Si-enabled self-lubrication and wear resistance of CrCoNiSi medium-entropy alloy films

High- or medium-entropy alloys have gained attention in coatings attributed to high wear resistance, with the formation of an amorphous phase. However, tailoring the crystal structure has limitations in improving both friction and wear characteristics due to the uncertain influence of amorphization on friction coefficients. In this respect, Si, known to increase glass-forming ability, can reduce the friction coefficient as a self-lubricating tribofilm formed by tribochemical reactions. However, tailoring Si content also affects crystal structures, defects, and mechanical properties, which require systematical investigations. Therefore, in this study, CrCoNiSi alloy films are fabricated with controlling Si content (0-13 at%) and sputtering power (5.8-13.7 W/cm2). The increase in Si content and sputtering power both produce identical microstructural evolutions, shifting from a crystalline towards an amorphous phase. In an amorphous film of high sputtering power, the high SiO2 fraction leads to dominant adhesive wear with delamination, thus contributing to the increase in wear rate. However, the Si addition alters the Si ion fraction of post-wear, forming Si-induced tribo-film, retaining abrasive wear, and consequently reducing the friction coefficient and wear rate. This Si addition would offer control over crystal structure and self-lubricating properties, enabling tailored mechanical and tribological properties.