Papers by Author: Bum Rae Cho

Abstract: The sapphire substrate have a significant impact on reducing the light scattering loss and in accordance with the crystalline surface and can improve the light emission efficiency is the surface roughness of the sapphire LED is an important factor. When the CMP process after measuring the transmittance, haze, and surface roughness had a transmittance of 84% or more transmittance at all wavelengths, regardless of the thickness it was confirmed that the haze also has a low value, regardless of the thickness. Was confirmed that the surface roughness is greater the thicker the thickness of the sapphire it affects the pressure of the CMP polishing and then take a lot of importing a low surface roughness value of the thickness of the sapphire CMP process the surface roughness value.

Abstract: Alumina has been widely used as a structural ceramic because of high hardness and chemical stability. However, due to the unique characteristics of low fracture toughness of ceramic materials, it has seen limited use as a dynamic structural material. Recently, zirconia toughened alumina (ZTA) has been receiving spotlight, which has various toughness mechanisms caused by the volume change associated in the phase transformation process of ZrO₂ particles dispersed in Al₂O₃ to increase the toughness of Al₂O₃. In this study, 8 mol% Y-ZrO₂ and 12 mol% Y-ZrO₂ with different crystal structures was dispersed in Al₂O₃ individually as a stabilizer and the mechanical properties of the ZTA were observed by differing the composition of the stabilizer. Experimental results show that the ZTA specimens with 12 mol% Y-ZrO₂ which contains a large amount of stable cubic crystal phases had relatively higher micro hardness values. Whereas, fracture toughness of ZTA specimens with 8 mol% Y-ZrO₂ which contains many unstable tetragonal crystal phases, was measured to have higher values than ZTA specimens with 12 mol% Y-ZrO₂, which was opposite to that of micro hardness.

Abstract: Sintered ZTA(zirconia toughened alumina) which has good mechanical properties at a low temperature was produced by milling and mixing with Al₂O₃ and ZrO₂(3Y-TZP). In order to examine the effect of sintering aids on the mechanical properties of ZTA, fracture toughness and hardness of the produced ZTA were observed in accordance with change of the added quantity of ZrO₂ Scanning electron microscopy and X-ray diffraction technique were applied to observe microstructural change and phase transformation during the process. Experimental results showed that the addition of sintering aids in ZTA at a low temperature induced densification and adding SiO₂ and talc lowered sintering temperature and promoted crystallization process of the compound. The mechanical strength of ZTA added ZrO₂ showed higher mechanical strength and SEM analysis revealed that Al₂O₃ and ZrO₂ during the sintering process restrained the grain growth each other. Especially, the 92% Al₂O₃ added sintering aids showed more than 98% of the theoretical density and more than 1500 Hv of hardness value at a low temperature of 1400. It was also showed that the fracture toughness is gradually increasing first and decreasing later in accordance with the quantity of ZrO₂.

Abstract: Indium tin oxide (ITO) used in many applications such as electronic and optical devices were deposited on the soda lime glass substrate by an electron beam evaporation techniques from a mixture of 90wt% of In2O3 and 10wt% of SnO2. The physical, electrical and optical properties of the ITO films were determined as a function of substrate temperature. The films deposited at 200
showed optimum properties with a strong diffraction peak having a preferred orientation along the [111] direction. Experimental results showed that sheet resistance and transmittance of the ITO film increased with an increase in substrate temperature. Surface roughness increased slightly as a function of substrate temperature because of grain growth.

Abstract: The effect of CaO, MgO and SiO2 as a flux on the sinterability of zirconia toughened alumina(ZTA) used for plasma reactors was investigated and the effect of zircon addition on thermal shock resistance of ZTA with 15wt.% of ZrO2 was also investigated. The resultant data revealed that ZTA shows the best sinterability at the composition of 2wt.% of CaO, 4wt.% of MgO and 2wt.% of SiO2 and at the sintering temperature of 1350°C. Thermal shock resistance of ZTA containing zircon was improved significantly. It is shown that ZTA containing 10wt.% of zircon shows better thermal shock resistance than others. This fact can be explained due to the low thermal expansion coefficient of zircon. It was concluded that zircon is an effective material to improve thermal shock resistance of alumina ceramics.

Abstract: Porous mullite with a porosity of about 78% was fabricated using Al(OH)3, SiO2 and AlF3 powders to develop non-asbestos friction materials for brake pads. A mullite composite was fabricated by infiltrating liquid phenolic resins to the porous mullite. The effects of the processing parameters (infiltration frequency, immersion duration and resin temperature) on the properties of the mullite composite obtained by the liquid infiltration technique were studied. SEM analysis reveals that the parameters affected the degree of infiltration and hardness of the mullite composite by changing the infiltration frequency and immersion duration. With increasing immersion duration and infiltration frequency, the resultant porosity had lower values. In the case of hardness, the measured values showed an opposite tendency.

Abstract: The effect of Al2O3 addition on sinterability of Tetragonal Zirconia Polycrystal powders including 3mol% Y2O3 (3Y-TZP) was investigated. Each 3Y-TZP powder dispersed with 0.3, 0.6, 0.9 and 1.2wt.% of Al2O3 was prepared by the spray-drying method. The prepared powders were pressed into a disk type and sintered at 1350
, 1400
, 1450
and 1500
for 2 hours in the air. Resultant microstructures and mechanical properties of specimens were investigated by using Vickers/Micro hardness Tester, FE-SEM and XRD. Most of the specimens showed high relative density over 99% and a higher fracture toughness than pure 3Y-TZP. Al2O3 particles dispersed in 3Y-TZP microstructure depressed grain growth of 3Y-TZP by the pinning effect. Increase in fracture toughness of 3Y-TZP was explained by the crack deflection due to dispersed Al2O3 particles.

Abstract: Mullite reinforced composites were produced by the injection molding technique to develop environmentally friendly friction materials for automotive applications. In order to examine the effect of mullite content on the friction and wear properties, two different specimens containing 10wt% and 30wt% of mullite were respectively fabricated and wear-tested by using the plate-on-disc type sliding friction and wear test machine. The sliding friction and wear test demonstrated that both specimens show similar tendencies at different sliding speeds under a low load of 2.9N. In comparison with common glass fiber reinforced composites, both of the mullite reinforced composites exhibited a lower wear rate at room temperature.

Abstract: Plate-on-disc type sliding friction and wear test was conducted to investigate the friction and wear properties of mullite reinforced composite (M composite), and compared with glass fiber reinforced composite (GF composite). The friction and wear test revealed that M composite has good wear resistance under mild sliding conditions, but the wear rate gradually increases under severe sliding conditions. M composite exhibited higher friction coefficient than GF composite at room temperature and maintained the friction coefficient stably at higher temperatures. The composite film formed on counter material against M composite showed influences on the friction and wear properties under severe sliding conditions.

Abstract: The porous microstructure with acicular mullites was fabricated using Al(OH) ₃, Al ₂O ₃, and amorphous SiO ₂ as starting materials by reaction sintering. Several molar ratios of Al ₂O ₃/SiO ₂ and Al(OH) ₃₂ were selected and AlF ₃ of 0, 4, 7, 10wt% was added to each composition to examine the effects of composition and the additives for the mullite formation. Fluorotopaz was produced at a temperature around 900°C under a constant SiF4 atmosphere and it was decomposed to produce the mullite at a relatively low sintering temperature around 1200°C. The acicular mullite was more easily fabricated in the case of the Al(OH) ₃₂ mixture with a larger amount of AlF ₃.