Elementary mechanisms of tribologically-induced oxidation in copper

在相对运动中相互作用表面的研究-称为摩擦学-在现代生活中非常重要，特别是对于在工程部件中普遍使用的金属材料。对于金属滑动伙伴，摩擦和磨损强烈地取决于其机械和表面性能，但由于摩擦诱导氧化，摩擦和磨损经常发生剧烈变化。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理解。这种迄今为止不完整的图片反过来不允许更耐摩擦氧化的材料的战略设计。为了解决这个在基础层面上理解摩擦诱导氧化的悬而未决的问题，KIT的Greiner小组凭借其材料摩擦学记录与马克斯普朗克铁研究所的Gault小组合作，拥有世界一流的原子探针断层扫描设施和专业知识。总之，我们的研究遵循这样的假设，即缺陷，主要是位错，是氧进入摩擦负载金属的核心途径。在那里-在材料内部-氧气然后反应形成氧化物。另一个研究问题，将接近的是确切的界面内发生氧化的材料。由于任何摩擦学接触的固有复杂性，这些问题只能通过模型系统来回答。因此，我们将三个不同的，战略选择的取向-（111），（110），（100）-与蓝宝石球体在干燥（无润滑），往复接触的单晶高纯度铜板配对。这些实验的环境，如环境气体（例如空气、干燥空气、干燥氮气）、湿度和温度将受到严格控制和密切监测。基本机制将通过高分辨率透射电子显微镜（HR-TEM），TEM内的能量色散X射线光谱和原子探针断层扫描（APT）进行非原位研究。在第二组实验中，样品将在测试期间或之后暴露于18 O2气氛。由于APT能够区分18 O和16 O，这些实验将揭示在摩擦学负载期间或之后发生了多少总氧化。通过系统地增加往复滑动循环的次数，从一个单独的通道，在一个完整的周期高达5000个周期，氧化和/或化学混合的各个阶段，以及确定这些阶段的基本机制将与两种类型的样品进行研究。阐明决定摩擦诱导氧化的基本机制预计将开辟有趣的基础研究途径，并从长远来看，允许战略性地设计材料及其微观结构，以提供更低的摩擦和减少的磨损。

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.