Pre-Ceramic Polymer (PCP) is a class of material that can be converted to ceramic by thermal treatment in an inert atmosphere; PCPs, generally silicon-based, are compatible with most of the additive manufacturing technologies used for polymers, including the Direct Ink Writing (DIW). DIW is part of the Additive Manufacturing (AM) technique, which is based on non-Newtonian fluid feedstocks, extruded through a nozzle according to a pattern computer-generated. The possibility of producing structures with ordered porosities and thin walls of DIW partially reduces one of the major drawbacks of the polymeric precursors, the formation of cracks during the thermal treatment, due to the shrinkage and gas release during the ceramic conversion.
The Polymer Derived Ceramics (PDCs) obtained from the pyrolysis in an inert atmosphere of a PCP possess high thermal and chemical properties and is generally used for thermo-structural applications.
In this work, PCP-based feedstocks containing chopped fibers is formed in complex shapes by DIW. Thermal treatments in inert atmospheres are used to convert the polymeric component into ceramic achieving Ceramic Matrix Composites (CMCs). In particular, the two PCPs used are silicon and silazane, to achieve respectively SiOC-based and SiCN-based composites, containing fillers and short carbon fibers. The fillers are used both as a rheological modifier, increasing the yield strength of the slurry and acting as an agent to reduce the shrinkage during the thermal treatment; they are divided into two main categories, passive and active. The first kind, usually a ceramic, reduces the shrinkage by reducing the volume ratio of the PCP; the second type, usually a metal, reacts with the atmosphere and with the decomposition gases. The carbon fibers are used to increase the mechanical properties of the final sample, by introducing a high energy-consuming mechanism, such as the fiber pull-out.
To increase the complexity of achievable shapes, Embedded-DIW was used to fabricate geometries generated by a custom-made python code. This technology uses the DIW technique to extrude the slurry in a viscous medium that supports the structure; in this way, it is possible to produce geometries with suspended bridges without the risk of collapsing. Curved structures can also be obtained.
The samples were first observed with a Scanning Electron Microscope (SEM) to investigate the eventual formation of cracks, due to the shrinkage during the thermal treatment, the distribution of the fibers in the cross-section, the presence of fiber pull-out and the presence of pores, created during the production of the slurries. Further characterization also includes the investigation of the thermal treatment effect on the slurries and their components by Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC), also used to investigate the thermal stability of the pyrolyzed samples.