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

该项目旨在开发一种机械的、基于微观结构的模型，用于计算MoS2固体润滑滚动轴承的使用寿命，重点是径向轴承。这些轴承通常用于恶劣的环境，如真空，例如用于外星行星探测的飞行器。文献中唯一可用的寿命计算模型依赖于经验方法，该方法没有考虑薄膜的微观结构。我们从第一个资助期的发现证明，用PVD在几何复杂的表面涂覆一定会导致树枝状-片层状的MoS2生长。在摩擦学加载时，这种最初的纳米多孔涂层变成致密的薄膜，显示出基面织构，这符合摩擦学目的最理想的条件。这种转变在很大程度上控制了薄膜随后的摩擦学行为，包括MoS2固体润滑剂的去除和传输过程。因此，最重要的是充分理解这一过渡步骤，并将其纳入模式。如果做不到这一点，将导致使用寿命预测出现较大误差。变形机制、微观结构和摩擦学载荷之间的强烈相互作用需要一种尺度桥接方法，从基本过程的原子模拟延伸到材料转变的微观表征和模型组件的宏观摩擦学试验。薄膜的原子结构将在FIB剥离后用高分辨电子显微镜来解析，随后将用密度泛函和半经验势来模拟。此外，纳米压痕和纳米级实验将在扫描电子显微镜中原位进行，而宏观摩擦学测试将在真空中对双盘和滚珠轴承进行。这些技术将使我们能够从根本上了解(A)过渡过程中发生的织构重新取向和微观结构变化；(B)MoS2的去除和传输、摩擦膜的形成、其附着力以及由此产生的有效固体润滑剂的体积，这些都是内在因素(微观结构、纹理、孔隙率)和外部因素(接触载荷、基材粗糙度)的函数。这将允许我们扩展Birkhofer的使用寿命计算模型。这些增强功能将通过试验台实验进行验证。新的知识将作为优化过渡行为的基础，从而延长轴承的使用寿命，例如通过沉积后处理或受控磨合。计划中的研究将有助于提高我们对摩擦学引起的微观结构变化与固体润滑剂磨损行为之间关系的总体理解。

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.