Elementary mechanisms of tribologically-induced oxidation in copper

研究相对运动中相互作用表面的摩擦学在现代生活中非常重要，特别是对于工程部件中普遍使用的金属材料。对于金属滑动伙伴，摩擦和磨损在很大程度上取决于其机械和表面性能，但由于摩擦学诱导的氧化，通常会发生剧烈变化。虽然这种现象在文献中广为人知，但氧化的确切基本机制尚未完全了解。到目前为止，这张不完整的图片反过来又不允许对更耐摩擦氧化的材料进行战略设计。为了解决这个在基本层面上理解摩擦学诱导氧化的开放问题，KIT的Greiner小组与具有材料摩擦学记录的Max Planck铁研究所的Gault小组合作，拥有世界一流的原子探针断层扫描设备和专业知识。总之，我们的研究遵循这样的假设，即缺陷，主要是位错，是氧气进入摩擦学负载金属的核心途径。然后，在材料内部，氧气反应形成氧化物。另一个将要解决的研究问题是材料内部发生氧化的确切界面。由于任何摩擦学接触的固有复杂性，这些问题实际上只能用模型系统来回答。因此，我们将把三种不同的、精心选择的方向（111）、（110）、（100）的单晶高纯度铜板与蓝宝石球在干燥（无润滑）的往复接触中配对。这些实验的环境，如环境气体（如空气、干空气、干氮气）、湿度和温度将被严格控制和密切监测。将通过高分辨率透射电子显微镜（HR-TEM）、TEM内部的能量色散x射线光谱和原子探针断层扫描（APT）对其基本机制进行原位研究。在第二组实验中，在测试期间或之后，样品将暴露在18O2的大气中。由于APT能够区分18O和16O，这些实验将揭示在摩擦学负载期间或之后发生的总体氧化程度。通过系统地增加往复滑动循环的次数，从一个单独的通道，在一个完整的循环中达到5000个循环，氧化和/或化学混合的各个阶段以及决定这些阶段的基本机制将在两种类型的样品中进行研究。揭示决定摩擦学诱导氧化的基本机制有望开辟有趣的基础研究途径，并且从长远来看，可以战略性地设计材料及其微结构，从而降低摩擦和减少磨损。

The study of interacting surfaces in relative motion - called tribology - is of great importance in modern life, particularly for metallic materials which are ubiquitously used in engineering components. For metallic sliding partners, friction and wear strongly depend on their mechanical and surface properties, but often undergo dramatic changes due to tribologically-induced oxidation. While this phenomenon is widely known in the literature, the exact elementary mechanisms for oxidation are not yet fully understood. This so far incomplete picture in turn does not allow for a strategic design of materials more resistant to tribo-oxidation. In order to address this open question of understanding tribologically-induced oxidation at a fundamental level, the Greiner group at KIT with its material tribology track record is teaming up with the Gault group at the Max Planck Institute for Iron Research, having a world-class atom probe tomography facility and expertise. Together, our research follows the hypothesis that defects, mainly dislocations, are the core pathways through which oxygen enters a tribologically loaded metal. There – inside the material – oxygen then reacts forming oxides. Another research question that will be approached is the exact interface inside the material at which oxidation takes place. Due to the inherent complexity of any tribological contact, these questions can – realistically – only be answered with model systems. We will therefore pair single crystalline high-purity copper plates of three different, strategically chosen orientations - (111),(110),(100) - with sapphire spheres in a dry (unlubricated), reciprocating contact. The environment of these experiments such as ambient gases (e.g. air, dry air, dry nitrogen), humidity and temperature will be strictly controlled and closely monitored. The elementary mechanisms will be investigated ex situ by high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy inside the TEM, and atom probe tomography (APT). In a second set of experiments, samples will exposed to an 18O2 atmosphere, either during or after the tests. As APT is able to differentiate between 18O and 16O, these experiments will reveal how much of the overall oxidation is taking place during or after the tribological load. By systematically increasing the number of reciprocating sliding cycles from one individual pass, over one full cycle up to 5000 cycles, individual stages of oxidation and/or chemical intermixing as well as the elementary mechanisms determining these stages will be investigated with both types of samples. Shedding light on the fundamental mechanisms determining tribologically-induced oxidation is expected to open up interesting fundamental research avenues as well as on the long run allow to strategically design materials and their microstructures offering lower friction and reduced wear.