# Preparation and Electron-Beam Surface Modification of Novel TiN Material for Medical Applications
## 3 Results and Discussion
### Structure and Composition
HCPEB treatment with higher energy of the material is seen in Figure 3c to result in further surface smoothening and homogenization. The size of surface features further increased and the average roughness of sample TN-30 decreased by 25% when compared with that of sample TN (Table 2). The changes occurring in the surface layer during HCPEB treatment are also presented in cross-sectional BSE images of the samples (Figure 4). In all panels of Figure 4, the matrix of materials consisting of TiNi (phase B2) incorporates small crystallites of Ti\({}_{3}\)Ni\({}_{4}\) which are not seen in the figure but were confirmed by XRD and well-known from the literature [32]. As a result of their distribution in the B2 matrix, internal stresses appear inside the particles of the coating [32; 33]. These stresses are known to originate from the parameter mismatch between the crystal lattice of small Ti\({}_{3}\)Ni\({}_{4}\) crystallites and the TiNi B2 matrix. In addition, the Ti\({}_{3}\)Ni\({}_{4}\) phase is not prone to martensitic transformations [34], which also contributes to stresses inside the material. The stresses inside phase B2 eventually cause its transformation to phase B19\({}^{\prime}\) with a packed-pyramidal morphology, which is seen as light lines inside the TiNi matrix (Figure 4 and insets therein). Thus, the cross-sections of obtained materials are characterized by disoriented martensitic relief. Finally, the dark-grey spots seen as inclusions into the TiNi matrix in Figure 4 indicate the presence of phase Ti\({}_{2}\)Ni, which is confirmed by EDX data (Figure 4d, Table 3). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **R\({}_{\text{z}}\) (\(\mu\)m)** & **R\({}_{\text{a}}\) (\(\mu\)m)** \\ \hline TN & 360 & 80 \\ TN-20 & 340 & 70 \\ TN-30 & 280 & 60 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Surface roughness (measured as R\({}_{\text{z}}\) and R\({}_{\text{a}}\) values) of samples as evaluated by surface profilometry. Figure 4: SEM (BSE mode) images of sample cross-sections (samples were fixed in epoxy polymer): (**a**) TN, (**b**) TN-20, and (**c**) TN-30. (**d**) EDX spectra from the matrix, its inclusions, and the surface layer of sample TN-30. After HCPEB treatment, a recrystallized layer appeared on the topmost surface of both samples TN-20 and TN-30 (insets in Figure 4b,c). This layer is seen in insets (Figure 4b,c) not to contain inclusions of different phases and seems to be structurally homogeneous. Depending on the curvature, which differs at different locations of the coating, this recrystallized layer demonstrated varied thickness between 5 and 20 \(\upmu\)m. Quite expectedly, this layer is seen to be generally thicker for sample TN-30, which was HCPEB-treated with higher energy beam. According to EDX spectra taken at different locations of sample TN-30 and shown in Figure 4d, the modified layer is enriched with Ti (Table 3). This can be explained by melting of Ti\({}_{2}\)Ni (inclusions in spectra and Table 3) at high temperatures caused by HCPEB and its further dissolution in the matrix (see Table 1). This observation is well consistent with works by others where homogenization of surface layer atop monolith materials based on TiNi caused by electron-beam processing was also reported [35; 36; 37; 38]. Thus analysis of structure and composition of untreated sample TN and samples TN-20 and TN-30 processed with HCPEB permits to conclude that electron-beam treatment caused structural changes and phase redistribution in the surface layer of the materials. The roughness of HCPEB-treated samples decreased as much as up to 25%, and their modified top layer was as thick as 5-20 \(\upmu\)m and enriched in Ti.