Preparation of Porous Si 3 N 4 Ceramics with Controlled Porosity and Microstructure by Fibrous α‐Si 3 N 4 Addition

Porous silicon nitride ceramics with various porosities were fabricated by liquid phase sintering of mixtures containing fibrous and equiaxed α‐ Si 3 N 4 powder with a various content ratios. The effects of the contents of the fibrous α‐ Si 3 N 4 powder (0%–100%) on the microstructure and mechanical properties of porous Si 3 N 4 ceramics were studied. As the increase of the fibrous α‐ Si 3 N 4 powder content, both the density of green bodies and the linear shrinkage decreased, resulting in increased porosity due to the inhibited densification by the fibrous Si 3 N 4 particle. XRD analysis proved the complete formation of single β‐ Si 3 N 4 phase. SEM analysis revealed that the microstructure of the low content of fibrous α‐ Si 3 N 4 porous ceramics was almost composed of fine elongated β‐ Si 3 N 4 grains with high aspect ratio while numerous coarse elongated β‐ Si 3 N 4 grains with low aspect ratio surrounding fine grains were formed as the content of the fibrous α‐ Si 3 N 4 powder increased. With the increase in content of the fibrous α‐ Si 3 N 4 powder from 0% to 100%, the porosity changed from 47.8% to 56.6%, and the flexural strength decreased from 146 to 62 MPa correspondingly, indicating a flexural adjustment of the porosity and mechanical properties.