High Entropy Alloy Bond Coats for Thermal Barrier Coatings: A Review

Thermal barrier coatings (TBCs) are commonly employed as thermal protection systems in harsh environments and are comprised of a ceramic top coat and a metallic bond coat. The traditional materials used for bond coats are MCrAlX alloys, where M represents metals or alloys (such as Ni, Co, or NiCo), and X represents reactive elements like Y, Hf, Ta, and Si. In high-temperature environments, high-entropy alloys (HEAs) have exhibited considerable potential as viable alternatives to bond coat materials, owing to their remarkable mechanical strength, thermal stability, and oxidation resistance. The distinctive compositional characteristics of HEAs can augment bond coat hot corrosion resistance by facilitating the formation of a thermally grown oxide (TGO) characterized by enhanced continuity, density, and uniformity, attributed to the synergistic effects arising from their unique alloying elements. Moreover, HEAs can modulate diffusion phenomena at the interface between the bond coat and substrate, particularly under elevated temperature conditions. This manuscript explores the thermodynamic, mechanical, and microstructural behavior of HEAs, as well as, the selection and application of HEAs as bond coats in comparison to conventional bond coats.