Microstructure evolution and interfacial failure characteristics of a NiAl/CoNiCrAlY bi-layer coating system at 1100 °C

This paper investigated the microstructure evolution and interfacial failure characteristics of a NiAl/CoNiCrAlY bi-layer coating system at 1100 °C. The multilayer system consisted of an Al-rich NiAl top layer, a CoNiCrAlY intermediate layer and a Ni substrate. The composition of the NiAl layer was selected to exhibit a single β phase structure. Commercial CoNiCrAlY composition with a two-phase γ+ β structure was used for the intermediate layer and pure Ni with a single γ phase was employed as the substrate. The NiAl layer and CoNiCrAlY layer were deposited by atmospheric plasma spraying (APS) to an approximate thickness of ∼200 μm for each. Interdiffusion experiments of this NiAl/CoNiCrAlY bi-layer system were performed at 1100 °C for periods up to 100 h in an argon atmosphere. The microstructure evolution, interdiffusion characteristics and interfacial failure behaviour were analysed. It was shown that the interdiffusion between CoNiCrAlY and Ni was very significant due to the large element concentration difference. The β phase in the CoNiCrAlY intermediate layer was rapidly consumed towards the substrate, whilst sufficient β phase was maintained in the NiAl top layer. It was also found that a band of Cr-rich precipitates was developed at the NiAl/CoNiCrAlY interface during interdiffusion, due to the limited solubility of Cr in β-NiAl. Interfacial voids were formed at the NiAl/CoNiCrAlY and CoNiCrAlY/Ni interfaces after 25 h interdiffusion, which was likely induced by the Kirkendall effect at the interface. In addition, microcracks and cavities were developed at the interfaces, which was possibly due to the volume strain caused by the phase transformation from β to γ in the system. These phase transformation-induced defects, together with the brittle α-Cr precipitates, primarily caused the interfacial degradation. And the interfacial delamination was further accelerated by the internal oxidation along the NiAl/CoNiCrAlY and CoNiCrAlY/Ni interfaces.