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Coating Residual Life Evaluation at Different Positions of Operated Components with EB-PVD TBCs

pubblicazioni - Articolo

Coating Residual Life Evaluation at Different Positions of Operated Components with EB-PVD TBCs

operated both in baseload and cycled conditions Claudia Rinaldi*, Letizia De Maria*, Marco Mandelli** International Conference Thermal Barrier Coatings II Kloster Irsee, Germania 12-17 Agosto, 2007 * CESI RICERCA ** PROING ITALIA Understanding the behaviour and reliability of thermal barrier coatings (TBCs) requires a multidisciplinary approach, as well known. Published literature about thermal barriers life evaluation methods is very wide, mainly in the fields of modelling and non destructive testing. From an engineering perspective, developments are required to obtain innovative NDT techniques applicable during periodic inspections of gas turbines and diagnostic parameters to quantify TBC spent life fraction. In this paper the results are presented of an Italian R&D programme devoted to develop an assessment methodology capable to use models and NDTs to gain insight into the spent life of operated components coated with EB-PVD TBCs. In the first phase of the four year activity furnace cycling of coated specimens allowed to study the coating/ substrate interaction and determine a quantitative description of the diffusion induced degradation phenomena of bond coat (BC) and BC/TBC interface (interdiffusion with the substrate, aluminum depleted regions evolution and TGO growth law). Inverse problem solution (IPS) methodologies were applied to obtain an estimation of the local mean operation temperature of the component surface from the measured values of depleted regions of the bond coat. A software was developed to obtain a quick temperature estimation starting alternatively from: ♦ metallographic analyses (BC depleted regions width), if the component is scrapped: ♦ the effective residual beta phase, EBT, which is the diagnostic parameter determined by the frequency scanning eddy current technique (F-SECT) /1/, ♦ In parallel a recently developed /2/ portable PLPS device was applied for in shop measurements of TGO residual stress level on components both before and after service operation. The different degradation level of the interface was evaluated at several positions of some ex-service components. The actual local effectiveness of film cooling could be evidenced. A correlation was found between the mean local operating temperature estimated by the IPS routine of the life prediction software and the interface degradation detected by PLPS. The metallographic analyses performed on sections confirmed the actual damage level at the BC/TGO/TBC interface. A literature correlation /3/ between the measured TGO internal compressive stress level and the spent life fraction of EB-PVD coated specimens is here extended to the data collected on components coming from plants operated in different conditions. With the proposed PLPS diagnostic method, differences in TBC damage levels could be quantified; in addition, different values of residual life were estimated at equivalent positions on different components (ex-service from base load and cyclic operated plants),. Confirmation of the NDT findings were obtained by metallographic analyses. Such results were also compared with the predictions of the coating life evaluation software, recently implemented with equations proposed in literature /4/ to take into account thermal cycling influence. /1/ C. Rinaldi et al., paper GT 95555 ASME ESDA 2006, Turin, Italy, July 7-10, 2006 /2/ L. de Maria et al., paper GT2006-90551 ASME Turbo Expo 2006, Barcelona, Spain, May 8-11, 2006 /3/ E. Jordan, M. Gell “ Measurement of Three Critical Parameters as a Basis for a Simple Thermal Barrier Coating Life Prediction Methodology”, SCIES Project 02- 01- SR 097. /4/ D. RENUSCH, H. ECHSLER e M. SCHÜTZE, “New approaches to the understanding of failure and lifetime prediction of thermal barrier coating systems”, in “Life time modelling of High Temperature Corrosion Processes”, ECF Pub. N°34, eds. M. Schuetze, W .J. Quaddakers and J. R. Nicholl, (2001), p. 324

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