• Korean Journal of Materials Research
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• v.28 no.9
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• pp.499-505
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• 2018

This investigates the microstructure and mechanical properties of Al hybrid material prepared by electromagnetic duo-casting to determine the effect of heat treatment. The hybrid material is composed of an Al-Mg-Si alloy, pure Al and the interface between the Al-Mg-Si alloy and pure Al. It is heat-treated at 373, 573 and 773K for 1h and T6 treated (solution treatment at 773K for 1h and aging at 433K for 5h). As the temperature increases, the grain size of the Al-Mg-Si alloy in the hybrid material increases. The grain size of the T6 treated Al-Mg-Si alloy is similar to that of one heat-treated at 773K for 1h. The interface region where the micro-hardness becomes large from the pure Al to the Al-Mg-Si alloy widens with an increasing heat temperature. The hybrid material with a macro-interface parallel to the tensile direction experiences increased tensile strength, 0.2 % proof stress and the decreased elongation after T6 heat treatment. On the other hand, in the vertical direction to the tensile direction, there is no great difference with heat treatment. The bending strength of the hybrid material with a long macro-interface to the bending direction is higher than that with a short macro-interface, which is improved by heat treatment. The hybrid material with a long macro-interface to the bending direction is fractured by cracking through the eutectic structure in the Al-Mg-Si alloy. However, in the hybrid material with a short macro-interface, the bending deformation is observed only in the limited pure Al.

• Elastomers and Composites
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• v.47 no.3
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• pp.238-245
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• 2012

Graphene oxide was synthesized from natural graphite, and its surface was modified using diisocyanatodicyclohexylmethane( $H_{12}MDI$). Isocyanate-graphene sheet(i-RGO) was obtained by reduction of surface modified GO. To select nanofiller having good dispersion with polyurethane, GO, i-RGO, natural graphite and thermal reduced graphite were analyzed, and then i-RGO was selected as a suitable nanofiller. PU/i-RGO nanocomposite was synthesized with various i-RGO contents to estimate effect of reinforcement on nanocomposite. Thermal stability, hardness, contact angle were increased with i-RGO contents due to i-RGO characteristic and crosslink bridge effect. But, tensile strength and elongation were decreased at i-RGO contents more than the 4 wt%. This phenomenon was interpreted by the excess formation of crosslink bridge.

• Elastomers and Composites
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• v.35 no.3
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• pp.173-179
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• 2000

Cure characteristics. tensile properties, and dynamic properties were investigated on the carbon black-filled natural rubber compounds, in which three typical vulcanization types conventional vulcanization(Conv), semi-efficient(Semi-EV), and efficient(EV) vulcanizations were used. The effects of vulcanization temperature on both the mechanical property and aging resistance of rubber compounds were also investigated. The Conv cure system showed a slightly slower rate of vulcanization than those of Semi-EV and EV ones. On the other hand, it showed a higher value in the maximum torque of cure curve. Higher tensile moduli were observed in Conv system than those in Semi-EV and EV ones, while lower elongation at break were obtained in Conv one. The tensile strength at break were found to be about the same for three cute systems. Hardness, modulus, and tensile strength decreased with increasing the vulcanization temperature, and the degree of changes in the properties was found to be smaller for EV and Semi-EV systems than that in Conv one. The EV system was found to be superior in thermal-aging resistance to Conv one.

• Applied Chemistry for Engineering
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• v.9 no.1
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• pp.58-65
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• 1998

Substituted dicyandiamide(Sub-DICY), Accelerated dicyandiamide(Acc-DICY), Trimellitic anhydride(TMA), Pyromellitic dianhydride(PMDA) and Phenolic curing agent(Ph.C.A.) are mainly used for epoxy powder coating curing agent. Various characteristics of epoxy films fully cured by optimum condition such as mechanical properties like $T_g$, tensile strength, elongation at break hardness, abrasion resistance and chemical properties like water absorption, acid resistance, alkali resistance and electrical properties, corrosion resistance are determined by various measuring devices and analyses devices. In conclusion, phenolic curing agent was shown excellent thoughness but severe color change as temperature increased. Acid anhydride has excellent insulation properties and color stability at elevated temperature but lower thoughness and adhesion to substrate. DICY curing agent was shown high water absorption and severe color chance as temperature increased.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.12 no.5
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• pp.2451-2456
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• 2011

Inorganic micropowder(Ce500)-filled polyurethane composite foams were fabricated and the effects of postcure on the mechanical properties were studied by the measurement of polymerization temperature, TGA, and UTM test. Temperature for the maximum reaction rate of 20wt% Ce500-filled sample reached upto ca. $100^{\circ}$ within 10min. and, for the same sample, double mode thermal decomposition was observed around two distinguished temperatures of $250^{\circ}$ and $350^{\circ}C$. The activation energies for the decomposition were calculated using Kissinger method as 117.4 and 139.4 kJ/mol, respectively. While break strength and hardness of the sample seemed nearly affected by postcure time at $160^{\circ}C$, elongation, however, was significantly changed upto 1.72 times after 7hrs treatment. As the results, the condition of 7hrs at $160^{\circ}$ was considered as the optimum postcure condition for the Ce500-filled PU composite foam samples.

• The Journal of Korean Academy of Prosthodontics
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• v.32 no.2
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• pp.249-267
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• 1994

The purpose of this study was to compare the fatigue, physical properties, flexibility and surface roughness of titanium used in removable partial dentures with those of a type IV and alloy and a cobalt- chromium alloy. Fatigue testing subjected the test specimen to rapid cycling at a given stress until failure occurred by using a small-sized, electrodynamic type bending fatigue testing machine. The S-N curves for the framework materials were generated. For tensile testing, a tensile bar as described in the ADA Specification No.14 was subjected to tensile loading until failure occurred. Load-displacement curves were generated for 18 gauge round specimen and tapered half round specimen. Then the flexibilities were calculated. The surface roughnesses were compared by analyzer. Through analyses of the data, the following conclusions were obtained. 1. The fatigue property of titanium was higher than that of a type IV gold alloy$(p\leq0.05)$, but there was no significant difference between titanium and a cobalt-chromium alloy $(p\geq0.05)$. 2. The yield strength, the ultimate tensile strength and Victors hardness of titanium were higher than those of a type IV gold alloy but lower than those of a coalt-chromium alloy$(p\leq0.05)$. 3. The percentage of elongation and reduction of area of titanium were the highest $(p\leq0.05)$. 4. The surface roughness of titanium was the greatest$(p\leq0.05)$. 5. The flexibility of titanium was lower than that of a type IV gold alloy but higher than that of a cobalt-chromium alloy$(p\leq0.05)$.

• Elastomers and Composites
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• v.33 no.3
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• pp.193-200
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• 1998

It is well known that EPDM(ethylene propylene diene monomer) rubber has inherently excellent resistance to the weathering, ozone, heat, cold and moisture, whereas crosslinked IIR (isobutylene isoprene divlnyl benzene terpolymer) shows proper resistance to the water and gas permeation. Various characteristics of EPDM blend with crosslinked IIR such as curing characteristics, mechanical properties, dispersion of minor component and gas impermeability were explored. The optimum curing time $(t_{90})$ examined with peroxide was decreased by adding small amount of crosslinked IIR to the EPDM rubber. Mechanical properties of blends such as tensile strength, hardness and elongation at break were enhanced by increasing EPDM content. These results might be explained with the affinity of carbon black to the EPDM rubber. On the other hand, the physical properties were not changed significantly after aging, and the increase of crosslinked IIR fraction caused the decrease of compression set to small rate. EPDM rubber shows different behavior with crosslinked IIR in oxygen permeability. By adding 30wt.% crosslinked IIR to the EPDM rubber, the resistance to the oxygen permeation was improved up to three times than that of pure EPDM rubber. Conclusively, EPDM blend containing 30wt.% crosslinked IIR might be commercially applied to the o-ring and electric parts because of its proper resistance to the weathering, ozone and oxygen permeability.

• Korean Journal of Materials Research
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• v.23 no.11
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• pp.607-612
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• 2013

A powder in sheath rolling method was applied to the fabrication of a carbon nano tube (CNT) reinforced aluminum composite. A 6061 aluminum alloy tube with outer diameter of 31 mm and wall thickness of 2 mm was used as a sheath material. A mixture of pure aluminum powder and CNTs with a volume content of 5% was filled in the tube by tap filling and then processed to an 85% reduction using multi-pass rolling after heating for 0.5 h at $400^{\circ}C$. The specimen was then further processed at $400^{\circ}C$ by multi-pass hot rolling. The specimen was then annealed for 1 h at various temperatures that ranged from 100 to $500^{\circ}C$. The relative density of the 5vol%CNT/Al composite fabricated using powder in sheath rolling increased with increasing of the rolling reduction, becoming about 97% after hot rolling under 96 % total reduction. The relative density of the composite hardly changed regardless of the increasing of the annealing temperature. The average hardness also had only slight dependence on the annealing temperature. However, the tensile strength of the composite containing the 6061 aluminum sheath decreased and the fracture elongation increased with increasing of the annealing temperature. It is concluded that the powder in sheath rolling method is an effective process for fabrication of CNT reinforced Al matrix composites.

• Elastomers and Composites
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• v.45 no.1
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• pp.32-39
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• 2010

The mechanical properties of PTFE 100%, PTFT 90% + carbon black 10%, PTFE 85% + glass fiber 15%, PTFE 80% + glass fiber 15% + molybdenum disulfide ($MoS_2$) 5%, PTFE 75% + glass fiber 25%, and PTFE 75% + carbon black 18% + graphite 7% composites were investigated in this study. The differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to examine the heat of fusion(${\Delta}H_f$) and thermal stability of the composites. Also, the wear surface and wear volume of PTFE lip seal were examined using the durability test. Wear surface was observed using scanning electron microscope (SEM). It was found that the hardness, wear resistance and durability were enhanced by adding glass fiber and molybdenum disulfide into pure PTFE, but tensile strength and elongation were decreased. According to the experimental results, the composite (PTFE + 15% glass fiber + 5% molybdenum disulfide) showed the best properties for applying to oil-seal among six types of PTFE composites.

• Journal of the Korean Society for Precision Engineering
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• v.16 no.9
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• pp.41-47
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• 1999

The scale of dendritic structure of a cast preform plays a key role in determining the mechanical properties of cast/forged products. In this study, casting experiments are carried out to reduce dendrite arm spacing (DAS) to smaller than 20 ${\mu}$m by increasing cooling rate of the mold and then to spheriodize dendritic structures by addition of alloying elements such as Zr and Ti-B. From the casting experiments, appropriate casting conditions for producing the cast preform of a motorcycle connecting rod are obtained. To obtain fine microstructures of the cast preform, mold temperature must set to be low whilst cooling rate being high. When cooling rate is 10 $^{\circ}C$/s, the size of DAS is 17.4 ${\mu}$m. And the degree of spheriodization of a grain in the cast preform is described by aspect ratio, which is defined as the ratio of major and minor radii of an elliptical grain. When 0.5% Zr and 0.24 % Ti+B are added to the molten aluminum alloy, the best aspect-ratio 0.75 is obtained. After forging the cast preform of a motorcycle connecting rod, the microstructure and mechanical properties of the cast preform are compared with those of the cast/forged product. Cast/forged products are superior in microstructure and in mechanical properties such as ultimate strength, elongation, and hardness.