Finite element simulation of stress evolution in thermal barrier coating systems
Finite element simulation of stress evolution in thermal barrier coating systems
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发表时间：
2006
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通讯作者：
P. Bednarz;L. Singheiser
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作者：
P. Bednarz;L. Singheiser
由Piotr Bednarz进行的热障涂层系统应力演变的有限元模拟燃气涡轮机材料暴露在极高的温度下需要保护涂层。为了设计可靠的组件，需要更好地了解涂层失效机制。热障涂层系统（TBC）的损坏与粘结涂层的氧化、陶瓷的烧结、材料成分的热失配、BC/TGO/TBC界面的复杂形状、通过蠕变和塑性变形的应力再分布以及抗裂性有关。在这项工作中，CMSX-4，MCrAlY（粘结涂层）和APS-TBC（部分稳定氧化锆）的热机械性能的实验数据，实施到一个FE模型，以模拟在金属/陶瓷界面的应力发展。有限元模型再现了相应实验中使用的试样几何形状。它包括一个周期性的单位细胞表示的一个切片的圆柱形试样，而单位细胞的周期长度等于一个理想的波长的粗糙的金属/陶瓷界面。实验加载条件下的热循环的形式与停留时间在高温和连续氧化的考虑进行了模拟。通过逐步考虑各种材料特性和工艺，获得了最真实地模拟材料行为的参考模型。系统的参数变化的应力发展和关键网站相对于可能的裂纹路径的影响。此外，计算了BC/TGO界面处粗糙峰处裂纹的萌生和扩展。可以得出结论，TBC中应力发展的现实建模至少需要以下的可靠数据：i）BC和TGO塑性，ii）BC和TBC蠕变，iii）连续氧化，特别是包括横向氧化，以及iv）界面（BC/TGO，TGO/TBC）和每层的临界能量释放速率。从所执行的材料性能变化的参数研究的主要结果表明，在TBC的孔隙率应增加和烧结减少，以防止或阻碍在TBC的山谷以上的拉伸应力的连续路径。结果表明，在BC的蠕变速率的变化影响在TBCs的边缘应力值。因此，无法推断对寿命的正面或负面影响。结果表明，TBC层中较高的蠕变速率导致较低的应力水平。热膨胀系数的极端变化（±50%）有助于更好地理解应力发展的这些变化。在纯热循环条件下，基体材料的蠕变对应力场的发展影响很小，因此在这种情况下可以忽略不计。随着拉伸应力随着相对高的横向氧化分数而增加，不仅平面外氧化动力学，而且其横向分量也应该低。的粗糙度的振幅和波长的修改表明，随着粗糙度的增加，连续的径向拉伸路径中的TBC和部分在TGO已经形成161个周期后。波长、振幅和形状的变化提高了对应力发展的理解。大量的各种参数的变化研究本工作在一个高度复杂的，而现实的有限元模型有助于显着提高理解的热障涂层。这得到了最终结论的支持，即一组关键参数可以简化为TBC和TGO的时间依赖性变形行为、氧化动力学（包括横向氧化）和界面粗糙度的形状函数。
Finite Element Simulation of Stress Evolution in Thermal Barrier Coating Systems by Piotr Bednarz Gas turbine materials exposed to extreme high temperature require protective coatings. To design reliable components, a better understanding of the coating failure mechanisms is required. Damage in Thermal Barrier Coating Systems (TBCs) is related to oxidation of the Bond Coat, sintering of the ceramic, thermal mismatch of the material constituents, complex shape of the BC/TGO/TBC interface, redistribution of stresses via creep and plastic deformation and crack resistance. In this work, experimental data of thermo-mechanical properties of CMSX-4, MCrAlY (Bond Coat) and APS-TBC (partially stabilized zirconia), were implemented into an FE-model in order to simulate the stress development at the metal/ceramic interface. The FE model reproduced the specimen geometry used in corresponding experiments. It comprises a periodic unit cell representing a slice of the cylindrical specimen, whereas the periodic length of the unit cell equals an idealized wavelength of the rough metal/ceramic interface. Experimental loading conditions in form of thermal cycling with a dwelltime at high temperature and consideration of continuous oxidation were simulated. By a stepwise consideration of various material properties and processes, a reference model was achieved which most realistically simulated the materials behavior. The influences of systematic parameter variations on the stress development and critical sites with respect to possible crack paths were shown. Additionally, crack initiation and propagation at the peak of asperity at BC/TGO interface was calculated. It can be concluded that a realistic modeling of stress development in TBCs requires at least reliable data of i) BC and TGO plasticity, ii) BC and TBC creep, iii) continuous oxidation including in particular lateral oxidation, and iv) critical energy release rate for interfaces (BC/TGO, TGO/TBC) and for each layer. The main results from the performed parametric studies of material property variations suggest that porosity in the TBC should be increased and sintering decreased, in order to prevent or hinder continuous paths of tensile stresses above the valleys in the TBC. It was shown that variations of creep rates in the BC influence marginaly stress values in TBCs . Therefore neither a positive nor a negative influence on the lifetime can be extrapolated. It was shown that higher creep rates in the TBC layer led to a lower stress level. The extreme variations of thermal expansion coefficient (±50%) help in better understanding of these variations on stress development. The creep of base material only slightly affects stress field development, under pure thermal cycling and can therefore be neglected in this case. As the tensile stresses increase with a relatively high fraction of lateral oxidation not only the out-of-plane oxidation kinetics, but also its lateral component should be low. The modification of amplitude and wavelength of the asperity showed that with increasing roughness a continuous radial tensile path in the TBC and partially in the TGO was formed already after 161 cycles. The variations of wavelength, amplitude and shapes improve the understanding of stress development. The large variety of parametric variations studied by the present work in a highly complex and rather realistic FE model contribute significantly to an enhanced understanding of TBCs. This is supported by the final conclusion, that the set of crucial parameters could be reduced to the time dependent deformation behavior of TBC and TGO, the oxidation kinetics, including lateral oxidation and the shape function of the interface asperity.