共查询到20条相似文献,搜索用时 31 毫秒
1.
Masaki Yasuoka Kiyoshi Hirao Manuel E. Brito Shuzo Kanzaki 《Journal of the American Ceramic Society》1995,78(7):1853-1856
Control of microstructure in the Al2 O3 /LaAl11 O18 system was performed. Elongated alumina grains were formed by doping with small addition of silica, and 20 vol% lanthana- luminate was formed in situ by the reaction of LaAlO3 - A12 O3 in an alumina matrix. Strengths of over 600 MPa and a high fracture toughness (6 MPa.m1/2 ) were achieved in the material with both elongated A12 O3 grains and LaAl11 O18 platelets. Generally antagonistic properties such as strength and fracture toughness have been made compatible in the same ceramic system. 相似文献
2.
The fracture toughness of 3 mol% Y2 O3 -ZrO2 (3Y-PSZ) composites containing 10–30 vol% Al2 O3 with different particle sizes was investigated. It was found that Al2 O3 dispersion of up to 30 vol% increased the fracture toughness by 17% to 30%, and the toughness increase was more remarkable in the composite dispersed with Al2 O3 particles of larger sizes. By combining the effects of the dispersion toughening and phase transformation toughening, the toughness change in the present materials was theoretically predicted, which was in good agreement with the experimental data. 相似文献
3.
LUTGARD C. DE JONGHE HERBERT SCHMID MEI CHANG 《Journal of the American Ceramic Society》1984,67(1):27-30
Poly crystalline and single-crystalline α-alumina were reacted with a eutectic CaO-Al2 O3 melt at 1530°C. A reaction zone develops in which a strongly textured CA6 layer, as well as a CA2 layer, forms, with a remaining layer of unreacted CaO-AI2 O3 melt. Silica, an impurity in the α-alumina, is rejected by the advancing CA6 phase and accumulates as calcium alumino-silicates in channels that assist in the reaction as fast transport paths. Reaction mechanisms and welding are briefly discussed. 相似文献
4.
Liquidus equilibrium relations for the air isobaric section of the system Y2 O3 –Fe2 O3 –FeO–Al2 O3 are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y 0.22 Fe 1.08 Al 0.70 O 2.83 * Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y 0.75 Fe 0.65 Al 0.60 O 3 garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y 0.75 Fe 1.25 O 3 and Y 0.75 Fe 0.32- Al 0.93 O 3 can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior. 相似文献
5.
A. Mark Thompson Helen M. Chan Martin P. Harmer Robert E Cook 《Journal of the American Ceramic Society》1995,78(3):567-571
The crack-healing behavior of A12 O3 and Al2 O3 -SiC nanocomposite was studied using Vickers indentations to generate precracks. After annealing in argon for 2 h at 1300°C, radial cracks in the nanocomposite healed: The cracks closed and there was a small degree of rebonding in the vicinity of the crack tip. In contrast, radial cracks in alumina grew when exposed to the same annealing treatment. The different responses are attributed to the fracture mode and toughening mechanism in each material: In the nanocomposite, the cracks close as the residual stresses surrounding the indentations relax. Radial cracks open and grow in A12 O3 because microstructural toughening is diminished during heating to the annealing temperature. An implication is that strength-limiting machining flaws in these materials behave similarly, thereby accounting for the strengthening effect of annealing in this "nanocomposite" system. 相似文献
6.
Isao Yamashita Koji Tsukuma Takafumi Kusunose 《Journal of the American Ceramic Society》2009,92(9):2136-2138
A translucent alumina composite containing 1 vol% LaAl11 O18 , prepared by the hot isostatic pressing (HIP) method, displays both high translucency and high fracture toughness. Its total forward transmission at 600 nm is 75% (thickness 1 mm), and its bending strength and fracture toughness are estimated to be 574±15 MPa and 5.9±0.46 MPa·m0.5 , respectively. Its high translucency is due to the similarity of refractive index between the additive phase (LaAl11 O18 ) and the matrix (alumina). 相似文献
7.
NILS CLAUSSEN 《Journal of the American Ceramic Society》1976,59(1-2):49-51
The fracture toughness of Al2 O3 is considerably increased by the incorporation of fine monoclinic ZrO2 particles. Hot-pressed composites containing 15 vol % ZrO2 yield Klc values of ∼ 10 MN/m3/2 , twice that of the A12 O3 matrix. It is hypothesized that this increase results from a high density of small matrix microcracks absorbing energy by slow propagation. The microcracks are formed by the expansion of ZrO2 during the tetragonal → monoclinic transformation. Since extremely high tensile stresses develop in the matrix, very small ZrO2 particles can act as crack formers, thus limiting the critical flaw size to small values. 相似文献
8.
Microstructure development in Bi0.5 (Na0.5 K0.5 )0.5 TiO3 prepared by a reactive-templated grain growth process was dependent on the sizes of platelike Bi4 Ti3 O12 (BiT) and equiaxed TiO2 particles used as starting materials. Calcined compacts were composed of large, platelike template grains and small, equiaxed matrix grains, the sizes of which were determined by those of the BiT and TiO2 particles, respectively. Texture was developed by the growth of template grains at the expense of matrix grains during sintering, and a new mechanism of grain growth was proposed on the basis of microstructure observation. The grain growth rate was determined by the template and matrix grain sizes, and a dense ceramic with extensive texture was obtained using small BiT and TiO2 particles. 相似文献
9.
NANCY J. DUDNEY 《Journal of the American Ceramic Society》1987,70(2):65-68
Composites of AgCl and A12 O3 were prepared by impregnating an alumina fiber paper with molten AgCl. The as-prepared composites showed no enhancement in the conductivity compared with pure AgCl samples prepared in a similar manner. When plastically deformed, the conductivity of both the pure and composite materials increased. After annealing at high temperatures, the conductivity of the pure AgCl sample decreased to the as-prepared value, whereas the composite maintained a 2- to 5-fold enhanced conductivity. This enhanced conductivity is due to deformation-induced defects which are stabilized by the alumina fibers rather than due to the AgCl/AI2 O3 interface. 相似文献
10.
A phase diagram of the system A12 O3 -A14 C3 is proposed. Two intermediate oxycarbides, A14 O4 C and A12 OC, were established. Eutectic melting between alumina and A14 O4 C occurred at 1840° C. No other low melting was observed. The alumina phase was not corundum but was similar to delta-alumina. Because of the high reactivity of aluminum carbide and all the intermediate compounds with moisture and oxygen, use of refractories based on the system A12 O3 -A14 C3 must be limited to applications where these agents are excluded. The behavior of high-alumina refractories in the presence of carbon is explained. 相似文献
11.
Haiyan Liu Morris E. Fine Herbert S. Cheng 《Journal of the American Ceramic Society》1991,74(9):2224-2233
Mineral oil lubricated sliding tests of Sic-whisker (SiCw, )/A12 03 composites and monolithic alumina against carburized 8620 steel were conducted on a cylinder-on-cylinder machine. The wear rate of the composites was one or two orders of magnitude less than that of pure alumina. Hot-pressed 25 wt% SiCw ,/A12 03 composite had a lower wear rate than sintered and HIPed 7.5 wt% SiC/Al2 O3 composite under the same conditions. The weight loss of the steel mating ring against the 25 wt% SiCw, composite was a factor of four lower than against the 7.5 wt% SiCw , composite, but the former was a factor of 50 to 60 less than that against pure alumina. The composites showed lower friction coefficients than alumina during the run-in stage. The friction coefficients decreased with initial wear. The steady-state friction coefficient decreased with increasing load up to 500 N for hot-pressed 25 wt% SiC,/Al2 03 composite. Further, SEM observation showed much less microfracture in composites than in alumina. EDAX analysis revealed less Fe transfer from the steel ring to the composites than to pure alumina. Wear by microfracture and by adhesion in composites was suggested to be suppressed by SIC whiskers. This in turn reduced wear of the steel because of the generation of fewer hard particles. 相似文献
12.
Gradient, porous alumina ceramics were prepared with the characteristics of microsized tabular α-Al2 O3 grains grown on a surface with a fine interlocking feature. The samples were formed by spin-coating diphasic aluminosilicate sol on porous alumina substrates. The sol consisted of nano-sized pseudo-boehmite (AlOOH) and hydrolyzed tetraethyl orthosilicate [Si(OC2 H5 )4 ]. After drying and sintering at 1150°–1450°C, the crystallographic and chemical properties of the porous structures were investigated by analytical electron microscopy. The results show that the formation of tabular α-Al2 O3 grains is controlled by the dissolution of fine Al2 O3 in the diphasic material at the interface. The nucleation and growth of tabular α-Al2 O3 grains proceeds heterogeneously at the Al2 O3 /glass interface by ripening nano-sized Al2 O3 particles. 相似文献
13.
A procedure for the formation of A12 O3 coatings as diffusion barriers between ductile reinforcements (e.g., Nb and Ta) and intermetallic matrices (e.g., MoSi2 and NiAl) is described. The coating technique involved sol-gel processing of alumina -forming sols with the addition of submicrometer-sized A12 O3 particles. Cracking in the coatings, a typical shortcoming of alumina sol-gel coating, was overcome by the addition of the fine particles into the sols. The surface charge of the A12 O3 particles was adjusted to be the same as the AIO(OH) colloids in the sols and electrophoresis was used to codeposit A12 O3 and AIO(OH) onto the surfaces of the reinforcements. The alumina gel derived from the sols acted as binder for the alumina particles, while the particles reduced the shrinkage of the sol-gel coatings and promoted the formation of dense coatings. The thickness of the coatings could be easily controlled without cracking and the effectiveness of the coatings as diffusion barriers was improved substantially. 相似文献
14.
The composite sol—gel (CSG) technology has been utilized to process SiC—Al2 O3 ceramic/ceramic particulate reinforced composites with a high content of SiC (up to 50 vol%). Alumina sol, resulting from hydrolysis of aluminum isopropoxide, has been utilized as a dispersant and sintering additive. Microstructures of the composites (investigated using TEM) show the sol-originating phase present at grain boundaries, in particular at triple junctions, irrespective of the type of grain (i.e., SiC or Al2 O3 ). It is hypothesized that the alumina film originating from the alumina sol reacts with SiO2 film on the surface of SiC grains to form mullite or alumina-rich mullite-glass mixed phase. Effectively, SiC particles interconnect through this phase, facilitating formation of a dense body even at very high SiC content. Comparative sinterability studies were performed on similar SiC—Al2 O3 compositions free of alumina sol. It appears that in these systems the large fraction of directly contacting SiC—SiC grains prevents full densification of the composite. The microhardness of SiC—Al2 O3 sol—gel composites has been measured as a function of the content of SiC and sintering temperature. The highest microhardness of 22.9 GPa has been obtained for the composition 50 vol% SiC—50 vol% Al2 O3 , sintered at 1850°C. 相似文献
15.
Tentative phase relations in the binary system BnOa-A12 O3 are presented as a prerequisite to the understanding of the system Li2 O-B2 O3 -Al2 O3 . Two binary compounds, 2A12 O3 .B2 O3 and 9A12 O3 .-2B2 O3 , melted incongruently at 1030° f 7°C and about 144°C, respectively. Two ternary compounds were isolated, 2Li2 O.A12 O3 .B2 O3 and 2Li2 O. 2AI2 O3 . 3B2 03 . The 2:1:1 compound gave a melting reaction by differential thermal analysis at 870°± 20° C, but the exact nature of the melting behavior was not determined. The 2:2:-3 compound melted at 790°± 20° C to Liz O.-5Al2 O3 and liquid. X-ray diffraction data for the compounds are presented and compatibility triangles are shown. 相似文献
16.
The phase equilibrium relations in the systems Y2 O3 -Al2 O3 and Gd2 O3 -Fe2 O3 were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd2 O3 -Fe2 O3 but only metastable in the system Y2 O3 -AI2 O3 . This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O3 . A new 2:1 molar ratio cubic phase has been found in the system Y2 O3 -A12 O3 . Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination. 相似文献
17.
SiO2 -Al2 O3 melts containing 42 and 60 wt% A12 O3 were homogenized at 2090°C (∼10°) and crystallized by various heat treatment schedules in sealed molybdenum crucibles. Mullite containing ∼78 wt% A12 O3 precipitated from the 60 wt% A12 O3 melts at ∼1325°± 20°C, which is the boundary of a previously calculated liquid miscibility gap. When the homogenized melts were heat-treated within this gap, the A12 O3 in the mullite decreased with a corresponding increase in the Al2 O3 content of the glass. A similar decrease of Al2 O3 in mullite was observed when crystallized melts were reheated at 1725°± 10°C; the lowest A12 O3 content (∼73.5 wt%) was in melts that were reheated for 110 h. All melts indicated that the composition of the precipitating mullite was sensitive to the heat treatment of the melts. 相似文献
18.
Mari Lou Balmer Fred F. Lange Vikram Jayaram Carlos G. Levi 《Journal of the American Ceramic Society》1995,78(6):1489-1494
Aqueous mixtures of zirconium acetate and aluminum nitrate were pyrolyzed and crystallized to form a metastable solid solution, Zr1- x Al x O2− x /2 ( x < 0.57). The initial, metastable phase partitions at higher temperatures to form two metastable phases, viz., t −ZrO2 +γ-Al2 O3 with a nano-scale microstructure. The microstructural observations associated with the γ- →α-Al2 O3 phase transformation in the t -ZrO2 matrix are reported for compositions containing 10, 20, and 40 mol% A12 O3 . During this phase transformation, the α-Al2 O3 grains take the form of a colony of irregular, platelike grains, all with a common crystallographic orientation. The plates contain ZrO2 inclusions and are separated by ZrO2 grains. The volume fraction of A12 O3 and the heat treatment conditions influence the final microstructure. At lower volume fractions of A12 O3 , the colonies coarsen to single, irregular plates, surrounded by polycrystalline ZrO2 . Interpenetrating microstructures produced at high volume fractions of A12 O3 exhibit very little grain growth for periods up to 24 h at 1400°C. 相似文献
19.
Koji Tsukuma 《Journal of the American Ceramic Society》2000,83(12):3219-3221
Composites of β-Ce2 O3 ·11Al2 O3 and tetragonal ZrO2 were fabricated by a reductive atmosphere sintering of mixed powders of CeO2 , ZrO2 (2 mol% Y2 O3 ), and Al2 O3 . The composites had microstructures composed of elongated grains of β-Ce2 O3 ·11Al2 O3 in a Y-TZP matrix. The β-Ce2 O3 ·11Al2 O3 decomposed to α-Al2 O3 and CeO2 by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce2 O3 ·11Al2 O3 divided into several grains of α-Al2 O3 and ZrO2 doped with Y2 O3 and CeO2 . High-temperature bending strength of the oxygen-annealed α-Al2 O3 composite was comparable to the β-Ce2 O3 ·11Al2 O3 composite before annealing. 相似文献
20.
Jing-Fong Tsai Cheng-Sheng Yu Dinesh K. Shetty 《Journal of the American Ceramic Society》1991,74(3):678-681
Stress–strain behaviors in three-point bending, transformation zones in single-edge-notch-bend specimens, and transformation toughening were studied in two types of Ce-TZP/Al2 O3 composites. A commercial grade exhibited yield-point behavior triggered by autocatalytic transformation and elongated zones. A new grade of Ce-TZP/Al2 O3 composite showed monotonic stress–strain behavior and a zone shape close to theoretical prediction based on a shear–dilatation yield criterion. The effects of zone sizes and shapes on fracture toughness of the two ceramic composites are shown to be qualitatively consistent with the predictions of transformation-zone shielding theories. 相似文献