【作者】 李颂；

【导师】 刘耀辉；

【作者基本信息】 吉林大学 ， 材料加工工程， 2007， 博士

【摘要】 本文在AZ91镁合金基体表面制备了微弧氧化陶瓷层,开发出高效、环保的新型镁合金微弧氧化电解液并系统优化了微弧氧化的电源参数;利用SEM、XRD、XPS、FTIR等手段对膜层的微观组织结构进行了分析;测试了膜层的耐腐蚀、耐磨损和硬度、抗拉强度等性能。实验发现,控制微弧氧化电源的电源参数,可以改变膜层表面的圆盘状结构及中间微孔的大小,同时疏松层和致密层的比例也发生改变。在微弧氧化过程中施加负脉冲电压,可明显减小疏松层的比例,促进O、P元素向基体内渗透。膜厚与氧化时间成抛物线关系。随着氧化时间的延长,致密层所占的比例呈现先升高又下降的趋势,而膜层向基体内生长的比例呈一直升高趋势。氧化时间过长会导致靠近基体的部分致密层中存在较多的气孔、裂纹等缺陷。首次采用Mat lab软件编程计算了微弧氧化膜层表面的孔隙率,为研究膜层特殊的多孔结构提供了新的表征手段。通过研究不同工艺条件下制备的镁合金微弧氧化膜层中尖晶石相和方镁石相的比例,首次提出该比例在0.6~1.0范围内时,膜层的耐蚀性明显提高。首次研究了镁合金微弧氧化膜层的润湿性问题。不同工艺条件下制得的微弧氧化膜层与水的接触角差别很大。适当增加膜层的孔隙率,有利于提高接触角。增加微弧氧化膜层表面的微孔数量的同时,减小孔径并使之均匀分布,对提高其表面的疏水性起着决定性的作用。无论是在点滴实验、电化学实验还是在盐雾腐蚀实验中,微弧氧化膜层都表现出很好的耐蚀性,和基体相比,微弧氧化膜层的耐蚀性大幅提高。首次研究了中性盐雾腐蚀环境中膜层的典型腐蚀形貌,并建立了丝状腐蚀区形成的机理模型。膜层的表面粗糙度、耐磨性、硬度及拉伸性能的测试结果表明铝酸盐体系中制备的微弧氧化膜层具有良好的综合性能。本论文所开发的镁合金微弧氧化工艺具有一定的实用价值,在工业应用上取得了初步的成功。更多还原

【Abstract】 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.更多还原