An exceptionally wear-resistant CoFeNi2 medium entropy alloy via tribo-induced nanocrystallites with amorphous boundaries

Single-phase face centered cubic (FCC) high entropy alloys (HEAs) usually display high coefficients of friction and poor wear resistance at ambient temperature, especially for CoCrFeMnNi HEA and its derived CoCrFeNi and CoCrNi medium entropy alloys (MEAs). Here, we reported a novel CoFeNi2 MEA exhibiting relatively low COFs (0.37–0.40); in particular, the wear rates (7.04 × 10−7 - 3 × 10−6 mm3 N−1 m−1) are two orders of magnitude lower than the other derivatives. This is primarily due to the unexpected occurrence of topmost micrometer-thick layer consisting of sub-10 nm nanocrystallites surrounded by the amorphous boundaries during steady-stage sliding. This study ignites some insights into designing exceptional friction and wear resistant MEAs/HEAs by linking to tribo-induced surface deformation mechanisms.