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1.
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
2.
M. Mitomo M. Tsutsumi H. Tanaka S. Uenosono F. Saito 《Journal of the American Ceramic Society》1990,73(8):2441-2445
The microstructures of gas-pressure-sintered materials from β-Si3 N4 powder were characterized in terms of the diameter and aspect ratio of the grains. The size distributions of diameters in materials fabricated by heating for 1 h at 1850° to 2000°C were nearly constant when they were normalized by average diameters because of normal grain growth. The rate-determining step in the densification and grain growth was expected to be the diffusion of materials through the liquid phase. The activation energy for grain growth was 372 kJ/mol. The average aspect ratio of the grains was 3 to 4, whereas that of large grains was smaller because of shape accommodation. The fracture toughness was about the same as that of material from α-Si3 N4 powder despite the smaller aspect ratio of the grains 相似文献
3.
Mikito Kitayama Kiyoshi Hirao Motohiro Toriyama Shuzo Kanzaki 《Journal of the American Ceramic Society》1999,82(10):2931-2933
A powder mixture of α-Si3 N4 , Y2 O3 , and SiO2 was heat-treated in a loose powder state in the temperature range of 1750°–1900°C for 2 h; then, the mixture was acid-rinsed to remove the glassy phase. The widths and lengths of the resulting β-Si3 N4 crystals were analyzed quantitatively. The width–aspect-ratio distribution of the β-Si3 N4 crystals initially showed a strong negative correlation, and then the aspect ratio of crystals with small widths quickly decreased. After a stage in which aspect ratio was almost constant, regardless of the width, the width-aspect-ratio distribution evolved to show a positive correlation in the final stage. This pattern of morphology evolution of the β-Si3 N4 crystals was in good agreement with that predicted by the anisotropic Ostwald ripening model. 相似文献
4.
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
5.
Microstructural Evolution in Liquid-Phase-Sintered SiC: Part I, Effect of Starting Powder 总被引:1,自引:0,他引:1
Huiwen Xu Tania Bhatia Swarnima A. Deshpande Nitin P. Padture Angel L. Ortiz Francisco L. Cumbrera 《Journal of the American Ceramic Society》2001,84(7):1578-1584
The effect of starting SiC powder (β-SiC or α-SiC), with simultaneous additions of Al2 O3 and Y2 O3 , on the microstructural evolution of liquid-phase-sintered (LPS) SiC has been studied. When using α-SiC starting powder, the resulting microstructures contain hexagonal platelike α-SiC grains with an average aspect ratio of 1.4. This anisotropic coarsening is consistent with interface energy anisotropy in α-SiC. When using β-SiC starting powder, the β→α phase transformation induces additional anisotropy in the coarsening of platelike SiC grains. A strong correlation between the extent of β→α phase transformation, as determined using quantitative XRD analysis, and the average grain aspect ratio is observed, with the maximum average aspect ratio reaching 3.8. Based on these observations and additional SEM and TEM characterizations of the microstructures, a model for the growth of these high-aspect-ratio SiC grains is proposed. 相似文献
6.
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
7.
Commercial β-SiAlON ceramics were joined using mixed Si3 N4 , Y2 O3 , Al2 O3 , and SiO2 powders. At a joining temperature of 1600°C and a hold time in excess of 10 min, the adhesive was converted to an approximate 60:40 vol% composite of β-SiAlON–glass-ceramic. The grain size of the acicular β-SiAlON grains precipitated in the joint (submicrometer diameter, average aspect ratio of 10) was significantly smaller than those in the adherend ceramic (1–5 μm diameter). Intergrowth of β-SiAlON grains at the joint interface resulted in high bond strengths. The chemistry and microstructure of the ceramic adhesives used are described. 相似文献
8.
A distinct bimodal microstructure has been obtained in a Si3 N4 –BAS (barium aluminum silicate) ceramic-matrix composite by pressureless sintering. It is shown that the addition of coarse β-Si3 N4 seeds causes abnormal grain growth in this composite, and hence encourages the formation of a bimodal microstructure. This abnormal grain growth is due to the nature of the heterogeneous nucleation mechanism in Si3 N4 α-to-β phase transformation, and is promoted by the transformation. After complete phase transformation, further abnormal grain growth is comparably slow and governed by the Ostwald ripening mechanism. Therefore, a stable bimodal microstructure can be easily achieved by pressureless sintering. 相似文献
9.
Feng Ye Michael J. Hoffmann Stefan Holzer Yu Zhou Mikio Iwasa 《Journal of the American Ceramic Society》2003,86(12):2136-2142
The yttrium–sialon ceramics with the composition of Y0.333 Si10 Al2 ON15 and an excess addition of Y2 O3 (2 or 5 wt%) were fabricated by hot isostatic press (HIP) sintering at 1800°C for 1 h. The resulting materials were subsequently heat-treated in the temperature range 1300–1900°C to investigate its effect on the α→β-sialon phase transformation, the morphology of α-sialon grains, and mechanical properties. The results show that α-sialons stabilized by yttrium have high thermal stability. An adjustment of the α-sialon phase composition is the dominating reaction in the investigated Y–α-sialon ceramics during low-temperature annealing. Incorporation of excess Y2 O3 could effectively promote the formation of elongated α-sialon grains during post-heat-treating at relatively higher temperature (1700° and 1900°C) and hence resulted in a high fracture toughness ( K IC = 6.3 MPa·m1/2 ) via grain debonding and pullout effects. Although the addition of 5 wt% Y2 O3 could promote the growth of elongated α grains with a higher aspect ratio, the higher liquid-phase content increased the interfacial bonding strength and therefore hindered interface debonding and crack deflection. The heat treatment at 1500°C significantly changed the morphology of α-sialon grains from elongated to equiaxed and hence decreased its toughness. 相似文献
10.
Mode I Fracture Toughness of a Small-Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects 总被引:1,自引:0,他引:1
The Mode I fracture toughness ( K I C ) of a small-grained Si3 N4 was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si3 N4 grains was <0.4 µm, with aspect ratios of 2–8. K I C at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in K IC was observed through 1200°C, to ∼4.1 MPa·m1/2 , before increasing rapidly to 7.5–8 MPa·m1/2 at 1300°C. The decrease in K IC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in K IC . Crack healing was observed in specimens annealed in air. No R -curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C. 相似文献
11.
Ya-Wen Li Pei-Ling Wang Wei-Wu Chen Yi-Bing Cheng Dong-Sheng Yan 《Journal of the American Ceramic Society》2002,85(10):2545-2549
Two calcium-doped α-SiAlON compositions (Ca0.6 Si10.2 Al1.8− O0.6 N15.4 and Ca1.8 Si6.6 Al5.4 O1.8 N14.2 ) were prepared by hot pressing at 1600° and 1500°C, respectively, for complete phase transformation from α-Si3 N4 to α-SiAlON. Both samples were subsequently fired at different temperatures for different periods of time to study the grain growth of α-SiAlON. Elongated α-SiAlON grains were developed in both samples at high temperatures. The kinetics of grain growth was investigated based on the variations in length and width of the α-SiAlON grains under different sintering conditions. Different growth rates were found between the length and width directions of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development. 相似文献
12.
A ∼50 nm thick alumina layer was deposited on an Ni-based superalloy substrate by a sol–gel method. α-AlOOH particles presented in the layer after drying at 140°C transformed mostly to α-Al2 O3 grains within ∼1 min at 1100°C under a low oxygen partial pressure annealing environment. During the same time period, the α-Al2 O3 grains grew significantly in the lateral direction, resulting in the aspect ratio of grain diameter to thickness of ∼20. The presence of a preferred orientation in the α-Al2 O3 layer suggested that the mechanism for the lateral growth was abnormal. The lateral growth mechanism appeared to become very slow when a critical thickness (∼100 nm) was reached. 相似文献
13.
Novel Two-Step Sintering Process to Obtain a Bimodal Microstructure in Silicon Nitride 总被引:5,自引:0,他引:5
Hai-Doo Kim Byung-Dong Han Dong-Soo Park Byong-Taek Lee Paul F. Becher 《Journal of the American Ceramic Society》2002,85(1):245-252
A two-step sintering process is described in which the first step suppresses densification while allowing the α-to-β phase transformation to proceed, and the second step, at higher temperatures, promotes densification and grain growth. This process allows one to obtain a bimodal microstructure in Si3 N4 without using β-Si3 N4 seed crystals. A carbothermal reduction process was used in the first step to modify the densification and transformation rates of the compacts consisting of Si3 N4 , Y2 O3 , Al2 O3 , and a carbon mixture. The carbothermal reduction process reduces the oxygen:nitrogen ratio of the Y-Si-Al-O-N glass that forms, which leads to the precipitation of crystalline oxynitride phases, in particular, the apatite phase. Precipitation of the apatite phase reduces the amount of liquid phase and retards the densification process up to 1750°C; however, the α-to-β phase transformation is not hindered. This results in the distribution of large β-nuclei in a porous fine-grained β-Si3 N4 matrix. Above 1750°C, liquid formed by the melting of apatite resulted in a rapid increase in densification rates, and the larger β-nuclei also grew rapidly, which promoted the development of a bimodal microstructure. 相似文献
14.
Marlyse Demartin Claude Hérard Claude Carry Jacques Lemaître 《Journal of the American Ceramic Society》1997,80(5):1079-1084
Dedensification (up to 10% of the theoretical density in some instances) occurs at temperatures of .1320°C during the sintering of commercial undoped BaTiO3 powders. The expansion of the rigid, interconnected network of large, rounded grains that grow by the Ostwald ripening effect can explain the dedensification that is observed. The isotropic anomalous grain growth results from the appearance of a liquid phase at } 1320°C and induces tensile stresses on the residual porous fine-grained matrix. No dedensification was observed at temperatures of, 1320°C, despite anisotropic anomalous grain growth. 相似文献
15.
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
16.
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
17.
Pulsed Electric Current Sintering of Silicon Nitride 总被引:1,自引:0,他引:1
Motohiro Suganuma Yasunori Kitagawa Shigetaka Wada Norimitsu Murayama 《Journal of the American Ceramic Society》2003,86(3):387-394
Pulsed electric current sintering (PECS) has been used to densify α-Si3 N4 powder doped with oxide additives of Y2 O3 and Al2 O3 . A full density (>99%) was achieved with virtually no transformation to β-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the α-to-β phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated β-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (∼500 kJ/mol), thereby causing the retention of α-phase during the densification by PECS. 相似文献
18.
Fabrication and Mechanical Properties of Silicon Carbide-Silicon Nitride Composites with Oxynitride Glass 总被引:1,自引:0,他引:1
Young-Wook Kim Young-Il Lee Mamoru Mitomo Heon-Jin Choi June-Gunn Lee 《Journal of the American Ceramic Society》1999,82(4):1058-1060
A microstructure that consisted of uniformly distributed, elongated β-Si3 N4 grains, equiaxed β-SiC grains, and an amorphous grain-boundary phase was developed by using β-SiC and alpha-Si3 N4 powders. By hot pressing, elongated β-Si3 N4 grains were grown via alpha right arrow β phase transformation and equiaxed β-SiC grains were formed because of inhibited grain growth. The strength and fracture toughness of SiC have been improved by adding Si3 N4 particles, because of the reduced defect size and the enhanced bridging and crack deflection by the elongated β-Si3 N4 grains. Typical flexural-strength and fracture-toughness values of SiC-35-wt%-Si3 N4 composites were 1020 MPa and 5.1 MPam1/2 , respectively. 相似文献
19.
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
20.
Jiuxin Jiang Bo Zhu Peiling Wang Weiwu Chen Dongsheng Yan Hanrui Zhuang Yibing Cheng 《Journal of the American Ceramic Society》2005,88(4):950-953
The Pr α-sialon powders prepared by self-propagating high-temperature synthesis (SHS), consisting of 55 wt% Pr α-sialon and 45 wt% of β-sialon (abbreviated as α' and β'), were hot-pressed at 1800°C for 1 h. The results showed that Pr α' phase would transfer to β' with the appearance of JEM phase (Pr(Si6− z Al z )(N10− z O z )) after sintering, thus resulting in the increase of β' phase to 86 wt%. The addition of Y2 O3 into SHS-ed Pr α' powders as the starting materials restrains the transformation of α' to β' and prevents the formation of JEM phase as well. The nucleation mechanism of Pr α' grain during hot-pressing was investigated in terms of transmission electron microscope and energy-dispersive spectrometer analysis. Two nucleation modes of Pr α' grains were found, i.e., nucleating on the undissolved Pr α' grains and on the nuclei of (Pr, Y) α' grains precipitated from liquid phase. 相似文献