Synthesis and evaluation of anisotropic porous silicon nitride

Anisotropic porous silicon nitride (Si3N4) was synthesized by using whiskers or seed crystals via tape casting. Evaluation of mechanical properties was performed in both directions parallel to and perpendicular to the grain alignment of anisotropic porous silicon nitride. In the parallel direction to the grain alignment, the fracture energy ranged from 300 to 500 J/m2 in the porosity range below 20%. High fracture toughness above 17 MPa m1/2 as well as high strength above 1.5 GPa was attained in the porosity range below 5%. The existence of pores around the aligned fibrous grains causes crack deflection. Crack deflection resulted in promoting debonding between the interlocking fibrous grains and increase of total numbers of the grains contributing to the toughening. Bridging the crack and /or pullout of aligned grains drawn apart without breaking caused increase of sliding resistance contributing large fracture energy. In the perpendicular direction to the grain alignment, tilted fibrous grains give rise to almost the same mechanisms of toughening.