| Biomedical magnesium alloy is one of the biodegradable biomedical materials,and after the complete healing of human skeleton,the magnesium alloy implants will degrade in the human body,which can avoid the pain and economic burden caused by the second surgery.But when it is used as an implant in the body,there will be two types of degradation,wear and corrosion.Local corrosion occurs first in the defects caused by wear,which leads to the rapid corrosion and rapid destruction of the magnesium alloy implants.This may not be consistent with the rate of human bone repair,so it is facing a challenge in the case of reliability.Therefore,it is necessary to explore a more effective way to improve the corrosion and wear properties of magnesium alloys,and to study the local corrosion mechanism.In this work sol-gel technology was applied to prepare the layer with bioactivity on micro arc oxidation coating(MAO)coating on AZ31 magnesium alloy surface.The wear and local corrosion properties of the coatings were studied.The protective effect of sol-gel coating was also discussed.In this paper,scanning electron microscopy(SEM),X ray diffraction(XRD),scratch tester and micro-hardness tester were used to observe the microstructure,the thickness,the phases,the adhesion strength and the hardness of the samples.The results show that the micro-pores on the surface of MAO were almost covered by the sol-gel coating and the thickness increased by one time.The adhesion strength of the coating was increased by 76%,the hardness of the coating increased from 236 HV to 340 HV,and the hydroxyapatite(HA)was found in the phase components.The sliding wear test is used to determine the influence of different loads(3N,4N,5N)on the friction coefficient.The results show that the friction coefficient of MAO and sol-gel/MAO samples decreases with the increase of applied load,but the friction coefficient and wear volume of sol-gel/MAO samples are less than those of MAO samples,showing good wear resistance.The wear mechanism is analyzed from the wear morphologies,mainly including wear,oxidation,plastic deformation and adhesive wear during the process of sliding wear.Finally,the physical model of the sliding wear of the composite coating is established according to the wear process.The electrochemical tests were conducted by open circuit potential(OCP),potentio-dynamic polarization(PDP)and electrochemical impedance spectroscopy(EIS)respectively,to evaluate the corrosion resistance of the coatings.The results showed that sol-gel/MAO sample has a better corrosion resistance than MAO sample.It is confirmed that sol-gel treatment on MAO samples can effectively improve the corrosion resistance of magnesium alloys.Then scanning vibrating electrode technique(SVET)and local electrochemical impedance spectroscopy(LEIS)were used to study the local corrosion behavior on the coated samples with defects.The results show that the sol-gel/MAO sample has low corrosion current density and high electrochemical impedance value,showing a good corrosion resistance.Meanwhile,it is found that the corrosion resistance of the scratch on the sol-gel/MAO coating is better than that of the MAO coating,which indicates that the sol-gel coating plays a certain role in alleviating the corrosion of the scratch.In addition,it is also concluded that the effect of scratches on the corrosion rate of MAO samples is more serious than that of sol-gel/MAO samples.Finally,a physical model for the local corrosion due to defects(artificial scratches)of coated AZ31 Mg alloy was proposed. |
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