Microstructure and anti-oxidation property of mullite oxidation protective coating prepared by hydrothermal electrophoretic deposition for SiC–C/C composites

Oxidation resistant mullite outer coating for SiC pre-coated carbon/carbon composites (SiC–C/C) was prepared by a novel hydrothermal electrophoretic deposition process using mullite powders, isopropanol and iodine as the start materials, solvent and charging agent, respectively. The phase composition, surface and cross-section microstructure of the as-prepared multilayer coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of hydrothermal deposition temperature on phase composition, microstructure and oxidation resistance of the as-prepared coatings was particularly investigated. Results show that the outer layer coating mainly composed of mullite phase can be achieved by hydrothermal electrophoretic deposition. The thickness, density and anti-oxidation property of the mullite coating were improved with the increase of deposition temperature from 353 K to 393 K. The mullite coating prepared at 393 K exhibits excellent anti-oxidation property, which can effectively protect C/C composites from oxidation in air at 1773 K for 172 h with a weight loss of 1.65%. The failure of the multilayer coatings is considered to be caused by the volatilization of the SiO2 glass and the formation of microholes and microcracks on the coating surface.

Research Highlights
► Oxidation resistant mullite outer coating is prepared by a novel hydrothermal electrophoretic deposition process.
► The homogeneous, dense and crack-free mullite outer coating is deposited at 393 K with thickness of ~90 nm.
► The as-prepared coatings can effectively protect C/C composites from oxidation in air at 1773 K for 172 h with a weight loss of 1.65%.
► The failure of the multi-layer coating is attributed to the generation of microholes and microcracks due to the CO and CO2 and volatilization of the silicates glass film.