The c-axis texturing of seeded Si₃N₄ with β-Si₃N₄ whiskers by slip casting in a rotating magnetic field

The c-axis texture development in seeded Si₃N₄ with β-Si₃N₄ whiskers by slip casting in a rotating magnetic field was investigated by powder X-ray diffraction, scanning electron microscopy and thermal anisotropy evaluation, including the effects of magnetic flux density, suspension rheological properties and rotational speed. By increasing the magnetic flux density from 1 to 12 T, the c-axis orientation of β-Si₃N₄ whiskers is achieved in the well-dispersed 30 vol.% suspension but hindered in the flocculated 40 vol.% suspension. The degree of orientation tends to decrease with increasing rotational speed from 5 to 30 rpm. The green body shows no density gradient but nonuniform orientation of β-Si₃N₄ whiskers regardless of the rotational speed, resulting in the similar nonuniform texture in the sintered body. The nonuniform texture is featured in three major layers: lower surface, inner region and upper surface, corresponding to the lower, higher and lower c-axis orientation, respectively. The resultant c-axis textured Si₃N₄ exhibits thermal anisotropy as high as 55%, slightly lower than the intrinsic thermal anisotropy (62%) of β-Si₃N₄. A theoretical analysis is performed to help understand the key factors affecting the orientation of β-Si₃N₄ whiskers by slip casting in the rotating magnetic field. The formation mechanisms of the nonuniform texture were discussed.