## 3 Results and Discussion  
### Characterization of HAp Layer Formed by FPP  
Figure 3 shows SEM micrographs and EDS maps of each specimen surface. EDS analysis was conducted with a working distance of 10 mm, an accelerating voltage of 20 kV, and a magnification of 100 for all specimens. As can be seen in the SEM micrograph of the FPP-treated specimen surface, a rough surface was formed by plastic deformation due to the collision of HAp particles with the specimen surface. As a result of measuring the surface roughness (arithmetic mean roughness, \(R_{\mathrm{a}}\), and maximum height, \(R_{\mathrm{z}}\)), it was found that \(R_{\mathrm{a}}\) = 0.02 \(\upmu\)m and \(R_{\mathrm{z}}\) = 0.16 \(\upmu\)m for the untreated specimen, while \(R_{\mathrm{a}}\) = 0.45 \(\upmu\)m and \(R_{\mathrm{z}}\) = 2.96 \(\upmu\)m for the FPP-treated specimen. As a result of EDS analysis, only titanium (Ti) was detected in the untreated specimen. In contrast, calcium (Ca), oxygen (O), and phosphorus (P), which were contained in the HAp particles, were also detected over the whole surface of the FPP-treated specimen. These results suggest that a HAp transfer layer was formed on the surface of the Ti-22V-4 Al alloy.  
Figure 4 shows the elemental composition of each specimen surface analyzed by EDS. Ca, P, and O were detected and accounted for 84% of the composition of the FPP-treated  
\begin{table}
\begin{tabular}{c c} \hline Tube voltage, kV & 40 \\ \hline Tube current, mA & 30 \\ \hline Diffraction angle 2\(\theta\), deg & 128 \\ \hline Diffraction plane & (321) \\ \hline Incident angle, deg & 0, 15, 22, 28, 33, 37, 41, 45 \\ \hline Beam diameter, mm & 1.0 \\ \hline Stress constant, MPa/deg & \(-\)361.62 \\ \hline \end{tabular}
\end{table}
Table 2: Residual stress measurement conditions.  
Figure 3: SEM micrographs and EDS maps for surface of untreated specimen and FPP-treated specimen.  
specimen surface. Therefore, the HAp was definitely transferred on the surface of the titanium alloy, because the amount of HAp constituent elements clearly increased.  
To investigate the thickness of the HAp layer, we observed the cross section of an FPP-treated specimen. Figure 5 shows the cross-sectional SEM micrograph and EDS maps of the FPP-treated specimen. Ca, O, and P, which are contained in the HAp shot particles, were detected on the Ti alloy substrate. Thus, it was again confirmed that the HAp layer was formed on the Ti-22V-4 Al alloy. Figure 5 also reveals that the HAp layer was inhomogeneously formed at the FPP-treated surface. The equivalent thickness of the HAp layer was calculated from Equation (1), assuming that the HAp layer, which consisted of HAp shot particles transferred onto the surface, was uniformly formed over the entire surface [23]:  
\[t_{\text{eq}}=area_{Ca}/b \tag{1}\]  
where \(t_{\text{eq}}\) is the equivalent thickness of the HAp layer (\(\upmu\)m), \(area_{Ca}\) is the area where calcium was detected (\(\upmu\)m\({}^{2}\)), and \(b\) is the width of the specimen in the analyzed area (42.3 \(\upmu\)m). The \(area_{Ca}\) and the \(b\) were calculated using image processing in the cross-sectional image of the FPP-treated specimen. As a result of the calculation, \(t_{\text{eq}}\) was 5.5 \(\upmu\)m. In our previous study, a HAp layer was formed by FPP on an \(\upalpha+\beta\) Ti-6 Al-4 Al alloy having a Vickers hardness of 340 HV [23], and the thickness was 7.0 \(\upmu\)m. There was no noticeable difference in the thickness of the HAp layer formed in the present study. This result indicates that the thickness of the HAp layer has no dependence on the Vickers hardness of the substrate in the range from 261 to 340.  
Figure 4: Elemental composition of untreated and FPP-treated specimen surface determined by EDS analysis.  
Figure 5: SEM micrographs and EDS maps for cross section of FPP-treated specimen.