Papers by Author: Hoon Kwon

Abstract: Beta-tricalcium phosphate(β-TCP) coating layer is known to be resorbed much faster than hydroxyapatite(HA), however, there has been no report to explain the exact reason of these results. Eighty titanium discs, coated with HA(n=40) or β-TCP(n=40) by dip and spin coating method, were divided into 2 subgroups respectively; Dissolution(D, n=20) and osteoclast culture(C, n=20). The coated discs in D group were immersed in the cell culture media for 5 days, whereas, in C group, osteoclasts were seeded on the specimens and cultured for 5 days. After simple dissolution test, β-TCP coating layer showed much more cracks and denudation as compared to HA. In osteoclast culture group, mean area fraction of resorption pits in HA-C group was 11.62%, which was significantly higher than that of 0.73% in β-TCP-C group(p=0.001). In conclusion, the resorption mechanisms of HA and β-TCP coating layers were different each other in vitro study. The coated β-TCP was degraded mainly by dissolution and separation from implant, on the other hand, the HA coating layer was resorbed by osteoclastic activity.

Abstract: In the API X65 steel, effects of rolling and cooling conditions on microstructure and mechanical properties were studied. In the case of accelerated cooling after multi-pass rolling in the high/low unrecrystallized range, the tensile strength was 574-670 MPa and the impact toughness was 74-109 J. In the case of accelerated cooled to 550°C and then interrupted by air cooling, on the other hand, those values were 524-538 MPa and 100-135 J, respectively. Whereas the former exhibited the continuous yielding, the latter showed discontinuous yielding. In addition, yield ratio increased from 0.59-0.67 to 0.85-0.87, accompanied with the enhancement of yield strength. Ultrafine ferrite grains formed by the strain induced dynamic transformation during the severe rolling and second phases formed during cooling were observed. In accelerated cooling and interrupted cooling conditions, main second phases formed after cooling were martensite and pearlite, respectively. Separation cracking mostly observed at interfaces of ferrite matrix and second phases, may be attributed to the intrinsic interfacial weakness.

Abstract: ZnO nanostructures have been synthesized on graphite substrates by thermal evaporation of ZnO powder without a metal catalyst at a temperature of 1300
. The colors of the as-synthesized products gradually change from white and brown to gray as the distance from the source material increases. ZnO particles were formed at higher temperature region. ZnO particles gradually changed into ZnO nanowire as the temperature decreased. Finally, ZnO nanowires disappeared completely and only Zn particles were observed at lower temperature region.

Abstract: Applying the severe deformation, the mechanical properties related to secondary hardening were investigated. The Mo-Cr-Co-Ni steels containing (5-13)wt% Co and (8-14)wt% Ni were severely rolled at 850 °C, followed by direct quenching(DQ). DQ specimens were then isothermally aged at 475°C. Ni additions promoted aging kinetics, accompanied by a little enhancement in hardness. In contrast, Co additions enhanced the peak hardness, but did not induce a remarkable aging acceleration. In the 13Co addition group, an actual secondary hardening, that is, the peak hardness is higher than the as-quenched hardness. Variation in peak hardness(Rc) in alloys is summarized as follows; 14Ni-13Co(57.2) ≒ 11Ni-13Co(57.1) > 8Ni-13Co(56.5) > 11Ni- 9Co(56.2) > 8Ni-9Co(54.4) > 11Ni-5Co(53.4).

Abstract: Effect of the severe deformation by multi-pass rolling on microstructure and tensile properties was analyzed in terms of rolling temperature, plate thickness, and cooling rate for a modified API X65 steel containing B. The plates, 80 and 50 mm thickness, were rolled six times by 20%/pass (total 75%) to 20 and 12 mm, at 1023 K of unrecrystallized γ region or 973 K of intercritical (α+γ) region, and then quenched in water or oil. All specimens except one oil-quenched condition showed relatively high UTS 700-830 MPa and the continuous yielding(YR～0.6), typical mode of the (ferrite + martensite (bainite)) dual phase microstructure. In contrast, one oil-quenched specimen with the 973 K-20 mm condition, exhibited the discontinuous yielding (YR～0.8), indicating that the microstructure basically consists of ferrite plus pearlite, as well as a relatively low UTS 660 MPa. The degree of deformation really occurring within materials, i.e., strain hardening seems to be enhanced with a decrease in deformation temperature. As the degree of deformation increases, the remaining austenite, not dynamically transformed to fine ferrite, becomes increasingly unstable. A lower hardenability of this remaining austenite thus would lead to a higher possibility to transform into the (ferrite + pearlite) structure of lower strength rather than the (ferrite + martensite (bainite)) of higher strength.

Abstract: 5052 Al matrix composites reinforced with carbide particles (SiC, TiC, and B4C) were fabricated by the pressureless infiltration and the tensile properties were analyzed. The strength values in the control Al were significantly increased over those of the commercial alloy while the strain to failure of the former decreased. Strength values in the composite reinforced with carbide particles were further increased compared to the control alloys. It was observed that strengthening effect by an addition of reinforcement varied with according to reinforcement types. By relative comparison, both TiC and B4C particles may be effective reinforcement compared to SiC particles in Al matrix composites.

Abstract: Secondary hardening and fracture behavior in the high Co-Ni steels containing W, has been studied in terms of severe deformation, Cr addition, and austenite condition. Two kinds of Co- Ni steels, containing the only W(W steel), and the W and Cr(WCr steel), were severely rolled and followed by direct quenching(DQ). In comparison with undeformed specimens, reaustenitizing( RA) was performed. DQ specimens exhibited higher impact toughness as well as higher hardness, as compared to RA specimens. The aging kinetics of the WCr steel was accelerated due to the Cr addition. While the W steel showed the intergranular embrittlement in the high temperature RA condition, in addition, the WCr steel exhibited the transgranular mode of mostly dimple type.

Abstract: BN/Al(-Mg) metal matrix composite (MMC) was fabricated by the pressureless infiltration technique. The phase characterizations of the composites were analyzed using the SEM, TEM, EDS and EPMA on reaction products after the electrochemical dissolution of the matrix. It is confirmed that aluminum nitride (AlN) was formed by the reaction of Mg ₃N ₂ and Al alloy melt. Plate type AlN and polyhedral type Mg(-Al) boride were formed by the reaction between Mg ₂N ₂, BN and molten Al in the composite. The reaction mechanism in the fabrication of BN/Al(-Mg)MMC was derived from the phase analysis results and the thermodynamic investigation.