Surface Fatigue of DLC-Coated Spur Gears

DLC coatings offer many benefits in machine component applications, such as reducing the tendency to adhesive wear and scuffing. However, in the case of surface fatigue wear (pitting), the reported results are contradictory. The authors present the results of model and component tests. The model tests were performed under single load using a cone-three ball tribosystem. The component tests were conducted on a back-to-back gear test rig under a wide range of Hertzian stresses. Both test types employed the W-DLC/CrN commercial antiwear coating for gear application. In model tests, the use of the coating resulted in a significant reduction in pitting resistance. A significant plastic deformation was visible on the wear surface. Similar tendency was observed when the W-DLC/CrN coating was deposited on one gear (pinion). The resistance to pitting decreased regardless of the applied load stage. However, it was observed that, with regard to the L50 life, the difference between an uncoated pair and a pair with a coated pinion became less noticeable when Hertzian stresses decreased. The results of the tests under different loads enabled the generation and analysis of the S-N curve and calculation of ϭHlim (the allowable stress number). The obtained values of allowable stress numbers for uncoated and DLC-coated gears were comparable. Thus, the coating influence on pitting resistance is complex, on one hand being highly dependent on the applied stresses, and on the other hand, very difficult to be properly determined during typical single load model tests.