Effect of Growth Temperature on Carbon Nanotube Grafting Morphology and Mechanical Behavior of Carbon Fibers and Carbon/Carbon Composites

High-purity carbon nanotubes (CNTs) with different orientation and lengths were grafted on carbon fibers (CFs) in woven fabrics by using double injection chemical vapor deposition and adjusting the growth temperature. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman investigations reveal that the grafted CNTs change from being predominantly aligned and uniform in diameter to absolutely disordered and variable in diameter, whilst they show significantly increased crystallinity, as the growth temperature is increased from 730 °C to 870 °C. In tensile tests of fiber bundles, much more strength degradation of CFs was observed after the growth process at higher temperature than that at lower temperature. These hybrid preforms produced at different growth temperatures were used to reinforce carbon/carbon (C/C) composites. An increment of 34.4% in out-of-plane compressive strength (OCS) was obtained for the composites containing CNTs grown at 730 °C, while the OCS increment exhibits an obvious decrease with increasing the growth temperature. Compared with the higher growth temperature, the lower temperature contributes to the decrease in the strength loss of reinforcing fibers and meanwhile the growth of large extending length of CNTs, which can provide long reinforcement to the pyrocarbon matrix, and thus increase the compressive strength better.