ICMCTF2013 Session F3-1: New Boron, Boride and Boron Nitride Based Coatings

The corrosion-wear material loss of metallic surfaces is a serious concern in many application sectors, ranging from bio-medical implants to marine, oil and gas field components to transport vehicle and nuclear reactor devices. To date little effort has focused on developing specific coating materials to combat corrosive-wear processes. The present paper explores the viability of using Cr-B-N based coatings and reports their performance when applied to a grade of super austenitic stainless steel and subjected to reciprocation sliding contact tests (against aluminium oxide) in an aqueous 0.9%NaCl solution under a normal force of 1N. (Super austenitic stainless steels are widely used in the oil & gas sector as well as in the nuclear and bio-medical device industries) The electrode potential and corrosion current was monitored (where possible) throughout the tests. Whilst in principle the formation of low friction layers based on BN was considered probable, in practice this did not happen, instead relatively high friction layers were produced. On the whole, increasing the N content of the coatings caused a deterioration in hardness and corrosion-wear resistance. In fact the best corrosion-wear protection was offered by Cr-B coatings containing no additions of N. The same coatings have also shown useful performance for the protection of ferritic stainless steel internal combustion engine piston rings in elevated temperature (circa 190˚C) high speed organic fluid lubricated reciprocation sliding contact tests.

Thin Me-B-C coatings (Me = early transition metal) have interesting mechanical and tribological properties. Only one ternary phase, Mo₂BC, is known and predicted to exhibit a unique combination of stiffness and ductility [1]. The stability of this ternary structure is considerably less for other Me-B-C systems and some explanations for this trend will be discussed. The thermodynamically most stable phase combination in most Me-B-C systems is therefore a mixture of binary phases. During magnetron sputtering at lower temperatures, the high quenching rate combined with low diffusion rates make it difficult to form the crystalline binary phases and amorphous growth is therefore frequently observed. A general overview of this behaviour will be discussed for different early transition metals. Some general trends in the correlation between materials properties and composition will be discussed with a special emphasis on the Cr-B-C system. Magnetron sputtering of Cr-B-C coatings using targets of CrB₂ and graphite leads in general to films with a B/Cr ratio < 2. With increasing carbon content the

coatings become amorphous. X-ray photoelectron spectroscopy (XPS) suggests a mixture of Cr-B, B-C and C-C bonds. The friction and wear of this material is dependent on the carbon content and a reduced friction coefficient is observed at higher carbon contents. The tribological behavior is, however, strongly dependent on the humidity. Finally, the possibility for formation of low friction boron oxide tribofilms on this type of materials will be discussed.

[1]. J. Emmerlich et al., J. Phys. D. Appl. Phys. 42 (2009) 185406

There is significant interest in inertial confinement fusion (ICF) and one of the institutions where research is being performed is the National Ignition Facility in Livermore. Thick film ablators serve a critical role in the targets and historically Plastic, Beryllium, and Diamond, have been considered as potential ablators. For Beryllium and Diamond, films several tens to hundreds of microns thick, are typically deposited on a spherical substrate that is subsequently removed to leave a hollow capsule that is filled with a deuterium-based gas. Due in part to the challenges in achieving ignition, there is a strong desire to have an alternative ablator candidate. And due to the fact that the film needs to be of a high quality and deposit uniformly on a small spherical capsule, PVD methods are usually preferred.

Based on simulations a new and potentially very promising ablator material is Boron Carbide. One challenge in employing this material as an ablator is that PVD Boron Carbide typically has a high film stress which limits the thickness achievable before delamination occurs. We have used a PVD process, e-beam evaporation, to deposit Boron Carbide films up to and exceeding 100 microns thick, which to our knowledge may be a record thickness for PVD deposited Boron Carbide. We shall briefly discuss efforts to use our Boron Carbide films for preliminary high pressure experimental studies at the Omega facility. We shall also discuss the process research/development that lead to these very thick films as well as present metrology results on the structure and properties of the films; for example, results showing that the Boron Carbide films were very hard.

Lately, the demands for ³He have increased, mainly due to the U.S. Homeland Security programs. At the same time the production of this rare gas has decreased since the end of the cold war, due to the main source being the radioactive decay of tritium. This had led to an urgent need for alternatives to ³He-based neutron detection techniques at large-scale neutron research facilities. New large area neutron detectors, including the ones that will be built at the European Spallation Source (ESS), or radiation scanners at, e.g., airports, would require much more than the complete U.S. supply of ³He.

We present a new generation of neutron detectors that use ¹⁰B-containing thin films as the neutron-absorbing material instead of ³He. The detectors comprise thin films of ¹⁰B₄C, which are deposited onto Al-blades or Si-wafers. A full-scale detector needs in total ~1000 m² of two-side coated Al-blades with ~1 μm thick ¹⁰B₄C films. Tough demands on film purity and thickness uniformities make it a big challenge to upscale such a process (total need > 7000 m²) to fulfill the demands of the ESS.

DC magnetron sputtering (PVD) in an industrial deposition system, from ^natB₄C and ¹⁰B₄C targets has been used for coatings on flat surfaces. For detector designs that need films on irregularly shaped substrates, chemical vapor deposition (CVD) is preferred. Since the substrate is Al, the temperature shall not exceed 600 °C, which is why both thermally activated and plasma enhance CVD are of interest.

The coatings have been characterized with scanning electron microscopy, elastic recoil detection analysis, X-ray reflectivity, and neutron scattering. Substrate temperatures of 400 °C result in PVD films with a density close to bulk values and good adhesion to film thicknesses above 3 μm. The ¹⁰B content is close to 80 at.%, i.e. full isotope enrichment, with impurity levels of less than 1 at.% of H, N, and O. Various relevant properties, including stress in the coatings and neutron radiation damage, have been looked into. Simulations to predict detector performance and optimal thin film properties have been supporting the experimental work.

Detector prototypes with ¹⁰B₄C thin films have been tested for their neutron performance and compared with existing flagship instruments, like the IN6 at the ILL in France. The prototypes yield a neutron detection efficiency of ~50%, which is in general agreement with simulated results. These new ¹⁰B₄C thin film based neutron detectors have a potential to replace most ³He-containing detectors. The development will continue far into the ESS construction phase, which lasts until ~2025.

Boron carbide (BC_x) coatings appear very attractive due to their high hardness and interesting tribological properties. In the present work, we systematically studied the effect of stoichiometry of BC_x films (0 < x < 1) as a means to tailor their hardness (H) and Young’s modulus (E) as well as the wear coefficient and corrosion resistance. The BC_x films were deposited on Si (100) and M2 high speed steel substrates using a pilot-scale closed-field unbalanced magnetron sputtering system equipped with one graphite and two boron targets. Different compositions were obtained by tuning the graphite target current. We found that the hardness of the BC_x films (measured on Si) decreases from 28 GPa to 18 GPa as the carbon content [C] increases from 22.1 at.% to 65.3 at.%, but it increases again up to ~22 GPa when reaching [C] = 82.0 at.%. The hardness variation is explained by changes in the film microstructure, namely formation of a nanocomposite structure formed by BC_x nanocrystals dispersed in an amorphous BC_y/a-C matrix as confirmed by a combination of XRD, TEM XPS, micro-Raman and ERD measurements. The friction coefficient of the BC_x films with a 200 nm TiB₂ interlayer on the high speed steel substrates decreased from 0.7 to 0.2, and the wear rate against alumina ball (6 mm diameter) decreased from 6.4×10^-5 mm³/N-m to 1.3×10^-7 mm³/N-m as [C] was increased. Subsequent surface analyses clearly indicated that improvement of the tribological properties of the BC_x films is primarily due to the formation of a graphitic tribolayer that acts as a solid lubricant during the wear process. Application of the BC_x coating possessing a high hardness (28 GPa) and 22.1 at.% of carbon improved the corrosion resistance of the M2 steel substrate by four orders of magnitude, documented by a decrease of the corrosion current from 3×10^-6 A/cm² to 8×10^-10 A/cm².

Recent progress in boron nitride nanotubes (BNNTs) syntheses opened new possibilities for utilization of the attractive combination of their excellent mechanical characteristics and superb thermal and chemical stabilities for the creation of new structural and functional reinforced materials. In this work we have applied metal ion implantation to prepare novel BNNTs/metal matrix composites. The resulting structures have thoroughly been studied by high-resolution transmission electron microscopy and Raman spectroscopy. The obtained results show that by changing the ion implantation parameters it is possible to fabricate BNNT/metal composites with different contents of BNNT crystalline phase and controlled morphologies, structures, and volume fractions of metal phase additives. Such novel composite nanomaterials are envisaged to be attractive for many structural and functional applications, in particular for reinforcing ultralight Al-based alloys.