摘要:

A method to measure short‐crackR‐curves is introduced which does not require an independent determination of calibration factors and which utilizes an equilibrium crack shape condition to compute the shape factor of a surface crack,R‐curves of an alumina with two different grain sizes and of an aluminum/alumina composite were measured in the crack length regime of below 100 μm up to almost 1000 μm. Calculated and experimentally obtained crack depth profiles were in good agreement with deviations being between −22% and +26% for the crack ellipticity and confirmed the reliability of t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08119.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Experimental studies were conducted to examine the fracture resistance orR‐curve response of fine‐grained (1 to 2 μm grain size) alumina‐based composites reinforced with SiC whiskers. A precracked applied moment double cantilever beam (DCB) specimen was employed where initial crack lengths of ≥50 μm were attained by back‐cutting the crack tip wake region. For a given crack length, the results show that the fracture resistance is shifted upward with whisker content in composites reinforced with 0.8 μm diameter SiC whiskers for all crack lengths ≥50 μm. When the whisker content is fixed at 20 vol% but the whisker size is varied, measurements show that theR‐curve response increases with the average diameter of the whiskers for all crack lengths ≥100 μm. These experimental results are consistent with analytical descriptions of theR‐curve contribution for frictional bridging and pullout processes. The frictional bridging component, involving the displacement within an intact but partially debonded bridging whisker, reflects the significant radial compressive stresses acting on the interface due to the larger thermal contraction of the matrix. Because of the large tensile or bending stresses imposed on the whisker during crack opening, frictional bridging is limited to the region immediately behind the crack tip. Pullout of such bridging whiskers can occur when they fracture or separate from the matrix away from the crack; this can occur close to the crack tip but should become more dominant with increasing distance behind the crack tip. Observations of crack wake zones support these conclusions. Both the experimental results and the constitutive models suggest approaches to tailor theR‐curve response of ceramics reinforced by discontinuous fibers (e.g., whisk

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08120.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Two batches of high‐purity, alumina ceramics having an equiaxed grain morphology were manufactured with a mean grain size of approximately 10 μm. A low‐temperature heat treatment performed prior to sintering on one batch of specimens resulted in a narrow grain size distribution. The other batch of specimens was fired in a conventional manner and exhibited a broader grain size distribution. Mechanical tests performed on these specimens indicated little difference in the flaw tolerance ofT‐curve behavior of the aluminas, despite the presence of coarser grains in the conventionally fired alumina. Observations of cracks in these high‐purity aluminas revealed that large grains ruptured transgranularly and therefore did not act as effective bridging sites. Strength tests on polished specimens indicated that the alumina with the broad grain size distribution exhibited greater strength variability than the alumina with a narrow grain size distribution. A simple analysis shows that, because of the shallowT‐curve behavior observed in both aluminas, stable crack extension should not occur from natural flaws. The strength of these high‐purity aluminas is therefore controlled by theinitialflaw size and theinitialtoughness. The increased strength variability in the alumina with the broad grain size distribution is rationalized in terms of a wider distribution of cr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08121.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The stress‐strain behavior of a continuous‐fiber‐reinforced ceramic matrix composite has been measured over a wide range of loading rates (0.01 to 500 MPa/s). It was found that the loading rate has a strong effect on almost every feature of the stress‐strain curve: The proportionality stress, the composite strength and failure strain increase with increasing loading rate. The microstructural damage varies also with the loading rate; with increasing loading rate, the average matrix crack spacing increases and the average fiber pullout length decreases. Using simple models, it is suggested that these phenomena are caused partly by time‐dependent matrix cracking (due to stress corrosion) and partly by an increasing interfacial shear stress with loa

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08122.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The local environment of the aluminum atoms in a series of metastable Zr(1−x)AlxO(2−x/2)crystalline materials (0.08 ≤x≤ 0.57), prepared by diffusion‐limited crystallization of amorphous precursors, has been determined by27Al magic angle spinning nuclear magnetic resonance (MAS NMR). Results show the existence of aluminum in 4‐, 5‐, and 6‐fold coordination in both the amorphous and crystalline states. Although the relative amounts of each type of coordination show no compositional dependence in the amorphous state, the results for the crystalline materials show a systematic decrease in the average aluminum coordination number with increasing aluminum content. Comparisons of MAS NMR results between pure Al2O3precursors and Zr(1‐x)AlxO(2‐x/2)crystalline materials processed under similar conditions show a profound effect of ZrO2on the coordination environment of the aluminum atom. Both a random distribution model and a model that assumes small‐scale clustering of aluminum ions are considered to explain the trends in the type of aluminum coordination as a fu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08123.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Photochemical hole burning (PHB) not only can be applied for data storage systems but also serves as a powerful method for studying the local structure around optical centers. The present work investigated the effects of aluminum, magnesium, and silicon ions on hole burning and the phonon sideband for borate glasses that exhibit PHB at room temperature. Hole burning was measured for the5D0−7F0transition of Sm2+and the phonon sideband spectrum for the5D0‐7F0transition of Eu3+. The hole width was closely related to local structural change, especially as it seemed to decrease with decreases in the number of nonbridging oxygens produced around the rare‐earth ions. In the case of sodium aluminoborate glasses, the hole width decreased considerably with increasing alumina content. The ratio Γih/Γhfor 85B2O3·10Al2O3·5Na2O·1Sm2O3glass, then, was 80 at room temperature, the largest value eve

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08124.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The rate of capillary rise of a liquid into a porous medium made up of consolidated particulates is analyzed. The infiltration distance is parabolic in time and can be modeled using the Washburn analysis. The effective pore radius is measured to be one to two orders of magnitude smaller than the particle size, particle spacing, and the median pore size as measured by mercury porosimetry. This result is interpreted using a modification of the Washburn model which models the porous medium as a single pore with varying diameter. Using a two‐sized single pore model, the predicted infiltration rate is consistent with the measured values. In applying the two‐sized single pore model to the reactive capillary infiltration of silicon into a carbonaceous preform in the Silcomp process, the effect of pore closure by the conversion of carbon to SiC is predicted. Using pore closure dimensions measured in a partial infiltration experiment, a decrease in the infiltration rate constant is predicted and is consistent with the measured infiltration r

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08125.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The Hamaker constants of eight different ceramic materials, 6H‐SiC, tetragonal, partially stabilized ZrO2(3% Y2O3), β‐Si3N4, α‐Al2O3, Y2O3, sapphire (single‐crystal α‐Al2O3), MgO, MgAl2O4, and fused silica, across air, water, andn‐dodecane at room temperature and across silica at 2000 K have been calculated from optical data using the Lifshitz theory. Spectroscopic ellipsometry was used to measure the photon energy dependence of the refractive index,n, and the extinction coefficient,k, in the visible and near‐UV range on several important ceramic materials. This relatively simple, nondestructive technique has proved to yield reliable optical data on sintered, polycrystalline materials such as Si3N4, SiC, ZrO2, Al2O3, and ZnO. For the other materials, Y2O3, sapphire, MgO, MgAl2O4, and fused silica, optical data from the literature were used to calculate the Hamaker constants. The calculated Hamaker constants were estimated to be accu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08126.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

29Si and31P MAS‐NMR spectra were measured for the Li2S‐SiS2‐Li3PO4glasses.29Si MAS‐NMR spectra revealed that a large number of silicon atoms in the glasses were coordinated with both sulfur and oxygen atoms.31P MAS‐NMR spectra showed that some phosphorus atoms were also coordinated with both sulfur and oxygen atoms. Such structural units resulted in the improvement of the conductivity and the glass stability against crystallization by the doping of Li3PO4to the Li2S‐

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08127.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

A thermodynamic evaluation of the ternary Bi‐Cu‐O system has been made and a set of parameters consistently describing the system at 1 bar total pressure has been determined. The ternary system contains a liquid phase which is continuous at high temperature and separates into a metal liquid and an oxide liquid at low temperature and the ternary compound Bi2CuO4in addition to the binary compounds. Concentration/temperature diagrams are given for the Bi2O3‐CuOxsystem at oxygen partial pressures of 1, 0.21 and 10−5bar, as well as a potential diagram. Selected isothermal sections at 1100 and 1473 K are also presented. Available experimental data are rather limited and often show a relatively large scatter, but nevertheless make it possible to determine a sufficient set of thermodynamic par

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08128.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY