Abstract: Many machine components are operated in dry sliding, elevated temperature, and oxidizing environments, leading to material failure or loss of functionality. Despite previous wear studies on conventional alloys, wear-related properties in high-entropy alloys (HEAs) and medium-entropy alloys (MEAs) up to 1000 °C are rarely reported. Here we systematically study the high-temperature hardness, wear behaviours and mechanisms of two popular MEAs, FeCoNi and CrCoNi, from room temperature to 1000 °C. We find that the wear resistance of FeCoNi surpasses that of CrCoNi at room temperature, 600 °C, and 800 °C. Contrarily, the wear resistance of CrCoNi surpasses that of FeCoNi at 400 °C and 1000 °C. By characterizing wear tracks, we identify that these wear-mechanism transitions are associated with alloy elements, oxidation rates, and oxide types. At room temperature, FeCoNi forms a spinel oxide layer with a lower wear rate than CrCoNi. At 400 °C, the wear rates of FeCoNi and CrCoNi are comparable because of temperature softening. At 600 °C and 800 °C, FeCoNi shows Co₃O₄ as the main constituent of the glaze layer, enhancing wear resistance compared to CrCoNi. At 1000 °C, such glaze layer in FeCoNi undergoes severe plastic deformation, reducing its wear resistance; the Cr₂O₃ oxide layer formed in CrCoNi remains hard and less deformable, contributing to its higher wear resistance. This study provides a fundamental understanding of the effect of principal elements on the wear performance in FeCoNi and CrCoNi-related MEAs and HEAs.

Key words: Wear, Tribo-oxidation, Medium-entropy alloys, Microstructures, Elevated temperatures