# Mechanical Behavior of Bi-Layer and Dispersion Coatings
## 4 Discussion  
The results can be discussed in two aspects. At first, the change in surface properties due to surface modification by either of the two tested coatings can be attributed to different phenomena and determinants. Secondly, the influence of substrate surface on observed topography and mechanical appearance can be considered. To summarize, two variables, the coating microstructure and composition, and substrate composition are two variables to discuss. The ceramic-carbon coating has roughness greater than the MWCNTs layer (Figures 1 and 2, and Table 2). It is a hybrid coating with inner CNTs and an outer titania layer. Highly rough topography is observed (Figure 3) with many separated nanoparticles of titanium dioxide or their aggregates. Figure 4 justifies the real presence of titanium in the layer. Unfortunately, the authors have failed to obtain a homogenous coating by simultaneous deposition of MWCNTs and TiO\({}_{2}\). On the other hand, the design and preparation of such bi-layer coatings on the titanium alloys appears a highly fruitful strategy if the inner layer is elastic (soft) and allows cracks to bridge, deviate or passivate, or absorb energy [52]. In other words, the layered structure (hybrid or sandwich structure) of a coating permits the simultaneous creation of the hard outer layer and the elastic, impact and high stress absorbent, resistant-to-damage inner layer. The last phenomenon results from preventing the occurrence of cracks and, in particular, their movement across the whole coating. There are opinions [53] that such a two-stage deposition process offers more flexibility in the composition of the coatings, but as it has been shown here, the EPD mechanisms are in such a case becoming complex, and to develop the coatings with optimal characteristics is difficult and needs several attempts, which will be made in the future for the most promising MWCNTs/TiO\({}_{2}\). In particular, an aggregation of nanoparticles is an unnecessary phenomenon that should be avoided or eliminated by chemical functionalization, solvent exchange, ultrasonication, or in another way [54; 55]. The composite carbon-metal coating also has roughness greater than the MWCNTs coating (Table 2), but lower than that of the hybrid coating, and a more homogenous surface (Figure 3). The Cu nanoparticles are well dispersed in the matrix, and in such a manner  
Figure 7: The diagrams for the H/E\({}_{\text{r}}\) (**A**) and the H\({}^{3}\)/E\({}_{\text{r}}\)\({}^{2}\) parameter (**B**), calculated for the MWCNTs, MWCNTs/TiO\({}_{2}\) bi-layer, and the MWCNTs_Cu dispersion coating, examined under indentation load of 10 mN. they do not seriously contribute to the roughness. Figure 4 proves the real presence of copper in the layer. When comparing the mechanical values for both tested coatings, the difference between hybrid MWCNTs-titania and composite MWCNTs_Cu coatings is apparent. The MWCNTs-oxide coating has greater hardness and Young's modulus than the carbon nanotube-metal coating. The first coating may be then considered in such applications in which the coating opposes high stresses and must be resistant to wear or cracking. The second coating, which includes antibacterial metal, can be recommended for particular coatings that should actively kill bacteria in their neighborhood [56], but with no excessive loads during surgery and further use of the implant. The obtained results are similar to those previously obtained for Ti alloy [47], but to some extent only. For hybrid coating, the network of carbon nanotubes forms an elastic sub-layer that immobilizes the titania nanoparticles; in such a way, the hardness is similar to that of titanium dioxide. Such a formula gives hard coating, but presumably, due to nanoform and the existence of inner layer, it is also not prone to brittle cracking which appears often in the case of ceramic coatings. For composite coating, low hardness can be attributed to small, relatively soft metallic nanoparticles, well-positioned in the CNTs network.