Friction of mesoscopic contacts - Analysis by manipulation of nanoparticles using atomic force microscopy

尽管摩擦是一种日常现象，但到目前为止，在从基本原理中找到一致的物理模型来推导摩擦方面，几乎没有取得什么成功。在宏观尺度上，阿蒙顿摩擦定律令人惊讶地很好地描述了事情，该定律指出，摩擦力与载荷成正比，但与表观接触面积无关。假设摩擦力与“真实”接触面积成正比，即接触中的界面原子数，这一定律是合理的。然而，目前的发现挑战了这一基本假设的普遍有效性。不幸的是，现有的测量纳米级摩擦力的技术，如传统的摩擦力显微镜，不适合解决与延长纳米接触有关的问题。因此，为了了解摩擦的基本原理，实验研究大多集中在仅有几纳米见方的接触面积上。因此，为了在更大尺度上测量摩擦力，我们将应用一种新的方法，使用原子力显微镜将纳米颗粒推到表面上，同时记录推力。利用这种方法，我们可以测量具有原子定义界面的介观接触的摩擦力。此外，还可以调整界面的关键实验参数，如结晶度、取向和形状。为了确定控制界面摩擦比例的关键参数，我们将对不同尺寸和结构的纳米粒子的摩擦与当前理论模型的预测进行系统的比较。根据特定的界面结构，这些模型预计会出现不同的次线性摩擦面积比例律，这种行为通常被称为“结构润滑性”或“超润滑性”。通过应用这些比例定律，甚至应该可以控制摩擦。更准确地说，我们打算利用热诱导的结构相变来改变Sb纳米颗粒的摩擦至少一个数量级。该项目的另一部分任务是分析颗粒和衬底之间的界面上的动态过程。我们将重点分析基片势能图景中相邻极小值之间的基本粒子跳跃。特别是，由此产生的粘滑运动的温度和速度相关性可以揭示动态界面过程的独特见解。众所周知，即使是轻微的亚单层界面污染也会导致界面摩擦的显著变化。因此，实验将在超高真空条件下进行。此外，还计划分析受控界面污染的影响。从技术的角度来看，这个话题特别相关，因为在任何现实世界的界面中，最小的污染都是不可避免的。这些研究应该有助于评估如何在技术设备中利用由于超润滑性而导致的摩擦消失。

Although friction is an everyday phenomenon, there has been little success so far in finding a consistent physical model to derive it from fundamental principles. On the macroscale things are surprisingly well described by Amontons law of friction, which states that friction is proportional to the load but is independent of the apparent contact area. This law is rationalized by the assumption that friction is directly proportional to the 'true' contact area, i.e. the number of interface atoms in contact. However, current findings challenge the general validity of this fundamental assumption. Unfortunately, established techniques to measure nanoscale friction, like conventional friction force microscopy, are unsuited to address questions related to extended nanocontacts. Therefore experimental studies to understand the fundamentals of friction have mostly focused on contact areas of only a few nanometers square. To measure friction on larger scales we will thus apply a new approach, using an atomic force microscope to push nanoparticles on a surface while the pushing force is simultaneously recorded. With this approach we can measure friction of mesoscopic contacts with atomically defined interfaces. Moreover, it is possible to tune crucial experimental parameters of the interface, like crystallinity, orientation and shape. To identify the key parameters governing the scaling of interfacial friction, we will perform a systematic comparison of the friction of nanoparticles with different sizes and structures to the predictions from current theoretical models. Depending on the specific interface structure those models anticipate distinct sublinear friction-area scaling laws, a behavior often referred to as 'structural lubricity' or 'superlubricity'. By applying those scaling laws, it should then be even possible to control friction. More precisely, we intend to utilize a thermally induced structural phase transition to change friction of Sb-nanoparticles by at least an order of magnitude. Another part of the project is assigned to the analysis of dynamic processes at the interface between particle and substrate. We will focus on the analysis of the fundamental particle jumps between adjacent minima on the substrate's potential energy landscape. Especially the temperature and velocity dependence of the resulting stick-slip motion can reveal unique insight into dynamic interface processes. It is known, that even a slight, sub-monolayer contamination of the interface can lead to significant changes of interfacial friction. Experiments will thus be done under ultra-high vacuum conditions. In addition, it is planned to analyze the influence of controlled interface contamination. This topic is especially relevant from a technological point of view, since minimal contamination is unavoidable in any real world interface. Those investigations should help to assess how vanishing friction due to superlubricity can be utilized in technological devices.