Effect of addition of silicon on the microstructures and bending strength of continuous porous SiC-Si3N4 composites

The microstructures and mechanical properties of continuous porous SiC-Si3N4 composites fabricated by multi-pass extrusion were investigated, depending on the amount of Si powder added. Si powder with different weight percentages (0%, 5%, 10%, 15%, 20%) was added to SiC powder to make raw mixture powders, with 6 wt% Y2O3-2 wt% Al2O3 as sintering additives, carbon (10-15 mu m) as a pore-forming agent, ethylene vinyl acetate as a binder, and stearic acid (CH3(CH2)(16)COOH) as a lubricant. In the continuous porous SiC-Si3N4 composites, Si3N4 whiskers like the hairs of nostrils were frequently observed on the wall of the pores. In this study, the morphology of Si3N4 whiskers was investigated with the nitridation condition and silicon addition content. In composites containing an addition of 10 wt% Si, a large number of Si3N4 whiskers were found at the continuous pore regions. In the sample to which 15 wt% Si powder was added, a maximum value of about 101 MPa bending strength and 57.5% relative density were obtained.