The Behaviour of Sliding Contacts Between Non-Conformal Rough Surfaces Protected by 'Smart' Films

Abstract

Many tribological contacts involve two components, each exhibiting finite roughness, sliding over one another for many repeated cycles of operation. Although the initial contact may be plastic, if low wear rate conditions are to be achieved, the steady state situation should be one of predominantly elastic stresses in each surface. When lubrication is present, the stress fields in the components must be consistent with the presence of a low shear strength film separating the surfaces. Commercial lubricants invariably contain a chemically complex additive package with one of the aims of the formulation being the production of boundary layers, which beneficially influence both friction and wear when conditions are not conducive to the formation of either a hydrodynamic or an elasto-hydrodynamic film. These boundary layers must be physically robust enough to survive prolonged service but slippery enough to maintain acceptably low coefficients of friction. Reductions in friction are not necessarily synonymous with improvements in anti-wear behaviour. Recent experimental evidence from studies using both atomic force microscopy and micro-tribometry suggest that boundary films produced by the action of commercial anti-wear additives, such as ZDTP, can exhibit mechanical properties which are affected by local values of pressure – the film is tribologically 'smart' becoming more robust just where it is needed at the most heavily loaded points of the conjunction. These effects have been explored in a numerical model of rough surface contact and the implications for the mechanisms of wear of real devices is discussed.