共查询到20条相似文献,搜索用时 515 毫秒
1.
Guo-Dong Zhan Mamoru Mitomo Hidehiko Tanaka Young-Wook Kim 《Journal of the American Ceramic Society》2000,83(6):1369-1374
The effect of annealing with and without applied pressure on the microstructural development and phase transformation was investigated in fine-grained β-SiC ceramics containing α-SiC seeds. Materials annealed without pressure had a microstructure consisting of elongated grains, while materials annealed with pressure showed a duplex microstructure consisting of small matrix grains and some of elongated grains. However, annealing with pressure (25 MPa) was found to greatly retard phase transformation from β→α polytypes and inhibit grain growth. This change in lattice parameter suggests that the retardation of phase transformation and grain growth might be attributed to a reduced mass transport rate, which is the result of Al being introduced into the SiC by the annealing pressure. 相似文献
2.
Young-Wook Kim Mamoru Mitomo Hideki Hirotsuru 《Journal of the American Ceramic Society》1997,80(1):99-105
Fine (}0.1μm) β-SiC powders, with 3.3 wt% large (}0.44μm) α-SiC or β-SiC particles (seeds) added, were hot-pressed at 1750°C and then annealed at 1850°C to enhance grain growth. Microstructural development during annealing was investigated using image analysis. The introduction of larger seeds into β-SiC accelerated the grain growth of elongated large grains during annealing, in which no appreciable β→α phase transformation occurred. The growth of matrix grains in materials with β-SiC seeds was slower than that in materials with α-SiC seeds. The material with β-SiC seeds, which was annealed at 1850°C for 4 h, had a bimodal microstructure of small matrix grains and large elongated grains. In contrast, the material with α-SiC seeds, also annealed at 1850°C for 4 h, had a uniform microstructure consisting of elongated grains. The fracture toughnesses of the annealed materials with α-SiC and β-SiC seeds were 5.5 and 5.4 MPa·1/2 , respectively. Such results suggested that further optimization of microstructure should be possible with β-SiC seeds, because of the remnant driving force for grain growth caused by the bimodal microstructure. 相似文献
3.
Guo-Dong Zhan Rong-Jun Xie Mamoru Mitomo Young-Wook Kim 《Journal of the American Ceramic Society》2001,84(5):945-950
Ultrafine β-SiC powders mixed with 7 wt% Al2 O3, 2 wt% Y2 O3 , and 1.785 wt% CaCO3 were hot-pressed and subsequently annealed in either the absence or the presence of applied pressure. Because the β-SiC to α-SiC phase transformation is dependent on annealing conditions, the novel processing technique of annealing under pressure can control this phase transformation, and, hence, the microstructures and mechanical properties of fine-grained liquid-phase-sintered SiC ceramics. In comparison to annealing without pressure, the application of pressure during annealing greatly suppressed the phase transformation from β-SiC to α-SiC. Materials annealed with pressure exhibited a fine microstructure with equiaxed grains when the phase transformation from β-SiC to α-SiC was <30 vol%, whereas materials annealed without pressure developed microstructures with elongated grains when phase transformation was >30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mechanical properties could be achieved through annealing with or without pressure. 相似文献
4.
Kyeong-Sik Cho Young-Wook Kim † Heon-Jin Choi June-Gunn Lee 《Journal of the American Ceramic Society》1996,79(6):1711-1713
A process based on liquid-phase sintering and subsequent annealing for grain growth is presented to obtain in situ -toughened SiC-30 wt% TiC composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiC powders with the liquid forming additives of A12 O3 and Y2 O3 by hot pressing. During the subsequent heat treatment, the β→α phase transformation of SiC led to the in situ growth of elongated α-SiC grains. The fracture toughness of the SiC-30 wt% TiC composites after 6-h annealing was 6.9 MPa-m1/2 , approximately 60% higher than that of as-hot-pressed composites (4.4 MPa-m1/2 ). Bridging and crack deflection by the elongated α-SiC grains appear to account for the increased toughness of this new class of composites. 相似文献
5.
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
6.
Cheol-Woo Jang Joosun Kim Suk-Joong L. Kang 《Journal of the American Ceramic Society》2002,85(5):1281-1284
We investigated the effects of the sintering atmosphere on the interface structure and grain-growth behavior in 10-vol%-YAG-added SiC. When α-SiC was liquid-phase-sintered in an Ar atmosphere, the grain/matrix interface was faceted, and abnormal grain growth occurred, regardless of the presence of α-seed grains. In contrast, when the same sample was sintered in N2 , the grain interface was defaceted (rough), and no abnormal grain growth occurred, even with an addition of α-seed grains. X-ray diffraction analysis of this sample showed the formation of a 3C (β-SiC) phase, together with a 6H (α-SiC) phase. These results suggest that the nitrogen dissolved in the liquid matrix made the grain interface rough and induced normal grain growth by an α→β reverse phase transformation. Apparently, the growth behavior of SiC grains in a liquid matrix depends on the structure of the grain interface: abnormal growth for a faceted interface and normal growth for a rough interface. 相似文献
7.
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
8.
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
9.
《Journal of the European Ceramic Society》2002,22(7):1031-1037
β-SiC powders containing 1.1 wt.% α-SiC particles as seeds were hot-pressed at 1800 °C and then annealed at 2000 °C under 25 MPa uniaxial pressure to enhance grain growth. Microstructural development during annealing with pressure was investigated quantitatively and statistically using image analysis. The bimodal grain-thickness distribution in samples annealed with pressure was obtained due to abnormal grain growth of some grains. In situ-toughened microstructure has been developed after 3-h annealing. The grain-thickness and aspect ratio of large grains increase with annealing time, but grain growth comes mainly from increases in thickness after 3-h annealing, owing to the impingement of large gains. Typical flexural strength and fracture toughness of 4-h annealed sample were ∼500 MPa and ∼7.5 MPa m1/2, respectively. 相似文献
10.
Rong-Jun Xie Mamoru Mitomo Wonjoong Kim Young-Wook Kim Guo-Dong Zhan Yoshio Akimune 《Journal of the American Ceramic Society》2002,85(2):459-465
Starting from three powder mixtures of 80 vol% SiC (100α, 50α/50β, 100β) and 20 vol% YAG, liquid-phase-sintered silicon carbide ceramics were prepared by hot pressing at 1800°C for 1 h under 25 MPa, and then by hot forging or annealing at 1900°C for 4 h under an applied stress of 25 MPa in argon. The phase transformation and texture development in the as-hot-pressed, hot-forged, and annealed SiC ceramics were investigated via X-ray diffraction (XRD) and the pole figure measurements. The 6H → 4H polytypic transformation was observed in samples consisting of both α- and β-SiC phases when subjected to compressive deformation but absent in the case of annealing, suggesting the deformation-enhanced solubility of aluminum in SiC. Deformation was also found to enhance the 3C → 4H transformation in the sample containing entirely β-phase, which is due to the accelerated solution-precipitation process assisted by grain boundary sliding. The current study showed that the β- →α-phase transformation had little effect on texture development in SiC. Hot forging generally produced the strongest texture, with the calculated maximum of 2.2 times random in samples started with pure α-SiC phase. The mechanism for texture development was explained based on the microstructural observations. 相似文献
11.
Hidehiko Tanaka Naoto Hirosaki Toshiyuki Nishimura Dong-Woo Shin Sam-Shik Park 《Journal of the American Ceramic Society》2003,86(12):2222-2224
α(6 H )- and β(3 C )-SiC powders were sintered with the addition of AlB2 and carbon. α-SiC powder could be densified to ∼98% of the theoretical density over a wide range of temperatures from 1900° to 2150°C and with the additives of 0.67–2.7 mass% of AlB2 and 2.0 mass% of carbon. Sintering of the β-SiC powder required a temperature of >2000°C for densification with these additives. Grains in the α-SiC specimens grew gradually from spherical-shaped to plate-shaped grains at 2000°C; the 6 H polytype transformed mainly to 4 H . On the other hand, grains in the β-SiC largely grew at >2000°C; the 3 C polytype transformed to 4 H , 6 H , and 15 R . The stacking faults introduced in grains were denser in β-SiC than in α-SiC. The rapid grain growth in the β-SiC specimen was attributed to polytype transformation from the unstable 3 C polytype at the sintering temperature. 相似文献
12.
Wonjoong Kim Young-Wook Kim Duk-Ho Cho 《Journal of the American Ceramic Society》1998,81(6):1669-1672
Quantitative texture analysis, which included calculation of the orientation distribution function, was used to demonstrate textures in hot-pressed and subsequently annealed silicon carbide (SiC). The results indicated that the hot pressing and annealing could produce strong textures in SiC. Grain rotation during hot pressing and preferred grain growth of plate-shaped α-SiC grains during annealing both apparently contributed to texture development in the SiC materials. The {111} pole figure in hot-pressed material (mostly ß-SiC) and the (004) pole figure in annealed material (mostly α-SiC) were consistent with the microstructural observations. The fracture toughness of hot-pressed and annealed material measured parallel to the hot-pressing direction (5.7 MPam1/2 ) was higher than that measured perpendicular to the hot-pressing direction (4.4 MPam1/2 ), because of the texture and the microstructural anisotropy. 相似文献
13.
Swarnima A. Deshpande Tania Bhatia Huiwen Xu Nitin P. Padture Angel L. Ortiz Francisco L. Cumbrera 《Journal of the American Ceramic Society》2001,84(7):1585-1590
The effects of planar-defect density in a β-SiC starting powder and the addition of α-SiC seeds to that powder on microstructural evolution in liquid-phase-sintered (LPS) SiC have been studied separately. Planar-defect density is altered by appropriate heat treatment of an as-received β-SiC starting powder. It was found that a decrease in the planar-defect density in the powder retards the β→α phase transformation rate. It is proposed that, because nucleation of α-SiC occurs on the planar defects present in the β-SiC starting powders, the nucleation rate and the attendant rate of transformation decrease with a reduction in planar-defect density. Consequently, this reduces the frequency of formation of elongated β/α composite grains, resulting in lower average aspect ratios, as the initial untransformed β-SiC grains coarsen in an equiaxed manner. In contrast, addition of external α-SiC seeds has no effect on the β→α phase transformation rate, although a significant reduction in the average aspect ratio occurs. It is proposed that preferential equiaxed coarsening of the α-SiC seeds over elongated coarsening of β/α composite grains occurs, resulting in a reduction of overall coarsening anisotropy. 相似文献
14.
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
15.
Young-Wook Kim Mamoru Mitomo Hideyuki Emoto June-Gunn Lee 《Journal of the American Ceramic Society》1998,81(12):3136-3140
By using α- and β-SiC starting powders with similar particle sizes, the effects of initial α-phase content on the microstructure and the mechanical properties of the liquid-phase-sintered and subsequently annealed materials were investigated. The microstructures developed were analyzed by image analysis. When β-SiC powder was used, the grains became elongated. The average diameter decreased with increasing α-SiC content and the aspect ratio showed a maximum at 10%α-SiC and decreased with increasing α-SiC content in the starting powder. Such results suggest that microstructure can be controlled by changing α-phase content in starting powders. The strength increased with increasing α-SiC content in the starting powder while the fracture toughness decreased with increasing α-SiC content. There may be a trade-off in improving both the strength and toughness in SiC ceramics sintered with oxide additives. 相似文献
16.
In Situ-Toughened Silicon Carbide 总被引:10,自引:0,他引:10
Nitin P. Padture 《Journal of the American Ceramic Society》1994,77(2):519-523
A new processing strategy based on atmospheric pressure sintering is presented for obtaining dense SiC-based materials with microstructures consisting of (i) uniformly distributed elongate-shaped α-SiC grains and (ii) relatively high amounts (20 vol%) of second-phase yttrium aluminum garnet (YAG). This strategy entails the sintering of β-SiC powder doped with α-SiC, Al2 O3 , and Y2 O3 . The Al2 O3 and Y2 O3 aid in the liquid-phase sintering of SiC and form in situ YAG, which has a significant thermal expansion mismatch with SiC. During a subsequent grain-growth heat treatment, it is postulated that the α-SiC "seeds" assist in controlling in situ growth of the elongated α-SiC grains. The fracture pattern in the in situ -toughened SiC is intergranular with evidence of copious crack-wake bridging, akin to toughened Si3 N4 ceramics. The elongate nature of the α-SiC grains, together with the high thermal-residual stresses in the microstructure, enhance the observed crack-wake bridging. This bridging accounts for a measured twofold increase in the indentation toughness of this new class of in situ -toughened SiC relative to a commercial SiC. 相似文献
17.
Microstructure and Fracture Toughness of Hot-Pressed Silicon Carbide Reinforced with Silicon Carbide Whisker 总被引:1,自引:0,他引:1
Dong-Han Lee Hyoun-Ee Kim Seong-Jai Cho 《Journal of the American Ceramic Society》1994,77(12):3270-3272
The effects of β-SiC whisker addition on the microstructural evolution and fracture toughness ( K IC ) of hot-pressed SiC were investigated. Most of the whiskers added disappeared during the densifcation process by transformation into the α-phase. The remaining whiskers acted as nuclei for grain growth, resulting in the formation of large tabular grains around the whiskers. The tabular grains around the whiskers were believed to be formed because of the extreme anisotropy of the interfacial energy between α- and β-SiC. The K IC of the material was improved significantly by the whisker addition. The increase in the K IC was attributed to crack bridging followed by grain pullout as a result of the formation of tabular grains in a fine matrix. 相似文献
18.
β-SiC powder containing 6 wt% A12 O3 and 4 wt% Y2 O3 as sintering additives was pressureless sintered at 2000°C for 1 h (AYE-SiC) and 3 h (AYP-SiC). AYE-SiC consisted of an equiaxed grain structure and AYP-SiC exhibited a micro-structure with platelike grains as a result of grain growth related to β→α phase transformation during sintering, R -curve behavior and flaw tolerance for these silicon carbides were evaluated by the indentation-strength technique. For comparison, the R -curve behavior of conventional sintered, boron- and carbon-doped SiC (SS-SiC) was evaluated. AYE-SiC and AYP-SiC exhibited rising R -curve behavior with toughening exponents of m = 0.042 and m = 0.135, respectively. AYP-SiC exhibited better flaw tolerance and more sharply rising R -curve behavior than AYE-SiC. The more sharply rising R -curve behavior and the better flaw tolerance of AYP-SiC were attributed mainly to grain bridging of crack faces by platelike grains. Because of the high degree of transgranular fracture, SS-SiC exhibited a flat R -curve despite a microstructural feature with platelike grains. 相似文献
19.
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
20.
《Journal of the European Ceramic Society》2000,20(9):1319-1325
The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y2O3, has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y2O3 composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y2O3-phase and grain-growth of the bulk AlN grains. 相似文献