Additive Manufacturing Utilizing a Novel In-Line Mixing System for Design of Functionally Graded Ceramic Composites

Exploring the development of a direct ink writing system with multimaterial and in-line mixing capabilities for printing inks composed of high solids-loaded ceramic particulate suspensions.

December 1, 2022

Army Research Laboratory, Aberdeen Proving Ground, Maryland

Exploded view of a print head to demonstrate component positioning.

The impetus for advancing brittle ceramic materials used for armor applications has been identified as both an increased mass efficiency for greater weight reduction and enhanced performance against ballistic threats through manipulating the physics of failure.

Development of functionally graded ceramic composites is a potential solution for enabling new dynamic fracture and deformation mechanisms in armor materials, surpassing that of the individual intrinsic material behaviors. However, engineering of these composite structures is currently limited due to the stochastic nature of traditional powder mixing and processing. Thus, additive manufacturing (AM) is being studied as an enabling technology for processing discrete mesostructural features, complex geometries, and compositional variation.

Various traditional ceramic forming technologies exist including pressing, extrusion, slip casting, tape casting, and injection molding. However, these traditional forming techniques often only allow 2-D design freedom, cannot create internal or multiscale features, and require complicated and expensive dies. On the other hand, AM enables the production of parts with complex, multiscale geometries including internal structures. Additionally, multi-material AM allows for the mimicking of biological composites that take advantage of hierarchical structures and compositional gradients to increase their strength and toughness.

Three main categories of AM exist: extrusion-based processes, liquid-bed processes, and powder-bed processes. Extrusion-based processes, such as fused deposition modeling (FDM) and direct ink writing (DIW), raster a print head across the print-bed layer by layer while extruding a continuous trace of material. Liquid- bed processes, such as stereo-lithography, use a laser or other energy source to cure resin layer-by-layer as the build plate further submerges. Powder-bed processes, such as selective laser melting/sintering, use a laser or other high-power energy source to melt/sinter powder in a layer-by-layer fashion as the print-bed moves downward and more powder is spread over the print area.

Exploration of next-generation ceramic armor composites will require high-resolution, multi-material, and mixing capabilities to print structures that have compositional gradients, internal structures, and hierarchical organization. Powder- and liquid-bed processes run into issues with multi-material applications because the powder/liquid vat must be switched for every composition variation. The extrusion-based FDM method of ceramic-filled polymer filament creates parts with low green density, leading to low percent theoretical density following sintering. Ink jetting, a popular commercial solution to ceramic AM, again produces porous parts with low density. DIW, on the other hand, shows significant promise for studying these composites because it fulfills each requirement.

The DIW technique, also known as robocasting, involves extrusion of a line of highly loaded colloidal suspension, termed “ink”, through a small nozzle in a specific pattern layer by layer to produce a 3-D part. Unlike FDM, where printed material solidifies directly after extrusion, DIW relies on the yield-pseudoplastic rheological behavior of the ink for structural integrity following extrusion.

Yield-pseudoplastic rheology gives the ink structural integrity up to its yield stress, followed by shear-thinning behavior. When the ink is designed with this rheological behavior, it can easily flow through the nozzle due to the high-shear environment, but after extrusion (yield stress goes to zero) it can retain its shape due to its yield strength.

Just as important as ink printability is post-print processing performance – namely shape retention and densification characteristics. A postprocessed part must be as close to 100 percent theoretical density as possible to have acceptable ballistic performance.

A prerequisite of high sintered density is high green density, necessitating high particulate-loading and low binder/dispersant content in the ink. However, high solids-loading and low binder/dispersant content leads to dilatant behavior and poor printability. Thus, a balance must be found between printability, yield-point, and post-print processing behavior. These competing factors result in a complex ink design process, where constituent type, content, and interaction are of paramount importance. Although complicated, understanding of colloidal ink processing has advanced to a point where there is flexibility available in terms of binder/dispersant selection, allowing selection of a variety of material systems.

This work was performed by Joshua Pelz, Nicholas Ku, Marc Meyers, and Lionel Vargas-Gonzalez for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) below.