Optimization of an aqueous, commercial silicon nitride slurry for colloidal isopressing

A commercial Si3N4-based aqueous slurry (AS800) was processed to enable the forming of shapes via a new method called colloidal isopressing. The as-received slurry was reformulated at pH 9–10 to contain 2 wt.% PEG-silane (N-[(triethoxysilyl)propyl]-O-polyethylene oxide urethane) and 0.5 M of either tetramethylammonium chloride or tetramethylammonium nitrate. The addition of the salt to an AS800 slurry formulated in the dispersed state at pH 9–10 changes the interparticle pair potential from fully repulsive to weakly attractive. These coagulated slurries were consolidated by pressure filtration at 2 MPa to a relative density of ≈0.51. It was shown that the consolidated bodies could be fluidized and used to fill a rubber mold. By isopressing a specific amount of consolidated slurry contained within a rubber mold at 200 MPa for one minute, bar-shaped powder compacts with a relative density of ≈0.60 were formed.