A COMPARISON OF THE OXIDATION-KINETICS OF SIC AND SI3N4

There is lingering disagreement over the relative oxidation kinetics o f SiC and Si3N4 in the regime of passive oxidation. Various oxidation enthalpies have been reported for these materials, in some cases with suggestions of an increase at higher temperatures. The result is uncer tainty over the respective diffusion mechanisms that limit oxidation r ates in these materials. It is suspected that this confusion mag be du e, in large part, to the effects of impurities from the oxidation envi ronment. Accordingly, this study was aimed at clarifying some of the i ssues by undertaking the simultaneous oxidation of CVD SiC and Si3N4 w ithin a furnace tube of fused silica known to be very pure. We discuss the results obtained in the interval 1200 to 1500 degrees C, and thei r implications on the respective rate-limiting mechanisms.