Modeling of fracture of functionally graded thermal barrier coatings under high heat fluxes

功能梯度热障涂层(FGCS)用于能源生产工程和飞机上燃气轮机发动机最热的部件，保护内部金属部件不受过热和熔化的影响。增加这些发动机的容量需要提高工作温度，这依赖于FGCS在提高温度耐久性和抗裂性方面的进一步改进。在制造和使用过程中，FGCS中会出现细小的缺陷，并由此产生裂纹，这些缺陷和裂纹改变了FGCS的导热系数和抗裂性。在这种背景下，研究高热流密度下均匀基片(FGC/H)上功能梯度涂层的热断裂是非常有必要的。本课题的主要目标是建立一个计算半解析模型来分析均匀基片(FGC/H)上的功能梯度涂层在热和机械载荷作用下的断裂(涂层与基片之间有一个氧化层)。将解决由于温度升高和高热流密度而产生的一些新问题，例如具有预先存在的相互作用的裂纹系统的FGC/H的导热问题；由于裂纹内气体的导热性能，这些裂纹将被认为是部分热渗透的。材料的性质随着温度的变化而显著变化，这将在模型中考虑在内。从FGCS在热载荷和机械载荷作用下裂纹的具体应用角度出发，对FGCS的热性能和弹性性能进行了模拟。热问题和热弹(塑性)问题将用积分方程组表示。将使用积分的特殊求积公式，以数值方式获得该解。此外，对于某些特殊情况，例如热渗透裂纹的相互作用，将得到新的近似解析解。这种半解析方法允许将结构材料参数(材料级配、裂纹参数)和热机械载荷参数与主要断裂特征关联起来。该模型与详细的参数分析相结合，有助于优化FGCS的级配和结构，从而提高高温下运行的FGC/H系统的抗裂性。在这方面，将分析FGCS潜在的所需的热和机械性能，以及可用于先进热障涂层应用的实际材料组合。

Functionally graded thermal barrier coatings (FGCs) are used in energy producing engineering and aircraft applications in the hottest parts of gas-turbine engines, and they protect the inner metallic parts from overheating and melting. Increasing the capacity of these engines requires increasing the operating temperature, and this relies on further improvement of FGCs with respect to improvement of temperature durability and fracture resistance. During manufacturing and service small defects appear in FGCs and then the cracks can initiate from these defects; the defects and cracks change both the thermal conductivity and fracture resistance of FGCs. In this context, a study of thermal fracture of functionally graded coatings on a homogeneous substrate (FGC/H) under the influence of high heat fluxes is of great demand.The main goal of the project is to develop a computational semi-analytical model for the fracture analysis of functionally graded coatings on a homogeneous substrate FGC/H (with an additional oxidation layer between the coating and substrate which is formed as a result of oxidation processes) under thermal and mechanical loading. A number of new issues arising with increased temperatures and with high heat fluxes will be addressed, such as, thermal conductivity problems for FGC/H with pre-existing systems of interacting cracks; the cracks will be considered as partially thermal permeable due to the thermo-conductivity of gas inside the cracks. The material properties vary significantly with temperature and this will be accounted for in the model. The thermal and elastic properties of FGCs will be modeled from the point of view of its practical application to the specific problem of cracks in FGCs under thermal and mechanical loadings. The thermal problem and thermo-elastic (plastic) problems will be formulated by means of integral equations. The solution will be obtained numerically, using special quadrature formulae for the integrals. Besides, new approximate analytical solutions will be obtained for some special cases, e.g. for the interaction of thermally permeable cracks. This semi-analytical approach allows to correlate the structural material parameters (material gradation, crack parameters) and the thermo-mechanical loading parameters with the main fracture characteristics. The model in combination with a detailed parametric analysis can help to optimize the gradation of the FGCs and their structure in order to improve the fracture resistance of FGC/H systems operating under elevated temperatures. In this regard, potentially desirable thermal and mechanical properties of FGCs will be analyzed as well as available real material combinations for advanced thermal barrier coating applications.