Hybrid and multi-material additive manufacturing technologies for ceramics

The parallel development and optimization of industrial manufacturing techniques have set new standards and increased the demand for highly performing, complex and cost-effective components. Novel techniques like those of additive manufacturing have lost their rapid prototyping reputation and have been more and more employed for the fabrication of functional components made of high-end materials. Nonetheless, the specific printing process requirements and their layer-by-layer construction approach have been limiting their unique advantages in terms of design freedom and especially of the achievable optical, mechanical and electrical properties of the final component. The aim of this research project has been the development of hybrid and multi-material additive manufacturing technologies for the fabrication of ceramic and ceramic-metal components. Specifically, three-dimensional parts with complex geometries have been fabricated using two different hybrid techniques, a UV-assisted Direct Ink Writing and a Layerwise Slurry Deposition coupled with Direct Ink Writing. The fundamental steps in the research project have regarded the development and optimization of the hybrid processes and feedstocks. Silica-based and silicon nitride-based inks were prepared and used to produce support-less structures, ultimately verifying the UV-DIW system’s ability to print ceramic particles that absorb the incident light. Particle surface properties and liquid-particle interaction forces were found to be the most critical parameters to control the ink particle loading and flow behavior. Adequate density and fracture strength were achieved for lattice-like structures of silicon nitride. Transparent silica glass components were obtained thanks to the optimization of the heating schedule and the introduction of a silicon alkoxide. The development of the hybrid technique evidenced the limitations induced by the 3-axis setup. Hence, the UV-DIW process was coupled with a 6-axis robotic arm: layer-less lattices were fabricated thanks to the use of a custom graphical user interface and control over the printing head orientation. A highly reactive resin was employed as the ink and loaded with silica particles in order to tailor its rheological properties. A good resemblance between the printed components and the digital model was achieved, while their mechanical properties were superior to the ones of the traditionally additive manufactured octets. The capabilities of the LSD-DIW approach were instead demonstrated by fabricating proofs-of-concept for micro-reactors and heating elements. An alumina water-based slurry was optimized as the ceramic matrix; hydroxy-methyl cellulose was identified as the best candidate to balance the water evaporation and absorption forces thanks to its gel-like behavior. Tungsten and graphite inks were employed to create electrically conductive paths and hollow channels, respectively.