共查询到20条相似文献,搜索用时 15 毫秒
1.
Microstructure and Grain-Boundary Composition of Hot-Pressed Silicon Nitride With Yttria and Alumina
The microstructure of two hot-pressed silicon nitrides containing Y2 O3 and Al2 O3 was examined by electron microscopy, electron diffraction, and quantitative, energy-dispersive X-ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y2 O3 +2 wt% Al2 O3 , as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high-resolution dark-field and lattice imaging. The second phase in a material with additives of S wt% Y2 O3 +5 wt% Al2 O3 was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot-stage experiments in a high-voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si3 N4 grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y2 O3 /Al2 O3 fluxing. 相似文献
2.
David R. Clarke 《Journal of the American Ceramic Society》1989,72(9):1604-1609
The outstanding question as to the microstructure of silicon nitride at temperatures associated with potential high-temperature applications of the material is addressed experimentally by quenching thin (transmission electron microscopy) samples from 1450°C and examining them in the microscope. The morphology of the microstructure is qualitatively unchanged compared to the materials slowly cooled, for example, after hot-pressing, to room temperature. The most significant difference is that the thickness of the intergranular phase is larger, typically 2 to 10 nm, as compared to the ∼ 1 nm observed in the hot-pressed material. In addition there is an apparent increase in the volume fraction of the intergranular phase at the three-grain junctions. On the basis of a number of supporting experiments including both hot-stage transmission electron microscopy (up to 1000°C) and Auger electron spectroscopy of material fractured and examined at 850°C, the change in microstructure is concluded to occur at temperatures above about 1000°C. 相似文献
3.
Youren Xu Avigdor Zangvil Martine Landon François Thevenot 《Journal of the American Ceramic Society》1992,75(2):325-333
A flexural strength of up to 1 GPa was achieved in SiC-AIN materials and is attributed to a dense, equiaxial grain structure of the 2H(δ) SiC-AIN solid solution, with a relatively uniform grain size of ∼ 1 μm. The strength was found to decrease with increasing grain size. While the β→α phase transformation and the formation of various metastable polytypes make microstructural control difficult in SiC materials, excellent control is facilitated in SiC-AIN materials as a result of the stable 2H solid solution. Several mechanisms of grain refinement during the β→ 2H transition were observed, most notably the direct formation of several 2H grains from a single β grain. In addition, grain growth is limited by the diffusion-controlled nature of the transition. These mechanisms could be utilized to achieve even higher strength values, with potentially higher reliability of the materials in structural applications. 相似文献
4.
The fracture-toughness-determining mechanism of silicon carbide with additions of yttria and alumina was studied. Observations of indentation crack profiles revealed that significant crack deflection had occurred. Median deflection angles increased with increased volume fractions of the second phases, which was accompanied by increased fracture toughness. 相似文献
5.
Oxidation of Silicon Nitride Hot-Pressed with Ceria 总被引:2,自引:0,他引:2
Short-term (<30 h) oxidation of CeO2 -doped hot-pressed Si3 N4 , studied at 773 to 1623 K in flowing dry air, resulted in parabolic-weight gain curves and in oxidation-rate constants nearly independent of the amount of grain-boundary second phase. Exolution of CeO2 crystals occurs from the oxide layer; their morphology depends on oxidation temperature. Either a direct redox reaction between CeO2 and Si3 N4 or solution of Si3 N4 in the glassy silica-rich oxide layer, followed by its oxidation by dissolved oxygen, have been proposed as possible oxidation mechanisms, both appearing strongly dependent on the low solubility of cerium oxides hi silicate melts. The value of the activation energy for oxidation of 385 kj·mol−1 suggests additive and impurity migration from the bulk to the Si3 N4 /oxide reaction interface as the most probable rate-limiting step. 相似文献
6.
Martin K. M. Hruschka Wenjie Si Samuele Tosatti Thomas J. Graule Ludwig J. Gauckler 《Journal of the American Ceramic Society》1999,82(8):2039-2043
A water-based route for processing ß-Si3 N4 from alpha-Si3 N4 , Al2 O3 , and Y2 O3 powder mixtures was established. The surface charges and isoelectric points of the three different powders were investigated within the pH range from pH 3 to pH 12. Citric acid diammonium salt was found to be an effective deflocculant for shifting the isoelectric points to pH 3.5 for Al2 O3 and to pH 6 for Y2 O3 . Aluminum hydroxide (Al(OH)3 ) showed strong interaction with the Si3 N4 powder, shifting the isoelectric point from pH 7 to pH 5.5. Low-viscosity, high-solids-loading suspensions (60-63 vol%) thus were possible at pH 9.7. The preparation of homogeneous and stable suspensions with a solids content of ≤61 vol% and a viscosity <1 Pa·s was limited to a pH regime between pH 9 and pH 10.5 because of the high solubility of yttria. The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease-catalyzed decomposition of urea at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green-density distribution was homogeneous throughout all bodies, even for complex geometries. 相似文献
7.
Hideki Hyuga Mark I. Jones Kiyoshi Hirao Yukihiko Yamauchi Katsumi Yoshida Hideki Kita 《Journal of the American Ceramic Society》2008,91(1):192-197
The wear behavior of Si3 N4 ceramics sintered with various rare earth additives was studied for nonlubricated sliding under different conditions, and scratch tests carried out in an attempt to correlate the wear behavior. When multiple scratch testing is used the results can be used to indicate the initial wear behavior under fracture-dominated wear of the materials. The additive system used in the sintering of the Si3 N4 ceramics affected the specific wear rate under nonlubricated sliding conditions, and under high load conditions, where fracture is dominant, the specific wear rate was shown to increase in samples sintered with lutetium as a consequence of a strong bonding strength between the grains and grain boundary resulting in a higher degree of brittle fracture. 相似文献
8.
Ellen Y. Sun Paul F. Becher Kevin P. Plucknett Chun-Hway Hsueh Kathleen B. Alexander Shirley B. Waters Kiyoshi Hirao Manuel E. Brito 《Journal of the American Ceramic Society》1998,81(11):2831-2840
Significant improvements in the fracture resistance of self-reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated ß-Si3 N4 seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria-to-alumina ratio in the sintering additives. It was found that the steady-state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R -curve and extensive interfacial debonding between the elongated ß-Si3 N4 grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the ß´-SiAlON growth layer formed on the elongated ß-Si3 N4 grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the ß´-SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct ß´-SiAlON growth layer forms. It is concluded that the fracture resistance of self-reinforced silicon nitrides can be improved by optimizing the sintering additives employed. 相似文献
9.
Jian-Feng Yang Tatsuki Ohji Koichi Niihara 《Journal of the American Ceramic Society》2000,83(8):2094-2096
The present study investigates the influence of the content of Y2 O3 –Al2 O3 sintering additive on the sintering behavior and microstructure of Si3 N4 ceramics. The Y2 O3 :Al2 O3 ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering-additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β-Si3 N4 grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction. 相似文献
10.
Martha A. Boling-Risser K. C. Goretta J. L. Routbort K. T. Faber 《Journal of the American Ceramic Society》2000,83(12):3065-3069
An experimental self-reinforced hot-pressed silicon nitride was used to examine the effects of microstructure on high-temperature deformation mechanisms during compression testing. At 1575–1625°C, the as-received material exhibited a stress exponent of 1 and appeared to deform by steady-state grain-boundary sliding accommodated by solution-reprecipitation of silicon nitride through the grain-boundary phase. The activation energy was 610 ± 110 kJ/mol. At 1450–1525°C for the as-received material, and at 1525–1600°C for the larger-grained heat-treated samples, the stress exponent was >1. Damage, primarily in the form of pockets of intergranular material at two-grain junctions, was observed in these samples. 相似文献
11.
Hot isostatic pressing was studied for densification of reaction-bonded Si3 N4 containing various levels of Y2 O3 . Near-theoretical density was achieved for com positions containing 3 to 7 wt% Y2 O3 . An Si3 N4 -5 wt% Y2 O3 composition had a 4-point flexural strength at 1375°C of 628 MPa and survived 117 h of stress rupture testing at 1400°C and 345 MPa . 相似文献
12.
By lamination of silicon nitride tapes, components with complex geometries can be produced. Unstructured tapes can be laminated by common thermal compression. Structured tapes, however, have to be joined by pressureless processes using e.g. pastes as lamination aids because deformation of the structures would occur. These pastes usually contain a binder for maintaining the mechanical contact between the tapes during processing. To prevent the high mass loss of typical organic binders during burnout, pre-ceramic polymers were used in this work. These ceramic precursors convert partly into an inorganic material during heat treatment with a significant reduced mass loss compared with common organic binders. Thus, the porosity in the interlayer of a laminated stack is strongly decreased, which should be favorable for the mechanical and thermal properties. This work discusses the resulting microstructure, strength, and thermal diffusivity data of stacks laminated with pastes containing various precursor contents. These results are compared with those obtained by samples prepared by compression of green tapes. It is found that except for some large pores, the microstructure of the precursor-derived interlayers is qualitatively the same as in the tape material. For stacks made by both lamination methods, strength measurements reveal that the properties parallel and perpendicular to the layers are different. It is shown that the same strength level can be obtained both by using the pressureless route and by the compression method. Unlike the strength, the thermal conductivity does not change with the direction of measurement. 相似文献
13.
Hot-pressed Si3 N4 samples were preoxidized and tested in stress rupture. The lifetime of preoxidized samples was compared to samples that were not pretreated; for the test conditions chosen, lifetime was shorter for the preoxidized samples. 相似文献
14.
Kevin M. Fox John R. Hellmann 《International Journal of Applied Ceramic Technology》2008,5(2):138-154
Microstructural evolution of silicon nitride (Si3 N4 ) and SiAlON materials and its influence on creep resistance is reviewed. Grain size, grain morphology, and the ratio of α- to β-phase grains play a part in resistance to creep. The glassy, intergranular phase typically has the strongest influence on creep. Creep data are usually obtained using uniaxial tensile or compressive tests, where creep in tension is controlled by cavitation and grain boundary sliding controls creep in compression. The impression creep methodology is also reviewed. An additional creep mechanism, dilation of the SiAlON grain structure, was found to be active in impression creep. 相似文献
15.
Paul F. Becher Gayle S. Painter Michael J. Lance Seiichiro Ii Yuichi Ikuhara 《Journal of the American Ceramic Society》2005,88(5):1222-1226
As in fiber-reinforced composites, debonding, which allows the elongated reinforcing grains to at least partially separate from the rest of the matrix, is a critical part of the toughening mechanism in self-reinforced silicon nitrides. In situ high-resolution electron microscopy observations reveal that the debonding path can occur at the interface between the grains and continuous nanometer-thick intergranular film (IGF) or within the IGF depending on the film's composition, which varies with the yttria to alumina ratio in the fixed total amount of sintering additives. Theoretical calculations reveal that the bonding across the interface can be weakened by decreasing the Al and O content ( z ) of the epitaxial Si6– Z Al Z O Z N8– Z layer on the grains, which is consistent with the observations of interfacial debonding. However, evidence also indicates that weakening of the amorphous network of the IGF occurs with increase in yttrium levels that can be responsible for the observed mixture of debonding by crack propagation along the interface and within the IGF when the sintering additive contains the highest yttria:alumina ratio. 相似文献
16.
G. G. TRANTINA 《Journal of the American Ceramic Society》1979,62(7-8):377-380
The strength of hot-pressed Si3 N, was evaluated for constant stress rate, linear cyclic stress, and constant-stress loading. A stress-rupture rig was used to simultaneously test 10 samples under constant-stress loading. Exponential crack-growth-rate expressions were numerically integrated for constant stress rate and cyclic-stress loading and an approximate expression for the time-to-failure for these loading cases was developed. For either the power law or the exponential crack-growth-rate formulation, linear cyclic-stress and constant-stress-rate loading can be treated with the same fracture stress-time-to-failure representation. This correspondence was demonstrated for hot-pressed Si3 N4 . The exponential crack-growth formulation provided a consistent interpretation of the strength-degradation results, whereas the power-law formulation was inadequate. 相似文献
17.
GUENTHER HIMSOLT HEINRICH KNOCH HEINZ HUEBNER † FRIEDRICH WILHELM KLEINLEIN 《Journal of the American Ceramic Society》1979,62(1-2):29-32
During hot-pressing of α-Si3 N4 powders, the equiaxed α micro-structure gradually transforms into a β structure characterized by needle-shaped prismatic grains which are closely entangled and linked together. With increasing amounts of the β fraction, the bend strength, fracture toughness, and work of fracture increase significantly, then decrease as grain growth occurs. The K lc , improves by a factor of >2 and the change in γF by a factor of >4. The crack resistance to achieve the same crack velocity in materials of different β contents shows a similar trend. The dependence of the mechanical properties on the microstructure is explained by linking and pullout of the β crystals and by grain coarsening. 相似文献
18.
Short-term exposure of hot-pressed silicon nitride to temperatures greater than 800°C in an oxidizing atmosphere causes an increase in the room-temperature strength and eliminates the truncated strength distribution produced by room-temperature proof-testing. Acid-etching the proof-tested samples restores the original truncated distribution. These strength changes are shown to be related to the formation of a glassy phase on the surface that smooths out the preexisting machining flaws. More extensive, long-term oxidation produces surface pits that lead to an irreversible change in strength 相似文献
19.
Erik E Luther Fred F. Lange Dale S. Pearson 《Journal of the American Ceramic Society》1995,78(8):2009-2014
Two different methods are used to coat silicon nitride particles with an alumina precursor to make Si3 N4 behave like Al2 O3 in aqueous slurries. The first method involves the precipitation of an aluminum hydroxycarbonate from dissolved Al(NO3 )3 during the decomposition of urea. In the second method, dry silicon nitride powder is reacted with aluminum tri- sec -butoxide in hexane at room temperature. Both methods produce a coated powder in which the electrophoretic and rheological properties of aqueous slurries mimic those of alumina. When salt is added to slurries consisting of coated Si3 N4 powder, all rheological evidence suggests the presence of a short-range repulsive potential that produces a weakly attractive particle network similar to that previously reported for Al2 O3 powder. Although electrophoretic and rheological data showed that the coated powder behaved like Al2 O3 , consolidation data indicated that slurries of coated powder with added salt did not pack to high density. In addition, these bodies were not plastic as found for bodies consolidated from dispersed and salt-added Al2 O3 slurries. 相似文献