Yeon-Wook Kim

Surface Modification of Ti Dental Implants by Grit-Blasting and Micro-Arc Oxidation

The osseointegration capability of titanium implants is related to their chemical composition and surface roughness. The combination of grit-blasting and micro-arc oxidation had been used to produce the improved implant surfaces. The ceramic particles were projected to titanium implants at high velocity to get high surface roughness. Then the surface of implants was modified by micro-arc oxidation treatment. 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 (R a = 0.182 µm) formed by grit-blasting maximizes the interlocking between mineralized bone and the surface of implants. Surface roughness in the manometer range formed by micro-arc oxidation would play an important role in the adsorption of proteins, adhesion of osteoblastic cell and thus the rate of osseointegration.

Microstructures and shape memory characteristics of Ti–25Ni–25Cu(at.%) alloy ribbons

Ti–25Ni–25Cu(at.%) alloy ribbons have been fabricated by melt spinning with a linear velocity of 51.0 m s−1, and then their microstructures and shape memory characteristics were investigated by means of optical microscopy, transmission electron microscopy, x-ray diffraction, thermal cycling tests under constant load and tensile tests. As-spun ribbon made at the melt spinning temperature of 1803 K was amorphous; below this it was a mixture of amorphous and crystal structure. The crystallization temperature was measured to be about 727 K, irrespective of the melt spinning temperature. The activation energy for crystallization obtained from the Kissinger plot was 136.63 ± 3.41 kJ mol−1. Annealing the as-spun ribbons with the melt spinning temperature of 1803 K at 738 K for 0.6 ks, they were crystallized with the average grain size of 130 nm. The B2–B19 transformation occurred in the ribbons and the B19 martensites with a single-pair variant were found in most of the grains. The transformation elongation and thermal hysteresis were found to be 2.6% and 5 K under the applied stress of 231 MPa, respectively. The ribbons showed a perfect superelasticity and their stress hysteresis was as small as 30 MPa. The small hysteresis was attributed to very small grain size and single-pair martensite variants formed in the small grain.

Monotonic and cyclic bond behavior of confined concrete using NiTiNb SMA wires

This study conducts bond tests of reinforced concrete confined by shape memory alloy (SMA) wires which provide active and passive confinement of concrete. This study uses NiTiNb SMA which usually shows wide temperature hysteresis; this is a good advantage for the application of shape memory effects. The aims of this study are to investigate the behavior of SMA wire under residual stress and the performance of SMA wire jackets in improving bond behavior through monotonic-loading tests. This study also conducts cyclic bond tests and analyzes cyclic bond behavior. The use of SMA wire jackets transfers the bond failure from splitting to pull-out mode and satisfactorily increases bond strength and ductile behavior. The active confinement provided by the SMA plays a major role in providing external pressure on the concrete because the developed passive confinement is much smaller than the active confinement. For cyclic behavior, slip and circumferential strain are recovered more with larger bond stress. This recovery of slip and circumferential strain are mainly due to the external pressure of the SMA wires since cracked concrete cannot provide any elastic recovery.

Behavior of NiTiNb SMA wires under recovery stress or prestressing.

The recovery stress of martensitic shape-memory alloy [SMA] wires can be used to confine concrete, and the confining effectiveness of the SMA wires was previously proved through experimental tests. However, the behavior of SMA wires under recovery stress has not been seriously investigated. Thus, this study conducted a series of tests of NiTiNb martensitic SMA wires under recovery stress with varying degrees of prestrain on the wires and compared the behavior under recovery stress with that under prestressing of the wires. The remaining stress was reduced by the procedure of additional strain loading and unloading. More additional strains reduced more remaining stresses. When the SMA wires were heated up to the transformation temperature under prestress, the stress on the wires increased due to the state transformation. Furthermore, the stress decreased with a decreasing temperature of the wires down to room temperature. The stress of the NiTiNb wires was higher than the prestress, and the developed stress seemed to depend on the composition of the SMAs. When an additional strain was subsequently loaded and unloaded on the prestressed SMA wires, the remaining stress decreased. Finally, the remaining stress becomes zero when loading and unloading a specific large strain.

Cytokine Array After Cyclosporine Treatment in Rats

OBJECTIVES Long-term treatment with cyclosporine (CsA) results in chronic nephrotoxicity, which is known to be mediated by several cytokines including transforming growth factor-betal. Cytokines are known to play an important role in innate immunity, apoptosis, angiogenesis, cell growth, and differentiation. They are known to be involved in most disease processes, including cancer, cardiac disease, and nephrotoxicity. To evaluate changes of cytokines in a rat model of CsA-induced chronic nephrotoxicity, we performed a cytokine array. METHODS Experiments were performed on two groups of rats; normal control group and CsA-treated group. Cytokine array in rat serum was performed using Cytokine Antibody Array I kit from RayBiotech. RESULTS Serum creatinine, urine creatinine, and creatinine clearance increased in the CsA-treated group. Among the several cytokines, the expressions of the lipopolysaccharide-induced CXC chemokine (LIX), monocyte chemoattractant protein 1 (MCP-1), nerve growth factor (beta-NGF), and tissue inhibitor of metalloproteinase-1 (TIMP-1) in the CsA-treated group were increased above that of cytokines in the control group. The density of the LIX in controls was 0.62, and in the CsA-treated group was 1.24. The density of the MCP-1 in controls was 0.68, and in CsA-treated, 1.43. The density of the beta-NGF in controls was 0.62, and that in CsA-treated, 1.24. The density of the TIMP-1 in controls 1.13, and in CsA-treated, 1.40. CONCLUSIONS Our data suggested that among several cytokines elevated levels of the LIX, MCP-1, beta-NGF, and TIMP-1 are the contributing factors to CsA-induced nephropathy.

Wear Properties of Nuclear Graphite IG-110 at Elevated Temperature

The high temperature gas-cooled reactor (HTR-10) is designed to produce electricity and hydrogen. Graphite is used as reflector, support structures, and a moderator in reactor core; it has good resistance to neutron and is a suitable material at high temperatures. Friction is generated in the graphite structures for the core reflector, support structures, and moderator because of vibration from the HTR-10 fuel cycle flow. In this study, the wear characteristics of the isotropic graphite IG-110 used in HTR-10 were evaluated. The reciprocating wear test was carried out for graphite against graphite. The effects of changes in the contact load and sliding speeds at room temperature and on the coefficient of friction and specific wear rate were evaluated. The wear behavior of graphite IG-110 was evaluated based on the wear surfaces.

Temperature effect of friction and wear characteristics for solid lubricating graphite

Graphite is one of the effective lubricant additives due to its excellent high-temperature endurance and self-lubricating properties. In this study, wear behavior of graphite used as sealing materials to cut off hot gas is evaluated at room and elevated temperature. Wear occurs on graphite seal due to the friction of driving shaft and graphite. Thus, a reciprocating wear test to evaluate the wear generated for the graphite by means of the relative motion between a shaft material and a graphite seal was carried out. The friction coefficient and specific wear rate for the changes of applied load and sliding speed were compared under different temperature conditions considering the actual operating environment. Through SEM observation of the worn surface, the lubricating film was observed and compared with test conditions.

Compression Fracture Behavior of ATJ Graphite for Rocket Nozzle throat

ABSTRACT Graphite is commonly used for rocket nozzle. The purpose of t he present study is to evaluate compression fracture behavior of ATJ graphite. Uniaxial compression test is conducted in accordance with ASTM C 695 in the range of R.T to 900°C. The size effects of specimen on the compressive strength and fracture behavior were investigated. Two types of cylindrical specimen, i.e., where the diameter to length ratio is 1:2 (ASTM C 695 specimen) or 1:1, w ere tested at room temperature초 록 본 연구의 흑연은 로켓 노즐목 재료로 많이 사용되는 것으로써 온도, 가공성 및 무게 측면에서 유리하다. 하지만 고온 및 고압상태에서 고속으로 취성파괴 되므로 설계 시 구조적 안정성을 충분히 확보해야 한다. 본 연구에서는 로켓 노즐목에 이용되는 ATJ 흑연의 압축 파괴 거동에 대해 연구했다. 상온 및 고온에서 ASTM C 695에 준하여 일축 압축 시험을 수행하였다. 또한 시편의 단면적 변화에 의한 압축강도 및 파단특성을 분석하였다. Key Words: ATJ Graphite(ATJ 흑연), Uniaxial Compression(일축 압축), Compressive Strength(압축 강도), Size Effects(크기 효과), Antioxidant(산화방지제)Received 9 December 2013 / Revised 4 July 2014 / Accepted 7 Jul y 2014 Copyright Ⓒ The Korean Society of Propulsion EngineerspISSN 1226-6027 / eISSN 2288-4548[이 논문은 한국추진공학회 2013년도 추계학술대회(2013. 12. 4-5, 경주 현대호텔) 발표논문을 심사하여 수정・보완한 것임.]

Sliding Wear Properties of Graphite as Sealing Materials for Cut off Hot Gas

Sealing structure to prevent flowing hot gas into the driving device, located between the driving shaft and the liner of On-Off valve for controlling the hot gas flow path was studied. Wear occurs due to the constant movement of the driving shaft controlled by actuator on graphite as the sealing material. In this paper, the dynamic wear behavior in high temperature of graphite(HK-6) to be used as sealing material was evaluated. Reciprocating wear test was carried out for the graphite(HK-6) to the relative motion between shaft materials(W-25Re). The results of friction coefficient and specific wear rate according to contact load, sliding speed at room temperature and considering the actual operating environment were evaluated. Through the SEM analysis of the worn surface, third body as lubricant films were observed and lubricant effect of third body was considered.

Wear Properties of Seal Graphite at Elevated Temperature

The graphites as airtight structure seals prevent high-pressure and high-temperature gas from flowing into actuator of propulsion system and generate lubricant film during wear procedure to assist lubricant and sealing. In this study, the tribological characteristics of the graphite in high-temperature are evaluated. In order to evaluate the tribological characteristics of high density graphite(HK-6), variables which are temperature, sliding speed and contact load are set. this study suggest optimized environment conditions through the wear properties of graphite. Consequeantly, high temperature is better than at room temperature to generate lubricant film, so that friction coefficient of graphite is lower at high temperature than at room temperature.