## 3 Results and Discussion
### Synergistic Effect of Inorganic Thermal Oxide Coating and Organic PVPA Coating  
In our previous work, we demonstrated the efficacy of organic PVPA coatings in the surface modification of artificial human joints [30]. Figure 7 shows the XPS survey scans of the modified surface. Lines I and III in Figure 7 represent the XPS survey of bare Ti6 Al4V and organic PVPA coated Ti6 Al4V, respectively. Compared to line I, the appearance of P, the increase in C, and the disappearance of Ti in line III demonstrate the successful modification of bare Ti6 Al4V with organic PVPA. Our previous studies showed that the PVPA coating of PVPA-modified Ti6 Al4V has a thickness of approximately 400 nm [24; 30]. Nanoindentation was performed on the sample to determine the elastic modulus, as shown in Figure 8. The elastic modulus of the organic PVPA-coated Ti6 Al4V was 11.2 \(\pm\) 0.2 GPa, which corresponds to a stress of 21.42 MPa at 2.5N. Friction experiments with different loads were conducted on the organic PVPA-coated Ti6 Al4V. The results are shown in Figure 9. Compared to bare Ti6 Al4V, the friction coefficient and wear of the organic PVPA-coated Ti6 Al4V were significantly lower. At stresses of 21.42 MPa and 42.84 MPa, the friction coefficient of the organic PVPA-coated surface was lower than 0.01. Thus, the organic PVPA coating can effectively reduce the friction coefficient and surface wear. Figure 8: Elastic modulus of the PVPA- and composite-coated Ti6 Al4V. Figure 7: XPS survey scans of I: bare Ti6 Al4V, II: Ti6 Al4V heat treated at 300 \({}^{\circ}\)C, III: organic PVPA-coated Ti6 Al4V, and IV: inorganic thermal oxide/organic PVPA composite-coated Ti6 Al4V. An organic PVPA layer can be added to the inorganic thermal oxide coating to simulate articular joints. As shown in Figure 7, lines II and IV represent the typical XPS survey scans of inorganic thermal oxide-coated Ti6 Al4V and composite-coated Ti6 Al4V, respectively. Compared to line II, the appearance of P, the increase in C, and the disappearance of Ti in line IV indicate the suitability of adding organic PVPA on top of the inorganic thermal oxide coating. The elastic modulus of the composite-coated Ti6 Al4V was 10.2 \(\pm\) 0.2 GPa, as shown in Figure 8. The corresponding stress of the composite-coated Ti6 Al4V at 2.5 N  
Figure 9: Tribological properties of organic PVPA-coated Ti6 Al4V under different loads: (**a**) friction coefficient versus time; (**b\({}_{1}\)**–**f\({}_{1}\)**) wear morphologies of organic PVPA-coated Ti6 Al4V under loads of (**b\({}_{1}\)**) 2.5 N, (**c\({}_{1}\)**) 5 N, (**d\({}_{1}\)**) 10 N, (**e\({}_{1}\)**) 20 N, and (**f\({}_{1}\)**) 40 N; (**b\({}_{2}\)**–**f\({}_{2}\)**) wear morphologies of PTFE balls (tribo-pairs of organic PVPA-coated Ti6 Al4V under loads of (**b\({}_{2}\)**) 2.5 N, (**c\({}_{2}\)**) 5 N, (**d\({}_{2}\)**) 10 N, (**e\({}_{2}\)**) 20 N, and (**f\({}_{2}\)**) 40 N.  
was 21.36 MPa. The influence of the inorganic thermal oxide coating on the bearing effect of the PVPA coating was studied by changing the applied load, and the results are shown in Figure 10. The composite coating maintained a low friction coefficient of less than 0.01 at stresses of 85.44 MPa. At stresses of 170.88 MPa and 341.76 MPa, the friction coefficients were 0.012 and 0.015, respectively. Compared to the organic PVPA-coated Ti6 Al4V, the bearing capacity of the composite-coated Ti6 Al4V was increased by the presence of the thermal oxide coating in the composite coating. In the range of superlubricity, the presence of the inorganic thermal oxide coating increased the bearing capacity of the composite coating from 42.84 MPa to 85.44 MPa. Organic PVPA surface modification can effectively reduce the friction coefficient and wear, at a stress of 42.84 MPa, the friction coefficient was less than 0.01. Compared to PVPA coating alone, the inorganic thermal oxide/organic PVPA composite coating had better bearing capacity.