Oxidation protection of C/C–SiC composites by an electrophoretically deposited mullite precursor

Electrophoretic deposition (EPD) is a suitable technique to produce mullite layers for acceptable oxidation protection of C/C–SiC composites. Combining sol–gel synthesis of 3Al2O3·2SiO2 mullite through hydrolysis and condensation of tetraethoxysilane (TEOS) and aluminum-tri-sec-butylate (Al-(OBu)3) with EPD yields sufficiently thick and homogeneous layers, which transform into mullite at 1300 °C. The protectiveness of the deposited mullite layers was tested in air in the temperature range 1300 °C≤T≤1550 °C by means of isothermal thermogravimetric analysis for up to 200 h. The experimental data can be described by a phenomenological model of the (reduced) oxidation rate of the SiC layer underneath the outer mullite layer, which suggests that transport of carbon monoxide through mullite and silica is rate determining. Comparing the oxidation rate of electrophoretically coated C/C–SiC samples to that of uncoated reference samples clearly demonstrates that mullite offers a significant improvement to the oxidation resistance of the reference material.