共查询到20条相似文献,搜索用时 46 毫秒
1.
Jing-Fong Tsai J. Daniel Belnap Dinesh K. Shetty 《Journal of the American Ceramic Society》1994,77(1):105-117
Crack shielding stress intensities in in situ loaded compact tension specimens of two types of ceria-partially-stabilized zirconia/alumina (Ce-TZP/Al2 O3 ) composites with prior histories of subcritical crack growth in sustained and tension-tension fatigue loading were directly assessed using laser Raman spectroscopy. Crack-tip stress fields within the transformation zones were measured by measuring a stress-induced frequency shift of a peak corresponding to the tetragonal phase. The peak shift as a function of the applied stress was separately calibrated using a ball-on-ring flexure test. Total crack shielding stress intensity was estimated from the far-field applied stress intensity and the local crack-tip stress intensity assessed from the measured near-crack-tip stresses. The shielding stress intensities were consistently lower in the fatigue specimens than in the sustained load crack growth specimens. The reduced crack shielding developed in the fatigue specimens was independently confirmed by measurements of larger crack-opening displacement under far-field applied load as compared to the sustained load crack growth specimens. Thus, diminished crack shielding was a major factor contributing to the higher subcritical crack growth rates exhibited by the Ce-TZP/Al2 O3 composites in tension–tension cyclic fatigue. Calculations of zone shielding considering only the dilatational strains in the transformation zones accounted for 81% and 86% of the measured values in the sustained load crack growth specimens, but significantly overestimated the shielding in the fatigue specimens. Possible reasons for the diminished crack shielding in the fatigue specimens are discussed. 相似文献
2.
Jing-Fong Tsai Cheng-Sheng Yu Dinesh K. Shetty 《Journal of the American Ceramic Society》1990,73(10):2992-3001
Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO2 -12 mol% CeO2 -10 wt% Al2 O3 ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs of different transformation yield stresses obtained by sintering for 2 h at temperatures of 1500°C (type A), 1475°C (type B), 1450°C (type C), and 1425°C (type D). The threshold stress-intensity range, ΔKth , for initiation of fatigue crack propagation increased systematically with decreasing transformation yield stress obtained with increasing sintering temperature. However, the critical stress-intensity range for fast fracture, ΔKc , as well as the stress-intensity exponent in a power-law correlation (log (da/d N ) vs log ΔK) were relatively insensitive to the transformation yield stress. The fatigue crack growth behavior was also strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. In particular, the threshold stess-intensity range, Δ K th , increased with increasing size of the transformation zone formed in prior quasi-static loading. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates. Calculations of crack shielding from the transformation zones indicated that the enhanced crack growth susceptibility of Ce-TZP ceramics in fatigue is not due to reduced zone shielding. Alternate mechanisms that can lead to reduced crack shielding in tension-tension cyclic loading and result in higher crack-growth rates are explored. 相似文献
3.
Jing-Fong Tsai Uong Chon N. Ramachandran Dinesh K. Shetty 《Journal of the American Ceramic Society》1992,75(5):1229-1238
Microstructure, phase stability, and mechanical properties of CeO2 -partially-stabilized zirconia (12 mol% Ce-TZP) containing 10 wt% Al2 O3 and 1.5 wt% MnO were studied in relation to the base Ce-TZP and the Ce-TZP/Al2 O3 composite without MnO. The MnO reacted with both CeO2 and Al2 O3 to form a new phase of approximate composition CeMnAl11 O19 . The reacted phase had a magnetoplumbite structure and formed elongated, needlelike crystals. The MnO-doped Ce-TZP/Al2 O3 composites sintered at an optimum temperature of 1550°C exhibited high strength (650 MPa in four-point bending) and rising crack-growth-resistance behavior, with fracture toughness increasing from 7.6 to 10.3 MPa.In12 in compact tension tests. These improved mechanical properties were associated with relatively high tetragonal-to-monoclinic transformation temperature ( M s =−42°C) at small grain size (2.5 μm), significant transformation plasticity in mechanical tests (bending, uniaxial tension, and uniaxial compression) and transformation zones at crack tips in compact tension specimens. The transformation yield stress, zone size, and fracture toughness were sensitive to the sintering temperature varied in the range 1500° to 1600°C. Analysis of the transformation zones using Raman microprobe spectroscopy and calculation of zone shielding for the observed zones indicated that a large fraction of the fracture toughness (∼70%) was derived from transformation toughening. 相似文献
4.
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
5.
Sang-Kee Lee Koji Takahashi § Masahiro Yokouchi Hiroyoshi Suenaga Kotoji Ando 《Journal of the American Ceramic Society》2004,87(7):1259-1264
Al2 O3 reinforced by SiC whiskers (Al2 O3 /SiC-W) was hot-pressed to investigate the fatigue strength of crack-healed specimens at high temperature. Semielliptical surface cracks of 100 μm surface length were introduced on each specimen surface. These specimens were crack-healed at 1300°C for 1 h in air, and static and cyclic fatigue strengths were systematically investigated at room temperature, 900° and 1100°C by three-point bending. The static and cyclic fatigue limits of the crack-healed specimens were more than 70% of the average bending strength at each testing temperature. Crack-healed specimens of Al2 O3 /SiC-W were not sensitive to static and cyclic fatigue at room temperature and high temperatures. Therefore, the combination of crack-healing and whisker reinforcement can play an important role in increasing static and cyclic fatigue strengths at high temperature. 相似文献
6.
James J. Hansen Raymond A. Cutler Dinesh K. Shetty Anil V. Virkar 《Journal of the American Ceramic Society》1988,71(12):501-C
Indentation fracture behavior of three-layer Al2 O3 -ZrO2 composites with substantial compressive residual stresses was compared with the behaviors of monolithic Al2 O3 and Al2 O3 -ZrO2 ceramics without intentionally introduced residual stresses. The indentation cracks were smaller in the three-layer specimens relative to the monolithic specimens in agreement with the predictions of indentation fracture mechanics theory. Indentation and strength testing were used to show that a residual compressive stress of approximately 500 MPa exists in the outer layers of the three-layer composites. The three-layer specimens showed excellent damage resistance in that the strength differential between the three-layer and monolithic indented specimens was maintained at indentation loads up to 1000 N, the maximum indentation load used in the experiments. 相似文献
7.
Mathias C. Knechtel Daniel E. Garcia Jürgen Rödel Nils Claussen 《Journal of the American Ceramic Society》1993,76(10):2681-2684
Crack velocity curves for Y-TZP and Al2 O3 -toughened Y-TZP were determined for long cracks in compact tension specimens with an in situ fracture device on the stage of an optical microscope. Indications for a crack velocity threshold were found for both materials. Above this threshold, at 2.6 MPa·m1/2 for Y-TZP and 3.6 MPa·m1/2 for Al2 O3 -toughened Y-TZP, chemically assisted subcritical crack growth occurs over an extended regime of applied stress intensity factors of width 2.1–2.8 MPa·m1/2 . It is recognized that the dependence of the shielding term on the crack-tip stress field renders transformation-toughened materials particularly susceptible to stress-corrosion cracking. This interrelation leads to the definition of a steady-state velocity at constant applied stress intensity factor. This velocity is obtained in the situation where the shielding term is fully defined by the present crack-tip stress field, not depending on prior loading history. 相似文献
8.
PAUL F. BECHER 《Journal of the American Ceramic Society》1983,66(7):485-488
Significant increases in the critical fracture toughness (K IC ) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO2 particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO2 particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO2 phase transformation by tailoring the Y2 O3 stabilizer content not only increases the critical fracture toughness (KIC ) but also the K Ia to initiate slow crack growth. For example, crack velocities ( v )≥10–9 m/s are obtained only at K Ia ≥5 MPa.m1/2 in transformation-toughened ( K IC =8.5 MPa.m1/2 ) composites vs K Ia ≥2.7 MPa.m1/2 for comparable velocities in composites where the transformation does not occur ( K IC =4.5 MPa.m1/2 ). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (≥1 μm) monoclinic ZrO2 particles which reside along the intergranular crack path. 相似文献
9.
Crack resistance characteristics and fatigue properties have been studied in four types of Y2 O3 –TZP ceramics including one containing Al2 O3 . The largely linear-elastic behavior connected with the very small transformation zone (<5 μm) explains the absence of any resistance-curve behavior and the flaw-controlled strength. The crack resistance shows high sensitivity to environment-induced subcritical crack growth. This influence is also operative in both types of fatigue experiments, i.e., under static and cyclic stresses, leading to reduced fatigue thresholds compared with K IC . While for static conditions a benefit is observed from enhanced t-m ZrO2 transformation, cyclic stresses provoke an additional fatigue effect. However, if the cyclic stresses are restricted to subthreshold values, cyclic stress-induced effects in the process zone provide an improvement of the materials being visible as a strengthening effect. 相似文献
10.
Takashi Kojima Wataru Sakamoto Toshinobu Yogo Takashi Fujii Shin-ichi Hirano 《Journal of the American Ceramic Society》2000,83(2):281-286
A Ce-TZP/platelike La(Co(Fe0.9 Al0.1 )11 )O19 composite was synthesized in situ while sintering from a mixture of Ce-TZP, La(Fe0.9 Al0.1 )O3 , Fe2 O3 , Al2 O3 , and CoO powders. Platelike La(Co(Fe0.9 Al0.1 )11 )O19 crystals were grown in a dense Ce-TZP matrix after sintering at temperatures of 1200°–1350°C. The temperature range for sintering Ce-TZP/La(Fe,Al)12 O19 composites was expanded widely by substituting Co2+ ions for Fe2+ ions in its structure. The highest value of the bending strength of the Ce-TZP/La(Co(Fe0.9 Al0.1 )11 )O19 composites was 880 MPa, which was higher than that of the Ce-TZP/La(Fe,Al)12 O19 composite (780 MPa) and Ce-TZP (513 MPa). The saturation magnetization of the Ce-TZP/La(Co(Fe0.9 Al0.1 )11 )O19 composite was a constant value of 7.7 emu/g after the composite was sintered at 1200°–1350°C. 相似文献
11.
Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH4 OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO2 in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO2 crystallites at elevated temperatures was strongly inhibited by Al2 O3 derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al2 O3 , and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO2 is lower than that of the monoclinic form. With increasing A12 O3 content, however, the transformation temperature gradually increased, although the growth of ZrO2 particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO2 . The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO2 particles was stabilized by the presence of water vapor, which probably decreases the surface energy. 相似文献
12.
Da Chen Kiroshi Shirato Michel W. Barsoum Tamer El-Raghy Robert O. Ritchie 《Journal of the American Ceramic Society》2001,84(12):2914-2920
The cyclic fatigue and fracture toughness behavior of reactive hot-pressed Ti3 SiC2 ceramics was examined at temperatures from ambient to 1200°C with the objective of characterizing the high-temperature mechanisms controlling crack growth. Comparisons were made of two monolithic Ti3 SiC2 materials with fine- (3–10 μm) and coarse-grained (70–300 μm) microstructures. Results indicate that fracture toughness values, derived from rising resistance-curve behavior, were significantly higher in the coarser-grained microstructure at both low and high temperatures; comparative behavior was seen under cyclic fatigue loading. In each microstructure, Δ K th fatigue thresholds were found to be essentially unchanged between 25° and 1100°C; however, there was a sharp decrease in Δ K th at 1200°C (above the plastic-to-brittle transition temperature), where significant high-temperature deformation and damage are first apparent. The substantially higher cyclic-crack growth resistance of the coarse-grained Ti3 SiC2 microstructure was associated with extensive crack bridging behind the crack tip and a consequent tortuous crack path. The crack-tip shielding was found to result from both the bridging of entire grains and from deformation kinking and bridging of microlamellae within grains, the latter forming by delamination along the basal planes. 相似文献
13.
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
14.
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
15.
Carmen Baudín Jonas Gurauskis Antonio Javier Sánchez-Herencia Victor M. Orera 《Journal of the American Ceramic Society》2009,92(1):152-160
Fracture features, residual stresses, and zirconia transformation are studied in indentation strength specimens of alumina-Y2 O3 -stabilized zirconia (3% mol of Y2 O3 , 3YTZP) ceramics in order to analyze the extension of the indentation damage in the bulk of the specimens. Two compositions, 5 vol% 3YTZP (A5) and 40 vol% 3YTZP (A40), have been prepared by stacking tape-casted tapes and sintering. After indentation with loads ranging from 50 to 300 N, samples were fractured in four-point bending and the fracture surfaces were characterized by scanning electron microscopy. Raman and piezospectroscopic techniques were used to determine the monoclinic zirconia fraction and the residual stresses through the fracture surfaces. In the A5 composition, the indentation damage morphology was clearly half-penny, whereas the A40 composition presented Palmqvist crack formation. Zirconia transformation was only observed in the plastically deformed zones underneath the imprints whereas there were significant residual compressive stresses outside the plastic zones due to the indentation damage. The intensity of this residual compressive field was dependent on the level of zirconia transformation due to indentation damage because zirconia transformation induced tensile stress fields superimposed on the compressive stresses. 相似文献
16.
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
17.
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
18.
Tsugio Sato Tadashi Endo Masahiko Shimada 《Journal of the American Ceramic Society》1989,72(5):761-764
Ceria-doped tetragonal zirconia (Ce-TZP)/alumina (Al2 O3 ) composites were fabricated by sintering at 1450° to 1600°C in air, followed by hot isostatic pressing (postsintering hot isostatic pressing) at 1450°C and 100 MPa in an 80 vol% Ar–20 vol% O2 gas atmosphere. Dispersion of Al2 O3 particles into Ce-TZP was useful in increasing the relative density and suppressing the grain growth of Ce-TZP before hot isostatic pressing, but improvement of the fracture strength and fracture toughness was limited. Postsintering hot isostatic pressing was useful to densify Ce-TZP/Al2 O3 composites without grain growth and to improve the fracture strength and thermal shock resistance. 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.
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