| Since the 1990s, the biodegradable implant, instead of the traditional biostable material, has been at the forefront of the metal based medical engineering world and has drawn unto itself significant attention. The application of magnesium and its alloys as potential biodegradable and bioabsorbable medical implants has gained significant attention in the field of novel metallic implants. However, the use of magnesium and its alloys as biomedical implants is still limited due to their latent toxic effects and rapid corrosion rate. Element alloying and surface modification are two effective ways to improve the corrosion resistance and biosafety of magnesium and its alloys. Element alloying can be used to improve the mechanical property and reduce the corrosion rate of magnesium alloys. Surface modification can be employed to control the degradation behavior and improve the surface biocompatibility of magnesium alloys. Among those surface modification technologies available for the preparation of magnesium based implants, micro-arc oxidation (MAO) is one of the most prospective methods for this application. The method of MAO has a lot of advantages over other surface treatments, e.g. low energy consumption, easy controlling for processing, ecology-friendly process and products, excellent adhesion between coating and substrate, and biocompatible and bioactive ceramic coatings with unique features including inner compact, outer porous, anticorrosive, etc. properties can be obtained by MAO technique.This paper aims to control the degradation rate, biocompatibility and bioactivity of magnesium based implant from two aspects, preparation of novel magnesium alloys and MAO treatment on these alloys. Firstly, KF, NH4HF2, C3H8O3 and H2O2 were added into the silicate electrolyte one by one. MAO coating with different performance were formed on ZK60 magnesium alloy. The effects of the additives on discharge phenomenon, coating appearance, phase composition and corrosion resistance were evaluated and a modified additive formula was obtained. Then three different phosphates (Na2HPO4·12H2O, Na3PO4H2O, (NaPO3)6) were added to the CH3COO)2Ca+NH4HF2+KOH+C3H8O3+H2O2 electrolyte system with equal calcium and phosphorus concentration ratio (Ca/P). The effects of these different phosphates on coating thickness, surface morphology, phase and elemental composition, corrosion resistance, bioactivity and degradability were studied. Bioceramic coatings were successfully prepared on the surface of ZK60 alloy by MAO in Ca- and P-containing electrolytes with different Ca/P ratio. The effect of negative voltage on microstructure and corrosion behavior of MAO coating was studied. Secondly, nutrient elements like Ca, Zn and Si with good biocompatibility were added into pure Mg to prepare novel Mg-Ca, Mg-Zn-Ca and Mg-Zn-Ca-Si alloys. Then CaP coatings were prepared on these magnesium alloys. The effect of alloying element Ca, Zn, Si on the microstructure, mechanical property, functional group, degradation behavior and in vitro bioactivity of magnesium alloys and their surface MAO coatings were studied. Besides, the formation and growth mechanisms of the MAO coating were studied. The dissolution and precipitation behaviors of the Si-containing coating in simulated body fluid (SBF) solution were investigated.Results showed that both KF and NH4HF2 promote discharge and accelerate reaction. C3H8O3 can effectively reduce the poor thermal effect produced by oxidation reaction as stabilizer. The introduction of H2O2 results in oxygen libration and makes coating surface rough. The CaP coatings mainly consist of MgO, MgF2, ZnO, ZnF2, CaO, CaF2 and Ca3(PO4)2 in spite of different phosphates in electrolyte. The formation and growth mechanism are influenced by formation of accumulation layer of anions at the nearest region to the coating in electrolyte. The fluoride released by samples immersion in SBF does not cause observed adverse effect.Pores with different shapes distribute all over the surface of CaP MAO coating prepared in electrolytes with different Ca/P ratio. The diameter of the pores, surface toughness and thickness of the coating and particles around the pores tend to increase with increasing Ca/P ratio. The coatings formed in higher Ca/P ratio electrolyte exhibit better apatite-inducing ability and slower degradation rate. The in vitro tests in SBF solution results indicate that the coatings have excellent bioactivity. The in vitro cytotoxicity tests and systemic toxicity texts demonstrate that the coating prepared in electrolyte with Ca/P ratio 3:1 have no toxicity to cell and living animal, indicating that the coatings are safety to serve as implants. The introduction and increase of negative voltage have some effects on the ions transferring in the oxidation process and degradation in SBF soaking. With the increase of negative voltage, the coating electrochemical corrosion property is improved, the coating degradation is delayed, and the combination property of coating corrosion resistance and apatites forming ability is enhanced. To some extent, the negative voltage has an effect on the controllability of MAO coating degradability in SBF in this study.Ca and Zn are effective grain refiners in pure Mg, especially for Ca. Adequate amounts of Ca and Zn elements can be added to pure Mg to control the amount and distribution of the second phases in alloys to improve the mechanical properties and degradation rate of magnesium alloys. Among all four binary Mg-xCa (x=0.05～1.56) alloys, Mg-0.60Ca alloy exhibits the highest corrosion resistance. Among all four ternary Mg-xZn-Ca (x=1.74～5.49) alloys, Mg-1.74Zn-0.55Ca alloy exhibits the best corrosion resistance. The corrosion resistance of MAO-coated pure Mg, Mg-0.6Ca and Mg-1.74Zn-0.55Ca samples is significantly better than that of bare alloys in SBF immersion tests, and the MAO coating provides the most effective protection for Mg-1.74Zn-0.55Ca alloy.When both Ca and Si elements were added into Mg-Zn-Ca-Si magnesium alloy, Ca will react with Si preferentially to form Ca-Si phase, so the coarse second phase Mg2Si can transform into homogeneously distributed fine particles, and the mechanical properties of magnesium alloy are improved significantly. With the increase of Si content in Mg-Zn-Ca-xSi (x=0.33,0.49,0.83,1.10) alloys, The corrosion potential of the alloys decreased basically, and the corrosion current density increased gradually, so the Si content in Mg-Zn-Ca-xSi alloys should not be too high. Among all four quaternary Mg-Zn-Ca-xSi alloys, Mg-2.06Zn-0.58Ca-0.83Si alloy exhibited the best corrosion resistance in Tris-HCl and SBF immersion test. The corrosion resistance of MAO-coated Mg-Zn-Ca-xSi is significantly better than that of bare alloys. The weight loss percentages for all MAO-coated Mg-Zn-Ca-xSi samples were lower than 6.5%, especially for the MAO-coated Mg-2.06Zn-0.58Ca-0.83Si, which presented the lowest weight loss percentage (4.48%) and degradation rate (0.024mg-cm’2-h"1)after 18 days immersion in SBF. So the MAO-coated Mg-2.06Zn-0.58Ca-0.83Si sample shows the best corrosion resistance, even better than that of MAO-coated pure Mg.The formation and breakdown of gas film at the initial stage of MAO process and ion transferring direction and speed are important factors influencing the coating formation mechanism. The electrochemical corrosion resistance changes with the prolongation of SBF immersion time, which is associated with the formation and dissolution of apatites. The consumption of H+ ions caused an increase of pH value at the nearby area of the coating, thus Ca2+ ions were attracted to the interface, which in turn led to the attraction of HPO42-, PO43- to form apatites eventually. Once the nuclei formed, they grew gradually at the expense of Ca2+, HPO42- and OH- ions absorbed in apatites/solution interface.The formation of apatites on Si MAO ceramic coating was faster than that on the CaP MAO ceramic coating in the SBF solution. The SiOxn- groups play an important role in accelerating the formation of apatites. The SiOxn- groups in the frontier liquid-solid interface make the whole interface negatively charged, positively charged Ca2+ ions are attracted to the interface, and the whole interface becomes positively charged, then OH-, PO43- and HPO42- ions are attracted to the interface as well. With the increase of ions concentration and interface specific area, the dissolution-precipitation reaction will reverse to precipitation. The SiOxn- groups in Si-containing MAO coating can induce the heterogeneous nucleation of amorphous HA by sorption of calcium and phosphate ions in SBF solution. |
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