Spinel formation in thermal barrier systems with a Pt-enriched γ–Ni + γ′–Ni3Al bond coat

The oxidation of electron beam physical vapour deposited thermal barrier coatings with a Pt-enriched γ–Ni + γ′–Ni3Al bond coat was investigated. Due to the growth of the thermally grown oxide (TGO), γ–Ni formed underneath the TGO as a result of Al depletion. Phase characterisation by X-ray diffraction, as well as microstructural observations, indicated that a NiAl2O4 spinel phase formed at the TGO/bond coat interface after prolonged oxidation. It is proposed that the formation of spinel occurs when local cracks present at the interface and the underlying bond coat is Al-depleted. The cracks provide a direct path for oxygen and nickel oxide forms at the bond coat surface. With further oxidation, the spinel forms at the interface through solid state reaction between the TGO and nickel oxide.