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1.
Xudong Sun Ji-Guang Li Shiwen Guo Zhimeng Xiu Kai Duan Xiao Zhi Hu 《Journal of the American Ceramic Society》2005,88(6):1536-1543
Al2 O3 /SiC ceramic nanocomposites were fabricated from nanocrystalline Al2 O3 (10 nm in diameter) and SiC (15 nm in diameter) powders, and a theoretical model of intragranular particle residual stress strengthening was investigated. The SiC nanoparticles in the Al2 O3 grains create a normal compressive stress at the grain boundaries and a tangential tensile stress in the Al2 O3 grains, resulting in the "strengthening" of the grain boundaries and "weakening" of the grains. The model gives a good explanation of the experimental results of the authors and others which are difficult to be explained by the existing strengthening models, i.e. the maximum strength is normally achieved at about 5 vol% of SiC particles in the Al2 O3 –SiC ceramic nanocomposites. According to the model, there exists an optimum amount of SiC for strengthening, below which the grain boundaries are not fully "strengthened" and the fracture is mainly intergranular, above which the grains are "weakened" too much and the fracture is mainly transgranular, and at which the fracture is a mixture of intergranular and transgranular. 相似文献
2.
Molecular dynamics computer simulations were used to study the fracture behavior of silica intergranular films (IGFs) between silicon nitride crystals as a function of film thickness. Results showed a significant increase in fracture stress with decreasing IGF thickness. IGFs that are 2 nm thick fracture similarly to bulk silica glass, while the 1 nm IGF fractured at a much higher value. The simulations show bond rupture and rearrangement of siloxane rings that coalescence to form larger rings, or "voids." The delineating difference in the systems is the significantly larger concentration of six-membered rings in the 1 nm IGF in comparison to the 2 nm IGF or bulk silica glass. The Si–O bond is more stable in six-membered rings than other ring sizes. Rupture is more catastrophic in the 1 nm IGF than the other systems where stress decays more slowly with strain. 相似文献
3.
P. ajgalík M. Hnatko Z. Lené J. Dusza M. Kaiarová 《International Journal of Applied Ceramic Technology》2006,3(1):41-46
Silicon nitride–silicon carbide nanocomposite has been prepared by an in situ method that utilizes the formation of SiC nanograins by carbothermal reduction of intentionally added fine SiO2 during the sintering process. The mean value of room-temperature four-point bending strength is 675 MPa with the Weibull modulus of 6.4 and an indentation fracture toughness of 7.4 MPa·m1/2 . A significantly enhanced creep resistance was achieved by the incorporation of SiC nanoparticles into the matrix up to 1400°C. The tribological properties of the material were tested using a ball-on-disk configuration and showed a friction coefficient of about 0.7. The cutting inserts machined from this composite had three times longer lifetime compared with those available on the market. On the other hand, the scatter of results is much larger compared with those measured for the commercial inserts. 相似文献
4.
Byung-Jin Choi Young-Hag Koh Hyoun-Ee Kim 《Journal of the American Ceramic Society》1998,81(10):2725-2728
The effects of microstructure and residual stress on the mechanical properties of Si3 N4 -based three-layer composite materials were investigated. The microstructure of each layer was controlled by the addition of two differently sized silicon carbides: fine SiC nanoparticles (∼200 nm) or relatively large SiC platelets (∼20 µm). When the SiC nanoparticles were added, the average grain size of Si3 N4 was reduced because of the inhibition of grain growth by the particles. On the other hand, when the SiC platelets were added, the microstructure of Si3 N4 was not much changed because of the large size of the platelets. Three-layer composites were fabricated by placing the Si3 N4 /SiC-nanoparticle layers on the surface of the Si3 N4 /SiC-platelet layer. The residual stress was controlled by varying the amount of SiC added. The mechanical properties of three-layer composites with various combinations of microstructure and residual stress level were investigated. 相似文献
5.
Guo-Jun Zhang Zhen-Yan Deng Naoki Kondo Jian-Feng Yang Tatsuki Ohji 《Journal of the American Ceramic Society》2000,83(9):2330-2332
A ZrB2 –SiC composite was prepared from a mixture of zirconium, silicon, and B4 C via reactive hot pressing. The three-point bending strength was 506 ± 43 MPa, and the fracture toughness was 4.0 MPa·m1/2 . The microstructure of the composite was observed via scanning electron microscopy; the in-situ -formed ZrB2 and SiC were found in agglomerates with a size that was in the particle-size ranges of the zirconium and silicon starting powders, respectively. A model of the microstructure formation mechanism of the composite was proposed, to explain the features of the phase distributions. It is considered that, in the reactive hot-pressing process, the B and C atoms in B4 C will diffuse into the Zr and Si sites and form ZrB2 and SiC in situ , respectively. Because the diffusion of Zr and Si atoms is slow, the microstructure (phase distributions) of the obtained composite shows the features of the zirconium and silicon starting powders. 相似文献
6.
Jingxian Zhang Mikio Iwasa Dongliang Jiang 《Journal of the American Ceramic Society》2005,88(4):1013-1016
It has been well accepted that polyethylene imine (PEI) is an effective dispersant for silicon carbide (SiC) in aqueous media. However, after the addition of sintering additives (Al2 O3 and Y2 O3 ), this dispersing effect is reduced significantly. In this work, a second dispersant, citric acid, was used to resolve this problem. It was found that citric acid could decrease the slurry viscosity (without sintering additives) and enhance the PEI adsorption on SiC particle surface. The optimal amount of citric acid required to achieve a minimum viscosity for 55 vol% SiC suspensions was equal to ∼0.87 wt% (at pH ∼6.8). With the aid of citric acid, well-stabilized SiC suspensions (containing sintering additives) were realized, which exhibited slight shear thinning rheologies. After tape casting and SPS sintering, dense SiC samples were obtained with a homogeneous fine-crystalline microstructure. Results showed that citric acid was an effective dispersant for improving the dispersion of SiC particles containing sintering additives. 相似文献
7.
Dense Sic ceramics were obtained by pressureless sintering of β-Sic and α-Sic powders as starting materials using Al2 O3 -Y2 O3 additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from α-SiC powder was composed of equiaxed grains, whereas SiC obtained from α-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the β→α phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using α-SiC powder increased slightly from 4.4 to 5.7 MPa.m1/2 with holding time, that is, increased grain size. In the case of the sintered SiC using β-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa.m1/2 with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains. 相似文献
8.
This communication demonstrates the role of cyclic compressive loads in inducing mode I fatigue crack growth at room temperature in Si3 N4 matrix–SiC whisker composite materials containing stress concentrations. The characteristics of stable, cyclic fracture are examined for several volume fractions of the SiC whisker and are compared with those of the matrix material. It is found that the composites with higher volume fractions of SiC whiskers exhibit an inferior resistance to fracture under cyclic compressive loads despite improvements in fracture toughness values. 相似文献
9.
The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al2 O3 ceramic and 29 vol% SiCw /Al2 O3 composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced. 相似文献
10.
The conditions necessary for synthesizing Al4 SiC4 from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al4 C3 ( s ) = Al4 SiC4 ( s ) changed from positive to negative at 1106°C. SiC and Al4 C3 formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al4 SiC4 then formed by reaction of the SiC and Al4 C3 at >1200°C. Al4 C3 , SiO2 , Al2 O3 , SiC, and Al4 O4 C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al4 SiC4 formed from a reaction of those intermediate products at >1600°C. 相似文献
11.
The fracture behavior of Al2 O3 /SiC nanocomposites has been studied as a function of the SiC volume fraction and compared to that of the pure Al2 O3 matrix. A pronounced strengthening effect was only observed for materials with low SiC content (i.e., ≤10 vol%) although no evidence of concurrent toughening was found. Assessment of near-tip crack opening displacement (COD) could not experimentally substantiate significant occurrence of an elastic crack-bridging mechanism, in contrast with a recently proposed literature model. Quantitative fractography analysis indicated that transgranular crack propagation in Al2 O3 /SiC nanocomposites depends on the location of the SiC dispersoids within the matrix texture; the higher the fraction of transgranularly located dispersoids, the more transgranular the fracture mode. Experimental evidence of remarkably high residual stresses arising from thermal dilatation mismatch (upon cooling) between Al2 O3 and SiC phases were obtained by fluorescence and Raman spectroscopy. A strengthening mechanism is invoked which merely arises from residual stress through strengthening of Al2 O3 grain boundaries. 相似文献
12.
José M. Córdoba María J. Sayagués María D. Alcalá Francisco J. Gotor 《Journal of the American Ceramic Society》2007,90(3):825-830
Titanium silicon carbide (Ti3 SiC2 ) and Ti3 SiC2 -based composite powders were synthesized by isothermal treatment in an inert atmosphere as a function of initial compositions (mixtures). A high content of TiC was obtained in the final product when the initial mixtures contained free carbon. The use of TiC as a reagent was unsuccessful in obtaining Ti3 SiC2 . High Ti3 SiC2 conversion was found for the initial mixtures containing SiC as the main source for silicon and carbon. An initial mixture with a large excess of silicon, 3Ti/1.5SiC/0.5C, was needed to obtain high-purity Ti3 SiC2 . A reaction mechanism, where Ti3 SiC2 nucleates on Ti5 Si3 C crystals and grows by long-range diffusion of Ti and C, is proposed. The reaction mechanism was proposed to be based on silicon loss during the formation of Ti3 SiC2 . 相似文献
13.
Molecular dynamics computer simulations were used to study the fracture behavior of silica, multicomponent silicate, and oxynitride intergranular films (IGFs) between silicon nitride crystals as a function of composition, film thickness, and crystallographic orientation. The maximum fracture strength is higher in the IGF between prism surfaces than between basal surfaces. This is caused by the preferential segregation of specific species to the basal surfaces in contrast to the prism surfaces, effectively modifying the composition within the glassy portion of the IGF, with a subsequent effect on strength. The ordering observed in the thinner IGFs causes an increase in strength in the linear portions of the stress/strain curves, effectively increasing fracture stress. The simulations show that the force/atom in the 1 nm SiO2 IGF in the direction of tensile strain is more similar to that in β-cristobalite than to v -SiO2 , whereas such data in the 2 nm SiO2 IGF is more similar to v -SiO2 . Simulations of the multicomponent silicate IGFs show the expected effect of lowering fracture strength with increasing modifier content in the silicate IGF, similar to that of bulk glasses. Similar to experimental studies, a decrease in strength is observed in silicon oxynitride IGFs with increasing oxygen concentration. 相似文献
14.
Hyun M. Jang Jong H. Moon Cheol W. Jang 《Journal of the American Ceramic Society》1992,75(12):3369-3376
Al2 O3 –ZrO2 –SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO2 phase on the kinetically stable colloid particles of Al2 O3 and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO2 and SiC whisker phases throughout the Al2 O3 matrix. The fracture toughness values of the Al2 O3 –15 vol% ZrO2 –20 vol% SiC whisker composites (∼12 MPa.m1/2 ) are substantially greater than those of comparable Al2 O3 –SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites. 相似文献
15.
James W. Zimmermann Gregory E. Hilmas William G. Fahrenholtz Ralph B. Dinwiddie Wallace D. Porter Hsin Wang 《Journal of the American Ceramic Society》2008,91(5):1405-1411
Thermophysical properties were investigated for zirconium diboride (ZrB2 ) and ZrB2 –30 vol% silicon carbide (SiC) ceramics. Thermal conductivities were calculated from measured thermal diffusivities, heat capacities, and densities. The thermal conductivity of ZrB2 increased from 56 W (m K)−1 at room temperature to 67 W (m K)−1 at 1675 K, whereas the thermal conductivity of ZrB2 –SiC decreased from 62 to 56 W (m K)−1 over the same temperature range. Electron and phonon contributions to thermal conductivity were determined using electrical resistivity measurements and were used, along with grain size models, to explain the observed trends. The results are compared with previously reported thermal conductivities for ZrB2 and ZrB2 –SiC. 相似文献
16.
Jianren Zeng Yoshinari Miyamoto Osamu Yamada 《Journal of the American Ceramic Society》1991,74(9):2197-2200
Fine Si3 N4 -SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si3 N4 , β-Si3 N4 , and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si3 N4 , β-SiC, and a few Si2 N2 O. 相似文献
17.
Takayuki Nagano Hui Gu Kenji Kaneko Guo-Dong Zhan Mamoru Mitomo 《Journal of the American Ceramic Society》2001,84(9):2045-2050
Nanocrystalline β-SiC with additions of 7 wt% Al2 O3 , 2 wt% Y2 O3 , and 1 wt% CaO was subjected to tensile deformation to study its microstructural behavior under the dynamic process. The liquid-phase-sintered body had a relative density of >97% and an average grain size of 170 nm. Tension tests were conducted at initial strain rates ranging from 2 × 10−5 to 5 × 10−4 s−1 , in the temperature range 1973–2023 K, in both argon and N2 atmospheres. Although grain-boundary liquids formed by the additions vaporized concurrently with the decomposition of SiC and extensive grain growth, the maximum tensile elongation of 48% was achieved in argon. Annealing experiments under the same conditions revealed that vaporization and grain growth were both dependent on experimental time. Therefore, high strain rates suffered less from the hardening effect when cavitation damage was more severe. Testing in an N2 atmosphere brought about crystallization of the grain-boundary phase and prevented severe vaporization; however, fracture occurred at only 8% elongation. Grain-boundary sliding was still the dominant mechanism for deformation. 相似文献
18.
F. F. LANGE 《Journal of the American Ceramic Society》1973,56(9):445-450
The Si3 N4 -SiC composite system was investigated to better understand the effect of microstructure on the strength-controlling factors, i.e. fracture energy, elastic modulus, and crack size. Silicon carbide dispersions with average particle sizes of 5, 9, and 32 μm were used to form 3 composite series within this system, each containing 0.10, 0.20, 0.30, and 0.40 vol fraction of the dispersed phase. These composites were fabricated by hot-pressing. Fracture energy and strength values were measured for each composite. A linear relation between the elastic modulus of the two phases was assumed. The crack size was calculated for each composite using the appropriate property values. The strength behavior of the 9- and 32-μm series was controlled by the crack size, which, in turn, was controlled by the particle size and volume fraction of the SiC phase. Particle size and volume fraction did not affect the crack size of the 5-μm series, in which strength was controlled by both fracture energy and elastic modulus. Strengths measured at 1400°C and thermal conductivity measurements indicate that several of these composites are promising as high-temperature structural materials. 相似文献
19.
Naoto Hirosaki Yoshinobu Yamamoto Toshiyuki Nishimura Mamoru Mitomo Junichi Takahashi Hisanori Yamane Masahiko Shimada 《Journal of the American Ceramic Society》2002,85(11):2861-2863
Phase relationships in the Si3 N4 –SiO2 –Lu2 O3 system were investigated at 1850°C in 1 MPa N2 . Only J-phase, Lu4 Si2 O7 N2 (monoclinic, space group P 21 / c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si3 N4 –SiO2 –Lu2 O3 system. The Si3 N4 /Lu2 O3 ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si3 N4 , and a new phase of Lu3 Si5 ON9 , having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si3 N4 –LuN–Lu2 O3 system. The phase diagram suggests that Lu4 Si2 O7 N2 is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications. 相似文献
20.
The strength, S , of ceramic and glass fibers often can be estimated from fractographic investigation using the fracture mirror radius, r m , and the relationship S = A m /( r m )1/2 , where A m is the "mirror constant." The present work estimates the value of A m for Tyranno® Si-Ti-C-O fibers in situ in a three-dimensional woven SiC/SiC-based composite to be 2.50 ± 0.09 MPa·m1/2 . This value is within the range of 2–2.51 MPa·m1/2 previously obtained for nominally similar Nicalon® Si-C-O fibers. 相似文献