• 터널과지하공간
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• 제5권3호
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• pp.240-250
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• 1995

Microcrack development in the macheon gabbro, the Sangju sandstone and the Jungsun marble due to fatigue stresses was investigated using differential strain analysis and microscopic observations from fluorescent-dyed thin sections. In the Macheon gabbro, various types of cracks, including gran boundary cracks, cleavage cracks, intragranular cracks adn intergranular cracks were developed. However, only grain boundary cracks were produced in the Sangju sandstone and the Jungsun marble due to fatigue stress. Many microcracks were produced due to fatigue stresses in the macheon gabbro. However, few microcracks were produced in the Sangju sandstone and the Jungsun marble. Fatigue stresses produced new microcracks in the Junngsun marble and the Macheon gabbro, but only pre-existing grain boundary cracks were lengthened in the Sangju snadstone. Most microcracks were produced within a few tens of numbers of cyclic loading, indicating that rocks under fatigue stresses will be damaged at the early stage of cyclic loading.

• 한국세라믹학회지
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• 제22권5호
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• pp.76-84
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• 1985

Mechanical property enhancing mechanisms of $Al_2O_3-ZrO_2$ two phase ceramic composites were studied for several compositions of different $ZrO_2$/$Al_2O_3$ ratio. Microstructural analysis of $Al_2O_3-ZrO_3$(pure) composites indicated that pre-existing microcrack due to larger $ZrO_2$ particle at grain boundary extended along alumina grain boundaries within process zone. Microcracks also nucleated when very small $ZrO_2$ particles at the grain boundaries transformed to monoclinic phase at near of main crack tip. These types of microcracks could contribute to the toughening achieved by creating additional crack surface area during crack propagation. Microstructural analyses also showed that the average grain size and abnormal grain size of $Al_2O_3$ were decreased with increasing $ZrO_2$ vol% in $Al_2O_3$ matrix. As a result it could be concluded as follows In TEX>$Al_2O_3-ZrO_3$(pure) system 1. Microcrack nucleation (stress-induced microcracking) and extension was effective mechanism for absorpiton of fracture energy 2, More narrow distribution and smaller grain size of $Al_2O_3$ due to $ZrO_2$particles mainly contributed to main-tatin the strength and hardness.

• 한국분말재료학회지
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• 제19권4호
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• pp.253-258
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• 2012

The characteristics of $Al_2TiO_5$ ceramics were influenced by the additives and the heat treatment that controls the microcrack behavior at grain boundaries. The effect of additives on $Al_2TiO_5$ ceramics were investigated in terms of mechanical properties and thermal expansion at high temperature. The $Al_2TiO_5$ were synthesized at $1500^{\circ}C$, $1550^{\circ}C$ and $1600^{\circ}C$ for 2h by reaction sintering. The formation of $Al_2TiO_5$ phase was increased by additives that enhanced the volume of the microcrack that can lead to low thermal expansion. The mechanical properties of the stabilized $Al_2TiO_5$ ceramics were increased remarkably at $1100^{\circ}C$, $1200^{\circ}C$ and $1300^{\circ}C$ due to the oneset of mechanical healing of grain-bondary microcracks at a high temperature. The amount of microcrack was decreased at lower sintering temperature that causes the increase of mechanical properties at high temperature.

• 한국전기전자재료학회논문지
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• 제11권12호
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• pp.1099-1107
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• 1998

In this study, the microstructure and electric conductivity of 5mol% $Gd_2O_3$-5mol% $Y_2O_3-ZrO_2$ system(5G5YZ) with a variation of sintering time at $1600^{\circ}C$ were investigated. By the result of TEM analysis of 5G5YZ sintered for 12h, a microcrack was observed near grain boundary. The change of the sintering time did not affect the lattice conductivity, but the grain boundary contribution was varied with the sintering time. The grain boundary conductivity of the sample sintered for 1h showed the highest value. Furthermore, the activation energy of the total conductivity was independent upon the sintering time and showed approximately 1.01eV. The highest conductivity measured at $1000^{\circ}C$ was 0.0197S/cm with the sample sintered for 1h. Comparing to 0h’s, the thickness ration of grain boundary as a function of sintering time were 0.88, 1.11 and 1.29 for 1h, 5h and 12h, respectively. In case of the sample sintered for 1h, the thickness of the grain boundary showed the lowest value. The increase of the sintering time over 1h made the decrease of the electric conductivity as well as the increase of the grain growth and the thickness of the grain boundary. As a result, it seemed that the proper sintering time for 5G5YZ composition was 1h.

• 한국결정성장학회지
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• 제2권1호
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• pp.70-75
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• 1992

강유전 요업체내에 존재하는 잔류응력에 대한 모델이 제시되었다. 모델로부터 다음과 같은 두가기 사실을 알 수 있었다. 첫째 잔류응력은 입계와 분역경계에 존재하며, 입계에 존재하는 응력이 분역경계에 존재하는 응력보다 컸다. 둘째는 입자내에 분역이 존재할 때, 잔류응력은 입자크기가 증가함에 따라 감소했다. 위와 같은 사실은 ($PbZr_{0.4}Ti_{0.6}O_3$)요업체에서 분극에 의해 유발된 미세균열과 입자크기에 따른 승온과 냉각시 유전율차이로부터 증명되었다.

• 한국분말재료학회지
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• 제9권5호
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• pp.307-314
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• 2002

The high temperature deformation behavior of the activated sintered W powder compacts was investigated. The W compact showed the relative density of 94% with the average W grain size of $23\mutextrm{m}$ by activated sintering at $1400^{\circ}C$ for 1 hour. Compression tests were carried out in the temperature range of $900-1100^{\circ}C$ at the strain rate range of $10^{0}$/sec - $10^{-3}$/sec. True stress-strain curve and microstructure exhibited the grain boundary brittleness which was dependent on the compression test temperature. The activated sintered W compact showed that the maximum stress as well as the strain at the maximum stress was abruptly decreased as the test temperature increase from $900^{\circ}C$ to 1000 and $1100^{\circ}C$ regardless of the strain rate. The discrepancy of the microstructure in the specimen center was obviously observed with the increase of the test temperature. After compression test at $900^{\circ}C$ the W grain was severely deformed normally against the compression axis. However, after compression test at $1000^{\circ}C$ and $1100^{\circ}C$ the W grain was not deformed, but the microcrack was formed in the W grain boundary. The Ni-rich second phase segregated along the W grain boundary could be partly unstable over $900^{\circ}C$ and affect the poor mechanical property of the activated sintered W compact.

• Progress in Superconductivity
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• 제6권2호
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• pp.133-137
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• 2005

We fabricated the Au/Ni template for YBCO coated conductors and evaluated texture formation and the microstructural evolution. The cube textured Ni substrate was fabricated by rolling and recrystallization annealing, and subsequently Au layer formed on the substrate by electroless-plating method. The texture was evaluated by pole-figure with x-ray goniometer with orientation distribution function (ODF) analysis. The surface roughness and grain boundary morphology of template were characterized by atomic force microscopy (AFM) We observed that Au layer deposited epitaxially on Ni substrate and formed a strong cube texture when plating time was optimized. The full-width at half-maximum (FWHM) was $8.4^{\circ}$ for out-of-plane and $9.98^{\circ}$ for in-plane texture for plating time of 30 min. Microstructural observation showed that the Au layer was homogeneous and dense without formation of crack/microcrack. In addition, we observed that root-mean-square (RMS) and depth of grain boundary were 14.6 nm and 160 $\AA$ for the Au layer, respectively, while those were 27.0 nm and 800 $\AA$ for the Ni substrate, indicating that the electoless-plated Au layer had relatively smooth surface and effectively mollified grain groove.

• 한국세라믹학회지
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• 제28권7호
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• pp.549-555
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• 1991

Mullite-zirconia (0∼4 mol% yttria) composites were obtained by In-situ sintering of zircon and alumina mixture, and their mechanical properties were studied in conjuction with microstructure observation. Martensitic transformation temperature (Ms) of zirconia dispersed in the mullite matrix decreased with Y2O3 contents and was about 600$^{\circ}C$ for ZrO2 containing 4 mol% Y2O3. On cooling of this composites, tetragonal to monoclinic phase transformation induced microcracks at the grain boundary of mullite matrix. The microcracks seemed to absorb the fracture energy in stress field during mechanical tests. Therefore, toughening mechanisms of this composite were considered to nucleation and extension of microcrack, and crack deflection mechanism due to the difference of thermal expansion coefficient between matrix and dispersed phase.

• 지질공학
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• 제9권3호
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• pp.243-251
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• 1999

일반적으로 등방→균질 재료로 취급되고 있는 화강암 내에는 많은 미세 결함들이 존재한다. 암석에서 이들 미세 결함들이 단계적으로 발생, 진전하여 취성파괴가 일어나는 것으로 알려져 있다. 본 논문에서는 새롭게 개발된 시험기를 이용하여 수침삼축압축시험중에 일어나는 미수크랙의 발생, 진전 및 상호작용을 연속적으로 관찰하고, 그 형태를 분류하였다.