Micrographs of two single-phase FeTiO5 laminates with textured and untextured layers. Image (a), top, shows crack bifurcation within the textured layers. Image (b) shows tunnel cracks in the textured layer. Source: International Journal of Applied Ceramic Technology
The current issue of the International Journal of Applied Ceramic Technology reports on a Northwestern University group’s work related to using improved gelcasting techniques that allow new possibilities for manufacturing of certain laminates.
Gelcasting, a technique perfected at the Oak Ridge National Lab, is a method used to create large, near-net-shape ceramic and metal components with complex shapes from low-viscosity slurries composed of powders suspended in a liquid binder system. The components begin to be solidified when a chemical initiator is added to the slurry. This starts the formation of a polymer gel network. The slurry is quickly poured into a molds and allowed to dry. The additives and binders are burned out before sintering.
The team of Noah Shanti, David Hovis, Michelle Seitz, John Montgomery, Donald Baskin and Katherine Faber describe the use of more flexible gelcasting system – thermoreversible gelcasting – that allows more opportunities to manipulate the materials during the molding stage, something they found useful, for example, in toughening laminates.
The advantage of TRG over traditional gelcasting is that it is not time constrained (as long as the slurry temperature is kept above the transition temperature). Lamination is possible during gelcasting by adding successive layers of slurries selected because the properties and interfaces being sought. The teams describes concepts of tailoring the porosity and texture of the layers to, for example, strengthen the final laminate material by crack deflection, crack bifurcation and taking advantage of residual compressive stresses.
In one case, they describe the using the enhanced manipulation time to introduce a magnetic field to the materials during casting. The magnetic field aligns ceramic particles and allows the development of highly textured microstructures. The would not be possible using traditional gelcasting techniques because of the relatively brief window of opportunity before solidification begins.
The groups notes that while the use of TRG requires a good understanding of polymer chemistry, physics, and slurry rheology, not to mention drying and sintering kinetics, they predict the technique will find much use in applications ranging from strong bioceramic materials to solid oxide fuel cells.