Catalytic chemical vapor infiltration of carbon nanofilament network reinforced carbon/carbon composites: Catalytic effect on the densification behavior and matrix microstructure

Carbon nanofilament (CFC) network reinforced carbon/carbon composites with the average density 1.72-1.75 g/cm³ were successfully produced by floating-catalytic film boiling chemical vapor infiltration from xylene pyrolysis at 1000 to 1100 °C. The catalytic effects of ferrocene content on the densification behavior were investigated such as the relative mass gain, the deposition rate, and the profiles of density and pyrocarbon (PyC) layer thickness. The microstructure of PyC matrix and CFC was observed using polarized light microscopy, scanning electron microscopy, and transmission electron microscopy. Results showed that, under the special floating catalyst method, the initial deposition rate of PyC firstly increased to a higher value at 0.8 wt% catalyst and then decreased as the catalyst content further increased to 2.0 wt%. The final density of the composites tended to decrease along both the axial and negatively radial directions resulted from the enhanced difficulty of mass transfer at the latter densification. A single layer of rough laminar (RL) PyC matrix was formed through heterogeneous nucleation and growth for the deposition at 0-0.8 wt% catalysts. But a hybrid matrix composed of isotropic (ISO) and RL PyCs was produced at 1.2 wt% catalyst, and the thickness of ISO layer increased at the higher catalyst content of 2.0 wt%. The reason for ISO PyC formation was mainly attributed to the deposition of iron encapsulated carbons and the possible homogeneous nucleation. A reinforced network consisted of numerous CFCs and vapor growth carbon fiber (VGCF) was formed probably through tip-growth mechanism. The direct deposition of PyC on CFC was the formation reason of VGCF. A transition of the CFC was generated from nanotube to nanofiber when the catalyst content increased to 0.5 wt% because of the enhanced diffusion rate of carbon atoms through and over the iron particles. The composites with a high density of 1.75 g/cm³ and uniform RL PyC matrix are rapidly produced at the catalyst content about 0.5 wt% resulting from the modified deposition front exhibiting a small thickness and large concave radius.