Pore structure and mechanical properties in freeze cast porous Si3N4 composites using polyacrylamide as an addition agent

Porous barium aluminosilicate (BAS)/Si3N4 composites were prepared by freeze casting process using liquid N2 as refrigerant. The pore morphologies, α → β-Si3N4 phase transformation, and mechanical properties of the resultant porous ceramics were strongly affected by the addition of polyacrylamide (PAM) into the slurries. The incorporation of 2 wt.%PAM changed the ice crystals structure from lamellar with dendrites into featureless columnar ones, the feature of which was duplicated by the pores in the sintered ceramics. The lamellar pores with dendrites could favor the α → β-Si3N4 phase transformation, but were detrimental to the mechanical properties. The sample with 1 wt.%PAM exhibited the highest strength and toughness in the resultant porous composites due to its columnar pore structure with less dendritic pores and higher aspect ratio of β-Si3N4 grains. A relative dense layer was formed at the bottom of the samples with PAM and its thickness increased with increasing PAM content.

Research highlights▶ Porous BAS/Si3N4 composites with unidirectional aligned channels were fabricated by freeze casting using liquid N2 as refrigerant. ▶ Increasing PAM content in the slurries strongly affect the pore structure, α → β-Si3N4 transformation and mechanical properties.