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

Development of Highly Durable Thermal Barrier Coating by Suppression of Thermally Grown Oxide

[+] Author and Article Information
Masahiro Negami

Kawasaki Heavy Industries, Ltd.,
1-1, Kawasaki-Cho,
Akashi City 673-8666, Japan
e-mail: negami_masahiro@khi.co.jp

Shinya Hibino

Kawasaki Heavy Industries, Ltd.,
1-1, Kawasaki-Cho,
Akashi City 673-8666, Japan
e-mail: hibino_shinya@khi.co.jp

Akihito Kawano

Kawasaki Heavy Industries, Ltd.,
1-1, Kawasaki-Cho,
Akashi City 673-8666, Japan
e-mail: kawano_akihito@khi.co.jp

Yoshimichi Nomura

Kawasaki Heavy Industries, Ltd.,
1-1, Kawasaki-Cho,
Akashi City 673-8666, Japan
e-mail: nomura_ym@khi.co.jp

Ryozo Tanaka

Kawasaki Heavy Industries, Ltd.,
1-1, Kawasaki-Cho,
Akashi City 673-8666, Japan
e-mail: tanaka_r@khi.co.jp

Kenichiroh Igashira

Kawasaki Heavy Industries, Ltd.,
1-1, Kawasaki-Cho,
Akashi City 673-8666, Japan
e-mail: igashira_kenichiroh@khi.co.jp

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 August 1, 2017; final manuscript received September 14, 2017; published online April 11, 2018. Editor: David Wisler.

J. Eng. Gas Turbines Power 140(8), 082101 (Apr 11, 2018) (8 pages) Paper No: GTP-17-1423; doi: 10.1115/1.4038607 History: Received August 01, 2017; Revised September 14, 2017

Durability of thermal barrier coating (TBC) systems is important because of recent rising of turbine inlet temperature (TIT) for improved efficiency of industrial gas turbine engines. However, high-temperature environment accelerates the degradation of the TBC as well as causes spalling of the top coat. Spalling of the top coat may be attributed to several factors, but evidently the growth of thermally grown oxide (TGO) should be considered as an important factor. One method for reducing the growth rate of TGO is to provide a dense α-Al2O3 layer at the boundary of the bond coat and top coat. This α-Al2O3 layer will suppress the diffusion of oxygen to the bond coat and consumption of aluminum of the bond coat is suppressed. In this study, we focused on thermal pre-oxidation of the bond coat as a means for forming an α-Al2O3 barrier layer that would be effective at reducing the growth rate of TGO, and we studied the suitable pre-oxidation conditions. In the primary stage, we analyzed the oxidation behavior of the bond coat surface during pre-oxidation heat treatment by means of in situ synchrotron X-ray diffraction (XRD) analysis. As a result, we learned that during oxidation in ambient air environment, in the initial stage of oxidation metastable alumina is produced in addition to α-Al2O3, but if the thermal treatment is conducted under some specific low oxygen partial pressure condition, unlike in the ambient air environment, only α-Al2O3 is formed with suppressing formation of metastable alumina. We also conducted transmission electron microscope (TEM) and XRD analysis of oxide scale formed after pre-oxidation heat treatment of the bond coat. As a result, we learned that if pre-oxidation is performed under specific oxygen partial pressure conditions, a monolithic α-Al2O3 layer is formed on the bond coat. We performed a durability evaluation test of TBC with the monolithic α-Al2O3 layer formed by pre-oxidation of the bond coat. An isothermal oxidation test confirmed that the growth of TGO in the TBC that had undergone pre-oxidation was suppressed more thoroughly than that in the TBC that had not undergone pre-oxidation. Cyclic thermal shock test by hydrogen burner rig was also carried out. TBC with the monolithic α-Al2O3 layer has resistance to >2000 cycle thermal shock at a load equivalent to that of actual gas turbine.

Copyright © 2018 by ASME
Topics: oxidation , Durability
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Figures

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Fig. 1

Configuration of in situ XRD test equipment (SPring-8 BL16XU)

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Fig. 2

Schematic representation of hydrogen burner rig test equipment

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Fig. 3

Appearance of hydrogen burner rig test equipment

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Fig. 4

Results of in situ XRD analysis of oxide formation process when NiCoCrAlY thermal-sprayed coat was heated in ambient air environment

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Fig. 5

Results of in situ XRD analysis of oxide formation process when NiCoCrAlY thermal-sprayed coat was heated in vacuum environment: (a) PO2 = 10−1 Pa and (b) PO2 = 10−12 Pa

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Fig. 6

XRD pattern of NiCoCrAlY surface after thermal oxidation at 1080 °C for 4 h

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Fig. 7

Cross section SEM image of NiCoCrAlY surface after thermal oxidation at 1080 °C for 4 h in ambient air

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Fig. 8

Cross section TEM image of NiCoCrAlY surface after thermal oxidation at 1080 °C for 4 h in vacuum conditions

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Fig. 9

(a) Thickness of TGO of various samples during isothermal oxidation test at 1050 °C and (b) thickness of TGO versus exposure time t1/2

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Fig. 10

SEM images of top coat/bond coat boundary of the TBC samples after isothermal oxidation at 1050 °C for 3000 h: (a) non-pre-oxidated sample and (b) pre-oxidated sample

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Fig. 11

EPMA mapping images of top coat/bond coat boundary of the pre-oxidated TBC samples after isothermal oxidation at 1050 °C for 3000 h

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Fig. 12

Typical temperature profile of TBC sample during burner rig test

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Fig. 13

Results of burner rig test of TBC samples applied pre-oxidation with different TGO thickness

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Fig. 14

The TBC sample spalled at 37 cycles burner rig test. The sample was prepared by isothermal oxidation at 1100 °C for 600 h: (a) appearance and (b) cross section SEM image.

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Fig. 15

(a) TBC sample before burner rig test and (b) TBC sample after completion of >2000 cycles burner rig test at a load equivalent to that of actual gas turbine, both samples applied pre-oxidation

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