Study On Microstructures And Properties Of Biomedical Mg-Zn-Ca Alloys And Micro-arc Oxidation Coatings

Magnesium alloy is a lightweight metal with higher specific strength and specific rigidity.Compared with metal materials widely used in clinical application,its elastic modulus is closer to human bone,which can reduce the stress shielding effect.Besides,it is degradable and can reduce the pain caused by the second surgery of the implant.As a biomedical material,it has a good application prospect.However,the mechanical properties of magnesium alloy are poor and the degradation rate is too fast to match with healing time of the bone tissue,which causes its osteogenesis is difficult to exert and greatly limits the clinical application of magnesium alloys.Among the surface treatment methods for reducing the degradation rate of magnesium alloys,micro-arc oxidation treatment has the advantages of simple process,easy operation,environmental protection,and better performance of the prepared film layers.In this paper,the as-cast Mg-2Zn-0.1Ca-0.2Sr and Mg-3Zn-0.1Ca alloys are selected to accept heat treatments include solid solution treatment and aging for different times and then morphology observation,microstructure analysis and mechanical performance test are adopted to study the microstructure and mechanical properties of Mg-Zn-Ca alloys after different heat treatments.The study shows that Mg-2Zn-0.1Ca-0.2Sr alloy is mainly composed of a-Mg matrix and a small amount of Ca2Mg6Zn3 phase after solution treatment at 360? for 4 h.After aging at 210 ? for different time,the amount of precipitated phase Ca2Mg6Zn3 increased in Mg-2Zn-0.1Ca-0.2Sr alloy,compared with that of solution treatment.When the aging time is 13 h,the comprehensive mechanical properties of the alloy are optimal.Its tensile strength reached 175.53 MPa,increased 18.86%compared to that of the solid solution state and the elongation reached 9.82%,increased 24.78%compared to that of the solid solution state.While its elastic modulus was reduced to 23.69 GPa,which was 26.57%lower than that of the solid solution state,and the strength and plasticity of the alloy were improved at the same time.In the Mg-3Zn-0.1Ca alloy,in addition to the Ca2Mg6Zn3 phase,a small amount of MgZn and MgZn2 phases also appeared.With the increasing of aging time,the tensile strength of the alloy remained stable,but its plasticity has been significantly improved.When the aging time is 24 h,the tensile strength and elongation of the alloy are optimal.Thereinto,the elongation reached 16.71%,which was 44.93%higher than that of the solid solution state.The comprehensive mechanical properties of Mg-3Zn-0.1Ca alloy are better than those of Mg-2Zn-0.1Ca-0.2Sr alloy after the same heat treatment.In the silicate and the phosphate solution systems,coatings were prepared on the surface of Mg-3Zn-0.1Ca alloy with good mechanical properties after solution treatmentby the micro-arc oxidation method,and the effects of different processing voltages,electrolyte components,and concentrations of components on the morphologies and their corrosion performances in simulated body fluids were studied.The study shows that in the silicate and phosphate system electrolytes,the micro-arc oxide coatings on the surface of the Mg-3Zn-0.1Ca alloy both own a porous structure,while the coatings prepared in phosphate system electrolytes have a larger difference in pore size,a larger pore distribution distance and a lower flatness,but the coatings are thicker.In the silicate system,the coatings are composed of MgO,MgSiO3,and a small amount of Mg?SiO4-After micro-arc oxide treatment in the basic silicate electrolyte at differnt processing voltages,the corrosion current densities of the Mg-3Zn-0.1Ca alloy are all reduced by 1 to 2 orders of magnitude,compared with those before micro-arc oxide treatment.When the micro-arc oxidation processing voltage is 350 V,the coatings obtain the best corrosion resistance.When the micro-arc oxidation process in the electrolytes of different concentration at same voltage(350 V),as the concentration of NaOH in the electrolyte increasing,the corrosion tendency and the corrosion current density first increase and then decrease and the corrosion resistance first decreases and then increases.When the concentration of NaOH is 6 g/L,the corrosion resistance is best.As the concentration of Na2SiO3·9H2O in the electrolyte increasing,the corrosion tendency and the corrosion current density increase and the corrosion resistance decreases.When the concentration of Na2SiO3·9H2O is 20 g/L,the corrosion resistance is best.In the phosphate system,the coatings are composed of MgO and Mg3(PO4)2.After micro-arc oxide treatment in the basic phosphate electrolyte at differnt processing voltages,the corrosion potential increases,the corrosion current density is reduced by 1 to 2 orders of magnitude,compared with that before micro-arc oxide treatment.When the micro-arc oxidation processing voltage is 340 V,the coatings obtain the best corrosion resistance.When the micro-arc oxidation process in the electrolytes of different concentration at same voltage(340V),as the concentration of NaOH in the electrolyte increasing,the corrosion tendency of the micro-arc oxide coatings in the SBF solution increases lightly and is relatively stable,thus the influence of the NaOH concentration on the corrosion tendency is light.As the concentration of Na3PO4·12H2O in the electrolyte increasing,the corrosion tendency and the corrosion current density increase and the corrosion resistance decreases.When the concentration of Na3PO4·12H2O is 20 g/L,the corrosion resistance is best.In summary,the coatings prepared in phosphate system electrolytes have lower corrosion tendency and better corrosion resistance.