High-strength porous Si3N4 ceramics prepared by freeze casting and silicon powder nitridation process

• Si3N4 ceramics with uniform morphology and high performance were obtained.
• We compare the nitridation degree and morphology with and without SiC addition.
• The increased aspect ratio of grains gave rise to the improved flexural strength.
• The growth and deflection of grains were restrained owing to the SiC addition.
• This technique showed near net-shape forming owing to the low linear shrinkage.

Porous Si3N4 ceramics with uniform morphology were fabricated via freeze casting and silicon powder nitridation process. Because of the interlocking grains, Si3N4 porous ceramics obtained high porosity of 70.51% and high flexural strength of 16.3 MPa despite of its nitridation degree 88.8%. With SiC addition, a much higher nitridation degree 97.9% was achieved. But the relatively low porosity of 66.7% with low flexural strength of 9.6 MPa was gained owing to the restrained growth and deflection of grains. After post-sintering, the increased aspect ratio of grains gave rise to the improved flexural strength for Si3N4 porous ceramics, while the contrary phenomenon was observed with SiC addition.

The rod-like grains of β-Si3N4 were particularly noticeable in the high magnification SEM micrographs after nitridation process for SN sample. As the nitridation process was gently conducted and the ice crystals in the samples were completely sublimated, there has been enough time and space for the formation of β-Si3N4 grains.Figure optionsDownload as PowerPoint slide