# Mechanical Behavior of Bi-Layer and Dispersion Coatings
## 3 Results
### Nanoindentation Studies  
The results of the nanoindentation test are illustrated as the load-displacement hysteresis curves in Figure 5. There are three stages distinguished in the nanoindentation test, which are pointed in the graph. Firstly, the increasing curve shows the force raising during the test until the maximum load is achieved. Secondly, the horizontal line demonstrates the pause, where the probe is stabilized at maximum depth. The third stage is the result of temperature drift, adjusted at the end of the nanoindentation measurement. The displacement, and thus maximum indent depth, was observed for the MWCNTs coating. Both the metal and metallic oxide additions to the MWCNTs based coating made it harder, as confirmed by mechanical properties (Table 3). Figure 4: EDS spectra of the (**A**) MWCNTs reference, (**B**) MWCNTs/TiO\({}_{2}\) bi-layer coating, and (**C**) MWCNTs_Cu dispersion coating. Figure 5: Nanoindentation load–displacement curves obtained for (**A**) the MWCNTs coating, (**B**) the MWCNTs/TiO\({}_{2}\) coating, and (**C**) the MWCNTs_Cu coating. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Nanohardness** & **Reduced Young’s** & \multicolumn{2}{c}{**Young’s**} & **Maximum** & **Plastic** & **Elastic** \\  & **H (GPa)** & **Modulus E\({}_{\text{F}}\) (GPa)** & **Modulus E\({}_{\text{Dept}}\)** & **Indent h\({}_{\text{max}}\) (\(\upmu\)m)** & **Deformation** & **Recovery** \\ \hline MWCNTs1 & 0.032 \(\pm\) 0.0003 & 4.5 \(\pm\) 0.05 & 3.41 \(\pm\) 0.03 & 3.58 \(\pm\) 0.04 & 7.06 \(\pm\) 0.07 & 0.958 \(\pm\) 0.0096 \\ MWCNTs/TiO\({}_{2}\) & 0.183 \(\pm\)  
Table 3 demonstrates the mean values and standard deviations of several mechanical properties of the MWCNTs, MWCNTs/TiO\({}_{2}\), and MWCNTs_Cu coatings. The hybrid and composite coatings have higher hardness and Young's modulus, and lower maximum indent depth compared to the reference MWCNTs coating, as the addition of nanoceramics and nanometals, in general, improves mechanical properties. However, we can observe the nanohardness increase when Young's modulus decreases (regarding the reference MWCNTs coating), differently than in our previous research [47], in which the same coatings have been deposited on Ti alloy substrate. The mechanical properties of the MWCNTs_Cu coating are pretty close to the reference sample, except plastic and elastic properties, while different for the MWCNTs/TiO\({}_{2}\) coating. The elastic recovery and plastic deformation energies are also shown by an area under load-displacement curves during the nanoindentation test. Figure 6 shows the comparison of elastic recovery and plastic deformation energy of the examined coatings. The MWCNTs_Cu coating was the most both plastically and elastically deformed, comparing to the others. It can be attributed to the nanoCu presence in the crossover sections of carbon nanotubes, which influences the coating's properties, whereas the MWCNTs/TiO\({}_{2}\) coating achieved better properties. After the nanoindentation test, the material was the least deformed both plastically and elastically, even less than the reference sample, which was certainly caused by hard TiO\({}_{2}\) particles strengthening the MWCNTs coating. Other crucial parameters, further explaining mechanical properties of the MWCNTs, MWCNTs/TiO\({}_{2}\), and MWCNTs_Cu, are the ratio of the nanohardness to reduced elastic modulus (H/E\({}_{\text{r}}\)), which shows the coating ability to accommodate substrate deflections under load (the coating endurance for chipping off under substrate bending), and the yield pressure (H\({}^{3}\)/E\({}_{\text{r}}^{2}\)), determining the resistance to plastic deformation under the nanoindenter probe load (the distortion of the coating material during nanoindentation test), as previously discussed in [47] for the Ti alloy.