Showing papers in "Journal of the American Ceramic Society in 1991"

Bioceramics: From Concept to Clinic

Abstract: Ceramics used for the repair and reconstruction of diseased or damaged parts of the musculo-skeletal system, termed bioceramics, may be bioinert (alumina, zirconia), resorbable (tricalcium phosphate), bioactive (hydroxyapatite, bioactive glasses, and glass-ceramics), or porous for tissue ingrowth (hydroxyapatite-coated metals, alumina). Applications include replacements for hips, knees, teeth, tendons, and ligaments and repair for periodontal disease, maxillofacial reconstruction, augmentation and stabilization of the jaw bone, spinal fusion, and bone fillers after tumor surgery. Carbon coatings are thromboresistant and are used for prosthetic heart valves. The mechanisms of tissue bonding to bioactive ceramics are beginning to be understood, which can result in the molecular design of bioceramics for interfacial bonding with hard and soft tissues. Composites are being developed with high toughness and elastic modulus match with bone. Therapeutic treatment of cancer has been achieved by localized delivery of radioactive isotopes via glass beads. Development of standard test methods for prediction of long-term (20-year) mechanical reliability under load is still needed.

Microstructural design of toughened ceramics

Abstract: On discute des mecanismes de renforcement des ceramiques par pontage des fissures a l'aide de trichites et d'autres types de renforcants discontinus. On montre que le renforcement par pontage des fissures peut etre combine avec d'autres mecaniques de renforcement (par exemple par transformation de phase) et que l'on peut obtenir des effets de synergie. La conception des materiaux renforces repose fortement sur le controle des proprietes des materiaux et des constituants microstructuraux

Theory of mechanical-properties of ceramic-matrix composites

Abstract: A theory is presented to predict the pullout work and ultimate tensile strength of ceramic-matrix composite (CMC) materials tested under uniaxial tension as functions of the underlying material properties. By assuming that the fibers fracture independently and that global load redistribution occurs upon fiber fracture, the successive fragmentation of each fiber in the multifiber composite becomes identical to that of a single fiber embedded in a homogeneous large-failure-strain matrix, which has recently been solved exactly by the present author. From single-fiber fragmentation, the multifiber composite distribution of pullout lengths, work of pullout, and ultimate tensile strength are easily obtained. The trends in these composite properties as a function of the statistical fiber strength, the fiber radius and fill fraction, and the sliding resistance-tau-between the fibers and the matrix easily emerge from this approach. All these properties are proportional to a characteristic gauge length-delta-c and/or the associated characteristic stress-sigma-c, with proportionality constants depending only very weakly on the fiber Weibull modulus: the pullout lengths scale with delta-c, the work of pullout scales with sigma-c-delta-c, and the ultimate strength scales with sigma-c. The key length-delta-c is the generalization of the "critical length," defined by Kelly for single-strength fibers, to fibers with a statistical distribution of strengths. The theory also provides an interpretation of fracture-mirror measurements of pulled-out fiber strengths so that the in situ key strength sigma-c and Weibull modulus of the fibers can be determined directly. Comparisons of the theoretical predictions of the ultimate tensile strength to literature data on Nicalon/lithium aluminum silicate (LAS) composites generally show good agreement.

Ceramic and Glass‐Ceramic Packaging in the 1990s

Abstract: A broad overview of packaging involving interconnecting, powering, protecting, and cooling semiconductor chips to meet a variety of computer system needs is presented. The general requirements for ceramics in terms of their thermal, mechanical, electrical, and dimensional control requirements are presented, both for high-performance and low-performance applications. Glass-ceramics are identified as the best candidates for high-performance systems, and aluminum nitride, alumina, or mullite are identified for low-performance systems. Glass-ceramic/copper substrate technology is discussed as an example of high-performance ceramic packaging for use in 1990s. Lower-dielectric-constant ceramics such as composites of silica, borosilicate, and cordierite, with or without polymers and porosity, are projected as potential ceramic substrate materials by the year 2000.

Gelcasting of Alumina

Abstract: Gelcasting is a novel method for molding ceramic powder based on a synthesis of concepts derived from traditional ceramics and polymer chemistry. Gelcasting of alumina is described in this paper. The process is based on the in situ polymerization of acrylamide monomers as the setting mechanism for forming the green body. It has the following features: slurries with high solids loading and low viscosity (1.8 Pa {center dot} s at 62 vol% solids), dried bodies containing less than 4 wt% binder, and the ability to fabricate complex-shaped bodies.

Mullite for Structural, Electronic, and Optical Applications

Abstract: Mullite (3AI2O3. 2Si02) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mot% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for lowtemperature (

Theoretical analysis of the fiber pullout and pushout tests

Abstract: The fiber pullout and pushout tests have been analyzed to predict the load-displacement behavior in terms of fiber/matrix interface parameters. The effects of residual axial strain in the fiber and fiber surface topography were included. The residual axial strain was found to be a significant parameter. It is shown that the interface failure can be progressive or catastrophic. In the case of a progressive failure of the interface, the load-displacement curve is nonlinear. The portion of the curve from above the first nonlinearity to near the peak load can be predicted in terms of parameters of the interface, viz., the friction coefficient, the radial stress at the interface, the fracture toughness of the interface, and the residual axial strain in the fiber. Values for these parameters can be obtained from a single loaddeflection curve. The peak load and load drop, which are usually reported, are found not to be directly relatable to any interface property, since the length of the last portion of the fiber to debond is influenced by end effects and hence not easily predicted. However, for data which describe the peak load as a function of initial embedded length, that factor can be eliminated and the data reduced to yield the relevant interface parameters. In pullout, the peak and friction loads saturate with large specimen thickness. Catastrophic failure is favored when the debond initiation load is high or when residual stress is low. Finally, a methodology to extract interface parameters from experimental data is suggested.

Bulk Conductivity and Defect Chemistry of Acceptor‐Doped Strontium Titanate in the Quenched State

Abstract: The grain bulk conductivity of acceptor-doped SrTiO3 ceramics was investigated by the impedance analysis method after quenching from high-temperature equilibria. The influence of the oxygen partial pressure during equilibration, PEqO2, is described in terms of a defect model for titanates. From a comparison between the experimental results and the predictions of this model, a predominant ionic conductivity is concluded for a wide PEqO2 range (approximately 10−11 to 105 Pa). The influence of the ionization energy of the acceptors and of possible defect association is discussed.

Phase Separation in Gelling Silica–Organic Polymer Solution: Systems Containing Poly(sodium styrenesulfonate)

Abstract: Phase separation which occurs in parallel to the hydrolysis and gelation of alkoxysilane solution containing poly(sodium styrenesulfonate) (NaPSS) has been investigated. Depending on the reaction conditions, gel morphologies such as isolated pores, particle aggregates, and interconnected continuous pores from 0.1 to 100 μm long have been observed. Time-resolved light scattering of gelling solution suggested the occurrence of spinodal phase separation through the polymerization of silica and the subsequent freezing of the developing structure by sol-gel transition. The effects of reaction parameters on the periodic size are explained mainly in terms of their influence on the “chemical cooling” rate which is determined by the polymerizatica rate of silica and the solubility of NaPSS in the reacting solution.

Chemical Processing and Microwave Characteristics of (Zr,Sn)TiO4 Microwave Dielectrics

Abstract: Monosized spherical particles of (Zr, Sn)TiO4 with an average diameter of about 0.3 μm were synthesized by the controlled hydrolysis of metal alkoxides. The as-prepared, amorphous, particles were compacted without any sintering aid, crystallized, and then sintered at 1600°C for 3 h into bodies with >96.0% of theoretical density. The dense sintered bodies of (Zr0.80Sn0.20)TiO4 showed good microwave characteristics; ɛr= 40.0, Q= 5000, and τt= 3 ppm/°C at 10 GHz. The dielectric constant was remarkably dependent upon the relative densities of the sintered bodies and the change of the lattice parameters, while Q value was mainly affected by the oxygen deficiency. An increase of the dielectric constant with the substitution of Sn4+ in ZrTiO4 was attributable to the enhancement of ionic polarization with the increase of the c-axis length.

Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders

Abstract: Mullite was fabricated by a process referred to as transient viscous sintering (TVS). Composite particles which consisted of inner cores of α-alumina and outer coatings of amorphous silica were used. Powder compacts prepared with these particles were viscously sintered to almost full density at relatively low temperatures (∼1300°C). Compacts were subsequently converted to dense, fine-grained mullite at higher temperatures (∼1500°C) by reaction between the alumina and silica. The TVS process was also used to fabricate mullite/zirconia/alumina, mullite/silicon carbide particle, and mullite/silicon carbide whisker composites. Densification was enhanced compared with other recent studies of sintering of mullite-based composites. This was attributed to three factors: viscous flow of the amorphous silica coating on the particles, avoidance of mullite formation until higher temperatures, and increased threshold concentration for the development of percolation networks.

Phase relations in the Barium Titanate—Titanium Oxide System

Abstract: Phase relations in the BaTiO3—TiO2 system were studied at temperatures above 1300°C in air. Quenching experiments were performed with high-purity reagents, and a new equilibrium phase diagram was constructed. Results include redetermination of the liquidus boundaries, the eutectic temperature, the melting or decomposition temperatures of the stable compounds in the system, the cubic—hexagonal transition in BaTiO3, and the solid solubility of TiO2 in BaTiO3.

Inorganic Fibers—A Literature Review

Abstract: A review of inorganic fibers other than asbestos, glass, and carbon fibers and optical and hollow fibers. Special emphasis is placed on high-performance fibers. The manufacture, structure, properties, and uses of the selected fibers are discussed.

Ionic Conduction in Phosphate Glasses

Abstract: Ionic conductivity in phosphate glasses has been known for almost 30 years. Recently these glasses have been shown to exhibit some of the highest ionic conductivities ever reported. In many cases, because of their ease of preparation, low melting points, strong glass-forming character, and simple composition, phosphate glasses have been studied more than any other ionically conducting glasses. However, no single review has ever been made of these glasses with the purpose of correlating the apparent widely disparate values of conductivity that these glasses exhibit. In this review, the conductivities of phosphate glasses are examined considering new structural studies of them. A systematic comparison of the dependence of the conductivity on glass chemistry reveals that, similar to other glass families (silicates and borates for example), the conductivity maximizes when the cation environments in the glass are minimized in their charge density and maximized in their site proximity.

Enhanced Fracture Toughness in Layered Microcomposites of Ce‐ZrO2 and Al2O3

Abstract: Laminar composites, containing layers of Ce-ZrO2 and either Al2O3 or a mixture of Al2O3 and Ce-ZrO2, have been fabricated using a colloidal method that allowed formation of layers with thicknesses as small as 10 μm. Strong interactions between these layers and the martensitic transformation zones surrounding cracks and indentations have been observed. In both cases, the transformation zones spread along the region adjacent to the layer, resulting in an increased fracture toughness. The enhanced fracture toughness was observed for cracks growing parallel to the layers as well as for those that were oriented normal to the layers.

Progressive Changes in the Structure of Hardened C3S Cement Pastes due to Carbonation

Abstract: The structures of partially carbonated hardened C3S cement pastes have been investigated by a combination of 29Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X-ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO2 for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C-S-H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C-S-H was, in the main, retained. A cross-linked silica-rich gel formed at later times in paste carbonated in CO2 but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.

Carbon–Nitrogen Pyrolyzates: Attempted Preparation of Carbon Nitride

Abstract: Precursors for the preparation of carbon nitride were selected from a number of relatively high nitrogen content organic materials. The precursors were pyrolyzed in a closed system at 700°C under 225 MPa. The resulting residues showed little nitrogen loss, but were amorphous with relatively low densities. Raman spectroscopy and 13C NMR indicate the presence of trigonal carbon.

Design Data for Engineering Ceramics: A Review of the Flexure Test

Abstract: The uniaxial strength of engineering ceramics is often measured by the well-known flexure strength test method there is a risk that flexure data are not representative of the properties of fabricated components. Reliability estimates for components based upon statistical extrapolation techniques from flexure data may not be valid. This paper reviews the problem and judges the usefulness of flexure data for design purposes. It is shown that some of the limitations of flexure data apply; to other modes of testing, including direct tension testing

Cyclic Fatigue from Frictional Degradation at Bridging Grains in Alumina

Abstract: Tension-tension cyclic loading tests have been conducted on a coarse-grained alumina ceramic that exhibits toughness-curve behavior by grain-interlock bridging Fatigue effects are observed in the regions of both short cracks, using indentation flaws, and long cracks, using compact-tension specimens A true mechanical fatigue effect is demonstrated by running the tests below the static fatigue limit A custom-made device for in situ observation of crack propagation in the scanning electron microscope enables us to identify bridge degradation as a cause of the fatigue process

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

Abstract: This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O2, CO, and N2 Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion

Boron Nitride Interphase in Ceramic‐Matrix Composites

Abstract: A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF3─NH3, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000°C. In a second step, the BN-treated preform was densified with SiC deposited from CH3SiCl3─H2 at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF3─NH3 gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO2/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO2 interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.

Microstructure of Solution‐Processed Lead Zirconate Titanate (PZT) Thin Films

Abstract: Lead zirconate titanate (PZT) thin films with a composition near the morphotropic phase boundary (Zr/Ti = 53/47) were fabricated by spin deposition of an alkoxide-derived solution and annealed at 650°C for 30 min. A complex microstructure is observed in which micrometer-scale rosettes of the desired perovskite phase are surrounded by nanocrystalline (10 to 15 nm) grains of pyrochlore structure. Transmission electron microscopy (TEM) demonstrates that the perovskite rosettes—features of approximately circular cross section which grow rapidly within the confined conditions of the thin film—are single crystals despite being highly porous. Pockets of lead-deficient pyrochlore extend throughout the thickness of the film. The only effects of Nb (2%) doping on the microstructure are to increase the fraction of the perovskite phase and the perovskite grain size. Despite the highly irregular shape of the perovskite particles and the presence of some pyrochlore, reasonable ferroelectric properties are measured (spontaneous polarization Ps∼ 0.2 C/m2).

Kinking of a Crack out of an Interface: Role of In‐Plane Stress

Abstract: A crack lying in the interface between two brittle elastic solids can advance either by continued growth in the interface or by kinking out of the interface into one of the adjoining materials. This competition can be assessed by comparing the ratio of the energy release rates for interface cracking and for kinking out of the interface to the ratio of interface toughness to substrate toughness. The stress parallel to the interface, σ0, influences the energy release rate of the kinked crack and can significantly alter the conditions for interface cracking over substrate cracking if sufficiently large. This paper provides the dependence of the energy release rate ratio on the in-plane stress. The nondimensional stress parameter which emerges is, σ0(a/E* Ti)1/2, where a is the initial length of the kink into the substrate, E* is a modulus quantity, and Ti is the fracture energy of the interface. An experimental observation of the cracking of reaction product layers in bonds between Ti(Ta) and Al2O3 is rationalized by the theory.

Preparation and Characterization of High‐Temperature Thermally Stable Alumina Composite Membrane

Abstract: A crack- and pinhole-free composite membrane consisting of an α-alumina support and a modified γ-alumina top layer which is thermally stable up to 1100°C was prepared by the sol–gel method. The supported thermally stable top layer was made by dipcoating the support with a boehmite sol doped with lanthanum nitrate. The temperature effects on the microstructure of the (supported and unsupported) La-doped top layers were compared with those of a common γ-alumina membrane (without doping with lanthanum), using the gas permeability and nitrogen adsorption porosimetry data. After sintering at 1100°C for 30 h, the average pore diameter of the La-doped alumina top layer was 17 nm, compared to 109 nm for the common alumina top layer. Addition of poly(vinyl alcohol) to the colloid boehmite precursor solution prevented formation of defects in the γ-alumina top layer. After sintering at temperatures higher than 900°C, the common alumina top layer with addition of poly(vinyl alcohol) exhibits a bimodal pore distribution. The La-doped alumina top layer (also with addition of poly(vinyl alcohol)) retains a monopore distribution after sintering at 1200°C.

Activation Energy for the Sintering of Two-Phase Alumina/Zirconia Ceramics

Abstract: In an earlier paper we reported measurements of the activation energy for sintering from constant-heating-rate experiments with alumina/5% zirconia. Here, results from a full range of compositions in this two-phase system are described. They show that the activation energy remains in the range 700 ± 100 kJ/mol when the composition changes from 5 to 95 vol% zirconia. In comparison, pure zirconia sinters with an activation energy of 615 ± 80 kJ/mol and pure alumina with the energy of 440 ± 45 kJ/mol. The addition of 2.8 mol% yttria to zirconia does not have a measurable effect on the activation energy. The grain size dependence of the sintering rate suggests boundary-diffusion-controlled sintering. These activation energies are phenomenologically correlated with the interfacial energies in alumina, zirconia, and two-phase alumina/zirconia, suggesting that the bonding at the interface influences diffusional transport.

Kinetics of Hydroxyapatite Formation at Low Temperature

Abstract: The influence of temperature on the kinetics of formation of calcium-deficient hydroxyapatite by an acid-base reaction has been determined. Calcium-deficient hydroxyapatite is compositionally similar to bone apatite. Reactants used in this study were tetracalcium phosphate, a calcium phosphate more basic than hydroxyapatite, and dicalcium phosphate, which is more acidic than hydroxyapatite. The kinetics of hydroxyapatite formation are initially controlled by the surface areas of the reactants; however, eventually the rate of hydroxyapatite formation becomes diffusionally controlled. The origin of diffusion control appears to be associated with the epitaxial formation of hydroxyapatite on the surface of the dicalcium phosphate reactant. The microstructure of hydroxyapatite formed at 5°, 15°, and 25°C were generally similar and are characterized by the formation of a porous acicular product which covers the surfaces of the reactants. The microstructure of the hydroxyapatite formed at 38°C was distinct from this and exhibited a pseudomorphic relationship with the dicalcium phosphate reactant. Seeding with hydroxyapatite accelerated the initial reaction but did not appear to have major long-term effects on the fractional degree of reaction or on microstructural development. Reaction was also accelerated in a neutral salt solution of high ionic strength.

Effect of Interparticle Potentials and Sedimentation on Particle Packing Density of Bimodal Particle Distributions During Pressure Filtration

Abstract: Effect of interparticle forces, bimodal particle size distribution, and slurry viscosity on particle packing in alumina bodies consolidated by pressure filtration is presented. The requirements for packing colloidal particles to their highest density are strong repulsive interparticle forces and optimum particle size distribution. Even though these conditions are met, the high packing density in consolidated bodies may be adversely affected by particle segregation resulting from sedimentation. Therefore, the slurry during consolidation must have a sufficiently high viscosity to prevent sedimentation.

Effect of Interfacial Roughness on the Frictional Stress Measured Using Pushout Tests

Abstract: A fiber which is partially pushed out of a surrounding matrix and subsequently pushed in the opposite direction exhibits a substantial decrease in sliding friction as it passes through its original position (its “origin”). This is manifest by a decrease in the load required to push the fiber. It is suggested that interfacial roughness causes this phenomenon and that the decrease in load (friction) is associated with the fiber seating back into its original position. The period of the drop has been correlated with the spatial extent of the interfacial surface roughness, and the magnitude of the drop (referred to hereafter as the seating drop) has been correlated with the amplitude of the interfacial roughness. Observation of the seating drop allows separation of the friction associated with interfacial irregularities from that resulting primarily from residual stresses at the interface. Implications for composite design and use are discussed. The effect of abrasion at the sliding interface is also addressed.

Carbon Additions to Molybdenum Disilicide: Improved High‐Temperature Mechanical Properties

Abstract: C addition (2 wt%) to MoSi2 acted as a deoxidant, removing the otherwise ubiquitous siliceous grain boundary phase in hot-pressed samples, and causing formation of SiC and Mo5Si3C1 (a variable-composition Nowotny phase). Both hardness and fracture toughness of the C-containing alloy were higher than those of the C-free (and oxygen-rich) material; more significantly, the fracture toughness of the MoSi2+ 2% C alloy increased from 5.5 MPa·m1/2 at 800°C to ∼11.5 MPa·m1/2 at 1400°C.

Preparation of Colloidal Boehmite Needles by Hydrothermal Treatment of Aluminum Alkoxide Precursors

Abstract: Fairly monodisperse colloidal boehmite fibrils with a high aspect ratio were synthesized by hydrothermal treatment at 150°C of an acidified aqueous alkoxide solution, prepared by adding an aqueous HCl solution to an aluminum alkoxide precursor. The average particle length could be controlled between about 100 and 500 nm by varying the initial amounts of alkoxide and acid. Using two different alkoxides in a 1:1 molar ratio yielded the most needlelike product, having a particle length standard deviation of 40%. The boehmite particles were polycrystalline and contained 0.14 mol of excess H2O per mol of AlOOH, bound to the particle surface.