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1.
Dense, single-phase β-sialon ceramics were sintered at 1700°C and 200 MPa using the glass-encapsulated hot isostatic pressing technique. The materials were very hard, 1500 to 1700 kg / mm2 (98 N load), but were fairly brittle, with an indention fracture toughness of about 3 MPa · m1/2. The addition of 1 wt% Y2O3 before sintering had a positive effect on the toughness, especially at the low x compositions of Si3-xAlxOxN4-x, where KIC∼4 MPa · m1/2.  相似文献   

2.
Dense sialon ceramics along the tie line between Si3N4 and Nd2O3·9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated in the temperature range 1300–1750°C and cooled either by turning off the furnace (yielding a cooling rate (Tcool) of ∼50°C/min) or quenching (Tcool≥ 400°C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase α-sialon forms for 0.30 x 0. 51 in the formula NdxSi12–4S x Al4.5 x O1. 5 x , N16–1.5 x . The α-sialon is stable only at temperatures above 1650°C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an α-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed α-sialon decomposes to the more stable β-sialon and either the melilite phase or a new phase of the composition NdAl(Si6-zAlz)N10-zOz. Nd-doped α-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22. 5 GPa) and a fracture toughness ( K lc= 4.4 MPa·m1/2) at room temperature. The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials.  相似文献   

3.
Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si3N4-Al2O3, containing β-sialon, X phase, β-Si3N4, and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si4+ migration for the temperature range studied. Pure X phase shows ionic conduction by Si4+ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si3N4-Al2O3 containing β-sialon, β-Si3N4 X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively.  相似文献   

4.
The yttrium–sialon ceramics with the composition of Y0.333Si10Al2ON15 and an excess addition of Y2O3 (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 Y2O3 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% Y2O3 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   

5.
Y-PSZ ceramics with 5 wt% Al2O3 were synthesized by a sol–gel route. Experimental results show that powders of metastable tetragonal zirconia with 2.7 mol% Y2O3 and 5 wt% Al2O3 can be fabricated by decomposing the dry gel powder at 500°C. Materials sintered in an air atmosphere at 1500°C for 3 have high density (5.685 g/cm3), high content of metastable tetragonal zirconia (>96%), and high fracture toughness (8.67 MPa.m1/2). Compared with the Y-PSZ ceramics, significant toughening was achieved by adding 5 wt% Al2O3.  相似文献   

6.
Partially crystalline Si3N4, with nanosized crystals and a specific surface area greater than 200 m2/g, is obtained by pyrolysis of a commercially available vinylic polysilane in a stream of anhydrous NH3 to 1000°C. This polymer does not contain N initially. Crystallization to high-purity α-Si3N4 proceeds with additional heating above 1400°C under N2. The changes in crystallinity, powder morphology, infrared spectra, and elemental compositions, for samples annealed from 1000° to 1600°C under N2, are consistent with an amorphous-to-crystalline transformation. Although macroscopic consolidation and local densification occur at 1400°C, volatilization and accompanying weight loss limit bulk densification. The effect of temperature on specific surface area is examined and related to the sintering process. These results are applicable to pyrolysis, decomposition, and crystallization studies of ceramics synthesized by polymeric precursor routes.  相似文献   

7.
High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO2 as additives using α-Al2O3-seeded nanocrystalline γ-Al2O3 powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al2O3 sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m1/2, respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m1/2.  相似文献   

8.
The fabrication of transparent Nd3+ ion-doped Lu2O3 ceramics is investigated by pressureless sintering under a flowing H2 atmosphere. The starting Nd-doped Lu2O3 nanocrystalline powder is synthesized by a modified coprecipitant processing using a NH4OH+NH4HCO3 mixed solution as the precipitant. The thermal decomposition behavior of the precipitate precursor is studied by thermogravimetric analysis and differential thermal analysis. After calcination at 1000°C for 2 h, monodispersed Nd3+:Lu2O3 powder is obtained with a primary particle size of about 40 nm and a specific surface area of 13.7 m2/g. Green compacts, free of additives, are formed from the as-synthesized powder by dry pressing followed by cold isostatic pressing. Highly transparent Nd3+:Lu2O3 ceramics are obtained after being sintered under a dry H2 atmosphere at 1880°C for 8 h. The linear optical transmittance of the polished transparent samples with a 1.4 mm thickness reaches 75.5% at the wavelength of 1080 nm. High-resolution transmission electron microscopy observations demonstrate a "clear" grain boundary between adjacent grains. The luminescent spectra showed that the absorption coefficient of the 3 at.% Nd-doped Lu2O3 ceramic at 807 nm reached 14 cm−1, while the emission cross section at 1079 nm was 6.5 × 10−20 cm2.  相似文献   

9.
Duplex αβ,-sialon ceramics with a minimum volume fraction of residual intergranular glass have been prepared using Dy or Sm as the α-sialon stabilizing element. These microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix. A number of the larger α-sialon grains contained dislocations and showed a core/shell structure. Dy gave an α-sialon which was stable over a wide temperature range (1350–1800°C) for long holding times, while the use of Sm resulted in less stable α-sialon structures at medium temperatures (1450°C) and the formation of melilite, R2Si3−xAlxO3+xN4−x, β-sialon, and the 21R sialon polytype during prolonged heating. High α-phase contents gave a very high hardness ( H V10 is approximately 22 GPa) but a comparatively low indentation fracture toughness (around 4.4 MPam1/2). Duplex sialons fabricated from powder mixtures corresponding to an α-to-β sialon ratio of around 50:50 resulted in a sialon material with a favorable combination of high hardness (around 22 GPa) and increased toughness (to around 5.5 MPam1/2).  相似文献   

10.
Plasma etching of β-Si3N4, α-sialon/β-Si3N4 and α-sialon ceramics were performed with hydrogen glow plasma at 600°C for 10 h. The preferential etching of β-Si3N4 grains was observed. The etching rate of α-sialon grains and of the grain-boundary glassy phase was distinctly lower than that of β-Si3N4 grains. The size, shape, and distribution of β-Si3N4 grains in the α-sialon/β-Si3N4 composite ceramics were revealed by the present method.  相似文献   

11.
Gas-pressure sintering of α-Si3N4 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-Si3N4 powder was similar to that in the materials from β-Si3N4 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 Si3N4 ceramics.  相似文献   

12.
Dense Sic ceramics were obtained by pressureless sintering of β-Sic and α-Sic powders as starting materials using Al2O3-Y2O3 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   

13.
The translucent Mg-α-sialon ceramics have been prepared by spark plasma sintering (SPS) α-Si3N4 powder with AlN and MgO as the additives at 1850°C for 5 min. The sample possesses a uniform, dense microstructure under the rapid densification of SPS process. The translucent Mg-α-sialon ceramics achieve the maximum transmittance of 66.4% for the sample of 0.5 mm in thickness in the medium infrared region, which could be attributed to the equiaxed microstructure and few glassy phase confirmed by the observation of transmission electron microscopy. The material also exhibits good mechanical properties of high hardness (21.4±0.3 GPa) and fracture toughness (6.1±0.1 MPa·m1/2).  相似文献   

14.
Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si3N4 powders as raw materials and MgSiN2 as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si3N4 phase left in the sintered material. α-Si3N4 was completely transformed to β-Si3N4 after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)−1 was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si3N4 ceramics with high thermal conductivity when appropriate additives are used.  相似文献   

15.
Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized β-sialon powder was monitored by X-ray powder diffraction, solid-state 29Si and 27Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but 29Si and 27Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJmol−1, suggesting the rate-limiting step to be due to dissolution of O2 in an amorphous silica surface layer on the β-sialon particles.  相似文献   

16.
Effects of N2 sintering atmosphere and the starting SiC powder on the microstructural evolution of liquid-phase-sintered (LPS) SiC were studied. It was found that, for the β-SiC starting powder case, there was complete suppression of the β→α phase transformation, which otherwise goes to completion in Ar atmosphere. It was also found that the microstructures were equiaxed and that the coarsening was severely retarded, which was in contrast with the Ar-atmosphere case. Chemical analyses of the specimens sintered in N2 atmosphere revealed the presence of significant amounts of nitrogen, which was believed to reside mostly in the intergranular phase. It was argued that the presence of nitrogen in the LPS SiC helped stabilize the β-SiC phase, thereby preventing the β→α phase transformation and the attendant formation of elongated grains. To investigate the coarsening retardation, internal friction measurements were performed on LPS SiC specimens sintered in either Ar or N2 atmosphere. For specimens sintered in N2 atmosphere, a remarkable shift of the grain-boundary sliding relaxation peak toward higher temperatures and very high activation energy values were observed, possibly due to the incorporation of nitrogen into the structure of the intergranular liquid phase. The highly refractory and viscous nature of the intergranular phase was deemed responsible for retarding the solution–reprecipitation coarsening in these materials. Parallel experiments with specimens sintered using α-SiC starting powders further reinforce these arguments. Thus, processing of LPS SiC in N2 atmosphere open the possibility of tailoring their microstructures for room-temperature mechanical properties and for making high-temperature materials that are highly resistant to coarsening and creep.  相似文献   

17.
Ultra-high-temperature ceramic composites of ZrB2 20 wt%SiC were pressureless sintered under an argon atmosphere. The starting ZrB2 powder was synthesized via the sol–gel method with a small crystallite size and a large specific surface area. Dry-pressed compacts using 4 wt% Mo as a sintering aid can be pressureless sintered to ∼97.7% theoretical density at 2250°C for 2 h. Vickers hardness and fracture toughness of the sintered ceramic composites were 14.82±0.25 GPa and 5.39±0.13 MPa·m1/2, respectively. In addition to the good sinterability of the ZrB2 powders, X-ray diffraction and scanning electron microscopy results showed that Mo formed a solid solution with ZrB2, which was believed to be beneficial for the densification process.  相似文献   

18.
The shrinkage behavior of isolated pores during the liquidphase sintering of MgO-CaMgSiO4 at 1650°C in O2 and N2 atmospheres has been studied. When 90MgO- 10Ca MgSiO4 specimens containing artificially produced large spherical pores are sintered in O2, the liquid and grains flow into the pores as oxygen diffuses out. When sintered in N2 the pores remain intact even after a long time, because the N2 gas entrapped in them does not diffuse out. The effect of the sintering atmosphere has also been studied in a fine powder mixture of 80MgO 20CaMgSiO4 composition. Changing the atmosphere from O2 to N2 during the sintering treatment reduces the porosity, probably because of the enhanced oxygen diffusion from the pores. The pores grow when the sintering atmosphere is changed from N2 to O2, probably because of oxygen diffusion into the pores from the specimen surface. The practical implication of these results is that changing the atmosphere from O2 to air during the liquid-phase sintering of oxide ceramics can greatly reduce the porosity.  相似文献   

19.
C and SiO2 of different types were mixed and heat-treated (1410° to 1550°C), according to a standardized temperature program, in flowing N2 at different pressures. It was found that a starting material combination of C (115 m2/g) and SiO2 (50 m2/g) yielded pure Si3N4 after about 2 h at 1550°C and 1.3 MPa. By adjustment of the pressure in the range 2 to 6 MPa, irreversible evaporation of SiO( g ) as well as formation of nonequilibrium phases was suppressed, and the amounts of residual C and O in the sample were controlled. Even a small amount of CO in the N2 was observed to retard the nitridation. Possible explanations are discussed, based on thermodynamic calculations on the Si-O-N-C system.  相似文献   

20.
α/β-Si3N4 composites with various α/β phase ratios were prepared by hot pressing at 1600°–1650°C with MgSiN2 as sintering additives. An excellent combination of mechanical properties (Vickers indentation hardness of 23.1 GPa, fracture strength of about 1000MPa, and toughness of 6.3 MPa·m1/2) could be obtained. Compared with conventional Si3N4-based ceramics, this new material has obvious advantages. It is as hard as typical in-situ-reinforced α-Sialon, but much stronger than the latter (700 MPa). It has comparable fracture strength and toughness, but is much harder than β-Si3N4 ceramics (16 GPa). The microstructures and mechanical properties can be tailored by choosing the additive and controlling the heating schedule.  相似文献   

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