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 degrees 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 degrees C to 1560 degrees C. After oxidation at 1250 degrees 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 degrees 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 degrees 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 degrees 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 degrees C. The degeneration path of the (Nb,X)Si2 coating is (Nb,X)Si2 -* (Ti,Nb)5Si4 -* gamma-(Nb,X)5Si3.