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Portable optical system for in-situ thermal barrier assessment of service operated blades

pubblicazioni - Articolo

Portable optical system for in-situ thermal barrier assessment of service operated blades

operated blades Letizia De Maria*, Claudia Rinaldi*, Piero Lupetin**, Valter Sergo** ASME TURBO 2006, Barcellona, 8-11 maggio 2006 *CESI RICERCA **CENMAT University of Trieste The integrity of thermal barrier coatings (TBC) is a major issue in life assessment of GT hot parts. An increasing interest is devoted to both life prediction models and non destructive techniques able to detect incipient failure of TBCs before spallation and consequent irreversible component damage. It is inherent to the interface between ceramic top coat and metallic bond coat that a thin Thermally Grown Oxide (TGO) layer develops and grows subjected to very high compressive residual stresses, due to both its volume increase and to thermal expansion mismatch among bond-coat/TGO/top coat. Photo-Stimulated Luminescence Spectroscopy (PLPS) was demonstrated to be able to provide information on the stress state of the TGO by monitoring the frequency shift of some suitable spectroscopic bands with respect to a stress- free reference. A significant decrease of such stress level is a useful indicator of incipient local detachment of the TBC. In this paper a new portable PLPS instrument is described, developed by CESI to allow the in situ detection of PLPS signals on service operated blades. A miniaturised probe was shaped to perform the PLPS measurement also at the leading edge. Instrument validation was performed through tests on a series of TBC coated specimens aged in cyclic and isothermal oxidation at different temperatures; the comparison with results of measurements with a commercial instrument was successful. Moreover the measurements of TGO stresses by PLPS were compared with microstructural analyses performed on the metallographic sections of the exposed samples. The different behaviour between isothermal and cycled specimens was evidenced. The results confirm the system capability of prematurely detect and localise initial “buckling” process on cycled samples, before EB-PVD TBC spallation. Secondly the applicability of the PLPS technique to pseudo columnar air plasma sprayed (APS)assessment was experimentally verified. Finally, the first in-shop application of this prototype is presented: the technique was used in the assessment of the residual life of TBC coated first stage blades of a V94.3A2 gas turbine. The TGO residual stresses beneath the thermal barrier were mapped on different areas of the blade. The system identified the regions of high stresses, near the leading edge. The measured values agree with those expected from the service operating condition. Some areas with an initial degradation of the ceramic/TGO/bondcoat interface were also detected and results confirmed by other NDT techniques.

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