Synthesis, Characterization and Applications of Boron Nitride, Carbon Nitride and Fullerene Structures
Tuesday, May 1, 2001 8:30 AM in Room Royal Palm 4-6
D1-2-1 Synthesis and TEM Analysis of Boron Nitride Nanotubes and Nanocones
Y. Bando (National Institute for Research in Inorganic Materials, Japan); D. Golberg (National Institute for Research in Inortanic Mateirals, Japan)
Nanotubes and nanocones made of boron nitride (BN) were synthesized at high yields through chemical reactions at 1700-2200 K. The nanostructures were studied by means of a JEM-3000F field emission high resolution transmission electron microscope (HRTEM) equipped with a parallel Electron energy loss detector (EELS) and Energy dispersion X-ray spectrometer (EDX). HRTEM and nanobeam electron diffraction allowed the orientation of the BN hexagonal rings to be determined. Preferential "zigzag" chirality of BN nanotubes and orderly stacked 240 degree disclinations in BN nanocones were found. 3D structural models of BN nanotubes and nanocones were constructed. The effects of synthesis parameters and chemical reaction promoters on the yields and morphologies of the nanostructures were investigated in detail. Nanostructure growth models were finally proposed.
D1-2-3 In-Situ Characterization of BN Film Growth
W. Fukarek (Leybold Optics, Germany); C. Fitz (FZ Rossendorf, Germany)
Real time ellipsometry provides information on the dielectric function and dynamic film thickness. Moreover, relative density depth profiles can be derived either from refractive index depth profiles or from dynamic growth rate data if the flux of film forming particles is measured in real time as well. Absolute density depth profiles can be derived by scaling the integral to the areal mass density as obtained from ion beam analysis. In-plane stress in thin amorphous or nanocrystalline films, where diffraction methods can not or only hardly be applied, can be measured in situ also under harsh conditions employing optical sampling of cantilever bending. From real time film thickness and curvature measurement instantaneous stress depth profiles are derived with a depth resolution in the nanometer range. The synergistic effects on the information obtained from ellipsometry, particle flux, and cantilever bending data recorded simultaneously are demonstrated exemplarily for ion beam assisted deposition of boron nitride films. In turbostratic (tBN) films the density is found to increase slightly with film thickness, whereas the compressive stress decreases, indicating an increasing quality and/or size of crystallites in the course of film growth. In the case of cubic (cBN) film growth the density starts to increase rapidly from the value characteristic of tBN up to the density of cBN (3.6 g/cm@super 3@). Comparison with the dark field TEM graph recorded from the cBN <111> reflex reveals that the increase in density coincides with the nucleation and increase in crystallite size of cBN. The density remains constant when coalescence of the cBN crystallites is observed in dark field TEM. In contrast to the growth of tBN the depth profile of the instantaneous compressive stress in cBN films is found to be quite complex and not only related to changes in the crystalline structure.
D1-2-5 Parameter Spaces for the Nucleation and the Subsequent Growth of Cubic Boron Nitride Films
W. Kulisch (University of Kassel, Germany); S. Ulrich (Forschungszentrum Karlsruhe, Germany)
The existing literature data on the deposition of cubic boron nitride thin films have been critically reviewed in order to establish the parameter spaces of c-BN deposition. The ion energy E@sub i@, the ion flux ratios @Phi@ = incoming ions/incoming atoms and @Phi@@super*@ = incoming ions/incorporated atoms, the ion mass (or the ratio Ar/N@sub 2@, respectively), the absolute values of the fluxed and the substrate temperature were identified as the decisive parameters. In agreement with earlier data collections on c-BN deposition@footnote 1@, irrespective of the deposition technique used a well-define c-BN region exists in the @Phi@/E@sub i@ parameter space, in which the deposition of c-BN is possible. However, in contrast to this older work it turns out that the boundaries of this c-BN region are different for nucleation and growth. After successful nucleation, the subsequent growth can take place either at reduced energy or reduced flux ratios (or a reduced Ar/N@sub 2@ ratio), and also at reduced temperatures. This means that the nucleation and growth of c-BN may rely on different (although in both cases ion-induced) mechanisms. The boundaries of the c-BN region depend also on the other important parameters, i.e. on ion mass and temperature. The existing models (subplantation model, stress model, quenching model, sputter model) on the ion-assisted formation of cubic boron nitride films, which are mainly based on the old boundaries in @footnote 1@, are critically reviewed with respect to this revised data collection. @footnote 1@Reinke et al., Diamond Relat. Mater 4, 272 (1995). }
D1-2-6 Synthesis and Characterization of Hexagonal/Cubic Boron Nitride Multilayer Films
M.U. Guruz, V.P. Dravid (Northwestern University)
Growth of cubic boron nitride films typically requires intense ion bombardment, which in turn results in films that are highly stressed and prone to delamination. Additionally, moisture is typically found to increase the tendency for delamination. To address these problems, attempts have been made to grow multilayer films consisting of alternating layers of hexagonal (h-BN) and cubic boron (c-BN) nitride. Here, the h-BN layers act to relieve the stress in the films. To promote adhesion, buffer layers of B@sub 4@C and Ti has been deposited prior to boron nitride growth. The films were synthesized by pulsed dc magnetron sputtering using a B@sub 4@C target. The switch from c-BN to h-BN was accomplished by reducing the substrate bias and/or incoming ion flux, which was controlled by an external coil assembly placed outside the chamber. The resulting film microstructure and properties were investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Fourier transform infrared spectroscopy (FTIR) and nanoindentation. The effects of these approaches on the formation of films with low-stress and high adhesion will be presented.
D1-2-7 Dependence of the Stress of c-BN Films on the Major Deposition Parameters: Theoretical and Experimental Studies
A. Klett, R. Freudenstein, W. Kulisch (University of Kassel, Germany)
High compressive stress is one of the major problems preventing at the present time any application of cubic boron nitride thin films. In order to solve this problem, a detailed understanding of the origins of stress formation in c-BN films is required. We will present a simple model which assumes that the stress is due to defects, especially interstitials, which are created by the strong ion bombard-ment which is necessary to obtain the cubic phase. However, via relaxation processes during the thermal spikes, energetic ions also lead to defect recombination; as a consequence, the resulting stress is determined by a balance of defect generation and recombination. By means of TRIM calculations, we investigated the interstitial generation as a function of important process parameters such as the ion energy and ion mass. From these results, our model predicts, for example, a stress reduction with increasing ion energy. In our experimental work, we investigated the stress of c-BN films depth-resolved by means of the bending of cantilevers on specially designed substrates. The dependence of the stress on the major deposition parameters (ion energy, ion mass (by varying the Ar/N@sub 2@ ratio), substrate temperature) was investigated; e.g. a decrease was found with increasing energy as predicted by the model. Furthermore, the incorporation of Ar ions within the films was investigated by means of ERD measurements. Although the argon content increases with increasing Ar fraction in the ion beam to maximum values of about 2%, it can be excluded from closer inspection that Ar yields a major stress contribution. Rather, the incorporated Ar is situated at the grain boundaries of the nanocrystalline films which play also an important role in stress release. From correlations between the stress of the h-BN nucleation layer and the thicknesses of this layer and that of the transition region from h-BN to c-BN, it can finally be concluded that stress plays an important role for c-BN nucleation but not for its subsequent growth.
D1-2-8 Active Cubic Boron Nitride Nucleation Sites at Different Stress Levels
Quan Li (City University of Hong Kong, China); L.D. Marks (Northwestern University); Y. Lifshitz (Soreq Nuclear Research Center, Israel); I. Bello, S.T. Lee (City University of Hong Kong, China)
Cubic boron nitride (c-BN) is an ideal candidate for hard protective coating because of its many superior properties such as high hardness and chemical inertness with Fe and Fe-based alloys. However, the ideal thickness required for most wear-protection applications should be greater than 0.5 µm. It is well known that c-BN coatings usually experience cracking and delamination for film thickness @>@ 200nm, due to high growth-induced compressive stress. Much effort has been made to reduce the stress level in c-BN films including separation of nucleation and growth during film deposition, i.e., reduce the ion-bombardment after c-BN nucleation. However, intensive ion-bombardment is still applied to ensure c-BN nucleation. The stress levels in most of the c-BN films are higher than 5 GPa, which is too high for thick film growth. In this study, c-BN films were deposited by rf magnetron sputtering with different substrate bias. Transmission electron microscopy (TEM) was performed to study the microstructure of c-BN upon nucleation. Transmission electron diffraction pattern was used to investigate the orientation relationship among Si substrate, turbostratic boron nitride (t-BN), and c-BN; high-resolution electron microscopy (HREM) was used to study the nucleation of c-BN on t-BN. The internal stress levels were determined by measuring the curvature of the as-deposited films. Two major types of nucleation relationships between c-BN and t-BN, associated with different stress levels were observed, i.e., “edge” nucleation at high stress level and “curvature” nucleation at low stress level. The latter type suggested great possibility to reduce the stress at the initial nucleation stage of c-BN, which could lead to thick and better quality film growth. Moreover, this possibility is strongly supported by the recent work of Matsumoto’s group in Japan, who has successfully grown c-BN films up to 10 microns with low internal stress. TEM study of their samples showed that the nucleation relationship between c-BN and t-BN is very similar to our samples with low internal stress.
D1-2-9 Aluminium-Doped c-BN Films by Magnetron Sputtering
F. Richter, V. Linss, T. Pfeifer, T. Welzel, H. Kupfer, E. Weissmantel, T. Halm, W. Hoyer (TU Chemnitz, Germany)
Cubic boron nitride thin films with an intentional aluminium incorporation (c-BN:Al films) have been deposited by r.f. magnetron sputtering of an h-BN target in pure nitrogen atmosphere. The aluminium flux was provided by an additional aluminium sputter cathode which was mounted all round the circular magnetron. Cubic BN films have been obtained up to a maximum aluminium incorporation of 1.3 at.% (SIMS measurement). Aluminium atoms in the gas phase have been observed by laser-induced fluorescence. In this manner, the relative flux of aluminium atoms in the vacuum chamber in dependence on position and electrostatic potential of the Al electrode could be analysed and optimum conditions for relatively homogeneous incorporation be found. The number density of Al atoms in the gas phase did monotonously increase with the electric power conversion at the Al electrode, in a similar manner as the aluminium incorporation concentration. Using 1.1 Å synchrotron radiation (ESRF Grenoble), the d@sub 111@ interplanar lattice spacings of the films have been measured in dependence on the inclination of the lattice planes relative to the substrate surface. We found a good linearity with sin@super 2@ @PSI@ (@PSI@ - angle of inclination) indicating a biaxial compressive film stress as anticipated. The stress values were about -10 GPa for all samples, i. e. relatively independent of the Al content. The unstressed lattice spacing showed a linear increase with the aluminium concentration. In addition, elastic properties of the films as well as results on the sites of incorporation of the Al in the cubic BN lattice are reported. Finally, conclusions are drawn on possible utilisation of Al-doped c-BN.
D1-2-10 Thick Ultra-hard Coatings Based on Cubic Boron Nitride
R. Clarke (University of Michigan); A. Inspektor (Kennametal, Inc.); D. Litvinov (Seagate Research); D. Barlett, C.A. Taylor II (k-Space Associates Inc.)
Cubic boron nitride (c-BN) has long been sought as an alternative to diamond for applications in ultrahard low-wear coatings technology. Its tribological properties are comparable to those of diamond with the added advantages of chemical inertness and better high temperature stability. In this paper we present a new approach to prepare 2@micron@m thick ultrahard coatings based on the versatile structural polymorphism of boron nitride and related ternary compounds. This is achieved by in-situ real-time monitoring of the film stress, using a high sensitivity optical stress monitor, for fine control of the deposition conditions while minimizing residual stress in the film. Such conditions include the use of compliant buffer layers high plasma density with reduced-bias growth techniques, and high-temperature annealing.
D1-2-11 Deposition and Tribological Properties of Thick c-BN Films
M. Keunecke (Fraunhofer Institute for Surface Engineering and Thin Films, Germany); K. Yamamoto (Kobe Steel LTD, Japan); K. Bewilogua (Fraunhofer Gesellschaft, Germany)
With a modified sputter technique we succeeded to deposit cubic boron nitride (c-BN) films with unique quality and a thickness of more than 2 µm on silicon substrates. Experiments were carried out using a r.f. diode sputtering device with boron carbide as target material. The improvement was reached with a complex process including a boron carbide interlayer and a B-C-N gradient layer and the cubic boron nitride (c-BN) top layer. Structure and composition of the films were investigated by IR spectroscopy, XRD, TEM and SEM images as well as by SIMS. Tribological and mechanical properties of thick (>1 µm) and long time stable films were investigated in detail. Quantities like friction coefficients against different materials, abrasive wear rates as well as hardness, Young`s modulus or surface energy data of our c-BN films were determined. The hardness measured with a nano-indentation technique was about 65 GPa and corresponds to the known cBN bulk value. Abrasive wear rates were clearly lower than for hard coating materials like TiN, TiCN. Furthermore the actual situation of research work and results concerning process transfer to technical substrates like hard metal (WC-Co) or high speed steel as well as alternative deposition methods, ion plating or magnetron sputtering, will be briefly presented.