Papers by Author: Seung Ho Kim

Abstract: Thermal conductivity and microstructure of 3YSZ/monazite-type LaPO4 composites were investigated. Powders were prepared by two kinds of preparation methods such as conventional ball-milled and chemical precipitation of LaPO4 on the zirconia powder surface. Thermal conductivity of 3YSZ/monazite-type LaPO4 composites was lower than that of 3YSZ. That of 3YSZ/monazite-type LaPO4 composites was influenced on the powder preparation methods and as a function of measured temperature. Microstructure of 3YSZ/monazite-type LaPO4 composites was not much affected by the amount of dispersed LaPO4 particles. 3YSZ/monazite-type LaPO4 composites were observed to pores after thermal etched. The pore of synthesized specimens was observed remarkably in compared with conventional ball-milled specimens.

Abstract: Multi-wall carbon nanotubes (MWCNTs) with a diameter of 20-30 nm were dispersed as a conductive phase into yttria stabilized tetragonal zirconia polycrystalline (3Y-TZP) to add electrical conductivity. The 3Y-TZP/MWCNT nanocomposites were fabricated by pressureless sintering under inert atmosphere. Electrical conductive function was successfully introduced by small amount of CNT addition. Critical volume fraction of the conductive phase for the percolation was analyzed and was found to be 0.390 vol% of CNT, which was much smaller than that for nano-sized carbon black dispersed 3Y-TZP (2.55 vol%). Microstructural investigation revealed that dispersed CNTs formed continuous 3-dimensional nano-network within the 3Y-TZP matrix, that contributed to the excellent conductive properties. Fracture strength was not improved much, while fracture toughness was increased by the CNT addition, due mainly to its crack bridging and/or pull-out mechanisms. It was considered that the use of anisotropic nano-sized conductive phase is advantageous to obtain electrically functionalized nanocomposite ceramics.

Abstract: Tribological behaviors of Al2O3/SiC nanocomposites were found to be related to the amount of nanosized SiC and initial Hertzian contact stress. As the applied load increased, the wear volumes of Al2O3/SiC nanocomposites slightly increased, but that of monolithic Al2O3 increased rapidly above 50 N. Tribological behavior of monolithic Al2O3 was transited from mild wear to severe wear.

Abstract: Silicon nitride with various amount of AlN as a sintering aid was sintered by a hot press method. Densified silicon nitrides were obtained, and it was found that the mechanical and wear properties were dependent on the contents of AlN. The effect of a/b phase on the mechanical and wear properties of silicon nitride was investigated. The properties were changed depending on the amount of a/b phase. In the brittle materials, tribological behaviors were dependent on the microstructure as well as hardness and fracture toughness. We focus on the relationship between the microstructure and mechanical/wear properties of silicon nitride including AlN additives.

Abstract: Mechanical and thermal properties of Si3N4 ceramics with various rare-earth oxides (La2O3, CeO2, Lu2O3, Dy2O3, Sm2O3, Nd2O3, Yb2O3, and RuO2) were investigated. Flexural strength of silicon nitride with addition of 5vol% Nd2O3, CeO2, Dy2O3, and Sm2O3 showed higher value than that of silicon nitride with Lu2O3 and La2O3 added because they form denser microstructure and smaller elongated grain. Thermal conductivity of silicon nitride with an addition of 5vol% RuO2 was more enhanced than that of silicon nitride added with Nd2O3, Sm2O3, and Dy2O3 because the addition of RuO2 depressed grain growth. It is also associated with lattice oxygen governing thermal conductivity of Si3N4 when added rare-earth oxides.

Abstract: Microstructure and mechanical behaviors of zirconia [ZR; ZrO2+3mol%Y2O3]–nano hydroxyapatite [HA; Ca10(PO4)6(OH)2] nanocomposites were studied. A pulse electric current sintering (PECS) method was applied to fabricate ZR-HA composites as biomaterials. The reaction between zirconia and hydroxyapatite was successfully avoided. It was a successful preparation of bioactive composites in the quasi-binary system Ca10(PO4)6(OH)2 + ZrO2. One of the most serious problems is that when a mixture of zirconia and nano-hydroxyapatite powders was sintered, extensive chemical reaction between zirconia and hydroxyapatite was unavoidable. By applying a novel super-fast consolidation technique, pulse electric current sintering (PECS), the deleterious reactions were inhibited kinetically. The specimens contain 0, 10, 20, and 30wt% of hydroxyapatite on zirconia as remainder. Samples were sintered using the PECS method at 1100°C , 1150°C, 1200°C, and 1250°C, with increasing 100°C /min, under a pressure of 50Mpa in vacuum atmosphere for 5min. Characterization of the samples was carried out using X-ray diffraction analysis at different sintering temperatures. Flexural strength was estimated with 3-point bending test. The relative density was acquired using an Archimedes method in toluene medium.