Influence of electricity on the traction and wear characteristics of ZDDP and MoDTC in a low-viscosity oil under different sliding-rolling conditions

Shaft currents, which are very common in electric vehicle (EV) drivelines, can trigger deleterious effects on the tribological performance of bearings and gears. However, a fundamental understanding of such effects on traction coefficient and wear characteristics of lubricants has not yet been well-established. Accordingly, this work aims to elucidate the traction characteristics of common lubricating additives such as ZDDP, and MoDTC when mixed with a low-viscosity oil (PAO4) on AISI 52100 steel test pairs under non-electrified and electrified sliding-rolling conditions. The parametric studies were carried out using an electrified mini-traction-machine (MTM) over a very broad range of slide-to-roll ratios and rolling speeds with and without electrification. The resulting wear tracks from each set of tests were examined by optical profilometry, Raman spectroscopy, and SEM to identify the key differences in their wear characteristics. According to the results obtained from these tests, applying electricity to the contact interface causes substantial alterations in the traction coefficients of additized oils and intensified the severity of abrasive wear on rubbing surfaces. The traction coefficients also increased substantially under electrification in almost all the sliding-rolling conditions. The oils containing ZDDP were the most affected by electricity, while the oil containing only MoDTC performed remarkably better under electrified conditions. Such an outstanding effect enabled by MoDTC was attributed to the formation of a more effective MoS₂-rich tribofilm under electrification.