Densification and Phase Transformation of β-SiAlON–Cubic Boron Nitride Composites Prepared by Spark Plasma Sintering

β-SiAlON–cubic boron nitride (cBN) composites were prepared from β-SiAlON and cBN powders at 1600°–1900°C under a pressure of 100 MPa by spark plasma sintering. The effects of cBN content and sintering temperature on densification and phase transformation of the β-SiAlON–cBN composites were studied. When 10–30 vol% cBN was added to β-SiAlON, the shrinkage rate of the compacts increased. The compacts of β-SiAlON–BN composites originally containing 10–30 vol% cBN ceased to shrink at a temperature lower than that of β-SiAlON and the density of the composites increased. The densification of β-SiAlON–BN composites originally containing >40 vol% cBN was suppressed. The phase transformation of cBN to hexagonal BN in the β-SiAlON–BN composite was inhibited to a greater degree than that in the cBN body.     

Reaction behavior during Spark Plasma Sintering of cBN-TiCN-Ni powders

Polycrystalline cubic boron nitride (PcBN) is the designation given to composites constituted by cBN hard particles within a ceramic/metallic matrix. These composites are normally processed in severe pressure and temperature conditions to achieve full densification and prevent hexagonal BN formation, which would decrease the composite hardness. In this work, the Spark Plasma Sintering (SPS) technique was investigated as an alternative to sinter cBN-TiCN based composites, with and without addition of metallic Ni. The initial compositions were selected according with calculated phase diagrams, using the Thermo-calc software. The thermal behavior during SPS was studied up to 2000°C, namely the densification, reactivity and phase transformations. A larger densification was achieved with Ni addition, but full removal of open porosity was only possible at 1700 °C, where the cBN phase transformation to hBN completely occurred. In agreement with the thermodynamic calculations, other matrix phases, as TiB₂ and Ni₃B, were formed during sintering.     

Dense, Shaped Ceramic/Metal Composites at lessthan equal to1000°C by the Displacive Compensation of Porosity (DCP) Method

The Displacive Compensation of Porosity method for fabricating dense, shaped ceramic/metal composites at modest temperatures is demonstrated. In this process, liquid-metal/solid-ceramic displacement reactions are used to generate more ceramic (by volume) than is consumed, so that pores within a ceramic preform can be filled with the new ceramic phase (i.e., densification without sintering). Dense, lightweight MgO/Mg-Al composites (74–86 vol% oxide) and higher-melting, co-continuous MgAl₂O₄/Fe-Ni-Al-bearing composites (42–59 vol% oxide) have been produced via the pressureless infiltration and reaction of magnesium-bearing liquids with porous preforms of Al₂O₃ and NiAl₂O₄+Fe, respectively, at temperatures of 900°−1000°C. The composites are relatively tough and retain the shapes and dimensions (to within a few percent) of the starting preforms.     

Processing and Properties of TiB2 with MoSi2 Sinter-additive: A First Report

The densification of non-oxide ceramics like titanium boride (TiB₂) has always been a major challenge. The use of metallic binders to obtain a high density in liquid phase-sintered borides is investigated and reported. However, a non-metallic sintering additive needs to be used to obtain dense borides for high-temperature applications. This contribution, for the first time, reports the sintering, microstructure, and properties of TiB₂ materials densified using a MoSi₂ sinter-additive. The densification experiments were carried out using a hot-pressing and pressureless sintering route. The binderless densification of monolithic TiB₂ to 98% theoretical density with 2–5 μm grain size was achieved by hot pressing at 1800°C for 1 h in vacuum. The addition of 10–20 wt% MoSi₂ enables us to achieve 97%–99%ρ_th in the composites at 1700°C under similar hot-pressing conditions. The densification mechanism is dominated by liquid-phase sintering in the presence of TiSi₂. In the pressureless sintering route, a maximum of 90%ρ_th is achieved after sintering at 1900°C for 2 h in an (Ar+H₂) atmosphere. The hot-pressed TiB₂–10 wt% MoSi₂ composites exhibit high Vickers hardness (∼26–27 GPa) and modest indentation toughness (∼4–5 MPa·m^1/2).     

Effect of Yttria and Yttrium-Aluminum Garnet on Densification and Grain Growth of Alumina At 1200°–1300°C

Densification and grain growth of alumina were studied with yttria or yttrium-aluminum garnet (YAG) additives at the relatively low temperatures of 1200°–1300°C. Yttria doping was found to inhibit densification and grain growth of alumina at 1200°C and, depending on dopant level, had a lesser effect at 1300°C. At 1200°C, yttria inhibits densification more than it hinders grain growth. The rate of grain growth increases faster with temperature than the rate of densification. Alumina-YAG particulate composites were difficult to sinter, yielding relative densities of only 65% and 72% after 100 h at 1200° and 1300°C, respectively. Pure YAG compacts exhibited essentially no densification for times up to 100 h at 1300°C.     

Sintering of in-Situ Synthesized Sic–TiB2 Composites with Improved Fracture Toughness

TiB₂-particle reinforcement is one of the most successful methods for improving the fracture toughness of SiC ceramics.^1–3 Commercially available TiB₂ powders, however, have a large particle size and/or are highly reactive so that they are not favorable as a starting powder. In the present work, TiB₂ particles are formed by an in situ reaction between TiC and boron. The reaction takes place during sintering between 1000° and 1600°C and is accompanied by a large volume expansion. Under optimum conditions, dense composites (> 98% of theoretical) can be obtained by pressureless sintering using B and C as sintering additives. The in situ reaction method enables, for the first time, a complete densification of SiC-particulate composites by pressureless sintering. The fracture toughness of the composites was approximately 30% higher than that of the monolithic SiC ceramic.     

Processing of Boron Carbide-Aluminum Composites

The processing problems associated with boron carbide and the limitations of its mechanical properties can be significantly reduced when a metal phase (e.g., aluminum) is added. Lower densification temperatures and higher fracture toughness will result. Based on fundamental capillarity ther modynamics, reaction thermodynamics, and densification kinetics, we have established reliable criteria for fabricating B₄C–Al particulate composites. Because chemical reactions cannot be eliminated, it is necessary to process B₄C–AI by rapidly heating to near 1200°C (to ensure wetting) and subsequently heat-treating below 1200°C (for microstructural development).     

Reactive Hot Pressing of Titanium Nitride–Titanium Diboride Composites at Moderate Pressures and Temperatures

Dense composites in the Ti-B-N system have been produced by reactive hot pressing of titanium and BN powders. The effect of the addition of a small amount of nickel (1–3 wt%) on the reaction kinetics and densification of TiN–TiB₂ (40 vol%) composite has been studied. Composites of ∼99% of theoretical density have been produced at 1600°C under 40 MPa for 30 min with 1% nickel. The hardness and fracture toughness of these composites are 24.5 ± 0.97 GPa and 6.53 ± 0.27 MPa·m^1/2, respectively. The microstructural studies on samples produced at lower temperatures indicate the formation of a transient liquid phase, which enhances the kinetics of the reaction and densification of the composite.     

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.     

Dielectric Behavior of Na0.5Bi0.5TiO3-Based Composites Incorporating Silver Particles

The dielectric properties of Na_0.5Bi_0.5TiO₃ (NBT) -based composites incorporating silver particles prepared by sintering at a low temperature of ∼900°C are reported. The dielectric constant increases with the amount of metal silver particles in the measured frequency range (150 Hz to 1 MHz), and could be enhanced up to ∼20 times higher than that of pure NBT ceramics, which was ascribed to the effective electric fields developed between the dispersed particles in the matrix and the percolation effect. Further investigation revealed that the dielectric constant of the composites has weak frequency and temperature dependence (−50°C to +50°C).     

High-Temperature Creep Deformation of Alumina — SiC-Whisker Composites

The creep resistance at temperatures between 1200° and 1300°C in air of alumina—SiC-whisker composites was investigated via four-point flexure to examine (1) the effect of whisker content and (2) the influence of densification additives (i.e., Y₂O₃ (plus MgO)). The creep resistance of polycrystalline alumina is greatly improved with the addition of ≤ 20 vol% SiC whiskers. The interlocking/pinning of grains by whiskers which limits grain-boundary sliding contributes to the improvement in creep resistance. However, the creep rates of alumina composites in air increase at whisker contents ≥ 30 vol%. Electron microscopy observations suggested that the degradation in creep resistance for whisker content ≥ 30 vol% originated from (1) the promotion of creep cavitation and subsequent microcrack generation from the higher number density of nucleation sites and (2) more extensive formation of grain-boundary amorphous phase(s) associated with an observed increased oxidation rate. Along this one, the excellent creep resistance of alumina composites containing 20 vol% SiC whiskers was significantly degraded by the presence of the intergranular amorphous phases introduced by the addition of the Y₂O₃ densification additive.     

Synthesis and Characterization of TiN‐coated Cubic Boron Nitride Powders

Titanium nitride‐coated cubic boron nitride (TiN/cBN) composite powders were prepared by nitridizing TiO₂/cBN powders in a NH₃ flow at 950°C. The TiO₂/cBN powders were synthesized via a sol‐gel process using tetra‐butyl titanate and concentrated‐HNO₃‐treated BN powders as starting materials. The techniques of XRD, SEM, TEM, FT‐IR, and TG‐DTA were used to characterize the products and their intermediates. The cBN powders were uniformly coated with TiN nanoparticles. During the nitridization, the morphology of the TiO₂/cBN powders is unchanged. The TiN/cBN powders can be used as starting materials to prepare polycrystalline cBN compacts, or as reinforcements to strengthen metal‐matrix composites.     

In Situ Processing and High-Temperature Properties of Ti3Si(Al)C2/SiC Composites

Ti₃SiC₂ has many salient properties including low density, high strength and modulus, damage tolerance at room temperature, good machinablity, and being resistant to thermal shock and oxidation below 1100°C. However, the low hardness and poor oxidation resistance above 1100°C limit the application of this material. The poor oxidation resistance at temperatures above 1100°C was because of the absence of protective layer in the scale and the presence of TiC impurity phase. TiC impurity could be eliminated by adding a small amount of Al to form Ti₃Si(Al)C₂ solid solutions. Although the high-temperature oxidation resistance was significantly improved for the Ti₃Si(Al)C₂ solid solutions, the strength at high temperatures was lost. One important way to enhance the high-temperature strength is to incorporate hard ceramic particles like SiC. In this article, we describe the in situ synthesis and simultaneous densification of Ti₃Si(Al)C₂/SiC composites using Ti, Si, Al, and graphite powders as the initial materials. The effect of SiC content on high-temperature mechanical properties and oxidation resistance were investigated. The mechanisms for the improved high-temperature properties are discussed.     

Zirconia–Mullite Composites Consolidated by Spark Plasma Reaction Sintering from Zircon and Alumina

Mullite–ZrO₂ composites have been fabricated by attrition milling a powder mixture of zircon, alumina, and aluminum metal with MgO or TiO₂ as sintering additives, heating at 1100°C to oxidize the aluminum metal, and consolidation by spark plasma sintering (SPS). The influence of the SPS temperature on the formation of mullite, and the density and the mechanical properties of the resulting composites have been studied. For the mullite–zirconia composites without sintering additives, the mullite formation was accomplished at 1540°C. In contrast, for the composites having MgO and TiO₂, the formation temperature dropped to 1460°C. The composites without sintering additives were almost fully dense (99.9% relative density) and retained a larger amount of tetragonal zirconia. Those materials attained the best mechanical properties ( E =214 GPa and K _{I C} =6 MPa·m^1/2). To highlight the advantages of using the SPS technique, the obtained results have been compared with the characteristics of a mullite–zirconia composite prepared by the conventional reaction-sintering process.     

Reaction Sintering and Mechanical Properties of Hydroxyapatite–Zirconia Composites with Calcium Fluoride Additions

The effects of calcium fluoride (CaF₂) additions on the densification and mechanical properties of hydroxyapatite–zirconia composites (HA–ZrO₂) were investigated. When small amount of CaF₂ was added, the density of the composites was markedly enhanced. The reactions of HA with CaF₂, which led to the formation of fluorapatite (FA), were attributed to the observed improvements in densification. When HA–20-vol%-ZrO₂ composites were sintered, with the addition of 5 vol% of CaF₂, in air at 1300°C, the density of the specimen approached 98% of the theoretical value. The flexural strength and fracture toughness of the composites were also improved, as a result of the enhanced densification.     

Reaction Processing of Al2O3 Composites Containing Iron and Iron Aluminides

Different Fe-Al₂O₃ and FeAl-Al₂O₃ composites with metallic contents up to 30 vol% have been fabricated via reaction processing of Al₂O₃, Fe, and Al mixtures. Low Al contents (<∼10 vol%) within the starting mixture lead to composites consisting of Fe embedded in an Al₂O₃ matrix, whereas aluminide-containing Al₂O₃ composites result from powder mixtures with higher Al contents. In both cases, densification up to 98% TD can be achieved by pressureless sintering in inert atmosphere at moderate temperatures (1450°-1500°C). The proposed reaction sintering mechanism includes the reduction of native oxide layers on the surface of the Fe particles by Al and, in the case of mixtures with high Al contents, aluminide formation followed by sintering of the composites. Density and bending strengths of the reaction-sintered composites depend strongly on the Al content of the starting mixture. In the case of samples containing elemental Fe, crack path observations indicate the potential for an increase of fracture toughness, even at room temperature, by crack bridging of the ductile Fe inclusions.     

Properties of (1−x)PZT–xSKN Ceramics Sintered at Low Temperature Using Li2CO3

The densification of a Sr, K, Nb (SKN)-doped PZT ceramic has been modified using different flux chemistries. The most successful flux is Li₂CO₃ of which a 0.9 mol% addition promotes almost complete densification at 900°C. Structure property relationships in the Li₂CO₃ flux-sintered (1− x )PZT– x SKN system (0.01< x <0.05) are compared with conventionally sintered material. Consequences of flux interactions include increased conductivity and a shift in the optimum (1− x )PZT– x SKN composition. The flux sintered PZT–0.03SKN composition exhibits a high field d ₃₃ piezoelectric coefficient of 640 pm/V and a Curie temperature of ∼350°C. The unipolar polarization hysteresis loss is ∼23 kJ/m³ and varies little with temperature up to 150°C. Finally, we demonstrate successful cofiring of a multilayer structure at 900°C using Li₂CO₃-fluxed PZT–SKN and pure silver electrodes.     

Low-Fire Processing of Microwave BaTi4O9 Dielectric with BaO–ZnO–B2O3 Glass

The effects of BaO—ZnO–B₂O₃ (BZB) glass addition on the densification and dielectric properties of BaTi₄O₉ (BT4) have been investigated. With increasing BaO content in the BZB glass, the softening and melting points of the resulting BZB glass decrease, but the wetting between BZB and BT4 improves cosiderably. Although the densification temperature is reduced from 1300°C for pure BT4 to 925°C for BT4+BZB dielectric ceramics, the enhancement in densification becomes less significant with increasing BaO content in the BZB glass. The above result is attributed to a chemical reaction taking place at the interface of BZB/BT4 during firing, which becomes less extensive with increasing BaO content in the BZB glass. For the BZB glass with a BaO content in the range of 0–20 mol%, the resulting 90 vol% BT4+10 vol% BZB microwave dielectric has a dielectric constant of 28–33, and a product ( Q × f _r) of quality factor ( Q ) and a resonant frequency ( f _r) of 15 000–20 000 GHz at 6.6 GHz.     

High-Temperature Strength and Creep Behavior of Melt-Infiltrated SiC-Mo≤5Si3C≤1 Composites

Molybdenum carbosilicide composites (SiC-Mo_≤5Si₃C_≤1) were fabricated via the melt-infiltration process. The fracture behavior of the composites was studied from room temperature up to 1800°C in 1 atm (∼10⁵ Pa) of argon. The bend strength of the composites slightly increased at ∼1200°C, because of the brittle-ductile transition of the intermetallic phase. The composites retained ∼90% of their room-temperature strength, even at 1700°C. Compressive creep tests were performed over a temperature range of 1760°-1850°C and a stress range of 200–250 MPa. The creep rate of the SiC-Mo_≤5Si₃C_≤1 composites was approximately an order of magnitude higher than that of reaction-bonded SiC.     

Synthesis, Densification, and Phase Evolution Studies of Al2O3–Al2TiO5–TiO2 Nanocomposites and Measurement of Their Electrical Properties

Alumina–aluminum titanate–titania (Al₂O₃–Al₂TiO₅–TiO₂) nanocomposites were synthesized using alkoxide precursor solutions. Thermal analysis provided information on phase evolution from the as-synthesized gel with an increase in temperature. Calcination at 700°C led to the formation of an Al₂O₃–TiO₂ nanocomposite, while at a higher temperature (1300°C) an Al₂O₃–Al₂TiO₅–TiO₂ nanocomposite was formed. The nanocomposites were uniaxially compacted and sintered in a pressureless environment in air to study the densification behavior, grain growth, and phase evolution. The effects of nanosize particles on the crystal structure and densification of the nanocomposite have been discussed. The sintered nanocomposite structures were also characterized for dielectric properties.