Effect of Al₂O₃ on the densification and oxidation behavior of SiC coating for carbon/carbon composites

A silicon carbide (SiC) coating was prepared on carbon/carbon (C/C) composites by pack cementation using Si, C and Al₂O₃ powers as raw materials. The role of Al₂O₃ on the densification of the SiC coating was investigated. The results show that the densification and thickness of the SiC coating obviously increase with the increase of Al₂O₃ content, which is attributed to the enhanced material transport through liquid Al₂O₃ at high temperature. Moreover, to study the effect of Al₂O₃ on the oxidation behavior of SiC coating itself at 1773 K in air, the synthetic SiC powders by pack cementation were sintered. The results present that after oxidation for 40 h, only a few holes are observed in the generated glass layer of the SiC ceramic without Al₂O₃, while more holes appear in the surface of the oxidized Al₂O₃ doped SiC ceramic, owning to the doped Al atom destroying the SiO₂ network and causing the permeability of oxygen to increase.