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 <5 wt %) can be removed quickly and the body sintered. Sintering to full density is promoted by the high solids loading that can be achieved in gel-casting slurries. Gelcasting should not be confused with sol-gel processing. In gelcasting, ceramic (or precursor) powders are suspended in a monomer or polymer solution to form slurries for casting. The monomer/polymer solution gels without reacting with the suspended powder, in essence locking the particles in place; the same gel would form in the absence of any ceramic. In sol-gel processing, ceramic precursors are integral to the gel formation process (through hydrolysis, polycondensation, etc.) Metal alkoxides, hydroxides, and the like form the backbone of the gel network and are converted to ceramic in later processing steps. In this chapter, we first describe the categories of gel-casting systems and the chemistry of gelation in each type. Following this is a description of the processing steps from gel preparation to densification. An account of the variety of structural classes that are afforded by gelcasting is then presented. In addition to the processing of conventional bulk ceramics, gelcasting of textured ceramics, porous bodies, and laminates is described. Finally, gel-casting challenges and opportunities are highlighted.