• 연구논문집
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• 통권29호
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• pp.141-149
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• 1999

Brazing of Fe-Cr-Al alloy was carried out at $1200^{\circ}C$ in vacuum furnace using nickel-based filler metals : BNi-5 powder(Ni-Cr-Si-Fe base alloy} and MBF-50 foil (Ni-Cr-Si-B). The effect of boron content on the stability of oxide scale on the brazed joint was investigated by means of cyclic oxidation test performed at $1050^{\circ}C$ and $1200^{\circ}C$. Apparently, the joints brazed with MBF-50 containing boron showed relatively stable oxidation rates compared to boron-free BNi-5 at both temperatures. However, it was considered that the slower weight loss of MBF-50 brazed specimen wasn’t resulted from the low oxidation rate but from the spallation of oxide layer. The oxide layer consisted of thick spinel oxide on the surface and $Al_2 O_3$ internal oxide layer along the interface between mother alloy and braze, the mother alloy was also eroded seriously by the formation of spinel oxides such as $FeCr_2 O_4$ and $NiCr_2 O_4$ on the surface, likely to be induced by the change of oxide forming mechanism due to diffusion of boron from the braze. On the contrary, the joint brazed with BNi-5 showed the good oxidation resistance during the cyclic oxidation test. It seems that the oxidation can be retarded by the formation of stable $Al_2 O_3$ layer at the surface.

• 한국표면공학회지
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• 제37권3호
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• pp.152-157
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• 2004

Ti-Al-N ($Ti_{75}$ $Al_{25}$ N) and Ti-Al-Si-N ($Ti_{69}$ $Al_{23}$ $Si_{8}$N) coatings synthesized by a DC magnetron sputtering technique were studied comparatively with respect to phase characterization and high-temperature oxidation behavior. $Ti_{69}$ $Al_{23}$ $Si_{ 8}$N coating had a nanocomposite microstructure consisting of nanosized(Ti,Al,Si)N crystallites and amorphous $Si_3$$N_4$, with smooth surface morphology. Ti-Al-N coating of which surface $Al_2$$O_3$ layer formed during oxidation suppressed further oxidation. It was sufficiently stable against oxidation up to about $700^{\circ}C$. Ti-Al-Si-N coating showed better oxidation resistance because both surface Ab03 and near-surface $SiO_2$ layers suppressed further oxidation. XRD, GDOES, XPS, and scratch tests were performed.

• 한국수소및신에너지학회논문집
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• 제12권3호
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• pp.231-238
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• 2001

Electrocatalytic activity of palladium for hydrogen oxidation and reduction was studied using AC impedance method. The system under study was arranged in electrolytic mode consisting of Pd electrode under study, Pt counter electrode and Nafion electrolyte between them. Two types of Pd electrodes were used - carbon-supported Pd (Pd/C) and Pd foil electrode. Pd/C anode contacting pure hydrogen showed a steady decrease of charge transfer resistance with the increase of anodic overpotential, which is an opposite trend to that found with Pd foil anode. But Pd foil cathode also exhibited a decrease of the resistance with the increase of cathodic overpotential. The relationship between imposition of overpotential and subsequent change of the charge transfer resistance is determined by the ratio of the rate of faradaic process to the rate of mass transportation; if mass transfer limitation holds, increase of overpotential accompanies the increase of charge transfer resistance. Regardless of the physical type of Pd electrode, the anode contacting hydrogen/oxygen gas mixture did not reveal any independent arc originated from local anodic oxygen reduction.

• Journal of Nutrition and Health
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• 제38권7호
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• pp.553-560
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• 2005

The purpose of this study was to examine the lipid peroxidation, plasma antioxidant status and insulin resistance in childhood obesity. To this end, we measured blood lipid profiles, glucose, insulin concentrations, plasma antioxidant vitamins, baseline conjugated diene formation as a measure of LDL oxidation in vivo and TRAP (total radical trapping antioxidant potential) of 93 school children (58 nonobese, 35 overweight-obese). Insulin resistance was estimated by homeostasis model assessment of insulin resistance (HOMA-IR). The overweight-obese children showed significantly higher levels of leptin (p < 0.0001) and triglyceride (p < 0.05) and significantly lower level of plasma Iycopene (p < 0.001) and $\gamma$-tocopherol (p < 0.05) compared with the normal weight children. Furthermore, the levels of TRAP were significantly lower in overweight-obese children (p < 0.05). Significant positive relationships between plasma leptin and conjugated dienes formation (p < 0.005) and inverse relationship between plasma leptin and lipid corrected levels of $\beta$-carotene (p < 0.05), Iycopene (p < 0.05) were observed. Our results showed an increased lipid peroxidation and decreased antioxidant capacity in childhood obesity which could be involved in the atherosclerotic process.

• 한국재료학회지
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• 제33권7호
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• pp.309-314
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• 2023

This study aimed to address the limitations of traditional plasma nitriding methods by implementing a short-term plasma oxy-nitriding treatment on the surface of AISI 420 martensitic stainless steel. This treatment involved the sequential formation of nitride and oxide layers, to enhance surface hardness and corrosion resistance, respectively. The process resulted in the formation of a 20 ㎛-thick nitride layer and a 3 ㎛-thick oxide layer on the steel surface. Initially, the hardness increased by 2.2 times after nitriding, followed by a subsequent decrease of approximately 31 % after oxidation. While the nitriding process reduced corrosion resistance, the subsequent oxidation process led to the formation of a passive oxide film, effectively resolving this issue. The pitting corrosion of the oxide passive film started at 82.6 mVssc, providing better corrosion resistance characteristics than the nitride layer. Consequently, the trade-off between surface hardness and corrosion resistance in plasma oxy-nitrided AISI 420 martensitic stainless steel is anticipated to be recognized as an innovative and comprehensive surface treatment process for biomedical components.

• 한국분말재료학회지
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• 제22권1호
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• pp.52-59
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• 2015

As well-known wrought stainless steel, sintered stainless steel (STS) has excellent high-temperature anti-corrosion even at high temperature of $800^{\circ}C$, and exhibits good corrosion resistance in air. However, when temperature increases above $900^{\circ}C$, the corrosion resistance of STS begins to deteriorate and dramatically decreases. In this study, the effects of phase and composition of STS on high-temperature corrosion resistances are investigated for STS 316L, STS 304 and STS 434L above $800^{\circ}C$. The morphology of the oxide layers are observed. The oxides phase and composition are identified using X-ray diffractometer and energy dispersive spectroscopy. The results demonstrate that the best corrosion resistance of STS could be improved to that of 434L. The poor corrosion resistance of the austenitic stainless steels is due to the fact that $NiFe_2O_4$ oxides forming poor adhesion between the matrix and oxide film increase the oxidation susceptibility of the material at high temperature.

• Composites Research
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• 제26권5호
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• pp.322-327
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• 2013

탄소섬유강화 SiC기지상 복합재는 우수한 산화저항성과 우수한 열충격저항성을 가진다. 그리고 이런 특성들은 탄소섬유강화복합재가 고온구조재로서 응용케하였다. 본 연구에서는 $C_f/SiC$ 복합재가 전구체 함침과 액상 함침이 동반된 열분해공정, Cyclohexene을 사용한 화학기상 경화공정을 통해 제조되었다. 최종 제조된 $C_f/SiC$ 복합재는 5회 함침을 통해 $0.43g/cm^3$ 밀도를 갖는 탄소섬유 프리폼에서 $1.76g/cm^3$의 밀도값을 나타내고 있다. 그리고 산화저항성 특성면에서 $C_f/SiC$ 복합재의 무게가 공기중 $1400^{\circ}C$에서 6시간 유지 후에 81%가 남았다. 결과적으로 Cyclohexene을 사용한 화학기상 경화공정은 효과적으로 높은 치밀화와 증가된 산화저항성을 보이고 있다.

• 폴리머
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• 제33권6호
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• pp.575-580
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• 2009

폴리카보실란에 탄소직물을 보강제로 이용하여 제조한 탄소/탄화규소 복합재는 좋은 내산화 특성과 열 충격에 강한 특성으로 인해 높은 온도의 구조체에 적용되고 있다. 본 연구에서는 고분자 함침 열분해법을 이용하여 탄소직물에 폴리카보실란 용액을 함침한 후, 전자선을 이용하여 가교하고, 열분해 과정을 통해 탄소/탄화규소 복합재로 제조하였다. 실험 결과 복합재 시료의 공극률과 밀도는 각각 13.5%와 $2.44\;g/cm^3$을 나타냈고, 내산화 특성은 지속적인 고온의 산화 분위기에서 95.9%의 잔류량을 나타내어 본 연구에서 제조한 탄소/탄화규소 복합재의 우수한 내산화 특성을 확인하였다.

• 한국표면공학회지
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• 제50권5호
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• pp.366-372
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• 2017

Plasma electrolytic oxidation(PEO) has attracted attention as a surface treatment which has high wear resistance and corrosion resistance. PEO is generally considered as cost-effective, environmentally friendly and superior in terms of coating performance. Most of studies about the PEO processes have been applied to light metals such as Al and Mg. Because the strength of Al and Mg is weaker than that of steel, there is a limit to the application. In this study, PEO process was used to form oxide coatings on Hot dipped aluminized(HDA) steel and the characteristics of the coating film according to the PEO current density were studied. The morphology was observed by SEM and component was analyzed by using EDS. The corrosion behaviors of PEO coating films were estimated by exposing salt spray test at 5 wt.% NaCl solution and measuring polarization curves in deaerated 3 wt.% NaCl solution. With the increase of PEO process current density, the pore size of the coating surface and the thickness of coating increased. It was confirmed that no Fe component was present on the coating surface. PEO coating films obviously showed good corrosion resistance compared with HDA. It is considered that the PEO coating acts as a barrier to protect the base material from external factors causing corrosion.

• 한국세라믹학회지
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• 제56권1호
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• pp.49-55
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• 2019

$Ti_{0.33}Al_{0.67}N/CrN$ nano-multilayers, which are known to have excellent wear resistance, were prepared using an unbalanced magnetron sputter to have various periods of 2-5 nm. $Ti_{0.33}Al_{0.67}N$ had a hexagonal structure in a single layer, but converted to a cubic structure by forming a multilayer with CrN, which has a cubic structure. Thus, $Ti_{0.33}Al_{0.67}N$ formed a superlattice in the multilayer. The $Ti_{0.33}Al_{0.67}/CrN$ multilayer with a period of 2.5 nm greatly exceeded the hardness of the $Ti_{0.33}Al_{0.67}N$ and the CrN single layer, reaching 39 GPa. According to the low angle X-ray diffraction results, the $Ti_{0.33}Al_{0.67}N/CrN$ multilayer maintained its as-coated structure to a temperature as high as $700^{\circ}C$ and exhibited hardness of 30 GPa. The thickness of the oxide layer of the $Ti_{0.33}Al_{0.67}N/CrN$ multilayered coating was less than one-tenth of those of the single layers. Thus, $Ti_{0.33}Al_{0.67}N/CrN$ multilayered coating had hardness and oxidation resistance far superior to those of its constituent single layers.