Study On Microstructure And Properties Of Micro-arc Oxidation Coating Of ZK60 Magnesium Alloy Assisted By Ultrasound

As one of the best degradable materials in metal materials,whose density and elastic modulus match human bones best,magnesium alloy is becoming a research hotspot in the field of orthopedic implants due to its safety and degradability.However,the magnesium alloy has poor biological activity and corrosion resistance.When used as an implant material,structural failure often occurs before the end of the service period and cannot meet the requirements of the implant.Therefore,Using surface modification technology to enhance the biological activity of magnesium alloy and improve its corrosion resistance has become the focus of current research.In this paper,the sodium silicate-calcium-phosphorus system biological electrolyte and constant voltage mode were used to prepare a micro-arc oxidation（MAO）coating on the surface of the ZK60 magnesium alloy.First,the effect of negative voltage on the MAO coating was studied,and the effect of ultrasonic waves on the MAO coating was further explored.Ultrasonic-assisted micro-arc oxidation（UMAO）coating was prepared by applying ultrasonic power in two stages.The time point at which the forward current drops from the highest point to 0.1A is used as the cut-off point,and the UMAO coating is prepared by applying ultrasonic waves with power of 60,100,140,and180W before（stage 1）and after（stage 2）respectively.（Marked as UMAO-1 and UMAO-2respectively）.Use scanning electron microscope（SEM）,energy spectrometer（EDS）,laser confocal microscope（CLSM）,X-ray diffractometer（XRD）to analyze the microstructure and phase of the film,and use nano-scratch tester and electrochemistry The workstation analyzes the adhesion and corrosion resistance of the coating,and finally simulates the body fluid immersion test to study the degradation behavior of the UMAO coating.The main research results are as follows:In the constant voltage mode,taking the calcium-phosphorus ratio as the evaluation index,through three factors and four levels orthogonal test,it is concluded that the negative voltage is the main factor affecting the calcium-phosphorus ratio and the total amount of calcium-phosphorus in the coating from the positive voltage,negative voltage and time.Then select the negative voltage 10,30,50V for univariate optimization test.The study found that the growth of the coating is divided into three stages:（a）anodizing stage;（b）MAO rapid growth stage（c）MAO slow growth stage.The phases of the MAO coating formed are Mg,Mg O,and Mg₂Si O₄.As the negative voltage increases,the peak value of the forward current of the reaction decreases,the rate of film thickening increases,the porosity of the coating decreases first and then increases,the maximum pore size increases,the surface cracks increase,the bonding force of the coating base increases,and the corrosion resistance of the coating increases first and then decreases.When the negative voltage is30V,the corrosion resistance of the coating is the best,and the corrosion current density is improved by two orders of magnitude compared to 10V and 50V.Comprehensive consideration,choose the negative voltage 30V as the optimal parameter.After applying ultrasound,the contact angle of UMAO coating is lower than MAO,and the calcium-phosphorus ratio and corrosion resistance are higher than MAO,showing better biological activity and corrosion resistance.Compared with UMAO-1 and MAO,after applying ultrasound,the peak value of the forward current of the reaction decreases and the peak current time delays.As the ultrasonic power increases,the breakdown voltage of the coating decreases.For UMAO-1,when the ultrasonic power is increased,the calcium-phosphorus ratio of the coating is increased,and a new phase Ca₃（PO₄）₂ is produced;the roughness of the coating and the bonding force of the coating substrate are continuously increased,and the corrosion resistance increases first and then decreases.When the ultrasonic power is 140W,the corrosion current density and the bonding strength of the coating base are higher than those of MAO,UMAO-2 and UMAO-1,showing the best corrosion resistance and bonding.For UMAO-2,when the ultrasonic power is increased,the calcium-phosphorus ratio of the film layer does not change much,and no new phase is produced;the roughness of the film layer continues to decrease,and the bonding strength and corrosion resistance of the film base increase first and then decrease.Considering comprehensively,applying 140W of ultrasonic power to UMAO-1 is the optimal parameter.After applying ultrasound,the peak value of forward current decreases,the peak current time delays,porosity increases,roughness decreases,and biocompatibility increases.When ultrasonic wave is applied in the stage 1,the cavitation effect accelerates the bubble generation-growth-collapse process,increases the average radius of the bubble,and reduces the breakdown voltage loss.Ultrasonic standing waves increase the density and uniform distribution of bubbles on the surface of the sample,increase the breakdown arc point,and reduce the destructive breakdown caused by a single large electric arc.When ultrasonic waves are applied in stages 2,the mechanical effect produces a shearing effect on the coating,which reduces the roughness of the coating,but may cause crack propagation.When the ultrasonic power is increased,the calcium-phosphorus ratio of the coating is increased and a new phase Ca₃（PO₄）₂ is produced.The bonding force of the coating base increases first and then decreases,and the bonding force is highest when the ultrasonic power is 140W.As the ultrasonic power increases,the corrosion resistance of the coating increases first and then decreases.When 140W ultrasonic power is applied in stage 1,the corrosion current density increases by an order of magnitude compared with other parameters.For comprehensive consideration,the 140W ultrasonic power applied in stage 1is selected as the optimal parameter.The degradation experiment of the coating shows that with the increase of time,the p H values of the MAO-neg and UMAO-Ⅱsamples continue to rise,and the p H of the UMAO-Ⅰsamples fluctuates continuously.The weight gain rate of the three samples increase,and the weight gain rate of UMAO-Ⅰwas much higher than that of MAO-neg and UMAO-Ⅱsamples.Phase examination showed that during the immersion period,the corrosion deposits Ca₃Mg₃（PO）₄ and Ca CO₃appeared in all three samples.Combined with the microscopic morphology and electrochemical impedance of the sample,the equivalent circuit models of MAO-neg and UMAO-Ⅰwere immersed in different days.