Papers by Author: Yeon Wook Kim

Abstract: The osseointegration capability of titanium dental implants is related to their chemical composition and surface roughness. In this study, the combination of grit-blasting and micro-arc oxidation had been used for producing the improved implant surfaces. The ceramic particles were projected to titanium dental implants through a nozzle at high velocity by means of compressed air to get high surface roughness. Then the surface of titanium implants was modified by micro-arc oxidation treatment. The current density, frequency and duty were 50-300 mA/cm2, 100 Hz, and 50%, respectively. A porous TiO2 layer was formed on the surface after the oxidation treatment. The surface structure of oxidized implants exhibited nanometer-sized pores with an average diameter of 0.2 µm. The TiO2 passive layer of the implant surface can attribute to the excellent biocompatibility. The high roughness (Ra=0.182 µm) formed by grit-blasting maximizes the interlocking between mineralized bone and the surface of the implant. Surface roughness in the manometer range formed by micro-arc oxidation treatment would play an important role in the adsorption of proteins, adhesion of osteoblastic cell and thus the rate of osseointegration.

Abstract: The shape memory alloy strips of Ti50Ni15Cu35 and Ti50Ni10Cu40 had been fabricated by arc melt overflow. Their microstructures and shape memory characteristics were investigated by means of X-ray diffraction, optical microscopy and differential scanning calorimetries. The microstructure of as-cast strips exhibited columnar grains normal to the strip surface. X-ray diffraction analysis showed that one-step martensitic transformation of B2-B19 occurred in the alloy strips. According to the DSC analysis, it was known that the martensitic transformation temperature (Ms) of B2→B19 was 71.2°C in Ti50Ni15Cu35 and 64.5°C in Ti50Ni10Cu40 alloy strip, respectively. During thermal cyclic deformation with the applied stress of 60 MPa, transformation hysteresis and elongation associated with the B2-B19 transformation were observed to be 4.9°C and 1.4% in Ti50Ni15Cu35 alloy strip. However, Ti50Ni10Cu40 alloy strip was so brittle that its mechanical properties could not be measured.

Abstract: One of the most important use of thermal spray coatings is for wear resistance. In this work, the tribological properties of plasma prayed Al83.7Fe7Cr6.3Ti3 quasicrystalline coatings have been studied. The quasicrystal powders of Al-base alloys were produced by gas atomization unit in vacuum. The plasma spray process was used to produce quasicrystalline coatings. Then, friction experiments were carried out on a pin-on-disc-type tribometer. The results indicated that the friction coefficient of Al83.7Fe7Cr6.3Ti3 quasicrystalline coating is about 50% lower than that of Al70Fe13Cu10Cr7 quasicrystalline coating.

Abstract: Transformation behaviors and shape memory characteristics of Ti–Ni45-Cu5 alloy ribbons prepared by melt spinning were investigated by means of DSC, XRD and OM. In these experiments particular attention has been paid to change the ejection temperature of the melt from 1400°C to 1600 °C. As the results, the thickness of ribbons could be controlled. An increase of the superheat of the melt leads to a reduced ribbon thickness and a refinement of grains. The microstructural refinement and the increased internal strains achieved by controlling the melt-spinning temperature decreased Ms significantly. It was also found that two-step transformation (B2-B19-B19’) occurred in the ribbons fabricated at higher melt-spinning temperatures than 1450°C.

Abstract: Transformation behaviors and shape memory characteristics of Ti-45Ni-5Cu alloy ribbons fabricated by melt spinning were investigated by means of optical microscopy, differential scanning calorimetries(DSC), X-ray diffraction and thermal cycling tests under constant load. They depended largely on temperatures of liquid metal. The B2-B19-B19’ two-step transformation occurred in the ribbons fabricated with the liquid whose temperature was higher than 1723 K, while the B2-B19’ one-step transformation occurred in the ribbons with the liquid at 1673 K. The stabilization of the B19 martensite in Ti-45Ni-5Cu alloy ribbons was ascribed to the high density of dislocations which made strong resistance to large lattice deformation associated with a formation of the B19’ martensite.