Microstructural characterization of thermal barrier coating on Inconel 617 after high temperature oxidation
Keywords: Oxidation, Atmospheric plasma spray, NiCrAlY, TGO and YSZ.
AbstractA turbine blade was protected against high temperature corrosion and oxidation by thermal barrier coatings (TBCs)using atmospheric plasma spraying technique (APS) on a Ni-based superalloy (Inconel 617). The coatings (NiCr6AlY/ YSZ and NiCr10AlY/YSZ) consist of laminar structure with substantial interconnected porosity transferred oxygen from Yittria stabilized Zirconia (YSZ) layer toward the bond coat (NiCrAlY). Hence, a thermally grown oxide layer (TGO) was formed on the metallic bond coat and internal oxidation of the bond coat occurred during oxidation. The TBC systems were oxidized in a normal electrically heated furnace at 1150 °C for 18, 22, 26, 32 and 40h.Microstructural characterization of coatings demonstrated that the growth of the TGO layer on the nickel alloy with 6wt. % Al is more rapid than TGO with 10wt. % Al. In addition, many micro-cracks were observed at the interface of NiCr6AlY/YSZ. X-ray diffraction analysis (XRD) showed the existence of detrimental oxides such as NiCr2O4, NiCrO3 and NiCrO4 in the bond coat containing 6wt. % Al, accompanied by rapid volume expansion causing the destruction of TBC. In contrast, in the bond coat with 10wt. % Al, NiO, Al2O3and Cr2O3 oxides were formed while very low volume expansion occurred. The oxygen could not penetrate into the TGO layer of bond coat with 10 wt. % Al during high temperature oxidation and the detrimental oxides were not extensively formed within the bond coat as more oxygen was needed. The YSZ with higher Al content showed higher oxidation resistance.
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