Saunders, S R J; Banks, J P (2002) An evaluation of thermal barrier coated specimens exposed to long-term ageing tests. NPL Report. MATC(A)113

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Abstract

Specimens coated with thermal barrier systems were exposed to long-term ageing tests and their behaviour monitored using piezospectroscopy in a non-destructive manner to determine the residual stress in the thermally grown oxide (TGO) layer that forms under the thermal barrier coating. An investigation of these specimens was carried out using destructive evaluation methods (mostly by examination of metallographically prepared cross-sections) in order to validate the observations made using the non-destructive piezospectroscopic method.
In general, for electron beam physical vapour deposition thermal barrier coatings (EB-PVD TBCs) the changes in stress state observed in plan were substantiated in cross-section, although the act of preparing the cross-section changed the stress state in the TGO. For air plasma sprayed (APS) TBCs systems, however, there was little change in stress of the TGO when examined in plan, while similar stresses to those observed for the EB-PVD TBC was found in cross-section examination. The effect of internal scattering of the laser light by cracks and pores in the APS systems such that the TBC was no longer transparent to the laser light is believed to be responsible for these observations. It was also demonstrated that scattering of the laser light occurred in the cross-section and fluorescence was observed in the TBC that originated in the TGO, and thus great care should be exercised when using the piezospectroscopic method to examine specimens in cross-section.
When the alumina layer began to fail increased levels of chromium were observed and this altered both the peak position and intensity of the fluorescence peaks. This effect should be considered when interpreting the spectra.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MATC(A)113
Subjects:	Advanced Materials Advanced Materials > Surface Engineering
Last Modified:	02 Feb 2018 13:17
URI:	http://eprintspublications.npl.co.uk/id/eprint/2444

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