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Development of innovative non destructive methods for TBC coatings and monitoring techniques in the frame of an Italian R&D programme

pubblicazioni - Articolo

Development of innovative non destructive methods for TBC coatings and monitoring techniques in the frame of an Italian R&D programme

and monitoring techniques in the frame of an Italian R&D programme Claudia Rinaldi*, Letizia De Maria*, Ada Del Corno*, Federico Cernuschi*, Iva Gianinoni* The Future of Gas Turbine Technology 4th International Conference Brussels, 15-16 Ottobre 2008 * CESI RICERCA In this paper results are presented of the activities recently performed in the continuation of an Italian National Research Program devoted to develop innovative NDTs and on-line monitoring methods, to increase the reliability of residual life evaluations of gas turbine hot parts. At first some innovative studies on NDTs for coatings are presented. The use of the residual stress of the Thermally Grown Oxide measured by Photo Luminescence Piezo-Spectroscopy is proposed as diagnostic parameter on components coated by EB-PVD TBCs. A literature correlation between the TGO residual stress and the TBC residual life was applied to operated components to evaluate the spent life fraction at different positions, near and far from cooling holes. A confirmation of the NDT’s findings was obtained both with destructive analyses and with a self-developed coating life prediction code. This code contains an inverse problem solution routine able to estimate the mean operating temperature from the β phase depletion of the bond coat. A good agreement was found between the evaluations made with the PLPS technique and the calculations with the life prediction model for the coatings of the first stage rotating blades of the V94.3A2 gas turbine. Secondly due to the limited applicability of PLPS technique to APS TBCs also thermographic methods are discussed, with particular attention on one side to an alogorithm which could help in distinguishing over thickness from delaminations and on the other side trying to use therml diffusivity measurements to gain insight in the damage level of the APS TBC near the BC/TBC interface. On this respect some recent results of thermal diffusivity measurements performed with thermography on groups of specimens aged in laboratory furnaces are shown to be able to give useful indications about the APS TBC degradation level. Finally an optical non intrusive on line monitoring technique to measure the TIT is presented, with the potential of an absolute measure of the gas temperature.

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