【作者】 李俊刚；

【导师】 魏尊杰； 孟祥才；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 镁锂合金具有良好的综合性能，是部件轻量化理想材料。耐腐蚀、耐磨性能差是制约镁锂合金应用的主要因素。对镁锂合金表面处理是改善其性能的最佳方法。采用微弧氧化处理提高镁锂合金的性能，对增强国家的航天航空水平，军事实力及在电子、汽车等工业领域的竞争力有重大意义。本文采用优化的微弧氧化（MAO）工艺，对镁锂合金进行表面处理，选择不同添加剂来提高涂层生长速率，改善涂层组织结构，获得了以植酸为添加剂制备的耐蚀、耐磨性能优异的涂层。利用TEM、AFM、SEM、EDS、XRD、XPS等手段分析微弧氧化涂层的微观组织和相成分，利用数码影像技术分析放电微弧形态，探讨微弧氧化涂层形成过程。采用动态极化曲线和电化学阻抗谱（EIS）研究合金及微弧氧化涂层的耐蚀性能、涂层的阻抗变化及腐蚀机理；采用球-盘式磨损试验机测试涂层的摩擦磨损性能。通过研究Mg-7Li合金微弧氧化电解液中硅酸钠浓度以及电参数对涂层结构的影响来优化微弧氧化工艺参数，结果表明：当硅酸钠浓度由10g/L提高至30g/L，微弧氧化涂层厚度由8μm增加到23μm。随着电压、占空比和时间的增加，涂层的生长速度加快，涂层厚度增大，微孔数量减少，表面变得粗糙；频率的升高则降低涂层厚度，使表面平整。硅酸钠电解液制备的微弧氧化涂层主要由MgO、Li₂O₂和Mg₂SiO₄组成。硅酸钠体系最佳微弧氧化工艺参数为：恒压方式，硅酸钠浓度15～20g/L、电压400～450V、时间30min、频率200～400Hz、占空比10～15%。为提高涂层厚度、调整涂层的组织结构，优化涂层耐蚀和耐磨性能，通过加入柠檬酸钠、硼酸钠、钨酸钠、氟化钠及植酸(C₆H₁₈O₂₄P₆)等添加剂研究微弧氧化涂层组织结构变化，结果表明：添加剂的加入均能提高涂层的厚度，降低涂层表面微孔数量，其中植酸添加剂的加入使涂层最大厚度达27μm。植酸的螯合作用吸附Mg²⁺、Li+等离子形成链状结构，提高涂层生长速率。微弧放电状态影响涂层生长过程，对普通数码影像分析结果表明：随着硅酸钠浓度、电源电压、加电时间的增加以及添加剂的加入，放电微弧数量减少，微弧发生合并，微弧直径变粗。高速影像分析表明：硅酸钠电解质溶液中，一个脉冲周期内，同一位置的放电击穿持续到脉冲结束；不同的脉冲周期中，放电击穿反复在同一位置进行，其断续时间达160ms；电压的升高加快了放电击穿位置的迁移；硼酸钠和1.5mL/L植酸添加剂的加入，促进放电击穿位置的快速迁移，而氟化钠和3mL/L植酸的加入使放电击穿反复在同一位置且断续时间增加。植酸的加入，使放电反复击穿断续时间达220ms。通过对镁锂合金微弧氧化涂层组织结构、相组成及元素结合能分析，结合放电微弧形态，得出微弧氧化涂层生长过程为：镁锂合金表面形成双电层；产生微弧等离子放电；发生高温高压化合反应；内部隐弧持续放电；溶液对熔融物冷淬形成涂层。极化曲线及阻抗谱是评价镁锂合金耐蚀性能优劣的依据。对Mg-7Li合金及其微弧氧化涂层在3.5wt.%NaCl溶液中的极化曲线和阻抗进行研究，结果表明： Mg-7Li合金的腐蚀电极电位和腐蚀电流密度分别为-1.5857V和2.235×10^-4A/cm²。Mg-7Li合金在3.5%NaCl溶液中最大阻抗值为250ohm·cm²。不同硅酸钠浓度和不同添加剂制备的微弧氧化涂层的腐蚀电极电位相对于基体均有所提高，腐蚀电流密度相对于基体均降低。其中，添加3mL/L C₆H₁₈O₂₄P₆的涂层腐蚀电极电位为-1.4761V，比基体合金正移了109.6mV，腐蚀电流密度为7.204×10^-7A·cm^-2，比基体合金降低3个数量级。硅酸钠系微弧氧化涂层在3.5%NaCl溶液中稳定0.5h后的阻抗值比基体阻抗值提高10倍以上；植酸-硅酸钠系微弧氧化涂层的阻抗值达到10050ohm·cm²。对合金基体和植酸-硅酸钠系微弧氧化涂层在3.5%NaCl溶液浸泡2～120h阻抗测试表明，随着浸泡时间延长，合金基体阻抗值越来越小，浸泡120h后阻抗值为70ohm·cm²，基体被腐蚀为粉末。微弧氧化涂层浸泡初期还能保持15倍于基体的阻抗，120h后仍维持300ohm·cm²。对极化曲线、阻抗谱及等效电路模型拟合的研究表明：有微弧氧化涂层合金的腐蚀机理为腐蚀溶液微孔渗透促溶。涂层腐蚀过程为溶液渗入涂层微孔—Cl-优先吸附于涂层—形成可溶性氯盐—涂层脱落—全面腐蚀。研究干摩擦磨损试验条件下Mg-7Li合金和表面微弧氧化涂层摩擦磨损性能，结果表明：Mg-7Li合金的摩擦系数保持在0.08～0.20。合金经微弧氧化处理后，涂层的摩擦系数相对于合金基体均降低，耐摩擦磨损性能显著提高。随着电解液中硅酸钠量增加，摩擦系数增大，并出现宽幅跳动。有添加剂的微弧氧化涂层摩擦系数小于0.15，比20g/L硅酸钠系微弧氧化涂层摩擦系数低。Mg-7Li合金微弧氧化涂层的摩擦磨损机理为：多孔涂层表面冲击振动—涂层微凸体磨损或断裂—涂层、脱落体与对磨物间磨粒磨损的反复作用。更多还原

【Abstract】 Mg-Li alloys, with good comprehensive properties, are suitable light weightfabrication materials. However, the poor corrosion resistance and wear performancebecome the main factor that has restricted their further applications. Surfacemodification for Mg-Li alloys is the best way to improve their performances.Using microarc oxidation treatment to improve the performance of Mg-Li alloy hasa great significance to enhance the national competitiveness in the field of aerospace,military power,electronic and automotive. In this study, the surface treatment wascarried out on Mg-7Li alloy by optimized microarc oxidation （MAO） process.Different additives were selected to enhance the growth rate and improve themicrostructure of the coatings. Finally, the ceramic coating prepared with phyticacid additive exhibits excellent corrosion and wear resistance. To discuss theformation mechanism of MAO coating, the morphology and phase compositionwere investigated by means of TEM, AFM, SEM, EDS, XRD and XPS. Themorphology of discharged microarc was analyzed by digital imaging technology.The corrosion resistance, impedance change and the corrosion mechanism of thebare alloy and the MAO coating were discussed by potentiodynamic polarizationcurves and Electrochemical Impedance Spectroscopy （EIS）. The friction and wearproperties of the coatings were measured using a ball-on-disk wear tester.In order to determine technological parameters of the MAO coating on Mg-7Lialloy, the influences of the Na2SiO3·9H2O concentration in the electrolyte andelectrical parameters on the structure of the coating were investigated. Resultsshowed that when the Na2SiO3·9H2O concentration varied from10g/L to30g/L, thethickness of the MAO coatings on Mg-7Li alloy increased from8to23μm. With arise in voltage and duty cycle as well as time, the growth rate and the thickness ofthe coatings increased, while the number of the micropores decreased and thecoating surface became coarse. On the contrary, the rise of frequency diminished thecoating thickness and made the surface smooth. The coatings prepared inNa2SiO3·9H2O electrolyte were mainly composed of nanoscale MgO, Li₂O₂andMg₂SiO₄. The optimized parameters are constant voltage of400-450V, oxidationtime of30min, frequency of200-400Hz, duty cycle of10-15%and Na2SiO3·9H2Oconcentration of15-20g/L.To adjust the structure and increase the thickness of the coating, the additivessuch as sodium citrate, sodium borate, sodium tungstate, sodium fluoride and phyticacid (C₆H₁₈O₂₄P₆) were added to the electrolyte, and effects of these additives on thecoating structure were discussed. Results showed that the use of additives improved the thickness of the coatings and reduced the quantity of surface micropores. Amongthem, the C₆H₁₈O₂₄P₆additive contributed maximum thickness of27μm to thecoatings. Chelation of phytic acid adsorbed Mg²⁺and Li+to form chain structure,which had improved growth rate of the coating.Morphology of microarc discharge affects growth process of the coatings.Results of ordinary digital imaging analysis showed that with increase inconcentration of sodium silicate, supply voltage, charging time and addition ofadditives, the discharged arc reduced, united and the diameter of micoarc increased.Analysis of high-speed video showed that discharge breakdown continued to the endof the pulse during a pulse-period in Na2SiO3solution, while discharge breakdownhappened at the same position repeatedly with intermittent time up to160ms indifferent pulse-period. The increase of voltage accelerated the migration ofdischarge location. Addition of sodium borate and1.5mL/L phytic acid resulted infast migration of discharge location, and sodium fluoride as well as phytic acidmade discharge occur at the same position repeatedly and extended intermittent time.Therefore, addition of phytic acid caused the repeated intermittent time of dischargebreakdown up to220ms.According to analysis of the microstructure, composition and element bindingenergy of the coatings in combination with morphology of discharged microarc, it isfound that the growth process of the coating involved the formation of doubleelectric layer, discharge of plasma, occurrence of compound reaction at hightemperature and pressure, continuous discharge of internal hidden arc and the coldquenching of the solution to the melts. Adding the additives to Na2SiO3·9H2Osolution could promote the growth rate of the coating during the breakdown process.The chelation of phytic acid made it absorb more Mg and Li ions to form chainstructure, improving the growth rate of the coating and causing the increase ofMg₂SiO₄content in the coating.Polarization curve and impedance spectroscopy are used to evaluate thecorrosion resistant of Mg-Li alloy. The researches on the polarization curve andimpedance of the Mg-7Li alloy and the coatings in3.5wt.%NaCl solution indicatedthat the corrosion potential（Ecorr） and corrosion current density（Icorr） of Mg-7Li alloywere-1.5857V and2.235×10^-4A/cm², respectively. The maximum impedance ofMg-7Li alloy was250ohm·cm². The Ecorrof the coating with both differentNa2SiO3·9H2O concentration and additive was higher than that of the substrate, andthe Icorrof the coating was lower than that of the substrate. Among them, the Ecorrofthe coating with3ml/L C₆H₁₈O₂₄P₆was-1.4761V which had shifted109.6mVtowards positive direction relative to the substrate, and its Icorrwas7.204×10-7A·cm-2which had reduced by3orders of magnitude compared to that of substrate. The impedance of the MAO coating prepared in Na2SiO3·9H2Oelectrolyte was10times higher than that of substrate, and the impedance of thecoating prepared in Na2SiO3·9H2O-C₆H₁₈O₂₄P₆electrolyte was10050ohm·cm²inNaCl solution after0.5h. The EIS test of the substrate and the coating immersed for2-120h indicated that the impedance of the substrate gradually decreased withincreasing time, and reached70ohm·cm²for120h when the substrate becamepowders due to corrosion damage. However, the impedance of MAO coating was15times that of substrate in the initial stage of immersion, and still maintained300ohm·cm²after120h immersion. Based on above data, the corrosion mechanism ofMAO coatings involved that the corrosion solution promoted the coating dissolutionby penetration into any porosity. The corrosion process included the penetration ofthe solution into the micropores, priority absorption of the Cl-ions on coating,formation of soluble chloride salt, coating peeling and overall corrosion.Under the dry friction test condition, the friction coefficient of Mg-7Li alloyranged from0.08to0.20. Its friction and wear process included the repeated actionsof plough wear, oxidation wear, adhesive wear as well as abrasive wear. After MAOtreatment, the friction coefficient of all the coating was lower than that of thesubstrate, improving the wear performance significantly. With the increase of theNa2SiO3·9H2O concentration in the electrolyte, the friction coefficient increased andwide-range fluctuations appeared. The friction coefficient of the additive-dopedcoatings less than0.15was lower than that of the coatings with20g/LNa2SiO3·9H2O.The friction and wear mechanism of the MAO coating on Mg-7Lialloy was resulted from the repeated actions of impact vibration on porous layersurface, wear or fracture of coating’s micro-protrusion as well as abrasive wearamong coating, peeling and SiO2ball.更多还原