2 research outputs found
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