Colloidal processing of fine ceramic powders enables the production of complex shaped ceramics with unique micro and macro structures which are not possible to produce via conventional dry processing routes. Because of this enhanced structural control and shaping capabilities, colloidal processing has been exploited to produce ceramic components with ever increasing complexity and functionalities. In this review, we revisit some of the research efforts on this topic to highlight its relevance and growing importance for the advanced manufacturing of functional ceramics. Selected examples of colloidal systems with increasing level of complexity are discussed to showcase the wide range of structures that can be generated through wet processing approaches. The historical development and background knowledge pertaining to colloids and surface interactions is first briefly reviewed. The major colloidal shape forming and additive manufacturing processes that utilize colloidal pastes and inks are then reviewed, highlighting the control of suspension rheology needed in these techniques. Next, methodologies that combine suspended particles with a pore‐forming phase are discussed as a means to produce porous ceramic materials. Further control over the interactions between anisotropic particles and their alignment in suspensions can be gained via externally applied fields (such as magnetic) to produce texturally aligned green bodies. This leads to bioinspired ceramics that can programmably morph into complex shaped objects upon sintering. Hierarchical porous structures with high mechanical efficiency are also shown as an example of the multiscale designs that can be generated through advanced colloidal processing. As drying of ceramic bodies is an inevitable consequence of wet colloidal processing, the current understanding of this critical processing step is reviewed. Finally, the gaps in knowledge in these fields are discussed to provide our perspective on where the field may support advances in ceramics in the future.

Porous cellular alumina ceramic green bodies have been produced by combining the particle stabilized foam method with gelcasting. The suspension foams were stabilized by particles rendered weakly hydrophobic with short chain sulfonate surfactants. Poly vinyl alcohol (PVA) and 2,5-dimethoxy-2,5dihydrofuran (DHF) were used as the gelcasting reagents. The microstructure (amount of porosity, average pore size, and morphology) of alumina (Al 2 O 3 ) ceramic green body foams has been studied as a function of surfactant concentration and chain length. The morphology of gelled ceramic foams changes from closed cell (bubble like morphology) to open cell (granular morphology) as the surfactant concentration is increased beyond a critical level. The density (and porosity) of the gelled alumina green body foams changes as a function of the surfactant concentration in a non-linear manner. Measurement of the suspension viscosity, contact angle and aggregate size are used to explain the changes in density and morphology. The transition from the bubble like structure to the granular structure is due to an increase in the particle aggregate size rather than phase inversion induced by an increase in the contact angle. The same behavior is observed with three different chain length surfactants (ranging from 4 to 10 carbon atoms on the hydrophobic portion of the surfactant) but at a lower surfactant concentration as the chain length increases.

Porous ceramic materials have been thoroughly developed due to the wide number of applications in different areas, such as filters for environmental clean-up and reuse, bioreactors, gas or chemical sensors and support materials for catalysts or absorbents. One of the forming methods that can be selected for these applications is the freeze drying of suspensions. This method has received increased attention for manufacturing near net shaped parts with directional porosity. This work deals with the manufacture of porous alumina bodies by a freeze casting route. The different parameters of the process, such as the solids content of the suspension, the addition of cryoprotector (glycerol) and the freezing conditions (temperature and device) have been studied to determine their influence on the microstructure, density and porosity of the green and sintered pieces. Materials with different degrees of porosity and distribution of pores (aligned in the freezing direction or homogeneously distributed) have been obtained.

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.