Papers by Author: Jong Kook Lee

Abstract: Dissolving behavior of hydroxyapatite ceramics prepared from bovine teeth (BTHA) was investigated. The bovine teeth were soaked in 0.1 M of NaOH at 80°C for 1 h. After soaking, the teeth were calcined at 800°C for 1 h, and then they were attritor-milled for 24 h. BTHA powder consisted of mainly HA and small amount of MgO. The powders obtained were cold isostatically pressed and sintered at 1200°C with a dwell time of 1 h. Sintered density of BTHA was about 70%. After polishing the surface of sintered specimen, BTHA was immersed in buffered water at 37°C for 3 and 7 days. After immersion period, there is no evidence of dissolution for BTHA at the end of the immersion time. In addition, there is no change of peak intensity in XRD after immersion time. It was found that hydroxyapatite from bovine teeth was more stable than commercial HA in liquid environment.

Abstract: A small amount of misch metal was added to CuZnAl alloy in order to study the effect of grain refinement and mechanical properties, phase transformation behavior and stabilization of martensite. It was found that the addition of misch metal was very effective for reducing the grain size. The coarse grains over 1000 μm have been refined to the size of 30 μm by the addition of 0.43wt% misch metal. The grain size of thermo-mechanically treated alloys was barely affected by cold working. The fracture strength and ductility have been significantly increased with the increase of misch metal content when tensile test is carried out below Mf temperature. Also, the fracture strength has been more increased in the case of post-quench ageing treatment than the direct quench ageing treatment. The fracture mode has been changed from transgranular brittle fracture to ductile fracture with void formation and coalescence by the addition of misch metal.

Abstract: Titanium-based nitride coatings on cutting tools, press molds and dies can prolong their life cycle because of superior corrosion and oxidation resistance of coatings. In this study, TiAlN/ZrN and TiCrN/ZrN multilayer coatings were prepared by R.F magnetron sputtering, and microstructural evolution and corrosion resistance of the coatings were investigated during heat treatment. The TiAlN/ZrN and TiCrN/ZrN multilayer coatings were degraded by heating up to 600 oC with formation of oxides particles on the surface. During the heat treatment, the TiCrN/ZrN and TiAlN/ZrN multilayer coatings showed the lowest corrosion current density and the highest polarization resistance at temperature range of 400 oC and 500 oC. Consequently, the TiAlN/ZrN and TiCrN/ZrN multilayer coatings showed good corrosion resistance at temperature range of 400°C and 500°C during heating.

Abstract: Calcium phosphate ceramics (CPC) have been attractively used in different areas of biomedical applications, such as substances of artificial hard tissue replacement implants, drug delivery system due to their biocompatibility and bioactivity. In this work, three calcium phosphate powders between Ca/P molar ratio 1.50-1.67 were synthesized by aqueous precipitation method, using the mixture of Ca(NO3)2·4H2O and H3PO4 solutions to ammonia solution. During the precipitation reaction, Ca/P molar ratio was adjusted by controlling pH of the solution between 8.0 and 10.5 by the addition of ammonium hydroxide solution. All powders were treated at 800 oC for 2 hrs. The calcined powders were immersed in pH 7.4 distilled water at 37°C for 3 and 7 days. Decomposition and related dissolution with the various Ca/P ratios were investigated by XRD, FTIR, and TEM observation.

Abstract: Hydroxyapatite derived from human teeth was sintered at 1200°C for 2 h. Dissolving behavior of the biologically derived HA (BHA) in distilled water was investigated and compared with an artificial hydroxyapatite (HA) made of synthetic HA powder. All disks were immersed in 40 ml of pH 7.4 distilled water (buffered using 0.05 M Tris.) for 7 and 14 days at 37°C. All detectable peaks in the HA are identical only to HA lattice planes, whereas BHA consisted of a mixture of HA and β-tricalcium phosphate (TCP). In the case of the HA specimen, the surface dissolution was initiated at grain boundaries followed by generated many separated grains and large defect like cavities. On the other hand, biologically derived HA showed that definite grains considered as β-TCP was predominantly dissolved and the grains were separated from the matrix leaving pores. In the mean time, the rest region, mainly consisting of HA, did not show any evidence of dissolution. It seems that BHA is more stable than the artificial HA in liquid environment.

Abstract: Dissolution of calcium phosphate powders with different compositions in simulated body fluid was investigated. Hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) powders were synthesized by a precipitation method and by controlling pH of the solution. Biphasic calcium phosphate (BCP) powders were prepared by mixing the HA and β-TCP powders with ratio of 80:20 and 60:40 in weight % in a planetary mill for 4 h. For dissolution test, 0.5g of each β-TCP, HA and BCP powders were immersed in 50 ml of SBF at 37°C for 3-28 days. HA powders with stoichiometric composition showed no significant morphological change regardless of the immersion time. β-TCP powders were not dissolved in SBF in spite of its high solubility. The surface of BCP powders became rough due to dissolution with increasing immersion time. This surface dissolution gradually extended into the core region of the particles resulting in disintegration of the dense particle into fine crystallites. It may be caused that the milling process during preparation of BCP powders may induce stress concentration or formation of dislocation on the surface of particles.

Abstract: Particle separations occurred frequently in sintered hydroxyapatite when immersed in distilled water or simulated body fluid. This dissolution initiated at grain boundary creating nano-size defects such as small pores and grew up to micro scale by increasing immersion time. The dissolution, probably due to the appearance of secondary phases in grain boundary, resulted in grain separation at the surfaces and finally in degradation and fracture. And the dissolution concentrated on those grains adjacent to pores rather than those in the dense region. Hydroxyapatite ceramics incorporated with calcium silicate glass were prepared by slip casting to enhance the sinterability as well as to reduce dissolution. Glass phase was incorporated into hydroxyapatite to act as sintering aids followed by crystallization in order to improve the mechanical properties without reducing biocompatibility. From dissolution test, significant damage was reduced even more than 7 days and the dissolution pattern somewhat changed than pure hydroxyapatite. X-ray diffraction and SEM showed no decomposition of secondary phases in grain boundary and fracture toughness somewhat increased.

Abstract: This research has been performed to investigate the effect of ageing onto the grain refinement of 82wt%Cu-13.5wt%Al-4.5wt%Ni-0.1wt% misch metal alloy with ageing temperature and time. The variation of transformation temperature with ageing has been found to be heavily dependent on ageing time and temperature. In the second reversed transformation cycle, the temperature of Ms point decreases with ageing time at 300
, but no variation at 100
. The temperature of As point increases with ageing time at both temperatures. After the second reversed transformation cycle, the transformation temperature does not show any significant changes in martensitic phase with ageing, the temperature of Ms point being the same afterwards. Parent phase after ageing appears to show M18R and N2H martensitic phases depending on the ageing time and temperature. It is also found that α and γ2 phases are formed only at the condition of 300
and 24h.

Abstract: Series of TiCrN wear resistant coatings have been known to be superior to other coatings in protecting tools which may be damaged corrosion, oxidization, abrasion and hardness of mold materials. The corrosion and oxidation resistance of TiN/CrN(TiCrN) multilayer coatings is better than that of single TiN and CrN coatings. TiCrN coated layer was formed by coating a layer of TixCr1-xN by non-stoichiometry. In this study, TiCrN coated layer by R.F magnetron sputtering was executed heat treatment each other temperature by tube furnace. And then surfaces of coated layer and cross sections were observed by FE-SEM and XRD. Corrosion resistance tester measured characterizations of coated layers. When the temperature increased, TiN was partially transformed to Ti2N and TiNX, and CrN was also transformed to Cr2N. The formation of non-stoichiometric phases decreased corrosion resistance.