Isothermal oxidation and TGO growth behavior of NiCoCrAlY-YSZ thermal barrier coatings on a Ni-based superalloy

Abstract

The isothermal oxidation of 8 wt% Y2O3 stabilized ZrO2 (YSZ) thermal barrier coating (TBC) and growth behavior of thermal growth oxide (TGO) have been investigated. The average thicknesses of oxide scale and internal oxidation zone of the substrate with coating are significantly smaller compared with the bare substrate. The TGO layer evolves from a black single-layer to a double-layer structure. The lower black layer is composed of α-Al2O3, and the upper gray layer consists of mixed oxides of (Ni,Co)O, (Co,Ni)Cr2O4 and (Ni,Co)Al2O4 at 1000 °C by TEM analysis. The formation of the solid solution oxides is probably related to the intense diffusion at high temperature. The growth stresses of α-Al2O3, NiCr2O4, NiO and CoO are calculated to be 30.88, 62.99, 34.52 and 35.85 GPa, respectively, indicating that mixed oxides, especially the spinel phase, are very detrimental to the stability of the TBC. The growth kinetics of TGO layer and the oxidation mechanism of YSZ TBC are illustrated.