共查询到20条相似文献,搜索用时 15 毫秒
1.
Anil V. Virkar Jan Fong Jue James J. Hansen Raymond A. Cutler 《Journal of the American Ceramic Society》1988,71(3):148-C
Al2 O3 –ZrO2 and MgO–ZrO2 layered composites were fabricated in such a way that the outer layers of bar-shaped specimens consisted of the oxide (Al2 O3 or MgO) and unstabilized ZrO2 , while the bulk consisted of the oxide and partially stabilized ZrO2 . During cool-down from the sintering or the annealing temperature, the outer layers expanded because of the tetragonal → monoclinic transition in ZrO2 , thereby creating compressive stresses in the outer layers and tensile stresses in the bulk. Residual stresses were determined using a strain gage technique in which a strain gage was mounted on one face while the opposing face was incrementally ground off. Measurement of the strain as a function of thickness permitted the evaluation of residual stresses using pertinent equations from simple beam theory. 相似文献
2.
Peter Z. Cai David J. Green Gary L. Messing 《Journal of the American Ceramic Society》1997,80(8):1940-1948
To analyze the inhibited densification during sintering and differential shrinkage during cooling of Al2 O3 /ZrO2 symmetric and asymmetric laminates, viscoelastic formulations, in which the viscosity and elastic modulus vary with time, have been developed. The viscoelastic mismatch stresses have been numerically computed over the entire processing cycle, including the heating period, the isothermal period, and the cooling period. The viscosity and free sintering rates that are needed for stress computation have been obtained by modifying the parameters that are measured for a normal isotropic densifying compact using cyclic loading dilatometry. The modification is based on the available sintering models to account for the effect of strain history on compact viscosity and sintering rates. The stress calculation shows that, with the exception of the initial heating period, the viscoelastic stress is identical to the viscous stress that is calculated solely from the strain rate mismatch. Sintering damage in the laminates is shown to occur during densification under conditions where the differential sintering stress is smaller than the intrinsic sintering pressure. The magnitude of residual stress in hybrid laminates on cooling is dependent on the cooling rate, and slower cooling rates are capable of almost completely relaxing the expansion mismatch stress at temperatures of >1200°C. 相似文献
3.
Philippe Boch Thierry Chartier Muriel Huttepain 《Journal of the American Ceramic Society》1986,69(8):191-C
Fracture toughness of ZrO2 -toughened alumina could he increased by macroscopic interfaces, such as those existing in laminated composites. In this work, tape casting was used to produce A/A or A/B laminates, where A and B can be Al2 O3 , Al2 O3 /5 vol% ZrO2 , and Al2 O3 /l0 vol% ZrO2 . An increase of toughness is observed, even in the Al2 O3 /Al2 O3 laminates. 相似文献
4.
Michael G. Pontin Masa P. Rao Antonio J. Sánchez-Herencia Fred F. Lange 《Journal of the American Ceramic Society》2002,85(12):3041-3048
Ceramic laminates have been fabricated with thin layers, containing a mixture of unstabilized zirconia (MZ-ZrO2 ) and alumina (Al2 O3 ), sandwiched between thicker layers of alumina that contain a small fraction of Y2 O3 -stabilized tetragonal ZrO2 to inhibit grain growth. The MZ-ZrO2 undergoes a tetragonal-to-monoclinic phase transformation during cooling to produce biaxial compressive stresses in the thin layers. Cracks that extend within the thicker alumina layers can be arrested by the compressive layers to produce a threshold strength, i.e., a strength below which the probability of failure is zero. Laminates composed of Al2 O3 layers 315 ± 15 μm thick and Al2 O3 /MZ-ZrO2 layers 29 ± 3 μm thick exhibit a threshold strength of 507 ± 36 MPa, regardless of the MZ-ZrO2 content, for volume fractions ≥0.35. These results, piezospectroscopic stress measurements, and microstructural observations suggest that microcracking produced during the transformation reduces the magnitude of the compressive stresses achieved, which in turn limits the magnitude of the threshold strength. 相似文献
5.
J. LLorca José Y. Pastor Pedro Poza José I. Peña Isabel de Francisco Angel Larrea Victor M. Orera 《Journal of the American Ceramic Society》2004,87(4):633-639
The effect of Y2 O3 content on the flexure strength of melt-grown Al2 O3 –ZrO2 eutectics was studied in a temperature range of 25°–1427°C. The processing conditions were carefully controlled to obtain a constant microstructure independent of Y2 O3 content. The rod microstructure was made up of alternating bands of fine and coarse dispersions of irregular ZrO2 platelets oriented along the growth axis and embedded in the continuous Al2 O3 matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y2 O3 in relation to ZrO2 (Y2 O3 ). Higher Y2 O3 content did not substantially modify the mechanical response; however, materials with 0.5 mol% presented a significant degradation in the flexure strength because of the presence of large defects. They were nucleated at the Al2 O3 –ZrO2 interface during the martensitic transformation of ZrO2 on cooling and propagated into the Al2 O3 matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y2 O3 retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y2 O3 dropped rapidly with temperature as a result of extensive microcracking. 相似文献
6.
Xun-Li Wang Camden R. Hubbard Kathleen B. Alexander Paul F. Becher Jaime A. Fernandez-Baca Steve Spooner 《Journal of the American Ceramic Society》1994,77(6):1569-1575
High-resolution neutron powder diffraction was used to study the residual stresses in Al2 O3 -ZrO2 (12 mol% CeO2 ) ceramic composites containing 10, 20, and 40 vol% ZrO2 (CeO2 ). The diffraction data were analyzed using the Rietveld structure refinement technique. The analysis shows that for all samples, the CeO2 -stabilized tetragonal ZrO2 particles are in tension and the Al2 O3 matrix is in compression. For both the ZrO2 particles and the Al2 O3 matrix, the average lattice strains are anisotropic and increase approximately linearly with a decrease in the corresponding phase content. It is shown that these features can be qualitatively understood by taking into consideration the thermal expansion mismatch between the ZrO2 and Al2 O3 grains. Also, for all composite samples, the diffraction peaks are broader than the instrumental resolution, indicating that the strains in these samples are inhomogeneous. From an analysis of the refined peak shape parameters, the average root-meansquare strain, which describes the distribution of the inhomogeneous strain field, was determined. Finally, the average residual stresses were evaluated from the experimentally determined average lattice strains and compared with recent results of X-ray measurements on similar composites. 相似文献
7.
Thin two-phase, Al2 O3 / t -Zr(3Y)O2 layers bounded by much thicker Zr(3Y)O2 layers were fabricated by co-sintering powders. After cooling, cracks were observed along the center of the two-phase, Al2 O3 / t -Zr(3Y)O2 layers. Although the Al2 O3 / t -Zr(3Y)O2 layers are under residual, biaxial compression far from the surface, tensile stresses, normal to the center line, exist at and near the surface. These highly localized tensile stresses can cause cracks to extend parallel to the layer, to a depth proportional to the layer thickness. A tunneling/edge cracking energy release rate function is developed for these cracks. It shows that for a given residual stress, crack extension will take place only when the layer thickness is greater than a critical value. A value of the critical thickness is computed and compared with an available experimental datum point. In addition, the behavior of the energy release rate function due to elastic mismatch is calculated via the finite element method (FEM). It is also shown how this solution for crack extension can be applied to explain cracking associated with other phenomena, e.g., joining, reaction couples, etc. 相似文献
8.
ANIL V. VIRKAR JOW LAY HUANG RAYMOND A. CUTLER 《Journal of the American Ceramic Society》1987,70(3):164-170
Alumina-zirconia composites were fabricated by isostatic pressing and sintering of powder mixtures in such a way that bar-shaped specimens consisted of three layers. The outer layers contained A12 O3 and unstabilized ZrO2 while the central layer contained A12 O3 and partially stabilized ZrO2 (with 2 mol% Y2 O3 ). When cooled from the sintering temperature, some of the zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation of zirconia in the outer layers led to the establishment of surface compressive stresses and balancing tensile stresses in the bulk. The existence of surface compressive stresses was verified by a strain gauge technique and bending strength measurements on samples with varying thickness of the outer layers. The layered composites exhibited greater strength in comparison with monolithic Al2 O3 -ZrO2 specimens. Further, variation of strength in bending with outer layer thickness (for a fixed total thickness) indicated that failure occurred from internal flaws. Scanning electron microscopy of fracture surfaces revealed that strength-limiting flaws were voids located in the central layer near the interface separating the central and the outer layers. 相似文献
9.
Strength Improvement in Transformation-Toughened Alumina by Selective Phase Transformation 总被引:2,自引:0,他引:2
RAYMOND A. CUTLER JEFFREY D. BRIGHT ANIL V. VITRKAR DINESH K. SHETTY 《Journal of the American Ceramic Society》1987,70(10):714-718
A12 O3 -15 vol% ZrO2 bar-shaped ceramic specimens were fabricated in the green state in such a way that the near surface regions consisted of A12 O3 and unstabilized ZrO2 while the bulk consisted of A12 O3 and partially stabilized ZrO2 . After sintering, specimens had macroscopic residual compressive surface stresses and balancing interior tensile stresses due to the tetragonal-to-monoclinic phase transformation in the outer layers which occurs during cooling. The depth of the surface region was controlled during green forming. Residual surface compressive stresses at room temperature varied between 100 and 400 MPa depending on the outer-layer thickness. The increased strengths of the three-layer specimens were obtained in the as-fired unground condition, demonstrating that the stresses introduced are the result of transformation of tetragonal zirconia into monoclinic polymorph which occurred upon cooling from the sintering temperature. Specimens with residual surface compressive stresses were 200 MPa stronger at 750°C than monolithic specimens, demonstrating the viability of this approach for improving elevated-temperature mechanical properties. 相似文献
10.
Large, hard ZrO2 agglomerates remained in an Al2 O3 /ZrO2 composite suspension after inefficient ball-milling. The ZrO2 agglomerates shrank away from the consolidated Al2 O3 /ZrO2 powder matrix during sintering, producing crack-like voids which were responsible for strength degradation. 相似文献
11.
Yu Jia Yuji Hotta Kimiyasu Sato Koji Watari 《Journal of the American Ceramic Society》2006,89(3):1103-1106
ZrO2 –Al2 O3 nanocomposite particles were synthesized by coating nano-ZrO2 particles on the surface of Al2 O3 particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al2 O3 surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO2 in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO2 particles, which contained about 12 wt% ZrO2 , were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO2 was well dispersed in the Al2 O3 matrix. 相似文献
12.
Crack Bifurcation in Laminar Ceramic Composites 总被引:2,自引:0,他引:2
Matthias Oechsner Craig Hillman Fred F. Lange 《Journal of the American Ceramic Society》1996,79(7):1834-1838
Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al2 O3 layers sandwiched between thicker layers of Zr(12Ce)O2 . The Al2 O3 layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O2 layers were nearly free of residual, tensile stresses because they were much thicker than the Al2 O3 layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al2 O3 layer on the bifurcation phenomena. Bar specimens were fractured in four-point bending. When the propagating crack encountered the Al2 O3 layer, it bifurcated as it approached the Zr(12Ce)O2 / Al2 O3 interface. After the crack bifurcated, it continued to propagate close to the center line of the Al2 O3 layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O2 layer, where it bifurcated again as it entered the next Al2 O3 layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al2 O3 layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm. 相似文献
13.
Koji Matsui Takanori Yamakawa Masato Uehara Naoya Enomoto Junichi Hojo 《Journal of the American Ceramic Society》2008,91(6):1888-1897
The isothermal shrinkage behavior of 2.9 mol% Y2 O3 -doped ZrO2 powders with 0–1 mass% Al2 O3 was investigated to clarify the effect of Al2 O3 concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al2 O3 addition increased the densification rate with increasing temperature. The values of apparent activation energy ( nQ ) and apparent frequency-factor term (β0 n ), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering-rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain-boundary diffusion (GBD) to volume diffusion (VD) by Al2 O3 addition and both nQ and β0 n increased with increasing Al2 O3 concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al2 O3 addition largely depends on the increase of β0 n , that is, the increases of n with GBD→VD change and β0 with an increase in Al2 O3 content, although the nQ also increases with Al2 O3 addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al2 O3 at ZrO2 grain boundaries. 相似文献
14.
Powder compacts consisting of Al, Al2 O3 , and ZrO2 were heated by microwave radiation. Tracing the phase evolution during reaction bonding revealed the reaction mechanism. In the case of conventional heating, the compacts expanded slightly at temperatures of <700°C due to Al surface oxidation and expanded sharply at temperatures greater than 700°C as oxidation proceeded from the surface to the interior. Then, the compacts shrank at 1550°C due to sintering. For the case of microwave heating, the compacts expanded at temperatures of <550°C due to the formation of Al3 Zr. This Al3 Zr formation was caused by the preferential heating of ZrO2 relative to Al and Al2 O3 by microwave radiation. Then, Al3 Zr was oxidized to form Al2 O3 and ZrO2 at temperatures of >1000°C. Finally, the compacts shrank at 1550°C due to sintering, similarly to conventional sintering. 相似文献
15.
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
16.
Toshio Kimura Yasuyuki Kaneko Takashi Yamaguchi 《Journal of the American Ceramic Society》1991,74(3):625-632
The relationship between the dispersion of colloidal powder particles in Al2 O3 –ZrO2 suspensions and the microstructures of consolidated compacts was examined. Suspensions were prepared from Al2 O3 powder and ZrO2 sol with average particle sizes of 390 and 62 nm, respectively. The dispersion was controlled by pH and salt concentration adjustments. The compacts composed of completely separated Al2 O3 and ZrO2 layers were obtained from well-dispersed suspensions with pH values below about 4 and salt concentration of 0.0527 M. An increase in pH or salt concentration resulted in macroscopically uniform compacts. The compacts made from suspensions with pH values above about 7, however, were composed of a mixture of Al2 O3 and ZrO2 agglomerates, with one acting as a matrix and the other a dispersed phase. Suspensions with a pH value of 4.5 and optimum salt concentrations resulted in compacts with microscopically uniform microstructure. Above or below these salt concentrations, ZrO2 agglomerates were distributed in an Al2 O3 matrix. The optimum concentration was dependent on solid content. In addition, the dispersion of mixed suspensions was compared with those of single-component suspensions. The ZrO2 particles formed three-dimensional networks during agglomeration, which reduced the component separation in suspensions and during consolidation. 相似文献
17.
Kevin P. Plucknett Carlos H. Cáceres Christopher Hughes David S. Wilkinson 《Journal of the American Ceramic Society》1994,77(8):2145-2153
This paper addresses the effect of thermocompression pressure, shear deformation of green laminates, and postsinter HIPing on the microstructural homogeneity of cast tapes and laminates prepared from fine Al2 O3 , and Al2 O3 / ZrO2 , powders. Green density increases with increasing thermo–compression pressure. Sintered densities, however, depend more on the macroscopic uniformity in the green tapes. When density gradients develop within the individual green tapes (because of improper drying), sintering is constrained in two dimensions and densities remain low. Postsinter HIPing does not significantly increase the sintered densities because of the retention of open porosity within the individual tape–cast layers. The use of a revised thermocompression process involving shear deformation results in higher sintered densities and complete densification after HIPing. Sintered densities increase with the degree of shear strain during green–state deformation processing. Thus, green-state deformation can improve homogeneity in laminates. A further variation of the shear deformation process has also been developed that allows the formation of complex shapes from tape–cast laminates in the green state, while retaining layer integrity. 相似文献
18.
High-temperature plastic flow in Al2 O3 -10 wt% ZrO2 (2.5 mol% Y2 O3 ) has been examined at temperatures between 1400° and 1500°C. Al2 O3 -10 wt% ZrO2 (2.5 mol% Y2 O3 ) exhibits much higher flow stress and smaller tensile elongation below about 1450°C than 0.1 wt% MgO-doped single-phase Al2 O3 . The suppression of grain growth with ZrO2 dispersion into Al2 O3 is not effective for improving the tensile ductility. The limited ductility in Al2 O3 -10 wt% ZrO2 (2.5 mol% Y2 O3 ) is associated with the increment of flow stress caused by ZrO2 . The ZrO2 dispersion or segregation in Al2 O3 /Al2 O3 boundaries suppresses the grain boundary sliding and hence results in the increased flow stress at high temperatures. 相似文献
19.
Both tetragonal ( t ) and monoclinic ( m ) ZrO2 particles in ZrO2 -toughened Al2 O3 can give rise to toughening. In the stress field of propagating cracks, the t -ZrO2 particles can undergo the stress-induced t → m transformation, and the residual stresses around already-transformed m -ZrO2 particles can cause microcracking. The t -ZrO2 particles transformed in crack tip stress fields do not, however, also cause appreciable microcracking. The toughening increments via these distinct mechanisms are comparable. It appears that optimally fabricated Zr02 -toughened Al2 O3 's should contain a mixture of t - and m -ZrO2 . 相似文献
20.
CHUN-HWAY HSUEH PAUL F. BECHER PETER ANGELINI 《Journal of the American Ceramic Society》1988,71(11):929-933
Toughening of whisker-reinforced (or fiber-reinforced) ceramics by whisker pullout requires debonding at the whisker/matrix interface. Compressive clamping stresses, which would inhibit interface debonding and/or pullout, are expected in composites where the matrix has a higher thermal expansion coefficient than the whisker. Because such mismatch in thermomechanical properties can result in brittle composites, it is important to explore approaches to modify the thermal stresses in composites. As a result, the effects of a film at the whisker/matrix interface on the stresses due to thermal contraction mismatch upon cooling are considered in this study. Analysis of various properties of the film are considered for the whisker/matrix systems, in particular for SiC/Al2 O3 , SiC/cordierite, and SiC/mullite composites. Reduction of thermomechanical stresses is shown to occur when the interfacial film has a low Young's modulus. Also, when the whisker has a lower thermal expansion coefficient than the matrix (e.g., SiC/Al2 O3 ), the interfacial stresses generated during cooling decrease as the thermal expansion coefficient of the film increases. 相似文献