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High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al 2 O 3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure. [ABSTRACT FROM AUTHOR]

We report the combined experimental and theoretical investigations on high pressure structural stability of niobium carbide (NbC). The compressibility of NbC has been measured using angle-dispersive synchrotron X-ray diffraction in a diamond anvil cell (DAC) at room temperature and high pressure (up to 38.0 GPa), complemented with first-principles density-function theory calculations. The results imply that NbC shows bulk modulus of = 281 (6) GPa with a pressure derivative = 6.2 (1.5) for its strong covalent bonding. In addition, indentation testing on the well-sintered bulk NbC obtained at 5.0 GPa/1400°C yielded a Vickers hardness of 19.2 GPa and fracture toughness of 7.7 MPa m1/2at applied load of 10 kgf, demonstrating that binderless NbC prepared under high pressure should be a prospective hard material. [ABSTRACT FROM PUBLISHER]

The diamond thick layers prepared using chemical vapor deposition (CVD) methods have been treated at high pressures and high temperatures (HP-HT). It was found that the Vickers hardness of the HP-HT treated bulks was nearly two times as high as that of the starting CVD diamond samples. The hardness of the samples treated at 8 GPa and 1800°C is ∼80 GPa at a loading force of 49 N, close to that of the single-crystal diamond (∼100 GPa). In addition, the oxidation resistance of CVD diamond samples was also enhanced greatly after HP-HT treatments, and the diamondization of sp2bonded carbon in the CVD diamond samples could occur at 7–8 GPa and 1600–1800°C. The HP-HT conditions of diamondization in our works are much lower than the case of the graphite-to-diamond direct transformation without the presence of catalyst (>15 GPa and 2000°C). [ABSTRACT FROM PUBLISHER]

Spherical iron nitride (ϵ-Fe3N) of 4 mm in diameter was synthesized by a high pressure solid-state metathesis reaction between alkaline or alkaline-earth metal ferrite (NaFeO2, MgFe2O4, and Ca2Fe2O5) and boron nitride. The Fe/N atomic ratio of the obtained crystals is close to 3:1 by means of energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, and chemical reaction analysis. Bulk modulusB0= 154(2) GPa with a fixed pressure derivativeis determined by high pressure synchrotron axial X-ray diffraction measurement. ϵ-Fe3N is thermally stable in the air below 500°C byin situhigh-temperature Raman spectroscopy. Vickers hardness tests show that theHvis 5.9 GPa with a load of 5 N, and vibrating sample magnetometer measurements show that saturation magnetization (σs) is 88.53 emu/g and coercivity value (Hc) is 466.67 Oe. [ABSTRACT FROM PUBLISHER]

High pressure and high temperature synthesized MoB2and WB2compacts were investigated using X-ray diffraction, energy dispersive spectroscope, scanning electron microscope, Vickers indentation test and ultrasonic measurements. Experiments showed that both MoB2and WB2compacts are phase pure and with a grain size of 100–200 nm. Vickers indentation test under a large loading force of 49 N showed that the Vickers hardness of MoB2and WB2are about 21 and 22 GPa, respectively. The bulk modulus and shear modulus are about 296 GPa, and 190 GPa for MoB2and 349 and 200 GPa for WB2through ultrasonic measurements. Our results indicate that MoB2and WB2are both hard materials with a hardness similar to that of tungsten carbide, which is widely used in industry. [ABSTRACT FROM PUBLISHER]

Abstract: In this work, we present a novel method to sinter tungsten bulk material under high pressure without any additives. The sintering temperatures were greatly reduced in our experiments. The sintered samples can achieve near-full densification (relative density of 99.5%), and have fracture toughness of about 50% higher than that from the traditional methods, without sacrificing hardness. Moreover, the grain sizes of the sintered samples maintained in the initio range. The sintering kinetics, microstructure evolutions, and relationships of mechanical properties versus pressure–temperature conditions were also discussed. [Copyright &y& Elsevier]

Hexagonal boron nitride (hBN) was used as the additive in improving the hardness and thermal stability of polycrystalline diamond compact under high temperature and high pressure. Indentation testing on diamond/cubic boron nitride (cBN) composites indicated a Vickers hardness of 96 ± 7 GPa, rivaling the hardness of monocrystal diamond. Thermal gravimetric analysis (TGA) suggested that the composites of diamond/cBN are thermally stable in air up to ∼910 °C which is higher than that of pure polycrystalline diamond compact (∼750 °C). Raman, TGA, x-ray photoelectron spectroscopic and microstructure analyses all revealed the features of diamond/cBN composition beyond both diamond and cBN. [ABSTRACT FROM AUTHOR]

In this study, high pressure and high temperature sintering (HPHT) is adopted in the cermet fabrication process, and the microstructure and mechanical properties of cermets with TiC 0.5 N 0.5 –15WC–10Mo 2 C–5TaC–10Ni–10Co (wt%) sintered under 5 GPa and different temperatures (900–1600 °C) using 6 × 14 MN cubic press are investigated. Results show that the densities of samples can reach up to 7.00 g/cm 3 . Vickers hardness and fracture toughness of the products are over 1727 HV30 and 7.2 MPa m 1/2 respectively. In addition, the sintering results are compared with the data that obtained from commercial samples which produced via conventional sintering technique. The conclusion is that high density and high hardness cermets can be obtained through HPHT sintering. [ABSTRACT FROM AUTHOR]