KINETICS OF SILICON-NITRIDE FORMATION BY GAS-PHASE ROUTES IN THE SI-C-O-N SYSTEM

With the purpose of elucidating gas-phase reaction routes to Si3N4 within a C-SiO2 powder mixture, pieces of carbon and silica have been heat-treated at different temperatures and pressures in the range 1550-1710-degrees-C, 0.5-6.0 MPa. The experiments were performed in gas autoclaves and with a controlled gas flow of N2 or N2-CO mixture through the sample container. Whether or not Si3N4 would form from the gas phase at specific T, p and CO content can be approximately predicted from thermodynamic calculations, based on a simple model including N2(g), SiO(g) and oxygen. The same model was found to be useful in predicting Si3N4 formation and growth in SiC powder heated in an N2-CO gas mixture. The nitridation rate in a fine-grained SiC powder, determined as a function of T and p, was found to increase with increasing p(N2) and p(SiO) but to decrease with increased oxygen activity. A tentative kinetic expression including these concentration variables has been formulated.