OPTIMIZATION OF THE INDUSTRIAL SYNTHESIS OF SILICON CARBIDE - REACTION BONDED SILICON NITRIDE (SiC-RBSN)

Si3N4 is a suitable material for many industrial applications because of its high thermomechanical properties and good chemical stability, especially the beta-Si3N4 phase. In this work the industrial production of SiC-RBSN has been optimized, focusing on the control of the alpha/beta Si3N4 ratio and on the understanding of the nitridation mechanism. The Si3N4 bonding phase has been synthesized in a flowing N-2 or N-2+H-2 atmosphere via Si(s)+N-2(g) -> Si3N4(s) reaction or Si-(1)+N-2(g) -> Si3N4(s) reaction, above Si melting point. The starting materials were mixtures of slip casted SiC and Si in a range of 15-60 % wt. The Si-N-O system has been investigated with both experimental runs and correlated industrial firings: different H-2 percentages (0-4%), solid mass/gas flow ratios and additives have been considered, and the main variables such as time, temperature and pO(2) have been monitored. For large solid mass/gas flow ratios, the reaction progresses in a self-buffering atmosphere, where most of the oxygen is consumed leading to the extremely O-2-depleted conditions required for the stability of the nitrides. Our results clearly show that the most important parameter is the sample mass/gas flow ratio, which allows to regulate the matrix composition and consequently the composite microstructure. This variable is often underrated in experimental tests but is extremely important on an industrial scale.