On the connection of thermal dilatation to protective layer formation in the fretting of metallic tribo-specimens

摘要

This paper studies the effects of the heat dissipation capacity of a rubbing material on wear resistance at high temperatures. These effects are studied with a special focus on the dilatation of the thermal energy in sliding. These results suggest a connection between wear transition and the change in the heat dissipation capacity of a rubbing material. The nature of change in the thermal properties before and after the transition influences the thermal environment within the contacting layers. This controls the kinetics of oxide formation and thereby controls wear. Thermal dilatation is a super position of three functions which represent the competing effects of the room temperature thermal properties and their respective variation with temperature. The possible relation between thermal dilatation and protective oxide formation is studied by examining the fretting wear data for two alloys a Ni-based alloy and a Co-based alloy. The results indicate a strong correlation between the formation of protective oxides and the change in the thermal dilatation of the examined alloys with temperature. Moreover, examination of the wear data suggests that a critical ratio between the effects of the conductivity and those of the diffusivity has to be established for favorable wear resistance. This ratio, which is controlled by the thermal diffusivity of the rubbing material, reflects on the intrinsic ability of the material to sustain an oxidative reaction of a controlled rate. So that, the protective glaze oxide layers are formed in a rate that is, approximately equal to the rate of oxide layer break down. Whence, continuous compensation for the removed oxide layer (self-repairing oxides) is established.