Oxidation mechanism of nitride and carbide powders by high-temperature, high-pressure water

The oxidation of the nitride and carbide of silicon, zirconium, and titanium by high-temperature high-pressure water has been systematically studied using pure powder samples. Obtained results indicate that the oxidation thermodynamics and kinetics are quite different from those by oxygen, even though similar oxide scales are produced. Under 10 and 100 MPa H2O, Si3N4, or SiC powders are oxidized at temperatures as low as 200 degrees or 500 degrees C to produce amorphous silica scale and NH3 or CH4, respectively. Since this silica scale plays the role of a protective film, the oxidation kinetics are controlled by diffusion of gaseous species in this film, i.e., H2O for Si3N4 below 400 degrees C, N2 for Si3N4, above 500 degrees C, or CH4 for SiC, respectively. On the other hand, the oxidation of ZrC and ZrN was reaction-controlled because the ZrO2 grains produced do not prepare a protective film. TiC and TiN indicated the formation of amorphous TiO2 film in the early stage of oxidation, then this film started to crystallize into anatase and rutile