Wear induced changes in surface topography during running-in of rolling-sliding contacts

Running-in occurs in rolling-sliding contacts under mixed-friction conditions and typically involves asperity smoothing through mild wear and plastic deformation. To improve the prediction of service life or friction of rolling-element bearings under mixed-friction conditions, knowledge of surface topography changes during running-in and their dependence on the operating condition is an important prerequisite. Therefore, this study aims to describe the surface mechanisms due to wear, during running-in and their dependence on one of the variables, slip. AISI 52100 steel specimens were tested in a mini traction machine (MTM) instrumented with friction and contact potential measurement in the presence of a PAO base oil, operating in the mixed lubrication regime. A novel method of pre and post-test surface relocation with 3-D optical profilometry and scanning electron microscopy (SEM) was implemented. Rolling-sliding tests were performed to study the effect of slide-roll ratio on the surface topography changes during running-in. Additional tests were conducted to track the changes to surface topography during this period. The results exhibit the rapid nature of running-in and how most of the surface topography changes occur in the first few load cycles. Surface topography transitions such as asperity removal due to wear, increase in the load-bearing area of the asperities and in tests with slip, plastic flow of material from peaks into adjacent valleys are shown at high magnification. Surface profilometry measurements show that the reduction in depth of the valleys due to plastic flow adds to the reduction in roughness, thus accurately identifying the surface mechanism, and redressing the consensus that the reduction in roughness during running-in is mainly due to wear and plastic deformation diminishing the height of the peaks, which is true for ideal running-in alone. •During running-in asperity peaks undergo wear and plastic deformation.•Slip is an aggressor of plastic deformation during running-in and results in asperity level material flow.