Gel casting and properties of porous silicon carbide/silicon nitride composite ceramics

Porous silicon carbide/silicon nitride (SiC/Si3N4) composite ceramics with flexural strength of 78 MPa, fracture toughness of 1.68 MPa m1/2, and porosity up to 63% were obtained by gel casting and two-step sintering using commercial phenolic resin as carbon source. Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and three-point bending methods were employed to characterize and analyze the microstructure and basic mechanical performance of the porous ceramics. The polymer was proved to be an effective source that could provide free carbon for fabrication of the SiC nanoparticles by in situ reaction with Si3N4 and SiO2. Sizes of the formed SiC particles were in the range of 50-300 nm. Low green density, significant weight loss, and little shrinkage led to high porosity. High aspect ratio of β-Si3N4, uniform porous distribution, and formation of proper amount of SiC particulates were the main reasons for good mechanical performance. However, only a limited amount of polymer additions had these beneficial effects. Excessive additions could cause agglomeration for residual carbon and inborn SiC, hence becoming fatal flaws for the materials.