Preparation and characterization of porous Si3N4-bonded SiC ceramics and morphology change mechanism of Si3N4 whiskers

Porous Si 3 N 4 -bonded SiC ceramics with high porosity were prepared by the reaction-sintering method. In this process, Si 3 N 4 was synthesized by the nitridation of silicon powder. The X-ray diffraction (XRD) indicated that the main phases of the porous Si 3 N 4 -bonded SiC ceramics were SiC, α-Si 3 N 4 , and β-Si 3 N 4 , respectively. The contents of β-Si 3 N 4 were increased following the sintering temperature . The morphology of Si 3 N 4 whiskers was investigated by scanning electron microscope (SEM), which was shown that the needle-like (low sintering-temperature) and rod-like (higher sintering-temperature) whiskers were formed, respectively. From low to high synthesized temperature, the highest porosity of the porous Si 3 N 4 bonded SiC ceramic was up to 46.7%, and the bending strength was ~11.6 MPa. The α-Si 3 N 4 whiskers were derived from the reaction between N 2 and Si powders, the growth mechanism was proved by Vapor–Solid (VS). Meanwhile, the growth mechanism of β-Si 3 N 4 was in accordance with Vapor–Solid–Liquid (VSL) growth mechanism. With the increase of sintering temperature, Si powders were melted to liquid silicon and the α-Si 3 N 4 was dissolved into the liquid then the β-Si 3 N 4 was precipitated successfully.