Preparation of MoSi₂-modified HfB₂-SiC ultra high temperature ceramic anti-oxidation coatings by liquid phase sintering

Liquid-phase sintering combining an in-situ reaction method with a slurry method was used to prepare HfB₂-MoSi₂-SiC coatings of controllable composition and thickness. The effect of the MoSi₂ content on the oxidation protection of HfB₂-MoSi₂-SiC composite coatings in a dynamic aerobic environment from room temperature to 1500 °C and a static constant temperature at 1500 °C in air was investigated. The relative oxygen permeability was used to characterize the oxidation resistance of the coatings. The results of dynamic oxidation test at room temperature~1500 °C show that the initial oxidation weight loss temperature of the samples is increased from 775 to 821 °C, and the maximum weight loss rate is decreased from 0.9×10⁻³ to 0.2×10⁻³ mg·cm⁻²·s⁻¹ with increasing MoSi₂ content, the lowest relative oxygen permeability is reduced to 12.2% with the weight loss of the sample being decreased from 1.8% to 0.21%. The mechanism of MoSi₂ improving the oxidation protection of the coatings is revealed. With an increase of the MoSi₂ content, the amount of SiO₂ glass phase in the coating is increased, and the dispersion of Hf-oxide on the surface is improved so that a Hf-Si-O glass layer with high stability is formed and the weight loss of the sample is reduced from 0.46% to 0.08% after 200 h oxidation at 1500 °C in air.