Synchrotron X-Ray Diffraction Study of Phase Transformation in CMAS Ingressed EB-PVD Thermal Barrier Coatings

摘要

Thermal barrier coatings (TBCs) protect turbine blades from the extreme temperatures found within the turbine section of jet engines during operation. Electron-beam physical vapor deposition (EB-PVD) of yttria-stabilized zirconia (YSZ) results in the coating forming a columnar microstructure that has a higher strain tolerance than other deposition methods, which is desirable for moving components in aerospace applications. However, the lifetime of a YSZ coating is greatly reduced when sand, in the form of calcium-magnesium-alumino-silicate (CMAS), enters into the engine from a sandy environment, becomes molten, and ingresses into the coating during operation. The Y and ZrO_2 within the coating are dissolved into the CMAS, whereas the coating reacts and causes undesirable phase transformations into a globular mon-oclinic phase upon cooling, introducing additional strain and accelerating the risk of failure. This work uses high-energy X-ray diffraction to measure the fhermochemical degradation on 7 wt% YSZ (7YSZ) coating as a result of CMAS ingression as a function of different annealing temperatures and times. This is achieved by measuring and quantifying the volume fraction of the monoclinic phase throughout the depth of the coating. The results show CMAS ingression at 1225 °C has about a 21% less monoclinic phases present in the coating as compared to ingression at 1250 °C. Additionally, the increase in annealing time allows for more Y and ZrO_2 to dissolve into the CMAS from the coating, resulting in a 21% increase in the monoclinic phase fraction and increased coating degradation.