摘要:

Cyclic fatigue‐crack propagation behavior in monolithic silicon nitride is characterized in light of current fatiguecrack growth models for ceramics toughened by grainbridging mechanisms, with specific emphasis on the role of load ratio. Such models are based on diminished cracktip shielding in the crack wake under cyclic loads due to frictional‐wear degradation of the grain‐bridging zone. The notion of cyclic crack growth promoted by diminished shielding is seen to be consistent with measured (long‐crack) growth rates, fractography,in situcrack‐profile analyses, and measurements of back‐face strain compliance. Growth rates are found to display a much larger dependence on the maximum applied stress intensity,Kmaxthan on the applied stress‐intensity range, ΔK, with behavior described by the relationship da/dN ∞ K29maxΔK1.3. Fatigue thresholds similarly exhibit a marked dependence on the load ratio,R=Kmin/Kmax; such effects are shown to be inconsistent with traditional models of fatigue‐crack closure. In particular, when characterized in terms ofKmaxgrowth rates below ∼10−9m/cycle exhibit an inverse dependence on load ratio, an observation which is consistent with the grain‐bridging phenomenon; specifically, with increasing R, the sliding disance between the grain bridges is decreased, leading to less frictional wear, and hence less degradation in shielding, per loading cycle. The microstructural origins of su

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08660.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Current (I)‐voltage (V) characteristics of porous ZnO varistors with different Bi2O3content have been investigated in air as well as in H2‐air mixtures in the temperature range room temperature (RT)‐600°C. TheI‐Vcharacteristics measured at RT remained unchanged in the presence of H2, but the breakdown voltage clearly shifted to a lower electric field in the temperature range 400–600°C. The breakdown voltage decreased with increasing H2concentration in air. The optimum amount of Bi2O3for the largest decrease was found to be 1.0 mol%. Thus, ZnO varistors can be used as a new type of H2sensor. The results presented in this study also suggest the important role of excess oxygen ions existing at the ZnO‐ZnO grain boundaries in developing the Schottky barrier as well as in the H2‐sensing mechanism o

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08661.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The rolling‐contact fatigue life and wear of CVD‐SiC coatings with surface compression were studied using a three‐ ball‐on‐rod rolling‐contact fatigue (RCF) tester. Two levels of surface compression in the coatings, approximately 190 μm thick, were introduced by using substrates of a liquid‐phase sintered SiC and a SiC‐30 vol% TiC to obtain desired thermal‐expansion mismatches with the CVD‐SiC. Residual surface compressions of 250 and 680 MPa were measured in the CVD‐SiC coatings on SiC and SiC‐30 vol% TiC substrates, respectively, by a strain‐gage technique. In the RCF test, the fatigue life of the CVD‐SiC coating with the moderate surface compression was limited by severe wear by a brittle fracture mechanism at a Hertzian contact stress of 5.5 GPa. The CVD‐SiC coating on the SiC‐30 vol% TiC composite, on the other hand, did not show measurable wear. A majority of the tests (11 out of 16) were suspended at 100 h or stopped due to failure of the steel balls. Five tests stopped due to spalling of the CVD‐SiC coating at weak interfaces 10 to 15 μm below the contact surface. Examination of the microstructure of the CVD‐SiC coating in cross sections revealed that the weak interfaces that led to the spalling were related to discontinuous growth of the CVD‐SiC in the form of nodules or growth regions. Elimination of these defective structures is likely to enhance the tribological p

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08662.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

In this work we have analyzed the conditions leading to a kinetic demixing of species in ceramics during cooling. Estimation of cation redistributions during cooling in a model material shows that a segregation maximum could be observed near surfaces at an intermediate cooling rate. This result is explained on the basis of two competing effects: the cation redistribution kinetics and the driving force of matter transport related to the cooling rate. Experimental results obtained with titanium and magnesium‐doped α‐alumina single crystals show that the segregation of titanium increases when the oxygen partial pressure increases and when the cooling rate and annealing temperature decrease. The opposite effect is generally observed for magnesium and other detected impurities. Complementary characterizations show that these kinetic reactions are also significantly influenced by the impurities present in the mate

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08663.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Indentation cracking under blunt indenters is analyzed using nonlinear fracture mechanics. The usual assumptions of linear elastic fracture mechanics have been replaced with a nonlinear load vs load‐point displacement curve while assuming the material is linear elastic. The load, the load‐point displacement, and a function of the crack area have been related to the crack driving force,J, while assuming a cone fracture under the Hertzian sphere. Experimentally, it was found that the load‐displacement curve during loading, cracking, and unloading is nonlinear. The crack length is empirically shown to be proportional to the load‐point displacement for several indenters. The experimentally measured relations between indenter load, load‐point displacement, and crack geometries are then analyzed with mechanical energy balances based on the similitude of crack lengths with load‐point displacements. The Hertz hardness that describes the nonlinear load vs load‐point displacement relation during cracking is derived on the constantJline in load‐displacement space. Finally, well‐known experimental expressions that relate load to crack length are shown to be indistinguishable from the load‐point displacemen

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08664.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Indentation cracking under sharp‐pointed indenters is analyzed using compliance‐based, nonlinear, fracture mechanics. The stress intensity factor,K, in linear elastic fracture mechanics is well known to be proportional to the load,P; in this nonlinear analysisKis proportional toP3/4. The observed relation between indenter load‐point displacement and crack length is based on similitude of crack lengths with load‐point displacements as a strain‐controlled fracture. The equations that relate load and a function of the crack length to the crack driving force,J, have been found for Vickers indentations. Analysis of the nonlinear load vs displacement assumes an equilibrium crack in the elastic material surrounding the indent. The hardness that describes the load vs load‐point displacement during cracking is derived on a constantJline in load‐displacement space. The crack length is shown experimentally to be proportional to the load‐point displacement after crack initiation for several different indenters in ZnS. The measured loading curves are nonlinear and display crack initiation during loading. TheKIcexpressions found here are very similar to correlations that have been applied to ind

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08665.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The secondary phase constitution in two sintered AIN ceramics (1.8% and 4.2% Y2O3additions) was studied as a function of heat treatment temperatures between 1750° and 1900°C under pure nitrogen atmosphere. The effect of the phase constitution on the physical properties, such as density, thermal conductivity (K), and lattice constants, and on the mechanical properties in three‐point bending, was also investigated. Y3Al5O12was found to getter dissolved oxygen from the AIN lattice below 1850°C, but evaporated at 1850°C and above. Y4Al2O9appeared to sublimate below 1850°C in the atmosphere used in this study. Depending on the secondary phase constitution, heat treatment affected thermal conductivity favorably or adversely. Occasionally, samples with similar lattice oxygen contents were found to have different thermal conductivities, suggesting that factors besides dissolved oxygen can also influenceK. Lattice parameter measurements indicated that, within the small range of lattice oxygen concentrations in the AIN samples studied, the c‐axis was more sensitive than thea‐axis to oxyg

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08666.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The effect of thermal expansion mismatch stress between silicon nitride and different grain boundary phases on the fracture toughness of silicon nitride was investigated. Different sintering aids in the Y‐Mg‐Si‐Al‐O‐N system produced silicon nitride specimens with very similar micro‐ structures but different grain boundary phase compositions and different values of fracture toughness. The fracture toughness of the silicon nitride increased as the thermal expansion coefficient of the grain boundary phase increased. The presence of tensile residual stress at the grain boundary caused by thermal expansion mismatch between the silicon nitride and the grain boundary phase enhanced crack deflection and gra

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08667.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Thin two‐phase, Al2O3/t‐Zr(3Y)O2layers bounded by much thicker Zr(3Y)O2layers were fabricated by co‐sintering powders. After cooling, cracks were observed along the center of the two‐phase, Al2O3/t‐Zr(3Y)O2layers. Although the Al2O3/t‐Zr(3Y)O2layers 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 c

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08668.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The crystalline phase development and microstructural changes with heating of sol‐gel‐derived lead titanate (PT) particles and films on silica with and without a fugitive (or removable) diffusion barrier layer were investigated. Amorphous gel‐derived PT particles were deposited on SiO2‐coated TEM grids with and without polyimide (PI) or carbon barrier layers between SiO2and PT. TEM analysis showed that PI or carbon barriers prevented reaction between the gel‐derived PT particles and SiO2. PT particles crystallize and then the PI or carbon film decomposes. Sol‐gel‐derived PT films were deposited on oxidized Si substrates (Si/SiO2) with and without a PI barrier layer. Perovskite PT films were prepared on Si/SiO2substrates with a PI barrier; however, some porosity remained in the films. Identically prepared films without the PI barrier formed a mixture of pyrochlore and perovskite. X‐ray photoelectron spectroscopy results indicate that the PI film pre‐ vents the diffusion of Si

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08669.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY