Abstract
Thermal barrier coating (TBC) is applied to high temperature components in gas turbines. The thermally grown oxide (TGO) layer, which forms at the interface between the topcoat and the bondcoat, is an origin of topcoat damage, which can lead to delamination of the topcoat. In order to detect the TGO layer in in-service components, a portable detection system based on photoluminesence was developed. The system consisits of a compact, continuous wave Nd:YAG laser of wavelength 532 nm to excite Cr3+ included as an impurity in Al2O3, and a compact spectrometer with a 2-D CCD array for enhanced sensitivity to detect the photoluminescence from Cr3+ at 694.3 nm (R1) and 692.9 nm (R2). An optical probe is used to illuminate the specimen with laser light and to collect the photoluminescence. The probing region is a circle of diameter 5 mm. The optical probe is coupled to the main body by an optical fiber, which allows flexibility to match specimens with curved surfaces or complex structures. The system performance was tested using TBC specimens heated at 1000°C and 1100°C, and an as-sprayed, unheated TBC specimen. Photoluminescence (R1, R2) was observed from heated specimens and was not observed from the unheated specimen, and agreed with previous measurement results. This system can be used for screening tests of topcoat delamination inspection, since delamination is likely to occur at locations where the TGO layer has grown.
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