Papers by Author: Young Do Kim

Abstract: Bulk Ti3(Al1-nGen)C2 (n=0, 0.3, 0.5, 0.7, 1.0)were synthesized by a reactive hot pressing of TiCX (x=0.6), Al and Ge powder mixture at 1400 °C for 1 h under 25 MPa. A series of Ti3(Al1-nGen)C2 was successfully synthesised by a hot pressing through a direct liquid-solid reaction between Al-Ge melt and TiCx, and simultaneous densification. Synthesized bulk Ti3(Al1-nGen)C2 was maintained a typical layered structure as shown in other ternary carbide such as Ti3AlC2 and Ti3GeC2. With increasing Al or Ge contents in Ti3(Al1-nGen)C2, both of Vickers hardness and flexural strength of Ti3(Al1-nGen)C2 were increased compared to those of pure Ti3AlC2 and Ti3GeC2 . The maximum flexural strength of Ti3(Al1-nGen)C2 (n=0.3) was about 600 MPa, which was about 3 times higher than that of pure Ti3GeC2. The Vickers hardnesses of Ti3(Al1-nGen)C2 and Ti3AlC2 were slightly decreased with increasing an indentation load.

Abstract: Al-Cr-N coatings were deposited on Si substrate by unbalanced magnetron (UBM) sputtering with Al and Cr targets and Ar and N2 reactive gases at substrate bias of -50V. At a fixed chamber pressure of 0.8 Pa, the microstructure of the coatings was changed from AlN/CrN nanoscale multilayered structure to (Al,Cr)N mixed single layered one with the increase of rotation speed of substrate holder. The residual compressive stress of AlN/CrN nanoscale multilayered coating was higher than that of (Al,Cr)N single layered coating. For the AlN/CrN nanoscale multilayered coating, the residual compressive stress was reduced with increase in total pressure of reactive gases. The AlN/CrN nanoscale multilayered coatings with higher residual compressive stress showed higher hardness and wear resistance.

Abstract: Compressive behavior of 7xxx series Al metal matrix composite (MMC) powders with different ceramic contents and different particle size were investigated. As a starting powder of the experiments, ceramic contents of each starting powder were 5 and 10 wt.% and ceramic particle size of starting powder were 20 and 100 ㎛, respectively. And 7xxx Al blended powder was used for comparison. The powders were uniaxially cold-compacted using cylindrical die with a compacting pressure 250 MPa and sintered at 620oC in a dry N2 atmosphere for 60 min with heating rate of 20oC/min. In case of heat treatment condition, sintered parts were solution treated at 475oC and aged at 175oC. To reveal the effect of Al2O3 particle content and particle size on the mechanical properties of composites, compression test were conducted with constant strain rate of 1×10-3/s using sub-size cylindrical samples of 9 mm diameter. Compression test was performed 5 times and its average value was used. Then fractography analysis was conducted using scanning electron microscope.

Abstract: Inconel 617 is a candidate tube material for high temperature gas-cooled reactors (HTGR). The microstructure and mechanical properties of Inconel 617 were studied after exposure at high temperature of 1050oC. The dominant oxide layer was Cr-oxide. The internal oxide and Crdepleted region were observed below the Cr-oxide layer. The major second phases are M23C6 and M6C types of carbides. The composition of M23C6 and M6C were determined to be Cr21Mo2C6 and Mo3Cr2(Ni,Co)1C, respectively, by EDS. These carbides are coarsened during exposure. M6C carbide is more stable than M23C6 at high temperature. There was not much change in mechanical properties after exposure at 1050oC for 1000 h.

Abstract: The effect of Cu on the hydrogen reduction of molybdenum oxide powders was investigated by measuring the humidity change during a non-isothermal process of hydrogen reduction. The presence of Cu induced a shift in the reduction temperature and strongly affected the reduction processes of MoO3→Mo4O11→MoO2, which comprised the contained chemical vapor transport of MoOx(OH)2. This study suggests that the surface of the Cu grains acts as a nucleation site for the reduction of MoOx(OH)2 to MoO2 particles from MoO3 or Mo4O11 phases. Such an activated reduction process results in the deposition of metallic Mo and MoO2 particles on the surface of the Cu grains.

Abstract: Cr2AlC, one of the ternary carbide families, was synthesized by a reactive hot pressing of CrCx (x=0.5) and Al powder mixture used as starting materials at the temperature range of 1200 oC~1400 oC under 25 MPa in Ar atmosphere. Fully dense Cr2AlC with high purity was synthesized by hot pressing CrCx and Al powder mixture at the temperature as low as 1200 oC. After synthesizing the bulk Cr2AlC, it was found that it has a typical layered structure as found in other ternary carbides such as Ti3AlC2 and Ti3SiC2. The average grain size of synthesized bulk Cr2AlC was varied in the range of 10-100 ㎛ depending on hot pressing temperatures. The Vickers hardness of bulk Cr2AlC was less than 4 GPa under the loading of 10N and this decreased with increasing indentation load. The maximum flexural strength of synthesized bulk Cr2AlC exceeded 600 MPa. Also, it was found that synthesized Cr2AlC was readily machinable by a conventional WC tool bit due to its low hardness as well as damage tolerance properties upon fracture.

Abstract: Magnetic properties of nanostructured materials are affected by the microstructures such as grain size (or particle size), internal strain and crystal structure. Thus, it is necessary to study the synthesis of nanostructured materials to make significant improvements in their magnetic properties. In this study, nanostructured Fe-20at.%Co and Fe-50at.%Co alloy powders were prepared by hydrogen reduction from the two oxide powder mixtures, Fe2O3 and Co3O4. Furthermore, the effect of microstructure on the magnetic properties of hydrogen reduced Fe-Co alloy powders was examined using XRD, SEM, TEM, and VSM.

Abstract: The Ti3SiC2 materials were synthesized by hot pressing TiCx and Si powder mixtures. The matrix grains were lamellar, having a small amount of TiCx. The high-temperature stability was investigated by subjecting Ti3SiC2 to high-temperature oxidation up to 1200oC in air. Ti3SiC2 began to oxidize appreciably above 850oC. The oxidation resulted in the formation of the oxide layer that consisted of TiO2 and SiO2. The scales formed were adherent.

Abstract: MgZn4.3Y0.7 alloy powders were prepared using an industrial scale gas atomizer, followed by warm extrusion. The powders were almost spherical in shape. The microstructure of powders as atomized and bars as extruded was examined as a function of initial powder size distribution using Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscope (EDS) and X-ray Diffractometer (XRD). The grain sizes were decreased with extruding as well as decreasing the initial powder sizes. Both the ultimate strength and elongation were enhanced as the initial powder sizes were decreased.

Abstract: Boron carbide is a very hard material with high abrasive wear resistance. It requires a very high sintering temperature of above 2200 oC to fabricate a monolithic B4C close to the theoretical density. However, the mechanical property of monolithic B4C is not good enough to use it directly to industrial applications. In this investigation, B4C based ceramic composites were fabricated by in-situ reaction hot pressing using B4C, TiC and SiC powder as starting materials. The reaction synthesized composites by hot pressing at 1950 oC was found to posses very high relative density. The reaction synthesized B4C composites comprise B4C, TiB2, SiC and graphite by the reaction between TiC and B4C. The newly formed TiB2 and graphite was embedded both inside grain and at grain boundary of B4C. The mechanical properties of reaction synthesized B4C-TiB2- SiC-graphite composites were more enhanced compared to those of monolithic B4C. The flexural strength and fracture toughness of these in-situ B4C synthesized composites were 400-570 MPa and 6-9.5 MPam1/2, respectively.