Aqueous and Nonaqueous Colloidal Processing of Difficult-to-Densify Ceramics: Suspension Rheology and Particle Packing

Aqueous and nonaqueous colloidal processing of zirconium diboride (ZrB2) and boron carbide (B4C) has been investigated. The aqueous and nonaqueous ZrB2 and B4C suspension formulations have been optimized. The suspensions were cast into green bodies using slip casting. The correlation between the state of dispersion with the rheological properties of the suspensions and the resulting packing density was observed in both aqueous and nonaqueous processing. The attractive interactions between powder particles in water were difficult to overcome with electrical double layer or electrosteric repulsion. Reasonably low viscosity aqueous ZrB2 suspensions up to 45vol% solids could be prepared. It was not possible to produce low viscosity (viscosity below 1Pas at shear rate of 100s(-1)) aqueous B4C suspensions with solid content above 30vol%. Slip casting of the weakly aggregated ZrB2 suspensions resulted in low packing densities (similar to 55% relative density) of the green bodies. On the other hand, dispersion of powder particles in nonaqueous media (cyclohexane and dodecane) enabled suspensions with lower viscosities and a higher maximum solid concentration (up to 50vol%) to be prepared. The well-dispersed nonaqueous suspensions promoted an efficient particle packing, resulting in higher green densities (64% and 62% relative density for ZrB2 and B4C, respectively) compared to aqueous processing. The significantly high green densities are promising to allow densification of the materials at lower sintering temperature.