Direct Coagulation Casting of Silicon Carbide Components

Abstract

Direct coagulation casting is a novel near‐net‐shape method for forming ceramic green bodies from homogenous high‐solids‐loaded particle suspensions. It is based on the principle of the in situcoagulation of a powder suspension via a reaction‐rate‐controlled internal‐enzyme(urease)‐catalyzed reaction after casting. Low‐viscosity (<3 Pas) suspensions with a high solids loading (>62 vol%) of SiC, boron, and carbon powder mixtures with a high surface area (>7‐10 m2/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 destabilize the suspensions. The coagulation kinetics and the strength of the wet green bodies have been investigated. The reaction rate is strongly dependent on the temperature (in the range of 5°‐30°C) and the enzyme concentration (for the range of 4‐16 units/g SiC) and is independent of the substrate (urea) concentration for urea concentrations of <2 wt%, based on the powder content. The resulting green bodies show no shrinkage during coagulation and 1%‐2% linear shrinkage during drying. The compressive strengths of the wet green bodies are as high as 60 kPa and increase as the coagulation time increases. The wet green strength of the coagulated suspensions scales with the solids content, according to a power law with an exponent of 11, in the range of 56‐61 vol% solids content. The possibilities of fabricating high‐solids‐containing complex SiC green and sintered components with homogenous microstructures and high sintered densities are demonstrated.