• The Korean Journal of Ceramics
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• 제5권3호
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• pp.211-216
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• 1999

Four different $\beta-Si_3N_4$ ceramics with silicon oxynitrides $[Y_10(SiO_4)_6N_2, Yb_4Si_2N_2O_7, Er_2Si_3N_4O_3, \;and La_{10}(SiO_4)_6N_2$, respectivley] as secondary phases have been fabricated by hot-pressing the $Si_3N_4-Re_4Si_2N_2O_7$ (Re=Y, Yb, Er, and La) compositions at $1820^{\circ}C$ for 2h under a pressure of 25 MPa. The high temperature strength and oxidation behavior of the hot-pressed ceramics were characterized and compared with those of the ceramics fabricated from $Si_3N_4-Si_2O_7$ compositions. The $Si_3N_4-Re_4Si_2N_2O_7$composition investigated herein showed comparable high temperature strength to those from $Si_3N_4-Re_2Si_2O_7$ compositions. Si3N4 ceramics from a $Si_3N_4-Y_4Si_2N_2O_7$ composition showed the highest strength of 877 MPa at $1200^{\circ}C$ among the compositions. All $Si_3N_4$ ceramics investigated herein showed a parabolic weight gain with oxidation time at $1400^{\circ}C$ and the oxidation products of the ceramics were $SiO_2$ and $Re_2Si_2O_7$. The $Si_3N_4-Re_4Si_2N_2O_7$ compositions showed inferior oxidation resistance to those from $Si_3n_4-Re_2Si_2O_7$ compositions, owing to the incompatibility of the secondary crystalline phases of those ceramics with $SiO_2$, the oxidation product of Si3N4.Si3N4 ceramics from a $Si_3N_4-Er_4Si_2N_2O_7$ composition showed the best oxidation resistance of 0.375mg/$\textrm{cm}^2$ after oxidation at $1400^{\circ}C$ for 102 h in air among the compositions.

• The Korean Journal of Ceramics
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• 제1권2호
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• pp.101-105
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• 1995

Electrical discharge machinable $Si_3N_4$ was fabricated with the addtion of 20-60 vol% TiN by gas pressure sintering. Their sinterability, microstructure, mechanical and electrical properties were characterized as a function of the TiN content. The addition of TiN up to 20 vol% increased the flexural strength and fracture toughness as compared with those of the monolithic Si3N4. For the TiN content higher than 40 vol%, the electrical resistivity was lower than $1062\Omega$.cm. The $Si_3N_4$ with the addition of 40 vol% of TiN appears to have the optimum considerable sinterability, mechanical and electrical properties, and machinability. A microstructural analysis showed that the enhanced toughening was due to the crack deflection.

• 한국세라믹학회지
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• 제36권4호
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• pp.432-443
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• 1999

The self-toughened Si3N4 ceramics where needle-like coarse ${\beta}$-Si3N4 grains were dispersed within fine-grain-ed matrix were prepared via hot-prssing at 1730$^{\circ}C$ for 2 h using 5 vol% ${\beta}$-Si3N4 whiskers as a seed. In this study the microstructures and mechanical properties of self-toughened Si3N4 ceramics were investigated. The flexural strength of self-toughened Si3N4 ceramics was increased from 600-800 MPa of the Si3N4 monolith to 830-1025 MPa. The KIC was also increased from 4.0-5.0MPa$.$m1/2 of the Si3N4 monolith to 5.8-6.5MPa$.$m1/2$.$The needle-like coarse Si3N4 grains in self-toughened ceramics were considered to induce various toughening mechanisms including the crack deflection pull-out and bridging and to contribute to KIC improvement. In ad-dition to toughening mechanisms the KIC improvement was considered to be partially indebted also to the orien-tation of large ${\beta}$-Si3N4 grains and to the promoting effect of ${\beta}$-Si3N4 whiskers on the ${\alpha}$ to ${\beta}$ transtion.

• 한국세라믹학회지
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• 제30권11호
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• pp.941-948
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• 1993

The powder mixture of Si3N4-AlN-Y2O3, Si3N4-AlN-CeO2 and Si3N4-AlN-Y2O3-CeO2 system was hot-pressed at 175$0^{\circ}C$ for 2h in N2 to prepare $\alpha$-Sialon ceramics. The mechanical property and oxidation behaviour of the prepared $\alpha$-Sialon ceramics were investigated. At 120$0^{\circ}C$, oxidation resistance was best for the Y2O3 added $\alpha$-Sialon ceramics and oxidation rate increased when the amount of CeO2 increased. But when the mixture of Y2O3 and CeO2 added $\alpha$-Sialon ceramics showed a good oxidation resistance. Fracture toughness of (Y2O3＋CeO2) added $\alpha$-Sialon ceramics was higher than Y2O3 added $\alpha$-Sialon ceramics.

• 한국세라믹학회지
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• 제29권5호
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• pp.410-418
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• 1992

Si3N4, AlN and Y2O3 powder mixtures of the Y0.1(Si, Al)12(N, O)16 composition were hot-pressed at 1900℃ for 0 to 60 min under 30 MPa in order to fabricate the partially-stabilized α-Sialon ceramics (X=0.1). Room and high temperature flexural strengths of the specimens were compared with those of Si3N4-5 wt%Y2O3, Si3N4-5 wt%Y2O3-2 wt%Al2O3, and β-Sialon (Z=0.5) ceramics. The flexural strength of the α-Sialon ceramics which was hot-pressed for 15 min showed the highest value of 820 MPa at 1400℃ that is relatively higher temperature. It is guessed that a little amount of glassy phase existed in grain boundary because Y2O3 and AlN components were incoperated in Si3N4 grains, or transient liquid phase sintering, and microstructure with the smaller grain size and the interlocked grains of α'-and β-Si3N4 was obtained by the hot-pressing at high temperature of 1900℃ for the short time (15 min).

• 한국정밀공학회지
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• 제21권4호
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• pp.48-56
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• 2004

Ceramics are very difficult-to-cut materials because of its high strength and hardness. Their machining mechanism is characterized by cracking and brittle fracture. In this paper, to give good machinability to the ceramics, h-BN powders are added to $Si_3N_4$, by volume of 20, 25 and 30%. And the machinability of the produced ceramics is tested using micro drilling system. Through required experimental works, it is shown that the micro drilling machinability is varied along with the volumetric percentage of h-BN powders. Also, it is verified that the obtained results can be used to develop new machinable ceramics of good material properties and machinability.

• 한국재료학회지
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• 제17권7호
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• pp.366-370
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• 2007

Silicon nitride ($Si_3N_4$) ceramics have been considered for various components of nuclear power plants such as the mechanical seal of a reactor coolant pump (RCP), the guide roller for a control rod drive mechanism (CRDM), and a seal support, etc. Corrosion behavior of $Si_3N_4$ ceramics in a high-temperature and high-pressure water must be elucidated before they can be considered as components for nuclear power plants. In this study, the corrosion behaviors of $Si_3N_4$ ceramics containing MgO and $Al_2O_3$ as sintering aids were investigated at a hydrothermal condition ($300^{\circ}C$, 9.0 MPa) in pure water and 35 ppm LiOH solution. The corrosion reactions were controlled by a diffusion of the reactive species and/or products through the corroded layer. The grain-boundary phase was preferentially corroded in pure water whereas the $Si_3N_4$ grain seemed to be corroded at a similar rate to the grain-boundary phase in LiOH solution. Flexural strengths of the $Si_3N_4$ ceramics were significantly degraded due to the corrosion reaction. Results of this study imply that a variation of the sintering aids and/or a control (e.g., crystallization) of the grain-boundary phase are necessary to increase the corrosion resistance of $Si_3N_4$ ceramics in a high-temperature water.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
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• pp.857-858
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• 2006

In order to clarify the wear resistance as cutting tools, the effect of oxygen addition on oxidation behavior of the ${\beta}-Si_3N_4$ ceramics with 5 mass% $Y_2O_3$ and 2 or 4 mass% $Al_2O_3$ was investigated by performing oxidation tests in air at $1300^{\circ}$ to $1400^{\circ}C$ and cutting performance tests. From test results, we could conclude that the mechanical properties of ${\beta}-Si_3N_4$ ceramics depending on oxygen introduction are much effective on cutting performance improvements of ${\beta}-Si_3N_4$ ceramics.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2006년 창립20주년기념 정기학술대회 및 국제워크샵
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• pp.166-168
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• 2006

$Si_3N_4/SiC$ composite ceramics was sintered in order to investigate their bending strength behavior after crack healing. $Y_2O_$ and $TiO_2$ power was added as sintering additives to enhance it's sintering property. A three-point bending specimen was cut out from sintered plates. About $100\;{\mu}m$ semi-circular surface cracks were made on the center of the tension surface of the three-point bending specimen using Vickers indenter. After the crack-healing processing from $500^{\circ}C$ to $1300^{\circ}C$, for 1 h, in air, the bending strength behavior of these crack-healed specimen coated with $SiO_2$ colloidal were determined systematically at room temperature. $Si_3N_4/SiC$ ceramics using additive powder ($Y_2O_3+TiO_2$) was superior to that of additive powder $Y_2O_3$. The additive powder $TiO_2$ exerted influence at growth of $Si_3N_4$. The optimum crack healing conditions coated $SiO_2$ colloidal were $1000^{\circ}C$ at $Si_3N_4/SiC$ using additive powder ($Y_2O_3+TiO_2$), and $1300^{\circ}C$ at $Si_3N_4/SiC$ using additive powder $Y_2O_3$.

• 한국분말재료학회지
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• 제24권6호
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• pp.431-436
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• 2017

SiAlON-based ceramics are some of the most typical oxynitride ceramic materials, which can be used as cutting tools for heat-resistant super-alloys (HRSA). SiAlON can be fabricated by using gas-pressure reactive sintering from the raw materials, nitrides and oxides such as $Si_3N_4$, AlN, $Al_2O_3$, and $Yb_2O_3$. In this study, we fabricate $Yb_{m/3}Si_{12-(m+n)}Al_{m+n}O_nN_{16-n}$ (m=0.3, n=1.9, 2.3, 2.7) ceramics by using gas-pressure sintering at different sintering temperatures. Then, the densification behavior, phase formation, microstructure, and hardness of the sintered specimens are characterized. We obtain a fully densified specimen with ${\beta}$-SiAlON after gas-pressure sintering at $1820^{\circ}C$ for 90 min. under 10 atm $N_2$ pressure. These SiAlON ceramic materials exhibited hardness values of ~92.9 HRA. The potential of these SiAlON ceramics for cutting tool application is also discussed.