## 3 Results and Discussion  
### Oxide Surface Characterization  
#### 3.1.1 Oxide Crystallinity Analyses  
The representative crystallinity results for each oxide group are provided in Figure 1. A full range of 20\({}^{\circ}\) to 90\({}^{\circ}\) two-theta scans are shown on the left side of Figure 1 to show all of the crystalline phases formed in each titanium alloy surface coating. The CPTi oxides showed alpha-phase titanium peaks, and the duplex TAV alloy oxides revealed both alpha- and beta-phase peaks, as expected. The thin-film XRD scans of the TiMo \(\beta\) and TiMo \(\alpha+\beta\) oxides, however, also revealed evidence of omega (\(\omega\)) phase formation. TiMo alloys have been previously reported to be subject to omega-phase formation when Mo levels are not sufficient to fully stabilize the beta phase [40; 41]. The \(\omega\) phase, an embrittling transformation that reduces the ductility of the alloy, may occur within the beta phase of Ti-15 Mo alloys under specific cooling rates [40; 41; 42]. While most of the peak positions of beta-phase titanium and omega-phase diffraction database files showed substantial overlap, making the phases difficult to distinguish, the omega phase had a relatively high-intensity diffraction peak at a 79\({}^{\circ}\) two-theta angle. The presence of a small peak at this two-theta diffraction angle for both the TiMo \(\beta\) and TiMo \(\alpha+\beta\) oxides confirmed the presence of the omega phase. In the present study, the formation of the omega phase at the TiMo substrate surfaces was attributed to the localized depletion of molybdenum from the substrate surfaces, as some molybdenum migration to the inner oxides was observed, as shown below. The TiMo \(\beta\) oxides also exhibited an alpha-phase peak at a two-theta diffraction angle of approximately 39\({}^{\circ}\) two-theta, which is indicative of alpha-case formation within the surface oxide layer. The alpha case is another surface embrittling phase that can form on titanium alloys in oxygenating environments [43; 44]. A zoomed-in 24\({}^{\circ}\)-to-36\({}^{\circ}\) two-theta diffraction angle region is also provided on the right side of Figure 1 to emphasize the formation of various crystalline calcium compound phases within the oxides. Each oxide exhibited evidence of hydroxyapatite, \(\alpha\)-tricalcium phosphate, and calcium titanate formation. The highest intensity diffraction peaks for each of these calcium compounds were within the 24\({}^{\circ}\)-to-36\({}^{\circ}\) two-theta diffraction angle range. Hydroxyapatite has been extensively explored for the formation of coatings on the surface of dental and orthopedic titanium implants to improve osseointegration [45]. However, the conventional techniques of forming hydroxyapatite coatings have limitations, necessitating the use of specialized or expensive processing equipment, long preparation periods, and inadequate adhesion strengths [46; 47; 48]. Anodization has previously been used to create hydroxyapatite coatings on titanium substrates using a single-step process [15; 24; 29; 30]. Some of these anodized oxides showed improved adhesion strengths between the titanium substrate and hydroxyapatite coatings [24; 29]. Hydroxyapatite and tricalcium phosphate are the most studied calcium phosphates and have been commonly used in bone cement and for bone substitution [49]. Hydroxyapatite-coated titanium implants have demonstrated superior osteoblast viability in MTT assays, enhanced differentiation in ALP assays, and increased mineralization in Alizarin Red assays in vitro [15; 24; 30; 50]. Furthermore, these coatings have been shown to promote earlier induction of new bone around the implant and improved BIC in rat and rabbit femur models in vivo, compared to their non-anodized counterparts [50; 51]. Previously, magnesium-sputtered \(\alpha\)-tricalcium phosphate coatings on titanium have shown increased bone-to-implant contact and peri-implant bone volume in rabbit femurs [52].