Aqueous injection moulding of silicon nitride

The fabrication of pressureless sintered silicon nitride parts by low pressure injection moulding of aqueous slurries is described. A basic requirement for this process is the preparation of a stable suspension with high solids content and appropriate rheological properties. The temperature has also a determining role in the stability conditions of the slip. In this work the effect of temperature on the rheological behaviour of silicon nitride aqueous slips was studied taking into account slip parameters such as the type and concentration of deflocculant, pH, ageing, etc. Once the stability conditions were determined, the effect of a gelling agent (agarose) on the slip rheology was also studied in order to optimize the injection conditions. The rheological behaviour of slips containing 1 wt.% agarose was studied as a function of temperature as well as the influence of sintering aids. Injection moulding was performed at temperatures of 60–65°C and a pressure of 0.4 MPa. Final densities of about 90% theoretical are obtained after sintering at 1750°C in N2 flow.