Fretting damage of high carbon chromium bearing steel
This thesis consists of four sections, the fretting wear properties of high carbon chromium bearing steel; the effect of debris during fretting wear; an introduction of a new fretting wear test apparatus used in this study; and the effects of fretting damage parameters on rolling bearings. The tests were operated under unlubricated conditions. Using a crossed cylinder contact arrangement, the tests were carried out with the normal load of 3N, slip amplitude of 50µm, and frequency of 30Hz at room temperature. The new fretting wear test rig consists of a sphere-on-plate arrangement, and the normal load and slip amplitude were variously changed. Using the new test rig, the tests were performed both at room temperature and 200℃, and tensile stresses were applied to the lower stationary specimens during the fretting wear tests. In the fretting wear tests after tempering at 200,230,260 and 350℃ in air, the high carbon chromium bearing steel showed low coefficients of friction due to a glaze type oxide film. In the fretting wear tests at 200℃, a very low coefficient of friction was obtained. Consequently, the oxide films on high carbon chromium bearing steel tempered at 200,230,260 and 350℃ were thought to be protective in fretting damage. Fretting wear volumes were measured using different specimen combinations and fretting oscillatory directions relative to the axes of the cylindrical specimens, although of the same material couples. It has been found that fretting wear volume is significantly governed by frictional energy (fretting damage per unit area) and frequency of metal-to-metal contact, as determined by electrical contact resistance measurements. Metal-to-metal contact was observed throughout the whole stage of fretting wear even in the case of full slip fretting wear. Fretting crack initiation is encouraged but fretting crack propagation rate is not significantly affected by high normal loads. Compressive residual stresses in the subsurface have little influence on crack initiation, but have a large influence on crack propagation rate. In the study of fracture induced by fretting wear, a critical slip amplitude which led to the shortest fracture life was identified. With the critical slip amplitude (35 µm), a higher coefficient of friction was obtained, and this result suggested a significant effect of coefficient of friction on fracture induced by fretting wear (or fretting fatigue). The mechanisms of fretting wear and fretting fatigue were also discussed. Fretting wear is predominantly governed by the total tangential shear strain due to fretting oscillation. In contrast, fretting fatigue is dominated by the maximum alternating tangential shear strain energy. As coefficient of friction affects significantly both the total tangential shear strain and the maximum alternating tangential shear strain energy, it is thought to be the most important factor which needs to be controlled to reduce damage by both fretting wear and fretting fatigue.