# Mechanical Behavior of Bi-Layer and Dispersion Coatings
## 3 Results  
### Structure and Morphology  
The surface topographies of the MWCNTs, MWCNTs/TiO\({}_{2}\), and MWCNTs_Cu coatings are shown in Figure 1. The reference MWCNTs coating is characterized with high ups and downs, while for the MWCNTs/TiO\({}_{2}\) and the MWCNTs_Cu coatings, the roughness is higher, but the distance between ups and downs is lower. The surface of the MWCNTs_Cu coating is significantly leaned--the left side of the sample is higher than the other, which could be the reason for an unevenly put force during the grinding process or the difference in the MWCNTs_Cu coating thickness on both sides of the sample. The values of the roughness parameter (S\({}_{\text{a}}\)) calculated both with the optical microscope and the atomic force microscope are demonstrated in Table 2. It could be observed that the S\({}_{\text{a}}\) parameter is higher for all surfaces when using the atomic force microscopy, which could be attributed to the use of the non-contact mode AFM and thus lower accuracy.  
Figure 2 shows the AFM images for tested surfaces, confirming the roughness Sa parameter values shown in Table 2. The surface of the MWCNTs/TiO\({}_{2}\) demonstrates the highest ups and downs, which is caused by the TiO\({}_{2}\) agglomerates deposited on the MWCNTs surface, while the MWCNTs surface is the smoothest, as assumed.  
Figure 3 demonstrates the SEM images of the surface topography of the MWCNTs, MWCNTs/TiO\({}_{2}\), and MWCNTs_Cu coatings at different resolutions. The surface of the MWCNTs coating shows uniformly distributed carbon nanotubes. There are light points/areas for high ups, as demonstrated in Figure 1A, supposedly places where the coating is thicker. There are many agglomerates on the combined MWCNTs/TiO\({}_{2}\) and MWCNTs_Cu coatings, however the structure of agglomerates is different. The MWCNTs/TiO\({}_{2}\) contains uniformly positioned angular particles of TiO\({}_{2}\), which adhere to the surface of the MWCNTs coating, while the Cu agglomerates in the MWCNTs_Cu coating are built into the MWCNTs structure. Therefore, they are almost invisible, but nevertheless influence mechanical properties of the coating, as previously observed in [47]. The areas, where the SEM image of the MWCNTs_Cu coating is blurred, are probably spaces in which the cracks of the MWCNTs coating occur.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Roughness S\({}_{\text{a}}\) Based on Optical** & **Roughness S\({}_{\text{a}}\) Based on Atomic** \\  & **Microscopy Tests (\(\mu\)m)** & **Force Microscopy Tests (\(\mu\)m)** \\ \hline MWCNTs & 0.29 * & 0.353 \(\pm\) 0.107 \\ MWCNTs/TiO\({}_{2}\) & 0.56 * & 1.033 \(\pm\) 0.107 \\ MWCNTs_Cu & 0.36 * & 0.495 \(\pm\) 0.034 \\ \hline \hline \end{tabular}  
* the experimental error has been estimated as less than 0.05 \(\mu\)m.  
\end{table}
Table 2: Surface roughness of the deposited coatings.  
Figure 1: Surface topography based on optical microscopy tests for (**A**) the MWCNTs coating, (**B**) the MWCNTs/TiO\({}_{2}\) coating, and (**C**) the MWCNTs_Cu coating.  
The EDS spectra of the examined coatings are shown in Figure 4, confirming the appearance of the MWCNTs, MWCNTs/TiO\({}_{2}\), and MWCNTs_Cu coatings. There are also present some impurities (silicon, chlorine, sodium, and fluorine) that result from the samples preparation.  
Figure 3: SEM surface topography of the MWCNTs coating (**A,B**), the MWCNTs/TiO\({}_{2}\) bi-layer coating (**C,D**), and the MWCNTs_Cu dispersion coating (**E,F**) at different resolutions.  
Figure 2: Surface topography based on atomic force microscopy tests for (**A**) the MWCNTs coating, (**B**) the MWCNTs/TiO\({}_{2}\) coating, (**C**) the MWCNTs_Cu coating.  
Moreover, the ASTM D3359-97 pressure-sensitive tape test was carried out on every single coating to check the thickness and the adhesion to the substrate. None of the examined coatings was removed during the test, which gives us valuable information of significantly strong coatings' adhesion to the Ti grade II substrate.