Microstructure characteristics and temperature dependent local elastic response of EB-PVD thermal barrier coatings examined by in-situ synchrotron X-ray diffraction

Abstract

Elastic properties of the constituents of a coating system are basic input data for stress analyses. In the case of EB-PVD thermal barrier coatings elastic properties of the ceramic topcoat depend on temperature and stress state, with the latter controlled by (1) the assembly of the different layers, (2) the mismatch of their elastic properties and thermal expansion coefficients, (3) their thermal conductivities, (4) the residual stresses and (5) the temperature field. Furthermore, elastic topcoat properties are non-uniform across the coating thickness due to gradients in microstructure parameters such as porosity and column spacing. For determining comprehensive temperature- and stressdependent elastic topcoat properties 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, 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 to microstructural evolution due to sintering a pre-aged specimen was investigated besides such in as-coated condition.