Consolidation of aqueous concentrated silicon nitride suspension by direct coagulation casting

Direct coagulation casting (DCC) process of silicon nitride has been investigated to achieve a near-net-shape forming and a high packing density of the green body. Various process variables such as the effect of dispersants, coagulation agent and sintering additives have been evaluated in order to achieve highly concentrated stabilized silicon nitride slurries. A high solid loading of dispersed silicon nitride slurry was prepared with 1.0 wt.% tetraethylammonium hydroxide (TEAH), which allowed a stable silicon nitride suspension with sintering additives (Al2O3 and Y2O3) in alkaline regions to a solid content of 51 vol.%. The addition of hydroxyaluminium diacetate into the suspension, which decomposed at elevated temperatures, led to coagulation of the silicon nitride suspension. Aluminum ions precipitated to aluminum hydroxide (Al(OH)3), decreased OH− concentration and, thus, coagulated the suspension. The resulting green bodies showed zero shrinkage during coagulation, which was also almost shrinkage-free after drying.