Microstructural Mechanics Model of Anisotropic‐Thermal‐Expansion‐Induced MicrocrackingSridhar, Narayanaswamy; Yang, Wuhua; Srolovitz, David J.; Fuller, Edwin R.
doi: 10.1111/j.1151-2916.1994.tb05384.xpmid: N/A
Thermal‐expansion‐induced microcracking in single‐phase ceramics has been simulated using a simple mechanics model based upon a regular lattice of brittle, elastic springs. Microcracks preferentially form at grain boundaries and propagate either into the bulk or along grain boundaries, depending on the toughness of the boundaries relative to the grain interiors. The present results show that anisotropic‐thermal‐expansion‐induced microcracking can be more severe for either large or small grain size samples depending on the damage measure employed. At very small misfit strains, the large grain microstructure develops microcracks before the small grain microstructure. However, over most of the misfit strain regime examined, the total length/area of all cracks in a sample is larger when the grain size is small. This is manifested in a larger decrement of the elastic modulus in small grain size samples as compared with large grain size samples at the same misfit (ΔT). However, large grain sizes are more detrimental with regard to fracture properties. This is because the fracture stress scales as inversely with the crack length and large grain samples exhibit larger microcracks than small grain samples. Unlike in the unconstrained samples, when a sample is constrained during a temperature excursion, the stress created by the overall thermal expansion can directly lead to fracture of the entire sample.
Thin Glass Film between Ultrafine Conductor Particles in Thick‐Film ResistorsChiang, Yet‐Ming; Silverman, Lee A.; French, Roger H.; Cannon, Rowland M.
doi: 10.1111/j.1151-2916.1994.tb05386.xpmid: N/A
Thick‐film resistors arc electrical composites containing ultrafine particles of ruthenate conductor (Pb2Ru2O7 in the present materials) distributed in a highly modified silicate glass. We show that conductor particles remain flocced in the absence of any applied or capillary pressures, but are separated at equilibrium by a nanometer‐thick film of glass. Microstructures show evidence for liquid‐phase sintering, i.e., contact flattening of particles, under van der Waals attraction alone. Titania addition, which in dilute concentrations markedly increases the resistivity, decreases the temperature coefficient of resistance, and improves voltage stability and noise, is found to increase the equilibrium film thickness between particles by a few angstroms. STEM analyses show that the added titania preferentially concentrates in the silicate‐rich grain boundary film, as well as at particle–glass interfaces. The roles of interparticle forces and adsorption on the glass film thickness with and without titania are discussed. The large increase in resistivity caused by titania additions is attributed to the increase in Film thickness as well as to local chemical changes of two possible types. Titania enrichment within the glass film itself is expected to decrease the local ruthenium ion solubility, and this along with the possible formation of a more insulating titania‐substituted surface layer on ruthenate grains will decrease the tunneling conductivity between conductor grains.
Mechanical and Environmental Factors in the Cyclic and Static Fatigue of Silicon NitrideJacobs, David S.; Chen, I‐Wei
doi: 10.1111/j.1151-2916.1994.tb05387.xpmid: N/A
The effects of environment on cyclic and static fatigue behavior were investigated with hot‐pressed silicon nitride materials. Tests were conducted at ambient temperature on standard compact tension specimens, and a dc electric potential technique was used to monitor crack lengths in situ. The results indicate that the environmental sensitivity of our materials under both cyclic and static loading mirrors that of durable glasses in static fatigue. The materials were most sensitive to water in the environment, while changes in pH had no significant effect in the range tested. In addition, NH3 was much less reactive with our materials than with vitreous SiO2. In some cases, the intergranular glass appears to be the site of environmental interaction. Evidence was also found that cyclic fatigue is not simply a manifestation of static fatigue. Cyclic fatigue was seen to occur in the absence of measurable static fatigue, and the data indicate that the mechanism of cyclic fatigue involves damage to the crack wake shielding zone.
Stability of Molybdenum Disilicide in Combustion Gak EnvironmentsLin, Wen‐yi; Hsu, Jen‐Yan; Speyer, Robert F.
doi: 10.1111/j.1151-2916.1994.tb05388.xpmid: N/A
The stability of MoSi2 in combustion gas at 1370° and 1600°C was evaluated using SOLGASMIX‐PV thermodynamic modeling, periodic weight measurements, and characterization via XRD, SEM, EDS, and image analysis. Passive oxidation occurred at both temperatures. During an initial stage of exposure, specimen surfaces oxidized to form MoO3(g) and amorphous SiO2 via reduction of CO2 and H2O. After a short time (<6.5 min at 1370°C, <1 min at 1600°C), the oxidation mechanism switched; Mo5Si3 and amorphous SiO2 formed as oxidation products. The first mechanism esulted in the formation of 46.1 vol% at 1370°C and 42.6 vol% at 1600°C of the amorphous silica surface coating. The attainment of a near‐terminal weight gain implied silica formation was limited by H2O and CO2 diffusion through the silica coating.
Internal Friction in Lithium Aluminosilicate Glass‐CeramicsVersteeg, Vera A.; Kohlstedt, David L.
doi: 10.1111/j.1151-2916.1994.tb05389.xpmid: N/A
This paper describes the results of an investigation of the anelastic behavior of two glass‐ceramic materials with similar compositions, one nucleated with TiO2 and the other with ZrO2. The influence of the grain size, residual glass fraction, and composition was examined. A low‐frequency torsion pendulum was constructed for this purpose. The as‐received TiO2‐nucleated. Corning Code 9608 ceramic exhibits an attenuation peak at about 750°C; when heat‐treated at 1200°C for 167 h, the attenuation peaks at approximately 850°C. The ZrO nucleated glass‐ceramic showed no such peak. Our results indicate that this anelastic behavior is due to grain boundary sliding, but the rates and magnitude of sliding are strongly dependent on boundary chemistry.
Elastic Properties of Two‐Phase CompositesRavichandran, Kakkaveri S.
doi: 10.1111/j.1151-2916.1994.tb05390.xpmid: N/A
Simple expressions were derived to predict the elastic properties of two‐phase systems containing discontinuous reinforcements, on the basis of approximation of composite microstructure to a unit cell incorporating isostrain and isostress type elements, arranged in two different ways. The bounds on elastic modulus obtained in this manner have been shown to accurately describe the variation of elastic modulus as a function of volume fraction of one of the phases, for a wide variety of two‐phase composites. The present expressions offer predictions of elastic modulus that are much closer to experimental data than the commonly used Hashin and Shtrikman bounds, particularly for composites with constituents having large differences in elastic moduli. Similarly, it has been shown that the shear moduli and the Poisson's ratios of composites as a function of second‐phase volume fraction can also be predicted. the present unit‐cell‐based method of calculation presents a promising approach for the prediction of properties of multiphase materials. especially for those consisting of more than two dissimilar phases.
Indentation Studies on Y2O3‐Stabilized ZrO2: II, Toughness Determination from Stable Growth of Indentation‐Induced CracksDransmann, Gerhard W.; Steinbrech, Rolf W.; Pajares, Antonia; Guiberteau, Fernando; Dominguez‐Rodriguez, Arturo; Heuer, Arthur H.
doi: 10.1111/j.1151-2916.1994.tb05392.xpmid: N/A
Stable indentation cracks were grown in four‐point bend tests to study the fracture toughness of two Y2O3‐stabilized ZrO2 ceramics containing 3 and 4 mol% Y2O3. By combining microscopic in situ stable crack growth observations at discrete stresses with crack profile measurements, the dependence of toughness on crack extension was determined from crack extension plots, which graphically separate the crack driving residual stress intensity and applied stress intensity factors. Both materials exhibit steeply rising R‐curves, with a plateau toughness of 4.5 and 3.1 Mpa·m1/2 for the 3‐ and 4‐mol% materials, respectively. The magnitude of the plateau toughness reflects the fraction of tetragonal grains contributing to transformation toughening.
Extended Defects in ZnO Ceramics Containing Bi4Ti3O12 AdditiveMakovec, Darko; Trontelj, Marija
doi: 10.1111/j.1151-2916.1994.tb05393.xpmid: N/A
Extended defects in ZnO ceramics containing, 6 wt% Bi4Ti3O12 were studied by analytical electron microscopy. Apart from basal plane condensation stacking faults, which are also present in as‐received ZnO, extended defects related to the presence of Bi4Ti3O12 were observed. In samples sintered at 900°C they lie in the basal or in the prismatic planes and they quite often form closed loops, whereas they form serpentine‐shaped boundaries in samples sintered at 1200°C. Evidence is given that they are inversion boundaries. Their TEM image characteristics, as well as the unambiguous presence of Ti at the boundaries, suggest that they are formed due to the presence of 2‐D coherent precipitates of Ti‐rich (possibly Zn2TiO4‐type spinel) phase.