Yo-Seung Song

Effect of post-oxidizing time on corrosion properties of plasma nitrocarburized AISI 1020 steel

Abstract Plasma nitrocarburized AISI 1020 steels are oxidized for 15, 30 and 60 min to evaluate the effect of post-oxidizing time on corrosion behavior. After plasma nitrocarburizing for 3 h at 570 °C, the compound layer composed of e-Fe2–3(N,C) and γ′-Fe4(N,C) phases and the diffusion layer above the matrix are observed. The oxide top layer, mainly consisting of magnetite (Fe3O4) and hematite (Fe2O3) phases, is formed after post-oxidation treatment at 500 °C. However, the oxide layer is degraded severely by spallation as a result of the prolonged oxygen plasma bombardment as the post-oxidizing time increases. The difference in corrosion resistance is mainly attributed to the thickness of the oxide top layer, which is governed by the post-oxidizing time.

Influence of sputtering parameters on microstructure and morphology of TiO2 thin films

Abstract Titanium dioxide thin films were sputtered on Si substrate by systematically varying the process conditions including O 2 /(Ar+O 2 ) ratio, working pressure, sputtering time and dc power, respectively, without heating the substrate, to evaluate the microstructure and the morphology of the films. The optimum condition for the films is 23% of O 2 /(Ar+O 2 ), 2×10 −3 Torr of working pressure, sputtering time longer than 10 min and dc power higher than 150 W, respectively. During sputtering, amorphous α-SiO 2 phase was firstly grown on Si substrate, amorphous α-TiO 2 structure close to α-SiO 2 phase was then formed to reduce the surface energy and columnar TiO 2 films normal to amorphous α-TiO 2 were finally developed. The surface morphology of the films was a typical island structure consisting of fine particles of 10–30 nm.

High-temperature properties of plasma-sprayed coatings of YSZ/NiCrAlY on Inconel substrate

Abstract Eleven-layered functionally gradient thermal barrier coatings (TBCs) were sprayed on Inconel substrate by varying the feeding ratio of YSZ/NiCrAlY in 0.1 intervals in the range of 0–1 linearly as well as nonlinearly (concave and convex type). The performance of the coatings was evaluated in terms of high-temperature fatigue, oxidation resistance, and bond strength. The fatigue behavior and the oxidation resistance were governed by the oxidation of Al in the NiCrAlY bond coating. The bond strength was determined by continuity of the composition and the microstructure between the layers. The linearly sprayed TBCs showed the highest endurance at high-temperatures among the coatings investigated, indicating that the linear feeding technique can be used for the high-temperature applications.

Effect of organic vehicle addition on service lifetime of Ti/IrO2–RuO2 electrodes

Abstract Ti/IrO 2 –RuO 2 electrodes, prepared by mixing the conductive IrO 2 –RuO 2 powders and organic vehicle (ethyl cellulose) followed by dissolution in isopropanol-α-terpineol solution, were evaluated with an aid of the Taguchi method and orthogonal arrays to elucidate the effect of the experimental parameters, such as the ratio of powders to ethyl cellulose, the ratio of isopropanol to α-terpineol and heat treatment temperature, on the service lifetime of the electrodes. Although the current density of the as-deposited IrO 2 –RuO 2 films was almost insensitive regardless of the processing conditions, it was found that the mixing ratio of IrO 2 –RuO 2 powders to ethyl cellulose was a critical factor to the service lifetime of the Ti/IrO 2 –RuO 2 anodes.

Chromaticity, hydrothermal stability, and mechanical properties of t-ZrO2/Al2O3 composites doped with yttrium, niobium, and ferric oxides

Abstract Tetragonal (t) zirconia–alumina composites were prepared by adding Al 2 O 3 into t-ZrO 2 solid solutions doped with Y 2 O 3 and Nb 2 O 5 ((Y,Nb)-TZP/Al 2 O 3 ). The effect of Fe 2 O 3 addition on chromaticity, hydrothermal stability, and mechanical properties of the composites was investigated by adding Fe 2 O 3 in 0.1 mol% intervals in the range of 0.1–0.5 mol%. Although monolithic (Y,Nb,Fe)-TZPs containing more than 0.2 mol% Fe 2 O 3 showed t→m phase transformation after autoclaving for 20 h at 180°C under 3.5 MPa water vapor pressure, no m-ZrO 2 was observed for the aged (Y,Nb,Fe)-TZP/Al 2 O 3 composites containing 0–0.5 mol% Fe 2 O 3 , probably due to the preferential solid solubility of Fe 2 O 3 in Al 2 O 3 , the presence of the rigid Al 2 O 3 particles, and the inherent phase stability of (Y,Nb)-TZP. The optimized strength and the fracture toughness of (Y,Nb,Fe)-TZP/Al 2 O 3 composites were 700 MPa and 8.5 MPa m 1/2 , respectively, when 0.1 mol% of Fe 2 O 3 was added. Both monoliths and composites showed a gradual color change from a slight white ivory to a pale yellowish brown with increasing the Fe 2 O 3 concentration.

Application of Taguchi method and orthogonal arrays for characterization of corrosion rate of IrO2–RuO2 film

Ti/TiO2/IrO2–RuO2 electrodes were evaluated with an aid of Taguchi method and orthogonal arrays to elucidate the effect of the experimental parameters, such as type of intermediate layer between Ti substrate and IrO2–RuO2 film, heat treatment temperature, heat treatment time, and flow rate of air, on the corrosion resistance of the electrodes. Although the chemical composition of the as-deposited IrO2–RuO2 films was almost identical regardless of the processing conditions, it was found that the presence and the type of the TiO2 intermediate layer was a critical factor to the anticorrosion properties of the Ti/TiO2/IrO2–RuO2 electrodes among four different experimental parameters investigated. The optimal condition was the dip-coated IrO2–RuO2 film having the TiO2 intermediate layer prepared by plasma spray and subsequently heat treated for 120 min at 450°C with air flow rate of 3 sccm.

Influence of Deposition Method on Refractive Index of SiO₂ and TiO₂ Thin Films for Anti-reflective Multilayers

Anti-Reflective (AR) thin film coatings of SiO₂ (n=1.48) and TiO₂ (n=2.17) were deposited by ion-beam assisted deposition (IBAD) with End-Hall ion source and conventional electron beam (e-beam) evaporation to investigate the effect of deposition method on the refractive indicies (n) of the films. Green-light generation using a GaAs laser diode was achieved via excitation of the second harmonic. The latter resulted from the transmission of the fundamental guided-mode wave of 1064 nm through periodically poled LiNbO₃. Large differences in the refractive indicies of each of the layers in the multilayer coating may improve AR performance. IBAD of SiO₂ reduced its refractive index from 1.45 to 1.34 at 1064 nm. Conversely, e-beam evaporation of TiO₂ increased its refractive index from 1.80 to 2.11. In addition, no fluctuations in absorption at the wavelength of 1064 nm were found. The results suggest that films prepared by different deposition methods can increase the effectiveness of multilayer AR coatings.

A study of the corrosion properties of plasma nitrocarburized and oxidized AISI 1020 steels

Plasma nitrocarburized AISI 1020 steels were oxidized for 15, 30 and 60 min to evaluate their corrosion and microstructural properties. After plasma nitrocarburizing for 3 h at 570°C in a gas mixture comprising 85 vol.% N2, 12vol.% H2 and 3 vol.% CH4, the compound layer composed of ɛ-Fe2–3(N,C) and γ’-Fe4(N,C) phases and the diffusion layer above the matrix were observed. The top oxide layer, consisting mainly of magnetite (Fe2O4) and hematite (Fe2O3) phases, forms after post-oxidation treatment at 500°C. However, the oxide layer was severely degraded by spallation as a result of increases in post-oxidizing time. The difference in corrosion resistance should be attributed to the thickness of the top oxide layer, which was governed by post-oxidizing time.

Microstructure and Hardness of TiC Particle-reinforced Fe Self-fluxing Alloy Powders Based Hybrid Composite Prepared by High Energy Ball Milling

The Fe-based self-fluxing alloy powders and TiC particles were ball-milled and subsequently compacted and sintered at various temperatures, resulting in the TiC particle-reinforced Fe self-fluxing alloy hybrid composite, and the microstructure and micro-hardness were investigated. The initial Fe-based self-fluxing alloy powders and TiC particles showed the spherical shape with a mean size of approximately 80 and the irregular shape of less than 5 , respectively. After ball-milling at 800 rpm for 5 h, the powder mixture of Fe-based self-fluxing alloy powders and TiC particles formed into the agglomerated powders with the size of approximately 10 that was composed of the nanosized TiC particles and nano-sized alloy particles. The TiC particle-reinforced Fe-based self-fluxing alloy hybrid composite sintered at 1173 K revealed a much denser microstructure and higher micro-hardness than that sintered at 1073 K and 1273 K.