Assessing the Structural Integrity of Plasma-Sprayed Multilayer Thermal Barrier Coatings

Abstract

The majority of lifetime models associates the failure of thermal barrier coatings to an oxidation of the bond coat. A thickening of the thermally grown oxide leads to a conversion of stresses at the undulated ceramic-metal interface, supporting the propagation of existing micro cracks. However, in plasma-sprayed multilayer thermal barrier coatings consisting of gadolinium zir-conate (GZO) and yttria-stabilized zirconia (YSZ) a shift of the failure site from the ceramic-metal interface to the GZO-YSZ interface has been observed. Thus, an exclusively oxide-based formulation is not sufficient to describe the damage transition phenomena. Therefore, this paper outlines a mechanism-based approach for assessing the structural integrity, considering all relevant thermally activated processes as well as the interaction between thermal and elastic misfits. Oxidation of bond coat, creep of composite materials and sintering of ceramics are modeled in terms of temperature and exposure time. Finite element analysis of GZO-YSZ pairings with different microstructures reveal a strong influence of the initial porosities on the sintering behavior and thus on the resulting mechanical stresses and potential crack driving forces at the bi-material interfaces.