• Journal of the Korean Ceramic Society
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• v.54 no.4
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• pp.314-322
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• 2017

This study investigates changes in the mechanical behavior, such as changes in indentation load-displacement curve, wear resistance and contact fatigue resistance of thermal barrier coatings (TBCs) by thermal cycling test and thermal shock test. Relatively dense and porous TBCs on nickel-based bondcoat/superalloy are prepared; the highest temperature applied during thermal durability test is $1350^{\circ}C$. The results indicate that the porous TBCs have relatively longer lifetime during thermal cycling and thermal shock tests, while denser TBCs have relatively higher wear and contact fatigue resistance. The mechanical behavior is influenced by sintering of the TBCs by exposure to high temperature during tests.

• Journal of Surface Science and Engineering
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• v.48 no.6
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• pp.315-321
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• 2015

Rare-earth zirconates, such as $La_2Zr_2O_7$ and $Gd_2Zr_2O_7$, have been investigated as one of the candidates for replacing conventional yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications at higher turbine inlet temperatures. In this study, double-ceramic-layer (DCL) TBCs of YSZ 1st layer and $La_2Zr_2O_7$ top coat layer are fabricated by suspension plasma spray with serial liquid feeders. Microstructures, hardness profiles, and thermal durability of DCL-TBCs are also characterized. Fabricated DCL-TBCs of YSZ/$La_2Zr_2O_7$ exhibit excellent properties, such as adhesion strength (>25 MPa) and electrical thermal fatigue (~1429 cycles), which are comparable with TBCs fabricated by atmospheric plasma spray.

• Ceramist
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• v.9 no.6
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• pp.24-29
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• 2006

Electron beam physical vapor deposition (EB-PVD) process has currently been applied to thermal barrier coatings (TBCs) for aircraft engines. Due to unique columnar structure, EB-PVD TBCs have advantages in resistances to thermal shock and thermal cycle for their applications, compared to films prepared by plasma spray By the EB-PVD equipment, we successfully obtained yttria-stabilized zirconia (YSZ) layer which has columnar and feather like structure including a large amount of nano size pores and gaps. The EB-PVD technique has been developed for coating functional perovskite type oxides such as (La, Sr)MnO3. Electrode properties have been improved by interface and structural control.

• Journal of the Korean Ceramic Society
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• v.55 no.5
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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.

• Journal of Ceramic Processing Research
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• v.20 no.5
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• pp.499-504
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• 2019

The effect of coating thickness on the contact fatigue and wear of thermal barrier coatings (TBCs) are investigated in this study. The same bondcoat material thickness (250 ㎛) are used for each sample, which allows the effect of the coating thickness of the topcoat to be investigated. TBCs with different coating thicknesses (200, 400, and 600 ㎛) are prepared by changing processing parameters such as the feeding rate of the feedstock, spraying speed, and spraying distance during APS(air plasma spray) coating. The damage size on the surface are strongly affected by the coating thickness effect. Although the damage size from contact fatigue using a spherical indenter diminish at a TBC of 200 ㎛, a high wear resistance such as a low friction coefficient and little mass change are found at a TBC of 600 ㎛. These results indicate that the coating thickness strongly affects the mechanical behavior in TBCs during gas turbine operation.

• Journal of the Korean Ceramic Society
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• v.53 no.6
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• pp.689-699
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• 2016

The effects of composition, structure design, and coating thickness of thermal barrier coating (TBC) on thermal barrier performance were investigated by measuring the temperature differences of TBC samples. TBCs with the thin and thick top coats were used for these studies, including TBCs with rare-earth (Gd, Yb, and La) compositions. The thermal barrier performance was enhanced with increasing the thickness of top coat even for thin TBCs, indicating that the thermal barrier performance was commensurate to the thickness of top coat. On the other hand, the bi-layered TBC, which was prepared with Yb-Gd-YSZ feedstock powder, with the buffer layer of high purity 8YSZ showed a better thermal barrier performance than that of regular purity 8YSZ. The interfaces in the bi-layered TBCs had a decisive effect on the thermal barrier performance, showing the performance enhanced with increasing numbers of interfaces. However, a new structural design and an additional process should be considered to reduce stress concentrations and to ensure interface stability, respectively, for improving thermal durability in the multi-layered TBCs.

• Journal of the Korean Ceramic Society
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• v.50 no.6
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• pp.353-358
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• 2013

The hot corrosion behavior of plasma sprayed 4 mol% $Y_2O_3-ZrO_2$ (YSZ) thermal barrier coatings (TBCs) with volcanic ash is investigated. Volcanic ash that deposited on the TBCs in gas-turbine engines can attack the surface of TBCs itself as a form of corrosive melt. YSZ coating specimens with a thickness of 430-440 ${\mu}m$ are prepared using a plasma spray method. These specimens are subjected to hot corrosion environment at $1200^{\circ}C$ with five different duration time, from 10 mins to 100 h in the presence of corrosive melt from volcanic ash. The microstructure, composition, and phase analysis are performed using Field emission scanning electron microscopy, including Energy dispersive spectroscopy and X-ray diffraction. After the heat treatment, hematite ($Fe_2O_3-TiO_2$) and monoclinic YSZ phases are found in TBCs. Furthermore the interface area between the molten volcanic ash layers and YSZ coatings becomes porous with increases in the heat treatment time as the YSZ coatings dissolved into molten volcanic ash. The maximum thickness of this a porous reaction zone is 25 ${\mu}m$ after 100 h of heat treatment.

• Journal of Surface Science and Engineering
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• v.49 no.6
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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.

• Journal of Surface Science and Engineering
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• v.39 no.4
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• pp.147-152
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• 2006

Failure mechanisms of electron beam physical vapor deposited thermal barrier coatings(EB-PVD TBCs) that occur during thermal cyclic oxidation were investigated. The investigations include microstructural degradation of NiCrAIY bond coat, thermally grown oxides(TGOs) along the ceramic top coat-substrate interface and fracture path within TBCs. The microstructural degradation of the bond coat during cyclic oxidation created Al depleted zones, resulting in reduction of NiAl and ${\gamma}$-Ni solid solution phase. It was observed that the fracture took placed primarily within the TGOs or at the interfaces between TGOs and bond coat.

• Journal of the Korean Ceramic Society
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• v.51 no.6
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• pp.598-604
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• 2014

Considerable research efforts have been explored attempting to enhance the thermal durability of thermal barrier coatings (TBCs) at the high operating temperatures of gas turbines. In this study, the suspension plasma spray (SPS) process was applied to produce TBCs with a segmented structure by using an yttria-stabilized zirconia (YSZ) suspension. Four different experiment sets were carried out by controlling the ratio between surface roughness of the bond coat and feed stock size ($R_a/D_{50}$) in order to examine the effect of $R_a/D_{50}$ ratio on the microstructure of SPS-prepared coatings. When the $R_a/D_{50}$ had a high value of 11.8, a deposited thick coating turned out to have a cone-type columnar microstructure. In contrast, at the low $R_a/D_{50}$ values of 2.9 and 0.18, a deposited thick coating appeared to have a dense, vertically-cracked microstructure. However, with the very low $R_a/D_{50}$ value of 0.05 the coating was delaminated.