【作者】 史国栋；

【导师】 杜善义； 梁军；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2009， 博士

【摘要】 金属/陶瓷微叠层材料能够综合陶瓷材料和金属材料的优点,在具有较高的高温强度的同时,还具有较好的室温韧性,从而在航空、航天领域具有光明的应用前景。通常的金属/陶瓷叠层材料是以金属增韧陶瓷基体,从而达到提高陶瓷材料韧性的目的,但金属/陶瓷材料间热膨胀系数相差很大,会在这些叠层材料中引入很大的热应力,造成材料在使用过程中容易失效和破坏。而电子束物理气相沉积（EB-PVD）的陶瓷涂层通常具有柱状晶结构,应变容限大,能在一定程度上跟随金属层的收缩和膨胀,从而能够大大提高金属/陶瓷叠层材料的耐热冲击性和使用寿命,但其柱状晶结构会严重损害涂层的强度。根据这些特点,本文提出了以层状陶瓷增强金属基体的结构和方法,并采用EB-PVD技术,成功制备了NiCoCrAl/YSZ金属基微叠层复合材料,同时借助一些现代化的分析测试手段分析了这些微层板在组织、结构和性能方面的特点。本研究的主要内容包括:NiCoCrAl/YSZ微层板的制备方法;微层板中金属层厚度对其再结晶行为的影响;工艺参数、层厚和热处理制度对微层板的组织结构和性能的影响;微层板的残余应力和断裂过程分析;微层板与单层NiCoCrAl薄板的力学性能和抗氧化性能的比较等。研究结果表明,采用EB-PVD技术制备的NiCoCrAl/YSZ微层板具有清晰的层结构,各层的微观结构与基板温度的关系遵从Movchen的经典模型。当基板温度为900℃时,金属层中的晶粒接近于等轴晶,金属相主要以Ni的固溶体的形式存在;陶瓷层为柱状晶结构,其相组成主要为t′-ZrO₂。当基板温度为650℃时,微层板的NiCoCrAl和YSZ层在形核初期都呈等轴晶结构,随着层厚的增加,柱状晶结构变得越来越明显了,柱状晶尺寸也增大了,此时,金属层的γ-Ni基体中,有少量的γ′相析出,而陶瓷层仍主要由t′-ZrO2相构成。当层厚和沉积时其他的工艺参数都相同时,较高的基板温度制备出的NiCoCrAl/YSZ微层板的力学性能要更好些。由于受到岛状生长模式和阴影效应的影响,在两种基板温度下,EB-PVD沉积的材料中,晶粒都成簇生长,晶粒簇沿板厚方向扩展,形成柱状。而且,晶粒簇间的间隙要大于簇内晶粒间的间隙,裂纹易沿晶粒簇界面扩展,造成该处材料发生脆性断裂。而陶瓷层对金属层中的孔或间隙,有一定的封堵作用,因而,通过金属/陶瓷微叠层结构,可以有效限制薄板中的柱状晶、晶粒簇及缺陷尺寸,因而有利于改善层板的力学性能。当基板表面较粗糙时,沉积出的NiCoCrAl/YSZ微层板中会形成较大更弯曲的层界面。大的晶粒簇间隙会降低微层板的室温力学性能,但弯曲的层界面却使微层板的高温力学性能变得更好。经1050℃真空热处理后,NiCoCrAl层发生了再结晶,柱状晶结构转变为等轴晶结构。利用有限元模拟的方法计算了多层厚尺度NiCoCrAl/YSZ微层板在热处理温度下,各种层厚的金属层中的热应力情况;并结合微观结构方面的观察,得出以下结论:在陶瓷层厚度一定的情况下,随着韧性相金属层厚度的增加,金属层所受的热应力减小,导致再结晶的形核率和核的长大速度降低,因此,在热处理过程中,过厚的金属层将不容易发生再结晶,而发生再结晶的金属层中的晶粒尺寸也随着层厚的增加而增加。对制备态微层板试样在弯曲载荷作用下的裂纹扩展情况的观察表明,陶瓷层强度较低,裂纹很容易在陶瓷层中萌生和扩展,如果此时相邻的金属层中的柱状晶较大（高度超过31μm）,裂纹则能够比较容易地沿着金属层上部的柱状晶边界扩展,但金属层中、下部分柱状晶尺寸较小的区域能够阻止裂纹在金属层中进一步扩展,并产生桥联;当遇到结合较弱的层界面时,裂纹很容易发生偏转,并沿界面扩展,这会加速表层桥联的金属层的断裂。对比NiCoCrAl单层薄板和NiCoCrAl/YSZ微层板的拉伸性能发现,在室温、700℃和1000℃时,微层板的强度高于单层金属薄板的,而且,测试温度越高,微层板保持强度的能力越突出。同时发现,微层板的密度远低于NiCoCrAl单层板的,因而,其比强度也要远高于后者的。制备态时,金属层厚度越大,微层板的强度越低,但室温断裂韧性越高,其中金属层厚为35μm的微层板的室温断裂韧性甚至高于NiCoCrAl单层板的。经热处理后,微层板和NiCoCrAl单层板的强度均有所提高,但随热处理时间的延长,NiCoCrAl单层板的强度单调增加,而微层板的强度却是先增加而后降低的。氧化试验表明,氧很容易通过YSZ层扩散到微层板内部的金属层界面,导致不仅靠近外侧的金属层表面发生氧化,而且板内部的金属层界面也发生氧化;而且,微层板中金属层厚度较小时,由于所含的Al量也较少,金属层的表面更不容易形成稳定的Al₂O₃膜。因此,所含层数越多,金属层厚度越小,微层板的抗氧化能力越差。较多的层数和较低的层厚,也导致了这些微层板的抗氧化性能要弱于NiCoCrAl单层薄板的。总的来说,在本文所研究的层厚范围内,金属层厚度越大,微层板的室温断裂韧性和高温抗氧化性越好,但密度和高温强度越小。因此,NiCoCrAl/YSZ微层板的金属层厚度应根据实际的使用要求来调节。更多还原

【Abstract】 Metal/ceramic laminates are increasingly being considered for high temperature applications in the aerospace industry. These composites have been chosen for development because they show superior high temperature creep resistance compared to metals, simultaneously having better toughness and structural integrity compared to monolithic ceramics. In general, the goal of the investigation on metal/ceramic laminates was to improve the poor intrinsic toughness of ceramic matrix. In this study, metal/ceramic composites in which NiCoCrAl alloy matrix was reinforced with modest volume fractions of YSZ ceramic were fabricated by EB-PVD, and their microstructures and properties were also studied by modern analysis and test methods. The main contents of the study include: fabrication of NiCoCrAl/YSZ microlaminates, influence of metal-layer thickness in microlaminates on their recrystallization behaviors, effect of operating technical parameters and laminated structures on microstructures and mechanical properties, influence of annealing treatment on mechanical properties of microlaminates, analysis of residual stress, nano-indentation properties and fracture process for the microlaminates, as well as comparison of tensile properties and oxidation resistance between the microlaminates and a monolithic NiCoCrAl foil, and so on.The results showed that there were flat and distinct interfaces between metal and ceramic layers of the microlaminates, and that the relationship between the microstructures of layers and the substrate temperature was consistent with classic model established by Movchen et al. As the substrate temperature was 900℃, NiCoCrAl layers were found to consist ofγphase and have a relatively large equiaxed grain structure; and YSZ layers had a small columnar grain structure, being composed of tetragonal phase. When the substrate temperature was decreased to 650℃, both NiCoCrAl and YSZ layers showed an equiaxed structure during the early stages of nucleation, and then the columnar structure became more evident with increasing coating thickness; at the same time, a few ofγ′phase separated out in NiCoCrAl layers. Whatever the substrate temperature was 900℃or 650℃, grains of EB-PVD NiCoCrAl and YSZ film formed clusters in which there was a relatively dense grain structure; however there were gaps or holes among grain clusters. As a result, cracks could easily develop along weak intercolumnar interfaces and caused film failure between columns. However, the smaller size scales resulted from laminated structures could increase the films cracking stresses by limiting the size of the flaw sizes. It was also found that the higher substrate temperature was propitious to improve the strength of the microlaminates, and that the coarser substrate could decrease their strength at room temperature and 700℃, but improve their strength at 1000℃.After annealing at 1050℃, recrystallization were observed in the NiCoCrAl layers produced by EB-PVD, and their columnar structures changed into equiaxed structures, which was beneficial to improve the strength of foils. The thermal stresses along the thickness direction of the NiCoCrAl/YSZ multiscalar microlaminates were studied by finite element analysis and numerical calculation method; and the microstructures of the ductile phase layers with different thicknesses were observed. The results showed, with the thickness of ductile phase layer increased, its thermal stress decreased, then, the rate of nucleation and growth of recrystallization decreased. As a result, over 20μm thick metal layers were relatively difficult to be recrystallized; however, the metal layers with thickness less than 13μm were rescrystallize, and the size of their recrystal grains increased with the increasing layer-thickness.The crack growth on cross-sections of as-deposited microlaminate samples was observed. It was found that the lower strength of YSZ layers made cracks nucleate and grow easily in them. If the adjacent NiCoCrAl layers under the YSZ layers was relatively thick, and had relatively evident columnar structure, cracks would keep up propagate along the boundaries between columnar grains of NiCoCrAl layers, but be blunt by the residual part of metal layers whose thickness less than 31μm, then a zone of bridging ligaments would be generated. Moreover, interfacial debonding was also been observed.Comparing with the monolithic NiCoCrAl foil, NiCoCrAl/YSZ microlaminates displayed greater tensile strengths; and with increasing temperature, they showed the better capability to retain strength. When metal layers were relatively thick, the as-deposited microlaminate had relatively low tensile strength, but relatively great fracture toughness. After annealing at 1050℃, the strengths of both the microlaminates and the monolithic NiCoCrAl foil were improved. And, with increasing annealing time, the strengths of the microlaminates increased at first, and then began to decrease; however, the strength of the monolithic NiCoCrAl foil increased uniformly.The oxidation behaviors of monolithic NiCoCrAl foil and NiCoCrAl/YSZ microlaminates were investigated at 1000℃in air. The microlaminates showed relatively weak oxidation resistance than monolithic NiCoCrAl foil. The reasons were analyzed. On one hand, oxygen could easily diffuse along YSZ layers to the interface of all metal layers in microlaminates, then the surface of metal phase be increased greatly, leading to an increasing oxidation rate. On the other hand, due to the relatively small thickness, Al content of metal layers in microlaminates was relatively low; as a result, stable Al2O3 oxide scales were more difficult to form on their surface. In a word, the more number and greater thickness of metal layers lead to the weaker oxidation resistances of microlaminates.In conclusion, when the layer-thickness being between 1.6μm and 35μm, the thicker metal layers were, the better the room-temperature fracture toughness and oxidation resistance of the microlaminate were, but the lower the high-temperature strength and density of microlaminate was. So, the metal-layer thicknesses of NiCoCrAl/YSZ microlaminates should be tailored according to application needs.更多还原