Abstract
An intact thermal barrier coating (TBC) specimen, consisting of electron-beam physical vapor deposited (EB-PVD) ZrO 2–7 wt.%Y 2O 3 (YSZ) topcoat, thermally grown oxide (TGO), grit-blasted (Ni,Pt)Al bond coat and CMSX-4 single crystalline superalloy, was characterized after 645 thermal cycles. Each thermal cycle was carried out in air, and consisted of 10-min heat-up to 1038 °C, 10-h hold at 1038 °C, and 10-min forced air-quench to ambient temperature. Characteristics of TGO scale were initially examined by photostimulated luminescence spectroscopy (PSLS) and scanning electron microscopy equipped with energy dispersive spectroscopy (EDS). Transmission electron microscopy (TEM) and scanning TEM (STEM) with nano-spot energy dispersive spectroscopy, high-angle annular dark field (HAADF) imaging, selected area diffraction (SAD), convergent beam electron diffraction (CBED), and electron energy loss spectroscopy was carried out for detailed microstructural characterization. A site-specific preparation of TEM specimen for the thermally cycled TBC was successfully carried out using focused ion beam in-situ lift out technique. Photostimulated luminescence corresponding to negligible residual stress in TGO scale was observed for this intact TBC. Microstructural analysis showed undulation of interface between TGO and bond coat, and the corresponding damage near YSZ/TGO interface. Extensive decohesion within TGO (nearly 9 μm thick), near the YSZ/TGO interface, was observed by TEM/STEM, and related to the observation made by PSLS. Microstructure and phase constituents of mixed-oxide zone containing Zr, Y, Ta, Ni, Co, Cr, Ti in Al 2O 3 matrix and continuous-columnar Al 2O 3 TGO are presented based on TEM/STEM and related analytical techniques.
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