EFFECT OF GAS-COMPOSITION ON SI2N2O FORMATION IN THE SI-C-N-O SYSTEM

Formation of silicon oxynitride was observed during silicon nitride whisker synthesis from silica, carbon and nitrogen. The silicon oxynitride formation was limited to a bottom area of the charged powders and found both in whisker and powder. The possible reason for this localized Si2N2O formation is analysed, based on the effect of gas phase composition on the phase stability among beta-Si3N4, beta-SiC and Si2N2O. The phase stability is closely related to the p(SiO)/P(N2) ratio in the gas phase. To suppress Si2N2O formation, the p(SiO)/P(N2) ratio must be lower than that of the phase boundary in the Si3N4/Si2N2O equilibrium, whereas it must be higher than that of the phase boundary in the SiC/Si2N2O equilibrium. The formation of silicon oxynitride during silicon nitride whisker formation was caused most likely by fluctuation in the p(SiO)/p(N2) ratio in the gas phase surrounding the lower area.