Solid Lubricants and Surfaces

290 Pages·1964·6.318 MB·English

by  E. R. Braithwaite (Auth.)

SOLID LUBRICANTS AND
SURFACES
BY
E. R. BRAITHWAITE
PERGAMON PRESS
OXFORD · LONDON · NEW YORK · PARIS
1964
PERGAMON PRESS LTD.
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PREFACE
THIS monograph is planned as a guide to the theory and use of solid lubri-
cants, particularly in the colloidal form. I have tried to keep in mind several
types of reader: the equipment designer who knows the value of solid
lubricants for a particular job but who is probably unaware of their wider
potentialities; the works engineer who is interested in the background
science underlying solid-lubricant technology but finds that the subject
matter is too widely scattered around for easy digestion; and, in particular,
final-year undergraduates who plan to enter the heavy or chemical engineering
industries. In addition, it is hoped that metallurgists may be interested, for
I am confident that they will be making further valuable contributions to
future progress in lubrication technology.
The choice of subject matter, although deliberate, inevitably reveals my
likes and dislikes. I do not claim to cover the whole field and plead guilty
here and there to placing greater emphasis on matters with which I have
been closely connected. For example, I have devoted a considerable amount
of space to graphite and molybdenum disulphide—these are the more
widely used solid lubricants, particularly in their colloidal form. There is an
extensive literature on the laboratory examination of hundreds of solids
as potential lubricants, but I feel that space devoted to such work which
as yet is unrelated to lubrication practice would not be helpful to the
reader. I have, however, tried to include research work which seems to be
pioneering, either because of results which are interesting or in my opinion
point to future trends in the development of the subject.
The chemist makes the lubricants, but the engineer uses them and he
must have the final say as to their usefulness. However far apart they are
in their skills and studies, they have a common interest in the interfaces
at the surface of solid lubricant and metal, and from this common interest
future progress can be made. The amount of published work on solid lubri-
cants since the war is so great as to be beyond the scope of a work of this
size, and I therefore hope that this book, the first on the subject and far from
a treatise, will at least act as a rough guide until the subject has settled down
and begins to take shape. I am well aware of my ignorance of many aspects
of progress in the field of high-temperature lubricants and can only make
the excuse that this is because much of the work is classed as secret in
military and industrial practice. It is true to say that solid lubricants now
occupy a worthy place in modern technology, a state of affairs that can
Vll
Vlll PREFACE
only still further improve with the almost impossible demands of techno-
logical developments.
In trying to bring the subject up to date, I have received valuable assistance
from a number of friends who have provided full notes relating to particular
sections and I gratefully acknowledge their help as follows:
Dr. G. E. Bacon: Structure of Graphite (part of Chapter V).
Dr. W. B. Jepson: Oxidation of Metals (part of Chapter II).
Dr. J. B. Peace: Techniques of Measuring Friction and Wear (part of
Chapter IV).
Dr. G. W. Rowe: Friction and Wear (part of Chapter I).
I would like to pay special tribute to my colleague Dr. J. Hickman for
reading the typescript and to my good friend Dr. F. W. Gibbs who made
a number of suggestions and volunteered to undertake the arduous task of
reading the proofs.
The author is grateful to the many individuals and publishers who have
given permission for] the reproduction of illustrations from their papers
and books: the sources are acknowledged in the captions.
Finally, I would like to thank the directors of Acheson Industries Inc.
(U.S.A.) and Acheson Colloids Ltd. (England) for their kind permission
to publish this work. Whilst they were encouraging throughout, they in
no way influenced the opinions expressed, which are entirely my own.
E. R. BRAITHWAITE
CHAPTER I
FRICTION, WEAR AND LUBRICATION
Introduction
Lubricants are used between surfaces which are in contact and moving
relative to one another to reduce the value of the coefficient of friction or
reduce the wear of the rubbing surfaces, or both. To facilitate the discussion
of lubrication by solids, it is necessary to review the theories, past and pre-
sent, which attempt to account for the phenomena of friction and wear.
Briefly, friction is a force of resistance to the relative motion of two
contacting surfaces; wear results when this resistance is overcome by applied
forces. Before entering upon the discussion of the relevant theories, it would
be as well to emphasize that the two properties of low friction and low
wear are not necessarily interconnected, for there are several examples in the
literature of materials which, though exhibiting high coefficients of friction,
do nevertheless give extremely low rates of wear under certain conditions.
Classical Theories of Friction
The classical theory of friction considers friction as the force required
to lift asperities over one another. This definition implies that "small-scale"
friction is involved in sliding one asperity over another, otherwise there
would be no nett energy loss in lifting. Amontons [1] in 1699 appears
to have compared this lifting of asperities past each other with the raising of
a load along an inclined plane, the asperities themselves acting as rigid
bodies in order to produce this lifting effect. Coulomb in 1785 supported
in part Amontons' ideas, but thought that the asperities could bend elasti-
cally. The four empirical laws which have been resolved from the classical
theory of friction are:
1. The frictional force is directly proportional to the load.
2. The frictional force for a constant load is independent of the apparent
area of contact.
3. The frictional force is independent of the velocity of sliding.
4. The frictional force depends upon the nature of the materials in contact.
1
2 SOLID LUBRICANTS AND SURFACES
The Electrostatic Theory of Friction
This theory was proposed by Schnurmann and Warlow-Davies [2]
and is based on the fact that a contact electrical potential which is generated
by the transfer of electrons from one rubbing surface to another results
in an electron deficiency in the one and electron excess in the other. The
theory suggests that different electrical potentials accumulate and the sur-
faces are held together by the electrical attraction of opposite charges.
Friction and Molecular Forces
There is considerable evidence that friction is some type of molecular
force; three theories based on this concept have been put forward and
will now be discussed in turn.
Molecular Force Theories
(a) Molecular theory of friction (Tomlinson)—Tomlinson's theory [3] is
based on the forces of molecular attraction and repulsion. It is assumed
that when molecules come into contact and then separate from the molecular
field, there is an energy loss which is manifest in friction. In this theory,
only those atoms which support the load pass through an irreversible
stage to involve friction. There must always be a sufficient number of these
repelling atoms to support the load; therefore, the friction is proportional
to this number and is independent of the load.
(b) Friction attributed to cohesive forces {Hardy)—Sir William Hardy [4]
who spent most of his life working on problems of friction and lubrication
put forward this theory. He considered that the friction of both lubri-
cated and clean surfaces is due to true cohesion. He postulates that this
cohesive force is that force which binds together the molecules of a solid
or fluid and stated that there would be no friction if there were no seizing
and that the function of a lubricant is to prevent seizing.
(c) Welding theory (Bowderi)—The most extensive recent study of
friction is the work done by Bowden and his colleagues at Cambridge
which supports a welding theory of friction. It is summed up in a book
by Bowden and Tabor [5]. According to these workers the area of real
contact is determined by the load, and the surface asperities deform plasti-
cally until the area of contact is just sufficient for the load to be supported
elastically. At these contacts it is assumed that welding (strong adhesion)
will occur, an idea which is supported by their radioactive transfer work.
As the load is increased, these areas deform further and so increase in size,
which in effect, brings the surfaces closer together. Whilst it has been shown
that the melting temperatures can be reached in these areas of intimate
contact, the deformation process does not depend on temperature, although
FRICTION, WEAR AND LUBRICATION 3
a rise in temperature is produced by the shearing of the junctions after
sliding. This makes it easier to break down the lubricant and form stronger
welds, but is significant only at high speeds. Frictional resistance is ascribed
to the force required to shear these junctions, which may break at the
interface or at a small distance from it.
Bowden and Tabor express the friction force mathematically as:
F= S + P
= As + A'p'
where S is the shearing force,
P the ploughing force,
A the real area of contact,
s the force per unit area to shear junctions,
A' the cross-sectional area of grooved track,
p' the mean pressure per unit area required to displace the metal
in the surface.
They further assume that, since the shearing term is considerably greater
than the ploughing term, the accuracy is not seriously affected when the
coefficient of friction is written
/ =— (1)
J W K)
= 1± (2)
= ^ (3)
Pm
where p is the mean yield pressure of the metal and W is the load. They
m
then write
shear strength of softer material
yield pressure of softer material
It should be emphasized, however, that whilst the ploughing term is in-
significant under most conditions, it may be important in systems which are
adequately lubricated. Several objections to the welding theory have been
raised and some of these will now be discussed.
1. If friction is the result of welded junctions, then it is difficult to under-
stand why the resisting force in the tangential directions is so large in re-
lation to the resisting force in the normal plane. This objection to the welding
theory has been voiced by Schnurmann [6], Bickerman [7], Gemant [8],
Feng [9] and others. Bowden and Tabor answer this objection by pointing
out the general inability to apply a force normal to the surface, so that
the small junctions tend to be broken one by one with only a slight tilting
4 SOLID LUBRICANTS AND SURFACES
or sideways motion. Furthermore, Bowden and Rowe [10] have since
pointed out that when using clean metals elastic recovery can break junc-
tions without the application of a severe load, and Rowe [11] has recently
confirmed this experimentally by roll-bonding of aluminium.
2. It has been suggested that the basic origin of the energy required for
welding is obscure in the welding hypothesis, but surely the very considerable
energy from loading which is sufficient to cause plastic deformation of the
metal is also large enough to cause cohesion of atoms.
3. Feng [9] has maintained that the friction of welded junctions would
not be expected to produce loose wear particles, as two breaks would then be
required; however, loose wear particles are generally found on sliding
surfaces. The welding theory does not, however, predict loose wear. For
example, it has been shown by Golden and Rowe [12] using an autoradio-
graphic technique, that single traversals of a tungsten carbide hemisphere
over a copper or steel surface produces only bonded wear deposits. On
well-prepared copper, about 50 x 10~12 g of tungsten carbide was trans-
ferred steadily to the track during a single traversal and the carbide was
still firmly embedded in the copper even after several traversals over the
same track. Multiple traversals on mild and stainless steel tracks did, how-
ever, give loose tungsten carbide wear debris due to abrasion. Golden
and Rowe [12] emphasize the importance of distinguishing between these
two types of wear. Whilst, therefore, the welding theory of wear explains
the junction formed at the first traversal, it is recognized that subsequent
sliding is much more complicated as it involves both adhesive and abrasive
wear (see pp. 9-15).
4. Resistance to normal loading can be shown in some cases for pure
metals, but cannot be shown when the same metals have an oxide film, even
though the frictional coefficient for some cases may be shown to be near
1Ό (see Feng [13]). This is, however, hardly acceptable, as the Cambridge
workers have shown [14] that in the ultrasonic welding of aluminium, the
oxide can be torn away from the metal (Fig. 1.1 and 1.2). Bonding was
tested by actually cutting through a piece of transferred oxide without
dislodging it, using an ultrasonic chisel.
5. According to Dismant [15] rolling friction is not directly accounted
for by the welding concept, and he emphasizes that at the time of the con-
ception of the welding theory, the present theory of plastic deformation
was at an undeveloped stage and, hence, too little information was then
available about deformation processes for many of the questions to be
answered. Consequently, the additional mechanism of friction by welding
was formulated to explain these problems, especially that of stick-slip.
It is contended that there is little reason for superimposing the welding
mechanism on the deformation mechanism in order to account for the
FRICTION, WEAR AND LUBRICATION 5
observations of frictional behaviour. However, it is by no means certain
that the elastic hypothesis has any definite bearing on the welding theory.
6. One difficulty with the predictions of the welding theory for a long
time was the discrepancy between the measured values of friction and the
corresponding values calculated from equation (3). It was shown by
Tabor [16] that for an ideal plastic material the local yield pressure (p )
m
is about five times the critical shear stress (s) of the metal. Thus for such
a material /= sjP = 02.
In practice, however, most unlubricated metals in air give/ = 1-0 which
is a serious stumbling block against full acceptance of the welding theory.
More recent work by Tabor [17] has gone a long way towards clearing
up this matter. He has shown that until the shear stress reaches the critical
shear stress of the interface, junction growth occurs with contaminated
metals as with clean metals, and beyond this point further junction growth
is impossible and gross sliding occurs within the interface layer itself.
Tabor found that if the interface is only 5 per cent weaker than the bulk
metal, junction growth ceases and gross sliding occurs when/ = 1; this is in
agreement with results of other workers who have found that small amounts
of contaminant gases or vapours reduce the high friction values for clean
metals to about unity. Tabor therefore concluded that
P
where S is the critical shear stress of the contaminant layer and this is a
t
more generalized form of the Bowden and Tabor concept; Tabor recognizes
that this system of "ideal" plastic metals will not necessarily be represen-
tative of real metals, whose behaviour will be more complex owing to the
effects of work-hardening and induced brittleness, and fairly summarizes the
present state of our knowledge in the light of his new theory as follows:
Because of these difficulties and because of other simplifications introduced here in
developing this theory of metallic friction, it is evident that it must not be pushed
too far nor must it be applied in too great a detail. Nevertheless it does explain
one of the most puzzling features of metallic friction; that for rigorously clean sur-
faces the coefficient of friction tends to infinity, whereas in the presence of only
small amounts of surface contamination the coefficient of friction falls to values
of the order of unity.
A new theory of friction based on Feng's theory of wear. Feng postulates
that when a body begins to slide over another there will be an initial surface
roughness due to the difficulty of preparing a perfectly flat surface; this has
been shown by Bastow and Bowden [18], Holm [19] Bowden and Tabor [5]
and others. However, it is assumed that new roughened surfaces are
6 SOLID LUBRICANTS AND SURFACES
continuously being formed while a tangential force is applied to one or both
of the bodies. These new surfaces are considered to be formed by several
different mechanisms. As proposed by Feng [20, 21, 9] one mechanism by
which new roughness could occur might be fracture taking place in the
bulk material, below the asperities, at the depth of several atomic diameters
from the point of applied shear. The existence of a tendency for breaking
in depth is also supported by Barwell [22] and Dokos [23]. Feng's major
point that surface roughness occurs by interlocking is an hypothesis that
he terms 'plastic roughening' of the interfaces. This term evidently refers
to the step wise nature of the surface that has been plastically deformed,
and will be larger in cases of deformation by slip. Brown [24] reports
that for aluminium the slip is of the order of 2000 Ä with 200 Ä between
individual slip planes. Deformation by twinning, however, is essentially a
shearing of atomic layers over one another producing truly homogeneous
shear, and so the roughness is of the order of 2 Ä. Kink bands would prob-
ably produce a roughness of the same order of magnitude as that for
twinning. Also, included in Feng's concept of plastic roughening of the
interface, is that of the rotation of the contacting crystals towards voids
of the surface, thus producing interlocked crystals. Feng specifically notes
this for plastic deformation, with illustrations, and makes no mention of
a similar effect for elastic deformation. Cottrell [25] discusses such rotations
and similar orientation movements for both elastic and plastic deformation.
It would seem reasonable to regard rotations of crystals that are being
elastically deformed as an additional cause of interlocking and this should
be just as effective as the others.
Feng considers that free or broken particles can be re-attached under the
proper conditions of temperature and diffusion (to which should be added
orientation), though it is not considered probable that these re-attached
broken particles could be another source of surface roughness. Whether
the particles re-attach themselves or simply remain free is considered to
be entirely fortuitous. There is a similarity between Feng's ideas and the
old Coulomb postulates, though Feng's are presented in a more developed
manner, and also appear to explain the friction of a hard metal on a soft
one, for, if the hard one deformed elastically no surface roughening would
occur. Feng falls back on the welding theory but implies that heat is neces-
sary and that surface roughness produces the heat to cause welding. This
would imply low friction at low speeds, which is contrary to experience.
Another method of producing continuous surface roughness can be
derived from the "punch" effect described by Smakala and Klein [26].
In this method the colliding asperities may produce this "punch" effect
and give rise to a series of multiple mounds at various points of each crystal
for each point of contact, the number and shape of the mounds depending
on the crystal structure. With the establishment of the potentials for an