Reactions of Silicon‐Based Ceramics in Mixed Oxidation Chlorination Environments

The reaction of silicon‐based ceramics with 2% Cl 2 /Ar and 1% Cl 2 /1% to 20% O 2 /Ar at 950 °C was studied with thermogravimetric analysis and high‐pressure mass spectrometry. Pure Si, SiO 2 , several types of SiC, and Si 3 N 4 were examined. The primary corrosion products were SiCl 4 ( g ) and SiO 2 ( s ) with smaller amounts of volatile silicon oxychlorides. The reactions appear to occur by chlorine penetration of the SiO 2 layer, and gas‐phase diffusion of the silicon chlorides away from the sample appears to be rate limiting. Pure SiO 2 shows very little reaction with Cl 2 . SiC with excess Si is more reactive than the other materials with Cl 2 , whereas SiC with excess carbon is more reactive than the other materials with Cl 2 /O 2 . Si 3 N 4 shows very little reaction with Cl 2 . These diferences are explained on the basis of thermodynamic and microstructural factors.