Cooling effects on photo-stimulated luminescence spectroscopy of ruby for 3-D stress characterization in thermal barrier coatings

Kara C Cunzeman, Purdue University

Abstract

The analysis and inspection of the cooling effects on photo-stimulated luminescence spectroscopy (PSLS) for stress measurements in chromium-doped alumina was motivated by the need to accurately characterize 3-D stress states in thermally grown oxide (TGO) layers in jet engine turbine blades. In recent studies, new techniques utilizing PSLS have successfully shown potential that the vibronic band region within chromium-doped alumina spectra can be used, along with the accompanying R-lines, to provide a 3-D stress characterization within the material. However, difficulty in accurately obtaining peak positions of the vibronic bands and tracking them as they shift with stress at room temperature has prevented this method from being fully utilized. This study investigates temperature effects on the R-lines and vibronic band regions of the principle axes of single crystal Al2O3Cr 3+, polycrystal specimen, and thermal barrier coating (TBC) samples. A genetic algorithm-based optimization methodology was used to perform curve fitting of the data, and the fitness factor obtained was higher for all cold temperature data compared to room temperature data. A temperature excursion test was conducted on a polycrystalline Al2O3Cr 3+ specimen, concluding experimental temperatures below -122°C provide the necessary spectral resolution to identify known peaks within the vibronic bands. Noise has been reduced in the spectral signal by more than 50% by lowering experimental temperatures from 23°C to -176°C. For the first time, thermally induced peak shifts of the R-lines and vibronic bands at zero load condition are presented for the a and m crystallographic orientations of single crystal at -176°C. The results from this study demonstrate that there are no significant differences in peak locations, under a zero load condition, among the three principal crystallographic orientations. The R-lines and vibronic band temperature-induced shifts suggests their movement is linear for all orientations of single crystal Al2O3Cr3+, however the magnitude of shifting may differ between the R-lines and vibronic bands. Stress-induced vibronic band peak shifts are also investigated for the m axis. For the first time, it can been seen that linear relationships of vibronic band peaks under applied uniaxial stress are more traceable using the piezospectroscopic technique at cold temperatures compared to room temperatures. Common peaks found among all crystallographic orientations at room and cold temperature in the vibronic bands have been identified. This suggests that they may provide a basis of correlation between room and cold temperature data, which is a necessary next step in determining the 3-D stress state within the TGO layer of the TBCs.

Degree

M.S.E.

Advisors

Imbrie, Purdue University.

Subject Area

Aerospace engineering

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