Evaluation of Anti-Micropitting of Gear Oil with Various Tribolgoy Test Modes

In order to efficiently evaluate the influence of gear oil and additives on micropitting, and minimize the cost associated with early-stage development of anti-micropitting gear oil, a four-ball testing machine, reciprocating friction testing machine (SRV), and sliding and rolling friction testing machine (MPR) were employed. These instruments enabled an examination of the performance of gear oil in terms of its frictional behavior and anti-micropitting capabilities when compared to PAO, PAO with commercially available anti-micropitting additive packages, and PAO with universal additive packages. The effects of these three types of gear oils on micropitting were analyzed post MPR testing. Experimental results indicated that the four-ball machine method did not effectively distinguish the differences in micropitting among the three lubricants. However, the friction test results from the SRV under low-frequency and heavy-load conditions aligned with the anti-micropitting performance of the three types of gear oils in real-world applications, making it beneficial for the efficient evaluation of the tribological properties of additives. Moreover, the MPR sliding-rolling friction test could more accurately simulate the state of micropitting under sliding rolling contact fatigue. By employing a metallographic microscope, SEM, and a 3D profilometer to analyze the friction morphology of the roller surface, it was concluded that the poor lubrication of the base oil had a mechanical polishing effect on the friction pair surface, which suppressed the propagation of cracks and reduced the micropitting area to a mere 1.36%. The inclusion of high-performance composite agents (Gear oil-1) resulted in a micropitting area of 1.45%, which was superior to the 4.67% observed under the action of general composite agents (Gear oil-2). In conjunction with the elemental analysis of additives using ICP and the roller surface using XPS, the combined effect of the boron-containing compound protective film on the friction surface and the sulfur/phosphorus additive was crucial in preventing micro-pitting. As a result, we proposed an efficient and cost-effective method for evaluating micropitting resistance performance, integrating SRV preliminary evaluation with MPR.