Transmission Electron Microscope Characterization of a Plasma-Sprayed ZrO2-Y2O3-TiO2 Thermal Barrier Coating

Some microstructural elements, such as microtwins in tetragonal ZrO2 grains (t-ZrO2), were identified in a plasma-sprayed ZrO2-Y2O3-TiO2 coating by using transmission electron microscopy. These elements are similar to those described in plasma-sprayed ZrO2-Y2O3 coatings and are characteristic of nontransformable t-ZrO2. This microstructure could promote some toughening mechanisms. The monoclinic ZrO2 phase (m-ZrO2), produced by a martensitic transformation of the metastable t-ZrO2 phase, also was observed. The variation in the concentration of Ti in the ZrO2-Y2O3-TiO2 coating body was found to be appreciable. The energy-dispersive spectroscopic analyses showed that a high concentration of Ti was associated with t-ZrO2 whereas a relatively lower content of Ti was found in m-ZrO2. ZrO2-Y2O3-TiO2 thermally treated at 1400 and 1600°C showed signs of grain growth. A higher incidence of m- and t-ZrO2 twins indicated higher constraints of the polycrystalline structure as a result of the grain growth and rapid cooling rate.