Search

We report here the high‐pressure and high‐temperature (HPHT) synthesis of well‐sintered B6O–diamond composites from B6O and carbon black nanopowders. The carbon black was transformed into diamond nanograins at HPHT conditions, and simultaneously formed high‐strength B6O–diamond interfaces. The ultrafine B6O and diamond nanograins and the high‐strength B6O–diamond interfaces synergistically construct excellent mechanical properties for the synthesized composites. The B6O–diamond composites possess a hardness (avg. 52 GPa) comparable to that of polycrystalline diamond (40–60 GPa), whereas the fracture toughness (avg. 7.2 MPa m1/2) is increased several times compared to previously synthesized polycrystalline B6O ceramics (1.7–3.1 MPa m1/2) and B6O‐based composites (3–4 MPa m1/2). Fracture behavior analysis demonstrates that the main toughening mechanisms in this B6O–diamond composite are nanotwin toughening, crack deflection, and crack bridging. [ABSTRACT FROM AUTHOR]

[Display omitted] A review of reported data on the crystal structure, synthesis, and properties of boron suboxide B 6 O, which is a superhard material (hardness of up to 55 GPa, as measured in single crystals), and composites based on it is presented. Methods for the synthesis of B 6 O with a icosahedral structure and for the growing of crystals at high temperature and high pressure are described. The mechanical and operational characteristics of composite materials based on B 6 O are summarized; the resistance of B 6 O to oxidation is analyzed, and the properties of B 6 O–diamond and B 6 O–B 4 C composite materials are described. [ABSTRACT FROM AUTHOR]

We report the atomic structure of coincident stacking faults (SFs) in superhard boron suboxide (B6O) by combining annular bright field scanning transmission electron microscopy and quantum mechanics (QM) simulations. Different from simple SFs, which only lead to the symmetry breaking, the coincident SF junctions in the complex B6O result in local chemical configuration changes by forming an abnormal three-oxygen-atoms chain linking boron icosahedra, instead of the regular two-oxygen-atoms chain in a perfect B6O crystal. QM studies demonstrate that coincident SFs lead to the decreased shear strength under pure shear and indentation conditions and are responsible to the initial failure and amorphization of B6O.

Using constant pressure ab initio calculations, we probe the high-pressure modifications in amorphous boron suboxide (B 6 O) consisting of glassy boron trioxide (B 2 O 3) and boron (B) domains up to a theoretical pressure of 100 GPa. At this pressure, the structure remains amorphous. We find a steady increase in the average coordination of both B and oxygen (O) atoms. O atoms mostly attain threefold coordination as in B 2 O 3 glass at high pressures. On the other hand, the mean coordination number of B-atoms reaches six at high pressures and the structural changes in B-rich regions are perceived to be quite analogous to those of amorphous B. B 12 clusters are found to persevere during the pressurizing process and the high-pressure modifications occur predominantly around O-atoms and the regions that connect the pentagonal pyramid-like motifs to each other. Upon pressure release, some high-pressure configurations persist in the model and another noncrystalline structure being about 10% denser than the original state is recovered, suggesting a permanent densification and a possible irreversible amorphous-to-amorphous phase transformation in B 6 O. The recovered network shows slightly better mechanical properties than the uncompressed model. During the compression and decompression processes, amorphous B 6 O remains semiconducting. The delocalization of some band tail states is seen at high pressures. [ABSTRACT FROM AUTHOR]

We study the atomic structure and the electronic and mechanical properties of amorphous boron suboxide (B6O) using an ab initio molecular dynamic technique. The amorphous network is attained from the rapid solidification of the melt and found to consist of boron and oxygen‐rich regions. In the boron‐rich regions, boron atoms form mostly perfect or imperfect pentagonal pyramid‐like configurations that normally yield the construction of ideal and incomplete B12 molecules in the model. In addition to the B12 molecules, we also observe the development of a pentagonal bipyramid (B7) molecule in the noncrystalline structure. In the oxygen‐rich regions, on the other hand, boron and oxygen atoms form threefold and twofold coordinated motifs, respectively. The boron‐rich and oxygen‐rich regions indeed represent structurally the characteristic of amorphous boron and boron trioxide (B2O3). The amorphous phase possesses a small band gap energy with respect to the crystal. On the bases of the localization of the tail states, we suggest that the p‐type doping might be more convenient than the n‐type doping in amorphous B6O. Bulk modulus and Vickers hardness of the noncrystalline configuration is estimated are be 106 and 13‐18 GPa, respectively, which are noticeably less than those of the crystalline structure. Such a noticeable decrease in the mechanical properties is attributed to the presence of open structured B2O3 glassy domains in the amorphous model. [ABSTRACT FROM AUTHOR]

We report the atomic structure of coincident stacking faults (SFs) in superhard boron suboxide (B6O) by combining annular bright field scanning transmission electron microscopy and quantum mechanics (QM) simulations. Different from simple SFs, which only lead to the symmetry breaking, the coincident SF junctions in the complex B6O result in local chemical configuration changes by forming an abnormal three-oxygen-atoms chain linking boron icosahedra, instead of the regular two-oxygen-atoms chain in a perfect B6O crystal. QM studies demonstrate that coincident SFs lead to the decreased shear strength under pure shear and indentation conditions and are responsible to the initial failure and amorphization of B6O. Combining ABF-STEM and MD simulations, we demonstrated that the coincident SFs lead to the decrease of shear strength and are responsible for the initial failure and amorphization of B6O. [ABSTRACT FROM AUTHOR]

The pure boron suboxide (B6O) powder admixed with 1.08 vol% of MgO, CaO, and CaCO3 (corresponding to 1.5, 1.4, and 0.7 wt%, respectively) were synthesized by hot-pressing and the mechanical properties were evaluated. Pure B6O powders were sintered at a temperature of 1900°C, while the B6O-alkaline earth metal oxide admixed powders were sintered at a temperature of 1850°C. All sintering was done under an applied load of 80 MPa in an argon environment. The microstructures and phase analyses were studied by scanning electron microscopy and X-ray diffraction techniques, respectively. The addition of the alkaline earth metal oxide additives resulted in the formation of boride and borate phases, depending on the oxide used. The mechanical properties of the hot-pressed materials were characterized based on their density, hardness, and fracture toughness. Between 95 and 98% of the theoretical densities were achieved and the result indicate a good combination of hardness (between 31.6 and 32.1 GPa) and fracture toughness (between 6.1 and 6.8 MPa.m0.5) in the B6O-alkaline earth metal oxide materials. The introduction of the additive enhances the fracture toughness of the pure B6O-materials, while the fracture mode observed in the sintered materials is mainly transgranular. [ABSTRACT FROM AUTHOR]

Recently, researchers discovered that in contrast to isolated twins, periodic twins with nanoscale spacing can dramatically improve mechanical properties. Ceramics engineers now seek to incorporate this “nanotwinning” into icosahedral solids because of their high strength, high stability, and low mass density. In this manuscript, we assert that boron suboxide, while far less studied than boron carbide (i.e., the most popular icosahedral solid), possesses higher propensity for nanotwinning and higher theoretical promise. For boron suboxide, the influence of processing on twin spacing is explored through mechanical testing and transmission electron microscopy. Quantum-mechanical simulations are then performed to suggest a critical twin spacing that would maximize performance and to show how to track experimental nanotwinning with x-ray diffraction. Finally, transmission electron microscopy and Raman spectroscopy show that amorphization, the localized loss of crystallinity, drives mechanical failure in ways unique to boron suboxide. [ABSTRACT FROM AUTHOR]

Tantalum diboride – boron suboxide ceramic composites were densified by spark plasma sintering at 1900 °C. Strength and fracture toughness of these bulk composites at room temperature were 490 MPa and 4 MPa m 1/2 , respectively. Flexural strength of B 6 O–TaB 2 ceramics increased up to 800 °C and remained unchanged up to 1600 °C. At 1800 °C a rapid decrease in strength down to 300 MPa was observed and was accompanied by change in fracture mechanisms suggestive of decomposition of boron suboxide grains. Fracture toughness of B 6 O–TaB 2 composites showed a minimum at 800 °C, suggestive a relaxation of thermal stresses generated from the mismatch in coefficients of thermal expansion. Flexural strength at elevated temperatures for bulk TaB 2 reference sample was also investigated. Results suggest that formation of composite provides additional strengthening/toughening as in all cases flexural strength and fracture toughness of the B 6 O–TaB 2 ceramic composite was higher than that reported for B 6 O monoliths. [ABSTRACT FROM AUTHOR]

Boron suboxide (B6O)−based materials have shown good properties which make them good candidates for cutting tool and other applications where abrasive wear resistance is important. The objective of this study was to optimize the production of B6O at ambient pressure by varying reaction temperature and time. B6O powders were produced from the reaction between boric acid and amorphous boron powders at the reaction temperatures between 300°C and 1400°C for 6 h. The powders produced were characterized in terms of particle size, phase analysis and composition, product yield, as well as morphology. Increase in temperature increased both the yield as well as the particle size of the produced powders. X-ray diffraction (XRD) pattern obtained also showed improved crystallinity of the produced powder as the temperature increases. Scanning electron microscopy (SEM) images of the synthesized powder at higher temperature clearly showed improved crystallinity (star-like crystals) as the reaction temperature was increased. The B6O powders synthesized at 1300°C for 6 h had the optimum yield of over 95%. [ABSTRACT FROM PUBLISHER]

Improvement on the densification and fracture toughness of ceramic materials based on boron suboxide (B6O) has been of great importance. The mechanical properties of B6O with and without chromium boride addition, hot pressed at a temperature range of 1850–1900°C and pressures of 50–80 MPa for 20 minutes, were investigated. The relative density, phase relationship, microstructures and mechanical properties of the processed ceramics were examined. More than 96% of the theoretical density was obtained for both ceramic systems. A good combination of mechanical properties was obtained with the B6O-CrB2material (HV = 32.1 GPa, KIC = 4.5 MPa · m0.5) compared to the pure B6O material (HV = 30.5 GPa, KIC = brittle). The addition of 1.7 wt.% CrB2resulted in a pronounced improvement in both the hardness and fracture toughness values. Crack bridging and deflection are some of the toughening mechanisms liable for the enhanced fracture toughness of the sintered ceramic materials. [ABSTRACT FROM AUTHOR]

Monolithic boron suboxide ceramic was densified by microwave sintering at 28 GHz using highly stoichiometric B 6 O powders. High heating rate of 200 °C min −1 was maintained owing to special insulation cell which acts as additional microwave absorber. This insured homogeneous heating during sintering and resulted in dense and crack-free specimens. Room-temperature strength and fracture toughness of bulk B 6 O ceramics were as high as 400 MPa and 2.8 MPa m 1/2 , respectively. [ABSTRACT FROM AUTHOR]

The spark plasma sintering ( SPS) of B6O ceramics using a highly crystalline boron suboxide powder with a low oxygen deficiency level is reported. The monolithic boron suboxide ceramic exhibited a room-temperature strength of 300 ± 20 MPa, which is comparable to the strength of monolithic boron carbide. With increasing flexural test temperature, the strength of the boron suboxide ceramics increased to 450 MPa at 1400°C. The increase in strength with the temperature is associated with the unique microstructure of boron suboxide grains, which allows intergranular 'brittle' fracture along subgrains even at 1400°C. This suggests that even higher strengths can be achieved. [ABSTRACT FROM AUTHOR]

Liquid phase-sintered B 6 O materials with Al 2 O 3 /Y 2 O 3 additives were recently developed to yield higher fracture toughness and strength levels than those of pure B 6 O-materials. However, the high additive contents of hot-pressed and spark plasma-sintered (FAST/SPS) materials led to the formation of strength-limiting boride precipitates. Systematic variation of the B 2 O 3 content of the grain boundary phase and thermodynamic calculations carried out in the present work showed that it is possible to avoid boride formation by controlling the decomposition of B 2 O 3 during densification and adjusting the B 2 O 3 content of the composition through direct addition of B 2 O 3 or through reaction of added transition metal oxides such as TiO 2 with the starting composition to form B 2 O 3 . [ABSTRACT FROM AUTHOR]

Crystalline boron suboxide B6O particles with size in the range of 1.5 µm – 2 µm and crystallite size in the range of 32 nm – 40 nm were prepared by calcination at 1400 °C for one or two hours of precursors obtained by mixing X-ray amorphous boron with water solution of B2O3 followed by evaporation and drying. Decrease of molar ratio B/B2O3 from 16 to 14 in the precursor mixture reduced nonstoichiometry of prepared B6O although simultaneously it increased admixture of B2O3. Particulate composites of B6O with TiN or Ni nanoparticles were prepared by mechanical mixing. The spark plasma sintering process intensified the densification of prepared boron suboxide nanoparticles at 1900 °C and allowed manufacturing of fully dense bodies (98 %) during five minutes. Additives of TiN or Ni nanoparticles reduced sintering temperature to 1700 °C and their promoted formation of Ti or Ni borides.DOI: http://dx.doi.org/10.5755/j01.ms.18.1.1345

The hardness and fracture toughness of boron suboxide ceramics synthesized under ambient pressure and consolidated by spark plasma sintering (SPS) are reported. The structure and characterization of ceramics with a hardness of up to 43 GPa under a 9.8 N load and almost 28 GPa under a load of 198 N are reported. It was shown that an increase in the stoichiometry of boron suboxide by promoting the occupancy of the 6c position in the rhombohedral cell is the effective for achieving both hard and tough pure B 6 O ceramics. SPS consolidation preserved the unique fivefold symmetry of the boron suboxide grains and thus promoted additional toughening due to numerous crack deflections on fine 100 nm subgrains. [ABSTRACT FROM AUTHOR]

Electron deficient and non-deficient boron compounds are discussed as potential thermoelectric generator materials. Particular attention is paid to carbon-doped beta-boron, high-carbon boron carbide, and the alpha-boron derivative compound boron suboxide. Stoichiometric B 6 O shows some promise, and may have a higher ZT than the other two compounds. Carbon saturated beta-boron appears to have a higher ZT than undoped samples. Carbon saturated boron carbide at B 12 C 3 does exist. Its thermoelectric behavior is unknown. [ABSTRACT FROM AUTHOR]

The article presents a study of the potential for the use of BO superhard boron suboxide as a binder in composite materials with a low cBN content. Superhard BO is characterized by higher mechanical properties than TiC widely used in commercial materials today. Composites containing 60 vol % cBN and different binder compositions that included BO and TiC have been sintered in a toroid-type high-pressure apparatus at a pressure of 7.7 GPa in a temperature range from 1500 to 2000 °C. The microstructure, phase and elemental composition of the sintered materials have been examined via electron microscopy and X-ray diffraction analysis. Mechanical properties have been analyzed via indentation techniques. The performance of the cutting tools produced from the sintered composites has been evaluated in turning hardened cold work tool steel. [ABSTRACT FROM AUTHOR]

Star-shaped boron suboxide (B 6 O) nanoplates were formed through a liquid-phase reaction of amorphous boron and boron oxide. The formation of the five-vertex B 6 O crystal is attributed to the oxygen-deficient state of B 6 O x , where x < 1, which correlates to the lattice parameters. The mechanical properties of spark plasma sintered B 6 O show a hardness of 34.8 GPa and a fracture toughness of 4.0 MPa.m 1/2 . The understanding of the relationship between crystal atomic structure and crystal morphology could be employed in the design of other boron-rich compounds. [ABSTRACT FROM AUTHOR]

B 6 O materials were prepared by a reactive and a non-reactive sintering routine using FAST/SPS and sintering temperatures of 1850 °C. Their sintering behavior, phase and microstructure formation and the mechanical properties are comparatively discussed. We demonstrate that the reactive sintering of B/B 2 O 3 -mixtures with oxide sintering additives is a promising and cost-effective alternative to the densification of pre-synthesized B 6 O with a resulting hardness of up to 22 GPa (HV 5 ) and a fracture toughness of up to 3   MPa   m (SEVNB) similar to non-reactive prepared B 6 O materials. The flexure strength is up to 360 MPa and 540 MPa for the reactive and non-reactive sintered materials, respectively, and substantially affected by the amount of B 2 O 3 in the sintering liquid. [ABSTRACT FROM AUTHOR]

The p-V-T equation of state of superhard boron suboxide BO has been measured up to 6 GPa and 2700 K using multianvil technique and synchrotron X-ray diffraction. To fit the experimental data, the theoretical p-V-T equation of state has been derived in approximation of the constant value of the Anderson-Grüneisen parameter δ. Bulk modulus B =181 GPa and its first pressure derivative B′ = 6 at 300 K; two parameters describing thermal expansion at 0.1 MPa, i.e., a = 1.4 × 10 K and b = 5 × 10 K as well as δ = 6 have been included in the model. The good agreement between fitted and experimental isobars has been achieved to the absolute volume changes up to 5% as compared to the volume at standard conditions, V. The fitted thermal expansion at 0.1 MPa well consistent with the experimental data as well as with ambient-pressure heat capacity c, bulk modulus B and δ describing its evolution with volume and temperature have been discussed. The fitted value of Grüneisen parameter γ = 0.85 in agreement with previous empiric estimations for BO and experimental values for other boron-rich solids has been reported. [ABSTRACT FROM AUTHOR]

Abstract: Nanoindentation has become a widely used and versatile means of characterising the near-surface nanomechanical properties of a wide variety of materials. Yet, the nanomechanical properties of the polycrystalline boron suboxide (B6O) material prepared by uniaxial hot-pressing are sparsely known. We recently reported on the nanoindentation profiles, nanoindentation hardness, and elastic modulus determined by using the Oliver–Pharr method. To complement our earlier reports on the analysis of the load–displacement indentation response of hot-pressed B6O and for the first time, we present and discuss results giving an insight into the temporal evolution of the nanoindentation creep behaviour in B6O ceramics. [Copyright &y& Elsevier]

Abstract: Boron suboxide (B6O) based materials containing TiB2 additions were produced in this study via two routes (non-reactive and reactive sintering) and were consolidated using a hot press, which was operated between the temperatures of 1800–1900°C and at an applied pressure of 50MPa for 20min dwelling time. Relationships between the formed phases, microstructures and mechanical properties of the sintered materials were established. The density was higher in the reactive sintered materials with finer microstructure than in the non-reactive sintered counterpart. Generally, the mechanical properties of the B6O–TiB2 materials were enhanced when compared to the pure B6O material. [Copyright &y& Elsevier]

Fully densified B6O materials with Al2O3/Y2O3 sintering additives amounts systematically varied between 0 and 15vol.% and Al2O3/(Al2O3 +Y2O3) molar ratios of 0.05–1 were prepared by FAST/SPS and HIP at sintering temperatures between 1725°C and 1900°C. Their densification and microstructure were correlated with measured mechanical properties. The addition of low additive amounts in the range of 2–3vol.% was found to increase the fracture toughness and strength from 2.0MPam1/2 (SEVNB) and 420MPa for pure B6O to about 3.0MPam1/2 and 540MPa, but it had no effect on the hardness, which remained at a high level of 30–36GPa (HV0.4). Higher additive contents did not yield a further improvement in the toughness but resulted in a reduction in hardness and strength. [ABSTRACT FROM AUTHOR]

Abstract: B6O is a possible candidate as a superhard material with a hardness of 45GPa measured on single crystals. The first dense superhard B6O-materials were produced under high pressure (1GPa). However, recently it was found that different oxides can be utilized as an effective sintering additive allowing the reproducible densification at 50–80MPa pressure at temperatures 1800–1900°C. The resulting materials have similar hardness as the pure B6O materials but strongly increased fracture toughness. This article summarizes the state of the art of B6O materials — the densification, microstructure and resulting room and high temperature properties. It compares the B6O materials with boron carbide and cBN based materials. [Copyright &y& Elsevier]

Abstract: B6O/TiB2 composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900°C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B6O material showed that the addition of TiB2 in a non-reactive sintering route promotes the B6O densification. Further improvement was obtained by sintering reactive B–TiO2 mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0MPam1/2 (SEVNB) and 2.6–5.0MPam1/2 (IF method) while simultaneously a high hardness of up to 36GPa (HV0.4) and 28GPa (HV5) could be preserved. The high temperature properties at 1000°C of hardness, thermal conductivity and CTE were up to 20GPa, 18W/mK and 6.63×10−6/K, respectively. [Copyright &y& Elsevier]

In the present study the analysis of available data on structural, thermodynamic and mechanical properties of BO has been performed. Although the compound is known for half a century and has been extensively studied, many properties of this boron-rich solid remain unknown or doubtful. A semi-empirical analysis of our experimental and literature data has allowed us to choose the best values of main thermodynamic and mechanical characteristics among previously reported data, to predict the thermo-elastic equation of state of BO, and dependence of its hardness on non-stoichiometry and temperature. [ABSTRACT FROM AUTHOR]

Abstract: B6O is a potential superhard material with a hardness of 45GPa measured on single crystals. Recently it was found that different oxides can be utilized as an effective sintering additive which allows densification under low pressures. In this work the effect of addition of Y2O3/Al2O3 on high temperature properties was investigated using impulse excitation technique (IET), hardness measurements and dilatometric measurements. The IET technique reveals the softening of the residual B2O3 in the materials without additives at 450°C; in the materials with Y2O3/Al2O3 the softening is observed at only about 800°C. This data agrees with the values found for different borate glasses. The materials showed no pronounced reduction of hardness at these temperatures. This is additional evidence, supporting previous observations that the material consists of pure grain boundaries between B6O grains. Hardness values (HV5) of up to 17GPa at 1000°C were observed. [Copyright &y& Elsevier]

Abstract: The oxidation resistance of B6O materials with and without Y2O3/Al2O3 additives and a typical cBN material were analysed in dry and wet air using TG measurements for up to 10h and static experiments in a muffle furnace for up to 50h. The results showed that the oxidation of the materials is sensitive to the composition of the atmosphere. In wet air with 17vol.% H2O a weight loss was measured for all materials. In dry air a parabolic increase of the mass and a formation of a glassy surface layer were observed for the B6O materials whereas the cBN-material is characterized by the formation of a mostly crystalline layer. The oxidation resistance is the lowest for the pure B6O and the highest for the investigated cBN material. [Copyright &y& Elsevier]

Abstract: The existence of the indentation size effect implies the absence of a single hardness value for the material under investigation especially at low applied loads. In this paper we present an investigation of the indentation size dependence behaviour of nanoindentation hardness in boron suboxide ceramic compacts prepared by uniaxial hot-pressing. Berkovich nanohardness indentations were conducted and analyzed accordingly. In addition to the ordinary Oliver and Pharr method of nanoindentation data analysis, a quantitative approach for the loading curve analysis is proposed. Using the proposed approach, the description and characterization of the observed indentation size effect through the application of the Meyer’s law, and the classical and the modified proportional specimen resistance models as well as the multi-fractal scaling law was conducted and is reported. The load-independent hardness values deduced from our quantitative approach are comparable to the results calculated with conventional methods, especially with the multi-fractal scaling law. [Copyright &y& Elsevier]

Abstract: The chemical interaction of boron suboxide (B6O) with compacted graphite cast iron (CGI) was investigated using static interaction diffusion couples between B6O and CGI at 700°C, 900°C and 1100°C for 1h. This interaction offers the possibility to evaluate the potential of B6O as a cutting tool. The microstructures and phase compositions of the interaction zones were investigated. At 700°C and 900°C the chemical interaction was minimal. However, at 1100°C, Fe2B and SiO2 were formed at the interface. Hence, machining at 1100°C is likely to result in chemical wear. [Copyright &y& Elsevier]

It has been previously demonstrated in a number of studies that the strength of boron carbide can be significantly reduced via introduction of a site vacancy. The site vacancies result in a discontinuous shock response at high pressure that is driven by bond formation between atoms which neighbor the vacancies. In this work, molecular dynamics simulations of boron suboxide (B 6 O) have been conducted to determine whether an analogous bond formation occurs at high pressure producing discontinuities in the B 6 O Hugoniot. Hugoniot data for ideal boron suboxide, a defect structure of boron suboxide, and a carbon doped structure are provided and the structural response of the oxygen atoms within the B 6 O crystal with increasing shock pressure is presented. [ABSTRACT FROM AUTHOR]

Abstract: The search for suitable additives for boron suboxide (B6O) materials which could improve densification, reduce sintering temperature and tailor the microstructure has been of great importance. In an earlier study it was shown that transition metal borides qualify as sintering aids for B6O, but partial segregations of the boride secondary phases were found. In this work, efforts have been made to understand the chemical interaction between the B6O and boride phase. A reaction couple of sintered B6O, nickel and green compact B6O were assembled and heat-treated at 1850°C for 20min. XRD and SEM examinations of the reaction zone show the formation of nickel boride, diffusing into the B6O matrix and a substantial grain growth of B6O at the interface. [ABSTRACT FROM AUTHOR]

Abstract: Difficulties in the densification as well as the low fracture toughness of the resulting polycrystalline materials have delayed the application of boron suboxide as a superhard material in industry. In this work, efforts have been made to improve the fracture toughness and densification of these materials by incorporating suitable secondary phases. Possible candidates are transition metals, which form under sintering conditions a liquid phase. Therefore different Co containing additives (metal, oxide and boride) were used to densify boron suboxide powder by hot pressing. The resulting materials have a significantly improved fracture toughness (3.9MPam1/2) and still high hardness (HV5 =28.6GPa.). [Copyright &y& Elsevier]

Abstract: Boron reacted with ball milled boron oxide under pressures between 1 and 5GPa and at temperatures between 1300 and 1700°C to afford boron suboxide (B6O). Icosahedral B6O grains with diameters ranging from 100nm to 1.3μm were prepared. The factors that affect the synthesis of B6O are investigated. The best sample with crystal size up to 1.3μm is obtained at 2GPa and 1400°C for 6h. The indentation experiment gave an average Vickers hardness of 32.3GPa for bulk B6O sample, which is consistent with previous reports. Bulk B6O sample exhibits oxidation resistance in air up to 1000°C and mild oxidation in the temperatures of 1000–1200°C, which is more oxidation resistant than diamond. It is possible that B6O could be used as a substitute for diamond in industry because of its relatively mild synthesis conditions, high thermal stability and high hardness. [Copyright &y& Elsevier]

The strength of boron suboxide (B6O) was measured at high pressure and temperature up to 8 GPa and 800°C using diffraction peak broadening analysis. The sample was synthesized at 6 GPa and 1750°C. In situ energy dispersive X-ray diffraction was conducted at the X17B2 beamline of the National Synchrotron Light Source. The differential stress in the powdered sample was derived through deconvolution of peak broadening of diffraction lines (0 2 1) and (1 1 3) during the compression and heating. The sample shows anisotropic yielding along the two orientations. The (0 2 1) orientation does not show yielding within the investigated pressure range (8 GPa) whereas the (1 1 3) orientation yields at 5 GPa. Upon heating, significant weakening (yielding) was observed at temperatures above 400°C. The yield temperature of boron suboxide is much lower than that of diamond, 1200°C, but similar to that observed in moissanite (SiC). [ABSTRACT FROM AUTHOR]

Abstract: Boron suboxide (B6O) powder was synthesized at temperatures of about 1400°C from the reaction of amorphous boron powder with boric acid. The synthesized B6O powders were hot pressed at temperatures up to 1900°C and at pressures of 50MPa. Additionally to pure B6O materials, composites with aluminium were prepared. The microstructure and properties of the sintered compacts were investigated. The addition of aluminium in the composites results in the formation of an additional aluminium borate phase. The composites showed a similar hardness (∼30GPa) as the pure B6O samples but increased fracture toughness (∼3.5MPam1/2). [Copyright &y& Elsevier]

Abstract: B6O composites were prepared by hot pressing of B6O powder with alumina, aluminium and carbon additions. Dense materials with hardness higher than 30GPa and fracture toughness of 3MPam1/2 were achieved at 1900°C. The sintering behaviour and the formed phases were discussed on the bases of thermodynamic calculations. The thermodynamic properties of boron suboxide (B6O) were approximated at high temperatures. The phase equilibria computed based on the obtained thermodynamic properties, agree with the available experimental data. The modelled thermodynamic properties of B6O have been applied to analyse interpretation of the sintering behaviour and the stability of these materials. The thermodynamic calculations and microstructural analysis showed that the densification takes place by liquid phase sintering. [Copyright &y& Elsevier]

Multiphase materials, including the B6N boron subnitride, B6O boron suboxide, and cBN cubic boron nitride, have been obtained from the initial β-rhombohedral or amorphous boron, hBN hexagonal graphite-like boron nitride and B2O3 amorphous boron oxide at a pressure of 8 GPa and temperatures from 2100 to 2900 K in a toroid high-pressure apparatus. The hardness of the resulting materials has been measured. The formation of ternary phases in the B-BN-B2O3 system has not been observed. The wettability of the resultant materials with copper-based melts has been studied. An addition of 10 mol % Ti to the copper melt has been found to decrease the contact angle to a value lower than 20 deg. [ABSTRACT FROM AUTHOR]

Structure, shear modulus and internal friction (IF) of compacted specimens of boron carbide and boron suboxide have been investigated. Microtwins and stacking faults were observed along the {100} plane systems of polycrystalline specimens of boron carbide. Electrical conductivity of the specimens was that of p-type. Concentration of holes varied from 1017 to 1019 cm−3. The IF was measured in the temperature range 80–300 K. It was shown that the IF of boron carbide and that of boron suboxide were characterized with a set of similar relaxation processes. Mechanisms of the relaxation processes in boron carbide and boron suboxide are discussed in terms of the Hasiguti model of interaction between dislocations and point defects. [Copyright &y& Elsevier]