Enzyme catalysis of alumina forming

In this work we describe a new ceramic green body fabrication method using ceramic colloidal suspensions that allow the production of complex shaped parts of high strength and reliability. The method uses an aqueous, electrostatically stabilized ceramic suspension free of agglomerates and of low viscosity. It is cast into a mold and then coagulates forming a stiff, wet green body. Coagulation is performed by changing the pH of the suspension and/or by creating salt directly inside the suspension using a controlled time-delayed reaction, Enzyme catalyzed reactions that are decomposing a substrate, or self-propagating decomposition reactions of a substrate, can be used. After the coagulation reaction in the suspension, the wet green body shows good mechanical properties and can be demolded, dried and sintered. This new forming process DCC (Direct Coagulation Casting) is characterized by near net shape capability of complex shaped parts with high quality and homogeneity in the green as well as in the sintered state. Only small amounts of organic additives which amount to less than 0.5 wt-% based on ceramic powder are needed for the catalytic reactions. Since the forming process takes place without pressure and at ambient temperatures, inexpensive molds and tools can be used. Ceramic bodies with homogeneous, defect poor microstructures can be cast using almost any ceramic powder. Complex shaped components with thin and thick cross sections can be cast as there is no gradient in density and/or temperature during consolidation of the green body. Alumina parts of average bend strength (680 MPa) and high reliability (Weibull-module m=40) have been demonstrated.