Electro-Tribological Behavior of PAO-Based Lubricants with ZDDP and CNT Additives

The electrification of mechanical systems imposes new demands on lubricants, which must simultaneously ensure effective tribological performance and adequate dielectric strength. This study investigates the combined influence of zinc dialkyldithiophosphate (ZDDP) and carbon nanotubes (CNTs) on the thermal, dielectric, and tribological behavior of a PAO6 base oil. Four lubricant samples were evaluated: neat PAO6, PAO6 + 1 wt% ZDDP, PAO6 + 0.1 wt% CNTs, and PAO6 + 1 wt% ZDDP + 0.1 wt% CNTs. Differential Scanning Calorimetry revealed additive-dependent thermal features, including a ZDDP-related transition that shifted to lower temperature in the presence of CNTs, indicating modified thermal activation behavior. Dielectric Breakdown Voltage (BDV) measurements at 21 °C and 75 °C showed that CNT addition markedly reduces dielectric strength due to conductive pathway formation, while ZDDP decreases BDV at elevated temperature, likely associated with increased molecular mobility and the formation of polar species. The combined formulation exhibited the lowest BDV, evidencing a clear trade-off between dielectric robustness and additive functionality. Tribological performance was assessed using electrified and non-electrified four-ball tests under 0 and 1.5 A, DC conditions. While the coefficient of friction remained largely unaffected, wear behavior was strongly formulation-dependent. The ZDDP-containing formulations exhibited lower mean wear volumes than the ZDDP-free samples under both electrical conditions. For PAO6 + CNTs, the mean wear volume decreased under electrical loading, although substantial between-test variability was observed. The combined formulation exhibited the lowest mean wear volume under electrical loading. Suspension stability and CNT redistribution during testing were not directly characterized; therefore, the wear response cannot be attributed to electrically induced deagglomeration. The results highlight the need to assess tribological performance and dielectric strength jointly in electrically loaded lubricated contacts.