발간논문

Home > KJMM 논문 > 발간논문

The formation of the secondary reaction zone (SRZ) at the interface between the CoNiCrAlY coating and experimental Ni-based single crystal superalloy substrate was investigated. Transmission electron microscope (TEM) observation revealed that the SRZ was composed of recrystallized γ` grains with a high density of fine topologically close-packed (TCP) precipitates. The TCP precipitate was identified as the faulted and twinned rhombohedral μ phase. TCP precipitation appears to be analogous to the discontinuous precipitation. The γ` grains nucleated epitaxially with grains in the coated layer. Preferential formation of the SRZ was observed along the <110> direction. This anisotropy of SRZ formation may be essentially related to the preferential <110> dislocation gliding direction where recrystallization would be most active due to high strain energy and the supply of fast interdiffusion pathways. Based on the suggested SRZ formation mechanism, the intermediate annealing is proposed to suppress SRZ formation in terms of mitigation of strain energy and interdiffusion. (Received September 26, 2013)

keyword : alloys, coating, secondary reaction zone, SRZ, recrystallization, electron backscattering diffraction, EBSD

The objective of this study is to investigate the reliability of tensile properties for aged cast stainless steels evaluated by small punch (SP) tests. This study conducted standard tensile and SP tests using thermally aged CF8M, which is a type of cast stainless steel, as well as unaged CF8M at ambient temperature. CF8M was thermally aged at 400 ℃ for up to 10000 hours. The test showed that load deflection curves of SP tests properly present the strain hardening and failure characteristics of aged CF8M observed from the engineering stress-strain curves. Total and necking elongations of aged CF8M were reasonably correlated with deflection to final failure and deflection of plastic-instability obtained from the SP tests, respectively. Thus, it is clear that those SP test parameters can be used to evaluate the thermal aging of cast stainless steels. However, yield and tensile stresses of aged cast stainless steel were not reliably correlated with the SP test parameters. (Received June 3, 2013)

keyword : Metals, Aging, Mechanical Properties, Tensile Test, Small-Punch Test

One of the candidates for metallic interconnects of solid oxide fuel cells is a ferritic stainless steel, Crofer22 APU. Crofer22 APU specimens are polished and thermally cycled at 800 ℃ 4, 8, 20, 30, and 40 times, with the periods of maintaining the temperature at 800 ℃ (corresponding to the oxidation times) of 100, 200, 500, 750, and 1,000 h, respectively. The variation in their oxidation behavior with cycling is then investigated. The EDX spectra of the polished Crofer22 APU show that the atomic percentages of the elements Cr, Mn, and Fe decrease, while the atomic percentage of oxygen increases, as the number of thermal cycles, n, increases. XRD patterns of the surfaces for the polished Crofer22 APU samples after thermal cycling show that as the number of thermal cycles increases, the content of the alloy substrate phase decreases and the content of the Cr2O3 phase remains almost constant. The content of (Cr, Mn 3O4 spinel phase increases from n=4 to n=20 and remains almost constant from n=20 to n=40. The slow increase in ASR from n=4 to n=20 is considered to correspond to the formation of the (Cr, Mn)3O4, Cr2O3, and SiO2 phases. The rapid increase in ASR from n=20 to n=40 is considered to occur because the connections between particles become weak due to the expansion and contraction of particles with thermal cycling. (Received July 18, 2013)

keyword : fuel cells, oxidation, microstructure, X-ray diffraction, thermal cycling

The process of secondary recrystallization in the Mg-6Sn binary alloy that initially had bimodal grain size distribution was observed. The prevalence of the {0001}//sheet plane orientation in the initial texture was destroyed and the texture was transformed to a very strong {2□□0}-type during solution heat treatment at 480 ℃. The grain boundary character was analyzed using EBSD, in which no preference of the CSL boundary among the grain boundaries of {2□□0}- oriented grains was evident. It was observed that some fine grains were isolated within coarse {2□□0} oriented grains due to limited boundary migration and these boundaries belonged to CSL boundaries. These analyses confirmed that the secondary recrystallization was driven by the general high-angle grain boundaries surrounding {2□□0}-oriented grains. (Received July 31, 2013)

keyword : metals, annealing, texture, electron backscattering diffraction, EBSD, secondary recrystallization

The types of atmospheric gases used in sputtering deposition are crucial in the formation of cubic boron nitride (c-BN) films. c-BN films were deposited on an Si wafer using radio frequency (RF) magnetron sputtering with a B4C target at N2/Ar+N2 ratios of 0.2, 0.4, 0.6, 0.8, and 1.0. The characteristics of the c BN layer were significantly enhanced at a nitrogen partial pressure ratio of 0.6. The highest growth rate and the lowest surface roughness were observed at this condition. The c-BN phase structure was clearly identified by the binding energy of the B1s and N1s along with X-ray diffraction patterns of the coated layer. These results indicate that the growth rate and crystal structure of the c-BN films are significantly affected under certain atmospheric gas conditions. (Received August 8, 2013)

keyword : Thin films, Sputtering, Crystal Structure, Scanning Electron Microscopy, SEM, Cubic boron nitride, c-BN

Solvent extraction equilibrium of manganese(II) using Cyanex 272 dissolved in kerosene was studied from chloride solutions. Distribution ratio data was collected in order to estimate the extraction equilibrium constant. A slope analysis method was used to verify the solvent extraction reaction. The equilibrium constant for the solvent extraction reaction was estimated by taking into account the complex formation reactions between Mn(II) and chloride ion and mass balance equations. The activity coefficients of species in the aqueous phase were estimated by applying the Bromley equation. A comparison was made between the calculated distribution ratios of Mn(II) and the experimentally measured values. Finally, a chemical model was successfully applied for the estimation of the equilibrium constant. (Received October 1, 2013)

keyword : Manganese(II), Cyanex 272, Chloride solutions, Equilibrium constant, Bromley equation

Ceramic abrasives are used widely as grinding materials because of their good wear resistance. In particular, alumina abrasives have other excellent mechanical properties, such as toughness, hardness, and durability. Nevertheless, relatively few studies have been carried out domestically in comparison with research in this field that has been conducted abroad. In this study, a fine-grained alumina abrasive that had a dense microstructure with a needle-type second phase, SrAl12O19,was successfully fabricated by impregnating a solution in corporating Sr and Y cations into a porous alumina gel prepared from boehmite sol. The final phase and microstructure as well as the porosity and density of the sintered alumina were dependent on the mixing ratio of Sr and Y nitrates, the concentration of the solution, the sintering temperature, and the heating rate. With a mixing ratio of 1:1 wt% of Sr and Y and above a 10 wt% concentration, a dense and homogeneous microstructure was achieved with needle-type grains with sizes of 1.0-2.0 μm. A denser microstructure with fine grains and improved hardness were obtained by an ultra-fast sintering process at relatively low sintering temperature of about 1400 ℃. (Received April 30, 2013)

keyword : Ceramics, Sintering, Microstructure, Hardness test, Sol-gel method

Through-silicon-via (TSV) is a core technology that enables vertical stacking of several semiconductor chips for 3-demensional packaging. However, thermo-mechanical problems of the TSV technology, such as Cu extrusion, cracks between the Cu and Si substrate, and voids within Cu become important issues. In this study, Cu-filled TSV with a diameter of 30 μm and depth of 70 μm was fabricated by electroplating, and the effect of thermal shock on Cu extrusion of the TSV was investigated. A thermal shock test was conducted from -65 ℃ to 150 ℃ for 250, 500, and 1000 cycles. The experimental results showed that a defect free Cu-filled TSV was obtained by electroplating using periodic pulse-reverse current form. Cu extrusion occurred after the thermal shock test, and the average height of the pumped Cu increased from -0.3 to 0.78 μm with increasing thermal shock cycles from 0 to 1000. An interfacial crack between Cu and Si was observed after 1000 cycles. The average grain size of Cu during the thermal shock test from 0 to 1000 cycles increased from 0.65 to 0.83 μm. (Received May 30, 2013)

keyword : electronic materials, plating, microstructure, focused ion beam,FIB, Cu extusion

The effect of additives, current density and plated temperature on the surface morphology and physical property, during copper electrodeposition on polyimide (PI) film was investigated. Two kinds of additives, Cl and leveler (additive B), were used in this study. Electrochemical experiments were performed in conjunction with SEM, XRD and four-point probe to characterize the morphology and mechanical characteristics of copper electrodeposited in the presence of the additives. The surface roughness, crystal growth orientation and resistivity was controlled by the concentration of additive B. High resistivity and lower peel strength were observed on the surface of the copper layer electroplated in the electrolyte without additive B. However, a uniform surface, lower resistivity and high flexibility were obtained with a combination of 20 ppm Cl and 100 ppm additive B. Large particles were observed on the surface of the copper layer electroplated using a current density of 25 mA/cm2, but a uniform surface and lower resistivity were obtained using a current density of 10 mA/cm2. One of the required important properties of FCCL is flexibility of the copper foil. High flexibility of FCCL was obtained at a low current density, rather than a high current density. Moreover, a reasonable current density is 20 mA/cm2, considering the productivity and mechanical properties of copper foil. (Received June 28, 2013)

keyword : Thin films, Deposition, Mechanical properties, resistivity, Flexibility, FCCL

Indium Tin Oxide (ITO) thin films are prepared by ion-beam assisted deposition at room temperature. An assisting oxygen ion beam (ion energy Ei=0 eV-600 eV) directly bombards the substrate surface to modify the properties of the ITO thin films. The effects of assisting ion beam energy on the characteristics of ITO thin films were investigated based on transmission scanning electron microscopy and atomic force microscopy. With an increasing assisting ion beam bombardment, the crystalline quality of the ITO films was improved and the oxygen vacancies were increased. SEM measurements showed that the film deposited at 300 eV had the characteristics of polycrystalline thin films with a larger grain structure than that of other conditions. The XRD spectra indicated that the ITO films that were produced in this study had polycrystalline structures. That means that surface ad atoms had enough mobility to migrate on the substrate with transferred kinetic energy by colliding secondary ions. However, too energetic secondary ion beam bombardments decreased grain size and also roughened surface morphology due to radiation damage. Optical transmittance of the ITO film in the visible region was related to the structural and surface morphological properties. In this study, the highest optical transmittance of 85% at a wavelength of 550 nm and the lowest resistivity of 3 × 10-4 cm were obtained at the oxygen Ion Beam Energy 300 eV, respectively.(Received September 4, 2013)

keyword : ITO thin film Ion beam assisted molecular beam epitaxy, Transparent conducting oxides