【作者】 袁亚；

【导师】 陈体军； 马颖；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2009， 硕士

【摘要】 镁合金微弧氧化技术是通过电解液中的高压放电作用,使镁合金表面形成硬质陶瓷膜层的一种新工艺,所形成的陶瓷膜层具有硬度高、绝缘性和耐蚀性好、与基体结合力强的优点,是一种极具发展前途的镁合金表面处理技术。本文在硅酸盐体系中,探索了AZ91D镁合金微弧氧化电解液的使用寿命问题。首先研究了试验次数对微弧氧化起弧电压和终止电压、电解液电导率、电解液离子浓度以及膜层微观结构和性能的影响,并通过对电解液的分析研究,探讨电解液老化的原因。利用TT260数字式涂层测厚仪、JSM-6700F扫描电子显微镜和2206型表面粗糙度测量仪等研究了膜层的微观结构,并利用化学分析的方法研究了电解液离子浓度的变化。在微弧氧化过程中,阳极的镁合金溶解到电解液中,和电解液中的元素反应生成沉淀物,且大部分为胶体沉淀。采用交流脉冲电源会使阴极不锈钢溶解到电解液中生成氢氧化铁沉淀,氢氧化铁沉淀亦为胶体沉淀。胶体沉淀的生成使电解液的粘度增大,离子扩散变慢,导致电解液电导率下降。随着试验次数的增加,电解液电导率逐渐减小,则电解液的电阻越大,作用在电解液中的电压越大,起弧电压随之增大。电导率逐渐下降,分配在试样上的电压也逐渐变小,发生击穿瞬间的能量越小,使得放电通道凝固后留下的微孔孔径越来越小,但孔的数量增多。由于微弧氧化膜层表面熔融物颗粒减小,孔洞减小的缘故,使膜层表面的粗糙度减小,但由于后期膜层的外观质量明显下降,而导致膜层表面粗糙度急剧增大。膜层的耐蚀性开始在一个稳定的范围内波动而后下降。在电解液失效前,膜层的厚度和致密度基本不受影响,且厚度和致密度成正比。膜层的成分不受试验次数的影响,主要由MgO,MgSiO₃,Mg₇F₂（SiO₄）₃相组成。过滤沉淀后,可以提高膜层的氧化速度,延长电解液的使用寿命。沉淀对电解液电导率有影响,过滤后电解液的电导率值比过滤前电解液的电导率有所增加,比最开始电解液的电导率低。同时,起弧电压也有所下降。氢氧根离子浓度增大,氢氧根离子浓度的大小对成膜及膜层的性能有重要的影响。更多还原

【Abstract】 Micro-arc oxidation（MAO） of magnesium alloys is a plasma discharge that occurs at the metal/electrolyte interface when the applied voltage exceeds a certain critical breakdown value.The process produces thick,hard and well-adhered ceramic-like coatings to be obtained with high corrosion resistance,thermal stability and dielectric properties.Therefore,MAO is one of the most prospective methods of surface treatment for magnesium and its alloys.Service life of electrolyte of micro-arc oxidation on AZ91D magnesium alloy in silicate solutions were probed into.First,effect of experiment periodicity on start voltage,final voltage,conductivity and concentration of electrolyte,microstructure and properties of micro-arc oxidation coating were studied,and the reason of electrolyte aging was discussed by analysis electrolyte.The microstructures of micro-arc oxidation coating were studied by means of TT260 digital coating thickness gauge,JSM-6700F scanning electron microscope（SEM） and 2206 surface roughness measuring instrument.Concentrations of electrolyte were studied by means of analytical chemistry.Magnesium alloy of anode dissolve into electrolyte and react with element in the electrolyte,the reactant is amorphous precipitate mostly.Stainless steel of cathode dissolve into electrolyte in the double pole pulse power,the reactant is hydrate of Iron. Hydrate of Iron is amorphous precipitate,as well.Amorphous precipitate decrease the Speed of ion moving and decrease conductivity of electrolyte.Resistance of electrolyte increases with the reducing of conductivity.Accordingly, voltage of electrolyte becomes smaller and start voltage increases,as well. Conductivity decreased gradually and voltage of sample becomes smaller,leading to energy decreased.The size of micro-pore in the micro-arc oxidation coating becomes smaller,but the amount of micro-pores increases.Accordingly,the roughness of coating becomes smaller.Finally,the macro-quality of MAO coating becomes rougher. Accordingly,the roughness of coating becomes larger quickly.Corrosion resistance of coating reduces with the increasing of roughness（the macro-quality of MAO coating descend）.Before electrolyte failure,thickness and increase weight of coating steadied. The phase of the coatings was hardly effect,the coatings was mainly composed of MgO,MgSiO₃,Mg₇F₂（SiO₄）₃. Filtrating Precipitate can quicken the rate of film forming and increase the service life of electrolyte.Conductivity of electrolyte becomes larger than without filtrating and becomes smaller than forepart.Simultaneity,start voltage declined than without filtrating.OH^-’s concentration in the electrolyte increases in evidence,OH^-’s concentration has effect on forming and properties of micro-arc oxidation coating.更多还原