Transfer during unlubricated sliding wear of selected metal systems

Abstract

Results of pin-on-disk sliding tests in vacuum, with copper, CuNi, nickel and molybdenum sliding against iron, indicate that initial transfer events involve discrete fragments. We propose that the process involved in the initial transfer events is a consequence of local shear instabilities which develop at large plastic strains. In copper samples the initial transfer elements are lamellar, with layer thickness equal to the cell thickness in the highly deformed base material adjacent to the sliding interface. Prolonged sliding gives rise to the formation of transfer layers or patches on the specimen surface. These layers or patches are composed of finely mixed material derived from the two sliding counterparts as well as scattered pieces of more recently transferred fragments. Typical wear debris particles are generated from the transfer layers or patches. Transfer tendencies for different materials combinations can be predicted from an adhesion point of view if geometrical effects are properly considered. Combining theoretical predictions of transfer tendencies and experimental observations of transfer and geometrical effects, one finds that for a pin-and-disk system the disk should be made of the material having the higher cohesive strength.