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71.
Antonio R. de Arellano-López Francisco L. Cumbrera Arturo Domínguez-Rodríguez Kenneth C. Goretta Jules L. Routbort 《Journal of the American Ceramic Society》1990,73(5):1297-3000
Compressive creep of SiC-whisker-reinforced Al2 O3 composites (0, 5, 15, and 25 wt% SiC) was measured in the temperature range of 1300° to 1500°C in air and argon. The creep resistance increased with increasing whisker concentration. The results indicated that the whiskers degraded in air, increasing strain rates compared to those in argon. Stress exponents between 1.0 and 2.0 and an activation energy of 620 ± 100 kJ/mol were measured. Transmission electron microscopy observations indicated that cavitation was minimal and that the deformed composites had the same dislocation structure as did the as-received samples. 相似文献
72.
Dong S. Park Michael J. McNallan Chan Park Winston W. Liang 《Journal of the American Ceramic Society》1990,73(5):1323-1329
The active corrosion of sintered α-silicon carbide from heat exchanger tubes in the temperature range 900° to 1100°C in gas mixtures containing 2% Cl2 by volume with additions of O2 or H2 has been investigated by thermogravimetric analysis and subsequent examination of the corrosion products. The presence of a small amount of oxygen accelerated rapid active corrosion in chlorine-containing gas mixtures, but the corrosion was suppressed by an active-to-passive transition when the concentration of oxygen in the gas mixture was too high. Low rates of attack were observed in the environments containing H2 even when the chlorine potential was high. The concentration of oxygen necessary to produce the active-to-passive transition was found to vary from one material to another and may be related to the amount of excess carbon in the sintered silicon carbide. 相似文献
73.
Toughening of a Particulate-Reinforced Ceramic-Matrix Composite by Thermal Residual Stress 总被引:11,自引:0,他引:11
Minoru Taya S. Hayashi Albert S. Kobayashi H. S. Yoon 《Journal of the American Ceramic Society》1990,73(5):1382-1391
The micromechanics involved in increased crack growth resistance, K R , due to the addition of TiB2 particulate in a SiC matrix was analyzed both experimentally and theoretically. The fractography evidence, in which, the advancing crack was attracted to adjacent particulates, was attributed to the tensile region surrounding a particulate. Countering this effect is the compressive thermal residual stress, which results in the toughening of the composite, in the matrix. This thermal residual stress field in a particulate-reinforced ceramic-matrix composite is induced by the mismatch in the coefficients of thermal expansion of the matrix and the particulate when the composite is cooled from the processing to room temperature. The increase in K R of the composite over the monolithic matrix, which was measured by using a hybrid experimental-numerical analysis, was 77%, and compared well with the analytically predicted increase of 52%. The increase in K R predicted by the crack deflection model was 14%. Dependence of K R on the volume fraction of particulates, f p , and of voids, f v , is also discussed. 相似文献
74.
Duane L. DeBastiani Michael F. Modest Vladimir S. Stubican 《Journal of the American Ceramic Society》1990,73(7):1947-1952
The mechanism of material removal from SiC by CO2 laser heating was studied using sintered and single-crystal α-SiC. Removal rate and width of the groove showed maxima when plotted as a function of translation speeds. Groove depth decreased as the translation speed of samples increased. Similar results were obtained if argon or air was used as gas assist, which indicated that the material removal mechanism is induced dissociation of SiC. Microstructure of the material deposited in and outside of the groove was studied by SEM. At low scanning speeds, columnar grains 10 to 50 μm long appeared. As the scanning speed increased, columnar grains became smaller and finally only irregular polycrystalline particles were observed. By using Raman spectroscopy, Auger analysis, and X-ray diffraction, phases inside and outside the groove were identified as Si, β-SiC, C, and SiO2 . Columnar grains were identified as β-SiC covered with thin layers of C, Si, and SiO2 . Slow scanning speeds enhanced the growth of β-SiC. At slow scanning speed, free silicon was always found in the grooves of lased single crystals but not in the grooves of lased sintered SiC. It can be concluded that the mechanism of material removal from silicon carbide by CO2 laser heating is a vaporization process, and material found in the groove and on the surface near the groove is formed by condensation from the vapor. 相似文献
75.
The present report explores the use of cross-polarization (CP) and single-pulse magic angle spinning nuclear magnetic resonance (MAS-NMR) as well as normal and surfaceenhanced Raman spectroscopy (SERS), in concert, for characterizing highly crystalline polymorphic silicon carbide ceramics. The combined use of these techniques provides a wealth of information regarding bulk, near-surface, and surface speciation. Both Raman and SERS are promising techniques for fracture surface characterization of these systems. The application of CPMAS-NMR and SERS to the study of ceramics is reported for the first time in this investigation. 相似文献
76.
Dynamic Fracture Responses of Alumina and Two Ceramic Composites 总被引:2,自引:0,他引:2
A hybrid experimental-numerical procedure was used to characterize the dynamic fracture response of alumina (Al2 O3 ) and TiB2 -particulate/SiC-matrix and SiC-whisker/Al2 O3 -matrix composites. Unlike metals and polymers, dynamic arrest stress intensity factors (SIFs) did not exist in the monolithic ceramics and the two ceramic composites considered. Thus a running crack in these materials cannot be arrested by lowering the driving force, i.e., the dynamic SIF. Fractography study of the alumina specimens showed that the area of transgranular failure varied from about 3% to about 16% for rapid crack extensions in statically and impact loaded specimens, respectively. The influence of kinematic constraints which enforces transgranular flat crack extension, despite the higher fracture energy of transgranular fracture, is discussed. 相似文献
77.
Nathan S. Jacobson rew J. Eckel Ajay K. Misra Donald L. Humphrey 《Journal of the American Ceramic Society》1990,73(8):2330-2332
The reactions of a sintered α-SiC with 5% H2 /H2 O/Ar at 1300°C were studied. Thermomchemical modeling indicates that three reaction regions are expected, depending on the initial water vapor or equivalently oxygen content of the gas stream. A high oxygen content ( P (O2 ) > 10−22 atm) leads to a SiO2 formation. This generally forms as a protective film and limits consumption of the SiC (passive oxidation). An intermediate oxygen content (10−22 atm > P (O2 ) > 10−26 atm) leads to SiO and CO formation. These gaseous products can lead to rapid consumption of the SiC (active oxidation). Thermogravimetric studies in this intermediate region gave reaction rates which appear to be controlled by H2 O gas-phase transport to the sample and reacted microstructures showed extensive grain-boundary attack in this region. Finally, a very low oxygen content ( P (O2 ) < 10−26 atm) is thermochemically predicted to lead to selective removal of carbon and formation of free silicon. Experimentally low weight losses and iron silicides are observed in this region. The iron silicides are attributed to reaction of free silicon and iron impurities in the system. 相似文献
78.
Tetsuya Suzuki Toyohiko Yano Takayoshi Iseki Tsutomu Mori 《Journal of the American Ceramic Society》1990,73(8):2435-2440
Silicon carbide ceramics fabricated by three different methods were neutron-irradiated in the Japan Materials Testing Reactor and were subsequently annealed free from stresses or under compressive external stresses. The macroscopic length monotonically decreased with annealing above the irradiation temperature, when annealing was performed below ∼1300°C. This decrease was not affected by the external stresses. However, annealing above ∼1300°C led to an increase in length in B-containing SiC. The expansion was caused by the formation and growth of He bubbles at grain boundaries. The growth occurred by flow of vacancies into bubbles. The compressive stress retarded the expansion along the loading direction. This retardation was compensated by a length incrase along the lateral direction. The effect of external stresses was discussed by considering differently oriented He bubbles. One bubble was stress favored and the other stress unfavored. The stress determined the diffusional flow of vacancy-He atom complexes between the favored and unfavored bubbles which were caused by anisotropic expansion under a directional stress. 相似文献
79.
David J. Maglev R. A. Winholtz K.T. Fabe 《Journal of the American Ceramic Society》1990,73(6):1641-1644
Residual stresses in a SiC-TiB2 particulate composite before and after stressing have been measured using X-ray diffraction. Tensile residual stresses in the TiB2 drop by 60% after bending stresses of 250 MPa are applied. Likewise, the compressive residual stresses in the Sic phase decrease accordingly. Such behavior is consistent with stress-induced microcracking. 相似文献
80.