• Journal of Powder Materials
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• v.30 no.6
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• pp.478-483
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• 2023

High-entropy alloys (HEAs) are characterized by having five or more main elements and forming simple solids without forming intermetallic compounds, owing to the high entropy effect. HEAs with these characteristics are being researched as structural materials for extreme environments. Conventional refractory alloys have excellent high-temperature strength and stability; however, problems occur when they are used extensively in a high-temperature environment, leading to reduced fatigue properties due to oxidation or a limited service life. In contrast, refractory entropy alloys, which provide refractory properties to entropy alloys, can address these issues and improve the high-temperature stability of the alloy through phase control when designed based on existing refractory alloy elements. Refractory high-entropy alloys require sufficient milling time while in the process of mechanical alloying because of the brittleness of the added elements. Consequently, the high-energy milling process must be optimized because of the possibility of contamination of the alloyed powder during prolonged milling. In this study, we investigated the high-temperature oxidation behavior of refractory high-entropy alloys while optimizing the milling time.

• 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.

• Journal of Korean Society for Atmospheric Environment
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• v.14 no.4
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• pp.313-320
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• 1998

The most desirable diesel oxidation catalyst (DOC) should have the properties of oxidibing CO and HC effectively at low exhaust gas temperature while minimizing the formation of sulfate at high exhaust gas temperature. Precious metals such as platinum and palladium have been known to be sufficiently active for oxidizing CO and HC and also to have high activity for the oxidation of sulfur dioxide (SO2) to sulfor trioxide (SO3). There is a need to develop a highly selective catalyst which can promote the oxidation of CO and HC efficiently, but, on the other hand, suppress the oxidation of SO2. One approach to solve this problem is to load a base metal such as vanadium in Pt-based catalyst to suppress sulfate formation. In this study, a Pt-V catalyst was prepared by impregnating platinum and vanadium onto a Ti-Si wash coated catalyst in a laboratory reactor by changing the formulations and reaction temperatures.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.923-927
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• 2017

Metal exposed to high temperature/high pressure/oxidizer-rich environment may cause rapid oxidation(ignition and combustion). Therefore, this study was performed for the selection of metal appropriate for high temperature/high pressure/oxidizer-rich environment. In order to make the high temperature, high pressure and oxidizer-rich environment, the test facility utilizing the catalytic reaction of hydrogen peroxide was constructed and the metal oxidation and ignition of the STS series metals were evaluated. The result showed that the change of the selected material (discoloration) and the surface roughness were observed, but the change in the weight and thickness of the specimen was not significant.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2001.06a
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• pp.89-95
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• 2001

The fretting wear characteristics of Zircaloy-4 tube at room and high temperature were Investigated experimentally. In this study, the number of cycles, slip amplitude and temperature were selected as main factors of fretting wear. The results of this research showed that the wear volume Increased with the Increase of slip amplitudes and the number of cycles but decreased with temperature and the coefficient of friction were observed different tendency between room and high temperature. According to SEM(EDS) only gross slip were observed on the surface of both specimens and compacted oxide were on worn surfaces. XRO patterns showed that the crystallization of ZrO$_2$ were observed on the worn surface at high temperature. The fretting wear were Investigated due to oxidation and accumulation of plastic flow.

• Korean Journal of Materials Research
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• v.13 no.12
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• pp.831-838
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• 2003

Anisotropic mesophase carbon fiber(AMCFs) was exposed to isothermal oxidation in air and $CO_2$atmosphere, and burn-off rates have measured by TGA. The microstructure changes of oxidized carbon fibers, were observed by SEM. It was observed that oxidation rate in the air is over 100 times faster than that in $CO_2$atmosphere. The activation energy obtained in air was about 43.4 Kcal/mole in the temperature range of $600∼800^{\circ}C$, and in $CO_2$was about 55.2 Kcal/mole in the temperature range of $950∼1200^{\circ}C$. Therefore, the oxidation reaction in both atmospheres was under chemical reaction regime in the above temperature ranges. It was shown that the oxidation of the AMCFs is initiated at the end of fibers at high temperature($1100^{\circ}C$) with developing the large pores, and the small pores are developed on the fiber surface at low temperature($900^{\circ}C$). In conclusion, the oxidation of the AMCFs is progressed through the imperfection.

• Journal of Power System Engineering
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• v.17 no.4
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• pp.109-114
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• 2013

In this study, the microstructure, mechanical properties and high temperature oxidation characteristics of HiSiMo and HiSiMoM ductile iron for exhaust manifold were investigated. The HiSiMoM ductile iron was developed by optimization of alloying element addition and casting design. The exhaust manifold prototype was fabricated using the HiSiMoM iron and this resulted in the weight saving of 0.73kg. The microstructures of the HiSiMo and HiSiMoM irons were similar each other and graphite nodularity was 89% and 93% respectively. Tensile strengths of them were 663.5 and 674.4 MPa and Brinell hardness were 235.3 and 243.9 respectively. Both irons showed parabolic weight gain behavior in high temperature oxidation atmosphere. Oxidation layer was divided into external and internal layers. The weight gain of the HiSiMoM iron was lower than that of the HiSiMo iron after isothermal oxidation test at $900^{\circ}C$. This should be rationalized by higher Si enrichment at the interface of the matrix and internal layer of the HiSiMoM iron.

• Journal of Powder Materials
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• v.22 no.4
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• pp.271-277
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• 2015

As wrought stainless steel, sintered stainless steel (STS) has excellent high-temperature anti-corrosion even at high temperature of $800^{\circ}C$ and exhibit corrosion resistance in air. The oxidation behavior and oxidation mechanism of the sintered 316L stainless was reported at the high temperature in our previous study. In this study, the effects of additives on high-temperature corrosion resistances were investigated above $800^{\circ}C$ at the various oxides ($SiO_2$, $Al_2O_3$, MgO and $Y_2O_3$) added STS respectively as an oxidation inhibitor. The morphology of the oxide layers were observed by SEM and the oxides phase and composition were confirmed by XRD and EDX. As a result, the weight of STS 316L sintered body increased sharply at $1000^{\circ}C$ and the relative density of specimen decreased as metallic oxide addition increased. Compared with STS 316L sintered parts, weight change ratio corresponding to different oxidation time at $900^{\circ}C$ and $1000^{\circ}C$, decreased gradually with the addition of metallic oxide. The best corrosion resistance properties of STS could be improved in case of using $Y_2O_3$. The oxidation rate was diminished dramatically by suppression the peeling on oxide layers at $Y_2O_3$ added sintered stainless steel.

• Nuclear Engineering and Technology
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• v.33 no.3
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• pp.307-314
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• 2001

The changes of weight gain, structure, morphology and uranium oxidation states in l0wt% G $d_2$ $O_3$-doped U $O_2$ during the oxidation below 475$^{\circ}C$ and heat treatment at 130$0^{\circ}C$ in air were investigated using TGA, XRD, SEM, EPMA and XPS. The room temperature ( $U_{0.86}$G $d_{0.14}$) $O_2$Cubic Phase Converted to highly distorted ( $U_{0.86}$G $d_{0.14}$)$_3$ $O_{8}$ -type sing1e Phase by oxidation at 475 $^{\circ}C$ in air. This oxidized phase was reduced by annealing at 130$0^{\circ}C$ in air. The room temperature XRD pattern of the 130$0^{\circ}C$ annealed powder revealed that ( $U_{0.86}$G $d_{0.14}$)$_3$ $O_{8}$ -type single phase was separated into Gd-depleted $U_3$ $O_{8}$ and Gd-enriched ( $U_{0.7}$G $d_{0.3}$) $O_2$$_{+x}$ type cubic phase. The reduction and phase separation by the high temperature annealing of kinetically metastable and highly deformed ( $U_{0.86}$G $d_{0.14}$)$_3$ $O_{8}$ -type phase are interpreted in terms of cation size difference between G $d^3$$^{+}$ and U according to the oxidation state of U.U.U.U.U.te of U.U.U.U.U.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2016.11a
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• pp.189-189
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• 2016

In this study, the Al-rich AlTiSiN thin films that consisted of TiN/AlSiN nano-multilayers were deposited on the steel substrate by magnetron sputtering, and their high-temperature oxidation behavior was investigated, which has not yet been adequately studied to date. Since the oxidation behavior of the films depends sensitively on the deposition method and deposition parameters which affect their crystallinity, composition, stoichiometry, thickness, surface roughness, grain size and orientation, the oxidation studies under various conditions are imperative. AlTiSiN nano-multilayer thin films were deposited on a tool steel substrate, and their oxidation behavior of was investigated between 600 and $1000^{\circ}C$ in air. Since the amount of Al which had a high affinity for oxygen was the largest in the film, an ${\alpha}-Al_2O_3-rich$ scale formed, which provided good oxidation resistance. The outer surface scale consisted of ${\alpha}-Al_2O_3$ incoporated with a small amount of Ti, Si, and Fe. Below this outer surface scale, a thin ($Al_2O_3$, $TiO_2$, $SiO_2$)-intermixed scale formed by the inwardly diffusing oxygen. The film oxidized slower than the $TiO_2-forming$ kinetics and TiN films, but faster than ${\alpha}-Al_2O_3-forming$ kinetics. During oxidation, oxygen from the atmosphere diffused inwardly toward the reaction front, whereas nitrogen and the substrate element of iron diffused outwardly to a certain extent.