MICROSTRUCTURAL INVESTIGATIONS OF PLASMA-SPRAYED YTTRIA PARTIALLY-STABILIZED ZIRCONIA TBC (IN RELATION TO THERMOMECHANICAL RESISTANCE AND HIGH-TEMPERATURE OXIDATION MECHANISMS)

This study deals with microstructural investigations of plasma-sprayed yttria partially stabilized zirconia thermal barrier coatings, perfor med by classical and analytical transmission electron microscopy. The aim of the study was to determine eventual relationships between coati ng microstructure and toughness. The ceramic/metal interface, which pl ays an important role during TBC thermomechanical solicitation, has al so been studied. In the 6-8 wt. percent Y2O3 range, the metastable tet ragonal t' phase is observed, showing special faulted microstructural features, such as grain twinning and antiphase boundary planes. Moreov er, after high-temperature annealing in air, a very fine and stable pr ecipitation of the equilibrium cubic phase appears. If is believed tha t these microstructural elements could act as crack deviation sites an d enhance the coatings' intrinsic toughness. Microstructural investiga tions of the alumina scales grown during high-temperature annealing re veal yttrium segregation at oxide grain boundaries as well as signific ant quantities of zirconium inside the alumina grains. The oxide growt h seems to be dominated by a classical grain boundary oxygen diffusion mechanism. The presence of zirconium inside the alumina grains sugges ts that Al2O3 also partially forms by chemical reduction of ZrO2 by Al .