The influence of lubricant rheology and surface topography in modelling friction at concentrated contacts

Abstract

The conventional picture of a boundary lubricant layer is of a film which exhibits mechanical properties more like a solid than a liquid. In this paper the influence that the form of the dependence of shear strength on pressure has on the overall friction coefficient of contacts exhibiting both model and measured surface profiles is examined quantitatively. The analysis can provide plots of coefficient of friction versus a service or load parameter which also includes some aspects of contact topography. The results indicate that variations in the macroscopic coefficient of friction which may occur as the load is varied have their roots in both the statistical nature of the surface and the mechanical response of the lubricating boundary film. The effect of running-in on friction and safe working loads can also be observed. The value of this analysis is that it attempts to combine the behaviour of films at molecular dimensions with the topography of surfaces measured at an engineering scale and so gives an indication of the full-size effects that can be achieved by chemical or molecular surface engineering.