Significantly lowering friction and wear of CoCrNi medium-entropy alloy film via massive interstitial carbon-induced amorphization

CoCrNi medium-entropy alloy (MEA) exhibits exceptional damage-tolerance, particularly at cryogenic temperatures, but demonstrates poor wear resistance. In this study, we propose a strategy to significantly reduce friction and wear of CoCrNi MEA film through the introduction of massive interstitial carbon-induced amorphization. The microstructure of as-sputtered CoCrNi film reveals an anisotropic columnar nanograin structure, featuring a primary face-centered-cubic (fcc) phase and a minor hexagonal-close-packed (hcp) phase. Upon the introduction of 16 at.% interstitial carbon (CoCrNi-C), a remarkable phase transformation occurs, resulting in an isotropic amorphous structure. Under low-load and low-sliding-velocity conditions, the CoCrNi-C film exhibits a 21% reduction in coefficient of friction (CoF) and 17% reduction in wear rate. More notably, when subjected to higher loads and sliding velocities, the CoCrNi film experiences rapid failure within the initial 100 cycles, manifesting as microcracks, breakage, and peeling. In contrast, the amorphous CoCrNi-C film demonstrates remarkable durability, sustaining over 5000 cycles without failure. The wear rate exhibits a negative correlation with load but a positive correlation with sliding velocity. This enhanced wear resistance can be attributed to the amorphous nature of the CoCrNi-C film, which inhibits local strain accumulation, preventing strain localization that typically leads to crack initiation and propagation in the CoCrNi film. Additionally, the formation of a protective oxide layer contributes to the improved wear resistance of CoCrNi-C film under higher loads and lower sliding velocities. This study provides valuable insights into the design of wear-resistant CoCrNi-based MEA coatings through massive interstitial carbon-induced amorphization. © 2024 Acta Materialia Inc.