Characterization of Molybdenum Based Coatings on 100Cr6 Bearing Steel Surfaces

Energy losses and friction locking are decisive factors in the conceptual design and sustainable realization of machine elements. Thus, the improvement of the tribological properties of rolling bearings by ceramic coatings on bearing surfaces represents a promising approach. These coatings are to be optimally adapted to the load case by minimizing the slip and resulting wear by rolling elements. For this purpose, molybdenum-based coatings were applied by means of magnetron sputtering in a vacuum atmosphere at controlled and adjusted oxygen partial pressure on 100Cr6 axial bearing washers. The effect of diffusing oxygen at near surface areas can be achieved during the physical vapor deposition (PVD) process itself as well as under adequate loading cases, so that a regenerative separation layer prevents high tribological wear at running surfaces. The generated layers were then characterized by high-resolution analysis with regard to morphology, attachment to the substrate and stoichiometry. The adjusted process parameters yielded pure molybdenum, as well as molybdenum oxide, dioxide and trioxide as a function of corresponding oxygen partial pressure. Scanning electron microscopy (SEM) was used for topographical evaluation, X-ray diffraction (XRD) for the characterization of stoichiometry and focussed ion beam cutting (FIB) for coating thickness determination. From selected surfaces, additional energy-dispersive X-ray spectroscopy (EDX) mappings were performed to quantify local oxygen contents at the border area of generated molybdenum layers. To record tribological characteristics, the layers were analyzed for their mechanical properties subsequently. Therefore, nanoindentational studies were carried out, which could provide information on the wear behavior in point contact in the form of nanoclay experiments. The results showed lower coefficients of friction for oxidized surfaces and thus a better resistance against sliding wear than uncoated specimen surfaces.