Liquid silicon infiltration is a cost-effective manufacturing route for C/C-SiC, which is a ceramic matrix composite. It is characterized by a high thermal shock resistance as well as a high specific stiffness associated with a quasi-ductile material behavior. Within this process, a carbon fiber reinforced polymer (CFRP) is manufactured using conventional polymer processing methods (e.g. warm pressing), pyrolyzed above 1000 °C in inert conditions into a C/C preform and infiltrated with liquid silicon above 1420 °C in vacuum.
To reduce the cost-intensive machining while improving the geometric freedom of design of the manufactured parts, a new additive manufacturing route of C/C-SiC was investigated. The fused filament fabrication (FFF) technology was used to manufacture CFRPs made of thermoplastic polyetheretherketone (PEEK) as the matrix with short carbon fibers as the reinforcement. First, a thermoplastic polymer filament is extruded by a heated nozzle. The softened strand is extruded and deposited layer by layer onto the build platform, forming a three-dimensional part. Due to this process, the manufactured green bodies show a characteristic grid structure with open pore channels. The open pore channels are necessary for degassing of the gaseous volatiles arising during pyrolysis. To prevent remelting of the CFRP during pyrolysis, a thermo-oxidative crosslinking prior to pyrolysis was introduced at 325 °C for 48 h in air atmosphere. According to literature, thermo-oxidative crosslinking of PEEK is dependent on the specific surface area (SSA). Therefore, the SSA of the green bodies was investigated via computer-aided image analysis as a function of the layer height, the infill density, and the printing direction during the FFF process using statistical design of experiments to enhance the stability of the CFRP during pyrolysis.
It was shown that it is possible to adjust the SSA selectively between 8.0 and 11.3 mm²/mg by varying the investigated process parameters due to changes in the mesostructure of the CFRP. Based on the process parameters resulting in a maximized SSA, the shrinkage after each process step and the flexural strength were measured. Although the carbon fibers were shorter than 250 µm in length, they do impede the shrinkage of the composite. The flexural strength increased with decreasing layer height and reached values of 80.3 ± 6.7 MPa. In general, it was shown that additive manufacturing offers the possibility to control the microstructure of the C/C-SiC and hence the mechanical properties by adjusting the process parameters during the printing process.