Processing of β-Silicon Nitride from Water-Based alpha-Silicon Nitride, Alumina, and Yttria Powder Suspensions

A water-based route for processing ß-Si₃N₄ from alpha-Si₃N₄, Al₂O₃, and Y₂O₃ powder mixtures was established. The surface charges and isoelectric points of the three different powders were investigated within the pH range from pH 3 to pH 12. Citric acid diammonium salt was found to be an effective deflocculant for shifting the isoelectric points to pH 3.5 for Al₂O₃ and to pH 6 for Y₂O₃. Aluminum hydroxide (Al(OH)₃) showed strong interaction with the Si₃N₄ powder, shifting the isoelectric point from pH 7 to pH 5.5. Low-viscosity, high-solids-loading suspensions (60-63 vol%) thus were possible at pH 9.7. The preparation of homogeneous and stable suspensions with a solids content of ≤61 vol% and a viscosity <1 Pa·s was limited to a pH regime between pH 9 and pH 10.5 because of the high solubility of yttria. The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease-catalyzed decomposition of urea at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green-density distribution was homogeneous throughout all bodies, even for complex geometries.