On the oxidation of Ni-23Co-17Cr-12Al-0.5Y-alloy serving as bond coat in thermal barrier coatings

Specimens of air plasma sprayed thermal barrier coatings with a Ni-23Co-17Cr-12Al-0.5Y bond coat have been oxidised in air at 900 degrees C for 300-3000 hours. The oxide scale formed between the ceramic and the bond coat consists of essentially pure Al2O3, except at the interface with the ceramic overlay, where the composition of the oxide reflects that of the bond coat. The oxidation rate decreases with time and reflects growth of Al2O3 through inward grain boundary diffusion of oxygen. An inter-metallic phase identified as NiAl, precipitated during high temperature exposure. The thickness of the inter-metallic zone decreased with increasing oxidation duration. This was interpreted to reflect diffusion of Al toward the oxide and into the substrate. The amount of transient oxides at the surface was found to depend on the surface topography.