Thermal cycling of EB-PVD/MCrAlY thermal barrier coatings: II. Evolution of photo-stimulated luminescence

Degradation of thermal barrier coatings (TBCs) was investigated as a function of thermal cycles by examining the structural integrity of the thermally grown oxide (TGO) and TBC/TGO/bond coat interfaces using photo-stimulated luminescence spectroscopy (PSLS). The disk-shaped specimens of a production TBC employed in this study consist of: an electron beam physical vapor deposited (EB-PVD) ZrO2–7 wt.% Y2O3 (YSZ); thermally grown oxide (TGO); MCrAlY bond coat; and IN-738 polycrystalline superalloy substrate. The furnace thermal cycling of the TBC consisted of a 10-min heat-up, a 40-min hold at 1121°C (2050°F) and a 10-min forced air quench to ambient temperature. Various characteristics of the photo-stimulated luminescence spectra from the TGO, including peak shifts (i.e. residual stress) and relative intensities of the R1–R2 photo-luminescence, were examined as a function of thermal cycles and related to the spallation failure of the TBC, which occurred at approximately 400 cycles. A gradual increase in the compressive residual stress in the TGO was observed with thermal cycling up to 200 cycles. The magnitude of compressive stress in the TGO remained constant (∼4.5 GPa) from 200 to 350 cycles, then decreased sharply near 400 cycles, associated with spallation of the TBC. Photo-stimulated, bimodal luminescence spectra, consisting of stressed and stress-free luminescence peaks were frequently observed for TBCs. Q1 and Q2 luminescence from metastable θ-Al2O3 and N-luminescence (in addition to R1–R2 luminescence) arising from Cr3+ concentration in the α-Al2O3 scale were also observed. The physical basis for the evolution of these photo-stimulated luminescence spectra is discussed and related to the progressive damage and the final spallation of the TBCs.