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This paper reviews the thermal barrier coatings (TBCs) for recent superalloy turbine blades produced by various techniques. Single-crystal turbine blades, which were recently introduced to enhance engine efficiency levels, with TBCs on the bond coats for application to turbine components are examined. To reduce the thermal conductivity of the blades for an improved function of the TBC, various parameters reported in recent papers, such as materials and processes for coatings, applications, durability issues, technical approaches and progress toward improved TBCs are introduced. The most recent research reports for new bonds with different compositions, thermal properties and void morphologies for longer lifetimes with delayed failure times and enhanced TBCs are explained. A suggestion is proposed for a strategy in the domestic research field for superalloys.

keyword : thermal barrier coatings, EB-PVD, bond coat, yttria stabilised zirconia, single crystal superalloys

In this study, the recrystallization volume fraction of the Al-Zn-Mg-Cu-Sc alloy after solid solution heat treatment varied with different temperatures (445℃-465℃). The highest elongation of the Al-Zn-Mg-Cu-Sc alloy was obtained at 465 ℃. Further, the hardness and strength of the solid solution heat treated Al-Zn-Mg-Cu-Sc alloy increased at room temperature due to G.P zone precipitates. The results confirmed that we can obtain advanced mechanical properties for the Al-Zn-Mg-Cu-Sc alloy from solid solution heat treatment and natural aging.

keyword : Al-Zn-Mg-Cu-Sc alloy, recrystallization, natural aging, G.P zone

Sliding wear behaviors of piercing plug tool steels were investigated at 900 ℃ against low Cr ferritic steels containing 2 %Cr and 9 %Cr using a pin-on-disc CETR tribometer under condition of applied normal load of 20N in air. Formation of glaze layers consisting of porous Fe-oxides wear debris with rapid growth on worn surface of piercing plug tool and smooth surface of steels containing 2 % Cr resulted in decreased friction coefficient at an initial period followed by exhibited constant value of about 0.3 during the elevated temperature wear tests. It was found on the cross-section underneath the worn surface of the tool steels against the steels containing 2 %Cr that coarse ferrite with equiaxed grains by phase transformation and dynamic recrystallization was formed showing the decrease in hardness. On the other hand, formation of the glaze layers consisting of compact Cr-rich Fe-oxide wear debris on the worn surface of the tool steels at slow rates of materials transfer from the steels containing 9 %Cr resulted in the fluctuated friction coefficient ranging from about 1.0~1.1 to 0.4 followed by exhibited almost constant value of 0.2~0.3. It was also found on cross-section underneath worn surface of tool steels against the steels containing 9 %Cr that severely deformed microstructures by cyclic applied loading was formed below compact and continuous oxide layers. The results showed that the elevated temperature sliding wear of the piercing plug tool steels was dominated by oxidation, materials transfer and subsurface degradation such as dynamic recrystallization softening or severe deformation.

keyword : elevated temperature sliding wear, metals, hot working, oxidation, scanning electron microscopy, SEM

High Mn TWIP steels were oxidized at 1150-1250℃ for up to 2h in 85%N2-10%CO2-5%O2 gas to determine their oxidation behavior in the reheating furnace. They oxidized fast to form thick, nonadherent oxide scales and the internal oxidation zone. The oxide scales consisted primarily of FeO, Fe2O3, MnO, MnAl2O4, and FeMn2O4, indicating that Mn, Fe, and Al were the dominant oxidizing species. They grew primarily through the outward diffusion of cations. Internal oxides formed intra- and inter-granularly through the inward diffusion of oxygen ions. They consisted of oxides of Mn, Fe, Al, Si, Cr and Cr.

keyword : metals, casting, oxidation, scanning electron microscopy, SEM, high Mn TWIP steel

The effect of atmospheric plasma treatment on change in the interlayer and the adhesion strength between the substrate and the coating layer was investigated. Soda-lime glass and indium-tin oxide (ITO) coated glass plates were used as the substrate, and the surface energy was calculated from the wetting angle of liquids on the substrates. To study the effect of surface treatment on the adhesion strength between the substrate and the coating layer, chrome (Cr) thin film was deposited on the surface treated substrates by sputtering. For the analysis of the interlayer, auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used. The surface energy of the substrates increased as a function of exposure time for atmospheric plasma surface treatment. When the exposure time exceeded 20 sec, the surface energy increased to a saturated value of 76 mJ/m2. The adhesion strength between the substrate and Cr coating layer according to the surface treatment showed a similar trend of the surface energy. The atmospheric plasma surface treatment using argon and oxygen gases promoted the formation of an interlayer that was identified as chrome oxide (Cr2O3). The thicker chrome-oxide interlayer contributed to the increase in adhesion strength between the substrates and the Cr coating layer.

keyword : thin films, surface modification, interfaces, auger electron spectroscopy, adhesion strength

Aluminum powders with average diameters of 24, 45, and 75 μm were homogeneously coated with nickel through an electroless plating process. The effect of nickel coating on the thermal oxidation behavior of the aluminum powders was clearly confirmed by observing the variation of the weight gain during thermal gravimetric analysis in an air atmosphere. The results obtained from differential scanning calorimetry showed that the exothermic enthalpy values of the aluminum powders increased significantly with decrease in the powder size from 75 μm to 25 μm. Furthermore, the nickel-coated aluminum powders exhibited much higher exothermic enthalpy values than those of pure aluminum powders. It was found that nickel coated onto aluminum powders plays an important role as a protective layer, preventing pure aluminum powders from undergoing surface oxidation while also providing exothermic heat through a self-propagating high-temperature synthetic reaction between the nickel and the aluminum. These results indicate that nickel coating should be considered to achieve high reactivity in aluminum powders for energetic applications.

keyword : aluminum, powder processing, coating, thermal oxidation

The sintering behaviors of commercial ZnS nanopowders (purity 99.999%) processed with the two different sintering techniques of hot pressing (HP) and spark plasma sintering (SPS) and optical properties of sintered material are investigated here. The results are compared between the two different sintering techniques in terms of the sintering parameters (density, microstructure, X-ray diffraction pattern) and the optical properties (infrared transmission in the range of 2~20 μm) of the sintered ZnS ceramics. The maximum density was obtained at 950℃ irrespective of the sintering method. The wurtzite (hexagonal) phase causes birefringence due to optical anisotropy which begins at a temperature of more than 920℃. The optimized optical properties were found to be ZnS ceramics sintered with HP at 950℃ and with SPS at 90 0℃. However, contamination from the graphite mold and its influence on the optical properties were confirmed to be very serious, especially in HP and SPS. In the SPS method, the optical properties by light scattering were found in this study to worsen considerably due to the refractive index differences among the ZnS, carbon impurities, and pores.

keyword : ZnS, hot press, spark plasma sintering, sintering, optical property

Al-doped ZnO thin films were deposited on glass substrates at room temperature by ion-beam-assisted molecular beam epitaxy (MBE) deposition. The crystallinity, microstructure, surface roughness, and electrical, optical and mechanical prosperities of thin films were investigated as a function of the deposition parameter and the ion energy. The microstructure of the Al-doped ZnO crystalline films on amorphous glass substrates was closely related to oxygen ion bombardment on the growing surface. The effects on the film may be divided into two categories: 1) the enhancement of atom mobility at low energetic ion bombardment and 2) the surface damage by radiation damage at high energetic ion bombardment. A large sized grain structure was obtained in the films deposited at 300 eV. At a high energy ion bombardment of 600eV, however, only a smaller grain structure with high hardness was observed. The electrical properties of the deposited films were significantly related to the change of microstructure and crystallinity. The Al-doped ZnO films with a large size grain structure have better electrical properties than those with a smaller grain structure because the grain boundary scattering decreased in the large size structure compared with the small size grains. The optical photoluminescence of Al-doped ZnO thin films was dependent on the grain size. And then the dye-sensitized solar cell (DSSC) fabricated on the AZO film grown at ion beam energy 300eV condition, it exhibits superior conversion efficiency than the other condition sample. Therefore, transparent conductive glass applying in DSSCs must have a low sheet resistance, a high transmittance in the ultraviolet-visible-infrared region and an excellent surface microstructure.

keyword : Al-doped ZnO, ion-beam-assisted molecular beam epitaxy, MBE, transparent conducting oxides, TCO, dye-sensitized solar cells, DSSCs

In this study, Ni and TaF5 were picked as additives to enhance the hydriding and dehydriding rates of Mg. A sample with a composition of 80 wt% Mg+14 wt% Ni+6 wt% TaF5 (named Mg-14Ni-6TaF5) was prepared by reactive mechanical grinding. Its hydriding and dehydriding properties were then examined and the effects of milling and hydriding-dehydriding cycling were investigated. To activate the prepared sample, two hydriding-dehydriding cycles were required at 573 K and 593 K. At the second cycle, the sample absorbed 5.74 wt% H for 60 min under 12 bar H2 at 573 K. At 623 K, the activated Mg-14Ni-6TaF5 absorbed 2.53 wt% H for 2.5 min, 4.20 wt% H for 10 min, and 5.31wt% H for 60 min under 12 bar H2, and desorbed 2.01 wt% H for 2.5 min, 4.76 wt% H for 15 min, and 4.83 wt% H for 60 min under 1.0 bar H2. The initial hydriding rate increased as the temperature increased from 423 K to 573 K (due to the predominance of the acceleration of the thermally activated process) and then decreased from 573 K to 623 K due to the predominance of the decrease in the driving force for the hydriding reactionunder 12 bar H2.

keyword : hydrogen absorbing materials, mechanical alloying/milling, scanning electron microscopy, SEM, X-ray diffraction, effects of milling and cycling

AlN/Cu composite powders with a core-shell structure were synthesized by a new technique using electroless plating of Cu on AlN particles and consolidated by spark plasma sintering (SPS). Dependence of the thermal conductivity and coefficient of thermal expansion (CTE) of the AlN/Cu composites on the particle size of AlN were studied. The thermal conductivity and coefficient of thermal expansion were significantly dependent on the existence of both boundary pores surrounding the Cu-coated AlN particles and the internal pores of the AlN particles. In order to eliminate the pores, the temperature and pressure of SPS were increased and the AlN particles were heat-treated at high temperature. Adoption of the core-shell AlN/Cu composite powders facilitated the fabrication of AlN/Cu composites with low porosity and uniform distribution in the AlN in Cu matrix.

keyword : composites, powder processing, thermal conductivity, coefficient of thermal expansion, CTE, scanning electron microscopy, SEM