## Results and Discussion
### Mechanical Property Characterization  
Evaluation of the mechanical properties of the WEP and PGT/WEP coatings by impact resistance, pencil hardness and adhesion are shown in Figure 9a-c. The impact resistance of the WEP coating is 45 cm (Figure 9a). The pencil hardness is 2H (Figure 9b) and the adhesion rating of 2 (Figure 9c). In contrast, The PGT/WEP coatings containing nano PDA@GO-TiO\({}_{2}\) composites have high impact resistance, pencil hardness, and adhesion. Among, the pencil hardness of the PGT-2%/WEP coating up to 5H, the impact resistance up to 60 cm and the adhesion rating of 1. Since nano-filler (nano-PDA@GO-TiO\({}_{2}\) composite) is completely coated by resin, it plays a reinforcing role in the spatial structure of resin. Filling the voids of the polymer segment itself and improving the Interfacial interaction of coating-metal. When the coating is impacted, the nanofiller absorbs part of the impact  
Figure 8: Scanning electron microscopy (SEM) surface image of (**a**) waterborne epoxy resin coating (WEP), (**b**) (PGT)-0.5%/ WEP, (**c**) PGT-1%/ WEP, (**d**) PGT-2%/ WEP and (**e**) PGT-3%/ WEP coating. SEM cross-sectional image of (**a’**) WEP, (**b’**) PGT-0.5%/ WEP, (**c’**) PGT-1%/ WEP, (**d’**) PGT-2%/ WEP and (**e’**) PGT-3%/ WEP coating.  
energy to improve the mechanical properties, such as adhesion and impact resistance of the coating. So, the PGT-2%/WEP coating exhibiting excellent physical and mechanical properties. However, it is obvious from Figure 9a-c, the physical functions such as impact resistance and pencil hardness of the PGT-3%/WEP coating has a tend to reduce. As a result of adding excessive amounts of nano-filler(nano-PDA-GO-TiO\({}_{2}\)) in coating, the nano-filler cannot be completely coated by the resin, and part of PDA-GO-TiO\({}_{2}\) is concentrated outside the void of the resin space structure, resulting in the bond between the coating and the substrate is affected, and the hardness of the coating, performance, such as impact resistance, is reduced. The mechanism analysis is as follows: The first, GO modified by dopamine is successfully reduced and stripped. Dopamine anchored between GO lamellar structures, which can effectively inhibiting the agglomeration between the GO sheets, and is beneficial to the encapsulation of the GO sheet by the resin, further improving the dispersing ability of the nano-PDA@GO-TiO\({}_{2}\) composites in resin matrix. On the other hand, depositing nano-TiO\({}_{2}\) on the surface of graphene oxide (GO) sheets can prevent graphene oxide (GO) nano sheets from restacking at the same time, the presence of TiO\({}_{2}\) nanoparticles in epoxy pores inducing the healing effect by sliding and filling pinholes, interstitial spaces of coating micro cracks regions, further to improve the hemechanical properties of PGT/WEP coating. The third, DA, forms a polydopamine layer by self-polymerization and deposits on the surface of GO. There are rich in active energy groups (such as burlyandum and midamin) of DA and PDA, it not only interact with the oxygen-containing metastases on the surface of GO to produce rGO tablets, but also reacts with the epoxy base of the epoxy resin, enhancing the binding force between the interface between nano-PDA@GO-TiO\({}_{2}\) composites and epoxy resin, and improving the mechanical properties of the coating.  
The physico-mechanical properties that figure out the Impact resistance, hardness and adhesion of the coating were estimated and tested according to following standards:  
* Adhesion refers to GB/T 1720--1979(89), Coating adhesion test method (circle method);
* Impact resistance refers to GB/T 1732--1993, Paint film impact resistance test (Impact test); and,
* Hardness refers to GB/T 6739--2006, Pencil method for measuring film hardness (Pencil hardness);