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Friction of surfaces carrying lubricant layers - modelling boundary lubrication
Many engineering contacts are lubricated by boundary films which exhibit properties which have
similarities to both liquids and solids: the classic example being that of ZDTP. While classical
elasto-hydrodynamic theory considers the entrapped lubricant to exhibit a piezo-viscous behaviour, the
conventional picture of more solid polymeric boundary lubricant layers views their shear strength as being
linearly dependent on local pressure. However, the properties of adsorbed or deposited surface films can be
more complex than this. Preliminary studies, supported by Shell Research Ltd, looked quantitatively at the
influence of such pressure on the overall friction coefficient of a contact which is made up of an array of
asperities. Individual contact points may be elastic or plastic and both analytic and experimental asperity
height and shape distributions have been considered. The analysis results in plots of coefficient of friction
versus a service parameter which involves the nominal load, the hardness of the metallic substrate and
appropriate surface topographic parameters. Although specifically concerned with cam and follower contact in
IC engines the value of this analysis is that it attempts to combine the behaviour of films on the molecular
scale with the topography of real engineering surfaces and so give an indication of the effects at the
full-size or macro-scale that can be achieved by chemical or molecular surface engineering [1-6].
This work is being extended to model the behaviour of multiple asperity contacts through surface films with
more complex rheological behaviour. Data from Georges' group at EC-Lyon suggests that ZDTP films are 'smart'
and have the capacity to react favourably to local conditions: this is crucial in their role as successful
anti-wear agents in commercial lubricant additive packages. By combining this sort of contact model with the
concept of shakedown it is possible to draw 'scuffing-maps' which delineate regions of 'safe' and 'unsafe'
operation; these appear to correlate successfully with data on valve train wear from fired engine tests [7].
Proposed legislative changes will reduce the allowed phosphorus content of oil additives and so the hunt is
one to find an acceptable alternative that is as successful. This work is supported by EPSRC and Shell
Research Ltd and is being carried out with some collaboration with the University of Birmingham and Ecole
Centrale de Lyon.
Relevant/recent publications
- Williams, J. A. and Xie, Y. 'Friction of sliding surfaces carrying adsorbed lubricant layers' Proc. 22nd
Leeds-Lyon Tribology Symposium, September 1995, Elsevier (1996).
- Blencoe, K. A. and Williams, J. A. 'Friction of sliding surfaces carrying boundary layers: the relation
between friction on the micro- and macro-scales' in Micro and Nano-tribology and its applications (ed.
Welland, M. E. and Bhushan, B.) Kluwer, Dordrecht (1997).
- Blencoe, K. A. and Williams, J. A. 'Friction of sliding surfaces carrying boundary films' Wear,
203-4, pp722-729 (1997).
- Blencoe, K. A. and Williams, J. A. 'Friction of sliding surfaces carrying boundary films' Trib.
Lett., 3, pp121-123 (1997).
- Blencoe, K. A., Roper, G. and Williams, J. A. 'The influence of lubricant rheology and surface topography
in modelling friction' J Engineering Tribology, 212, pp391-400 (1998).
- Williams, J. A. 'Advances in the modelling of boundary lubrication' to be presented at the World
Tribology Congress, Vienna, Sept 2001.
- Bell, J. C. and Willemse, P. J. 'Midlife scuffing failure in automotive cam-follower contacts' J
Engineering Tribology, 212, pp259-269 (1998).
Contact Details
For further information on the work summarised above contact Dr J A
Williams at Cambridge University Engineering Department, Trumpington St, Cambridge, CB2 1PZ, UK. Tel:
01223 332641, Fax: 01223 332662. E-mail: jaw@eng.cam.ac.uk
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