Atomic Wear

轴承或齿轮等机械部件的磨损限制了机器的使用寿命，并导致操作成本和原材料成本增加，工厂停机时间延长。避免各种应用中的磨损和磨损具有巨大的经济和生态价值。经典的表面磨损理论区分了粘着磨损和磨粒磨损。前者描述了部分表面的剥离，而后者的特点是较软的表面被较硬的对接表面的粗糙度犁掉。对潜在的原子磨损过程的一般微观理解还不存在。分析这些过程的一个有用的方法是分析单个粗糙度的磨损情况。特别是，原子力显微镜已被证明是一种有价值的工具，因为它的几何形状非常类似于理想的单一粗糙面接触。近年来，摩擦力显微镜研究的实验结果推动了新的磨损模型。纳米级接触的原子磨损值非常低，每毫米滑动距离只有一个原子。因此，最近描述该过程的方法是将其建模为单个原子从表面分离，其中克服能垒的速度是用速率方程建模的。尽管速率理论首次得到了成功的应用，但它只是几种可能的描述之一。验证原子磨损速率模型的关键实验将是系统地测量纳米级触头的磨损量作为温度的函数。到目前为止，还没有进行过这样的试验，这是这项提案的主要重点。一个核心问题是：在多高的温度下，原子磨损是由热激活主导的？在很低的温度下，人们预计会发生向非热磨损的转变，遵循机械力化学的原理，即通过力的相互作用形成/断裂化学键。这些测量将用三个模型材料系统进行，以关注原子磨损的不同方面。利用超高真空条件下的摩擦力显微镜，研究离子晶体、聚合物以及金刚石表面的原子磨损机制。为了直接验证速率模型，磨损量将被量化为样品温度的函数。样本冷却/加热阶段允许评估从30K到700K的大温度范围。我们的中心项目目标是发现和了解预期的从热激活到非热原子磨损过程的转变。

The wear of mechanical components like bearings or gears limits the life time of machines and results in increased costs of operation and raw materials as well as down times of factories. Avoiding wear and abrasion in various applications is of enormous economical as well as ecological interest. The classic theory of surface wear distinguishes between adhesive and abrasive wear. The former describes the detachment of parts of the surface, while the latter process is characterized by the plowing of the softer surface by the asperities of the harder counter surface. A general microscopic understanding of the underlying atomic wear processes does not exist. A useful approach for the analysis of those processes is to analyze the wear of individual asperities. In particular the atomic force microscope has proven to be a valuable tool, since its geometry closely resembles the ideal single asperity contact.New wear models have in recent years been motivated by experimental results from friction force microscopy studies. The atomic wear for nanoscale contacts has extremely low values of only one atom per millimeter sliding distance. As a consequence recent approaches to describe the process model it as the detachment of individual atoms from the surface, where the rate of overcoming the energy barrier is modelled with the rate equation. Despite the first successful applications of the rate theory, it is only one of several possible descriptions.The key experiment to verify the rate model for atomic wear would be the systematic measurement of the wear of nanoscale contacts as a function of temperature. Such an experimental has not been conducted so far, and is the main focus of this proposal. One central question is: Up to which temperatures is atomic wear dominated by thermal activation? At very low temperatures one would expect a transition to athermal wear, obeying the principles of mechano-chemistry, i.e. the formation/breaking of chemical bonds by force interactions. The measurements will be conducted with three model material systems, to focus on different aspects of atomic wear. Using a friction force microscope in ultrahigh vacuum conditions the atomic wear mechanisms will be studied on ionic crystals, polymers as well as diamond surfaces. For the direct verification of the rate model the wear will be quantified as a function of sample temperature. A sample cooling/heating stage allows assessing a large temperature range from 30 K to 700 K. Our central project goal is to find and understand the anticipated transition from thermal activated to athermal atomic wear processes.