Formation mechanism of elongated β-Si3N4 crystals in Fe-Si3N4 composite via flash combustion

Elongated β-Si3N4 crystals have a significant influence on the mechanical property of Fe-Si3N4 composite. In this paper, the formation mechanism of elongated β-Si3N4 crystals in Fe-Si3N4 composite was investigated. During the preparation process, β-Si3N4 crystals developed in a spiral and layer growth mechanism in the dense areas. They kept growing from the dense areas and formed radially distributed elongated crystals with hexagonal prismatic morphology as time went on. As for the formation mechanism, the (100) crystal plane of β-Si3N4 from Si-N-O melt is mainly the vicinal crystal planes growth with different angles from the (100) crystal plane. At the later stage, the crystallization and the diffusion forces in Si-N-O molten phase decreased. However, the short range diffusion remained active and resulted in the gradient distribution of N content near the boundary. With the temperature decreasing, the disappearance of the short range diffusion implied the end of the crystallization process of the elongated β-Si3N4 crystals.