摘要:

A review of the observations of indentation‐induced fracture suggests that there is no simple generalization which may be made concerning crack initiation sequences. Here, we investigate the material dependence of the initiation sequence of indentation cracks (cone, radial, median, half‐penny, and lateral) using an inverted tester allowing simultaneous viewing of the fracture process and measurement of the indeter load and displacement during contact. Two normal glasses, two anomalous glasses, and seven crystalline materials are examined. Key results include (i) direct evidence that the surface traces of cracks observed at indentation contacts are those of radial cracks, rather than median‐nucleated half‐penny cracks (at least for peak contact loads<40 N) and (ii) that, in crystalline materials, radial cracks form almost immediately on loading of the indenter, in anomalous glasses at somewhat greater loads, but in normal glasses during unloading. A detailed consideration of the stress fields arising during indentation contact predicts material‐dependent initiation sequences, in agreement with observations, particularly those of radial crack formation on loading for materials with large modulus‐to‐hardness ratios. In addition, a new, unexplored crack system is demonstrated, the shallow lateral cracks, which appear to be responsible for material removal at s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05119.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The temperature dependence of electrical resistance of several spinel‐type oxides was measured in exhaust gases of air‐methane combustion, to evaluate the potential of the oxides as an air/fuel stoichiometric oxygen sensor. The results are discussed in terms of sensitivity and response time at the stoichiometric combustion point. It was found that the sensitivity as well as the response time is markedly affected by point defect structures of the oxide specimens. The response time is also related to the catalytic activity of the oxides for complete oxidation of methane. Among the specimens studied, the ZnCr2O4+ Cr2O3(1.0:0.1 molar ratio) is the most promising as the oxygen sensor, because it exhibits the largest resistance changes of more than 3 orders of magnitude and shortest response times of about 0.5 s when the combustion mixture is switched between the air‐rich versus fuel‐rich comp

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05120.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Near‐phase pure perovskite lead magnesium niobate (PMN) with MgO or PbO additives was produced by reacting PbO with MgNb2O6at 800°C and sintering at 1200°C. Dense ceramics were characterized by scanning electron microscopy, X‐ray diffraction, and dielectric measurements. The microstructural studies showed that excess MgO exists as micrometer spherical particles either in the grain boundary as a discrete particle or in the perovskite grain as an inclusion. The pyrochlore phase exists in large isolated grains in the microstructure. The 10 mol% MgO excess composition had a peak dielectric constant of 19 500 at 100 Hz, which suggests very “clean” or uninhibiting grain boundaries. The excess addition of PbO did not improve the yield of perovskite PMN phase and decreased the dielectric constant. PMN grain boundaries are the dominant path of fracture. This paper, to a certain degree, explores the chemistry and characteristics of these grain b

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05121.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Surface diffusion in sapphire and sintering of Al2O3have been studied under identical firing conditions (1593°C, N2) as a function of MgO solute additions. The effect of MgO on surface diffusion has been investigated directly using the technique of multiple scratch smoothing on the (1123) surface of sapphire. For these conditions MgO additions enhance the decay of periodic profiles by a factor of 4; that is interpreted as reflecting an increase in the surface diffusivity by the same amount. Measurements of the grain sizedensity trajectory during final stages of sintering of Al2O3reveal that MgO enhances the densification rate/coarsening rate ratio by a modest factor of 1.8. Since an increase in surface diffusivity alone would decrease the ratio, it is deduced that the primary role of MgO in the sintering of Al2O3is other than the influence on surface diffusion. These observations in conjunction with independent measurements of the effect of MgO on grain‐boundary mobility confirm that the primary role of MgO is the suppression of grainboundary moti

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05122.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The reactions of titanium carbide and hafnium carbide with iridium have been studied in thin film couples fabricated by vapor deposition processes. The reaction product layers after exposure in the temperature range of 1923 to 2400 K are dependent on the stoichiometry of the metal carbide layers and range from simple solid solutions to MIrxcompounds. The observed microstructures are predictable from available thermochemical data. The morphology of residual carbon in the reacted metal carbide‐iridium product layer varies from interfacial deposits to uniform carbon dispersion and depends upon exposure temperature and metal carbide stoichiometr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05123.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Infrared spectroscopic studies of lead and alkaline‐earth aluminosilicate glasses in the seriesxMO‐xAl2O3(1 ‐ 2x)SiO2M = Mg, Ca, Sr, Ba, and Pb; 0.05x0.275] were carried out in the range 2000 to 200 cm−1. Three major absorption bands were observed in the 1100‐, 800‐, and 500‐cm−1regions. The frequency and the intensity of the 1100‐cm−1band varied linearly with composition. For a specific value ofx, the changes in the frequency, intensity, and bandwidth of this band decreased in the order Mg>Ca>Sr>Pb>Ba and the apparent disorder in the glass structure, effected by the substitution of aluminum, increased in the direction Mg

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05124.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

On the basis of the topotaxy between BaTiO3and Ba6Ti17O40found recently, a model of a nonconservative (111) twin in TiO2‐rich BaTiO3was constructed. The model consists of several (001) layers of Ba6Ti17O40intergrown between (111) layers of BaTiO3, the core of the twin being a slightly modified double layer of Ba6Ti17O40containing face‐sharing octahedra. Using this model, anomalous grain growth below the eutectic temperature and preferential growth of (111) twins in a reducing atmosphere were explained, as well a nucleation of butterfly tw

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05125.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Nuclear magnetic resonance pulse techniques were used in situ during creep of single crystals of NaCl to evaluate the contribution of mobile dislocations to spin relaxation.23Na spin‐lattice relaxation rates were measured in the rotating frame (T1p) during compression creep of single crystals of NaCl along the [110] direction at 473 K at an applied stress of 20 MPa. The relaxation rates were evaluated from the spin‐echo height following π/2, locking, and 64° pulse sequence. We describe here the experimental setup and methodology in evaluating the dislocation jump distance from NMR‐creep experiments. The height of the free induction decay decreased as soon as the load was applied followed by a gradual increase until the steady state was reached, at which point a saturation value was observed corresponding to theconstantsteady‐state creep rate. The mean jump distance of the mobile dislocations, evaluated from the ratio of the signal heights without deformation and during creep, decreased with time/strain, reaching a constant value during the steady‐state creep regime. The results are compared with the dislocation‐dislocation spacing, subgrain size, as well as the jump distance predicted from

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05126.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The electrical properties of rare‐earth oxide doped CeO2‐ZrO2ceramics are evaluated by impedance spectroscopy. Doping of the trivalent metal oxide decreases both the grain and grain‐boundary resistivities of the CeO2‐ZrO2system. The grain and grain‐boundary resistivities as well as the activation energy for conduction increase with the dopant cation radius. The grain conductivity is fairly constant for 2 mol% YO1.5‐10 mol% CeO2‐88 mol% ZrO2samples sintered under different conditions, while lower grain‐boundary resistivity is obtained for samples sintered at higher temperature for longer times. Possible mechanisms for the electrical behavior of the rare‐earth oxide doped CeO2‐ZrO2ceramics are pro

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05127.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The kinetics of formation of calcium aluminates was studied by firing the reaction mixes in the temperature range 12000° to 1460°C for reaction times from 15 to 360 min. Phases formed were determined by taking X‐ray diffractograms of the samples. It was observed that all stable calcium aluminates were formed and that monocalcium aluminate (CA) grew with calcium dialuminate (CA2) in a 1:2 reaction mix of CaO and Al2O3. CA reacted further with Al2O3to form CA2. The formation of CA2obeyed the rate law equation 1 ‐ (1 ‐x)1/3=Kt/r2. The activation energy for the system (140 kJ·mol−1(33.4 kcal · mol−1)) was determined by the Arrhe

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05128.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY