Models for the generation of oxides during sliding wear

Abstract

The sliding wear of metals in air or oxygen, even at relatively low temperatures, is generally characterized by a period of relatively severe wear, followed by a transition to a period of mild wear, the transition being associated with the generation of sufficient oxide to limit metal–metal interactions. Consideration is given to some of the factors that can influence this oxidation process and it is shown that, in addition to preoxidation, there are three main causes of oxide generation. In situations where surface flash-temperature rises are significant, particularly where the number of load-bearing contacts is few, the formation of oxide on those contacts can be important. If there are many load-bearing contacts, particularly under low-speed conditions and at high ambient temperatures, flash-temperature rises are small and oxidation of the asperity tips may be negligible in comparison with the general oxidation of the entire areas of the specimens. Quantitative models are presented to account for these processes. However, under certain conditions, neither asperity nor general surface oxidation is important in initiating the transition to mild wear; rather, metal debris particles are generated in the severe wear period. As discussed theoretically, these are fractured and refractred during sliding under the loading forces until eventually the particles become so small that they oxidize almost completely spontaneously, even at room temperature. These oxide particles can agglomerate in the wear tracks, there by reducing metal-metal contact and causing a considerable reduction in wear.