Microstructure and mechanical properties of dense Si3N4 ceramics prepared by direct coagulation casting and cold isostatic pressing

Complex shaped dense Si3N4 ceramics were produced by using direct coagulation casting technique via dispersant reaction method of Si3N4 suspension, followed by gas pressure sintering. The effects of solid content of the suspension, additional cold isostatic pressing of the cast parts, and sintering behaviour and on the mechanical reliability of silicon nitride ceramics were investigated. It was observed that all slurries exhibited rheological properties suitable for casting in the range of 44–50 vol.% solid concentrations. Nevertheless, higher solid concentration suspensions resulted in smaller floc size and thus better green microstructures. Parts shaped by direct coagulation casting at all the solid loadings had relatively low strength and reliability after sintering. However, application of additional cold isostatic pressing to the cast parts increased the strength and, particularly, reliability. Dense Si3N4 ceramics with relative density above 99.5%, average bending strength 760 ± 39 MPa and Weibull module 23.5 had been obtained with 50 vol.% solids content after DCC + CIP process.