Improvements to Optical Diagnostics of Thermal Barrier Coatings: Contaminant Identification during Laser Cleaning and Time Gated Photoluminescence Piezospectroscopy

Laser ablation has previously been demonstrated as a technique for removal of contaminants from engine-run thermal barrier coatings (TBC). A method for monitoring the concentrations of some of the known constituents of the contaminants is developed for paralleled quantification during laser cleaning. The major constituents of these contaminants, CaO, MgO, Al2O3, and SiO2, also termed CMAS, are prepared in pressed powder samples for which Laser-induced Breakdown Spectroscopy (LIBS) is applied. During LIBS, unique spectra characterizing the contents of the ablated material are obtained. LIBS is applied to the individual CMAS constituent pressed powder samples to obtain reference spectra which are used to construct a set of orthonormal modes via Proper Orthogonal Decomposition (POD). Although these orthogonal modes are mathematically uncorrelated, they may not necessarily be entirely representative of the correspondent constituent references that may cause inaccurate quantifications. Thus, these modes are renormalized by taking advantage of orthogonality between the modes and using that property as a constraint; by doing so, the modes can be reshaped while retaining orthogonality. The resulting orthogonal modes were calibrated using known mixtures, and then applied to engine-run blades for measurements. The missing constituents were quantified by a calculation of residual data. Photoluminescence Piezospectrocopy (PLPS), a nondestructive evaluation (NDE) technique that has been previously used to assess remaining lifetime of TBCs, can suffer from fluorescence interference due to dopants in the coating composition. Unlike that of CMAS obstruction, which can be cleaned via LIBS, the source of interference within the coating could not be physically removed. A different approach was taken to enable evaluation of TBC health; the collected fluorescence was time gated (or time resolved) prior to being spectrally resolved for meaningful measurements. Results showed that variations in the stress across the blade could be resolved