발간논문

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A rate model for the growth of multi sulfide layers during the sulfidation of a metal was proposed by considering the partitioning of cations between sulfides, their relative stabilities, rate of growth and molar volume of each sulfide. From this model, the theoretical ratios of thicknesses of sulfide layers formed were estimated as a function of temperature. Theoretical values obtained from the proposed model were compared with experimental results for the sulfidation of pure nickel by Bastow in order to verify excellence of this model. Sulfides formed during the sulfidation of pure nickel were three phases, such as Ni₃S₂, Ni_7S_6 and NiS, and the equilibrium sulfur pressures at the interfaces between sulfides were calculated by thermodynamic data. Good agreement was found between the theoretical and experimental values of the ratio of sulfide thicknesses. The parabolic rate constants for the growth of the entire sulfide layers could be theoretically obtained from the parabolic rate constants for the growth of only Ni₃S₂and also Ni_7S_6 layer, but values obtained were only in good agreement with experimental values at high temperatures. This may be due to the fact that the thicknesses of NiS are extremely small at high temperatures.

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The reduction rate of TiO₂ in the CaO-SiO₂-Al₂O-MgO-TiO₂ slags system by saturated carbon and silicon in iron melt was measured at 1350-1600℃. The mechanism of transport of titanium from slag to metal was dicussed. 1. The reduction rate of TiO₂ was strongly dependent on the temperature. It was concluded that the rate-determining step of the transport rate of titanium from slag to metal was chemical reaction at slag/metal interface. 2. The ratio of amount of TiO₂ reduced by silicon to overall TiO₂ reduced and the transport rate of titanium from slag to metal were increased greatly with increasing initial content of silicon in the metal. In this case, the transport rate of titanium from slag to metal was controlled by chemical reaction and mass transport rate. 3. The apparent activation energy of TiO₂-C, SiO₂-C, TiO₂-Si system were measured to be 2.2×10², 2.4×10², 2.5×10² (kJ.mol^(-1)). 4. The reduction rate of TiO₂ decreased with increasing ratio of CaO/SiO₂ in the acid slags(CaO/SiO₂< 1), whereas the reduction rate increased at the range of CaO/SiO₂> 1.

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Friction welding was conducted on the dissimilar alloys, Nimonic 80A and a martensitic heat resisting steel. Effects of specimen diameter and post heat-treatment on the microstructure and mechanical properties of weld joints were investigated. Metallographic and microhardness tests indicate that the weld region is composed of interaction layer, dynamically recrystallized zone, and heat affected zone. Interaction layer was composed of γ-(Fe, Ni) solid solution formed by mechanical mixing and interdiffusion. Hardness of the interaction layer reaches HV 700 due possibly to fine microstructure and high dislocation density. Post heat-treatment reduces hardness through decomposion of γ(Fe, Ni). The post heat-treatment improves tensile properties through modifying microstructures of the HAZ where flow pattern is formed in as-welded condition.

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Effects of plating conditions such as electropolishing, an organic solution in electrolyte and vacuum aging on the wear and corrosion resistance of hard chrome deposits on the AISI 1024 steel were studied to improve the performance of the deposits. For the plating conditions of 0.6A/㎠ at 53℃, electropolishing in the electrolyte with phosphoric acid for 1 minute followed by 30 seconds in plating bath showed the best neutral salt fog spray life among the chrome coatings prepared for this study. The contamination of the plating bath with the phosphoric acid reduced the cathodic current efficiency. Chrome plated in modified Sargent bath containing Formic acid had amorphous structure supersaturated with carbon up to 2.0wt.%, which precipitated as chrome carbide such as Cr_(23)C_6 after vacuum aging above 200℃ for 1 hour. This resulted in improvement of hardness and wear resistance of the chrome layer. Considering the reduction of corrosion resistance due to the thermal effect, optimum vacuum aging condition of the chrome layer was 1 hour at 200℃

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The residual stresses in Ti films, deposited on (001) Si substrate via dc magnetron sputtering, have been measured using XRD method as a function of Ar pressure and bias voltage. Tensile stress is observed in Ti thin films deposited at relatively high Ar pressure and high deposition rate, while compressive stress is observed in Ti thin films deposited at low Ar pressure. The application of bias voltage also induces a compressive stress This is interpreted as due to the shot peening effect by energetic Ti atoms and/or Ar ions. The study of the reaction of Ti thin films with Si substrate using XRD and sheet resistance measurements indicated that the compressive stresses in Ti thin films persist up to TiSi₂ formation temperatures, And these compressive stresses can facilitate the formation of C54 structure by lowering the C49→C54 transition temperature.

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Electrochemical redox reactions of electrochemically precipitated nickel hydroxides were studied by using Electrochemical Quartz Crystal Microbalance and Cyclic Voltammetry. Cyclic voltamassograms showed the increase and decrease of mass of α-Ni(OH)₂ film during its oxidation and reduction, respectively. The mass change of α-Ni(OH)₂ film during redox reactions was explained by the intercalation/deintercalation of K^+ and H^+ and CO₃^(2-) ions. β-Ni(OH)₂ film, however, showed mass change behaviors completely opposite to those of α-Ni(OH)₂. Its mass change was explained by intercalation/deintercalation of H^+ ions. Phase transformaton of α-Ni(OH)₂ to βNi(OH)₂ proceeded through the formation of intermediate phases, which showed the cyclic voltammetric behavior of a single phase but behaved as a microscopic mixture of α-Ni(OH)₂ and β-Ni(OH)₂ in cyclic voltamassogram.

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