Comparison and Selection of Data Processing Methods for the Application of Cr3+ Photoluminescence Piezospectroscopy to Thermal Barrier Coatings

Abstract

Thermal barrier coatings (TBCs) are an indispensable part of the blades used in aeroengines. Under a high-temperature service environment, the thermal oxidation stress at the interface is the main cause of thermal barrier failure. Cr3+ photoluminescence piezospectroscopy has been successfully used to analyze the thermal oxidation stress of TBCs, but systematic and quantitative analysis results for use in data processing are still lacking, especially with respect to the identification of peak positions. The processing methods used to fit spectral data were studied in this work to accurately characterize TBC thermal oxidation stress using Cr3+ photoluminescence spectroscopy. Both physical and numerical experiments were carried out, where Cr3+ photoluminescence spectra were detected from alumina ceramic samples under step-by-step uniaxial loading, and the simulated spectra were numerically deduced from the measured spectral data. Then, the peak shifts were obtained by fitting all spectral data by using Lorentzian, Gaussian and Psd-Voigt functions. By comparing the fitting results and then discussing the generation mechanism, the Lorentzian function—not the Psd-Voigt function that is most widely utilized—was regarded as the most applicable method for the application of Cr3+ photoluminescence piezospectroscopy to TBCs because of its sufficient sensitivity, stability and confidence for quantitative stress analysis.

Characterization of the Internal Stress Evolution of an EB-PVD Thermal Barrier Coating during a Long-Term Thermal Cycling

“…The thermal oxidation stress in TGO can be expressed as: where Π U is the piezospectroscopic coefficient under a uniaxial stress state. The piezospectroscopic coefficients Π U of TGO were 2.71 ± 0.04 cm −1 /GPa and 2.50 ± 0.05 cm −1 /GPa for R1 and R2, respectively [ 30 ].…”

“…PLPS technology has a wide application in stress characterization and debonding detection of TBCs [ 29 , 30 , 31 , 32 , 33 , 34 ]. Cr 3+ PLPS technology has been widely used to characterize the TGO stress [ 35 , 36 , 37 , 38 , 39 ].…”

Characterization of the stress distribution and evolution inside a thermal barrier coating after initial thermal cycles

“…As the TGO can be regarded as a thin film, the stress state is presumed to be a biaxial in‐plane equal biaxial stress state (i.e., σ 11 = σ 22 , σ 33 = 0). Therefore, the simplified relationship between the wavenumber shift of the ruby fluorescence peak and the stress of the TGO layer is almost homologous with that of Eu 3+ photoluminescence given in Equation () 35,65 . For ruby photoluminescence piezospectroscopy, the piezo‐spectroscopic coefficients under a uniaxial stress state (П U ) were calibrated as in a previous study 65 and were −2.71 ± 0.04 cm −1 /GPa and −2.50 ± 0.05 cm −1 /GPa for R 1 and R 2 , respectively.…”

“…Therefore, the simplified relationship between the wavenumber shift of the ruby fluorescence peak and the stress of the TGO layer is almost homologous with that of Eu 3+ photoluminescence given in Equation ( 6). 35,65 For ruby photoluminescence piezospectroscopy, the piezo-spectroscopic coefficients under a uniaxial stress state (П U ) were calibrated as in a previous study 65 and were −2.71 ± 0.04 cm −1 /GPa and −2.50 ± 0.05 cm −1 /GPa for R 1 and R 2 , respectively. Three measuring points were randomly selected on the surface of TBC specimens after thermal cycles to collect ruby fluorescence spectra, as shown in Figure 6.…”

Optimal Data Processing Method for the Application of Eu3+ Photoluminescence Piezospectroscopy in Thermal Barrier Coatings

“…Data processing methods (Lorentzian, Gaussian, pseudo-Voigt (Psd-Voigt) functions, and the barycenter method) were used to identify the positions and intensities of the characteristic peaks in the spectra taken from samples with external or internal stress. For example, Lu et al [45] compared the fitting results using different functions and found that the Lorentzian method was the most applicable and contributed to improving the stress measurement precision in the TGO layer based on Cr 3+ photoluminescence piezospectroscopy. However, there is still a lack of specific studies on the effectiveness and applicability of data processing methods for the high-resolution stress analysis of the TBC layer using Eu 3+ photoluminescence piezospectroscopy.…”