Life Time Prediction Model for Plasma‐Sprayed Thermal Barrier Coatings Based on a Micromechanical Approach

Abstract

Typical plasma-sprayed thermal barrier coating systems consist of a vacuum plasma-sprayed MCrAlY-bondcoat and an atmospheric plasma-sprayed yttria stabilized zirconia top coat. The bondcoat layer has a surface roughness in the order of 5-7 μm, which guarantees sufficient bonding of the ceramic top coat. This roughness has a significant effect on the stresses within the thermal barrier coating close to the interface. Finite element calculations have been performed to get a deeper insight in the stress developement. In this calculations also the effect of bondcoat oxidation has been introduced. It could be shown that the formation of a thermally grown oxide (TGO) on the bondcoat promotes crack extension across roughness valleys. Taking into account also the effect of substrate curvature it was possible to outline a model to describe the failure of thermally cycled thermal barrier coatings. The predictions of this model were in good agreement with the results of thermal cycling tests. Introduction Thermal barrier coating (TBC) systems are widely used in gas turbines to improve the performance of the engines. On the one hand the use of TBCs can lead to a reduction of the metal temperature and hence to an increase of the components life time, on the other hand it enables an increase of the operating temperature resulting in a higher efficiency of the engine [1]. From all coating technologies for thermal barriers atmospheric plasma spraying (APS) has the potential to become the most widely used. Commonly used TBCs consist of a 7-8 wt. % yttria stabilized zirconia (YSZ) topcoat and a vacuum plasma sprayed (VPS) MCrAlY-bondcoat (BC). The latter protects the substrate from oxidation and enables a good linkage with the topcoat because of its rather rough surface. More details are found in several review articles [2, 3, 4, 5, 6]. TBC failure is essentially initiated by stresses which are induced during thermal cyclic loading due to thermal expansion mismatch and due to oxidation of the rough bondcoat. So the efficient use of TBC systems in industrial applications demands for a reliable life time prediction model. For the development of a microstructural based life time model we evaluate the stress state in the plasma-sprayed TBC system. Here the roughness of the bondcoat, the oxide layer thickness as well as the curvature of the substrate are considered. The results of the stress evaluation are then used to describe the microstructural mechanism for crack propagation in the TBC. The predictions of the model are compared with experimental results from thermal cycling tests.