## 3 Results
### Characterization of the Substrate and Pre-Cating Layer  
Prior to the deposition of HAp, the Ti-6 Al-4V alloy substrate underwent surface pre-treatment to improve coating adhesion, addressing one of the major limitations of electrodeposition or electrophoretic deposition of HAp on metal surfaces--namely, the poor adhesion of the HAp layer to the implant surface. Typically, interfacial delamination of the HAp layer is initiated by vertical cracking throughout the coating thickness, a phenomenon already reported for the HAp-Ti-6 Al-4V interface. Such delamination can lead to inflammation in surrounding tissues, resulting in bone loss and even implant loosening [5]. This drawback can be overcome by creating a stable intermediate layer that bridges the titanium alloy surface and the HAp coating. In this regard, a TiO\({}_{2}\) layer is a favorable option as an interfacial layer between the HAp coating and the titanium substrate. The mechanical compatibility of TiO\({}_{2}\) with HAp, along with its chemical affinity for both HAp and titanium, enhances the bonding strength of the HAp coating to the metallic substrate [5; 6]. As mentioned before, to form this interfacial TiO\({}_{2}\) layer, the Ti-6 Al-4V plates were subjected to a thermal oxidation process. Figure 4 illustrates the spectral reflectivity curves and the simulated colors derived from these reflectivity values, represented by the coordinates L, a, and b* in the CIELAB colorimetric space, to estimate the TiO\({}_{2}\) pre-coating thickness. Additionally, the figure includes photographs of the Ti-6 Al-4V surface before and after thermal oxidation, providing a visual comparison of the color changes. These three numerical values uniquely define the color of a surface for a given illuminant and observer. Specifically, L* indicates the lightness of the sample, while a* and b* are the chromatic coordinates: a* varies from green (\(-\)a*) to red (\(+\)a*), and b* varies from blue (\(-\)b*) to yellow (\(+\)b*). Together, these parameters determine the perceived color of the material examined [32]. The Ti-6 Al-4V substrate, shown in its original unoxidized state, appears silver in color and exhibits reflectivity rates ranging from approximately 30% at 360 nm to about 52% at 750 nm. This smooth increase in reflectivity with wavelength is characteristic of a metallic surface with minimal oxide interference. The relatively low overall reflectivity confirms  
Figure 4: Spectral reflectivity curves of the Ti-6 Al-4V surface before and after thermal oxidation, along with the corresponding simulated colors and optical images of the Ti-6 Al-4V surface pre- and post-thermal oxidation. the alloy's classification as a high-loss metal, consistent with descriptions found in the literature [33]. After heat treatment at 40 \({}^{\circ}\)C for 1 h, the titanium alloy exhibits a distinctive U-shaped spectral reflectance curve--starting at approximately 24%, dropping to a minimum of around 5% at 440 nm, and gradually increasing to about 38% at 750 nm. This pattern is typical of thin-film interference resulting from the formation of an oxide layer, as seen in anodized and thermal-oxidized titanium alloys. The selective absorption near 440 nm corresponds to the visually observed yellow-orange coloration. This minimum at 440 nm corresponds to destructive interference. The wavelength at which this minimum occurs can be related to the oxide film thickness by the expression [32; 34; 35]:  
\[2 nd=\left(m+\frac{1}{2}\right)\lambda \tag{1}\]  
where \(\lambda\) is the wavelength of the incident light, d is the thickness of the oxide layer, n is the refractive index of the oxide, and m is the order of interference. Assuming normal incidence and considering first-order interference (m = 0) with a refractive index n = 2.4 [32] for the TiO\({}_{2}\) layer, the oxide thickness is estimated to be approximately 46 nm. The impact of this oxide layer on surface color is further demonstrated by the shift in CIELAB color coordinates.