발간논문

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The sintering process in alumina (Al2O3) specimens for a functional ceramic anchor was investigated and the physical and mechanical properties of the sintered samples were analyzed. MgO (from 0.25 wt% to 3.0 wt%) as an additive was added and sintered at various temperatures (1450 ℃, 1550 ℃, and 1650 ℃) using a conventional solid state sintering process. The sintered density, flexural strength and compressive strength were investigated for the mechanical strength evaluation. The 2.0 wt% MgO added Al2O3 samples (sintered at 1650 ℃) showed a maximum flexural strength of 354.82 MPa and compressive strength of 1615.2 MPa. These experimental results suggested that the sintering temperature and the amount of additives played an important role in enhancing the density and mechanical strength because of the micro-structure modification by MgO addition in the Al2O3 sintered samples. We expect to be able to produce ceramic anchors using the experimental conditions with the best mechanical and physical properties.

keyword : ceramics, sintering, mechanical properties, X-ray diffraction

AM30 + (0, 0.5, 1, 1.5) wt%CaO magnesium alloys were cast and oxidized at 625 ℃ and 650 ℃ in atmospheric air. The CaO particles that were added during casting allowed casting the AM30 alloys in air without using environmentally hazardous SF6 or SO2 gases. They decomposed to dissolve in the α-Mg matrix and also to precipitate along the matrix grain boundaries as Al2Ca during casting. The ignition temperatures were 584.6 ℃ for AM30, 604.5 ℃ for AM30 + 0.5 wt%CaO, and 691.7 ℃ for AM30 + 1 wt%CaO. No ignition occurred for AM30 + 1.5 wt%CaO up to 700 ℃. During oxidation, CaO-rich oxide scales formed on the alloy surface, which prevented direct contact of the alloy with air and fast oxidation of the alloy in air.

keyword : magnesium alloys, AM30, CaO, oxidation

The microstructure and corrosion properties of an as-cast Mg-4Al-2Sn alloy with different levels of Sr content (0, 1, 2 wt%) were investigated. A new rod-like MgSnSr ternary phase was identified in the alloys containing Sr. The grain size decreased with an increasing Sr content. The corrosion properties were improved by increasing the Sr content. The improved corrosion properties of the Mg-4Al-2Sn-Sr alloys is related to MgSnSr phase and grain refinement. The uniformly dispersed MgSnSr phase within the grain and grain boundary perform the role of an effective barrier for retarding corrosion propagation.

keyword : magnesium alloy, casting, microstructure, corrosion test

A porous titanium structure was fabricated by investment casting with a pressurization/depressurization system, and its mechanical properties were studied. A Micro-Vickers hardness profile revealed that hardness gradually increased from the matrix to the metal/mold interface. A compression test was conducted on a single cell of the porous Ti structure. The theoretical and experimental values of yield strength were in good agreement. Such agreement suggested that the reaction layer did not affect the macro-mechanical properties of the porous Ti structure.

keyword : porous materials, casting, mechanical properties, hardness test, titanium

To investigate the effect of tungsten substitution of molybdenum on the formation of the second phase in Super Duplex Stainless Steel Weldments, welding wires with a composition of 3 wt% Mo, 2.2 wt% Mo-2.2 wt% W were designed for the flux cored arc welding process. As a result, the precipitation of the χ phase and σ phase increased in proportion to the decrease in the amount of δ ferrite content because the reaction, δ ferrite → σ + γ2, proceeded as the temperature rose. Under the same experimental conditions, the precipitation of the second phase, which degrades the properties of the material, was significantly reduced in the W substitution specimens compared to the Mo-only specimens. A polarization test conducted in a salt solution revealed that the pitting potential of the W substitution specimens was higher than that of the Mo-only specimens.

keyword : metals, welding, precipitation, electron backscattering diffraction(EBSD), super duplex stainless steel

This paper describes the effect of the solidification behavior of camphene on the pore structure of porous W fabricated via a freeze-drying process. Camphene slurries with 10 and 15 vol% WO3 were prepared by milling at 50 ℃ with a small amount of dispersant. Porous W with unidirectionally aligned pore channels was prepared by freeze-drying of slurry and heat treatment in a hydrogen atmosphere. The sintered specimen with 10 vol% WO3 showed large pores with a size of about 100 μm which were aligned parallel to the growth direction of camphene. The internal wall of large pores had relatively small pores. In the slurry with 15 vol% WO3, large pores were not observed in the sintered body, and only small pores existed. The difference in the pore size in the two specimens is explained on the basis of the growth characteristics of the camphene dendrite depending on the amount of particles in the slurry.

keyword : porous materials, powder processing, solidification, scanning electron microscopy, W system

A cast iron axe from the second century BC and a forged iron axe from the fourth century AD were excavated from the Gongju Suchon-ri site. Metallurgical examination shows that the cast iron axe was made of white cast iron alloys of hyper-eutectic and pearlite of near eutectoid composition with temper carbon. This axe was decarburized to produce steel in an oxidizing atmosphere for a long time. However, cementite and malleable cast iron coexisted inside the axe because of an insufficient heat treatment time for decomposing cementite into ferrite and temper carbon. By contrast, the microstructure of the blade of the forged iron axe consisted of martensite formed by quenching. The cast iron axe is evidence that the Gongju area imported ironware from China in the second century BC. The forged iron axe supports the idea that the forging technique was used throughout Southern Korea in the fourth century AD.

keyword : Cast iron, carbon and graphite, annealing, microstructure, optical microscopy

Sn-doped In2O3 (ITO)/TiO2 bi-layered films were deposited by radio frequency magnetron sputtering under different substrate temperatures, 100, 200, and 300 ℃, and the effects of substrate temperature on the structural, electrical and optical properties of the films were investigated. For all depositions, the thicknesses of the ITO and TiO2 films were kept constant at 100 and 5 nm, respectively, by controlling the deposition times. The films deposited at room temperature showed a relatively low optical transmittance of 76.0%, while the films deposited at 300 ℃ showed a higher transmittance of 80.7%, compared to the other films. In addition, the electrical resistivity of the films was influenced by substrate temperature and the lowest resistivity of 3.0 × 10.4 Ω·cm was observed in the films deposited at 300 ℃. From the experimental results, we concluded that increasing the substrate temperature enhanced the optical and electrical properties of the ITO films.

keyword : ITO, TiO2, optical properties, electrical properties, X-ray diffraction, sputtering

A Mo/Pt bilayered catalytic layer on a flat glass substrate was used as a counter electrode to improve the energy conversion efficiency of a dye-sensitized solar cell device with the structure of effective area of 0.45 cm2 glass/FTO/blocking layer/TiO2/N719(dye)/electrolyte/50 nm Pt/50 nm Mo/glass. For comparison, 100 nm-thick Pt and Mo counter electrodes on flat glass substrates were also prepared using the same procedure. The photovoltaic properties, such as the short circuit current density, open circuit voltage, fill factor, energy conversion efficiency, and impedance, were characterized using a solar simulator and poten-tiostat. The phases and microstructures of the catalytic layers were examined by X-ray diffraction and field emission electron microscopy. The measured energy conversion efficiency of the dye-sensitized solar cell devices with only Pt and Mo/Pt bilayer counter electrodes was 4.60% and 6.30%, respectively. The interface resistance at the interface between the counter electrode and electrolyte decreased when a Mo/Pt bilayer thin film was used. The new phase of Pt3Mo led to an increase in catalytic activity. This suggests that the Mo/Pt bilayered catalytic layers may provide better efficiency in the dye-sensitized solar cells than the conventional Pt layers.

keyword : solar cells, sputtering, optical properties, solar simulator, bilayer

Adhesion of osteoblast cells to TiCrAlSiN thin films was evaluated in vitro. Ti and TiCrAlSiN thin films were deposited on glass substrates by cathodic arc deposition. Surface roughness and chemistry of the TiCrAlSiN thin films was characterized by AFM and EPMA, respectively. Ti and TiCrAlSiN thin films and glass coverslips were cultured with human osteoblast cells (hFOB 1.19). The cell cytoskeleton was analyzed by observing the organization of actin stress fibers and microtubules. Cell proliferation was investigated by MTT assay and visualization. Focal contact adhesion was studied by observing the vinculin density. The results indicated that the TiCrAlSiN coating significantly influenced the actin cytoskeleton and microtubule organization. Human osteoblasts hFOB attached and proliferated better on TiCrAlSiN thin films with more focal contact adhesions than on Ti thin films or glass surfaces. These results suggest that TiCrAlSiN thin films can be an implantable material where the maximum cell adhesion is required.

keyword : TiCrAlSiN thin film, biomaterials, surface modification, surface, optical microscopy