Gelcasting of Ceramic Bodies

Net-shape and near-net-shape forming techniques have long been appealing in the production of ceramic materials. The high hardness and low toughness of ceramics make post-densification machining both costly and time-consuming, providing strong incentive for the development and optimization of net-shape techniques. The oldest of these forming techniques is slip casting. However, extrusion of cylindrical shapes, tape casting of laminates, and gelcasting, freeze casting, and injection molding of complex shapes have received considerable attention. Selective laser sintering, where shapes are determined via a computer-controlled localized heating profile, and robocasting, where material from a syringe or fine extruder is deposited in robotically controlled patterns, have garnered more recent interest. Each of these techniques relies on the suspension of a ceramic powder in a liquid vehicle or binder system for the forming stage of the operation. The shaped component is solidified through drying, cooling, or gelling. Once residual liquid is evaporated and binders are burned out, traditional densification methods, such as sintering, are employed. Treated here is gelcasting, one of the more promising forming methods for complex-shaped ceramic and powdered metal components. This method was patented by Janney and Omatete in the early 1990s and was explored in detail by them and their coworkers at Oak Ridge National Laboratory. In their originally conceived method, a low-viscosity slurry is produced by mixing a ceramic powder into an aqueous-based monomer solution, while Venkataswamy et al. used monomers that required organic solvents. The slurries have characteristically high solids loadings, often greater than 50 vol %, but have sufficiently low viscosity to flow easily. Through the addition of a chemical initiator and, in some cases, a catalyst, polymerization commences, at which point the slurry should be cast. The chemically cross-linked network that is formed through polymerization renders the ceramic powder particles immobile. The filled gel conforming to the shape of the mold is rigid enough to be removed for further processing. The high water content makes a controlled drying process critical to prevent warping and cracking. Low binder concentrations (generally