Ductility in Crystalline Boron Subphosphide (B₁₂P₂) for Large Strain Indentation

Our studies of brittle fracture in B₄C showed that shear-induced cracking of the (B₁₁C) icosahedra leading to amorphous B₄C regions induced cavitation and failure. This suggested that to obtain hard boron-rich phases that are ductile, we need to replace the CBC chains of B₄C with two-atom chains that can migrate between icosahedra during shear without cracking the icosahedra. We report here a quantum mechanism (QM) simulation showing that under indentation stress conditions, superhard boron subphosphide (B₁₂P₂) displays such a unique deformation mechanism. Thus, stress accumulated as shear increases is released by slip of the icosahedra planes through breaking and then reforming the P–P chain bonds without fracturing the (B₁₂) icosahedra. This icosahedral slip may facilitate formation of mobile dislocation and deformation twinning in B₁₂P₂ under high-stress conditions, leading to high ductility. However, the presence of twin boundaries (TBs) in B₁₂P₂ will weaken the icosahedra along TBs, leading to the fracture of (B₁₂) icosahedra under indentation stress conditions. These results suggest that crystalline B₁₂P₂ is an ideal superhard material to achieve high ductility

Boron Suboxide and Boron Subphosphide Crystals: Hard Ceramics That Shear without Brittle Failure

Boron suboxide (B₆O), boron carbide (B₄C), and related materials are superhard. However, they exhibit low fracture toughness, which limits their engineering applications. Here we show the shear deformation mechanism of B₆O using density functional theory along the most plausible slip system (01̅11)/<101̅1>. We discovered an unusual phenomenon in which the highly sheared system recovers its original crystal structure, which indicates the possibility of being sheared to a large strain without failure. We also found a similar structural recovery in boron subphosphide (B₁₂P₂) for shearing along the same slip system. In contrast, for components of B₄C, we found brittle failure. These novel deformation mechanisms under high shear deformation conditions suggest that a key element to designing ductile hard materials is to couple the icosahedra via one- or two-atom chains that allow the system to shear by walking the intericosahedral bonds and chain bonds alternately to accommodate large shear without fracturing the icosahedra

Structure and Properties of Boron-Very-Rich Boron Carbides: B₁₂ Icosahedra Linked through Bent CBB Chains

The atomic structures of boron carbide in the regime below ∼13.3 at. % C (known as boron-very-rich boron carbide, BvrBC) have not previously been reported due to the complexity of the structure and bonding. We report here the atomistic crystal structures for stoichiometry B₁₄C, with only 6.7 at. % C, predicted using quantum mechanics (QM) at the PBE level. We find that B₁₄C consists of one B₁₂ icosahedral cluster and one C–B–B chain per unit cell. The C–B–B chain can be linear or bent, leading to two different space groups for (B₁₂)­CBB. Our bonding analyses show that both structures satisfy the electron counting rule (Wade’s rule). However, the bent CBB chain which has lower crystal symmetry, leading to an energy substantially more stable (0.315 eV per molecular unit) than that of the linear CBB chain structure, which has high crystal symmetry. This is because the bent CBB chain structure requires only one three-center–two-electron (3c-2e) bond, while the linear CBB chain structure requires three 3c-2e bonds. We predicted the mechanical properties of both structures from QM simulations. We found that shearing the linear CBB chain structure transforms first to the bent CBB chain structure under both pure and biaxial shear deformations as the shear proceeds the icosahedra deconstruction due to the interaction of the CBB chains with the icosahedra. This suggests that the bent CBB structure is responsible for the failure processes of B₁₄C

Structure and Properties of Boron-Very-Rich Boron Carbides: B₁₂ Icosahedra Linked through Bent CBB Chains

The atomic structures of boron carbide in the regime below ∼13.3 at. % C (known as boron-very-rich boron carbide, BvrBC) have not previously been reported due to the complexity of the structure and bonding. We report here the atomistic crystal structures for stoichiometry B₁₄C, with only 6.7 at. % C, predicted using quantum mechanics (QM) at the PBE level. We find that B₁₄C consists of one B₁₂ icosahedral cluster and one C–B–B chain per unit cell. The C–B–B chain can be linear or bent, leading to two different space groups for (B₁₂)­CBB. Our bonding analyses show that both structures satisfy the electron counting rule (Wade’s rule). However, the bent CBB chain which has lower crystal symmetry, leading to an energy substantially more stable (0.315 eV per molecular unit) than that of the linear CBB chain structure, which has high crystal symmetry. This is because the bent CBB chain structure requires only one three-center–two-electron (3c-2e) bond, while the linear CBB chain structure requires three 3c-2e bonds. We predicted the mechanical properties of both structures from QM simulations. We found that shearing the linear CBB chain structure transforms first to the bent CBB chain structure under both pure and biaxial shear deformations as the shear proceeds the icosahedra deconstruction due to the interaction of the CBB chains with the icosahedra. This suggests that the bent CBB structure is responsible for the failure processes of B₁₄C

Experiment Methodology.

<p>Panel A shows the physical set-up of the experiment. Participants interacted with the virtual environment by placing their right index finger into a custom splint attached to the PHANTOM. Participants sat in front of the projection system and their hand was free to move about the 3D workspace. The PHANTOM was located inside a projection system, consisting of a frame above the PHANTOM, supporting an inverted video monitor. Panel B shows the force-displacement curves for the stiffnesses of box 1 and box 2. Panel C shows a schematic of the visual feedback supplied to the participant during a single successful trial. Participants attempted to move a box from the left of the screen to a target position by pushing down on the box and sliding it to the right. Finger position was indicated by a small sphere and was occluded during penetration of the box. Deformations of the box were not shown.</p

ANOVA for Box Velocity.

<p>ANOVA for Box Velocity.</p

Effects of box, cognitive task, and session on box displacement and velocity.

<p>Participants were asked to move one of two possible boxes (box 1 or box 2) from the left of the screen to a target position by pushing down on the box and sliding it to the right. Half of trials were performed while participants were completing a simultaneous cognitive task (Cog ON) and during the rest participants completed the motor task alone (Cog OFF).Box displacement was defined as the total distance toward the target that the participant was able to translate the box during the trial and average box velocity was defined as the box displacement normalized by trial duration. Markers indicate data means and error bars mark 95% confidence bands of the mean. During sessions 1–7 participants were provided with vibrotactile feedback proportional to the normal force they were applying to the box as well as visual feedback. During session 8 they completed the task using visual feedback alone (NF = no feedback). The shaded area corresponds to data from a previous study <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0032743#pone.0032743-Stepp1" target="_blank">[22]</a> in which participants trained using visual feedback alone in a single session.</p

ANOVA for Box Displacement.

<p>ANOVA for Box Displacement.</p

Significant (<i>p</i><0.05) Pairwise Effect Sizes in Box Velocity between Sessions.

†<p><i>p_adj</i><0.001.</p

Hierarchical Composite Structures Prepared by Electrophoretic Deposition of Carbon Nanotubes onto Glass Fibers

Carbon nanotube/glass fiber hierarchical composite structures have been produced using an electrophoretic deposition (EPD) approach for integrating the carbon nanotubes (CNTs) into unidirectional E-glass fabric, followed by infusion of an epoxy polymer matrix. The resulting composites show a hierarchical structure, where the structural glass fibers, which have diameters in micrometer range, are coated with CNTs having diameters around 10–20 nm. The stable aqueous dispersions of CNTs were produced using a novel ozonolysis and ultrasonication technique that results in dispersion and functionalization in a single step. Ozone-oxidized CNTs were then chemically reacted with a polyethyleneimine (PEI) dendrimer to enable cathodic EPD and promote adhesion between the CNTs and the glass-fiber substrate. Deposition onto the fabric was accomplished by placing the fabric in front of the cathode and applying a direct current (DC) field. Microscopic characterization shows the integration of CNTs throughout the thickness of the glass fabric, where individual fibers are coated with CNTs and a thin film of CNTs also forms on the fabric surfaces. Within the composite, networks of CNTs span between adjacent fibers, and the resulting composites exhibit good electrical conductivity and considerable increases in the interlaminar shear strength, relative to fiber composites without integrated CNTs. Mechanical, chemical and morphological characterization of the coated fiber surfaces reveal interface/interphase modification resulting from the coating is responsible for the improved mechanical and electrical properties. The CNT-coated glass-fiber laminates also exhibited clear changes in electrical resistance as a function of applied shear strain and enables self-sensing of the transition between elastic and plastic load regions