Thin Films in Tribology, Proceedings of the 19th Leeds-Lyon Symposium on Tribology held at the Institute of Tribology, University of Leeds

688 Pages·1993·24.212 MB·iii-vi, 3-749\688

by  D. Dowson, C.M. Taylor, T.H.C. Childs, M. Godet and G. Dalmaz (Eds.)

TRIBOLOGY SERIES, 25
THIN FILMS IN
TRIBOLOGY
edited by
D. DOWSON, C. M.TAYLOR,T. H. C. CHILDS, M. GODETand G. DALMAZ
Proceedings of the 19th Leeds-Lyon Symposium on Tribology
held at the Institute of Tribology, University of Leeds, U.K.
-
8th 11th September 1992
E LSEVlE R
Amsterdam -London -New York -Tokyo 1993
For the Institute of Tribology, Leeds University
and
The lnstitut National des Sciences Appliquees de Lyon
ELSEVIER SCIENCE PUBLISHERS B.V.
Sara Burgerhartstraat 25
P.0 Box 21 1,1000A E Amsterdam, The Netherlands
Library of Congress Cataloging-ln-Publlcatlon Data
Leeas-Lyon Symposlum on Tribology (19th 1992 Unlverslty of Leeds)
Thln films in trlbology proceedings of the 19th Leeds-Lyon
Symposium on Trlbology held at the Institute of Tribology.
University of Leeds. U.K. 8th-11th September 1992 / edited by D.
...
Dowsopn. cn[.e t- -a l.(1T.r lbology series . 25)
Includes bibliographical references.
ISBN 0-444-89789-5 lalk. paper1
1. Trlbology--Congresses. 2. Thin fllms--Congresses. I. Dowson.
D. 11. University of Leeds. Institute of Tribology. 111. Institut
national des sciences appliquies de Lyon. IV. Title. V. Series.
TJ1075.AZL43 1992
621.8'9--dC20 93-5673
CIP
ISBN 0444 89789 5 (Vol 25)
01 993 Elsevier Science Publishers B V All 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.
mechanical. photocopying. recording or otherwise, without the prior written permission of the publisher, Elsevier Science
Publishers B V, Copyright & Permissions Department, PO Box 521,lO OOAM Amsterdam, The Netherlands
Special regulations for readers in the U S A -This publication has been registered with the Copyright Clearance Center Inc (CCC).
Salem, Massachusetts Information can be obtained from the CCC about conditions under which photocopies of parts of this
publication may be made in the U S A All other copyright questions, including photocopying outside of the U S A should be
referred to the copyright owner, Elsevier Science Publishers B V, unless otherwise specified
No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability,
negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material
herein
This book is printed on acid-free paper.
Printed in The Netherlands
V
INTRODUCTION
The nineteenth Leeds-Lyon Symposium on Tribology was held at Bodington Hall, University of Leeds, from 8th
to 11th September 1992. The focus of the meeting and its title was 'Thin Films in Tribology'. Some one
hundred and fifty delegates from twenty countries gathered to listen to and discuss seventy papers presented in
nineteen sessions. This number of papers is an increase on numbers at previous meetings and indicates the
continuing popularity of the Symposium.
It was again pleasant to welcome Professor Maurice Godet and a group from our sister institution, INSA de
Lyon. On this occasion we also particularly welcomed Professor K L Johnson from the University of
Cambridge, both to mark our respect for his wide-ranging achievements in tribology and on his retirement, and
as Guest of Honour and after dinner speaker at the Queens Hotel, Leeds: these Proceedings are dedicated to
him.
The nineteenth Symposium was special for another reason too, as the last occasion on which Professor Duncan
Dowson, co-founder with Maurice Godet of the Symposia, could attend the Symposium at Leeds as a serving
member of staff. In addition to the traditional Thursday afternoon tour which this year was to Chatsworth
House and The Abbeydale Industrial Museum and culminated with dinner and a display of the silver at the
Cutlers' Hall, Sheffield, the UK champion Grimethorpe Colliery Brass Band was invited to play in the
University's Parkinson Court on the Wednesday evening. One could well believe that the walls of Jericho once
fell to trumpet blasts, although it must be reported that Leeds survived.
Professor Dowson's impending retirement to some extent determined the theme of the meeting. From Victorian
days when working lubricant films were calculated in tens of micrometres, to today when molecular dynamics
simulations and even experiments are beginning to look at nanometre, single molecule thick, films, the study of
surfaces which is the tribologists' challenge has moved to finer and finer scales. It was felt appropriate to invite
reviews from across the tribological field with thin films as their theme. Thus, in addition to the Keynote
Lecture, delivered by Professor Dowson on Thin Films in Tribology, most halfday working periods were
started by an invited review session. The meeting finished with a panel discussion. We would like to thank the
following for their reviews and panel contributions: K L Johnson, T Bell, C M Myllerup and B J Hamrock, H S
Cheng, T H C Childs, H A Spikes, M 0 Robbins, R C Coy, M Godet and B Jacobson.
The further working sessions of the Symposium were as lively as ever, concentrating on contact, friction and
traction (session 3), coatings and surface modification (Sessions 4 and 12), analysis and design of highly
stressed components (Session 5), elastohydrodynamics (Sessions 7 and ll), lubricated wear (Session 8),
Hardness and metal cutting (Session 9), molecular scale thin films (Session 13), bio-tribology (Sessions 15 and
18), compliant surfaces (Session 16) and coating wear (Session 17). We would like to thank the Chairmen of
Sessions and the papers' authors, as recorded in these proceedings, for the success of the Symposium.
To mount such a Symposium in an effective style needs the help of many and we would like to record sincere
appreciation of the great contribution of ours colleagues - Mrs Cath Goulbourn, Mr Ron Harding, Mr Brian
Jobbins, Mr David Jones, Mrs Sheila Moore, our technicians and research fellow/research students. It is once
again a pleasure to record our thanks to Elsevier Science Publishers B.V., Amsterdam for their impressive
production of the volume of proceedings. We were fortunate, as in the past, to attract the financial support of a
numkr of concerns, support which helps to encourage younger researchers to attend the meeting and reduce
the registration fee. We gratefully acknowledge the following organisations in this regard.
vi
BP International Ltd
Castrol Ltd
Mitchell Bearings
Shell Thornton Research Centre
SKF Engineering and Research Centre
US Army Research, Development and Standardisation Group (UK)
The wide range of topics covered by the Leeds-Lyon series of tribology symposia is evidenced by the following
sequential list:-
1974 Cavitation and Related Phenomena in Lubrication
1975 Super Laminar Flow in Bearings
1976 The Wear of Non-Metallic Materials
1977 Surface Roughness Effects in Lubrication
1978 Elastohydrodynamics and Related Topics
1979 Thermal Effects in Tribology
1980 Friction and Traction
1981 The Running-In Process in Tribology
1982 Tribology of Reciprocating Engines
1983 Numerical and Experimental Methods in Tribology
1984 Mixed Lubrication and Lubricated Wear
1985 Mechanisms and Surface Distress
-
1986 Fluid Film Lubrication Osborne Reynolds Centenary
1987 Interface Dynamics
1988 The Tribological Design of Machine Elements
1989 Mechanics of Coatings
1990 Vehicle Tribology
1991 Wear Particles: From the Cradle to the Grave
1992 Thin Films in Tribology
We look forward with anticipation to the twentieth meeting in Lyon from 7th to 10th September 1993. The
Symposium will focus on energy dissipation processes and the origins of friction in tribology at all scales, from
all points of view and the title will be "Disspative Processes in Tribology".
Chris Taylor
Duncan Dowson
Tom Childs
Thin Films in Tribology / D. Dowson et al. (Editors)
1993 Elsevier Science Publishers B.V. 3
Thin Films in Tribology
D Dowson
Institute of Tribology, Department of Mechanical Engineering, The University of Leeds, Leeds LS2 9JT, United
Kingdom
In this Keynote Address the author reviews briefly the full spectrum of thin film tribology, ranging from surface
modification and coatings to the behaviour of thin, protective lubricating films. Attention is drawn to the
remarkable way in which the effective minimum film thickness in highly loaded, reliable and efficient
lubricated machine elements has fallen from tens of microns to tens of nano-metres throughout the 20th
century, The near coincidence of studies of 'solidified' lubricants and 'boundary' lubricants in tribology is noted.
1. INTRODUCTION lubricated machme components. Some
illustrations of thin film dominance in tribology
The short definition of Tribology, introduced more include;
....
than a quarter of a century ago, is that it is "The
science and technology of interacting surfaces in chemical reactions with the lubricant or the
relative motion and the practices related environment which might create surface films
thereto....". Since the subject was defined there on the solids with properties which dominate
has been much research and not a little success in the tribological properties of the conjunctions.
understanding the response of the bulk materials,
both solids and fluids, which form the tribological lubricants might solidify and cease to respond
conjunctions, to the severe loading, kinematic, as Newtonian fluids.
thermal and environmental conditions which they
experience in performing their tribological the effective thickness of the lubricating film
functions. Much of this progress has been might fall to the level of the roughness scale of
developed from firm foundations in continuum the solid surfaces, and might even approach
mechanics and concepts of fracture mechanics and molecular proportions.
fatigue in the case of solid mechanics. Analysis
has nevertheless generally rested upon idealisations the surfaces of the solids might have been
of the materials considered and in many ways it is modified, either deliberately or by pre-
remarkable that the predictions of performance treatment, or as a result of exposure to the
have been so consistent with practical experience. operating environment of the tribological
conjunction.
Notwithstanding the impressive progress
recorded in the understanding of the tribological wear debris (third bodies) might accumulate
performance of bulk materials, the most within the conjunction in thin layers and
spectacular developments in recent times have dominate the tribological performance of the
resulted from the recognition of the distinctive system.
roles of thin films, both solid and fluid, in
tribology. The operating surfaces of solids might Molecular rather than continuum mechanics
exhibit physical properties quite different from might be necessary to account for the detailed
those of the underlying bulk materials, while the solid-to-solid and solid-to-fluid interactions in
fluid nature of the lubricant itself becomes thin film tribology.
questionable in the exceptionally thin films which
appear to determine success or failure in many Many of these topics form the subject of review
and research papers in the 19th Leeds-Lyon
4
Symposium on Tribology, while the recognition of Case hardening, or carburising, based upon the
their importance accounts for the title of the diffusion of carbon into the surface when the
Symposium itself. In the present paper brief metals are heated in the presence of a carbonaceous
mention will be made of some of the major fields of material. Other thermal processes for the
progress reported fully elsewhere in the enhancement of hardness based upon induction
Symposium, but particular attention will be given flames, lasers and electron beams are widely
to studies of thin fluid film lubrication. employed, depending upon the materials and sizes
of the specimens.
2. THE SPECTRUM of THIN FILM
TRIBOLOGY Diffusion. Thermo-chemical action can be
used to prepare harder surfaces in relatively thin
Initially the selection of sliding pairs of layers through the use of elements such as carbon,
materials for machinery was gwded by sulphur and nitrogen.
consideration of the properties of the bulk
materials. However, in due course it was An alternative to the modification of the
recognised that in many components, like plain surface layers of the bulk material is to apply a
bearings, it was adequate to provide a low shear coating of either a very hard material or a solid
strength, low melting point bearing material on the lubricant.
bearing surface to support the relatively hard and
expensive journal or shaft. As the unit loads on Electroplating is a well known and
machinery increased, the relatively thick layers of established procedure for coating materials such as
soft material, like white metal, gave way to the thin iron, chromium and nickel.
multiple layers of materials capable of resisting
fatigue and operating in the ever more hostile Welding is used to provide protective alloy
environments that are familiar today in many plain coatings based upon materials such as cobalt, iron,
bearings. General reviews of these developments nickel and tungsten carbide.
were published in [1,2,3],w hile valuable guidance
to wear resistance now appears in handbooks [4,5]. Flame-spraying is widely used where metals,
alloys or ceramics are to be applied to enhance the
It can be argued that the spectacular resistance to wear. The coatings are relatively
developments in plain bearing technology did thick and difficulties can be encountered with
much to promote interest in many aspects of thin awkwardly shaped components.
film tribology. It is, however, the emphasis on
wear reduction, particularly in unlubricated Ion Plating and Plasma deposition are the
conjunctions, in recent years that has been coating processes that have attracted the most
responsible for much of the progress evident today. attention in tribological circles in recent years.
They are used for hard and soft coatings alike and
Perhaps the greatest advance followed the generally yield dense structures with good
recognition that the bulk materials could be tribological properties. The two major categories
selected for their structural integrity, general of coating systems under these headings are
mechanical properties and relatively low cost while Chemical Vapour Deposition (0an)d
the surfaces of rubbing solids could be modified by Physical Vapour Deposition (PVD). In the
a variety of techniques to enhance their resistance former, gaseous reagents provide the source of the
to corrosion or wear. In general wear resistance is coating materials, while in the latter the coating
enhanced by increasing the hardness of the surface, material is generally evaporated from solid placed
through such processes as; within the vacuum chamber. The CVD process
generally involves high temperatures, which may
Heat Treatment. Perhaps the best known be disadvantageous, whereas PVD minimises this
method of preparing relatively thick layers of problem. The coatings are very thin, such that the
harder material in low and medium carbon steels is original surface topography is not impaired and
5
adequate bond strength is often claimed. The Plasma sprayed alumina ceramic coatings
assurance of adequate bonding is nevertheless a having thicknesses in the range 200-300 pm were
matter of the utmost importance in many applied to the surfaces at velocities up to 500 m/s
tribological situations, such as in medical implants, using argon with a small amount of hydrogen as
and much effort is currently being applied to the the camer and plasma gas. The bond strength
evaluation and improvement of coatings produced specimens were prepared on the circular ends of
by ion plating techniques. cylindrical metallic specimens of diameter 25 mm.
The ends of the metal cylinders were cleaned by
2.2 The Integrity of Coatings pressure grit blasting and the ceramic coatings
were applied either directly or by interposing a Ni,
If it is true that protective coatings, both hard Cr, A ! bond coat some 75-100 pm thick.
and soft, have been responsible for much of the
progress in wear protection under rubbing Normal loads were applied to the coating in a
conditions [6],i t is equally true that much of the tensile testing machine by sticking the flat face of a
concern about their use has been related to the carefully prepared stainless steel cylinder to the
issue of their integrity on the substrates. alumina with Araldite. A constant strain rate of
0.5 mdmin was adopted and some specimens
In some cases it is not a disaster if the coating were tested in air and others afler soaking for
eventually wears away, since many substrates are various periods in deionised water.
themselves reasonably good wear resisting
materials. However, in other cases it might be
disastrous if the coating detaches itself from the
L
substrate, since the debris might be hard, abrasive
and destructive to the tribological system. It is the E
responsibility of the tribologist to ensure that the
protective coatings which he or she recommends
have adequate bond strength at the interface with
the underlying material throughout the lifetime of
operation of the sliding pairs of materials.
The point was brought to my attention most r0n -
vividly almost ten years ago, when we were .a-l
d
concerned with the development of improved Icn -
al
counterfaces for the ultra-high molecular weight I- I I I I
polyethylene acetabular cups used in total 1 10 100 1000 10000
replacement hip joints [7]. The wear studies had Soak Period ( hours 1
already indicated that high density alumina
ceramic provided an excellent counterface to the Fig. 1. Variation of Tensile Bond Strength of
polyethylene, with wear of the latter taking place at Coatings of Alumina on Metals.
a lower rate than when well prepared metals were ( control; + withbondcoat;
used [8,9]. The mechanical strength of metallic o without bond coat)
femoral stems and heads nevertheless presented
desirable features for the severe loads and bending The findings are shown in Figure 1. The
moments encountered by implants. Interest thus initial bond strengths are recorded in the ordinate
developed in the possibility of retaining the and it is evident that little deterioration took place
mechanical strength of metallic stems and heads, during the first 1,000 hours or so. However, the
and the tribological advantages of ceramic bond strength of the specimens soaked in water
counterfaces by applying alumina coatings to the decreased rapidly thereafter, in some cases by
metallic heads. almost 90% after 10,000 hours (say 14 months).
There were no significant differences between the
6
results for the two different metallic substrates and lubricated by either Newtonian or Non-Newtonian
the bond coat had little influence on the bond fluids. The analyses became increasingly
strength. sophisticated and in recent times a number of the
initial restrictions on the analysis have been
While survival times of about 1,000 hours may relaxed to permit solutions to be obtained which
be satisfactory in many engineering situations, they take account of the surface roughness of the solids;
are clearly inadequate for implant applications. It point (elliptical) and line contacts; isothermal and
is only now, with the considerable improvements in thermal conditions and the realistic rheological
coating technology and the development of characteristics of the lubricants. As the available
alternative surface treatments, that surface solutions multiplied, and the learned society
modifications are once again attracting the literature on tribology still devotes a good deal of
attention of bioengineers engaged in the space to the subject, empirical film thickness
advancement of total replacement joints. equations were developed, verified by experiment
and incorporated into design procedures.
3. FLUID FILM LUBRICATION
Experimental techniques based primarily upon
In the previous section surface modification of interferometry and capacitance systems not only
the solids, often in thick films of micron on sub- verified the theoretical predictions but also revealed
micron proportions, was discussed in relation to the effectiveness of very thin elasto-hydrodynamic
developments in thin film tribology. We now move films. Roberts and Taylor [ll] had revealed water
from the solids to the fluids in tribological films having thicknesses of about between
conjunctions to find evidence of equally spectacular rubber and glass as early as 1971, while Spikes
advances in thin film tribology. [12] more recently developed an ultra thin film
interferometry system capable of measuring
3.1 The Thinning Film effective elastohydrodynamic films between glass
I have outlined elsewhere [lo] the manner in and metals of similar film thicknesses. Flying
which advances in analysis, design and heads float above computer discs with great
mandame have enabled fluid-film lubricated effectiveness and remarkable reliability on films of
systems to operate with films of ever decreasing air generated by hydrodynamic actions which have
thickness throughout the 20th century. At the thicknesses in the range to
beginning of the century minimum film thicknesses
in reliable fluid film bearings were probably of We now recognise that the thin, tenacious and
order m and maybe even as high as lo4 m. very effective films of fluid developed beneath the
Development of the internal combustion engine rugosities on surfaces as they pass through elasto-
provided the spur for improvements of big end and hydrodynamic conjunctions can maintain fluid film
main crankshaft bearings wluch regularly operate lubrication between many real (rough) surfaces
today with minimum film thicknesses of order which might otherwise be expected to touch each
10-6 m. other. This appears to explain why many elasto-
hydrodynamic conjunctions are effective even
In the second half of the 20th century the though the predicted film thicknesses, based upon
remarkable tenacity of fluid-films in highly smooth surface analysis, are but modest compared
stressed, lubricated machine elements was revealed to the composite surface roughness of the solids.
through the development of understanding of the This is known as micro-elasto-hydrodynamic
phenomenon now known as elasto-hydrodynamic action or simply asperity lubrication and it appears
lubrication. The initial measure of such films was to be particularly effective in low elastic modulus
the micron, but as experience grew we began to conjunctions such as elastomeric bearings, seals
talk confidently of minimum film thicknesses of and synovial joints. Much remains to be done on
sub-micron proportions; say m. Analytical the analysis and experimental study of micro-
procedures for smooth solids were developed for elasto-hydrodynamic lubrication, but it is already
the prediction of such films in conjunctions clear that such studies are enhancing our
7
understanding of the lubrication of highly the roughness profiles upon pressure distribution,
deformed rough surfaces and also exposing the film thickness and stresses within the solids.
inadequacy of the simple (A) ratio concept as a
guide to the performance and long term survival of The important outcome of micro-elasto-
certain elastohydrodynamic conjunctions. The hydrodynamic action is that the surface rugosities
essentials of micro-elasto-hydrodynamic actions are are partially flattened by the enhanced local
reviewed in the next section. pressures which they generate. This may allow the
surfaces to be separated by an effective film of
3.2 Micro-Elasto-Hydrodynamics lubricant when they would otherwise be expected to
The well known features of smooth surface make contact, thus extending the regime of fluid
elasto-hydrodynamtcally lubricated conjunctions film lubrication to very thin film situations.
are shown in Figure 2. In this illustration the
moving plane surface represents a rigid solid while
a cylinder of appropriate radius and elasticity is
shown as the deformable solid.
-15 -10 - 5 0 5 10 15
(a) Pressure
- u+-
-15 -10 -5 0 5 10 15
(b) Film Thickness
Fig. 2. Elasto-Hydrodynamic Conjunction
I I
If transverse roughnesses modelled by a
sinusoidal profile are considered on the deformable
surface, the usual smooth surfaced elasto-
hydrodynamic conjunction pressure curve will be
perturbed by the local asperity hydrodynamic
action associated with the peaks and troughs as I
1
shown in Figure [3]. Several numerical solutions 2
to the micro-elastohydrodynamic lubrication
problem have now been reported [13,14], with (0 Deformed Roughness
attention being focused upon the influence of the
amplitude, wave length and orientation or lay of Fig. 3. Features of Micro-Elastohydrodynamic
(Asperity) Lubrication.
8
3.3. The Lambda (A) Ratio. 4. THE STRIBECK CURVE
It has been useful in the past to ascertain the
likely effectiveness of fluid film lubrication by The transitions from 'fluid film' to 'boundary'
calculating the ratio of the calculated lubrication via an intermediate zone known as
elastohydrodynamic film thickness (h) for smooth 'mixed' lubrication are often represented by a
surfaces to the composite surface roughness [(a1)2 Stribeck [15] curve. The first President of the
+ (02)~]" of the solids. Experience has shown that American Society of Mechanical Engineers, Robert
if the resulting lambda (A) ratio exceeds about H Thurston [16] reported in 1885 that the friction
three, the conjunction enjoys effective fluid film in a journal bearing passed through a minimum as
action as indicated by Figure 4. While a value of the load increased; a transition now associated with
unity will inevitably lead to substantial surface-to- a change from fluid-film to mixed lubrication.
surface contacts, wear and a reduced fatigue life, Gumbel [17] urufied the observations of Thurston
there is increasing evidence that conjunctions with and Stribeck when he represented journal bearing
intermediate lambda values can survive much friction in a given bearing as a function of a single
better than expected due to micro-elasto- dimensionless group (qdp), where (q) is the
hydrodynamic effects. lubricant viscosity, (a)t he angular velocity of the
shaft and @) the projected load per unit area.
This raises doubts about the real value of the
lambda ratio. It is clearly a usefid initial guide, to The general form of the Stribeck curve is
conjunction survival, but if circumstances permit shown in Figure 5. It was one of the earliest forms
micro-elastohydrodynamic action, a more detailed of lubrication regime charts, distinguishing as it
investigation of the actual minimum film thickness did between fluid-film; mixed and boundary
should be undertaken. A modified lambda ratio in lubrication. The minimum coeficient of friction is
which the composite roughness term is adjusted to of order while the level representative of
reflect the remarkable elastic smoothing action boundary lubrication in engineering systems is
which sometimes takes place might still be useful, about 10-I. The mixed lubrication curve
but as the resulting minimum film thicknesses connecting these extremes is steep, thus offering
become smaller and smaller and approach attractive possibilities for a significant reduction in
molecular proportions, account must also be taken the coefficient of friction by the promotion of a
of the absolute separation of the solids. little hydrodynamic action, or a severe penalty on
fluid-film bearing performance when asperity
interaction occurs.
The possibility of extending fluid-film
lubrication into regimes that would appear to be
mixed or boundary lubricated, through micro-
elastohydrodynamic action has already been noted
in the previous section. These suggestions are by
no means recent, since Michell [ 181 reviewed some
of the issues in his excellent book pu5lished in
1950. He recalled that Professor Heidebroek [19]
had discussed a paper by Tenot, published in 1937,
and declared that the rising friction in the mixed
lubrication regimes was not necessarily due to dry,
or solid, friction. He pointed out that if regard was
paid to the rugosities on the bearing surfaces,
convergent films could be envisaged between them
which would contribute to the local load canying
1 2 ( A ) 3
capacity and greatly to an increase in the friction.
Fig. 4. Influence of Lambda Ratio (A) Upon Life.