Microstructure And Properties Of Mg-Zn-Ca-Mn Alloys And Micro-arc Oxidation Coatings For Biomedical Application

As a biodegradable light metal with superior physical and mechanical properties, similar Young’s modulus to that of human bone which can prevent stress shielding effect, and nontoxicity in human body, magnesium has the potential to serve as biocompatible implants. Also, the patients won’t have to bear a second surgery for the degradability of magnesium and its alloys. But the poor corrosion behavior makes magnesium and its alloys limited as biomedical implants. Alloying and surface modification are effective methods to improve the corrosion resistance of magnesium. Micro-arc oxidation (MAO) is considered to be an effective surface modification treatment to improve the corrosion resistance of magnesium and its alloys by producing a layer of protective coating. The properties of the MAO coatings mainly depend on the substrate constitution, electrolyte composition and electric parameters.In this study, two different Mg-Zn-Ca-Mn alloys with different manganese (Mn) contents were prepared and the effect of Mn element on the microstructure* mechanical properties and corrosion resistance of the Mg-Zn-Ca-Mn alloys was-studied as well. Then MAO coatings were prepared and the effect of substrate constitution on the microstructure and corrosion resistance of the MAO coatings was studied. According to the above researches, one of the Mg-Zn-Ca-Mn alloys with better comprehensive properties was chosen as the substrate of the following MAO experiments. In order to prepare MAO coatings containing calcium (Ca) and phosphorus (P) compounds with better bioactivity, three different calcium compounds (calcium citrate ((C6H5O7)2Ca3-4H20), calcium acetate ((CH3COO)2Ca), calcium gluconate (Ca(C6H11O7)2·H2O)) and three different phosphate compounds (sodium phosphate (Na3PO4), sodium hexametaphosphate ((NaPO3)6), sodium hydrogen phosphate (Na2HPO4)) were chosen to prepare nine different electrolytes. The microstructure, corrosion resistance and biodegradation behavior of the MAO coatings prepared in different electrolytes were studied.Results showed that the increment of Mn content in the as-cast Mg-Zn-Ca-Mn alloys contributed to grain refinement, which further improved the tensile strength and elongation. However, the effect of Mn on the corrosion resistance of the magnesium alloys wasn’t obvious. Mg-2Zn-1Ca-0.2Mn alloy with lower Mn content showed better corrosion behavior even with coarser grain. That can be attributed to less amounts of grain boundary which is prone to corrosion. Homogenously distributed micro-pores were observed on the surface of Mg-2Zn-1Ca-0.8Mn alloy. The MAO coating on the surface of Mg-2Zn-1Ca-0.2Mn alloy was coarser and inhomogenously distributed. The corrosion resistance of the MAO coated magnesium alloy was improved as the Mn content in the substrate was increased.The effects of electrolyte composition with different calcium and phosphate compounds on the microstructure and biodegradation behavior of MAO coated Mg-2Zn-1Ca-0.8Mn alloy were studied. Results showed that Ca(C6H1107)2-H20 and (NaPO3)6 can accelerate the formation of Ca and P compounds, increasing the contents of Ca and P in the coatings. The MAO coating prepared in the electrolyte with (CH3COO)2Ca and Na2HPO4 showed the best corrosion resistance for its compact microstructure. The Simulated body fluid (SBF) immersion tests showed that in the early stage of immersion, the MAO coated alloys prepared in the electrolyte with Ca(C6H11O7)2-H2O, (NaPO3)6 or Na2HPO4 exhibited good biodegradability, which would be beneficial to the growth of bone tissue. The pH values of the SBF with the MAO coated alloys prepared in the electrolyte with Ca(C6H1107)2·H2O, (CH3COO)2Ca, (NaPO3)6 or Na2HPO4 were relatively stable and almost below 8.0 during the whole immersion test, which demonstrated better biosafety. After immersion in SBF for 21 days, deposits containing Ca and P of different shapes were detected on the surface of both MAO coated and uncoated magnesium alloys. The Ca and P compounds deposited on the MAO coated alloys prepared in the electrolyte with Ca(C6H11O7)2-H2O and (NaPO3)6, (CH3COO)2Ca and Na2HPO4 were demonstrated to be Ca3(PO4)2 and Hydroxyapatite (HA), indicating better bioactivity.