Effects of additives on the nitridation process of foamed materials containing silicon and silicon carbide powders

Without using any additive, the nitridation process of silicon powder was slow and the main product was α-Si3N4 due to the cycling production of SiO species. The addition of Al2O3 and Y2O3 could facilitate the nitridation process resulting in a higher β-Si3N4 content presumably due to the liquid phase formed between Al2O3, Y2O3 and surface silica on silicon powder. When a small amount of 1.76% Fe2O3 was added, the accelerated nitridation process was attributed to the FeSi2 liquid phase produced by reaction Fe element with surface silica at a lower temperature of 1212 °C, but the Al2O3 and Y2O3 additives could still be active for sustaining the nitridation process at higher temperature. At a higher Fe2O3 concentration of 3.46%, the nitridation process was mainly controlled by the formed FeSi2 liquid phase. This study has demonstrated the active role of using Al2O3 and Y2O3 combination and Fe2O3 on the nitridation process, which could be helpful for further investigation on reaction bonding of SiC and Si3N4 ceramics.