Identification of delamination through TGO stresses due to indentation testing of an EB-PVD TBC
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Yutaka Kagawaa) Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo 153-8904, Japan; and National Institute for Materials Science, Tsukuba-shi, Ibaraki 305-0047, Japan (Received 11 April 2009; accepted 3 September 2009)
The Rockwell indentation test was used to generate interface delamination in an EB-PVD thermal barrier coating (TBC) system that was thermally cycled between 300 C and 1050 C. Luminescence spectroscopy was applied to measure the thermally grown oxide (TGO) stresses in indentation-affected and perfectly bonded regions. Comparison between experimental stress mapping in bonded and delaminated regions by luminescence spectroscopy and finite element analysis was shown and the effect of delamination paths was presented. Zigzag delamination paths alternating between the top TBC-TGO interface and TGO-bond coat interface were shown to induce TGO stress releases and fluctuations in the delaminated region. Luminescence spectroscopy measurements successfully detect such behavior, making it possible to identify nondestructively the size of the delaminated region, the delamination crack front position, and the delamination paths in part. I. INTRODUCTION
Thermal-barrier coatings (TBCs) consisting of yttriastabilized zirconia (YSZ) are widely used in most turbine engines to protect metallic components. The achievable capability is limited by delamination and the delamination is strongly affected by the evolution of a thermally grown oxide (TGO), typically a-alumina formed in the service condition.1–4 There is a strong need for nondestructive detection of TBC delamination in industries as well as laboratory tests for assessing TBC bonding durability and delamination resistance. Various nondestructive evaluation methods have been developed such as thermal wave imaging,5,6 ultrasonic scanning,7 and luminescence spectroscopy.8–11 Thermal wave imaging shows promise to monitor TBC surface temperature and to detect delamination, while ultrasonic scanning requires immersion of the specimen in water. The spatial resolution of both methods is limited to the order of 0.1 mm. Luminescence spectroscopy of laser photo-stimulated Cr3+ ions incorporated within the TGO has provided a reliable way to measure the TGO stresses. This stress measurement technique not only offers a potential nondestructive method but also provides an effective means to assess various TGO stress evolution mechanisms such as TGO rumpling and cyclic instabilities,12,13 intera)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0441 J. Mater. Res., Vol. 24, No. 12, Dec 2009
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face degradation,8 effect of TBC delamination,14 buckling and spallation,9,15 and TBC life.10,11 Luminescence spectroscopy can be applied to the TBC system through the top YSZ with the best spatial resolution of tens of microns, compared to about 1 mm measured on cross sections.10 The rough resolution through the top ceramic layer represents a d
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