Boron Nitride Phase Research Articles

Hybrid CO2 laser/waterjet (CO2-LWJ) cutting of Polycrystalline Cubic Boron Nitride (PCBN) blanks with phase transformation induced fracture

The present paper investigates a transformation induced fracture mechanism for the cutting of Polycrystalline Cubic Boron Nitride (PCBN) sample by a hybrid CO2 laser/waterjet (CO2-LWJ) manufacturing process. In CO2-LWJ machining, a laser was used for local heating followed by waterjet quenching leading to fracture propagation along the sample surface. Cutting results indicate that as line energy of the laser was increased the sample response transitioned from scribing to through cutting. Raman spectroscopy analysis of the cut surface indicates that laser heated PCBN undergoes chemical phase transformation from sp3-bonded cubic Boron Nitride (cBN) into hexagonal Boron Nitride (hBN) and other sp2-bonded phases. The sp2-bonded structure occupies more volume than sp3-bonded structure such that the transformed material has a tendency to expand the original material and leads to surface deformation around the cutting path. Surface profile of the cut samples was experimentally measured using profilometry and compared with numerical predictions in order to estimate the expansion strain and dimensions of transformation region. Based on the obtained expansion strain and transformation zone, stress fields and crack driving forces were computed for channeling cracks that result in material separation. Comparison of the crack driving forces with fracture toughness of PCBN shows that transformation induced crack propagation is the feasible mechanism for cutting during CO2-LWJ machining.

Effect of the hexagonal phase interlayer on rectification properties of boron nitride heterojunctions to silicon

Rectification properties of boron nitride/silicon p-n heterojunction diodes fabricated under low-energy ion impact by plasma-enhanced chemical vapor deposition are studied in terms of the resistive sp2-bonded boron nitride (sp2BN) interlayer. A two-step biasing technique is developed to control the fraction of cubic boron nitride (cBN) phase and, hence, the thickness of the sp2BN interlayer in the films. The rectification ratio at room temperature is increased up to the order of 104 at ±10 V of biasing with increasing the sp2BN thickness up to around 130 nm due to suppression of the reverse leakage current. The variation of the ideality factor in the low bias region is related to the interface disorders and defects, not to the sp2BN thickness. The forward current follows the Frenkel-Poole emission model in the sp2BN interlayer at relatively high fields when the anomalous effect is assumed. The transport of the minority carriers for reverse current is strongly limited by the high bulk resistance of the thick sp2BN interlayer, while that of the major carriers for forward current is much less affected.

Thermodynamic study on the chemical vapor deposition of boron nitride from the BCl3–NH3–H2 system

The gas-phase reaction thermodynamics in the chemical vapor deposition (CVD) process of preparing boron nitride with the precursors of BCl3–NH3–H2 was investigated with a relatively complete set of 144 species, in which the thermochemistry data were calculated with accurate model chemistry at G3(MP2) and G3//B3LYP levels combined with standard statistical thermodynamics. The data include the heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation. Three different modifications of condensed phase boron nitride (hexagonal h-BN, cubic c-BN, and wurtzite w-BN) were taken into consideration. Based on these data, the distribution of the equilibrium concentration of the 144 species was obtained with the principle of chemical equilibrium. It was concluded that c-BN is formed at temperatures up to 1,800 K, h-BN is the most stable above this temperature, and w-BN is unstable under considerable conditions.

Spark plasma sintering of cBN(core)/SiO2(shell) powder prepared by rotary chemical vapor deposition

SiO2 nanolayer coated cubic boron nitride (cBN), cBN(core)/SiO2(shell) powder, was prepared by rotary chemical vapor deposition. The cBN/SiO2 powder was densified by spark plasma sintering at 1873 K for 0.3 ks. The hexagonal boron nitride (hBN) phase was not observed in the cBN–SiO2 composites, indicating that the SiO2 nanolayer depressed the phase transformation from cBN to hBN. The relative density of cBN–SiO2 increased with increasing SiO2 content (). The highest hardness of the cBN–SiO2 composite was 17.5 GPa at and a load of 0.98 N.

Kinetic considerations of gas-phase adsorption of growth species on the c-BN(100) surface

The cubic phase of boron nitride (c-BN) is an extremely promising multifunctional material. However, to exploit all possible applications, a successful route for large area chemical vapor deposition (CVD) of c-BN films is required. Adsorption of gaseous growth species onto the c-BN surface is one of the key elementary reactions in CVD growth of c-BN. The present study has therefore focused on, by using density functional theory (DFT), the kinetics of the adsorption processes for BFx, and NHx species (x=0, 1, 2, 3) occurring on the H- of F-covered B- and N-terminated c-BN(100) surfaces.It was shown that BFx and NHx (x=0, 1, 2, 3) adsorption occurs without any energy barrier for all x. Adsorption of growth species on a monoradical surface site on the F-covered B-terminated c-BN(100)-(2×1) surface and the H-covered N-terminated c-BN(100)-(1×1) surface is therefore expected to be a highly facile process. However, decomposition of the incoming BF3 and NH3 growth species is needed for CVD growth of c-BN to occur.

Effect by Doping and Surface Termination on c-BN Surface Reactivity: A Theoretical DFT Approach

The cubic phase of boron nitride, c-BN, is a very promising material due to its very interesting and extreme properties, of which some are comparable or even superior to diamond. However, there are severe problems with the initial nucleation during vapor phase synthesis of c-BN thin films. Investigations of new possibilities for growth mechanisms are therefore of greatest importance to perform. It has experimentally been shown that the film quality will increase when a small amount of oxygen is added into the film during deposition. In the present study, the adsorption and abstraction energies of the surface terminating species, the geometrical structures, and the resulting electronic structures were studied for the situation when N (or B) was substituted by O (or C) atom in combination with different terminating species (H, F, or Cl) on the c-BN (100) surface. This theoretical study was performed by using first-principles functional theory (DFT) calculations under periodic boundary conditions. The results indicate that the reactivity of the B-rich c-BN (100)-2 × 1 surface is more pronounced when doping an H-terminated surface with O (or C). For a corresponding F-terminated surface, only the C-doped surface will result in a more exothermic adsorption process. Doping with O gives a slightly less exothermic adsorption process, when compared to the nondoped situation. The situation was completely different for the N-rich c-BN (100)-1 × 1 surface, where the H and F terminating species were found to chemisorb stronger for the nondoped situation than for the O (or C)-doped one. When Cl-terminating the surfaces, all the adsorption processes became endothermic and hence not possible to perform. The results from the DFT calculations also indicate that the extra electron density induced in the system by doping with O or C atoms will not stay in the nearest vicinity to the respective dopant, but will instead move closer to the nearest surface atoms. This overall increase in electron density was observed to either strengthen the surface-terminating bonds for the B-rich surface or weaken these bonds for the N-rich surface. The reactivity of the terminated surfaces, as obtained from the Fukui functions, revealed a much larger variation in electrophilic and nucleophilic susceptibility for the doped N-rich surfaces, as compared with the doped B-rich ones.

Plasma-Induced Erosion on Ceramic Wall Structures in Hall-Effect Thrusters

A 60 wt% boron nitride (BN)–40 wt% silica () hot-pressed composite, denoted as M26, was used as the insulating chamber wall for a xenon plasma Hall-effect thruster operated for approximately 2000 h at power levels between 1.5 and 5 kW. The chamber wall showed a range of erosion microstructures. In the heavily eroded regions, striations in the surface topology were evident with surface protrusions in the generic shape of conical tips consisting of stratified BN and silica phases. Microcracking along the long axis of the BN basal plane was prevalent. Through a two-dimensional finite element model, the microcracking has been determined to occur because of the anisotropic thermal expansion of BN in the amorphous silica matrix. Exfoliation accompanied the microcracking in BN and resulted in the preferential loss of BN as compared with silica in these heavily eroded regions.

Tribological Characterization of Boron Nitride Films against Pure-titanium for Microforming die Application

Boron nitride films were deposited onto Si substrates by hot filament chemical vapor deposition. The Tris(dimetylamino)borane (B[N(CH3)2]3) was used as source precursor, and the ammonia gas was used as the extra nitrogen source to increase the N concentration in the films. The bias was applied on the substrate stage to obtain the cubic phase of boron nitride. Structures and compositions of the films were investigated by energy dispersive X-ray (EDX) analysis and Raman spectroscopy. BN films contain hexagonal structures were deposited. To verify whether the films can be applied to the microforming die for forming pure titanium, the ball-on-disk test was carried out with pure titanium as the counterpart. The interfacial behavior between the films and pure titanium was also investigated by observe the wear track after the ball-on-disk test. The result shows that there was no titanium adhesion on the wear track when BN films were remained.

Three-Dimensional Metallic Boron Nitride

Boron nitride (BN) and carbon are chemical analogues of each other and share similar structures such as one-dimensional nanotubes, two-dimensional nanosheets characterized by sp(2) bonding, and three-dimensional diamond structures characterized by sp(3) bonding. However, unlike carbon which can be metallic in one, two, and three dimensions, BN is an insulator, irrespective of its structure and dimensionality. On the basis of state-of-the-art theoretical calculations, we propose a tetragonal phase of BN which is both dynamically stable and metallic. Analysis of its band structure, density of states, and electron localization function confirms the origin of the metallic behavior to be due to the delocalized B 2p electrons. The metallicity exhibited in the studied three-dimensional BN structures can lead to materials beyond conventional ceramics as well as to materials with potential for applications in electronic devices.

Development of self-lubricating composite materials of nickel with molybdenum disulfide, graphite and hexagonal boron nitride processed by powder metallurgy: preliminary study

The purpose of this work was to develop nickel self-lubricating composites using solid lubricant as second phase and obtained by powder metallurgy. The work involved the study of chemical stability and chemical interaction between the nickel alloy matrix and solid lubricating additives (molybdenum disulfide, graphite and hexagonal boron nitride). The results showed a thermodynamic stability during sintering for the composites containing the phases of graphite and hexagonal boron nitride. The presence of molybdenum disulfide phase permits a reaction with the matrix, the formation of a liquid phase of nickel sulfide and the solubilization of molybdenum in the matrix.

Investigation on mechanochemical synthesis of Al2O3/BN nanocomposite by aluminothermic reaction

Alpha-alumina–boron nitride (α-Al2O3–BN) nanocomposite was synthesized using mixtures of aluminum nitride, boron oxide and pure aluminum as raw materials via mechanochemical process under a low pressure of nitrogen gas (0.5MPa). The phase transformation and structural evaluation during mechanochemical process were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential thermal analysis (DTA) techniques. The results indicated that high exothermic reaction of Al–B2O3 systems under the nitrogen pressure produced alumina, aluminum nitride (AlN), and aluminum oxynitride (Al5O6N) depending on the Al value and milling time, but no trace of boron nitride (BN) phases could be identified. On the other hand, AlN addition as a solid nitrogen source was effective in fabricating in-situ BN phase after 4h milling process. In Al–B2O3–AlN system, the aluminothermic reaction provided sufficient heat for activating reaction between B2O3 and AlN to form BN compound. DTA analysis results showed that by increasing the activation time to 3h, the temperature of both thermite and synthesis reactions significantly decreased and occurred as a one-step reaction. SEM and TEM observations confirmed that the range of particle size was within 100nm.

Fabrication and characteristics of melt-spun Al ribbons reinforced with nano/micro-BN phases

Melt-spun aluminum ribbons with up to 5wt.% of embedded nano/micro boron nitride (BN) phases, namely multiwalled BN nanotubes (BNNTs) or BN microparticles (BNMPs), were fabricated by melt-spinning in an argon atmosphere. The comparative structural characteristics were analyzed using X-ray diffraction, scanning and transmission electron microscopy, and internal friction measurements as a function of temperature within an 80–800K range. Room temperature tensile tests were carried out on ribbons. These revealed reinforcing effects on pure Al-matrices after nano/micro-BN embedment for both added phases with the notably higher numbers peculiar to the BNNT-containing samples. The intrastructural interactions between BN additions and Al-matrices are discussed based on the structural analysis and the internal friction data.

Kinetic considerations of gas-phase abstraction of H and F from the c-BN(100) surface

The cubic phase of boron nitride (c-BN) is an extremely promising multifunctional material. To exploit all possible applications, large area chemical vapor deposition (CVD) of c-BN films is required. To maintain the cubic (sp3) structure of the surface atoms, the growing surface is covered with surface stabilizing species. However, the surface stabilizing species must also be able to undergo abstraction reactions with gaseous species and, hence, leave room for an incoming B- or N-containing growth species for a continuous c-BN growth to occur. The abstraction process is therefore a central elementary reaction step in CVD growth of c-BN. Hydrogen, H, and fluorine, F, have earlier been found to be promising as surface stabilizing species for both the B- and N-terminated c-BN(100) surfaces — the sp2 structure is maintained and both H and F bind strongly to the surface. In addition, of highest importance is the chemical capability to remove these terminating species from the surface, leaving a highly reactive surface site (i.e., gas-phase abstraction). The present study has therefore focused on, by using density functional theory (DFT), the kinetics of the H- or F-abstraction processes from the B- or N-terminated c-BN(100) surface.The energetic and structural evolution for gaseous F approaching a surface-binding F species, show that F radicals are not able to abstract chemisorbed F atoms, i.e., a gas-phase containing only F is unfavorable for growth of c-BN(100). On the other hand, H radicals are able to abstract chemisorbed H atoms. However, a minor barrier of energy was observed for the N-terminated surface (+13kJ/mol). The abstraction of F from the B-terminated surface, with gaseous H radicals, was found to be highly probable from both thermodynamic and kinetic considerations, being also the situation for the F-covered N-terminated surface (with a minor energy barrier of +8kJ/mol). In addition, the energy evolution for the approaching F to a surface H, clearly shows that any abstraction reaction will never take place. Hence, the results within the present study clearly show that at realistic deposition temperatures, it is only gaseous H that will have the capacity to remove H or F from the c-BN(100) surface.

Nanostructures of mixed-phase boron nitride via biased microwave plasma-assisted CVD

Boron nitride films and nanostructures are deposited by the reaction of B2H6 and NH3 in H2 via biased microwave plasma-assisted chemical vapor deposition. One-dimensional boron nitride nanostructures (BNNSs) are observed on the BN layers by the processing of diamond/BN multilayer under ion-bombardment and plasma etching. At a substrate bias of −100 V, the morphologies of the BNNSs evolve from dense film to nanocorns to nanotips, when the number of the diamond/BN bi-layer increases. Various phases of boron nitride including amorphous-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion E-BN, wurtzitic w-BN and cubic c-BN are detected in the BNNSs. The BN nanotips observed are considered to result from the tip structure growth from the combined effects of ion bombardment and plasma etching.

Erratum to: Growth Behavior of Cubic Boron Nitride (cBN) Phase in B-C-N Film Deposited on Si Substrate with Non-Uniform Ion Flux

B-C-N film with a ∼450 nm thickness was prepared on a (100) oriented silicon wafer (4 cm × 4 cm) by unbalanced magnetron sputtering of a B4C target with a unipolar pulsed DC substrate bias. At a substrate bias of −250 V, a cubic boron nitride (cBN) phase appeared in the B-C-N film when the nitrogen content was over 6.7 vol% in Ar-N2 reactive gases. For the B-C-N film deposited on unipolar pulsed DC biased Si substrate with Ar-16.7%N2 reactive gas, the content and formation behavior of the cBN phase along with the film thickness were critically dependent on the substrate position due to the difference in ion flux. In contrast to the circular central region of a substrate 2.5 cm in diameter, where cBN was nucleated and grown on hexagonal boron nitride (hBN) in a layered manner, the cBN phase was co-grown with the hBN phase and was surrounded by hBN at the circumference region of the circle. Only hBN was observed at the outer region of the circle. The microstructure of B-C-N film in which cBN is surrounded by hBN is believed to be effective in reducing residual stress developed by the nucleation of a cBN phase.

Improving performance of inoculating alloy wires by coating a boron nitride layer

The paper reports the deposition(by magnetron sputtering) and properties of polycrystalline boron nitride (BN) layers on commercial inoculating alloy wires. As is characterized by means of Fourier transform infrared(FTIR) spectroscopy, electron energy dispersive X-ray(EDX) spectroscopy and scanning electron microscopy(SEM), the thin BN layers consist of hexagonal and orthorhombic BN phases and are smooth without cracks. Organism transferring-circles experiments reveal that the adhesion between the BN layer and alloy wire is very good after tens of cycles. It is demonstrated that the BN layers covered wires are biomaterial lubricious and self-cleaning. As a result, BN layer would effectively enhance the function and efficiency of inoculating alloy wires, which could be widely applied to bio-experimentation and biomedicine apparatuses.

Growth behavior of cubic boron nitride (cBN) phase in B-C-N film deposited on Si substrate with non-uniform ion flux

Growth behavior of cubic boron nitride (cBN) phase in B-C-N film deposited on Si substrate with non-uniform ion flux

Fluid synthesis and structure of a new polymorphic modification of boron nitride

A new previously unknown phase of boron nitride with a hardness of 0.41–0.63 GPa has been pre-pared by the supercritical fluid synthesis. The presence of a new phase is confirmed by the X-ray spectra and IR absorption spectra, where new reflections and bands are distinguished. The fundamental reflection of the X-ray diffraction pattern is d = 0.286–0.291 nm, and the characteristic band in the infrared absorption spectrum is observed at 704 cm−1. The X-ray diffraction pattern and the experimental and theoretical infrared absorption spectra show that a new synthesized boron nitride phase can be a cluster crystal (space group 211) with a simple cubic lattice. Cage clusters of a fullerene-like morphology B24N24 with point symmetry O are arranged in lattice sites.

A novel superhard BN allotrope under cold compression of h-BN

Using first-principles calculations, we identify a new orthorhombic boron nitride (BN) phase (namely, P-BN; space group: Pmn21), which has similar topological structure to Bct-BN and Z-BN, but without the six-membered ring. This P-BN allotrope is energetically more favorable than previously reported Pnma-BN, Bct-BN and Z-BN phases. With only 0.06 eV/atom less stable than h-BN at ambient pressure, it can be formed from h-BN under cold compression at a low pressure of 4 GPa. The theoretical hardness and bulk modulus of the P-BN crystal are 403 GPa and 60.5 GPa, respectively, comparable to those of c-BN. Moreover, the P-BN phase along with Bct-BN and Z-BN are suggested as possible intermediate phases between h-BN and w-BN, which can be qualitatively explained by two empirical rules of Ostwald and Ostwald–Volmer.

Growth and Optical Properties of Explosion Phase Boron Nitride Octahedron Crystals

A low temperature and high pressure synthetic route assisted with ball milling has been established to prepare high quality orientated explosion phase boron nitride (E-BN) octahedron crystals using H3BO3, iron powders, NH4Cl, and Mg as starting materials. FSEM, TEM, HRTEM, SAED, XRD, Raman, FTIR, and XPS were employed for characterization of the morphology and structure of as-synthesized E-BN samples. The E-BN was identified as the face-centered cubic structure with a lattice parameter of 8.245 A. The typical particle sizes were mainly in the range of 30–120 nm. A UV spectrum indicated that the product has an obvious absorption band (5.61 eV). Photoluminescence spectra revealed that it has a dominant ultraviolet emission peak at 383 nm. The possible chemical reactions as well as the proposed mechanism of the growth of E-BN octahedron crystals were also discussed.