Substantially enhancing tribological resistance of CoCrNi medium-entropy alloy via CFSP-induced ultrafine-twinned microstructure

Cryogenic friction stir processing (CFSP) was applied to tailor the near-surface microstructure of a CoCrNi medium-entropy alloy and its tribological response. CFSP produced an ultrafine, twin-rich surface layer that significantly modified oxidation behavior and subsurface deformation during sliding. The CFSP sample formed a thin, dense, and stable Cr-rich amorphous tribo-oxide film, while the base material and air-processed alloy developed thick, cracked oxides prone to delamination. Beneath the CFSP tribo-layer, severe shear was accommodated through rapid grain subdivision, stacking-fault formation, and nano-twinning, effectively suppressing crack initiation. In contrast, the other samples experienced pronounced microcracking and abrasive wear. The results reveal that wear resistance in low-SFE alloys is controlled not by initial hardness but by tribo-oxide stability and the microstructural capacity to withstand friction-induced deformation. This work provides mechanistic guidance for microstructure-engineered wear optimization in medium-entropy alloys.