Atomic Friction - (AFRI)

在这个提议中，我们打算了解原子尺度摩擦的极端情况下的摩擦和磨损性能，其中只有少数原子构成针尖-样品接触。尽管最近在理解原子摩擦过程方面取得了成功，其中速度依赖性，负载依赖性和新的效应，如超润滑性（结构和外部诱导）已经成为目标，其中许多现象仍处于争议之中。在这一点上，大量的实验和理论工作存在，但是，只有少数论文报告的实验和理论的直接重叠。原子尺度摩擦特别适合直接比较，因为接触尺寸尽可能小，因此比传统的摩擦学实验更好地定义。这就要求直接比较原子摩擦实验与基于离散原子几何的第一性原理和分子动力学模拟（MD）。一个核心问题是缺陷和界面的作用。我们将研究在双真空条件下缺陷附近的原子摩擦，并与理论研究进行比较。热驱动的重要作用是间接地从原子摩擦的速度依赖性导出的。对玻璃态聚合物进行了温度研究，发现受阻旋转是相关机制。然而，在双真空下原子摩擦的温度研究仍然缺失。因此，该项目的一个重要目标将是研究温度从25 K到1000 K的原子摩擦。最近，高温摩擦进行了理论研究，其中一个强大的减少预测的滑冰效果。在非常低的速度下，由于热激发，理论上也预测了摩擦力的下降，即热鲁比效应。我们将尝试用实验来验证这些效应。Nc-AFM测量表明，在近接触中发现了每周期1 eV的量级的耗散。第一性原理和MD模拟与现实的针尖几何形状进行了比较与nc-AFM实验，这表明，由于针尖配置变化的粘附滞后是起源耗散。我们的目标是进行nc-AFM实验与小振幅，并直接与模拟比较。滑动纳米尺寸接触的另一个极端状态是在高法向力下，在那里发生磨损。在这里，我们计划进行实验作为负载和速度的函数，并扩大接触面积与使用的超高压微摩擦仪，探索多粗糙接触的行为。

In this proposal we intend to understand friction and wear properties in the extreme case of atomic scale friction, where only a few atoms constitute the tip-sample contact. Despite recent successes in the understanding atomic friction processes, where the velocity dependence, load dependence and new effects like superlubricity (structural and externally induced) have been targeted, where many phenomena are still under dispute. At this point, a multitude of experimental and theoretical work exists, however, only a few papers report on the direct overlap of experiments and theory. Atomic scale friction is particularly well suited for direct comparison, since the contact size is as small as possible, and thus is much better defined than in conventional tribology experiments. This invites direct comparison of atomic friction experiments with first principles and molecular dynamics simulations (MD) based on discrete atom geometries. A central question is the role of defects and interfaces. We will investigate atomic friction in the vicinity of defects under ultrahigh vacuum conditions and compare with theoretical studies. The important role of thermal actuation was indirectly derived from the velocity dependence of atomic friction. Temperature studies were performed on glassy polymers, where hindered rotation was found to be the relevant mechanism. However, temperature studies of atomic friction under ultrahigh vacuum studies are still missing. Therefore, an important goal of this project will be the study of atomic friction at temperature from 25K up to 1000K. Recently, high temperature friction was studied theoretically, where a strong reduction is predicted by the skating effect. At very low velocities, a drop of friction due to thermal excitation is also predicted theoretically, the thermolubricity effect. We will try to verify these effects experimentally. Nc-AFM measurements have shown that dissipation of the order of 1 eV per cycle is found in near contact. First principles and MD-simulations with realistic tip geometries were compared with nc-AFM experiments, which demonstrated that adhesion hysteresis due to tip configuration changes is the origin for dissipation. Our aim is to perform nc-AFM experiments with small amplitudes and to directly compare with simulations. The other extreme regime of sliding nanometer-sized contacts is at high normal forces, where the onset of wear occurs. Here, we plan to perform experiments as a function of load and speed and to extend contact areas with the use of a UHV-microtribometer to explore the behavior of multi-asperity contacts.