Fundamental multiscale investigations for improved service life calculation of solid lubricated rolling bearings in vacuum

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The project aims at developing a mechanistic, microstructure based model for calculating the service life of MoS2 solid lubricated rolling bearings, with emphasis on radial bearings. These bearings are typically used in severe environments, such as vacuum, e.g. in vehicles used for extraterrestrial planetary exploration. The only service life calculation model available in the literature relies on an empirical approach, which does not take into account the microstructure of the films. Our findings from the first funding period evidence that coating geometrically complex surfaces by PVD invariably results in dendritic-lamellar MoS2 growth. Upon tribological loading, this initially nanoporous coating is turned into a compact film exhibiting a basal texture, which corresponds to the most desirable condition for tribological purposes. This transformation largely governs the subsequent tribological behavior of the film, including the processes of removal and transport of MoS2 solid lubricant. It is therefore paramount to fully apprehend this transition step and integrate it into the model. Failure to do so would cause large errors inservice life prediction. The strong interplays between deformation mechanisms, microstructure and tribological loading call for a scale bridging approach, stretching from atomistic simulations of fundamental processes, to microscale characterization of material transformations and macroscale tribological tests on model components. The atomistic structure of the films will be resolved by HRTEM following FIB lift-out and will be subsequently modelled by DFT and semi-empirical potentials. Furthermore, nanoindentation and nanowear experiments will be performed in situ in the SEM, whilemacroscale tribological tests will take place in vacuum on twin-discs and ball bearings. This arsenal of techniques will allow us to gain a fundamental understanding of (a) the textural reorientation and the microstructural changes taking place during the transition step; (b) the removal and transport of MoS2, the formation of a tribofilm, its adhesion and the resulting volume of effectively available solid lubricant, as a function of intrinsic (microstructure, texture, porosity) and extrinsic (contact loading, substrate roughness) factors. This will allow us to extend the service life calculation model from Birkhofer. These enhancements will be validated by test bench experiments. The new knowledge will serve as a basis for optimizing the transition behavior so as to extend the service lifetime of the bearings, e.g. through post-deposition treatments or controlled running-in. The planned investigations will contribute to improving our general understanding of the relationship between tribologically induced microstructural changes and the wear behavior of solid lubricants.