Study Of Structure Degradation Of EB-PVD YSZ Thermal Barrier Coatings Annealed At High Temperature

Thermal barrier coatings(TBCs) are extensively applied in engines, gas turbine motor and turbine blade in an aggressive environment for improving the engine efficiency by increasing operating temperatures. It plays a key role in protecting the superalloy substrate from the high temperature oxidation, corrosion, abrasion. The TBCs in the harsh in-service environments need to withstand very high temperature. Annealing of TBCs at high temperature for a long time may result in significant changes in its structures, even failure. The study of structure degradation of YSZ TBCs annealed at high temperature may provide some scientific basis for improving the performance of TBCs.In this thesis, EB-PVD YSZ thermal barrier coatings(TBCs) grown on a polycrystalline alumina substrate are annealed for 4h, 8h, 12 h and 48 h at 1250°C, respectively. Structure degradation of YSZ is investigated by scanning electron microscopy(SEM), X-ray diffraction(XRD), Raman spectroscopy and transmission electron microscopy(TEM). The obtained results are as follows:The phase composition of the TBCs is investigated by means of X-ray diffraction(XRD) and Raman spectroscopy. The as-deposited sample and annealed sample both consist of t’-ZrO2 and t-Zr O2. The preferred orientation exist in the columns of the TBCs. The main growth direction of thermal barrier coatings is <100> direction.The microstructure of the TBCs is investigated by means of scanning electron microscopy(SEM) and transmission electron microscopy(TEM). The columnar crystals of as-deposited coatings show the “feathery morphology” structures at edges of columns. The feathery-like grain boundaries are along the growth direction of the feather. The columns consist of some columnar grains with grain boundaries parallel to the growth direction of the columnar grains. After annealed, the “feathery morphology” structures develop as the "zigzag" structures, and the “white spots” arrange along the feathery-like grain boundaries and the grain boundaries in the columns. It is confirmed that the “white spots” are actually some vacancy clusters. With the increase of the annealing time, the "zigzag" structures smooth and the adjacent columns fuse together. The density and the size of vacancy clusters increases. The vacancy clusters may promote smoothness of the feathery morphology and fusion of the adjective columns. The developent of the vacancy clusters may lead to propagation of the crack along the vacancy clusters and failure of the thermal barrier coatings.