共查询到20条相似文献,搜索用时 31 毫秒
1.
Tzer-Shin Sheu 《Journal of the American Ceramic Society》1994,77(9):2345-2353
The in situ β-Si3 N4 /α'-SiAlON composite was studied along the Si3 N4 –Y2 O3 : 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β-Si3 N4 /α'-SiAlON composite were found to vary with composition. Because of the cooperative formation of α'-Si AlON and β-Si3 N4 during its phase development, this composite had equiaxed α'-SiAlON (∼0.2 μm) and elongated β-Si3 N4 fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30–40%α', which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m1/2 . α'-SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°-SiAlON grains were affected by the sintering conditions. 相似文献
2.
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
3.
Mamoru Mitomo Yoh-ichiro Sato Nobuo Ayuzawa Isamu Yashima 《Journal of the American Ceramic Society》1991,74(4):856-858
Plasma etching of β-Si3 N4 , α-sialon/β-Si3 N4 and α-sialon ceramics were performed with hydrogen glow plasma at 600°C for 10 h. The preferential etching of β-Si3 N4 grains was observed. The etching rate of α-sialon grains and of the grain-boundary glassy phase was distinctly lower than that of β-Si3 N4 grains. The size, shape, and distribution of β-Si3 N4 grains in the α-sialon/β-Si3 N4 composite ceramics were revealed by the present method. 相似文献
4.
Naoto Hirosaki Yoshio Akimune Mamoru Mitomo 《Journal of the American Ceramic Society》1994,77(4):1093-1097
Fine β-Si3 N4 powders with or without the addition of 5 wt% of large β-Si3 N4 particles (seeds) were gas-pressure sintered at 1900°C for 4 h using Y2 O3 and Al2 O3 as sintering aids. The microstructures were examined on polished and plasmaetched surfaces. These materials had a microstructure of in situ composites with similar small matrix grains and different elongated grains. The elongated grains in the materials with seeds had a larger diameter and a smaller aspect ratio than those in the materials without seeds. A core/rim structure was observed in the elongated grains; the core was pure β-Si3 N4 and the rim was β-SiAION. These results show that the large β-Si3 N4 particles acted as seeds for abnormal grain growth and the rim was formed by precipitation from the liquid containing aluminum. 相似文献
5.
Silicon nitride (Si3 N4 ) and SiAlONs can be self-toughened through the growth of elongated β-Si3 N4 /β-SiAlON grains in sintering. α-SiAlONs usually retain an equiaxed grain morphology and have a higher hardness but lower toughness than β-SiAlONs. The present work has demonstrated that elongated alpha-SiAlON grains can also be developed through pressureless sintering. alpha-SiAlONs with high-aspect-ratio grains in the calcium SiAlON system have exhibited significant grain debonding and pull-out effects during fracture, which offers promise for in-situ -toughened α-SiAlON ceramics. 相似文献
6.
Gas-pressure sintering of α-Si3 N4 was carried out at 1850 ° to 2000°C in 980-kPa N2 . The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si3 N4 powder was similar to that in the materials from β-Si3 N4 powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si3 N4 ceramics. 相似文献
7.
Dong-Duk Lee Suk-Joong L. Kang Gunter Petzow Duk N. Yoon 《Journal of the American Ceramic Society》1990,73(3):767-769
By using α-Si3 N4 and β-Si3 N4 starting powders with similar particle size and distribution, the effect of α-β (β') phase transition on densification and microstructure is investigated during the liquid-phase sintering of 82Si3 N4 ·9Al2 O3 ·9Y2 O3 (wt%) and 80Si3 N4 ·13Al2 O3 ·5AIN·5AIN·2Y2 O3 . When α-Si3 N4 powder is used, the grains become elongated, apparently hindering the densification process. Hence, the phase transition does not enhance the densification. 相似文献
8.
L. K. V. LOU T. E. MITCHELL A. H. HEUER 《Journal of the American Ceramic Society》1978,61(9-10):392-396
Impurity phases in commercial hot-pressed Si3 N4 were investigated using transmission electron microscopy. In addition to the dominant, β-Si3 N4 phase, small amounts of Si2 N2 O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 Å thick between, β-Si3 N4 grains and ∼ 30 Å thick between Si2 N2 O and β-Si3 N4 grains. 相似文献
9.
Youren Xu Chao M. Huang Waltraud M. Kriven Avigdor Zangvil 《Journal of the American Ceramic Society》1994,77(8):2213-2216
The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO2 doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si3 N4 inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO2 -doping and its high doping level as well as the high Al2 O3 concentration were thought to be the main reasons for accelerating the reaction between the α-Si3 N4 and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si3 N4 from the reacting liquid, resulting in a delay in the α→β-Si3 N4 transformation. The large O' grains and the α-Si3 N4 inclusions have a pronounced effect on the strength degradation of O'+β' ceramics. 相似文献
10.
Naoto Hirosaki Yoshio Akimune Mamoru Mitomo 《Journal of the American Ceramic Society》1993,76(7):1892-1894
β-Si3 N4 powder containing 1 mol% of equimolar Y2 O3 –Nd2 O3 was gas-pressure sintered at 2000°C for 2 h (SN2), 4 h (SN4), and 8 h (SN8) in 30-MPa nitrogen gas. These materials had a microstructure of " in-situ composites" as a result of exaggerated grain growth of some β Si3 N4 grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa-m1/2 because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas-pressure sintering of β-Si3 N4 powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure. 相似文献
11.
The microstructure, crystal structure, and chemical composition of reaction-sintered Si3 N4 containing iron were studied using conventional and scanning transmission electron microscopy. It was found that the grains of β -Si3 N4 were large and blocklike with well-developed facets, a series of voids along some grain boundaries, a subgrain of iron silicide near the periphery, and penetration of iron silicide into the three-grain junctions and grain boundaries. At some distance from each β -Si3 N4 grain was a region of small α-Si3 N4 grains, with no evidence of iron silicide. Between this region and the β -Si3 N4 grain was a zone containing both α- and β -Si3 N4 and iron silicide. These observations suggest that the large β -Si3 N4 grains grow in liquid iron silicide, that the smaller α-Si3 N4 grains grow from the vapor, and that the latter are converted to the β form by solution in, and reprecipitation from, liquid iron silicide. 相似文献
12.
Effect of Sintering Additives on Microstructure and Mechanical Properties of Porous Silicon Nitride Ceramics 总被引:2,自引:0,他引:2
Jun Yang Jian-Feng Yang Shao-Yun Shan Ji-Qiang Gao Tatsuki Ohji 《Journal of the American Ceramic Society》2006,89(12):3843-3845
Porous silicon nitride (Si3 N4 ) ceramics with about 50% porosity were fabricated by pressureless sintering of α-Si3 N4 powder with 5 wt% sintering additive. Four types of sintering aids were chosen to study their effect on the microstructure and mechanical properties of porous Si3 N4 ceramics. XRD analysis proved the complete formation of a single β-Si3 N4 phase. Microstructural evolution and mechanical properties were dependent mostly on the type of sintering additive. SEM analysis revealed the resultant porous Si3 N4 ceramics as having high aspect ratio, a rod-like microstructure, and a uniform pore structure. The sintered sample with Lu2 O3 sintering additive, having a porosity of about 50%, showed a high flexural strength of 188 MPa, a high fracture toughness of 3.1 MPa·m1/2 , due to fine β-Si3 N4 grains, and some large elongated grains. 相似文献
13.
Martin Krämer 《Journal of the American Ceramic Society》1993,76(6):1627-1629
α-Si3 N4 core structures within β-Si3 N4 grains have been studied by transmission electron microscopy. The grains were dispersed in an oxynitride glass which was previously melted at 1600°C. The cores were topotactically related to the as-grown β-Si3 N4 crystallites and are related to epitactical nucleation during heat treatment as the most probable mechanism. 相似文献
14.
high-strength Si3 N4 with elongated β-Si3 N4 and equiaxed α-sialon was tested in cyclic and static fatigue at 1400°C. At low stress intensity factors and high frequencies, the pullout process of the elongated grains was enhanced, which suppressed the crack growth. This provides a possible explanation for the increased lifetime under cyclic leading conditions reported for ceramics by several investigators. While crack-healing by high-temperature annealing was found to greatly reduce the subsequent static fatigue crack growth rate, it had only a modest effecf on cyclic fatigue and none at high frequencies. 相似文献
15.
Soo Young Lee 《Journal of the American Ceramic Society》1998,81(5):1262-1268
Si3 N4 /SiC composite materials have been fabricated by reaction-sintering and postsintering steps. The green body containing Si metal and SiC particles was reaction-sintered at 1370°C in a flowing N2 /H2 gas mixture. The initial reaction product was dominated by alpha-Si3 N4 . However, as the reaction processed there was a gradual increase in the proportion of β-Si3 N4 . The reaction-bonded composite consisting of alpha-Si3 N4 , β-Si3 N4 , and SiC was heat-treated again at 2000°C for 150 min under 7-MPa N2 gas pressure. The addition of SiC enhanced the reaction-sintering process and resulted in a fine microstructure, which in turn improved fracture strength to as high as 1220 MPa. The high value in flexural strength is attributed to the formation of uniformly elongated β-Si3 N4 grains as well as small size of the grains (length = 2 μm, thickness = 0.5 μm). The reaction mechanism of the reaction sintering and the mechanical properties of the composite are discussed in terms of the development of microstructures. 相似文献
16.
The abnormal grain growth of β-Si3 N4 was observed in a 70% Si3 N4 –30% barium aluminum silicate (70%-Si3 N4 –30%-BAS) self-reinforced composite that was pressureless-sintered at 1930°C; Si3 N4 starting powders with a wide particle-size distribution were used. The addition of coarse Si3 N4 powder encouraged the abnormal growth of β-Si3 N4 grains, which allowed microstructural modification through control of the content and size distribution of β-Si3 N4 nuclei. The mechanical response of different microstructures was characterized in terms of flexural strength, as well as indentation fracture resistance, at room temperature. The presence of even a small amount of abnormally grown β-Si3 N4 grains improved the fracture toughness and minimized the variability in flexural strength. 相似文献
17.
The kinetics of anisotropic β-Si3 N4 grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si3 N4 grains follows the empirical growth law Dn – D0 n = kt , with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si3 N4 growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time. 相似文献
18.
Zdenk Pánek 《Journal of the American Ceramic Society》1995,78(4):1087-1088
Experimental thermochemical data (temperature, pressure) corresponding to the equilibrium conditions between finegrained β-SiC and β-Si3 N4 for carbon activity a (C) = 1 are presented. Based on these data, the temperature dependence of ΔG°f (β-Si3 N4 ) has been expressed for standard states Si( s ), C( s ), and p(N2 ) = 0.1 MPa by the equation ΔA°f (β-Si3 N4 ) = (-995.9 + 0.4547 T/K) kJ mol for T/K ε〈1650; 1968〉. 相似文献
19.
Elongated β'-SiAlON grains grown from several finegrained Ym/3 Si12(m+n) Alm+n On N16–r compositions with α-Si3 N4 , AlN, Al2 O3 , and Y2 O3 starting materials have been examined. These grains have large aspect ratios and are oriented along the [0001] axis. TEM structural and chemical analysis suggests that they are nucleated from various seed crystals, which can be α-Si3 N4 , β-Si3 N4 , or other β'-SiAlON. The β'-SiAlON seed and the initial precipitation on β-Si3 N4 show a higher content of Al and O, indicating that a large transient supersaturation of Al and O in the liquid is instrumental for β'-SiAlON formation, whereas subsequent growth proceeds under a much lower driving force. The misfit between phases is accommodated by interfacial dislocations ( c -type and a -type). Fully grown β'-SiAlON grains usually contain several variants independently nucleated from the same seed. In particular, the two alternative α/β phase-matching possibilities result in two [0001] growth habits separated by a twin boundary. 相似文献
20.
Shock Synthesis of Cubic Silicon Nitride 总被引:2,自引:0,他引:2
Toshimori Sekine 《Journal of the American Ceramic Society》2002,85(1):113-116
The phase transitions of α-Si3 N4 and β-Si3 N4 have been investigated by shock compression through the recovery technique and Hugoniot measurements. α- and β-Si3 N4 are transformed into a cubic spinel structure ( c -Si3 N4 ). The yield of c -Si3 N4 increases with increasing shock pressure and reaches 100% at 63 GPa. The shock-synthesized c -Si3 N4 powders are nanocrystals and display a high-temperature metastability up to about 1620 K. c -Si3 N4 is one of the hard materials based on the measured equation of state. c -Si3 N4 powders have been characterized by electron microscopy and 29 Si magic angle spinning NMR spectroscopy. The purification and separation method has been developed to obtain pure c -Si3 N4 powders. 相似文献