Showing papers in "Journal of the American Ceramic Society in 1980"
TL;DR: In this article, a theory for describing the evolution of the median/radial crack system in the far field of sharp-indenter contacts is developed, based on a model in which the complex elastic/plastic field beneath the indenter is resolved into elastic and residual components.
Abstract: A theory for describing the evolution of the median/radial crack system in the far field of sharp-indenter contacts is developed. Analysis is based on a model in which the complex elastic/plastic field beneath the indenter is resolved into elastic and residual components. The elastic component, being reversible, assumes a secondary role in the fracture process: although it does enhance downward (median) extension during the loading half-cycle, it suppresses surface (radial) extension to the extent that significant growth continues during unloading. The residual component accordingly provides the primary driving force for the crack configuration in the final stages of evolution, where the crack tends to near-half-penny geometry. On the hypothesis that the origin of the irreversible field lies in the accommodation of an expanding plastic hardness impression by the surrounding elastic matrix, the ensuing fracture mechanics relations for equilibrium crack growth are found to involve the ratio hardness-to-modulus as well as toughness. Observations of crack evolution in soda-lime glass provide a suitable calibration of indentation coefficients in these relations. The calibrated equations are then demonstrated to be capable of predicting the widely variable median and radial growth characteristics observed in other ceramic materials. The theory is shown to have a vital bearing on important practical areas of ceramics evaluation, including toughness and strength.
1,862 citations
TL;DR: In this article, a generalized model of the toughness of ceramics is developed, which indicates the specific roles of the transformation parameters on the expected magnitude of the toughening, and the implications of the analysis for the development of high-toughness ceramic materials are discussed.
Abstract: The toughness of ceramics can be substantially enhanced by inducing a martensitic transformation in the stress field of the dominant crack. The specific characteristics of martensitic transformations pertinent to this toughening process are examined in this paper. A generalized model of the toughening is developed, which indicates the specific roles of the transformation parameters on the expected magnitude of the toughening. Finally, the implications of the analysis for the development of high-toughness ceramics are discussed.
427 citations
TL;DR: In this paper, the deformation dynamics of fine-grained alumina polycrystals (grain size 1 to 15 μm) were studied and diffusional creep, basal slip, and unaccommodated grain-boundary sliding were investigated.
Abstract: Plastic deformation in fine-grained alumina polycrystals (grain size 1 to 15 μm) was studied. At least three distinct deformation mechanisms are important: diffusional creep, basal slip, and unaccommodated grain-boundary sliding. The first and most important of these processes is addressed in this paper. Analysis of the deformation dynamics suggests that both lattice and grain-boundary diffusion are important in the diffusional creep. Aluminum, rather than oxygen, lattice and grain-boundary diffusion are rate-controlling because oxygen diffusion is very slow in the lattice but very rapid in grain boundaries. Significantly, the diffusional creep can become interfacecontrolled at low stresses, causing the often-reported non-Newtonian creep behavior of fine-grained alumina.
321 citations
213 citations
TL;DR: In this article, fracture toughness of sintered A12O3 specimens with straight-through or chevron notches was determined with four-point-bend specimens.
Abstract: Fracture toughness of a sintered A12O3 was determined with four-point-bend specimens having either straight-through or chevron notches. For the straight-through notched specimens, measured KIc decreased with decreasing notch width. For the smallest notch width (66 μm) KIc= 3.42±0.13 MN m−¾. For specimens with chevron notches, a crack initiates and extends from the tip of the notch under increasing load. KIc is calculated from the maximum load without measuring crack length, under the assumption that the derivative of the compliance is the same as that for a specimen with a straight-through crack. A refined calculation accounts for the truncated chevron crack shape at maximum load using Bluhm's slice model. For the chevronnotch configuration, a value of KIc= 3.49±0.11 MN m−¾ was measured, which appears to be independent of the initial notch length a0 (distance from the crack mouth to the tip of the triangular notch). An effect of a1 (length of the chevron notch at the surface) on KIc was observed, independent of whether the calculation of KIc was based on the straight-through crack assumption or on the slice model.
206 citations
TL;DR: In this article, the authors measured infrared absorption on an NajO-K2O-ZnO-Al2O3-SiO2 glass with up to ∼11 wt% water.
Abstract: Infrared absorption was measured on an NajO-K2O-ZnO-Al2O3-SiO2 glass with up to ∼11 wt% water. Fundamental and overtone-combination bands were observed at 1.41, 1.91, 2.22, 2.87, and 6.1 μm. Molar absorptivities were determined for the hydrated glass for H2O and OH levels using the molar absorptivity calculated for the 2.22-μm band in the as-melted glass. By this procedure the concentration of molecular water and OH groups could be determined separately. These results show that the OH content of the hydrated glass, as determined from the 2.22-μm band, is constant at >7 wt% H2O; additional H2O is attributed to molecular water only. Good agreement was found between these data and H2O/OH molecular ratios obtained from NMR.
188 citations
TL;DR: In this paper, the crystalline phase, microstructure, semiconduction, and humidity-sensitive electrical conduction of MgCr2O4-TiO2 ceramics were studied.
Abstract: The crystalline phase, microstructure, semiconduction, and humidity-sensitive electrical conduction of MgCr2O4-TiO2 ceramics were studied. A solid solution with TiO2 up to 30 mol% occurs as a single phase with a pure MgCr2O4-type spinel structure. The humidity-sensitive electrical conduction of the MgCr2O4-TiO2 porous ceramics is the most promising for humidity-sensing devices.
159 citations
Abstract: Plastic deformation was studied in fine-grained alumina polycrystals (grain sizes 1 to 5 μm); several deformation mechanisms occur simultaneously. This paper is concerned with basal slip and unaccommodated grain-boundary sliding (GBS). The basal slip can give rise to a deformation texture. Cavitation occurs due to the occurrence of both unaccommodated GBS and basal slip, because of the marked plastic anisotropy of alumina.
153 citations
TL;DR: In this paper, a numerical procedure is described for modeling the number and distribution of microcracks around a crack tip as a function of the applied stress intensity, and the results indicate that texture can strongly influence the slope of the R curve, and that a positively sloped R curve can be produced by the operation of micro-cracking mechanisms.
Abstract: A numerical procedure is described for modeling the number and distribution of microcracks around a crack tip as a function of the applied stress intensity. The procedure accounts, approximately, for microcrack-microcrack and microcrack-crack interactions. Starting with an array of sites where microcracks can form, the model computes the generation rate of microcracks with increasing stress intensity. The interaction between several neighboring sites leading to extension of the main crack is also apparent and, as a result, an R curve emerges from the calculation. The effects of changing the nature of the microstructural variables inserted into the model can be examined; the results of several calculations with differing microstructural properties are compared. These variables include texture and the statistical distribution of strength over the microcrack sites. The results indicate that texture can strongly influence the slope of the R curve, and the calculations demonstrate that a positively sloped R curve can be produced by the operation of microcracking mechanisms.
145 citations
TL;DR: In this paper, the formation of chemically coprecipitated barium ferrite was studied using differential thermal analysis, static magnetic measurements, X-ray analysis, and ferromagnetic resonance measurements.
Abstract: The formation of chemically coprecipitated barium ferrite was studied using differential thermal analysis, static magnetic measurements, X-ray analysis, and ferromagnetic resonance measurements. The investigation showed that the formation process of coprecipitated BaFe12O19 differs significantly from that of conventionally prepared BaFe12O19. The DTA diagram showed a large exothermal peak at 760°, indicating hexaferrite formation. Intermediate phases, such as α-Fe2O3and BaFe2O4 were not observed during the reaction process.
139 citations
Abstract: Subsolidus phase relations were established in the system Si3N4-SiO2-Y2O3. Four ternary compounds were confirmed, with compositions of Y4Si2O7N2, Y2Si3O3N4, YSiO2N, and Y10(SiO4)6N2. The eutectic in the triangle Si3N4-Y2Si2O7-Y10(SiO4)6N2 melts at 1500°C and that in the triangle Si2N2O-SiO2-Y2Si2O7 at 1550°C. The eutectic temperature of the Si3N4-Y2Si2O7 join was ∼ 1520°C.
TL;DR: In this article, cavity nucleation and growth were analyzed in an attempt to identify microstructures and/or strain-rate regimes that would suppress cavity evolution and hence allow superplasticity.
Abstract: Ceramics exhibit macroscopic stress/strain rate relations that should lead to superplastic extension. However, premature fracture is normally encountered, due to the formation and growth of grain-boundary cavities. Thus, cavity nucleation and growth were analyzed in an attempt to identify microstructures and/or strain-rate regimes that would suppress cavity evolution and hence allow superplasticity. Analysis of cavity nucleation indicates that fine-grained materials devoid of grain-boundary amorphous phases and inclusions should sustain substantial deformation rates without nucleating cavities, especially if solid-solution additions that encourage rapid grain-boundary diffusion (while not excessively decreasing surface energy) are identified. The analysis of void growth indicates that high relative surface diffusivities are also desirable, indicating that alloy additions that do not depress (and probably enhance) the relative surface diffusivities must be selected.
TL;DR: The self-diffusion coefficients of 14C in high purity, essentially intrinsic and N-doped, n-type α-SiC single crystals were measured at 2123 to 2453 K as mentioned in this paper.
Abstract: The self-diffusion coefficients of 14C in high-purity, essentially intrinsic and N-doped, n-type α-SiC single crystals were measured at 2123 to 2453 K. The Si14C tracer was chemically bonded by a traveling solvent technique. The Dc* in the doped crystals is smaller than that of the pure materials because of the decreased concentration of the charged donor-type C vacancies in the presence of the donor N species.
TL;DR: Anion self-diffusion coefficients normal to (1 anti 102) were obtained for single-crystal Al/sub 2/O/sub 3/ in a 1.3 x 10/sup -3/ N/m/sup 2/ (10/sup −5/ torr) vacuum.
Abstract: Anion self-diffusion coefficients normal to (1 anti 102) were obtained for single-crystal Al/sub 2/O/sub 3/ in a 1.3 x 10/sup -3/ N/m/sup 2/ (10/sup -5/ torr) vacuum at 1585/sup 0/ to 1840/sup 0/C. Tracer was supplied from an initial 650 to 1300 A Al/sub 2//sup 18/O/sub 3/ layer produced by the oxidation of vapor-deposited Al metal films in an /sup 18/O/sub 2/ atmosphere at 520/sup 0/C. Concentration gradients extended over depths of 3000 to 5000 A and were measured by mass spectrometry of material sputtered from the samples with a beam of Ar/sup +/ ions. Crystals which had not been preannealed to remove surface damage displayed enhanced diffusion. Diffusion coefficients from preannealed crystals may be described by D/sub -0/ = 6.4 x 10/sup 5/ cm/sup 2//s, with an activation energy of 188 +- 7 kcal/mol. The diffusion is interpreted as an extrinsic vacancy mechanism.
TL;DR: The dc conductivity and ionic transference number of MgO single crystals doped with Al, Fe, and Sc were measured as a function of temperature (1200° to 1600°C) and oxygen pressure (10−1 to 10−13 MPa) as discussed by the authors.
Abstract: The dc conductivity and ionic transference number of MgO single crystals doped with Al, Fe, and Sc were measured as a function of temperature (1200° to 1600°C) and oxygen pressure (10−1 to 10−13 MPa). Magnesium oxide is a mixed conductor with its ionic component directly proportional to the trivalent solute concentration. The activation energy for ionic conduction equals 202 kj/mol and is independent of the specific trivalent solute used. This behavior results from a defect structure for a single trivalent solute for which the charge balance condition is 2[VMg'] =[IMg], where [VMg”] is the Mg vacancy concentration and [IMg] the trivalent solute concentration. At lower temperatures and high Sc concentrations, effects attributed to Sc ionvacancy association are observed. Ionic conduction results from Mg vacancy motion with a diffusion coefficient of VMg' given by
Deduced values of the Mg-ion diffusion coefficient lead to the conclusion that the scatter in the published DMg data largely results from differences in impurity content.
TL;DR: In this article, the effect of residual contact stresses on the dynamic fatigue response of surfaces containing indentation-induced flaws is studied, and a general formulation is obtained for the fatigue strength at constant stress rate.
Abstract: The effect of residual contact stresses on the dynamic fatigue response of surfaces containing indentation-induced flaws is studied. Indentation fracture mechanics is used to analyze the growth of well-defined “median/radial” cracks in combined residual (elastic/plastic) contact and applied (uniform) tensile fields, and thence to determine strength characteristics. In this way a general formulation is obtained for the fatigue strength at constant stress rate. Experimental confirmation of the essential predictions of the theory is obtained from strength tests on Vickers-indented soda-lime glass disks in water environment. It is thereby implied that residual stresses can have a significant deleterious influence on the fatigue behavior of any brittle solid whose controlling flaws have a contact history. Such effects need to be considered in the design of structural ceramics, most notably where fracture-mechanics calibrations of crack-velocity parameters are used for lifetime predictions.
TL;DR: In this paper, thermophoresis is conclusively established as the particulate deposition mechanism in the MCVD process by comparing experimental measurements and quantitative theoretical predictions, and it is determined that the deposition efficiency is a strong function of the torch traverse velocity, the traverse length, the temperature of the ambient environment, and the tube wall thickness but only a weak function of gas flow rate.
Abstract: Thermophoresis is conclusively established as the particulate deposition mechanism in the MCVD process by comparing experimental measurements and quantitative theoretical predictions. The deposition efficiency, E, is defined as the fraction of the silica in the gas stream (initially as SiCl4) that is deposited. For normal MCVD operating conditions, the deposition efficiency is only a function of the equilibrium temperature, Te, at which the gas and walls equilibrate downstream of the torch and the temperature, Tr, at which reaction occurs. The deposition efficiency is ∼0.8[1-(Te/Tr)]. It is determined that Te is a strong function of the torch traverse velocity, the traverse length, the temperature of the ambient environment, and the tube wall thickness but only a weak function of the gas flow rate. At high gas flow rates, the efficiency is limited by incomplete reaction.
TL;DR: In this article, the R-T and I-V characteristics of single grains and grain boundaries in large-grained BaTiO3 PTC ceramics were studied with a two-probe technique using a micromanipulator and fine Al wire.
Abstract: R-T and I-V characteristics of single grains and grain boundaries in large-grained BaTiO3 PTC ceramics were studied with a two-probe technique using a micromanipulator and fine Al wire. The PTC originates in the grain boundary only and behaves differently in each boundary. Even below Tc, the ceramic resistance depends almost entirely on the boundary. I-V characteristics in the boundary follow Ohm's law and conduction by a space-charge-limited current with a trap, using different applied voltages. The PTC anomaly relates to activation of the trap in the boundary, not to barrier height. A band model in the intergranular layer, with dielectric BaTiO3 and the trap, is proposed.
TL;DR: In this article, the transition from single-crystal to polycrystalline fracture energies was studied as a function of the flaw-size to grain-size ratio by two methods.
Abstract: The transition from single-crystal to polycrystalline fracture energies was studied as a function of the flaw-size to grain-size ratio by two methods. The primary method was calculating fracture energies from observed flaw sizes found at fracture origins in strength-test specimens. Some measurements were also made by varying the number of grains across the web in the applied-moment DCB test. Both methods agreed and generally snowed the transition to polycrystalline fracture energies being completed at flaw-size to grain-size ratios of ∼1 to ∼ 6 for the cubic materials studied. It is estimated that cracks less than ∼½ to ¼ of the grain size cannot be arrested at grain boundaries and that single-crystal fracture energies can be applied below this limit. The grain-size range over which this fracture-energy transition occurs was shown to be a function of extrinsic factors, such as texture, as well as intrinsic factors, such as the number and multiplicity of low-energy single-crystal fracture planes.
TL;DR: In this article, the small differences in thermal expansion behavior between stoichiometric cordierites fabricated by different methods were measured using an XRD film method which records the complete diffraction patterns, at two temperatures, on one film.
Abstract: The small differences in thermal expansion behavior between stoichiometric cordierites fabricated by different methods were measured using an XRD film method which records the complete diffraction patterns, at two temperatures, on one film. Examples of the extensive chemical family that can be obtained with this structure type are given and their widely varying expansions and distribution of diffracted intensities are demonstrated.
TL;DR: In this article, the authors investigated the oxidation resistance at 1400°C of silicon nitride alloys hot-pressed with magnesia and found that the extent of oxidation, the products of oxidation and the continuity of the oxide scale are all dependent on the initial composition.
Abstract: The oxidation resistance at 1400°C of silicon nitride alloys hot-pressed with magnesia was investigated as a function of the phase content of the alloys. The extent of oxidation, the products of oxidation, their stratification, and the continuity of the oxide scale are all dependent on the initial composition. Also, the compositions of the alloys themselves change during oxidation. The findings can be interpreted on the basis of compositional paths in the equivalence phase diagram. Reasons for the outward diffusion of Mg and impurities are given and the effect of the secondary phase on oxidation kinetics is discussed.
TL;DR: In this paper, first-order kinetics were observed and an activation energy of 118±4 kcal was calculated for LaAl11O18 in an N2-gas atmosphere.
Abstract: Synthesis of LaAl11O18 by the solid-state reaction of La2O3 and Al2O3 occurs in two stages, i.e. LaAlO3 forms immediately at 1450°C in air but LaAl11O18 formation is very slow and requires up to 141 days. First-order kinetics were observed and an activation energy of 118±4 kcal was calculated. Formation of LaAl11O18 depends on external O2-gas partial pressure. In an N2-gas atmosphere, the very slow formation of LaAl11O18, observed in air, is slowed even further. Possible mechanisms are also discussed.
TL;DR: In this article, a paired interstitialcy model is used as a basis for qualitative comparisons of conductivity and dielectric phenomena in β-alumina crystals and in glass.
Abstract: A paired interstitialcy model is used as a basis for qualitative comparisons of conductivity and dielectric phenomena in β-alumina crystals and in glass. Thus, the increase in the conductivity of sodium silicate glasses with increasing Na2O activity can be explained if the concentration of (Na2*)2+ interstitial pairs increases with increased polarizability of O2- ions, expressed in terms of the optical basicity parameter, Δ. Similarly, the occurrence of the pronounced minima in conductivity isotherms (the mixed-alkali effect in glass) is attributed to disappearance of mobile interstitial pairs, e.g. (Li2*)2+ or (K2*)2+, and the stabilization (by polarization interactions) of apparently immobile mixed-alkali pairs, (LiK*)2+. The phenomenon of coionic conduction in certain β-alumina crystals is an interesting departure from this general pattern. The orientation dependence of the electrical modulus spectrum of monocrys-talline β-alumina highlights the presence of a bimodal distribution of relaxation times, in which the low-frequency component (v0=1011 Hz) may arise from the rearrangement of interstitial pairs and the high-frequency component (v0=2×1012 Hz) may arise from less hindered ionic motions. It is suggested that the motions of interstitial pairs and surrounding cations are mutually catalytic and that some form of combined motion is responsible for both the electrical and mechanical relaxations in β-alumina and glass.
TL;DR: In this article, the porosity of CaO and Ca(OH)2 powders in vacuum has been investigated, and it has been shown that the linear dimensions of the 1 to 20 μm particles of h-CaO and sr-CAO are ∼5% smaller than those of the parent particles.
Abstract: The solid products of decomposing CaCO3 powder in vacuum at 510°C (sr-CaO) and of decomposing Ca(OH)2 powder at 320°C in vacuum (h-CaO) are particles which have approximately the same exterior dimensions as the parent CaCO3 or Ca(OH)2 particles. N2 adsorption and desorption isotherms show that sr-and h-CaO have high internal surface areas which for sr-CaO have cylindrical symmetry, with the most common diameters being ∼ 10 nm, and for h-CaO are slit-shaped, with the most common slit width being ∼ 2.7 nm. The conclusions reached in earlier investigations, i.e that these decomposition reactions in vacuum initially yield a form of CaO which has the same unit cell dimensions as the parent solid, were in error, probably because water vapor converts much of the CaO to poorly crystalline Ca(OH)2 before XRD measurements can be completed in air. From the volume of N2 adsorbed by the porous powders, the porosity of h-CaO is calculated to be 36±5% and of sr-CaO 41.5±5%. These porosities imply that the linear dimensions of the 1 to 20 μm particles of h-CaO and sr-CaO are ∼5% smaller than those of the parent particles. XRD measurements made as a function of time show that particle shrinkage must occur by cooperative, diffusionless movement of crystallites of sr-CaO or h-CaO as they form.
TL;DR: In this paper, the critical stress intensity factor (Ka) was measured on the surfaces of materials fabricated with 5 to 30 vol% ZrO2 and subsequently oxidized at 600° to 800°C.
Abstract: Silicon nitride/zirconium oxide hot-pressed materials contain ZrO2-2xN4x/3 (0.25≤x≤0.43), which readily oxidizes at ≥ 500°C to monoclinic ZrO2. The molar volume increase (∼4 to 5%) of this reaction was used to develop compressive surface stresses. The development of these useful surface stresses was demonstrated with an indentation technique used to measure the apparent critical stress intensity factor (Ka) on the surfaces of materials fabricated with 5 to 30 vol% ZrO2 and subsequently oxidized at 600° to 800°C. The increase in Ka was directly related to the oxidation kinetics and the initial volume content of the unstable phase.
TL;DR: In this article, electrical conductivity and relaxation data were measured for glasses of composition 0.242[XK2O+(1-X)Na2O]-0.758SiO2 from 25° to 205°C and in the dilute Na+ ion composition range X= 0.9 to 1.
Abstract: Electrical conductivity and relaxation data were measured for glasses of composition 0.242[XK2O+(1-X)Na2O]-0.758SiO2 from 25° to 205°C and in the dilute Na+ ion composition range X= 0.9 to 1. The decrease in electrical conductivity σ when the majority alkali K+ is replaced with the foreign alkali Na+ is most rapid in the very dilute range X= 0.98 to 1 and becomes less rapid at larger Na+ contents. The electric field relaxation times agree well with the single-alkali mechanical relaxation times. The electric field relaxation function becomes more exponential with increasing foreign alkali content, indicating a corresponding decrease in the concentration of mobile ions. Attempts to account for the decrease in σ with increasing foreign alkali content by assuming that the single-alkali glass is a strong electrolyte and that the foreign alkali complexes with and immobilizes the majority alkali were unsuccessful. Calculated values of the number of majority alkali ions complexed by a foreign-alkali ion were unreasonably large.
TL;DR: In this paper, a new type of internal friction experiment was used to investigate the kinetics of grain-boundary sliding in hot-pressed Si3N4 materials, and the measured sliding resistance in NC-132 was greater than in HS-110 and HS-130 materials.
Abstract: A new type of internal friction experiment was used to investigate the kinetics of grain-boundary sliding in hot-pressed Si3N4. The measured sliding resistance in NC-132 was greater than in HS-110 and HS-130 materials.
TL;DR: In this article, a study of the near infrared spectra and differential thermogravimetric analysis of hydrated glasses was performed to determine the response patterns and the factors determining these patterns.
Abstract: Silicate glasses show a wide range of responses to exposure to water or steam at high temperatures and pressure. The response patterns and the factors determining these patterns are discussed. The manner in which water is incorporated in the silicate glasses is elucidated by a study of the near infrared spectra and differential thermogravimetric analysis of hydrated glasses. Factors affecting relative amounts of tightly bonded and loosely bonded water are discussed.