Effect of raw material ratio on microstructure and oxidation resistance of SiO₂-B₄C-glass coating on the surface of Al₂O₃-C refractories

To improve the high temperature (1100 ℃) oxidation resistance of Al₂O₃-C refractories, SiO₂-B₄C-glass composite coating was prepared on its surface by slurry brushing method. Effects of SiO₂ content, B₄C content, type and content of binder on microstructure and oxidation resistance of the coating were studied. Results showed that SiO₂ could increase high temperature viscosity of the glass phase, when the SiO₂ content was low (50.0wt.%), fluidity of the glass was large, and the molten glass was easy to lose at high temperature. When the SiO₂ content was over high (65.0wt.%), the glass phase with high viscosity and low fluidity could not uniformly cover the matrix surface, which was not conducive to forming continuous glass film with excellent oxygen barrier performance. The molten B₂O₃ generated by B₄C oxidation could be dissolved in borosilicate glass to form a composite glass phase, which contained SiO₄ and BO₄ tetrahedral structure framework. The formed glass phase exhibited a more compact structure. As B₄C content increased, the glass structure became denser. However, when B₄C content was excessively high (15.0wt.%), a "boron anomaly" phenomenon occurred, which reduced strength of the glass phase structure and reduced oxidation resistance of the coating. Compared with silica sol, sodium tripolyphosphate and sodium hexametaphosphate, using water glass as a binder could make the slurry with moderate viscosity, the prepared coating with high compactness was uniformly adhered to the substrate. In conclusion, when contents of SiO₂, B₄C and glass powder were 55.0wt.%, 15.0wt.% and 30.0wt.%, respectively, and content of water glass binder was 60.0wt.% of the total powder mass, the coating exhibited uniform particle distribution and minimal pores. After isothermal oxidation at 1100℃ for 80 h, weight loss rate of the coating was only −0.064%.