Maximizing solids loading for aqueous slurry robocasting of silicon carbide

Direct ink writing (DIW) of silicon carbide (SiC) enables the fabrication of net-shaped ceramic components, but achieving slurries with high solids loading is a challenge. In this work, we develop a monomodal, sub-micron SiC slurry by tuning colloidal interactions through zeta potential analysis, dispersant selection, and rheological characterization. A 2 vol% addition of 25k MW polyethyleneimine (PEI) was identified as the optimal surface-coverage condition, enabling strong electrostatic stabilization without pH adjustment. Solids loading was increased from 35 to 56 vol%, approaching the percolation threshold (~57 vol%) predicted by viscosity trends. This shows significant improvement compared to literature where 51 vol% was found to be the point where shear thickening behavior was exhibited and could no longer work for a gel casting system. The resulting inks exhibited pronounced shear-thinning behavior and yield strengths suitable for DIW up to ~49 vol%, with higher-loading slurries processed through casting. After pressureless sintering at 2200 °C, final densities reached ~93.5% theoretical for the 56 vol% formulation, accompanied by reduced porosity and improved microstructural uniformity. This study establishes a quantitative framework for maximizing solids loading in aqueous SiC DIW slurries and demonstrates the highest reported solids loading for monomodal sub-micron SiC suitable for pressureless sintering.