| Medical magnesium and its alloys are called third-generation biomedical materials because of their advantages of excellent biological safety,biocompatibility,density and elastic modulus close to human bones,degradability and absorbability.Owing to its strong chemical activity and poor corrosion resistance,they are degradable in body fluids.Thus,they can degrade spontaneously within a certain period when implanted into the human body,and the patients can avoid the pain and medical costs of secondary surgery.However,the fast degradation rate caused by poor corrosion resistance is also a serious problem that restricts its clinical application.Problems such as large amount of hydrogen evolution,local alkalization around the implant site,and premature reduction or even loss of mechanical strength of the implant material may lead to the failure of the implant surgery and cause new harm to the patients.Based on the above problems,in this paper,bioactive coating containing calcium phosphate was successfully prepared on the surface of pure magnesium by micro-arc oxidation.The MAO coating can control the corrosion degradation rate of pure magnesium and improve its bioactivity.Subsequently,the MAO coating was sealed in order to further improve its corrosion resistance and biocompatibility.First of all,MAO coating containing calcium phosphate was successfully prepared on the surface of pure magnesium in an alkaline phosphate electrolyte containing calcium gluconate,sodium tripolyphosphate,sodium fluoride and sodium hydroxide.The effects of current density on the surface morphology,thickness,phase composition,chemical composition and corrosion resistance of the MAO coatings were systematically studied.And the formation mechanism of the MAO coatings containing calcium phosphate was also discussed.The results show that the surface pore diameter,roughness and thickness of the coating gradually increase with the increase of current density,and the coating fabricated at appropriate medium current density(0.06 A/cm2)has a relatively uniform and dense structure.In addition,all MAO coatings are mainly composed of Mg O,Ca F2,Ca2P2O7 and Ca3(PO4)2 phase,indicating that the change of current density will not influence the chemical elements,phase and functional group compositions of the coatings.The MAO coatings formed at different current densities all improve the corrosion resistance of the pure magnesium substrate,and the most excellent corrosion protection was associated with the MAO coating fabricated at the current density of 0.06 A/cm2with the lowest corrosion current density value(6.05×10-7 A/cm2),hydrogen evolution rate(0.061 m L·cm-2·day-1)and the highest impedance modulus,which can be attributed to its internal dense barrier layer and relatively uniform dense structure.After soaking in simulated body fluids(SBF)solution for two weeks,the MAO coating began to degrade,and the content of precipitate containing calcium phosphate salt on the surface of the sample gradually increased with prolonging the immersion time.This was a dynamic equilibrium process in which the Ca and P elements were deposited continually while the MAO coating continued to dissolve constantly.Throughout the 28 days immersion period,the weight loss rate of the MAO-0.06 coating sample,as well as the p H and Mg2+concentration value of the immersion solution were lower than the pure magnesium,but the Ca and P contents were more than the pure magnesium,which indicating that the MAO coating containing calcium phosphate can not only control the degradation rate of pure magnesium but also increase its bioactivity.The existence of micropores on the surface of the MAO coating make the corrosive medium easily to penetrate and destroy the internal dense layer and substrate,so three different concentrations of polylactic acid are used to seal the single porous MAO-0.06coating.The results show that the micropores are completely sealed by polylactic acid,and the roughness and corrosion resistance of the coating are increased.Among them,the 6%concentration MAO/PLA composite coating exhibits the best corrosion resistance.Compared with the pure magnesium sample,the corrosion current density(9.90×10-8 A/cm2)is reduced by nearly four orders of magnitude,and is one order of magnitude lower than that of the MAO-0.06 coating. |
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