Diffusion of 110mAg Tracer in Polycrystalline and Single-Crystal Lead-Containing Piezoelectric Ceramics

We have conducted diffusion measurements of radioactive ^110mAg tracer in single-crystal PbMgNbO₃–PbTiO₃ (PMN-PT) and in polycrystalline 50Pb(Ni_1/3Nb_2/3)O₃·35PbTiO₃·15PbZrO₃ (PNN-PT-PZ) piezoelectric ceramics. Both materials measured belong to the perovskite family. Diffusion in PMN-PT is characterized by an activation energy of 277 kJ/mol and pre-exponential factor of 0.0034 m²/s and compares well with cation diffusion in high-temperature superconducting YBa₂Cu₃O_7–δ. Diffusion in polycrystalline PNN-PT-PZ, on the other hand, is many orders of magnitude faster and is attributed to grain boundaries. PNN-PT-PZ has a lower activation energy, 168 kJ/mol, and a combined pre-exponential factor ( s δ( D _b)_o, where s is the segregation factor of silver, δ the thickness, and ( D _b)_o the pre-exponential factor for grain boundaries) of 3.7 × 10⁻⁹ m³/s. The unusually large combined pre-exponential factor infers large segregation of silver at the grain boundaries and small solid solubility within the grains. It is possible, using a semiempirical model, to compute metal– (silver–) ceramic interface energies as a function of temperature, from which values of 90 kJ/mol and 0.9 R are obtained for enthalpy and entropy, respectively, for grain-boundary segregation.     

Fracture of an Aluminosilicate Fiberboard

Mode-I fracture of aluminosilicate fiberboard that is used in large mirror casting molds was studied. The material was idealized as a transversely isotropic, layered composite that was composed of planar sheets of crosslinked fibers. Elastic constants, the toughness ( K_R ) curve, and the fracture work were measured at room temperature. The observed rising K_R behavior was attributed to crack bridging. Experimental measurements of the bridging stress were made using a specimen-renotching technique. Relationships between the bridging stress, K _R , and fracture work were explored and shown to be consistent.     

Fracture Resistance Characteristics of a Metal-Toughened Ceramic

The fracture characteristics of an Al₂O₃/Al composite are examined. Measurements of resistance curves and work of rupture are compared with predictions of a micromechanical model, incorporating the effects of crack bridging by the Al reinforcements. The bridging traction law is assumed to follow linear softening behavior, characterized by a peak stress, σ_c, and a critical stretch-to-failure, u _c. The values of σ_c and u _c inferred from such comparisons are found to be broadly consistent with independent measurements of stretch-to-failure, along with the measured flow characteristics of the Al reinforcement. The importance of large-scale bridging on the fracture resistance behavior of this class of composite is also demonstrated through both the experiments and the simulations.     

Enhancement of Fracture Properties of Wustite by Precipitation

Both the fracture-initiation energy, γ_i, and the fracture strength, σ _f , of 99.5% dense wustite (grain size 23 μm) were substantially increased by (1) precipitating coherent-coplanar Fe₃O₄, and (2) decomposing the wustite eutectoidally to α-Fe+Fe₃O₄, forming a continuous α-Fe network at the former wustite grain boundaries. The increase in γ_i, and σ_f resulting from the presence of 13 vol% Fe₃O₄ in wustite was attributed to the difference in cleavage habits between the precipitates and matrix and to crack-front pinning. The continuous α-Fe network increased the fracture stability in that fracture was not catastrophic but progressed in steps. The improvement in γ _i and σ _f was attributed mainly to the plastic work in fracturing the α-Fe.     

Anisotropy of Grain Growth in Alumina

Grain growth in theoretically dense undoped and MgO-doped polycrystalline Al₂O₃ was studied, and average grain-boundary migration rates were compared with those of a -plane and c -plane sapphire during migration into the same undoped and MgO-doped materials. The results are discussed in terms of a grain-size-dependent grain-boundary mobility-grain-boundary energy product, M _bγ_b. The grainsize dependencies of the M _bγ_b products for seed and matrix grains differ. Seed orientation appears to affect the nature of solute-boundary interactions. The importance of grain-boundary structure on migration characteristics is also indicated by a demonstration of twin-formation-enhanced grain growth.     

Influence of ZrO2 Grain Size and Content on the Transformation Response in the Al2O3─ZrO2 (12 mol% CeO2) System

Based on experimental and modeling studies, the rate of increase in the martensite start temperature M _s for the tetragonal-to-monoclinic transformation with increase in zirconia grain size is found to rise with decrease in ZrO₂ content in the zirconia-toughened alumina ZTA system. The observed grain size dependence of M _s can be related to the thermal expansion mismatch tensile (internal) stresses which increase with decrease in zirconia content. The result is that finer zirconia grain sizes are required to retain the tetragonal phase as less zirconia is incorporated into the alumina, in agreement with the experimental observations. At the same time, both the predicted and observed applied stress required to induce the transformation are reduced with increase in the ZrO₂ grain size. In addition, the transformation-toughening contribution at temperature T increases with increase in the M _s temperature brought about by the increase in the ZrO₂ grain size, when T > M _s. In alumina containing 20 vol% ZrO₂ (12 mol% CeO₂), a toughness of ∼10 MPa. √m can be achieved for a ZrO₂ grain size of ∼2 μm ( M _s∼ 225 K). However, at a grain size of ∼2 μm, the alumina–40 vol% ZrO₂ (12 mol% CeO₂) has a toughness of only 8.5 MPa. √m ( M _s∼ 150 K) but reaches 12.3 MPa. ∼m ( M _s∼ 260 K) at a grain size of ∼3 μm. These findings show that composition (and matrix properties) play critical roles in determining the ZrO₂ grain size to optimize the transformation toughening in ZrO₂-toughened ceramics.     

Polytypes, Grain Growth, and Fracture Toughness of Metal Boride Particulate SiC Composites

In order to understand the relation between microstructure and toughening behavior in SiC materials, NbB₂, TaB₂, TiB₂, and ZrB₂ particulate SiC composites were fabricated with pressureless sintering. In the composites, 3(cubic)-SiC powder was used as starting material for the matrix. The p-SiC powder transformed to a(noncubic) phase during sintering. The transformation, the behavior of which was influenced by the existence of metal boride particles, was accompanied by normal or exaggerated grain growth. The metal boride particles suppressed large-scale exaggerated grain growth of SiC, and it had a tendency to simulate grain growth with a high aspect ratio of the SiC grains. Increase in the fracture toughness of the composites was observed when the grain size and the aspect ratio of the SiC grains increased together. The toughening behavior is discussed based on a grain bridging mechanism.     

High-Strength Zirconium Diboride-Based Ceramics

Zirconium diboride (ZrB₂) and ZrB₂ ceramics containing 10, 20, and 30 vol% SiC particulates were prepared from commercially available powders by hot pressing. Four-point bend strength, fracture toughness, elastic modulus, and hardness were measured. Modulus and hardness did not vary significantly with SiC content. In contrast, strength and toughness increased as SiC content increased. Strength increased from 565 MPa for ZrB₂ to >1000 MPa for samples containing 20 or 30 vol% SiC. The increase in strength was attributed to a decrease in grain size and the presence of WC.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).     

Novel Fabrication Route to Al2O3–TiN Nanocomposites Via Spark Plasma Sintering

A novel method for the preparation of Al₂O₃–TiN nanocomposites was developed. A mixture of TiO₂, AlN, and Ti powder was used as the starting material to synthesize the Al₂O₃–TiN nanocomposite under 60 MPa at 1400°C for 6 min using spark plasma sintering. X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy were used for detailed microstructural analysis. Dense (up to 99%) nanostructured Al₂O₃–TiN composites were successfully fabricated, the average grain size being less than 400 nm. The fracture toughness ( K _{I C} ) and bending strength (σ_b) of the nanostructured Al₂O₃–TiN composites reached 4.22±0.20 MPa·m^1/2 and 746±28 MPa, respectively.     

Effects of α-Sic versus β-Sic Starting Powders on Microstructure and Fracture Toughness of Sic Sintered with Al2O3-Y2O3 Additives

Dense Sic ceramics were obtained by pressureless sintering of β-Sic and α-Sic powders as starting materials using Al₂O₃-Y₂O₃ additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from α-SiC powder was composed of equiaxed grains, whereas SiC obtained from α-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the β→α phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using α-SiC powder increased slightly from 4.4 to 5.7 MPa.m^1/2 with holding time, that is, increased grain size. In the case of the sintered SiC using β-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa.m^1/2 with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.     

Relationship between Microstructure and Fracture Toughness of Toughened Silicon Carbide Ceramics

Different microstructures in SiC ceramics containing Al₂O₃, Y₂O₃, and CaO as sintering additives were prepared by hot-pressing and subsequent annealing. The microstructures obtained were analyzed by image analysis. Crack deflection was frequently observed as the toughening mechanism in samples having elongated α-SiC grains with aspect ratio >4, length >2 μm, and grain thickness ( t ) <3 μm (defined as key grains 1). Crack bridging was the dominant toughening mechanism observed in samples having grains with thickness of 1 μm < t < 3 μm and length >2 μm (key grains 2). The values of fracture toughness varied from 5.4 to 8.7 MPa·m^1/2 with respect to microstructural characteristics, characterized by mean grain thickness, mean aspect ratio, and total volume fraction of key grains. The difference in fracture toughness was mainly attributed to the amount of key grains participating in the toughening processes.     

Effects of Damage on Thermal Shock Strength Behavior of Ceramics

When subjected to severe thermal shock, ceramics suffer strength degradation due to the damage caused by the shock. A fracture-damage analysis is presented to study the effects of damage on the thermal shock behavior of ceramics. It is assumed that a narrow strip damage zone is developed at the tip of a preexisting crack after a critical thermal shock and the damage behavior can be described by a linear strain-softening constitutive relation. Damage growth and strength degradation are determined based on fracture and damage mechanics. Numerical calculations are carried out for two ceramic materials, and the strength degradation agrees quite well with experimental results. The effects of bridging/damage stress, the fracture energy of the bridging/damage zone, and specimen size on thermal shock strength behavior are studied. A higher fracture energy can enhance the residual strength of thermally shocked ceramics and, for a given fracture energy, a higher bridging stress is needed to reduce the strength degradation. It is also shown that the thermal shock strength behavior is size-dependent, and a high value of ( K _IC/O_b)², where K _IC is the intrinsic fracture toughness and O_b is the bending strength, can improve significantly the residual strength.     

Critical Microstructures for Microcracking in Al2O3-ZrO2 Composites

The internal strains asSociated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al₂O₃/ZrO₂ composites. The degree of transformation was found to be dependent on the volume fraction of ZrO₂ and its size, the latter of which could be controlled by suitable heat treatments. The microstructural changes that occurred during the heat treatments were studied using quantitative microscopy and X-ray diffraction. For materials containing more than 7.5 vol% Zr0₂, the ZrO₂ particles were found to pin the Al₂O₃ grain boundaries, thus limiting the Al₂O₃ grain growth. The limiting grain size was found to be dependent on size and volume fraction of ZrO₂. Heat treatments for the higher volume fraction materials (>7.5 vol% ZrO₂) caused micro-structural changes which resulted in increased amounts of monoclinic ZrO₂ at room temperature; elastic modulus measurements indicated that this was occurring concurrently with microcracking. By combining the ZrO₂ grain-size distributions with the X-ray analysis it was possible to calculate the critical ZrO₂ size required for the transformation. The critical size was found to decrease with increasing amounts of ZrO₂. Hardness and indentation fracture toughness were measured on the composites. Grain fragmentation was observed at the edge of the indentations and microcracks were observed directly, using an AgNO₃ decoration technique, near the indentations.     

R-Curve Behavior of a Polycrystalline Graphite: Microcracking and Grain Bridging in the Wake Region

The contributions of nonlinear fracture processes both in the microcracking frontal process zone and in the following wake region and of grain bridging to crack-growth resistance parameters are discussed in terms of the R-curve behavior of an isotropic polycrystalline graphite. The R-curve behavior of the graphite is characterized by rapidly increasing values at the initial stage of crack extension (Δa≤1 to 2 mm) followed by a steady-state plateaulike region and then a distinct decrease when the primary crack tip approaches the end surface of the test specimen. Scanning electron microscopy of fracture mechanics specimens revealed a dominant role of grain bridging in the following wake regions on the rising R-curve behavior and confirmed the significant size effect of the large-scale microcracking process zone on the falling R-curve behavior. The stress-derived fracture toughness (K_R) and the energy fracture toughness (R_c) are discussed in relation to the micro-cracking residual strain.     

Fabrication of Al-Added TiN Materials by the Combination of Double Self-Propagating High-Temperature Synthesis and Pulsed Electric-Current Pressure Sintering

Crystalline phases of Al-added TiN, denoted as (Ti_{1− x} Al _x )N _y (0≤ x ≤0.10, 0.8< y <1.0), prepared from a mixture of Ti and Al powders by self-propagating high-temperature synthesis (SHS) in a nitrogen atmosphere, have been investigated. By repeating SHS twice, in the region of 0.0< x ≤0.02 cubic (Ti_{1− x} Al _x )N _y solid solutions, and in the region of 0.02< x ≤0.10 composites consisting of (Ti_{1− x} Al _x )N _y and hexagonal Ti₂AlN were formed. After powder characterization, they were consolidated to dense materials (>97% of theoretical) by pulsed electric-current pressure sintering. With increasing Al addition, the optimum sintering temperatures were lowered, followed by reduction of grain size. Their mechanical properties, that is, three-point bending strength σ_b, Vickers hardness H _v, and fracture toughness K _{I C} were evaluated as a function of Al content.     

Preparation, Microstructure, and Mechanical Properties of Silicon Carbide–Dysprosia Composites

Composites of silicon carbide (SiC) with up to 30 vol% of dysprosia (Dy₂O₃) were fabricated by hot pressing and hot isostatic pressing. The effects of Dy₂O₃ dispersions on the microstructure and on selected mechanical properties of the composites were investigated. When 10-15 vol% of Dy₂O₃ was dispersed in the SiC matrix, the fracture toughness increased by ∼40%, whereas the flexural strength was comparable to that of unreinforced SiC. The increased fracture toughness was due to crack deflection, in conjunction with crack-interface grain bridging, and was not related to a phase transformation of Dy₂O₃ in the matrix.     

Effect of Grain Growth of β-Silicon Nitride on Strength, Weibull Modulus, and Fracture Toughness

β-Si₃N₄ powder containing 1 mol% of equimolar Y₂O₃–Nd₂O₃ was gas-pressure sintered at 2000°C for 2 h (SN2), 4 h (SN4), and 8 h (SN8) in 30-MPa nitrogen gas. These materials had a microstructure of " in-situ composites" as a result of exaggerated grain growth of some β Si₃N₄ grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa-m^1/2 because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas-pressure sintering of β-Si₃N₄ powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.     

(Pr, Co, Nb)-Doped SnO2 Varistor Ceramics

The effect on microstructure and electrical properties of (Co, Nb)-doped SnO₂ varistors upon the addition of Pr₂O₃ was investigated by scanning electron microscopy and by determining I – V , ɛ– f , and R – f relations. The threshold electric field of the SnO₂-based varistors increased significantly from 850 to 2280 V/mm, and the relative dielectric constants of the SnO₂-based varistors decreased greatly from 784 to 280 as Pr₂O₃ concentration was increased up to 0.3 mol%. The significant decrease of the SnO₂ grain size, from 4.50 to 1.76 μm with increasing Pr₂O₃ concentration over the range of 0–0.3 mol%, is the origin for the increase in the threshold voltage and decrease of the dielectric constants. The grain size reduction is attributed to the segregation of Pr₂O₃ at grain boundaries hindering the SnO₂ grains from conglomerating into large particles. Varistors were found to have a superhigh threshold voltage and a comparatively large nonlinear coefficient α. For 0.15 mol% Pr₂O₃-doped sample, threshold electric field and nonlinear coefficient α were measured to be 1540 V/mm and 61, and for 0.3 mol% Pr₂O₃-doped sample, V and α were 2150 V/mm and 42, respectively. Superhigh threshold voltage and large nonlinear coefficient α qualify the Pr-doped SnO₂ varistor as an excellent candidate for a high voltage protection system.     

Effect of the Growth Treatment on Two-Stage Nucleation Experiments

Numerical simulations are presented that document the strong effect of a previously underappreciated portion of two-stage thermal treatments used in the study of nucleation processes: the "heat-up" process whereby samples are heated from "nucleation" conditions to "growth" conditions. The simulations indicate that two limiting regimes exist, dependent on (a) the cluster size distribution of as-quenched glasses, (b) the temperatures used for nucleation and growth, and (c) the rates of heating and cooling: (1) all clusters larger than the critical size at growth conditions ( n ^*_gr) will grow to macroscopic size (the "standard" case); and (2) all clusters larger than the critical size at nucleation conditions ( n ^*_nuc) will grow to macroscopic size. In addition, cases in which the "effective critical size" ( n ^*_eff) is intermediate between n ^*_gr and n ^*_nuc can also occur. Cases in which n ^*_eff < n ^*_gr is manifested during nucleation experiments as an abrupt boost in crystal number density during the heat-up from nucleation to growth conditions, as all clusters larger than n ^*_eff are rapidly "flushed" past n ^*_gr. For the system studied herein, this can lead to a 10⁶-fold increase in final number density within seconds to a few minutes. Finally, the importance of structural relaxation for this process is demonstrated by examining a case in which the nucleation temperature is below the nominal glass transition temperature.