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A method for estimating the threshold stress intensity for crack growth is presented. The technique requires prior knowledge of the flaw population of a material and uses applied static loads followed by fast fracture to assess the effect of initial applied stress intensity on flaw behavior. The technique was applied to a hot-pressed Sic at 1200° and 1400°C in a nonoxidizing atmosphere. At 1400°C with a static load time of 4 h, the threshold stress intensity was determined to be ∼ 1.75 MPa·m1/2 with a slight tendency toward higher fracture stress with increasing initial stress intensity below the threshold. At 1200°C for a static load time of 4 h, apparent strengthening was observed below a threshold stress intensity of ∼2.25 MPa·m1/2 . This strengthening effect appears to result from stress relaxation in the crack-tip region, probably by plastic deformation which involves the oxide grain-boundary phase. 相似文献
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Crack Healing Behavior of Silicon Carbide Ceramics 总被引:1,自引:0,他引:1
Jan Korou Min Cheol Chu Masahiko Nakatani Kotoji Ando 《Journal of the American Ceramic Society》2000,83(11):2788-2792
This study focuses on the crack healing behavior of three kinds of commercial SiC ceramics. Specimens with and without cracks were subjected to thermal treatment at different temperatures, and their strengths were measured by a three-point bending test in accordance with JIS standards. The tests were performed in air at both room temperature and elevated temperatures between 600° and 1500°C. The healed specimens showed a complete recovery of strength at room temperature for the investigated crack sizes of 2 c ≅ 100 μm and 2 c ≅ 200 μm, and their strength increased in accordance with the healing temperature. The behavior of the healed specimens at elevated temperatures was influenced by the material used and the test temperature. Generally, the strength decreased at a high temperature, but the degree of strength reduction was determined by the kind of ceramic. The most important difference between the healed and smooth specimens was exhibited in material A. It was observed that at 1400°C, the bending strength of the healed specimens made from this ceramic was about 37% of the value for specimens in an as-received state. Static fatigue tests were also performed for ceramic B at 900° and 1000°C. The experiment demonstrated that the static fatigue limit of a healed specimen is about 75% of the monotonic bending strength at the same temperature. 相似文献
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Crack Healing in Silicon Carbide 总被引:1,自引:0,他引:1
Min-Cheol Chu Seong-Jai Cho † Yun-Chul Lee Hyun-Min Park Duk Y. Yoon 《Journal of the American Ceramic Society》2004,87(3):490-492
Crack healing in silicon carbide was investigated by introducing cracks into specimens and subsequently heat-treating the specimens. It was observed that cracks were healed, dramatically increasing the strength, by being filled with amorphous silica produced by the oxidation of silicon carbide. It was shown that the residual stress produced by the thermal expansion mismatch between silica within cracks and surrounding silicon carbide played a major role in the increase of the strength. Our results imply that a simple oxidation heat treatment can improve the reliability of silicon carbide components. 相似文献
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Guo-Dong Zhan Mamoru Mitomo Young-Wook Kim 《Journal of the American Ceramic Society》1999,82(10):2924-2926
Fine-grained (<1 μm) silicon carbide ceramics with high strength were obtained by using ultrafine (∼90 nm) β-SiC starting powders and a seeding technique for microstructural control. The microstructures of the as-hot-pressed and annealed ceramics without α-SiC seeds consisted of fine, uniform, and equiaxed grains. In contrast, the annealed material with seeds had a uniform, anisotropic microstructure consisting of elongated grains, owing to the overgrowth of β-phase on α-seeds. The strength, the Weibull modulus, and the fracture toughness of fine-grained SiC ceramics increased with increasing grain size up to ∼1 μm. Such results suggested that a small amount of grain growth in the fine grained region (<1 μm) was beneficial for mechanical properties. The flexural strength and the fracture toughness of the annealed seeded materials were 835 MPa and 4.3 MPa·m1/2 , respectively. 相似文献
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碳化硅陶瓷的液相烧结及其研究进展 总被引:3,自引:0,他引:3
本文对碳化硅液相烧结添加系统及其烧结机理作了论述。有氧化物参与的碳化硅的液相烧结可以降低碳化硅的烧结温度,促进碳化硅的致密化,提高碳化硅陶瓷的性能。沿晶断裂和穿晶断裂混合断裂机理是液相烧结碳化硅陶瓷强度和韧性提高的原因,表面强化和韧化可以进一步提高碳化硅陶瓷材料的性能。 相似文献
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Effective Sintering Aids for Silicon Carbide Ceramics: Reactivities of Silicon Carbide with Various Additives 总被引:2,自引:0,他引:2
Effective sintering aids for SiC ceramics are discussed on the basis of their reactivities with various sintering aids around sintering temperatures (2300 to 2400 K). The free energy consideration of the reactions suggests that metals and metal oxides which do not decompose SiC in the sintering process are effective as sintering aids for SiC ceramics. 相似文献
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This study deals with tensile creep and crack growth behavior of silicon carbide doped with alumina at 1400°C. Excellent creep resistance was observed for stresses from 150 MPa to 200 MPa. From the creep exponent of 1.4 and the activation energy of 320 kj/mol, the principal creep mechanism was Coble creep. The creep failure was caused by slow crack growth from a preexisting flaw. The crack was found to grow subcritically along grain boundaries almost in isolation. The relation between the time–to–failure and the applied stress was well treated by a diffusive crack growth model, and the threshold stress of this material at 1400°C was estimated at 165 MPa. 相似文献
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Young-Wook Kim Kotoji Ando Min Cheol Chu 《Journal of the American Ceramic Society》2003,86(3):465-470
Crack-healing behavior of liquid-phase-sintered (LPS) SiC ceramics has been studied as functions of heat-treatment temperature and crack size. Results showed that heat treatment in air could significantly increase the indentation strength. The heat-treatment temperature has a profound influence on the extent of crack healing and the degree of strength recovery. The optimum heat-treatment temperature depends on the softening temperature of an intergranular phase in each material. After heat treatment at the optimum temperature in air, the crack morphology almost entirely disappeared and the indentation strength recovered to the value of the smooth specimens at room temperature for the investigated crack sizes up to ∼200 μm. In addition, a simple heat treatment of SiC ceramics sintered with Al2 O3 –Y2 O3 –CaO at 1100°C for 1 h in air resulted in even further improvement of the strength, to a value of 1054 MPa (∼150% of the value of the unindented strength). Crack closure and rebonding of the crack wake due to oxidation of cracked surfaces were suggested as a dominant healing mechanism operating in LPS-SiC ceramics. 相似文献
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Ibram Ganesh D. C. Jana Shamshad Shaik N. Thiyagarajan 《Journal of the American Ceramic Society》2006,89(10):3056-3064
An aqueous gelcasting process for the preparation of dense as well as porous-sintered SiC ceramics has been described in this paper. A commercial silicon carbide powder coated with phenolic resin was used in this investigation. For the purpose of comparison, a pure SiC powder was also studied. ς potential and viscosity studies revealed that the pure SiC powder requires an electro-steric stabilization, whereas the phenolic resin-coated powder requires an electrostatic stabilization in order to produce their corresponding aqueous slurries with high solids content. Thermogravimetry and differential thermal analysis techniques have been used to study the decomposition behavior of phenolic resin. Aqueous slurries containing 25–50 vol% SiC powder were gelcast and sintered at 2150°C for 1 h. The sinterability of gelcast SiC samples was found to be highly influenced by the SiO2 formed on the surface of SiC during aqueous processing, as confirmed by the Fourier transform infrared spectroscopy study. The results obtained from various characterization techniques suggest that in order to make dense SiC parts with >3.13 g/mL bulk density (a theoretical density of 97.5%) by an aqueous gelcasting process, the starting phenolic resin (∼5%)-coated SiC powder should possess a median particle size of <11.0 μm, surface area of >3.2 m2 /g, a compact (green) density of >1.67 g/mL, and a B content of >0.5%. Further, by using polyethylene granules and organic foaming agents, sintered SiC foam with a porosity of >80%, a compressive strength of >16 MPa and a coefficient of thermal expansion of 4.574 × 10−6 /°C between 30° and 700°C can be prepared by an aqueous gelcasting process, followed by sintering at 2150°C for 1 h. 相似文献
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By using an oxynitride glass composition from the Y-Mg-Si-Al-O-N system as a sintering additive, the effect of atmosphere on densification was investigated during the liquid-phase sintering of SiC, and the resulting microstructure and mechanical properties of the sintered and subsequently annealed materials were investigated. SiC ceramics that were densified with 10 wt% oxynitride glass showed higher sinterability in a nitrogen atmosphere. Oxynitride glass enlarged the stability region of β-SiC and suppressed β→ alpha phase transformation, which resulted in an equiaxed microstructure. Grain growth of fine-grained SiC in some extent (up to ∼300 nm) was beneficial in improving both room-temperature strength and toughness. The best results were obtained when the ceramics were hot-pressed at 1800°C for 1 h in a nitrogen atmosphere and subsequently annealed at 1900°C for 3 h in an argon atmosphere. The room-temperature flexural strength and fracture toughness of the material were 847 MPa and 3.5 MPa·m1/2 , respectively. 相似文献
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Raymond Brennan Richard Haber Dale Niesz James McCauley 《International Journal of Applied Ceramic Technology》2008,5(2):210-218
Nondestructive ultrasound testing has been evaluated as a technique for analyzing isolated bulk defects and microstructural inhomogeneities in silicon carbide (SiC). Three SiC samples of varying thickness, two of which were fabricated by hot pressing and a third that was fabricated by chemical vapor deposition (CVD), were characterized using pulse–echo ultrasound characterization at a frequency of 75 MHz. Point analysis techniques were utilized to measure variations in time-of-flight (TOF), or ultrasound travel time through each sample, for calculation of regional differences in material velocity and elastic properties. C-scan imaging was used to evaluate differences in both TOF and reflected signal amplitude over the area of each sample. Area-under-the-curve (AUTC) and full-width at half-maximum (FWHM) data were obtained from normalized histograms to establish trends for direct sample comparison. It was determined that lower AUTC and FWHM values correlated to higher density samples with fewer inhomogeneities. However, the histogram tail area and distribution were also important features, providing information about specific inhomogeneities and their distributions. 相似文献
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采用自主研发的冲蚀磨损试验机,以碳素钢为参照,对碳化硅陶瓷进行冲蚀磨损实验.通过控制冲蚀角度、冲蚀时间和粒子冲蚀速度,测定碳化硅陶瓷的冲蚀磨损量,并借助于SEM对冲蚀磨损后试样的显微结构进行观察.实验结果表明:碳化硅陶瓷体积冲蚀率随角度增大而增大,在90°下达到最大;碳化硅陶瓷的冲蚀磨损机理为:在小角度下主要以切削磨损为主,在大角度下主要以脆性断裂为主. 相似文献
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Model for Fatigue Crack Growth in Grain-Bridging Ceramics 总被引:1,自引:0,他引:1
A model for fatigue crack propagation based on sliding wear of bridging grains is analyzed for polycrystalline ceramics. Taking into account damage development and crack tip energy balance, we have obtained rigorous solutions for equilibrium and compatibility equations in the crack wake under monotonic and cyclic loading/unloading conditions. Fatigue mechanics in ceramics is found to be formally similar to elastic-plastic mechanics of a path-dependent hardening material, due to the frictional resistance to reverse sliding. It features a load-displacement hysteresis causing energy dissipation and wear, and a longer cohesive zone required for supporting the same peak load with the wear-reduced bridging stresses. The unloading crack opening displacement is more strongly dependent on K max than on Delta K ; such displacement causes wear on the bridging grains. Meanwhile, incremental crack growth brings in new bridging grains that has a shielding effect on the crack tip stress field; such an effect is strongly dependent on K max but independent of Delta K . At steady state, when shielding accumulation and shielding degradation are balanced, the fatigue crack growth rate has a form d a /d N = A ( K max ) b (Delta K ) c , where A, b , and c are material-dependent parameters. Fatigue is predicted to have a very high b , a modest c , a higher fatigue resistance for tougher ceramics, and a stronger K max dependence for less tough ceramics. These predictions are in agreement with experimental observations. 相似文献
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Thermal Shock Behavior of Porous Silicon Carbide Ceramics 总被引:1,自引:0,他引:1
Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications. 相似文献