Hot-pressed Materials Research Articles

Hot‐Pressed Mica Glass‐Ceramics with High Strength and Toughness

A new type of mica glass‐ceramics has been prepared by hot pressing. X‐ray diffractometry (XRD) has confirmed that (001) planes of mica crystals are almost perpendicular to the direction of hot pressing. Flexural strength and fracture toughness of the materials increase with increasing hot‐pressing deformation. The orientation of the hot‐pressed materials is attributed to the combination of viscous flow of the residual glass phase and rotation of mica plates under pressure at high temperature.

Strengthening, loss of strength and embrittlement of beryllium under high temperature neutron irradiation

The `dose' dependence of short-term mechanical properties of beryllium DIP-600 with 2.3 wt% of oxygen was determined. Irradiation temperature was 650°C, post-irradiation test temperature was 20°C. The influence of the main structural factors on ultimate strength of warm- and hot-pressed materials, characterized by oxygen content 0.7–4.9 wt%, grain size 8–50 μm, initial porosity 0.81–2.26% was revealed. The materials were irradiated to a fast fluence of 4.67 × 10 21 n/cm 2 ( E ⩾ 0.85 MeV) at 620°C and that of 5.69 × 10 21 n/cm 2 ( E ⩾ 0.85 MeV) at 680°C.

Postindentation slow growth of cracks in structural ceramics

Different forms of representing data on subcritical crack growth are discussed, which makes it possible to compare materials and (or) interpret the physical meaning of the parameters of slow crack growth (SCG). The SCG parameters are determined for hot-pressed OTM-914 materials and an Si3N4 — SiC (NK) composite. The scattering of the results can be explained by errors of the method, variation of the properties over the bulk of the material, surface oxidation, and other causes.

MoSi2–ZrO2 composites—fabrication, microstructures and properties

Abstract In this study, dense monolithic MoSi2 and MoSi2–ZrO2 composites were fabricated by hot-pressing from mechanically alloyed (MA) and commercial MoSi2 powders, unstabilized zirconia (UZ) and yttria partially stabilized zirconia (YPSZ) powders. The microstructures and properties of these materials were characterized. Hot-pressed monolithic materials and composites were consolidated to 96.1–99.5% density. Zirconia phases identified in the composites MoSi2–UZ and MoSi2–YPSZ were monoclinic and tetragonal, respectively. The MA–MoSi2 powder prepared materials have a higher hardness due to the refinement of the matrix. The toughness of the composites UZ and YPSZ were 2.4 times and 20% higher than that of MoSi2. The predominating fracture modes have been identified. High density of microcracks and dislocations were observed in UZ composites but not in YPSZ composites. Microcracking is suggested to be the main toughening mechanism for the former; while for the latter, the improvement in toughness could be attributed, at least partly, to crack deflection.

NdDyFeBZr -HDDR Processing

Cast alloys with a range of compositions based on Nd15Fe77B8 with additions of Dy and Zr were produced using the hydrogen decrepitation. disproportionation. recombination (HDDR) technique. Magnetic measurements on HDDR powders indicated that additions of Zr tended not to affect the recombination temperatures at which the material began to develop high coercivity but was clearly observed to extend the temperature range. by up to 60°C, over which high coercivity material could be produced. Microstructural analysis has revealed that the Zr doped samples were less prone to the explosive grain growth exhibited by the Zr free samples at these temperatures. Long term corrosion studies carried out on hot pressed materials using HDDR powders have revealed that small additions of Zr or ZrO2 to the melt are able to substantially increase the resistance to corrosion of the resulting magnets.

Carbon nanotubes–Fe–Alumina nanocomposites. Part II: microstructure and mechanical properties of the hot-Pressed composites

Carbon nanotubes–Fe–Al 2O 3 massive composites have been prepared by hot-pressing the corresponding composite powders, in which the carbon nanotubes are arranged in bundles smaller than 100 nm in diameter and several tens of micrometers long, forming a web-like network around the Fe–Al 2O 3 grains. In the powders, the quantity and the quality of the carbon nanotubes both depend on the Fe content (2, 5, 10, 15 and 20 wt%) and on the reduction temperature (900 or 1000°C) used for the preparation. Bundles of carbon nanotubes are present in the hot-pressed materials but with a decrease in quantity in comparison to the powders. This phenomenon appears to be less pronounced for the powders containing higher-quality carbon, i.e. a higher proportion of nanotubes with respect to the total carbon content. The presence of carbon as nanotubes and other species (Fe carbides, thick and short tubes, graphene layers) in the powders modifies the microstructure of the hot-pressed specimens in comparison to that of similar carbon-free nanocomposites: the relative densities are lower, the matrix grains and the intergranular metal particles are smaller. The fracture strength of most carbon nanotubes–Fe–Al 2O 3 composites is only marginally higher than that of Al 2O 3 and are generally markedly lower than those of the carbon-free Fe–Al 2O 3 composites. The fracture toughness values are lower than or similar to that of Al 2O 3. However, SEM observations of composite fractures indicate that the nanotubes bundles, which are very flexible, could dissipate some fracture energy.

Elevated temperature mechanical properties of MoSi2/Si3N4, MoSi2/SiC composites produced by self-propagating high temperature synthesis

MoSi2/Si3N4 and MoSi2/SiC composite powders were produced via a novel self-propagating high temperature synthesis technique (SHS) and consolidated both by hot-pressing and plasma spraying. Mechanical testing subsequently was performed under four-point bending at a variety of elevated temperatures and strain rates. Damage accumulation in the hot-pressed materials, either by particle cracking or particle/matrix separation, resulted in composite mechanical properties that were reduced below the level of the unreinforced material. Plasma sprayed materials showed power-law hardening behaviour caused by Mo5Si3 precipitates and increased strain tolerance because of the fine grain size and reduced yield strength of the matrix.

Solid state synthesis and properties of monoclinic celsian

Monoclinic celsian of Ba0.75Sr0.25Al2Si2O8 (BSAS-1) and Ba0.85Sr0.15Al2Si2O8 (BSAS-2) compositions have been synthesized from metal carbonates and oxides by solid state reaction. A mixture of BaCO3, SrCO3, Al2O3, and SiO2 powders was precalcined at ∼900–940 °C to decompose the carbonates followed by hot pressing at ∼1300 °C. The hot pressed BSAS-1 material was almost fully dense and contained the monoclinic celsian phase, with complete absence of the undesirable hexacelsian as indicated by X-ray diffraction. In contrast, a small fraction of hexacelsian was still present in hot pressed BSAS-2. However, on further heat treatment at 1200 °C for 24 h, the hexacelsian phase was completely eliminated. The average linear thermal expansion coefficients of BSAS-1 and BSAS-2 compositions, having the monoclinic celsian phase, were measured to be 5.28 × 10−6 °C−1 and 5.15 × 10−6 °C−1, respectively, from room temperature to 1200°C. The hot-pressed BSAS-1 celsian showed room temperature flexural strength of 131 MPa, elastic modulus of 96 GPa and was stable in air up to temperatures as high as ∼1500 °C.

Viscoelastic sliding and diffusive relaxation along grain boundaries in polycrystalline boron nitride

Dense hexagonal boron nitride (BN) materials were prepared via two different processing routes: (i) hot-pressing with the addition of a Ca/B-containing glass and (ii) chemical vapor deposition (CVD). The resulting microstructure of both materials was studied by scanning and transmission electron microscopy. While the hot-pressed BN material shows, apart from large BN matrix grains, an inhomogeneous distribution of residual glass at room temperature, the CVD deposition yields a homogeneous fine grained microstructure with no amorphous residue detectable. Internal-friction experiments were performed to study the micromechanical response of the materials when exposed to high temperatures. The CVD material revealed no relaxation peak during testing up to 2300°C, while the glass-doped sample showed a pronounced relaxation peak at a peak-top temperature of about 600°C. This temperature corresponds to the softening temperature known for bulk Ca/B-glasses and it is, therefore, concluded that the glass homogeneously wets the BN grains at elevated temperatures. The results presented are seen as the first clear evidence that the internal friction peak monitored for various glass-containing ceramics is indeed related to a viscous sliding process along grain boundaries.

Microstructure and mechanical properties of sputter-deposited Cu1−x Ta x alloys

The microstructures and mechanical properties of a family of sputter-deposited Cu1−xTax (0 50-nm diameter) in addition to the much smaller fcc Ta particles. Annealing the low and high Ta composition films at 900 °C for as long as 100 hours produced no observed change in either the Cu matrix grain size or the size and distribution of the fcc and bcc Ta particles. Microhardness and nanoindentation mechanical property evaluations of bulk hot-pressed materials indicated that the high strengths of the composites were unchanged, even after annealing for 100 hours at 900 °C.

Solid-particle erosion of hot-pressed silicon carbide and SiC-TiB2 composite

The erosion behaviour of SiC has been studied for hot-pressed materials [1–3]. An anomalous particle size dependence [1], a low velocity exponent of less than 2 [1, 2] and fracture other than by lateral cracking have been reported in the erosion of hotpressed, monolithic SiC [3]. Most of these works were performed on commercial SiC of limited compositions and hot-pressing conditions were not reported. Compared to monolithic SiC, much less information is available on the erosion of composite materials. The erosion behaviour of composites has been reported only for SiC–SiC whiskers [4] and the SiC–Al2O3 system [5]. The objective of the present work is to investigate and compare the erosion behaviour of monolithic SiC and SiC–TiB2 particulate composite, both hot-pressed with 5 vol % Al2O3. These materials are chosen because SiC is toughened by the addition of TiB2 and the erosion rate is greatly affected by the fracture toughness [6, 7]. Two hot-pressed SiC materials were used for this work. One, designated SA, was monolithic SiC containing 5 vol % Al2O3, and the other, designated SBA, was SiC composite containing 5 vol % Al2O3 and 14 vol % TiB2. Specimens were produced from commercial powders of mean particle size of less than 0.9 μm. The combined powders were vibromilled in ethanol using SiC balls with a diameter of 8 mm and hot-pressed in graphite dies lined with graphite foil at 2000 to 2050 8C at 30 to 38 MPa under vaccum. In all cases the density of the hotpressed pieces exceeded 97% of theoretical. After hot pressing, specimens were cut and mechanically ground to dimensions of 20.0 mm by 20.0 mm by 50 mm. Erosion tests were conducted at room temperature using an air blast type apparatus. The erodent used was commercial angular SiC, average particle size of 50, 100 and 150 μm. Erodent velocities were 40, 70 and 100 m sy1 and angles of impact were 30, 60 and 908. The specimens were weighed to an accuracy of 10y5 g, subjected to erosion, cleaned with an air blast and then reweighed. Steady-state erosion rates were determined as the mass lost from the target per mass of impacting particle. Fig. 1 presents the angular dependence of the steady-state erosion rate of the materials SA and SBA for particles 100 μm in diameter eroding at a rate of 40, 70 and 100 m sy1. The shape of the curve is representative of both materials obtained for all particle sizes and velocities. Maximum erosion occurs for α ˆ 608, unlike the previous results on hot-pressed SiC [1–3]. The shift of peak in erosion rate indicates that the erosion of these materials occurs in a more ductile manner [8] compared to materials showing peak erosion for normal incidence. The erosion rate is higher for SA under more test conditions compared to the composite SBA, though the difference in erosion rate is not large under some test conditions. A higher erosion rate is obtained for SA in 13 conditions out of 18 test conditions, as shown in Fig. 1. Thus, a higher erosion rate is obtained for SA, having lower fracture toughness under more test conditions, compared to SBA. The fracture toughnesses of the materials measured by the single-edge pre-cracked beam (SEPB) method are 4.0 and 4.8 MPa m1=2 for SA and SBA, respectively [9]. Toughening observed for SBA is due to crack deflection caused by the interaction of the crack front with a residual stress field that surrounds the TiB2 particles [10]. The origin of this stress field is the difference in thermal expansion and elastic moduli between SiC and TiB2. The effect of fracture toughness on the erosion rate found in this work is roughly consistent with the theoretical predictions

Dense zirconia-SiC platelet composites made by pressureless sintering and hot pressing

2.6 mol% Y-TZP composites with up to 30 vol% SiC platelet additions were fabricated by hot pressing and pressureless sintering to densities above 93%. Borosilicate glass additions aided densification, reduced the thermal mismatch stress between the matrix and the platelets, and reduced grain growth rates in the mildly reducing atmosphere employed. Strength was reduced (up to 50% in hot-pressed material and 60% in pressurelesss-intered composites) by the addition of up to 20 vol% platelets as a result of increased flaw size and tensile thermal mismatch stresses in the matrix. With a matrix grain size of 0.71 μm, platelets increased toughness (from 7.0 to 8.3 MPa √m for a 30 vol% addition) while the amount of transformation decreased from 33 to 22%, demonstrating that mechanisms such as crack deflection were present. Conversely, where the composite was processed to give a matrix grain size which optimised transformation toughening (≥0.85 μm), the addition of the platelets led to a reduction in toughness (from 14.0 to 8.8 M Pa √m for a 30 vol% addition). The volume fraction tetragonal which transformed during fracture to monoclinic zirconia decreased with platelet addition, for all processing conditions. The composite toughness was always lower than the maximum in the monolith arising from transformation toughening alone, due to a low fracture stress in the composite and modulus load transfer, both of which reduce the transformation zone size.

Nitrogen sintering of high vanadium high speed steels : J. Mascarenhas, C.S. Wright (University of Bradford, Bradford, UK)

NiAl intermetallic powder has been synthesized by mechanical alloying of elemental powders in an attritor mill using controlled atmospheres. The powders contain a bimodal distribution of dispersoids with sizes ranging from 5 to 200 nm. Analytical transmission electron microscopy has been used to establish the types of dispersoid phase present in the powder and the development of the microstructure with processing. In the finished powder the majority of the dispersoids present are either α-alumina or aluminum nitride depending on the process conditions. The grain size in the as-milled powder is nanocrystalline. Two types of hot pressing have been used for consolidation: a simple pressing using an argon cover gas and a vacuum hot pressing. After hot pressing to greater than 97% density, both grain and dispersoid growth are observed to have occurred and the larger dispersoids appear to coarsen in a coupled manner with the grains via a dragging mechanism. Hot-pressed materials show much improved yield strength and ductility compared with the ingot-cast NiAl, indicating the effectiveness of mechanical alloying in improving the mechanical properties.

Characterization of sintering furnace mesh belt materials : G.D. Smith et al. (Inco Alloys Int., Huntingdon, USA)

NiAl intermetallic powder has been synthesized by mechanical alloying of elemental powders in an attritor mill using controlled atmospheres. The powders contain a bimodal distribution of dispersoids with sizes ranging from 5 to 200 nm. Analytical transmission electron microscopy has been used to establish the types of dispersoid phase present in the powder and the development of the microstructure with processing. In the finished powder the majority of the dispersoids present are either α-alumina or aluminum nitride depending on the process conditions. The grain size in the as-milled powder is nanocrystalline. Two types of hot pressing have been used for consolidation: a simple pressing using an argon cover gas and a vacuum hot pressing. After hot pressing to greater than 97% density, both grain and dispersoid growth are observed to have occurred and the larger dispersoids appear to coarsen in a coupled manner with the grains via a dragging mechanism. Hot-pressed materials show much improved yield strength and ductility compared with the ingot-cast NiAl, indicating the effectiveness of mechanical alloying in improving the mechanical properties.

Attritor technology — Optimization of charging and discharging : H. Zoz (Zoz GmbH, Germany)

NiAl intermetallic powder has been synthesized by mechanical alloying of elemental powders in an attritor mill using controlled atmospheres. The powders contain a bimodal distribution of dispersoids with sizes ranging from 5 to 200 nm. Analytical transmission electron microscopy has been used to establish the types of dispersoid phase present in the powder and the development of the microstructure with processing. In the finished powder the majority of the dispersoids present are either α-alumina or aluminum nitride depending on the process conditions. The grain size in the as-milled powder is nanocrystalline. Two types of hot pressing have been used for consolidation: a simple pressing using an argon cover gas and a vacuum hot pressing. After hot pressing to greater than 97% density, both grain and dispersoid growth are observed to have occurred and the larger dispersoids appear to coarsen in a coupled manner with the grains via a dragging mechanism. Hot-pressed materials show much improved yield strength and ductility compared with the ingot-cast NiAl, indicating the effectiveness of mechanical alloying in improving the mechanical properties.

Effect of the composition of Si3N4 powders and of the charge based on them on the strength and wear resistance of hot-pressed material

Effect of the composition of Si3N4 powders and of the charge based on them on the strength and wear resistance of hot-pressed material

Grain Growth and Fracture Toughness of Fine‐Grained Silicon Carbide Ceramics

Fine-grained silicon carbide ceramics with an average grain size of 0.11 {micro}m were liquid-phase sintered from fine {beta}-SiC powder by hot pressing. The hot-pressed materials were subsequently annealed to enhance grain growth. The diameters and aspect ratios of grains in the hot-pressed and annealed materials were measured on polished and etched surfaces. The bimodal grain size distribution in annealed materials was obtained at 1,850 C without appreciable phase transformation. The average diameter and average aspect ratio increased with annealing time. The fracture toughness of a fine-grained silicon carbide ceramic determined by the Vickers indentation method was 1.9 MPa {center_dot} m{sup 1/2}. The fracture toughness increased to 6.1 MPa {center_dot} m{sup 1/2} after grain growth by annealing at 1,850 C for 12 h. Higher fracture toughness of annealed materials is due to bridging by elongated grains as evidenced by R-curve-like behavior.

Hot-pressed composite materials in the silicon nitride-boron nitride system

A composite material developed in the Si3N4-BN system is characterized by the possibility of control over the properties of the ceramics by changing the ratio of BN and Si3N4. Since the range of the properties of the initial components is very wide and they can give unique combinations of properties, the material is usable in the aircraft, glass, machine-building, and other industries.

Electrochemical aspects of corrosion in sintered and hot-deformed NdFeB magnets

General corrosion behaviour of NdFeB alloys and magnets was studied by electrochemical tests in acid environment. Sintered and hot-deformed magnets of different compositions were tested. Stoichiometric alloys such as NdCu and Nd 6Fe 13Cu, corresponding to intergranular phases present in hot-pressed material, were also examined. The results indicate reduction in critical corrosion currents in doped materials. The significant influence of porosity and the fraction of the intergranular Nd-rich phase in the sintered sample is reported.

Properties of very fine aluminum boride and borocarbide powders and also hot-pressed materials based on them

Using very fine (<2 µm) AlB2, α-AlB12, Al3B48C2 and Al8B4C7 powders synthesized from simple substances a study is made of the oxidation of aluminum borides and borocarbides in air and liquid reagents. Their microhardness and electrical conductivity are measured using hot-pressed specimens. In order of decreasing reducibility the boron-containing aluminum phases are placed as follows: AlB2-Al8B4C7-B-AlB12-Al3B48C2. In AlB12 and Al3B48C2 specimens after high-temperature annealing their electrical conductivity increases and the effect of Mott conductivity is revealed.