Local strain response of a TBC system under thermal mechanical loading by in-situ synchrotron X-ray Diffraction

Abstract

For stress analyses and lifetime assessments of TBC-systems, it is necessary to know the mechanical properties of each constituent, depending on the temperature and stress state. For determining comprehensive temperature- and stress-dependent mechanical properties of the thin layers of an EB-PVD coating, local strains were measured by means of high energy X-ray diffraction. Tubular coated specimens were loaded with systematically varying thermal and mechanical loads, while recording the strains in situ. Test parameters were surface temperature, applied mechanical load, and heat flux through the specimens’ wall, the latter controlled by internal cooling of the tubular specimen. The topcoat surface was heated by infrared radiation up to 1000°C. For capturing ageing effects associated with microstructural evolution, e.g. due to sintering or oxidation, a pre-aged specimen was investigated besides such in as-coated condition. Exemplary results of the investigations are residual strain data for the constituents as a function of temperature, gradients of elastic top coat properties across the coating thickness due to gradients in microstructure parameters such as porosity and column spacing, and data on creep behavior of the thermally grown oxide between bond coat and top coat.