| 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.
|
PDF Full Text Request