Effect of phase composition on the oxidation resistance of ZrB2-SiC coatings

ZrB2-SiC-Al2O3 (ZSA) and ZrB2-SiC-Si (ZSS) coatings were prepared on the surface of C/C composites by atmospheric plasma spraying. During oxidation at 1200 ℃ for 10 h, the formation of B2O3 in the ZSA coating volatilizes gradually, no glass phase is able to seal cracks and holes, resulting in poor oxidation resistance. On the other hand, the ZSS coating produces a large amount of borosilicate glass phase and still gains 0.8% weight after 10 h. During oxidation at 1500 ℃, a low viscosity SiO2 glass phase forms on the ZSA coating, due to the presence of Al2O3 and this glassy phase has a good ability to seal defects. However, the rapid volatilization of B2O3 in the ZSS coating leads to the formation of large holes in the coating, and to progressively increased viscosity of SiO2 glass phase, resulting in decreased oxidation resistance.