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中燃气涡轮发动机最热的部分，它们保护内部金属部件免受过热和熔化。提高这些发动机的性能需要提高工作温度，这依赖于FGCs在耐温性和抗断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，然后这些缺陷会引发裂纹；缺陷和裂纹改变了FGCs的导热系数和抗断裂性能。在这种背景下，研究在高热流通量影响下均匀基底（FGC/H）上功能梯度涂层的热断裂是很有必要的。该项目的主要目标是开发一种计算半分析模型，用于分析均匀基板FGC/H（由于氧化过程而在涂层和基板之间形成额外的氧化层）上功能梯度涂层在热和机械载荷下的断裂。随着温度升高和高热通量而产生的一些新问题将得到解决，例如，具有预先存在的相互作用裂缝系统的FGC/H的导热性问题；由于裂缝内气体的热导性，裂缝将被认为是部分热渗透的。材料的性质随温度变化很大，这将在模型中考虑。FGCs的热弹性特性将从其实际应用的角度来建模，以解决FGCs在热载荷和机械载荷下的裂纹问题。热问题和热弹（塑性）问题将用积分方程的方法表述。解将得到数值，使用特殊的积分公式。此外，对于一些特殊情况，如热渗透裂纹的相互作用，将得到新的近似解析解。这种半解析方法可以将结构材料参数（材料级配、裂纹参数）和热-机械载荷参数与主要断裂特征联系起来。该模型与详细的参数分析相结合，有助于优化FGCs的级配及其结构，从而提高FGC/H系统在高温下的抗断裂能力。在这方面，将分析FGCs潜在的理想热学和机械性能，以及用于高级热障涂层应用的可用实际材料组合。

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.