## 3 Results and Discussion  
### Modification of Surface Bearing Capacity of Ti6 Al4V Using Inorganic Thermal Oxide Coating  
Previous studies have shown that performing inorganic thermal oxidation on biomedical titanium alloys is beneficial to the preparation of composite coatings [27]. [PERSON] et al. also indicated that inorganic thermal oxidation can effectively improve the surface properties of biomedical titanium alloys for application in artificial cervical discs [28]. The effect of the characteristics of inorganic thermal oxide coatings on the friction characteristics of the surface of Ti6 Al4V were investigated herein. The Ti6 Al4V substrates were heat treated for 5 h at temperatures of 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C. The surface of the bare Ti6 Al4V substrate and the inorganic thermal oxide coatings were verified using XPS. Figure 1 presents the typical XPS survey scans over a binding energy of 0-1200 eV. As shown, the ordinate values gradually shifted upwards from the second line by 80,000, indicating a stark contrast. Line I in Figure 1 represents the typical XPS survey of the bare Ti6 Al4V substrate. Lines II, III, IV, V, and VI represent the typical XPS surveys of Ti6 Al4V after heat treatment at temperatures of 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C, respectively. Compared to line I, the O1 peaks of lines II-VI were significantly higher, which indicates that inorganic thermal oxide coatings were formed on the surface. Table 1 shows the relative oxygen content on the surface of Ti6 Al4V before and after heat treatment. The oxygen content on the Ti6 Al4V surface was highest at 300 \({}^{\circ}\)C, which can lead to changes in its surface mechanical properties. The surface morphologies of the Ti6 Al4V substrates, before and after heat treatment at different temperatures, were examined using scanning electron microscopy (SEM) to explore the characteristics of the inorganic thermal oxide coatings. As shown in Figure 2(b\({}_{1}\)-d\({}_{1}\)), the surface morphologies of the inorganic thermal oxide coatings after heat treatment at 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C are smooth, and are similar to the surface morphology of the bare Ti6 Al4V substrate (Figure 2(a\({}_{1}\))). The thickness of different inorganic thermal oxide coatings was investigated with the help of stereo light microscope. Cross-sections of inorganic thermal oxide-coated Ti6 Al4V were polished by sandpaper. Images that displays the cross section of the inorganic thermal oxide-coated Ti6 Al4V were shown in Figure 2(a\({}_{2}\)-f\({}_{2}\)). The corresponding thickness of the inorganic thermal oxide coatings formed at 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C were 2 \(\upmu\)m, 3 \(\upmu\)m, and 2 \(\upmu\)m, respectively. This indicates that the inorganic thermal oxide coatings formed at 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C have a similar density and thickness. The structure of the inorganic thermal oxide coating that formed at 700 \({}^{\circ}\)C appeared uneven, as shown in Figure 2(e\({}_{1}\)). The corresponding thickness is 3 \(\upmu\)m (Figure 2(e\({}_{2}\))). The surface inhomogeneity became more obvious for the inorganic thermal oxide coating formed at 900 \({}^{\circ}\)C (Figure 2(f\({}_{1}\))), while the thickness increased to 7.2 \(\upmu\)m (Figure 2(f\({}_{2}\))). Therefore, above a certain temperature, the surface morphology and thickness of the inorganic thermal oxide coating changes significantly with the increase in temperature, which is possibly due to the change of phase of the inorganic thermal oxide coating.