ICMCTF2011 Session F3: New Boron, Boride and Boron Nitride Based Coatings
Wednesday, May 4, 2011 1:30 PM in Sunset
F3-1 Hardness, Thermal Stability and Oxidation Resistance of Cr5B3 Films
David Pilloud, Jean-François Pierson (Ecole des Mines de Nancy, France)
The binary Cr-B system contains several defined compounds: Cr2B, Cr5B3, CrB, Cr3B4, CrB2 and CrB4. Although the literature reports information on the deposition of CrB or CrB2 films using sputtering processes, there is a dramatic lack of detail about the synthesis of others chromium boride films. Furthermore, the properties of borides such as Cr5B3 are seldom described in the literature. Then, the aim of this presentation is to bring relevant information to the synthesis and the properties of Cr5B3 films.
Cr-B films were deposited on silicon, stainless steel and glass substrates by pulsed DC sputtering of a Cr/B (60/40 at.%) target synthesized by SHS method. The films were deposited without external heating and the deposition temperature was close to 50 °C . X-ray diffraction analyses clearly evidenced that as-deposited films were X-ray amorphous. In addition, as-deposited films were highly reflecting and a value of 166 µW cm was measured for their electrical resistivity at room temperature. The crystallisation of the films was studied by in situ XRD during annealing in an inert atmosphere. The formation of the crystalline Cr5B3 phase started at 700°C . SIMS analyses performed on as-deposited and annealed films showed that the chromium to boron atomic ratio is kept constant in the whole film depth, indicating that chromium and boron atoms did not diffuse into the gas phase or in the substrate. Air annealing treatments were also performed on the coatings to evaluate their oxidation resistance. After a 2 hours annealing at 700°C, the oxide layer thickness was estimated at approximately 50 nm. SIMS analyses showed that the oxide layer contained only chromium oxide, indicating that boron atoms diffused into the gas phase. As-deposited films exhibited a hardness of 20 GPa. In contrast, the annealing of the films and their crystallisation induced a softening of the material, values of 15 and 12 GPa were measured on amorphous film annealed at 500°C and crystalline film annealed at 700°C, respectively.
F3-2 Direct Current Magnetron Sputtering of ZrB2 Thin Films from a Compound Target in an Industrial Scale Deposition System
Hans Högberg (Linköping University, Sweden); Mikael Ottosson (Uppsala University, Sweden); Jun Lu, Jens Jensen, Mattias Samuelsson, Lars Hultman (Linköping University, Sweden)
Thin films of the refractory, oxidation resistant, and conductive ceramic ZrB2 have been sputtered from a compound target, using an industrial scale deposition system, CemeCon, CC 800®/9 ML, operated at a fixed target to substrate distance of 7 cm. Elastic recoil detection analysis show that close to stoichiometric films with a B to Zr ratio of 2 to 2.1 and total level of contaminants, including H, of less than 5% can be deposited on Si(100) at a growth rate of ~ 100 nm per minute and for Ar pressures ranging from 0.4 to 1.3 Pa. X-ray diffraction patterns show that 0001- oriented films are grown on Si(100) and Al2O3(0001) substrates without external heating or pulsed plasma cleaning and using a substrate bias of -80 V. Transmission electron microscopy show that the films deposited on Al2O3(0001) consist of an ~100 nm thick amorphous layer closest to the substrate followed by the nucleation of an ~300 nm thick 0001- oriented layer. Scanning electron microscopy (SEM) confirm such a layered growth mode also on Si(100), and for substrate temperatures ranging up to ~150oC. A more intense external heating up to ~500oC, gradually alters the 0001 orientation on Si(100) to a random orientation. For these films, the SEM cross section images reveal a microstructure consisting of broken columns, thus indicating re-nucleation. Four point probe measurements on films deposited on 1000 Å SiO2/ Si(100) substrates with or without external heating yield resistivity values in the region of 200 to 300 µΩ cm; with no clear trend for the resistivity with respect to the amount of applied external heating. The measured values are considerably higher than the resistivity reported for bulk ZrB2 with 4.6-9 µΩ cm, but reflects the properties of as deposited films. Growth at ~100oC and using either higher substrate bias, up to -200 V, or at lower, down to floating potential, yield little impact on the preferred 0001 orientation, but result in delamination of the films deposited with bias voltages above -120 V
F3-3 A Combinatorial Effect of Substrate and Surface Terminating Species on Phase Pure Growth of c-BN
Karin Larsson (Uppsala University, Sweden)
Cubic boron nitride (c-BN) has been in focus for several years due to its interesting and extreme properties, of which some are even superior to diamond. These important properties include extreme hardness, chemical stability, large band gap, wear resistance and chemical inertness. The possibility for large area deposition of c-BN is a requirement for realization of these different properties in various applications. Unfortunately, there are at present severe problems in the vapour phase synthesis of c-BN, which makes the possibility to grow large-area c-BN considerably smaller. It is, hence, urgent to investigate the possibility for new chemical growth pathways. This can be accomplished in a multidisciplinary way by combining experimental efforts with theoretical modeling (e.g., density functional theory under periodic boundary conditions).
Adsorption of growth and surface termination species onto the c-BN surface has been found to be two of the key elementary reactions during growth of c-BN. In addition, the choice of substrate is decisive for an ideal cubic phase to be formed directly onto the substrate surface. The purpose with the here presented study is to theoretically investigate the possibility for a layer-by-layer growth of c-BN, using a careful combinatorial choice of substrate material, surface terminating agent and growth species.
F3-5 Chemical Vapor Deposition of MB2 and M-B-N Alloys Below 300°C: Highly Conformal, Hard and Wear-Resistant Films
Andrew Cloud, John Abelson (University of Illinois at Urbana-Champaign)
Transition metal diborides are metallic ceramics with high electrical and thermal conductivities, melting temperatures around 3000°C, high hardnesses, and strong resistances to chemical attack. Chemical vapor deposition (CVD) of high quality thin films proceeds readily at low substrate temperatures from single source borohydride precursors that contain no O, C, N, or halogens. The coatings are smooth and extremely conformal; the step coverage is up to 90% in trenches or vias with aspect ratios up to 30:1. Reentrant and interior surfaces of components can thus be protected from mechanical wear or chemical attack using CVD diboride coatings. As-deposited films are X-ray amorphous with a nanoindentation hardness of more than 20 GPa; upon annealing at 700°C they obtain a fine-grained nanocrystalline state exhibiting ‘superhardness’ of 40 GPa.
Incorporation of nitrogen affords films that are a mixture of HfB2, HfN, and BN. Growth in the presence of ammonia produces films that contain BN and HfNx (x > 1), but no Hf-B bonds; at 1 mTorr of NH3 pressure all the boron is displaced, resulting in HfNx films with a bandgap of ~2.6 eV. N-containing alloys are softer than pure HfB2 films and do not crystallize upon annealing. The growth of HfB2 / Hf-B-N multilayers affords a means to adjust the effective elastic modulus of a coating while retaining high hardness.
Tribological measurements are performed using pin-on-disc, nanoscratch, and nanowear methods. HfB2 films show favorable friction behavior with respect to TiN. The wear rate is significantly reduced for annealed HfB2 and the critical energy for onset of steady-state wear is enhanced. Coatings on steel substrates have performed very well in dry cutting and aluminum die-casting applications under real-world industrial conditions.
F3-6 The Effect of Deposition Parameters on the Structure, Chemistry and Physical Properties of Deposited B-C-N Films
Mustafa Fatih Genisel, Erman Bengu (Bilkent University, Turkey)
In this study, we investigated the effect of process parameters during synthesis on the properties of the films in the B-C-N phase diagram. These films were synthesized using a reactive magnetron sputtering (RMS). The bonding characteristics of the deposited B-C-N films were investigated using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Our findings implied that the chemical environment of the bonds in the B-C-N films is very sensitive to synthesis parameters. We were not able to deduce the crystal structure of the films using X-Ray diffraction (XRD) technique due to the low range order in the films as the transmission electron microscope (TEM) images suggested. Hence, we have used evidence provided from FTIR, TEM, and XPS and using these in complement to clues obtained from some of the measurable bulk physical properties (hardness, optical band gap, electrical conductivity) to understand the chemical environment of the atoms and atomic structure of the films. The, nano-indentation measurements indicated that fully hybridized films produced by grounded substrates during synthesis to have a superior hardness to those deposited using higher substrate bias values. And also optical band gap analysis in which the UV-Vis spectroscopy data was used, shows that the band gap of the films were decrease to certain value when the applied radio frequency generated substrate bias was increase at same atomic composition. In addition to these electrical conductivity analysis shows the applied substrate bias effect the conductivity which is an important clue of the change atomic structure in the film. These suggests a change in the atomic structure of the films as well, which has been confirmed by TEM analysis. As a conclusion, in this study we investigated structural and chemical variations in the B-C-N films and their effect on the material properties.
F3-7 Bonding Structure of B-C-N Ternary Compounds and Their Tribomechanical Properties
Ignacio Caretti, Ignacio Jiménez (Instituto de Ciencia de Materiales de Madrid, Spain)
In the last three decades, research on B-C-N solid solutions has received considerable attention, inspired by the promising combination of the excellent chemical, thermal and mechanical properties of the hexagonal and cubic allotropic forms of BN (h-BN, c-BN) and C (graphite, diamond). From the very beginning, special interest has been paid to the BCxN stoichiometry, understood as a substitution of BN atomic pairs by isoelectronic CC pairs that satisfies the charge neutrality condition expected for a stable ternary compound. However, the synthesis of B-C-N compounds in thin film form often produces a broad range of compositions far from BCxN. Actually, several theoretical and experimental studies have pointed to the metastable character of B-C-N materials, a fact that is frequently translated into elemental and/or binary phase segregation.
In this work, we present a detailed and systematic study of the X-ray absorption near edge structure (XANES) of B-C-N ternary compounds with B2CxN3, BCxN and B2CxN (0<x<6) compositions. A large set of thin films representing these compositional lines has been synthesized at room temperature by ion beam assisted deposition (IBAD) using different ratios of the incoming B, C and N independent atomic fluxes. In this way, the influence of the deposition parameters on the composition and the resulting bonding structure of the BCN coatings can be thoroughly addressed.
On the one hand, the B-rich BCN compounds (B2CxN) showed clear signs of icosahedral BCx-like and a-C phase segregation in a hexagonal B-C-N matrix, whilst N-rich samples (B2CxN3) exhibited CNx segregated phases. On the other hand, the spectral features of BCxN layers could be correlated to a graphitic-like B-C-N single ternary phase, with increasing number of structural defects for increasing nominal B/N ratios of the relative impinging atomic fluxes used for the synthesis.In addition, the friction and wear resistance of all samples were determined by pin-on-disk experiments, which were performed at 25% humidity, using a WC/Co(6%) tip under 3N normal load and 375 rpm rotation speed. Moreover, nanoindentation measurements were done on the BCxN ternary thin films. It was found that BCxN layers with low concentration of defects exhibited the best tribomechanical performance. In this sense, the presence of defects was clearly counterproductive, as it was also the presence of segregated phases for C concentrations below ~50%. As a whole, a comprehensive discussion of the tribomechanical properties of B-C-N coatings with respect to their composition and bonding structure is provided.
F3-8 Influence of Nitrogen and Oxygen Addition on the Energy Flux in a rf-Magnetron Discharge for the Deposition of Superhard c-BN Coatings
Sven Bornholdt (Christian-Albrechts-Universität zu Kiel, Germany); Jian Ye, Sven Ulrich (Karlsruhe Institute of Technology (KIT), Germany); Holger Kersten (Christian-Albrechts-Universität zu Kiel, Germany)
R.f. magnetron sputtering of a hexagonal boron nitride target in an Ar-N2-O2 gas atmosphere can be used for the deposition of thick, superhard, oxygen-containing cubic boron nitride coatings. Under these deposition conditions the magnetron plasma is investigated by energy flux measurements with a calorimetric probe and by Langmuir double probe measurements. The results of the energy flux measurements are compared with estimations which are calculated on the basis of related plasma parameters (electron density, electron temperatures and ion fluxes) taken from Langmuir double probe measurements. In addition, SRIM simulations were performed to determine the contribution to the energy influx by sputtered boron and nitrogen atoms. The contributions of energetic plasma species, surface reactions and film growth to the resulting substrate temperature are discussed and the influence of nitrogen and oxygen addition to the process gas as well as the effect of substrate bias on the total energy flux have been determined. The largest contribution to the total energy influx in an argon-nitrogen sputtering atmosphere is caused by sputtered atoms. The variation by the nitrogen admixture, which causes a decrease of electron and ion densities, electron temperature and plasma potential are rather small. The typical hysteresis effect which can be observed during magnetron sputtering in oxygen containing gas mixtures can also be detected in the energy influx measurements.
F3-9 Electrochemical Boriding and Characterization of AISI D2 Tool Steel
Vivekanand Sista (Argonne National Laboratory); Ozgenur Kahvecioglu (Istanbul Technical University, Turkey); Osman Eryilmaz, Ali Erdemir (Argonne National Laboratory); Servet Timur (Istanbul Technical University, Turkey)
D2 is an air-hardening tool steel and provides excellent resistance to wear and corrosion, especially at elevated temperatures. Boriding of this steel can further enhance its surface mechanical and tribological properties but with the use of traditional pack boriding it was very difficult to achieve very dense and uniformly thick boride layer. In this study, we explored electrochemical boriding of AISI D2 tool steel in a molten borax electrolyte at 950°C for durations ranging from 15 minutes to 1 hour. We found that depending on the boriding time, a single phase Fe2B layer or a composite layer of FeB+Fe2B is produced on the surface. The microstructural characterization and phase analysis was performed using microscopy and x-ray diffraction methods, respectively. The boride layers formed after shorter durations (i.e., 15 min) mainly consisted of Fe2B phase and was about 30µm thick. The microhardness values of the boride layers varied between 25 and 30 GPa, depending on the phase composition.
F3-10 Ultra Fast Boriding of Nickel Aluminide
Ozgenur Kahvecioglu (Istanbul Technical University, Turkey); Vivekanand Sista, Osman Eryilmaz, Ali Erdemir (Argonne National Laboratory); Servet Timur (Istanbul Technical University, Turkey)
As an intermetallic material, nickel aluminide combines properties similar to both ceramics and metals. In particular, it can offer great mechanical strength and high resistance to high temperature corrosion and oxidizing environments. In this study, in order to further enhance its surface properties, we performed ultrafast boriding in a molten borax electrolyte for 15 minutes at 950°C at a current density of 0.1-0.5 A/cm2. The resultant boride layer formed on the test samples was about 50µm thick despite the very short boriding duration, i.e., 15 minutes . The mechanical, structural, and chemical characterization of the boride layer was carried out using a Vickers microhardness test machine, optical and scanning electron microscopes and x-ray diffraction. The hardness of boride layer was in the range of 1570 to 1700 ± 20 HV, while x-ray diffraction confirmed that the layer was primarily composed of Ni3B and Ni4B3 phases. Structurally, the boride layer was very homogenous and uniformly thick across the borided surface area.