## 3 Results and Discussion
### Characterization of Coatings
#### 3.1.1 SEM of the Coatings  
The surface morphologies of the deposited coatings, as characterized by SEM, are shown in Figure 2. The Cu coating showed the presence of micro- and macropores along with splats [34; 35], which led to the formation of severe defects (Figure 2a). There were many semi-molten metal particles of different shapes, such as globular, disk, and plate, which caused irregularities in the morphology of the coatings. By contrast, a comparison of the Ti coating morphology with that of Cu showed a uniform and compact structure [19] with inflight particles, as shown in Figure 2b. The melting point of Ti is very high; therefore, the completely molten metal particles become suspended in the atmosphere, settle during the cooling process, and partially oxidize the coating. Some splat particles are observed in Figure 2b, owing to the sudden cooling of the molten metal particles, which were later deposited on the surface and caused defects. The 85 Zn-15 Al alloy coating exhibited severe defects and pore formation, as shown in Figure 2c. This is attributed to the different metals used in the coating, that is, Zn and Al, which have different melting points and densities. Al has a high melting point and low density, compared with Zn; therefore, it melts at high temperatures and is deposited later owing to its low density [34], leading to the creation of defects. Moreover, if the molten Zn particles cool down with Al, it is possible to obtain a less defective coating; however, owing to atmospheric conditions, they are difficult to cool down simultaneously. Therefore, this coating exhibited severe defects along with splats and inflight particles of different sizes. The surface morphology of the coatings was correlated with cross-sectional SEM images, as shown in Figure 3. The thickness of the coating was found to be 200 (\(\pm\)10) \(\upmu\)m, which correlated well with the thickness measured by using an Elcometer. Some defects were observed in the Cu coating (Figure 3a), while the Ti coating exhibited a dense morphology (Figure 3b). Therefore, this coating could exhibit a greater bonding strength (the details are discussed in Section 3.1.3). However, 85 Zn-15 Al revealed severe defect formation (Figure 3c) along with a color contrast owing to the Zn and Al metals. Due to the uneven morphology of the Cu, Ti, and 85 Zn-15 Al coatings, the porosity of the coatings was determined using the ImageJ software by considering the cross-sectional SEM images (Figure 3). The 85 Zn-15 Al coating exhibited a surface porosity of 14.59%, followed by Cu (11.56%) and Ti (1.69%) coatings. The Ti coating reduced the porosity by 8.63 and 6.84 times the porosity of the 85 Zn-15 Al and Cu coatings, respectively. This is due to the high melting point of Ti, causing all metal particles to be completely melted, and the metal droplets become very small; therefore, they fill the open pores and enhance the properties of the coating.  
The EDS analysis results of the coatings are listed in Table 1. As the melting point of the metal increased, the O (oxygen) content increased. The Ti coating exhibited 19.85% O, whereas 85 Zn-15 Al showed 2.63% O. It is possible that during the melting of the metals during the twin-wire arc thermal spray process, they were partially oxidized. The Zn and Al contents after the deposition of the 85 Zn-15 Al alloy coating were 84.15% Zn and 13.24% Al. The Zn content was identical to that present in the 85 Zn-15 Al alloy wire (feedstock). This result suggests that the O content in this coating was due to the partial oxidation of Al rather than Zn. This means that the metal with a higher melting point was oxidized during this process.  
Figure 2: SEM of (**a**) Cu, (**b**) Ti, and (**c**) 85 Zn-15 Al coatings at 1000\(\times\) and 10,000\(\times\) (inset SEM images) deposited on the concrete surface.