Study On The Influence Mechanism Of CO₂ Gas On The Micro-nanostructure Of Mg-Nd-Zn-Ca Surface MAO Coating

Magnesium alloy has good biocompatibility and elastic modulus similar to human bone,which is widely used in the field of biomedical materials.However,the degradation rate of magnesium alloy in the human body is too fast,which seriously limits its widespread use and popularity.Modifying magnesium alloy by surface treatment technology to obtain a composite functional coating with biocompatibility and corrosion resistance is a hot issue in biomedical materials in recent years.The micro-arc oxidation method can generate in situ ceramic film with high bonding force,dense and smooth,high hardness,wear resistance,and good corrosion resistance on the metal surface,which has received wide attention in the field of magnesium alloy surface modification.Numerous studies have shown that the morphology and properties of the ceramic coating can be changed by adjusting the electrolyte composition,additive type,and electrical parameters to meet different application requirements.Among them,the flexible and easy adjustment of solution additive composition is the key and breakthrough point for functional adjustment of the coating.Previous studies have mostly used solid or liquid additives to change the electrolyte composition and thus influence the film formation,but the role of gas additives in the micro-arc oxidation process has rarely been reported.In the film reaction,a large number of gas precipitates from the metal surface,which has a significant effect on the film formation quality.When gas is introduced into the solution,the interaction between self-generated and applied gas on the film formation must be explored.At the same time,the effects of gas additives and conventional solid additives on the coating of micro-arc oxide ceramics also need to be studied and revealed.The study of these problems has important theoretical significance and technical value.In this paper,a CO₂ gas-assisted micro-arc oxidation method is proposed to prepare composite coatings,i.e.,CO₂ gas is introduced from the outside while micro-arc oxidation is in progress,and the study reveals the mechanism of the influence of CO₂gas on the initial structure of the coatings and the influence law of corrosion resistance of different coatings;on the basis of gas-assisted micro-arc oxidation,micro-nano hydroxyapatite（HA）is then added to the electrolyte,and it is clear that the gas and micro-nano On the basis of this,the composite film was prepared on the Mg-Nd-Zn-Ca surface by hydrothermal reaction of different specimens,and the mechanism of the combined effect of CO₂-assisted micro-arc oxidation and post-treatment on the microscopic morphology,phase composition,chemical composition and electrochemical properties of the film was elucidated.As a result of the study,the following main conclusions were obtained:（1）Study on the growth mechanism of CO₂ involved in Mg-Nd-Zn-Ca micro-arc oxidation coatingComposite coatings were prepared in a single Na OH solution by introducing CO₂ gas in the micro-arc oxidation process.The results show that CO₂ can participate in the micro-arc oxidation process and its introduction helps to form Mg（OH）₂ on the coating surface.however,the electrolyte composition is too simple,the generated Mg（OH）₂ is loose and porous,and the short reaction time leads to a thin coating thickness,which cannot protect the substrate effectively in the long term.the introduction of CO₂ is beneficial to the coating growth,but the long pass time leads to microcracks on the outside of the coating.The collision of CO₂ molecules with electrons moving at high speed under plasma discharge conditions generates active CO₂^*which reacts with Mg²⁺to form nano-Mg CO₃,thus improving the corrosion resistance of the coating.Since the amount of active CO₂^*is limited,the amount of Mg CO₃ produced is also small.At the early stage of the reaction,the introduction of CO₂ is beneficial to improve the corrosion resistance of the coating,while at the later stage of the reaction,the gas blocks the pathway for electrical conductivity,and the current path becomes narrower,resulting in a loose and porous coating resulting in reduced corrosion resistance.（2）Mechanistic study of the effect of CO₂ and HA together on the microstructure of MAO coatingWhen CO₂ was introduced together with micro-and nano-hydroxyapatite,the coating mainly contained Mg O,Mg（OH）₂,HA,and a small amount of Mg CO₃.In a single Na OH solution,the joint action of gas and micro-and nano-hydroxyapatite was not conducive to the growth of the coating and produced a large number of cracks on its surface.In Na₂Si O₃+Na OH solution,the porous morphology typical of micro-arc oxidation starts to appear on the surface of the coating,and the passage of CO₂ can close the holes left by micro-arc oxidation to a certain extent.In addition,CO₂ will generate CO₃^2-and HCO₃^-under alkaline conditions,and part of CO₃^2-and HCO₃^-will replace or fill the defective parts of hydroxyapatite crystals,resulting in smaller crystal size,reduced crystallinity,and increased lattice distortion,and then the crystal structure of hydroxyapatite is damaged,and this part of replaced hydroxyapatite exists more in the electrolyte,and only a small part is involved in the growth of the coating,a large amount of CO₂ preferentially reacts with hydroxyapatite resulting in less Mg CO₃ generated,thus reducing the corrosion resistance of the coating.（3）Mechanistic study of the effect of hydrothermal treatment on the involvement of CO₂ gas in MAO coatingsAfter the hydrothermal reaction,the coating prepared by CO₂ gas-assisted micro-arc oxidation generates a dense layer dominated by lamellar Mg（OH）₂ on the surface of the coating,and partially closes the holes left by the micro-arc oxidation discharge,which can prevent contact between the substrate and the corrosive medium to a certain extent,and the Mg（OH）₂ on the outside of the coating and Mg CO₃ on the inside work together to improve the overall corrosion resistance of the coating.In contrast,the coating prepared in a single Na OH solution is poorly bonded to the substrate and peels off during the hydrothermal reaction and other phenomena,leading to a decrease in the corrosion resistance of the coating.The coating is prepared by passing CO₂ in Na₂Si O₃+Na OH solution after a hydrothermal reaction,on the one hand,a large amount of Mg（OH）₂ will be generated on the surface,which seals the pores and reduces the channels for the corrosive medium to enter into the coating/Mg substrate interface;on the other hand,Mg（OH）₂ can extend the path for the corrosive medium to enter into the channel,delaying the corrosive medium such as Cl^-ions to the substrate,thus improving the corrosion resistance of the substrate.