Solid State Phenomena Vols. 124-126

Abstract: The effect of chemical pretreatments on the electrochemical etching behavior of aluminum was investigated with the topographic studies of surface and the analysis of initial potential transients. Two-step pretreatments with H3PO4 and H2SiF6 result in a high density of pre-etch pits on aluminum surface by the incorporation of phosphate ion inside the oxide film and the removal of surface layer by aggressive fluorosilicic acid solution. It generates a high density of etch pits during electrochemical etching and results in the capacitance increase of etched Al electrode by expanding the surface area, up to 61.3 μF/cm2 with the pretreatment solution of 0.5M H3PO4 at 65°C and 10 mM H2SiF6 at 45°C.

Abstract: The condition of a heat transfer surface with boiling is composed of three environmental components; high temperature, high pressure and water vapor. In general an autoclave or a high temperature loop system is used for maintaining the required condition. The thermodynamic relationship of chemical reactions states that the free energy difference (ΔG) is clearly dependent on the reaction temperature and independent of the reaction pressure if the reaction has no volume change: (∂ΔG/∂P)T = ΔV ~ 0. Based on the above relationship, the high-pressure term was removed from the environmental components of the heat transfer surface. A vacuum quartz capsule with a moisturized metal oxide powder was used to study the formation of nickel ferrite. The feasibility of this simplified method to simulate a heat transfer surface was confirmed by an analysis of the FT-IR spectra and XRD pattern during the transformation of a nickel and iron mixed oxide into nickel ferrite.

Abstract: The bonding strength of top coat in thermal barrier coatings (TBCs) with different bond layers has been measured with thermal fatigue and its fracture behavior has been investigated considering the effects of a thermally grown oxide (TGO) layer and resintering of the top coat. Two kinds of TBCs with different thicknesses of approximately 80 and 280 μm in bond layer were prepared by two different methods of air plasma spray (APS) and high velocity oxygen flow (HVOF). Top layer (3Y-TZP) was coated on both bond layers using the APS process. Thermal fatigue tests were conducted at temperature of 1100oC with dwell time of 10 hr, which tests were done till 20 cycles. The HVOF process provides a relatively homogeneous microstructure in the bond layer, compared with the APS process. The formation of TGO layer is influenced by the exposed time, independent of the bond layer thickness and the bond layer species. TBCs with the bond layer prepared by the APS process show higher bonding strength than those by the HVOF process in as-prepared TBCs (before thermal fatigue), showing 15 – 18 MPa for the APS process and 13 – 16 MPa for the HVOF process. Whereas the fracture in TBCs with the HVOF bond coat are originated at the interface between the top coat and the bond coat, the fracture within the top coat in TBCs with the APS bond coat However, after the thermal fatigue, the bonding strength value is increased to 18 – 20 MPa in both cases even though the TGO layer is formed, showing the fracture paths within the top coat in both cases.

Abstract: The tribological behavior of TiC-based metal matrix composites was investigated via measuring counterpart weight. The composites were prepared using powder metallurgy technique. Wear of counterpart by the composites varying from 35 to 45 % by volume Titanium carbide were observed over a load ranges of 9.81 to 49.05 N and sliding velocities of 2.0, 2.2, 2.4, 2.6, 2.8 and 3.0 m/sec. Detailed wear track micro-graphy was done to verify the effect of sliding condition on wear mechanism. Observations indicate that wear rate of counterpart increases with the increase in load and the sliding velocity and discontinuous wear rate change occurs at a certain load.

Abstract: The separator of the molten carbonate fuel cells (MCFC) has the major corrosion problems, especially wet-seal area, because this part contacts directly with the corrosive molten carbonate electrolyte. For the protection of the wet-seal area, aluminization method is developed recently. Coated aluminum reacts with the molten carbonate and forms the LiAlO2 having high corrosion resistance, high melting point, no electrical conductivity and no dissolution in molten carbonate. In this study, 50㎛ thickness aluminum foil was applied to form the Al-Fe coating layer on the AISI316L by using the laminating method. Laminating of aluminum foil on AISI 316L was performed at 630°C for 5h in Ar atmosphere using a jig and diffusion heat treatment was carried out at 700°C, 750°C and 800°C for 3h in Ar atmosphere respectively. This method simplified the existing complicated process and has a great advantage of low-cost mass production compared to other methods like PVD, CVD and spray coating.

Abstract: Single pulse of 2.0 to 3.5 kJ of input energy from 150 μF capacitor was applied in a N2 atmosphere to the cp Ti rod for its surface modification and heat treatment. Under the conditions of using 2.0 and 2.5 kJ of input energy, no phase transformation has been occurred. However, the hardness and tensile strength decreased slightly after a discharge due to a grain growth. Using greater than 3.0 kJ of input energy, the electro discharge process made a phase transformation and the hardness especially at the edge of the cross-section of the Ti rod increased significantly. The Ti rod before the discharge was lightly oxidized and was primarily in the form of TiO2. However, the surface of the Ti rod has been instantaneously modified by the discharge into the major form of TiN from TiO2. The electro discharge process can thus modify the Ti rod’s surface in times as short as 200 μsec by manipulating the input energy, capacitance, and discharging ambience.

Abstract: The corrosion behavior of glass and glass-ceramics fabricated with silicate glass frit mixed with 50~70 wt% EAF dust in the acidic solution was analyzed by both heavy metal leaching test and microstructural observation. The crystallization temperature, Tc of glassy specimens was around 850
measured by DTA and the heat treatment temperature to crystallize a glassy specimen was selected as 950
/ 1 hr. The spinel crystal peaks were found in XRD analysis for the glass containing dust > 60 wt%. For the glass-ceramics, however, the spinel peaks in a specimen containing dust > 50 wt% was found with weak willemite peaks. The glass and glass-ceramic specimens showed the first stage of corroding reaction according to Clark models in acidic solution. The glass-ceramic specimens showed much lower a heavy metal leaching concentration than that of glass specimens in the corrosion test in acidic solution of pH=2.95. Especially, the glass-ceramics containing dust 60 wt% showed a heavy metal leaching concentration of 66 % Pb, 60 % Zn and 98 % Fe lower than that of glass specimens due to crystal phases formed, thermodynamically more stable than a glass network structure. From the leaching test that more Zn ion leached out than Fe ion, the spinel crystal phase [ZnFe2O4] showed better corrosion resistant in the acidic solution than the willemite [Zn2SiO4].

Abstract: Nickel tungsten (Ni-W) alloy coating was electroplated to increase its mechanical properties. Tungsten cannot be electroplated by itself, however, it is codeposited with other iron group metals especially with nickel. The morphologies of nickel tungsten coating were varied with current density. To eliminate the formation of cracks, pulse plating was employed. Crackless nickel tungsten alloy were obtained in pulse reverse electroplating. Hardness of nickel tungsten coating has twice higher compared to the normal electroplated nickel.

Abstract: Surface modification of polyimide(PI) films was treated with oxygen RIE by varying ion doses from 1x1016 to 1x1018 ions/cm2 at an ion beam energy of 250 eV. Following the modification of PI surface, metal films consisting of NiCr/Cu and Cu were deposited on modified PI films by D.C. magnetron sputtering and electroplating, respectively. The surface modified PI film was characterized by XPS, AFM, SEM and contact angle measurement, respectively. The water contact angle of PI film decreased significantly from 64° to 4.4° with an increase of ion dose, indicating that the surface energy of PI film increased. The XPS spectrum showed that functional group, particularly C-O bonding, on modified PI surface was significantly increased by interaction between scissored unstable chains and reactive ions. The modified PI film surfaces by oxygen RIE showed significant improvement in adhesion to a overcoated metal film of NiCr/Cu.

Abstract: This paper reports the fabrication of indium oxide (In2O3) films using a triethylindium and oxygen mixture. The deposition has been carried out on TiAlN substrates (200-350°C). We have established the correlation between the substrate temperature and the structural properties. The films deposited at 300-350°C were polycrystalline, whereas those deposited at 200°C was close to amorphous. XRD analysis and SEM images indicated that the films grown at 350°C had grained structures with the (222) preferred orientation. The room-temperature photoluminescence spectra of the In2O3 films exhibited a visible light emission.