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.     

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.     

On the low-temperature threshold for cubic boron nitride formation in energetic film deposition

The sharp threshold in substrate temperature below which cubic boron nitride (cBN) cannot be formed in energetic film-deposition processes was investigated. We found that cBN could be synthesized below the threshold temperature on top of cBN that had been previously formed above the threshold temperature. That the initial nucleation of cBN is more strongly dependent on temperature than its subsequent growth is suggested. How the structure of the sp²-bonded BN that accompanied cBN growth changed with temperature was also investigated. Lowering the substrate temperature decreased the local ordering within the graphitic planes, and below the threshold temperature the separation of the graphitic planes increased dramatically. How these structural changes may influence the nucleation of cBN is discussed.     

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.     

Micro-sized polycrystalline cubic boron nitride with properties comparable to nanocrystalline counterparts

High-performance polycrystalline cubic boron nitride (PcBN) was sintered without binders at 1500 °C in a pressure range from 11 to 15 GPa using commercial micrometre cubic boron nitride (cBN) with a diameter of approximately 2–4 μm. The results demonstrated that the sample sintered at 12 GPa and 1500 °C had the best mechanical properties and thermal stability. Its average Vickers hardness, fracture toughness, and thermal stability was 63 GPa, 15 MPa m^1/2, and 1315 °C, respectively. The considerable improvement in the mechanical properties was mostly attributed to the high compactness, close bonding between grains, and the sample's internal defect structures. The relatively small specific surface area of the micron grains provides an advantage due to its high thermal stability. The amorphous regions observed in the sample's local areas may provide a new strengthening mechanism under high pressure and temperature.     

A novel route to superhard nanocrystalline cubic boron nitride: Emulsion detonation and high-pressure high-temperature transformation-assisted consolidation

Synthesizing bulk nanocrystalline materials is challenging since grain growth should be suppressed whereas densification promoted. Here, we demonstrate a novel route to synthesize superhard bulk nanocrystalline cubic boron nitride (cBN), which combines the use of emulsion detonation and high-pressure high-temperature transformation-assisted consolidation. The emulsion detonation process activates BN to possess unique structure and chemistry, i.e. wurtzitic BN nanograins in hexagonal BN matrix with enhanced structural disordering and oxygen impurity, a combination that enhances the nucleation rate of cBN and its densification leading to the formation of bulk nanocrystalline cBN at reduced conditions. The cBN, synthesized at 7.5 GPa and 1800 °C, displayed Vickers hardness values of 50?62 GPa for 5?20 N loads. The findings in the study suggest a feasible solution to synthesize bulk nanocrystalline cBN in a more scalable way, while also providing design insights on how to refine grain growth while enhancing densification to synthesize bulk nanocrystalline materials.     

Near-edge X-ray absorption fine structure spectroscopy of arcjet-deposited cubic boron nitride

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was employed to help determine the structure of boron nitride films grown by bias-enhanced chemical vapor deposition in a low-density supersonic arcjet flow. BN films containing 0.90% cubic boron nitride were analyzed by NEXAFS and compared with c-BN and h-BN reference spectra. The mainly cubic films have been shown previously to be nanocrystalline, which leads to the inability to obtain structural information from Raman scattering spectra. However, with NEXAFS, the nanocrystalline nature of the films does not strongly affect the structural interpretation. It is shown that films deposited with a bias of −75 V are primarily sp³ bonded. This high sp³ bonding character agrees with previous measrements based on infraredtransmission and reflectance spectroscopy, as well as X-ray photoelectron spectroscopy.     

Cubic boron nitride film residual compressive stress relaxation by post annealing

The authors demonstrate that the residual compressive stress in cubic boron nitride films could be relaxed by 1500 K post annealing in H₂ atmosphere. According to the IR peak shifting, approximately 4.5 GPa stress was relaxed after 4 hours annealing. Thus film adhesion was improved significantly, cubic boron nitride films with a cubic phase concentration of 90% (vol%) and a thickness of more than 200 nm showed excellent stability and no delaminations were observed even after annealing for over 30 months in the open air, while films without annealing delaminated from substrates within 1 week. Moreover, the relaxation of the compressive stress is accompanied with cubic boron nitride d (111) interplanar distance broadening and corresponding IR peak intensities increasing.     

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.     

Joining interface and compressive strength of brazed cubic boron nitride grains with Ag-Cu-Ti/TiX composite fillers

Ag-Cu-Ti/TiX (TiX=TiB₂, TiN or TiC) composite filler materials, instead of pure Ag-Cu-Ti alloy, were developed to improve the comprehensive mechanical performance of brazed joints of cubic boron nitride (CBN) grains/bonding layer/steel matrix. This article mainly concerns the effects of TiX addition on the joining interface and compressive strength of brazed CBN grains. The results demonstrate that, due to the variation of chemical activity of Ti atoms induced by TiX addition into the brazing system, the brazing reactions, especially chemical resultants produced at the joining interface between CBN grain and Ag-Cu-Ti alloy, are restrained to some extents. In general, the TiN particles show the greatest suppression effect on the brazing resultants, while the TiC particles have the weakest effect, and TiB₂ particles have the medium effect. The optimum reinforcement of the composite filler is finally determined as the TiB₂ particles with the content of 8 wt%, with which the average compressive strength of brazed CBN grains reaches 15 N, which is nearly the same high as that of original CBN grains.     

Processing and properties of polycrystalline cubic boron nitride reinforced by SiC whiskers

Dense polycrystalline cBN (PcBN)–SiCw composites were fabricated by a two-step method: First, SiO₂ was coated on the surface of cubic boron nitride (cBN) particles by the sol-gel method. Then, silicon carbide whisker (SiC_w)- coated cBN powder was prepared by carbon thermal reaction between SiO₂ and carbon powders at 1500°C for 2 hour. Then, cBN–SiC_w complex powders were sintered by high-pressure and high-temperature sintering technology using Al, B, and C as sintering additives. The phase compositions and microstructures of cBN–SiC_w composites were investigated by X-ray diffraction and scanning electron microscopy, respectively. It was found that the SiC_w and Al₃BC₃ had been fabricated by in situ reaction, which cannot only promote densification but also improve mechanical properties. The relative density of PcBN composites increased from 96.3% to 99.4% with increasing SiC_w contents from 5 to 20 wt%. Meanwhile, the Vickers hardness, fracture toughness and flexural strength of as-obtained composites exhibited a similar trend as that of relative density. The composite contained 20 wt% of SiC_w exhibited the highest Vickers hardness and fracture toughness of 42.7 ± 1.9 GPa and 6.52 ± 0.21 MPa•m^1/2, respectively. At the same time, the flexural strength reached 406 ± 21 MPa.     

3D characterization of cubic boron nitride (CBN) composites used as tool material for high precision abrasive machining processes

Cubic boron nitride (CBN) composites are widely used as cutting tool materials for high precision abrasive machining processes. They are composed of super hard CBN abrasives and a softer binder. CBN abrasives are one of the hardest materials. They are embedded in the binder which can be metallic, polymeric or ceramic. The binder supports the abrasives and offers suitable toughness. The two components are consolidated by sintering processes under high pressure and temperature. Hence, abrasive particles exhibit an irregular spatial distribution in terms of size, location and orientation. In this work, X-ray computed tomography (CT scan) is used to investigate the geometrical properties of CBN abrasives in the volume regarding quantity, dimension and shape. A three-dimensional (3D) model is generated and the CBN abrasives are correspondingly characterized. The contribution includes both detailed explanation of CT scan and 3D modeling implementation, as well as quantification analysis of the key microstructural features for CBN composites.     

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

High‐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.m^1/2. Vickers hardness of 49 ± 1 GPa and fracture toughness of 4.6 ± 0.1 MPa.m^1/2 was achieved with a precursor that contained milled hBN and 50 vol. % of 0.5 μm diameter cBN crystals.     

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.     

Chemical vapor deposition of pyrolytic boron nitride ceramics from single source precursor

Pyrolytic boron nitride ceramics were prepared on graphite substrates from borazine as the single source precursor by hot-wall chemical vapor deposition in deposition temperature range from 1300 °C to 1600 °C with a total pressure of 200 Pa. The chemical composition and the effect of deposition temperature on the morphology, phase, and structure of the pyrolytic boron nitride were investigated. A high purity product with stoichiometric B/N ratio is obtained. The deposition surface of the product exhibited a pebble-like structure, and the fracture surface showed an apparent laminar structure having a preferential (002) orientation parallel to the surface of the substrate at temperatures above 1400 °C. The product contained some turbostratic and amorphous boron nitride as evidenced from XRD and FTIR examinations. With the increase of deposition temperature, the crystallinity of the pyrolytic boron nitride increased with the turbostratic and amorphous boron nitride turned into hexagonal structure, and the crystallinity of the product became higher.     

Pressure infiltration of boron nitride preforms with molten aluminum

The infiltration of compacted cubic BN (cBN) with molten aluminum has been investigated as a potential route for a cheap and easy method of manufacturing cBN/metal composites. CBN compacts have been infiltrated with molten Al at a temperature between 670 and 800 °C and pressure of 15 MPa in vacuum. At these temperatures no pronounced interactions between hexagonal and cubic BN with Al was observed, allowing the complete infiltration of cBN with 12 μm mean grain size. After infiltration at 800 °C, the temperature was increased without pressure to convert aluminum into borides and AlN. The hardness of the resulting materials depends on the content of hexagonal, cubic BN and the rate of conversion of Al into borides and AlN. The infiltration height of less than 1 mm obtained from infiltrating the 3 μm cBN powder green compacts gave a hardness of 22.0 ± 0.6 GPa after heat treatment.     

Synthesis of ultrafine nano-polycrystalline cubic boron nitride by direct transformation under ultrahigh pressure

Using a turbostratic pyrolytic boron nitride as a starting material, we synthesized a variety of ultrahard polycrystalline cubic boron nitride (PcBN) as a function of the heating duration changing from 1 to 60?min under a constant temperature and pressure conditions (1950?°C and 25?GPa) using a multi-anvil apparatus. When the heating duration was less than 13?min, ultrafine nano-polycrystalline cBN (U-NPcBN) with the mean grain size of <50?nm was produced. Among these U-NPcBNs those synthesized with 11–13?min were found to have a uniform texture composed purely of cBN (i.e. with no wurzite BN residue) and a Knoop hardness of >53?GPa, which is 20% higher than that of the hardest conventional binderless PcBN in practical use. Furthermore, the PcBNs synthesized with 18–20?min showed a unique nanocrystalline texture composed of relatively coarse grains dispersed in a fine grained matrix and even higher Knoop hardness (54.5–55.2?GPa).     

Micromorphology and structure of pyrolytic boron nitride synthesized by chemical vapor deposition from borazine

Pyrolytic boron nitride (PBN) plates were synthesized by chemical vapor deposition (CVD) with temperatures of 900–1900?°C and total pressures of 50–1000?Pa on graphite by using borazine as the precursor. The effects of temperature and pressure on the micromorphology and crystal structure of the PBN were investigated. The as-deposited PBN possessed three typical types of micromorphologies depending on the deposition condition. PBN with dense and laminated structure (Type A) were deposited at temperatures of 1150–1900?°C with relative low pressures of 50–200?Pa, and PBN with porous and isotropic structure (Type C) was deposited at temperatures above 1100?°C with higher pressures above 250?Pa. PBN with dense and glass-like fracture structure (Type B) was obtained at the other range of the deposition condition. The interlayer spacing (d₍₀₀₂₎) and the preferred orientation (PO) of the crystallite were calculated by using XRD data of the PBN plates. The degree of the preferred orientation tended to be higher with the increase of temperature and decrease of pressure, and higher temperature led to smaller value of d_(002). The crystal growth mechanism of the three types of PBN was discussed.     

Enhanced out-of-plane thermal conductivity of polyimide composite films by adding hollow cube-like boron nitride

Boron nitride nanosheet (BNNS) is widely used in electronic thermal management due to its excellent planar thermal conductivity and insulating properties. However, it is challenging to improve the out-of-plane thermal conductivity of BNNS-doped composites due to the anisotropy of the thermal conductivity of BNNS. Therefore, the BNNS in the matrix must be oriented to obtain composites with high out-of-plane thermal conductivity. In this study, BNNS powders with directional structures were synthesized directly using sodium chloride templates. The as-obtained BNNS powders have a unique hollow cube-like structure with an ultra-low density of 2.67 × 10⁻² g/cm³ and nearly 8 times the volume of the same mass of two-dimensional (2D) BNNS, making it easy to form the out-of-plane thermal conductivity paths in the polymer matrix. In addition, the high out-of-plane thermal conductivity of 4.93 W m⁻¹ K⁻¹ at 23.3 wt% loadings was obtained by doping it into a polyimide (PI) matrix. This value is 9.7 times higher than that of 2D BNNS-doped PI at the same loadings, 17.6 times higher than pure PI, and 6.1 times higher than the thermally conductive PI film sold by DuPont. Therefore, the prepared composite film has great potential for application in electronic thermal management.     

Improving the composition and multifunctional properties of amorphous boron nitride films prepared by post-annealing assisted femtosecond pulsed laser deposition method

Amorphous boron nitride (aBN) materials have similar density to crystalline phases and retain many unique electronic properties, valuable chemical inertness and high thermal stability characteristics. However, the current research on aBN materials has mainly focused on the synthesis and electrical properties of ultrathin aBN films. In this study, we developed a post-annealing assisted femtosecond laser deposition route towards stoichiometric, continuous, and multifunctional aBN films with thickness values of ∼1 μm. A series of boron nitride films were deposited on silicon wafers using a 1030 nm, 300 fs laser with a pulse energy of ∼1 mJ and a high repetition rate of 2 kHz to ablate a hexagonal boron nitride target. The deposited films were then annealed at 900 °C in a nitrogen atmosphere. The structures and chemical compositions of these obtained films were analysed by X-ray diffraction and X-ray photoelectron spectroscopy. Fourier transform infrared and nano-scratch tests were performed to measure the infrared optical and frictional properties of the adhered films. An infrared thermal imager was used to investigate the heat-dissipation performance of these films. The results indicate the components of the aBN film are further purified, the number of large heterogeneous particles is effectively reduced, and the surface becomes smooth after post-annealing treatment. This improvement promotes the transfer of heat flux and increases the transmittance in the mid-infrared light band. The significant effect mechanisms of the post-annealing treatment on the enhancement of the composition and multifunctional properties of aBN films prepared by the femtosecond pulsed laser were provided. The uniform coverage of the aBN films on the substrates, as well as the mid-infrared optical transparency and the protective performance are highly valuable and practical for infrared window protection applications.