In-situ synchrotron X-ray strain measurements in TBC systems during thermal mechanical cycling

Abstract

Thermal barrier coatings (TBC) are applied on cooled gas turbine components. Between the heated and cooled surfaces a thermal gradient develops, resulting for constraint components in high multiaxial stresses, which may exceed stresses due to mechanical loading. In order to investigate the damage behaviour of TBC systems under close to reality conditions, thermal mechanical fatigue tests with controlled thermal gradients (TGMF-tests) were performed on coated tubular specimens. The specimen substrate was made from directionally solidified nickel base superalloy IN100 DS, and the coating system comprised a NiCoCrAlY bond coat and a ceramic top coat from partially stabilized zirconia. In TGMF testing specific damages occurred underneath the adherent ceramic top coat, evolving into fatigue cracks, which propagated primarily in the metallic bond coat parallel to the surface. To explain the initiation and evolution of these fatigue cracks, the thermo-mechanical response of the bond coat and the thermally grown oxide (TGO) between bond coat and ceramic top coat is examined and quantified through finite element analyses. The models include non-linear and time-dependent behaviour such as creep, TGO growth stress, and thermo-mechanical cyclic loading. The simulations suggest that stress redistribution due to creep can lead to tensile stresses in the TGO during TGMF testing that are large enough to initiate the cracks investigated. In order to get experimental data of the strain in the different layers of the coating system during TGMF-loading a test device has been assembled allowing in-situ strain measurements in the entire TBC system by means of synchrotron X-rays. Data analyses so far have shown that all layers could be identified, for all layers it is possible to measure strain qualitatively, and the TGO showed a texture. Goal of the ongoing work is the calculation of the full strain tensor in each layer for the entire thermal mechanical load cycle.