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Factors affecting fluorescence of thermally grown oxide in the presence of a thermal barrier coating.

Nunn, J W; Banks, J P; Chunnilall, C J; Saunders, S R J (2004) Factors affecting fluorescence of thermally grown oxide in the presence of a thermal barrier coating. NPL Report. DEPC-MPE 006

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Adhesion of a thermal barrier coating (TBC) to the bondcoat has been shown to be dependent upon a good bond being maintained between the thermally grown oxide (TGO) and bondcoat. The fluorescence of chromium impurities in the TGO is used as a measure of the residual stress and thus the adherence of the TGO. This non-destructive method is now used to determine the mechanical integrity of the TBC system itself. To better understand this phenomenon the factors affecting the fluorescent signal have been investigated.
The intensity of fluorescent signal is a direct function of the chromium content of the thermally grown oxide (TGO). In some cases where the intensity increases at a rate faster than expected for oxidation of the bondcoat, this may indicate growth of non-protective TGO. The lateral resolution of spectra from TGO without a thermal barrier coating (TBC) present is about 1-3 µm when using a x100 magnification objective. The effect of adding a TBC layer is a degradation of this resolution and further work is required to quantify this. However, it is clear that with APS TBC systems considerable scatter occurs and indeed it is unclear whether the laser light is transmitted through the 300 µm thick APS layer investigated in this work. For EBPVD TBC it is highly likely that the columnar structure may act as a light-pipe to aid transmission through the layer. While it is clear that lateral resolution will be degraded by the presence of the EBPVD layer, it cannot be quantified at present. EBPVD thickness has a marked effect on signal intensity, and it was estimated that the intensity decreases by half for each 20-25 µm additional thickness. For the laser conditions used with this work, approximately 200 µm was the maximum TBC thickness that could be examined to guarantee a reliable fluorescent signal.

Item Type: Report/Guide (NPL Report)
NPL Report No.: DEPC-MPE 006
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/3823

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