Studies On Preparation And Properties Of Micro-arc Oxidation Coating On Biomedical AZ31B Magnesium Alloy

Magnesium (Mg) and its alloys have shown potential as biodegradable bone implants materials due to their mechanical properties similar to the natural bones, good biocompatibility and in vivo degradability via corrosion in the electrolytic environment of the body, attracting the attention of many researchers in recent years. Magnesium is an exceptionally lightweight metal, with a density of 1.73g/cm³, 1.5 and 4.5 times less dense than aluminum and steel, respectively. Moreover, magnesium is an essential nutrient element for the body's metabolism and other vital physiological functions. It is the fourth most abundant cation in the human body, with an estimated 1mol of magnesium stored in the body of a normal 70kg adult, and approximately half of the total physiological magnesium is stored in bone tissue. However, the faster degradation of pure magnesium in the body and earlier failure in chloride containing solutions including the human body fluid has limited its clinical application. Therefore it is important to consider the surface modification to improve the corrosion resistance of magnesium alloys, and the micro-arc oxidation surface treatment was used in this dissertation.Recently, micro-oxidation (MAO), which is also usually called plasma electrolytic oxidation (PEO), is a relatively new and promising surface treatment method used to form an oxide coating on aluminum, titanium and magnesium alloys in a suitable electrolyte with a high anodic applied voltage. MAO has become a hotspot of international research. The MAO coating could improve the corrosion resistance and wear resistances greatly.This dissertation studied AZ31B magnesium alloy with surface micro-arc oxidation treatment, and optimized the micro-arc oxidation process. While micro-arc oxidation of magnesium alloy, the microstructure, phase composition, mechanism of MAO coatings, corrosion resistance and biocompatibility were systematically studied.The main conclusions of the dissertation are summarized as follow: Micro-arc oxidization of magnesium alloys was studied in silicate solutions containing other alkaline solution, NaOH or KOH for example. SEM and XRD analyses were used to examine the morphology and phase constitute of the MAO coatings. SEM observation on both the surface and the cross section of the coating showed that there were a few of circular pores and micro-cracks on the MAO coating surface, and the coating was about 20³0μm thickness and was compact in the inner layer. XRD analysis indicated that the ceramic coatings fabricated on the surface of magnesium alloys by micro-arc oxidization were composed of MgO, Mg₂Si₄ and a few of MgF₂. EDS and XPS analyses indicated that the MAO coatings were mainly composed Mg, O, Si and F.The results obtained by electrochemical tests showed that the corrosion resistance of magnesium alloys after micro-arc oxidation treatment had a greater increase than that of the untreated ones, and the same result could also be seen through the immersion test. After the micro-arc oxidation treated magnesium alloy samples were immersed in a simulated body fluid for some time, the samples surface remained intact without significant damage. Bond strength of the coatings could meet the requirements for medical implant materials. Hemolysis test results showed that the hemolytic ratio of the MAO treated magnesium alloy achieved the requirement for medical implant materials.