In-situ observation of molecular behavior of lubricant during application of electric field

Abstract

Electric vehicles (EVs) have achieved considerable efficiency gains in terms of energy consumption, but further improvements remain a challenge. One example is electrical failure of bearings. The electrical environment of bearings, such as the sources of shaft voltage and bearing current and the different components, as well as typical modes of bearing electrical failure, such as various localized damage and poor lubrication, have been reported, but the mechanism of lubrication failure remains unclear. Therefore, the objective of this study was to investigate the molecular behavior of lubricating oil when an electric current is applied, especially molecular orientation and physical and chemical adsorption characteristics on sliding surfaces. A microscopic FT-IR in-situ observation system was applied to clarify the molecular behavior of the lubricating oil. Oleic acid was used as a typical lubricant. It was found that the friction coefficient increased on the charged surface. FT-IR spectra suggested that the dissociation of dimers of oleic acid due to surface charge. As a result, friction coefficient increased since molecular orientation of dimers was reduced. Moreover, lubricant molecules became difficult to adsorb on a negatively charged surface, thereby decreasing the boundary lubricant film thickness.