【作者】 吴振强；

【导师】 夏原；

【作者基本信息】 中国科学院研究生院（力学研究所） ， 工程力学， 2008， 博士

【摘要】 等离子体电解氧化（Plasma electrolytic oxidation,PEO）是在Al、Mg、Ti等金属及其合金表面原位制备陶瓷层的新方法,但在钢表面难以直接形成陶瓷层。针对这一难题,本文采用PEO方法对热浸镀铝钢进行陶瓷化处理,在钢表面制备了包含Al₂O₃层、Al层和Fe-Al层的多层涂层。本文对该复合涂层的组织结构、微观力学性能和失效行为等方面进行了研究,并采用有限元方法对复合涂层体系结构进行优化设计。研究结果对明确铝镀层陶瓷化演变规律、优化涂层性能和揭示拉伸失效机理具有重要的意义。通过对铝镀层PEO陶瓷化过程的电压特征、陶瓷层厚度生长规律、组织形态和成分等进行研究,揭示出铝镀层转化为陶瓷层的演变规律。研究发现,在PEO初期,铝镀层阳极电压的变化趋势与纯铝试件相同,而在PEO后期,铝镀层陶瓷化的阳极电压出现下降趋势。铝层消耗和陶瓷层增长近似为线性关系,当铝层完全消耗后,陶瓷层生长速率减缓。陶瓷层的主要相结构是γ-Al₂O₃和莫来石相,处理后期出现a-Al₂O₃相。PEO陶瓷层内部包含有许多微米/亚微米尺度的放电孔洞。当陶瓷化处理至Fe-Al层参与反应阶段,PEO过程更加复杂。此时,陶瓷层表面可观察到相对较大的放电孔洞。截面形态分析表明,它们贯穿到Al₂O₃/Fe-Al层界面,并在界面处出现较多裂纹。界面孔洞附近的EDS结果显示,Fe含量可达7.6at.%,远大于陶瓷层非界面孔洞区域的Fe含量。涂层表面元素分布同时显示,Fe-Al层参与反应后,由于Al原子供应减少,导致陶瓷层表面放电孔洞周围的氧化物内Al含量下降,Si含量提高,Fe含量约从0.5at.%上升到1.5².6at.%。本文采用显微硬度计和纳米硬度计表征了复合涂层微区间的硬度、弹性模量和断裂韧性等力学性能,讨论了放电孔洞对陶瓷层微观力学性能的影响。结果表明,陶瓷层纳米硬度约为19.6GPa,比钢基体提高了近15倍。由于放电孔洞影响,陶瓷层的弹性模量和硬度随压入深度出现异常变化情况,纳米压痕中同时出现了侧边裂纹和径向裂纹。陶瓷层放电孔洞的存在,促使纳米压痕裂纹的扩展方向发生偏转、裂纹尖端钝化和应力强度降低,同时产生了多重微裂纹,抑制了裂纹扩展,一定程度上提高了陶瓷层的断裂韧性。通过测量径向裂纹长度,依据Anstis公式可得陶瓷层断裂韧性约为1.75MPa·m1/2。本文建立了铝镀层陶瓷化复合涂层的有限元模型,讨论了在法向均布接触载荷作用下涂层/基体系统的应力场分布状态,创建了复合涂层的归一化厚度配比图,侧重分析厚度配比关系对复合涂层表面应力和界面应力的影响。计算表明,复合涂层各分层均发挥着特定的功能特性,Al₂O₃层厚度决定着涂层内最大剪应力的位置,铝层的存在可降低界面处的剪切应力。当Al₂O₃层与铝层厚度相等时,Al₂O₃/Al界面处的剪应力最小。增加Al₂O₃层厚度,可增强涂层支撑能力,减缓涂层表面的拉应力。当选择较厚的陶瓷层和较薄的铝层和Fe-Al层时,复合涂层具有较低的表面和界面应力,可提高涂层整体的抗接触载荷性能。复合涂层的承载力响应实验,实验结果与有限元分析结果基本吻合。保持Fe-Al层厚度不变,通过改变陶瓷层与铝层厚度比（tAl₂O₃/tAl）,研究了复合涂层的拉伸裂纹扩展及失效行为。结果显示,Fe-Al层内首先出现垂直拉伸方向的横向裂纹,陶瓷层与铝层厚度比（tAl₂O₃/tAl）的变化影响着横向裂纹的扩展方向。当陶瓷层与铝层厚度比（tAl₂O₃/tAl）较小时,横向裂纹易沿着Fe-Al层/基体界面扩展。当tAl₂O₃/tAl较大时,横向裂纹则穿透铝层向涂层表面扩展。为定量描述铝层对横向拉伸裂纹扩展到表面的阻挡能力,本文引入临界裂纹张开位移δc。随铝层厚度增加,δc也随之增加。通过观察拉伸断裂试件的表面横向裂纹分布,发现厚度比tAl₂O₃/tAl增大时,陶瓷层表面的拉伸裂纹间距随之减小。研究证实,厚度配比对铝镀层陶瓷化复合涂层的拉伸失效行为影响较大,铝层能够阻止或延缓横向拉伸裂纹向涂层表面的扩展行为。更多还原

【Abstract】 Plasma electrolytic Oxidation （PEO） is a new method of forming ceramic coatings on light metals such as Al, Mg, Ti, etc. and their alloys. But, it is very difficult in fabricating ceramic coatings on steel using a single PEO technique. To overcome this problem, hot-dipped aluminum （HDA） was treated by PEO technique in this paper, and multilayer composite coatings which consisting of Fe-Al layer, Al layer, and Al₂O₃ layer were formed on steel substrate. Structures, micro-mechanical properties, and failure behavior of in-situ tension of composite coatings were investigated. Moreover, the structure system of composite coating was also analyzed by finite element method （FEM）. These studies are of great significance for exploring the evolution of Al layer transformed into ceramic coating, optimizing coating properties and revealing failure mechanism of these coatings under in-situ tension test.Characteristics of the average anodic voltage, growth regularity of ceramic thickness, surface and cross-sectional morphologies and element distribution were studied by using many test techniques. Results show that the anodic voltages of aluminized steel and pure Al vary similarly at the early PEO stage, but the voltage of aluminized steel decreases at later PEO stage. The thickness of ceramic coating increases approximately linearly with the Al consumption. Once hot-dipped Al layer is completely transformed, the aluminized steel voltage descends. Ceramic coatings are mainly composed ofγ-Al₂O₃, mullite, and a-Al₂O₃ phases, but a-Al₂O₃ appears only at the end PEO stage.There are many discharging pores with micron scale or sub-micron scale within ceramic coatings. When Fe-Al layer participates in PEO reaction, PEO process becomes more complicated than before. Some big discharging holes, which get to Al₂O₃/Fe-Al interface, are within ceramic coatings and many micro-cracks are also observed at the interface. EDS results reveal that the atomic percent of Fe element of these big discharging holes near interface is about 7.6 at.%. It is much larger than that the regions far from discharging holes at interface. Element distribution at the surface of ceramic coating shows that when Fe-Al layer partic ipates in PEO process, and the percent of Al element of the oxides near the big discharging hole decreases. At the same time, the percent of Si element increases and the percent of Fe increases from 0.5at.% to 1.5².6at.% because of the shortage of Al atoms.Mechanical parameters of coatings such as hardness, elastic modulus, fracture toughness, etc. were measured by using a microhardness tester and a nanohardness instrument. The effects of discharging pores on mechanical properties were discussed. Results show that nanohardness of ceramic coating is about 19.6GPa which is 15 times that of the substrate. Due to the effect of discharging pores, some abnormal curves of elastic modulus vs. depth and hardness vs. depth appear and radial and lateral cracks also occur at nanoindentation. It can be found that the crack path of ceramic layer is deflected and the crack tip becomes dull. The stress intensity at crack tip decreases. Many multiple mirco-cracks are also generated by these discharging pores. Therefore, indentation cracks of ceramic layer can be restrained and the fracture toughness of them is improved to a certain extent. Fracture toughness of ceramic coating can be calculated on Ansits formula by measuring the length of radial cracks, which is about 1.75MPa·m1/2 .FEM models of composite coating on aluminized steel were established by using FEM software. And stress fields of these coating/substrate system subjected to uniform normal contact load were computed and investigated. A triangle of normalized layer thickness was created for describing thickness ratios of composite coatings. Then, the effect of thickness ratio on stress fields at the surface or the interfaces was analyzed. It is seen that every layer of composite coating has a special function. For example, the thickness of Al₂O₃ layer determines the location of maximum shear stress within coating. The advantage of Al layer is that interfacial stresses can be reduced greatly. As Al₂O₃ layer and Al layer have the same layer thickness, shear stress at the Al₂O₃/Al interface is minimal. As the thickness Al₂O₃ layer increases, the support performance of coating is improved and tensile stresses at the surface are relieved. When thick ceramic layer and thin Al, Fe-Al layers are chosen, there are fine stress distributions at surface and interface of coating and the supporting performance of composite coatings under contact load can be improved. It is found that the data of supporting test are in accord well with FEM results.As Fe-Al layer thickness was invariable and the thickness ratio of ceramic coating to aluminium layer （tAl₂O₃/tAl） was changed, tension failure and propagating behavior of tensile cracks of composite coating on aluminized steel were studied. The results show that the transverse cracks firstly appear in Fe-Al layer. The thickness ratio tAl₂O₃/tAl can affect the propagating direction of these transverse cracks. When thickness ratio tAl₂O₃/tAl is small, these transverse cracks favor to grows along the Fe-Al/substrate interface. However, when thickness ratio tAl₂O₃/tAl is large, Al layer is very thin and these transverse cracks are easy to penetrate Al layer and arrive at the surface of coating. In this thesis, the critical crack opening displacementδc was introduced to express quantitatively the crack resistance of Al layer, which increases as the Al thickness increases. Through observing the distribution of tensile cracks of samples, we also found that the space between tensile cracks decreases as the thickness ratio tAl₂O₃/tAl increases. It can be concluded that thickness ratio of coating has an important influence on its tension failure and the propagation of tensile cracks is resisted or delayed by Al layer.更多还原