Travis Patterson

Degradation of Yttria Stabilized Zirconia Thermal Barrier Coatings by Molten CMAS (CaO-MgO-Al2O3-SiO2) Deposits

In advanced gas turbine engines that operate in a dust-laden environment causing ingestion of siliceous debris into engines, thermal barrier coatings (TBCs) are highly susceptible to degradation by molten CMAS (calcium-magnesium alumino silicate) deposits. In this study, the degradation mechanisms other than the commonly reported thermomechanical damage are investigated with an emphasis on the thermochemical aspects of molten CMAS induced degradation of TBCs. Free-standing yttria stabilized zirconia (8YSZ) TBC specimens in contact with a model CMAS composition were subjected to isothermal heat treatment in air at temperatures ranging from 1200°C to 1350°C. Phase transformations and microstructural development were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Starting at 1250°C, the molten CMAS readily infiltrated and dissolved the YSZ coating followed by reprecipitation of zirconia with a different morphology and composition that depends on the local melt chemistry. Significant amount of Y2O3 depleted monoclinic ZrO2 phase evolved from CMAS melt that dissolved ť-ZrO2 was evident. Thus the mechanism of dissolution and reprecipitation due to molten CMAS damage resulted in destabilization of the YSZ with disruptive phase transformation (t’ f + m).

Electrophoretic Deposition of Environmental Barrier Overlay Coatings for Yttria-Stabilized Zirconia Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are increasingly susceptible to environmental degradation due to molten deposits of fuel impurities and air ingested sand. At elevated temperature, these deposits adhere, melt and degrade the TBCs via repeated freeze-thaw action and, to a certain extent, direct chemical reaction with TBC constituents. In order to protect TBCs from such melt ingression, a thin, dense and continuous environmental barrier overlay coating can be applied on the surface of TBCs. In this study, dense, continuous and crack-free overlay coatings of YSZ, Al2 O3 and MgO were fabricated by electrophoretic deposition (EPD) on air-plasma sprayed (APS) YSZ. APS YSZ specimens of 300 μm thickness were prepared from 8YSZ powders on graphite substrates. A stable colloidal suspension of YSZ, Al2 O3 and MgO powders in an organic solvent mixture of acetone and ethanol with Iodine additive was used for EPD. At a controlled applied DC voltage with deposition time within minutes, a crack-free green compact can be obtained. After drying and controlled sintering at high temperature, thin, dense, crack-free overlay coatings of controlled thickness on free-standing YSZ were produced. The coatings were characterized by x-ray diffraction and scanning electron microscopy. The degradation resistance of overlay coatings fabricated by EPD and sintering for YSZ TBCs against melt ingression of CMAS and V2 O5 is highlighted.Copyright