Magnetron sputter deposition of ultrathick boron carbide coatings on spherical substrates for inertial confinement fusion
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Merlo, J. B. (ORCID:0009000092502072)↗
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Results for “boron carbide”
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The effects of carbonaceous inclusions and their distributions on dynamic failure processes in boron carbide ceramics
Commercially available boron carbide ceramics typically have heterogeneous microstructures that contain distributions of processing-induced inclusions. The inclusions that are rich in carbon (i.e., carbonaceous) govern the underlying mechanisms of brittle fracture through wing crack formation, and thus dictate the mechanical response of the ceramic. In this study, we investigate the dynamic failure of five boron carbide ceramic materials with different inclusion populations. All of the materials were prepared by hot-pressing; four of these boron carbides contained different sizes and concentrations of carbonaceous inclusions, while one contained no carbonaceous inclusions. The heterogeneity distributions were characterized in some detail for statistical analysis using scanning electron microscopy and quantitative image analysis. A modified compression Kolsky bar setup with in situ ultra-high-speed microscopic imaging (10 million frames per second) was then used to study the influence of the inclusion distributions on the dynamic failure processes in these materials, at nominal high strain rates of 10 2 -10 3 s -1 . The in situ ultra-high-speed microscopy highlighted the link between micro and macroscale failure processes and demonstrated that the carbonaceous inclusions are indeed the preferential sites for nucleation of wing cracks, as previously hypothesized based on post-mortem observations. The relative orientation of an inclusion with respect to the compression axis was shown to affect the likelihood that it would participate in crack nucleation. In conclusion, all of the ceramics were also found to have orientation-dependent peak compressive stress, regardless of the presence of carbonaceous inclusions, suggesting that grain orientation distributions are also important.
36 MATERIALS SCIENCE↗
Synthesis and Thermal Oxidation Resistance of Boron-Rich Boron–Carbide Material
A boron-rich boron–carbide material (B4+δC) was synthesized by spark plasma sintering of a ball-milled mixture of high-purity boron powder and graphitic carbon at a pressure of 7 MPa and a temperature of 1930 °C. This high-pressure, high-temperature synthesized material was recovered and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Vickers hardness measurements, and thermal oxidation studies. The X-ray diffraction studies revealed a single-phase rhombohedral structure (space group R-3m) with lattice parameters in hexagonal representation as a = 5.609 ± 0.007 Å and c = 12.082 ± 0.02 Å. The experimental lattice parameters result in a value of δ = 0.55, or the composition of the synthesized compound as B4.55C. The high-resolution scans of boron binding energy reveal the existence of a B-C bond at 188.5 eV. Raman spectroscopy reveals the existence of a 386 cm−1 vibrational mode representative of C-B-B linear chain formation due to excess boron in the lattice. The measured Vickers microhardness at a load of 200 gf shows a high hardness value of 33.8 ± 2.3 GPa. Thermal gravimetric studies on B4.55C were conducted at a temperature of 1300 °C in a compressed dry air environment, and its behavior is compared to other high-temperature ceramic materials such as high-entropy transition metal boride. The high neutron absorption cross section, high melting point, high mechanical strength, and thermal oxidation resistance make this material ideal for applications in extreme environments.
36 MATERIALS SCIENCE↗
Synthesis of boron-carbide aerogels
We present the synthesis of boron carbide aerogels utilizing nano-boron powder and resorcinol–formaldehyde (RF) organic aerogels as precursors. Monolithic aerogels were fabricated from suspensions of boron nanoparticles and RF via an organic sol-gel process, enabling effective distribution of boron in the gel network. The resulting gels underwent supercritical drying, thermal reduction, and subsequent heat treatment to yield boron carbide aerogels with densities ranging from 37 to 55 mg/cm³. By tuning the boron-to-carbon ratio, heat treatment temperature, and dwell time, surface areas up to 53 m²/g were obtained. X-ray diffraction analysis confirmed the formation of the boron carbide phase and detected the presence of residual carbon within the structure.
Materials science↗
Additive manufacturing process for producing aluminum-boron carbide metal matrix composites
A method for additive manufacturing of a composite object containing a bonded network of boron carbide particles and aluminum occupying spaces between boron carbide particles, the method comprising: (i) producing a porous preform constructed of boron carbide by an additive manufacturing process in which particles of boron carbide are bonded together; and (ii) infiltrating molten aluminum, at a temperature of 1000-1400° C., into pores of said porous preform to produce said composite object constructed of boron carbide particles within an aluminum matrix, wherein the boron carbide is present in the composite object in an amount of 30-70 wt. %. The resulting composite material is also herein described.
Cramer, Corson L.↗
Thermostructural evolution of boron carbide characterized using in-situ x-ray diffraction
Boron carbide, with a nominal stoichiometry of B 4 C, is a highly desired ceramic candidate for armor applications due to its high hardness derived from the complex crystal structure. However, stress-induced local amorphization can lead to failure and is a known challenge for this material which must be addressed for applications in ballistic environments. Understanding boron carbide's atomic structural behavior and bonding environment is critical in determining effective strategies to mitigate these issues. Here, in this work, the thermo-structural behavior of B 4 C has been studied in detail using a conical nozzle levitator system coupled with in-situ synchrotron X-ray diffraction. Lattice expansion and the resulting thermal expansion coefficients (CTEs) were determined from 25-2100 °C. Rietveld refinements showed anisotropic atomic displacement for each of the 4 unique sites as a function of temperature. An exceptionally large z-axis displacement for the boron chain center is linked to bond weakness and may be linked to faster expansion of the α 33 relative to α 11 CTEs. Thermally induced lattice changes can inform the use of boron carbide at elevated temperatures as well as help develop strategies for mitigating structural failure for armor applications.
36 MATERIALS SCIENCE↗
Ultrathick Boron Carbide Coatings for Nuclear Fusion Targets
Boron carbide is an attractive ablator for next-generation inertial confinement fusion (ICF) targets. Here we describe several aspects of our ongoing systematic studies of the deposition and processing of B 4 C coatings for ICF targets. We show that residual compressive stress in films can be reduced and the deposition rate increased by N-doping. Further, we also demonstrate successful Si substrate etching and surface polishing and discuss remaining challenges and offer potential solutions to the buildup of particulates in the deposition chamber during prolonged coating runs, control of nodular growth defects, and lateral nonuniformity of film properties for deposition conditions with relatively low target-to-substrate distances.
36 MATERIALS SCIENCE↗
High-rate magnetron sputter deposition of low-stress boron carbide films on tilted substrates
Boron carbide is attractive for several applications, including nuclear fuel capsules for inertial confinement fusion (ICF). The fabrication of ICF capsules involves the deposition of ultrathick coatings with density and thickness uniformity on submicron length scales on non-planar (spherical) substrates. Such a deposition requires control of the deposition rate, residual stress, and film microstructure. Here, we systematically study the direct-current magnetron sputter deposition of B 4 C coatings with a full-face-erosion magnetron source as a function of substrate tilt and Ar working gas pressure. Film properties are correlated with results of plasma diagnostics and predictions of Monte Carlo simulations of ballistic sputtering and gas-phase transport. The fabrication of low-stress amorphous B 4 C films with deposition rates of 7μm/h is demonstrated.
Kawasaki, K. [Lawrence Livermore National Laborato↗
Effect of electronic transitions on near edge optical properties of off-stoichiometric boron carbide thin films
In the present study, soft x-ray optical properties of off-stoichiometric boron carbide thin films are investigated, and the structure and chemical composition of the film is analyzed using angle dependent x-ray reflectivity and x-ray photoelectron spectroscopy techniques. Energy dependent soft x-ray reflectivity measured at a fixed grazing angle of 1.5° is used to determine the optical constants in the boron K edge region by applying the Kramers–Kronig technique. The measured optical constants show near edge fine features corresponding to σ* and π* resonances. Furthermore, the electronic transitions corresponding to σ* resonance cause a 40%–75% increase in the delta value in the above boron K edge region. The π* transitions corresponding to off-stoichiometric nature of the boron carbide are observed in the absorption spectra near ~ 192.7 eV. Details of the measured soft x-ray optical properties of the off-stoichiometric boron carbide thin film are discussed.
36 MATERIALS SCIENCE↗
High-Rate Sputter Deposition of Ultrathick Boron Carbide Coatings on Rolling Spherical Substrates
Amorphous boron carbide (B 4 C) is a promising next-generation ablator material for inertial confinement fusion. However, the deposition of ultrathick B4C coatings with submicron-scale density uniformity on spherical substrates, as required for ablator shell fabrication, remains challenging. Here, in this study, we use direct current magnetron sputtering to deposit B 4 C onto 2-mm-diameter Si spheres rolling in a dish-shaped substrate holder. High deposition rates of ∼3 μm/h are achieved. Ultrathick films have a columnar microstructure, with the column width determined by the density of nodular defects. Nodular defect nucleation is dominated by the pickup of particulates from the holder during substrate rolling. We demonstrate the fabrication of a hollow B 4 C spherical shell with a wall thickness of 80 μm. Also demonstrated is the laser machining of holes in the B 4 C coating, which is necessary for both the chemical removal of the Si template and the attachment of a fusion fuel fill tube.
Inertial confinement fusion targets↗
Activating dislocation mediated plasticity in boron carbide through Al-doping
Dislocation slip, deformation twinning, phase transformations, and fast fracture are energy dissipation mechanisms that accommodate mechanical loading in materials. The energetically unfavorable formation of dislocation attributes to the ease of cracking and the low damage tolerance observed in superhard ceramics, notably boron carbide. Here, this work demonstrates that room temperature dislocation slip can be enabled in boron carbide by altering its chemistry through Al doping. The activation of dislocation slip is mechanistically explained by quantum mechanics simulations and electron microscopy, which indicate that strain energy is released through basal icosahedral slip facilitated by icosahedral rotation and chain bond breaking and reconfiguring. The new insight gained through this work suggests that atomic doping could be an effective strategy to tune deformation mechanisms in boron carbide, which provides a significant potential for limiting amorphization and catastrophic failure, and opens a new strategy to enhance damage tolerance in brittle ceramics.
36 MATERIALS SCIENCE↗
Effects of substrate bias and tilt on magnetron sputtered boron carbide films
Maintaining high density and good mechanical properties at oblique angles for magnetron sputtered amorphous boron carbide films remains a challenge. In this study, we investigate the effect of the substrate bias on density, residual stress, mechanical properties, and morphology of boron carbide films deposited via direct current magnetron sputtering with a full-face erosion magnetron source in a mixed Ar-Ne plasma. Results show that films deposited at oblique substrate tilt angles on substrates biased at -50 V possess significantly lower oxygen content, superior mechanical properties, and higher density compared to their electrically grounded counterparts. In conclusion, these enhancements observed at oblique angles, while promising, come at the cost of larger residual compressive stress and higher Ne incorporation in films.
Materials science↗
High pressure Raman spectroscopy of boron-rich boron carbides up to 50 GPa
We investigated the effects of increased boron content on the resistance to non-hydrostatic stress-induced (NHSI) local amorphous zones in boron-rich boron carbide (BxC) compounds. Using high-pressure Raman spectroscopy, we subjected B 4.3 C, B 6.4 C, and B 10.4 C to pressures up to 50 GPa and monitored their responses. Our results show that higher boron content delays the onset of NHSI local amorphous zone formation, shifting it from 35 GPa in B 4.3 C to approximately 50 GPa in B 10.4 C. This enhanced resistance is attributed to a reduction in the formation of the B 12 (CCC) polytype, which is susceptible to amorphization, and the greater flexibility of B–B–B chains. Furthermore, alternative mechanisms, such as boron vacancy-driven C–C bond formation, provide additional insights into defect-mediated structural changes that may influence the amorphization process. In conclusion, these findings highlight the dual role of boron content and defect mechanisms in improving the structural stability of B x C materials under extreme environments.
36 MATERIALS SCIENCE↗
High-Power Impulse Magnetron Sputter Deposition of Boron Carbide with Full-Face Erosion Magnetron and Mixed Ar-Ne Plasma
Boron carbide (B 4 C) is an attractive inertial confinement fusion ablator material. The fabrication of B4C ablators by magnetron sputtering requires process optimization. To increase process flexibility, here we explore high-power impulse magnetron sputter (HiPIMS) deposition of B 4 C in pure Ar and mixed Ar-Ne plasmas. Here, the results show that higher plasma discharge currents can be reached with a mixed Ar-Ne plasma in the entire working pressure range studied (5 𝑡𝑜 50 mTorr). At 45 mTorr with 10% of Ne in the Ar-Ne mix, high peak target current densities of ~1 A cm −2 were demonstrated. Films deposited with such a mixed Ar-Ne plasma with a full-face erosion magnetron source on substrates biased at −25 V exhibited higher density and improved mechanical properties, albeit with higher compressive residual stresses compared to the case of HiPIMS deposition in a pure Ar plasma. This work demonstrates additional process flexibility of the HiPIMS discharge mode for the deposition of B 4 C coatings.
Ablator capsule↗
Boron carbide films with reduced nodular defect density deposited by full-face erosion radio-frequency magnetron sputtering
Boron carbide coatings deposited by sputtering with conventional planar magnetrons exhibit nodular growth defects. Here, we demonstrate that the density of nodular defects in B 4 C films can be greatly reduced by using a full-face-erosion magnetron source that achieves a more uniform target erosion by sweeping the confining magnetic field over the target surface. Here, we systematically study properties of B 4 C films deposited on substrates with different tilt angles and demonstrate deposition of ultrathick films with compressive residual stress below 200MPa.
36 MATERIALS SCIENCE↗
Preshot Report for Carbonara: Dynamics of a Boron Carbide Sandwich
The main purpose of the Carbonara experiment is to provide much-needed data to inform physics modeling of boron carbide (B 4 C) subjected to a sweeping high explosive (HE) detonation. The proposed experiment will include three separate shots, each employing a rectangular slab of HE (4” × 3” × 1”) sandwiched between two wedges of B 4 C, see Fig.1. In each case the top B 4 C wedge tapers from a height of 2.5 cm to 0.5 cm, while the bottom wedge tapers from 1.5 cm to 0.5 cm. The wedges are 4” in the direction of the proton beam, and 3” in the direction transverse to the beam. Three shots are currently planned, each one using a different HE: PBX 9501, PBX 9502, and PBX 9701. The HE will be initiated using a line wave generator (LWG) to preserve planar symmetry, in conjunction with a Comp. B booster to smooth out the profile for the initial condition on the main charge and to aid in initiation of the PBX 9502 shot. In each case the booster spans the entire length and height of the main HE slab and extends 0.5” in toward the LWG.
36 MATERIALS SCIENCE↗
Optimization of boron carbide density using direct current sintering for use as sputtering sources at the National Ignition Facility
Several experimental trials were conducted to determine optimal sintering parameters and compositions for purchased boron carbide powder using spark plasma sintering/direct current sintering. Sputtering targets used to create ablators for the National Ignition Facility have been unable to survive post processing procedures suggesting that in-house fabrication may yield better results due to extended control.
36 MATERIALS SCIENCE↗
P-V-T equation of state of boron carbide
We report the P-V-T equation of state measurements of B 4 C to 50 GPa and approximately 2500 K in laser-heated diamond anvil cells. We obtain an ambient temperature, third-order Birch–Murnaghan fit to the P-V data that yields a bulk modulus K 0 of 221(2) GPa and derivative, (dK/dP) 0 of 3.3(1). These were used in fits with both a Mie–Grüneisen–Debye model and a temperature-dependent, Birch–Murnaghan equation of state that includes thermal pressure estimated by thermal expansion (α) and a temperature-dependent bulk modulus (dK 0 /dT). The ambient pressure thermal expansion coefficient (α 0 + α 1 T), Grüneisen γ(V) = γ 0 (V/V 0 ) q and volume-dependent Debye temperature, were used as input parameters for these fits and found to be sufficient to describe the data in the whole P-T range of this study.
36 MATERIALS SCIENCE↗