## 4 Conclusions  
In this study, the effect of the aramid fiber surface treatment on the mechanical performance and interfacial bonding of composites was investigated. The aramid fiber surface roughness was improved by performing a hybrid treatment of ZnCl\({}_{2}\) concentration, treatment time and PDA. All samples were manufactured by the VARTM process. As a result, the hybrid treatment of ZnCl\({}_{2}\) and PDA had an excellent effect on the mechanical properties of the composites. 1. The best tensile strength of 423.74 MPa was obtained at a ZnCl\({}_{2}\) concentration of 6 wt%, which is 3.51% higher than the untreated specimens. In addition, the tensile strength obtained at a ZnCl\({}_{2}\) concentration of 6 wt% was higher than that obtained at 10 wt%. Since the tensile strength is related to the properties of the fabric itself, the surface treatment did not have a significant effect on the tensile strength. It was also found that ZnCl\({}_{2}\) and PDA had negative and positive opposite effects on elongation, respectively. Additionally, these two treatments can effectively compensate for the mutual effects on AFRP, thus ensuring the mechanical properties of AFRP. 2. The bending strength under PDA-treated conditions reached the most excellent 247.85 MPa, which was 13% higher than that of the untreated specimens. Among the single treatments, the ZnCl\({}_{2}\) treatment showed the lowest improvement in performance while producing an etching effect on the surface fabric. All mixed treatments showed a decrease in performance in bending strength. 3. A concentration of 6 wt% ZnCl\({}_{2}\) and PDA treatment had the highest impact energy of 1.52 J was obtained for the hybrid treatment, which was more than 20% higher than the untreated condition. The impact power of the treated aramid  
Figure 12: Fracture surface of each composites after the Impact test. (**a**) Untreated, (**b**) 6 wt% ZnCl\({}_{2}\), (**c**) 6 wt% ZnCl\({}_{2}\) PDA, (**d**) 6 wt% PDA ZnCl\({}_{2}\), (**e**) **10 wt% ZnCl\({}_{2}\) PDA. fiber composites was improved, where the mixed treatment condition increased by more than 8% compared to the single treatment condition. In the mixed treatment condition of ZnCl\({}_{2}\) and PDA, the higher impact work was obtained in the PDA treatment condition. 4. The Lewis acid treatment increased the adhesion to the resin by etching the surface of the aramid fiber, but it had no effect on the tensile strength because it would damage the fabric itself. In the blend, the treatment had a positive effect on the impact work but had little effect on the tensile and flexural strengths. 5. According to the SEM observations, the higher the concentration of ZnCl\({}_{2}\) and the longer the treatment time, the better the etching effect on the fiber surface, but fiber separation occurs and reduces the mechanical properties. By observing the cross-sections of the composites, it was confirmed that the cross-sections under mixed treatment conditions were not treated or that the aromatic polyamide fibers were made denser with resin than the material with only a single treatment. The results show that the mixed treatment of ZnCl\({}_{2}\) and PDA can improve the mechanical properties of the composites. Conceptualization, G.X. and Y.J. ; methodology, G.X. ; software, G.X. ; validation, G.X. ; formal analysis, G.X. ; investigation, G.X. and Y.J. ; resources, J.S. ; data curation, G.X. ; writing--original draft preparation, G.X. ; writing--review and editing, G.X., Y.J. and J.S. All authors have read and agreed to the published version of the manuscript. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF- 2018 RIDIA1B07043528). The authors thank the Center for University-wide Research Facilities (CURF) at Jeonbuk National University for providing the characterization equipment and its instructions. Not applicable.