C/C-SiC Materials Based on High Performance C Fibres with Tailored Fibre-Matrix Bonding

The Liquid Silicon Infiltration (LSI) process is used to manufacture carbon fiber based C/C-SiC composites. A CFRP-preform (carbon fibre reinforced polymer) is manufactured by infiltrating C fibre rovings with a phenolic resin, wet filament winding and warm pressing. After curing, the polymer matrix is converted to carbon by a high temperature treatment. Due to matrix shrinkage, the resulting C/C preform is characterized by a high amount of porosity. The pore morphology includes fine, closed pores as well as open porosity in form of segmentation microcracks (SMC), fragmenting the fibre tows into discrete C/C bundles and forming a 3D-network of interconnected microchannels. Depending on the fibre matrix bonding (FMB) and interaction, also shrinkage cracks between individual filaments and matrix (FMC) are observed in the C/C bundles all over the composite. During siliconization, the molten silicon is infiltrating the open pores and microcracks and SiC matrix is built up by a chemical reaction of Si and C. In order to obtain damage tolerant C/C-SiC materials, the fibres have to be protected from Si contact and conversion to SiC. Therefore, FMI has to be tailored to form dense C/C bundles and to avoid shrinkage cracks between filaments and matrix. In the current work, the formation of SMC and FMC is studied on CFRP-preforms based on standard high tenacity as well as on intermediate and ultrahigh modulus C fibres and is simulated in a FE-model. The influence of fibre surface treatment on FMB and microstructure development as well as on the material processing and, finally, on the mechanical properties of the resulting C/C-SiC materials is presented.