# Mechanical Behavior of Bi-Layer and Dispersion Coatings
## 4 Discussion
Therefore, in this case, they do not play an important role in creating hardness and strength. As concerns Young's modulus value, it is close to the values observed for human bones: about 19 GPa for cortical bone [57] and 6 GPa for mature bone [58]. It may be expected that the hardness and Young's modulus either both increase or decrease as observed for CNTs-Ti6 Al4V nanocomposite [59], MWCNTs-graphene-Al alloy [60], and HAp-CNTs on Ti [61]. The mechanical behavior can also be characterized by the elastic recovery energy, plastic deformation energy, a ratio of the nanohardness to the reduced elastic modulus (H/E\({}_{\text{r}}\)), and the yield pressure (H\({}^{3}\)/E\({}_{\text{r}}\)\({}^{2}\)). The deep plastic deformation during the nanoindentation test is, based on these results, observed for composite coating MWCNT_Cu. The thickness of obtained coatings has not been here determined, but comparing to previous reports and applied voltage and time of deposition it is likely to be in the range of 50-80 \(\upmu\)m [53]. Therefore, the nanoindentation results have not been influenced by the substrate. Finally, it is interesting to assess whether the chemical and phase composition of a substrate may affect the mechanical appearance after deposition of coatings at the same conditions. The appropriate values are listed in Table 4. Considering the above results, almost all of them can be satisfactorily explained. In particular, the higher hardness, the greater Young's modulus. However, it is true only for the same substrate, either titanium or its alloy, and with the exception of MWCNTs-titania coating, likely due to the highly nonhomogeneous surface. More importantly, the chemical and phase composition for substrate seems to play a significant role. For titanium substrate, the MWCNTs-oxide coating has a greater hardness, Young's modulus, and lower indent depth than the MWCNTs-nanocopper coating. For Ti alloy, the opposite relation was observed. These results might be considered as not fully reliable as the standard deviations approach even 30-50%. Therefore, this issue needs further experiments. On the other side, observed phenomena can be explained by: (i) deposition of coating dependent on surface characteristics, and (ii) interaction between nanoindenter tip and substrate (size effect) [62]. The mechanical properties of hybrid carbon-ceramic coatings on both substrates are, within the limits of an experimental error, close to one another. It is acceptable, as the MWCNTs are deposited in both cases as inner layers, and become adherent to the surface by chemical bonding and physical adsorption. They both depend on surface chemical composition [18]. The weaker coating observed on the Ti alloy as a substrate may be attributed to a general tendency to form more homogenous coatings on pure metals than on alloys with their chemical segregation and two or more phase constituents. The elastic and plastic resistances indicate that the outer titania layer is strong and brittle, which may be expected for the ceramic layer. The opposite effect is observed for composite coatings. It is postulated that, in this case, the much harder surface for coating deposited on the alloy, as compared to the pure titanium, may contribute to the hardness value at relatively deep indents. Table 4 shows the mechanical parameters of coatings obtained on two different substrates. The values to compare the previous results to those obtained in the present work have not been found in the literature, except the MWCNTs coating. The elastic modulus of MWCNTs ranged between 270 and 2400 GPa [55], while in other work it was 467-507 GPa [63]. For TiO\({}_{2}\) reinforced with CNTs composites [64], hardness was about 250 GPa and elastic modulus about 190 GPa, and for fluorapatite-TiO\({}_{2}\) and fluorapatite-TiO\({}_{2}\)-CNT(Cu) coatings hardness was only 0.72 and 0.58 GPa, and [PERSON]'s modulus 14.5 and 19.3 GPa, respectively [65].