Abstract
Plasma-sprayed ceramic coatings are often used as thermal barrier or abradable coatings in high-pressure stages of gas turbines. They are exposed to high thermo-mechanical loadings, due to the harsh operating conditions. Today, a material typically used in engines as thermal barrier coating material is yttria-stabilised zirconia (YSZ). This material has a low conductivity and a high thermal expansion coefficient, but a limited temperature capability of about 1200°C in long-term applications. For the use as abradable coatings, thicker coatings with a thickness above one millimetre are necessary. However with increasing coating thickness and limited cooling efficiency there is a risk of premature failure. As a result new ceramic materials have been developed. For the lifetime analysis they were tested by thermal gradient cycling tests. In the present work an APS ceramic double-layer topcoat composed of 7YSZ and a top layer of non-stoichiometric magnesia alumina spinel (Mg-Al-Spinel) was used. The layer was sprayed on disc-shaped IN738 superalloy substrates which were coated with a VPS bondcoat. Under specific thermal cycling conditions with temperatures above 1400°C, these samples showed a typical failure mechanism with exfoliation of thin coating lamellae, starting from the coating surface. This failure mechanism was analysed in detail, e.g. by scanning electron microscopy (SEM), X-ray diffraction, and chemical analysis. From these findings, a description of the failure mechanism was developed.
