## 3 Results and Discussion
### Oxide Layer Adhesion Results  
The representative VDI 3198 standard layer adhesion results for each oxide group are illustrated in Figure 9. The cross-sectional oxide analyses revealed each oxide group to be bi-layered. Previously, bi-layered hydroxyapatite-containing anodized oxides have been shown to have higher adhesion strengths compared to plasma-sprayed hydroxyapatite coatings [29, 67]. The higher strengths of the bi-layered coatings were attributed to stronger interlocking at the interface between the inner titanium dioxide-rich layer and the outer hydroxyapatite-rich layer [29, 67]. The CPTi oxide group revealed the thinnest oxide and exhibited only a small number of microcracks in the adhesion quality testing, as shown in Figure 9A. It has been widely reported that the adhesion strengths of anodized titanium oxides generally decrease with increasing oxide cross-sectional thickness values [23, 56, 70]. However, the thicker oxides shown on the duplex TiO \(\alpha+\beta\) and the TAV oxides still  
Figure 7: Representative EDS line scans of dopant element distributions across the cross-sections of each oxide group. The Ti, O, and substrate alloying element distributions are shown in the leftmost column, while distributions of elements incorporated from the anodization electrolyte (Ca, P, and Mg) are shown in the rightmost column. The alloy substrate is located on the left side of each image, followed by the inner oxide layer and then the outer oxide layer. The presence of a bi-layered oxide structure is clarified for each oxide, with the concentration of substrate (Ti) and alloying elements (Mo, Al, V) decreasing across the inner oxide layer and being absent in the outer oxide layer. The uptake of Ca, P, and Mg begins in the inner oxide layer and continues through each outer oxide layer. exhibited acceptable oxide adhesion qualities, as shown in Figure 9C,D. This finding was especially surprising for the TiMo \(\alpha+\beta\) oxide, which was found to undergo substantial omega-phase formation in the XRD analyses (Figure 1). While some microcracking and interface chipping were observed after the indentation test for the TiMo \(\alpha+\beta\) oxide, this image is still an acceptable adhesion level according to the VDI adhesion quality reference images [15; 24; 37; 38; 39; 71; 72]. In contrast, the thicker TiMo \(\beta\) oxide, which, in the XRD analyses, revealed evidence of both omega-phase and alpha-case embrittling phases, was found to almost completely delaminate from the substrate surface during the VDI adhesion testing, as shown in Figure 9B. The presence of the omega phase indicates embrittlement caused by thermal or stress-induced transformation occurring during the anodization process as the molybdenum concentration is reduced near the substrate-oxide interface. Furthermore, the alpha-case layer formed on the substrate surface at the oxide interface is also a brittle layer, and it occurs in titanium alloys when exposed to higher temperatures in oxidizing environments [43]. The combination of the \(\omega\) phase and the alpha-case layer formation at the substrate-oxide interface, the TiMo \(\beta\) oxide group in the present study may have weakened the substrate-oxide adhesion strength of the anodized coating. As stated earlier, the formation of the omega phase in the TiMo alloy anodization processes would likely weaken the implant ductility of the substrate alloys and thus would not be desirable for implant applications, even though the TiMo \(\alpha+\beta\) oxides showed good adhesion strengths. However, the CPTi and TAV oxides also showed good adhesion strengths, and these two substrate alloys constitute the vast majority of titanium alloys used for commercial implants. While hydroxyapatite coatings on titanium implants have often been shown to promote faster osseointegration compared to uncoated titanium counterparts [15; 24; 30; 50; 73; 74], it is important to note some limitations that lead to debates over their use in certain clinical applications.