## 3 Results
### Characterization of the Substrate and Pre-Cating Layer
Prior to thermal oxidation, the alloy exhibits values of \(L\) = 73.01\({}^{\star}\), \(a\) = 0.96\({}^{\star}\), and \(b\) = 7.21\({}^{\star}\), indicating a relatively high lightness and near-achromatic character, consistent with its visually perceived silver appearance. These low a\({}^{\star}\) and b\({}^{\star}\) values correspond to a desaturated tone, typical of metallic surfaces with minimal oxide interference [36]. Following heat treatment, the color coordinates shift significantly to \(L\) = 49.65\({}^{\star}\), \(a\) = 9.57\({}^{\star}\), and \(b\) = 36.09\({}^{\star}\), indicating a substantial reduction in lightness and a marked increase in chromaticity. This transition reflects the development of a colored interference film on the surface. The positive a\({}^{\star}\) and b\({}^{\star}\) values confirm a shift toward a warmer hue, dominated by red and yellow components [33]. To further characterize the perceived color, the hue angle (h\({}^{\circ}\))--calculated from the arctangent of the b\({}^{\star}\)/a\({}^{\star}\) ratio--yields a value of 75.15\({}^{\circ}\), placing the color firmly within the yellow-orange region of the CIELAB color space [32; 37]. This hue corresponds well with the visually observed coloration and aligns with the interference pattern described, where destructive interference at 440 nm selectively reduces blue reflectance. This analysis supports the conclusion that the observed color change arises from thin-film interference effects associated with the oxide layer formed during thermal oxidation. From this point forward, the thermally oxidized Ti-6 Al-4V surface will be referred to as the \"TiO\({}_{2}\) pre-coating\" condition, denoting the substrate state prior to the electrophoretic deposition of HAp. Figure 5a presents the SEM image of the untreated Ti-6 Al-4V substrate, which exhibits a relatively smooth and homogeneous surface. The corresponding EDS spectrum (Figure 5b) confirms the elemental composition of the substrate, establishing a baseline for comparison with the thermally oxidized surface. After thermal treatment at 400 \({}^{\circ}\)C for 1 h, a thin TiO\({}_{2}\) layer is formed on the surface of the substrate. Although the surface morphology, shown in Figure 5c, appears similar to that of the untreated substrate in Figure 5a, the EDS analysis in Figure 5d, including the corresponding quantitative data table, indicates an increase in oxygen content, confirming the formation of the oxide layer. Further confirmation is provided by Raman spectroscopy, as shown in Figure 6, which displays characteristic peaks of the rutile phase of TiO\({}_{2}\) at approximately 142, 440, and 611 cm\({}^{-1}\) corresponding to the symmetries B\({}_{\rm 1g}\), E\({}_{\rm g}\), and A\({}_{\rm 1g}\), respectively [38; 39; 40; 41]. Additionally, a peak at 250 cm\({}^{-1}\) is observed, which is attributed to second-order or disorder-induced scattering, associated with multi-phonon processes [39; 40; 41]. The presence of these well-defined peaks confirms the formation of a crystalline rutile phase. The experimental Raman band assignments and corresponding literature values are summarized in Table 2. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Experimental** & **[PERSON] et al.** & **[PERSON] et al.** & **[PERSON] et al.** & **[PERSON] et al.** & **Band** \\
**(cm\({}^{-1}\))** & **[40]** & **[39]** & **[38]** & **[41]** & **Assignment** \\  & **(cm\({}^{-1}\))** & **(cm\({}^{-1}\))** & **(cm\({}^{-1}\))** & **(cm\({}^{-1}\))** & **(cm\({}^{-1}\))** & **Assignment** \\ \hline
142 & 140 & 144 & - & 143 & B\({}_{\rm 1g}\) \\
250 & 230 & 235 & - & 235 & - \\
440 & 430 & 445 & 447 & 447 & E\({}_{\rm g}\) \\
611 & 590 & 610 & 612 & 612 & A\({}_{\rm 1g}\) \\ \hline \end{tabular}
\end{table} 
Table 2: Assignment of Raman bands along with the literature values. Figure 5: _Cont._  
Figure 5: (**a**) SEM image and (**b**) EDS spectrum of the substrate; (**c**) SEM image and (**d**) EDS spectrum of the thermal-oxidized substrate. Figure 6: Raman spectra of the thermal-oxidized substrate.