| Recently, magnesium (Mg) and its alloys have attracted much attention as potential biodegradable implant materials owing to their good biocompatibility and better mechanical properties combination such as high strength and the elastic modulus close to that of natural bones, compared with those of biodegradable polymers. However, problems such as alkalization, hydrogen release and high concentration of magnesium ions, caused by high corrosion rate in body's fluid, will affect their clinical applications. Rare earth conversion coating, manganese oxide conversion coating, eletrogalvanizing zinc coating, fluoride conversion coating were respectively prepared on the surfaces of AZ31B magnesium alloy in order to better control the degradation of the alloy and further improve its biocompatibility. Surface characterizations, corrosion resistance and biocompatibility of the different coatings were systematically studied. The corrosion resistance and biocompatibility of fluoride conversion coating were specially studied by both in vitro and in vivo tests. The main conclusions of the dissertation were summarized as following:(1) In order to improve the corrosion resistance of biodegradable AZ31B magnesium alloy, a rare earth conversion coating was prepared on the surface of AZ31B alloy. An orthogonal experiment was designed to optimize the processing parameters for formation of dense conversion coating. The SEM study revealed that the thickness of the coating was 2-4μm. The XRD analysis indicated that the composition of the coating was composed of CeO2 and MgO. It was found that the coating prepared from optimized processing possessed good corrosion resistance in both of physiological saline and SBF, as well as good anti-clotting property.(2) A manganese oxide contained coating was prepared on biodegradable AZ31B magnesium alloy in order to control the degradation of AZ31B and improve its biocompatibility. Morphology, composition and corrosion resistance of the coating were studied. The SEM observations showed that the coating was approximately 4-6μm in thickness with net-like microcracks. The XPS analysis indicated that the coating was mainly composed of MgO, Mg(OH)2, MnO2, Mn2O3 and Mn3O4. It was found that AZ31B with such coating behaved better corrosion resistance in SBF through electrochemical and immersion tests. The hemolytic assay indicated that the treated AZ31B had no hemolytic effect.(3) An eletroglavanizing zinc coating was prepared on the surface of AZ31B alloy by pulse plating treatment. XRD and SEM analyses were used to examine the composition and morphology of the zinc film. The zinc coating was compact and bonding well to the substrate of AZ31B. The deposition of Ca-P on the surface of zinc coating revealed that the coating behaved bioactivity to a certain extent. Immersion and electrochemical tests indicated that the zinc coating could not effectively improve the resistance of AZ31B and not be suitable to be singly used as the protectable coating for magnesium alloy.(4) A compact fluoride conversion coating was prepared on AZ31B magnesium alloy by reaction with hydrofluoric acid. The SEM observation showed that a compact film with some irregularly distributed pores was formed on the surface of sample. The TF-XRD and XPS analyses indicated that the coating was mainly composed of MgO and MgF2. The relationship between treatment parameters and anti-corrosion property or thickness of the coating was systematically studied. Electrochemical and immersion tests revealed that the corrosion type of fluoride treated AZ31B was homogenuous corrosion and the fluoride conversion coating could effectively improve the corrosion resistance of AZ31B in SBF. The corrosion type, combined with the anti-corrosion property, could ensure a constant mechanical property of AZ31B in the corrosion environment. In vitro biocompatibility tests showed that fluoride treated AZ31B had superior cell compatibility, blood compatibility and antibacterial capability. In vivo study indicated that fluoride treated AZ31B exhibited better corrosion resistance than the bare one. The pathological examination demonstrated that the fluoride treated AZ31B exhibited significantly good osteogenesis, which would benefit the early healing process of bone tissue. |
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