Elementary mechanisms of tribologically-induced oxidation in copper

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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.