발간논문

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13%Cr martensitic stainless steel was alloyed with 2∼6%Ni and 2%Mo, and impact properties and hardness have been tested after austenitizing and tempering treatment. Ni addition decreased austenitization (A_s, A_f) and martensitic transformation temperature(M_s). The steels with Ni showed high hardness after tempering treatment at the temperature up to 500℃, and decreased drastically after tempering at 600℃, which was caused by the decomposition of martensitic structure and the decrease of high dislocation density. After tempering at 700℃ the hardness of steels with 4-6% Ni content increased again, because martensite phase was transformed from austenite phase, which was formed during tempering. Ni addition enhanced impact energy of tempered steels. The steels with Ni content more than 4% did not show any tempered embrittlement, while the steels with 2%Ni showed a remarkable tempered embrittlement phenomenon after tempering at 500℃.

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The microstructure of eutectic MnBi/Bi alloy was investigated after unidirectional solidification under centrifuge process in Bridgman-Stockbarger furnace. Microstructure of the crystals with growth velocity 100 ㎜/h, 25 ㎜/h showed cellular structure and evenly dispersed second phase morphology of MnBi intermetallic compound. The second phase MnBi rod size was varied with acceleration in all growth condition. Numerical analysis was produced to expect fluid flow in rotating system using flow 3d. According to numerical analysis, the fluid flow increased with centrifugal acceleration under constant temperature condition and abrupt increase was expected near centrifugal force is 5G (G = 9.8 ｍ/s: territorial acceleration).

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Semi-solid metal was obtained by semi-solid continuous casting apparatus which consisted of melting furnace, cooling plate and continuous casting part. Primary α size, aspect ratio and hardness of an A356 Al alloy have been investigated. Crystal seeds of the primary α were nucleated on a cooling plate. The seed of primary crystals existing in the molten alloy grew granular grain. The minimum size of primary α at pouring temperature 700℃ were 65 ㎛ in BN coated cooling plate, 53 ㎛ in BN uncoated cooling plate. BN coated cooling plate prevented molten metal from becoming hard. The primary α size increases from 53 ㎛ to 61 ㎛ as casting temperature increase from 630℃ to 650℃. The aspect ratio was increased with increasing primary α size. To investigate the effect of addition of Al-5Ti-1B master alloy is added to A356 Al alloy. The primary α size and aspect ratio are about 42 ㎛ and 1.3 at the content of additional 0.5% grain refiner. The solid fraction of the α phase are about 60∼70%.

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To improve the creep resistance of the anode for molten carbonate fuel cell, the fabrication processes of Ni anode with different Al contents were investigated, where four types of sintering condition have been examined at different temperatures and gas atmospheres. The effects of such different sintering conditions on the secondary phases, and the morphologies of secondary phase of the anode were examined. If Al is added over the solid solution limit, Ni₃Al or Al₂O₃ are formed as a secondary phase according to the sintering conditions and degrees of creep strain and conductivity are changed as results. The creep test and porosimeter analysis certificated that the Ni₃Al intermetallic compound precipitation and Al₂O₃ dispersion hardened structure had good creep resistances with suitable pore size distributions for anode, which resulted in the stable mechanical performances.

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Composite films have been one of approaches for new hard coating development and provided properties which are unachievable in uniform materials. Structurally, composite films tended to have dense and fine granular structure because column propagation was periodically interrupted by the nucleation of another phase. In this study, WC-TiN composite films were deposited on cemented carbide substrates by cathodic arc ion plating process. The chemical compositions and crystal structures were evaluated by energy dispersive spectroscopy(EDS) and X-ray diffractometry(XRD), respectively. Film thickness and fracture morphologies were investigated by using scanning electron microscopy(SEM). The mechanical properties such as film adhension and microhardness were evaluated by scratch test, microknoop hardness test and nano indentation hardness test. The microhardness of the WC-TiN composite film on cemented carbide was measured to be 4000(㎏/㎟) which is 1.5 times higher than that of the WC and TiN single layer. The critical load(Lc) of WC-Ti/WC-TiN multi-layer film was measured to be 48.5N in scratch test.

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Magnetic field influences the electrodeposition of ferromagnetic metals into the pores of anodic film. In this study, the magnetic properties and morphology of the anodic film deposited with Fe or Co were examined, and a particular emphasis was placed on the effect of magnetic field on the crystallographic orientation of the deposited crystals. In the case of Fe-deposited anodic film the intrinsic coercivity was not influenced significantly by the magnetic field, and no preferred orientation of the deposited crystals was observed regardless of the application of magnetic field. On the other hand, the coercivity and the crystallographic orientation in Co-deposited film were significantly influenced by magnetic field. Co crystals in the film deposited even in absence of magnetic field showed preferred orientation to some extent, in which c-axis of the deposited Co crystals was more or less parallel to the electric field. This preferred orientation was further enhanced by the application of magnetic field, and his was closely related the enhancement of coercivity.

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MoSi₂ is a very promising high temperature structure material for its excellent oxidation resistance and mechanical properties at high temperatures with relatively low weight. For the extensive use of MoSi₂, however, problems such as pesting, disintegration of MoSi₂ during oxidation, should be taken care of. The initial step of posting in MoSi₂ is reported to occur by a simultaneous oxidation of Mo and Si, with some variation on the oxidation temperature. To investigate the posting in MoSi₂, this study focused on the simultaneous oxidation temperature of Mo and Si and the nucleation and growth of oxides formed at 593∼793 K. In this study, MoSi₂ was produced by spark plasma sintering(SPS) and then oxidized at 593∼1093K. The simultaneous oxidation of Mo and Si took place below 893 K while selective oxidation of Si occurred above 993 K. The weight gain of MoSi₂ was the highest at about 693 K. SEM analyses of the specimens oxidated at 693 K showed a lot of MoO₃, most of which were believed to be preicipitated at or around defects in matrix including SiO₂ particles.

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Multi-phase composite coating which provides mechanical properties unachievable in uniform materials is a promising technique for new hard coating development. Structural1y, multi-phase composite films tend to be dense and fine granular structure because column propagation is periodically interrupted by the nucleation of another phase. Recently, we have synthesized WC-TiN multi-phase films by cathodic arc process. The microhardness of the WC-TiN films on cemented carbide were measured to be up to 4,000 ㎏/㎟ and these values were 1.5 times as hard as that of the WC and TiN single layer. Arc spot movement on WC cathode, however, is so slow that explosive particle emissions occur, resulting in lots of macro-particles and damages on the coating surface. In this study, super-hard WC-(TiAl)N coatings were deposited by arc-magnetron hybrid process and content of WC in the films were varied with power density of WC magnetron source. Hybrid process allowed compositionally gradient WC-(TiAl)N multi-phase coatings to be deposited with low macro-particles and good adhesion. It is noted that compositionally gradient WC-(TiAl)N coatings show super hardness of 5,000 ㎏/㎟. The present study introduces the hybrid process parameters involved and the formation behavior of WC gradient WC-(TiAl)N coatings were investigated by SEM, AES and XRD.

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This investigation was conducted to clarify the effects of process variables such as spraying distance and fuel flow rate on the formation of the new amorphous coating using Zr_(65)Al_(7.5)Ni_(10)Cu_(17.5) alloy by a HVOF(High Velocity Oxygen-Fuel) spraying process. Powder used for spraying was prepared by vacuum gas atomization and centrifugal-mill. HVOF spraying experiments were carried out using a Tafa JP-5000 spraying gun. The amorphous content was measured up to about 65% depending on the spraying parameters. The amorphous fraction is decreased with increasing the spray distance. Microstructural observations and XRD analysis reveal that the change of amorphous fraction is attributed to the degree of solidification of droplets and the oxide (ZrO₂) formation in coatings. The higher fuel flow rate resulted in the decrease of amorphous fraction, and this decreased amorphous fraction was caused by the increase of oxide fraction. Therefore, flame temperature and spray distance that can control the oxide formation in coating and undercooling effects of droplets are the most crucial factors for the formation of amorphous coating using this alloy system.

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We proposed a rapid thermal annealing process with Mo heat absorption layer for the purpose of an enhancement of Metal-Induced Lateral Crystallization(MILC) rate of amorphous silicon (a-Si) thin films on glass substrates. MILC rate was enhanced by using Mo absorption layer with relatively low lamp power than without Mo layer. With Mo layer, amorphous silicon was laterally crystallized about 20 ㎛ only 1 minute at 620℃. This is because Mo film can absorb the light more effectively than a-Si thin film and can be higher temperature about 290℃ than that of a-Si thin film. Enhancement of MILC rate by Mo layer was found to be more effective as thickness of Mo increase, because the thicker Mo layer can absorb the more light. MILC rate, however, decreased when insulating layer(SiO₂) between Mo and a-Si layer was inserted.

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