Dry lubrication of rolling contacts by self-regenerating molybdenum-oxide coatings

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Due to the changing global economical and ecological scenarios, the use of innovative environmental friendly and resource-saving processes is becoming increasingly important. The shortage of resources, such as fossil fuels and rare alloying elements, e.g. because of the emerging industrial nations such as China and India, requires an optimal and sustainable use of these resources. This can be achieved by an innovative combination of the design and the production processes for technical goods as well as mechanical engineering products. In this research project, the resource efficiency of rolling contact elements is to be achieved for dry lubrication regarding extended bearing life, avoiding early failures and a reduction of frictional losses. The aim of this project is to realize a reliable dry lubrication applying molybdenum-oxides as solid lubricants with self-regenerating properties. The coating of the dry lubricant offers a hard and wear-resistant surface and elastic properties in the matrix. In case of material removal, the underlying layer reacts with the atmospheric oxygen hence regenerates the wear-resistant surface. The application limits of the coatings needs to be investigated and their performance will be compared with the conventional grease-lubricated bearings. This joint research is performed as collaboration between three different institutes of the Leibniz Universität Hannover namely the Institute of Materials Science (IW), the Institute of Machine Design and Tribology (IMKT), and the Institute of Forming Technology and Machines (IFUM). The goal is to develop a self-regenerating solid lubricant that ensures its availability for highly loaded rolling surfaces.For this purpose, PVD coatings based on molybdenum oxide are used as a dry lubricant on the heavily loaded rolling surfaces. Particularly, molybdenum-tri-oxide (MoO3) is suitable as a solid lubricant due to its layer structure. Molybdenum-di-oxide (MoO2) serves as a reservoir between the rolling bearing steel and the functional surface. These properties distinguish MoO2 and MoO3 as possible coating materials for rolls in contact. During the operation of the bearings, a continuously layer degradation takes place in the highly loaded rolling contact zone, due to wear processes. At the same time, the solid lubricant layer is generated from the reservoir layer. The processes in the rolling contact are analysed in bearing test rigs. The wear processes have to be investigated in-depth by means of an FE model in combination with a numerical wear calculation. This research work will allow for a better comprehension of the wear behaviour of the coatings. In addition, it will be possible to draw conclusions about the wear resistance of the coatings in operation.