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

Cobalt-based (Co-based) alloys are widely used in aerospace and machinery applications because of 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 cobalt metal building block, especially under different humidities. Insight into the wear mechanism of Co under different humidities is crucial for studying the tribological performance of Co-based alloys as well as their potential applications under various working conditions. Here, we report an investigation of the effect of humidity on the wear behavior of Co. The results revealed that Co exhibited ultralow wear characteristics in a humid air environment (relative humidity (RH) 70%), with a wear rate of 2.15×10^–7 mm³/(N·m), and dramatically increased by three orders of magnitude to 1.47×10^–4 mm³/(N·m) in the dry environment (~5% RH). Surface analysis revealed that tribochemistry dominated the whole wearing process, with the worn surface almost fully covered by cobalt oxide, Co₃O₄, when subjected to a humid environment, whereas a small number of oxide layers were observed only within the wear grooves under the RH 5% testing condition. The stripe test results revealed the evolution of this protective oxide generation, and the focused ion beam scanning electron microscopy (FIB-SEM) images of the cross-sections at different sliding stages revealed the role of tribochemistry in triggering such self-protection behavior. Our work provides a fundamental understanding of the wear mechanisms of Co metal, and we anticipate that these findings can offer valuable guidance for further improving the wear performance of cobalt-based alloys in the future.