Preparation And Characterization Of Micro-arc Oxidation Bio-ceramic Coating On ZK60Magnesium Alloy

With the closest biomechanical properties to those of human bone, magnesium and itsalloys are considered to have ideal biomechanical compatibility among all the metallicmaterials. However, due to the poor corrosion resistance of it, the excessive degradation ratelimits its clinical application. Micro-arc oxidation （MAO）, a new promising surfacetreatment technique, has been significantly focused on in-situ grown particular functionalceramic coatings or strong corrosion resistance on aluminum, magnesium, titanium and theiralloys. In this paper, MAO method was used to prepare lower degradation rate and higherbioactivity bio-ceramic coatings on ZK60Mg alloy under different power modes.MAO process was conducted on ZK60Mg alloy in a basic biologic electrolytecomposed of Na₂SiO₃and CaAc as the main film former by addition of （NaPO₃）6,NaH₂PO₄and C6H5Na3O7respectively under constant current power mode firstly. A optimumcomposite biologic electrolyte system screened by analysis of voltage rise of MAO process,the microstructure and active element content of coating with contrast experiment, whichwas quaternary system Na₂SiO₃-CaAc-（NaPO₃）6-NaH₂PO₄.Then the effects of concentration of electrolyte component in the quaternary biologicelectrolyte system on morphology, corrosion resistance of MAO coatings was studied bymeans of single variable and L9（34） orthogonal experiments. And as a result an optimizedelectrolyte with a composition of6.0g/L Na₂SiO₃+0.8g/L （NaPO₃）6+0.5g/L NaH₂PO₄+0.5g/L CaAc was developed.Microstructure, bioactivity and corrosion resistance of the bio-ceramic coatingprepared by optimized electrolyte were investigated and analyzed by scanning electronmicroscopy （SEM） coupled with an energy dispersive spectrometer （EDS）, X-raydiffraction （XRD） and et al. The results revealed that the growth of the coating presented anallometry mode and the process can be divided into four stages: passivation film formingstage, spark discharge stage, rapid growth stage and periodic arc-discharge stage underconstant current mode. At last the coating with a good uniformity and many micro blindholes on the surface and few holes in the uniform and dense cross-section was fabricated.Bioactive elements Mg, Ca, P and Si were remained in the optimized bio-ceramic coatingand the Ca/P molar ratio of it reached to1.3. Also, the coating consisted of a small amountof Mg2Si and Mg2SiO4phases and some amorphous calcium-phosphorus compounds, which shows a certain biological activity.Subsequently, the bio-ceramic coating were prepared on ZK60Mg alloy by constantvoltage mode, two-stage voltage increasing mode and constant current-constant voltagemode respectively. And a proper power mode was selected by studing effects of forwardvoltage on microstructure and bioactive element content of the coatings with single variableexperiments. The results showed that increasing forward voltage is conducive to thecoating’s thickening and the introduction of bioactive elements. The order of three powermodes that affect the coating from good to bad is constant current-constant voltage mode,two-stage voltage increasing mode and constant voltage mode.Bioactivity of the coating constructed by constant current-constant voltage mode with thefinal voltage of480V was tested in vitro simulated body fluid （SBF） soaking at37℃for7days and14days respectively. The results indicated that some granules were induced on thecoating after7days’ immersion. And calcium-phosphorus compounds with the Ca/P molarratio reached1.3were found on the surface after14days’ immersion which presents goodbioactivity and osteoinduction.