Formation and separation of the fragmented surface structure of low-carbon steel and copper under friction

The surface layer structures of low-carbon steel and copper specimens under friction were studied. The advantages and limitations of the principal methods of transmission electron microscopy (TEM) investigation of surface layer structures are discussed. Two different models for the dislocation structures of surface layers of metal are suggested. The models are based on two types of fragmented structure under friction, as distinguished by TEM studies. One of them shows a steady increase of the dimensions of the surface layer fragments with increasing depth from the surface. It corresponds to the gradual decrease of shear deformation in the sliding direction relative to the distance from the surface. The other model is characterized by the stepwise increase of the fragment dimensions within several thicknesses of fragments corresponding to the surface layer. In this case, the moderately plastically deformed metal seems to be covered with a "crust" of the ultimately deformed layer. The models are examined in the light of experiments performed under contact conditions of friction between two materials of different strengths. The type of fragmented structure formed depends on a physical model flattening and plastic flow of asperities under sliding friction. The mechanisms of formation of sheet-like wear particles under flow wear are discussed. An approach to quantitative prediction of wear rate is advanced.