Thick cBN Research Articles

On the solubility of boron nitride in supercritical ammonia-sodium solutions

Cubic boron nitride (cBN) is considered to be one of the leading candidates for next generation electronic devices yet has no current synthesis route for single crystals greater than a few mm in size leading to a lack of suitable substrates. New synthesis methods need to be explored, with the ammonothermal method being a potential candidate for synthesis of large area, thick cBN and hexagonal BN (hBN) single crystals. This paper investigates the solubility of BN in supercritical ammonia using sodium amide (NaNH2) as a mineralizer and cBN crystals as feedstock. Solubilities were measured based on mass loss of the feedstock at temperatures from 450—600 deg. C, with pressures of 150–190 MPa, using durations from 24—96 h and with varying amounts of Na. A positive solubility trend is demonstrated with respect to temperature with solubility varying from 0.017—0.050% molBN/molNH3. Solubilities did not vary as a function of time or when>0.07 molNa/L mineralizer content was used suggesting both that equilibrium was reached and the solutions were saturated. These values are high enough to motivate further investigation for solution-based growth of BN.

Low threshold field emission from high-quality cubic boron nitride films

Field emission performance of materials with mixed sp2/sp3 phases often depends upon the phase composition at the surface. In this study, the emission performance of high-quality cubic boron nitride (cBN) films is studied in terms of phase purity. Thick cBN films consisting of micron-sized grains are prepared from boron trifluoride gas by chemical vapor deposition in a plasma jet and an inductively coupled plasma. Both the bulk and surface phase purities as well as crystallinities of cBN evaluated by visible and ultraviolet Raman spectroscopy, glancing-angle x-ray diffraction, and x-ray photoelectron spectroscopy are the highest when the film is deposited in a plasma jet under an optimized condition. The emission turn-on field decreases with increasing the phase purity, down to around 5 V/μm for the highest cBN purity, due to the larger field enhancement, while it is higher than 14 V/μm without cBN (sp2-bonded hexagonal BN only). The results indicate that the total field enhancement for the high phase purity film is governed by the internal field amplification related to the surface coverage of more conductive cBN, rather than the external one related to the surface topology or roughness.

Growing cubic boron nitride films at different temperatures

Cubic boron nitride (cBN) films have been synthesized by both physical and chemical vapor deposition (PVD and CVD) methods at a wide range of substrate temperature. Some works conclude that the cBN growth is insensitive to the temperature parameter while few works suggest that substrate temperature plays a considerable role at cBN deposition, furthermore, the different temperatures used for the nucleation and growth make the situation more complex. In this work, we investigated systematically the growth of cBN films on CVD diamond surfaces at variable temperatures (from 200 to 1100 °C) using electron cyclotron resonance microwave plasma CVD (ECR-MPCVD) and radio-frequency magnetron sputtering (RF-MS) methods. The role of substrate temperature is discussed in the view of controlling phase purity, crystallinity, growth rate, and residual stress of cBN films deposited. Under optimized conditions, several-micrometers thick films are prepared by ECR-MPCVD as demonstrated on the example of a 3-μm thick cBN film grown at 900 °C with faceted surfaces. The sizes of crystallites are fairly large (∼0.4μm) to yield visible Raman spectra characteristic to cBN.

Cubic Boron Nitride (cBN) based nanocomposite coatings on cutting inserts with chip breakers for hard turning applications

Deposition of cBN (5∼20 μm) is of great technological importance but poses significant difficulty in terms of depositing it from vapor phase(s). Initial proof-of-concept work has indicated that a thick cBN–TiN composite coating can be deposited in a practical hybrid deposition process using electrostatic spray coating (ESC) and chemical vapor infiltration (CVI), and that coated inserts can provide significant improvement in machining performance. In this paper, the development of a nanocomposite cBN–TiN coating especially for hard turning is presented. Aspects of the coating on chip-breaker inserts, repeatability, and process optimization for the given application are discussed. Characterization of the coating shows a repeatable uniform deposition. Encouraging tool life and workpiece surface roughness have been achieved in machining of AISI 4340 hardened steels (50∼53 HRC) using the coated inserts.

Boron containing combination tool coatings—characterization and application tests

The requirements for durable tool coatings continuously increase. In many cases, tool coatings combining different phases or several layers could provide an improvement in tool life. The broad range of mechanical properties of materials in the B–C–N and Ti–B–N ternary systems, from very soft to superhard, presents many possibilities to generate various combination coatings. Such coatings were prepared using reactive sputter techniques with different target materials. An outstanding example is a superhard 3 μm thick coating system with a 0.5 to 0.8 μm thick cBN top layer deposited on cutting inserts. Soft coatings like hexagonal boron nitride were found to be essential for machining operations under dry conditions. The coatings were characterized with respect to hardness, abrasive wear rate and friction coefficients. The correlation between properties and composition was revealed. Application test results of B–C–N and Ti–B–N coating systems on tools obtained under near production conditions will be reported. Specifically, turning tests performed with cemented carbide cutting inserts coated with a superhard coating system with a cBN top layer will be discussed.

Growth and characterization of thick cBN coatings on silicon and tool substrates

Recently some research groups have achieved progress in the deposition of cubic boron nitride (cBN) coatings with a thickness of 2 μm and more, which is necessary for cutting tool applications. In our laboratory, thick cBN coatings were sputter deposited on silicon substrates using a boron carbide target. Following a boron carbide interlayer (few 100 nm thick), a gradient layer with continuously increasing nitrogen content was prepared. After the cBN nucleation, the process parameters were modified for the cBN film growth to a thickness of more than 2 μm. However, the transfer of this technology to technically relevant substrates, like cemented carbide cutting inserts, required some further process modifications. At first, a titanium interlayer had to be deposited followed by a more than 1-μm-thick boron carbide layer. The next steps were identical to those on silicon substrates. The total coating thickness was in the range of 3 μm with a 0.5- to nearly 1-μm-thick cBN top layer. In spite of the enormous intrinsic stress, both the coatings on silicon and on cemented carbide exhibited a good adhesion and a prolonged stability in humid air. Oxidation experiments revealed a stability of the coating system on cemented carbide up to 700 °C and higher. Coated cutting inserts were tested in turning operations with different metallic workpiece materials. The test results will be compared to those of well-established cutting materials, like polycrystalline cubic boron nitride (PCBN) and oxide ceramics, considering the wear of coated tools.

Synthesis of Boron Nitride Films Using Electron–Cyclotron Resonance Microwave Plasma and an H2–BF3–N2–Ar–He Gas System

AbstractBoron nitride (BN) films with high crystallinity and phase purity (>80 %) in the cubic phase were synthesized over large areas using fluorine chemistry and electron–cyclotron resonance (ECR) microwave plasma. Plasma‐enhanced fluorine chemistry was provided by a complex H2–BF3–N2–Ar–He gas mixture. Each component of the mixture was optimized in accordance with its function. Such a procedure allowed the preparation of thick cBN films at a bias voltage as low as –30 V. Micrometer‐thick cBN films exhibiting columnar growth were obtained. However, ion bombardment was still indispensable for the cBN growth. The effects of deposition time and ion bombardment on the phase purity and crystallinity of the cBN films were investigated.

The growth of thick cBN films employing fluorine chemistry and ECR deposition

Energetic ion bombardment in N 2 and Ar plasmas conventionally applied to cubic boron nitride (cBN) deposition restricts the size of cBN crystallites and induces high internal stress levels, limiting the non-delaminating film thickness obtainable (to ∼100 nm). The introduction of fluorine chemistry via a complex He–Ar–N 2–BF 3–H 2 plasma produced in an electron cyclotron resonance (ECR) system overcomes such limitations and enables the preparation of thick cBN films (>1 μm) with low stress. The films were deposited using a low substrate bias (−40 V) and high substrate temperature (900 °C). Fourier-transform infrared (FTIR) spectroscopy shows that the films are composed of >80% cBN. Detailed analysis of BN film structures employing high-resolution transmission electron microscopy (HRTEM) and transmission electron energy loss spectroscopy (EELS) shows that a cBN layer with high phase purity is formed on top of an initial turbostratic BN (tBN) layer, which accounts for the small hexagonal BN (hBN) signal in FTIR spectra. The appearance of characteristic transverse optical (TO) phonon and longitudinal phonon (LO) modes of cBN in visible Raman spectra demonstrates the larger crystalline size (∼100 nm, also confirmed by HRTEM) in contrast to the films reported previously. The ion bombardment, gas composition, substrate temperature and growth time significantly affect the phase purity and crystallinity of the cBN films formed, as further elucidated in this paper.

A nanoindentation study of thick cBN films prepared by chemical vapor deposition

The mechanical properties of high-quality cubic boron nitride (cBN) films were systematically investigated by nanoindentation measurements performed in both cross-sectional and plan-view directions. The large film thickness (∼5 μm) allows the effective ruling out of both substrate and indenter size effects. The hardness and elastic modulus values were found to be 70 and 800 GPa, respectively, which are the highest values ever obtained on cBN films deposited by either PVD or CVD methods so far (comparable to those reported for cBN crystals synthesized by high-pressure high-temperature methods). The variation of hardness across the cBN film thickness was investigated. In conjunction with the transmission electron microscopic observations, the relationship of the hardness measured with the crystallinity and crystal size/grain boundaries was discussed.

Orientation effects in tBN/cBN interfaces: A transmission electron microscopic study

The orientation between cubic boron nitride (cBN) crystallites and the tBN layers on which they grow was studied using high-resolution transmission electron microscopy. BN films were prepared by radio-frequency magnetron sputtering under conditions leading to the formation of ∼100% cBN films grown on a preceding tBN layer. Two types of orientations were observed: (i) cBN layers grown on the edges of the tBN(0002) planes so that the cBN{111} planes are parallel to the tBN(0002) planes (as reported previously by many authors), (ii) cBN layers grown on curved tBN(0002) planes with no orientation to the tBN planes. The first type of cBN growth is associated with stress leading to delamination of cBN films thicker than 100 nm. The second type is associated with reduced stress enabling the growth of much thicker (∼500 nm) films at a relatively low (450 °C) substrate deposition temperature. These results may be helpful in the fabrication of thick cBN films and improving our understanding of the cBN nucleation process.

The introducing of fluorine into the deposition of BN: a successful method to obtain high-quality, thick cBN films with low residual stress

Cubic boron nitride films were synthesized on silicon substrates by DC-bias-assisted DC jet chemical vapor deposition in an Ar–N 2–BF 3–H 2 system. By this method, the deposition of cBN at high gas pressure of 50 torr became possible, and the conditions of cBN CVD approached to those of diamond CVD. cBN films with low residual stress (1–2 GPa) and with large crystal size of up to several hundred nanometers were obtained and clear Raman peaks of cBN appeared. Furthermore, the deposition rate was as high as 0.3 μm/min at the initial stage and over 20-μm-thick BN films were obtained for a 3-h deposition. These remarkable improvements are attributed to the preferential etching effect of fluorine to sp 2 bonds and the decrease of the bombarding energy of ions.

Microstructure and nanomechanical properties of cubic boron nitride films prepared by bias sputter deposition

Nanoindentations, using a triangular based cube-corner shaped diamond tip, were tried to study the mechanical properties of cubic boron nitride ( cBN) films. Since the cBN film has a layered structure consisting of an initial sp 2-bonded BN (amorphous BN and turbostratic BN) layer and a sp 3-bonded BN ( cBN) layer, which was previously studied by cross-sectional transmission electron microscopy (TEM), cBN films in different growth stages were prepared for the measurements. Hardness and elastic modulus were evaluated as a function of the penetration depth from the obtained loading-unloading curves using a method described by Oliver et al. The maximum hardness and elastic modulus of a 110-nm thick cBN film (including an approximately 50-nm thick initial layer) were approximately 10 and 3 times, respectively, larger than the values evaluated for a silicon wafer substrate. The dependence of the evaluated hardness and the elastic modulus on film thickness and penetration depth can be explained in terms of the layered structure of the cBN film, i.e. soft initial sp 2-bonded BN layer with low elastic modulus, and hard cBN layer with high elastic modulus.