A eutectic high-entropy alloy coating of novel hard-surface and soft-core structure with improved tribological properties

Structures with a hard surface and soft core have always been the goal of surface-coating technology. In this study, this goal was achieved in a eutectic high-entropy alloy (EHEA) surface by using aging treatment at elevated temperature. The EHEA surface structure comprised L21 and B2 phases, and a substantial amount of nanoprecipitates were observed inside the B2 and L21 phases. Interestingly, the structure near the interface resembled a “flowerpot seed”, whereas the structure away from the interface resembled a “flower-like” one. After aging treatment, Al diffused to the area away from the interface, resulting in increased volume fraction of L21. Cr and Fe diffused toward the interface, resulting in increased B2 phase. The hardness gradually decreased from the surface to the interior, creating a structure with a hard surface and soft core. The tribological properties of the surface also significantly improved, with decreased coefficient of friction from 0.514 to 0.471. Therefore, this EHEA is a promising candidate as a coating material for long-term applications at elevated temperatures.