## 3 Results and Discussion
### Modification of Surface Bearing Capacity of Ti6 Al4V Using Inorganic Thermal Oxide Coating
The results of the bare and inorganic thermal oxide-coated surfaces are shown in Figure 4. The friction coefficients of the inorganic thermal oxide coatings formed at 700 \({}^{\circ}\)C and 900 \({}^{\circ}\)C are 0.288 and 0.240, respectively, which are significantly higher than those formed at the lower temperatures. The friction coefficients of the inorganic thermal oxide coatings formed at 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C were 0.025, 0.027, and 0.072, respectively. Comparing the surface morphologies after sliding--as shown in Figure 4f,g--and combining the results of XRD, the rutile phase of the inorganic thermal oxide coatings would significantly increase the friction coefficient. The surface wear of the inorganic thermal oxide coating formed at 300 \({}^{\circ}\)C after the sliding test was minimal. Therefore, the inorganic thermal oxide coating formed at 300 \({}^{\circ}\)C has good friction and wear properties. The inorganic thermal oxide coatings formed at 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C had the same phase and micro-morphology, but different tribological properties, primarily due to the differences in the mechanical properties of the inorganic thermal oxide coatings formed at different temperatures. Nanoindentation experiments were performed to determine the hardness and elastic modulus of the bare Ti6 Al4V and the thermal oxide-coated surfaces formed at three different temperatures, as shown in Figure 5a,b. The hardness of the bare Ti6 Al4V and the inorganic thermal oxide-coated surfaces formed at 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C was 4.34 \(\pm\) 0.27 GPa, 5 \(\pm\) 0.29 GPa, 4.86 \(\pm\) 0.44 GPa, and 7.84 \(\pm\) 0.61 GPa, respectively. The corresponding elastic modulus were 134.2 \(\pm\) 4.9 GPa, 124.6 \(\pm\) 4.4 GPa, 116.5 \(\pm\) 4.8 GPa, and 130 \(\pm\) 7 GPa, respectively. Therefore, heat treatment increases the surface hardness and decreases the elastic modulus. Considering the inorganic thermal oxide coating, the lower the hardness and the elastic modulus, the lesser the corresponding friction and wear. Therefore, compared to heat treatment temperatures of 100 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, a heat treatment temperature of 300 \({}^{\circ}\)C offers better tribological properties. Figure 3: XRD patterns of bare Ti6 Al4V and inorganic thermal oxide coatings formed at 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C.  
Figure 4: Tribological properties of bare Ti6 Al4V and inorganic thermal oxide coatings formed at 100 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C: (**a**) friction coefficient versus time, (**b1**–**g1**) wear morphologies of (**b1**) bare Ti6 Al4V, (**c1**–**g1**) inorganic thermal oxide coatings formed at (**c1**) 100 \({}^{\circ}\)C, (**d1**) 300 \({}^{\circ}\)C, (**e1**) 500 \({}^{\circ}\)C, (**f1**) 700 \({}^{\circ}\)C, and (**g1**) 900 \({}^{\circ}\)C, (**b2**–**g2**) wear morphologies of PTFE balls ((**b2**) tribo-pair of the bare Ti6 Al4V, (**c2**–**g2**) tribo pairs of inorganic thermal oxide coatings formed at (**c2**) 100 \({}^{\circ}\)C, (**d2**) 300 \({}^{\circ}\)C, (**e2**) 500 \({}^{\circ}\)C, (**f2**) 700 \({}^{\circ}\)C, and (**g2**) 900 \({}^{\circ}\)C). An interesting phenomenon was that the friction coefficient and wear of the bare Ti6 Al4V were minimal with an initial load of 2.5 N, as shown in Figure 4a. The friction coefficient of the bare Ti6 Al4V increased with the increase in stress, as shown in Figure 6a. However, the friction coefficient of the inorganic thermal oxide coating formed at 300 \({}^{\circ}\)C did not change significantly with the increase in stress, as shown in Figure 6b.