• Journal of the Korean Society for Heat Treatment
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• v.36 no.3
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• pp.128-136
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• 2023

In this study, the effects of heat treatment on the nano-scale creep behavior of CoCrFeMnNi high-entropy alloy (HEA) processed by high-pressure torsion (HPT) was investigated through nanoindentation technique. Nanoindentation experiments with a Berkovich indenter were performed on HPT-processed alloy subjected to heat treatment at 450℃, revealing that the hardness of the HPT-processed alloy (HPT sample) significantly increased with the heat treatment time. The heat treatment-induced microstructural change in HPT-processed alloy was analyzed using transmission electron microscopy, which showed the nano-sized Cr-, NiMn-, and FeCo-rich phases were formed in the HPT-processed alloy subjected to 10 hours of heat treatment (HPT+10A sample). To compare the creep behavior of HPT and HPT+10A samples, constant load nanoindentation creep experiments were performed using spherical indentation indenters with two different radii. It was revealed that the predominant mechanism for creep highly depended on the applied stress level. At low stress level, both HPT and HPT+10A samples were dominated by Coble creep. At high stress level, however, the mechanism transformed to dislocation creep for HPT sample, but continued to be Coble creep for HPT+10A sample, leading to higher creep resistance in the HPT+10A sample.

• The korean journal of orthodontics
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• v.29 no.5 s.76
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• pp.589-597
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• 1999

The purpose of this study was to evaluate the effects of fluoride releasing orthodontic sealants, light-cured (Group L1&L2) and self-cured (Group S1&S2) $FluoroBond^{\circledR}$, on enamel microhardness under artificial carious solution in vitro.112 extracted human premolar teeth were collected for experiments and divided into seven groups. A Tukon microhardness tester equipped with a Knoop diamond indenter was employed to determine microhardness. Tukon 23 program converted the number of microhardness into KHN (Knoop hardness number). The results were as follows: 1. The microhardness of enamel depth of all groups were the least at the depth of $50{\mu}m$ and that of all groups except L2 group, the greatest at the depth of $200{\mu}m$, were the greatest at the depth of $300{\mu}m$. And as the enamel depth of all groups except L2 and S2 group increased, the microhardness value also increased. 2. There was a little preventive effect in enamel decalcification both light-and self-cured orthodontic sealant groups, but had no statistical significance between the groups(p>0.05). 3. Light-cured orthodontic sealant groups had a progressive inhibiting effect in enamel decalcification at the depth of $100{\mu}m,150{\mu}m,\;and\;200{\mu}m$ (p<0.05). 4. Self-cured orthodontic sealant groups had a progressive inhibiting effect in enamel decalcification at the depth of $150{\mu}m$ (p<0.05). 5. There was no difference of the anti-enamel demineralization effect between light- and self-cured orthodontic sealant groups (p>0.05).

• Journal of the Korean Vacuum Society
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• v.20 no.4
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• pp.266-270
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• 2011

This paper suggest tungsten (W)-carbon (C)-nitrogen (N) thin films for diffusion barrier that W is main material and C and N are additives. W-C-N thin films are deposited with fixed rates of W and C but with a variation of $N_2$ gas flow and W-C-N thin films are heated at $600^{\circ}C$. From the experimental results, the variation of elastoplastic region for W-C-N thin film measured by tribological property is larger than that of elastic region with a variation of $N_2$ gas flow. These results show that the $N_2$ gas flow is more directly related with the elastoplastic region of W-C-N thin film. Nanoindenting test executed 16 times consecutively and we got the stress-strain curve graphs and hardness datas at each sample. Through the stress-strain curve graphs, the standard diviation of stress-strain curve for $N_2$ gas flow rate of 2.0 sccm is smaller than that of 0, 0.5, 1.5 sccm. Consequently, the physical stability of W-C-N thin film depends on the flow rate of $N_2$ gas.

• Composites Research
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• v.31 no.5
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• pp.260-266
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• 2018

This study investigates thermal and mechanical characterization of Hafnium carbide coating on the $C_f-C$ composites. The hafnium carbide coatings by vacuum plasma spray on the C/C-SiC composites are prepared to evaluate oxidation and wear resistance. We perform the thermal durability tests by thermal cycling at $1200^{\circ}C$ for 10cycles in air and investigates the weight change of each cycle. We also evaluate the wear and indentation behavior using tungsten carbide ball indenter as a mechanical evaluation. As a result, the HfC coating is beneficial to reduce of weight loss during thermal cycling test and improve the elastic property of C/C-SiC composite. Especially, the HfC coating improves the wear resistance of C/C-SiC composite.

• Journal of the Korean Society for Nondestructive Testing
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• v.26 no.5
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• pp.306-314
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• 2006

Instrumented indentation technique is a new way to evaluate nondestructive such mechanical properties as flow properties, residual stress and fracture toughness by analyzing indentation load-depth curves. This study evaluated quantitatively the flow properties of steels and residual stress of weldments. First, flow properties can be evaluated by defining a representative stress and strain from analysis of deformation behavior beneath the rigid spherical indenter and the parameters obtained from instrumented indentation tests. For estimating residual stress, the deviatoric-stress part of the residual stress affects the indentation load-depth curve, so that by analyzing the difference between the residual-stress-induced indentation curve and residual-stress-free curve, the quantitative residual stress of the target region can be evaluated. The algorithm for flow property evaluation was verified by comparison with uniaxial tensile test and the residual stress evaluation model was compared to mechanical cutting and ED-XRD results.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.2
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• pp.165-172
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• 2013

Power generating unit structures are designed and built to meet standard to secure its safety for expected life time. As the structures have been exposed to combined environment, degradation of structure material is accelerated and it can cause unexpected damage; evaluating precise mechanical properties of weak site like welded area is an essential research area as it is directly connected to safety issues. Existing measuring technique like tensile test requires specific size in testing specimen yet it is destructive method which is hard to apply on running structures. To overcome above mentioned limitation, IIT is getting limelight as it is non-destructive and simple method. In this study, latest technique is introduced to evaluate tensile property and residual stress by analyzing stress field occurs under the indenter while IIT is performed. Test on welded area, the weak site of nuclear structures have been practiced and confirmed that IIT can be usefully applied to evaluate integrity in industry.

• Proceedings of the Korean Vacuum Society Conference
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• 1999.07a
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• pp.112-112
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• 1999

고분자 표면에 MeV급 이온을 주입하게 되면 화학적, 광학적, 물리적, 전기적 특성 등 기존의 재래식 공정으로는 불가능한 다양한 특성변화가 일어나게 된다. 본 실험에서는 이온 조사에 따른 micro-hardness의 변화에 중점을 두어 관찰하였다. 수 MeV로 가속시킨 Cl과 C 이온을 Kapton, Teflon, PMMA에 조사량을 바꾸어가며 조사하였다. 이때 조사 전후 고분자 시료의 표면 경도는 depth-sensing nanoindentation technique (Nano Indenter II, USA)을 이용하여 측정하였다. 측정깊이는 극표면 효과와 기측효과의 영향을 무시할 수 없는 100nm로 결정하였다. 또한 이와 같은 경도변화를 규명하기 위하여 각각의 시료에 대해 FTIR, Raman, UV-VIS, RBS, XPS 스펙트럼을 측정하였으며, 조사중에 발생되는 가스상 분자들을 RGA를 이용하여 측정하였다. 1 MeV Cl 이온을 Kato에 4x1015ions/cm2 조사하면 경도가 0.35 GPa에서 7.1GPa로 약 20배 정도 증가하게 되며, 이 경도는 stainless steel 경도 2~3 GPa, martensite steel 8~12와 비교하여 보면 상당히 높은 수준임을 알 수 있다. Teflon과 PMMA 시료의 경우 1MeV Cl 이온 4X1015ions/cm2를 조사시키면 각각 경도가 0.31GPa에서 4.1a, 0.30 GPa에서 3.9GPa로 변화하였으며 Kapton에 비하여 상대적으로 경도의 변화가 적음을 알 수 있었다. 이온 주입된 Kapton의 경우 FT-IR측정에 의하여 이온 조사량이 증가함에 따라 C=O 진동, 이미드그룹의 CONH, tertiary amine의 C-N 흡수 피크가 감소되며, 1X1014ions/cm2 이상의 양이 조사되어야 개질변화가 일어남을 알 수 있었다. XPS 측정 결과 Kapton에 조사되는 이온양이 증가할수록 C=O, C-O 및 C-N의 탄소는 감소하고 페닐고리 탄소가 증가함을 알 수 있었다. 또한 이온 조사 중 측정한 RGA에서도 CO 가스가 대량 방출되는 현상을 관찰하였으며 FTIR 및 XPS에서의 C=O 결합의 감소와도 일치하였다. RBS에 의한 CNO 원소의 변화에서도 다른 원소보다 O의 감소가 현저하게 나타남을 확인하였다. UV-VIS 측정을 통해 조사 이온량에 따라 에너지 준위가 변동하여 흡수스펙트럼이 장파장으로 확대됨을 알 수 있었으며, 이는 공액 2중 결합의 형성에 의한 $\pi$-전자 및 기타 결함에 기인한다. PMMA 및 Teflon의 경우 FTIR 측정에 의하여 이온 조사됨에 따라 function group 들의 peak가 세기가 감소되면서 완만해지는 경향을 보이며 원래의 구조를 상실하게 된다. PMMA와 Teflon은 이온 조사가 되면 가교형서보다는 사슬절단이 주로 일어나므로 가교형성이 잘 일어나는 Kapton보다는 상대적으로 경도의 증가가 적음을 알 수 있었다.

• Journal of the Korean institute of surface engineering
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• v.46 no.2
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• pp.68-74
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• 2013

For the coating of diamond films on WC-Co tools, a buffer interlayer is needed because Co catalyzes diamond into graphite. W and Ti were chosen as candidate interlayer materials to prevent the diffusion of Co during diamond deposition. W or Ti interlayer of $1{\mu}m$ thickness was deposited on WC-Co substrate under Ar in a DC magnetron sputter. After seeding treatment of the interlayer-deposited specimens in an ultrasonic bath containing nanometer diamond powders, $2{\mu}m$ thick nanocrystalline diamond (NCD) films were deposited at $600^{\circ}C$ over the metal layers in a 2.45 GHz microwave plasma CVD system. The cross-sectional morphology of films was observed by FESEM. X-ray diffraction and visual Raman spectroscopy were used to confirm the NCD crystal structure. Micro hardness was measured by nano-indenter. The coefficient of friction (COF) was measured by tribology test using ball on disk method. After tribology test, wear tracks were examined by optical microscope and alpha step profiler. Rockwell C indentation test was performed to characterize the adhesion between films and substrate. Ti and W were found good interlayer materials to act as Co diffusion barriers and diamond nucleation layers. The COFs on NCD films with W or Ti interlayer were measured as less than 0.1 whereas that on bare WC-Co was 0.6~1.0. However, W interlayer exhibited better results than Ti in terms of the adhesion to WC-Co substrate and to NCD film. This result is believed to be due to smaller difference in the coefficients of thermal expansion of the related films in the case of W interlayer than Ti one. By varying the thickness of W interlayer as 1, 2, and $4{\mu}m$ with a fixed $2{\mu}m$ thick NCD film, no difference in COF and wear behavior but a significant change in adhesion was observed. It was shown that the thicker the interlayer, the stronger the adhesion. It is suggested that thicker W interlayer is more effective in relieving the residual stress of NCD film during cooling after deposition and results in stronger adhesion.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.11
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• pp.1715-1720
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• 2010

In this study, we analyzed the crack-healing characteristics of specimens; different crack lengths and coating methods of $Si_3N_4$ ceramic structures with long cracks were analyzed. Cracks with lengths of about $100-500\;{\mu}m$ were obtained using a Vickers indenter for a load of 24.5-98 N. In the case of a crack obtained by applying a load of 24.5 N, the crack-healed specimen with $SiO_2$ nanocolloid coating exhibited the highest bending strength, which was higher than that of a smooth specimen by 140%, but the bending strength of a crack-healed specimen that had a $SiO_2$ nanocolloid coating and originally had multiple cracks was lower than that of a smooth specimen. However, when compared to the cracked specimens, the bending strength of most specimens with multiple cracks increased slightly. On the basis of these results, the crack-healing characteristics of $Si_3N_4$ ceramic structures with multiple indentations were studied for different coating methods. The most effective coating method for long-crack specimens was hydrostatic pressure coating.

• Journal of Korean Ophthalmic Optics Society
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• v.14 no.1
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• pp.57-62
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• 2009

Purpose: TiN films were deposited on sus304 by unbalanced magnetron sputtering system which was designed and developed as unbalancing the strength of the magnets in the magnetron electrode. The color and hardness of deposited TiN films was investigated. Methods: The cross sections of deposited films on silicon wafer were observed by SEM to measure the thickness of the films, the components of the surface of the films were identified by XPS, the components of the inner parts of the films were observed by XPS depth profiling. XPS high resolution scans and curve fittings of deposited films were performed for quantitative chemical analysis, Vickers micro hardness measurements of deposited films were performed with a nano indenter equipment. Results: The colors of deposited films gradually changed from light gold to dark gold, light violet, and indigo color with increasing of the thickness. It could be seen that the color change come from the composite change of three compound,$TiO_{x}N_{y}$, $TiO_2$, TiN. Especially, the composite change of$TiO_{x}N_{y}$ compound was thought to affect the color change with respect to thickness. Conclusions: Deposited films had lower than the value of general TiN film in Vickers hardness, which was caused by mixing three TiN, $TiO_2$,$TiO_{x}N_{y}$ compound in the deposited films. The increasing and decreasing of micro hardness with respect to thickness was thought to have something to do with the composite of TiN in the films.