Influence of Slide-Roll Ratio on Micropitting under Rolling-Sliding Contact in Base Oil and E-Axle Fluid

The efficiency of electric vehicles (EVs) can be further enhanced by increasing the rotational speed of their motors. However, higher motor speeds may reduce the fatigue life of gear teeth, particularly through the formation of micropitting. While the slip-roll ratio (SRR) is known to influence micropitting, its precise effect on this type of surface damage remains insufficiently understood. In this study, we investigated the influence of SRR on the occurrence of micropitting in disk specimens lubricated in base oil (PAO4) and e-axle fluid environments. Friction tests were conducted using a ball-on-disk setup, in which steel disks with two different surface roughness levels were tested under various slip ratios. A binarization method was employed to quantify the extent of micropitting. The results showed that both micropitting and wear are highly dependent on SRR and surface roughness. In the PAO4 environment, significant micropitting occurred at 0% SRR and decreased with increasing SRR, suggesting that sliding wear may suppress fatigue wear. In the e-axle fluid environment, significant micropitting was also observed at 0% SRR, but the wear pattern showed a broader distribution, indicative of chemical wear.