An Experimental Study of Micropitting of Bearing Steel Under Oil Lubricated Rolling/Sliding Conditions

Micropitting, as a type of surface-originated early failure, restricts the precision and service performance of rolling bearings. The formation mechanism of micropitting has not been fully revealed. This study used a micropitting rig to investigate the influences of slide-roll ratio (SRR), rolling speed, and anti-wear additives on the micropitting of GCr15 bearing steel under synthetic oil lubrication conditions. The evolution process of micropitting in terms of variations of surface morphology, micropitting area proportion, friction coefficient and vibration acceleration was investigated under millions of stress cycles, and analyzed the causes of micropitting and the influence of anti-wear additives on micropitting. First, the effect of SRR on micropitting was studied experimentally. It was found that when approaching pure rolling, slight micropitting damage would occur due to the smaller shear stress at the tribological interfaces. With an increase in SRR, the interfacial shear stress increased, and the micropitting damage increased. Further increasing the SRR led to the wear and polishing effects of the roller surface, which inhibited micropitting. Micropitting exhibited a trend of initially increasing followed by a gradual decrease, ultimately stabilizing, which was influenced by the damage and the slide-roll ratio. Second, the effect of entrainment velocities on micropitting had been studied in boundary/mixed lubrication regimes. It was found that after initial surface crack formation, high rolling speed would cause the lubricant sealed in the crack to be difficult to discharge, resulting in high oil pressure and thus accelerating crack propagation and spalling of the surface material into micro-pits. Finally, the effects of sulfided isobutylene (SIB) and tricresyl phosphate (TCP) as additives on micropitting of bearing steel were carried out. And the film formation phenomenon of the early additives on the rollers was observed and measured using an optical microscope and an optical interference film thickness measurement instrument. It was found that both additives promoted the generation of micropitting under the used experimental conditions. However, there were large differences in the morphology of the generated micropitting. SIB produced isolated distributed shallow pits with large delaminating areas of micropitting, and it was thought that the sulfur element may accelerate crack propagation and delamination of surface material after crack formation. Through scanning electron microscopy and EDS elemental analysis, it was found that there were a considerable amount of sulfur (S) elements present within the micropitting, whereas the areas without micropitting did not contain sulfur (S) elements. TCP produced dense and more regular micro-cracks and later micro-pits, and its formed metal phosphate film on the bearing steel surface was thin and uneven, measured by the optical interference test apparatus, the formed phosphate film was approximately 40 nm thick, which may fail to separate direct contact between rough peaks, thereby promoting the generation of micropitting. This paper explored the operating conditions leading to micropitting and delved into the mechanism of S and P additives on micropitting, providing theoretical support for improving micropitting phenomena in engineering applications.