# 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  
It is seen in Figure 5a that the initial OCP value for sample TN was the lowest of all the tested samples (black solid circles), while both HCPEB-modified materials exhibited somewhat higher OCP values (open circles and crossed circles for TN-30 and TN-20, respectively). This implies that HCPEB treatment increases the material's resistance against corrosion in saline environment. The E\({}_{\text{OC}}\) values observed for HCPEB-modified TN-20 and TN-30 were slightly different, which could be related to their different phase composition (Section 3.1). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Analyzed Point** & **Element** & **Content (at.\%)** \\ \hline \multirow{2}{*}{Matrix} & Ti & 45 \\  & Ni & 55 \\ \hline \multirow{2}{*}{Surface layer} & Ti & 53 \\  & Ni & 47 \\ \hline \multirow{2}{*}{Inclusions} & Ti & 64 \\  & Ni & 36 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Elemental content at different points of cross-sectional sample TN-30 according to EDX data. Storage in saline environment for 10 days is seen in Figure 5a to lead to a steady increase in OCP values for all the samples. The highest value was exhibited by sample TN-20, but the smallest overall increment was demonstrated by sample TN-30, implying that increased beam energy during HCPEB treatment had a positive effect on the stability of formed TiNi coatings (Figure 5b). This can be explained by a thicker modified topmost layer observed for sample TN-30 in comparison with TN-20, since this layer is believed to block corrosive processes. Thus, the stability of the surface properties of the three samples in contact with saline was found to increase within the row: TN < TN-20 < TN-30. EIS data were recorded in a three-electrode cell filled with the same saline solution, and a typical experimental Nyquist plot is presented in Figure 6a. Results of ZView simulation and a corresponding equivalent circuit are also presented in Figure 6. Note that all the samples prepared in this study showed similar Nyquist plots, which is why Figure 6a only presents data obtained for sample TN-30, while all values of simulation parameters calculated for each sample can be found in Supplementary Materials Table S1. The equivalent circuit exhibited in Figure 6b consists of two parts, which is why the processes on the surface of tested samples are characterized by two time constants. This is consistent with the earlier described model for a double-layered structure of a passive film  
Figure 5: (**a**) Evolution over time of OCP values for samples stored in saline (inorganic fraction of DMEM) for 10 days. (**b**) The difference between initial and final OCP values observed for the samples after 10 days of immersion. Figure 6: Typical EIS data obtained for samples: (**a**) Nyquist plot and ZView simulation; (**b**) equivalent circuit. on a TiNi surface [39]. For comparison, another group used a simpler equivalent circuit (with one time constant) to describe their TiNi presented as a thin film [40]. Based on the above discussed SEM results, we suggested that the first part of the equivalent circuit (parallel elements CPE\({}^{1}\) and R\({}_{\text{p}}\)\({}^{1}\)) with the relaxation time of \(10^{-6}\)-\(10^{-7}\) s (Table 4) represents the outer layer of the coating and surface of open shallow pores. Meanwhile, the second part (a part of a simplified Randles cell, CPE\({}^{2}\), R\({}_{\text{p}}\)\({}^{2}\) and W) with its relaxation time around \(10^{-5}\) s, was ascribed to the more difficult-to-reach surface of deep and/or difficult-to-permeate pores and the monolithic TiNi sublayer that remained uncovered with the particle-based coating. For simplicity, hereafter they will be referred to as the 'external' (first part in Figure 6b) and 'internal' (second part in Figure 6b) layers of the material.