Abstract:
This work concentrates on developing oxidation resistant coatings using novel high entropy alloys (HEAs) via thermal spray (TS) processing. The intended application is as bond coats in thermal barrier coatings (TBCs), employed in turbojet engines. Bond coats are the intermediate layer between the superalloy substrate and the overlying thermally insulating topcoat, responsible for protecting the substrate from high temperature oxidation during service. Typical bond coats are made of Al-containing alloys such as MCrAlY (M=Ni/Co), or Ni/Pt aluminides. The advent of HEAs has rekindled interest in the field of bond coat research, which was long considered saturated.

As the starting point for this coating development program, AlCoCrFeNi, a thoroughly studied HEA with proven oxidation resistance capabilities was selected. Atmospheric plasma spray (APS), the most well-established TS technique was the chosen route of coating generation. APS AlCoCrFeNi coatings were characterized in-depth to understand their microstructural evolution and their governing parameters. Significant in-flight oxidation (IFO) was also observed, which was expected to deteriorate the HEA coating’s oxidation resistance abilities. Several feedstock and TS parameters were identified which affect the coating quality to various degrees. Two major routes were found to reduce the extent of IFO, (1) changing the feedstock to a more oxidation resistant HEA composition; (2) changing the coating technique to one with less active environment than APS.

Towards the former, four variants of AlCoCrFeNi with different Al, Cr and Fe contents were developed, of which AlCoCr0.5Ni and Al2CoCrFeNi were found to exhibit better oxidation resistance than AlCoCrFeNi. Further, TBCs with AlCoCr0.5Ni HEA was used as feedstock and sprayed as bond coat via APS. This APS-HEA-TBC exhibited oxidation rates on par with conventionally sprayed CoCrAlY coatings, along with development of a dual layer TGO (thermally grown oxide). Cross section microstructural analysis of the oxidized coatings also indicated development of internal oxides, attributed to defects and porosities in the coating, which can be optimized to improve the coating performance further.

Towards the latter proposition, APS was replaced by cold spray technique for generating AlCoCrFeNi coatings, and the HEA was found to undergo zero IFO, completely preserving the feedstock phases in the coating. The advantage of this was observed in the growth of a protective alumina layer upon oxidation.

Oxidation resistant HEA compositions were thus developed successfully, as well as an understanding of various governing parameters which can be optimized to obtain desired coating characteristics. While individual parameters were targeted in this study to clearly discern their effect, learnings gained from this thesis can be applied collectively to extract the potential offered by TS HEA coatings.
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