【作者】 骆海贺；

【导师】 魏伯康； 蔡启舟；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2009， 博士

【摘要】 镁合金具有许多优良的物理和力学性能，在航空航天、汽车制造、电子通讯、军事及核能等诸多领域应用广泛。然而，镁合金的耐蚀、耐热及耐磨性能较差，严重阻碍了其进一步应用。微弧氧化（MAO）是在普通阳极氧化基础上发展而来的一种表面处理新技术，因能在Al、Mg、Ti等阀金属及其合金表面原位形成陶瓷膜层，极大地提高金属的耐蚀、耐磨等性能，而被公认为最有前途的镁合金表面处理技术。目前，对镁合金MAO的研究多集中在提高耐蚀性能方面，且采用的电解液主盐多为硅酸盐、磷酸盐或铝酸盐，膜层成分单一、综合性能一般。此外，对镁合金MAO的工艺原理、膜层的形成机制以及膜层的组成、结构与性能之间的内在联系尚缺乏系统、深入的研究。因此，开发新的电解液体系、改善膜层的组成与结构，深入探讨MAO的处理工艺、膜层的形成机制，系统研究膜层的组成、结构与性能之间的关系，对镁合金MAO的进一步发展和应用具有十分重要的意义。本文在综合分析国内外电解液特性的基础上，开发出了两种新型的电解液体系（Si-Zr-Y体系和Al-Zr-Y体系），首次在碱性电解液中于AZ91D镁合金表面原位制备了ZrO₂-Y₂O₃陶瓷膜层，并与已开发的硅酸盐-磷酸盐复合电解液体系（Si-P体系）作对比，系统研究了三种膜层（Si-P film、Si-Zr-Y film和Al-Zr-Y film）的厚度、表面粗糙度、组成、结构以及耐蚀、耐热、耐磨等性能之间的关系，为开发新的MAO膜层、进一步提高AZ91D的综合性能奠定了良好的基础。研究了添加剂（NaPO₃）₆的浓度对硅酸盐电解液中AZ91D镁合金MAO膜层的组成、结构及耐蚀性能的影响。结果表明，随着（NaPO₃）₆浓度的增加，膜层中MgO和Mg₂SiO₄的含量分别在0～3 g·L^-1和0～7 g·L^-1浓度范围内增加，在3～10 g·L^-1和7～10g·L^-1浓度范围内减少；而MgAl₂O₄含量逐渐减少。添加（NaPO₃）₆后，MAO膜层（Si-Pfilm）的耐电化学腐蚀性能提高，这主要是因为（NaPO₃）₆增加了MAO反应过程中基体镁合金表面的“氧空位”和溶液中PO₄^3-含量，促使Mg在金属／膜层界面上快速形成相应的化合物，增加了陶瓷膜层的厚度和致密性，从而提高了其耐腐蚀性能。系统研究了Si-Zr-Y film和Al-Zr-Y film的生长过程、膜层结构和化学组成。结果表明，随着氧化时间的延长，膜层的厚度逐渐增加，但生长速率逐渐减小。两种膜层均具有双层结构，即外部的多孔层和内部的致密层。Si-Zr-Y film主要由t-ZrO₂、Y₂O₃、SiO₂、MgO、MgF₂和Mg₂SiO₄组成，膜层表面的陶瓷颗粒尺寸较大，分布不均匀，粗糙度相对较大（～3.495μm），但孔隙率很低（～0.9％）；而Al-Zr-Y film主要由c-ZrO₂、t-ZrO₂、Y₂O₃、Al₂O₃、MgO、MgF₂和MgAl₂O₄组成，膜层表面的陶瓷颗粒多而细小，分布均匀，粗糙度相对较小（～2.128μm），但孔隙率较高（～39.4％）。研究分析了三种膜层的耐蚀性能。结果表明，Si-P film和Si-Zr-Y film耐点滴腐蚀的能力较强，而Al-Zr-Y film的较差，但均好于未处理的AZ91D镁合金。三种膜层在3.5 wt．％NaCl溶液中均表现出很好的耐蚀性，其耐蚀能力较基体合金提高了3个数量级。分析表明在较长时间的电化学反应过程中，膜层的耐蚀性能主要取决于内部致密层的厚度和致密性，而对于短时间的电化学反应过程来说，膜层表面的孔隙率和孔洞的大小对其耐蚀性能也存在较大影响。高温氧化及热冲击研究结果表明，三种膜层的高温抗氧化能力明显好于未处理的AZ91D镁合金；Si-Zr-Y film和Al-Zr-Y film耐热冲击性能好于Si-P film，其主要原因是膜层中含有Y₂O₃稳定的ZrO₂以及Al₂O₃等高温稳定相，在加热过程中其相组成变化较小，对温度变化敏感度较低；此外，膜层的内层具有一定厚度且相对致密，与基体间的结合强度较高，因而在热冲击过程中不易产生裂纹和膜层脱落。系统研究了三种膜层的硬度、耐磨及拉伸性能。结果表明，微弧氧化处理后，镁合金的表面硬度明显增加，这主要是因为MAO膜层的主体相为陶瓷相，其硬度高于基体，因而能显著提高基体的硬度。由于MAO膜层的存在，使得镁合金的磨损机制发生了改变，粘着磨损和磨粒磨损损伤减轻，因而其耐磨性能显著提高。镁合金在拉伸实验中发生了准解理断裂和韧窝断裂相结合的混合断裂，但以准解理断裂为主；而MAO膜层拉伸实验后在膜层／基体的界面结合处未出现明显的分层现象，且无大的、贯穿性的裂纹出现，抗拉伸性能良好。更多还原

【Abstract】 Magnesium and its alloy have many excellent physical and mechanical properties,which make it valuable in a number of applications including aeronautics and astronautics,automobile manufacture, electronic communications, military affairs and nuclear power, etal. However, their corrosion resistance, heat resistance and wear resistance are rather poor,which seriously hinder their further applications. Microarc oxidation （MAO） is a novelsurface treatment technique developed from the common anodic oxidation. By use of thistechnique, a ceramic coating can be in situ formed on Al, Mg, Ti and their alloys. Thus,this method has been generally recognized as the most promising surface treatmenttechnique for magnesium alloy. At present, the researches on MAO for Mg alloy aremainly focused on the improvement of corrosion resistance, and the electrolytes used aremainly silicate, phosphate and aluminate, the coating components are single and thecomprehensive properties of the coating are not good enough. In addition, there are nosystematic and in-depth researches on the fundamental theory of MAO process, theformation mechanism of the coating, as well as the internal relations between the coatingcompositon, structure and its properties. Therefore, the development of new electrolytesystem for improve coating composition and structure, the further study of the MAOtreatment process and the coating formation mechanism, the systematic study of therelations between the coating composition, structure and its integrated performaces, willhave a very important significance for the further development and application of MAOtechnique.Based on the comprehensive analysis of the electrolyte characteristics at home andabroad, two novel electrolyte systems （Si-Zr-Y system and Al-Zr-Y system） have beendeveloped and the ZrO₂-Y₂O₃ composite ceramic coatings have been firstly prepared on AZ91D magnesium alloy in the alkaline electrolyte. For comparison, a silicate-phosphatecomposite electrolyte system （Si-P system） has been developed. This work systematicallystudied the thickness, surface roughness, composition and microstructure of the threedifferent coatings, as well as the relations between them and the corrosion resistacne, heatresistance and wear resistance, et al. A good foundation for the development of novelMAO coatings and further improvement of the integrated performances of AZ91Dmagnesium alloy was established by this research.Effect of （NaPO₃）₆ concentrations on composition, microstructure and corrosionresistance of MAO coatings formed on AZ91D Mg alloy in silicate electrolyte was studied.The results showed that with the increase of （NaPO₃）₆ concentration, the contents of MgOand Mg₂SiO₄ was increased in the concentratin range of 0～3 g·L^-1 and 0～7 g·L^-1, and thendecreased in the range of 3～10 g·L^-1 and 7～10 g·L^-1, respectively. But the content ofMgAl₂O₄ was decreased gradually. After adding （NaPO₃）₆ to the electrolyte, theelectrochemical corrosion resistance of Si-P films had been significantly improved. Themain cause was that （NaPO₃）₆ increased oxygen vacancies on the Mg substrate surfaceand the content of PO₄^3-, which speeded up the formation of magnesium compounds at theinterface between metal and coating, increased the thickness and compactness of thecoatings, and then improved their corrosion resistance.The gowth process, microstructure and chemical compositon of Si-Zr-Y film andAl-Zr-Y film have been systematically studied. The results showed that the coatingthickness incresed, but the gowth rate was decreased as the oxidation time increased. Thetwo coatings both had double-layer structures, namely the outer porous layer and the innerdense layer. The Si-Zr-Y film was mainly composed of t-ZrO₂, Y₂O₃, SiO₂, MgO, MgF₂and Mg₂SiO₄, the ceramic particales of the coating were relatively large and distributedunevenly. It has a high surface roughness （～3.495μm） and a low porosity （～0.9%）.However, the Al-Zr-Y film was mainly composed of c-ZrO₂, t-ZrO₂, Y₂O₃, Al₂O₃, MgO,MgF₂ and MgAl₂O₄, the ceramic particales of the coating were numerous and fine and distributed evenly. It had a low surface roughness （～2.128μm） and a high porosity（～39.4%）.The corrosion resistances of three coatings were studied. The results showed that thedropping corrosion resistance of the Si-P film and Si-Zr-Y film were better, but that of theAl-Zr-Y film was poor. However, they all were better than untreated AZ91D magnesiumalloy. The three coatings showed a good corrosion resistance in 3.5 wt. % NaCl solution,the corrosion resistances of which were improved by 3 orders of magnititude comparedwith that of untreated AZ91D alloy. The analysis results showed that the corrosionresistance of coating was mainly determined by thickness and compactness of the innerlayer in the long-time electrochemical reaction process, but in the short-time reactionprocess, the corrosion resistance of the coating was also influnced by coating surfaceporosity and hole size.The results of high temperature oxidation and thermal shock test showed that the threeMAO coatings had better high temperature oxidation resistance than that of untreatedAZ91D magnesium alloy. The thermal shock resistance of Si-Zr-Y film and Al-Zr-Y filmwere better than that of Si-P film, which was mainly attributed that the coatings containedthe high-temperature stable phsases of Y₂O₃ stablized ZrO₂, Al₂O₃ and so on. During theheating process, the coatings had little change in the phase composition and less sensitvieto temperature change. In addition, the inner layer of the coating was thick and compact,and with a high bonding strength, which made the formation of crackes and abhesion ofthe coating difficult during the thermal shock process.The hardness, wear resistance and tensile properties of three coatings weresysmatically studied. The results showed that the coating hardness was significantlyimproved after MAO treatment. This was mainly attributed that the main phases of thecoating were ceramic phases, the hardness of which were higher than that of Mg, whichmade the hardness of Mg substrate improved. Due to the presence of MAO coating, thetribological mechanism of Mg alloy was changed, the adhesive wear and abrasive wear were reduced, and then the wear resistance was significantly improved. During the tensiletest, a mixed fracture including quasi-cleavage fracture and dimple fracture had takenplace in AZ91D magnesium alloy. But quasi-cleavage fracture was the main fracture form.However, there was no obvious layer seperation occurring in the interface of coating andsubstrate, and there was no large and perfoliate crack in the interface. Therefore, the MAOcoatings have very good anti-tensile properties.更多还原