Refractory High-Entropy Alloy Coatings for High-Temperature Aerospace and Energy Applications

Refractory high-entropy alloys (RHEAs) were first developed a decade ago for aerospace applications, with the goal of manufacturing high-strength materials having higher structural performance than high-nickel superalloys. Herein, RHEAs were investigated as protective coatings that can provide increased erosion and corrosion resistance for high-temperature components. This is a step to demonstrate their use as a viable, cost-effective solution for both aerospace and energy industry needs. Two nearly equiatomic-composition RHEAs based on HfNbTaZr and MoNbTaVW are examined. A methodology for RHEA coating composition selection, manufacturing, and characterization is presented. It is shown that HfNbTaZr is suitable for harsh environments that do not include nuclear reactor radiation, while MoNbTaVW is suitable for harsh environments that include radiation. The air plasma spray (APS) and high-velocity oxygen-fuel (HVOF) thermal spray coating process is used to deposit 50 to 200-mu m thick functional coatings on stainless steel (SS) 321 and Inconel 718 substrates. Contact force-dependent friction and wear rates, as well as depth- and strain rate-dependent hardness, were obtained using spheroconical scratch-based and nanoindentation methods. The data show excellent adhesive properties, high strength, and reasonable homogeneity.