Preparation And Properties Research On Micro-Arc Oxidation Ceramic Coating On Pure Magnesium And Mg-Zn Alloy For Biomedical Applications

Pure magnesium and its alloys have excellent biocompatibility and biodegradation which make them the most potential biodegradable implants. However, the degradation rate of pure magnesium and its alloys is so rapid in aqueous solutions, especially in chloride-containing media such as human physiological environments that the implants can be corroded seriously before the diseased or damaged bone tissue becomes healed. Therefore, the inhibition of high corrosion rate is the most urgent problem to be solved for magnesium and its alloys as degradable implants.Current approaches to slow down the degradation rate of magnesium and its alloys include high purity alloys, element alloying and surface coating modification technology. In the present thesis, a protective micro-arc oxidation (MAO) coating has been fabricated on pure magnesium and alloys. The coating can isolate the substrate from corrosive media as a physical shield and inhibit the degradation rate of the metal substrate.The MAO process was performed in aqueous solutions without toxic elements to keep good biocompatibility of MAO coatings. The effects of electrolyte composition, current density, electrolyte temperature and MAO time on corrosion resistance of ceramic coating were investigated systematically. The growth characteristics of MAO coating was investigated in the optimized electrolyte of NaOH (30 g/L), Na2SiO3?9H2O (160 g/L) and Na2B4O7?10H2O (160 g/L) at 20 and 25℃under the condition of 40 mA/cm2.In order to improve the corrosion resistance of MAO coating, some additives such as triethanolamine, calcium oxide, calcium carbonate and hydroxyapatite were added to the optimized electrolyte during MAO process. The results indicate that the additives participate in the growth reactions of ceramic coating. SEM micrographs show that the thickness of the coatings are about 30～50μm. XRD patterns of the coatings show that the coatings are amorphous. After sealing in Na2SiO3 solution, the surface roughness of MAO coating improved and porosity of MAO coating also decreased. The corrosion resistance of Mg and Mg4Zn alloy covered with anodic coating was tested by potentiodynamic polarization tests and immersion tests in simulated body fluid (SBF). The results show that the MAO coatings can efficiently prevent metal substrate from contacting with corrosive media and the corrosion resistance of the samples was improved remarkably.In addition, the immersion tests also show that Mg and Mg4Zn alloy covered with anodic coatings have a good ability for inducing Ca and P deposition.Homolysis tests were used to evaluate the blood compatibility of Mg and Mg4Zn alloy covered with MAO coatings. The results confirm that the hemolysis ratio of the samples was less than 5% and the samples have excellent blood compatibility.