Investigation of scale formation and degeneration of silicide coating on Nb–Si based alloy in the temperature range from 1250 °C to 1560 °C

Nb silicide coatings modified with active elements have good oxidation resistance, but there is a lack of systematic research on their oxidation behavior at high temperatures above 1250 °C. The aim of this work is to investigate the oxidation behavior of an Al–Y modified silicide coating on Nb–Si based alloy in the temperature range from 1250 °C to 1560 °C. After oxidation at 1250 °C, the scale displays a typical layered structure consisting mainly of a TiO₂ outer layer, an amorphous SiO₂ matrix layer embedded with TiO₂ and ZrSiO₄, and an inner layer consisting of SiO₂ and Cr₂O₃. Above 1350 °C, the evaporation reaction of Cr₂O₃ into CrO₃ (g) is significantly accelerated and Cr₂O₃ disappeared gradually with temperature or time. The TiO₂ layers covering on the scales formed at 1250–1500 °C grow along the crystallographic orientation of [100]. The micropores formed on the surface of TiO₂ should be attributed to the further oxidation of Cr₂O₃ into volatile oxide CrO₃ (g). The scales exhibit a similar structure consisting mainly of a SiO₂ glass matrix embedded with ZrSiO₄ and a surface TiO₂ layer until 1500 °C. Above the eutectic temperature of SiO₂–TiO₂ system, the scale is mainly composed of a matrix layer of SiO₂–TiO₂ eutectic embedded with block TiO₂ particles, and a thin SiO₂ interface layer at 1560 °C. The degeneration path of the (Nb,X)Si₂ coating is (Nb,X)Si₂ → (Ti,Nb)₅Si₄ → γ-(Nb,X)₅Si₃.