Amorphous BN Research Articles

Effects of B4C and BN additions on formation of NbB2–Al2O3 composites from reduction-based combustion synthesis

Formation of NbB2–Al2O3 composites was investigated by reduction-based combustion synthesis in the SHS mode. Amorphous boron, B4C, and BN were adopted as the source of boron in three reaction systems composed of Nb2O5–Al–B, Nb2O5–Al–B–B4C and Nb2O5–Al–B–BN. In addition, the effect of excess boron up to 30 at% was examined. For the B- and B/BN-based reaction schemes, the synthesis process involved not only aluminothermal but borothermal reduction of Nb2O5. The latter caused a significant loss of boron in the form of gaseous oxides, and therefore, their resulting products contained NbB, Nb3B4, and NbB2 even if the samples were formulated with boron of 30 atom.% in excess. Besides acting as a boron precursor, B4C was showed to effectively suppress the borothermal reaction for the B/B4C-adopted sample. Consequently, a small amount of excess boron of 10 atom.% was sufficient for the B/B4C-adopted sample to produce the NbB2–Al2O3 composite with trivial minor phases.

Direct nucleation of hexagonal boron nitride on diamond: Crystalline properties of hBN nanowalls

Hexagonal boron nitride (hBN) nanowalls were deposited by unbalanced radio frequency sputtering on (100)-oriented silicon, nanocrystalline diamond films, and amorphous silicon nitride (Si3N4) membranes. The hBN nanowall structures were found to grow vertically with respect to the surface of all of the substrates. To provide further insight into the nucleation phase and possible lattice distortion of the deposited films, the structural properties of the different interfaces were characterized by transmission electron microscopy. For Si and Si3N4 substrates, turbostratic and amorphous BN phases form a clear transition zone between the substrate and the actual hBN phase of the bulk nanowalls. However, surprisingly, the presence of these phases was suppressed at the interface with a nanocrystalline diamond film, leading to a direct coupling of hBN with the diamond surface, independent of the vertical orientation of the diamond grain. To explain these observations, a growth mechanism is proposed in which the hydrogen terminated surface of the nanocrystalline diamond film leads to a rapid formation of the hBN phase during the initial stages of growth, contrary to the case of Si and Si3N4 substrates.

Nanosegregated amorphous AlBN2 alloy

We use ab initio molecular dynamics simulations to create an amorphous AlBN2 model and find that it consists of nanosegregated two-dimensional hexagonal BN-like and tetrahedral AlN-like domains. These domains are somewhat homogenously distributed in the network. There exist no chemical disorder and Al–B bonding. Amorphous AlBN2 is a semiconductor having a theoretical band gap energy of ∼2.24 eV, larger than that of amorphous AlN and BN systems. This amorphous nitride might find some applications as an electronic material.

ZrB2 thin films deposited on GaN(0001) by magnetron sputtering from a ZrB2 target

ZrB2 films were deposited on 900°C-preheated or non-preheated GaN(0001) surfaces by direct current magnetron sputtering from a compound target. Analytical transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy revealed a 0001 fiber textured ZrB2 film growth following the formation of a ~2nm thick amorphous BN layer onto the GaN(0001) at a substrate temperature of 900°C. The amorphous BN layer remains when the substrate temperature is lowered to 500°C or when the preheating step is removed from the process and results in the growth of polycrystalline ZrB2 films. The ZrB2 growth phenomena on GaN(0001) is compared to on 4H-SiC(0001), Si(111), and Al2O3(0001) substrates, which yield epitaxial film growth. The decomposition of the GaN surface during vacuum processing during BN interfacial layer formation is found to impede epitaxial growth of ZrB2.

Crystallization behavior of cubic boron nitride from an amorphous BN precursor via high-pressure, high-temperature treatment with controlled water addition

Crystallization behavior of cubic boron nitride (c-BN) from amorphous boron nitride (a-BN) by high-pressure, high-temperature (HP-HT) treatment with the intentional addition of a controlled amount of water was studied. The a-BN precursor was prepared by pyrolysis of a boric acid-urea complex with urea/boric acid=2 at 1000°C for 3h under an ammonia atmosphere. Hexagonal BN (h-BN) were initially crystallized from a-BN containing 2 mass% of water after HP-HT treatment at 7.7GPa and 1200°C, and the formation of c-BN was observed after 5min. The crystallization of c-BN from a-BN containing 8 mass% of water after HP-HT treatment at 7.7GPa and 1200°C was observed even after 1min, indicating that the crystallization of c-BN was promoted by increasing the amount of water added. In addition, since neither h-BN nor c-BN was crystallized from a-BN without intentional water addition after the same HP-HT treatment, the addition of water promoted the crystallization of both h-BN and c-BN from a-BN. Since no transformation from h-BN to c-BN was observed during HP-HT treatment at 7.7GPa and 1200°C, it was concluded that c-BN was directly crystallized from a-BN. After HP-HT treatment at 7.7GPa and 1650°C for 1min, both h-BN and c-BN were crystallized from a-BN containing 8 mass% of water. These results clearly indicate that the addition of water has a positive effect on crystallization of c-BN from a-BN.

Influence of boron content on the microstructure and tribological properties of Cr-B-N coatings in water lubrication

Cr-B-N coatings with different boron contents were deposited on Si(1 0 0) wafers and 316L stainless steels using unbalanced magnetron sputtering system by way of adjusting the CrB2 target currents. The microstructure and mechanical properties of Cr-B-N coatings were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), white light interferometric three dimensional profilometer and nano-indentation tester, respectively. The tribological properties of Cr-B-N/SiC tribopairs in water were studied using ball-on-disk tribometer. The results showed that the Cr-B-N coatings showed a fine nanocomposite structure consisted of CrN nanograins and amorphous BN phase regardless of boron contents, and the typical columnar structure became featureless with increasing the CrB2 target current. The hardness and reduced elastic modulus first increased to 28.9GPa and 330GPa at the CrB2 target current of 2A, and then decreased gradually with further increasing the CrB2 target current to 4A. As compared with the CrN/SiC tribopairs, the lowest friction coefficient of Cr-B-N/SiC ball tribopairs in water was 0.15, and the wear resistance of Cr-B-N coatings was effectively enhanced.

Effects of modulation period on microstructure, mechanical properties of TiBN/TiN nanomultilayered films deposited by multi arc ion plating

TiBN/TiN multilayered coating with different modulation periods (bilayer thickness) have been synthesized using typical cathodic arc plasma deposition equipment. The objective of this work is to study the influence of modulation period on the microstructure, mechanical and tribological properties. TiBN/TiN multilayer coatings exhibited a TiN (111), (200), (220) and TiB2 (220) crystallographic orientations and preferred TiN (111) orientation. With the decrease of modulation period from 12 nm to 1.9 nm, the mole ratio of B increased from 0.079 to 0.162 in addition to the average mole ratio of N was about 0.39 steadily, while the mole ratio of Ti decreased from 0.506 to 0.407. The microstructure of TiBN/TiN multilayered coating evolves form nanocrystalline TiN/a-BN to nanocomposite Ti(B,N) with an dense amorphous BN phase, The maximum values of hardness and elastic modulus reached 31.6 GPa and 336 GPa with a bilayer period of 1.9 nm and B:Ti ratio of 2:5.

Hexagonal nanosheets in amorphous BN: A first principles study

Amorphous boron nitrite is modeled by means of first principles molecular dynamics simulations and found to be almost chemically ordered in a stark contrast to the previous predictions. Its average coordination number is 2.97. The main building unit of the amorphous network is hexagonal rings as in the most stable boron nitrite phase but chain-like structures and tetragonal-like rings also exist in amorphous network. The model consists of partially hexagonal nanosheets and hence it is not entirely disordered. Amorphous boron nitrite has a band gap energy of about 2.0eV.

Low Friction-Coefficient TiBCN Nanocomposite Coatings Prepared by Cathode Arc Plasma Deposition**supported by National Natural Science Foundation of China (Nos. 11350110206, 11375133) and the Fundamental Research Funds for the Central Universities of China (No. 11275141)

TiBCN nanocomposite coatings were deposited on cemented carbide and Si (100) by a cathode arc plasma system, in which TiB2 cathodes were used in mixture gases of N2 and C2H2. X-ray diffraction shows that TiB2 and Ti2B5 peaks enhance at low flow rates of C2H2, but they shrink when the flow rate is over 200 sccm. An increase of deposition rate was obtained from different TiBCN thicknesses for the same deposition time measured by scanning electron microscopy. Atomic force microscopy shows that the surface roughnesses are ˜10 nm and ˜20 nm at C2H2 flow rates of 0-100 sccm and of 150-300 sccm, respectively. High resolution transmission electron microscopy and X-ray photoelectron spectroscopy show that the coatings consist of nanocrystal phases Ti2BB, TiB2 and TiN, and amorphous phase carbon and BN. The average crystal sizes embedded in the amorphous matrices are 200 nm and 10 nm at C2H2 flow rates of 200 sccm and 300 sccm, respectively. In Raman spectra, the D- and G-bands increase with C2H2 flows at low flow rates, but weaken at high flow rates. The microhardness of the coatings decreases from 28.6 GPa to 20 GPa as the C2H2 increases from 0 sccm to 300 sccm, and the ball-on-disk measurement shows a dramatic decrease of the friction coefficient from 0.84 to 0.13. The reason for the reduced hardness and friction coefficient with the change of C2H2 flow rates is discussed.

Impact of low-energy ions on plasma deposition of cubic boron nitride

Plasma deposition of cubic boron nitride (cBN) films under low-energy (a few eV to ~40eV) ion impact with the chemistry of fluorine is studied in terms of ion energy, ion flux, and ion to boron flux ratio onto the substrate. The ion energy and the ion to boron flux ratio are determined from the sheath potential and the ratio of incident ion flux to net deposited boron flux, respectively. For negative substrate biases where a mixture of turbostratic and amorphous BN phases only or no deposit is formed, both the ion energy and the ion to boron flux ratio are high. For positive substrate biases where cBN phase is formed, the ion energy and the ion to boron flux ratio are in the range of a few eV to 35eV and 100 to 130, respectively. The results indicate that the impact of positive ions with high ion to boron flux ratios makes a substantial contribution to the formation of cBN phase, while that of negative ions and electrons makes only a minor contribution.

Synthesis of few-layer, large area hexagonal-boron nitride by pulsed laser deposition

Pulsed laser deposition (PLD) has been investigated as a technique for synthesis of ultra-thin, few-layer hexagonal boron nitride (h-BN) thin films on crystalline highly ordered pyrolytic graphite (HOPG) and sapphire (0001) substrates. The plasma-based processing technique allows for increased excitations of deposited atoms due to background nitrogen gas collisional ionizations and extended resonance time of the energetic species presence at the condensation surface. These processes permit growth of thin, polycrystalline h-BN at 700°C, a much lower temperature than that required by traditional growth methods. Analysis of the as-deposited films reveals epitaxial-like growth on the nearly lattice matched HOPG substrate, resulting in a polycrystalline h-BN film, and amorphous BN (a-BN) on the sapphire substrates, both with thicknesses of 1.5–2nm. Stoichiometric films with boron-to-nitrogen ratios of unity were achieved by adjusting the background pressure within the deposition chamber and distance between the target and substrate. The reduction in deposition temperature and formation of stoichiometric, large-area h-BN films by PLD provide a process that is easily scaled-up for two-dimensional dielectric material synthesis and also present a possibility to produce very thin and uniform a-BN.

Mechanical, tribological and corrosion performance of WBN composite films deposited by reactive magnetron sputtering

Abstract WBN composite films with various boron contents ranging from 25.1 at.% to 46.5 at.% were deposited by a multi-target magnetron sputtering system. The microstructure, mechanical, tribological and corrosion behavior of films were studied using XRD, SEM, FTIR, HRTEM, nano-indentation, Ball-on-disc dry sliding wear tester, Bruker 3D Profiler and compared to W2N. All the films exhibited face-centred cubic (fcc) structure W2N; bcc α-W phases appeared as the B content was 25.1 at.% and amorphous BN appeared as the B content was 31.9 at.%. The hardness and compressive stress of WBN films first increased and then decreased with increasing the B content. As the B content was 38.1 at.%, they reached the maximum values of 36.1 GPa and 2.6 GPa, respectively. The best wear resistance at room and elevated temperature was found for the film which was shown to exhibit the highest hardness and compressive stress. The corrosion resistance of the substrate which coated with W2N films or WBN films was superior to the uncoated substrate. The corrosion resistance of the substrate which coated with W2N films was improved slightly by doping some boron content.

Chemical Sharpening, Shortening, and Unzipping of Boron Nitride Nanotubes

Boron nitride nanotubes (BNNTs), the one‐dimensional member of the boron nitride nanostructure family, are generally accepted to be highly inert to oxidative treatments and can only be covalently modified by highly reactive species. Conversely, it is discovered that the BNNTs can be chemically dispersed and their morphology modified by a relatively mild method: simply sonicating the nanotubes in aqueous ammonia solution. The dispersed nanotubes are significantly corroded, with end‐caps removed, tips sharpened, and walls thinned. The sonication treatment in aqueous ammonia solution also removes amorphous BN impurities and shortened BNNTs, resembling various oxidative treatments of carbon nanotubes. Importantly, the majority of BNNTs are at least partially longitudinally cut, or “unzipped”. Entangled and freestanding BN nanoribbons (BNNRs), resulting from the unzipping, are found to be ∼5–20 nm in width and up to a few hundred nanometers in length. This is the first chemical method to obtain BNNRs from BNNT unzipping. This method is not derived from known carbon nanotube unzipping strategies, but is unique to BNNTs because the use of aqueous ammonia solutions specifically targets the B‐N bond network. This study may pave the way for convenient processing of BNNTs, previously thought to be highly inert, toward controlling their dispersion, purity, lengths, and electronic properties.

Fabrication and characterization of BN/AlGaN/GaN metal‐insulator‐semiconductor heterojunction field‐effect transistors with sputtering‐deposited BN gate dielectric

AbstractWe investigated BN/AlGaN/GaN metal‐insulator‐semiconductor heterojunction field‐effect transistors (MIS‐HFETs) with sputtering‐deposited BN gate dielectric. Amorphous BN thin films obtained by RF magnetron sputtering deposition were applied to MIS‐HFET device fabrication. The MIS‐HFET devices exhibit very low gate leakage current indicating good insulating properties of BN, and almost no negative drain conductance. The device characteristics and its temperature dependence were investigated, where we observe decreasing drain current and increasing gate current with increase in temperature. The relative decrease in the drain current for the low field region is larger than that for the high field region, according to the temperature dependence of low‐ and high‐field electron velocities. The temperature dependence of the gate leakage current is explained by a mechanism with thermally activated tunneling and direct tunneling. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure and mechanical property evaluation of Cr–Ti–B–N coatings

In this study, five Cr–Ti–B–N thin films with different B/(Cr+Ti) ratios ranging from 0.1 to 0.69 were fabricated on silicon substrates and AISI420 stainless steel disks by pulsed DC magnetron sputtering using the Cr and TiB2 targets. It was found that the B content had strong influences on the structures and mechanical properties of Cr–Ti–B–N coatings. When the boron content reached 17at.%, the microstructure changed from the nanocolumnar structure to a nanocomposite structure with nanograins around 4nm in diameter embedded in the amorphous matrix. As compared with Cr–B–N coating, the solution hardening effect was found to result from the addition of Ti element. The average surface roughness, hardness and elastic modulus of Cr–Ti–B–N film decreased with increasing B content due to the amorphous BN phase. No chipping failures were found adjacent to the scratch track or indentation craters of each coating. In this study, the 37.3 Cr-2.3 Ti-4.0 B-56.0N (in at.%) coating provided an optimal combination of hardness, wear resistance and adhesion.

Structures and magnetic properties of Fe–N alloys formed by ball milling of mixture of Fe and BN

Diffusion reaction between Fe and BN and structural and magnetic properties of Fe–N alloys are investigated by ball milling of mixture of the α-Fe and hexagonal BN powders with volume ratio of 1:12.5, 1:5.4, 1:1.2 and 1:0.38 up to 72h. It is found that Fe can react with N of the BN to form amorphous Fe–N alloy (α-Fe–N) when the grain size of the Fe decreases to 8nm and the hexagonal BN transforms into amorphous BN; further milling, α-Fe–N can crystallize to ε-FexN. The formation of ε-FexN is related to the volume ratio of Fe to BN, when the ratio is larger than 1:0.38, no ε-FexN forms. The N content of ε-FexN increases with milling time and the volume ratio increased, resulting in decrease in saturation magnetization of ε-FexN. ε-FexN transforms from ferromagnetism to paramagnetism, when the N content exceeds 43at%.

Effect of B2O3 Additives on GaN Powder Synthesis from GaOOH

In this study, GaN powders were synthesized from gallium oxide-hydroxide (GaOOH) through an ammonification process in an <TEX>$NH_3$</TEX> flow with the variation of <TEX>$B_2O_3$</TEX> additives within a temperature range of <TEX>$300-1050^{\circ}C$</TEX>. The additive effect of <TEX>$B_2O_3$</TEX> on the hexagonal phase GaN powder synthesis route was examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transformation infrared transmission (FTIR) spectroscopy. With increasing the mol% of <TEX>$B_2O_3$</TEX> additive in the GaOOH precursor powder, the transition temperature and the activation energy for GaN powder formation increased while the GaN synthesis limit-time (<TEX>$t_c$</TEX>) shortened. The XPS results showed that Boron compounds of <TEX>$B_2O_3$</TEX> and BN coexisted in the synthesized GaN powders. From the FTIR spectra, we were able to confirm that the GaN powder consisted of an amorphous or cubic phase <TEX>$B_2O_3$</TEX> due to bond formation between B and O and the amorphous phase BN due to B-N bonds. The GaN powder synthesized from GaOOH and <TEX>$B_2O_3$</TEX> mixed powder by an ammonification route through <TEX>${\beta}-Ga_2O_3$</TEX> intermediate state. During the ammonification process, boron compounds of <TEX>$B_2O_3$</TEX> and BN coated <TEX>${\beta}-Ga_2O_3$</TEX> and GaN particles limited further nitridation processes.

Syntheses and Properties of Quaternary Cr-Ti-B-N Coatings by a High Power Impulse Magnetron Sputtering Technique

Cr-Ti-B-N coatings were synthesized by a hybrid coating system combining high power impulse magnetron sputtering (HIPIMS) and DC pulse magnetron sputtering from a <TEX>$TiB_2$</TEX> and a Cr target in argon-nitrogen environment, respectively. By changing the power applied on the Cr and <TEX>$TiB_2$</TEX> cathodes, the Cr-Ti-B-N coatings with various Ti/Cr ratio and B content were deposited. The phase structure, microstructure and chemical compositions of the Cr-Ti-B-N coatings were studied by X-ray diffraction (XRD), transmission scanning electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). With increase of Cr element in the coatings, the nanocomposite microstructure consisting of nano-sized (Cr, Ti) N crystallites and amorphous BN phase were obtained in the coatings. The microhardness of the Cr-Ti-B-N coatings exhibited a peak value of ~41 GPa for the <TEX>$CrTi_{0.1}B_{0.4}N_{1.3}$</TEX>, and then decreased with further increase of Cr content in the coatings, and all the coatings exhibited low friction coefficient. The oxidation and corrosion behavior of the Cr-Ti-B-N coatings revealed better properties due to the formation of a nanocomposite microstructure.

Microstructure evolution and mechanical properties of Ti–B–N coatings deposited by plasma-enhanced chemical vapor deposition

Ternary Ti–B–N coatings were synthesized on AISI 304 and Si wafer by plasma-enhanced chemical vapor deposition (PECVD) technique using a gaseous mixture of TiCl4, BCl3, H2, N2, and Ar. By virtue of X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscope, and high-resolution transmission electron microscope, the influences of B content on the microstructure and properties of Ti–B–N coatings were investigated systematically. The results indicated that the microstructure and mechanical properties of Ti–B–N coatings largely depend on the transformation from FCC-TiN phase to HCP-TiB2 phase. With increasing B content and decreasing N content in the coatings, the coating microstructure evolves gradually from FCC-TiN/a-BN to HCP-TiB2/a-BN via FCC-TiN+HCP-TiB2/a-BN. The highest microhardness of about 34 GPa is achieved, which corresponds to the nanocomposite Ti–63%B–N (mole fraction) coating consisting of the HCP-TiB2 nano-crystallites and amorphous BN phase. The lowest friction-coefficient was observed for the nanocomposite Ti–41%B–N (mole fraction) coating consisting of the FCC-TiN nanocrystallites and amorphous BN phase.

Synthesis of multiwall boron nitride nanotubes dependent on crystallographic structure of boron

Synthesis and growth of multiwall boron nitride nanotubes (BNNTs) under the B and ZrO2 seed system in the milling–annealing process were investigated. BNNTs were synthesized by annealing a mechanically activated boron powder under nitrogen environment. We explored the aspects of the mechanical activation energy transferred to milled crystalline boron powder producing structural disorder and borothermal reaction of the ZrO2 seed particles on the synthesis of BNNTs during annealing. Under these circumstances, the chemical reaction of amorphous boron coated on the seed nanoparticles with nitrogen synthesizing amorphous BN could be enhanced. It was found that amorphous BN was crystallized to the layer structure and then grown to multiwall BNNTs during annealing. Especially, bamboo-type multiwall BNNTs were mostly produced and grown to the tail-side of the nanotube not to the round head-side. Open gaps with ∼0.3 nm of the bamboo side walls of BNNTs were also observed. Based on these understandings, it might be possible to produce bamboo-type multiwall BNNTs by optimization of the structure and shape of boron coat on the seed nanoparticles.