Preparation And Micro-arc Oxidation Surface Modification Of Biodegradable Mg-based Bulk Metallic Glass

Magnesium and its alloys,due to their excellent biocompatibility,density and mechanical properties similar to those of natural bones,stand out among many materials to become the most promising biodegradable bone graft materials.Unfortunately,due to its rapid corrosion rate,it can not guarantee the integrity of the structure at the early stage of implantation,limiting the clinical application of the material.Alloying and surface modification are two effective ways to reduce the corrosion rate of magnesium alloy,the researchers have done extensive studies,and made some progress,but still unsatisfactory.In recent years,the emergence of Mg-based bulk metallic glasses provides a new method to reduce the corrosion rate of Mg-based biodegradable metallic materials.Amorphous alloys have better corrosion resistance than conventional crystalline counterparts because of their chemically homogeneous structure and absence from crystalline defects such as grain boundaries,dislocations and stacking faults.In this paper,two amorphous alloy systems of MgZnCa and MgZnCaSi were designed and prepared,and the surface of the amorphous alloy was treated by micro-arc oxidation to improve the its bioactivity and corrosion resistance.A MgZnCa bulk metallic glass?BMG?with a critical casting size of 5 mm was successfully developed by copper-mold injection casting.The Mg-based metallic glass was subjected to micro-arc oxidation treatment by using Na OH,Na₂ SiO₃·9H₂O,and Na₂B₄O₇·10H₂O as the electrolyte.It was found that the film is composed of Mg O,Ca O,MgSiO₃,and Zn₂ SiO₄ phases.The coating has a porous surface and a dense inner layer and no micro-crack observed resulted from the thermal stress.Compared to the bare BMG,the corrosion potential and corrosion resistance are increased by 101 m V and 3.246×10⁵ ?respectively,and the corrosion density is decreased by two orders.In immersion test,the spontaneous formation of hydroxyapatite?HA?on the porous layer demonstrated that the prepared MAO coating has a high bioactivity.Next,we added a small amount of Si on the basis of MgZnCa ternary alloy to prepare another quaternary alloy system Mg₆₆Zn_29-xCa₅Si_x?at.%,x=0,0.25,0.5,0.75,1?.The results show that when the Si content is 0.5%,the corrosion resistance of the alloy is greatly improved,and the glass forming ability and room-temperature compressive strength are also excellent.In order to further improve the bioactivity of Mg-based amorphous alloy surface,we prepared a micro-arc oxidation ceramic coating containing Ca and P on the surface of Mg₆₆Zn_28.5Ca₅Si_0.5amorphous alloy in an alkaline electrolyte system consisting of Na OH,Ca-Gp,Ca?H₂PO₄?2·H₂O),NH₄HF₂ and EDTA-2Na.The effects of voltage,time and Na OH concentration in electrolyte on the structure and properties of the coating were studied by using the control variable method.Electrochemical tests show that the corrosion current density of Ca,P coating reduced to one-sixth of the matrix,the polarization resistance increased by an order of magnitude.The previously prepared silicon-containing coating can increase the polarization resistance by two orders of magnitude.It is clear that the corrosion resistance of Ca and P coatings is not as good as that of Si-containing coating,but the number and crystallinity of hydroxyapatite induced by Ca,P coatings are better than that of Si-containing coating,suggesting a better biological activity.