Preparation and anti-oxidation performance of Al2O3-containing TaSi2–MoSi2–borosilicate glass coating on porous SiCO ceramic composites for thermal protection

In order to improve the thermal oxidation resistance of carbon fiber-reinforced porous silicon oxycarbide (SiCO) ceramic composites, an Al₂O₃-containing TaSi₂–MoSi₂–borosilicate glass coating was formed on the surface of the composites via brushing and sintering. The anti-oxidation property of the coated composites at 1873 K was investigated. Microstructures and chemical compositions of the sample before and after anti-oxidation test were determined using XRD, SEM and EDS. After heating in air at 1873 K for 20 min, the Al₂O₃-containing TaSi₂–MoSi₂–borosilicate glass coating effectively protects the SiCO ceramic composites and the coated sample kept its appearance well without obvious defects on the surface. The cross-sectional SEM images show that the coating is covered by a film of oxidation products with a thickness of about 40 μm, which is dense and crack free. Inside the A-TMG coating, irregular-shaped silicides are surrounded by continuous borosilicate glass and no penetrating holes or visible cracks are found. Al₂O₃ increases the viscosity of the borosilicate glass, which improves oxidation resistance of the coated sample by enhancing gas-penetration resistance of the glass. In contrast, the sample without Al₂O₃ in the coating slurry is severely oxidized and exhibits lots of open pores on the surface after oxidation test.

Al₂O₃ improves the oxidation resistance of TaSi₂–MoSi₂–borosilicate glass coating through increasing the viscosity and inhibiting gas penetration into matrix at high temperature, thus prevents porous SiCO ceramic composites from being oxidized.