High Entropy Alloy (HEA) Bond Coats for Thermal Barrier Coatings (TBCs)—A Review

Abstract Due to the aggressive operation conditions of turbine hot sections, protective coatings are required to provide oxidation and hot corrosion resistance for superalloy components. Thermal barrier coatings (TBCs) are comprised of a ceramic top coat and a metallic bond coat (BC) and are typically used as thermal protection systems against these aggressive environments. Conventional BC materials are MCrAlX, with M being metals or alloys (e.g., Ni, Co or NiCo) and X being reactive elements such as Y, Hf, Ta, Si. Due to their strength, thermal stability, and oxidation resistance, high-entropy alloys (HEAs) have presented promise for use as BC materials in hightemperature applications. Owing to its cocktail effect, optimally chosen HEAs could help to enhance the hot corrosion resistance of BCs by forming a more continuous, dense, and uniform thermally grown oxide (TGO). Furthermore, HEAs could help to control the diffusion between the bonding layer and substrate in elevated temperature environments. This paper will discuss the thermodynamic, mechanical, and microstructural behaviour of HEAs. Furthermore, the selection and usage of HEAs as BCs will be explored and compared to conventional BCs in TBC systems.