Deformation and Damage Behaviour of an EB-PVD Thermal Barrier Coating System on Superalloy during Thermal Mechanical Testing under Near-Service Conditions

Thermal mechanical fatigue tests simulating near-service loading conditions of internally cooled gas turbine blades have been conducted on superalloy specimens with a protective coating system. The coating comprised a partially yttria stabilized zirconia (YSZ) topcoat and a MCrAlY bond coat both applied by electron physical vapour deposition (EB-PVD) and a thermally grown oxide (TGO) evolving with time at high temperature. The tests aimed at capturing realistic damage behaviour of the coated system for further understanding the role of temperature, hold time, thermal gradients and mechanical loading on the damage process. Besides fatigue tests until spallation of the coating and subsequent post-mortal microscopic investigations, in-situ thermal mechanical tests with simultaneous monitoring X-ray diffraction patterns of the strained coatings were performed at the synchrotron source at Argonne National Laboratory. Varying thermal and mechanical load schemata were applied intending to determine material properties from the respective strain response. Strain data of the coating layers were acquired in plane parallel to the specimen’s length axis and out of plane. Exemplary results are: - The YSZ strain was – below the deposition temperature - in plane compressive and out of plane tensile, which is a consequence of the higher thermal expansion coefficient of YSZ with respect to the substrate and the cylindrical specimen geometry with the YSZ at the outer surface. - Elastic properties of the YSZ showed a gradient across the coating thickness reflecting the microstructure gradient of the YSZ resulting from the EB-PVD process. - TGO strain did relax during high temperature hold due to creep mechanisms. The achieved in-situ data have confirmed hypotheses derived from numerical analyses, which provided explanations for the observed damage behaviour of the TBC-system. Especially, the in-situ observation of TGO creep was the missing link to explain accumulation of tensile stresses in the TGO by a ratcheting process entailed by crack initiation in the TGO.