| Magnesium(Mg)and its alloys,as biodegradable metals,have attracted great interest as promising implants in orthopedic application by virtue of their biodegradability excellent biocompatibility and mechanical compatibility.Unfortunately,poor corrosion resistance as an intrinsic response of these metals with corrosive fluids in body,leading to a huge hydrogen release,the increase of local pH value and a quick decline of mechanical integrity and,hinders their clinical application.The main approach to improve the corrosion resistance is alloying and surface treatment.The surface treatment includes chemical conversion,anode oxidation,micro arc oxidation(MAO),electrochemical deposition,sol-gel,ion implantation,laser surface modification,organic coating and so on.Contrasted with others method,MAO coating possesses high hardness,corrosion resistance,insulativity,good wear resistance.The method is easy to operate and the equipment used is rather simple.However,micropores and microcracks exist on the surface of MAO coatings,which provides corrosion channels for the corrosive medium and further studies are required.Therefore,MAO sealing is the main method to improve the defect,and the main sealing methods include sol-gel,organic polymer,electrophoresis and so on.In this article,Mg-Zn-Ca alloys with different Ca content were prepared by using Mg-1.75Zn alloy as the matrix,and the Ca elements with good biocompatibility were selected as alloying elements.MAO coatings containing Ca-P was prepared on Mg alloy surface.The effect of voltage and different content in the electrolyte of glycerin calcium phosphate on the microstructure,phase composition,binding force,corrosion resistance and in vitro hemolysis rate of MAO coatings were studied.In this paper,the structure,phase composition,degradation property and corrosion resistance of the MAO containing the Ca-P and Si were studied,and the mechanism of the difference between the two coatings was analysed.The MAO coating containing Ca-P-Si was prepared by a two-step method,and the structure and properties of the MAO coating containing Ca-P were compared.The formation mechanism of MAO coating was discussed.On the basis of this,sol-gel and chitosan were respectively used to seal the MAO coating,and the influence of the dipping times on the MAO coating was analysed.The results show that the Ca element has the effect of refining the grain of Mg alloy.The hardness,tensile strength and yield strength increased with the increase of Ca element content,and then decreased with the first increase,and reached the maximum when the Ca content was 0.97wt.%.The simulated body fluid(SBF)immersion test showed that adding Ca elements,the weightlessness rate decreased with the increase of Ca content,but it was larger than the alloy that Mg-1.75Zn alloy.This is mainly due to with the increase of Ca content,the continuous enhancement of the network structure formed by Ca2Mg5Zn13 in Mg alloy,which impedes the corrosion process.However,with the addition of Ca elements,the grain refinement,grain boundary and grain number increased,which resulted in the increase of the corrosion point of Mg alloy and accelerated the corrosion of Mg alloy.Along with the rising of the voltage preparation containing Ca-P MAO keeps increasing,the content of calcium phosphate in the thickness of the coating is also increasing,voltage by influencing the discharge energy and the electric field change of membrane layer structure.The thickness of the MAO prepared under 450V is 30μm,and the pore diameter of the coating is 3-10μm.The immersion and electrochemical experiments show that the MAO coating containing calcium phosphate has biological activity and can effectively improve the corrosion resistance of Mg alloy.The coating prepared in 450V has the best corrosion resistance.With the increase of the content of glycerin phosphate in the electrolyte,the surface of the MAO is becoming smoother,and the average pore diameter of the micropore is smaller,and the adhesion between the MAO coating and the matrix is also increasing.When the content of calcium glycerophosphate was 7.5g/L,the binding force of the MAO coating and the matrix was 20.46N.The SBF immersion test results showed that the pH value of the MAO coating was not increased sharply.Due to the increase of calcium and phosphorus content in the MAO coating,the weight loss rate decreased with the increase of the content of glycerin phosphate in the electrolyte.A large amount of calcium and phosphorous deposits appeared on the surface of the Mg coating.The hemolytic test and the rabbit heat test showed that when the content of calcium phosphate was 7.5g/L,the coating met ISO 10993-4:2002 standard.Compared with the MAO coatings prepared in the Ca-P electrolyte,the Si coating thickness is thinner and the micropores are smaller and the microcracks are less.Due to the relatively few microcracks and small micropores on the MAO coating containing Si,the corrosion resistance in the SBF solution was better than that of the MAO coating containing the Ca-P.The MAO coating containing Ca-P has better corrosion resistance in prolonged soaking because of its better bioactive Ca-P phase.MAO coating containing Ca-P-Si was prepared on the surface of Mg-Zn-Ca alloy by a two-step method.The first step in the preparation MAO coating containing Ca-P and the second step preparation containing Ca-P-Si MAO coating has similar surface and cross-section morphology,but MAO coating containing Ca-P-Si has a thicker layer of film thickness and more tiny pores.Through EDS,XRD and FT-IR analyse,the MAO containing Ca-P-Si was confirmed to contain calcium phosphate and magnesium silicate.Electrochemical and immersion tests show that the MAO coating containing Ca-P-Si can significantly reduce the corrosion rate and have excellent biomineralisation ability.Through the analysis of two-step micro arc oxidation,the formation process of MAO coating is a process of continuous destruction of the old coating and the continuous growth of the new coating.The microstructure analysis showed that the micro-crack of the composite film was reduced with the increase of the number of dipping.The composite coating was obtained by dipping and pulling three times in the sol,and its surface was uniform and the crack was less.Because sol can enter the MAO pores,effectively slowed the MAO coating in the early immersion in SBF solution,pH rapidly rising phenomenon,effectively reduce the soak of degradation rate.In the case of chitosan acetic acid solution,the composite coating was obtained twice,and its surface was smooth and uniform without cracks.This effectively prevents the corrosion ions from entering the micropores of the MAO coating and improves the corrosion resistance of the MAO coating. |
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