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Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

Bending Fatigue of Thermal Barrier Coatings

[+] Author and Article Information
Robert Eriksson

Division of Solid Mechanics,
Linköping University,
Linköping 58183, Sweden
e-mail: robert.eriksson@liu.se

Zhe Chen

Division of Engineering Materials,
Linköping University,
Linköping 58183, Sweden
e-mail: zhe.chen@liu.se

Krishna Praveen Jonnalagadda

Division of Engineering Materials,
Linköping University,
Linköping 58183, Sweden
e-mail: praveen.jonnalagadda@liu.se

1Corresponding author.

Contributed by the Manufacturing Materials and Metallurgy Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 3, 2017; final manuscript received July 4, 2017; published online September 13, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(12), 122101 (Sep 13, 2017) (6 pages) Paper No: GTP-17-1256; doi: 10.1115/1.4037587 History: Received July 03, 2017; Revised July 04, 2017

Thermal barrier coatings (TBCs) are ceramic coatings used in gas turbines to lower the base metal temperature. During operation, the TBC may fail through, for example, fatigue. In this study, a TBC system deposited on a Ni-base alloy was tested in tensile bending fatigue. The TBC system was tested as-sprayed and oxidized, and two load levels were used. After interrupting the tests, at 10,000–50,000 cycles, the TBC tested at the lower load had extensive delamination damage, whereas the TBC tested at the higher load was relatively undamaged. At the higher load, the TBC formed vertical cracks which relieved the stresses in the TBC and retarded delamination damage. A finite element (FE) analysis was used to establish a likely vertical crack configuration (spacing and depth), and it could be confirmed that the corresponding stress drop in the TBC should prohibit delamination damage at the higher load.

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Figures

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Fig. 1

The test setup. Fatigue tests were performed in tensile four-point bending.

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Fig. 2

Load–displacement curves for as-received and oxidized uncoated substrates. The load levels were chosen to remain in the linear-elastic region.

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Fig. 3

As-sprayed TBC tested in bending fatigue at 250 N for 10,000 cycles. (a) and (b) Display delamination (arrows) at different regions along the interface.

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Fig. 4

An oxidized TBC system tested in bending fatigue at 250 N for 10,000 cycles. (a) Delamination damage close to the interface (arrows) and (b) higher magnification of delamination damage (arrows).

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Fig. 5

As-sprayed TBC tested in bending fatigue at 400 N for 10,000 cycles. (a) Vertical cracking in the TBC (arrows), (b) higher magnification of a vertical crack (arrows), and (c) only very minor indications of delamination damage were found (arrows).

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Fig. 6

As-sprayed TBC tested in bending fatigue at 400 N for 50,000 cycles. (a) Vertical cracking in the TBC (arrows) and (b) only very minor indications of delamination damage were found (arrows).

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Fig. 7

FE model with vertical cracks of different lengths in the TBC: (a) 0.1 mm, (b) 0.5 mm, and (c) 1 mm. The color mapping shows the in-plane stress. (The grayed out region is in the substrate.)

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Fig. 8

FE model with vertical cracks of different crack densities in the TBC: (a) 0 mm−1, (b) 1 mm−1, and (c) 2 mm−1. The color mapping shows the in-plane stress. (The grayed out region is in the substrate.)

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Fig. 9

Mode I stress intensity factors, KI, for vertical cracks as function of crack spacing and crack length. The values should be compared to the assumed fracture toughness of 3 MPam. (a) KI at 250 N and (b) KI at 400 N.

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Fig. 10

In-plane stress for: (a) TBC loaded at 250 N without vertical cracks, (b) TBC loaded at 400 N without vertical cracks (for reference), and (c) TBC loaded at 400 N with vertical cracks 1 mm apart. The color mapping shows the in-plane stress. (The grayed out region is in the substrate.)

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Fig. 11

Values of a nondimensional failure parameter, Ω, assuming an in-plane fracture toughness of 1.5 MPam. The TBC tested at 400 N would have had a higher propensity for delamination failure, but the vertical cracks lowered Ω due to relief of the in-plane stress.

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