• Korean Journal of Materials Research
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• v.25 no.10
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• pp.571-576
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• 2015

The effects of coating thickness on the delamination and fracture behavior of thermal barrier coating (TBC) systems were investigated with cyclic flame thermal fatigue (FTF) and thermal shock (TS) tests. The top and bond coats of the TBCs were prepared by electron beam-physical vapor deposition and low pressure plasma spray methods, respectively, with a thickness ratio of 2:1 in the top and bond coats. The thicknesses of the top coat were 200 and $500{\mu}m$, and those of the bond coat were 100 and $250{\mu}m$. FTF tests were performed until 1140 cycles at a surface temperature of $1100^{\circ}C$ for a dwell time of 5 min. TS tests were also done until more than 50 % delamination or 1140 cycles with a dwell time of 60 min. After the FTF for 1140 cycles, the interface microstructures of each TBC exhibited a sound condition without cracking or delamination. In the TS, the TBCs of 200 and $500{\mu}m$ were fully delaminated (> 50 %) within 171 and 440 cycles, respectively. These results enabled us to control the thickness of TBC systems and to propose an efficient coating in protecting the substrate in cyclic thermal exposure environments.

• 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 Korea Academia-Industrial cooperation Society
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• v.21 no.8
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• pp.248-255
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• 2020

The effects of the coating thickness on the thermal durability and thermal stability of thermal barrier coatings (TBCs) with a gradient coating thickness were investigated using a flame thermal fatigue (FTF) test and thermal shock (TS) test. The bond and topcoats were deposited on the Ni-based super-alloy (GTD-111) using an air plasma spray (APS) method with Ni-Cr based MCrAlY feedstock powder and yttria-stabilized zirconia (YSZ), respectively. After the FTF test at 1100 ℃ for 1429 cycles, the bond coat was oxidized partially and the thermally grown oxide (TGO) layer was observed at the interface between the topcoat and bond coat. On the other hand, the interface microstructure of each part in the TBC specimen showed a good condition without cracking or delamination. As a result of the TS test at 1100 ℃, the TBC with gradient coating thickness was initially delaminated at a thin part of the coating layer after 37 cycles, and the TBC was delaminated by more than 50% after 98 cycles. The TBCs of the thin part showed more oxidation of the bond coat with the delamination of topcoat than the thick part. The thick part of the TBC thickness showed good thermal stability and oxidation resistance of the bond coat due to the increased thermal barrier effect.

• Journal of the Korean Ceramic Society
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• v.43 no.8 s.291
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• pp.498-503
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• 2006

A gas turbine blade with thermal barrier ceramic coating is operated at high temperature to increase engine efficiency. Recently, thermal barrier characteristics have been improved by advanced coating technology through microstructure control and increase of adhesion force of the coating layer. More advanced coating materials, rare earth zircon ate ceramics have been studied for replacing YSZ coatings as thermal barrier coatings. In this study, $La_2O_3,\;HfO_2,\;CeO_2,\;Gd_2O_3$ and pure or yttria stabilized zirconia were prepared. Microstructure analysis and the evaluation of mechanical properties such as Hertzian indentation and hardness test were performed.

• Journal of the Korean Ceramic Society
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• v.54 no.6
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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.

• Corrosion Science and Technology
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• v.2 no.3
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• pp.155-160
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• 2003

Plasma spraying technique was used to fabricate functionally graded coating (FGC) of NiCrAIY/YSZ 8wt%$Y_2O_3-ZrO_2$ on a Co-base superalloy (HAYNES 188) substrate. Six layers were coated on the substrate for building up compositionally graded architecture. Conventional thermal barrier coating (TBC) of NiCrAIY/SZ with sharp interface was also fabricated. As-coated FGC and TBC samples were exposed at the temperature of $1100^{\circ}C$ for 10, 50, 100 hours in air. Microstructural change of thermally exposed samples was examined. Pores and microcracks were formed in YSZ layer due to evolution of thermal internal stress at high temperature. The amount of pores and microcracks in YSZ layer were increased with increasing exposure time at high temperature. High temperature oxidation of coatings occurred mainly at the NiCrAIY/YSZ interface. In comparison with the case of TBC. the increased area of the NiCrAIY/YSZ interface in FGC is likely to attribute to forming the higher amount of oxides.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2011.06a
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• pp.1249-1250
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• 2011

• Journal of the Korean Ceramic Society
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• v.53 no.6
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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 $400{\sim}600{\mu}m$.

• Journal of the Korean institute of surface 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.

• International Journal of Korean Welding Society
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• v.3 no.2
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• pp.29-39
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• 2003

The durability and reliability of thermal barrier coatings (TBCs) play an important role in the service reliability, availability and maintainability (RAM) of hot­section components in advanced turbine engines for aero and utility applications. Photostimulated luminescence spectroscopy (PSLS) and electrochemical impedance spectroscopy (EIS) are being concurrently developed as complimentary non­destructive evaluation (NDE) techniques for quality control and life­remain assessment of TBCs. This paper overviews the governing principles and applications of the luminescence and the impedance examined in the light of residual stress, phase constituents and resistance (or capacitance) in TBC constituents including the thermally grown oxide (TGO) scale. Results from NDE by PSLS and EIS are discussed and related to the microstructural development during high temperature thermal cycling, examined by using a variety of microscopic techniques including focused ion beam (FIB) in­situ lift­out (INLO), transmission and scanning transmission electron microscopy (TEM and STEM).