Investigation of the effect of humidity on the wear of cobalt metal

Cobalt-based alloys are widely used in aerospace and machinery due to their excellent mechanical properties, where extraordinary wear performance is also desirable to ensure stable operation. However, there is still scarce information on the tribological mechanism of the building block, the cobalt metal, especially under different humidity. The insight into the wear mechanism of Co under different humidity is crucial for the study of the tribological performance of Co-based alloys as well as exploring their potential applications under various working conditions. Here, we report the investigation of the humidity effect on the wear behavior of Co. The results showed that the Co exhibited an ultralow wear characteristic under the humid air environment (RH 70%) with the wear rate of 2.15 × 10^-7 mm³/Nm and dramatically increased by three orders of magnitude to 1.47 × 10^-4 mm³/Nm for dry ambient (~5% RH). Surface analysis revealed that the tribochemistry dominated the whole wearing process, with the worn surface almost fully covered by cobalt oxide, Co₃O₄, when subjected to the humid environment, whilst a small amount of oxide layers was only observed within the wear grooves under RH 5% testing condition. The stripe test results unraveled the evolution of this protective oxide generation, and the FIB/SEM of the cross-sections at different sliding stages bore out the role of tribochemistry for triggering such self-protection behavior. Our work provides a fundamental understanding of the wear mechanisms of Co metal, and we anticipate that this finding can offer valuable guidance for further improving the wear performance of cobalt-based alloys in the future.