Table Of ContentCOMPOSITE MATERIALS SERIES
Series Editor: R. Byron Pipes, Center for Composite Materials, University of Delaware,
Newark, Delaware, U.S.A.
Vol. 1 Friction and Wear of Polymer Composites (K. Friedrich, Editor)
Vol. 2 Fibre Reinforcements for Composite Materials (A.R. Bunsell, Editor)
Vol. 3 Textile Structural Composites (T.W. Chou and F.K. Ko, Editors)
Vol. 4 Fatigue of Composite Materials (K.L. Reifsnider, Editor)
Vol. 5 Interlaminar Response of Composite Materials (N.J. Pagano, Editor)
Vol. 6 Application of Fracture Mechanics to Composite Materials (K. Friedrich, Editor)
Vol. 7 Thermoplastic Composite Materials (L.A. Carlsson, Editor)
Vol. 8 Advances in Composite Tribology (K. Friedrich, Editor)
Vol. 9 Damage Mechanics of Composite Materials (R. Talreja, Editor)
Vol. 10 Flow and Rheology in Polymer Composites Manufacturing (S.G. Advani, Editor)
Vol. 11 Metal and Ceramic Based Composites (S.T. Mileiko, Editor)
Vol. 12 Sheet Forming (D. Bhattacharyya, Editor)
The background photo on the book cover is taken from a fracture surface of a carbon
fiber/polyetheretherketone (PEEK) matrix composite (material APC2, from ICI, Wilton,
U.K.).
Fracture was performed with DCB specimens at room temperature, under mode I condi
7 1
tions and a crack opening velocity of ν = 10"" m s" . The corresponding fracture energy
2
measured was G\ = 1.5 kJ m~ (courtesy T. Smiley, R.B. Pipes, Center for Composite
c
Materials, University of Delaware, Newark, Del. 19716, U.S.A., September 1985).
Composite Materials Series,!
FRICTION
AND WEAR
OF POLYMER
COMPOSITES
edited by
Klaus Friedrich
Technical University Hamburg-Harburg, Hamburg, F.R.G.
ELSEVIER
Amsterdam - Lausanne - New York - Oxford - Shannon - Tokyo 1986
ELSEVIER SCIENCE B.V.
Sara Burgerhartstraat 25
P.O. Box 211, 1000 AE Amsterdam, The Netherlands
First printing: 1986
Second edition: 1996
Library of Congress Cataloging-in-Publication Data
Friction and wear of polymer composites.
(Composite materials series ; l)
Bibliography: p.
Includes indexes.
1. Polymeric composities. 2. Friction.
I. Friedrich, Klaus, 19^5- . II. Series.
TAU18.9.C6F75 1986 620.1'92 66-2059
ISBN 0-UUU-U252U-1
ISBN 0-444-42524-1 (VoL 1)
ISBN 0-444-425 25-X (Series)
© 1986 Elsevier Science B.V. All rights reserved.
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ν
PREFACE
Over recent decades, the field of tribology, i.e. the study of friction, lubrication
and wear of materials, has received increasing attention from the scientific, technical
and practical points of view. It has become evident that the operation of many
mechanical systems depends on friction and wear values. In addition, the annual
dissipation of energy and waste of valuable raw materials resulting from high
friction and wear are of great economical significance. Correspondingly, potential
savings can be expected from improved tribological knowledge. In fact, in recent
years, a variety of books have appeared which cover fundamental principles as well
as application-related aspects of friction, lubrication and wear of materials [1-5].
Polymers and polymer composites are being used increasingly often as engineer
ing materials for technical applications in which tribological properties are of
considerable importance. In addition to the traditional fields (i.e. the use of
reinforced rubbers for tires and transmission belts or of filled thermosetting resins
as brake materials) polymers and polymer composites are well established in the
field of bearings and slider materials. They are encountered in many different
industries, e.g. automotive, aerospace, mining or biomedical. In all of these exam
ples, a fundamental understanding of the interactions between the polymer-based
components and the possible counterfaces can further enhance their potential
application.
In an attempt to meet this objective, several books have already dealt with the
subject of polymer tribology [6-9]. However, more work needs to be done to reveal
the physical and chemical nature of their tribological characteristics and to generate
reliable data for design. This is particularly true for high-performance, polymer
composites, a very young group of materials specifically formulated for advanced
applications in the future. Although several hundred publications have appeared,
mostly concerned with the phenomenology of friction and wear of composites, and
although some progress has been made in developing composites systematically for
special friction and wear applications, there are many important questions still
unanswered today.
In such a situation, it seems appropriate to try to establish a frame of reference. I
believe that this can best be done not only by presenting the crucial experimental
results and the conclusions drawn from them, but also by displaying the new
analytical and experimental tools being used. Obviously, such a wide-ranging task
cannot be accomplished by one or two authors.
However, I have been privileged to collaborate with a distinguished group of
researchers who are playing a prominent part in the development and application of
new techniques and fields of activity. Thus, it is hoped that this volume will not only
provide a useful summary of current knowledge on the friction and wear properties
of polymer composites, but also promote further studies in this important field.
vi
Special consideration is given, first, to the fundamental aspects of tribology in
general and of polymer composites in particular; second, to the effects of the
micro-structure of composites on their friction and wear behavior under different
external loading conditions; and third, to the problem of the control of friction and
wear behavior in practical situations. Although polymer composites associated with
bearing-type applications dominate the contributions, part of the volume is also
devoted to the friction and wear of metal-based composites and rubber compounds.
In an introductory chapter, H. Czichos presents definitions and terminology used
in friction and wear research and deals with general aspects of friction and wear
mechanisms. In addition, he explains why friction and wear must be considered as
system properties. The fundamental principles of interfacial friction of polymers
and polymer composites are discussed by B.J. Briscoe. Special attention is focussed
on the individual components of the frictional coefficient under abrasive and
non-abrasive dry-sliding conditions, on other wear models, and on friction in
lubricated systems. A concept to describe friction and wear properties of materials
with heterogeneous microstructures in terms of tribological tensors, introduced by
E. Hornbogen, can be considered as a new approach to describing the tribological
behavior of anisotropic materials more systematically. It will help towards a better
understanding of the effects of individual components and their geometrical
arrangement in a composite on its tribological properties, thus leading to basic rules
for systematic material development.
The second part of this volume is reserved for a discussion of tribological
properties of polymeric and composite materials tested under different conditions.
J.M. Thorp analyzes the tribological properties of selected polymer matrices with a
special tri-pin-on-disc tribometer and against abrasive counterparts. His wear tests
largely endorse manufacturers' recommendations regarding applications for the test
materials. The friction and wear properties of PTFE, well known as a potential
candidate for polymer bearing application, are surveyed by K. Tanaka. In particu
lar, the author outlines the wear-reducing action of fillers in PTFE-based com
posites and the effects of water lubrication. A special chapter on friction and wear
of metal matrix/graphite fiber composites is presented by Z. Eliezer. Typical wear
mechanisms as a function of sliding direction are discussed, along with some
potential tribological applications of this type of material.
T. Tsukizoe and N. Ohmae give an overview of the performance of unidirection-
ally oriented glass, carbon, aramid, and steel-fiber-reinforced plastics sliding against
smooth steel counterparts. The authors propose a law-of-mixture approach for
calculating the composites' coefficient of friction and they have developed a model
wear equation for fiber-reinforced plastics. In a contribution by K. Friedrich,
differences in wear behavior of short fiber-reinforced thermoplastics and continu
ous-fiber woven-fabric-reinforced thermosets against various abrasive counterparts
(steel, severe abrasive paper) are discussed as a function of fiber volume fraction.
Methods are suggested for correlating the wear rates of materials obtained under
different conditions with the properties of the sliding partners and the conditions in
the contact area. The mild wear processes of rubber-based compounds are eluci-
vii
ο ο
dated by J.A. Schweitz and L. Ahman. The authors concentrate on micro-mechani
cal detachment events with particular reference to modified surface layers, ageing,
fatigue, crack formation and growth.
In a third section, specific friction and wear problems encountered in various
practical applications of polymer composites are pointed out. In discussing the
subject of commercially available polymer bearing composites, J.C. Anderson
subdivides candidate materials into several categories. The classification considers
the structural composition and appearance of the materials, their performance
attributes, and other characteristics, and contributes towards the optimum selection
of polymer composites for particular bearing applications. J.K. Lancaster, a pioneer
in the study of friction and wear of polymer composites, reviews in detail the
influences of operating parameters on the friction and wear properties and the
load-carrying capacity of polymer composites for aerospace dry bearing applica
tions. He also outlines some principles of materials development and life prediction
for this particular application and discusses future trends in materials for airframe
bearings. In a final highlight, M.N. Gardos reviews typical compositions of self-
lubricating composites used for extreme environmental conditions. It was shown
that the most important and decisive factor in composite versus counterface
performance is the effectiveness of a preferentially accumulated, low shear strength
surface layer on the worn composite.
For whom is this volume intended? Undoubtedly, it will be of interest to three
groups of specialists: (1) those who are active or intend to become active in research
on some aspects of polymer composite tribology (material scientists, physical
chemists, mechanical engineers); (2) those who have encountered a practical friction
or wear problem and wish to learn more methods of solving such problems
(designers, engineers and technologists in industries dealing with selection, re
processing and application of polymer engineering materials); (3) teachers and
students at universities, in view of the fact that, today, the average mechanical
engineer receives less than two hours of instruction on wear during his university
studies.
Finally, many thanks are due to Professor R.B. Pipes, editor-in-chief of the new
Elsevier composite materials series, for the invitation to organize this particular
volume. In addition, my sincere thanks go to the chapter authors, who so kindly
agreed to collaborate on this book, for their hard and efficient work.
January 1986 K. Friedrich
References
1 F.P. Bowden and D. Tabor, Friction and Lubrication of Solids, Claredon Press, Oxford, 1964.
2 D.S. Moore, Principles and Applications of Tribology, Pergamon Press, Oxford, 1975.
3 H. Czichos, Tribology, Elsevier, Amsterdam, 1978.
4 K.H. Habig, Verschleiss und Harte von Werkstoffen, Hanser Verlag, Munchen, 1980.
viii
5 D.A. Glaeser, Fundamentals of Friction and Wear of Materials, American Society for Metals, Metals
Park, OH, 1981.
6 L.H. Lee, Advances in Polymer Friction and Wear, Plenum Press, New York, 1974.
7 D. Dowson, M. Godet and CM. Taylor, The Wear of Non-Metallic Materials, Mechanical Engineer
ing Publications, London, 1978.
8 G.M. Bartenev and V.V. Lavrentev, Friction and Wear of Polymers, Elsevier, Amsterdam, 1981.
9 H. Uetz and J. Wiedemeyeir, Tribologie der Polymere, Carl Hanser Verlag, Munich, Vienna, 1985.
1
Chapter 1
Introduction to Friction and Wear
HORST CZICHOS
Bundesanstalt fur Materialpr'ufung (BAM), Berlin - Dahlem (F.R.G.)
Contents
Abstract, 1
1. Friction and wear: sub-areas of tribology, 2
2. Terminology, 2
3. Mechanisms of friction, 3
3.1. General considerations, 3
3.2. Real area of contact, 5
3.3. Adhesion component of friction, 6
3.4. Ploughing component of friction, 7
3.5. Deformation component of friction, 8
3.6. The complexity of friction processes and the mechanisms of energy dissipation, 10
4. Mechanisms of wear, 11
4.1. General considerations, 11
4.2. Surface fatigue and delamination wear mechanisms, 12
4.3. Abrasive wear mechanisms, 13
4.4. Tribochemical wear mechanisms, 15
4.5. Adhesive wear mechanisms, 15
4.6. Classification of wear phenomena, 17
5. Friction and wear as system properties, 19
List of symbols, 21
References, 22
Abstract
In the introductory section, a brief general description of the fundamentals of
friction and wear is given.
The chapter opens with the definitions of basic terms concerning friction, wear,
and lubrication. The section on the mechanisms of friction includes the following
topics: real area of contact, adhesion component of friction, ploughing component
of friction, and deformation component of friction. The section on the mechanisms
of friction is concluded with a consideration on the complexity of friction processes
and the mechanisms of energy dissipation.
The section on the mechanisms of wear includes the following topics: surface
fatigue and delamination wear mechanisms, abrasive wear mechanisms, tribochemi-
2
cal wear mechanisms, and adhesive wear mechanisms. The section on the mecha
nisms of wear is concluded with a classification of wear phenomena.
Finally, it is pointed out that friction and wear are not intrinsic material
properties but depend on so many influencing factors that, in any given situation,
the "whole tribological system" must be considered, including the following basic
groups of parameters: (i) the "structure" of the tribological system, i.e. the material
components of the system and the tribologically relevant properties of the system's
components; (ii) the operating variables, including load (or stress), kinematics,
temperature, operating duration, etc.; and (iii) the tribological interactions between
the systems components. To assist the systematic analysis of relevant tribological
parameters in a given friction and wear situation, a methodology and data sheet are
outlined.
1. Friction and wear: sub-areas of tribology
Friction and wear are nowadays considered as sub-areas of tribology. The term
tribology was coined by a British committee in 1966 from the word tribos, which
means rubbing in classic Greek [1]. Tribology is defined as the science and
technology of interacting surfaces in relative motion and of related subjects and
practices. Since its definition, tribology has been widely recognized as a general
concept embracing all aspects of the transmission and dissipation of energy and
materials in mechanical equipment including the various aspects of friction, wear,
lubrication and related fields of science and technology.
In this chapter, a brief general introduction to the basic principles of friction and
wear is given. The discussion concentrates on fundamental tribological mechanisms;
the friction and wear behaviour of specific materials is considered in the following
chapters.
2. terminology
Owing to the multidisciplinary nature of tribology, the terms used in this field
have many origins. Thus, some introductory remarks on terminology should be
made in order to avoid misleading or ambiguous terms. In 1969, the International
Research Group on Wear of Engineering Materials, working under the auspices of
OECD, compiled a glossary of terms and definitions for the field of tribology. This
glossary has been recognized as a key word index and is included in the new ASME
Wear Control Handbook published in 1980 [2]. It contains definitions of 500 general
tribological terms, 93 terms on bearing types, and 58 terms on oils and greases. In
addition, it contains an eight-language index consisting of translations of the
tribological terms into English, French, German, Italian, Spanish, Japanese (Romaji
and Kanji), Arabic, and Portuguese.
With this glossary on terms and definitions, the basis for a multilingual terminol
ogy for the field of tribology has been laid. Because it is not possible to repeat details
3
of the glossary within the frame of this chapter, only the definitions of the three
most important tribological terms will be cited.
Friction: The resisting force tangential to the common boundary between two
bodies when, under the action of an external force, one body moves, or tends to
move, relative to the surface of the other.
Wear: The progressive loss of substance from the operating surface of a body
occurring as a result of relative motion at the surface.
Lubrication: The reduction of frictional resistance and wear or other forms of
surface deterioration between two load-bearing surfaces by the application of a
lubricant.
Because of the great variety of types, modes, or processes of friction, lubrication
and wear which determine the behaviour of any tribological entity, it is necessary to
have a sub-classification of these basic terms. For these sub-classifications, different
criteria such as
(a) kinematics or type of motion (sliding, rolling, impact, etc.),
(b) type of material (solid, fluid, metal, polymer, etc.), and
(c) type of interfacial tribological process (hydrodynamics, adhesion, abrasion,
etc.)
are used.
In this chapter, the discussion is restricted to friction and wear of unlubricated
solids in sliding motion.
3. Mechanisms of friction
3.1. General considerations
If two bodies are placed in contact under a normal load, F , a finite force is
N
required to initiate or maintain sliding; this is the force of friction, F . In this
F
situation, the following phenomenological rules have been observed experimentally.
(i) When relative motion between the contacting bodies occurs, the friction force,
F , always acts in a direction opposite to that of the relative velocity of the surfaces.
F
(ii) The friction force, F , is proportional to the normal force, F
F N
Through this relationship, it is possible to define a coefficient of friction
(iii) The friction force is independent of the apparent geometric area of contact.
These relations, known as "Amontons-Coulomb laws" of dry sliding friction are
used as simple guiding rules in tribological applications.
Despite the considerable amount of work that has been devoted to the study of
friction since the early investigations of Leonardo da Vinci, Amontons, Coulomb
