Stages in the tribologically-induced oxidation of high-purity copper

Surface modification through tribological loading is understood as a mechanically driven process. Yet, potentially complex chemical reactions can be critical since their products influence contact mechanics and affect friction and wear. Here we investigate the tribologically driven surface oxidation of pure copper in contact with a sapphire sphere. Different electron microscopy techniques reveal the formation of (amorphous) patches of copper oxide after very few loading cycles and their growth to semicircular amorphous/nanocrystalline cuprous oxide clusters under tribological loading. Understanding this elementary mechanism is of the utmost importance for a strategic tailoring of materials' microstructures for superior tribological performance.