Significant improvements in the fracture resistance, fracture toughness and thermal properties of silicon nitride ceramics are obtained by tailoring the microstructure. Combined use of seeding and tape casting techniques allowed the production of highly anisotropic microstructures. The seeded silicon nitrides exhibited a distinct bimodal microstructure, with large elongated β-Si3N4 grains, grown from seeds, dispersed within a fine-grained matrix. These large grains in the seeded silicon nitrides lie in the casting planes and self-align along the casting direction during tape forming process. It is here, when due to the high degree of alignment that “special boundaries” without the, otherwise, ubiquitous amorphous phase occurs. These “special” boundaries, hardly seen in three dimensionally random microstructures, are the object of the present study.

Silicon nitride with high thermal conductivity of up to 120 W/mK (ref. 3) is produced by hot-pressing at 1800 °C for 2 h. powders with the following nominal composition: α-Si3N4 ;5 wt% Y203; 5 vol.% (β-Si3N4 seeds.