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.

C/C-SiC parts manufactured from woven, braided of winded fabrics have a specific crack pattern that evolves during pyrolysis and is infiltrated in the step of liquid silicon infiltration. Using the thermoset injection molding in the first step of the processing chain, the microstructure is looking totally different. During compounding of the Novolak and mixing it with the hardener and carbon fibers, the fibers are shortened drastically (approx. average 300 µm) and homogeneously distributed. While filling of the injection cavity, the fibers are orientated depending on the flow, which effects the mechanical properties of the composite parts. The cracks that are generated in the pyrolysis step form a totally different network like e.g. those parts with woven fabrics.

This works shows the microstructural changes throughout the manufacturing process with special focus on the correlation between microstructure and properties. Light and scanning electron microscopy images show the structural development in CFRP, C/C and C/C-SiC state of the samples in cuts and the fracture behavior is analyzed using the fracture surfaces is comparison with the stress-strain curves and in-situ micro-bending in the SEM.