Co-32Ni -21Cr- 8Al-0.5Y powder was sprayed onto Inconel 718 alloy substrate by LPPS, HVOF, and APS. An improved HVOF with a gas-shroud attachment was also employed. Oxidation test was conducted in air at 1373K and the weight gain due to oxidation was measured up to 100 hours.  Microstructure and oxidation behavior of the CoNiCrAlY coatings were studied using SEM and XRD. The results were compared with the phase diagram calculated by a commercial program for Ni-base alloys.
Even though the feed stock powder consisted of γ and β phases, LPPS and APS coatings consisted of only γ phase. HVOF coatings consisted ofγ+β phases. After 5 hours of oxidation, however, LPPS coating retained the two-phase structure indicating that the initial structure was a metastable structure due to rapid solidification but APS coating remained in the γ-only structure. The calculated phase diagram suggests that Al content more than 6.8 mass % is necessary to form β but the Al in the metal matrix of the APS coating was expected to be below this level due to the formation of oxides during spraying.

The oxide scales formed on these coatings consisted of alumina (α and γ), spinel (NiCo₂O₄), and NiO. In terms of mass gain due to oxidation, LPPS and HVOF were comparable but the growth of the spinel oxide on the alumina was faster with the HVOF coating. APS coating exhibited a significantly greater mass gain during oxidation as compared to the other coatings.