Preparation of carbon nanotube by rotary CVD on Ni nano-particle precipitated cBN using nickelocene as a precursor

Ni nano-particles were deposited on hexagonal boron nitride (hBN) and cubic boron nitride (cBN) powders by rotary chemical vapor deposition (RCVD) using nickelocene as a precursor. Ni nano-particles precipitated on hBN and cBN powders were about 20 nm and 10 to 50 nm in diameter, respectively. Carbon nanotubes (CNTs) were grown from relatively large Ni particles about 50 nm in diameter on cBN powder, whereas carbon layers surrounded Ni nano-particles on the surface of hBN powders.     

Magnetron sputter deposition of low-stress, carbon-containing cubic boron nitride films using Ar―N2―CH4 gas mixtures

Cubic boron nitride (c-BN) films produced by PVD and plasma-assisted CVD techniques typically exhibit undesired high compressive stresses. One of the effective and feasible methods to reduce stress and hence improve film adhesion has been a controlled addition of a third element into the film during deposition. In the present study, BN films were grown on to silicon substrates using reactive magnetron sputtering with a hexagonal BN target. An auxiliary flow of methane was mixed into argon and nitrogen as the working gas. The deposition was conducted at various methane flow rates at 400 °C substrate temperature, 0.2 Pa total working pressure, and − 250 V r.f. substrate bias. The microstructure of the deposited films was then examined in dependence of the methane flow rate. With increasing methane flow rate from 0 to approx. 2.0 sccm, the fraction of the cubic BN phase in the deposited films decreased gradually down to approx. 75 vol.%, whereas the film stress was reduced much more rapidly and almost linearly in relation to the methane flow rate. At 2.1 sccm methane, the stress became approx. 3 times reduced. Owing to the significantly decreased film stress, adherent, micrometer thick, cubic-phase dominant films can be allowed to form on silicon substrate. The microstructure of the films will be illustrated through FTIR and XRR.     

Spark plasma sintering of Co-WC cubic boron nitride composites

25 vol.% cubic boron nitride (cBN) added tungsten carbide (WC) powders containing 6 wt.% Co (WC-6Co) were densified by spark plasma sintering (SPS) technique under different experimental conditions and the effect of cBN addition on the microstructure, mechanical properties and thermal conductivity were investigated. Over 99.5% theoretical density was achieved for WC-6Co-cBN composites sintered at 1300 °C, under 75 MPa pressure for 7.5 min. Under these conditions, cBN → hBN phase transformation was not observed.     

Effect of oxygen on the growth of cubic boron nitride using Mg3N2 as catalyst

Cubic boron nitride (cBN) was synthesized from hexagonal boron nitride (hBN) under high pressure and high temperature using Mg₃N₂ as catalyst. The yield and morphology of cBN were investigated in relation to the oxygen impurity of the BN-Mg₃N₂ system. Magnesium oxide precipitated as a by-product in this system and the amount of the precipitate increased with an increase in the oxygen content of the starting materials. The morphology and surface patterns of cBN crystals synthesized using an hBN which contained oxygen showed unusual features. It was confirmed that the precipitation of MgO interfered with the free growth of cBN crystals. Purification of starting materials and addition of zirconium powder to the catalyst as an oxygen getter increased the yield of cBN crystals showing smooth surfaces.     

Phase formation in the B-BN-B<Subscript>2</Subscript>O<Subscript>3</Subscript> system at high pressures and temperatures: The wettability of the phases with copper-based melts

Multiphase materials, including the B₆N boron subnitride, B₆O boron suboxide, and cBN cubic boron nitride, have been obtained from the initial β-rhombohedral or amorphous boron, hBN hexagonal graphite-like boron nitride and B₂O₃ amorphous boron oxide at a pressure of 8 GPa and temperatures from 2100 to 2900 K in a toroid high-pressure apparatus. The hardness of the resulting materials has been measured. The formation of ternary phases in the B-BN-B₂O₃ system has not been observed. The wettability of the resultant materials with copper-based melts has been studied. An addition of 10 mol % Ti to the copper melt has been found to decrease the contact angle to a value lower than 20 deg.     

Enhanced deposition of cubic boron nitride films on roughened silicon and tungsten carbide-cobalt surfaces

We report the influence of substrate surface roughness on cubic boron nitride (cBN) film deposition under low-energy ion bombardment in an inductively coupled plasma. Silicon and cemented tungsten carbide-cobalt (WC-Co) surfaces are roughened by low-energy ion-assisted etching in a hydrogen plasma, followed by deposition in a fluorine-containing plasma. Infrared absorption coefficients are measured to be 22,000 cm⁻¹ and 17,000 cm⁻¹ for sp²-bonded BN and cBN phases, respectively, for our films. For the silicon substrates, the film growth rate and the cBN content in the film increase with increasing the surface roughness, while the amount of sp²BN phase in the film shows only a small increase. A larger surface roughness of the substrate results in a smaller contact angle of water, indicating that a higher surface free energy of the substrate contributes to enhancing growth of the cBN film. For the WC-Co substrates, the film growth rate and the cBN content in the film increase similarly by roughening the surface.     

Preparation and properties of unidirectional boron nitride fibre reinforced boron nitride matrix composites via precursor infiltration and pyrolysis route

The unidirectional boron nitride fibre reinforced boron nitride matrix (BN_f/BN) composites were prepared via the precursor infiltration and pyrolysis (PIP) route, and the structure, composition, mechanical and dielectric properties were studied. The composites have a high content and fine crystallinity of BN. The density is 1.60 g cm⁻³ with a low open porosity of 4.66%. The composites display good mechanical properties with the average flexural strength, elastic modulus and fracture toughness being 53.8 MPa, 20.8 GPa and 6.88 MPa m^1/2, respectively. Lots of long fibres pull-out from the fracture surface, suggesting a good fibre/matrix interface. As temperature increases, both of the flexural strength and elastic modulus exhibit a decreasing trend, with the lowest values being 36.2 MPa and 8.6 GPa at 1000 °C, respectively. The desirable residual ratios of the flexural strength and elastic modulus at 1000 °C are 67.3% and 41.3%, respectively. The composites have excellent dielectric properties, with the average dielectric constant and loss tangent being 3.07 and 0.0044 at 2-18 GHz, respectively.     

The binary system of BN-Mg3N2 under high pressures and temperatures

The phase relations in the binary system of BN-Mg₃N₂ were investigated in regions of pressure, P, from 3.0 GPa to 8.0 GPa and temperature, T, up to 1900 K by means of in situ differential thermal analysis (DTA) and X-ray diffraction (XRD) of the quenched products. It was found that the succession in formation of the intermediate compounds Mg₃BN₃ (HP-phase) and Mg₃B₂N₄ depends on the molar ratios of hexagonal boron nitride (hBN) and Mg₃N₂ and on the P-T conditions. In the P-T region of cubic boron nitride (cBN) growth, the system has three metastable eutectics such as Mg₃N₂-hBN, Mg₃BN₃-hBN and Mg₃B₂N₄-hBN. It was found that eutectic temperatures are pressure dependent. The difference in the lower-temperature limits of cBN growth regions is explained by cBN crystallization from different eutectic melts.     

Diamond and cBN hybrid and nanomodified cutting tools with enhanced performance: Development,testing and modelling

The potential of enhancement of superhard steel and cast iron cutting tool performance on the basis of microstuctural modifications of the tool materials is studied. Hybrid machining tools with mixed diamond and cBN grains, as well as machining tool with composite nanomodified metallic binder are developed, and tested experimentally and numerically. It is demonstrated that both combination of diamond and cBN (hybrid structure) and nanomodification of metallic binder (with hexagonal boron nitride/hBN platelets) lead to sufficient improvement of the cast iron machining performance. The superhard tools with 25% of diamond replaced by cBN grains demonstrate 20% increased performance as compared with pure diamond machining tools, and more than two times higher performance as compared with pure cBN tools. Further, cast iron machining efficiency of the wheels modified by hBN particles was 80% more efficient compared to the tool with the original binder. Computational model of hybrid superhard tools is developed, and applied to the analysis of structure-performance relationships of the tools.     

Structural and mechanical properties of multi-element (AlCrMoTaTiZr)Nx coatings by reactive magnetron sputtering

(AlCrMoTaTiZr)N_x high-entropy films were deposited on silicon wafer and cemented carbide substrates from a single alloy target by reactive RF magnetron sputtering under a mixed atmosphere of Ar and N₂. The effect of nitrogen flow ratio R_N on chemical composition, morphology, microstructure, and mechanical properties of the (AlCrMoTaTiZr)N_x films was investigated. Nitrogen-free alloy film had an amorphous structure, while nitride films with at least 37 at.% N exhibited a simple NaCl-type FCC (face-centered cubic) structure. Mixed structures occurred in films with lower nitrogen contents. Films with the FCC structure were thermally stable without phase decomposition at 1000 °C after 10 h. The (AlCrMoTaTiZr)N film deposited at R_N = 40% exhibited the highest hardness of 40.2 GPa which attains the superhard grade. The main strengthening mechanisms for this film were grain-size and solid-solution strengthening. A residual compressive stress of 1.04 GPa was small to account for the observed hardness. The nitride film was wear resistant, with a wear rate of 2.8 × 10^− 6 mm³/N m against a loaded 100Cr6 steel ball in the sliding wear test. These high-entropy films have potential in hard coating applications.     

Hexagonal boron nitride epitaxial layers as neutron detector materials

Micro-strip metal-semiconductor-metal detectors for thermal neutron sensing were fabricated from hexagonal boron nitride (hBN) epilayers synthesized by metal organic chemical vapor deposition. Experimental measurements indicated that the thermal neutron absorption coefficient and length of natural hBN epilayers are about 0.00361 μm⁻¹ and 277 μm, respectively. A continuous irradiation with a thermal neutron beam generated an appreciable current response in hBN detectors, corresponding to an effective conversion efficiency approaching ∼80% for absorbed neutrons. Our results indicate that hBN semiconductors would enable the development of essentially ideal solid-state thermal neutron detectors in which both neutron capture and carrier collection are accomplished in the same hBN semiconductor. These solid-state detectors have the potential to replace ³He gas detectors, which faces the very serious issue of ³He gas shortage.     

Mechanical properties of high purity polycrystalline cBN synthesized by direct conversion sintering method

Mechanical properties of high purity polycrystalline cBN sintered bodies synthesized by the direct conversion method under high pressure and high temperature have been investigated by hardness and transverse rupture strength (TRS) measurement in the temperature range of 20–1200 °C. It was confirmed that the hardness and TRS of the polycrystalline cBN depends strongly on the cBN grain size and the amount of residual (compressed) hBN in the sintered body. The fine-grained (<0.5 m) and high purity (cBN > 99.9%) polycrystalline sintered body synthesized at 7.7 GPa, 2200–2400°C has highest hardness and TRS at any temperature. The TRS of the sintered body shows a positive temperature dependence up to 1200 °C. The high hardness and high TRS at high temperature of the fine-grained high purity polycrystalline cBN suggest that the sintered body has high potential in cutting tool uses.     

Influence of the ion-atom flux ratio on the mechanical properties of chromium nitride thin films

In this paper we report the mechanical properties of chromium nitride (CrN) thin films deposited at different levels of ion bombardment and their relationship with the microstructural parameters, such as grain size, preferred orientation and residual stress. The samples were deposited by unbalanced magnetron sputtering changing the substrate-target distance and the substrate bias, keeping other deposition condition fixed. The mechanical properties were obtained by nanoindentation performed on 1.8 μm thick samples. Under the different deposition conditions all of the CrN films were approximately stoichiometric, but clear variations in the microstructure were seen. The hardness was nearly constant at 24-27 GPa even when the grain size, residual stress and crystalline orientation changed. However, the elastic modulus showed a steady increase from 300 to 350 GPa, proportional to the variations in grain size and the residual stress level.     

The synthesis of cBN using Ca3B2N4

The phase relation of the system Ca-B-N was investigated at a pressure of 2.5 GPa by both differential thermal analysis (DTA) and a quenching method. When BN reacted with Ca at temperatures higher than 1150° C, Ca₃B₂N₄ was formed together with small amounts of Ca₃N₂ and CaB₆. The system of Ca₃B₂N₄-BN has a eutectic relationship at 1316° C and 2.5 GPa. The synthesis of cBN was established by using Ca₃B₂N₄ under the thermodynamic stable conditions of cBN. It could be seen that cBN crystals grew repeatedly in Ca₃B₂N₄ melt, which advanced into the hBN layer. The cBN crystals obtained in this way were of outstanding purity and of good quality. This indicated that hBN dissolved and precipitated as cBN in Ca₃B₂N₄ melt without any appreciable change of molar boron/nitrogen ratio. In the system of Ca-B-N, the low-temperature limit of cBN formation was closely related to the eutectic relationship between BN and Ca₃B₂N₄.     

Solid-state interfacial reactions and compound morphology of cBN grain and surface Ti coating

In order to research thoroughly the mechanism of the solid-state interfacial reactions and the induced compound morphology of cubic boron nitride (cBN) abrasive grains and the surface Ti-coating layer, annealing experiments of Ti-coated cBN grains were conducted at different temperatures of 550-950 °C for dwell times of 60 min under high-vacuum conditions. The corresponding interfacial characteristics were investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopy. The results show that the relevant interfacial reactions and compound morphology between cBN and Ti are significantly influenced by the heat-treatment temperature. No reaction occurs below 550 °C, and TiN is the sole reaction product during heating from 650-750 °C. Three kinds of compounds, TiN, TiB₂ and TiB, can exist in the interfacial region at 950 °C. Here, the favorable interfacial structures, cBN/TiB₂/TiB/(TiB+TiN)/TiN/Ti, are developed for the excellent mechanical and chemical transition effects between cBN grains and Ti coating. The thermodynamic analysis finally predicts that there is a reasonable probability for the formation of such a special interfacial transition layer.     

Mid-frequency PECVD of a-SiCN:H films and their structural,mechanical and electrical properties

The mid-frequency pulsed plasma enhanced chemical vapour deposition (PECVD) of hydrogenated amorphous silicon carbonitride (a-SiCN:H) was investigated to prove the suitability of these films as a mechanical stiff insulator for the integration of piezoelectric fibres in microstructured aluminium plates. For the a-SiCN:H deposition trimethylsilane (SiH(CH₃)_3; 3MS) and nitrogen in mixture with argon were used. The films were characterised regarding their deposition rate, elastic modulus and hardness (nanoindentation), mechanical stress, elemental composition (ERDA) and electrical insulating properties.The breakdown field strength of μm-thick a-SiCN:H films is in the range of 2–4 MV/cm. At pressures of a few Pa the deposition rate reached values up to 6 μm/h. It is limited by the power absorption in the 100 kHz bipolar-pulsed discharge. Varying the pressure from 2 Pa to 15 Pa has only little influence on the film composition. With increasing pressure during deposition the elastic modulus of the films decreases from about 146 GPa to 100 GPa and the compressive film stress from 1.2 GPa to 0.55 GPa. By reducing the 3MS flow rate from 50 sccm to 10 sccm (at 8 Pa deposition pressure), the carbon and the hydrogen concentrations in the films were reduced by about 10 at. %. The Si-content is only slightly reduced but the N-content is more than tripled. In contrast, the changes in the mechanical film properties are comparatively small. The mechanical properties of a-SiCN:H films are not simply correlated to the stoichiometry but are rather controlled by the ion bombardment during growth.     

The effect of the backscattered energetic atoms on the stress generation and the surface morphology of reactively sputtered vanadium nitride films

During the reactive magnetron sputtering of transition metal nitrides in an Ar-N₂ ambient, Ar⁺ and N₂⁺ plasma ions are neutralized upon impingement on the target and are backscattered towards the growing film as neutral Ar and N species, respectively. Based on simulations, as well as on plasma and on film characterization techniques we manifest the relationship between the bombardment by the backscattered energetic atoms and the properties of reactively sputtered vanadium nitride (VN) films. Depending on the N₂ flow (q_N2) two bombardment regimes are established. In the first regime, (q_N2 < 20 sccm) the contribution of the N species to the energetic bombardment is insignificant. The major bombarding species in this regime are the backscattered Ar species, as well as positive plasma ions and sputtered atoms. These species have relatively low energies and subplantation ratios and thus, their energy is transferred to the surface of the growing film. In the second regime (q_N2 > 20 sccm) the backscattered N atoms are the major bombarding species and their flux to the growing film increases with increasing the N₂ flow. We argue that the backscattered N atoms have higher energy and subplantation ratio in comparison to the other bombarding species. As a result, a higher part of their energy is dissipated in the bulk of the film. The two bombarding regimes correlate well with the residual compressive stresses and the surface roughness of the films. Films grown at q_N2 < 20 sccm exhibit low compressive stresses and their roughness drops when q_N2 is increased. This consistent with the low subplantation ratio and the transfer of the energy of the bombarding species to surface the growing film. The compressive stresses of films grown at q_N2 > 20 sccm are higher, than those of the films grown in the first regime, and increase with increasing N₂ flow. This is attributed to the subplantation of the bombarding N species in the growing film.     

Depth-resolved residual stress analysis of thin coatings by a new FIB-DIC method

A new methodology for the measurement of depth sensitive residual stress profiles of thin coatings with sub-micrometer resolution is presented. The two step method consists of incremental focused ion beam (FIB) ring-core milling, combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and a full field strain analysis by digital image correlation (DIC). The through-thickness profile of the residual stress can be obtained by comparison of the experimentally measured surface strain with finite element modeling using Schajer's integral method. In this work, a chromium nitride (CrN) CAE-PVD 3.0 μm coating on steel substrate, and a gold MS-PVD 1.5 μm on silicon were selected for the experimental implementation. Incremental FIB milling was conducted using an optimized milling strategy that produces minimum re-deposition over the sample surface. Results showed an average residual stress of σ = −5.15 GPa in the CrN coating and σ = +194 MPa in the Au coating. These values are in reasonable agreement with estimates obtained by other conventional techniques. The depth profiles revealed an increasing residual stress from surface to the coating/surface interface for both coatings. This observation is likely related to stress relaxation during grain growth, which was observed in microstructural cross sections, as predicted by existing models for structure-stress evolution in PVD coatings. A correlation between the observed stress gradients and the in-service mechanical behavior of the coatings is proposed. Finally, critical aspects of the technique and the influence of microstructure and elastic anisotropy on stress analysis are analyzed and discussed.     

A novel synthetic route to prepare cubic BN nanorods

Cubic boron nitride nanorods were prepared at 450°C by a low pressure benzene thermal synthesis method using BCl₃ and Li₃N as the reactants. The nanorods with various lengths and diameters were observed by transmission electron microscopy. The analysis of X-ray diffraction pattern and electron diffraction pattern revealed that the nanorods are cBN with a lattice constant of 3.62 Å. The Fourier transformation infrared spectra showed that the dominant phase is cBN, and the B:N ratio of 1.15:1 can be obtained from the X-ray photoelectron spectrum.     

Pressure infiltration of boron nitride preforms with molten aluminum for low density heat sink materials

Cubic boron nitride (cBN) has outstanding mechanical and thermal properties. The previous research focused on mechanical properties, to data, the thermal property of cBN has rarely been reported. In this work, a wide range of aluminum/cubic boron nitride (Al/cBN) composites were fabricated by pressure infiltration at 5.0 GPa and 960–1600 °C. The microstructure, phase composition, thermal conductivity and coefficient of thermal expansion of the Al/cBN composites were investigated. The results showed that a maximum thermal conductivity of 266 W/mK and the coefficient of thermal expansion of 4–6 × 10^?6 K^?1 which matches well to semiconductors, indicating that the Al/cBN composites are promised heat sink materials of high efficiency for the wide band gap semiconductors.