Three-dimensional carbon/carbon composites with vertically aligned carbon nanotubes: Providing direct and indirect reinforcements to the pyrocarbon matrix

Vertically aligned carbon nanotubes (CNTs) were grown in situ on the surface of carbon cloths coated by a thin SiO₂ layer and then the hybrid cloths were stacked and densified by chemical vapor infiltration to obtain three-dimensional (3D) C/C composites. Effects of the length (5.2-21.8μm) of aligned CNTs on the microstructures and mechanical properties of the composites were investigated. Results showed that aligned CNTs not only directly stiffened the matrix within the reach of CNTs, but also gave indirect reinforcement to the matrix out of the reach of CNTs by inducing the formation of small pyrocarbon grains that interlocked with each other. Both the direct and indirect reinforcements on the matrix could be dramatically increased by extending CNT length. Therefore, 3D C/C composites with 21.8μm-long CNTs showed the most notably improvements of matrix-dominated mechanical properties: 63% and 275% improvements in out-of-plane and in-plane compressive strength; meanwhile, 13% improvements in fiber-dominated flexural strength. Compared with z-pinning and stitching, the use of vertically aligned CNTs would pave a meaningful way for effectively improving the global mechanical performance of woven-fabric C/C composites.