Elastic Properties of Model Porous Ceramics

The finite-element method (FEM) is used to study the influence of porosity and pore shape on the elastic properties of model porous ceramics. Young's modulus of each model is practically independent of the solid Poisson's ratio. At a sufficiently high porosity, Poisson's ratio of the porous models converges to a fixed value independent of the solid Poisson's ratio. Young's modulus of the models is in good agreement with experimental data. We provide simple formulas that can be used to predict the elastic properties of ceramics and allow the accurate interpretation of empirical property–porosity relations in terms of pore shape and structure.     

Intrinsic Elastic, Dielectric, and Piezoelectric Losses in Lead Zirconate Titanate Ceramics Determined by an Immittance-Fitting Method

The material coefficients of "soft" and "hard" lead zirconate titanate (PZT) ceramics were determined as complex values by the nonlinear least-squares-fitting of immittance data measured for length-extensional bar resonators. The piezoelectric d -constant should be a complex value to obtain a best fitting between observed and calculated results. Because the elastic, dielectric, and piezoelectric losses determined in this process were not "intrinsic" losses, a calculation process to evaluate the "intrinsic" losses was proposed. It was confirmed that the intrinsic losses were smaller than the corresponding extrinsic losses. The intrinsic piezoelectric loss existed in both soft and hard PZTs; ∼50% of the loss of piezoelectric d -constant was derived from the elastic and dielectric losses. The most notable difference between the soft and hard PZTs was observed in their elastic losses.     

Evaluation of Slow Crack Growth Resistance in Ceramics for High-Temperature Applications

The slow (subcritical) crack growth (SCG) resistance of Si₃N₄ and SiC ceramics has been evaluated by a stepwise loading test on bending bars precracked by Vickers indentation. Three highly refractory materials were selected for the evaluation: i.e., (1) high-purity Si₃N₄ sintered by hot isostatic pressing (HIP) without additives and (2,3) α - and β - SiC pressureless sintered with B and C addition. Under the hypothesis of linear elastic behavior at high temperature, which was found satisfied in the present materials, the SCG resistance was expressed in terms of initial stress intensity factor critical for SCG failure within a predetermined lifetime. The present method was found useful in shortening the testing time and consistent with other traditional fatigue tests (e.g., static-fatigue test): It is recommended as a screening test for materials under research and development. Among the materials tested in the present study, the highest SCG resistance up to 1440°C was found in the high-purity Si₃N₄ without additives.     

Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites

The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.     

High-Temperature Mechanical Properties of a Uniaxially Reinforced Zircon-Silicon Carbide Composite

Mechanical properties of a monolithic zircon ceramic and zircon-matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide monofilaments were measured in flexure between 25° and 1477°C. Monolithic zircon ceramics were weak and exhibited a brittle failure up to about 1300°C. An increasing amount of the plastic deformation was observed before failure above about 1300°C. In contrast, composites reinforced with either uncoated or BN-coated SiC filaments were stronger and tougher than the monolithic zircon at all test temperatures between 25° and 1477°C. The ultimate strength and work-of-fracture of composite samples decreased with increasing temperature. A transgranular matrix fracture was shown by the monolithic and composite samples tested up to about 1200°C, whereas an increasing amount of the intergranular matrix fracture was displayed above 1200°C.     

碳化硅耐火材料的发展与性能

中国从50年代,人们就开始研究先进的结构陶瓷,SiC耐火制品也有40多年的研究历史,在50年代初,研制成功并迅速建成车间投产,满足炼锌竖罐精馏的特殊要求。苏联、日本、美国对SiC耐火材料的研究更早一些。SiC耐火材料具有优良的高温性能,广州应用于化工、冶金、能源、机械、建材、刀具等领域。本文简要介绍SiC耐火材料的发展种类及性能。     

Elastic-Property Measurements on CVI SiC/SiC Minicomposites by Acoustic Microscopy

In situ elastic properties of the fiber and matrix in composites obtained via chemical vapor infiltration/chemical vapor deposition (CVI-CVD) have been determined using acoustic microscopy. An unusually high frequency was used to attain the resolution that was required by the structure and small size of the constituents in these nonhomogeneous materials. A coupling liquid was prepared with acoustic properties that were chosen with consideration of the high frequency and the material characteristics. Local measurements on the fibers and matrix were achieved by studying the acoustic signature that was processed on very small areas. The elastic modulus of the fibers and matrices was deduced from the velocity measurements.     

Microwave Processing of Actively Seeded Precursor for Fabrication of Polymer Derived Ceramics

An innovative, faster and nonconventional processing technique is demonstrated for preparing silicon carbide‐based ceramics derived from polymer precursor. The technique is based on microwave‐induced pyrolysis of an actively seeded, high‐purity preceramic polymer that leads to rapid fabrication of silicon carbide components. It is successfully demonstrated that it is feasible to carry out microwave‐induced pyrolysis by seeding the polymer precursor with very low volume fractions of micrometer and nanometer‐sized metal and dielectric fillers. This process allows for rapid, net‐shape, and potentially low‐cost fabrication of silicon carbide‐based materials. Mechanical properties and microstructure of the silicon carbide‐based composites fabricated using this process are characterized.     

Elastic Properties of Nickel-Based Anodes for Solid Oxide Fuel Cells as a Function of the Fraction of Reduced NiO

The changes in porosity and elastic moduli of YSZ-containing nickel-based anode materials for solid oxide fuel cells were studied as a function of the fraction of reduced NiO. Anode samples were reduced in a gas mixture of 4% hydrogen and 96% argon for different periods of time at 800°C and their Young's and shear moduli were determined afterward at room temperature using resonant ultrasound spectroscopy and impulse excitation. It was found that the magnitude of Young's and shear moduli decreased significantly with increasing fraction of reduced NiO and that the magnitude of the elastic moduli of a fully reduced Ni–YSZ anode was ∼45% lower than that of unreduced NiO–YSZ. Because the elastic moduli of NiO are close to those of Ni, the observed decrease in the magnitude of the elastic moduli was found to be caused mainly by the significant increase in the porosity of the sample as a result of NiO reduction. Expressions are presented for the amount of porosity and the magnitude of the elastic moduli as a function of the fraction of reduced NiO.     

Evaluation of Electric Fracture Properties of Piezoelectric Ceramics Using the Finite Element and Single-Edge Precracked-Beam Methods

Single-edge precracked-beam (SEPB) tests were performed on a commercial lead zirconate titanate (PZT) ceramic. Mechanical loading was applied by the crosshead displacement control of a screw-driven electromechanical test machine. The fracture toughness parameter K _C was determined for various electric fields. A finite element analysis was also done to calculate the total potential energy release rate, mechanical strain energy release rate, and stress intensity factor for three-point flexure piezoceramic specimens with permeable and impermeable cracks under displacement and load control conditions. Numerical investigation and comparison with test data indicate that the energy release rate, upon application of the permeable model, is useful for predicting crack growth in PZT ceramic under electromechanical loading. Based on current findings, we suggest that the energy release rate criteria for the permeable crack are superior to fracture criteria for the impermeable crack.     

Ultrasonic Imaging of Porosity Variations Produced During Sinteriing

A silicon carbide disk was sintered from 2090° to 2190°C in 25°C steps. After each sintering step, the disk was examined using a precision acoustic scanning system to determine acoustic attenuation and velocity. The bulk density was found to vary non-monotonically with sintering temperature. The density varied as much as 10% from its value at 2090°C during the sintering process. Local density fluctuations occurred in an organized and history-dependent way. These local density fluctuations varied up to ±7% of the bulk density and were made visible by acoustic attenuation and velocity mapping.     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

High-Temperature Mechanical Properties of SiC–Mo5(Si,Al)3C Composites

SiC–Mo₅(Si,Al)₃C composites were fabricated by the melt infiltration process, and the infiltration characteristics were studied in detail. Fracture strength and toughness were measured up to 1600°C using a three-point bending test and indentation strength method, respectively. Both fracture strength and toughness significantly increased at 1400°C with respect to the values at room temperature. These increases were mainly attributed to plastic deformation of the infiltrated Mo₅(Si,Al)₃C phases at elevated temperatures, which acted as ductile toughening inclusions. Compressive creep tests were used to study the creep behavior of the composite in the range of 1550°–1650°C and 150–200 MPa. The stress exponent and activation energy were 1.3 and 277 kJ/mol, respectively. Preliminary oxidation tests showed that the composites exhibited good oxidation resistance at 1500°C because of the formation of a dense oxide scale.     

Preparation and Properties of Porous Aluminum Nitride–Silicon Carbide Composite Ceramics

Microporous two-phase AlN–SiC composites were prepared using Al₄C₃ and either Si (N₂ atmosphere) or Si₃N₄ (Ar atmosphere) as precursors. The reaction mechanisms of the two synthesis routes and the effect of processing conditions on reaction rate and the material microstructures were demonstrated. The exothermic reaction between Si and Al₄C₃ under N₂ atmosphere was shown to be a simple processing route for the preparation of porous two-phase AlN–SiC materials. The homogeneous two-phase AlN–SiC composites had a grain size in the range of 1–5 μm, and the porosity varied in the range of 36%–45%. The bending strength was 50–60 MPa, in accordance with the high porosity.     

Thermal Shock Damage Assessment in Ceramics Using Ultrasonic Waves

Structural ceramics are susceptible to microcrack damage by thermal shock. There is a critical temperature for thermal shock damage initiation with damage severity increasing at greater shock temperatures. In this work the applicability of an ultrasonic method to determine the critical temperature and the accumulated damage is demonstrated in alumina. Information is obtained via velocity and attenuation measurements using surface and obliquely incident bulk ultrasonic waves. The elastic anisotropy effect due to preferred crack orientation has been estimated. The critical temperature for the alumina is about 200°C. The damage increases steeply from 200° to 400°C and grows significantly above 400°C. Changes of up to 17% from the original values in the effective shear moduli and up to 45% in the longitudinal effective modulus in the direction transverse to crack orientation are measured at high thermal shock temperatures.     

Synergistic Effects of Water Vapor and Alkali Chloride Vapors on the High-Temperature Corrosion of SiC-Based Ceramics

The high-temperature oxidation of SiC-based structural ceramics is accelerated in the presence of KCl vapors. The corrosion products are primarily alkali silicates and the rate of attack is approximately proportional to the vapor pressure of KCl in the atmosphere. When both KCl and H₂O are present in the oxidizing atmosphere, the rate of attack is further accelerated. The accelerated attack is related to the more negative free energy change of the fluxing reaction when the Cl can be removed as the more stable HCl molecule. Addition of Cl₂ to the environment reduces the rate of attack by keeping the alkali bound to the chlorine rather than forming alkali silicates.     

High-Strength Porous Silicon Carbide Ceramics by an Oxidation-Bonding Technique

Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO₂ glass. Because of the crystallization of amorphous SiO₂ glass into cristobalite during sintering, the fracture strength of oxidation-bonded SiC ceramics can be retained to a relatively high level at elevated temperatures. It has been shown that the mechanical strength is strongly affected by particle size. When 0.6 μm SiC powders were used, a high strength of 185 MPa was achieved at a porosity of ∼31%. Moreover, oxidation-bonded SiC ceramics were observed to exhibit an excellent oxidation resistance.     

Stepwise-Graded Si3N4–SiC Ceramics with Improved Wear Properties

The processing of stepwise graded Si₃N₄/SiC ceramics by pressureless co-sintering is described. Here, SiC (high elastic modulus, high thermal expansion coefficient) forms the substrate and Si₃N₄ (low elastic modulus, low thermal expansion coefficient) forms the top contact surface, with a stepwise gradient in composition existing between the two over a depth of ∼1.7 mm. The resulting Si₃N₄ contact surface is fine-grained and dense, and it contains only 2 vol% yttrium aluminum garnet (YAG) additive. This graded ceramic shows resistance to cone-crack formation under Hertzian indentation, which is attributed to a combined effect of the elastic-modulus gradient and the compressive thermal-expansion-mismatch residual stress present at the contact surface. The presence of the residual stress is corroborated and quantified using Vickers indentation tests. The graded ceramic also possesses wear properties that are significantly improved compared with dense, monolithic Si₃N₄ containing 2 vol% YAG additive. The improved wear resistance is attributed solely to the large compressive stress present at the contact surface. A modification of the simple wear model by Lawn and co-workers is used to rationalize the wear results. Results from this work clearly show that the introduction of surface compressive residual stresses can significantly improve the wear resistance of polycrystalline ceramics, which may have important implications for the design of contact-damage-resistant ceramics.     

Tensile Strength Degradation of Ceramics from Prior Compression

Both theory and studies on models predict that cracks extend stably form flaws in brittle materials subjected to compression. Once cracks have originated and extended, the tensile strength transverse to the cracks should be reduced. Tests on siliconized carbide verify this prediction; however, tests on hot-pressed silicon nitride refute it. Possible morphologies of the inherent flaws are speculated.     

Determining the Toughness of Ceramics from Vickers Indentations Using the Crack-Opening Displacements: An Experimental Study

Recently, a method for evaluating the fracture toughness of ceramics has been proposed by Fett based on the computed crack-opening displacements of cracks emanating from Vickers hardness indentations. To verify this method, experiments have been conducted to determine the toughness of a commercial silicon carbide ceramic, Hexoloy SA, by measuring the crack-opening profiles of such Vickers indentation cracks. Although the obtained toughness value of K _o= 2.3 MPa·m^1/2 is within 10% of that measured using conventional fracture toughness testing, the computed crack-opening profiles corresponding to this toughness display poor agreement with those measured experimentally, raising concerns about the suitability of this method for determining the toughness of ceramics. The effects of subsurface cracking and cracking during loading are considered as possible causes of such discrepancies, with the former based on direct observations of lateral subsurface cracks below the indents.