Atomic Friction - (AFRI)

在这个建议中，我们打算了解原子尺度摩擦的极端情况下的摩擦和磨损特性，在这种情况下，只有几个原子构成尖端-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、载荷依赖和超润滑性(结构和外部诱导的)等新效应一直是研究的目标，其中许多现象仍然存在争议。在这一点上，存在大量的实验和理论工作，然而，只有几篇论文报道了实验和理论的直接重叠。原子级摩擦特别适合于直接比较，因为接触尺寸尽可能小，因此比传统的摩擦学实验定义得更好。这引发了原子摩擦实验与第一性原理和基于离散原子几何的分子动力学模拟(MD)的直接比较。一个中心问题是缺陷和界面的作用。我们将在超高真空条件下研究缺陷附近的原子摩擦，并与理论研究进行比较。热驱动的重要作用间接来源于原子摩擦的速度依赖性。对玻璃态聚合物进行了温度研究，发现旋转受阻是相关机制。然而，超高真空下原子摩擦的温度研究仍然缺乏。因此，本项目的一个重要目标将是研究从25K到1000K的原子摩擦。最近，人们从理论上对高温摩擦进行了研究，其中通过滑行效应预测了高温摩擦的强烈减少量。在很低的速度下，还从理论上预测了由于热激发而产生的摩擦降，即热溶度效应。我们将尝试通过实验来验证这些效应。NC-AFM测量表明，在近接触处，每周耗散约1 eV量级。First原理和具有真实针尖几何形状的MD模拟与NC-AFM实验进行了比较，结果表明，针尖结构变化导致的粘合滞后是耗散的根源。我们的目标是进行小幅度的NC-AFM实验，并直接与模拟进行比较。另一种极端的纳米尺寸滑动接触是在高法向力下，在那里发生磨损。在这里，我们计划进行作为负载和速度的函数的实验，并使用特高压微型摩擦试验机扩展接触区域，以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.