Applications of a non-contacting Kelvin probe during sliding

Abstract

Sliding contact causes dramatic changes near the interface in metals. These include the development of large plastic strains and associated changes in defect concentration, microstructure, crystallographic texture and phase content. In addition, interactions with the counterface material and the environment and mechanical mixing processes can modify the material on both sliding surfaces. Structural and chemical characterization techniques have provided some insight into friction and wear processes, but many questions remain concerning mechanisms. A non-contacting Kelvin probe which detects changes in electron work function offers a useful tool for in situ monitoring of changes associated with sliding. Use of such a probe allows improved design of friction and wear experiments and provides new information on wear mechanisms. Earlier results reported by Zharin were interpreted in terms of damage accumulation and fatigue-like processes. The present results for the unlubricated sliding of various metal combinations confirm the experimental observations of Zharin but suggest that chemical reactions and changes in composition influence the changes detected by the Kelvin probe. For copper-based alloys, the Kelvin probe signal (KPS) can vary with a well-defined period. Images from a video system and from post-test observations with SEM and EDS indicate that the KPS correlates well with the amount of oxygen incorporated in the surface material. Results from materials which do not exhibit nearly-periodic behavior are also described. Differences in the development of the friction signal and the KPS with time can be correlated with differences in the development, accumulation and removal of material affected by tribochemical processes.