The oxidation of silicon carbide and nitride

The technical problem in the high temperature application of Si3N4 is that unlike the pure material, which can be prepared in small quantities by CVD for example, the commercial material is made by sintering the nitride with additives, such as MgO. The presence of the additive increases the rate of oxidation, when compared with the pure material, by an order of magnitude, probably due to the formation of liquid magnesia-silica solutions, which provide short-circuits for oxygen diffusion. These solutions are also known to reduce the mechanical strength at these temperatures. 

Kofstad. High Temperature Oxidation of Metals. J. Wiley Sons. New York (1966) TA 462. K57. 

Birks and G.H. Meier. Introduction to High Temperature Oxidation of Metals. Edward Arnold, London (1983) QD 501. 

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