Ceramic matrix composites (CMC) are often manufactured through cost effective polymer infiltration and pyrolysis. Thereby a fibre reinforced polymer is pyrolyzed under inert atmosphere and converted to a ceramic material. Due to the shrinkage of the polymer precursor during pyrolysis and the mismatch of thermal expansion between fibres and matrix, a characteristic crack pattern is formed. The formation and morphology of this resulting crack pattern is crucial for following polymer impregnation steps or a reactive melt infiltration.
This work investigated the formation of the pyrolysis crack pattern for carbon fibre reinforced phenolic resins used for the production of C/C or C/C-SiC materials. Unidirectional carbon fibre reinforced polymers (CFRP) were manufactured via warm pressing of a novolak type resin and high-tensile carbon fibres. The fibre volume content of the CFRP was varied between 35 % to 55 %. A thermal pre-treatment of carbon fibres was carried out on selected samples prior to warm pressing to alter the fibre-matrix adhesion strength.
Thermal expansion / shrinkage during pyrolysis was measured using a dilatometer. Mass loss during pyrolysis was investigated using thermogravimetry. The microscopic crack formation and growth of cracks up to 1000 °C was observed using a microscopic heating cell which was placed under a digital microscope with high magnification. The crack patterns of pyrolyzed materials were characterized by the measurement of crack width, distance between cracks and overall crack density (cracks per millimetre) using light microscopy. For the crack pattern analysis large samples (> 50 mm x 120 mm) were pyrolyzed under nitrogen atmosphere up until 4 different temperatures (550 °C, 650 °C, 750 °C and 1000 °C). Furthermore, micro-computer tomography (µCT) was performed on pyrolyzed samples to investigate the spatial orientation of cracks.
It was found that for all investigated materials the values for crack width and crack spacing were distributed in a way which could be modelled by statistical distribution functions with high accuracy. For the investigated materials, increasing fibre volume contents led to an elevated number of finer cracks parallel to the reinforcement direction and to overall less cracks perpendicular to the fibre orientation. Furthermore, the fibre-matrix adhesion strength also influenced the formation of cracks and determined the resulting crack pattern. The visible microscopic expansion and shrinkage during pyrolysis in the heating cell was correlated to the macroscopic dilatometer measurements and ultimately to the resulting crack pattern and crack morphology. The µCT measurements exhibited an interconnected network of longitudinal and transversal cracks for all investigated materials once pyrolysis was completed at 1000 °C.
The presented statistical functions, microscopic investigations and general findings are valuable tools for predicting and simulating the crack pattern and led to a deeper understanding on the crack formation during pyrolysis of fibre reinforced materials.