Fabrication, Structure And Property Of Bioactive Ceramic Coating On Magnesium Alloy

Magnesium (Mg) is an essential element to the body and beneficial to the growth of new bone tissue. Furthermore, Mg alloy is a distinctive lightweight metal, whose compressive yield strength, density and elastic modulus are very closed to those of natural bone. Thus, Mg alloys is one kinds of promising implant material in clinic application. Unfortunately, the major weakness of Mg alloy is low corrosion resistance. In that the corrosion rate is very quick in the chloride environment in body. So, the improvement of the corrosion resistance of Mg alloy is an urgent task at present.In this thesis, the ceramic coatings were firstly produced on AZ91D Mg alloy by micro-arc oxidation (MAO) technique in three different electrolytic solutions. Based on this, the hydroxylapatite (HA) coating was subsequently prepared jointly by electrodeposition technique and alkaline treatment. The morphology, microstructure and composition of the prepared ceramic coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The bonding strength between ceramic coatings and AZ91D Mg alloy were evaluated with a MFT-4000 tester. The corrosion properties of coatings were investigated with a potentiodynamic polarization tester. Moreover, the MTT test was performed to investigate the proliferation of osteoblasts on the different sample surfaces. The main results are drawn as follows:The SEM images indicate that the surface morphologies of oxidation coatings are different in different electrolyte systems. The MAO coating on AZ91D Mg alloy in Na2SiO3-KOH electrolytic solution has a typical appearance with many micropores and the size of pores is at the range of 1～10μm; the MAO coating formed in Na3PO4-NaOH electrolytic solution displays some micropores and microcracks on the surface and the size of pores is smaller; the coating on AZ91D Mg alloy in NaAlO2-KF electrolytic solution has a diversification structure with many strips on the surface. After electrodeposition and alkaline treatment, a flake-like structure is grown on the MAO coating. The XRD results demonstrate that the MAO coating on AZ91D Mg alloy in Na2SiO3-KOH electrolytic solution is mainly composed of MgAl2O4,Mg2SiO4 and MgO; the coating formed in Na3PO4-NaOH electrolytic solution is mainly composed of MgAl2O4, MgO and Mg; while the coating formed in NaAlO2-KF electrolytic solution is mainly composed of MgAl2O4, MgO and Mg. After electrodeposition and alkaline treatment, the obtained coating is mainly made up of HA. The bonding strength between ceramic coatings and AZ91D Mg alloy after being oxidated for 15 min is the biggest among the prepared samples. The corrosion resistance of ceramic coating formed by MAO technique or the combining treatments of MAO and electrodeposition increases obviously. Moreover, it also can be obtained that the corrosion current density for the sample with HA coating is obvious lower than that of MAO coating and two orders lower than that of Mg alloy. The MTT test indicates that the cell reproduction rate of each group has statistical significance after being incubated for 2,4 and 6 days (P<0.01). In conclusion, the corrosion resistance of the AZ91D Mg alloy is obviously improved after the treatments of MAO and electrodeposition, and the coating formed by MAO or MAO-electrodeposition is beneficial to the proliferation of osteoblasts, which also has the better biocompatibility to osteoblasts than that of Mg alloy.