Effect of Microstructure on Oxidation Resistance and TGO Formation in FeCoNiCrAl HEA Coatings Deposited by Low-Temperature HVAF Spraying

The effects of microstructure, density, and porosity of a FeCoNiCrAl high-entropy alloy (HEA) coating, fabricated using an internal diameter high-velocity air fuel (ID-HVAF) torch (model: i7 ID), on the isothermal oxidation behavior were investigated. This study pioneers the use of the ID-HVAF i7 ID system for HEA bond coat manufacturing, achieving a highly dense microstructure because of its low-operating spray temperature technique. To elucidate these effects, the microstructure and chemistry of the coating, the growth of the thermally grown oxides (TGOs), the phase transformation of alumina, and the oxidation rate were investigated at different temperatures. After 50 h at 1000 °C, 1100 °C, and 1150 °C, a dense, uniform, and thin alumina TGO layer (1.8 μm) was observed. The results demonstrate that the oxidation resistance of the HEA coating is enhanced because of the dense microstructure achieved via HVAF-i7, characterized by low porosity and uniform phase distribution, which contribute to improved barrier properties against oxygen diffusion. The growth of the TGO layer is controlled, resulting in a dense and continuous TGO layer. However, with increasing temperature and time, the alumina TGO layer becomes spalled, which is attributed to the absence of reactive elements. Overall, this study reveals that the FeCoNiCrAl HEA exhibits significant potential for enhancing oxidation resistance at high temperatures.