Near-net shaping of silicon nitride via aqueous room-temperature injection molding and pressureless sintering

Silicon nitride (Si3N4) is of interest because of its high inherent fracture toughness due to interlocking and elongated β-Si3N4 grains, but it is difficult to economically produce into near-net and complex shapes. In this study, the difficulties were overcome with the use of a novel injection molding process where highly loaded (up to 45 vol%) suspensions were loaded into a syringe and injected at a controlled rate into a mold of a desired shape. The suspensions have carefully tailored yield-pseudoplastic rheology such that they can be injection molded at room temperature and low pressures (<150 kPa). Four suspensions were studied; two different commercially available concrete water-reducing admixtures (WRAs) were used as dispersants with and without a polymer binder (Polyvinylprolidone, PVP) added for rheological modification and improved green body strength. Test bars formed via this process were sintered to high densities (up to 97% TD) without the use of external pressure, and had complete conversion to the desirable β-Si3N4 phase with high flexural strengths up to 700 MPa. The specimen sets with the smallest average pore size on the fracture surface (77 µm) had the highest average flexural strengths of 573 MPa. The hardness of all specimens was approximately 16 GPa. The ease and low cost of processing of these water-based suspensions, and the robust mechanical properties reported, demonstrate this as a viable process for the economical and environmentally friendly production of Si3N4 parts.