## 3 Results and Discussion
### Morphology  
The morphology of the fractured surfaces of the specimens was analyzed to understand the toughening mechanism. Figure 8 shows the optical micrographs of the baseline and buckypaper-interleaved specimens. We observed that the buckypaper filled almost the entire resin-rich area, which was likely because of the roller compaction. As seen from Figure 8, the gap between the carbon fiber layers of the specimen containing buckypaper was approximately 10 \(\upmu\)m larger than the gap between the carbon fiber layers of the specimen without buckypaper. In general, resin-rich composites exhibit a high interlaminar  
Figure 6: Load–displacement curves from the delamination test: (**a**) mode I and (**b**) mode II. Figure 7: \(G_{IIc}\) of UD and WN. delamination toughness. The thick resin-rich interlaminar layer of composites allowed plastic deformation to extend over a larger volume than in high-fiber composites. However, the interlaminar fracture toughness of the buckypaper-interleaved composites was affected more by the CNT content and fiber-CNT interface than by the thickness of the interlaminar layer. The toughening mechanisms of the specimens were investigated by examining the fracture surfaces from the mode I and mode II tests. The fracture surfaces of the tested specimens were cut into 10 \(\times\) 10 mm\({}^{2}\) samples for convenience in measurements. The samples were sputtered with platinum to avoid electron charging. SEM images of the fracture surface of the mode I delamination specimens are shown in Figure 9. The specimen observed in the SEM images depicted in Figure 9a,c are without buckypaper because the surface of the fibers was clean and did not have an attached matrix. Moreover, the fiber and matrix were separated in the baseline specimens. In other words, the baseline specimen had weak interfacial adhesion between the fiber and matrix, and debonding of the weak interfacial adhesion was the primary mode of failure for the specimen without buckypaper interleaving. In contrast, the fiber and matrix were attached after failure in the buckypaper-interleaved specimens, demonstrating strong interfacial adhesion, as shown in the SEM images depicted in Figure 9b,d. Moreover, the interlaminar failure for the specimen with buckypaper interleaving occurred due to fiber peeling and fiber breaking. Therefore, buckypaper interleaving modified the morphology of the interphase area of CFRP, leading to significant toughening. An increase in the mode I interlaminar fracture toughness was another application of CNT reinforcement using the same toughening mechanism as was used for buckypaper interleaving [29]. The fracture toughness of WN increased more than that of UD as the delamination length increased. This may have been due to the increase in the fracture surface area caused by the deflection of crack propagation by warp and weft, as well as the presence of CNTs in the surface area of the woven carbon fibers, as shown in Figure 9b. Figure 8: Optical micrographs of cross-sections of the (**a**) baseline specimen and (**b**) buckypaper-interleaved specimen. The fracture surfaces of the baseline and buckypaper-interleaved specimens from the ENF tests are shown in Figure 10. The SEM images in Figure 10a,d show that the  
Figure 9: SEM images of the fracture surface of the mode I test specimens: (**a**) WN baseline, (**b**) WN interleaved with buckypaper, (**c**) UD baseline, and (**d**) UD interleaved with buckypaper. fiber-matrix interface in the specimen with buckypaper interleaving was firmly attached after the fracture, as opposed to the interface in the baseline specimen. This indicates that strong interfacial adhesion and delamination occurred because of the CNT matrix in the interlayer. A major difference between the baseline and buckypaper-interleaved specimens was the amount of resin cover on the delaminated carbon fiber surface. The resin between the carbon fibers became rougher due to the presence of CNTs because of higher energy absorption during delamination [30].