Atomic Scale Mechanisms of Contact Ageing

术语接触老化描述了一种基本的摩擦学效应，其中摩擦的显著变化与基板和滑块之间的接触时间相关。接触老化可以通过静摩擦随时间的增加而表现出来，同时，通过考虑取决于滑动速度的动态接触时间，滑动摩擦的变化也可以与接触老化联系起来。这两种影响对技术和生态系统都很重要。由于持续接触而产生的明显的静摩擦峰值会导致磨损增加，并且地震活动的静态和动态阶段的交替可以与接触老化效应相关联。因此，科学家和工程师长期以来一直试图将接触老化纳入摩擦学模型。在大多数情况下，这些模型将接触老化与有效接触面积的持续增加联系起来，有效接触面积是由连接表面凹凸不平形成的。然而，必须考虑到老化过程不一定限于增加总接触面积。相反，现有接触的接触加强也可以发挥重要作用。在这里，特别是原子尺度的过程是相关的，例如化学键的形成或原子水平上的结构弛豫。然而，这些过程还没有得到很好的探索或理解，因此有助于问题，即接触老化通常需要用唯象模型来描述。本项目的主要目标现在在于实验分析接触老化的原子尺度机制。基于理论考虑，我们可以预测不同关键参数（如温度、载荷或剪切应力）的主要影响。通过对各种不同材料组合的这些参数进行系统变化，我们将能够识别不同的老化通道，并评估它们与技术相关系统的相关性。在此过程中，该项目还探讨了原子尺度老化过程是否与热激活的既定概念兼容以及机械应力如何影响原子水平上的界面演变的基本问题。所有这些问题都将通过基于扫描探针显微镜的实验技术来解决，其中一种特别为该项目开发的新方法允许分析接触老化，而无需考虑接触断裂期间的复杂过程。基于第一部分的结果，将对界面应用不同的机械致动手段，以增加或抑制接触老化。这两个方面都与纳米或微机电系统（NEMS，MEMS）高度相关，但尚未进行系统分析，因此必须考虑未解决的技术问题。

The term contact ageing describes a fundamental tribological effect, where significant changes in friction are associated with the contact time between substrate and slider. Contact ageing can manifest itself by an increase of static friction with time, while at the same time also changes in sliding friction can be linked to contact ageing by considering a dynamic contact time depending on the sliding velocity. Both effects can be important for technological and ecological systems. Pronounced static friction peaks resulting from continuous contact can lead to increased wear and also the alternating static and dynamic phases of seismic activity can be correlated to contact ageing effects.As a consequence, scientists and engineers are have long since been trying to incorporate contact ageing into tribological models. In most cases, these models associate contact ageing with a continuous increase of the effective contact area, that is formed by connecting surface asperities. However, it has to be taken into account that ageing processes are not necessarily restricted to an increasing overall contact area. Instead, contact strengthening of existing contacts can also play an important role. Here, especially atomic scale processes are relevant, like e.g. the formation of chemical bonds or structural relaxation on the atomic level. However, these processes not yet well explored or understood and therefore contribute to the problem, that contact ageing usually needs to be described by phenomenological models.The main goal of this project now lies in experimentally analyzing the atomic scale mechanisms of contact ageing. Based on theoretical considerations we can anticipate major influences by different key parameters like temperature, load or shear stress. By systematic variation of these parameters for a variety of different material combinations, we will be able to identify the different ageing channels and assess their relevance for technologically relevant systems. In doing so, this project also approaches the fundamental questions if atomic scale ageing processes are compatible with the established concepts of thermal activation and how mechanical stress can influence the interface evolution on the atomic level. All these questions will be approached by experimental techniques based on scanning probe microscopy, where a novel approach, that was developed especially for this project, allows the analysis of contact ageing without the need to consider the complex processes during contact rupture.A second part of the project then deals with active control of nanoscale contact ageing. Based on the results of the first part, different means of mechanical actuation will be applied to the interface in order to increase or suppress contact ageing. Both aspects are highly relevant for nano- or microelectromechanical systems (NEMS, MEMS), but have not yet been analyzed systematically and must therefore be considered unsolved technological problems.