• Proceedings of the Korean Society of Precision Engineering Conference
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• 1995.10a
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• pp.14-18
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• 1995

Although structural ceramics have excellent mectanical properties, it is very difficult to grind with high efficiency and high quality because of their high strength, hardness, and brittleness. Unfortunately machined ceramics often contain surface damages such as micro fracture and crack on account of brittle fracture. Therefore, is is important to minimize the brittle fracture. The present paper examines grinding characteristics of representative structural ceramics,such as Al /sab 2/O /sab 3/, SiC, Si /sab 3/ N /sab 4/. Effects of grinding variables including table speed and depth of cut on the grinding performance were investigated. Experimental results show that the surface quality is related to the specific grindings energy. The higher specific energy results in the better surface quality.

• Proceedings of the Materials Research Society of Korea Conference
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• 1994.11a
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• pp.1-1
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• 1994

;The electronic state calculations for various types of ceramic materials have beell performed by the use of $DV-X\;{\alpha}$ cluster method. The molecular orbital levels and wave functions for model clusters have been computed to study the electronic properties ami chemical bonding of the ceramics. For ${\beta}-sialon(Si_{6-z}Al_zO_zN_{8-z})$ which is a high temperature structural material based on ${\beta}-Si_3N_4$, we have made model cluster calculations to estimate the strength of chemical bonding between atoms by the Mulliken population analysis. It is found that the covalent bonding between Si and N atoms is very strong in pure ${\beta}-Si_3N_4$, but the covalency around solute atom is considerably weakened when Si atom is substituted by AI. This tendency is enhanced by an additional substitution of oxygen atom for N. The result calculated can well explain the experimental data of changes in mechanical properties such as the reductions of Young's modulus and Vickers hardness with increment of z-value in ${\beta}-sialon$. Various model clusters for transition metal oxides which show many interesting physical and chemical properties have also been calculated. High-valent perovskite-type iron oxides EMFe0_3E(M=Ca and Sr) possess very interesting magnetic and chemical properties. In these oxides, iron exists as $Fe^{4+}$ state, but the experimental measurement of Mossba~er effect suggests that disproportionation $2Fe^{4+}=Fe^{3+}+Fe^{5+}$ takes place for $CaFe0_3$ at low temperatures. The model cluster calculations for these compounds indicated the existence of considerably strong covalent bonding of Fe-O. The calculations of hyperfine interaction at iron neucleus show very good agreement with the experimental Mossbauer measurements. The result calculated also implies that the disproportionation reaction is strongly possible by assuming the quenching of breathing phonon mode at low temperatures.tures.

• Journal of the Korean Ceramic Society
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• v.47 no.3
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• pp.205-209
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• 2010

In the last (fourth) section, the discussion will entail a silicon-nitride/silicon-carbide nanocomposite, produced by pyrolysis of liquid polymer precursors, demonstrating one of the lowest creep rates reported so far in ceramics at the comparable temperature of $1400^{\circ}C$. This was first achieved by avoiding the oxynitride glass phase at the intergrain boundaries. One important factor in the processing of these nanocomposites was the use of the electrical field assisted sintering method.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1994.10a
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• pp.156-160
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• 1994

Use of engineering ceramics has been increasing due to the outstanding physical and chemical properties. Conventional machining processes, however, are not applicable due to their hardness and brittleness. Laser cutting is a promising alternative for these ceramics. In this study, experimental data of CO $_{2}$ laser cutting of $Al_{2}$ $O_{3}$ and Si $_{3}$ N $_{4}$ are obtained to give a guide in the industry. Results of $Al_{2}$ $O_{3}$ cutting showed extreme weakness to thermal crack and it was found that pulsed beam has to be used for thick $Al_{2}$ $O_{3}$ specimen. Si $_{3}$ N $_{4}$ showed good results for both CW and pulsed beams. Using pulsed beam resulted narrower kerf width with increased surface roughness a nd reduced cutting speed. It was also found that a parameter call path energy is useful for representing minimum threshold value for possible cutting range with pulsed beam.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.325-326
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• 2006

Using $6wt%Y_2O_3-2wt%Al_2O_3$ as sintering additives and Si as a raw powder, the continuously porous in-situ $Si_2N_2O-Si_3N_4$ bodies were fabricated by multi-pass extrusion process and their microstructures were investigated depending on the addition of carbon (0-9wt%) in the mixture powder. The introduction of $Si_2N_2O$ fibers observed in the unidirectional continuous pores as well as in the pore-frame regions of the nitrided bodies can be an effective method in increasing the filtration efficiency. In the case of no carbon addition, the network type $Si_2N_2O$ fibers with high aspect ratio appeared in the continuous pores with diameters of 150-200 nm. However, in the case of 9wt% C addition, the fibers were found without any network type and had diameters of 200-250 nm.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1997.10a
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• pp.187-187
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• 1997

Silicon nitride is the most excellent materials among structural ceramics. It has been reported that fracture toughness was improved with adding second phase particles, whisker, fiber etc. However, containing of second phase particles enhanced fracture toughness, however flexural strength was degraded. As adding nanosize SiC particles into silicon nitride, the physical properties of fluxural strength, fracture toughness, the modulus of elasticity. In this study, 2wt% $Al_2$O$_3$ and 4 wt% $Y_2$O$_3$ were added into UBE E-10 and 0, 10, 20, 30, 40, 50 vol% nano-SiC powder (Sumitomo T1 powder) were added, respectively. It is hot pressed at 185$0^{\circ}C$ for 1 hour. Most of structural ceramics for engineering application are wear resistance. In this study, wear behaviors (in water) of silicon nitride with varying the amount of nano-size silicon carbide were investigated, and was compared to physical properties. Simultaneously wear mechanism will be found out.

• The Korean Journal of Ceramics
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• v.5 no.4
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• pp.324-330
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• 1999

$Si_3N_4$-$TiB_2$ with 2 wt% $Al_2O_3$ and 4 wt% $Y_2O_3$ additives was hot pressed in a flowing $N_2$ environment with varying $TiB_2$ content from 10 to 50 vol%. Variations of mechanical (hardness, fracture toughness, and flexual strength), and tribological properties as a function of $TiB_2$ content were investigated. As the content of $TiB_2$ increased, relative density decreased due to the chemical reaction of $TiB_2$in $N_2$ environment. The reduction of density causes mechanical properties to be degraded with an increase of $TiB_2$ in $Si_3N_4$. Tribological properties were dependent of microstructure as well as mechanical properties, however, they were degraded strongly by the chemical reaction of $Si_3N_4$-$TiB_2$ during hot pressing in $N_2$ environment. SEM and TEM observations, and X-ray diffraction analysis that the chemical reaction products at the interface are TiCN, Si, and $SiO_2$. Also, the comparison of XRD patterns of the $Si_3N_4$-40 vol% $TiB_2$ composites hot pressed at $1,750^{\circ}C$ for 1 hour between in $N_2$ and in Ar gas was made. The XRD peaks of Si and $SiO_2$ were not found in Ar, but still a weak peak of TiCN was presented.

• Korean Journal of Materials Research
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• v.7 no.4
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• pp.295-302
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• 1997

금속분말 Fe와 Si에 KNO$_{3}$(Fe+Si)무게비=0.2로 점화촉매 KNO$_{3}$를 혼합하고 50MPa로 성형한 후 점화시키는 비기체연소합성(SHS; Self propagating High temperature Synthesis)법으로 출발 분말을 얻었다. 점화분위기를 공기 및 Ar으로 한 경우 XRD결과에서 특별한 차이가 없었고 두 경우 모두 SiO$_{2}$피크가 검출되었다. 합성된 분말을 성형한 후 119$0^{\circ}C$환원분위기에서 소결하고 포석온도이하에서 열처리하여 반도성 FeSi$_{2}$가 주상인 Fe-Si계 열전재료를 제조하였다. Fe/Si무게비=46/54,44/56 및 42/58시편의 제벡계수는 Si함량이 증가할수록 증가하였다. 점화후의 세척처리를 2단계로 하는 경우 제벡계수의 부호가 변화하여 p-type에서 n-type으로 변화하며 소결밀도가 크게 상승하였다. 조성에 관계없이 공통적으로 발견되는 SiO$_{2}$는 점화시의 분위기보다는 점화촉매에 포함된 K성분이 소결 및 열처리시 산화제로 작용하여 형성되는 것이 확인되었다.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.12 no.4
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• pp.9-16
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• 2003

In this study, the decision of dressing time for diamond wheel was analyzed by observing with acoustic emission signals and surface roughness, and also obtained the machining characteristics by weibull distribution plot for the values of bending strength. From the experimental study, it was possible to predict the time of re-dressing for the diamond grinding wheel with the analysis of acoustic emission signals and surface roughness values, and following conclusions were obtained. The root-mem-square values of acoustic emission signals were obtained low as the increased of table speed for different abrasive grain size. This is caused by the lack of grinding power which is not able to get rid of all real grinding mass of depth as the table speed is increased. The values of bending strength for ground $Si_3 N_4$ specimens were decreased for gain size of #400 than that of #60, but it was found that the surface roughness values for gain size of #60 were better than that of #400. As compared the shape parameter of weibull distribution plot for the values of bending strength, it was found that the reliability of bending strength for grain size of #60 increased than that of #400.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.10
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• pp.2597-2605
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• 1994

The wear behavior of two types of $Si_3N_4$ exposed to high and low humidity was examined at various sliding speeds, using bearing steel as disk material under pin-on-disk type sliding conditions. Higher wear rates were obtained at a high humidity than at a low humidity. As the sliding speed was increased, the wear rates were decreased and the effect of humidity on the wear rates of $Si_3N_4$ was reduced. The result that the $Si_3N_4$ pin showed higher wear rate under the high humidity condition was explained by the decrease in microhardness of $Si_3N_4$ due to the chemisorbed moisture on the pin and plowing action by the hard particles of $Fe_2O_3$ from the disk. An increase in the sliding speed is supposed to reduce the effect of humidity on the wear rate of $Si_3N_4$ by raising the average temperature of the disk surface and the local temperature at pin-disk contact point.