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.     

Cubic boron nitride films for industrial applications

The effects of kinetic energy, chemical nature of substrates and temperature on the synthesis of cBN films are explored to obtain cBN films with industrial quality. Carbon including amorphous carbon, nanocrystalline and polycrystalline diamond enables deposition of stable, thick and adherent cBN films with characteristic Raman signature. Although temperature has been designated as an unimportant parameter, the deposition at higher temperatures yields higher quality of cBN films. The higher temperature (800 °C) was also employed at cBN deposition on diamond coated tungsten carbide (WC) cutting inserts using plasma enhanced chemical vapor deposition (PECVD). The quality of cBN films grown by PECVD significantly overcomes that prepared by physical vapor deposition (PVD) which is affected in large extent by the lower kinetic energies of particles used in PECVD. The low kinetic energy of particles induces surface growth mechanism which differs from the growth models previously proposed.     

纳米六方氮化硼负载离子液体润滑添加剂的摩擦学特性

制备了超声剥离的六方氮化硼负载油酸咪唑离子液体(OL-IL/h-BN)润滑添加剂,利用SEM、TEM、FTIR、TG及XRD等对其结构和形貌进行了表征。利用Zeta电位测试方法表征了其在聚乙二醇400中的分散性能,结果发现分散稳定性得到大幅提升。利用四球摩擦磨损试验机测试了摩擦学性能,结果显示减摩抗磨性能都优于h-BN。利用SEM和XPS对其磨斑表面形貌和成分进行了分析,XPS分析结果显示表面膜中含有化学吸附膜以及Fe2O3和B2O3等化学反应膜,这些膜的形成对减摩抗磨性能起到至关重要的作用。     

Optical and electrical properties of boron nitride oxide films

Polycrystalline boron nitride oxide (BNO) films are synthesized by RF magnetron sputtering. It is found that the bandgap of the BNO film increases with increasing oxygen composition. The bandgap energy as wide as 5.5 eV is obtained with oxygen composition of 17%. The electrical resistivity is estimated to be as high as 10¹³ Ωcm. Metal/BNO/metal structures are fabricated with various metals such as Ni, Cu and Al, and electrical characterization are performed for metal/BNO contacts. The true Schottky barrier heights are estimated for metal/BNO contacts. The true Schottky barrier height decreases with increasing metal work function. This behavior of the Schottky barrier height suggests that the BNO film has p-type electrical conduction.     

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.     

Thermal conductivity of polyimide/boron nitride nanocomposite films

The thermal conductivity of polyimide/boron nitride (PI/BN) nanocomposite thin films has been studied for two sizes of BN nanofillers (40 and 120 nm) and for a wide range of content. A strong influence of BN particle size on the thermal conduction of PI has been identified. In the case of the largest nanoparticles (hexagonal‐BN), the thermal conductivity of PI/h‐BN (120 nm) increases from 0.21 W/mK (neat PI) up to 0.56 W/mK for 29.2 vol %. For the smaller nanoparticles (wurtzite‐BN), PI/w‐BN (40 nm), we observed two different behaviors. First, we see a decrease until 0.12 W/mK for 20 vol % before increasing for higher filler content. The initial phenomenon can be explained by the Kapitza theory describing the presence of an interfacial thermal resistance barrier between the nanoparticles and the polymer matrix. This is induced by the reduction in size of the nanoparticles. Modeling of the experimental results allowed us to determine the Kapitza radius a_K for both PI/h‐BN and PI/w‐BN nanocomposites. Values of a_K of 7 nm and >500 nm have been obtained for PI/h‐BN and PI/w‐BN nanocomposite films, respectively. The value obtained matches the Kapitza theory, particularly for PI/w‐BN, for which the thermal conductivity is expected to decrease compared to that of neat PI. The present work shows that it seems difficult to enhance the thermal conductivity of PI films with BN nanoparticles with a diameter <100 nm due to the presence of high interfacial thermal resistance at the BN/PI interfaces. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42461.     

Improvement of the adhesion of sputter-deposited cubic boron nitride films

Cubic boron nitride (cBN) thin films were grown on Si(100) and high-speed steel substrates by reactive r.f. sputtering in an Ar/N₂ discharge using an electrically conducting boron carbide (B₄C) target. The substrate electrode was grounded or operated either with a d.c. or an r.f. power supply. The deposition of cBN can be subdivided into three steps: (1) the growth of a thin, textured, hexagonal boron nitride (hBN) film, (2) the nucleation of cBN and (3) the growth of the cBN phase. As a measure of the cBN content, the ratio of the infrared absorption bands near 1100 cm⁻¹ (cBN) and 1400 cm⁻¹ (hBN) was used. The adhesion of cBN films is still an unsolved problem. Two aspects have to be considered: (1) the high intrinsic stress of the film and (2) the reactivity under humid conditions. We investigated the influence of the thickness, structure and surface roughness of hBN on the adhesion of cBN films. To modify the hBN films, the pressure, substrate bias and Ar/N₂ mixture was varied. Another way of improving the adhesion is plasma treatment of the cBN film directly after deposition. The process variations mentioned above increase the thickness of the adhering cBN films.     

Infrared and Raman analysis of plasma CVD boron nitride thin films

We present an optical investigation, by means of polarized IR reflectivity and Raman scattering, of the quality and stability of PECVD grown BN films obtained on the anode and on the cathode of a parallel-plate radio-frequency reactor by glow discharge decomposition of two different gas mixtures, B₂H₆-H₂-NH₃ and B₂H₆-N₂. The addition of Ar to B₂H₆-N₂ has also been investigated. It is shown that polarized IR reflection spectra of thin films have a complex optical behaviour, which is a function of the oblique angle of incidence and the substrate material used in the deposit. However, these spectra are predictable when optical theory is applied. The results obtained show the capabilities of polarized IR reflection and Raman scattering to perform detailed investigations on the microstructure and aging of BN thin films, independently of the substrate material.     

Microstructure and mechanical properties of pulsed laser deposited boron nitride films

Boron nitride films were prepared by pulsed laser ablation from a boron nitride target using a KrF-excimer laser, where the growing films were deposited in nitrogen atmosphere or bombarded by a nitrogen/argon ion beam. Films deposited without or at weak ion bombardment (such films will be called l-BN in this paper) are hexagonal with amorphous to turbostratic microstructure (l-BN) and show high adhesive strength to silicon and stainless steel substrates. By using them as intermediate layers, the adhesion of pure cubic boron nitride films (c-BN) can significantly be improved. l-BN films and l-BN/h-BN/c-BN layer systems have been investigated by in-situ ellipsometry, infrared spectroscopy and cross-section and plan-view high-resolution transmission electron microscopy, including diffraction. The mechanical properties, i.e. stress and hardness, of these films and layer systems are presented. l-BN films deposited at higher laser energy densities have compressive stresses as high as 11.5 GPa. Films deposited at lower laser energy densities have stresses in the range of 4.7 to 1.3 GPa and a Vickers hardness in the range of 18.6 to 7.5 GPa depending on substrate temperature and ion bombardment. The compressive stresses of 400 nm thick adherent c-BN films were estimated to be 4.5 GPa.     

Growth,doping and applications of cubic boron nitride thin films

This article reviews the growth techniques and growth parameters for the formation of cubic boron nitride (c-BN) thin films. It is generally accepted that the impact of energetic ions or neutral atoms is crucial to achieve c-BN film growth. Furthermore, c-BN nucleation is only observed within certain thresholds for the deposition parameters, including the substrate temperature. However, the temperature threshold exists only for the nucleation and not for the growth of c-BN. We will show that three independent characterization methods are necessary for the non-ambiguous identification of the c-BN phase within BN films. The cylindrical thermal spike model developed to describe the formation of diamond-like phases by ion deposition can be used to explain the c-BN nucleation and growth. A short introduction and the basic ideas of this model will be given. Finally, possible tribological applications of c-BN films and the doping of c-BN films will be discussed.     

Removal of sp2-boron nitride transition layer in the growth of cubic boron nitride films

Practical application of cubic boron nitride (cBN) films is known to be hindered by its bad adhesion to substrates. One reason is that an sp²-bonded boron nitride (sp²-BN) transition layer is formed on the substrate surface at the early stage of deposition prior to the nucleation of cBN, which weakens the link between the cBN-rich layer and the substrate. In this study, we demonstrated the feasibility of removing this sp²-BN layer, by replacing it with a zirconium- (Zr-) rich composite layer. The method is to deposit a multilayer of Zr layer/sp²-BN layer/cBN-rich layer at 680 °C on a tungsten carbide substrate, followed by annealing the structure at 850 °C for 1 h. X-ray photoelectron spectroscopy, transmission electron microscopy and electron energy loss spectroscopy analyses showed that the Zr metal layer and sp²-BN layer reacted completely, to produce a composite interfacial layer consisting of metal Zr, Zr nitride, boride and oxide. The depth profiles of the elemental distributions and chemical states were investigated and discussed to reveal the diffusion of the elements associated with this deposition process.     

Hard boron rich boron nitride nanoglasses

Boron‐rich amorphous boron nitride (B_xN_1?x, 0.55 ≤ x ≤ 0.95) alloys are generated by means of ab initio molecular dynamics simulations and their local structure, mechanical properties and electronic structure are exposed. BN:B phase separations are perceived in all amorphous networks, suggesting that these materials can serve as nanoglass ceramics. The sp² hybridization is the main building unit in the BN‐rich regions for low boron concentrations, and the models carry locally the signature of the two‐dimensional hexagonal BN structure. The amorphous states having both sp² and sp³ hybridizations form for boron contents between 70% and 80%. At higher boron concentrations, sp³ hybridization with a fraction of ~90%‐98% is detected as seen in the cubic or wurtize BN crystals. In the boron rich regions, the ideal and defective pentagonal pyramids emerge at 60% boron content, and the first complete B₁₂ molecule develops at 70% boron concentration. In addition to the B₁₂ icosahedron, the formation of a cage‐like B₁₆ molecule is, for the first time, discovered in some amorphous alloys. The isolated B₁₆ molecule is, however, found to be unstable. The Vickers harness calculations reveal that some of these amorphous alloys can serve as hard materials. When their electron properties are considered, all amorphous materials are predicted to be semiconducting.     

The electrical and thermal properties of polyimide/boron nitride nanocomposite films

In the paper, the polyimide (PI)/boron nitride (BN) nanocomposites were prepared by in situ polymerization and exhibited enhanced electrical property and thermal stability. The structure of synthesized PI was confirmed by scanning electron microscopy, energy dispersive spectrometer, and Fourier transform infrared. The influence of doping concentrations on the relative permittivity, electrical conductivity, loss tangent, corona-resistant lifetime, and thermal stability of PI composites was investigated. Results showed that the relative permittivity of PI/BN composites increases after doping BN nanoparticles. It was noteworthy that both the electrical conductivity and loss tangent of PI composites were enhanced in low frequency (0–3000 Hz) and the situations were shifted in high frequency (>3000 Hz). It was observed that the corona-resistant lifetime of PI/BN composite with 20 wt% BN increases more than eight times. Moreover, significant improvements in the thermal stability of PI composites were achieved by addition of only a small amount of BN. The decomposition temperatures at 5 and 10% weight loss were 518.7 and 551.6 °C for 15 wt% doped PI/BN composite, respectively, which increases by 37.3 and 40.5 °C compared to those of pure PI. The resulting properties expand further the application range of polyimides.     

Tribological and thermal properties of hexagonal boron nitride filled high‐performance polymer nanocomposites

High‐performance polymer nanocomposites based on poly(aryletherketone) (PAEK) as matrix and hexagonal boron nitride (h‐BN) nanopowder as reinforcement were fabricated using planetary ball mill followed by hot pressing. The addition of h‐BN (0–5 wt %) to the matrix enhanced the microhardness and thermal stability compared to pure matrix. For a constant sliding speed, the wear rate of the nanocomposites determined by using Pin‐on‐Disk tribometer was reduced approximately 22 times compared to pure matrix. The coefficient of friction of the nanocomposites is slightly increased but it is stable compared to that of pure matrix. It was also investigated that the thermal stability of the debris of the nanocomposites was decreased compared to the pure matrix and its nanocomposites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44409.     

Deposition of cubic boron nitride films on diamond-coated WC:Co inserts

Cubic boron nitride (cBN) thin films were deposited on diamond-coated tungsten carbide (WC) cutting inserts using electron cyclotron resonance (ECR) microwave plasma chemical vapor deposition (MPCVD). The effects of gas flow rate and substrate bias on the phase composition and structure of the BN films deposited on diamond surfaces were studied. It was revealed that both the cubic phase formation and the selective etching of hexagonal phase were controlled by modulating the hydrogen and boron trifluoride flow rate ratio. By the trial and error method the gas flow rate ratio and substrate bias voltage were optimized. Moreover the phase composition of the BN film was found to be affected by the thickness of diamond buffer layer and interrelated to the effective substrate bias. The hardness of the resulting cBN films reached the value of 70 GPa. In the synthesized coatings, the diamond beneath renders the best mechanical supporting capacity while the top cBN provides the superior chemical resistance and extreme hardness. The cBN/diamond bilayers deposited on WC inserts may serve as universal tool coatings for machining steels and other ferrous metals.     

Ab initio study of boron nitride at high pressures

The results of first-principles calculations of the elastic, dielectric and dynamical properties for wurtzite BN are presented. The determination of the ground-state properties was performed within the local density approximation to the density functional theory, while the density functional perturbation theory is employed to derive the Born effective charges, high-frequency dielectric constants, and the phonon frequencies and eigenvectors. The influence of the pressure on the elastic and dielectric properties is examined and discussed. The pressure dependence of vibration modes was also investigated, leading to the determination of pressure coefficients. Our results agree generally well with the available data in the literature.     

Ion implantation induced phase transformation in carbon and boron nitride thin films

The mechanism behind energetic ion impact induced stress reduction in highly stressed tetrahedral amorphous carbon and cubic boron nitride thin films is investigated by real time in situ spectroscopic ellipsometry and ex situ electron microscopy. Highly stressed carbon and boron nitride films were grown by filtered cathodic vacuum arc and RF magnetron sputtering, respectively. The films were then implanted by 5–10 keV argon ions and the film optical properties and thickness monitored in situ by spectroscopic ellipsometry. In both cases the films were observed to expand due to a reduction in the density of the ion-modified layer. Cross-sectional transmission electron microscopy and electron energy loss spectroscopy of the carbon films showed that this reduction in density is associated with a conversion of diamond-like bonding to graphite-like bonding. In situ stress measurements performed on the boron nitride films revealed a simultaneous reduction in stress with expansion of the material.     

Room temperature synthesis of boron nitride thin films by dual-ion beam sputtering deposition

Boron nitride (BN) films are prepared by dual-ion beam sputtering deposition at room temperature (~25 °C). An assisting argon/nitrogen ion beam (ion energy E_i=0–300 eV) directly bombards the substrate surface to modify the properties of the BN films. The effects of assisting ion beam energy on the characteristics of BN films were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectra, atomic force microscopy, and optical transmittance. The density of the B–N bond in the film increased with the increase in assisting ion beam energy. The highest transmittance of more than 95% in the visible region was obtained under the assisting ion beam energy of 300 eV. The band gap of BN films increased from 5.54 eV to 6.13 eV when the assisted ion-beam energy increased from 0 eV to 300 eV.     

Carbothermic formation of boron nitride

Formation of boron nitride by reaction of boric oxide with carbon and nitrogen was studied. It was found from the results of experiments conducted by holding B₂O₃-activated C mixtures under a flowing nitrogen atmosphere that formation of boron nitride was complete in 120 min at 1500 °C. After cleaning the reaction product from the ash of the activated carbon and from the unreacted B₂O₃ pure BN powder was obtained. B₄C was found to exist as an intermediate species in the reaction products of the experiments in which BN formation was not complete. The results of experiments conducted with the objective of gaining an insight into the reaction mechanism by using different geometrical arrangements show that liquid B₂O₃ and solid carbon need not be in contact in the formation of BN from B₂O₃, C and N₂ and indicate that the reaction proceeds through a gaseous boron containing species which is most probably B₂O₃(g).     

High-temperature oxidation of boron nitride

The oxidation resistance of boron nitride samples obtained by various methods and having different crystalline structures has been studied by thermogravimetric, differential thermal and IR spectral analyses. The real structure of a material being oxidized is of the greatest importance for the steady-state process of high-temperature oxidation. During the boron nitride oxidation, secondary processes are observed, associated with the rearrangement of the wurtzite-like BN lattice into a thermodynamically more stable hexagonal lattice of the graphite-like boron nitride. Nitrogen dioxide formation in the gas phase has been shown. Physico-chemical properties of the scale (B₂O₃ vaporization) impose limitations on the use of boron nitride-based materials in oxidizing media at temperatures higher than 1300°C.