Papers by Author: Si Young Chang

Abstract: Ceramic coatings were synthesized on pure Mg and binary Mg-7.1wt%Al alloy by plasma electrolytic oxidation (PEO) technique, and the effect of current ratio, C2/C1, ranging from 0.2 to 0.85 on their mechanical and electrochemical properties was investigated. As the C2/C1 ratio increased, the thickness of the coating layer increased, while surface roughness was almost unchanged. The hardness and wear resistance had a marked tendency to increase with increasing C2/C1 ratio and Mg-Al alloy showed higher hardness and wear resistance at all C2/C1 ratios compared to pure Mg. The weight loss of the pure Mg and Mg-Al alloy in 3.5%NaCl solution was reduced by coatings. However, it had no dependence on the C2/C1 ratio. The wear resistance of the coated Mg-Al alloy was better than that of the coated pure Mg.

Abstract: Pure Mg, binary Mg-5.6wr%Al, Mg-1.0wt%Si and Mg-7.5wt%Zn alloys were coated by plasma electrolytic oxidation (PEO) method, and their wear properties were investigated and discussed based on surface roughness, thickness and component of coatings. All coatings showed porous layer with some volcano top-like pores on the surface and the non-uniform thickness. The coatings on Mg-Al, Mg-Si and Mg-Zn alloys were composed of primarily MgO and Al2O3, SiO2, ZnO, respectively. The coating layer on Mg-Si alloy was thicker than that of Mg-Al and Mg-Zn alloys, while it was less rough. The wear resistance of pure Mg and Mg alloys was improved by PEO method and the coated Mg alloys showed better wear resistance compared to the coated pure Mg. In particular, the coated Mg-Zn alloy revealed the best wear resistance, while the coated Mg-Si alloy had poor wear resistance despite much thicker coating layer.

Abstract: The hybrid materials with Al-Mg alloy and its composites reinforced with SiC and Al2O3 particles were prepared by conventional powder metallurgy method. The Al-5wt%Mg and composite mixtures were compacted under a pressure of 400MPa and sintered at 873K for 5h. The obtained hybrid materials with Al-Mg/SiCp composite showed the higher relative density than those with Al-Mg/Al2O3 composite after compaction and sintering. In the composite side of hybrid materials, the SiC particles were densely distributed compared to the Al2O3 particles. The hybrid materials with Al-Mg/SiC composite showed higher micro-hardness than those with Al-Mg/Al2O3 composite. The mechanical properties were evaluated by the compressive test. The hybrid materials revealed almost the same value of 0.2% proof stress with Al-Mg alloy. However, their compressive strength was lower than that of Al-Mg alloy, resulting from the fracture occurring along the microinterface between matrix and reinforcements in the macro-interface.

Abstract: Ceramic coatings were synthesized on pure magnesium, binary Mg-Al and Mg-Zn alloys by plasma electrolytic oxidation (PEO) technique, and their mechanical and electrochemical properties were investigated. The coatings showed porous microstructure and some volcano top-like pores on the surface. The coatings on Mg-Al alloys consisted of MgO, MgAl2O4 and Al2O3. In the coatings on Mg-Zn alloys, MgO and ZnO were detected. The hardness and wear resistance showed slight dependence on Al content but no tendency to increase with increasing Zn content. The coated Mg alloys with Al, Zn showed better corrosion resistance in 3.5%NaCl solution but poor corrosion resistance in methyl alcohol, compared to the coated pure Mg.

Abstract: The effect of working temperature on microstructure and mechanical properties of ultrafine grained monolithic Al and Al-5vol.%SiCp composite processed by accumulative roll bonding (ARB) was studied. The ARB was performed up to eight cycles (an equivalent strain of ~6.4) without lubricant. The working temperature was varied from ambient temperature to 200
C. The samples processed at temperatures below 100
C exhibited an ultrafine grained structure over almost all regions. However, the samples processed at 200
C showed an inhomogeneous structure in which a few coarse grains due to an occurrence of conventional recrystallization is partially seen. The tensile strength of both the monolithic Al and the composite decreased with increasing the ARB working temperature. The variation of microstructure and mechanical properties of the composite with the working temperature was compared to that of the monolithic aluminum.

Abstract: The aging behavior of sintered Al composites with various ceramic contents was investigated. 2xxx series blended powder was used as the starting powder. Ceramic contents were 0 wt.% and 5 wt.%. The blended powders were compacted at 250 MPa. The sintering process was performed at 620oC for 60 min in a N2 atmosphere. After sintering, each part was solution-treated at 518oC for 60 min and aged at 180oC. The Rockwell hardness value at the peak aging time increased with increased ceramic contents. However, the peak aging time at maximum hardness was reduced with increased ceramic contents. The changes in aging behavior may have been caused by the acceleration of diffusion due to the strain induced by differences in the thermal expansion coefficients of the matrix and the ceramic phases.

Abstract: The bimodal-structured materials composed of Al-5wt%Mg alloy and its composites reinforced with SiC and Al2O3 particles were prepared by ball-milling and subsequent compaction under the pressure of 350MPa and sintering at temperatures ranging from 973K to 1173K for 1h, 3h and 5h. The macro-interface between Al-Mg and Al-Mg/ Al2O3 composite was macroscopically well-bonded compared to that between Al-Mg and Al-Mg/SiC composite under same sintering conditions. The bonding of macro-interface became better as the sintering time and temperature increased, resulting in that the bimodal-structured materials sintered at 1173K for 5h showed the relative density of nearly 100%. In addition, the higher sintering temperature was, the more irregular macro-interface was. The microhardness of macro-interface area was in between the Al- Mg and composites, which was independent of the sintering temperature.

Abstract: Fe/MgO nanocomposites, which are applicable to high frequency electronic devices, were fabricated by ultrasonic spray pyrolysis and selective reduction processes. Transmission electron micrographs showed that nano ferromagnetic Fe particles were isolated by MgO insulating matrix. With the increase of the reduction temperature, the particle size and saturation magnetization of the nanocomposites increased, which resulted in the decrease of the coercive force and the increase of the permeability. Furthermore, the ferromagnetic resonance peak of the nanocomposites was not observed up to 9 GHz.

Abstract: Deformation characteristics of a newly developed wave-typed heat sink during mechanical assembly of fin-base were analyzed by using the Ansys program. The original points a and b marked in the fin and base moved to the opposite y direction with increasing pressure, while their movements to x direction were relatively small. It was evident that at least 207㎫ must be applied to well bond the fin with the base. Under a constant pressure, the smaller the pressured area was, the better the bonding between fin and base was, despite a small uniform deformation in the base area as the pressured area increased.

Abstract: Copper via filling is an important factor in 3D stacking interconnection of SiP (system in package). As the packaging density is getting higher, the size of via is getting smaller. When DC electroplating is applied, a defect-free hole cannot be obtained in a small size hole. To prevent the defects in holes, pulse and pulse reverse current was applied in copper via filling. The size of 50, 70, 100
in diameter and 100
in height. The holes were prepared by DRIE method. TaN and Ta was sputtered for copper diffusion barrier. Via specimen were filled by DC, pulse and pulse-reverse current electroplating methods. The effects of additives and current types on copper deposits were investigated. Vertical and horizontal cross section of the via were observed by SEM to find the defects in via. When pulse-reverse electroplating method was used, defect free via were uccessfully obtained.