This study reports on the main cause of the reduced tensile strength of carbon fibers (CFs) by investigating the microstructural changes in the CFs that are undergoing mainly two processes: catalyst nanoparticle formation and chemical vapor deposition (CVD). Interestingly, the two processes oppositely influenced the tensile strength of the CFs: the former negatively and the latter positively. The catalysts coating and nanoparticle formation degraded the CF surface by inducing amorphous carbons and severing graphitic layers, while those defects were healed by both the injected carbons and interfaced CNTs during the CVD process. The revealed degradation and healing mechanisms can serve as a fundamental engineering basis for exploring optimized processes in the manufacturing of hierarchical reinforcements without sacrificing the tensile strength of the substrate CFs.
The formation of carbon nanotubes (CNTs) through precipitated carbons emerging from supersaturated metal catalysts is an established mechanism for their growth during the CVD process. Here, the CNT growth mode is determined by the interaction between the substrate and the catalyst nanoparticle, e.g., the tip-growth mode for the weak adhesion between them and the base-growth mode for the strong adhesion case. With microscopic evidence, this study reports another factor that governs the growth mode of CNTs on carbon-based substrates. Catalyst nanoparticles after only sputtering and annealing processes before the chemical vapor deposition (CVD) process are fully or partially wrapped with some graphitic layers, which are formed by carbons escaping from the carbon substrate. The formation of the wrapping graphitic layers is initiated by catalyst atoms diffusing into the carbon substrate during the catalyst sputtering process. The diffused catalyst atoms later coalesce into the nanoparticles, during which carbon atoms escape from the carbon substrate, forming the graphitic layers which wrap around the catalyst nanoparticles for energy minimization. Then, the carbon atoms generated from the catalytic reactions during the CVD process interact with the carbons in the graphitic layers wrapped around the catalyst nanoparticles, bringing about clear tip-growth of CNTs on carbon-based substrates and a stable interface (carbon-carbon bonding) between CNTs and carbon-based substrates.
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