Effect of high temperature treatment on the microstructure and mechanical properties of binary layer textured 2D C/C composites

X-ray diffraction (XRD), scaning electron micrsocopy and transmission electron microscopy were used to study the changes of the pyrolytic carbon microstructure and fiber-matrix interface of binary layer textured 2D C/C composites before and after heat treatment. The XRD results show that the d₀₀₂ decreased and the graphitization degree increased significantly with the increase of heat treatment temperature. No obvious change of the fracture surface morphology and lattice fringes of the inner layer, which correspond to low textured pyrolytic carbon, was observed. However, the lattice fringes of the outer layer (high textured pyrolytic carbon) are regularly parallel, and the density of microcracks in the outer layer increases obviously after heat treatment. These microcracks make cracks propagate and deviate easily, leading to an increase of the toughness of the C/C composites. Moreover, the weakening of the fiber-matrix interface bonding is another mechanism for the toughness enhancement. The results of three point bending tests confirmed the proposed mechanism.