Abstract The presence of shaft voltages and currents in electrical machines is known to cause damage and premature failure of lubricated rolling-sliding contacts such as those in rolling element bearings. Damage due to high current discharge events in contacts operating in full-film lubrication is relatively well documented and understood. However, for tribological contacts operating in mixed lubrication, where the load is supported by both hydrodynamic oil film and surface asperity contacts, the mechanism of electrical damage is more complex and poorly understood. This study presents a systematic comparison of the effects of direct current (DC) and alternating current (AC) voltages on surface damage in rolling-sliding contacts operating in the mixed lubrication regime. A modified ball-on-disc MTM-SLIM rig is used to apply controlled DC or AC voltages across a rolling-sliding lubricated ball-disc contact. Ball and disc are both made of AISI 52100 bearing steel and tests are performed with three oils, a polyalphaolefin (PAO) base oil blended with ZDDP anti-wear additive (PAO+ZDDP), PAO base oil blended with organic friction modifier oleylamine (PAO+OAm), and a commercial automatic transmission fluid (ATF). The results show that the wear in electrified contacts is strongly dependent on oil chemistry and voltage type, DC or AC. For PAO+ZDDP, which forms a relatively thick, electrically insulating but patchy tribofilms, DC voltage leads to increased wear on the anodic surface that is concentrated in specific regions or bands; this is thought to be due to the current density increasing in the regions where tribofilm is not present or is thin The wear on cathode is relatively unaffected by presence of voltage. Under AC voltages, the wear with PAO+ZDDP is distributed equally across the two contacting surfaces; this is likely due to continuous polarity switching so that each surface spends half the time as anode and half the time as the cathode. For PAO+OAm and ATF, a DC voltage leads to a two orders of magnitude increase in wear on the cathodic surface compared to non-electrified conditions; this is thought to be due to tribo-electrochemical reactions leading to the formation of hard surface compounds on the anodic surface and a softer layer on the cathodic surface, causing polishing wear of the cathodic surface. Under AC voltages, the wear with PAO+OAm and the ATF is much reduced; this is thought to be because the relevant tribo-electrochemical reactions that took place under DC voltages are disrupted under AC voltages, in turn suppressing the associated electrified wear. Novel phase-synchronised AC tests, where a given part of the disc is always either an anode or a cathode despite AC voltage signal, confirm that under relatively low frequencies (35 Hz) these differences in wear between AC and DC are not because the tribo-electrochemical reactions do not have time to occur under AC, they still occur within individual contact cycles, but polarity reversal in successive cycles effectively cancels their net effect. At higher AC frequencies (10-20 kHz), the reduction in damage under AC voltages is because rapid polarity switching the time available for the relevant chemical reactions to take place on either surface; however at these higher frequency a limited amount of discharge-type surface damage, resembling frosting, is observed in thin tribofilm regions with PAO+ZDDP and ATF. Overall, the application of DC voltage causes significant surface damage, but the application of AC voltage or completely suppresses the relevant damage mechanisms, resulting in less damage. The AC signal frequency and oil chemistry play a critical role in determining the resulting wear and surface damage.
Yousuf et al. (Fri,) studied this question.