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

陶瓷双层热障涂层与传统单层涂层相比，顶层为锆酸钆（GZO），底层为部分氧化钇稳定的氧化锆（YSZ），可以显著提高表面温度，从而实现更高的能效。在第一个项目期间，实现了以下主要目标：(i)建立可靠的GZO涂层生产路线，（ii）确定基本物理性能，（iii）确定相关损伤机制，（iv）表征损伤演变和机械性能，作为终身建模的输入参数，以及(v)开发终身模型。经过系统的参数研究，生产出具有定制微观结构的化学计量涂层，与标准YSZ涂层相比，其热循环性能显着提高。然而，涂层的性能高度依赖于初始条件和工艺条件。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，还详细研究了热梯度加载条件和约束烧结对涂层性能的影响。特别是残余应力状态的发展是影响损伤演化和寿命的一个主要问题。在了解损伤机理的基础上，对寿命模型进行了修正，成功地适应了双层结构。开发的寿命模型可以通过比较临界应变和组件加载周期来预测失效的临界应变和寿命评估。然而，到目前为止，在寿命模型中没有考虑复杂的几何边界条件对寿命的影响，而这些条件会对层系中的应力分布和损伤演化产生重大影响。因此，在项目二期将研究不同直径的圆柱形试件和凹凸形状的更复杂的试件。利用这种试样，可以系统地研究几何边界条件对层性能和寿命的影响。此外，还将进行输入参数对临界应变和寿命的统计灵敏度分析，以量化涂层性能散射的影响。在此基础上，将寿命模型扩展为一个几何因子。临界应变作为宏观损伤参数，是载荷和几何边界条件的函数。最后，确定的涂层系统的机械稳定性极限将集成在一个有限元模型中，以便预测涂层系统的故障位置以及失效时间。

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.