Robocasting of Ceramics and Composites Using Fine Particle Suspensions

Solid freeform fabrication is the near-net-shape manufacturing of components by sequentially stacking thin layers of material until complicated three dimensional shapes are produced. The operation is computer controlled and requires no molds. This exciting new field of technology provides engineers with the ability to rapidly produce prototype parts directly from CAD drawings and oftentimes little or no machining is necessary after fabrication. Techniques for freeform fabrication with several types of plastics and metals are already quite advanced and maybe reviewed in references 1 and 2. Very complicated plastic models can be fabricated by stereolithography, selective laser sintering, fused deposition modeling, or three-dimensional ink jet printing. Metals may be freeformed by the LENS{trademark} technique and porous ceramic bodies by three dimensional printing into a porous powder bed. However, methods for freeform fabrication that utilize particulate slurries to build dense ceramics and composites are not as well developed. The techniques that are being developed for the freeform fabrication of dense structural ceramics primarily revolve around the sequential layering of ceramic loaded polymers or waxes. Laminated Object Manufacturing and CAM-LEM processing use controlled stacking and laser cutting of ceramic tapes [2,3]. Similar to fused deposition modeling, ceramic loaded polymer/wax filaments are being used for the fused deposition of ceramics [2,4]. Extrusion freeform fabrication uses high pressure extrusion to deposit layers of ceramic loaded polymer/wax systems[1]. Modified stereolithographic techniques are also being developed using ceramic loaded ultraviolet curable resins [2]. Pre-sintered parts made with any of these techniques typically have 40-55 vol.% polymeric binder. In this regard, these techniques are analogous to powder injection molding of ceramics. Very long and complicated burnout heat treatments are necessary to produce a dense ceramic, free of organics. Heating rates of 0.2 degrees Celsius per minute are common. [5] Thus, while a part maybe rapidly prototype within a few hours, it takes several days to densify. In contrast, robocasting is a freeform fabrication technique developed at Sandia National Labs that utilizes particulate slurries but does not require organic binders. Since binder burnout is not an issue, a dense ceramic part maybe freeformed, dried, and sintered in less than 24 hours. In some regards, robocasting is analogous to the ceramic near-net-shape processing techniques, slip casting and gel casting [6]; however, robocasting is moldless and fabrication times can be quicker