• 한국표면공학회지
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• 제49권6호
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• pp.595-603
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• 2016

Rare-earth zirconates, including lanthanum zirconate and gadolinium zirconate, have been investigated as ones of the most promising candidates for next-generation thermal barrier coating (TBC) materials due to their excellent properties such as low thermal conductivity, chemical stability at high temperature and so on. In this study, TBCs with three compositions, in the $La_2O_3-Gd_2O_3-ZrO_2$ system with reduced rare-earth contents from $RE_2Zr_2O_7$ compositions, were fabricated by using suspension plasma spray with different suspension preparation methods. The phase formation, microstructure, and thermal properties of TBCs were examined. In particular, each coating exhibited single fluorite phase and a dense, vertically-separated microstructure. The potential of coatings with rare-earth zirconates for TBC applications was also discussed.

• 한국정밀공학회지
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• 제27권12호
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• pp.99-106
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• 2010

Gas turbines operation for power generation increased rapidly since 1990 due to the high efficiency in combined cycle, relatively low construction cost and low emission. But the operation and maintenance cost for gas turbine is high because the expensive superalloy hot gas path parts should be repaired and replaced periodically This study analyzed the initiation and propagation of the crack at the gas turbine blades which are coated with MCrAIY as a bond coat and TBC as a top coat. The sample blades had been serviced at the actual gas turbines for power generation. Total 7 sets of blades were analyzed and they have different EOH(equivalent operation hour). Blades were sectioned and the cracking distribution were measured and analyzed utilizing SEM(scanning electron microscope) and optical microscope. The blades which had 52,000 EOH of operation had cracks at the substrate and the maximum depth was 0.2 mm. Most of the cracks initiated at the boundary layer between TBC and bond coat and propagated down to the bond coat. Once bond coat is cracked, the base metal is exposed to the oxidation condition and undergoes notch effect. Under this environment, the crack branched at the inter-diffusion layer and propagated to the substrate. Critical cracks affecting the blade life were analyzed as those on suction side and platform.

• 비파괴검사학회지
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• 제33권1호
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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.

• 대한기계학회논문집A
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• 제22권1호
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• pp.73-79
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• 1998

The development of a new material which should be continuously usable under severe environment of very high temperature has been urgently requested. The conventional thermal barrier coating(TBC) is a two layer coating, but a composition and a microstructure of functionally graded material(FGM) are varied continuously from place to place in ways designed to provide it with the maximum function of mitigating the induced thermal stress. The purpose of this study is to evaluate the heat-resistant characteristics by thermal shock of laser and furnace heating. The fracture behaviors of non-FGM(NFGM) and FGM were investigated based on acoustic emission(AE) technique during thermal shock test. Therefore, it can be concluded that FGM gives higher thermal resistance compared to NFGM by AE signal and fracture surface analysis.

• 대한기계학회논문집A
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• 제34권4호
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• pp.475-480
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• 2010

가스터빈 블레이드는 터빈 가동 시 발생하는 고온화염으로부터 블레이드를 보호하고, 구조물의 표면 온도를 안전한 수준으로 낮추기 위하여 블레이드 표면에 열차폐 코팅(TBC; Thermal barrier coating)을 하여 사용하고 있다. 본 논문에서는 가스터빈 1단 블레이드에 적용되는 코팅 방식을 이용하여 코인형 시험편을 제작하였고 열화 온도 및 유지 시간의 변화에 따른 코팅 계면 산화물의 성장 거동을 분석하였다. 코팅 단면에 대하여 코팅 계면 산화물의 두께와 마이크로 비커스 경도를 측정하여 열화 특성을 평가 하였다. 또한 성분분석을 통하여 미세조직의 변화를 관찰함으로써 열차폐 코팅의 열적 열화특성을 평가하였다.

• 한국세라믹학회지
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• 제51권6호
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• pp.554-559
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• 2014

Gadolinium zirconate, $Gd_2Zr_2O_7$, is one of the most promising candidates for replacing yttira-stabilized zirconia (YSZ) in thermal barrier coating (TBC) applications due to its low thermal conductivity and chemical stability at high temperature. In this study, rare-earth zirconate ceramics in the $GdO_{1.5}-ZrO_2$ system with reduced gadolinia contents were fabricated via solid-state reaction as well as hot-pressing at $1800^{\circ}C$. The phase formation, microstructure, and thermo-physical properties of these oxides were examined. The potential application of $GdO_{1.5}-ZrO_2$ ceramics for TBC was also discussed.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2015년도 제51회 KOSCO SYMPOSIUM 초록집
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• pp.271-272
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• 2015

Gas turbine combustors is critical part due to high temperature operating conditions and the optimization of cooling design is required to avoid combustor failure. In gas turbine combustor, effusion cooling, impingement cooling and thermal barrier coating (TBC) are commonly used to improve cooling characteristics. In conceptual design, these cooling schemes are designed by 1D heat transfer calculation. Therefore, these design should be validated ted by nemurical or experiment methods. In this study, Conjugate Heat Transfer (CHT) analysis is performed for validation of gas turbine combustor cooling design.

• 한국세라믹학회지
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• 제55권5호
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• pp.452-460
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• 2018

This study investigate changes in mechanical behaviors such as indentation load-displacement and hardness of thermal barrier coatings (TBCs) using cycling of thermal shock test. Relatively dense and porous TBCs on nickel-based bondcoat/super alloy are prepared using different starting granules, 204C-NS and 204NS commercial powers, and the effect of double layers of 204C-NS on 204NS and 204NS on 204C-NS are investigated. The highest temperature applied during thermal shock test is $1100^{\circ}C$ and the maximum number of cycles is 1,200. The results indicate that bilayered TBC showed a relatively mechanically resistant property during thermal shock cycles and that the mechanical behavior is influenced by the microstructure of TBCs by exposure to high temperature during tests or different starting granules.

• 한국세라믹학회지
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• 제55권4호
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• pp.344-351
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• 2018

The effects of bond coat species on the growth behavior of thermally grown oxide (TGO) layer in thermal barrier coatings (TBCs) was investigated through furnace cyclic test (FCT). Two types of feedstock powder with different particle sizes and distributions, AMDRY 962 and AMDRY 386-4, were used to prepare the bond coat, and were formed using air plasma spray (APS) process. The top coat was prepared by APS process using zirconia based powder containing 8 wt% yttria. The thicknesses of the top and bond coats were designed and controlled at 800 and $200{\mu}m$, respectively. Phase analysis was conducted for TBC specimens with and without heat treatment. FCTs were performed for TBC specimens at $1121^{\circ}C$ with a dwell time of 25 h, followed by natural air cooling for 1 h at room temperature. TBC specimens with and without heat treatment showed sound conditions for the AMDRY 962 bond coat and AMDRY 386-4 bond coat in FCTs, respectively. The growth behavior of TGO layer followed a parabolic mode as the time increased in FCTs, independent of bond coat species. The influences of bond coat species and heat treatment on the microstructural evolution, interfacial stability, and TGO growth behavior in TBCs are discussed.

• 한국표면공학회지
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• 제46권6호
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• pp.258-263
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• 2013

4 mol% Yttria-stabilized zirconia (4YSZ) coatings are fabricated by Air Plasma Spray (APS) and Electron Beam Physical Vapor Deposition (EB-PVD) with top coating of thermal barrier coating (TBC). NiCrAlY based bond coat is prepared as 150 ${\mu}m$ thickness by conventional APS (Air Plasma Spray) method on the NiCrCoAl alloy substrate before deposition of top coating. Each 4YSZ top coating shows different tribological behaviors based on the inherent layer structures. 4YSZ by APS which has splat-stacked structure shows lower friction coefficient but higher wear rate than 4YSZ by EB-PVD which has columnar structure. For 4YSZ by APS, such results are expected due to the sliding wear accompanied with local delamination of splats.