cBN Crystals Research Articles

Constructing coherent/semi-coherent phase boundaries to enhance toughness of superhard cBN-B composite

Harsh synthesized pressure is required in constructing of excellent grain boundary (GB) connection for nano-polycrystalline cubic boron nitride (nPcBN) to yield high hardness and toughness. Therefore, it is urgent to fabricate robust nPcBN composite under relatively mild pressure by developing the hard nonmetal binders to form coherent/semi-coherent phase boundary (cPB, scPB) with cBN. Here, the slight content of nano‑boron was composited with nPcBN to achieve superhard property of 45.2 GPa and high fracture toughness 8.4 MPa‧m0.5 under a relatively mild condition of 5 GPa and 1800 °C. The hardness is comparable with single crystal cBN and the toughness is about three times higher than single crystal cBN. The nano boron connects the nano cBN grains by cPB and scPB, and nano boron filled the triangular space of cBN crystals, which eliminates the defects to prohibit the crack trigger sources and enhances the strength. This work provides a reference and a new strategy on composite engineering in superhard materials with high toughness by relatively mild synthesized pressure.

Microstructure of <111>‐Textured Cubic Boron Nitride Film Deposited under Oxygen‐Containing Atmosphere

The cross‐sectional and planar microstructures of a cubic boron nitride (cBN) thin film with a <111>‐preferential orientation are observed using transmission electron microscopy. The cBN films are deposited by unbalanced magnetron sputtering under the condition that oxygen is added to a 20 sccm Ar–N2(25%) gas mixture. Thecross‐sectional view of the cBN film deposited with the addition of 0.4 sccm of oxygen shows a dual‐phase structure: turbostratic boron nitride (tBN) layers are filled between cBN columns, and the planar view shows that cBN crystals were surrounded by a tBN matrix. The films deposited under less than 0.4 sccm of oxygen addition show a single‐phase structure with no tBN layers between the cBN columns. The difference between dual‐ and single‐phase structures is that they have preferred <111> and <220> orientations. This texture variation is interpreted as being due to the low residual stress of the dual‐phase‐structured cBN film and the low surface energy of the cBN (111) plane. The residual stress of the dual‐phase structure is significantly lower than that of the single‐phase structure. This is attributed to the compressive residual stress relieved by the tBN layers formed between the cBN columns.

On the solubility of boron nitride in supercritical ammonia-sodium solutions

Cubic boron nitride (cBN) is considered to be one of the leading candidates for next generation electronic devices yet has no current synthesis route for single crystals greater than a few mm in size leading to a lack of suitable substrates. New synthesis methods need to be explored, with the ammonothermal method being a potential candidate for synthesis of large area, thick cBN and hexagonal BN (hBN) single crystals. This paper investigates the solubility of BN in supercritical ammonia using sodium amide (NaNH2) as a mineralizer and cBN crystals as feedstock. Solubilities were measured based on mass loss of the feedstock at temperatures from 450—600 deg. C, with pressures of 150–190 MPa, using durations from 24—96 h and with varying amounts of Na. A positive solubility trend is demonstrated with respect to temperature with solubility varying from 0.017—0.050% molBN/molNH3. Solubilities did not vary as a function of time or when>0.07 molNa/L mineralizer content was used suggesting both that equilibrium was reached and the solutions were saturated. These values are high enough to motivate further investigation for solution-based growth of BN.

The use of Preston equation to determine material removal during lap-grinding with electroplated CBN tools

Grinding executed in a lapping configuration is an alternative finishing process benefiting from both grinding and free-abrasive machining, while minimizing the heat effect impact. Electroplated tools can be effectively used in different abrasive processes, including high-speed grinding, however, the assessment of machining performance over time is a key factor in their correct use to achieve satisfactory technological results. In conventional grinding, bigger grain-coverage provides better results due to the higher bonding strength of grains. In lap-grinding, fracturing and crushing of the abrasive particles initially covered by the plating result in a suspension which is typically dosed continuously in free-abrasive machining. Due to this, the process transforms from two-body (grinding) to three-body abrasion (free abrasive machining) which may result in reduced grinding performance approaching asymptotically a specific value while improving surface finish. The main aim of the presented study is the evaluation of electroplated CBN tools used in lap-grinding of 40H alloy steel workpieces whose hardness was 54 HRC. The obtained results and observations of the working surface of CBN wheels and workpieces allowed for the identification of the wear characteristics for three nickel plating thicknesses corresponding to 35%, 50% and 65% of the nominal CBN crystal size and for specific process parameters. The surface roughness Ra parameter decreased gradually from the initial value 1.9 μm to the values below 0.7 μm for bigger B107 grains and for all plating thicknesses. For smaller B64 grains, the surface roughness and waviness parameters reached similar values to those obtained for the larger B107 grains at corresponding processing times. The lowest value of Ra parameter below 0.4 μm was obtained for B64 grains and for the thinnest plating but with a 50% reduction in material removal comparing to B107 grains. The Preston equation was employed to calculate the material removal as a function of time under variable process conditions in the machining zone due to the tool wear. This attempt extended the range of the material removal modeling, despite the fact that the electroplated wheels were subject to wearing down resulting in the gradual reduction in the efficiency, as well as in the change of the working conditions in the workpiece-tool contact zone.

Electronic and optical characterization of bulk single crystals of cubic boron nitride (cBN)

Cubic boron nitride (cBN) is a relatively less studied wide bandgap semiconductor despite its many promising mechanical, thermal, and electronic properties. We report on the electronic, structural, and optical characterization of commercial cBN crystal platelets. Temperature dependent transport measurements revealed the charge limited diode behavior of the cBN crystals. The equilibrium Fermi level was determined to be 0.47 eV below the conduction band, and the electron conduction was identified as n-type. Unirradiated dark and amber colored cBN crystals displayed broad photoluminescence emission peaks centered around different wavelengths. RC series zero phonon line defect emission peaks were observed at room temperature from the electron beam irradiated and oxygen ion implanted cBN crystals, making this material a promising candidate for high power microwave devices, next generation power electronics, and future quantum sensing applications.

Production of nano- and micropowders of cubic BN from mechanically activated graphite-like BN under high pressures and temperatures using aluminium as a phase convertion initiator

An effect of mechanical activation of graphite-like (hexagonal) boron nitride (hBN) on catalytic synthesis of nano- and micropowders of cubic boron nitride (cBN) under conditions of high pressures and temperatures is shown in the article. It has been suggested that nuclei of dense phases of boron nitride (cubic and wurtzite) are formed in the hBN structure during mechanical activation they serve as crystallization centers stimulating the formation of cBN crystals under subsequent thermobaric treatment. Additional chemical-thermal modification of hBN with aluminum as a catalyst (initiator) for the phase transformation of hBN into cBN leads to a significant increase in the content of cBN along with an increase in the synthesis pressure. Thus, the introduction of Al additives in the amount of 10 wt.% leads to an increase in the content of the cBN phase from 10–15 % under synthesis pressure of 2.5 GPa up to 90 % under pressure of 5.5 GPa. In this case, the grain size of cBN, estimated by scanning and atomic force microscopy, is mainly 200–400 nm and 40–120 nm for the samples synthesized under pressure of 2.5 and 5.5 GPa, respectively. The increase in the content of the initiator from 10 to 40 wt.% with an increase in isothermal holding time from 15 to 60 s in the studied range of pressures and temperatures leads to the formation of intergrowths of cBN grains in nano- and submicron sizes and individual cBN single crystals of a cubic habit with a grain size of 1–4 µm, as well as polycrystalline cBN particles from 10 to 50 µm. In this case, the maximum size distribution of cBN micropowders in the range up to 50 µm falls on particles up to 5 µm (~ 70 %). In the submicron grain size range the maximum yield (~ 50 %) is marked for cBN particles 0.5–0.7 µm in size. The obtained powders can be used to make abrasive and cutting tools.

Transfer of Abrasive Material at Grinding a Titanium Alloy with a Wheel of Cubic Boron Nitride

The article summarizes the results of the of the titanium alloy surface morphology and chemical composition study after grinding with a wheel of cubic boron nitride on a ceramic bond. The titanium alloy was treated using the method of cut-in grinding in the finishing mode using a synthetic water-soluble lubricant-cooling liquid that does not contain mineral oil. The research was carried out using the FEI Versa 3D LoVac electron microscope. Digital photos of the titanium alloy surface at different magnifications are given. Individual objects’ morphology allows us to identify them as wear products of abrasive tools. The chemical composition of the selected objects was studied by local x-ray spectral analysis. CBN crystals are partially or completely pressed into the treated surface and covered with a layer of the treated material. On the surface of CBN crystals, there are chemical elements that are part of the abrasive tool bond.

The Analysis of the Transformation Mechanism of cBN Crystals with the First-Principle Calculation

To clarify the synthesis mechanism of cubic boron nitride (cBN) with catalysts at high temperature and high pressure, we calculate the surface energy of the main phases in the Li-N-B synthesis system using the first-principle method. Based on the density functional theory, the surface energy of low-index surfaces of cBN, hexagonal boron nitride (hBN), and lithium boron nitride (Li3BN2) at the cBN synthetic temperature of 1700 K and synthetic pressure of 5.0 GPa is calculated. The surface energy of the main low-index surfaces of cBN is σ (111) > σ (001) > σ (110), that of hBN is σ 10 1 ¯ 0 > σ 11 2 ¯ 0 > σ (0001), and that of Li3BN2 is σ (100) > σ (110) > σ (001). The energy orders of the main low-index surfaces were well contrary to the corresponding orders of the valence electron density of the low-index surfaces of cBN, hBN, and Li3BN2, which were calculated by the empirical electron theory (EET) of solids and molecules. The result shows that the calculation results in this paper are well consistent with the previous results of the EET theory and support for the results of the “direct transformation of hBN to cBN under the catalysis of Li3BN2” obtained by the EET theory.

Process optimization of large-sized cubic boron nitride single crystal synthesis with Li3N as catalyst

Cubic boron nitride (cBN) was synthesized using cubic press with lithium nitride as the catalyst and hexagon boron nitride as the source material under high temperature and high pressure in this study. The high quality big-sized cBN single crystal can be synthesized with the optimized synthesis process. Comparing two different pressurization process, two different power process and three different heating times, the influence for the quality of cBN single crystal were studied. The results show that the process of gradually elevated pressure, the constant power and 15 minutes heating time are conducive to big-sized cBN crystals synthesis under high temperature and high pressure. Using the optimized synthesis process, the single block output reaches up to more than 120ct, the proportion of cBN crystal bigger than 50 mesh reaches to more than 16%, the static pressure strength of 60/70 mesh single crystal reaches to 30 N. Meanwhile, the cBN single crystals have fewer surface defects. The final result is high quality large-sized cBN single crystal was obtained.

Effect of sintering temperature on synthesis of PCBN in cBN-Ti-Al-W system

Polycrystalline cubic boron nitride (PCBN) was prepared under high-temperature and ultra high-pressure. The influence of sintering temperature on composition, micro-structure, relative density, porosity, micro-hardness and flexural strength of the PCBN was investigated by means of X-ray diffraction (XRD), field scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the phase components of PCBN have no significant difference, which are composed of BN, TiN, TiB2, W, AlN, W2B and Al3Ti when the sintering temperature at 1500 °C–1600 °C. The cBN crystals connect with each other by the reaction product.With the increase of sintering temperature, the porosity of PCBN decreased from 1.05% to 0.53%, and the relative density increased from 94.5% to 99%. When the sintering temperature increasing to 1600 °C, the PCBN exhibit optimal comprehensive mechanical properties with a micro-hardness of 34.64GPa and a flexural strength of 1005.23 MPa.

Effect of tungsten content on microstructure and mechanical properties of PCBN synthesized in cBN-Ti-Al-W system

Polycrystalline cubic boron nitride (PCBN) compacts were prepared using the mixture of cubic BN and Ti-Al-W powders at 5.5 GPa and 1550 °C for 10 min. The influence of different Tungsten (W) content on composition, microstructure, porosity, mechanical property and cutting performance of the PCBN is investigated. The results show that, with the addition of tungsten, the cubic boron nitride (cBN) crystals are connected with each other by the new product phases TiB2, TiN, Al3Ti and W2B under the pressure of 5.5 GPa and the temperature of 1550 °C. The rod-shaped crystals in the PCBN are expanded from the surface portion of the cBN. As the W content increases, the amount of rod-shaped crystals and the length-diameter ratios decrease in the system. When the tungsten content is 6 wt%, PCBN presents the best comprehensive performance and cutting performance, the porosity, the hardness, the flexural strength and the flank wear are 0.55%, 30.71 GPa, 972.3 MPa and 292 μm, respectively.

The properties of planar Na:CBN waveguide with “well” + “barrier” type by multiple energy helium ion implantation

Sodium doped Calcium Barium Niobate (Na:CBN) is of potential use in integrated optical devices with high-power laser operation because of its unique nonlinear optical properties and the high Curie temperature. In our work, we research the fabrication and properties of planar Na:CBN waveguide by multiple energy He ion implantation. The sample is implanted with (450 + 500 + 550) keV He ions at room temperature with fluence of (3 + 3 + 3) × 1016 ions/cm2. Prism-coupling and end-face coupling measurements are used to investigate the optical properties of ion-implanted Na:CBN crystal. SRIM and FD-BPM simulations have been used to study the damage formation and light propagation process on the Na:CBN waveguide. Micro-Raman imaging is a powerful nondestructive approach to examine lattice modification and local micro-structural change. We perform confocal micro-Raman measurement, which is used to provide the information about the crystal structure of Na:CBN crystal after multiple energy He ion implantation. This work could provide useful information for integrated optical device based on Na:CBN crystal.

High hardness cubic boron nitride with nanograin microstructure produced by high‐energy milling

AbstractHigh‐energy shaker milling of hexagonal boron nitride (hBN) powders was used to produce powders rich in sp3 bonding. The powders contained up to 68% sp3 bonding and were found to nucleate nanosize cBN grains during consolidation at 5.5 GPa and 1400°C. The effect of hBN starting particle size, milling time, and powder‐to‐milling ball ratio were studied. The amount of sp3 bonding for milled hBN powders was determined, using 11B solid‐state NMR. The milled material was also analyzed by XRD, Raman spectroscopy, and HRTEM. The results indicate that the material has a nanosized microstructure comprised of a disordered hBN matrix and cBN nuclei in the form of sp3‐rich domains. Eight different milled powders were produced and consolidated at pressures of either 5.5 or 6.5 GPa and temperatures of either 1400°C or 1450°C into 12 mm diameter and 5 mm thick pellets. Consolidated pellets formed from milled hBN with 68% sp3 bonding had Vickers hardness of 42 ± 1 GPa and fracture toughness 3.8 ± 0.1 MPa.m1/2. Vickers hardness of 49 ± 1 GPa and fracture toughness of 4.6 ± 0.1 MPa.m1/2 was achieved with a precursor that contained milled hBN and 50 vol. % of 0.5 μm diameter cBN crystals.

Raman study of color-zoning cubic boron nitride single crystals

Unintentionally doped cubic boron nitride (cBN) single crystals with color zoning were investigated by Raman spectroscopy. Besides the TO mode at 1053cm−1 and the LO mode at 1304cm−1, some little vibrational bands at 925cm−1, 955cm−1and 1247cm−1 corresponding to the amber regions in the {1¯ 1¯ 1¯}N sectors of cBN crystals were observed. These vibrational bands probably result from the disorder-activated Raman scattering (DARS) due to the high defect densities. It is shown that these defects and impurities are nonuniform, easily formed and incorporated in {1¯ 1¯ 1¯}N growth sectors during the growth process of cBN crystals. These defects and impurities are color centers in cBN, therefore, the cBN crystal exhibits symmetrical color zoning relative to the {111} growth sectors.

IM (Indentation Method) Based Impact Fracture Characteristics of cBN Crystal and Their Influence on High Speed Wheel Performance

Cubic Boron Nitride (cBN) is considered a superabrasive due to its excellent material behavior, and is commonly used in the manufacturing of high end grinding tools, in particular for machining hard-to-machine materials, in order to achieve high productivity in combination with high precision. During the actual grinding process, the performance of grinding tools is significantly affected by the fracture response of employed cBN crystals to the external impact load related to the grinding speed. Therefore, two types of cBN crystals are selected in this paper and their fracture resisting behaviors are measured under various impact loading rates through the Indentation Method (IM). In order to establish the fracture behavior of cBN crystals and the overall grinding performance of cBN wheels, high speed surface grinding experiments are conducted and grinding performances are evaluated regarding the evolution of grinding force and power consumption, wheel wear and grinding efficiency, as well as the chemical stability of cBN materials on affinity. The experimental results show a good agreement of cBN crystal’s impact fracture properties on the cBN grinding wheel performance, which helps to understand cBN crystal’s high speed grinding characteristics.

Growth and spectral properties of Yb:Ca0.28Ba0.72Nb2O6 disordered crystal

The growth, XRD patterns, spectral properties, and fluorescence decays of Yb:Ca0.28Ba0.72Nb2O6 (Yb:CBN) with doping concentration of 1at.% and 5at.% were studied. The peak absorption cross-section and the emission cross-section were calculated. Larger Stark splitting of Yb:CBN offers the prospect of the quasi-four level laser operation.The fluorescence lifetimes are 0.64 and 0.73ms for 1at.% and 5at.%, respectively. The FWHM of absorption and fluorescence spectra of 5at.% Yb:CBN are 62 and 68nm, which are broader than other ultrashort laser media. The results prove that Yb:CBN crystal is a good candidate as an ultrashort laser medium.

XPS analysis for cubic boron nitride crystal synthesized under high pressure and high temperature using Li3N as catalysis

Cubic boron nitride (cBN) single crystals are synthesized with lithium nitride (Li3N) as catalyst under high pressure and high temperature. The variation of electronic structures from boron nitride of different layers in coating film on the cBN single crystal has been investigated by X-ray photoelectron spectroscopy. Combining the atomic concentration analysis, it was shown that from the film/cBN crystal interface to the inner, the sp2 fractions are decreasing, and the sp3 fractions are increasing in the film at the same time. Moreover, by transmission electron microscopy, a lot of cBN microparticles are found in the interface. For there is no Li3N in the film, it is possible that Li3N first reacts with hexagonal boron nitride to produce Li3BN2 during cBN crystals synthesis under high pressure and high temperature (HPHT). Boron and nitrogen atoms, required for cBN crystals growth, could come from the direct conversion from hexagonal boron nitride with the catalysis of Li3BN2 under high pressure and high temperature, but not directly from the decomposition of Li3BN2.

Incommensurate modulations of relaxor ferroelectric Ca0.24Ba0.76Nb2O6 (CBN24) and Ca0.31Ba0.69Nb2O6 (CBN31).

CBN crystals show a one- and a two-dimensionally modulated modification. The former is isotypic with orthorhombic Ba4Na2Nb10O30 and the latter with the tetragonal tungsten bronze type of crystal structure. The orthorhombic form irreversibly transforms to the tetragonal polymorph at the ferroelectric phase transition near 603 K. Orthorhombic and tetragonal CBN24 slightly differ in the distribution of the Ba and Ca atoms over the incompletely filled Me1 and Me2 sites. The tetragonal symmetry is further broken in orthorhombic CBN24 by different amplitudes of the positional modulations of O atoms which are symmetrically equivalent in the TTB structure. A similar orthorhombic phase of CBN31 could be obtained by quenching from 1473 K.

Investigation on cubic boron nitride crystals doped with Si by high temperature thermal diffusion

Abstract The method of high temperature thermal diffusion was successfully applied for doping Si impurities into cubic boron nitride (cBN) crystals. X-ray photoelectron spectra (XPS) and the current–voltage ( I – V ) characteristics at different temperatures were respectively used for analyzing the chemical states and the activation energy of Si impurity in cBN. According to the XPS results, Si impurities mainly replace B atoms bonding with the adjacent N atoms and become donors in cBN. Without surface cleaning, there are a lot of C and O contaminations on the surface of cBN, so a small quantity of C–Si and Si–N–O bonds also exist at the surface of cBN. Most Si impurities distribute in the shallow layer underneath the surface of cBN. Based on the electric measurement, Si impurities in cBN usually have the activation energy beyond 0.4 eV, and they can only be slightly ionized at room temperature, therefore the resistivity of Si-doped cBN is still high, and the space charge limited current becomes the main conductive mechanism in cBN. However, the conductivity of Si-doped cBN can rapidly increase with the temperature. In addition, the activation energy and the concentration of Si impurity in cBN can be affected by the temperature and the time of thermal diffusion, which needs to be verified further.

Floating-zone growth and characterization of CaxBa1−xNb2O6 crystals

Transparent and pale yellow calcium barium niobates (CaxBa1−xNb2O6) single crystals with larger Ca content ranging from 0.224 to 0.366 have been grown out by floating zone technique. The growth of CBN crystals in air is oriented along the 〈001〉 direction. The CBN crystals have been characterized by X-ray diffraction and X-ray single crystal diffraction. The composition uniformity of CBN-35 crystal rod has been checked by EPMA measurement. The deviations of Ca mole fraction between initial melt and the crystal are 0.1–1.4mol%, which is much lower than crystals prepared by the Czochralski method. All the as grown crystals had high transmittance above 80% with increasing wavelength from about 500nm. With increasing Ca content in crystal, symmetric bending mode v5 and symmetric stretching mode v2 broadened and moved to the low wave number. The relative intensity of Raman peak v5 compared to v2 increasing gradually indicates that the NbO6 octahedral distortion degree increased.