Effects of process parameters of vacuum pre-oxidation on the microstructural evolution of CoCrAlY coating deposited by HVOF

The CoCrAlY coatings deposited using the high velocity oxygen fuel process (HVOF) onto the Inconel 718 superalloy were employed by three different vacuum pre-oxidation heat treatments, and subsequently, the specimens were subjected to isothermal oxidation exposure testing in air at 1050 °C for up to 100 h. The spinel oxides were found in the CoCrAlY coatings during the high velocity oxygen fuel process, and subsequently, after isothermal oxidation exposure in air, both the spinel and CoO oxides were formed in the TGO of the NVHT specimen. The evolution of both the phase transformation and the thermally grown oxides' (TGO) growth rate could be significantly influenced by adjusting certain process parameters of vacuum pre-oxidation heat treatments. The TGO of the VHT specimen was predominantly composed of θ-Al2O3 and α-Al2O3; however, Co(Cr, Al)2O4 and alumina were observed in the TGO of the VHT950 and VHT1000 specimens. The θ-Al2O3 to α- Al2O3 phase transformation in the TGO of the VHT specimen could be delayed after isothermal oxidation exposure. The TGO growth rate was significantly suppressed after the vacuum pre-oxidation heat treatments, and the order of the TGO growth rate was as follows: NVHT