Enhancement of surface hardness and self-lubrication of CoCrFeNiMn high entropy alloy through laser boriding

Abstract

High entropy alloys (HEAs), such as CoCrFeNiMn, are garnering significant attention due to their exceptional mechanical properties. However, their relatively low hardness and wear resistance limit their applicability in demanding industrial environments. This study investigates the enhancement of the surface properties of CoCrFeNiMn HEA through laser boriding—a technique that combines laser cladding with boriding to create a hard laser boriding layer. FeB powder was employed as the boron source during the laser boriding process, where it reacted with the substrate to form boriding layer. The effects of various process parameters, including laser power and scan speed, on the microstructure, phase composition, mechanical properties, and tribological behavior of the modified surface were systematically evaluated. A laser boriding layer ranging from 472 to 1250 μm in thickness was successfully fabricated, exhibiting a biphasic reticulated structure consisting of Fe₂B-type and FCC phases. The surface hardness of the modified alloy increased up to seven times that of the original HEA. Additionally, the ratio of plastic work to total work (W_pl/W_total) of 54.9 % further suggested excellent plastic deformability, indicating superior mechanical properties of the laser boriding layer. Tribological testing demonstrated outstanding self-lubricating properties under water lubrication, with a 35.5 % reduction in the coefficient of friction and an 82.9 % decrease in wear rate. XPS analysis revealed the formation of a boron-containing tribofilm, which contributes to improved self-lubrication, reducing friction and wear. The findings provide a promising approach for rapidly modifying HEAs to improve their industrial performance.