• Journal of the Korean Society for Nondestructive Testing
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• v.18 no.5
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• pp.381-388
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• 1998

The ultrasonic method, which is well known as non-destructive test method, is widely used to evaluate the material damage caused by degradation practically. However, this method is just used for measuring the crack size and the thickness loss of tube. The purpose of this study is to investigate the applicability of the ultrasonic technique for the evaluation of carburized material and to suggest the correlations between the ultrasonic characteristics and carburized degradation. The miniaturized specimens($40{\times}20{\times}6.3mm$) are adopted from the HK-40 (25Cr-20Ni-0.4C) centrifugal cast tube after carburization treatment. Carburization was carried at $1200^{\circ}C$ by the pack method. The results of ultrasonic test present that the longitudinal wave velocity increased with the increase of carburized depth. The correlation between the longitudinal wave velocity and carburization was changed with the density and Young's modulus. Therefore, the average velocity in the materials carburized for 336 hours and the unused one were 5,840 m/s and 5,755 m/s at 5 MHz, respectively. With the obtained results from this study, it can be recognized that the technique using the ultrasonic velocity property is very useful method to evaluate the degree of carburized material non-destructively.

• Journal of the korean academy of Pediatric Dentistry
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• v.44 no.4
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• pp.412-418
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• 2017

During orthodontic treatment, formation of white spot lesions (WSLs) around brackets has long been recognized as a potential risk. This study performed to investigate effect of preventing enamel demineralization and remineralization by application of fluoride-containing orthodontic primer. Fifty extracted bovine incisors teeth were randomly allocated to 3 groups: (I) Non-preparation specimens, (II) Application of Light Bond$^{TM}$ as fluoride containing orthodontic primer, (III) Application of Transbond$^{TM}$ XT Primer as traditional orthodontic primer without fluoride. Each group is demineralized under artificial carious solution. The demineralization pattern was evaluated using a Q-ray view, Vickers hardness test and polarized light microscope. The remained primer was calculated as 35 - 50%. The highest surface microhardness was shown on Light Bond$^{TM}$ surface. There were statistically significant differences in Vickers microhardness number between adjacent areas of Light Bond$^{TM}$ and non-prepared area. There was almost no demineralization of the enamel surface under the Light Bond$^{TM}$. At the adjacent site of Light Bond$^{TM}$, the shallow caries pattern and remineralization appearance were also observed. These results suggest that the use of fluoride-containing primers may be useful for bracket attachment to reduce enamel demineralization during orthodontic treatment.

• Journal of the Institute of Electronics Engineers of Korea TE
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• v.39 no.1
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• pp.1-8
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• 2002

The effects of the addition of either monoclinic $ZrO_2$($ZrO_2$(m)) or tetragonal $ZrO_2$($ZrO_2$(t)) containing 5.35wt% $Y_2O_3$ on the physical properties and electrical conductivity of TEX>$Al_2O_3$ were investigated. The addition of $ZrO_2$(m) and $ZrO_2$(t) increased sintered density of $Al_2O_3$. The Vickers hardness also increased as addition of >($ZrO_2$(t) increased going through a maximum at 20wt% and the hardness of the specimens was found to be dependent on the sintered density. The addition of $ZrO_2$(t) improved the hardness of $Al_2O_3$-$ZrO_2$ systems and the $ZrO_2$(m) addition showed the better effect on the thermal shock property of $Al_2O_3$-$ZrO_2$ systems than that of the $ZrO_2$(t) addition. Above 15wt% addition of $ZrO_2$(t), the electrical conductivity is gradually increased with increasing applied voltage but not effects by addition of $ZrO_2$(m).

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.1
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• pp.85-90
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• 2010

Friction welding is a common practice to join axially symmetrical parts for automobile industry applications. The shaft for automobile reverse idle gear is generally produced by forging steel, SF 45. This method is not so good because of high cost of material and production. In this study, in order to investigate the possibility of application of SM 45C to SF 45 dissimilar friction welding, the dissimilar friction welded joints were performed using 20 mm diameter solid bar in forging steel(SF 45) to carbon steel(SM 45C). The optimal friction welding parameters were selected to ensure reliable quality welds on the basis of visual examination, tensile test, micro-Virkers hardness surveys of the bond of area and optical microstructure investigations for welded joint parts. Finally, post weld heat treatment(PWHT) of the high-frequency induction hardening was performed for the friction welded specimens under the optimal welding conditions. And then, the mechanical properties were compared for as-welded and PWHT in SM 45C to SF 45.

• The Journal of Engineering Geology
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• v.26 no.1
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• pp.117-128
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• 2016

Chemical reaction tests were performed to assess the properties of hardened specimens of cement pastes (KS-1 Portland and Class G) exposed to supercritical CO₂ for 1, 10, and 100 days. After exposure, the samples' measured permeability and strength were compared with values measured for pristine samples. The pristine cements had permeabilities of 0.009~0.025 mD, which increased by one order of magnitude after 100 days of exposure (to 0.11~0.29 mD). The enhancement of permeability is attributed to the stress release experienced by the samples after removal from the pressure vessel after exposure. Despite its enhancement, the measured permeability mostly remained lower than the API (American Petroleum Institute) recommended maximum value of 0.2 mD. The degradation of the cement samples due to exposure to supercritical CO₂ led to a layer of altered material advancing inwards from the sample edges. The Vickers hardness in the altered zone was much higher than that in the unaltered zone, possibly owing to the increase in density and the decrease in porosity due to the carbonation that occurred in the altered zone. Hardness close to the edge within the altered zone was found to have decreased significantly, which is attributed to the conversion of C-S-H into less-strong amorphous silica.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2002.05a
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• pp.23-28
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• 2002

This paper describes the machining characteristics of the $MoSi_2$ based composites by electric discharge drilling with various tubular electrodes, besides, Hardness characteristics and microstructures of $Nb/MoSi_2$ laminate composites were evaluated from the variation of fabricating conditions such as preparation temperature, applied pressure and pressure holding time. $MoSi_2$ -based composites has been developed in new materials for jet engine of supersonic-speed airplanes and gas turbine for high- temperature generator. Achieving this objective may require new hard materials with high strength and high temperature-resistance. However, With the exception of grinding, traditional machining methods are not applicable to these new materials. Electric discharge machining (EDM) is a thermal process that utilizes a spark discharge to melt a conductive material, the tool electrode being almost non-unloaded, because there is no direct contact between the tool electrode and the workpiece. By combining a nonconducting ceramics with more conducting ceramic it was possible to raise the electrical conductivity. From experimental results, it was found that the lamination from Nb sheet and $MoSi_2$ powder was an excellent strategy to improve hardness characteristics of monolithic $MoSi_2$. However, interfacial reaction products like (Nb, Mo)$SiO_2$ and $Nb_2Si_3$ formed at the interface of $Nb/MoSi_2$ and increased with fabricating temperature. $MoSi_2$ composites which a hole drilling was not possible by the conventional machining process, enhanced the capacity of ED-drilling by adding $NbSi_2$ relative to that of SiC or $ZrO_2$ reinforcements.

• Journal of the Korean Ceramic Society
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• v.42 no.1
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• pp.43-49
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• 2005

YSZ layer containing nano-sized particles has been deposited on the commercial A1203 substrate by Electron Beam Physical Vapor Deposition (EB-PVD). The role of coating rnjcrostructures of YSZ to indentation damage is studied. The different coating microsouctures are prepared by varying the substrate temperatures from $600^{circ}C$ to $800^{circ}C$ during the deposition. Microhardness test and Hertzian indentation are conducted on the $YSZ/Al_{2}O_{3}$ layered systems. The damage and flilure behaviors have been investigated according to the effect of microstructures and indentation loads. With increasing the substrate temperature during EB-PVD, the overall grain sizes are coarser and more faceted, which microsoucture ultimately influences on the indentation behavior, thus, YSZ/Al_{2}O_{3}$ layered system prepared at the substrate temperature of $800^{circ}C$ shows relatively higher damage tolerance.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.3
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• pp.267-272
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• 2011

We analyzed the wear characterization of $Si_3N_4$ ceramics according to the amount of added $SiO_2$ nanocolloid. The test specimen was prepared by hot-press sintering at 35 MPa and 2123 K in an $N_2$ gas atmosphere for 1 h. A wear test was performed with a block-on-ring tester, and the test conditions were as follows: (1) the ring with a diameter of 35 mm had a rotational speed of 50 rpm; (2) the load was 9.8 N; and (3) the temperature was $25^{\circ}C$. The test results show that $Si_3N_4$ ceramics have a friction coefficient of about 1.0 and a wear loss of about 0.02 mm. Of the specimens used this study, the test specimen with 1.3 wt% of added $SiO_2$ nanocolloid has the best wear resistance because it has the lowest friction coefficient and the smallest wear loss. This specimen also has the highest Vickers hardness and bending strength. In this study, the friction coefficient is inversely proportional to the hardness and bending strength.

• Journal of the Korean Society for Nondestructive Testing
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• v.34 no.3
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• pp.220-225
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• 2014

Generally, the nonlinearity of ultrasonic waves is measured using a nonlinear parameter ${\beta}$, which is defined as the ratio of the second harmonic's magnitude to the power of the fundamental frequency component after the ultrasonic wave propagates through a material. Nonlinear parameter ${\beta}$ is recognized as an effective parameter for evaluating material degradation. In this paper, we evaluated the nonlinear parameter of Al6061-T6 which had been subjected to an artificial aging heat treatment. The measurement was using the transmitted signal obtained from contact-type transducers. After the ultrasonic test, a micro Vickers hardness test was conducted. From the result of the ultrasonic nonlinear parameter, the microstructural changes resulting from the heat treatment were estimated and the hardness test proved that these estimates were reasonable. Experimental results showed a correlation between the ultrasonic nonlinear parameter and microstructural changes produced by precipitation behavior in the material. These results suggest that the evaluation of mechanical properties using ultrasonic nonlinear parameter ${\beta}$ can be used to monitor variations in the mechanical hardness of aluminum alloys in response to an artificial aging heat-treatment.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.8
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• pp.603-611
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• 2020

A porous material which can contain liquid perfume was manufactured by adding 1~4 wt% of polysiloxane into a composite containing 20 ㎛ Ag powder and 30 wt% of 53 ㎛ salt, sintering for 60 min at 750℃, and melting salt selectively. The changes in pore, hardness, and microstructure were confirmed according to the polysiloxane content both before and after sintering. The manufactured silver liquid container was formed with open pores both before and after sintering, and the container shrunk by 2~7 % in both perpendicular and parallel directions after sintering. Vickers hardness was increased after sintering and was doubled when 2 wt% of polysiloxane was added. In case of the microstructure, the surface condition of the silver liquid container became darker according to the polysiloxane content, and the pore size was decreased from 50 ㎛ to under 10 ㎛. The composition distribution result revealed an even distribution when 2 wt% of polysiloxane was added but uneven distribution when over 3 wt% of polysiloxane was added due to decreased hardness by cluster. Therefore, the addition of an appropriate amount of 2 wt% polysiloxane reinforced the porous silver with open pores to offer application for jewelry usage.