Carbon nanotubes (CNTs) have been identified as excellent nanoreinforcements for carbon fiber (CF)-reinforced polymers regarding a wide range of engineering applications. The outstanding properties of CNTs, such as their large surface area, high mechanical strength, and low manufacturing cost bring them to be distinguished nanoreinforcements for carbon fiber-reinforced polymers to form multifunctional and multiscale composites. Electrophoretic deposition of graphene oxide for CNTs onto the CF surface was conducted. The presence of graphene oxide-CNTs may effectively increase both the roughness and wettability of the CF surface, resulting in an improvement to the interfacial bonding strength between the CF and the polyimide (PI).

The poor interfacial adhesion between carbon fibers (CFs) and polyimide (PI) resin has seriously hampered the application of CF/PI composites. In this work, the interfacial adhesion was efficiently enhanced by grafting on the CF surface. Surface morphology and surface composition of modified carbon fibers were characterized, which indicated that acrylamide was grafted successfully on the CF surface and the surface roughness was increased slightly. After grafting, the interface shear strength of modified carbon fibers/PI composites was significantly improved by 86.96%, and the interlaminar shear strength was enhanced by 55.61% due to the covalent bonds in interphase and the toughening effect of sizing agent. Moreover, the mechanical properties of composites with different interfacial adhesion were measured, which further confirmed the effect of the grafting modification.

Carbon fibers were coated in an attempt to improve the interfacial properties between carbon fibers and ultra-high molecular weight polyethylene resin matrix. Atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy were performed to characterize the changes of carbon fiber surface. Atomic force microscopy results show that the coating of carbon fiber significantly increased the carbon fiber surface roughness. X-ray photoelectron spectroscopy indicates that silicon containing functional groups obviously increased after modification. Interlaminar shear strength was used to characterize the interfacial properties of the composites.

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Mechanical properties of carbon fiber (CF) and carbon nanotube (CNT)-filled thermoplastic high-density polyethylene (HDPE) composites were studied with particular interest on the effects of filler content and fiber surface treatment by coupling agent. Surface-treated CF-filled HDPE composites increased their tensile strength and impact strength, which is further increased with the addition of CNT. SEM showed that CNT-coating-treated CF-HDPE composites show better dispersion of the filler into the matrix, which results in better interfacial adhesion between the filler and the matrix.