Corrosion resistance characteristics of silicide coating on Nb–Si based alloy exposed to CMAS at high temperature

Niobium disilicide shows good oxidation resistance in high-temperature oxidizing atmosphere and can be used as oxidation resistant coatings. However, these coatings are also required to have good environmental resistance, where corrosion by calcium‑magnesium-alumina-silicate (CMAS) represents a potential threat to the coating during the application. The aim of this work is to clarify the interaction between the Al–Y modified silicide coating on Nb–Si based alloy and the CMAS. The results indicate that the coating consists mainly of a disilicide layer and a transitional layer. The disilicide layer is mainly composed of (Nb,X)Si₂ with (Zr,X′)Si₂ particles dispersed in it. During the CMAS exposure, both the silicide coating and the as-formed SiO₂-based composite oxide scale can be dissolved by the melted CMAS. The increase of temperature accelerates the CMAS corrosion process. The accumulation of dissolved Si at the interface between the CMAS and coating leads to the in-situ crystallization of Cristobalite-SiO₂, which can arrest the melted CMAS, offering a good protection against CMAS attack. The (Nb,X)Si₂ coating exhibits a two stage of CMAS corrosion process. The first one is the dissolution of the coating until SiO₂ crystallization. The second one is the growth of the Cristobalite-SiO₂ layer, during which the coating elements will be oxidized and can then diffuse into the melted CMAS through the Cristobalite-SiO₂ layer.