Do electrification-temperature effects deteriorate ZDDP tribofilms in electric vehicles transmission? Insights into antiwear mechanisms using low-SAPS oils

Abstract

This study aims to provide the answer to the crucial question regarding the combined effects of electrification and temperature on ZDDP tribofilm durability. The current investigation aims to understand the tribofilm formation mechanism on electrified steel surfaces, helping mitigate the adverse effects of current leakage in electric vehicles (EVs) transmission. A low-SAPS oil was formulated using polyalphaolefin oil (PAO6) and 0.5 wt% zinc isopropyl-isooctyl-dithiophosphate (ZDDP, type T204). Additionally, the commercially fully formulated transmission oil for EVs was used as a reference oil to compare. The current-carrying friction tests were conducted under various DC currents (2–8 A) and temperatures (50–200 °C) at 250 N and 10 Hz using an electrified SRV-IV tribometer. The results demonstrated that the electrical contact resistance (ECR) and coefficient of friction (COF) signals were dynamically compatible, indicating the tribofilm behavior. Notably, the antiwear properties of ZDDP oil were enhanced by up to 58 % compared to the reference oil under various currents at 150 °C. The HRTEM results revealed that ZDDP oil produces a thick antiwear tribofilm up to 654.8 nm, while the reference oil forms a thin tribofilm up to 37.1 nm at 8 A and 150 °C. The electrification effects do not deteriorate antiwear ZDDP tribofilms due to their higher formation rate than the scrape-off rate. Accordingly, these findings will help synthesize new organic oil additives (low-SAPS) and open future opportunities for discovering green alternatives to enhance the durability of EVs transmission.