Compositional analysis of boron carbide on nanometer length scales to examine or interpret atomic mechanisms, for example, solid-state amorphization or grain-boundary segregation, is challenging. This work reviews advancements in high-resolution microanalysis to characterize multiple generations of boron carbide. First, zeta-factor microanalysis will be introduced as a powerful (scanning) transmission electron microscopy ((S)TEM) analytical framework to accurately characterize boron carbide. Three case studies involving the application of zeta-factor microanalysis will then be presented: (1) accurate stoichiometry determination of B-doped boron carbide using zeta-factor microanalysis and electron energy loss spectroscopy, (2) normalized quantification of silicon grain-boundary segregation in Si-doped boron carbide, and (3) calibration of a scanning electron microscope X-ray energy-dispersive spectroscopy (XEDS) system to measure compositional homogeneity differences of B/Si-doped arc-melted boron carbides in the as-melted and annealed conditions. Overall, the improvement and application of advanced analytical tools have helped better understand processing-microstructure-property relationships and successfully manufacture high-performance ceramics.

Boron carbide and boron suboxide are attractive for their low densities and ultrahigh hardness values, yet they exhibit poor fracture resistance. Strategies to improve mechanical performance aim to control bulk microstructures and properties by incorporating Si-based additives. A related approach to toughen boron carbide is tailoring the phase-like states of grain boundaries by applying the concept of grain boundary complexion engineering. This work directly investigates the potential of grain boundary complexion engineering to improve fracture resistance by systematically evaluating grain boundary segregation behavior of a polycrystalline silicon hexaboride / boron carbide diffusion couple using aberration-corrected scanning transmission electron microscopy and ζ-factor microanalysis. A main result was that all grain boundaries in the diffusion couple exhibited Si segregation that, depending on the bulk Si concentration, approached up to 3 monolayers of coverage, thereby resulting in nanolayer complexions (oftentimes known as intergranular films). Furthermore, upon analyzing 110 distinct grain boundaries throughout the diffusion zone and an impurity boron suboxide region, free energies of Si segregation in boron carbide and boron suboxide were experimentally determined using the Brunauer, Emmett and Teller (BET) multilayer grain boundary segregation theory. Subsequently, changes in grain boundary energy due to Si segregation were quantified and a maximum energy reduction of 51% and 81% were observed in boron carbide and boron suboxide, respectively, which led to decreases in works of adhesion of 18% and 17%, respectively. In summary, this work uncovers grain boundary processing-structure-property relationships that can be used to improve the fracture resistance in icosahedral boron-based ceramics.

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Journal Article Recent Developments of ζ-factor Microanalysis and Its Application to Armor Ceramics Get access Christopher Marvel, Christopher Marvel Lehigh University, Bethlehem, Pennsylvania, United States Search for other works by this author on: Oxford Academic Google Scholar Kristopher Behler, Kristopher Behler SURVICE Engineering; CCDC Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States Search for other works by this author on: Oxford Academic Google Scholar Vladislav Domnich, Vladislav Domnich Rutgers University; Ferro Corporation, King of Prussia, Pennsylvania, United States Search for other works by this author on: Oxford Academic Google Scholar Jerry LaSalvia, Jerry LaSalvia CCDC Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States Search for other works by this author on: Oxford Academic Google Scholar Richard Haber, Richard Haber Rutgers University, Piscataway, New Jersey, United States Search for other works by this author on: Oxford Academic Google Scholar Masashi Watanabe, Masashi Watanabe Lehigh University, Bethlehem, Pennsylvania, United States Search for other works by this author on: Oxford Academic Google Scholar Martin Harmer Martin Harmer Lehigh University, Bethlehem, Pennsylvania, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 26, Issue S2, 1 August 2020, Pages 982–985, https://doi.org/10.1017/S1431927620016566 Published: 01 August 2020

A Density Functional Theory (DFT) model for the (0001) oriented boron suboxide surface is developed. The effect of surface termination (boron- vs oxygen-terminated) and icosahedral stacking (alpha-ABCABC vs beta-ABAB) on the structural and electronic reconstructions of the surface were determined. Icosahedral stacking sequence did not change the electronic structure of the surface and therefore both alpha and beta boron suboxide phases are expected to behave chemically similar. The adsorption of yttrium atoms is highly favorable for both oxygen and boron terminated surfaces with a 0.57 eV preference for the oxygen termination. Three unique adsorption sites for yttrium atoms were found. Site 1 is a bridging site over the interstitial site between three surface icosahedra on a boron terminated surface. Site 2 is a bridging site between two surface icosahedra on an oxygen terminated surface. Site 3 is tetrahedrally bonded to a surface icosahedra on an oxygen terminated surface. Using HAADF-STEM imaging, we were able to experimentally identify yttrium occupying a site 2 configuration along a (0001) oriented grain boundary interface in hot-pressed boron suboxide which had been doped with Y2O3:SiO2 powder. The spacing between yttrium atoms along the grain boundary as viewed in projection was 4.74 angstrom which compared well to the DFT prediction of 4.67 angstrom.

Accurate quantification of light elements which produce only soft X-ray lines via X-ray energy dispersive spectrometry (XEDS) has been traditionally difficult due to poor X-ray emission and detector efficiencies at low energies and significant X-ray absorption effects. The ζ-factor microanalysis method enables one to correct for these shortcomings; however, ζ-factor microanalysis has not yet been thoroughly applied to inorganic materials which are entirely or mostly composed of light elements such as boron carbide, boron nitride, or boron suboxide. This work successfully extended ζ-factor microanalysis to boron-rich ceramics and accurately determined stoichiometries of multiple boron carbides and measured grain boundary compositions of a boron carbide mixed with additives consisting of rare-earth ions. Various strategies were employed to experimentally determine a full range of ζ-factors and measurements were validated using materials of known composition including silicon hexaboride and silicon carbide. Overall, this work has shown that XEDS is a viable technique for light element quantification in (scanning) transmission electron microscopy, in terms of both the accuracy and precision, which is comparable or superior to the complementary electron energy loss spectrometry.

Transparent polycrystalline ceramics are used in various applications, with specific interest in infrared windows for this study. For this application, superior optical and mechanical properties of the material are required, and the intrinsic properties of many traditional monolithic material systems are lacking. Using composites in lieu of monolithic systems offers a potential engineering solution to this material problem by limiting grain growth during sintering to reduce the overall grain size. In this study, a calcium fluoride (CaF2) and zinc sulfide (ZnS) composite system was investigated for use as an infrared (IR) window material. A colloidal processing technique was employed to ensure intimate mixing of the powders. These powders were then loaded into a die for hot pressing. The subsequent processing challenges for producing a sintered ceramic were also discussed, as well as the effect of the composition on the final properties.

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The well-documented formation of amorphous bands in boron carbide (B4C) under contact loading has been identified in the literature as one of the possible mechanisms for its catastrophic failure. To mitigate amorphization, Si-doping was suggested by an earlier computational work, which was further substantiated by an experimental study. However, there have been discrepancies between theoretical and experimental studies, about Si replacing atom/s in B12 icosahedra or the C-B-C chain. Dense single phase Si-doped boron carbide was produced through a conventional scalable route. A powder mixture of SiB6, B4C, and amorphous boron was reactively sintered, yielding a dense single phase Si-doped boron carbide material. A combined analysis of Rietveld refinement on XRD pattern coupled with electron density difference Fourier maps and DFT simulations were performed in order to investigate the location of Si atoms in the boron carbide lattice. Si atoms occupy an interstitial position, between the icosahedra and the chain. These Si atoms are bonded to the chain end C atoms, which result in a kinked chain. Additionally, these Si atoms are also bonded to the neighboring equatorial B atom of the icosahedra, which is already bonded to the C atom of the chain, forming a bridge like structure. Owing to this bonding, Si is anticipated to stabilize the icosahedra through electron donation, which is expected to help in mitigating stress-induced amorphization. Possible supercell structures are suggested along with the most plausible structure for Si-doped boron carbide.

The effect of 5 vol.% Al2O3 additive on the densification behavior and resulting microstructures of a hot-pressed commercial B4C powder has been investigated. A maximum density of 95% TD was achieved for the as-received B4C powder by hot-pressing at 2000 degrees C and 17,5 MPa. With Al2O3, densities > 99% TD were obtained between 1900 degrees C - 2000 degrees C. Densities increased linearly with temperature between 1700 degrees C - 1900 degrees C and 1800 degrees C - 2000 degrees C for B4C powders with and without Al2O3, respectively. Densification behavior was monitored during hot-pressing by measuring the hot-press ram displacement using an LVDT. During the temperature ramp, Al2O3 had a negligible effect on the densification onset temperature, but increased the average densification rate by 83% in the rapid densification regime. Only B4C and Al2O3 phases were detected in the densified samples. Al2O3 was observed as a distinct secondary grains or as smaller particles located at triple junctions and grain boundaries. B4C grain morphology and size remained essentially constant over the range of hot-pressing temperatures for both the as-received and Al2O3 containing powders. Experimental procedures and results are presented.

Analytical electron microscopy was used to examine the grain boundary chemistry in boron carbide containing Al-O-rich phases at triple junctions. In this study, SiO2, Al2O3, and B2O3 additives were used to synthesize an aluminoborosilicate glass on the grain boundaries with the long-term goal to enhance fracture resistance. Nanolayer films were not observed using high-angle annular dark field imaging; however, energy-dispersive spectroscopy quantification revealed grain boundaries with varying excess of Si and Al. Overall, it was concluded variations in grain boundary chemistry depend upon the intrinsic grain boundary character and spatial heterogeneity of the oxide additives.

Filament-induced periodic surface structures are generated on a wide variety of materials with near-IR and UV lasers. The surface structure features demonstrate the relation to laser wavelength and polarization and energy distribution in a filament.

This chapter examines the impact of 5 vol.% Al2O3 additive on the densification behavior and resulting microstructures of a hot-pressed commercial B4C powder. Al2O3 was observed as a distinct secondary grains or as smaller particles located at triple junctions and grain boundaries. B4C grain morphology and size remained essentially constant over the range of hot-pressing temperatures for both the as-received and Al2O3 containing powders. Solid-state sintering is a challenge in B4C due to a high melting point, strong covalent bonds, low plasticity and resistance to grain boundary sliding. A number of oxide and non-oxide ceramics such as Al2O3 and Si3N4 have been developed with greatly improved fracture toughness values based on the creation of nanoscale amorphous oxide intergranular films (IGFs). Attempts to create IGFs in B4C have had no apparent success. Maximum density is reached at 1900°C, with density slightly decreasing for higher hot-pressing temperatures. This decrease may not be real due to the formation of other phases not assumed in calculating the theoretical density. Both materials exhibit an essentially linear increase in density with increasing hot-pressing temperature. The effect of 5 vol.% Al2O3 powder on the densification behavior and microstructure of hot-pressed B4C was investigated.

Chemical interactions in B4C and WC powder mixtures (50/50 by volume) heated under inert and oxidizing atmospheres were investigated. As-received powders and B4C/WC powder mixtures were examined by DSC/TGA up to 1000 degrees C at 10 degrees C/min under flowing UHP Ar and Ar/20O(2) atmospheres. No significant mass change or thermal activity was observed for flowing UHP Ar conditions. Under flowing UHP Ar/20O(2), as-received powders and powder mixtures displayed significant mass change and thermal activity associated with oxidation reactions. Based on mass gain, only the as-received WC powder was fully oxidized under the experimental conditions. SEM revealed distinct changes in particle morphologies and phases for powder specimens heated under flowing UHP Ar/20O(2). Experimental procedures and results are presented and discussed.

Boron-based ceramics are appealing for lightweight applications in both vehicle and personnel protection, stemming from their combination of high hardness, high elastic modulus, and low density as compared to other ceramics and metal alloys. However, the performance of these ceramics and ceramic composites is lacking because of their inherent low fracture toughness and reduced strength under high-velocity threats. The objective of the present article is to briefly discuss both the challenges and the state of the art in experimental and computational approaches for engineering grain boundaries in boron-based armor ceramics, focusing mainly on boron carbide (B4C) and boron suboxide (B6O). The experimental challenges involve processing these ceramics at full density while trying to promote microstructure features such as intergranular films to improve toughness during shock. Many of the computational challenges for boron-based ceramics stem from their complex crystal structure which has hitherto complicated the exploration of grain boundaries and interfaces. However, bridging the gaps between experimental and computational studies at multiple scales to engineer grain boundaries in these boron-based ceramics may hold the key to maturing these material systems for lightweight defense applications.

Solid-state shock-wave propagation is strongly nonequilibrium in nature and hence rate dependent. Using high-power pulsed-laser-driven shock compression, unprecedented high strain rates can be achieved; here we report the directional amorphization in boron carbide polycrystals. At a shock pressure of 45∼50 GPa, multiple planar faults, slightly deviated from maximum shear direction, occur a few hundred nanometers below the shock surface. High-resolution transmission electron microscopy reveals that these planar faults are precursors of directional amorphization. It is proposed that the shear stresses cause the amorphization and that pressure assists the process by ensuring the integrity of the specimen. Thermal energy conversion calculations including heat transfer suggest that amorphization is a solid-state process. Such a phenomenon has significant effect on the ballistic performance of B4C.

Journal Article TEM Characterization of the Deformed Region Beneath Knoop Indents in Boron Carbide Get access S D Walck, S D Walck TKC Global, RDRL-WMM-C, Aberdeen Proving Ground, MD 21005, USA Search for other works by this author on: Oxford Academic Google Scholar J C LaSalvia, J C LaSalvia U.S. Army Research Laboratory, RDRL-WMM-E, Aberdeen Proving Ground, MD 21005, USA Search for other works by this author on: Oxford Academic Google Scholar K D Behler K D Behler TKC Global, RDRL-WMM-E, Aberdeen Proving Ground, MD 21005, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 22, Issue S3, 1 July 2016, Pages 1880–1881, https://doi.org/10.1017/S1431927616010242 Published: 25 July 2016