Investigation of the structural and mechanical properties of micro-/nano-sized Al2O3 and cBN composites prepared by spark plasma sintering

Alumina-cubic boron nitride (cBN) composites were prepared using the spark plasma sintering (SPS) technique. Alpha-alumina powders with particle sizes of ∼15 µm and ∼150 nm were used as the matrix while cBN particles with and without nickel coating were used as reinforcement agents. The amount of both coated and uncoated cBN reinforcements for each type of matrix was varied between 10 to 30 wt%. The powder materials were sintered at a temperature of 1400 °C under a constant uniaxial pressure of 50 MPa. We studied the effect of the size of the starting alumina powder particles, as well as the effect of the nickel coating, on the phase transformation from cBN to hBN (hexagonal boron nitride) and on the thermo-mechanical properties of the composites. In contrast to micro-sized alumina, utilization of nano-sized alumina as the starting powder was observed to have played a pivotal role in preventing the cBN-to-hBN transformation. The composites prepared using nano-sized alumina reinforced with nickel-coated 30 wt% cBN showed the highest relative density of 99% along with the highest Vickers hardness (H_v2) value of 29 GPa. Because the compositions made with micro-sized alumina underwent the phase transformation from cBN to hBN, their relative densification as well as hardness values were relatively low (20.9–22.8 GPa). However, the nickel coating on the cBN reinforcement particles hindered the cBN-to-hBN transformation in the micro-sized alumina matrix, resulting in improved hardness values of up to 24.64 GPa.     

Enhanced thermal conductivity of boron nitride epoxy‐matrix composite through multi‐modal particle size mixing

Castable particulate‐filled epoxy resins exhibiting excellent thermal conductivity have been prepared using hexagonal boron nitride (hBN) and cubic boron nitride (cBN) as fillers. The thermal conductivity of boron nitride filled epoxy matrix composites was enhanced up to 217% through silane surface treatment of fillers and multi‐modal particle size mixing (two different hBN particle sizes and one cBN particle size) prior to fabricating the composite. The measurements and interpretation of the curing kinetics of anhydride cured epoxies as continuous matrix, loaded with BN having multi‐modal particle size distribution, as heat conductive fillers, are highlighted. This study evidences the importance of surface engineering and multi‐modal mixing distribution applied in inorganic fillered epoxy‐matrix composite. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Densification of Alkoxide-Derived Fine Silica Powder Compact by Ultra-High-Pressure Cold Isostatic Pressing

Powder compacts of alkoxide-derived fine silica powders were consolidated into a highly dense and uniform structure by ultra-high-pressure cold isostatic pressing of granules with controlled structure. The diameters of spherical and nearly monosized amorphous silica particles, prepared from metal alkoxide, were successfully controlled in the range of 9 to 760 nm by varying the concentration of ammonia. Close-packed granules of these powders were produced by spray drying. These powders were isostatically pressed up to 1 GPa at room temperature. Although the average particle diameter was less than 100 nm, the maximum relative density of the compacts was more than 78% of theoretical density. The optimum particle size to obtain highly dense compacts was in the range of 30 to 300 nm at 1 GPa. Furthermore, the ratio of mode pore diameter in these compacts to particle diameter was less than 0.155, which corresponded to the minimum ratio of calculated three-particle pore channel radii for hexagonal close packing. Viscous deformation of particles under ultra-high isostatic pressure played an important role in the densification of the compacts.     

Morphology of cubic boron nitride crystals synthesized using (Fe,Co, Ni)–(Cr,Mo)–Al alloy solvents under pressure

The growth region of the cubic boron nitride (cBN) using (Fe, Ni)–Cr–Al and Co–(Cr, Mo)–Al solvents were presented in the pressure range of about 4–6 GPa and the temperature range between 1200 and 1700 °C. The minimum pressure for cBN formation was confirmed at about 4–4.1 GPa for both (Fe, Ni)–Cr–Al and Co–(Cr, Mo)–Al solvents. Based upon this pressure–temperature condition of the cBN growth region, the morphology of cubic boron nitride crystals was examined under various compositions of the solvents. The morphology of cBN crystals was affected by not only the reaction pressure and but also the composition of the solvents. It was found that the variation of alloy composition provides various morphologies and grain sizes of cBN crystals.     

Densification and Phase Transformation of β-SiAlON–Cubic Boron Nitride Composites Prepared by Spark Plasma Sintering

β-SiAlON–cubic boron nitride (cBN) composites were prepared from β-SiAlON and cBN powders at 1600°–1900°C under a pressure of 100 MPa by spark plasma sintering. The effects of cBN content and sintering temperature on densification and phase transformation of the β-SiAlON–cBN composites were studied. When 10–30 vol% cBN was added to β-SiAlON, the shrinkage rate of the compacts increased. The compacts of β-SiAlON–BN composites originally containing 10–30 vol% cBN ceased to shrink at a temperature lower than that of β-SiAlON and the density of the composites increased. The densification of β-SiAlON–BN composites originally containing >40 vol% cBN was suppressed. The phase transformation of cBN to hexagonal BN in the β-SiAlON–BN composite was inhibited to a greater degree than that in the cBN body.     

Recrystallization behaviour of cubic boron nitride under high pressure

The recrystallization behaviour of micron-sized cubic boron nitride (cBN) was studied by analysing the grain size and morphology of samples treated at 8−16 GPa/1500–2200 °C. The results show that the recrystallization temperature of cBN under a pressure of 8 GPa is approximately 1650 °C and increases by approximately 100 °C with every 2 GPa increase in pressure. Once grain recrystallization starts, the grains grow abnormally quickly as the temperature rises, and the strengthening effects of grain refinement and defect structure are greatly weakened. The recrystallization behaviour of cBN at high pressure is helpful to understand the sintering mechanism and control the microstructure and mechanical properties of sintered polycrystalline cBN compacts. In addition, the melting curve for cBN under high pressure is inferred according to the empirical relationship between recrystallization temperature and melting temperature, and the phase diagram for boron nitride is revised based on this new melting curve.     

Role of the particle size of graphite-like boron nitride in nucleation of cubic boron nitride

The influence of the reduced radius of grains of the graphite-like hexagonal boron nitride (h-BN) on the nucleation of the cubic boron nitride (c-BN) during synthesis from an initiator solution at a high pressure is analyzed. The colloidal mechanism of nucleation is confirmed experimentally. It is shown that there is a correlation between the nucleus sizes of the hexagonal boron nitride and the pressure of the onset of nucleation of the cubic boron nitride. The effect of these sizes of the hexagonal boron nitride on the concentration of crystal nuclei of the cubic boron nitride is studied. The kinetic nucleation curves are obtained. It is demonstrated that the concentration of crystallization centers depends on the thermodynamic and kinetic parameters, as well as on the particle size of the graphite-like hexagonal boron nitride. Original Russian Text ? S.P. Bogdanov, 2008, published in Fizika i Khimiya Stekla.     

Synthesis and Characterization of TiN‐coated Cubic Boron Nitride Powders

Titanium nitride‐coated cubic boron nitride (TiN/cBN) composite powders were prepared by nitridizing TiO₂/cBN powders in a NH₃ flow at 950°C. The TiO₂/cBN powders were synthesized via a sol‐gel process using tetra‐butyl titanate and concentrated‐HNO₃‐treated BN powders as starting materials. The techniques of XRD, SEM, TEM, FT‐IR, and TG‐DTA were used to characterize the products and their intermediates. The cBN powders were uniformly coated with TiN nanoparticles. During the nitridization, the morphology of the TiO₂/cBN powders is unchanged. The TiN/cBN powders can be used as starting materials to prepare polycrystalline cBN compacts, or as reinforcements to strengthen metal‐matrix composites.     

Dynamic Compaction of Cubic Boron Nitride Powders

Compacts of cubic boron nitride with 94% of theoretical density and a Vickers microhardness of 30.3 GPa were produced from coarse c-BN powder by a shock compaction technique. The density and microhardness of these compacts depend strongly on the grain size of the starting powders.     

Synthesis of Cubic Boron Nitride under Relatively Lower Pressure and Lower Temperature via Chemical Reaction

In this paper, we present a new cubic boron nitride synthesis method via introduction of highly reactive nitrogen and boron atoms generated from chemical reactions as the raw materials of cubic boron nitride, so the system pressure and temperature are reduced significantly compared with the traditional phase transformation method at super high-pressure and high-temperature, and then the production cost of cubic boron nitride can be reduced greatly and the equipment service life can be extended significantly. Experiments have shown that by this method the cBN can be synthesized with high yield at pressures as low as 2.5 GPa and temperatures as low as 450°C. Compared with the conventional phase transformation method, the pressure decreases by about 50% and the temperature decreases by about 64%. Analyses have shown that the crystal structure of the cBN synthesized is perfect and the impurity content is extremely low.     

Studying cubic boron nitride by Raman and infrared spectroscopies

In this paper, optical properties of cubic boron nitride (cBN) in forms of films, powders and monocrystals are studied in vibrational spectral regions. For cBN single crystals, the reststrahlen peak reflectivity of about 95% has been obtained. The second-order Raman scattering spectra induced by visible light excitation is also presented. The infrared two-phonon absorption bands are suggested as additional fingerprints for identification of the cBN nature.     

Synthesis and characterization of novel Ti3SiC2–cBN composites

In this paper, synthesis of novel super hard and high performance composites of titanium silicon carbide–cubic boron nitride (Ti₃SiC₂–cBN) was evaluated at three different conditions: (a) high pressure synthesis at ~ 4.5 GPa, (b) hot pressing at ~ 35 MPa, and (c) sintering under ambient pressure (0.1 MPa) in a tube furnace. From the analysis of experimental results, the authors report that the novel Ti₃SiC₂–cBN composites can be successfully fabricated at 1050 °C under a pressure of ~ 4.5 GPa from the mixture of Ti₃SiC₂ powders and cBN powders. The subsequent analysis of the microstructure and hardness studies indicates that these composites are promising candidates for super hard materials.     

The influence of Li-based catalysts/additives on cBN crystal morphologies synthesized under HPHT

In this paper, various cubic boron nitride (cBN) crystal morphologies were synthesized using hexagonal boron nitride (hBN) as raw materials and Li₃N, LiH, and LiNH₂ as catalysts/additives under high pressure and high temperature (HPHT).These crystal shapes contain thick-plate, spherical, octahedral or hex-octahedral, flat cone and flaky hexagonal morphologies. The reasons of various crystal shapes synthesized can be summarized as follows: various catalysts/additives take on distinct properties under HPHT, which have crucial effects on the cBN crystal morphologies synthesized. Catalyst Li₃N tends to grow cBN with thick-plate morphology, catalyst LiH would induce the growth of cBN tending to integrated octahedral morphology, and catalyst LiNH₂ would play diverse roles for cBN crystal morphologies in various systems.     

Consolidation of cubic and hexagonal boron nitride composites

Consolidating cubic boron nitride (cBN) typically requires either a matrix of metal bearing materials that are undesirable for certain applications, or very high pressures within the cBN phase stability field that are prohibitive to manufacturing size and cost. We present new methodology for consolidating high stiffness cBN composites within a hexagonal boron nitride (hBN) matrix (15–25 vol%) with the aid of a binder phase (0–6 vol%) at moderate pressures (0.5–1.0 GPa) and temperatures (900–1300 °C). The composites are demonstrated to be highly tailorable with a range of compositions and resulting physical/mechanical properties. Ultrasonic measurements indicate that in some cases these composites have elastic mechanical properties that exceed those of the highest strength steel alloys. Two methods were identified to prevent phase transformation of the metastable cBN phase into hBN during consolidation: 1. removal of hydrocarbons, and 2. increased cBN particle size. Lithium tetraborate worked better as a binder than boron oxide, aiding consolidation without enhancing cBN to hBN phase transformation kinetics. These powder mixtures consolidated within error of their full theoretical mass densities at 1 GPa, and had only slightly lower densities at 0.5 GPa. This shows potential for consolidation of these composites into larger parts, in a variety of shapes, at even lower pressures using more conventional manufacturing methods, such as hot-pressing.     

Granularity-induced plastic deformation mechanism of pure polycrystalline cubic boron nitride

Combined X-ray diffraction(XRD) profile analysis and HRTEM observation, a method for exploration of plastic deformation in pure polycrystalline cubic boron nitride (PcBN) samples with sizes of primary cBN powders was developed. XRD profile results showed that the coarse-grained PcBN exhibited a larger micro-strain ε, a greater deformation stacking faults probability fD, which was an order of magnitude larger than that of fine-grained PcBN, but a smaller twin stacking faults probability fT. It was deduced that the plastic deformation of the coarse-grained PcBN was dominated by stacking faults and mechanical twins mode, which would result in strain strengthening and then recrystallization. While the manner of growth twins was the mainly modes of fine-grained PcBN by phase transform, especially utilized the curled SP2 structure as a basis for a cubic structure nucleation. Fundamental plastic deformation principles of the ultrafine polycrystalline cBN was crucial for the field of high-precision cutting tools.     

Non-stoichiometry of composition and dissolution of oxygen in the cubic boron nitride crystal lattice

Powders of cubic boron nitride (cBN), synthesized from multicomponent melts, were studied with a crystallization medium containing excessive nitrogen or boron. X-ray diffraction analysis was used for precision determination of the crystal lattice constants, space factor of regular sets of points of the F43m space group, dislocation density, and stacking fault concentration. Cubic boron nitride has been found to crystallize as a variable-composition compound having vacancies in the boron sublattice. These vacancies are due to a heterovalence substitution of oxygen for nitrogen, i.e. formation of the oxygen solid solution in the cBN crystal lattice. With an increase in the number of vacancies in the boron sublattice, the cBN lattice constant decreases, and the stacking fault concentration increases.     

Reactive Laser Ablation Synthesis of Nanosize Alumina Powder

An aluminum (Al) target was laser ablated in an oxygen (O₂) atmosphere, producing nanosize alumina (Al₂O₃) powder. The powder surface area decreased (and the particle size increased) with both increasing oxygen pressure and laser fluence. All powders produced had surface areas between 135 and 250 m²/g, corresponding to primary particle sizes ranging from 7 to 3 nm in radius. Phase evolution with temperature was studied via X-ray diffraction. These powders showed a direct transformation from γ- to α-alumina at approximately 1200°C, bypassing other transition alumina phases, while still maintaining small particle size ( 30 nm). Despite the nanosize particles, green densities equal to 54% of the skeletal density (i.e., true density of the solid phase) were obtained by uniaxial pressing at 40 MPa.     

Crushing characteristics

A method of measuring the basic characteristics of comminution was developed. These characteristics are expressed by the major comminution functions: crushing probability function, energy function and breakage function. The crushing probability function is the strength distribution of particles of a given size. The energy function is the strength of the particles as a function of their sizes. And finally, the breakage function is the size distribution of the crushed material. The functions are defined mathematically. Several natural minerals were tested by drop tests in order to determine their individual comminution functions. From the tests, several crushing properties of the particulate materials can be derived. The comminution functions given in this paper would be the basic elements in developing mathematical models for various crushing and grinding processes.     

Theoretical investigation of the hBN(0001)/cBN(111) interface

The interface between the (0001) surface of the hexagonal boron nitride (hBN) and the (111) surface of the cubic boron nitride (cBN) is studied through first principles plane-wave pseudopotentials within the density functional theory. Four different structural models for the pseudomorphic growth of the cBN on hBN have been investigated, two with tetrahedral and two with hexagonal arrangements of the atoms at the interface. The interfaces with N-terminated cBN(111) surface are seen to have the lowest formation energies. The studied interface models present a metallic character, with the levels at Fermi energy spatially confined in the interface region. The band offsets show type I band lineups, with large valence and conduction band discontinuities.