Papers by Author: Jin Kook Yoon

Abstract: A dense nanostuctured Fe3Al was consolidated by high frequency induction heated sintering method within 2 minutes from mechanically synthesized powders of Fe3Al and milled powders of 3Fe+Al. The consolidation was accomplished under the combined effects of a induced current and mechanical pressure. The grain size, sintering behavior and hardness of Fe3Al sintered from horizontally milled Fe+Al powders and high energy ball milled Fe3Al powder were compared. Keywords: Combustion synthesis; Nanomaterials; Mechanical properties; Rapid sintering

Abstract: Two methods, High-Frequency Induction-Heated Sintering (HFIHS) and Pulsed Current Activated Sintering (PCAS), were utilized to consolidate WC-8wt.%Ni hard materials. The demonstrated advantages of these processes are rapid densification to near theoretical density in a relatively short time and with insignificant change in grain size. The hardness, fracture toughness, and the relative density of the dense WC–8Ni composites produced by HFIHS and PCAS were investigated. And the effect of variation in particle size of WC powder on the sintering behavior and mechanical properties were investigated.

Abstract: Dense nanostructured ZrSi2-SiC composite was synthesized by high frequency induction heated combustion synthesis (HFIHCS) method within 1 minute in one step from powders of ZrC and 3Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of an induced current and mechanical pressure. Highly dense ZrSi2-SiC with relative density of up to 98% were produced under simultaneous application of a 60MPa pressure and the induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.

Abstract: Effects of 0.5 wt% Sn-addition to the dental casting Au-12Pt-0.6Cu alloy on the interfacial microstructures and bonding strength between porcelain and the alloy were investigated. Porcelain powders (SiO2-based oxides) are bonded through a thermal schedule consisting of preoxidation, 1st firing, and 2nd firing. Interfacial microstructures were examined after pre-oxidation and 2nd firing, respectively, by scanning and transmission electron microscopy. The bonding strength of the Au-12Pt-0.6Cu and Au-12Pt-0.6Cu-0.5Sn alloys with porcelain was about 24.6 MPa and 46.2 MPa, respectively. The higher bonding strength of the Sn-added alloy compared with that of the alloy without Sn is attributed to the SnO2 formed at the interface between porcelain and the alloy during pre-oxidation. SnO2 layer is thought to enhance chemical bonding with various oxides in the porcelain and, accordingly, improve bonding strength.

Abstract: The microstructure and oxidation resistance of MSi2-SiC or MSi2-Si3N4 nanocomposite coatings (M = Mo, W, Nb, Ta) on M substrates formed by displacement reactions between M-carbides or M–nitrides and silicon, respectively, was investigated. Present study demonstrated that the crack density formed in the MSi2-base nanocomposite coatings due to mismatch in the coefficient of thermal expansion between nanocomposite coatings and M substrates could be controlled by adjusting the volume fraction of the SiC or Si3N4 reinforcing particles with the low CTE values. The high- and low-temperature oxidation resistance of nanocomposite coatings was superior to that of monolithic MSi2 coatings.

Abstract: By the high frequency induction heated combustion synthesis (HFIHCS) method, dense nanostructured TaSi2-SiC composite was synthesized within 2 minutes and in a single step from powders of TaC and 3Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of an induced current and mechanical pressure. Highly dense TaSi2-SiC with relative density of up to 97% were produced under a 60MPa pressure and induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.

Abstract: The effect of indium on the microstructures and mechanical properties of a Au-Pt-Cu alloy was investigated. The Au-Pt-Cu-xIn alloys heat-treated at 550°C for 30 min revealed a maximum hardness value of 207 HV, irrespective of the heat temperature and In contents. Also, the hardness of the Au-Pt-Cu-xIn alloys (x = 0.5, 1.0, 1.5, 2.0) aged at 550 °C rapidly increased with increasing aging time, and it reached an almost constant value after 30 min. The hardness of the Au- Pt-Cu-xIn alloys aged at 550°C for 30 min increased with increasing In content until 1.5wt%, but it slightly decreased with more increasing In content. Also, a variation of the tensile strength of the alloys with In contents showed a similar trend of hardness change with In contents. Analysis of EDS and TEM revealed that the microstructure of Au-Pt-Cu-xIn alloys is composed of solid solution with fcc structure and intermetallic InPt3 precipitate with L12 structure. Based on this investigation, it can be concluded that an increase in hardness of Au-Pt-Cu-xIn alloys is ascribed to a complex effect of the precipitation hardening of InPt3 and the grain size refinement.

Abstract: Effects of 0.5 wt% Indium addition on the oxidation of Au-Pt-Cu alloy, the interfacial microstructure and bonding strength between porcelain and Au-Pt-Cu-xIn alloys(x = 0, 0.5wt%) were investigated using scanning electron microscopy, X-ray diffractometry, transmission electron microscopy, and a tensile tester. The bonding strength of the Au-Pt-Cu and Au-Pt-Cu-0.5In alloys with porcelain was about 24.6 MPa and 49.5 MPa in average, respectively. This higher bonding strength in the Au-Pt-Cu-0.5In alloy compared with the Au-Pt-Cu alloy without In is ascribed to the formation of In2O3 at the interface between porcelain and Au-Pt-Cu-0.5In alloy. Especially, the formation of In2O3 at the interface between porcelain and Au-Pt-Cu-0.5In alloy leads to enhancing chemical bonding between In2O3 and various oxides in porcelain, and also to improving the anchoring effect.