Research Papers: Gas Turbines: Structures and Dynamics

Synchrotron X-Ray Diffraction Measurements Mapping Internal Strains of Thermal Barrier Coatings During Thermal Gradient Mechanical Fatigue Loading

[+] Author and Article Information
Kevin Knipe, Albert C. Manero, II

Department of Mechanical
and Aerospace Engineering,
University of Central Florida,
Orlando, FL 32816

Stephen Sofronsky

Department of Mechanical and
Aerospace Engineering,
University of Central Florida,
Orlando, FL 32816

John Okasinski, Jonathan Almer

X-Ray Science Division,
Advanced Photon Source,
Argonne National Laboratory,
Argonne, IL 60439

Janine Wischek

Head of Mechanical Testing of Materials Group
Institute of Materials Research,
German Aerospace Center (DLR),
Cologne 51147,Germany

Carla Meid

Institute of Materials Research,
German Aerospace Center (DLR),
Cologne 51147,Germany

Anette Karlsson

Dean of Engineering,
Cleveland Stand University,
Cleveland, OH 44115

Marion Bartsch

Head of Experimental and Numerical Methods,
Institute of Materials Research,
German Aerospace Center (DLR),
Cologne 51147,Germany

Seetha Raghavan

Associate Professor
Department of Mechanical
and Aerospace Engineering,
University of Central Florida,
Orlando, FL 32816

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received October 4, 2014; final manuscript received November 23, 2014; published online February 3, 2015. Editor: David Wisler.The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States government purposes.

J. Eng. Gas Turbines Power 137(8), 082506 (Aug 01, 2015) (5 pages) Paper No: GTP-14-1570; doi: 10.1115/1.4029480 History: Received October 04, 2014; Revised November 23, 2014; Online February 03, 2015

An understanding of the high temperature mechanics experienced in thermal barrier coatings (TBC) during cycling conditions would be highly beneficial to extending the lifespan of the coatings. This study will present results obtained using synchrotron X-rays to measure depth resolved strains in the various layers of TBCs under thermal mechanical loading and a superposed thermal gradient. Tubular specimens, coated with yttria stabilized zirconia (YSZ) and an aluminum containing nickel alloy as a bond coat both through electron beam-physical vapor deposition (EB-PVD), were subjected to external heating and controlled internal cooling generating a thermal gradient across the specimen's wall. Temperatures at the external surface were in excess of 1000 °C. Throughout high temperature testing, 2D high-resolution XRD strain measurements are taken at various locations through the entire depth of the coating layers. Across the YSZ, a strain gradient was observed showing higher compressive strain at the interface to the bond coat than toward the surface. This behavior can be attributed to the specific microstructure of the EB-PVD-coating, which reveals higher porosity at the outer surface than at the interface to the bond coat, resulting in a lower in plane modulus near the surface. This location at the interface displays the most significant variation due to applied load at room temperature with this effect diminishing at elevated uniform temperatures. During thermal cycling with a thermal gradient and mechanical loading, the bond coat strain moves from a highly tensile state at room temperature to an initially compressive state at high temperature before relaxing to zero during the high temperature hold. The results of these experiments give insight into previously unseen material behavior at high temperature, which can be used to develop an increased understanding of various failure modes and their causes.

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Fig. 1

Experimental setup

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Fig. 2

Synchrotron beam orientation

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Fig. 3

Loading conditions for testing strain versus applied mechanical load

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Fig. 4

Phase identification of image data

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Fig. 5

YSZ (111) through thickness strain (e22) for (a) 25 °C, (b) 600 °C, and (c) 1000 °C

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Fig. 6

YSZ (111) in-cycle strain for in-plane (e22)

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Fig. 7

NiAl (110) in-cycle strain in-plane (e322)



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