Effect of carbon nanotubes electrophoretically-deposited on reinforcing carbon fibers on the strength and toughness of C/SiC composites

The present work reports on the strength and toughness enhancement due to carbon nanotube (CNT) presence in silicon carbide-matrix composites reinforced by CNT-coated carbon fibers (CNT-C/SiC). The composites were prepared via a chemical vapor infiltration/electrophoretic deposition methodology which enables CNT-coating of reinforcing fibers at varying load fractions. Both the tensile strength and toughness were found to first increase and then decrease with CNT electrophoretic deposition time, hence also CNT loading. The optimized deposition time turned out to be 10 min where excellent mechanical properties were obtained and for instance, tensile strength and work of fracture reached their maximum of 392 MPa and 1869.4 kJ/m3, respectively. Scanning electron microscopy observations of CNT-C/SiC fracture surfaces showed that the magnitude of the fiber pull-out phenomenon increased significantly compared with pristine C/SiC. Abundant CNT pull-out within the CNT-rich layers around the fibers acted complementary to carbon fiber pull-out and was identified as the main toughening mechanism.