발간논문

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In this study, microscratch test was combined with fracture-mechanical analysis to characterize the interfacial adhesion of catalyst coating on electrolyte polymer in a direct methanol fuel cell (DMFC). Scratch test was used to determine the critical load for interfacial failure, while fracture-mechanical analysis was used to quantify the adhesion between Nafion (the electrolyte polymer substrate) and Pt-Ru alloy (catalyst coating). Especially, it is appropriate to establish a relationship between interfacial toughness and adhesion failure by scratching because the failure mechanism by electrochemically induced swelling and shrinking at this interface is very similar to that of scratch testing. We also proposed a key of solving ambiguous problems in indentation cracking test by determining geometric information from crack propagation and critical points, as for a hard porous coating on a soft substrate. Finally, three kinds of process manufacturing catalyst layer for anode were compared to verify our new test algorithm qualitatively.

keyword : interfacial toughness, failure mechanism, microscratch analysis, DMFC, critical strain energy release rate

To increase the toughness of amorphous alloys and to verify the processibility of their composites, Zr41.2Ti13.8CU12.5Nil0.0Be22.5 composites reinforced with continuous fibers of tungsten and SUS304 have been developed by the liquid pressing process. The thermal stress of fiber/matrix interfaces and the cooling behavior of composites during the process were predicted. Observation by X-ray diffraction and scanning electron microscopy showed the amorphous state of matrices, which confirmed the process feasibility of amorphous alloy composites. In order to identify the toughness enhancement, compression tests have been conducted. SUS304 fiber composites resulted in the drastic increase of compressive strain (over 40%) compared to that of unreinforced Zr41.2Ti13.8CU12.5Ni10.0Be22.5 alloy.

keyword : composite, amorphous alloy, Liquid pressing process, toughness

Dispersed particles of (3 and 10) wt.% Si3N4 were incorporated into Ti-50 at alloys via a mechanical alloying-spark plasma sintering (MA-SPS) technique. Since Si3N4 was chemically incompatible with TiAl, Si3N4 decomposed into silicon and nitrogen during sintering and the subsequent exposure to air at 1100℃ for 100 hr. The decomposition of Si3N4 resulted in the formation of Ti5Si3, nitrides such as Ti2AlN and TiN, and voids in the matrix. Although Si3N4 was unstable in the TiAl matrix at high temperatures, Si3N4 increased the oxidation resistance, forming a thin scale on the surface of the Ti-50 at.% Al-(3, 10) wt.% Si3N4 composites.

keyword : titanium aluminide, silicon nitride, sintering, oxidation

In order to improve efficiency, performance and lifetime of hot components of gas turbine at high temperature, thermal barrier coating (TBC) systems on the superalloy are widely used. TBC systems mainly consist of superalloy substrate, MCrAlY bond coat(BC) and yttria stabilized zirconia top coat(TC). Thermally grown oxide (TGO), mainly α-Al2O3, is formed and grown at BC/TC interface during gas turbine operation and/or fabrication. Stable growth of TGO, especially α-Al2O3, is very important for the lifetime of TBC systems because the formation of α-Al2O3 at the interface increases the adherence of TBC and BC. In order to improve the high temperature stability of TBC systems, a small amount of Pt was added at the interface between BC and TBC. TBC systems, which were coated with only NiCoCrAlY (with or without a Pt layer) BC and both BC and TC (with or without a Pt layer) on Co-Base Superalloy ECY768, were prepared by air plasma spraying process (APS). Isothermal and cyclic oxidation tests were performed at 1100℃ in air, respectively. TBC systems with a Pt layer, showed the formation of more stable TGO and then longer lifetime than TBC systems without a Pt layer. We might conclude that the addition of a Pt layer in TBC systems, consisted with Cobased ECY768 substrate, NiCoCrAlY BC and YSZ was effective to enhance the durabilitsy of TBC system at high temperature.

keyword : thermal barrier coating, NiCoCrAlY, Pt, Air plasma spray, thermally grown oxide

The corrosion fatigue properties of organic coated automotive steel were investigated in 3.5 wt.% NaCl solution by polarization test, corrosion test and SEM analysis. The corrosion fatigue lifetime of bare steel is longer than that of organic coated steel, indicating that the organic coated steel is more susceptible to corrosion fatigue cracking to the bare steel. The shallow surface penetration of bare steel associated with cyclic stress is not related to corrosion fatigue cracking. On the contrary, the organic coating is ruptured by cyclic stress and environmental degradation, thereby permitting the anodic dissolution of the freshly exposed bare which facilitates fatigue crack initiation.

keyword : automotive, organic coating, accelerated test, DSC, potentiodynamic test, corrosion fatigue, SEM

The effects of heat-treatment on the tensile strength and corrosion resistance of extruded Zr-Nb alloy were evaluated. The Zr-Nb alloy was extruded at 700℃, and then heat-treated at 580℃ for up to 32 hours. The results of tensile tests at room temperature and 400℃ showed that the tensile strength was continuously reduced with increasing heat-treatment time, mainly due to the grain growth. The results of corrosion test in an ammonia aqueous solution of pH 10 at 360℃ for 100 days indicated that the heat-treatment led to enhance the corrosion resistance due to the decomposition of β-Zr phase to α-Zr + β-Nb phases. However, no more enhanced corrosion resistance was observed when the heat-treatment time more than 8 hours was applied. It is thus concluded that the longer heat-treatment time induce the lower tensile strength without enhancing the corrosion resistance.

keyword : Zr-Nb alloy, tensile strength, corrosion resistance

An ultra-low carbon bake-hardening steel bearing 0.0031 weight percent titanium was temper-rolled in the mill to investigate exactly and effectively the strain aging behavior. Correlation between the natural aging at room temperature and the accelerated aging at elevated temperatures of 70℃ and 100℃ was also studied. Accelerated aging at 100℃ for 60 minutes, which has been extensively used as a criterion for occurrence of ambient strain aging, showed a very different aging behavior from the steel aged at 20℃ for 2 months. It was found that the ambient strain aging behavior could be better evaluated by the upper yield point or the difference between upper and lower yield points rather than the 0.2 percent yield point elongation measured in the tensile test after aging at 100℃ for 60 minutes. Increasing the temper rolling extension from 0.8 to 1.5 percent retarded the occurrence of the ambient strain aging of bake hardening steel.

keyword : cold-rolled steel sheet, bake hardening steel, strain aging, equivalent aging time, yield point elongation

CNT-supported Pt catalysts were prepared by impregnation with H2PtCl6 precursor and H2 reduction, and the effects of drying and reduction conditions on Pt particles mean size and distribution were investigated. In experiments, drying and reduction processes were conducted using a thermal balance type furnace and changes of gravity were measured. Sizes of Pt particles were estimated by powder XRD and TEM analysis. From the results of preparations at 150℃, it was found that residual water before reduction start affects the particles size and distribution. The size of Pt particles decreased with decrease in the amount of residual water. Also, fine and well dispersed Pt particles on CNTs supports showed rare increase in the sizes by additional H2 reduction at 350℃.

keyword : carbon nanotubes, Pt/CNTs, catalyst preparation, impregnation, hydrogen reduction

ITO/Ag/ITO multilayer thin films on PET and coming 1737 glass were prepared by D.C. magnetron reactive sputtering method. Ag layer was inserted for applying ITO to a flexible substrate at low temperature or room temperature. The crystallinity and the electrical-optical property of ITO/Ag/ITO thin films were investigated. ITO/Ag/ITG thin films were at room temperature and grew to be crystalline with (400) preferred plane as the substrate temperature increased. ITO/Ag/ITO thin films on PET showed the transmittance of above 80% when the thickness of Ag was 5 nm. The carrier concentration increased more than 10 times due to the effect layer, and the resistance was below 15 Ω/□ much lower than that of ITO thin film without As the deposition temperature increased, Ag oxide was formed at the boundary between ITO and is supposed to be a disturbing factor for the motion of electrical charge.

keyword : ITO thin film, microstructure, multilayer, transmittance, sheet resistance