Direct coagulation casting of silicon carbide components

Direct coagulation casting is a novel near-net-shape method for forming cer amic green bodies from homogenous high-solids-loaded particle suspensions. It is based on the principle of the in situ coagulation of a powder suspens ion via a reaction-rate-controlled internal-enzyme(urease)-catalyzed reacti on after casting. Low-viscosity (<3 Pa.s) suspensions with a high solids lo ading (>62 vol%) of SiC, boron, and carbon powder mixtures with a high surf ace area (>7-10 m(2)/g) have been prepared at pH = 10. Salt ions (up to 1-2 mol/L) are created by the urease-catalyzed decomposition of urea, to desta bilize the suspensions. The coagulation kinetics and the strength of the we t green bodies have been investigated. The reaction rate is strongly depend ent on the temperature (in the range of 5 degrees-30 degrees C) and the enz yme concentration (for the range of 4-16 units/g SiC) and is independent of the substrate (urea) concentration for urea concentrations of <2 wt%, base d on the powder content. The resulting green bodies show no shrinkage durin g coagulation and 1%-2% linear shrinkage during drying. The compressive str engths of the wet green bodies are as high as 60 kPa and increase as the co agulation time increases. The wet green strength of the coagulated suspensi ons scales with the solids content, according to a power law with an expone nt of 11, in the range of 56-61 vol% solids content. The possibilities of f abricating high-solids-containing complex SiC green and sintered components with homogenous microstructures and high sintered densities are demonstrat ed.