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.