• 동력기계공학회지
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• 제6권1호
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• pp.88-95
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• 2002

본 연구에서는 균열 치유 거동을 가지는 소결된 Mullite/SiC의 모재, 열처리재, 균열재, 치유 균열재의 기계적 특성이 논의되었다. 반타원형 균열의 치수는 $100{\mu}m$ 과 $200{\mu}m$이다. 얻어진 결과는 다음과 같다. (a) Mullite/SiC 복합 세라믹스는 균열 치유 능력이 있었다. (b) 최적의 균열 치유 열처리 조건은 $1300^{\circ}C$, 1시간이었다. (c) 치유 가능한 최대 균열 길이는 직경 $100{\mu}m$의 반타원 균열이다. (d) 균열 치유부는 $1200^{\circ}C$이상에서 충분한 강도를 가졌고, 대부분의 시험편은 균열 치유부 이외의 영역에서 파단 하였다. (e) 공기중에서 예열처리는 본 재료의 피로강도 향상에 유용하였다.

• 한국세라믹학회지
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• 제32권5호
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• pp.626-634
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• 1995

SiC를 51.9wt% 함유한 SiC-자기질 혼합 분말이 자기 코아의 표면연마 공정에서 부산물로 발생한다. 이 원료 분말을 SiC질 세라믹스를 제조하기 위한 출발물질로 하였다. 원료자체로 성형 $1350^{\circ}C$에서 열처리하면 SiC, $Al_{2}O_{3}$, cristoblite, mullite 결정상이 존재하면 미세구조는 SiC 입자를 유리질 매트릭스가 덮고 있고 구형 기공이 존재한다. 원료 성형체를 열처리시 시편 무게 증가에 의해 판단되는 SiC의 산화는 600~$800^{\circ}C$ 범위에서 시작하나 XRD 분석에 의해서는 $1000^{\circ}C$ 열처리에 이르러서야 $SiO_{2}$의 cristobalite 결정상을 확인하였다. 예비 열처리 후에 소성한 시편은 mullite화 반응이 촉진되는데, $1000^{\circ}C$의 예비 열처리 후 $1350^{\circ}C$에서 소성한 시편은 SiC, cristobalite, mullite가 결정상으로 존재하며, 밀도 2.24g/$cm^{3}$, 흡수율 11.73%이고 유리상이 적고 다공성인 미세구조를 갖는다. 원료 자체에서 51.9wt%이던 SiC 함량은 $1350^{\circ}C$ 소성시편에서는 예비 열처리 조건에 따라 약 37~22%로 감소한다.

• 한국세라믹학회지
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• 제47권6호
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• pp.509-514
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• 2010

Porous mullite-bonded SiC (MBSC) ceramics were fabricated at temperatures ranging from 1400 to $1500^{\circ}C$ for 2 h using silicon carbide (SiC), alumina ($Al_2O_3$), strontium oxide (SrO), and poly (methyl methacrylate-coethylene glycol dimethacrylate) (PMMA) microbeads. The effect of template content on porosity, pore morphology, and flexural strength were investigated. The porosity increased with increasing the template content at the same sintering temperature. The flexural strength showed maximum after sintering at $1450^{\circ}C$/2 h for all specimens due to small pores and dense strut. By controlling the template content and sintering temperature, it was possible to produce porous MBSC ceramics with porosities ranging from 30% to 54%. A maximum flexural strength of ~51MPa was obtained at 30% porosity when no template were used and specimens sintered at $1450^{\circ}C$/2 h.

• 한국세라믹학회지
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• 제36권7호
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• pp.773-781
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• 1999

A new technique to fabricate complex-shaped mullite has been developed through gelcasting without organic binder. The suspension of a commercial mullite powder and boehmite-SiO2 sol as binder was elecrtrosterically stabilized. Ammonium polymetharylyate was employed as a dispersant for the electrosteric stabilization. Complex-shaped bodies were made through casting and gelation at room temperature. The green bodies were dried at a temperature of 30$^{\circ}C$ and relative humidity of 98% without crack development. The dried bodies were then pressureless-sintered at 1650$^{\circ}C$ for 2 h to get a relative density of 98% The Vickers hardness and 4-point flexural strength of sintered parts were 11.0 GPa and 150MPa respectively which are similar to those of commercially available sintered mullite.

• 한국세라믹학회지
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• 제48권6호
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• pp.604-609
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• 2011

Additives for low temperature sintering of mullite ceramics were investigated for matching Mo-Cu conducting paste with that ceramics at 1,400$^{\circ}C$. $SiO_2$, MgO and $Y_2O_3$ were chosen as the additives for low temperature sintering, and the amounts of those additives were varied with sintering temperature of 1,400$^{\circ}C$ to 1,500$^{\circ}C$. With additives of 1.0 wt% of $SiO_2$, 1.0 wt% of MgO, and 1.5 wt% of $Y_2O_3$, the densest sintered body of 3.12 g/$cm^3$ was obtained at 1,400$^{\circ}C$ in reducing atmosphere. The flexural strength of that was 325 MPa and the CTE (Coefficient of thermal expansion) was 4.33 ppm/$^{\circ}C$.

• 한국세라믹학회지
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• 제51권5호
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• pp.481-486
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• 2014

Environmental barrier coatings (EBCs) are used to protect SiC-based ceramics or composites from oxidation and corrosion due to reaction with oxygen and water vapour at high temperatures above $1000^{\circ}C$. Mullite ceramics have been studied for environmental barrier coatings for Si-based ceramics. More recently, rare earth silicate ceramics have been identified as more water vapour-resistant materials than mullite for environmental barrier coatings. In this study, we fabricate mullite and yttrium silicate ceramics by an atmospheric plasma spray coating method using spherical granules fabricated by spray drying. As a result, EBCs with thicknesses in the range of $200-300{\mu}m$ are successfully fabricated without any macroscopic cracks or interfacial delamination. Phase and microstructure analysis are conducted, and the basic mechanical properties, such as hardness and indentation load-displacement curves are evaluated.

• 한국재료학회지
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• 제28권12호
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• pp.719-724
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• 2018

Cordierite composed of an alumina-silica-magnesia compound has a low coefficient of thermal expansion(CTE) and excellent thermal shock resistance. It also has a low dielectric constant and high electrical insulation. However, due to low mechanical strength, it is limited for use in a ceramic heater. In this study, $ZrO_2$ is added to an 80 wt% cordierite-20 wt% mullite composition, and the effect of $ZrO_2$ addition on the mechanical strength and thermal shock resistance is investigated. With an increasing addition of $ZrO_2$, cordierite-mullite formed $ZrO_2$, $ZrSiO_4$ and spinel phases. With sintering conducted at $1400^{\circ}C$ with the addition of 5 wt% $ZrO_2$ to 80 wt% cordierite-20 wt% mullite, the most dense microstructure forms along with an excellent mechanical strength with a 3-point flexural strength of 238MPa. When this composition is quenched in water at ${\Delta}T=400^{\circ}C$, the 3-point flexural strength is maintained. Moreover, when this composition is cooled from $800^{\circ}C$ to air, the 3-point flexural strength is maintained even after 100 cycles. In addition, the CTE is measured as $3.00{\times}10^{-6}{\cdot}K^{-1}$ at $1000^{\circ}C$. Therefore, 80 wt% cordierite-20 wt% mullite with 5 wt% $ZrO_2$ is considered to be appropriate as material for a ceramic heater.

• 한국전기전자재료학회논문지
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• 제30권12호
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• pp.762-767
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• 2017

Mullite ($3Al_2O_3{\cdot}2SiO_2$) has emerged as a promising candidate for high-temperature structural materials due to its erosion resistance, chemical and thermal stabilities, relatively low thermal expansion coefficient, excellent thermal shock and creep resistances, and low dielectric constant. However, since the pure mullite sintering temperature is as high as $1,600{\sim}1,700^{\circ}C$, there is an increasing need for a sintering additive capable of improving the strength characteristics while lowering the sintering temperature. Herein we have tried to obtain the optimal sintering additive composition by adding MgO, $Cr_2O_3$, and $Y_2O_3$ to mullite, followed by sintering at $1,325{\sim}1,550^{\circ}C$ for 2 h. With additives of 2 wt% of MgO, 2 wt% of $Cr_2O_3$, 4 wt% of $Y_2O_3$, A density of $3.23g/cm^3$ was obtained for the sintered body at $1,350^{\circ}C$ upon using 2 wt% MgO, 2 wt% $Cr_2O_3$, and 4 wt% $Y_2O_3$ as additives. The three-point flexural strength of that was 275 MPa and the coefficient of thermal expansion (CTE) was $4.15ppm/^{\circ}C$.

• 한국재료학회지
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• 제10권3호
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• pp.233-240
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• 2000

소결 촉진제 첨가없이 치밀한 코디어라이트 세라믹스를 얻기 위해 Mg-Al-Si계의 열적거동을 검토하였다. 수용액 매체에서 코디어라이트 현탁액의 분산안정성을 ESA를 이용하여 조사하였으며, 코디어라이트 졸의 응집 방지와 분산성을 부여하기 위해 2N $HNO_3$와 2N $NH_4OH$ 용액으로 pH 1.03과 pH 8.30로 조절하였다. pH 8.30인 코디어라이트 졸을 $150^{\circ}C$에서 12시간 건조한 시료에서 magnesium-aluminum-silicate 복합 겔이 존재하였다. 평형상태의 코디어라이트 겔의 결정화는 $1300^{\circ}C$ 이상의 소결 온도에서 존재하였으며, $1300^{\circ}C$ 이하의 조건에서는 ${\mu}-cordierite(Mg_2Al_4Si_5O_18)$, spine($MgAl_2O_4$) 및 mullite($Al_6Si_2O_{13}$)와 같은 다양한 준안정상이 복합적으로 혼재하였다. pH 1.03과 pH 8.30로 조절된 현탁액에서 코디어라이트 겔의 핵반응은 유사하게 일어났으나, 치말화거동은 졸의 pH에 상당한 차이를 보였다. pH 1.03인 졸 보다 pH 8.30인 졸의 치밀화가 더욱 진전되었다.

• 한국세라믹학회지
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• 제35권4호
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• pp.311-318
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• 1998

The reaction bonded alumina ceramics with reinforced particles which have low shrinkage were pro-duced by blending of SiC or TiC or ZrO2 powders to the mixture of Al metal and Al2O3 powder. The powd-ers were attrition milled isostantically pressed and preheated tio 110$0^{\circ}C$ with a heating rate of $1.5^{\circ}C$/min The specimens were then sintered at the temperature range 1500 to 1$600^{\circ}C$ for 5 hours with a heating rate of 5$^{\circ}C$/min. The specimens showed 5-9% weight gain and 2-9% dimensional expansion through the complete oxidation of Al after preheating up to 11--$^{\circ}C$ the overall dimensional change of the specimens after the reaction sintering at 1500-1$600^{\circ}C$ was 6-12% The maximum densities were 92% theoretical. The fine grain-ed(average grain size :0.4 ${\mu}{\textrm}{m}$) microstructure were observed in the specimen with ZrO2 and SiC. But the microstructure of specimen with TiC was relatively coarse.(average grain size : 2.1 ${\mu}{\textrm}{m}$) The mullite phase was formed by the reaction of Al2O3 and SiO2 in a specimen with SiC. In the TiC contained specimen TiC was oxidized into TiO2 and finally reacted with Al2O3 to form Al2TiO5 during sintering.