An aqueous gelcasting process for sintered silicon carbide ceramics

An aqueous gelcasting process for the preparation of dense as well as porous-sintered SiC ceramics has been described in this paper. A commercial silicon carbide powder coated with phenolic resin was used in this investigation. For the purpose of comparison, a pure SiC powder was also studied. sigma potential and viscosity studies revealed that the pure SiC powder requires an electro-steric stabilization, whereas the phenolic resin-coated powder requires an electrostatic stabilization in order to produce their corresponding aqueous slurries with high solids content. Thermogravimetry and differential thermal analysis techniques have been used to study the decomposition behavior of phenolic resin. Aqueous slurries containing 25-50 vol% SiC powder were gelcast and sintered at 2150 degrees C for 1 h. The sinterability of gelcast SiC samples was found to be highly influenced by the SiO2 formed on the surface of SiC during aqueous processing, as confirmed by the Fourier transform infrared spectroscopy study. The results obtained from various characterization techniques suggest that in order to make dense SiC parts with > 3.13 g/mL bulk density (a theoretical density of 97.5%) by an aqueous gelcasting process, the starting phenolic resin (similar to 5%)-coated SiC powder should possess a median particle size of < 11.0 mu m, surface area of > 3.2 m(2)/g, a compact (green) density of > 1.67 g/mL, and a B content of > 0.5%. Further, by using polyethylene granules and organic foaming agents, sintered SiC foam with a porosity of > 80%, a compressive strength of > 16 MPa and a coefficient of thermal expansion of 4.574 x 10(-6)/degrees C between 30 degrees and 700 degrees C can be prepared by an aqueous gelcasting process, followed by sintering at 2150 degrees C for 1 h.