Densification behavior and microstructure of carbon/carbon composites prepared by chemical vapor infiltration from xylene at temperatures between 900 and 1250 °c

Carbon/carbon composites were prepared by film boiling chemical vapor infiltration from xylene pyrolysis. Their densification behaviors such as the mass gain, the deposition rate and the density profile were investigated. The microstructure was studied by polarized light microscopy and characterized quantitatively with average extinction angle (Ae). Results showed that, under the particular experimental equipment and process, the initial deposition rate and the average Ae of pyrocarbon (PyC) increased with the increasing deposition temperature (T_d). The structural transition of PyC from rough laminar (RL) to smooth laminar along both the axial and radial directions of the composites was retarded as T_d increased from 900 to 1100 °C; PyC was deposited by the heterogeneous nucleation and growth. The homogeneous nucleation was generated producing isotropic PyC at the bottom of the composites for 1200-1250 °C deposition. The matrix produced at 1100-1250 °C was dominated by RL PyC, and the composites with high average density and uniform RL matrix were rapidly produced for T_d around 1100 °C.