Three regimes in the tribo-oxidation of high purity copper at temperatures of up to 150 $^\circ$C

TL;DR

This study investigates how temperature influences tribo-oxidation of high-purity copper under sliding conditions, revealing three distinct regimes and the formation of CuO at elevated temperatures, which impacts surface properties.

Contribution

It provides new insights into temperature-dependent tribo-oxidation mechanisms and microstructural changes in high-purity copper during sliding contact.

Findings

Oxide layer formation occurs at 150°C but not at lower temperatures.

Increased tribo-oxidation depth and porosity with rising temperature.

CuO formation begins at temperatures ≥ 90°C.

Abstract

Surface oxidation of high-purity copper is accelerated under tribological loading. Tribo-oxide formation at room temperature is associated with diffusion processes along defects, such as dislocations or grain boundaries. Here, we embark on investigating the additional influence of temperature on the tribo-oxidation of copper. Dry, reciprocating sliding tests were performed with a variation of the sample temperature between 21 - 150 $^\circ$C. Microstructural changes were monitored and analyzed with state-of-the-art electron microscopy techniques. Oxide layer formation through thermal oxidation was observed for 150 $^\circ$C, but not for lower temperatures. As the temperature increases from room temperature up to 100 $^\circ$C, a significantly stronger tribo-oxidation into deeper material layers and an increase in the amount of formed pores and oxides was detected. Up to 75 $^\circ$C, diffusional processes along grain boundaries and dislocation pipes were identified. Starting at 100 $^\circ$C, CuO was detected. Hence, tribological loading significantly alters the CuO formation in comparison with static oxidation. Along with the CuO formation at temperatures >= 90 $^\circ$C, the oxide layer thickness decreased while the friction coefficient increased. The observations broaden our understanding of the elementary mechanisms of tribo-oxidation in high-purity copper. Eventually, this will allow to systematically customize surfaces showing tribo-oxidation for specific tribological applications.