Search

Laser ablation has previously been demonstrated as a technique for removal of contaminants from engine-run thermal barrier coatings. In this paper, laser-induced breakdown spectroscopy (LIBS) was applied during ablation cleaning to quantify the contaminant concentrations. Calibration LIBS spectra were acquired from pressed powdered samples of calcium oxide, magnesium oxide, alumina, and silica, which are major components of surface contaminants (referred to as CMAS) in gas turbine engines. Proper orthogonal decomposition (POD) was applied to the calibration spectra from the CMAS components to generate an orthogonal mode set for reducing measured data. Since these POD modes can represent mixed compositions, an approach was derived to separate the modes into chemically unique spectra while retaining their orthogonal and normal properties. Results using these renormalized modes show that the calcium and magnesium oxide composition in CMAS can be generally quantified. Quantification for alumina and silica is limited due to their inherently lower signal. Measurements were applied to turbine blades run in a gas turbine engine to examine the distribution of CMAS contaminants across the surface. Results showed the CMAS contaminants on the blades were abundant in calcium and magnesium oxides, which represented over 85% of the LIBS spectra. Approximately 14% of the measured CMAS contaminant spectra were residual, representing noise and unidentified species. [ABSTRACT FROM AUTHOR]

Abstract: A method for cleaning thermal barrier coatings (TBCs) contaminated during engine operation has been developed using laser ablation. Surface contamination on the turbine blades hinders nondestructive remaining life evaluation using photoluminescence piezospectroscopy (PLPS). Real time monitoring of the removed material is employed to prevent damage to the underlying coating. This method relies on laser induced breakdown spectroscopy (LIBS) to compute the cross correlation coefficient between the spectral emissions of a sample TBC that is contaminated and a reference clean TBC. The ablation process is stopped when the LIBS signal indicates the presence of the underlying TBC. It is shown that it is possible to remove targeted contaminants and cease ablation at top surface of the TBC. Subsequent microscopy images and PLPS measurements indicate that the integrity of the TBC has been maintained during the removal of surface contaminants. [Copyright &y& Elsevier]

Abstract: Optical measurements of stress in a thermally grown oxide (TGO) layer that has formed between a metallic bond coat and thermal barrier coating (TBC) have been previously demonstrated and shown useful in understanding aspects of TBC failure. These measurements have promise for nondestructive evaluation of coated turbine components. However, engine-run turbine parts collect significant surface contamination consisting of calcium-magnesium-alumina-silicate (CMAS). The deposited CMAS both blocks optical measurement of stress in the TGO and produces false stress signals. A recently developed laser ablation technique has enabled contaminant removal with minimal TBC damage. The locally cleaned engine run parts can then have their TGO stresses optically determined. In the present paper, it is shown that sporadic signals from contamination are still present due to CMAS infiltration into the TBC and due to local regions that cannot be cleaned without damaging the TBC. Methods for separating the contamination signal from the TBC signals are presented as needed for successful use of optical stress measurement on engine run turbine components. The stress measurements censored for false signals are compared to other destructive approaches and the methods are shown to provide robust nondestructive evaluation of the coating stress. [Copyright &y& Elsevier]