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

该项目旨在开发一种基于微观结构的机械模型，用于计算二硫化钼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中，例如真空，例如用于地外行星探索的车辆中。文献中唯一可用的使用寿命计算模型依赖于经验方法，该方法不考虑薄膜的微观结构。我们的研究结果从第一个资金周期的证据表明，涂层的几何复杂的表面PVD总是导致树枝状层状MoS2的增长。在摩擦学加载时，这种最初的纳米多孔涂层转变成表现出基底纹理的致密膜，这对应于摩擦学目的最理想的条件。这种转变在很大程度上决定了膜的后续摩擦学行为，包括MoS2固体润滑剂的去除和传输过程。因此，至关重要的是要充分理解这一过渡步骤，并将其融入模型。如果不这样做，将导致使用寿命预测的大误差。变形机制，微观结构和摩擦学负载之间的强烈相互作用要求一个规模的桥接方法，从原子模拟的基本过程，延伸到微观尺度表征的材料转换和宏观尺度的模型组件的摩擦学测试。薄膜的原子结构将通过高分辨透射电子显微镜（HRTEM）在FIB剥离后进行解析，并随后通过DFT和半经验势进行建模。此外，纳米压痕和磨损实验将在SEM中原位进行，而宏观摩擦学测试将在真空中在双盘和球轴承上进行。这些技术将使我们能够对（a）在转变步骤中发生的织构重新取向和微观结构变化有一个基本的了解;（B）MoS2的去除和输送、摩擦膜的形成、摩擦膜的粘附性和有效可用的固体润滑剂的所得体积，作为固有摩擦系数的函数，（微观结构、纹理、孔隙率）和外在（接触载荷、基底粗糙度）因素。这将使我们能够扩展Birkhofer的使用寿命计算模型。这些改进将通过测试台实验进行验证。新知识将作为优化过渡行为的基础，以延长轴承的使用寿命，例如通过沉积后处理或受控磨合。计划的调查将有助于提高我们的摩擦引起的微观结构变化和固体润滑剂的磨损行为之间的关系的一般理解。

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.