방전 플라즈마 소결법에 의한 다공성 알루미나 세라믹스의 제조

신현철,조원승,신승용,김준규,Shin, Hyun-Cheol,Cho, Won-Seung,Shin, Seung-Yong,Kim, Jun-Gyu 한국세라믹학회 2002 한국세라믹학회지 Vol.39 No.12

고강도의 다공성 알루미나 세라믹을 제조하기 위하여, 기공 전구체로서 흑연을 첨가한 후, 방전 플라즈마 소결시의 제조조건을 체계적으로 조사하였다. 제조한 다공성 알루미나에 대하여, 기공률과 기공분포 및 압축강도 등을 조사하였다. 알루미나에 흑연을 10∼50 vol%까지 첨가하여 방전 플라즈마 소결법으로 1차 소결체를 제조한 후, 공기 중 흑연을 열분해시켜 최종적으로 다공성 알루미나를 제조하였다. 그 결과, 소결온도 1000${\circ}C$, 유지시간 3 min, 압력 30 MPa, 승온속도 80${\circ}C$/min, 펄스방법 12:2의 조건에서 높은 기공률의 다공성 소결체를 얻을 수 있었다. 일례로, 10∼30 vol%의 흑연을 첨가하여 제조한 다공성 알루미나의 경우, 약 50∼57%의 높은 기공률을 나타내었다. 또한, 흑연의 함량이 증가함에 따라 개기공률은 지속적으로 증가하였다. 이러한 기공특성과 흑연함량의 상호관계는 클러스터의 수와 크기에 의존하는 퍼콜레이션 모델에 의해 설명될 수 있다. 한편, 10∼30 vol%의 흑연을 첨가하여 제조한 다공성 알루미나의 경우, 55∼200 MPa의 높은 강도값을 나타내었다. 이와 같은 강도의 향상은 펄스전원에 의한 분말 입자간 방전 및 자기발열작용의 영향으로 생각된다. In order to develope the porous alumina ceramics with high strength, the pore characteristics and compressive strength were investigated in terms of relation to the conditions of spark-plasma sintering and the contents of graphite as a pore precursor. Porous alumina bodies were successfully prepared by spark-plasma sintering and burning out graphite in air. High porous bodies were fabricated by sintering at 1000${\circ}C$ for 3 min under a pressure of 30 MPa, heating rate of 80${\circ}C$/min and on-off pulse type of 12:2. For example, alumina bodies prepared by the addition of 10∼30 vol% graphite showed high porosity of 50∼57%. Also, the open porosity increased with graphite content. The relationship between pore characteristics and graphite contents could be explained by percolation model depending on cluster number and size. Porous alumina bodies prepared by the addition of 10∼30 vol% graphite showed the high compressive strength of 55∼200 MPa. This great improvement in strength was considered to be mainly due to the spark-plasma discharges and the self-heating action between particles.

카본블랙을 첨가하여 제조한 다공성 BaTiO₃계 세라믹스의 미세구조 및 PTCR 특성 변화

이기주,당동욱,조원승,Lee, Ki-Ju,Tang, Dongxu,Cho, Won-Seung 한국분말야금학회 2011 한국분말재료학회지 (KPMI) Vol.18 No.1

As a pore precursor, carbon black with different content of 0 to 60 vol% were added to (Ba,Sr)$TiO_3$ powder. Porous (Ba,Sr)$TiO_3$ ceramics were prepared by pressureless sintering at $1350^{\circ}C$ for 1h under air. Effects of carbon black content on the microstructure and PTCR characteristics of porous (Ba,Sr)$TiO_3$ ceramics were investigated. The porosity of porous (Ba,Sr)$TiO_3$ ceramics increased from 6.97% to 18.22% and the grain size slightly decreased from $7.51\;{\mu}m$ to $5.96\;{\mu}m$ with increasing carbon black contents. PTCR jump of the (Ba,Sr)$TiO_3$ ceramics prepared by adding carbon black was more than $10^5$, and slightly increased with increasing carbon black. The PTCR jump in the (Ba,Sr)$TiO_3$ ceramics prepared by adding 40 vol% carbon black showed an excellent value of $9.68{\times}10^5$, which was above two times higher than that in (Ba,Sr)$TiO_3$ ceramics. These results correspond with Heywang model for the explanation of PTCR effect in (Ba,Sr)$TiO_3$ ceramics. It was considered that carbon black is an effective additive for preparing porous $BaTiO_3$ based ceramics. It is believed that newly prepared (Ba,Sr)$TiO_3$ cermics can be used for PTC thermistor.

방전 플라즈마 소결법에 의한 다공성 육티탄산 칼륨 휘스커 프리폼의 제조

이장훈,조동철,조원승,이지환,Lee, Chang-Hun,Cho, Dong-Choul,Cho, Won-Seung,Lee, Chi-Hwan 한국세라믹학회 2002 한국세라믹학회지 Vol.39 No.12

방전 플라즈마 소결법을 이용하여 건전한 강도를 갖는 다공성 육티탄산 칼륨 휘스커 프리폼을 제조하기 위해서, 여러 소결온도 조건하에서 휘스커 프리폼을 제조한 후 기공률과 압축강도 등을 조사하였다. 그 결과, 소결 온도 900∼950${\circ}C$, 유지시간 10분, 압축하중 40 MPa, 승온속도 50${\circ}C$/min, on-off pulse type 12:2의 조건에서 높은 기공률을 갖는 프리폼을 제조할 수 있었다. 이상의 최적조건에서 제조한 프리폼은 기공률이 15∼37% 범위임에도 불구하고 174∼266 MPa의 비교적 높은 압축강도를 나타내었다. 이와 같은 강도의 향상은 휘스커간의 스파크 플라즈마 방전 및 자기 발열작용의 영향에 기인한 것으로 생각된다. 900${\circ}C$ 이하에서 제조한 휘스커 프리폼의 강도 레벨은 80∼100MPa로 900∼950${\circ}C$의 온도조건에서 제조한 프리폼에 비해 2배 이하의 낮은 값을 나타내었다. 1000 ${\circ}C$에서의 압축강도는 523 MPa로 가장 높은 강도값을 나타내었으나, 약 5%의 낮은 기공률을 나타내었다. 이상의 결과로부터, 비교적 강도가 높고 다공성의 티탄산 칼륨 휘스커를 제조하는데 있어 방전 플라즈마 소결법이 효과적인 것으로 생각된다. In order to develope the porous $K_2Ti_6O_13$ whisker preform with good strength, the pore characteristics and compressive strength were investigated as a function of spark plasma sintering temperature. As a result, high porous whisker preform were successfully fabricated by sintering at 900∼950${\circ}C$ for 10 min under a pressure of 40 MPa, heating rate of 50${\circ}C$/min and on-off pulse type of 12:2. The whisker preform prepared under above optimum condition showed relatively high compressive strength of 174∼266 MPa, despite of high porosity ranging from 15% to 37%. This improvement in strength was considered to be mainly due to the spark-plasma discharges and the self-heating action between whiskers. The compressive strength of whisker preform, fabricated at sintering temperature less than 900${\circ}C$, showed 80∼100 MPa. This is low strength level less than one half times compared with whisker preform fabricated at 900∼950${\circ}C$. The whisker preform fabricated at 1000${\circ}C$ showed the highest compressive strength of 523 MPa, but resulted in low porosity of ∼5%. Based on above results, it was considered that spark plasma sintering was an effective method for developing high strength and porosity of whisker preform.

MR Polsihing에서 알루미나 슬러리를 이용한 Si₃N₄세라믹스의 고정도 표면향상에 관한 연구

이정원(Jung-Won Lee),김동우(Dong-Woo Kim),조명우(Myeong-Woo Cho),조원승(Won-Seung Cho),신영재(Young-Jae Shin) 한국생산제조학회 2008 한국생산제조시스템학회 학술발표대회 논문집 Vol.2008 No.5

Silicon nitride Si3N4 is a leading ceramic for structural applications due to its superior properties at high temperature. It is very difficult-to-grind ceramics with high efficiency because of their high strength, hardness and brittleness. In this paper, ultra-precision surface for Si₃N₄ ceramic were using MR polishing method. MR Polishing experiment were using the magnetorheological fluid and Polishing abrasives are Al₂O₃. Polishing experiments were under various polishing conditions by changing a magnetic field force, wheel speed and polishing time. Polishing surfaces were observed analyzed by non-contact type 3-dimensional surface roughness measurement equipment. As a result, MR Polishing method can give excellent surface roughness, and surface roughness was obtained Ra=1㎚.

AIN-hBN계 머시너블 세라믹스의 기계적 특성 및 엔드밀링 가공성 평가

백시영,조명우,조원승,Beck, Si-Young,Cho, Myeong-Woo,Cho, Won-Seung 한국세라믹학회 2008 한국세라믹학회지 Vol.45 No.1

In this study, machining characteristics of AIN-hBN composites were evaluated in end-milling process. As a first step, AIN-hBN composite specimens with various hBN contents were prepared using hot press method. Material properties of the composites, such as relative density, Young's modulus and fracture toughness, were measured and compared. Then, a series of end-milling experinients were performed under various cutting conditions by changing cutting speed, depth-of-cut and feed rate. Cutting force variations were measured using a tool dynamometer during the cutting experiments. Machined surfaces of the specimens were observed using SEM and a surface pro filer to investigate the surface integrity changes. The cutting force decreased with an increases of hBN content. The cutting process was almost impossible for monolithic AIN, owing to severe chipping. In contrast, at high content of hBN, surface damage and chipping decreased, and better surface roughness can be obtained.

AlN/hBN 복합재료의 기계적 성질

이재형,안현욱,윤영식,조명우,조원승,Lee, Jaehyung,Ahn, Hyun-Wook,Yoon, Young-Sik,Cho, Myeong-Woo,Cho, Won-Seung 한국세라믹학회 2005 한국세라믹학회지 Vol.42 No.8

AlN-BN ceramic composites were fabricated and their mechanical properties were investigated. The relative density of hot-pressed composites decreased with increasing BN content, but over $99\%$ could be obtained with 30 $vol\%$ BN in AlN. YAG was formed in the composites and monolithic AlN as a second phase by the reaction between $Y_2O_3$, added as sintering aid, and $Al_2O_3$. As expected, Vickers hardness and Young's modulus decreased with increasing BN content. The three-point flexural strength also showed similar behavior decreasing from 500 MPa of monolith down to 250 MPa by the addition 30 $vol\%$ BN. However, interestingly, the standard deviation of the strength values decreased significantly as BN was added to AlN. As a result, the Weibull modulus of the AlN-30 $vol\% BN composite was 21.3, which was extremely high. Fractography and crack path studies revealed that BN platelets induced grain pull-out and crack bridging in a bigger scale during crack propagation. Consequently, fracture toughness increased as more BN was added, reaching 4.5 $MPa\sqrt{m}$ at 40 $vol\%$ BN.

치과 임플란트용 지르코니아-알루미나 복합체의 생 가공 및 경 가공 특성

임형봉,당동욱,이기주,조원승,Lim, Hyung-Bong,Tang, Dongxu,Lee, Ki-Ju,Cho, Won-Seung 한국세라믹학회 2011 한국세라믹학회지 Vol.48 No.2

The green and hard machining characteristics of dental ceramics are of great interest to dental industry. The green bodies of TZP/$Al_2O_3$ composites were prepared by the cold isostatic pressing, and machined on the CNC lathe using PCD (polycrystalline diamond) insert under various machining conditions. With increasing nose radius of PCD insert, surface roughness initially increased due to increased cutting resistance, but decreased by the onset of sliding fracture. The lowest surface roughness was obtained at spindle speed of 1,300 rpm and lowest feed rate. Hard bodies were prepared by pressureless sintering the machined green bodies at several temperatures. The grinding test for sintered hard body was conducted using electroplated diamond bur with different grit sizes. During grinding, grain pull out in the composite was occurred due to thermal expansion mismatch between the alumina and zirconia. The strength of the composite decreased with alumina contents, due to increased surface roughness and high monoclinic phase transformed during grinding process. The final polished samples represented high strength by the elimination of a phase transformation layer.

Si₃N₄-hBN 머시너블 세라믹의 R-curve 거동분석과 가공성 평가

장성민(Sung Min Jang),조명우(Myeong Woo Cho),조원승(Won Seung Cho),이재형(Jae Hyung Lee) Korean Society for Precision Engineering 2004 한국정밀공학회지 Vol.21 No.1

파우더 플라스팅에 의한 Si₃N₄-hBN계 머시너블 세라믹스의 미세패턴 가공성 평가

박동삼(Dong Sam Park),조명우(Myeong Woo Cho),김동우(Dong Woo Kim),조원승(Won Seung Cho) 한국생산제조학회 2004 한국생산제조학회지 Vol.13 No.2

Sandblasting has recently been developed into a powder blasting technique for brittle materials. In this study, the machinability of Si₃N₄-hBN based machinable ceramics are evaluated for micro - pattern making processes using powder blasting. Material properties of the developed machinable ceramics according to the variation of h-BN contents give a good machinability to the ceramics. The effect of scanning times, the size of patterns and variation of BN contents on the erosion depth of samples without mask and samples with different mask patterns in powder blasting of Si₃N₄-hBN ceramics are investigated. The Parameters are the impact angle of 90°, the scanning times of nozzle up to 40, and the stand-off distances of l00㎜. The widths of masked pattern are 0.l㎜, 0.5㎜ and 1㎜. The powder used is Alumina particles, WA#600. and the blasting pressure of powder is 0.2㎫ Through required experiments, the results are investigated and analyzed. As the results, the machinability of the developed ceramics increases as the BN contents in the ceramics.

초경공구를 사용한 Si₃N₄-hBN 머시너블 세라믹 가공에서 절삭 파라미터 분석과 결정

장성민(Sung Min Jang),조명우(Myeong Woo Cho),조원승(Won Seung Cho),박동삼(Dong Sam Park) 한국생산제조학회 2003 한국생산제조학회지 Vol.12 No.6