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

# Effect of Time and Temperature on Thermal Barrier Coating Failure Mode Under Oxidizing Environment

[+] Author and Article Information
N. S. Cheruvu, K. S. Chan

Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238

D. W. Gandy

Electric Power Research Institute, Charlotte, NC 28262

J. Eng. Gas Turbines Power 131(2), 022101 (Dec 19, 2008) (7 pages) doi:10.1115/1.2979747 History: Received December 22, 2007; Revised June 19, 2008; Published December 19, 2008

## Abstract

Thermal barrier coatings (TBCs) have been recently introduced to hot section components, such as transition pieces and the first two stages of turbine blades and vanes of advanced F, G, and H class land-based turbine engines. The TBC is typically applied on metallic-coated components. The metallic bond coat provides oxidation and/or corrosion protection. It is generally believed that the primary failure mode of TBCs is delamination and fracture of the top ceramic coating parallel to the bond coat in the proximity of the thermally grown oxide (TGO) between coatings. One of the concerns associated with the use of a TBC as a prime reliant coating is its long-term stability. The effect of long-term operation at typical land based turbine operating temperatures of below $1010°C$ (1850°F) of the failure mode of TBCs is unknown. Long-term isothermal tests were conducted on the thermal barrier-coated specimens at three temperatures, $1010°C$ (1850°F), $1038°C$ (1900°F), and $1066°C$ (1950°F), to determine the effects of long term exposure on the TBC failure location (mode). Following the isothermal testing, the samples were destructively examined to characterize the degradation of the TBC and determine the extent of TGO cracking, TGO growth, bond coat oxidation, and TBC failure location after long term exposure for up to $18,000h$. Optical microscopy and a scanning electron microscope (SEM) attached with an energy dispersive spectroscopy (EDS) system were used to study the degradation of the TBC and bond coatings. The results showed that long term isothermal exposure leads to a change in the TBC failure mode from the delamination of the TBC at the $TGO∕TBC$ interface to the internal oxidation of the bond coat and bond coat delamination. In this paper, the effect of long-term exposure on the delamination of TBC and the bond coat failure mode is discussed.

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## Figures

Figure 1

An as-coated specimen

Figure 2

Optical micrographs of the as-coated TBC on a NiCoCrAlY-coated GTD-111 specimen. The arrows point to delamination cracks at the TBC/bond coat interface.

Figure 3

Backscattered electron micrographs of mixed oxides in the TGO on the coated specimen after 2015h of exposure at 1010°C (1850°F)

Figure 4

(a) SEM micrograph of mixed oxides in the TGO on the coated specimen after 600h exposure at 1066°C (1950°F), and (b) EDS obtained from the mixed oxide shown by the short arrow in (a)

Figure 5

Optical micrographs showing the variation of the TGO thickness after (a) 1510h and (b) 2785h of exposure at 1066°C (1950°F)

Figure 6

Optical micrographs showing the variation of the TGO thickness on the NiCoCrAlY-coated GTD-111 specimen after (a) 2015h and (b) 9850h of exposure at 1010°C (1850°F)

Figure 7

Kinetics of the TGO growth at the interface of the APS TBC/NiCoCrAlY bond coat on GTD-111DS

Figure 8

Optical micrographs of the TBC/CoNiCrAlY interface of GTD-111 specimens after (a) 2015h and (b) 12,030h of exposure at 1010°C (1850°F) showing the variation of the interface cracking. Arrows on the micrographs point to interface cracks.

Figure 9

Average and maximum delamination crack lengths at the TGO/TBC interface on the CoNiCrAlY-coated GTD-111 specimens as a function of the exposure time at 1010°C (1850°F)

Figure 10

Optical microstructure of the CoNiCrAlY bond coat on the GTD-111 specimens after 9850h of exposure at 1010°C (1850°F) showing the early stages of internal oxidation

Figure 11

Optical microstructure of the CoNiCrAlY bond coat on the GTD-111 specimens after 6470h at 1038°C (1900°F)

Figure 12

Optical microstructure of the NiCoCrAlY bond coat on GTD-111 after (a) 18,000h of exposure at 1010°C (1850°F), (b) 6470h of exposure at 1038°C (1900°F), and (c) 2925h of exposure at 1066°C (1950°F). Note the onset of bond delamination and void coalescence at the bond coat/substrate interface.

Figure 13

Optical micrographs of the NiCoCrAlY bond coat/GTD-111 interface after (a) 2015h and (b) 8155h of exposure at 1010°C(1850°F). Note the variation of the interface voids with the exposure time.

Figure 14

Optical micrographs of the NiCoCrAlY bond coat/GTD-111 interface after (a) 600h, (b) 1510h, and (c) 2925h of exposure at 1066°C (1950°F). Note the variation of the interface voids and void coalescence with the exposure time.

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