Investigation On Preparation, Characterization And Performances Of Microarc Oxidation Coating On Magnesium Alloy

Magnesium alloys, with low density, high special strength and rigidity, good heat conductibility, high damping characteristics, good electromagnetic shielding characteristics and other virtues, have become promising materials for manufacturing light structures and have a bright future in aviation, aerospace, walking machine and 3C etc. However, the poor corrosion resistance of magnesium alloys has been greatly restricting their further application. Various surface treatment method is still the necessary procedure before the practical application of magnesium component. Micro arc oxidation (MAO) processing is an emerging, environmentally friendly and simple surface technique, which gets rid of the disadvantages of the common surface treatment methods and has a wide application future. Although, a lot of research have been made about the formation mechanism, microstructure and performances of the MAO coatings, there is still a great many of important scientific questions to be answered about the above items, which is of great significance theoretically and actually. Based on the above insufficiencies in MAO researching of magnesium alloys, some valuable researches have been done and some innovative results have been obtained in this dissertation.Firstly, in the way of the preparation of the MAO coating, a new kind of MAO electrolyte with high efficiency and without pollution has been developed, and the effect of the electronic parameters on the thickness, microstructure and corrosion resistance of the MAO coatings has been systematically investigated.The compositions of the MAO electrolyte are: NaAlO2 (~20g/l) as the main component, NaOH(4~8 g/l) as additive, also can be added by KMnO4 (3~8g/l) as accelerant and sodium citrate(3~8g/l) with or without sodium phosphate(3~8g/l) as stabilizator, the pH is adjusted in the range of 13~14 by adding H3PO4 or ammonia into the electrolyte. The optimized electronic parameters are 4.7~5.6A/dm2 current density, 500~900 Hz frequency and 20~30% duty in view of the present MAO equipment and corrosion resistance is given priority.Secondly, in the way of the characterization of the MAO coating, the microstructure and composition of the coating are analyzed by modern surface analysis methods such as SEM, XRD, XPS, TEM and FTIR. The results are as followed:The surface morphology of the coating is dense and uniformity. The average diameter of the pancake is about 21μm and the central micro-pore size is in the range of 1~10μm. A lot of 10~40 nano-structure exist in the surface of pancake. The phase composition of the coating is the main MgAl2O4 spinel and litter dissociative MgO periclase.By comparing the cross-section morphology and the elements distribution of the MAO coatings prepared with and without negative voltage, it can be seen that the ratio of outer porous layer decreases and the interface between inner dense layer and the substrate gets zigzag, as well as, the penetration of O and P elements into the substrate more equably under the condition of negative voltage. The Mat lab program can meet the need of reflecting the porosity of the MAO coating surface, which provides a new approach to characterize the MAO coating.It is found that the micro-arc oxidation process can be divided into three stages according to the spark and voltage variation, the initial passive film stage, the coating rapid growth stage and the coating parcel growth stage. Variation of the surface morphology with the oxidation time reveals that the ceramic coating growth is such a course that ceramic particles form first on the surface of substrate, and then the particles gather into island, at last, the island ripens further in dimension of X-Y. At the same time, the phases with high temperature breaking forth from the discharge channel stack continuously and modify the appearance of the island, as well as seal the surface micro-pores.The coating thickness increases according to a formula of parabola with the oxidation time depending on the heat diffusing under the condition of keeping current density constant. The ratio of dense layer thickness to the total thickness increases first and then decrease along with the oxidation time, while the ratio of the coating growing inward into the substrate increases all through. Long oxidation time causes many flaws such as pores and crack in the dense layer near the substrate. The quantitative estimations for the mass percent of MgAl2O4 and MgO and the crystal size of MgAl2O4 are achieved by the XRD results, consequently, the corresponding response of phase composition to the corrosion resistance is proposed. The results show that the corrosion resistance of the MAO coating improves significantly when the mass percent ratio of MgAl2O4 to MgO is in the range of 0.6~1.0. A conclusion can be drawn that the mass percent ratio of MgAl2O4 to MgO in the coating is another main factor besides the thickness and microstructure. The crystal size of MgAl2O4 in the coatings obtained with different oxidation time is in the range of 40~50nm. In addition, the crystal size is decreasing and then increasing with increasing the process time. The lattice constant of MgAl2O4 is less than the standard and increases with the longer processing time.Thirdly, the properties of the coating such as surface roughness, wettability, corrosion and wear resistance are measured by various apparatus and test. The results are as follows:The surface roughness of the MAO coating increases with increasing current density and decreases with increasing duty. The main frequency has no significant effect on the surface roughness. While the pH value is in the range of 11~12, the coating surface is good.The water contact angles of the coating prepared under different parameters have great difference. Increasing the porosity of the coating can improve the water contact angles to some extent and whereas, decreasing the porosity can reduce the angle and improve the wettability. Increasing the number of the micro-pore and synchronously reducing homogenizing its diameter reveals a decisive role to get hydrophobic coatingThe corrosion test results in different corrosion condition show that MAO treatment improves sufficiently the corrosion resistance of AZ91substrate in despite of the corrosion medium. After salt spray test, the coating surface is analyzed by SEM and EDS. The results show that Cl- absorbed firstly in such area that aluminum content is low, the micro-cracks and micro-pore occur. The traits of the corrosion are: the diameter of micro-pore enlarges, the micro-crack broadens and lengthens, the deep corrosion pit or the long filiform corrosion area comes into being.The microhardness test result shows that the microhardness from substrate to coating increases gradually. The coating's microhardness is the higher than 1000HV and much higher than the substrate. The mechanical properties of AZ91 magnesium alloy have slightly decrease after MAO treatment. The rupture of the coating is brought by the pulling crack The wear resistance test results show that MAO treatment can greatly improve the wear resistance of the die cast AZ91 under the condition of dry sliding wear. Under the condition of oil lubrication, the wear resistance can improve further because of the saving-oil speciality of the micro-pore in the coating.Finally, the new developed MAO electrolyte and the optimized electronic parameters have been applied successfully to automobile die-casting of magnesium alloy. The coating on the surface of the casting is uniform in thickness and color. Therefore, the MAO process was of some practical value to the industry application.