High entropy alloy coatings
High Entropy Alloy Coatings
My work is centered around finding a HEA composition that can be a possible alternative to present day bond coats used in thermal barrier coatings. Traditionally, TBCs are thermally insulating ceramics bonded onto superalloy components via metallic bond coats, which are usually Ni based dual phase alloys having sufficient Al to form an oxidation resistant alumina layer during service. However, over time this oxide layer thickens and spalls off under thermal cycling strains, thereby compromising the entire TBC. At the same time, diffusion of rare earth elements from the bond coat into the superalloy is another serious issue that deteriorates the mechanical properties of the component. HEAs can thus be possible candidates for bond coats, owing to sluggish diffusion through them. A slow growing yet resilient oxide layer, or possibly a high entropy oxide layer can provide the desired protection from oxygen ingress. Towards this, we are looking at alloy compositions based on Al, Ni, Co, Cr, Ti, Fe with minor additions of reactive elements such as Y, Zr, Yb, etc. Powders are synthesized by mechanical alloying, done here at IITM, and are coated via (i) Atmospheric Plasma Spray (APS) at Swinburne University of Technology with Prof. C.C. Berndt, and (ii) Cold Spray, done at ARCI, Hyderabad. APS results in a composite microstructure, with alumina and Al-Cr rich oxides interlaced with metallic splats, whereas Cold Spray does not see any oxidation or melting of feedstock. The plasma sprayed coatings showed a hardness of about 4GPa while the conventional NiCrAlY coatings usually give a hardness of about 2.5GPa.
Fig - XRD patters of mechanically alloyed AlCoCrFeNi powder vs. APS coated AlCoCrFeNi
Fig - BSE image of APS coating cross section. Dark grey phases are oxides, while light grey ones are metallic phases