## 3 Results
### Characterization of Coatings  
To evaluate the effect of an additional surface pre-treatment on the quality of HAp coatings, thermally oxidized Ti-6 Al-4V substrates were subjected to two different pre-electrophoretic deposition conditions: one set was previously immersed in SBF, while the other received no SBF treatment. It is hypothesized that the initial HAp formed during SBF immersion may act as a nucleation layer, thereby enhancing the coverage and uniformity of the subsequently deposited HAp coating. Following these pre-treatment steps, both sets of samples underwent HAp electrophoretic deposition, followed by surface characterization. Figure 7 shows SEM micrographs of HAp coatings deposited via EPD onto thermal-oxidized substrates subjected to two different surface treatments. In Figure 7a, the SBF-treated surface is presented, which exhibits a low HAp content, with a dispersed and uneven deposition. In contrast, Figure 7b reveals a surface with significantly higher HAp content, characterized by a denser and more uniform HAp layer on the oxidized substrate that did not undergo SBF pre-treatment. The surface coverage is more extensive, and the HAp particles are clearly visible. Figure 7: SEM images of HAp coatings on thermal-oxidized substrates: (**a**) SBF-treated and (**b**) untreated. These results clearly demonstrate that the sample without SBF pre-treatment achieved better EPD performance, due to the formation of a weak interface that reduced the final adhesion of HAp to the thermally oxidized substrate. Therefore, this condition was selected for further development and characterization due to its superior HAp adhesion and more substantial coating formation. To further analyze the properties of the coating formed, a comprehensive characterization was carried out on the non-SBF-treated sample. Figure 8 presents the SEM image of the HAp coating electrophoretic deposited on the Ti-6 Al-4V thermal-oxidized substrate without prior SBF treatment. The surface displays a dense and heterogeneous layer composed of well-distributed HAp agglomerates. To evaluate the chemical composition of the coating, EDS analyses were conducted in three distinct regions, as marked in the image. The corresponding EDS data, presented in Table 3, indicate a Ca/P ratio ranging from 1.87 to 1.90, which is slightly higher than the stoichiometric ratio of 1.67. This enrichment in calcium is commonly observed in electrodeposited coatings and suggests the formation of a calcium-rich surface layer [42; 43]. Additionally, calcium-rich HAp is commonly observed in syntheses performed in a highly alkaline pH medium [44]. Figure 9a shows an SEM micrograph of the HAp coating electrophoretic deposited on the Ti-6 Al-4V oxidized substrate, while Figure 9b provides higher magnification of the green highlighted section, where elemental quantification measurements by EDS were performed, as reported in Table 3. Figure 9c,d display the elemental mapping of the elements Ca and P, respectively, confirming their uniform distribution across the analyzed area. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Region** & **Atomic \% Ca** & **Atomic \% P** & **Ca/P Ratio** \\ \hline
1 & 65.16 & 34.84 & 1.87 \\
2 & 65.30 & 34.70 & 1.88 \\
3 & 65.62 & 34.38 & 1.90 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: EDS analysis of HAp coating at selected regions. Figure 8: SEM image of HAp coating on thermal-oxidized substrate without SBF treatment, showing the regions analyzed by EDS. To further confirm the presence of HAp on the TiO\({}_{2}\) pre-coated substrate, Raman analysis was performed. Figure 10 shows the Raman spectrum of the HAp coating, in which the characteristic bands at 432 cm\({}^{-1}\), 451 cm\({}^{-1}\), 580 cm\({}^{-1}\), 593 cm\({}^{-1}\), 609 cm\({}^{-1}\), 963 cm\({}^{-1}\), and 1046 cm\({}^{-1}\), highlighted in yellow, clearly indicate the deposition of HAp [45; 46; 47; 48]. Figure 10: Raman spectrum of the HAp coating with the bands assigned to HAp.