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

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The project aims at gathering the fundamental knowledge required for improving the calculation of the service life of solid lubricated rolling bearings, which are typically used in vacuum pumps and rotating anodes of medical X-Ray devices. The project will specifically focus on MoS2 tribological coatings. The only service life calculation model available up to date relies on an empirical approach, based on the assessment of the wear rate from macroscopic contact parameters such as load and sliding distance. This model does not capture important processes, such as the progressive transfer and redeposition of coating material onto the uncoated counterpart. The proposed investigations will enable the development of a mechanistic model taking into account the influence of the microstructure, texture and stoichiometry of the coating and relying only marginally on empirical parameters. The primary challenge consists into identifying the fundamental mechanisms governing the deformation of the coating during rolling contact and understanding the consequences in term of material removal, as well as its transfer and deposition onto the uncoated counterpart. This goal will be met by implementing a multi-scale approach closely combining experimental characterizations with computer simulations. The investigations will focus on two different MoS2 PVD coatings: the first one featuring a strongly basal texture associated with a coarse microstructure and the second one a weak texture associated with a fine columnar microstructure. The tribological behavior of these coatings will be investigated at the macroscopic scale by twin-disc tests performed under realistic service conditions. Interrupted monitoring by electron microscopy and atom probe tomography will allow assessing the structural and crystallographic evolution of the coating at the micro and nanoscale as a function of the loading duration. Micromechanical testing will furthermore provide direct insights into the associated deformation and material removal processes. The underlying fundamental mechanisms will finally be revealed by atomistic simulations. As these account for the observed transformation of the coating, they are ultimately responsible for its lifetime, which implies that their knowledge will enable formulating a mechanistic model for the service life prediction of MoS2 solid lubricated rolling bearings.