Texture development in Si₃N₄ ceramics by magnetic field alignment during slip casting

The effects of α-Si₃N₄ powder, β-Si ₃N₄ seed particles and sintering conditions on the texture development in Si₃N₄ ceramics, which were prepared by slip casting in a strong field of 12 T, followed by pressureless sintering. In the case of the replacement of the fine powder (UBE SN-E10) with the coarse powder (UBE SN-E05), no alignment of α-Si₃N₄ crystals was observed, but the alignment of β-Si₃N₄ crystals was observed in the green body, despite the low amount of β-Si₃N₄ (∼1 wt%). This suggests that the grain alignment during slip casting is predominant by the grain alignment of the minor β-Si₃N₄ phase present in the α-Si ₃N₄ raw powder, which is featured by the rod-like shape and less-agglomerated form. The alignment of both α-Si₃N ₄ and β-Si₃N₄ crystals occurs by the c-axis of the crystals perpendicular to the magnetic field. The addition of 5 wt% β-Si₃N₄ particles leads to the enhanced grain orientation of the β-Si₃N₄ particles in the green body for both raw α-Si₃N₄ powders. It is shown that the crystallographic orientation of β-Si₃N₄ is enhanced by the substitution of SN-E10 for SN-E05 powder, the addition of β-Si₃N₄ particles and prolonged sintering, but the phase transformation of α to β may play a dominant role in the texture development in Si₃N₄ during sintering, compared to the grain growth after the complete phase transformation. Owing to the crystallographic orientation, the samples exhibited anisotropic shrinkage behavior during sintering and the anisotropy becomes strong with enhanced grain orientation. The present study suggests that the control of the alignment of β-Si₃N₄ is crucial for the texture development in Si₃N₄ by the magnetic field alignment.