A general approach to discontinuous transfer films: Influence of sliding speed and stick-slip phenomena

Material transfer is a common phenomenon in dry friction, generally associated with a change in surface topography and in frictional interactions. In previous work, the actual influence of adhesion, cohesion and interfacial shear stresses on the extent of transfer (the transfer being assessed experimentally by image analysis) was established both on a theoretical and experimental basis, but at constant sliding speed. In this work emphasis is put on the influence of sliding speed. Both the frictional forces and the transfer characteristics (i.e., the area fraction as well as the particle density), as determined after identical sliding distances, decreased linearly with the logarithm of the sliding speed. The possible existence of stick-slip phenomena suggested by these experimental relationships was investigated further by studying the influence of various parameters affecting either the mechanical characteristics of the tribometer (stiffness, inertial moment) or the physicochemical characteristics of the compacted graphite (granulometry of the graphite powder prior to compaction, crystallinity of graphite, relative humidity during sliding, etc.). The results are interpreted by introducing a complex time of stick accounting for the coexistence of both macrostick-slip and microstick-slip, and by introducing this time dependence into the various adhesive interactions involved in the previous model of transfer stability. (C) 1997 Elsevier Science S.A.