| Because the density of magnesium alloy is close to natural bone,it has the characteristics of good biocompatibility,easy degradation,and low cost.In recent years,it has been favored in the field of implant medicine.However,it is very easy to dissolve in the human body,the corrosion rate is too fast,and the surface needs to be modified.By adjusting the composition and structure of the magnesium alloy surface,the corrosion rate can be controlled,so that the magnesium alloy can be more widely used in bone implantation.Calcium-based phosphate coating has good biocompatibility and is mostly used in biological protective coatings.In this thesis,a composite conversion coating of calcium hydrogen phosphate,hydroxyapatite and polydopamine hydroxyapatite was prepared on the surface of AZ31 magnesium alloy by chemical deposition,hydrothermal,electrochemical deposition and other methods,studied conversion coating morphology,composition and corrosion resistance.CaHPO4 conversion coating was prepared on AZ31 magnesium alloy by chemical deposition method,the growth process and coating formation mechanism of CaHPO4 coating on the surface of magnesium alloy were studied,and the initial stage of CaHPO4 coating growth was innovatively proposed with the growth of Mg(OH)2.The coating is composed of a microsphere structure containing a small amount of Mg(OH)2 formed in the initial stage of the conversion and a lamellar structure CaHPO4 formed in the later stage,and the formed coatingr has better compactness.In the 3.5%NaCl solution,the corrosion potential of the coating is positively shifted by 0.2097 V compared to the substrate,and the corrosion current density is reduced by two orders of magnitude.After being soaked in the human body bionic fluid for 7 days,the coating remains intact.The coating prepared by the hydrothermal method is composed of microspherical hydroxyapatite,which has the strongest corrosion resistance.In a 3.5%NaCl solution,the corrosion potential of the coating is positively shifted by 0.3269 V compared to the substrate,and the corrosion current density is also reduced by 2 orders of magnitude.The electrochemical deposition method is faster to prepare hydroxyapatite.Adding H2O2 to the electrolyte makes the membrane contain spherical structure,and adding NaNO3 makes the membrane surface smoother.Adding the above two additives can greatly improve the compactness and integrity of the coating,and the corrosion resistance is also significantly improved.The corrosion potential is positively shifted by 0.1643 V,the corrosion current density is only 1/50 of the substrate,and the fading time in the dropping test nearly fifty minutes.The polydopamine coating was prepared on the surface of AZ31 magnesium alloy by chemical deposition method.The formed polydopamine coating can evenly cover the magnesium alloy substrate.The polydopamine-hydroxyapatite composite coating was prepared by the secondary chemical deposition method on the polydopamine coating.The hydroxyapatite coating can significantly improve the corrosion resistance of the magnesium alloy.The fading time reaches about one hour,and it has shown optimal protective ability and self-healing ability in the human body bionic liquid immersion experiment. |
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