Structure Design,Preparation And Performance Research Of Gradient Composite Coating On The Surface Of Medical Mg-Zn Alloy

As one of the most promising lightweight materials internationally recognized,magnesium alloys have been widely concerned by many scholars because of their good degradability,which is different from other traditional metal materials.However,it has been found that its inherent defects limit its clinical application in the field of implants to a large extent with the deepening of research.The degradation rate of magnesium alloys is too fast and the process is uncontrollable under normal conditions,which will lead to the obvious phenomenon of hydrogen accumulation in the subcutaneous tissue during the degradation process.In order to overcome this defect,the current common practice is to modify the surface of magnesium alloys.The method of preparing coating on its surface can not only improve the corrosion resistance of magnesium alloy and the coating can also bring new functions to magnesium alloy that it does not have in a certain extent.This greatly promotes the application of magnesium alloys in the field of biomaterials.Mg-Zn alloy is a simple and easy-to-obtain magnesium-based alloy material,and the Mg²⁺and Zn²⁺produced during its degradation are necessary elements for the human body and will not be toxic to cells.Therefore,Mg-Zn alloy has broad application prospects as a new medical biomaterial.In this thesis,Mg-Zn alloys with different zinc contents were prepared by gravity casting,and they were solution heat treated.The solution treated Mg-6Zn alloy with the best corrosion resistance was selected as the matrix material by electrochemical experiments.Then,the Ca-P pre-coating was prepared on the selected alloy surface using the more effective hydrothermal method,and PCL coatings doped with different nanoparticles were prepared on the pre-coating by dip coating technique subsequently,thus forming a composite coating with double-layer structure.Finally,a multilayer composite coating template with the gradient structure was designed and fabricated on the basis of the double-layer coating.The performance differences between different types of coatings were analyzed by X-ray diffractometer,environmental scanning electron microscope,tensile test,electrochemical test,in vitro immersion test,antibacterial test and cytotoxicity test.The experimental results show that the maximum solid solubility of zinc in Mg-Zn alloy is about 6.5wt.%,and the residual magnesium-zinc phase that is not solid-dissolved into the matrix will lead to the occurrence of galvanic corrosion,which will reduce the corrosion resistance of the alloy.Therefore,the solution treated Mg-6Zn alloy is the best matrix material in the Mg-Zn alloy.The solution heat treatment of the alloy will generate in-situ zinc oxide in the Ca-P pre-coating,which is beneficial to the combination of the pre-coating and the substrate.At the same time,the pre-coating itself increases the bonding strength between the PCL coating and the substrate by about 5 times.This shows that it is very necessary to preferentially prepare a pre-coating on the surface of the alloy,and it can be used as an intermediate coating to effectively avoid the peeling,blistering and loss of the barrier effect of the PCL coating.For polycaprolactone（PCL）,a polymer material whose degradation cycle is too long,nanoparticles play an important role in improving its function.Firstly,doping HA and Zn O nanoparticles help to increase surface roughness of the composite coating,which is beneficial to cell attachment and proliferation.Secondly,the formation of porous structure in the composite coating enable it the ability to control the degradation rate of the matrix by adjusting the porosity to a certain extent.Thirdly,the composite coating doped with Zn O nanoparticles can enhance the antibacterial properties of magnesium alloy implants.Among them,the solution treated Mg-6Zn alloy obtained the best comprehensive properties after coating with Ca-P pre-coating and（2wt.%n HA+2wt.%n Zn O）/PCL composite coating.Its antibacterial rate exceeds 90%,and the cell proliferation rate is also close to 90%,which can be used as a potential multifunctional medical implant material.Experiments also show that the designed gradient composite coating is not weaker than other double-layer composite coatings in terms of comprehensive performance.At the same time,the composition and structure of the coating can theoretically be adjusted with the use requirements,and the different arrangement and combination of each layer structure of gradient composite coating can make it have adjustable anti-corrosion performance.This provides new ideas and methods for further research on medical materials.