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

尽管摩擦是一种日常现象，但迄今为止，在从基本原理推导出摩擦的一致物理模型方面，几乎没有取得成功。在宏观尺度上，令人惊讶的是，阿蒙顿摩擦定律很好地描述了这一点，该定律指出，摩擦与载荷成正比，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，即接触界面原子的数量，这一定律是合理的。然而，目前的研究结果对这一基本假设的普遍有效性提出了挑战。不幸的是，现有的测量纳米级摩擦的技术，如传统的摩擦力显微镜，不适合解决与扩展纳米接触有关的问题。因此，了解摩擦基本原理的实验研究主要集中在只有几平方纳米的接触区域上。因此，为了在更大的尺度上测量摩擦，我们将采用一种新的方法，使用原子力显微镜在表面上推动纳米颗粒，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。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尺度的关键参数，我们将对不同尺寸和结构的纳米颗粒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，这些模型预测了不同的亚线性摩擦面积缩放规律，这种行为通常被称为“结构润滑”或“超润滑”。通过应用这些比例定律，就有可能控制摩擦。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。项目的另一部分是对颗粒和基材之间界面的动态过程进行分析。我们将重点分析基材势能格局中相邻最小值之间的基本粒子跳跃。特别是粘滑运动的温度和速度依赖关系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、亚单层的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。从技术角度来看，这个主题尤其相关，因为在任何现实世界的界面中，最小的污染是不可避免的。这些调查应有助于评估如何在技术装置中利用由于超润滑而消失的摩擦。

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.