An Examination of Thermionic Emission Due to Frictionally Generated Temperatures

Abstract

The emission of electrons from a surface due to heating-referred to as thermionic emission-is examined theoretically for sliding contact. High local temperatures generated by friction at the contacts between rubbing surfaces can activate the emission of electrons and influence tribochemical reactions. A thermal model previously developed by Vick and Furey [1,2] for sliding contact is used to predict the temperature rise over the surface. This predicted temperature rise, along with the bulk temperature of the material, is used in the Richardson--Dushman equation for thermionic emission to predict the current density from the surface. The total current discharged from the surface is obtained from an integration of the current density. Results demonstrate that high local temperatures generated by friction at the contacts between rubbing surfaces can activate the emission of electrons, with local spikes in the current density occurring in the vicinity of the peak temperature. In addition, relatively large changes in both current density and total current result from relatively small changes in either material properties or sliding conditions, such as velocity or applied load. Since the true area of contact is likely to evolve in a highly dynamic fashion, a study using time varying, multiple contacts was also conducted. Results suggest that the locations of high local temperatures and thermionic activity are likely to be short lived and random.