Processing and creep performance of silicon carbide whisker-reinforced silicon nitride

Abstract

Silicon carbide whisker reinforcement can significantly reduce creep rates in polycrystalline alumina [1], but the system SiC/Al₂O₃ is thermodynamically unstable in air and oxidizes to mullite during creep testing [2]. The system SiC/Si₃N₄ was investigated as a potentially more stable, high temperature structural composite. Silicon carbide whiskers were successfully incorporated into a silicon nitride matrix doped with alumina and yttria. Processing involved mixing dispersed slurries of silicon carbide and silicon nitride, adding the dopants as a solution of their nitrates and subsequently increasing the pH to precipitate the additive hydroxides. The resulting slurries were filter pressed at room temperature and hot pressed at 1650° C in graphite dies to full density. X-ray diffraction and transmission electron microscopy confirmed the presence of β-Si₃N₄, α-SiC and trace quantities of α-Si₃N₄, confirming that the -β Si₃N₄ reaction occurred. An additional, as yet unidentified, minor phase was also detected. Whisker reinforcement was shown to increase the room temperature flexural strength and fracture toughness but high temperature creep performance was unaffected by whisker reinforcement.