• Corrosion Science and Technology
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• 제19권6호
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• pp.310-317
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• 2020

This study investigated the damage to the specimen due to liquid droplet impingement erosion corrosion, which improved the corrosion resistance and durability via hard anodization of 5083-H321 aluminum alloy, which is widely used for small ships and marine structures. The experiment combined liquid droplet impingement erosion and electrochemical equipment with the flow rates in natural seawater solution. Subsequently, Tafel extrapolation of polarization curves was performed to evaluate damage due to the liquid droplet impingement erosion corrosion. The damaged surface was observed using a 3D microscope and a scanning electron microscope. The degree of pitting damage was measured using the Image J program, and the surface hardness was measured using the micro-Vickers hardness tester. The corrosion current density, area, depth, and ratio of the damaged areas increased with the increase in flow rate. The grain size of the damaged area at a flow rate of 20 m s-1 showed fewer and minor differences in height, and a smooth curved shape. The hardness of the damaged surface tended to decrease with increase in flow rate.

• 한국기계가공학회지
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• 제12권6호
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• pp.3-9
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• 2013

Aluminum Oxide($Al_2O_3$) ceramics are excellent candidates for such applications due to their outstanding mechanical, thermal, and tribological properties. However, they are difficult to machine using conventional mechanical methods. Carbon fillers, such as carbon nanotubes(CNT) and graphene nanoplatelets(GNP)can be dispersed in a ceramic matrix to improve the mechanical and electrical properties. In this study, CNT and Graphene reinforced hybrid ceramic composites were fabricated using the spark plasma sintering method at a temperature of $1,500^{\circ}C$, pressure of 40 MPa, and soaking time of 10min. Besides this, the material properties such as microstructure, crystal structure, hardness, and electrical conductivity were analyzed using FE-SEM, XRD, Vickers, and the 4-point probe method. A micro machining test was carried out to compare the effects of the material properties and the machining performance for CNT and Graphene reinforced ceramic composites.

• Restorative Dentistry and Endodontics
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• 제37권2호
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• pp.90-95
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• 2012

Objectives: The purpose of this study was to determine the effect of resin infiltration technique on color and surface hardness of white spot lesion (WSL) with various degrees of demineralization. Materials and Methods: Ten human upper premolars were cut and divided into quarters with a $3{\times}4mm$ window on the enamel surface. Each specimens were separated into four groups (n = 10) and immersed in demineralization solution to create WSL: control, no treatment (baseline); 12 h, 12 hr demineralization; 24 h, 24 hr demineralization; 48 h, 48 hr demineralization. Resin infiltration was performed to the specimens using Icon (DMG). $CIEL^*a^*b^*$ color parameters of the enamel-dentin complex were determined using a spectroradiometer at baseline, after caries formation and after resin infiltration. Surface hardness was measured by Vickers Micro Hardness Tester (Shimadzu, HMV-2). The differences in color and hardness among the groups were analyzed with ANOVA followed by Tukey test. Results: Resin infiltration induced color changes and increased the hardness of demineralized enamel. After resin infiltration, there was no difference in color change (${\Delta}E^*$) or microhardness among the groups (p < 0.05). Conclusion: There was no difference in the effect of resin infiltration on color and hardness among groups with different extents of demineralization.

• 열처리공학회지
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• 제32권6호
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• pp.263-269
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• 2019

The mechanism of microstructure and hardness changes during mill annealing of Ti-6Al-4V alloy was investigated. The annealing heat treatments were performed at $675{\sim}795^{\circ}C$ in vacuum for 2 hours, followed by air cooling. The microstructure was observed by using an optical microscope and X-ray diffraction, and hardness was measured by using a Rockwell hardness tester and micro Vickers hardness tester. The average grain size becomes smaller at $675^{\circ}C$ to $735^{\circ}C$ due to the formation of new grains rather than grain growth, but becomes larger at $735^{\circ}C$ to $795^{\circ}C$ due to growth of the already-formed grains rather than formation of new grains. The mill annealing temperature becomes higher, the ${\beta}$ phase fraction decreases and ${\alpha}$ phase fraction increases at room temperature. This is because the higher annealing temperature, the smaller amount of V present in the ${\beta}$ phase, and thus the ${\beta}$ to ${\alpha}$ transformation occurs more easily when cooled to room temperature. As the mill annealing temperature increases, the hardness value tends to decrease, mainly due to resolution of defects such as dislocations from $675^{\circ}C$ to $735^{\circ}C$ and due to grain growth from $735^{\circ}C$ to $795^{\circ}C$, respectively.

• 대한기계학회논문집A
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• 제34권4호
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• pp.475-480
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• 2010

가스터빈 블레이드는 터빈 가동 시 발생하는 고온화염으로부터 블레이드를 보호하고, 구조물의 표면 온도를 안전한 수준으로 낮추기 위하여 블레이드 표면에 열차폐 코팅(TBC; Thermal barrier coating)을 하여 사용하고 있다. 본 논문에서는 가스터빈 1단 블레이드에 적용되는 코팅 방식을 이용하여 코인형 시험편을 제작하였고 열화 온도 및 유지 시간의 변화에 따른 코팅 계면 산화물의 성장 거동을 분석하였다. 코팅 단면에 대하여 코팅 계면 산화물의 두께와 마이크로 비커스 경도를 측정하여 열화 특성을 평가 하였다. 또한 성분분석을 통하여 미세조직의 변화를 관찰함으로써 열차폐 코팅의 열적 열화특성을 평가하였다.

• 한국전기전자재료학회논문지
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• 제30권1호
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• pp.7-12
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• 2017

3YSZ + (x) $Al_2O_3$ composites (x = 20, 40, 60, 80 wt%) were fabricated and the influences of particle sizes of $Al_2O_3$ on their microstructures and mechanical properties were investigated with XRD, SEM, vickers hardness and fracture toughness. $Al_2O_3$-3YSZ composites containing $Al_2O_3$ powder of a $0.3{\mu}m$ and an $1.0{\mu}m$, which are here in after named as $Al_2O_3$($0.3{\mu}m$)-3YSZ and $Al_2O_3$($1.0{\mu}m$)-3YSZ, respectively, were made by mixing raw materials, uni-axial pressing and sintering at $1,400^{\circ}C$, $1,500^{\circ}C$, and $1,600^{\circ}C$. $Al_2O_3$($0.3{\mu}m$)-3YSZ composites show the higher density and the better mechanical properties than $Al_2O_3$($1.0{\mu}m$)-3YSZ composites. The Vickers hardness of the $Al_2O_3$($0.3{\mu}m$)-3YSZ composites show a peak value of 1,997 Hv at the content of 60 wt% $Al_2O_3$, which is a slightly higher value in comparison with 1,938 Hv of the $Al_2O_3$($1.0{\mu}m$)-3YSZ composite. However, the fracture toughness of $Al_2O_3$-3YSZ composites monotonically increases with decreasing the content of $Al_2O_3$ without any peak values. $Al_2O_3$($0.3{\mu}m$)-3YSZ and $Al_2O_3$($1.0{\mu}m$)-3YSZ composites sintered at $1,600^{\circ}C$ have a maximum value of a $6.9MPa{\cdot}m^{1/2}$ and a $6.2MPa{\cdot}m^{1/2}$, respectively at the composition of containing 20 wt% $Al_2O_3$. It should be noticed that the mechanical properties and the sintering density of the $Al_2O_3$-3YSZ composites can be enhanced by using more fine $Al_2O_3$ powder due to their denser microstructure and smaller grain size.

• Tribology and Lubricants
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• 제33권2호
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• pp.59-64
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• 2017

This study investigates friction and wear characteristics of anodized aluminum (Al) alloy 6061 by using a reciprocating tribotester. The diameter and height of the specimen are 30 mm and 10 mm, respectively. The surface roughness of the mirrored-surface is approximately $0.01{\sim}0.02{\mu}m$, and it is used throughout the current study. As a result of anodizing, the depth and diameter of the nanopore are approximately $25{\mu}m$ and 30-40 nm, respectively. The testing conditions are as follows: loads of 1, 3, and 5 N; a frequency of 1 Hz; a stoke of 3 mm; and a duration of 1800 s. We use deionized water with a volume of approximately $25{\mu}l$, as the lubricant. Micro Vickers hardness measurements show that mirrored-surface specimens had lower hardness values than anodized specimens. Further, their coefficients of friction are lower than those of the anodized samples, and the width of their wear track increases with load, as expected. The anodized specimens' coefficients of friction increase with stable frictional behavior and exhibit insignificant load dependence. Further, we observe that the width of the wear track is less than that of the mirrored-surface specimens, and micro cracks are present near it. Moreover, the anodizing process increases the hardness of the samples, improving their wear resistance. These results indicate that nanoporous structures are not effective in lowering friction under the water-lubricated condition.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2010년도 춘계학술발표대회 초록집
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• pp.18-18
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• 2010

최근에 건조되는 선박이나 구조물들은 점차 대형화 되어가고, 이에 사용되는 판재들은 점차 고강도 극후판재화 되어가고 있다. 극후판재의 용접성을 향상시키기 위해서는 대입열 용접이 주로 적용되고 있는 실정인데, 30 mmt 이상의 후판을 1 pass로 용접하기 위해서는 EGW(Electro-gas welding) 기법을 사용한다. 대입열 용접은 용접입열(heat input)이 매우 높아 용착금속과 열영향부의 냉각속도가 매우 느려 용접열영향부에서 특히 fusion line 근처의 열영향부는 결정립 조대화 및 취약한 미세조직을 형성함으로서 저온인성을 크게 저하시키고, 연화 현상(softening effect)을 발생시켜 강도가 저하되는 문제점이 주로 발생하였다. 하지만 이런 문제점을 해결하기 위해 대입열용접에 사용된는 강재의 미세조직을 제어하여 AlN, TiN, $TiO_2$ 등의 석출물을 이용한 용접열영향부의 저온인성을 향상시켰다. 이러한 문제점이 발생하는 대입열용접에서 저온인성 시험은 주로 fuison line + 1, 2mm에서 수행한다. 하지만 대입열 용접시 용착금속의 냉각속도도 매우 느리기 때문에 용착금속의 위치에 따라 저온 인성 특성이 다르게 나타날 수 있다. 본 연구에서는 EGW 용착금속의 위치에 따른 저온인성 특성을 평가하기 위해 EH-36N, 40mmt 판재를 사용하여 1pole EG 용접 하였다. 용착금속의 저온인성 특성을 평가하기위해 충격 시편의 노치 위치가 fusion line - 2mm와 용접부 중앙을 기준으로 4곳을 선정하여 충격시험을 수행하였다. 또한 용착금속의 경도 분포를 알아보기 위해 micro vickers hardness tester(mitutoyo UR-501)을 사용해 hardness mapping 시험을 하였다. 용착금속의 저온인성은 미세조직과, 산소량에 따라 변화 할 수 있기 때문에 용착금속 위치를 달리하여 미세조직과 산소량도 각각 분석하였다. 용착금속의 저온인성을 향상시킬 수 있는 침상형페라이트와 비금속개재물의 상관관계에 관해 검토 하였다.

• 한국치위생학회지
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• 제10권5호
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• pp.947-953
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• 2010

Objectives : The purpose of this investigation was to evaluate the remineralization and acid resistance in fluoride varnish by Micro-computed tomography (micro CT). Methods : Specimens of bovine teeth enamel were embedded in resin, polished and randomly divided into 3 groups (a control group, a NaF solution group, a fluoride varnish group). Each group has 3 specimens that was standardized according to Vickers hardness number (VHN). Specimens were immersed in demineralization solution for 72 hours. The control group had no treatment, the NaF solution group was treated by a 5% NaF solution for 4 minutes, and the fluoride varnish group was treated by a fluoride varnish for one hour. All specimens were subjected to a chemical pH cycling method for 14 days. After a chemical pH cycling method, the density were measured using micro CT. Then, specimens were immersed in each demineralization solution for 72 hours. After demineralization processed, the density were measured using micro CT. Results : 1. The density was significantly higher in the fluoride varnish and 5% NaF solution group than that of the control group after 14 days cycling (p<0.05). And the density value of the fluoride varnish group was higher than that of the 5% NaF solution, with no significant difference. 2. The differences of density after acid resistance treatment were statistically significant among 3 groups(p<0.05). Conclusions : It is suggested that fluoride varnish showed the remineralizing effect and acid resistance effect on the enamel, and micro CT could be used to evaluate the change of enamel lesion.

• 한국공작기계학회논문집
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• 제17권4호
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• pp.104-111
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• 2008

The objective of this study is to evaluate micro end-milling characteristics and tool wear behavior of AlN-hBN composites. First, AlN based composites with hBN contents in the range of 10 to 20vol% were prepared by hot-pressing. Vickers hardness and flexural strength of the prepared composite specimens were measured and compared according to the vol% of hBN variations. Then, cutting force variations were measured and analyzed using a tool dynamometer during the micro end-milling experiments; and machined surface shapes and roughness were investigated using a 3D non-contact type surface profiler. After micro end-milling, worn tools were investigated using a tool microscope and SEM images. From the experimental results, it can be observed that the cutting forces decreased, and surface qualities were improved with increasing hBN contents. At low content of hBN, tool chipping was observed; and tool wear rate decreased with increasing hBN contents. The results of this study insist that proper machining conditions, including tool wear behavior investigation, should be determined for the micro end-milling of AlN-hBN composites for its further application.