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

虽然摩擦是一种日常现象，但迄今为止，在找到一个一致的物理模型来从基本原理中推导出它方面几乎没有成功。在宏观尺度上，阿蒙顿摩擦定律令人惊讶地描述了事情，该定律指出摩擦与载荷成比例，但与表观接触面积无关。这个定律是合理的假设，摩擦是直接成正比的“真正的”接触面积，即接触的界面原子的数量。然而，目前的研究结果挑战了这一基本假设的普遍有效性。不幸的是，已建立的技术来测量纳米级的摩擦，如传统的摩擦力显微镜，是不适合解决有关的问题，延长nanocontacts。因此，了解摩擦基本原理的实验研究主要集中在只有几纳米平方的接触面积上。为了在更大的尺度上测量摩擦力，我们将采用一种新的方法，使用原子力显微镜将纳米颗粒推到表面上，同时记录推力。用这种方法，我们可以测量与原子定义的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缩放的关键参数，我们将对具有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，这些模型预期不同的次线性摩擦面积比例定律，通常被称为“结构润滑性”或“超润滑性”的行为。通过应用这些比例定律，甚至可以控制摩擦。更确切地说，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。该项目的另一部分被分配到粒子和衬底之间的界面的动态过程的分析。我们将着重分析基片势能面上相邻极小值之间的基本粒子跳跃。特别是温度和速度的依赖性所产生的粘滑运动可以揭示独特的洞察动态界面过程。众所周知，即使是界面的轻微亚单层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，计划分析受控界面污染的影响。从技术的角度来看，这个主题特别重要，因为在任何真实的世界界面中，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.