## 3 Results and Discussion
### Mechanical Properties
#### 3.3.4 FE-SEM Observation  
Figure 10 is SEM images of surface treatment with ZnCl\({}_{2}\) for 0-12 h treatment times. We obtain different surface images with ZnCl\({}_{2}\) treatment times. (a) is the untreated aramid fiber surface image. It was clean and smooth. (b), (c) and (d) are surface images of aramid fibers treated with ZnCl\({}_{2}\) for 2 h, 6 h and 10 h, respectively. A similar image was obtained by (a) and (b). Because the 2 h treatment was too short, the fabric surface was not treated enough. Conditions (c) and (d) showed a remarkable etching effect on the fabric as the surface was changed. However, in condition (d), which is treated for 10 h, the fabric surface was separated by the destruction of the fabric itself as well as etching. This destruction phenomenon can affect the mechanical performance of the fabric composites. It can be seen that the effect received by the fabric surface grows with the treatment time. Because the Lewis acid treatment causes an etching effect on the fabrics, the long treatment time will have some effect on the fabrics themselves. In the surface treatment, the (c) condition, in which etching was appropriate and the fabric itself was not destroyed, was selected as an excellent condition.  
Figure 11 shows a ZnCl\({}_{2}\)-treated SEM image on the surface of the fiber. Comparing the conditions of (a) and (b), the material treated on the fiber surface under condition (b) was remarkably observed. This proves that PDA was coated on the fiber surface under condition (b). The difference between (b) and (c) is the difference in the treatment order of polydopamine and ZnCl\({}_{2}\). The amount of PDA adhering to the surface in condition (c)was smaller than condition (b). Because dopamine could be drained in the process of ZnCl\({}_{2}\) treatment if the processing order is ZnCl\({}_{2}\) first and then PDA. In (b) and (d), the fiber surface etching phenomenon of the high concentration (d) condition is more pronounced than the low concentration (b) condition. It can be seen that the concentration affects the degree of fiber surface treatment. However, the fiber surface under condition (d) has a high etching effect, and the fiber itself may be damaged, resulting in a decrease in performance.  
Figure 12 shows the fracture surface of each composite after the Impact test. As in image (a), the impact fracture surface of the untreated aramid composites was often cracked between the fiber and the resin. However, under the conditions of (b), (c), (d) and (e), the cracking phenomenon is significantly reduced. Among them, there was still some cracking in condition (b) with only ZnCl\({}_{2}\). (c) and (d) conditions, in which the hybrid treatment was performed, were not only cracked but also had fewer gaps between the fibers and the resin. Condition (e), which was treated at a concentration of 10 wt%, was no cracking after receiving an impact force between the fiber and the resin, but it was found that the fiber itself was destroyed. It is judged that the performance of the composites decreases due to the separation and destruction of the fiber itself because of a high concentration.  
Figure 11: ZnCl\({}_{2}\)-treated 6 h SEM image on the surface of the fiber. (**a**) 6 wt% ZnCl\({}_{2}\), (**b**) 6 wt% ZnCl\({}_{2}\), PDA, (**c**) 6 wt% PDA ZnCl\({}_{2}\) and (**d**) 10 wt% ZnCl\({}_{2}\) PDA.  
Figure 10: SEM images of the surface treatment with the ZnCl\({}_{2}\) treatment time. (**a**) Untreated, (**b**) 2 h, (**c**) 6 h (**d**) 10 h.