Interaction of Silicon Carbide and Nitride with Halogen Containing Compounds

Abstract

SiC, Si3N4, and other silicon-based ceramics are generally considered to be resistant to oxidation because they form a protective silica film when exposed to air, which limits the rate of the reaction. Silicon halide compounds are volatile at elevated temperatures, however, and silicon reacts rapidly with chlorine without formation of a protective film. In mixed environments containing a halogen, the competition between formation of volatile and condensed products leads to complex reaction kinetics. In chlorine containing mixtures, if the ratio of chlorine to oxygen is low, a protective film forms which reduces the rate of the reaction to that which would be expected in clean air. If the chlorine to oxygen ratio is high enough to produce volatile species, the rate of attack is very rapid, and corrosion products may consist of volatile products and/or silica with a non-protective morphology. The active corrosion under this condition is several orders of magnitude faster than would be expected in clean air. The transition between these regimes of behavior depends on temperature and on the microstructure and sintering aids in the SiC. Silicon nitride is much more resistant to this attack than SiC.