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

Abstract

Aqueous and nonaqueous colloidal processing of zirconium diboride (ZrB 2 ) and boron carbide (B 4 C) has been investigated. The aqueous and nonaqueous ZrB 2 and B 4 C 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 ZrB 2 suspensions up to 45 vol% solids could be prepared. It was not possible to produce low viscosity (viscosity below 1 PaÁs at shear rate of 100 s -1 ) aqueous B 4 C suspensions with solid content above 30 vol%. Slip casting of the weakly aggregated ZrB 2 suspensions resulted in low packing densities (~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 50 vol%) to be prepared. The well-dispersed nonaqueous suspensions promoted an efficient particle packing, resulting in higher green densities (64% and 62% relative density for ZrB 2 and B 4 C, respectively) compared to aqueous processing. The significantly high green densities are promising to allow densification of the materials at lower sintering temperature.

Additive manufacturing of boron carbide via continuous filament direct ink writing of aqueous ceramic suspensions

“…Previous research on aqueous colloidal processing of B 4 C has been limited to slip and tape casting processes [6,7,10,21,22]. Leo et al [6] reported that aqueous slip casting slurries of B 4 C were only stable up to 40 vol.% when using 2.6 m powders and Darvan C-N, an ammonium salt of poly(methacrylic acid) (PMAA), as a dispersant.…”

“…Leo et al [6] reported that aqueous slip casting slurries of B 4 C were only stable up to 40 vol.% when using 2.6 m powders and Darvan C-N, an ammonium salt of poly(methacrylic acid) (PMAA), as a dispersant. As stated previously, higher solids loading must be used in a direct writing approach to achieve the desired rheological properties and higher green body packing densities [6,17].…”

“…Leo et al [6] reported that aqueous slip casting slurries of B 4 C were only stable up to 40 vol.% when using 2.6 m powders and Darvan C-N, an ammonium salt of poly(methacrylic acid) (PMAA), as a dispersant. As stated previously, higher solids loading must be used in a direct writing approach to achieve the desired rheological properties and higher green body packing densities [6,17]. Furthermore, other studies have reported that suspensions with higher solids loading minimize drying-induced crack formation and phase segregation while promoting higher sintered densities [6,12,17].…”

“…First, increased powder loading tends to increase the yield stress of the suspension, necessary for building parts with multiple stacked layers to avoid slumping. Second, increasing the powders loading increases the green body density, thus there is less porosity to remove during sintering [6,16,17].…”

“…B 4 C has historically been sintered using externally applied pressure, via hot pressing or hot isostatic pressing, due to the challenges associated with densification [2][3][4][5]. Hot pressing limits the geometries of the final pieces to simple shapes such as pellets, plates, disks, and rods [6]. Hot isostatic pressing (HIP), on the other hand, allows for the sintering of more complex shapes formed through processes such as slip, tape, or gel casting [7,8].…”