Preparation of Alumina-Zirconia Powders by Evaporative Decomposition of Solutions

Fine-grained, homogeneously dispersed alumina-zirconia and zirconia powders were prepared by evaporative decomposition of solutions. The pure metastable tetragonal zirconia powder transformed to the monoclinic form when it was heated to 1150°C. The zirconia in the alumina-zirconia powder, which was also in the tetragonal form, did not transform when the powder was heated to 1150° C. This result is explained in terms of inhibition of coarsening of the zirconia grains by the alumina particles.     

Strengthening of Porous Mullite and Zirconia CMC Matrices by Evaporation/Condensation

A processing method using evaporation/condensation sintering in an HCl atmosphere was developed for strengthening porous materials without shrinkage. Strengthening without shrinkage is useful in preventing voids and cracks that might be formed during constrained densification, e.g., a porous matrix in a continuous fiber reinforced ceramic composite. Mixtures of mullite and zirconia (monoclinic, tetragonal (3 mol% Y₂O₃), and cubic (8 mol% Y₂O₃)) were studied and exposed to HCl vapor at temperatures up to 1300°C. It was observed that the evaporation–condensation mass transport process produced a porous material with minimal shrinkage. As the crystal structure of the starting tetragonal and cubic zirconia powders did not change after extensive coarsening, it appeared that zirconium and yttrium were transported in the same proportion via evaporation/condensation. The process produced significant coarsening of the zirconia grains, which made the material resistant to densification when heated to 1200°C in air. Because the sintering produced coarsening without shrinkage, the pores also coarsened and a porous microstructure was retained. Mixtures of mullite and zirconia were used because mullite does not densify under the processing conditions used here, namely, heat treatments up to 1300°C. The mullite particles acted as a non-densifying second phase to further inhibit shrinkage when the mullite/zirconia composite was heated up to 1200°C in air. The coarsened cubic zirconia plus mullite mixture had the least densification after heat treatments in air of 100 h at 1200°C.     

Effect of Twin-Plane Reentrant Edge on the Coarsening Behavior of Barium Titanate Grains

When BaTiO₃ ceramics were sintered at relatively low temperatures (≤1250°C), the grains with reentrant edges caused by a (111) double twin grew exclusively. As a result, a microstructure with a bimodal grain-size distribution composed of platelike large grains and fine matrix grains was obtained. In contrast, at the usual sintering temperature between 1250° and 1350°C, grains containing a (111) double twin did not exhibit any growth advantage. In this case, a coarse and uniform microstructure was obtained. When this coarse-grained specimen was further heat-treated at 1365°C, the grains possessing a double twin were observed to grow exclusively again. The results were explained in terms of a coarsening process controlled by two-dimensional nucleation.     

Precipitation and Coarsening of Magnesioferrite in Dilute Solutions of Iron in MgO

The precipitation and coarsening of magnesioferrite in MgO containing 1% Fe, prepared at 1440°C and aged at 700° and 800° C in air, were investigated by quantitative electron microscopy and magnetic analysis. The particles are superparamagnetic. The results of the two methods agree. The volume fraction of precipitate quickly reaches essentially its final value; then the particles grow according to the law of diffusion-controlled coarsening. The precipitate becomes anisotropic at long aging times.     

Coarsening Process of Penetration-Twinned Grains in PMN-35 mol% PT Ceramics

In a Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) specimen with 5 mol% excess PbO, the pores trapped inside grains by heat treatment in a double cycle at 1150°3C have revealed the coarsening pattern of the grains. Using this technique, the coarsening process of abnormal large grains with typical characteristics of penetration twins could be determined. The results suggest that two-dimensional nucleation and lateral growth enhanced at the reentrant edges are the cause of zigzag-shaped σ3 incoherent twin boundaries.     

Mechanism of Grain Growth and α-β'Transformation During Liquid-Phase Sintering of β'-Sialon

The rates of grain coarsening and α-β'transformation during the liquid-phase sintering of Si₃N₄-β'₆₀-YAG sialon have been measured at varying liquid fractions and z values in order to determine the rate-controlling mechanism. The average β'-grain size after sintering for 16 h at 1650°C shows no variation with the liquid-matrix fraction if the z value is fixed and a marked increase with the z value if the liquid fraction is fixed. Similarly, the amount of untransformed α-phase after sintering for 2.5 or 3.5 min at 1600°C shows no variation with the liquid-matrix fraction if the z value is fixed and a marked decrease with the z value if the liquid fraction is fixed. These results show that the grain coarsening and the α-β'transformation are controlled by the interface reaction. This conclusion is consistent with the observations in carbide-Co systems and with the theoretical predictions that the growth of faceted grains is controlled by interface reaction and that of spherical grains by diffusion. A general rule between the shape and the growth mechanism of grains in a liquid matrix is thus proposed.     

Transformation, Microstructure Development, and Densification in α-Fe2O3-Seeded Boehmite-Derived Alumina

Addition of α-Fe₂O₃ seed particles to alkoxide-derived boehmite sols resulted in a 10-fold increase in isothermal rate constants for the transformation of γ- to α-Al₂O₃. Changes in porosity and surface area with sintering temperature showed no effect of seeding on coarsening of the transition alumina gels, but the 200-fold decrease in surface area associated with transformation to α-Al₂O₃ occurred ∼ 100°C lower in seeded gels compared with unseeded materials. As a result of high nucleation frequency and reduced microstructure coarsening, fully transformed seeded alumina retained specific surface areas >22 m²/g and exhibited narrow pore size distributions, permitting development of fully dense, submicrometer α-Al₂O₃ at ∼ 1200°C.     

Effect of β-Seed Addition on the Microstructural Evolution of Silicon Nitride Ceramics

When a small amount of β-Si₃N₄ seed particles is added during the preparation of Si₃N₄ ceramics, a bimodal microstructure is obtained by sintering at 1760°C. When the specimen is further heat-treated at 1900°C to enhance the bimodal characteristic, the growth of large β grains is limited. The addition of a controlled amount of β seeds of uniform and large size is suggested to obtain the intended bimodal microstructure of Si₃N₄ ceramics.     

Nanostructured Dense ZrO2 Thin Films from Nanoparticles Obtained by Emulsion Precipitation

Nonagglomerated spherical ZrO₂ particles of 5–8 nm size were made by emulsion precipitation. Their crystallization and film-forming characteristics were investigated and compared with nanosized ZrO₂ powders obtained by sol–gel precipitation. High-temperature X-ray diffraction indicated that the emulsion-derived particles are amorphous and crystallize at 500°C into tetragonal zirconia, which is stable up to 1000°C. Crystallite growth from 5–20 nm occurred between 500°–900°C. Films of 6–75 nm thickness were made by spreading, spin coating, and controlled deposition techniques and annealed at 500°–600°C. The occurrence of t -ZrO₂ in the emulsion-precipitated powder is explained by the low degree of agglomeration and the corresponding low coarsening on heating to 500°–800°C, whereas the agglomerated state of the sol–gel precipitate powder favors the occurrence of the monoclinic form of zirconia under similar conditions.     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

Microstructural Evolution in Ca-PSZ and the Room-Temperature Instability of Tetragonal ZrO2

Microstructural evolution of a 7.8 mol% Ca-PSZ, fired at 1870°C and subsequently aged at 1300°C, was studied and correlated with the mechanical properties. The morphology, rafting, and subsequent coarsening of the tetragonal ( t ) ZrO₂ precipitates were of particular interest, as was the instability of some of the t -ZrO₂ particles during room-temperature aging. Precipitate coarsening kinetics (Ostwald ripening) was also studied. Over the limited time range studied (136 h), the interface reaction dominated the kinetics.     

Discontinuous Coarsening of Tetragonal Precipitates in Partially Stabilized Zirconia Induced by Diffusional Coherency Strain under Applied Stress

When partially stabilized zirconia with 6 mol% MgO and 4 mol% CaO is aged at 1450°C, intragranular precipitation occurs and concurrently the boundaries between the grains migrate, forming a Ca-enriched precipitate-free cubic phase and large tetragonal precipitates behind them. At these compositions and temperature the boundary migration is rapid and shows the characteristics of a discontinuous coarsening. A uniaxial compressive stress applied to this specimen during the aging treatment increases or decreases the migration rate of the boundaries parallel or perpendicular to the stress axis, respectively, in agreement with the prediction that a compressive coherency strain due to the diffusion of Ca atoms is produced at the surface of the retreating grains and drives the migration. The diffusional coherency strain energy is thus expected to be the dominant driving force for the discontinuous coarsening in this solid.     

Kinetics of {001} Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 Single Crystals Grown by Seeded Polycrystal Conversion

The kinetics of {001}-oriented Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN–35PT) single crystals grown by seeded polycrystal conversion were systematically quantified as a function of excess PbO liquid phase. The coarsening behavior of the corresponding matrix grains was similarly quantified. Single-crystal seed plates were embedded in a matrix of PMN-35PT with varying amounts of liquid phase (PbO) content in the range of 0 to 5 vol% and annealed at 1150°C for 0–10 h. Apparent maxima in the growth rates were observed at a PbO content of ∼3 vol% for both the single crystal and matrix grains. In both cases, the growth data were found to most closely follow cubic growth kinetics. Implications regarding the effect of PbO volume fraction on the matrix and single-crystal growth mechanisms are discussed.     

Oxygen Vacancies in Pure Tetragonal Zirconia Powders: Dependence on the Presence of Chlorine during Processing

We have used several experimental methods to study how a large extrinsic oxygen vacancy density in pure tetragonal ZrO₂ powders depends on details of how those powders are made. Samples were made from oxychloride and nitrate precursor solutions. We used perturbed angular correlation spectroscopy to determine in situ phase structure and the density of oxygen vacancies at 1200°C, XRD and SEM to determine the grain size and morphology of samples annealed at temperatures ranging from 200°–1200°C, and neutron activation analysis (NAA) to investigate purity of samples. NAA results showed that samples contain cation impurities at levels <<100 ppm. The XRD and SEM measurements showed that grains were nanometer-size, had a broad distribution, and grew from ∼10 nm at 200°C to ∼1 μm at 1200°C. The most striking process dependence is on presence of chlorine during processing. The grain size and phase above 600°C, and both the morphology and the density of oxygen vacancies at 1200°C were strongly affected by presence of chlorine-containing vapor during annealing. Samples processed in a chlorine-free atmosphere had large well-sintered grains and large (>500 ppm) oxygen vacancy concentrations at 1200°C, whereas samples processed in flowing H₂O/HCl vapor had smaller grains, porous morphology, and small (<100 ppm) vacancy density. All samples were loose powders consisting of single grain particles at <1000°C and multiple-grain particles at 1200°C.     

Effect of Carbon Addition on Elevated Temperature Crack Growth Resistance in (Mo,W)Si2–SiCp Composite

Experimental results on subcritical crack growth behavior of hot-pressed MoSi₂–50 mol% Wsi₂ alloy reinforced with 30 vol% SiC particles in the temperature range 1200°-1300°C are presented. The effect of 2 wt% C addition on the stable crack growth resistance of this composite was investigated under both static and cyclic loading conditions. The results indicate that the addition of carbon to the composite improves the subcritical crack growth resistance under both static and cyclic loads and increases the elevated temperature capabilities of the (Mo,W)Si₂ composite. Increasing the temperature from 1200° to 1300°C is found to increase the crack growth velocities with a concomitant decrease in the crack growth initiation thresholds. Electron microscopy of the crack-tip region indicates that the stable crack growth process is influenced primarily by interfacial cavitation. At 1300°C, deformation processes such as twinning of the SiC particles and dislocation motion within the matrix grains appear to play an active role in determining the crack growth kinetics. The role of glassy phase in influencing the high-temperature fracture behavior and its implications for design of the microstructures of the brittle materials are discussed.     

Bismuth Titanate from Mechanical Activation of a Chemically Coprecipitated Precursor

Most of the chemistry-based preparation routes for bismuth titanate (BIT) involve calcination at elevated temperatures in order to realize precursor-to-ceramic conversion. In a completely different approach using an amorphous BIT hydroxide precursor, nanocrystalline particles of layered perovskite BIT are synthesized by mechanical activation, skipping the detrimental crystallite coarsening and particle aggregation encountered at high temperatures. Mechanical activation leads to nucleation and steady growth of BIT crystallites in the amorphous precursor matrix, while Bi₂O₃ is involved as an intermediate transitional phase. The activation-derived BIT particles demonstrate a rounded morphology of ∼50 nm in size. This is in contrast to the BIT derived from calcination of the coprecipitated precursor at 600°C that is dominated by coarsened platelike particles. The former is sintered to a density of >95% theoretical at 875°C for 2 h, leading to a dielectric constant of ∼1260 when measured at 1 MHz and the Curie temperature of 646°C.     

SiC/ZTM Nanocomposite with Needlelike Mullite Grains and Enhanced Mechanical Properties

Zirconia-toughened mullite (SiC/ZTM) nanocomposites were prepared by a chemical precipitation method. The samples showed good sinterability and could be densified to >98.7% of the theoretical density at 1350°–1550°C. Because of the addition of mullite seeds in the starting powder and the pinning effects of ZrO₂ and SiC particles on mullite grain growth, a fine-grained microstructure formed. Mullite grains were generally equiaxed for the sample sintered at 1400°C; whereas, for the sample sintered at 1550°C, most mullite grains took a needlelike morphology, and SiC particles were primarily located within mullite grains. The strength and toughness increased with the increasing sintering temperature, and reached their respective maximum of 780 MPa and 3.7 MPa·m^1/2 for the sample sintered at 1550°C.     

Ferrimagnetic Resonance Study of the Growth Rate of Super-paramagnetic Precipitates of Magnesioferrite in MgO

Superparamagnetic magnesioferrite particles were precipitated from single-crystal MgO containing °2.2 cation% Fe, by aging between 700° and 1000°C. The anisotropy field of the precipitates was measured as a function of aging time and aging temperature. Superparamagnetic analysis was used to calculate the precipitate growth rate for several aging temperatures. It was found that from the earliest times studied, the particles grow according to the law of diffusion-controlled coarsening. An activation energy of 72.5 kcal/mol was calculated for this growth process.     

Thermal Stability of Air Plasma Spray and Solution Precursor Plasma Spray Thermal Barrier Coatings

Yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) were produced by conventional air plasma spray (APS) and solution precursor plasma spray (SPPS) processes. Both TBCs were isothermally heat treated from 1200° to 1500°C for 100 h. Changes in the phase content, microstructure, and hardness were investigated. The nontransformable tetragonal ( t ') phase is the predominant phase in both the as-sprayed APS and SPPS TBCs. APS and SPPS coatings exhibit similar thermal stability behavior such as densification rate, hardness increase, and grain coarsening rate. Both the as-received and heat-treated APS and SPPS TBCs show a bimodal pore size distribution with nano- and micro-size pores. After 1400°C/100 h heat treatment, equiaxed grains replace the columnar structure in APS TBCs and the splat structure disappears. Vertical cracks remain after the 1500°C/100 h exposure in SPPS TBCs. The monoclinic phase appears in APS TBCs after a 1400°C/100 h exposure and in SPPS coatings after a 1500°C/100 h exposure.     

Initial Coarsening and Microstructural Evolution of Fast-Fired and MgO-Doped Al2O3

The effect of an initial coarsening step (50-200 h at 800°C) on the subsequent densification and microstructural evolution of high–quality compacts of undoped and MgO–doped Al₂O₃ has been investigated during fast–firing (5 min at 1750°C) and during constant–heating–rate sintering (4°C/min to 1450°C). In constant–heating–rate sintering of both the undoped and MgO–doped Al₂O₃, a refinement of the microstructure has been achieved for the compact subjected to the coarsening step. A combination of the coarsening step and MgO doping produces the most significant refinement of the microstructure. In fast–firing of the MgO–doped Al₂O₃, the coarsening step produces a measurable increase in the density and a small refinement of the grain size, when compared with similar compacts fast–fired conventionally (i.e., without the coarsening step). This result indicates that the accepted view of the deleterious role of coarsening in the sintering of real powder compacts must be reexamined. Although extensive coarsening after the onset of densification must be reduced for the achievement of high density, limited coarsening prior to densification is beneficial for subsequent sintering.