Rheology of organics-free aqueous ceramic suspensions for additive manufacturing of dense silicon nitride ceramics

A dense structure of silicon nitride ceramic was fabricated by direct ink writing using aqueous suspensions. The rheology of the suspensions was carefully tailored by the particle chemical state and the ion concentrations, without the use of any organics. The surface chemical states of the Si₃N₄ powder were modified by calcination at 600 °C at various times. The minimum absolute value of zeta potential, calculated by the DLVO theory, was 19 mV at which the interaction was attractive interaction. Suspensions with solid volume fraction of 25–40 vol% exhibited pseudoplastic behavior with yield stresses ≥55 Pa and equilibrium storage moduli ≥ 10⁴ Pa. These enabled suspensions to flow through the nozzle and retain the shape of the printed parts. The flexural strength of Si₃N₄ ceramics produced using a 40 vol% suspension was 348 MPa. This strategy provides a simple process for fabricating high-performance ceramics based on the DIW.