Key governing factors for the tribochemical changes in the interface films

摘要

Tribochemical reactions between ceramics and metals under various sliding or fretting conditions have been reported many times to critically determine the wear and friction behaviour. Although their strong dependence on environmental and operating conditions was confirmed, details of the mechanisms and chemical reactions are still not fully understood. In particular, different and contradictory assessments of the importance of thermal and mechanical effects on the tribochemical reactions have been reported in literature. In our comprehensive study of fretting wear between steel and silicon nitride ceramics tribochemical reactions were determined to play the critical role under both dry and oil-lubricated conditions. By using SEM, EDS, AES and TEM analysis severe oxidation of ceramics and steel thermal cracking of oil and extreme carbonisation of steel resulting in the carbon content at the tribological interface as high as 10 wt.% were confirmed under selected conditions. As a consequence the new properties of the changed tribological interfaces were much different than the original and govern the wear and friction behaviour. In this paper various possible effects on resulting tribological interfaces are discussed based on the experimental evidences of microstructure changes, phase transformations various thermodynamic and flash temperature calculations and interaction couple experiments in which chemical wear was isolated by controlled mechanical and thermal parameters. It is shown that extremely high temperatures must be considered as a key governing factor in tribochemical reactions, even under conditions, which are usually considered as a low speed-low temperature conditions. Furthermore, discrepancies between the macro-scale evidences including the most advanced contact temperature measurement techniques and the micro-scale changes of the tribological interfaces are explained.