Ceramic matrix composites (CMC) with a combined matrix of carbon and silicon carbide reinforced with carbon fibers (C/C-SiC) have exceptional heat, thermal shock, creep and wear resistance at a low density. Compared to monolithic ceramics, CMCs are quasi-ductile and damage tolerant, making them suitable for difficult applications. CMCs were originally developed for space applications, but are now used as friction materials for high-performance brakes, e.g. in elevators and sports cars. However, due to the high production costs, the applications are limited, which is why brake discs made of CMC have not yet found their way into standard passenger cars, despite their lower weight, better braking performance and longer service life. The industrial manufacturing process is the three-step LSI route: First, a carbon fiber-reinforced plastic (CFRP) part is manufactured, which is then pyrolysed into a porous carbon composite (C/C) before being infiltrated with liquid silicon (LSI - liquid silicon infiltration) to the C/C-SiC CMC.
For the first stage, the thermoset injection molding process is used in this work. For improved freedom of design, lost cores made of epoxy resin are put in the injection tool and fully embedded during the injection of the carbon fiber reinforced phenolic resin that crosslinks in the mold. During the pyrolysis process step, the epoxy resin is removed from the samples and the liquid silicon infiltration is carried out via the wick method.
By using it as a highly automated manufacturing process from plastics processing with upstream compounding, the aim is to achieve mass production. However, the carbon fibers are greatly shortened by the screws and the high shear forces in compounders and injection molding machines, which reduces the ductility of the CMC. To overcome this issue, additional chopped carbon fibers are given in the hopper together with the granulate compound.
This work gives an overview of our research to date, starting with the basic process development, the improvements and optimizations to the process and the material, and finally the infiltration of textile semi-finished products in thermoset injection molding is given. The material research is carried out by microstructure investigations such as computer tomography and scanning electron microscopy, for the analysis of the mechanical properties the bending method is used as an example.