Phase equilibria in the B–BN–B2O3 ternary system at pressures up to 5 GPa have been studied by in situ probing with synchrotron X-ray powder diffraction, microscopic and X-ray diffraction ...characterization of recovered samples, and thermodynamic analysis. The phase diagram of the system has been constructed by combining the previously reported data for binary subsystems and a conventional phenomenological model describing ternary interactions. The interaction parameter of the model of the ternary liquid phase has been determined by adjustment of experimental melting points to the calculated monovariant eutectic curve. The phase diagram is characterized by two ternary eutectics, one ternary transition-type equilibrium, and the maximum in the monovariant eutectic curve.
The aluminum–boron–nitrogen phase diagram is calculated at a pressure from 0.1 MPa to 9 GPa by the models of phenomenological thermodynamics in the Thermo-Calc software package with interaction ...parameters from literature data and equation of states for competing phases. The comparison of our calculation results with the experimental studies of the phase composition of polycrystalline boron nitride samples sintered ay high pressures and temperatures shows their agreement.
The dynamic tensile strength limit
of superhard BL-group ceramic materials synthesized by HPHT sintering in the cBN–TiC–Al, cBN–TiCN–Al, cBN–TiC–WC–Al, cBN–Ti
4
WC
5
–Al, and cBN–NbC–Al systems has ...been investigated. It has been shown that
of these composites is ranged within 271–378 MPa, being quite sufficient for their use in cutting tools for metalworking.
The effect of wSiC and wSi
3
N
4
whiskers on the ultimate dynamic tensile strength
of superhard BL group cBN composites synthesized in the cBN–NbN–Al and cBN–NbN–Al
2
O
3
systems is considered. It is ...demonstrated that
is ranged from 182 to 333 MPa and depends on the character of a filler. It is pointed out that the addition of wSiC whiskers to the initial sintering mixture increases
of sintered ceramic averagingly by 10–15% and, at the same time, the presence of Al
2
O
3
leads to a slight decrease in the material strength.
Three types of cBN-based composites with TaN binder—without whiskers, with Al
2
O
3
whiskers, and with SiC whiskers—have been produced by the high pressure-high temperature (HPHT) sintering. Density, ...hardness, Poisson ratio, and fracture toughness of the composites have been studied. The whisker-reinforced cBN-based composites exhibit a higher level of mechanical properties (hardness, fracture toughness) in comparison with the non-reinforced materials.
The phase diagram of the Al–B–C system has been calculated at pressure 7.7 GPa using models of the phenomenological thermodynamics with the interaction parameters derived from the experimental data ...on the phase equilibria at high pressures and temperatures.
A concept is proposed to produce Y2O3:Eu3+ optical nanograined ceramics by the transformation-assisted consolidation of nanospheres under 8GPa pressure. The nanoceramics were prepared by ...high-pressure low-temperature sintering (~0.04–0.2Tm, where Tm is the melting temperature) accompanied by the cubic-to-monoclinic phase transition. The effects of sintering conditions upon phase composition, grain size evolution, density, morphology, optical and luminescent properties of sintered ceramics have been studied. It has been shown that Y2O3:Eu3+ nanograined ceramics consisting of individual (cubic or monoclinic) phases or their mixture can be obtained by variation of the sintering temperature. The use of transformation-assisted consolidation makes it possible to prepare Y2O3:Eu3+ nanoceramics with average grain size three times smaller (12nm) than that of the starting nanopowders (37nm), which corresponds to extremely low grain growth factor of 0.3. The grain size refinement is related to numerous nucleation events in the parent phase of cubic yttrium oxide. The preparation conditions of translucent (T=50%) composite Y2O3:Eu3+ nanograined ceramics with a relative density of 99±1 % have been determined. The obtained two-phase ceramics show high optical transparency due to negligible birefringence at extremely small (~12nm) average grain size.
Ceramic-matrix composites based on cubic boron nitride (cBN) with binders, containing refractory compounds of transition metals and aluminum, are widely used for high-speed (300 – 500 m/min) and ...finishing metal processing of hardened alloy steels and heat resistant alloys. PcBN composite materials of the BL group obtained in the cBN–NbC–Al and cBN–TaC–Al systems (60:35:5 vol. % charge content) have good application prospects. The purpose of this work was to study the effect of high pressure – high temperature (HPHT) sintering conditions (7.7 GPa, 1600 – 2450 C) on the crystal structure of NbC and TaC carbides, which form together with aluminum the binder of the charge. As a result of a detailed X-ray diffraction study, it was shown that the original NaCl type crystal structure of these carbides is modified under barothermal exposure. Namely, an additional position for the placement of small atoms (nitrogen and/or carbon) is formed. During HPHT sintering, the interaction of NbC and TaC with Al atoms in the charge, as well as with the nitrogen flow formed during partial decomposition of cBN, results in the formation of solid solutions. At the same time, the dissolution of aluminum takes place in accordance with the type of Nb or Ta atoms substitution by it (up to 2.5 at.% Al). Accumulation of nitrogen atoms in the TaC structure occurs by filling vacancies in the carbon sublattice (dissolves up to 1 at.% N). In the NbC structure this process goes through interstitial of nitrogen atoms with their placement on the additional position of a modified NaCl-type structure (dissolves up to 5 at.% N). Besides, the carbon atom also partially moves from its basic positions. This accumulation of defects leads to a significant increase in the lattice parameter of NbC carbide (a relative increase of 0.28% for NbC versus 0.11% for TaC). It is shown that the carbide compositions can be described as Nb0,96Al0,04(C, N)1,17 and Ta0,95Al0,05(C, N)1. for composites of the cBN–NbC–Al and cBN–TaC–Al systems with the best operational characteristics (HPHT sintering at 7.7 GPa and 2150 C).
Y2O3:Eu3+ (1 at%) translucent nanostructured ceramics with a total forward transmission of about 70% of the theoretical limit were obtained by the transformation-assisted consolidation of custom-made ...cubic Y2O3:Eu3+ nanopowders under high pressure. Sintering under a pressure of 7.7 GPa and temperatures in the range 100-500 C led to the partial cubic-to-monoclinic phase transition that resulted in two-phase Y2O3:Eu3+ nanoceramics. The average grain size of the ceramics was up to 50 nm for both Y2O3:Eu3+ polymorphs and was comparable with the crystallite size of the initial nanopowders (about 40 nm), indicating that the grain growth factor was near unity. The phase compositions, morphology, densities, preliminary optical and luminescent properties of the synthesised nanostructured ceramics were studied.
The sintering of cubic boron nitride (cBN) based ceramic matrix materials with zirconium carbide and aluminum has been carried out under high pressures and temperatures. The performed study has ...revealed that the interaction between mixture components with the formation of a new phase, namely, zirconium diboride (ZrB
2
) occurs at sintering temperatures above 1750°C and an applied pressure of 7.7 GPa. A further increase in the sintering temperature leads to a growth in the volume ZrB
2
content and reduces the microhardness and crack toughness of the studied materials. The flank wear of specimens under the conditions used for the processing of AISI316L stainless steel (
v
c
= 300 m/min,
t
= 300 s) grows with an increase in the sintering temperature to attain the critical value
VB
= 325 µm, when the specimen sintered at temperature of 2300°C is used, and
VB
= 200 µm for the specimen sintered at 1750°C.
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