Detailed Analysis of the Crack Behavior around the Surface Dent under Rolling Contact Fatigue

Abstract

With recent trends toward carbon neutrality, represented by the progress in the electrification of automobiles, there is an increasing demand for higher-efficiency power transmission parts. Given this background, lubricants have tended to decrease in viscosity. Therefore, it is expected that the low-viscosity state will be promoted by the rise in lubricant temperature during operation, thereby resulting in an increasingly harsher operating environment for bearings. For example, in electric vehicles, the rotation speed of the motor is increased by weight reduction and the greater efficiency of the drive units. Under such harsh environments, the working parts come into contact with one another and generate wear particles that can bring surface dents on the raceway. Furthermore, iron powder and scale may be accidentally brought to the surface of bearings for steel rolling mills, forming dents on the raceway. Surface dents are well known as a root cause of surface-originating premature flaking under rolling contact fatigue (RCF) in bearings. Previously reported results indicate that cracks are generated around the edge of the dent and then cause flaking because of crack propagation. Several previous studies proposed hypotheses on crack initiation mechanisms. However, there are few investigations of crack initiation mechanisms under a contaminated lubricating condition. In this study, the details of the crack generation process around dents were investigated so as to devise new countermeasures for further improvements in bearing steel materials. A rolling fatigue test was conducted in which hard steel particles were artificially introduced into the lubricant as foreign matter. Observation of the surface and cross sections of the specimen showed a causal relationship between the microscopic plastic deformation and crack initiation near the surface region around the dent. These findings are useful for developing countermeasures to strengthen steel against flaking that originates from a dent.