American Ceramic Society

The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers‐produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment‐sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well‐behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.

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.

Ferroelectric ceramics were born in the early 1940s with the discovery of the phenomenon of ferroelectricity as the source of the unusually high dielectric constant in ceramic barium titanate capacitors. Since that time, they have been the heart and soul of several multibillion dollar industries, ranging from high‐dielectric‐constant capacitors to later developments in piezoelectric transducers, positive temperature coefficient devices, and electrooptic light valves. Materials based on two compositional systems, barium titanate and lead zirconate titanate, have dominated the field throughout their history. The more recent developments in the field of ferroelectric ceramics, such as medical ultrasonic composites, high‐displacement piezoelectric actuators (Moonies, RAINBOWS), photostrictors, and thin and thick films for piezoelectric and integrated‐circuit applications have served to keep the industry young amidst its growing maturity. Various ceramic formulations, their form (bulk, films), fabrication, function (properties), and future are described in relation to their ferroelectric nature and specific areas of application.

A ceramic fuel cell in an all solid‐state energy conversion device that produces electricity by electrochemically combining fuel and oxidant gases across an ionic conducting oxide. Current ceramic fuel cells use an oxygen‐ion conductor or a proton conductor as the electrolyte and operate at high temperatures (>600°C). Ceramic fuel cells, commonly referred to as solid‐oxide fuel cells (SOFCs), are presently under development for a variety of power generation applications. This paper reviews the science and technology of ceramic fuel cells and discusses the critical issues posed by the development of this type of fuel cell. The emphasis is given to the discussion of component materials (especially, ZrO 2 electrolyte, nickel/ZrO 2 cermet anode, LaMnO 3 cathode, and LaCrO 3 interconnect), gas reactions at the electrodes, stack designs, and processing techniques used in the fabrication of required ceramic structures.

A large body of work has been reported in the last 5 years on the development of lead‐free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead‐free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead‐free piezoceramics is then critically assessed.

Ceramics used for the repair and reconstruction of diseased or damaged parts of the musculo‐skeletal system, termed bioceramics, may be bioinert (e.g., alumina and zirconia), resorbable (e.g., tricalcium phosphate), bioactive (e.g., hydroxyapatite, bioactive glasses, and glass‐ceramics), or porous for tissue ingrowth (e.g., hydroxyapatite‐coated metals). 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 repair after tumor surgery. Pyrolytic carbon coatings are thromboresistant and are used for prosthetic heart valves. The mechanisms of tissue bonding to bioactive ceramics have resulted in the molecular design of bioceramics for interfacial bonding with hard and soft tissue. Bioactive composites are being developed with high toughness and elastic modulus that match with bone. Therapeutic treatment of cancer has been achieved by localized delivery of radioactive isotopes via glass beads. Clinical success of bioceramics has led to a remarkable advance in the quality of life for millions of people.

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.

This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB 2 ) and hafnium diboride (HfB 2 ) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre‐ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine‐grained ZrB 2 and HfB 2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B 2 O 3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi 2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB 2 and HfB 2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re‐entry, and rocket propulsion.

The recognition of the potential for enhanced fracture toughness that can be derived from controlled, stress‐activated tetragonal ( t ) to monoclinic ( m ) transformation in ZrO 2 ‐based ceramics ushered in a new era in the development of the mechanical properties of engineering ceramics and provided a major impetus for broader‐ranging research into the toughening mechanisms available to enhance the fracture properties of brittle‐matrix materials. ZrO 2 ‐based systems have remained a major focal point for research as developments in understanding of the crystallography of the t → m transformation have led to more‐complete descriptions of the origins of transformation toughening and definition of the features required of a transformation‐toughening system. In parallel, there have been significant advances in the design and control of microstructure required to optimize mechanical properties in materials developed commercially. This review concentrates on the science of the t → m transformation in ZrO 2 and its application in the modeling of transformation‐toughening behavior, while also summarizing the microstructural control needed to use the benefits in ZrO 2 ‐toughened ceramics.

Polycrystalline bulk samples of Ti 3 SiC 2 were fabricated by reactively hot‐pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room‐temperature failure was brittle, the high‐temperature load‐displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10 −8 and 2 × 10 −5 kg 2 ‐m −4 .s −1 . The activation energy for oxidation is thus =300 kJ/mol. The room‐temperature electrical conductivity is 4.5 × 10 6 Ω −1 .m −1 , roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room‐temperature thermal conductivity, and the heat capacity are respectively, 10 × 10 −6 °C −1 , 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti 3 SiC 2 does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.

This paper deals with the application of ZnO varistors—an area which has not been treated in a systematic way in the literature. The paper starts with a brief description of the fundamental properties comprising the electrical behavior as well as the physics, chemistry, and microstructure of the varistor. These properties then form the basis for defining the application parameters that are directly related to the nonlinear current‐voltage characteristics of the varistor. This paper provides a detailed description of these parameters and their relation to microstructure and the processing of the varistor. Finally, a discussion is presented on the reliability of the varistor by considering a grain‐boundary defect model which explains both the instability and the stability under use conditions.

During the past five years, we have developed in our laboratory a new type of solar cell that is based on a photoelectrochemical process. The light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor (i.e., titanium oxide (TiO 2 )). When excited by a photon, the dye has the ability to transfer an electron to the semiconductor. The electric field that is inside the material allows extraction of the electron, and the positive charge is transferred from the dye to a redox mediator that is present in solution. A respectable photovoltaic efficiency (i.e., 10%) is obtained by the use of mesoporous, nanostructured films of anatase particles. We will show how the TiO 2 electrode microstructure influences the photovoltaic response of the cell. More specifically, we will focus on how processing parameters such as precursor chemistry, temperature for hydrothermal growth, binder addition, and sintering conditions influence the film porosity, pore‐size distribution, light scattering, and electron percolation and consequently affect the solar‐cell efficiency.

The mid‐, near‐, and far‐infrared (IR) spectra of synthetic, single‐phase calcium silicate hydrates (C‐S‐H) with Ca/Si ratios (C/S) of 0.41–1.85, 1.4 nm tobermorite, 1.1 nm tobermorite, and jennite confirm the similarity of the structure of these phases and provide important new insight into their H 2 O and OH environments. The main mid‐IR bands occur at 950–1100, 810–830, 660–670, and 440–450 cm −1 , consistent with single silicate chain structures. For the C‐S‐H samples, the mid‐IR bands change systematically with increasing C/S ratio, consistent with decreasing silicate polymerization and with an increasing content of jennite‐like structural environments of C/S ratios >1.2. The 950–1100 cm −1 group of bands due to Si‐O stretching shifts first to lower wave number due to decreasing polymerization and then to higher wave numbers, possibly reflecting an increase in jennite‐like structural environments. Because IR spectroscopy is a local structural probe, the spatial distribution of the jennite‐like domains cannot be determined from these data. A shoulder at ∼1200 cm −1 due to Si‐O stretching vibrations in Q 3 sites occurs only at C/S lessthan equal to 0.7. The 660–670 cm −1 band due to Si‐O‐Si bending broadens and decreases in intensity for samples with C/S > 0.88, consistent with depolymerization and decreased structural order. In the near‐IR region, the combination band at 4567 cm −1 due to Si‐OH stretching plus O‐H stretching decreases in intensity and is absent at C/S greater than ∼1.2, indicating the absence of Si‐OH linkages at C/S ratios greater than this. The primary Si‐OH band at 3740 cm ‐1 decreases in a similar way. In the far‐IR region, C‐S‐H samples with C/S ratio greater than ∼1.3 have increased absorption intensity at ∼300 cm −1 , indicating the presence of CaOH environments, even though portlandite cannot be detected by X‐ray diffraction for C/S ratios <1.5. These results, in combination with our previous NMR and Raman spectroscopic studies of the same samples, provide the basis for a more complete structural model for this type of C‐S‐H, which is described.

Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot‐zone components made entirely from ceramics. The ceramic engine programs have had only partial success, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties, which in many respects is more advanced than of more widely used technical ceramics. This review examines from the historical standpoint the development of silicon nitride and of its processing into a range of high‐grade ceramic materials. The development of understanding of microstructure–property relationships in the silicon nitride materials is also surveyed. Because silicon nitride has close relationships with the SiAlON group of materials, it is impossible to discuss the one without some reference to the other, and a brief mention of the development of the SiAlONs is included for completeness.

The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close‐packed oxygen sublattice (either face‐centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face‐centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.

Zirconates with high melting points were investigated for application as materials for thermal barrier coatings at operating temperatures >1300°C. SrZrO 3 , BaZrO 3 , and La 2 Zr 2 O 7 powders were synthesized and sintered to compacts with various levels of porosity. The sintering results indicated a promising low‐sintering activity of the investigated materials. Thermal properties of these dense materials were determined. Thermal expansion coefficients were slightly lower than those of Y 2 O 3 ‐stabilized ZrO 2 (YSZ); thermal conductivities of SrZrO 3 and BaZrO 3 were comparable or slightly higher than those of YSZ. La 2 Zr 2 O 7 had a lower thermal conductivity. SrZrO 3 was not suitable for application as a thermal barrier coating because of a phase transition at temperatures between 700° and 800°C. Mechanical properties (hardness, fracture toughness, and Young's modulus) of dense BaZrO 3 and La 2 Zr 2 O 7 samples were determined by indentation techniques and showed lower hardness and Young's modulus compared to YSZ. BaZrO 3 and La 2 Zr 2 O 7 powders were optimized for application as powders for plasma spraying. Plasma‐sprayed coatings were produced and characterized. Thermal cycling with a gas burner at 1200°C showed a massive attack of the BaZrO 3 coating, with loss of BaO. On the other hand, the La 2 Zr 2 O 7 coating showed excellent thermal stability and thermal shock behavior.

Colloidal processing of ceramics is reviewed with an emphasis on interparticle forces, suspension rheology, consolidation techniques, and drying behavior. Particular attention is given to the scientific concepts that underpin the fabrication of particulate‐derived ceramic components. The complex interplay between suspension stability and its structural evolution during colloidal processing is highlighted.

The mechanisms of electrophoretic deposition (EPD) are discussed and their shortcomings identified. The kinetics of the processes involved are analyzed for constant‐current and constant‐voltage conditions. A method of determining the Hamaker constant of suspended particles is developed by modeling the relationship between the particle inter‐action energy and the suspension stability. A three‐probe dc technique is used to map the voltage profile around the depositing electrode, and the results are used to explain discrepancies between the calculated and experimentally observed voltage drops during deposition. A mechanism of deposition is proposed based on DLVO theory and particle double‐layer distortion/thinning on application of a dc field to the suspension. Kinetic equations are developed for constant‐current and constant‐voltage EPD using mass balance conditions; these are verified by experiments. After the phenomenon is introduced and discussed, a critique of the application of EPD to the synthesis of ceramic shapes and coatings is given.

An equation derived by Ritland relating the cooling rate and fictive temperature for glasses without memory is extended to those with memory, i.e. those which exhibit a spectrum of relaxation times. Provided that the spectrum of relaxation times is temperature‐independent, the limiting fictive temperature, T′ f , obtained when a glass is cooled through the transition region, is shown to be related to the cooling rate, q , by d In | q |/ d (1/ T'f )=‐Δ h ★/ R where R is the ideal gas constant and Δ h ★ the activation enthalpy for the relaxation times controlling the structural relaxation. Values of T′ f vs q obtained from enthalpy measurements by differential scanning calorimetry are presented for B 2 O 3 , 0.4Ca(NO 3 ) 2 —0.6KNO 3 , and borosilicate crown glasses; Δ h ★ is equal, within experimental error, to the activation enthalpy for shear viscosity. Values of T′ f from volume and enthalpy measurements obtained at the same cooling rate for the borosilicate crown glass are equal.

An examination is made of the sharp‐indentation technique of strength‐test precracking for toughness evaluation. The experimental approach follows that proposed by other workers but the theoretical analysis contains one vital new feature; the residual‐stress term discussed in Part I of this study is now introduced explicitly into the strength formulation. This modification overcomes a major systematic discrepancy evident in the previous models and at the same time, by virtue of attendant changes in the nature of the crack stability prior to attaining a failure configuration, eliminates the need for frac‐tographic measurements. Other advantages are also apparent, notably an insensitivity to postindentation radial crack extension. The main disadvantage is that only one result is obtained per specimen. Indentation/strength data from ceramics listed in Part I confirm the essential features of the theory and provide a suitable calibration factor. The method has special application to those materials which do not necessarily produce a well‐defined radial crack pattern, in which case an “effective” K c appropriate to fracture properties at the flaw level is obtained.