Jin Kook Yoon

Fabrication of Nanocrystalline TaSi2-SiC Composite by High Frequency Induction Heated Combustion Synthesis and Its Mechanical Properties.

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.

Growth behavior and microstructure of oxide scale formed on MoSi2 coating at 773 K

Growth behavior and microstructure of oxide scale formed on MoSi 2 coating by cyclic oxidation testing in air at 500 °C were investigated using field emission scanning electron microscopy, cross-sectional transmission electron microscopy, glancing angle x-ray diffraction, and x-ray photoelectron spectroscopy. MoSi 2 coating was prepared by chemical vapor deposition of Si on a Mo substrate at 1100 °C for 5 h using SiCl 4 –H 2 precursor gas mixtures. After the incubation period of about 454 cycles, accelerated oxidation behavior was observed in MoSi 2 coating and the weight gain increased linearly with increasing oxidation cycles. Microstructural analyses revealed that pest oxide scale was formed in three sequential processes. Initially, nanometer-sized crystalline Mo 4 O 11 particles were formed with an amorphous SiO 2 matrix at MoSi 2 interface region. Inward diffusing oxygen reacted with Mo 4 O 11 to form Mo 9 O 26 nano-sized particles. At final stage of oxidation, MoO 3 was formed from Mo 9 O 26 with oxygen and growth of MoO 3 took place forming massive precipitates with irregular and wavy shapes. The internal stress caused by the growth of massive MoO 3 precipitates and the volatilization of MoO 3 was attributed to the formation of many lateral cracks into the matrix leading to pest oxidation of MoSi 2 coating.

Mechanical Properties and Consolidation of WC-8wt.%Ni Hard Materials by HFIHS and PCAS

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.

Formation of WSi2–SiC nanocomposite coating by carburizing process followed by reactive diffusion of Si on W substrate

Abstract A WSi2/(19.3–32.9) vol.% β-SiC nanocomposite coating was formed by chemical vapor deposition of Si on the W-carbide layers. The nanocomposite coating consisted of the equiaxed WSi2 grains with the average size of 88–153 nm and the β-SiC particles with the average size of 37–65 nm, which were mostly located at the grain boundaries of WSi2.

Properties and Rapid Consolidation of Nanostructured Fe3Al Compound by High Frequency Induction Heating

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

One Step Synthesis and Consolidation of Nanostructured ZrSi2-SiC Composite and its Mechanical Properties

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.

Manufacturing and Oxidation Behavior of Silicide-Based Nanocomposite Coatings on Refractory Metals by Displacement Reaction

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.

Effect of Sn-Addition on the Bonding Strength between Casting Au-Pt-Cu Alloy and SiO2-Based Porcelain

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.

Effect of indium on the microstructures and mechanical properties of dental gold alloys

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.

Effect of Indium on Bonding between Porcelain and Au-Pt-Cu Alloy

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.