발간논문

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Dry sliding wear behavior of gas nitrided and nitrocarburized Cr-Mo steel was investigated using ring-on-square arrangement. Nitrocarburized specimens with compact compound layer are showing better wear performance than nitrided specimens with precracked compound layer for all experimental conditions. Wear performance of brittle compound layer are inferior to that of diffusion zone. The principal wear mechanism for ring-on-square arrangement is considered to be adhesion and hardness increase was observed in diffusion zone after wear testing. Fracture of compound layer is increased as the applied load is increased, thus wear resistance is decreased. Wear resistance of nitrocarburized specimens with compact compound layer that is showing good wear performance at lower applied load, is relatively decreased as the applied load is increased.

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The effects of Cu on the magnetic properties and crystallization behavior of Fe_(84-x)B_9Nb_7Cu_x(x=0.5∼2.0 (at%) alloys, prepared by a single roll melt-spinning method, were investigated. The soft magnetic properties were found to improve significantly by the addition of Cu, which played the role of reducing the grain size from about 10 ㎚ for Cu-free ternary alloys down to 6∼7㎚. Particularly, the effective permeability of 25,000, dc coercive force of 1.83 A/m and the core loss of 60 W/㎏ at 100 ㎑ -0.2 T were obtained for the alloy with Cu = 1 at%.

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To develop inexpensive bearing steel with improved resistance to rolling contact fatigue, the influence of applied load on rolling contact fatigue of C-Mn induction-hardened bearing steel has been investigated under the axial load conditions of 310㎏f∼605㎏f(3040N∼5933N). Fatigue tests were conducted at a shaft speed of 1,700 rpm with circulating lubricant through filtered-pump feed system. It was found that microhardness in the subsurface below the raceway of rolling contact fatigued specimens was increased in comparison with that of induction-hardened layers. As the applied load during rolling contact increased from 310㎏f to 605 ㎏f, the depth of microhardness-increased region was increased from about 300㎛ to 500㎛ from surface, respectively. This resulted from both strain hardening and transformation induced plasticity of retained austenite. It was also found that increased amount of retained austenite decomposed during rolling contact fatigue resulted in improved resistance to rolling contact fatigue under the different loaded conditions in the range of 310㎏f∼605㎏f. (Received July 19, 1994)

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Effect of substrate pretreatment materials on diamond nucleation density on Si substrate was investigated. The powder used for substrate pretreatment were diamond, c-BN, SiC and Al₂O₃ whose size were about Sum. The substrate was vibrated ultrasonically being immersed in powder-acetone mixture solution. The diamond was deposited on the substrate by hot filament CVD method. With the increase pf pretreatment time, the nucleation density of diamond increased and saturated at values which decreased in the order of diamond, c-BN, SiC and Al₂O₃. The density of residue of pretreatment powder was measured by AFM and compared with the nucleation density. The nucleation density of diamond varied as the same tendency with the residue density of pretreatment powder. However the ratio of diamond nuclei to the residue decreased rapidly in the order of diamond, c-BN, SiC, Al₂O₃. This was related to the similarity of lattice between the diamond and pretreatment material. As the lattice structure and size of pretreatment powder deviated from diamond, the nucleation appeared more difficult. The possible effect of surface roughness and defect near the surface on the nucleation was also discussed.

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Tensile and wear-resistance properties were studied on SiC particle reinforced Al-12Si eutectic alloy(AC8A) composites. The Al-Si alloy was produced by gas atomization process and the composites were fabricated by powder metallurgy route. On hot extrusion process, at an extrusion ratio of 25 : 1 and at 500℃, the Si precipitates and SiC particles were not fractured. During tensile tests the Si precipitates which were Larger than 4㎛ were fractured and the degree of fracture of SiC particles was small. As the Vol.% of SiC increased the fracture sources of the composites were varied from internal cracks of Si precipitates or decohesion of interfaces between SiC and matrix to separation of SiC particles within agglomeration regions. After thermal expousure the width of wear was increased with increasing exposure temperature and time. The wear mode was varied from abrasive wear to delamination wear with increasing SiC volume fraction.

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The formation of cladding induced microstructures was studied in the explosively cladded copper/copper sheets. The variation of the distance between the flyer and base plates during the cladding provided different morphologies of bonded interface in cladded Cu/Cu sheets. The fine equiaxed cells having random orientations were found to form at the bonded interface regions. The development of these cells was attributed to the rapid solidification of the thin melt layer immediately after the cladding. The microbands and the mechanical twins were observed in the deformed grains of the wave zone, where the grains suffered a large plastic deformation. The twinning was one of the dominant deformation mechanisms in the grains of the shocked zone. It is therefore suggested that the hardening rate under an extremly high strain rate is higher than the softening rate to raise the stress level over the critical shear stress for twinning.

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On the basis of microscopic observations of the fracture process, the material is modelled as a homogeneous binder phase containing pre-existing discontinuous cracks. Application of linear elastic fracture mechanics gives K_(IC)=[α/βEσ_Yλ_c/d+λ_cλ_c]^½ where α and β are constants, E and σ_Y are the elastic modulus and in situ yield strength, respectively, of the binder phase, λ_c is the binder true mean free path, and d is the mean intercept length of the carbide particles. Fair agreement is shown with published fracture toughness data.

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