Experimental Methods in Tribology

708 Pages·2004·49.323 MB·1-354\708

by  Gwidon W. Stachowiak, Andrew W. Batchelor and Grazyna B. Stachowiak (Eds.)

PREFACE
Upon completion of our first book, Engineering Tribology, it became evident that
many important topics in tribology still remained to be presented and discussed.
So a sequel to Engineering Tribology was immediately initiated to address this
problem. Several different subject areas appeared suitable for a new book and the
question was to choose the most important topic.
After being engaged over the years in several test programs dedicated to study
various forms of wear and lubrication, the complexity of tribological
experimentation became increasingly evident. When introducing students to
tribological testing, it became obvious that hurried sketches and oral descriptions
of experimental methods to students were inadequate. A book containing
carefully prepared diagrams and exact descriptions of the experimental problems
in tribology was required. There are manv problems facing a student or researcher
who has to perform tribological experiments for the first time. For example, what
test rigs to use? How to prepare the samples for the experiments? How to assess
the wear mechanism occurring? What type of experimental data need to be
collected, and more importantly, what to do with it? How to check the validity of
the data and whether any true information could be obtained? For instance, in
one laboratory, mysterious lubricating effects from a highly purified mineral oil
were claimed. The reasons for this strange result were never fully investigated
but it was widely suspected that airborne sulphur from vehicle exhaust fumes
contaminated the oil. This perhaps trivial example shows how essential is a
thorough understanding of tribology and its experimental methods.
This book, Experimental Methods in Tribology, is intended to provide a basic
guide for young or newly inducted researchers. The subjects covered range from
the basic technology of experimental tribometers, to the theory of friction and
wear testing. Much of the information on tribological experimentation is gained
by experience and is not usually discussed fully in research papers or textbooks.
Careful application of the principles outlined in this book should ensure valid
test data and accelerate progress in tribology. If some doubts over the tribological
experimentation still remain please let us know so that the book can be updated.
Gwidon W. Stachowiak, Andrew W. Batchelor and Grazyna .B Stachowiak
ACKNOWLEDGEMENTS
Any book depends on the efforts of many different people and this book is no
exception. We would like to thank Professor Duncan Dowson for his great
personal input, enthusiasm, encouragement and meticulous checking of the
manuscript and very many constructive comments and remarks. Thanks are to
Professor Irwin Singer for useful discussions on the effects of tilt angle on the
apparent friction force, A/Professor Brett Kirk for Figure 8.2, Dr Simon
Graindorge for Figure 8.9, Dr Ksenija Topolevec-Miklozic for Figure 8.21 and Dr
Pawel Podsiadlo for help with dimensional analysis, images and finding useful
references. Special thanks are to Dr. Nathan W. Scott for the preparation of the
illustrations for the book. Without Nathan's illustrations the book would not be
the same. We would also like to thank Professor Nic Spencer for useful
comments and providing nice environment at the Swiss Federal Institute of
Technology during the final checking of the manuscript. Finally we would like to
thank the School of Mechanical Engineering, University of Western Australia
and School of Engineering, Monash University Malaysia, for their help and
unfailing support during the preparation of the manuscript.
We would also like to thank the following publishers for granting us permission
to reproduce the figures listed below:
Figure 8.21: Austrian Society of Tribologists. From the Proceedings of the 2nd
World Tribology Congress, 3-7 September 2001, Vienna, Austria, (editors: .F
Frank, W.J. Bartz, A. Pauschitz), 2001, pp. .681-971
Figures 10.22 and 10.23: Japanese Society of Tribologists. From the Proceedings of
the International Tribology Conference in Nagasaki, 2000, Japanese Society of
Tribologists, Vol. ,1 2001, pp. 251-256.
INTRODUCTION
1.1 HISTORICAL ORIGINS OF EXPERIMENTATION IN TRIBOLOGY
Long before the initiation of historical records, palaeolithic people experimented
with a basic frictional phenomenon, the temperature rise in sliding contact.
Sticks, stones and any other available materials were rubbed together to create
sparks, sound and debris [1]. The palaeolithic people then realized that the sparks
generated might create that invaluable commodity, fire, which gave heat, light
and protection from wild animals. Elementary methods of generating fire by
banging two stones together were superseded by more advanced methods
involving sliding surfaces which were probably developed by trial and error. The
importance of high sliding speed was recognized by rotating the round end of a
stick against another piece of wood, in order to create fire. To raise the sliding
speed and frictional heat generated at the end of the stick still further, the string
of a bow was threaded around the stick and the bow was pulled backwards and
forwards to rotate the stick faster. Small splinters of wood or dried leaves were
then placed around the rotating end of the stick. The leaves or wood fragments
were known to catch fire when touching the hot surfaces of the rotating contact.
When the wood fragments or leaves had started to burn, more wood or leaves
were added to build up a fire of useful size. Fire ignition by a rotating stick is
illustrated in Figure 1.1.
The process of deduction that led to the rotating stick method to start a fire is
unknown. Perhaps some adults observed children playing with sticks or perhaps
they noted that hands warm up when rubbed together and then made a few tests
of their own.
The pre-Christian history contains many other examples of the applied study of
friction and wear, for example, the development of a wheel or use of sledges and
lubricants to move large masses for the construction of pyramids at Giza [1,2].
Although the development of a wheel is often heralded as the greatest invention
2 LATNEMIREPXE SDOHTEM NI YGOLOBIRT
in the human history it seems that the development of an axle with a set of
bearings was a far greater and more practical invention. Contrary to the early
progress of practical applications the scientific study of friction and wear in terms
of abstract concepts and theories is far more recent. The earliest systematic studies
of the laws of friction are ascribed to Leonardo da Vinci in the 16th century [2].
Starting with a notion of force represented by weight and a test apparatus
consisting of a smooth surface, some weights, cord and a pulley, da Vinci
observed and recorded the amount of weight hanging by a cord that was required
to move a block placed on the smooth surface. This experiment led to the
deduction of the proportionality between friction force and weight of the block
and the independence of friction force on contact area. These experiments were
refined later by Amonton and Coulomb. It is important to realize that these early
observations are merely the approximations to the modern view of frictional
mechanics and that many complex phenomena bet3veen two interacting surfaces
would have occurred in this apparently simple experiment. Similar experiments
performed more recently have revealed the presence of micro-sliding when a
frictional force insufficient to cause gross sliding is applied .]31 The basic principles
of tribological experimentation are, however, best illustrated by da Vinci's
experiments where a usefully simplified model of complex phenomena was
obtained.
Figure 1.1 Palaeolithic method of igniting a fire by using a rotating stick as an
early example of tribological experimentation.
Tribology, like any other field of science, provides the researcher with only a
limited number of concepts or models that can be tested with available
experimental techniques. In friction and wear experiments the observed friction
and wear phenomena are usually interpreted in terms of these concepts even
though an incomplete interpretation results. Until now, most of the
experimental studies were conducted on a macro scale aiming at explaining the
tribological phenomena related to everyday engineering problems. Although
satisfactory solutions were often found, the theoretical explanation provided was
1retpahC NOITCUDORTNI 3
often wrong or not complete. Technological improvements to the experimental
techniques have subsequently led to the gradual modification or changes to the
fundamental theories of friction and wear.
In recent years there was a great leap from macro to nano scale observation
resulting in the development of a new field of nanotribology [4]. The new
technologies developed allow the study of friction and wear processes at the
fundamental level involving a single molecular contact interface. The techniques
include the surface force apparatus [5], the atomic force microscope (AFM) [6,7],
the scanning tunnelling microscope [8,9], the quartz-crystal microbalance [10] and
nonlinear optical techniques [11]. Among these new techniques the AFM is the
most widely used. The operating principle of the AFM involves the traversal of a
microscopic stylus across the surface of a specimen. The tip of the stylus is so fine
that it can contact individual atoms on the specimen surface where quantum
effects between the stylus and the specimen enable information about the nature
of the surface atoms to be obtained. By measuring the forces on the stylus it is also
possible to measure frictional forces on an atomic scale. Localised force
measurements of this nature have revealed that frictional forces vary
considerably for even very small movements of the stylus and that most surfaces
have a heterogeneous frictional characteristic. As a consequence of new
discoveries in nanotribology classical tribological laws of friction are being
modified or re-written as they are found not to be applicable on the atomic scale
.]01[
Recent developments in computer technology allow to confirm and explain
many experimental observations and also to advance theoretical calculations of
complex phenomena occurring within the tribological interface. One such an area
where computer modelling is widely employed is molecular-dynamics (MD)
simulation [12].
ecnacifingiS of lacigolobirT noitatnemirepxE
In the past, progress in tribology has usually been preceded by some experiment
or experimental observation. One reason for the slower development of
theoretical concepts in tribology is its close relationship to technology, which uses
a more empirical methodology than science [2]. In this sense tribology is different
from, for example, fundamental physics where theoretical predictions are often
stated long before their experimental confirmation. For example, the electro-weak
theory discovered in the 1970s is still used today to search for the Higg's boson
.]41,31[
A classical example of the experimentally based process of discovery in tribology
is provided by the first observations of a hydrodynamic pressure field operating
in a lubricated bearing. The original axle bearings for railway carriages were fitted
with numerous oil holes in an effort to supply as much oil as possible to the
bearing. During the operation, however, these bearings often became very hot
due to excessive friction. As a result some of them caught fire or else ceased to
rotate causing the wheel to develop a flat spot on its rolling surface. Both these
phenomena were rather undesirable and much research effort was directed
towards rectifying this problem. There was no shortage of ideas for an improved
4 LATNEMIREPXE SDOHTEM NI YGOLOBIRT
bearing but little progress was made in finding a real solution until the engineer,
Beauchamp Tower, performed a detailed study of the friction in these bearings. A
test bearing was constructed and a study of friction as a function of the lubrication
condition was competently performed. The bearing was meticulously constructed
with oil holes to feed the vital lubricant to the sliding surfaces. The problem of
high friction in these bearings was, however, not solved even though all the
experiments were carefully executed. As sometimes happens in research, chance
intervened with a lucky discovery. One day the technician, frustrated with the
mess the leaking bearings were making, first plugged the persistently leaking oil
holes with rags and then with bungs but this failed to stop the leakage. Tower
then realised that the oil in the bearing must be under a considerable pressure.
When pressure gauges were fitted around the bearing, it was found that the oil
pressure was capable to support the bearing load [15], i.e. the pressure generated
was sufficient to separate the axle from its bearing as schematically illustrated in
Figure 2.1 [16].
Figure 2.1 Accidental discovery of lubrication mechanism of axle bearings
(adapted from [16]).
The publication of Tower's results led in consequence to the development of the
hydrodynamic theory of lubrication by Osborne Reynolds [17]. Although the
concept of a film of lubricant separating two sliding surfaces was not new (it had
been earlier proposed by Leupold in 1735, Leslie in 1804, Rennie in 1829, Adams
in 1853 and Hirn in 1854 [2]), it was virtually impossible to resolve the
mechanism of lubrication until Tower's precise measurements were applied by
Chapter 1 NOITCUDORTNI 5
Reynolds to a detailed theoretical analysis. A combination of Tower's and
Reynolds efforts replaced innumerable empirical ideas on railway axle bearing
lubrication and effectively solved the problem. It was realized only later that
Tower's measurements did not include other fundamental bearing parameters,
such as operating temperatures and elastic deformation of the bearing under load
[18]. Tower also performed an extensive series oL relatively unknown,
experiments on thrust bearings with parallel surfaces. For some reason during
the experiments conducted no pressures generated were measured [19]. Why were
these pressures not measured? Nobody knows. Perhaps Tower, being a proud
engineer, feared that his carefully prepared parallel surfaces might be deemed by
his academic colleagues to be converging [19]. A precise model of the lubrication
mechanism of ostensibly parallel surface bearings was only achieved in 1918,
some thirteen years after Tower's death, by Lord Rayleigh [19,20].
A similar process of experimental observation forcing the development of basic
tribological concepts is illustrated by the discovery of elastohydrodynamic
lubrication. It was known for many years that a viable oil film existed between
lubricated gear teeth. However, there appeared to be a contradiction between the
oil film thickness predicted by classical hydrodynamic theory and the oil film
thickness actually required for effective lubrication of gear teeth [21]. For speed
and contact conditions operating in gears the predicted hydrodynamic films were
so thin that it was inconceivable for the contacting surfaces to be separated by a
viscous liquid film. In fact, the film thicknesses calculated suggested that the
surfaces were lubricated by films only one molecule in thickness, clearly far too
thin to be effective. The entire problem acquired an aura of mystery and
remained unsolved for some time. There was an urgent need for a new theory
explaining the lubrication mechanism operating under these conditions and
many elaborate experiments and theories were developed as a result. The
mysterious lubrication mechanism was solved by Ertel and Grubin [22], who
provided a quantitative model of gear teeth lubrication in terms of a theory that
is now known as elastohydrodynamic lubrication. Ertel and Grubin were
however unable to fully describe the nature of the elastohydrodynamic film and
the remarkable nature of the elastohydrodynamic film remained poorly
understood until the observations of Cameron and Gohar [23,24]. The
elastohydrodynamic film was rendered visible by an optical interference pattern
where prismatic colours indicated the thickness of oil trapped between a rolling
ball and glass disc. The experimental measurements were later analysed by
Dowson and Higginson who were able to reproduce the elastohydrodynamic film
as a numerical model [25].
Further historical review confirms the converse argument that not only the
utility of tribological experiments rest on adequate modelling but also that
scientific models of friction and wear depend on detailed experimental
confirmation. In the 18th century, Desaguliers hypothesized that friction and
wear between clean surfaces depended on the mutual adhesion of the contacting
solids [26]. However, it was not until the middle of the 20th century that the
experiments of Bowden and Tabor [27] into adhesion and friction between clean
metals enabled this theory to advance understanding of friction and wear. Despite
its initial universal acceptance, the theory linking friction to adhesive bonding
6 LATNEMIREPXE SDOHTEM NI YGOLOBIRT
between contacting surfaces and wear has since been modified in the light of new
experimental evidence conducted at the atomic level [10].
In 1804, Leslie provided the first model of the friction of contaminated surfaces
where waves of deformed material are pushed across the surface by asperities
from the opposing surface [28,29]. This theory remained obscure until
experimental confirmation of deformed material surface waves formation was
provided by Challen, McLean and Oxley [30] in 1984. Slow-moving slabs of
material that resembled the model waves generated by Challen and co-workers
were recently observed by X-ray microscopy of a sliding contact [31]. It was found
that the formulation of models of wear is greatly facilitated by observational
studies, in a manner directly comparable to the experiments of Tower, Cameron
and Gohar described above.
2.1 COMPLEXITY OF TRIBOLOGICAL PHENOMENA
The reason for interdependence between experiment and theory in tribology may
lie in the fact that friction and wear are essentially chaotic processes [32,33]. While
it might be possible to model the deformation of two contacting asperities from
basic mechanical principles, the chaotic nature of tribological processes prevents
extension of this model to predict wear and friction on a macroscale. Until the
theory of complex systems is fully established [33] it is necessary to apply a
phenomenological approach to the study of tribology. Phenomenological
approach means that friction and wear processes are described in terms of a set of
specific phenomena which are systematically analyzed to provide an engineering
model of friction, wear and lubrication.
When reviewing the history of tribology it becomes apparent that the
development process of an effective means of friction or wear control depends on
the following steps. First, an unequivocal experimental observation of the
underlying phenomena is required. Data from experimental observation then
allows the formulation of an appropriate theoretical model. Thirdly and finally,
refinement of the theory, which involves further experimentation, leads to the
specification of effective methods of controlling friction or wear. This process is
illustrated schematically in Figure 1.3.
Most tribological phenomena, e.g. friction, wear, frictional heating and
triboemission of electrons, are not intrinsic material properties but depend on a
complex balance between many competing factors. The simplest example of the
complexity of tribological problems is provided by the hydrodynamic bearing,
where it is necessary to consider viscous heating of the lubricant, cavitation and
turbulent lubricant flow as well as elastic deformation of the bearing structure
before an accurate value of load capacity can be calculated. This basic characteristic
of all tribological phenomena imposes two restrictive conditions on most
analyses or investigations:
(cid:12)9 our limited capacity to analyse friction, wear and related phenomena,
and
a limited program of experimental tests during which the variability of
friction and wear can easily be neglected.
Chapter 1 NOITCUDORTNI 7
The latter condition is less readily appreciated than the former. Even a brief
survey of existing knowledge reveals the need for experimentation in tribology
but it is often assumed, without justification, that experimental results will
provide the required answers. The following example illustrates the problem
associated with the validity of results obtained from the tribological tests.
Lubrication engineers are frequently asked to evaluate effectiveness of a lubricant
and in many cases this means finding the lowest wear rate from a range of
available lubricants.
I Observation ro tnemerusaem e.g. Tower's observation q[oil
( fo gnillortnoc sretemarap pressun' in bearing a
":-; .<-"
/
II Formulation fo ,yroeht e.g. Reynolds' use ol:Tower's pressure [
data ot establish the hydrodynanlic
theou! of lubrication
,~,-7
III tnempoleveD fo lortnoc e.g. Design of hydrodynamic bearings
dohtem rof noitcirf dna wear
Figure 1.3 Experimental development of applied tribology.
In one investigation, the wear rate of steel discs lubricated by oil with and
without lubricant additives was measured. The experiment involved two steel
discs loaded against each other and rotated with one of the discs being constrained
to produce combined sliding and rolling at the contact. This apparatus is often
referred to as the 'twin disc' or 'two disc' test apparatus. An example of the
measured wear rate on the discs versus sliding speed in the contact is shown in
Figure 1.4 for an oil without additives and an oil containing dibenzyl disulphide
(a mild E.P. additive) [34].
Without discussing the details of possible lubrication and wear mechanisms
involved in this experiment, it can be seen from Figure 1.4 that at low sliding
speeds less than 0.17 [m/s], the common additive dibenzyl disulphide (DBDS) is
effective in reducing wear rate compared to plain oil. However, the same additive
accelerates wear compared to plain oil above a sliding speed of 0.2 [m/s]. If the
effect of sliding speed was not investigated and instead a sliding speed arbitrarily
chosen during the lubricant testing, then it would be possible that the result
obtained would be entirely misleading as to the merits of various additive-
enriched lubricating oils. In most cases, there is no uniquely superior lubricant or
wear-resistant material. The optimum combination of a lubricant and material
will vary according to the characteristics of the tribosystem. Apart from material
properties, parameters such as load, contact stress, temperature, sliding speed and
8 EXPERIMENTAL METHODS NI TRIBOLOGY
environment (see the effects of replacing air with nitrogen in Figure 1.4) will all
influence wear and friction.
lest temperature C'-06
.
,~.c...-.~:~,,, ~ ..... - liO with additive
" - ? ~- ni atmosphere ria
3
2 ~"x\ .,< _ . Plain lio
- - - - in ria atmosphere
. " .... .-- : liO with additive
-- : ~- ni nilrogen atmosphere
00 1.0 2.0 3.0 4.1( 5.0 6.11 7.)1 8.0
speed Sliding ]s/m[
Figure 1.4 Example of effect of sliding speed on the wear rate of additive
enriched oil compared to plain oil [34].
esopruP of noitatnemirepxE lacigolobirT
Tribological experiments can be very expensive and time-consuming, e.g. for
testing of the wear resistance of orthopaedic endoprostheses (artificial knee and
hip joints), a series of identical test machines must be operated for several
months at a time. A question could be asked - what is the precise purpose of this
large effort? The purpose of tribological experimentation can range from pure
research, e.g. investigation of how wear and friction mechanisms change as
sliding speed is increased from several meters per second to about 1000 [m/s] or
the prediction of wear rates, to practical industrial problems. An engine
manufacturer may wish to know whether an improved microstructure of
cylinder liner material will allow extended engine life to be obtained. A computer
memory disk manufacturer may wish to know what is the minimum surface
roughness required on a ceramic disc to avoid high friction at the
commencement of sliding by a recording head. Very often a researcher in
tribology is required to interpret the complex needs of users who usually know
very little of the specialised experimental techniques required to obtain reliable
tribological data. To summarise, the purpose of tribological investigation can be
assigned to two categories which are:
(cid:12)9 fundamental research into the basic mechanisms of friction and wear,
and
applied research to resolve specific industrial, scientific and medical
friction and wear problems.