Multi-scale simulations of the contact mechanics of lubricated, rough surfaces

UWO和加拿大通用汽车公司（GM）的申请者将开展与铝发动机磨损相关的合作研究。磨损限制了带有运动部件的机械装置的寿命，如汽车发动机中的气缸和活塞。了解和控制磨损对公司和消费者都有明显的经济利益。然而，令人惊讶的是，我们对磨损的微观起源知之甚少。造成这种缺陷的一个原因是工程接触中局部压力的分布难以预测。即使是高度抛光的表面，在足够高的分辨率下也会变得非常粗糙。更复杂的是，很难评估浸入在运动部件之间的润滑剂的性能。例如，很难预测当两个相对表面的两个凸起（小凸起）碰撞时润滑剂是否会被挤出，或者它是否会在这些微观接触点的巨大局部压力下凝固。对于现代的、节油的、基于铝的发动机来说，情况变得更加复杂。不仅表面粗糙，而且为了硬化材料，在铝基体中嵌入了小硅晶体。这些晶体从表面伸出，据说在滑动接触中承担了大部分载荷。不幸的是，人们对真正的压力分布知之甚少。描述接触力学的传统方法不适用于这些系统。通过将原子计算技术与更传统的方法相结合，我们打算评估哪些因素导致有利的机械行为。更具体地说，在粗糙的Al-Si表面上模拟的应力分布有望提供有价值的指导，例如如何优化表面的几何形状，哪种润滑剂性能是优先的（对粘度的压力依赖性强或弱），以及是否希望润滑剂在铝和/或硅为主的晶粒上具有良好的粘附条件。这些信息可以用于设计最小滑动摩擦和高耐磨性的铝发动机。

The applicants at UWO and General Motors of Canada (GM) will carry out collaborative research related to wear in aluminum engines.  Wear limits the life span of mechanical devices with moving parts such as cylinders and pistons in car engines.  Understanding and controlling wear is of obvious economic interest to companies and consumers alike. However, surprisingly little is known about the microscopic origins of wear.  One reason for this deficiency is that the distribution of local pressures in engineering contacts is difficult to predict.  Even highly polished surfaces turn out to be very rugged when looked at with sufficiently high resolution.  To complicate things, it is difficult to assess the behaviour of the lubricant immersed between the moving parts.  For example, it is very difficult to predict whether the lubricant gets squeezed out when two asperities (little bumps) of two opposed surfaces collide or whether it solidifies in response to the immense local pressures in these microscopic points of contact.  Things become even more complicated for modern, fuel-efficient, aluminium-based engines.  Not only are the surfaces rugged, but in order to harden the materials, little silicon crystals are embedded into the aluminium matrix.  These crystalites stick out of the surfaces and supposedly carry most of the load in sliding contacts. Unfortunately, very little is known as yet about the true pressure distribution. Conventional methods to describe contact mechanics break down for these systems.  By combining atomistic computation techniques with more conventional methods, we intend to assess what factors lead to favourable mechanical behavior. More specifically, the simulated stress distribution on a rough Al-Si surface promises to give valuable guidelines as to how one might want to optimize the geometry of surfaces, which lubricant properties are preferential (strong or weak pressure dependence of viscosity) and whether one would want good stick conditions of a lubricant on the aluminum and/or on the silicon-dominated grains.  This information can then be used to design aluminum engines for minimal sliding friction and high wear resistance.