The nonoxide technical ceramics of the silicon carbide and silicon nitride groups have been developed specifically for a number of high temperature applications, of which gas turbine engine components and heat exchangers provide 2 well studied examples. In these and similar applications the materials are exposed to substances, already well known in the metallurgical area, likely to cause accelerated oxidation through interaction with protective oxide films. 2 examples are sodium sulphate and vanadium pentoxide. For this reason considerable effort has been put into studying silicon nitride based materials in oxidising environments at high temperature in the presence of fluxing metal salts and oxides.
Chemical stability is determined by 2 sets of factors, the thermodynamic and the kinetic, which must be examined separately.
Specific aspects in the present context are :
the thermodynamic stability at high temperature of a system prepared by equilibration under conditions of low oxygen potential but now exposed to a high oxygen potential;
the rate at which a new state of equilibrium can be attained, initially at least at the surface of the material.
Because of the importance of silicon dioxide, as the primary oxidation product for the corrosion process, this material is examined first.
Funding SchemeCSC - Cost-sharing contracts