Carbon fiber/SiOC ceramic matrix composite truss-based structures fabricated via UV-assisted robot direct ink writing

Ceramic matrix composites (CMCs) are composites that contain inorganic fibers with ceramics as the matrix. The ceramic substrate in CMCs helps maintain the use of ceramics under load at very high temperatures (i.e., > 1000 °C) and improves the toughness and reliability of ceramics, particularly overcoming their intrinsic brittleness. Traditional manufacturing techniques for CMCs involve liquid- or gas-phase infiltration of carbon or ceramic fiber preforms with a precursor, followed by thermal cross-linking in an autoclave and pyrolysis. However, such manufacturing processes make it difficult, expensive, and time consuming to obtain complex-shaped CMCs that meet the requirements of engineering applications such as in aerospace (i.e., in turbine blades, heat shields), transportation (i.e., brake discs, engine components), and armor (i.e., ballistic protection panels, body armor). In this sense, additive manufacturing (AM) techniques represent the best solution: thanks to their layer-by-layer process and the selective deposition of material, AM allow the fabrication of custom and complex geometries, thus avoiding the use of expensive molds and/or the need of post-machining steps. On the other hand, conventional AM techniques, such as Direct Ink Writing (DIW) and vat photopolymerization, are limited in the fabrication of CMCs due to the poor control on the fibers orientation and the need of supports and/or sacrificial materials for the fabrication of fine features or severe overhangs. Furthermore, AM technologies are based on layer-by-layer approach, generating in this way interfaces, thus leading to lower strength and mechanical response of the printed structures. From such perspective, the best approach is represented by the combination of two or more AM technologies into a unique hybrid technique. UV-assisted DIW (UV-DIW), for example, is based on the extrusion of a photo-curable suspension through a nozzle which is consequently cured enabling retention of its shape in thin air, thus avowing the use of supports and allowing the fabrication of support-less features. As a further step forward in overcoming the limitations of traditional AM, UV-DIW been proven able to be coupled with a 6-axis robot arm; in this way, it is possible not only to increase the degree of freedom of the printable structures but especially to orient the printing head in the direction of the extruded filament, thus selectively align the fibers. This is particularly advantageous in the fabrication of strut-based lattice structures which are characterized by a regular pin-jointed frame made of trusses and surrounded by a void space. Thanks to their connectivity, such structures are able to distribute the stresses and to create a rigid and un-foldable geometry with a strength-to-weight ratio suitable for lightweight applications. This thesis project explored the fabrication and mechanical enhancement of lightweight, truss-based structures using UV-assisted Direct Ink Writing (UV-DIW) technology coupled with a 6-axis robot arm. The research activity focused the preparation of a silicon-based photocurable ink reinforced with chopped carbon fibers, achieving a final fiber volume of 20%. The suspension needs to maintain adequate fluidity under shear force to ensure smooth delivery to the print point and exhibit good UV light reactivity, despite the black color of the carbon fibers, which does not reflect UV light. The pre-ceramic matrix contained an organic component, which was eliminated during the sintering process. The presence of the fibers caused constrained matrix shrinkage and crack formation during pyrolysis. To address this, different steps of pre-ceramic polymer infiltration and pyrolysis were performed to compensate for these cracks and ensure structural integrity. An analysis of the flexural strength was performed.