Preparation And Performance Of Nb₂O₅ Based Coatings On Magnesium Alloys

Magnesium alloys are regarded as the third generation of biomedical materials because of their excellent biodegradability,biocompatibility,and biomechanical properties.However,the high degradation rate of magnesium alloy in physiological media has greatly limited its clinical applications.The preparation of protective coatings is widely recognized as an effective way to improve the corrosion resistance of magnesium alloys.However,the mismatch in performance between the coating and magnesium alloy substrate causes poor bonding between these.In addition,bacterial infection can easily occur during the service-life of implant materials,leading to complications and implant failure.In order to solve these problems,Nb₂O₅-based coatings with different structures and Zn content were prepared on the surface of AZ31 magnesium alloy,by using a combination of multi-layer structures coating and element doping,achieved through magnetron sputtering technology.The design,preparation,microstructure,and performance of the coatings are systematically studied,providing a theoretical basis and guidance for using multi-functional coatings for the modification of implant materials.The Nb₂O₅coating was prepared on AZ31 magnesium alloy by magnetron sputtering.The influence of process parameters on the microstructure and properties of the coating were investigated.The results show that the Nb₂O₅layer is an amorphous columnar structure.With increases in sputtering power,the coating exhibits an increased deposition rate,greater thickness,and improved corrosion resistance.However,when the sputtering power exceeds 250 W,the layer shows poor compactness and reduced corrosion resistance.Increasing the argon flow rate can cause an increase in the deposition rate,thickness,and corrosion resistance of the coating.But,these decrease when the argon flow rate is above 40 sccm.The deposition rate,thickness,corrosion resistance,and adhesion of the coating all increase with increased sputtering time.In order to enhance the interface binding between the Nb₂O₅coating and the magnesium alloy substrate,an intermediate transition layer of Nb₂O₅-Mg/Mg was inserted between the coating and the substrate,and a Nb₂O₅/Nb₂O₅-Mg/Mg multilayer coating（abbreviated to MNb₂O₅）was prepared.The introduction of the intermediate transition layer achieves a gradual change of the composition from the substrate to the coating surface,resulting in a reduction in the performance difference between the coating and the substrate and a significant improvement in the interface adhesion of the coating/substrate system.The hardness（H）,elastic modulus（E）,and H³/E²of the MNb₂O₅coating are,in order,12%,10.6%,and 14.3%higher than those of Nb₂O₅monolayer coating.The intermediate transition layer blocks the connection between the penetrating defects in the Nb₂O₅coating and the substrate,delays the attack of the corrosion medium on the substrate,and reduces the corrosion current density of the Nb₂O₅coating sample from 3.08×10^-7A/cm²to 1.04×10^-7A/cm²,charge transfer resistance increases from1.82×10³Ωcm²to 5.42×10³Ωcm²,significantly improving the corrosion resistance of Nb₂O₅coating.In order to improve the antibacterial properties of the Nb₂O₅coating,the Zn element was doped in the outer layer of the MNb₂O₅coating,and the Zn-Nb₂O₅/Nb₂O₅/Nb₂O₅-Mg/Mg multilayer coating（abbreviated to Zn-MNb₂O₅）was prepared.The influence of the doping amount of Zn on the microstructure and properties of the Zn-MNb₂O₅coating samples was investigated.The results show that the Zn element exists in the coating in the form of Zn O,and that incorporating the Zn element can improve the surface compactness and corrosion resistance of MNb₂O₅coating.However,higher Zn content will lead to surface roughness and pore formation,resulting in decreased compactness and reduced corrosion resistance.Compared with the ZM0 sample without Zn doping,the ZM1sample doped with 3.91 wt.%Zn showed a bactericidal rate of 92.5%,and the bactericidal rate increased with increasing Zn content.The Zn-MNb₂O₅coating significantly improves the biocompatibility of AZ31magnesium alloy,but excessive Zn incorporation weakens cell activity and reduces the biocompatibility of the coating.Among the samples,the ZM1 specimen shows the best biocompatibility,indicated by the highest cell proliferation rate.In order to further improve the interface adhesion between the Nb₂O₅coating and the magnesium alloy substrate,based on the functional gradient material theory,the idea of using G-Nb₂O₅gradient coating was proposed.The physical model,composition distribution model,and physical property parameter model of the G-Nb₂O₅gradient coating were established,and the influence of structural parameters on the residual thermal stress of the coating was determined.The optimal structural parameters of the gradient coating were obtained through the orthogonal test method,with 7 interlayer layers,outer layer thickness of 2μm and bond layer thickness of 0.45μm.G-Nb₂O₅gradient coating was deposited by magnetron sputtering technology,and the effect of gradient structure on the microstructure and properties of Nb₂O₅coating was investigated by comparing a Nb₂O₅monolayer and M-Nb₂O₅multilayer coating with the same thickness.The results show that the interface of multiple interlayers blocks the column continuity of the Nb₂O₅coating.The incorporation of Mg fills the pinholes and pores in the growth of the Nb₂O₅coating,making the density of the G-Nb₂O₅coating significantly better than that of Nb₂O₅and M-Nb₂O₅coatings.The gradient change in composition along the thickness direction of the coating greatly reduces the residual stress in the coating/substrate system.It enhances the interface bonding,thereby increasing the hardness and elastic modulus of the coating and enhancing the wear resistance and corrosion resistance of the coating.The G-Nb₂O₅coating exhibited an obviously reduced residual stress than that of Nb₂O₅and M-Nb₂O₅coating.The interfacial adhesion of G-Nb₂O₅coating is 11.86 N,which is 0.5 times higher that of M-Nb₂O₅coating（7.91 N）and 16.4 times that of Nb₂O₅coating（0.68 N）.Among the three coating samples,G-Nb₂O₅coating has the highest H,E and H³/E²values,followed by M-Nb₂O₅coating,while Nb₂O₅coating has the lowest values.The wear rate of the G-Nb₂O₅sample is 0.031×10^-3mm³/Nm,29.5%,97.7%,and 98.8%lower than M-Nb₂O₅,Nb₂O₅,and AZ31 specimens respectively,meaning that it has the best wear resistance.The G-Nb₂O₅sample exhibits a corrosion current density of 1.5×10^-7A/cm²,1 and 2 orders of magnitude lower than the samples of M-Nb₂O₅and Nb₂O₅respectively.It has a polarization resistance of 1.263×10⁴Ωcm²,0.5 times higher than the M-Nb₂O₅specimen,and 25.8 times higher than the Nb₂O₅specimen,thus having the best corrosion resistance.