Mechanism underlying brittle fracture of a low-density unidirectional carbon-carbon composite

We used CVD (chemical vapor deposition) method to prepare a low density (1.4g/cm³) 1D-C/C (unidirectional carbon/carbon) composite. Tension tests revealed that it had a high tensile strength of 526MPa and low percentage elongation of 0.61%. SEM (Scanning Electron Microscopy) study confirmed that the fracture was brittle fracture. Stick-like granular matrix in the outer carbon fibers and isotropic matrix in the inner carbon fibers were easily found by using polarizing microscope. TEM study revealed a PyC (pyrolytic carbon) thin layer intervened between the carbon fibers and the granular and isotropic matrix. The mechanical locking action between soft PyC and hard, rough carbon fiber surface was a kind of strong interface. It is believed that this strong interface action is responsible for high tensile strength and brittle fracture.