Direct nucleation of cubic boron nitride on silicon substrate

Cubic boron nitride thin films were deposited on silicon substrates by plasma chemical vapor deposition, and the nanostructure of the interface between the film and substrate was investigated by cross-sectional high-resolution transmission electron microscopy. Cubic boron nitride was found to nucleate directly on the properly pretreated silicon substrates in some local areas; in particular, cBN nuclei with a size of approximately 3 nm are nucleated on (111)–(100) step surfaces with an epitaxial relationship. This suggests the possibility of direct growth of cubic boron nitride in special surface environments of silicon substrates.     

Raman and photoluminescence spectra of indented cubic boron nitride and polycrystalline cubic boron nitride

The use of Raman spectroscopy, and in particular Raman line shifts, to measure stress in diamond and nitrides such as gallium nitride (GaN), is well known. In both diamond and GaN the application is principally to study stresses in thin films and at the substrate–thin film interface. Stresses in polycrystalline diamond composites have also been measured by this method. Typically stresses of the order of GPa can be determined with a spatial resolution of a few micrometers. In this paper, Raman spectra of indentations on cubic boron nitride (cBN) crystals and polycrystalline cubic boron nitride (PcBN) composites are presented. Shifts of the cBN Raman lines from their unstressed positions quantify the residual stresses in the boron nitride due to the deformation brought about by the indentation. Making use of the measured coefficient of shift of 3.39 cm⁻¹/GPa for the transverse optical Raman peak, these are of the order of 1 GPa. These measurements illustrate, for the first time, the use of Raman spectroscopy to study residual stresses in boron nitride. Plastic deformation is usually associated with the creation of vacancies. To investigate the possible presence of vacancy defects and vacancy-related defects, the indented boron nitride samples were also studied with photoluminescence spectroscopy.     

Tribological behaviors of polycrystalline cubic boron nitride sliding against silicon nitride in air and vacuum conditions

In order to evaluate the friction and wear properties of polycrystalline cubic boron nitride (PCBN) based on the drilling tools cutting, the ball-on-disk tribological experiments of PCBN sliding against silicon nitride (Si₃N₄) were carried out in air and vacuum conditions. The tribological behaviors were investigated by Scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and Nanomap-D three-dimensional White Light Interferometer. The results demonstrate that the coefficient of friction (CoF) is closely interrelated with the changing tendency of loads, where the CoFs gradually decrease with the growth of the load whether in air or in vacuum on account of a transfer film in air and a change from sliding friction to rolling friction in vacuum. Moreover, the CoF in vacuum condition is invariably greater than that in air under the similar load owing to friction heat. Furthermore, no observable abrasion appears on Si₃N₄ in air while severe abrasive wear is dominant on Si₃N₄ in vacuum. In addition, there is more intense adhesion on PCBN in vacuum than that in air. The reason is that the friction heat is gathered in vacuum condition with a confined environment.     

Nucleation of cubic boron nitride thin films

High-energy electrons (300 keV to 1 MeV) in a transmission electron microscope have been used to cause ballistic atomic displacements in hexagonal boron nitride. The high-resolution imaging capabilities of the TEM have allowed us to study the effect of the atomic displacements on the crystal structure of the BN. We report the formation of nanoarches — fullerene structures consisting of half of a BN nanotube capping the ends of the planar BN graphitic sheets. To form a basis of comparison between the high-energy electron bombardment and the ion bombardment typically used for cubic BN film growth, TRIM calculations were also performed to simulate Ar⁺ ion bombardment of hexagonal BN. A model is presented, indicating a process through which the nanoarches can serve as nucleation sites for the cubic phase of BN. The nucleation model is consistent with current experimental reports on the formation of cubic BN thin films.     

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.     

Influence of superstoichiometric boron on the synthesis of cubic boron nitride

Initiators of the synthesis of cubic boron nitride in the Mg-B-N and Li-B-N-O systems are compared. The influence of boron not bound in metal boronitrides and metal borates is investigated. It is proposed to distinguish “excess” boron introduced in the elemental form from “active excess” boron formed in the course of chemical transformations of initiators.     

Thermal shock resistance of polycrystalline cubic boron nitride

The effect of thermal shock on the flexural strength has been investigated experimentally. It was found that the variation in flexural strength with quench temperature was influenced by the CBN grain size. Polycrystalline material containing small CBN grains showed a discontinuous drop in measured flexural strength above a material dependent critical quench temperature difference, ΔT_c. The sharp decrease in measured strength is accompanied by unstable crack propagation. Material containing a significantly larger CBN grain size exhibited a gradual decrease in strength above the critical quench conditions. The experimental observations agreed with an established theory developed for thermal shock of alumina. The theoretically calculated critical temperatures agree well with the observed experimental data for each material when a flaw size equal to the CBN grain size is employed.     

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si₃N₄) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH₂-BNNSs) and Si₃N₄ powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si₃N₄ composites were obviously enhanced by adding NH₂-BNNSs. The fracture toughness and bending strength of Si₃N₄ composites added 0.75 wt% NH₂-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si₃N₄. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si₃N₄ ceramics are also improved after the addition of NH₂-BNNSs.     

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.     

Influence of boron impurities on the crystal structure of cubic boron nitride

The influence of boron impurities on the unit cell parameters of the boron nitride with a sphalerite structure is investigated. It is demonstrated that the presence of boron in amounts exceeding the stoichiometric composition leads to a distortion of the cubic lattice and reduces its symmetry. The revealed oriented distortion of the crystal lattice of the cubic boron nitride is explained by the incorporation of excess boron atoms not in a random manner but between paired corrugated hexagonal layers. Original Russian Text ? S.P. Bogdanov, 2008, published in Fizika i Khimiya Stekla.     

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.     

From design to characterization of zirconium nitride/silicon nitride nanocomposites

ZrN/Si₃N₄ nanocomposites have been prepared by chemically crosslinking two polysilazanes with a zirconium-based compound and subsequent heat-treatment at temperatures ranging from 1000 to 1600 °C. The polymer synthesis has been systematically investigated using FT-IR, solid-state NMR, and elemental analyses. Then, the pyrolysis under ammonia at 1000 °C trigering the thermo-chemical polymer-to-ceramic conversion was examined, leading to X-ray amorphous ceramics with yields governed by the chemistry of the neat polysilazane. Investigations of the structural evolution of the single-phase amorphous ceramic network above 1000 °C by X-ray diffraction and Raman spectroscopy pointed out that the ZrN phase already segregated at 1400 °C and formed highly crystalline ZrN/Si₃N₄ nanocomposites at 1600 °C. HRTEM investigations validated the unique nanostructural feature of the nanocomposites made of ZrN nanocrystals distributed in α- and β-Si₃N₄ phases. Our preliminary investigations of the optical properties showed that these structural changes allowed tuning the optical properties of ZrN/Si₃N₄ nanocomposites.     

Effects of silicon incorporation on composition,structure and electric conductivity of cubic boron nitride thin films

We have achieved in-situ Si incorporation into cubic boron nitride (c-BN) thin films during ion beam assisted deposition. The effects of silicon incorporation on the composition, structure and electric conductivity of c-BN thin films were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and electrical measurements. The results suggest that the content of the cubic phase remains stable on the whole with the incorporation of Si up to a concentration of 3.3 at.%, and the higher Si concentrations lead to a gradual change from c-BN to hexagonal boron nitride. It is found that the introduced Si atoms only replace B atoms and combine with N atoms to form Si–N bonds, and no evidence of the existence of Si–B bonds is observed. The resistance of the Si-doped c-BN films gradually decreases with increasing Si concentration, and the resistivity of the c-BN film with 3.3 at.% Si is lowered by two orders of magnitude as compared to undoped samples.     

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.     

Reaction bonded aluminum oxide composites containing cubic boron nitride

The properties of alumina can be improved by incorporating second phases like zirconia or carbides. It has also been reported that reaction bonded aluminum oxide (RBAO) process can be used as a host matrix for large scale second phase reinforcement particles without causing harmful residual stresses normally encountered with shrinking matrix materials. The potential of using cubic boron nitride in reinforcing alumina has not yet been reported. This work reports some improvements in mechanical properties of alumina achieved by incorporating cubic boron nitride particles in a reaction bonded aluminum matrix. Attrition milled aluminum, alumina and cubic boron nitride powders were heat treated to 800 °C in air followed by sintering to 1300 °C in argon. Sintered samples were found to have better density, hardness and fracture toughness compared to conventionally sintered samples of same composition.     

Ionoluminescence in CVD diamond and in cubic boron nitride

Using the new ion beam-induced luminescence (IBIL) apparatus in National Legnaro Laboratories, Italy, a series of measurements concerning both wide-area luminescence spectra and monochromatic luminescence maps with a space resolution of a few μm has been carried out on several CVD diamond and c-BN samples. Protons of 2 MeV with a penetration depth of approximately 25 μm have been used in order to investigate the materials in the bulk. These measurements have been correlated with particle-induced X-ray emission (PIXE) and EPR data. The measurements have been performed at increasing proton doses in order to also investigate the radiation hardness of luminescence peaks. The results indicate that ionoluminescence of CVD diamond is dominated by three bands at approximately 2, 2.4 and 2.9 eV, with the intermediate band being very radiation-hard, and the other two radiation-weak. The band at 2 eV is correlated with N content, and is particularly high in samples with poor electronic properties. IBIL in c-BN is also dominated by three bands, one at approximately 2 eV, and the other two at higher energies with respect to CVD diamond. All these three bands seem to be relatively radiation-hard with respect to CVD diamond, and to be related to defects induced by doping.     

Electron-beam-induced currents on beryllium-doped cubic boron nitride single crystal

The electrical properties of a beryllium-doped cubic boron nitride (c-BN) single crystal grown on (111) diamond were investigated by using electron-beam-induced current measurements as well as current–voltage (I–V) and capacitance–voltage measurements. The I–V measurements through silver electrodes on c-BN showed non-linear characteristics, revealing Schottky behavior. From the temperature dependence of resistivity, the activation energy of 0.24 eV was obtained. We observed electron-beam-induced currents under the silver electrode, which means that a depletion region was formed due to the Schottky barrier. The diffusion length of minority carriers was determined by scanning the electron beam across the Schottky contact. The transient current after the electron-beam pulse was also measured at various temperatures.     

Interfacial characterization of silicon nitride/silicon nitride joints brazed using Cu-base active metal interlayers

Silicon nitride/silicon nitride joints were vacuum brazed at 1317 K for 5 min and 30 min using ductile Cu-base active metal interlayers. The joints were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron back scattered diffraction (EBSD), and transmission electron microscopy (TEM). An inhomogeneous Ti-rich reaction layer (～2–3 μm thick) formed in 5 min at the Si₃N₄/braze interface. The inhomogeneity disappeared after brazing for 30 min and was replaced with a compact and featureless reaction zone. TEM studies revealed fine grains in the reaction layer, and larger grains in the inner part of the joint interfaces. The joints were crack-free and presented features associated with plastic deformation, which indicated accommodation of strain associated with CTE mismatch. Electron Backscatter diffraction (EBSD) revealed a highly textured braze alloy interlayer and its crystallographic orientation was determined. The formation of additional phases at the joint interface during brazing is discussed.