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FRICTION AND WEAR
IN
POLYMER-BASED MATERIALS
by
V . A . B E L Y , Α . I. S V I R I D Ë N O K , M . I. P E T R O K O V E T S
a n d V . G . S A V K I N
Translated from the Russian
by
P. GRANVILLE-JACKSON
PERGAMON PRESS
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FEDERAL REPUBLIC Pergamon Press GmbH, 6242 Kronberg-Taunus,
OF GERMANY Hammerweg 6, Federal Republic of Germany
Copyright © 1982 Pergamon Press Ltd.
English language Rights Reserved. No part of this
publication may be reproduced, stored in a retrieval
system or transmitted in any form or by any means:
electronic, electrostatic, magnetic tape, mechanical,
photocopying, recording or otherwise, without
permission in writing from the publishers
First edition 1982
British Library Cataloguing in Publication Data
Friction and wear in polymer-based materials.
1. Polymers and polymerization
2. Friction
3. Mechanical wear
I. Bely, V A II. Granville-Jackson, P.
621.8'9 TA45.P58 80-41825
ISBN 0-08-025444-6
In order to make this volume available as economically and
as rapidly as possible the authors' typescripts have been
reproduced in their original forms. This method unfor-
tunately has its typographical limitations but it is hoped that
they in no way distract the reader.
Translation of TRENTE i iznos materialovna osnove polimerov,
©Nauka i Tekhnika, Minsk, 1976
Printed in Great Britain by A. Wheaton & Co. Ltd., Exeter
PREFACE
In recent years, friction and wear problems have attracted increased
attention in many technically and economically advanced countries of the
world, where active national associations of scientists and tribological
engineers have been formed. Increased international co-operation has led to
intergovernmental scientific, technical and economic agreements on the
problems of friction and wear.
This is due mainly to the world energy crisis caused by intensified product-
ion and waste of resources. The world can no longer allow thousands of
millions of kilowatt-hours of energy to be dissipated to overcome harmful
friction, and millions of skilled technicians and engineers to be employed
repairing machinery. Most operational breakdowns are due to loss of
capacity in friction assemblies. Scarce metal resources can no longer be
wasted in their manufacture and repair.
As now understood, friction and wear in solid bodies are quite complex multi-
functional processes, which involve the interaction of their fine surface
layers and which are accompanied by a change in the structure and properties
of materials under the influence of load, temperature, the active ingredients
in the surrounding medium, and of many other factors.
For this reason, the most important scientific results obtained in this
field of research in recent years have been associated with attempts to
penetrate to the atomic and molecular levels of the phenomena and processes
being studied with the aid of modern physical and physico-chemical research
methods. For the modern science of friction and wear there is a natural
transition from examining processes in static positions to studying them
dynamically with direct interaction between bodies; from a wide conception
to a detailed analysis and to a more profound research programme.
We must again emphasize the predominating role played by theoretical work in
our study of the nature and mechanisms of friction and wear.
Theory determines the basic lines of research and shows us the best methods
of organizing experiments and treating the results obtained from them. In
the problem we are examining it is perfectly natural that theoretical
research frequently contains information which makes it possible to solve
ν
vi Friction and Wear
complex practical problems in the briefest periods of time.
The appearance of polymers has shown the wide opportunities which exist for
solving urgent problems, for improving the reliability and durability of
friction assemblies, and for developing a new technology.
It is particularly effective to use different combinations of polymers
together with classical materials - metals, wood, silicates, etc. But at
the same time a new set of scientific problems has arisen which is associated
with the peculiar features of the frictional behaviour of composition
materials based on polymers.
The present work appeared as a result of the wide response to the author's
first monograph, 'Friction in VolymQAS' (Moscow, Nauka, 1972). Our aim is
therefore to take into account the wishes expressed in reviews and published
both in the Soviet Union and abroad.
We have given particular attention to such important problems as the part
played by polymer structure in the frictional interaction mechanism, the
account in formulae used for calculation of the specific Theological
properties of the materials we are examining, and the attempt to find methods
of applying the results of basic research to solve the concrete practical
problems of manufacturing new materials and using them effectively in
friction assemblies. As we explain in the text, if we use data from the
literature we have preserved the units of measurement given in the originals.
We confess to having examined certain problems by way of discussion, and
the authors will be grateful to any readers who find it possible to send in
their comments and wishes. The authors hope that their work will prove
useful to specialists who are interested in the problems of using polymer
materials, and will appear as a means of stimulating deeper research into
the field of friction and wear in polymer-based materials.
C H A P T E R 1
FRICTIONAL INTERACTION DURING
METAL-POLYMER CONTACT
The nature of friction is quite complex, and for this reason attempts to
formulate a universal theory of friction as well as simplified, ill-founded
interpretations of frictional phenomena, are of no effect. The history of
friction research provides us with convincing evidence also of the limitation
of a narrow approach to describing these phenomena. As technology develops,
and as the specifications for the frictional assemblies of machines and
mechanisms have improved, our ideas of the nature of friction have evolved
from mechanical to molecular representations which affect, as we currently
understand them, both the combined physico-chemical, mechano-chemical, and
other aspects of friction research. It is clear that if we examine the
peculiar features of a friction mechanism which appear when it is tested or
put into operation in given conditions, we must take into account that
frictional properties are certain integral characteristics which embrace a
whole complex of phenomena - both mechanical, physical, physico-chemical,
and otherwise in origin, the result of the combined effect of which is far
from additive. Nevertheless, from a systematic point of view it is
convenient at the present stage to examine two basic groups of phenomena
which characterize external friction: the mechanical phenomena associated
with deformation processes and the adhesion phenomena caused by the atomic-
molecular and intermolecular interaction of the bodies in contact.
Examination of the problem of friction in solid bodies and polymers is
vastly complicated by the fact that friction research requires the applica-
tion of the methods of various natural sciences whose special and applied
divisions frequently prove insufficiently developed to solve many frictional
problems.
1. THE FRICTION MECHANISM OF SOLID BODIES AND POLYMERS
Polymer-based materials, to a greater extent than metals, are sensitive to
the influence of numerous factors caused by friction and by the effect of an
external medium. Let us note certain common aspects of the friction
mechanism of polymer materials which are associated with the basic lines of
development in the theory of friction in solid bodies.
3
4 Friction and Wear
Mechanical Aspect
The mechanical aspect is essential to determine the "boundary" conditions
within which physical and chemical frictional phenomena operate.
For bodies of simple geometric shape, solutions to contact problems are
known which are widely applied in friction theory [A-9, 149]. For example,
for the case of contact between a sphere and an elastic semi-infinite body,
an estimate for the stressed-deformed state is obtained by solving the
second basic problem of the theory of elasticity [139, 3 4 4].
If the normal and tangential loads are proportional [139], then the stresses
at points which coincide with the axes of the coordinates (the tangential
load is directed along the y axis and the ζ axis is perpendicular to the
contact region) will be:
1. Within the contact region (z = 0 ) :
τ ( - ί )
—/ — ( ι — —
' 4 \ 2
1 — 2μ J 3 / 2
[1 — (1 — * ) ] —fYl—x*
3**
| A r Z r ? + i [ 1 ( 1 i 2 ) 3 / 2 j
^? - - '
- ' f ( - f ) « "
_ 2 μ ν Ί - ί , 3 - - ^ [ 1 - ( 1 - ^ ) 3 / 2 ] y\—f ji
f V i - y *
Frictional Interaction 5
2. Beyond the contact region (z = 0)
1 — 2μ
3*2
2
x 2 — x \ Vx* — l 1
f- - 2 1 ~ — Ì arc tg—z2
2x J χ 2 / γ χ
χ
2
' 7 I V 2JC y χ \ 2 / / j f 2 - 1
1 — 2μ
2
3*
2
1 — 2μ μ Γ + ¥
h /
3|/2 " 4 ^
0
1 — 2μ
2 • arctg 2 y —
3y ]fy -l
2 —
Formulae to compute the component for the arbitrary point in the semi-
infinite body (in a case where only normal loads act) are given, for example,
in reference [l39].
Analysis of stresses Τ χ - Tt, shows that at the contour of contact there is
a plane stressed state for which the maximum of tensile stress is determined
by the equation
The maximum tangential stress is attained at a certain depth which is
reduced as the value of the coefficient of friction increases. This forms
one of the peculiar features of the stressed state during frictional
contact. Its other peculiar feature is caused by the discrete nature of the
6 Friction and Wear
actual contact, to account for which the concept of actual area of contact
is introduced. Measurements taken with the aid of an optical method have
shown that in the environs of each peak of projections there exists a field
of Hertzian contact microstresses. It has been discovered that roughness
leads to an increase (when compared with contact between smooth surfaces)
in the maximum tangential Hertzian macrostresses at great depth, and to a
concentration of stresses near the surface [174]. Unfortunately, theoretical
research in this field is not developing quickly enough [70, 181, 21*f].
In a number of cases, an important part in friction and wear processes may
be played by the formation of a bead beyond the contact region of the
indentor [lA-9, 250, 326]. The presence of such a bead has been discovered
experimentally and demonstrated theoretically [37, 178]. An attempt to
associate the undulatory strain at the polymer surface with the fact of
frictional resistance [390] is known. It should be noted that a bead of
strained material may appear even when friction forces are absent. In
actual fact, it follows from the solution to the contact problem for a
rigid cylinder and an elastic semi-infinite body that the relative displace-
ments of points at the surface of an elastic body are determined by [37j:
h = 1
2(1+P) - d - ^ " iti +
+ 4 - f\ (1 -2^arctg -L- Sign<-(1 + 2μ) τ ^ τ ] > 0>
where
τ 2G
h = w
bq0
is the maximum pressure on contact, b is the half-width of the area of
contact, μ and G are Poisson's ratio and the shear modulus respectively, ω
are the displacements of points at the surface of elasticity of the body, £
is the coefficient of friction, t = ψ/ζ2 = y/z^, z 2 is the depth of the
layer to which integration is applied.
Analysis of eq. (1) shows (Fig. 1.1) that even when i = 0, a bead of
deformed material is formed. It is also known that an analogous phenomenon
is observed for laminated systems [207].
Fig. 1.1. Curves for displacements in the surface points of an
elastic body for different values of friction
coefficient.
Frictional Interaction 7
The friction force exerts an effect on the value and shape of the bead of
strained material. In front of the moving indentor the height of this bead
increases as the coefficient of friction increases, but behind the indentor
a reduction in the profile of the surface is observed.
Polymer-based materials exhibit a series of peculiar features in the friction
process. In the first place it should be noted that the properties of the
polymers exhibit a time-temperature relationship which leads to the result
that metal-polymer contact is quite sensitive to the shear rate, to the
temperature of the surrounding medium, and to the loading period. The
problem is further complicated by the fact that the physico-mechanical
properties of polymers depend in their turn on the type of stress-strain
state and in particular on the tensor of hydrostatic compression [4, 57] .
Thus, the material in the contact zone is in conditions which differ from
the conditions in which the material exists in bulk and forms "as it were a
third body from two bodies in contact" [14-9] . It is possible that the
tenacious resistance offered by a "third" body when it is sheared is one of
the dissipation mechanisms which make a substantial contribution to the
effect of friction.
Also of interest are problems of research into the nature of strain in the
surface layers of polymers. The presence of three physical states for
amorphous thermoplastic polymers (vitreous, rubbery, and viscous flour) and
the specific conditions on frictional contact due to which displacement in
the second-order phase transitions may take place also complicate research
on metal-polymer contact. In a series of articles the elastic or visco-
elastic nature of the contact is assumed [3, 49, 358]. For viscous polymers
it has been shown experimentally that when the shear rates are not high,
elastic and forced elastic strain takes place in the surface layers and
asparities of polymers, leading at certain pressures to brittle fracture of
the surface layer of the polymer [114] . Yet the absence of experimental
work on research into the micropicture of the strain mechanism during shear
friction does not make it possible for us to give a simple answer to the
question of metal-polymer contact.
Physical Aspect
When solid bodies are brought together and subsequently relatively displaced,
atomic and molecular interactions take place which differ in their nature
and effective range. These interactions are considered responsible for
adhesion in solid bodies and in particular for the adhesion component of
friction force. Practically all modern friction theories in one form or
other are considered to be molecular interactions [15, 97, 149].
Let us examine some examples of the characteristic appearance of the forces
of molecular interaction. In a normal case, the forces of adhesion operate
not only at the points of actual contact, but also in adjacent regions where
there is no direct contact, but the distance between the surfaces of bodies
in contact does not exceed the radius of the effect of molecular attraction.
This forms the essence of so-called extra-contact adhesion, the concept of
which was first introduced in 1961 [272]. It is considered that extra-
contact adhesion is the result of the action of van der Waals forces, which
when there is a clearance between the uneven parts of surfaces in contact
2
of between 50 and 100 A exert a force in the order of 1 kg/cm [388]. The
result of this additional force, as experiments with a rubber sphere and a
rigid smooth surface have shown, is that the area of contact for small loads
is always more than the area predicted by Hertzian theory [348] .
