ICMCTF2011 Session F3: New Boron, Boride and Boron Nitride Based Coatings

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.

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 ~150^oC. A more intense external heating up to ~500^oC, 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 ~100^oC 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

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.

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 HfN_x (x > 1), but no Hf-B bonds; at 1 mTorr of NH₃ pressure all the boron is displaced, resulting in HfN_x 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.

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.

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 BC_xN 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 BC_xN. 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 B₂C_xN₃, BC_xN and B₂C_xN (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 (B₂C_xN) showed clear signs of icosahedral BC_x-like and a-C phase segregation in a hexagonal B-C-N matrix, whilst N-rich samples (B₂C_xN₃) exhibited CN_x segregated phases. On the other hand, the spectral features of BC_xN 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.

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.

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.