Microstructure and mechanical behavior of carbon/carbon composites prepared from xylene pyrolysis

Two-dimensional carbon fiber felts were densified by film boiling chemical vapor infiltration from xylene pyrolysis at 950~1150°C. Within 30~35 hour densification, carbon/carbon composites with average density of 1.72~1.74g/cm³ were produced. The density and mechanical property of the composites were measured by Archimedes method and three-point bending test, respectively. The microstructure of pyrocarbon and the morphology of fracture surface were studied using polarized light microscopy and scanning electron microscopy. The results show that the microstructure of pyrocarbon transits gradually from rough laminar (RL) to smooth laminar along both the axial and radial direction of the composites for 950°C deposition. RL pyrocarbon dominates the matrix of the composites for the deposition at 1050~1150°C, and the composites produced at 1050°C have more uniform density. As the deposition temperature increases from 950°C to 1150°C, the flexural strength of the composites decreases from 158.9 MPa to 133.6 MPa, and the fracture behavior transits from brittle failure mode to quasi-ductile failure one.