Effect of temperature on the tribo-oxidation of high-purity copper
The oxidation and corrosion of metals can occur in many applications such as hip implants, micro-mechanical systems or wind turbines. Tribo-oxidation is a mechanism, taking place by tribochemical reactions of the sliding partners or with the surrounding medium. Often, tribo-chemical reactions are accelerated with increasing temperatures. The formation of tribo-oxides, particularly in the very early stages of the sliding contact, is insufficiently understood. Our research aims to elucidate the elementary mechanisms of tribologically-induced oxidation by paring sapphire spheres with polycrystalline high-purity copper plates. Previous investigations reported the formation of copper oxides (Cu2O) during tribological loading . This oxidation process took place at rates order of magnitudes faster than the native oxidation of copper in contact with the same environmental conditions without tribological loading.
This work intends to understand the influence of the experimental temperature on the formation of these oxides. We vary the experimental temperature (22, 50, 75, 90, 100 to 125 °C) and investigate the resulting microstructure after 1,000 sliding cycles. The experiments were performed under mild tribological loading in a strictly controlled atmosphere with dry reciprocating linear sliding.
By applying state of the art scanning electron microscopy techniques, we investigated the resulting oxides within the plastically deformed subsurface. No oxide layer formation through thermal oxidation was observed in cross sections next to the wear track in the scanning electron microscope. Inside the wear track, the oxide depth with increasing temperature followed an Arrhenius law which is associated to diffusion processes. We believe the oxidation process is governed by diffusion processes along defects generated while sliding such as phase and grain boundaries.