Influence of Electric Potentials on Surface Damage in Rolling–Sliding Contacts Under Mixed Lubrication

Abstract

Shaft voltages in electric vehicle drivetrains are believed to be capable of causing significant damage to rolling bearing surfaces. While surface damage due to high current discharge in full-film lubrication is relatively well understood, the effects of electric potentials on surface damage and tribofilm formation in thin-film, mixed lubrication are rarely studied and remain poorly understood. This paper investigates the influence of relatively low DC potentials (1 V, 5 V) and currents (<10 mA) on wear and tribofilm growth in rolling–sliding contacts in operating mixed lubrication. A suitably modified ball-on-disc MTM-SLIM rig is used to apply an electric potential across a lubricated ball-on-disc contact. Tests were conducted with bearing steel specimens and three oils: PAO base oil, PAO with antiwear additive ZDDP, and a commercial automatic transmission fluid (ATF). The effect of electric polarity was an integral part of the study. The results reveal that even small electric potentials and currents significantly affect wear. For the oil containing ZDDP, electric potential both suppressed the formation of tribofilm and significantly increased the wear on the anodic surface, be it the ball or the disc. The wear was localised in discrete bands within the rubbing track. ZDDP film was shown to be electrically resistive and it is postulated that the non-uniform nature of the ZDDP film leads to the concentration of current within the thin-film regions and this high current density causes wear in these local areas (wear bands) via electric discharge. In contrast, with the ATF, electric potential promoted tribofilm on the anode but resulted in higher wear on the cathode disc. The study highlights a complex interplay between the electric potential, tribofilm formation, the consequent distribution of electrical resistance within the contact, and the resulting wear evolution.

Graphical abstract