Reaction and formation mechanism of Fe-Si3N4 composite prepared by flash combustion synthesis

In this paper, in order to improve the performance of Fe-Si3N4 composite synthesized by flash combustion, the detailed nitridation mechanism and formation process were discussed. In the process of high temperature nitriding, xi phase rapidly melted to form Fe-Si melt and the cycle of rapid surface nitriding -> rupture of nitridation shells -> new melt exposing and nitriding occurred continually on the surface of Fe-Si melt. Since ions have different activities on the surface and internal, Fe ions near the surface migrated inwards and Si ions inside moved out; meanwhile, the Fe-Si melt kept shrinking. The nitriding reaction of the Fe-Si melt finished till the overall activity a(Si) approached 0, leaving the atom ratio of [Fe] to [Si] at 3:1. During the falling of the formed Si3N4 and Fe3Si melt in the N-2 flow, the surface of Si3N4 was oxidized to form a SiO2 film. The nitridation product fell into the product pool, loosely stacking and adhering together by the SiO2 film. alpha-Si3N4 dissolved and precipitated to form beta-Si3N4 crystals, and the beta-Si3N4 crystals kept growing to form radioactive elongated crystals. As the temperature decreased, the Fe3Si melt cooled down; the Si-N-O melt, alpha-Si3N4 and the roots of elongated beta-Si3N4 crystals formed the dense areas.