Isothermal oxidation behavior of HVAF-sprayed NiCoCrAlY coatings

The functionality of the NiCoCrAlY coatings as oxidation protection or as a bond coat for thermal barrier coatings depends significantly on the oxide content and the microstructure. This study focuses on the development of NiCoCrAlY coatings with reduced porosity and oxide content using the AC-HVAF process, utilizing both coarse and fine powders. A CFD simulation model of HVAF process, validated by LDA measurements, was utilized to study the particle in-flight behavior of coarse and fine NiCoCrAlY powders. The process parameters derived from the simulation were applied for the deposition of the coatings on nickel-based superalloy substrates. Subsequently, the coatings were subjected to isothermal oxidation at 1000 degrees C in air. The microstructure of the coatings was analyzed both in the as-sprayed state and after the oxidation test. In addition, the morphology and thickness of the oxide scale were characterized by SEM, EDS and XRD. The oxidation kinetics were determined by measuring the thickness of the thermally grown oxides (TGO). The results showed that employing a relatively short stand-off distance ds = 200 mm determined from the simulation results resulted in dense coatings for both powder fractions. Both coatings exhibited higher oxide content compared to conventional HVAF processes due to the near-stoichiometric ratio required for process stability, which resulted in elevated process temperatures. Nevertheless, the developed NiCoCrAlY coatings displayed good oxidation resistance even without vacuum heat treatment, as indicated by the sub-parabolic growth rate of the TGO.