An Analysis of the Plastic Interaction of Surface Asperities and its Relevance to the Value of the Coefficient of Friction

Surface asperities are considered as wedge-shaped bodies which are plastically deformed wherever relative motion occurs between mating surfaces. This plastic interaction produces interfacial forces between the surface asperities which are considered, in the collective sense, to represent the total applied normal and tangential forces acting on the sliding surfaces. A solution is proposed which enables the values of the interfacial forces to be obtained at each time interval during the life history of a junction interaction. Furthermore, it is shown that the nature of these forces is markedly dependent on both the initial asperity geometry and on the nature of any surface films which may be present at the asperity interfaces. From such results it is possible to predict the macroscopic values of the coefficient of sliding friction. Such results suggest that an earlier solution due to Tabor should be considered as the special case for perfectly plane surfaces. The solution also indicates the nature of the plastic deformation of the asperities and displays the phenomena of junction growth. Furthermore, it is demonstrated that very large values of the macro-scopic coefficient of friction are associated with very strong molecular adhesion of the surface asperities particularly for materials having high ductility.