**4. Discussions**

The nature of the electrolyte, scanning rate, temperature, impurities, anode material and surface state of the samples are a part of the parameters, which can influence the electrochemical reactions. For example, the surface texture of the working electrode (investigated sample) is one of the most important parameters, which has an influence on the Tafel slopes, and consequently on the corrosion rate. Surface roughness has an important effect on general or pitting corrosion and the nucleation of metastable pitting. Skewness and kurtosis were used to identify the corrosion mechanism. Reid et al. have patented a method and apparatus for the identification of corrosion in metal objects and defining typical values of skewness and kurtosis for the identification of corrosion mechanism [59]. The pitting mechanism appears when the Sk < −2. In all our cases, the Sk had values greater than −2, indicating that a general corrosion mechanism can be found for all investigated surfaces. Regarding the kurtosis, if the value is < 3, general corrosion can be observed, while for kurtosis > 3, pitting corrosion can be found. Considering this, it can be seen that for all investigated surfaces, a general corrosion mechanism has been identified. If the surface is rough, then a larger area could contribute to the increase in the corrosion current or to the corrosion rate. Therefore, a decreased surface roughness will lead to a better corrosion resistance [60]. According to this statement, the TiCN-coated surface was rougher than TiSiCN, and this is probably the reason for its better corrosion resistance. For the uncoated CoCr, the roughness is not a factor which has a major influence on the corrosion resistance. Compared to the coated samples, the CoCr uncoated substrate had a smaller roughness, while its corrosion resistance was worse than for both coatings. Clearly, the surface roughness a ffects the corrosive behavior of materials (i.e., metals, alloys, coatings) and the nature of its e ffects (increase or decrease in the degree of corrosion) depends on the nature of the material.

To the best of our knowledge, there is no direct relation between hardness and corrosion resistance. Hard coatings can be subjected to surface microcracks, and then a localized penetration of corrosive solution will take place, leading to a galvanic cell, which accelerates the corrosive process. Hardness is important for load-bearing implants because a hardened material can have the ability to withstand wear. Taking into account the results of the present study, TiSiCN was harder than CoCr and the TiCN coatings and was more adequate for the proposed application.

For load-bearing implants, resistance to plastic deformation is an important factor and it can be described by the H<sup>3</sup>/E<sup>2</sup> ratio. Moreover, a material with a high H<sup>3</sup>/E<sup>2</sup> ratio resists plastic deformation during low load contact events and exhibits a higher yield strength [61,62]. It is also generally accepted that the H/E ratio can be considered an important indicator of a good wear resistance of the surface [63–65]. Thus, the improvement of the H/E ratio and, consequently, of the resistance to plastic deformation (H<sup>3</sup>/E<sup>2</sup> ratio) of the load-bearing implant may o ffer advantages, such as less surface damage and increased durability. In this study, the TiSiCN coatings have an H<sup>3</sup>/E<sup>2</sup> ratio equal to 1.1, which is higher than the one for TiCN (0.63), indicating that TiSiCN has a superior toughness and it can o ffer a better resistance to plastic deformation and good wear resistance.

The addition of Si to TiCN coatings leads to a grain refinement, and the crystallite size (d) was decreased to 16.4 nm in the case TiCN and to 14.6 nm in the case of TiSiCN. The formation of new defects, especially dislocations, is also responsible for the reduction in the crystallite size. The strain in the TiSiCN coatings (ε = 0.012) was lower compared to the TiCN coatings (ε = 0.053). The reason for this decrease could be attributed to di fferent factors. One reason could be due to the addition of Si, which has atomic radii (0.111) smaller than that of Ti (0.146 nm), leading to a disorder of the crystal lattice, which is also evident by XRD di ffraction (peaks were shifted when compared with the TiCN standard). The second reason could be attributed to amorphous phases in which Si can be found (Si, SiNx, SiCN) or C =C phases at grain boundaries, which are detrimental for crystallite development. TiSiCN has a higher C content than TiSiC. It is di fficult to separate these factors and to know their contribution. However, the crystalline disorder becomes more pronounced by an increase in carbon content, which is also suggested by the decrease in the crystallite size and by the decrease in microstrain. Moreover, Franceschini et al. reported a strong dependence of stress on the nitrogen content in a-C:H films; at a low N content, the stress is high [66]. This e ffect is di fficult to see in our coatings, because the N content is reduced after the addition of Si, but it is a minor reduction. This result can also have a major influence on the corrosion resistance of TiSiCN. This coating probably presents fewer defects and it is more compact than TiCN.

When the crystallite size decreases, the corrosion current density decreases and polarization resistance increases, which means that the corrosion resistance of the coatings increases with decreasing grain size. Thus, the TiSiCN coatings, which have the smallest crystalline size, were more resistant to corrosive attack. The dependence of corrosion resistance on crystallite size can be ascribed to the BOLS mechanism [67]. In the grain boundaries, there are undercoordinated atoms with lowered residual cohesive energy which possess high energy, these atoms exist in unstable states and an increase in their percent will lead to an increase of corrosion resistance [68]. This finding is also sustained by the strain ε value. In both cases, the strain was low, but after the addition of Si, the strain was significantly reduced. When the strain decreases, the corrosion resistance of the coatings increases. Thus, the correlation between high corrosion resistance and low strain and small crystallites can be explained in terms of the "bond-order-length-strength correlation mechanism", meaning that the undercoordinated atoms found on the surface or in grain boundaries take the responsibility of the good corrosion resistance. In the current paper, along with the addition of Si, the Qdl parameter was also decreasing, and this result could be due to a smaller crystallite size obtained by the TiSiCN coating. Thus, the decreased generation of defects, in the case of this coating, had a beneficial e ffect on the protective properties. It was shown that defects within a structure can cause localized corrosion at the coating–substrate interface, due to the electrolyte ingress [69]. In addition, Rpore indicated that the resistivity of the electrolyte in the pores had the highest value in this case, which can be also correlated with the lack of defects. The α values ranged from about 0.87 to 0.90, and the deviation from an ideal capacitor was ascribed to di fferences in roughness, as was shown. The dependence between roughness, capacitance and associated α values was demonstrated [69], although there are also some other factors which can be influences, such as thickness and the dielectric constant of the material.
