Hot-pressed Bodies Research Articles

Fabrication of transparent MgAl2O4 spinel via spray freeze drying of microfluidized slurry

Abstract Spherical granules were prepared from a monodispersed slurry by combining the microfluidization (MF) method and the spray freeze-drying (SFD) process. Starting with the prepared granules, transparent MgAl 2 O 4 ceramics were fabricated through pressureless sintering followed by hot isostatic pressing. A comparison with a polydispersed slurry prepared by the ball-milling method showed successful fabrication of a mono-dispersed state by microfluidization method, and 80% visible in-line transmittance was obtained at a 600-nm wavelength from a process starting with a monodispersed slurry of low solid content. Microstructural analysis of the green bodies, the pre-sintered bodies, and the hot isostatic pressed bodies of the prepared MgAl 2 O 4 ceramics revealed that the slurry dispersion should be controlled to a high level in order to suppress scattering sources such as pores and microcracks, which affect the in-line transmittance of visible light.

Anisotropy and temperature stability of parameters of Bi3TiNbO9-based high-temperature piezoceramics

We have studied the effect of preparation conditions on the structural, dielectric, piezoelectric, and other properties of ceramic samples cut from large, bulk hot-pressed bodies based on modified Bi3TiNbO9 and found conditions favorable for obtaining high piezoelectric parameters that ensure long-term functioning of Bi3TiNbO9-based ceramics at a temperature of 750°C.

Effect of sintering temperature and sintering additives on ionic conductivity of LiSi2N3

The effect of sintering temperature and sintering additives on the ionic conductivity of LiSi2N3 was studied by performing complex impedance measurements. LiSi2N3 materials were fabricated by the reaction of Li3N, Si3N4, and sintering additives at temperatures of 1873–2073 K. Dense hot-pressed bodies were obtained at 1973–2073 K in the case of undoped LiSi2N3 and at 1873 K in the case of Y2O3, CaF2, and B2O3 addition. The ionic conductivity increased greatly with increasing sintering temperature and exhibited a strong dependence on the type of sintering additive. When the sintering temperature was constant at 1873 K, although the conductivities of Y2O3-doped LiSi2N3 and CaF2-doped LiSi2N3 were lower than that of undoped LiSi2N3, the conductivity of B2O3-doped LiSi2N3 was higher than that of undoped LiSi2N3. The enhanced conductivity of B2O3-doped LiSi2N3 can be attributed to the increase in the density of the sintered material without the formation of a phase of significant resistance at the grain boundaries.

Processing and properties of ultra-high temperature ceramics for space applications

The processing and the properties of two ultra-high temperature ceramics (UHTCs) designed for the manufacturing of aerospace sharp-shaped hot-structures are presented, along with the results obtained in the electrical discharge machining (EDM) of these UHTCs into sharp hot-structure components. The powder mixtures in the (ZrB 2–SiC)-based systems were brought to full density by hot-pressing. The hot-pressed bodies were characterized by fine and uniform microstructures (typical grain size < 5 μm), along with controlled thermo-mechanical properties. Average flexure strength and fracture toughness at room temperature (RT) up to about 900 MPa and 4.1 MPa√m, respectively, were measured. Elastic moduli at RT approaching 500 GPa were also measured. At 1500 °C in air, flexure strength decreased to 250 MPa. Beyond basic mechanical properties, the machinability of UHTC blocks into a more complex shape by means of the EDM was also assessed. Sharp-shaped hot-structures in the form of a nose-cone were produced from 12 cm × 10 cm hot-pressed cylindrical blocks. The machined surfaces showed limited roughness Ra < 1 μm. With the exception of the typical size of critical flaws induced by EDM seeming slightly larger than those of specimens machined with conventional diamond-loaded tools, the EDM technique proved to be as effective in machining UHTC sintered pieces into more complex components.

Hot-Pressed ALON-Based Composite Ceramics and Their Mechanical Properties.

Microstructure and mechanical properties of hot-pressed ALON-based composite ceramics were investigated. Microstructure of hot-pressed bodies prepared from a starting mixture of Al2O3, AlN and BN, consisted of γ-ALON phase as a matrix and a homogeneously distributed BN phase. The γ-ALON phase was discontinuous in a composite with BN amount >20vol%. However, addition of 2vol% Y2O3 as sintering aid made the microstructure to maintain a continuous γ-ALON network even for BN addition >20vol%. It is considered that grain growth of the γ-ALON phase was enhanced by the addition of Y2O3. The flexural strength of the hot-pressed bodies decreased with increasing the amount of BN addition up to 20vol%, however, addition of BN >50vol% resulted in an increase of strength up to 80-100MPa. The thermal shock resistance of all samples tested was about 200°C. However, strength degradation due to thermal shock decreased with increasing the BN fraction. It was considered that the mechanical properties of hot-pressed ALON-based composite ceramics are significantly affected by the formation of BN phase through the pressurized sintering.

Fabrication of silicon nitride–Silicon oxynitride in-situ composites

Fine-grained β-Si 3N 4 ceramics were fabricated from fine and uniform β-powder by hot-pressing at 1700–1750°C using different sintering aids (cordierite, Y 2O 3–MgO, and Y 2O 3–Al 2O 3). The microstructural development of hot-pressed bodies was studied in annealing at 1750°C for 0·5 to 8 h. When Y 2O 3–MgO and Y 2O 3–Al 2O 3 were used as sintering aids, fine and uniform microstructures were maintained because of the low driving force for grain growth of β-Si 3N 4. When cordierite was used as an aid, β-Si 3N 4 reacted with the SiO 2 in the additive and developed a bimodal microstructure with large Si 2N 2O grains in fine β-Si 3N 4 matrix grains after annealing. Local segregation of β-Si 3N 4 grains was observed after hot-pressing. Nucleation of Si 2N 2O occurred at a part of the segregation surface in the early stage of annealing. Si 2N 2O grains grew selectively with annealing time, resulting in improved fracture toughness.

Fabrication of Net Shape High Strength TiB&lt;sub&gt;2&lt;/sub&gt;/SiC Composite by Slip Cast Molding

The strength and the size accuracy of TiB2/SiC composite sintered bodies, which were shaped by the slip cast molding using gypsum molds and hot isostatic pressing (HIP), were investigated. Near net shaped bodies with no cracks were obtained after optimizing the dispersibility of the slurry by adjusting the amount of a dispersant and the pH value. The increase in the solid concentration of the slurry improved the dispersibility of TiB2 particles that acted as fracture origins in the HIPed body. The four-point flexural strength of the sintered body by HIP was 710MPa, which was same level of that made by hot-pressing. Combustors were fabricated by using this slurry. The standard deviation of each parts, 110mm in diameter was 0.1mm. The coefficient of variation in the size among eleven parts was 0.9%. The flexural strength of the specimen cut from the combustor was also equivalent to that of the hot-pressed body.

Hot Isostatic Pressing in Oxygen of Ferrite Magnet

The effect of hot isostatic pressing in 5°%,O 2 -95%Ar on the magnetic properties of Sr ferrite magnet was studied. It was found that the size of voids at grain boundaries are decreased by hot isostatic pressing without substantial grain growth and the density of isostatically hot-pressed body is about 5.03x10 3 kg/m 3 . Magnetic measurement revealed that the hot isostatic pressing does not signiticantly decrease coercive force and keeps the good squareness of B-H curve of pre-sintered body unchanged. The typical value of residual flux density Br of the isostatically hot-pressed magnet is 0.438T at MHe=318KA/m.

Effects of Ball-Milling Time on the Properties of Low Purity β-Phase Silicon Nitride Powder and the Hot-Pressed Body

Y2O3-doped low purity β-Si3N4 powder was ball-milled for 6 to 100h. The effects of ball-milling time on the properties of the powder and the hot-pressed body were investigated. As increasing ball-milling time, average particle size and crystallite size decreased and the amount of SiO2, carbon, some metallic impurities and lattice strain increased for the ball-milled powder. The increase of SiO2 was ascribed to the mechanochemical reaction of H2O contained in the milling fluid with newly formed powder surfaces. The source of carbon was both the milling fluid and the container. The increase of Al and Mg was explained by the wear of milling balls. Sintering behavior and microstructure of the sintered bodies were influenced by the powder properties modified by different ball-milling time. Mechanical properties were strongly dependent on their microstructures.

PZT/ZnO actuator responding to reducing gases

The bending behaviour of a piezoelectric actuator consisting of a dense PZT (Pb 0.995Nb 0.01(Zr 0.53Ti 0.47) 0.99O 3) plate and a porous ZnO (ZnO + 10 mol% NiO + 0.2 mol% Li 2O) plate in response to reducing-gas (CO, H 2) atmospheres has been investigated. The actuator is prepared by bonding the dense PZT and press-formed ZnO disks using a hot-press technique with glass frits at the interface. The PZT and ZnO plates, cut off from the hot-pressed body, have high resistivities of the same level in air but only the ZnO showed a decrease in resistivity on introduction of reducing gas at 250 °C. An applied d.c. electric field is confirmed to concentrate on the PZT part of the PZT/ZnO actuator on introduction of reducing gases. The shrinkage caused by a d.c. bias at the PZT side of the actuator increases due to the concentration of electric field, and then the actuator shows a change of bending displacement from 3 μm in air to 7 μm in 0.40 vol.% air-balanced CO gas under 250 kV m −1. It is found that the applied voltage is distributed to both plates in air, but concentrates in the PZT part in reducing-gas atmosphere due to a decrease in resistance of the ZnO part.

Mechanical Properties and Cutting Performance of Titanium Carbide Whisker/Alumina Composites

Composite ceramics of alumina mixed with up to 50vol% titanium carbide whiskers were fabricated by hot-pressing at 1600°C under 30MPa for 30min. The mechanical properties and the cutting perfomance for cast iron or carbon steel of the hot-pressed bodies were studied. Dense composites with relative densities above 98% were obtained at whisker content with up to 40vol%. The composite containing 20vol% whiskers showed the highest bending strength of around 560MPa. Increasing whisker content decreased the elastic modulus, but increased the Vickers hardness and the fracture toughness. The composite mixed with 20vol% or 30vol% titanium carbide whiskers was found to be capable of having the cutting performance similar to the available ceramic composite cutting tools such as titanium carbide particle/alumina composite and silicon carbide whisker/alumina composite.

Sintering and Mechanical Properties of Titanium Carbide Whisker/Alumina Composites

Composite ceramics of alumina mixed with up to 30 vol% titanium carbide whiskers were fabricated by hot-pressing at 1400 to 1700°C under 30MPa for 30 min. The mechanical properties of the hot-pressed bodies were studied. Dense composites with relative densities above 98% were obtained at 1600°C when the whisker content was up to 30vol%. The composite containing 10vol% whiskers and fired at 1500°C showed the highest bending strength of around 510MPa, which is lower than the maximum strength of alumina, around 650MPa. These composites are superior to monolithic alumina in high temperature bending strength and Vickers hardness. Increasing whisker content decreased the elastic modulus monotonously, but increased the fracture toughness from around 3.5MPa·m1/2 for alumina to around 5.7MPa·m1/2 for composite ceramics containing 30vol% whiskers.

Fabrication and Microstructure of Al2O3–SiC–YAG Hybrid Composites Prepared by Particulate Dispersion

Stable Al2O3–SiC–YAG hybrid composites were successfully fabricated by reaction of Al2O3 and Y2O3 and incorporation of SiC. The hot‐pressed bodies consisted of uniformly dispersed grains of microsized YAG particulates and nanosized SiC particulates in an Al2O3 matrix. Although the grain size of monolithic A12O3 increases markedly with increased temperature, the grain size of the Al2O3–SiC–YAG hybrid composites was effectively restrained due to grain‐boundary pinning by the particulates.

ドクターブレード法で作製したＳｉＣウィスカー／Ａｌ２Ｏ３複合材料中のウィスカー配向

Whisker orientation of an sic-whisker/Al2O3 composite fabricated by doctor blade molding was investigated. The orientation of whiskers in a green sheet became higher with increase in viscosity and shear strain rate of slurry. Because, in the slurry, whiskers were rotated by the moment due to the distribution of viscous resistance which increase with viscosity and shear strain rate of slurry. Shrinkage of the thickness of a green sheet was also affected by whisker orientation. Hot-pressed body with the same degree of whisker orientation as that of green sheet could be fabricated. However, the orientation of whisker was lower than that in hot-pressed SiC-whisker/Al2O3 composite after extrusion molding with high squeeze ratio.

Sintering of Zircon-Silicon Carbide Whisker Composites and Their Mechanical Properties

Zircon composites reinforced with 20vol% silicon carbide whiskers were prepared by hot-pressing. Three kinds of silicon carbide whiskers with different diameters were used as a reinforcement agent. The mechanical properties of the hot-pressed bodies, such as bending strength, elastic modulus, hardness, and fracture toughness, were measured. Dense composites were obtained at the sintering temperatures of 1600° and 1650°C. At these sintering temperatures, a few percent of zircon decomposed. The room temperature bending strength was improved largely by an addition of whiskers with small and middle diameters. In all composites, the high temperature strength measured at 1200°C was 1.8 times larger than monolithic zircon. Although elastic modulus and hardness of dense bodies increased with the addition of whiskers, the composite containing whiskers with large diameter had lower values compared to the other composites. Fracture toughness was improved by the addition of whiskers. Notable crack deflection by whiskers was not observed, but some pull-out of whiskers occurred in the composites.

Mechanical Properties of Alumina-Mullite Whisker Composites

Composite alumina ceramics with a mullite whisker content of up to 30vol% were fabricated by hot-pressing at 1400 to 1600°C under 30MPa for 30min. The bending strength, hardness, elastic modulus and fracture toughness of the hot-pressed bodies were investigated. Considerably dense alumina-mullite whisker composite ceramics were obtained by sintering at 1600°C with a whisker content of up to 30vol%. The composite with a whisker content of 10vol% and sintered at 1600°C exhibited the highest room temperature bending strength of around 750MPa. This value is still lower than the maximum strength of alumina ceramic which is about 850MPa. However, the composites exhibited greater high temperature strength and hardness than alumina. The elastic modulus decreased with an increase in whisker content. No improvement was ohserved in the fracture toughaness of the alumina by the addition of mullite whiskers, as very little crack deflection by the whiskers and pull-out of the whiskers occurred in these composites.

Mechanical Properties of Spinel-Silicon Carbide Whisker Composites

Spinel composite ceramics reinforced with 0-40vol% β-silicon carbide whiskers were prepared by hot-pressing at 1800°C under 30MPa for 30min. The mechanical properties (bending strength, hardness, modulus of elasticity, fracture toughness) of the hot-pressed bodies were measured. Fully dense composites were obtained in the whisker content up to 30vol%. The composite mixed with 30vol% whiskers showed the highest room temperature bending strength (about 440MPa), which was about two times as large as that of monolithic spinel. These composites had more excellent high temperature strengths than monolithic spinel. The hardness and modulus of elasticity of dense sintered bodies increased with an increase in whisker content. The fracture toughness increased monotonously with increasing whisker content and showed maximum values (3.5-4.0 MPa⋅m1/2) at 40vol%, which were 2.1-2.7 times as large as those of monolithic spinel due to crack deflection and pull-out of whiskers.

A study on the c-axis alignment of Nd-Fe-B die-upset magnets (abstract)

It is necessary to get higher c-axis alignment of Nd2Fe14B grains in Nd-Fe-B magnets for higher-energy products. During the hot working process, where melt-spun flakes were hot pressed followed by die-upsetting, hot-pressed bodies showed roughly isotropic structure while c-axis alignments were developed during die-upsetting. In this report, we studied the relationship between the reductions during upsetting and the degree of the c-axis alignments and also tried to get the formula of this relationship. We prepared samples having different reductions and evaluated remanence, c-axis alignment and strains of each flake in the samples. c-axis alignments were evaluated using x-ray diffraction.1 Strains were measured using the sizes of each deformed melt-spun flake. In the range of 50%–70% reduction, remanence was lower in the surface layer than in the center. The difference in remanence in these two regions became smaller as the reduction became larger. The distribution of c-axis alignments and strains showed similar phenomena. Then we could propose the following formula showing the relationship between reduction and c-axis alignment: (σ − σ0)/(σi − σ0) = (1 − εz)4, where σ is standard deviation of c axis alignment and σ0, σi denotes the σ value at enough large strain and initial value of σ, respectively. εz means strain parallel to pressing axis.

Effect of Forming Techniques on the Orientation of Platelet Composites

ABSTRACTTwo forming techniques, i.e., slip casting and dry compaction, have been employed to make SiC(platelet)-A1203 composites. The orientation of SiC platelets in the composites, before and after hot pressing, was studied using optical microscopy and a pole-figure X-ray device. It was found that the preferred orientation in the dry-pressed green bodies increased with increasing pressure, whereas the platelets in the slip cast bodies were close to a random orientation. The pressure during the hot pressing process did, however, align the platelets of slip-cast composites during densification. Optical micrographs confirmed the platelets tend to align with their faces perpendicular to the hot pressing direction. The effect of platelet size on the orientation was also investigated. X-ray analysis showed that small platelets (∼ 1.6 μm) do not give rise to preferred orientation both in green and hot pressed bodies; whereas larger platelets (∼24 μm) with larger aspect ratio do yield a stronger preferred orientation. The causes for the platelet alignment will be discussed.

An Effort on Preparing Sintered Bodies of Fe&lt;sub&gt;4&lt;/sub&gt;N

Sintered bodies of Fe4N were prepared by way of a hot-press, followed by firing in an atmosphere containing NH3 and H2 gases. After the hot-press, Fe4N was not sintered and impurities of an iron oxide and a paramagnetic compound coexisted. The sintered bodies were obtained by firing the hot-pressed bodies in the atmosphere. It was found that, in firing, Fe4N was reduced and sintered above the decomposition temperature (approx. 670°C) and then the sintered bodies were rapidly renitrified in the atmosphere to Fe4N through open pores in cooling. X-ray diffraction and Mossbauer spectra showed that the sintered bodies were Fe4N single phase.