Optimizing Dispersants for Direct Ink Writing of Alumina Toughened Zirconia (ATZ) Ceramics: Insights into Suspension Behavior and Rheological Properties

Alumina toughened zirconia (ATZ) ceramics combine high biocompatibility with remarkable mechanical properties, making them suitable for dental and orthopedic implant applications. Producing these ATZ ceramics using slurry-based additive manufacturing necessitates homogeneous, stable suspensions with controlled particle sizes. Stabilizing such systems with the appropriate type and amount of dispersant is challenging, particularly since multi-component systems are prone to hetero-coagulation. In this study, ATZ powders with different surface areas were investigated to determine the optimum concentration of three commercially available dispersants: Darvan CN, Darvan 821A, and Dolapix CE64, which have been successfully used to stabilize Al₂O₃ and 3Y-TZP suspensions. Based on zeta potential (0.01 vol% suspensions), agglomerate size (0.01 vol% suspensions), sedimentation (10 vol% slurries), and rheological (40 vol% slurries) characterization, the optimum dispersant concentrations were found to be 0.50 mg/m² for Dolapix CE64, 0.75 mg/m² for Darvan 821A, and 1.50 mg/m² for Darvan CN. Among the studied dispersants, Dolapix CE64 was the most effective in terms of reduced sedimentation, smaller agglomerate size (0.70 μm), flow behavior, and low resistance to structure breakdown. The rheological assessment showed that slurries prepared with ATZ powder featuring a smaller specific surface area (7.3 m²/g) resulted in lower viscosity, critical stress, and equilibrium storage and loss moduli compared to those prepared with higher specific surface area (13.3 m²/g) starting powder. The sedimentation analysis however revealed that the larger specific surface area ATZ powder exhibited higher slurry stability. While 38 vol% ATZ pastes without dispersant showed inhomogeneous extrusion and the presence of aggregates, the filaments extruded from 45 vol% paste with 0.50 mg/m² Dolapix CE64 had a homogeneous and smooth structure and were free of aggregates, highlighting the importance of the dispersant addition for DIW.