Silicon nitridation mechanism in reaction‐bonded Si3N4–SiC and Si3N4‐bonded ferrosilicon nitride

Reaction‐bonded Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride, with Si powder, SiC particles and Fe₃Si–Si₃N₄ particles as raw materials, respectively, are prepared in flame‐isolation nitridation shuttle kiln with flowing N₂ at 1723K. There is columnar β‐Si₃N₄ in both Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride. However, fibrous α‐Si₃N₄ is only observed in Si₃N₄–SiC and Si₃N₄‐bonded ferrosilicon nitride contains much more Si₂N₂O than Si₃N₄–SiC. By analyzing the oxidation thermodynamics of Si and Si₃N₄, it is known that in the process of producing Si₃N₄–SiC, Si is oxidized first to gaseous SiO and fibrous α‐Si₃N₄ is generated with SiO and N₂. The existence of SiO is the reason of low silicon nitridation rate. But in the process of producing Si₃N₄‐bonded ferrosilicon nitride, Si₃N₄ is easier to be oxidized than Si and Si₂N₂O is generated on the surface of Si₃N₄ hexagonal prisms in ferrosilicon nitride particles. Meanwhile, Si in raw materials forms new ferrosilicon alloys with Fe₃Si, which decreases the temperature of liquid appearance and blocks some open pores in the samples, which stops the matter loss of nitridation. Liquid ferrosilicon alloys favors β‐Si₃N₄ generation from Si direct nitridation and fibrous α‐Si₃N₄ transformation, which used to exist in ferrosilicon nitride raw materials.