共查询到20条相似文献,搜索用时 46 毫秒
1.
The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB2 –SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB2 –SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB2 –SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB2 –SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO2 –SiO2 –B2 O3 (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO2 precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO2 because they have less B2 O3 , which results in a smaller amount of precipitated ZrO2 and consequently fewer convection cells. 相似文献
2.
Tetragonal Zirconia Polycrystals Reinforced with SiC Whiskers 总被引:2,自引:0,他引:2
The microstructure and the mechanical properties of hot-pressed tetragonal ZrO2 polycrystals (TZP) reinforced with up to 30 vol% SiC whiskers were studied. The SiC whisker-TZP composites were stable under the hot-pressing conditions at 1450°C. Annealing in an oxidizing atmosphere at ∼1000°C resulted in glass formation and microcracking caused by whisker oxidation and transformation of the ZrO2 grains near the whiskers to monoclinic symmetry. The fracture toughness was markedly improved by the dispersed whiskers (∼12 Mpa·m1/2 at 30 vol% SiC) compared to the values measured for the matrix (∼6 Mpa·m1/2 ). The flexural strength of the hot-pressed TZP-30 vol% SiC whisker composite at 1000°C (∼400 MPa) was twice that of the TZP matrix. 相似文献
3.
Chien-Cheng Lin Avigdor Zangvil Robert Ruh 《Journal of the American Ceramic Society》1999,82(10):2833-2840
The oxidation of SiC whiskers, contained in alkoxide-derived mullite-based matrices and exposed in air at 1000–1350°C for up to 1000 h, has been studied by analytical TEM, high-resolution SEM, and XRD. Silicon carbide whiskers were effectively protected from oxidation when embedded in a pure mullite matrix, but oxidized considerably when embedded in mullite/ZrO2 matrices. The oxidation mechanisms varied with matrix composition and exposure temperature. At 1350°C the amorphous layer first crystallized as cristobalite, then gradually incorporated alumina. At later times, the mullite portion of the mullite/ZrO2 matrix dissolved extensively into the layer. Also, the zirconia particles reacted with silica to form zircon. At 1200°C less extensive interdiffusion and chemical reaction occurred, and the silica layer devitrified into cristobalite and quartz. At 1000°C no interdiffusion or chemical reaction was seen, and the silica layer tended to devitrify into quartz. The thickness of the oxide layer around a SiC whisker in a particular matrix depended on the morphology and composition of grains abutting it or adjacent to it. 相似文献
4.
Equilibrium relations in the system NiO–TiO2 –SiO2 in air have been investigated in the temperature range 1430° to 1660°C. The most conspicuous feature of the phase relations is the existence of a cation-excess spinel-type phase, in addition to NiO and NiTiO3 , on the liquidus surface and at subsolidus temperatures down to 1430°C. Three invariant points have been located on the liquidus. There is a peritectic at 1540°C characterized by coexisting NiO ( ss ), spinel( ss ), cristobalite, and liquid of composition 47 wt% NiO, 29 wt% TiO2 , and 24 wt% SiO2 . Two eutectics are present, one at 1480°C, with spinel( ss ), NiTiO3 , cristobalite, and liquid (42 wt% NiO, 43 wt% TiO2 , and 15 wt% SiO2 ), as the coexisting phases. The other is at 1490°C with NiTiO3 , rutile, cristobalite, and liquid (32 wt% NiO, 56 wt% TiO2 , and 12 wt% SiO2 ). A liquid miscibility gap extends across the diagram from the two bounding binary systems NiO–SiO2 and TiO2 –SiO2 . 相似文献
5.
The oxidation of SiC at 1200°C in a slowly flowing gas mixture of either air or air + 15 vol% H2 O at 10 atm (1 MPa) was studied for extended times to examine the effects of elevated water-vapor pressure on oxidation rates and microstructural development. At a water-vapor pressure of 1.5 atm (150 kPa), distinct SiO2 scale structures were observed on the SiC; thick, porous, nonprotective cristobalite scales formed above a thin, nearly dense vitreous SiO2 layer, which remained constant in thickness with time as the crystalline SiO2 continued to grow. The pore morphology of the cristobalite layer differed depending on the type of SiC on which it was grown. The crystallization and growth rates of the cristobalite layer were significantly accelerated in the presence of the high water-vapor pressure and resulted in rapid rates of SiC surface recession that were on the order of what is observed when SiO2 volatility is rate controlling at high gas-flow velocities (30 m/s). The recession process can be described by a paralinear kinetic model controlled by the conversion of dense vitreous SiO2 to porous, nonprotective SiO2 . 相似文献
6.
Glasses of composition 3ZrO2 O · 2SiO2 were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO2 was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO2 , reaching ∼5.0 MN/m3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO2 transformation toughening. 相似文献
7.
Ana M. Espino Luis A. Díaz Adolfo Fernández Ramón Torrecillas José S. Moya 《Journal of the American Ceramic Society》2005,88(4):1000-1003
The wettability of binary and ternary glasses belonging to SiO2 –Al2 O3 –ZrO2 diagram has been studied using the sessile drop technique at 1750° and 1800°C. The ternary SiO2 –Al2 O3 –ZrO2 (90–5–5 wt%) glass has proved to be well appropriated as a molybdenum oxidation barrier coating. The addition of 5 wt% of MoO2 slightly improves its wettablity at higher temperatures without affecting its oxidation barrier properties. The Mo comes into the glass network as a mixture of Mo5+ , Mo4+ , and Mo6+ . After oxidation at 1000°C in oxygen atmosphere, the molybdenum remains in the glass network as Mo6+ . 相似文献
8.
The interfacial reaction layers in the Ti/ZrO2 diffusion couples, isothermally annealed in argon at temperatures ranging from 1100° to 1550°C for 6 h, were characterized using scanning electron microscopy and transmission electron microscopy, both attached with an energy-dispersive spectrometer. Very limited reaction occurred between Ti and ZrO2 at 1100°C. A β'-Ti(Zr, O) layer and a two-phase α-Ti(O)+β'-Ti(Zr, O) layer were found in the titanium side after annealing at T ≥1300°C and T ≥1400°C, respectively. A three-phase layer, consisting of Ti2 ZrO+α-Ti(O, Zr)+β'-Ti (O, Zr), was formed after annealing at 1550°C. In the zirconia side near the original interface, β'-Ti coexisted with fine spherical c- ZrO2− x , which dissolved a significant amount of Y2 O3 in solid solution at T ≥1300°C. Further into the ceramic side, the α-Zr was formed due to the exsolution of Zr out of the metastable ZrO2− x after annealing at T ≥1300°C: the α-Zr was very fine and dense at 1300°C, continuously distributed along grain boundaries at 1400°C, and became coarsened at 1550°C. Zirconia grains grew significantly at T ≥1400°C, with the lenticular t -ZrO2− x being precipitated in c -ZrO2− x . Finally, the microstructural development and diffusion paths in the Ti/ZrO2 diffusion couples annealed at various temperatures were also described with the aid of the Ti–Zr–O ternary phase diagram. 相似文献
9.
Dennis S. Fox Elizabeth J. Opila Raiford E. Hann 《Journal of the American Ceramic Society》2000,83(7):1761-1767
The oxidation kinetics of CVD SiC were measured by thermogravimetric analysis (TGA) in a 4H2 ·12H2 O·10CO·7CO2 ·67N2 gas mixture flowing at 0.44 cm/s at temperatures between 1300° and 1450°C in fused quartz furnace tubes at 1 atm total pressure. The SiC was oxidized to form solid SiO2 . At ≥1350°C, the SiO2 was in turn volatilized. Volatilization kinetics were consistent with the thermodynamic predictions based on SiO formation. These two simultaneous reactions resulted in overall paralinear kinetics. A curve fitting technique was used to determine the linear and parabolic rate constants from the paralinear kinetic data. Volatilization of the protective SiO2 scale resulted in accelerated consumption of SiC. Recession rates under conditions more representative of actual combustors were estimated from the furnace data. 相似文献
10.
Masashi Mori Masahiro Yoshikawa Hibiki Itoh Toshio Abe 《Journal of the American Ceramic Society》1994,77(8):2217-2219
The sintering behavior and electrical conductivity of high-purity 8-mol% Y2 O3 -stabilized ZrO2 (8YSZ) with Al2 O3 additions were investigated. The addition of 1 wt% AI2 O3 to 8YSZ provided dense, sintered samples with 9.1% relative density at 1400°C without a holding time. Addition of 1 wt% SiO2 enhanced the sinterability of 8YSZ. Na2 O addition of 0.1 wt% remarkably lowered it. Electrical conductivity at 1000°C in air increased slightly with increased Ai2 O3 content up to 1 wt% and then monotonously decreased. 8YSZ with 1 wt% AI2 O3 showed the maximum conductivity of 0.16 S/cm at 1000°C. 相似文献
11.
The vertical section Ti-ZrO2 within the Ti-Zr-O system was investigated by metallographic, X-ray diffraction, electron probe, and melting point studies. Analyses were conducted using arcmelted specimens which had been equilibrated and quenched from temperatures of 600° to 1600°C. The Ti-ZrO2 section is similar to the Zr-ZrO2 system. At high temperatures, considerable amounts of Zr and O go into solid solution in Ti, stabilizing α-Ti to 30 wt% ZrO2 . From 30 to 98 wt% ZrO2 an α-Ti+ZrO2 region is defined, and at compositions above 98 wt% ZrO2 , single-phase ZrO2 ( ss ) exists. At low temperatures an α-Ti+(Ti,Zr)3 O field exists from 22 to 32 wt% ZrO2 ; this region decreases in size with increasing temperature until it disappears at 1200°C. Above 32 wt% ZrO2 , a three phase α-Ti+ (Ti,Zr)3 O+ZrO2 field exists; its stability extends from 1200°C at 30 wt% 相似文献
12.
Siddhartha Bandyopadhyay Joydeb Mukerji Kalpana Roychoudhury 《Journal of the American Ceramic Society》1989,72(6):1061-1064
The synthesis of dense sintered sialon with external additives selected from the system Y2 O3 –AIN–SiO2 is reported. The highest density (3.21 g/cm3 ) was achieved at 1750°C at 90 min of sintering with 5 wt% additive. The degree of sialon substitution increased with the amount of liquid; the YSiO2 N crystalline phase formed concurrently. Strength degradation occurred above 1000°C. The fracture toughness of the material sintered with a lower amount of sintering aid remained relatively unchanged to 1200°C. The material with more additive exhibited decreased toughness above 1000°C. 相似文献
13.
Strength of Nicalon Silicon Carbide Fibers Exposed to High-Temperature Gaseous Environments 总被引:1,自引:0,他引:1
The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H2 with various P H2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO2 layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO2 layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO2 inherent in the fibers. 相似文献
14.
Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure
Ya-Li Li Fen Zheng Yong Liang Xian-Feng Ma Suo-Jing Cui Takamasa Ishigaki 《Journal of the American Ceramic Society》2001,84(4):875-877
Oxidized amorphous Si3 N4 and SiO2 powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si2 ON2 ) was observed from amorphous silicon nitride (Si3 N4 ) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si2 ON2 coexisted with β-Si3 N4 with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si2 ON2 . Pressing a mixture of amorphous Si3 N4 of lower oxygen (1.5 wt%) and SiO2 under 1.0–5.0 GPa between 1000° and 1350°C did not give Si2 ON2 phase, but yielded a mixture of α,β-Si3 N4 , quartz, and coesite (a high-pressure form of SiO2 ). The formation of Si2 ON2 from oxidized amorphous Si3 N4 seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure. 相似文献
15.
The phase relations of the systems ZrO2 –TiO2 and ZrO2 –TiO2 –SiO2 were investigated. X-ray diffraction techniques served as the principal means of analysis. The binary system ZrO2 –TiO2 was found to be one of partial solid solutions with no intermediate compounds. A eutectic point was found to exist at 50 to 55 weight % ZrO2 and 1600°C. A preliminary investigation of the ternary system ZrO2 –TiO2 –SiO2 , although not extensive, resulted in a better understanding of this system, with a fairly accurate location of some of its boundary lines. A eutectic point was located at 2% ZrO2 , 10% TiO2 , and 88% SiO2 at approximately 1500°C. 相似文献
16.
Cheng-Yuan Tsai Chien-Cheng Lin Avigdor Zangvil Ai-Kang Li 《Journal of the American Ceramic Society》1998,81(9):2413-2420
The oxidation of hot-pressed SiC-particle (SiCp )/zirconia (ZrO2 )/mullite composites with various ZrO2 contents, exposed in air isothermally at 1000° and 1200°C for up to 500 h, was investigated; an emphasis was placed on the effects of the ZrO2 content on the oxidation behavior. A clear critical volume fraction of ZrO2 existed for exposures at either 1000° or 1200°C: the oxidation rate increased dramatically at ZrO2 contents of >20 vol%. The sharp transition in the oxidation rate due to the variation of ZrO2 content could be explained by the percolation theory, when applied to the oxygen diffusivity in a randomly distributed two-phase medium. Morphologically, the composites with ZrO2 contents greater than the critical value showed a large oxidation zone, whereas the composites with ZrO2 contents less than the critical value revealed a much-thinner oxidation zone. The results also indicated that the formation of zircon (ZrSiO4 ) at 1200°C, through the reaction between ZrO2 and the oxide product, could reduce the oxidation rate of the composite. 相似文献
17.
A furnace for use in conjunction with the X-ray spectrometer was developed which was capable of heating small powdered specimens in air to temperatures as high as 1850°C. This furnace was also used for the heating and quenching of specimens in air from temperatures as high as 1850°C. An area of two liquids coexisting between 20 and 93 weight % TiO2 above 1765°± 10°C. was found to exist in the system TiO2 –SiO2 , which is in substantial agreement with the previous work of other investigators. The area of immiscibility in the system TiO2 –SiO2 was found to extend well into the system TiO2 –ZrO2 –SiO2 . The two liquids were found to coexist over a major portion of the TiO2 (rutile) primary-phase area with TiO2 (rutile) being the primary crystal beneath both liquids. The temperature of two-liquid formation in the ternary was found to fall about 80°C. with the first additions of ZrO2 up to 3%. With larger amounts of ZrO2 the change in the temperature of the boundary of the two-liquid area was so slight as to be within the limits of error of the temperature measurement. Primary-phase fields for TiO2 (rutile), tetragonal ZrO2 , and ZrTiO4 were found to exist in the system TiO2 –ZrO2 –SiO2 . SiO2 as high cristobalite is known to exist in the system TiO2 –ZrO2 –SiO2 . 相似文献
18.
Michael K. Cinibulk Geoff E. Fair Ronald J. Kerans 《Journal of the American Ceramic Society》2008,91(7):2290-2297
The stability of lanthanum orthophosphate (LaPO4 ) on SiC was investigated using a LaPO4 -coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO4 is reduced in the presence of SiC and reacts to form La2 O3 or La2 Si2 O7 and SiO2 as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO2 film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO4 coating. Continuous LaPO4 coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO4 coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO2 oxidation product to penetrate the coating. 相似文献
19.
Koji Takahashi Masahiro Yokouchi Sang-Kee Lee Kotoji Ando 《Journal of the American Ceramic Society》2003,86(12):2143-2147
Al2 O3 reinforced by SiC whiskers (Al2 O3 /SiC-W) was hot-pressed to investigate the crack-healing behavior. Semielliptical surface cracks of 100 μm in surface length were introduced using a Vickers indenter. The specimens containing precracks were crack-healed at temperatures between 1000° and 1300°C for 1 h in air, and their strengths were measured by three-point bending tests at room temperature and elevated temperatures between 400° and 1300°C. The results show that Al2 O3 /SiC-W possesses considerable crack-healing ability. The surface cracks with length of 2 c = 100 μm could be healed by crack-healing at 1200° or 1300°C for 1 h in air. Fracture toughness of the material was also determined. As expected, the SiC whiskers made their Al2 O3 tougher. 相似文献
20.
Mullite ceramics were fabricated according to the recently developed reaction-bonded Al2 O3 (RBAO) technology. Green compacts consisting of mechanically alloyed Al, SiC, and Al2 O3 were heat-treated in two steps. During the first hold at 1200°C, Al and SiC were oxidized to form Al2 O3 and SiO2 . On further heating, mullite was formed which then sintered during the second hold at 1550°C. All reactions involved in the process were associated with volume expansions that almost compensated for the shrinkage on sintering. Processing details and microstructure development are discussed. Reaction-bonded mullite ceramics exhibit high fracture strength, e.g., 290 MPa at a density of 97% of theoretical density. 相似文献