• Journal of the Korean Ceramic Society
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• v.43 no.7 s.290
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• pp.415-420
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• 2006

The residual stresses in the interface region of the Thermal Barrier Coating (TBC)/Thermally Grown Oxide (TGO)/Bond Coat (BC) were calculated on the TBC-coated superalloy samples using a Finite Element Method (FEM). It was found that the stress distribution of the interface boundary was dependent upon mainly the geometrical shape or its aspect ratio and the thickness of TGO layer, which was formed by growth and swelling behavior of oxide layer. Maximum compressive residual stress in the TBC/TGO interface is higher than that of the TGO/bond coat interface, and the tensile stress had nothing to do with change of an aspect ratio. The compressive residual stresses in the TBC/TGO and TGO/bond coat interface region increased gradually with the TGO growth.

• Journal of the Korean institute of surface engineering
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• v.45 no.2
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• pp.70-74
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• 2012

In thermal barrier coating (TBC) systems, the life of the coating depends on thermally grown oxide (TGO) layer because most of the failure of TBCs occurs when TGO growth increases. In order to inhibit TGO growth, process was additionally carried out before the heat treatment of the TBC coating layer at $1200^{\circ}C$ in air. In the additional process, heat treatment in vacuum furnace of < $10^{-5}$ torr was conducted for 7 h and 14 h before the heat treatment. The area and length of TGO, as well as the crack length in the TBC were characterized using a scanning electron microscope (SEM). The TGO thickness and crack of specimens pre-heat treated in vacuum furnace were reduced by 45% compare to those heat treated in furnace. Consequently, pre-heat treatment in a vacuum furnace process lead to effective inhibition of growth of the TGO.

• Journal of the Korean Society for Precision Engineering
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• v.31 no.12
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• pp.1155-1159
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• 2014

Thermal barrier coating (TBC) which is used to protect the substrate of gas turbine is exposed to high temperature environment. Because of high temperature environment, thermally grown oxide (TGO) is grown at the interface of thermal barrier coating in operation of gas turbine. The growth of TGO critically affects to durability of TBC, so the evaluation about durability of TBC with TGOs of various thickness is needed. In this research, TGO was inserted by aging of TBC specimen to evaluate the effect of the TGO growth. Then thickness of TGO was defined by microstructure analysis, and thermal fatigue test was performed with these aging specimens. Finally, the relation between thermal fatigue life and the TGO growth according to aging time was obtained.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.12
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• pp.1431-1434
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• 2014

The thermal barrier coating (TBC) applied to a gas turbine can be damaged by repeated thermal fatigue during operation, so an evaluation of its durability is needed. Thermally grown oxide (TGO) is generated inside the TBC in a high-temperature environment. The growth of TGO is known to be the main cause of damage to the TBC. Therefore, the durability of TBC should be evaluated according to the growth of TGO. In this research, Kim et al.'s work on the growth of TGO with aging was used as a basis for finite element analysis. The relationship between stress and aging was derived from the finite element analysis results. The durability of the TBC with aging was evaluated through a comparison between the results of the finite element analysis and a bond strength test.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.1
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• pp.46-53
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• 2013

Impedance spectroscopy is a non-destructive evaluation (NDE) method first proposed and developed for evaluating TGO layers with compressive stress inside thermally degraded plasma-sprayed thermal barrier coatings (PS TBCs). A bode plot (phase angle (h) vs. frequency (f)) was used to investigate the TGO layer on electrical responses. In our experimental study, the phase angle of Bode plots is sensitive for detecting TGO layers while applying compressive stress on thermal barrier coatings. It is difficult to detect TGO layers in samples isothermally aged for 100 hrs and 200 hrs without compressive stress, and substantial change of phase was observed these samples with compressive stress. Also, the frequency shift of the phase angle and change of the phase angle are observed in samples isothermally aged for more than 400 hrs.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.6 s.261
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• pp.665-670
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• 2007

Alpha-phase alumina TGO(Thermally Grown Oxide) forms on the interface between zirconia top coat and bond coat of thermal barrier coating system for superalloys during exposure to high temperature over $1000^{\circ}C$. It is known to provide a good protection against hot corrosion and to cause surface failure such as rumpling and cracking due to difference in thermal expansion coefficient from the substrate metal and the lateral growth. Consequently, mechanical properties of the alumina TGO at the high temperature are the key parameters determining the integrity of TBC system. In this work, by using Fecralloy foils as the alumina forming substrate, creep tests and tensile tests have been performed with various TGO thicknesses$(h=0{\sim}4{\mu}m)$ and yttrium contents(0, 200ppm) at $1200^{\circ}C$. Displacement-time curves and load-displacement curves for each TGO thickness(h=1,2,..) were measured from the creep and tensile tests, respectively, and compared with the curves without TGO thickness(h=0). As the result, the intrinsic tensile and creep properties of TGO itself were determined.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.26 no.6
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• pp.549-559
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• 2008

This study verified the limitations of commercial GPS data processing software and the applicability on precise positioning through comparing the processing results between Bernese and TGO under various conditions. To achieve the goal, we selected three nationwide station data and two smaller local data to constitute networks. By using Bernese and TGO, those networks are processed through the baseline analysis and the network adjustment. The comparative analysis was carried out, in terms of software, baseline length and network scale, observation duration, and number of fixed points. In the comparison between softwares, the scientific software was excellent in accuracy. It was confirmed that, as GPS-related technology is developed, the performance of the receiver was enhanced. And, in parallel with this, even the functionalities of the commercial software were tremendously enhanced. The difference, however, in result between the scientific and commercial software are still exist even if it is not big. Therefore, this study confirms that the scientific software should be used when the most precise position is necessary to be computed, especially if baseline vectors are big.

• Journal of the Korean Ceramic Society
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• v.43 no.8 s.291
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• pp.479-485
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• 2006

The residual stress changes on thermo-mechanical loading in the interface region of the Thermal Barrier Coating (TBC)/Thermally Grown Oxide (TGO)/Bond Coat (BC) were calculated on the TBC-coated superalloys using a Finite Element Method (FEM). It was found that the residual stress of the interface boundary was dependent upon mainly the oxide formation and the swelling rate of the oxide by creep relaxation. During an oxide swelling, the relaxation of residual stress which is due to creep deformation increased the TBC's life. In the case of the fine grain size of TGO scale, the TBC stresses piled up by oxide swelling could be relaxed by diffusional creep effect of TGO.

• Korean Journal of Metals and Materials
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• v.46 no.6
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• pp.363-369
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• 2008

Effects of top coat morphology and thickness on thermal fatigue behavior of thermal barrier coatings (TBC) were investigated in this study. Thermal fatigue tests were conducted on three coating specimens with different top coat morphology and thickness, and then the test data were compared via microstructures, cycles to failure, and fracture surfaces. In the air plasma spray specimens (APS1, APS2), top coat were 200 and $300{\mu}m$ respectively. The thickness of top coat was about $700{\mu}m$ in the perpendicular cracked specimen (PCS). Under thermal fatigue condition at $1,100^{\circ}C$, the cycles to top coat failure of APS1, APS2, and PCS were 350, 560 and 480 cycles, respectively. The cracks were initiated at the interface of top coat and thermally grown oxide (TGO) and propagated into TGO or top coat as the number of thermal fatigue cycles increased. For the PCS specimen, additive cracks were initiated and propagated at the starting points of perpendicular cracks in the top coat. Also, the thickness of TGO and the decrease of aluminium concentration in bond coat do not affect the cycles to failure.

• Journal of the Korea Institute of Military Science and Technology
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• v.23 no.5
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• pp.435-443
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• 2020

Thermal barrier coating(TBC) applied to fighter and turbine engines is a technology that improves the durability of core parts by lowering the surface temperature of base material. The thermal stress caused by mis-match of the coefficient of thermal expansion between the top coating and the TGO interface is the main cause of TBC breakage. Since the thermal stress is dependent on the microstructure of the TBC, designing microstructure of TBC can improve the durability as well as lower the thermal stress. In this study, the effect of coating thickness, volume of porosity and vertical cracking on the thermal stress was analyzed through finite element analysis. Through the analysis results, a design range of a microstructure that can improve the durability of thermal barrier coating by lowering thermal stress is proposed.