• 한국표면공학회지
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• 제47권6호
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• pp.316-322
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• 2014

With increase in demand for higher operating temperatures of gas turbines, extensive research efforts have been carried out to enhance the performance of thermal barrier coatings (TBCs) in the field of coating processing as well as materials. In this study, thermal barrier coatings in lanthanum/gadolinium zirconate system, which is one of the most promising candidates for replacing yttira-stabilized zirconia (YSZ) in thermal barrier coating applications, are fabricated via suspension plasma spray. Dense, $300{\sim}400{\mu}m$ thick coatings of fluoritephase zirconate with modest amount of segmented microstructures are obtained by using suspension plasma spray with suspensions of planetary-milled mixture between lanthanum and/or gadolinium oxide and nano zirconia. These coatings exhibit thermal conductivities of 1.6 ~ 1.7 W/mK at $1000^{\circ}C$, which is relatively lower than that of YSZ.

• 한국분말재료학회지
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• 제18권6호
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• pp.568-574
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• 2011

As operating temperatures of engines or turbines continually increase for higher efficiency, significant amounts of researches have been focused on finding new materials, which would be alternatives to conventional yttria-stabilized zirconia (YSZ) for thermal barrier coatings (TBCs). In this study, phase evolution and thermo-physical properties of $La_2(Zr_{1-x}Hf_x)_2O_7$ pyrochlore systems are investigated for TBC applications. $La_2(Zr_{1-x}Hf_x)_2O_7$ systems are comprised by selecting $La^{3+}$ as A-site ions and $Zr^{4+}/Hf^{4+}$ as B-site ions in $A_2B_2O_7$ pyrochlore structures. For the developed phases in $La_2(Zr_{1-x}Hf_x)_2O_7$ compositions, thermo-physical properties such as thermal conductivity, thermal expansion coefficient are examined. The potential of these $La_2(Zr_{1-x}Hf_x)_2O_7$ compositions for TBC application is also discussed.

• 한국세라믹학회지
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• 제53권6호
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• pp.652-658
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• 2016

Gadolinium zirconate, $Gd_2Zr_2O_7$, is one of the most versatile oxides among the new thermal-barrier-coating (TBC) materials for replacing conventional yttira-stabilized zirconia (YSZ). $Gd_2Zr_2O_7$ exhibits excellent properties, such as low thermal conductivity, high thermal expansion coefficient comparable with that of YSZ, and chemical stability at high temperature. In this study, bulk and coating specimens with $Gd_{2-x}Zr_{2+x}O_{7+0.5x}$ (x = 0.0, 0.5, 1.0) compositions were fabricated in order to examine the characteristics of this gadolinium zirconate system with different Gd content for TBC applications. Especially, coatings with $Gd_{2-x}Zr_{2+x}O_{7+0.5x}$ (x = 0.0, 0.5, 1.0) compositions were produced by suspension plasma spray (SPS) with suspension of raw powder mixtures prepared by planetary milling followed by ball milling. Phase formation, microstructure, and thermal diffusivity were characterized for both sintered and coated specimens. Single phase materials with pyrochlore or fluorite were fabricated by normal sintering as well as SPS coating. In particular, coated specimens showed vertically-separated columnar microstructures with thickness of $400{\sim}600{\mu}m$.

• 한국세라믹학회지
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• 제54권6호
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• pp.492-498
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• 2017

A thermal barrier coating (TBC) with self-healing property for cracks was proposed to improve reliability during gas turbine operation, including structural design. Effect of healing agent on crack propagation behavior in TBCs with and without buffer layer was investigated through furnace cyclic test (FCT). Molybdenum disilicide ($MoSi_2$) was used as the healing agent; it was encapsulated using a mixture of tetraethyl orthosilicate and sodium methoxide. Buffer layers with composition ratios of 90 : 10 and 80 : 20 wt%, using yttria stabilized zirconia and $MoSi_2$, respectively, were prepared by air plasma spray process. After generating artificial cracks in TBC samples by using Vickers indentation, FCTs were conducted at $1100^{\circ}C$ for a dwell time of 40 min., followed by natural air cooling for 20 min. at room temperature. The cracks were healed in the buffer layer with the healing agent of $MoSi_2$, and it was found that the thermal reliability of TBC can be enhanced by introducing the buffer layer with healing agent in the top coat.

• 한국표면공학회지
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• 제50권6호
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• pp.465-472
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• 2017

The ingot fabrication conditions related with the thermal shock bearing phase and microstructure have investigated for the rare earth zirconate ceramic material, lanthanum gadolinium zirconate, as a thermal barrier coating using electron beam evaporation method. The thermal shock resistance of the prepared ingot was evaluated by high energy electron beam irradiation. The rare earth zirconate ceramic powder was prepared by controlling the raw material powder composition of $La_2O_3$, $Gd_2O_3$ and $ZrO_2$ so as to have a composition of $(La_{0.3}Gd_{0.7})_2Zr_2O_7$ which was selected from the former study. Ingot samples were prepared under two conditions. The first condition is prepared by sintering the prepared powder mixture to form an ingot. The second condition is prepared by calcining the prepared powder mixture to form a composite phase and then sintering to form an ingot. X-ray diffraction(XRD) and Scanning Electron Microscope(SEM) were used to analyze phase forming behavior and microstructure of ingot samples. Nanoindentation method used to obtain elastic modulus and hardness of each ingot specimen. Also the stress distribution of ingot was simulated by using FEM method assuming the ingot surface was exposed to electron beam. As a results, in the case of an ingot having a network-shaped microstructure in which relatively coarse pores are included, it seems that the thermal shock resistance was higher than in the case of an ingot having a microstructure composed of relatively fine grains only or particles with the similar level size when the high energy electron beam irradiation.

• 한국안전학회지
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• 제23권3호
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• pp.8-11
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• 2008

In the present work, the measurement method of residual stresses in thermal barrier coatings(TBCs) which are received the thermal shock is performed numerically. For this, the internal residual stresses are predicted by commercial FEM software ABAQUS because the hole drilling strain gage method measures residual stresses only near the surface of a material. As the results of this study, the residual stresses are linearly increased when the surface temperatures are over $1,200^{\circ}C$. It is also found that the values of residual stress are increased as the coating thickness is thin.