Development of a mechanism-based lifetime model for bi-layer thermal barrier coatings, part 2

陶瓷双层热障涂层的锆酸钆（GZO）的顶层和部分氧化钇稳定的氧化锆（YSZ）的底层允许与传统的单层涂层相比，表面温度的显着增加，从而实现更高的能量效率。在第一个项目期间，追求的主要目标如下：（一）确定GZO涂层的可靠生产路线，（二）确定基本物理性能，（三）确定相关的损坏机制，（四）表征损坏演变和机械性能，作为寿命建模的输入参数，以及（五）开发寿命模型。经过系统的参数研究，与定制的微观结构的化学计量涂层的生产与标准YSZ涂层相比，热循环性能显着增加。然而，涂层性能高度依赖于初始和工艺条件。因此，后续项目的一个主要目标是开发一种工艺工具，以生产具有特定性能范围的可靠涂层。此外，热梯度加载条件和约束烧结对涂层性能的影响将被详细研究。特别是残余应力状态的发展是一个主要的问题，可以有显着的损伤演化和寿命的影响。随着知识的相关损伤机制的寿命模型进行了修改，并成功地适应了双层。该寿命模型能够预测失效临界应变，并通过比较失效临界应变与构件加载循环次数来评估寿命，但迄今为止，该寿命模型没有考虑复杂几何边界条件对寿命的影响，而复杂几何边界条件会显著影响层间应力分布和损伤演化。因此，在第二个项目期间，将研究不同直径的圆柱形试样和具有凹凸形状的更复杂试样。利用这类试件，可以系统地研究几何边界条件对涂层性能和寿命的影响。此外，还对输入参数对临界应变和寿命的统计敏感性进行了分析，以量化涂层特性的散射对寿命的影响，并在此基础上对寿命模型进行了几何扩展。作为宏观损伤参数的临界应变是载荷和几何边界条件的函数。最后，将确定的涂层系统的机械稳定性极限整合到有限元模型中，以预测涂层系统的失效位置以及失效时间。

Ceramic bi-layer thermal barrier coatings with a top layer of Gadoliniumzirconate (GZO) and a bottom layer of partially yttria stabilized zirconia (YSZ) allow a significant increase of surface temperature in comparison with conventional monolayer coatings, thus achieving a higher energy efficiency. In the first project period the following main objectives were pursued: (i) establishing a reliable production route for GZO coatings, (ii) determination of the basic physical properties, (iii) identification of the relevant damage mechanisms, (iv) characterization of the damage evolution and mechanical properties acting as input parameter for lifetime modelling as well as (v) development of a life time model. Following a systematic parametric study, stoichiometric coatings with a tailored microstructure were produced which showed a significant increase in thermo cyclic performance in comparison with standard YSZ coatings. Nevertheless the coating properties are highly dependent on the initial and process conditions. Consequently, one main objective of the follow-up project is to develop a process tool to produce reliable coatings with properties in a specific property range. Moreover, the influence of thermal gradient loading conditions and the influence of constraint sintering on coating properties will be investigated in detail. Especially the development of residual stress state is one main issue which can have a significant influence on damage evolution and lifetime. With the knowledge of the relevant damage mechanisms the lifetime model was modified and successfully adapted to the bi-layer. The developed lifetime model enables a prediction of the critical strain to failure and a lifetime assessment by comparing the critical strain with component loading cycles.However, up to now, the influence of complex geometric boundary conditions, which can significantly affect the stress distribution in the layer system and the damage evolution, on lifetime is not considered in the lifetime model. Consequently, in the second project period, cylindrical specimens with varied diameter and more complex specimens with concave and convex shape will be studied. Using this kind of specimens, the influence of geometric boundary conditions on layer properties and lifetime can be investigated systematically. Moreover a statistical sensitivity analysis of input parameters on critical strain and lifetime will be performed, to quantify the influence of the scattering of coating properties.On the basis of the results the lifetime model will be extended with a geometry-factor. The critical strain, acting as a macroscopic damage parameter, is then a function of loading and geometric boundary conditions. Finally, the determined mechanical stability limits of the coating system will be integrated in a finite- element model to enable a prediction of the failure location as well as the time to failure of the coating system.