connection is to allow the uninterrupted passage of electrical

current 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 not

fully explained within the limits of present knowledge. Although the nature of these processes may

differ, they are all governed by the same fundamental phenomena, the most important being the

degradation 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 1.1

represents 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 stationary

unit of a device to provide the permanent joint. Stationary contacts are divided into nonseparable or

all-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 contact

with 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-resistant

working 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 or

using intermediate parts, specifically, clamps. These contacts may be assembled or disassembled

without damaging the joint integrity. The simplest case of a clamped contact is the joint of two

massive 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 are

used for joining wire conductors in transmission lines.

The nature of clamped and all-metal contacts is different. This is because in the all-metal

contacts 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

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Electric contacts

Stationary Moving

Sliding Commutating

Binding Brush Slider Trolley Separable Relay BreakingSolderedweldedbonded

Current-carryingbusses

Currentpickoffs ofelectrical

and weldingmachines

Rheostats,potentio-meterscode

senders

Currentpickoffs ofcranes andtransport

Plugconnectorsand circuitbreakers

Operate under conditions of friction and wear

FIGURE 1.1 Classification of electrical contacts.

Electrical Contacts: Fundamentals, Applications and Technology4

deformation. The lower the specific resistance and hardness of a material, the higher its corrosion

resistance and, consequently, the lower the contact transition resistance. For this reason, contact

surfaces are usually covered with soft, corrosion-resistant materials such as tin, silver, cadmium, or

similar materials. Different surface cleaning techniques are often used to improve the

joint connectability.

In moving contacts, at least one contact member is rigidly or elastically connected to the

moving unit of a device. Depending on their operating conditions, these contacts are divided

into two categories: commutating and sliding. Commutating contacts intermittently control the

electric 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 breaking

power, current, and voltage, there is a great variety of breaking contacts. The breaking contacts can

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

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† Light-duty contacts carry very low currents, operate at voltages up to 250 V, and display no

appreciable arc-related electrical wear. The successful operation of these devices depends

mainly 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 to

oxidize (tarnish); presence of dirt, dust or other contaminants on the contact surface; and

contact design (form, size, contact pressure, and finish). Light-duty contacts are intended

for use in instrument controls, general automation, radio and data communication, and

telecommunication systems.

† Medium-duty contacts carry appreciably higher currents (see 5A above) and operate at

voltages up to 1000 V. For this group, electrical wear is of prime importance. The factors

governing contact material selection to meet the very severe operating conditions

include tendency to welding, material transfer, and erosion (pitting). Applications of

medium-duty contacts are control devices for industrial, domestic, and distribution

network applications.


FIGU

Introduction to Electrical Contacts 5

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† 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, and

circuit breakers.

In sliding contacts, the contacting parts of the conductors slide over each other without separ-

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

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 the

current passing through the contact, the applied voltage, operating conditions, and contact

materials.4,5 The physical processes occurring in the contact zone of sliding contacts can be

classified as follows:

† Sliding contacts with a heavy electrical contact load are contacts whereby currents or

voltages are commutated, inducing mechanical, thermal or electrical effects including

sparking and arcing. These effects thus produce changes in the state (properties) of the

contact members. The necessary condition of such an operating regime is that the voltage

across disclosed contacts exceeds the minimal electric arc voltage for the materials used;

† Sliding contacts with a moderate electrical contact load are contacts where mechanical,

thermal or electrical effects, excluding sparking and arcing, change the state of the mated

surfaces. The voltage across opened contacts is between the softening voltage and the

minimal electric arc voltage for the material used;

† Sliding contacts with a low electrical contact load are contacts where no additional

physical phenomena and changes are induced in the state of the mated surfaces. In

this 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 electrical

machines, 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 transportation

machines are intended to commutate currents of a moderate and high intensity while those for

radio-electronic devices and control and automatic systems are usually low-current level contacts.

Physical effects

ThermalElectricalElectromechanical

Electroplastic effect Sparking and arcing Softening/melting ofsurface layers

Electrodynamicrepulsion

Fritting

Electrotransport

Effect of electricalfield on oxidation

RE 1.2 Possible effects on the passage of electrical current through the interface.


Electrical Contacts: Fundamentals, Applications and Technology6

For electrical machines, there are two types of sliding electrical contacts: brush-collector and

brush-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 of

electrolytic copper or copper with small additions of cadmium, silver, magnesium, zirconium, or

tellurium. Collector rings are made of copper alloys with zinc, lead, and aluminum and, in some

cases (very high peripheral velocities), of ferrous metals and their alloys. Brush materials are based

mainly on multicomponent composites of graphite, soot, copper, and coke powders.7

Sliding contacts are very important components in many devices used in automatic, teleme-

chanical, and communication equipment, such as different potentiometers serving as

electromechanical sensors.8,9 Their 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 in

their production. From a mechanical viewpoint, the operating conditions of low-current sliding

contacts are quite favorable because sliding velocities are low and loads on the contact members are

light; thus, as a rule, these devices are protected against harmful environment factors.

1.2 SUMMARY OF BASIC FEATURES

It has been established that real surfaces are not flat but comprise many asperities.1 Therefore, when

contact is made between two metals, surface asperities of the contacting members will penetrate the

natural 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–metal

contact spots will increase as a result of the rupturing of the oxide film and extrusion of metal

through the ruptures.

These spots, termed a-spots, are small cold welds providing the only conducting paths for the

transfer of electrical current. A direct consequence of this is a porous contact where infiltrating

oxygen and other corrosive gases can enter to react with the exposed metal and reduce the metallic

contact areas. This will eventually lead to disappearance of the electrical contact, although the

mechanical contact between the oxidized surfaces may still be preserved. The real contact area Ar is

only a fraction of the apparent contact area Aa, as illustrated in Figure 1.3.

Rm

2a

Conductor resistance

Apparent (nominal) contact areaReal contact areaLoad-bearing areaQuasi-metallic contact areaConducting contact area (a-spots)

Constriction resistanceDiameter of a-spot

Rc

Rm

Rc

a

F

FIGURE 1.3 Schematic of current constriction and real contact area.


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