Effects Of Micro-arc Oxidation On The Properties And Micro Structure Of Magnesium Alloy Wires

Magnesium alloys are considered as the promising biodegradable materials for biomedical applications and surface modifications are commonly applied to slow down the degradation rate of magnesium alloys in the physiological environment. In this work, micro-arc oxidation (MAO) was used for surface treatment of AZ31B wires. The effect of MAO parameters including MAO time and current densities on the properties and microstructure of the wires was investigated. Moreover, the degradation behaviors of the wires with different MAO time, in different phosphate buffers with various concentration of chloride ion were studied. Furthermore, poly-L-lactic acid (PLLA) was applied to seal the surface pores ofMAO coating and its effect was reported.The results show that the main phase of the MAO coating is MgO and the mean grain size of the wires with small diameter has a little increase after MAO treatment. With the increasing of MAO time, the porosity of the coating increases firstly and then decreases, meanwhile, more cracks are observed at the longer MAO time. The porosity of the coating gets the largest value at MAO time of 30 min, and however, the average pore size has no apparent change with the changing of MAO time. Similarly, with the increasing of the current density (CD), the coating grows with a constant rate and more cracks would appear in the coating, and correspondingly, the tensile strength of the wire decreases. Meanwhile, the values of surface porosity and average pore size of the coating firstly increase and then decreases and at CD of 8 A/dm~2, they have the largest values, which are respectively 6.4% and 2.59?m. While the diameter of the wire increases, the surface porosity and average pore size of the coating has an inverse decrease and the loss of the strength caused by MAO decreases, however, the morphology ofthe coating has no obvious change.The degradation behavior of the wires in the phosphate buffer with the same chloride ion concentrations in the case of flood improves significantly after MAO treatment. During short time immersion (less than 10 days), the MAO time has little effect on the degradation behavior. After 10 days immersion, the remaining mass percentage of 10 min MAO-treated wire kept the highest. For the un-MAO wire, it corroded seriously after 10 days immersion while coating of 10 min MAO-treated wire kept integrated. The loss ratio of the strength of un-MAO wire and remained tensile strength of the wire with 10 min MAO treatment both kept the highest after 4 days immersion. The results of the immersion tests in the phosphate buffer with different chloride ion concentration denote that in the predetermined immersion time, the number of the corrosion cracks and the loss of strength increase with the increasing of chloride ion concentration.With the incorporation of PLLA coating, the effect of MAO time on the loss ratio of mass and diameter of the wire is improved. After 33 days immersion, the 20 min and 30 min MAO-treated wire kept integrated after sealing with PLLA, and the latter one has the largest strength afler immersion.