Characterization of ZrO2-7 wt% Y2O3 thermal barrier coatings with different porosities and FEM analysis of stress redistribution during thermal cycling of TBCs

Superallog samples (IN 738) were coated with thermal barrier coatings (TBC). This TBC-system consisted of two layers. The first layer was a vacuum-plasma-sprayed, corrosion-resistant layer (MCrAlY) which also acted as a bond coat. The ceramic top layer was atmospheric-plasma-sprayed Y2O3 partially stabilized ZrO2. in order to produce different microstructures, the plasma-spraying parameters for the production of the ceramic coatings were varied. The different ceramic coatings were characterized in terms of porosity and mean elastic modulus. The porosity distribution was also investigated due to its influence on the measured elastic modulus. One series of TBC-coated specimens was cyclically oxidized at a maximum temperature of 1100 degrees C. After 500 h of thermal cycling, creep within the MCrAlY-bond coat led to a coating failure at both the internal beveled edge and free edge around the specimen. A finite element analysis study of the cyclic oxidation experiment was performed to gain insight into the stress redistributions within the bond coat as a function of time. During the initial temperature increase, critical tensile normal stresses developed within the zirconia coating at the free edge. However, these normal stresses became compressive for all following cooling cycles. On the other hand, large tensile normal stresses developed within the zirconia coating at the beveled edge during all the cooling cycles. Therefore, high normal stresses responsible for debonding were present within the zirconia coating during all cooling cycles, with the most critical stresses occurring at the free edge during the first cooling cycle and near the beveled edge for all the following cooling cycles. (C) 1997 Elsevier Science S.A.