Combination of Fourier-Domain Optical Coherence Tomography and Photo-Stimulated Luminescence Piezo-Spectroscopy as an NDE Tool for Thermal Barrier Coatings

Abstract

A combination of two optical methods — Fourier-domain optical coherence tomography (FD-OCT) and photo-stimulated luminescence piezo-spectroscopy (PLPS) is used as a non-destructive evaluation (NDE) tool for thermal barrier coatings (TBC). This research is focused on NDE of electron beam physical vapor deposition (EB-PVD) TBC’s. FD-OCT is an interferometric technique, which uses spectrally broadband visible or infrared light to obtain spectrally resolved interferograms of the light that is back-scattered from subsurface structures and defects (e.g., interfaces, cracks, voids) in optically translucent material. When the Fourier transform is applied to the interferogram, a depth-resolved image of the back-scattering sites is obtained. FD-OCT is shown to be a useful NDE tool that can profile the top coat-metal substrate interface and measure the top coat thickness. Also, it has the potential of assessing microcracking and spallation damage. PLPS provides quantitative information on stress in the thermally grown oxide (TGO) by measuring the spectral shifts in the laser-induced luminescence spectra of the Cr3+ ions present in the TGO. When combined, the PLPS and FD-OCT methods can provide a set of important input parameters for the TBC remaining life predicting model. Ultimately they will collect spatially resolved data on matching spatial domains. The two optical methods are applied to thermally cycled EB-PVD TBC samples. The experimental results are compared to destructive inspection data.