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 degrees 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 theta-Al2O3 and alpha-Al2O3; however, Co(Cr, Al)(2)O-4 and alumina were observed in the TGO of the VHT950 and VHT1000 specimens. The theta-Al2O3 to alpha-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<VHT950<VHT<VHT1000. The tensile stress at the alumina area of the TGO in the VHT and VHT950 specimens reduced considerably compared to that in the NVHT specimen, and finally, for the VHT1000 specimen, the tensile stress turned to compressive stress. The elemental interdiffusion behaviour between the HVOF-coating and nickel-based superalloy was characterized, and the diffusion activation energy of cobalt, aluminium and chromium were attempted to be estimated. (C) 2017 Elsevier B.V. All rights reserved.