A general approach to discontinuous transfer films: The respective role of mechanical and physico-chemical interactions

Often in dry friction, material transfer occurs as a discontinuous film consisting of discrete particles, strongly adhering to the substrate. On smooth surfaces this leads to the formation of asperities interacting both mechanically and physico-chemically with the counterface. A simple analysis of the forces acting on a single asperity allows the definition of the conditions of stability and hence of transfer particle existence. The model developed is illustrated by transfer particle density variations (assessed by image analysis) as a function of experimental conditions for various materials whose surface energies have been determined by inverse gas chromatography or wetting contact angles.