Microstructure, thermostability and tribological behavior of composite CoCrFeNiTix high-entropy alloy coatings fabricated by laser cladding

CoCrFeNiTix (x = 0,0.25,0.5,0.75,1.0) high-entropy alloy (HEA) coatings were prepared on 45 steel by laser cladding, to explore the microstructural evolution, thermostability and the high-temperature tribological behavior of the coating. Ti-doping promotes more intergranular hard BCC phase and TiO2 intermetallics to precipitate from the ductile matrix of FCC phase, resulting in an increase in the averaged microhardness. The favorable thermostability of the heterogeneous microstructure and the oxide layer induced by friction endow the laser cladded HEA coating with excellent dry sliding wear-resistance in a wide temperature range from room temperature to 800 °C. Taking the place of the adhesive wear at room temperature, the oxidative wear becomes dominant in the wear process when the temperature exceeds 200 °C, contributing to the decrease in the wear rate before 600 °C. The formation of spherical particles on the wear track, as well as the oxide layer at elevated temperature, are revealed from the perspective of thermodynamic. It is found that the protective oxide layer is susceptive to delaminate due to the synergetic effect of strength reduction, internal stress accumulation and the destruction of the bonding, which explains the rise of the wear rate from its lower value of 1.84 × 10−5 mm3/(N·m) at 600 °C to 9.64 × 10−5 mm3/(N·m) at 800 °C.