Fabrication of fine-grained Si₃N₄-Si₂N₂O composites by sintering amorphous nano-sized silicon nitride powders

Abstract

Si₃N₄-Si₂N₂O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering (LPS). The Si₂N₂O phase was generated by an in-situ reaction 2Si₃N₄(s)+1.5 O₂(g)=3Si₂N₂O(s)+N₂(g). The content of Si₂N₂O phase up to 60% in the volume was obtained at a sintering temperature of 1650°C and reduced when the sintering temperature increased or decreased, indicating the reaction is reversible. The mass loss, relative density and average grain size increased with increasing the sintering temperature. The average grain size was less than 500 nm when the sintering temperature was below 1700°C. The sintering procedure contains a complex crystallization and a phase transition: amorphous silicon nitride→equiaxial α-Si₃N₄→equiaxial β-Si₃N₄→rod-like Si₂N₂O→needle-like β-Si₃N₄. Small round-shaped β-Si₃N₄ particles were entrapped in the Si₂N₂O grains and a high density of staking faults was situated in the middle of Si₂N₂O grains at a sintering temperature of 1650°C. The toughness increased from 3.5 MPa·m^1/2 at 1600°C to 7.2 MPa·m^1/2 at 1800°C. The hardness was as high as 21.5 GPa (Vickers) at 1600°C.

Keywords

Si₃N₄−Si₂N₂O composite / in-situ reaction / amorphous nano-sized silicon nitride

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