Electric polarity: A key factor in lubricated wear of bearing steel

Abstract

Electric vehicles (EVs) have garnered considerable global attention due to their significant contribution to reducing carbon emissions. As EV adoption accelerates, understanding the effects of electrical currents on lubricant performance under varying operational conditions has become essential. This study used an electrified ball-on-disc tribometer to investigate two commercialized lubricants designed for transmission systems under both electrified and non-electrified conditions, focusing specifically on how the current direction between the ball and the disc influences lubricants behavior and consequently wear of the tribo-pairs. Compared to the non-electrified tests, the application of current increased 5–10-fold wear on the anode (connected to +). This wear on anode decreased by 50 % at a current of 3 A compared to the wear observed at 1 A and 2 A. The adherence of wear debris to the cathode during friction tests was observed at low current levels (1 A and 2 A), diminishing or disappearing at higher currents (3 A). This debris adherence effectively reduces wear on the negative electrode (connected to the - pole) but produces abrasive wear on the anode simultaneously. The transfer layer with patchy like morphology on the cathode were characterized using X-ray Photoelectron Spectroscopy, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy and confirmed the presence of metallic iron and iron oxide, together with sulfide, sulfate and phosphate derived from the extreme pressure/anti-wear additives in the lubricants. The aggregation of charged wear debris under electric field can partially explain the phenomenon observed. These results offer important implications for the design of innovative additives with enhanced resistance to electric current effects, as well as protective coatings aimed at reducing the impact of stray currents.