Friction of Sliding Surfaces Carrying Boundary Layers: The Relation between Friction on the Micro- and Macro-Scales

Abstract

Commercial lubricants invariably contain a chemically complex additive package, one of whose roles is to produce protective boundary layers on the opposing solids: the aim of the lubricant technologist is to produce films which are physically robust enough to survive prolonged service conditions, and so prevent damaging metal to metal junctions, but slippery enough to maintain low coefficients of friction. The conventional picture of such a solid boundary lubricant layers views their shear strength as being linearly dependent on local pressure — this relation is consistent with Amonton’s laws irrespective of the geometric details of surface topography. However, the properties of adsorbed, deposited or reacted surface films, or indeed other ‘third bodies’ can be much more complex than this. In the present study we have looked quantitatively at the influence of the pressure dependence of the shear strength of a surface layer on the overall, or macroscopic, friction coefficient of a contact which is made up of an array of asperities whose heights vary in a statistical manner representative of real surfaces. The analysis results in plots of coefficient of friction versus a service or load parameter which also includes information on surface topography and the hardness of the softer surface.