• Journal of Welding and Joining
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• v.28 no.6
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• pp.40-44
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• 2010

Laser assisted machining (LAM) has been researched in order to machine the silicon nitride ceramics economically and effectively. LAM is an effective machining method by local heating of the cutting part to the softening temperature of the silicon nitride using laser beam. When silicon nitride ceramics is heated using a laser beam, the surface of silicon nitride ceramic is softened, oxidized and decomposed. And then surface hardness is decreased. Through machining in low viscosity and hardness conditions, silicon nitride was machined effectively and the life span of tool was increased. The plastic deformation was occurred due to softening of amorphous YSiAlON above $ 1,000^{\circ}C$. Transgranular fracture of ${\beta}-Si_3N_4$ was occurred when YSiAlON was not softened, but mostly intergranular fracture was occurred by the plastic deformation of softened YSiAlON.

• Journal of the Korean Ceramic Society
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• v.49 no.4
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• pp.337-341
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• 2012

Mechanochemical processing (MCP) involves several high-energy collisions of powder particles with the milling media and results in the increased reactivity/sinterability of powder. The present paper shows results of mechanochemical processing (MCP) of silicon nitride powder mixture with the relevant sintering additives. The effects of MCP were studied by structural changes of powder particles themselves as well as by the resulting sintering/densification ability. It has been found that MCP significantly enhances reactivity and sinterability of the resultant material: silicon nitride ceramics could be pressureless sintered at $1500^{\circ}C$. Nevertheless, a degree of a silicon nitride crystal lattice and powder particle destruction (amorphization) as detected by XRD studies, is limited by the specific threshold. If that value is crossed then particle's surface damage effects are prevailing thus severe evaporation overdominates mass transport at elevated temperature. It is discussed that the cross-solid interaction between particles of various chemical composition, triggered by many different factors during mechanochemical processing, including a short-range diffusion in silicon nitride particles after collisions with other types of particles plays more important role in enhanced reactivity of tested compositions than amorphization of the crystal lattice itself. Controlled deagglomeration of $Si_3N_4$ particles during the course of high-energy milling was also considered.

• Journal of the Korean Society for Precision Engineering
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• v.19 no.8
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• pp.141-146
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• 2002

This paper has studied to obtain the grinding characteristics and optimal grinding conditions of ceramic materials in the grinding with diamond wheel by design of experiments. The load on wheel by varying the feed rate was related with the surface roughness due to the minute destruction phenomenon of grains for the $Si_3\;N_4\;and\;ZrO_2$. The depth of cut is related with the surface roughness because the grinding is carried out by grain shedding process due to the brittle fracture phenomenon for the $A1_2\;O_3$. The major factors affecting the surface roughness and the optimum grinding conditions were obtained with minimum experiments using design of experiments.

• Proceedings of the Materials Research Society of Korea Conference
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• 2010.05a
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• pp.51.1-51.1
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• 2010

$Y_2O_3$ $Er_2O_3$ and $Nd_2O_3$ doped polycrystalline silicon nitride were prepared by hot pressed sintering at $1850^{\circ}C$ and their optical transmittance were investigated in visible and in infrared region. Mechanical and tribological properties were also investigated. Grain growth in silicon nitride was reduced with addition of $Y_2O_3$ and $Nd_2O_3$. 1 wt.% of each rare earth metal were sintered with 3 wt.% MgO, 9wt.% AlN and 87 wt.% of ${\alpha}-Si_3N_4$. Adding these rare earth metal oxides shows good mechanical properties as high strength and toughness and also shows low friction coefficient.

• Journal of Mechanical Science and Technology
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• v.16 no.9
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• pp.1093-1101
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• 2002

Continuous fiber ceramic composites (CFCCs) have advantages over monolithic ceramics : Silicon Nitride composites are not well used for application because of their low fracture toughness and fracture strength, but CFCCs exhibit increased toughness for damage tolerance, and relatively high stiffness in spite of low specific weight. Thus it is important to characterize the fracture resistance and properties of new CFCCs materials. Tensile and flexural tests were carried out for mechanical properties and the fracture resistance behavior of a SCS6 fiber reinforced Si$_3$N$_4$ matrix CFCC was evaluated. The results indicated that CFCC composite exhibit a rising R curve behavior in flexural test. The fracture toughness was about 4.8 MPa$.$m$\^$1/2 , which resulted in a higher value of the fracture toughness because of fiber bridging. Mechanical properties as like the elastic modulus, proportional limit and the ultimate strength in a flexural test are greater than those in a tensile test. Also a numerical modeling of failure process was accomplished for a flexural test. This numerical results provided a good simulation of the cumulative fracture process of the fiber and matrix in CFCCs.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.10a
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• pp.223-226
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• 2002

Appropriate design/manufacturing conditions, to give outstanding material properties to the $Si_3$$N_4$-BN and AIN-BN based composite materials, will be investigated using the experimental design methods. Ultra-precision machinability of the developed ceramics will be systematically studied in the viewpoint of microstructure and material properties. Also, finite element methods will be applied to define basic principles to significantly improve machinability and various properties. Basic experiments will be performed to develop optimum ultra-precision machining technologies for the developed ceramics. For ultra-precision lapping machining, need to develop a ultra-precision lapping system, suitable metal bonded diamond wheel, and appropriate condition of ultra-precision lapping machining.

• Proceedings of the KAIS Fall Conference
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• 2001.05a
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• pp.40-46
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• 2001

The thermal stability of $Al_2TiO_{5}$ ceramics was improved by formation of solid solution with MgO, such as $MgAl_2O_4$ spinel through electrofusion in an arc furnance, and by limitation of grain size and microcracks with $SiO_2$, $ZrO_2$ and ${\alpha}$-$Al_2O_3$. The low thermal expansion properties of $Al_2TiO_{5}$ composites show the thermal hysteresis curves due to the strong anisotropy of $Al_2TiO_{5}$. These phenomena are explained by the opening and closing of microcracks. The relation between thermal hysteresis, microstructures and sintering temperature were studied by dilatometry.

• Journal of Ocean Engineering and Technology
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• v.23 no.5
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• pp.32-38
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• 2009

Silicon nitride samples with aligned whisker seeds were prepared with different amounts of yttria and alumina as the sintering additives. Their sintering behaviors and the microstructural developments between $1850^{\circ}C$ and $2050^{\circ}C$ were examined. The sample with larger amount of the sintering additives showed faster densification and grain growth. Even though addition of the aligned whisker seeds slightly retarded densification of silicon nitride, it improved the flexural strength and the fracture toughness. Both the flexural strength and the fracture toughness of silicon nitride with the aligned whisker seeds were increased as the amount of the sintering additives was increased.

• Transactions on Electrical and Electronic Materials
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• v.12 no.3
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• pp.119-122
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• 2011

A series of mesoporous silicalites was synthesized using different compositions of tetraethylorthosilicate and methyltriethoxysilane (MTES) as the silica source. Cetyltrimethylammonium bromide was used as the organic template. Their detailed pore structures were investigated by transmission electron microscopy, X-ray diffraction, and N2 adsorption method. The thermal properties of these silicalites were studied by thermogravimetric analysis. The increased amount of MTES destroyed mesoporous channels and reduced pore sizes from 3.4 nm to 2.8 nm in calcined silicalites. The calcined silicalite transformed completely into an amorphous state at 30% MTES loading. Methyl pending groups of MTES hindered the structural ordering of ≡Si-O- frameworks, resulting in an amorphous structure. This was caused by the insufficient formation of supramolecular assembly with the organic template. No capillary condensation step was found in MS 7/3 silicalite. The other capillary condensation steps shifted toward the lower relative pressure with increasing MTES content, indicating the reduction of pore sizes.

• Journal of the Korean Ceramic Society
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• v.34 no.2
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• pp.157-162
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• 1997

Silicon Nitride samples with different microstructure were prepared by hot pressing and subsequent heat treatment under N2 gas pressure. The fracture toughness (KIC)of Si3N4 increased with the increase of grain size, but the bending strength of plain specimen($\sigma$F) decreased. The relation between fracture stress($\sigma$c) and equivalent crack length(ae) agreed well with the calculated values by process zone size failure criterion. A probabilistic failure assessment curve is proposed based on both statistical character of $\sigma$F and KIC.