# Preparation and Electron-Beam Surface Modification of Novel TiN Material for Medical Applications
## 3 Results and Discussion
### Electrochemical Tests and Cell Growth
#### 3.2.1 Effect of Saline Medium on Samples
Comparing the CPE-T and R\({}_{\text{p}}\) values of the two parts (see Table S1), one can see that the CPE\({}^{1}\)-T parameter is much lower than CPE\({}^{2}\)-T, whereas R\({}_{\text{p}}\)\({}^{1}\) is larger than R\({}_{\text{p}}\)\({}^{2}\). This is well consistent with the assumption about the external and internal layers. As the internal layer has a larger surface area, its capacitive parameter should be higher. At the same time, its resistance to corrosion is lower because this part of the coating was less exposed to HCPEB treatment (lower access to deeper layers). Vice versa, the external part of the coating is better protected from the corrosion (exhibiting higher R\({}_{\text{p}}\) values) since its modified layer was enriched in Ti and is normally covered with a very thin TiO\({}_{\text{x}}\) film [28] (see Section 3.1). As for its capacitance, it is smaller as a result of a lower surface area of the external layer of the material. As can be seen in Table S1, all the parameters remained within one order of magnitude during all the period when the materials were immersed in saline medium. This implies no dramatic changes of surface properties were observed for as long as ten days. Even parameter R\({}_{\text{p}}\)\({}^{1}\), which is attributed to the overall corrosion resistance of the material, did not change significantly. However, the second component of the circuit, which reveals behavior of the internal layer, was found to show steady changes in parameter R\({}_{\text{p}}\)\({}^{2}\) (see Figure 7). The resistance of the internal layer is seen in Figure 7 to decrease steadily for sample TN over immersion time. It also decreased for sample TN-30, although to a smaller extent. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Time (Days)** & \multicolumn{2}{c}{**Lifetime (s)**} \\  & \(\boldsymbol{\tau_{1}}\) & \(\boldsymbol{\tau_{2}}\) \\ \hline
0 & \(10^{-6}\) & \(10^{-5}\) \\
1 & \(10^{0}\) & \(10^{-6}\) \\
3 & \(10^{0}\) & \(10^{-4}\) \\
5 & \(10^{-1}\) & \(10^{-4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Time components \(\tau_{1}\) and \(\tau_{2}\) obtained from EIS data for sample TN with cells growing on its surface. Figure 7: Evolution of resistance parameter R\({}_{\text{p}}\)\({}^{2}\) from the second part of equivalent circuit for samples during their storage in saline (inorganic fraction of DMEM) for 10 days. For sample TN-20, however, after 10 days its \({\rm R_{p}}^{2}\) was found to be very close to its initial value. Note that after exposure to saline for 10 days, the \({\rm R_{p}}^{2}\) value observed for sample TN-20 is the highest resistance of all the samples tested (see crossed circles in Figure 7). Since parameter \({\rm R_{p}}^{2}\) is a characteristic of a deep-pore surface and that of uncoated TiNi monolith substrate, sample TN-20 seemed to have this component of its surface better protected against corrosion. This could be related to the difference in phase content of this material in comparison with the other two samples (the highest content of TiNi and the lowest of \({\rm Ti_{3}Ni_{4}}\) and \({\rm Ti_{2}Ni}\) phases). Paying attention to the lifetime parameters presented in Table S2, one finds that, in general, the processes occurring in the external layer (first part of the circuit) proceeded faster than those in the internal layer (second part of the circuit). This is reasonable as the internal layer is more difficult to access for electrolyte. The lowest values of \({\rm T_{1}}\), implying the fastest rates of surface processes in comparison with the other samples, are observed for sample TN-20. Similar to the above discussion for parameter \({\rm R_{p}}^{2}\), this could be also related to the highest content of TiNi and the lowest contents of phases \({\rm Ti_{3}Ni_{4}}\) and \({\rm Ti_{2}Ni}\) on this sample. Thus, our electrochemical studies revealed that HCPEB-modified TiNi coatings increased their stability against corrosion.