4. Generation, Development, Inheritance, and Control of the Defects in the Transformation from Suspension to Solid

This chapter focuses on the generation, development, inheritance, and control of the defects in colloidal forming process, by which the complex near-net shape ceramic products with a homogeneous microstructure and high reliability were consequentially achieved. The rheological performance of the ceramics suspension is very important to the colloidal forming processes. In our studies, the rheological behaviors of aqueous alumina suspensions were investigated, and the various ways of modifying the rheological behaviors of the suspensions were discussed. A critical solid volume fraction between the shear-thinning and shear-thickening behavior was found in an aqueous alumina suspension. Moreover, the gelation of the suspension with high solid volume fraction of alumina (SVF) got delayed and the strength of the green body decreased, which implies that the fast polymerization of monomers in high SVF alumina suspension was inhibited, and the flexibility of the gelcasting was improved. We found that there was an optimal SVF for the ZTA suspension to obtain the highest bending strength and the highest Weibull modulus for the sintering bodies. Micro mechanisms of agglomerations’ generation in ceramic suspensions used in colloidal forming processing were investigated. On the basis of the potential energy between particles in liquid, a microscopic model of agglomerations in ceramic suspensions was presented. Two types of agglomerations, tight and loose, were observed by using an environmental scanning microscope as a quasi-direct method. It was observed that their generation is caused by the deviation of the solid-phase content in the slurry from the stable solid loading. The influences of the idle time on the microstructures and the mechanical properties of the green bodies are also described in this chapter. Vertically laminated cracks were observed in the coagulated green bodies after casting. The cracks were attributed to the shrinkage of the particle network (syneresis) during coagulation. This process in combination with sedimentation during the reaction affected the vertical cracks, which was not observed when a certain idle time before casting was introduced. The idle time diminished the final shrinkage still having a castable slurry of reasonably low viscosity. The study indicated that the solids volume loading in the suspension depended on high valence counterions and ion conductivity constants. Further research revealed that the inner stress in the ceramic green bodies originates from the nonuniform shrinkage during the solidification of the precursor suspension and the drying of the green bodies. The gradients of temperature, the initiator concentration, and the moisture are important original factors causing the inner stress. It is found that a proper amount of a water-soluble polymer, such as hydroxyethyl acrylate (HEA), polyethylene glycol (PEG), and polyvinylpyrrolidone (PVP) added into the concentrated suspension can adjust the polymer network structure and thus reduce the inner stress and cracking in the ceramic green body. The debindering time of large ceramic parts can be significantly shortened by reducing the harmful inner stresses in the green body.