The Science of Engineering Ceramics III

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Authors: Yoshinobu Yamamoto, Toshiyuki Nishimura, Naoto Hirosaki, S. Guo, J. Cao, Mamoru Mitomo
Abstract: High-temperature properties of silicon nitride ceramics with Lu-silicon-oxynitride grain boundary phases were investigated. Si3N4 powder with 1.2 mol% (SN12) and 4.8 mol% (SN48) of Lu2O3 were gas-pressure hot-pressed at 1950°C for 2 h under 20 MPa in 1 MPa N2. SN12 consisted of elongated β-Si3N4 and a secondary phase, Lu4Si2O7N2, whilst SN48 consisted of elongated β-Si3N4 and Lu4Si2O7N2 + Lu2SiO5. At 1500°C and 1600°C, the stress-strain curve of SN48 was nonlinear, whilst that of SN12 was linear, indicating that SN12 broke as a brittle fracture at these temperatures. SN12 had excellent oxidation resistance and weight gain during the oxidation at 1500°C for 1000 h was 4 g/m2. Creep lifetime of SN12 at 1500°C under tensile stress of 137 MPa exceeded 1678.5 h.
Authors: Shuji Sakaguchi
Abstract: Temperature dependence of internal friction on silicon nitride ceramics with yttria and alumina sintering aids was discussed using 4-elements visco-elastic model. It is known that the internal friction of this ceramic material shows a characteristic peak at around 1040 oC and an exponential increase over 1200 oC. This temperature dependence of internal friction was well approximated with using parallel Maxwell model. The parameters for this model and the relaxation time on this model were obtained. The relaxation time was very short, compared to the transient creep deformation of this ceramics. The analyzed results on internal friction could not be related with the creep deformation.
Authors: Michiyuki Yoshida, Yutaka Shinoda, Takashi Akatsu, Fumihiro Wakai
Abstract: The deformation behavior of SiO2 doped nanocrystalline monoclinic zirconia (MZP) was studied at 1323-1223 K in compression tests. The strain rate of SiO2 doped nanocrystalline MZP was slower than that of high-purity MZP by one order of magnitude. SiO2 doped nanocrystalline MZP exhibited a stress exponent n ≈ 2. The apparent activation energy for the deformation of SiO2 doped nanocrystalline MZP was characterized by a higher value than that observed for high-purity MZP. 1wt% SiO2 doped nanocrystalline MZP was deformed at constant flow stress, while the flow stress of high-purity MZP increased significantly with the strain (strain hardening). While no grain growth was observed after the compressive deformation of 1wt % SiO2 doped nanocrystalline MZP, remarkable grain growth was observed after the deformation of high-purity MZP. The addition of SiO2 into nanocrystalline MZP is effective in limiting grain growth at low temperatures
Authors: Hideki Ohtsubo, Narihito Nakagawa, Kazutoshi Shimizu, Koji Shibata, Atsuyuki Mitani, Yoshiharu Waku
Abstract: Enploying unidirectional solidification, Al2O3/GdAlO3(Gadolinium Aluminum Perovskite:GAP) eutectic composite was fabricated. The Al2O3/GAP eutectic composite obtained has a flexural strength of over 600Mpa above 1500C. The composite also showed plastic deformation above 1550C on the flexural test. The effect of microstructure on mechanical strength at high temperature was investigated. Al2O3/GAP eutectic composites having different microstructure were fabricated by controlling the temperature gradient on unidirectional solidification. It was found that the flexural strength at high temperature of the composite became higher with refining the microstructure.
Authors: Yoshimasa Kobayashi, Toru Hayase, Naohito Yamada
Abstract: Grain boundary microcracking in sliding test using a silicon pin was evaluated on several kinds of AlN to investigate the behavior of damage generation and the influence of material properties on it. The grain boundary fracture toughness (KIC gb ) was estimated from a percentage of the intergranular fracture as a grain boundary property for each specimen. KIC gb was greatly influensed by an amount and kinds of additives. After the sliding test, silicon debris accumulation and grain boundary microcracking of AlN were observed at the wear traces on the surface of AlN. The density of grain boundary microcracks increased linearly with increasing the contact load. Empirical relationship indicated that its density depended on KIC gb and the AlN grain size, and decreased mainly with increasing KIC gb. The density of grain boundary microcracks greatly degreased at the KIC gbvalue above 1.9 (MPa·m1/2).
Authors: J.E. Pitchford, E. Lidén, Stefan Gustafsson, L.K.L. Falk, Elis Carlström, W.J. Clegg
Abstract: The tensile creep behaviour of a mullite-SiC nanocomposite containing 5 vol% of SiC particles deformed under stresses from 4 to 50 MPa at 1400 °C has been studied. After grain-size effects had been accounted for, the creep-rate of the nanocomposite was found to be approximately 30× less than that of the monolithic mullite. It is suggested that this reduction is caused not by a threshold stress but by the extra work required to drive diffusion in the low diffusivity SiC particles so that they can move with the grain boundaries during creep. A model is presented which predicts the rate of creep under these conditions and gives reasonable agreement with the experiments at low stresses.
Authors: Hassan El Attaoui, Malika Saâdaoui, Jérôme Chevalier, Gilbert Fantozzi
Abstract: A compliance function is used to quantify the shielding capacity of grain bridging, the degradation of which is the main cyclic fatigue mechanism in alumina. Materials with different grain sizes were processed and the fatigue experiments were performed using the double torsion test. Significant degradation is observed in the coarse grain material and a marked sensitivity to the loading level is outlined. At moderate loads, bridging degradation occurs prior to fatigue crack growth during an incubation period. At low cyclic loads, the shielding capacity can be entirely degraded, leading to a cyclic fatigue threshold equivalent to that of the fine grain material.
Authors: Koji Takahashi, Kotoji Ando, Shinji Saito
Abstract: Si3N4/SiC composite ceramics were hot-pressed in order to investigate their crack-healing behavior under cyclic stress and the resultant static fatigue strength. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under a cyclic bending stress of 210MPa in air at 900, 1000, 1100, and 1200 °C. The bending strength and static fatigue strength of the crack-healed specimens were systematically investigated at each healing temperature. The specimens which has been crack-healed and static fatigue-tested at 900 and 1000 °C showed lower static fatigue strength than those tested at 1100 and 1200 °C. Detailed investigation on the fracture surface of static fatigue-tested specimens showed that oxidation of the base material had strong effects on the static fatigue strength. It was found that when the specimens were pre-oxidized in air at 1300 °C, the surface was covered by a protective oxide layer, leading to a significant improvement of static fatigue strength at 900 and 1000 °C.
Authors: Jae Won Kim, Seong Hwan Park, Yeon Gil Jung, Hee Soo Lee
Abstract: Layered structures of dense porcelain/porous alumina and dense porcelain/porcelainalumina/ porous alumina are designed and their crack propagation behaviors are investigated. As a substrate, the porous alumina, which is prepared by a gel-casting process using the binary slip of alumina powder and PMMA spherical micro-bead, is dried at room temperature for 24 h and then sintered at 1600 for 2 h. Porcelain is coated on the porous alumina substrate and then re-sintered at 987. Bi- and tri-layered structures are produced by the different dwell times (2 min, 10 min) at re-sintering temperature. There is no delaminating or cracks observed after re-sintering the layered samples. The crack propagation behaviors in the bi- and tri-layered structures are evaluated by micro-indentation. The indentation cracks do not propagate into the porous alumina through interface (porosity; 36~62%) except for another one (porosity; 70%) in the bi-layered samples. In the case of the high porosity bi-layered sample (porosity; 70%), cracks are scattered along the 3-D open-pore channels. However, cracks do not propagate through the interface in the tri-layered samples with a porcelain-alumina buffer layer, because the porcelain-alumina buffer layer plays an important part such as a barrier layer in crack propagation.
Authors: Masato Ono, Wataru Nakao, Koji Takahashi, Kotoji Ando
Abstract: Mullite reinforced by SiC whiskers, Mullite(W), was developed with the aim of improving fracture toughness and to add crack-healing ability. For determining the threshold stress during crack-healing, a composite having a semi-elliptical surface crack of 100 μm was crack-healed at 1473 K for 8 h in air under elevated static and cyclic stresses and the bending strength at 1473 K of the crack-healed composite under stress were also investigated. Mullite(W) crack-healed under static stress below 100 MPa were never fractured during crack-healing and had the same bending strength as the specimens crack-healed under no-stress. Therefore, the threshold static stress during crack-healing of Mullite(W) has been determined to be 100 MPa. The threshold cyclic stress has also been determined to be 150 MPa. Considering that the crack growth is time-dependent, the threshold stress for all conditions during crack-healing of Mullite(W) has been concluded to be 100 MPa.

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