발간논문

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Al₂O₃, Chin films were fabricated by a dipping method. A Sol-Gel process, which comprises precipitation of aluminum monohydroxide, was used to produce coating solutions. Thickness of the films increased with increasing the viscosity of solution and the number of coating cycle. The relationship of the film thickness and viscosity using Nevier-Stokes equation was proposed, and it showed good agreements with film thickness measured in the range of low viscosity. The adhesion strength of Al₂O₃films coated on stainless steel was measured with the acoustic emission scratch tester. The critical load of Al₂O₃ films was measured to be about 14 N. Surface hardness could be enhanced up to 3.4 times that of uncoated substrate by increasing the thickness of Al₂O₃films to 12㎛. Electric resistance was 60Ω for layers of 13㎛ thick. This value is corresponding to that of an insulator. According to the corrosion test, it turned out that thicker the coating layer became, less was the total corrosion rate. This was attributed to the decrease in porosity.

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Microstructures of two turbine blades were observed; one of them is as cast product. The other was actually served in a real engine for 2150 hours after aluminizing coating treatment. Concluded: the as cast blade contains a lot of casting pores as casting defects, but also MC carbides of chinese script types. These casting defects were removed through volume expansion, which was given rise to precipitation and growth of γ` and carbides of M_(23)C_6 and M_6C during operation of blade at service temperature of about 650-900℃. In addition to former described effect, MC carbides of Chinese script types were not found.

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Adhesion is one of the most important properties of coated TiN layer for the wear resistant application. This study is focused on improving the adhesion of a TiN film grown by PECVD method. The adhesion was significantly affected by the microstructure of a Ti interlayer. In order to investigate a relationship between the adhesion and the microstructure of a Ti interlayer, the analyses of XRD, EDS and SEM were performed. As a pressure was decreased or the negative bias voltage was applied to the substrate while sputter-depositing a Ti interlayer, the degree of preferred orientation increased. The Ti film exhibited a preferred growth orientation of(002) which matches with TiN (111) plane coherently. It was found that the adhesion of a TiN layer decreased with increasing the degree of preferred orientation of a Ti interlayer. This tendency was explained with the phenomena of stress accumulation and relaxation at the interfaces. The adhesioin of the TiN film grown by PECVD was improved up to 45N when the crystallization and the preferred orientation of a Ti interlayer were suppressed by depositing at a relatively high pressure.

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The in-situ composites produced by unidirectional solidification are known by its fabrication at lower cost and its higher wettability between matrix and fibers, compared with the other composites such as F.R.M and F.R.P. The present work is aimed to examine transitional behaviour of Al-Fe based in-situ composites. Microstructure and mechanical properties at room and elevated temperatures have been studied on unidirectionally solidified Al-Fe alloy which contained additional elements such as Ti and Cr. As the solidification rate increased, Al-Al₃Fe eutectic phase changed into Al-Al_6Fe eutectic phase and morphology of the composite structure changed from the plate type into rod type in off-eutectic Al-2.23% Fe alloy. Cellular eutectic phase was formed in the coupled growth region except at extremely slow rates of solidified alloys have been improved as compared with the normally cast alloy. The Al-Al_6Fe eutectic became stronger than the Al-Al₃Fe eutecticalloy by addition of Ti and Zr elements to Al-Fe binary alloy and the Al-Al₃Fe eutectic transfered into Al-Al_6Fe at the faster solidification rate.

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The microstructures and effect of the residual stresses on the stress induced martensitic transformation in plasma-sprayed ZrO₂(8wt% Y₂O₃) coatings were discussed in this study. Acoustic counts and hardness decreased with an increase of compressive axial(or radial) residual stress since martensitic transformation was repressed. The critical stress for inducing phase transformation increased and was influenced greatly by the residual stress as the deposition rate decreased. This is because stress intensification decreases as well with decreasing the deposition rate. Transformable equilibrium tetragonal phase precipitated only at bundaries, which are believed to have small cooling rates due to a collision between solid/liquid interfaces, and was transformed readily by applied stress. It was revealed that the critical stress depended not on the effective stress of Von mises criterion but on the residual stress(σ_(xy) σ_y or σ_z). This implies that martensitic transformation occurs predominently at these boundaries.

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To investigate the applicability of a coarse powder to the metal injection molding, coarse iron podwers were combined up to 60% in weight fraction with fine iron powders in the injection molding process. With increasing relative fraction of a coarse powder, the green density of the compacts increased slowly up to 97% at 60 wt. % addition of coarse powder and it became possible to remove a binder completely out of the compact through the broader channels of binder flow during the debinding stage. The increase in the porosity of the compact owing to the increasing portion of a coarse powder resulted in the decrease in the sintered density and volume shrinkage. And then it brought the tensile strength and elongation of the final product down. In this experiment the maximum amount of a coarse powder replacing a fine powder with the equivalence in densification behavior, microstructure and tensile property to injection molded-fine carbonyl iron powder was found to be 20% in weight fraction.

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Three dimensional finite element analysis was employed using the elasto-plastic model with work hardening characteristics to study the tensile behaviour of unidirectional short fiber composite. The internal stress and matrix yielding behaviour were studied and stress-strain curve of composite was predicted. Matrix yielding was greatly influenced by the presence of non-axisymmetrical part of matrix. There was the region of no yielding near fiber end even for higher applied stress than matrix tensile strength, which was associated with high triaxial stress state in that region. Load transfer through fiber end surface should not be ignored for the short fiber composite, because matrix fracture can be largely affected by load transfer through the fiber end. The predicted tensile strength of composite was obtained by the fracture criterion, when maximum effective stress of the matrix reaches tensile strength of matrix, composite material are fractured. The predicted tensile strengths of composite with various volume fraction and aspect ratio were in good agreement with experimental results.

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A new Plasma Transferred Arc(PTA) hardfacing process has been developed to obtain an overlay weld metal having excellent wear resistance. Effect of Ti, Ni and Cu metal powders addition on the surface characteristics of Aluminum alloy 5083 has been investigated with PTA process. Optimum overlaying condtions under which bead appearance is superior and porosities in overlaid metal are lesser are determined. This paper describes the results of tests on the performance of the overlay weld metal. The results are as fallow: (1) The mean hardness of the overlay weld metal is adjustable from Hv200 to Hv600 by PTA process. (2) Concerning defect of overlaid metals, blow holes were seen in Ni and Ti addition. Residual gas contents in blow hole for overlaid metals were confirmed as mainly H₂and a little of N₂. (3) The welding performance of this process is improved by use of DCRP plasma arc, which performs cleaning action on the base metal and weld metal. These results is an increase in welding efficiency. (4) The results of wear tests show that the weld metal has high wear resistance by the PTA process.

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The effects of cold working and annealing temperature on the shape memory effect(SME) and tensile properties of Fe-30Mn-5Si-1Co alloy have been studied. Neel temperature(T_N) was decreased by the addition of 1wt% Co. After the specimens were cold rolled and annealed at various temperatures, the recovery of each specimens was measured. A maximum recovery was obtained at 600℃. Above 600℃, the SME decreased with the increase of annealing temperature due to the decrease of matrix strength by the recrystallization. Below 600℃, retained α` martensite and ε martensite diminished the SME. The M_d temperature decreased as the reduction ratio increased, which means that the γ→ε martensite transformation was suppressed. As a consequence, the SME decreased. The deformation mode changed from the usual slip to the stress-induced γ→εmartensite transformation as the deformation temperature lowered. And as the deformation temperature lowered, the SME became enhanced because of the increasing driving force of γ→εmartensite transformation.

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The structural relaxation process of amorphous F_(40)Ni_(40)B_(20), Fe_(60)Ni_(20)B_(20) and Fe_(38)Ni_(38)B_(24) alloys were investigated by means of DSC and Mo¨ssbauer measurements. Upon heating the samples annealed at the temperature below Tg, an excess endothermic reaction occur above Ta. The ΔC_(p.endo) evolves reversibly in a continuous manner with In(t_a). The change in the magnitude of ΔC_(p.endo) and ΔH_(endo) with Ta show clealy increased at about Ta=450∼550K. The activation energy of structural relaxation increases with Tm from 1.8eV at Tm=525K to 2.OeV at Tm=600K for Fe_(40)Ni_(40)B_(20) alloy. The first-neighbor iron atoms were changed and the average number of first-neighbor iron atoms increased during isothemal annealing with chemical short range order (CSRO).

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