Development and characterization of additive manufactured thermoplastic-based discontinuous- and continous fiber-reinforced C/C-SiC composites

• Applicant: Dr. Nico Langhof
• Subject Area: Glass, Ceramics and Derived Composites
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 461938713

The demand of fiber reinforced ceramic matrix composites (CMC) is increasing continuously, due to their favorable thermomechanical properties. During the most important fabrication process (LSI-Liquid Silicon Infiltration), a carbon fiber reinforced polymer laminate (CFRP resp. green body) is transformed in to a fiber reinforced C/C-SiC ceramic. A suitable polymer matrix has to show more than 50 wt.% residual carbon after the pyrolysis step in inert conditions (> 900 °C). The carbon matrix reacts finally with liquid silicon (> 1420 °C) to SiC. The thermophysical properties of C/C-SiC depends mainly on the fiber orientation, the fiber volume content (FVC), the fiber-matrix-bonding and the fiber length. Currently, those four factors can be modified for CMC only very limited. Therefore, CMC materials in terms of design and shape are not very application related and cannot fulfill the requirements of specific thermomechanical loads. With the use of short fiber reinforcement (> 6 mm length) net-shape fabrication is possible, but the strength is moderate, due to the low FVC and isotropic fiber orientation. High strengths, FVC but only few fiber orientations and no net-shape are possible by applying C-fiber fabrics. A combination of these advantages is desired and feasible by introducing additive manufacturing. Hence, according the thermomechanical loads, fibers of the required length, volume and orientation can be placed and modified within the CFRP and each laminate layer.In order to develop continuous and non-continuous CMC, fiber reinforced PEEK is used and additively manufactured by the FDM/AFP-process. After the pyrolysis of PEEK more than 50 wt.-% amorphous carbon remained, but unfortunately, PEEK as a thermoplastic polymer melts > 340 °C. Melting of the matrix polymer causes delaminations, deformations and bloating during pyrolysis of the composites. Therefore, a process that prevent the complete melting of PEEK will be developed within the project. The process is based on crosslinking PEEK. The approach is based on several successful preliminary experiments. Combined with the possibility to print tailored microstructures in the CFRP, that enable the creation of degassing channels during pyrolysis, there will be a possibility to develop additive manufactured, three-dimensional net-shape composites, that don’t need axial loadings or other form fixing tools during pyrolysis, compared to the state of the art. PEEK without fibers, PEEK with C-fibers < 0.5 mm length and PEEK with C-fiber bundles > 25 mm lengths will be evaluated.The crosslinking process will be studied by rheological, calorimetrical, thermogravimetrical and dynamic thermomechanical methods up to 600 °C in order to understand the crosslinking mechanisms and the kinetic effects. Furthermore, microstructural and mechanical properties will be determined and some prototype manufactured, that show the potential of additive manufactured CMC.

• DFG Programme: Research Grants