Microstructure and oxidation behavior of sol–gel mullite coating on SiC-coated carbon/carbon composites

A dense, uniform, crack-free and well-bonded sol–gel mullite coating with thickness of about 178 μm was successfully fabricated on SiC-coated carbon/carbon (C/C) composites. Mullite coating is produced by airbrush spraying a 3/2-mullite coating precursor solution consisting of fine mullite powder dispersed in a mullite precursor sol and air sintering the as-deposited coating. This method has several benefits such as simplicity of the process, high efficiency, ability to coat complex geometries and cost-effectiveness. The mullite/SiC coating was found to protect C/C composites from oxidation at 1773 K for 100 h with a weight loss rate of only 6.85 × 10−5 g cm−2 h−1 and retained compression strength of 97.44%. The oxidation rate was found to be significantly decreased with oxidation time, possibly due to the densification of mullite coating during the oxidation process. The corresponding high temperature oxidation activation energy of mullite/SiC coated C/C in the temperature range of 1573–1773 K is calculated to be 110.63 kJ/mol.