LAUSR

DOI: 10.1002/admi.202001673 with a temperature increase in the contact, resulting in oxide formation.[13,14] Normal and shear stresses are capable to accelerate the rates of chemical reactions and sometimes even their path and with that influence tribo-oxide-formation, friction, and occurring wear mechanisms.[1,10,15–18] Here, the rate of oxidation is accelerated by orders of magnitude through tribological loading in comparison with native oxidation.[19] The influence of the formed oxides on the friction and wear response is quite different, depending on the generated oxides: harder and stiffer layers can lead to an increase in wear as well as higher friction.[20] By contrast, a mechanically softer layer might deform plastically, eventually leading to less wear.[21,22] In addition, such softer layers can dissipate some of the sliding energy by their growth. Formed oxide films can reduce friction and wear by preventing metal–metal contact,[23,24] known as mild oxidational wear.[25] As sliding progresses, a wide and complex range of possible processes can occur: protective “glaze-layers” may form,[26] wear particles can cause abrasive wear of the metal surfaces,[27,28] generated oxide films can be removed through tribological loading, and freshly exposed clean metal can be (re)oxidized.[29] Against the background of these manifold observations of tribo-oxidation, we embark on a systematic investigation with a tribological model setup. Copper was chosen as sample material since it serves as a model system to understand the formation of metal oxides in general.[30] For native oxidation, copper was found to readily oxidize in two main common forms of oxides: cubic, purple Cu2O, and monoclinic, black CuO.[30] To this day, native oxidation of copper is being extensively investigated.[30] Valladares et al. described a possible oxidation mechanism of a copper surface in an atmosphere with constant oxygen partial pressure:[31] oxygen is chemically Tribochemical reactions in many applications determine the performance and lifetime of individual parts or entire engineering systems. The underlying processes are however not yet fully understood. Here, the tribological properties of copper and its oxides are investigated under mild tribological loading and for dry sliding. The oxides represent the late stages of a copper–sapphire tribo-contact, once the whole copper surface is covered with an oxide. For this purpose, high-purity copper, thermally-oxidized and sintered Cu2O and CuO samples are tribologically loaded and eventually formed wear particles analyzed. The tribological behavior of the oxides is found to be beneficial for a reduction of the coefficient of friction (COF), mainly due to an increase in hardness. The results reveal tribochemical reactions when copper oxides are present, irrespective of whether they form during sliding or are existent from the beginning. Most strikingly, a reduction of copper oxide to metallic copper is observed in X-ray photoelectron spectroscopy measurements. A more accurate understanding of tribo-oxidation will allow for manufacturing welldefined surfaces with enhanced tribological properties. This paves the way for extending the lifetime of contacts evincing tribo-oxidation.