Evaluating Deformation Behavior of a TBC-System during Thermal Gradient Mechanical Fatigue by means of High Energy X-Ray Diffraction

Kurzfassung

Applications of TBC-systems involve complex thermal mechanical loading pattern including transient thermal gradients across the coated system, which result in multiaxial stresses and stress gradients affecting the damage behavior. In an ongoing research, starting more than 10 years ago, the authors developed laboratory test facilities for evaluating the damage behavior of TBC-systems for gas turbine blades in aeroengines under realistic thermal mechanical loading conditions [1]. Fatigue tests involving thermal gradients have been conducted and damage behavior in dependence of load pattern and pre heat treatment has been intensively investigated on TBC-systems comprising a partially yttria stabilized zirconia (YSZ) topcoat and a MCrAlY bond coat both applied by electron physical vapor deposition (EB-PVD) onto nickel based super alloys serving as substrate [2]. Numerical analyses by means of FE-calculations did provide hypotheses explaining the observed damage behavior [3], but even though the results are plausible they did depend on reasonable assumptions on materials properties since reliable data on the properties of the thin coating layers are still lacking, especially for high temperatures. High energy X-ray diffraction can provide the requested information since it is possible to achieve information on the local deformation processes in each layer with high spatial resolution, and short acquisition times allow for in situ investigation of time dependent deformation processes. A new test facility based on concepts after [1] for cyclic thermal loading of tubular specimens and applying a controlled thermal gradient across the coated specimen’s wall has been developed for implementation into an electro-mechanical test machine at the advanced photon source (APS) at Argonne National Laboratory. A precision positioning rig allows for exact µm-positioning of the entire test machine with respect to the focused X-ray beam, and X-ray diffraction patterns were taken using a 2D detector, giving accurate 360° lattice parameter data [4]. Tests have been performed with varying thermal and mechanical load schemata intending to determine material properties from the respective strain response. The beam energy was 65 keV, and throughout all experiments the beam scanned through the coating layers with a window and step size of 30 µm. Strain data were acquired in plane parallel to the specimen’s length axis and out of plane. Results of the strain data evaluation will be presented and discussed. Exemplary results are: - 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. - The YSZ strain was – below the deposition temperature - in plane compressive and out of plane tensile, which is a consequence of (i) the higher thermal expansion coefficient of YSZ with respect to the substrate and (ii) the cylindrical specimen geometry with the YSZ at the outer surface. [1] M. Bartsch, G. Marci, K. Mull, C. Sick, Adv. Eng. Mater. (1999), 1(2), 127–9 [2] M. Bartsch, B. Baufeld, S. Dalkilic, L. Chernova, M. Heinzelmann, Int. J. Fatigue (2008) 30, 211–8 [3] M. T. Hernandez, A. M. Karlsson, M. Bartsch, Surf. Coat. Technol. (2009) 203, 3549–58 [4] S.F. Siddiqui, K. Knipe, A. Manero, C. Meid, J. Wischek, J. Okasinski, J. Almer, A.M. Karlsson, M. Bartsch, S. Raghavan, Review of Scientific Instruments (2013) 84, 083904