Oleg Vasylkiv

Consolidation of B4C-TaB2 eutectic composites by spark plasma sintering

Abstract The in situ synthesis/consolidation of B4C-TaB2 eutectic composites by spark plasma sintering (SPS) is reported. The microstructure–property relations were determined for composites with the B4C-TaB2 eutectic composition as functions of TaB2 content, and TaB2-TaB2 interlamellar spacing. A clear maximum in fracture toughness was identified (∼4.5 MPa m1/2) for eutectic composites with interlamellar spacing between 0.9 and 1.1 μm. The composites with the hypereutectic composition of 40 mol.% TaB2 obtained by SPS exhibited lower Vickers hardness (25–26 GPa) but higher indentation fracture toughness (up to 4.9 MPa m1/2) than eutectic composites with 30–35 mol.% of TaB2.

A novel non-catalytic synthesis method for zero- and two-dimensional B13C2 nanostructures

Boron carbide nanoparticles and nanoflakes represent nano-building blocks for complex hierarchical assembly of nanoscale structures that exhibit ideal mechanical robustness. These nano-building blocks were synthesized by simply changing the mixing ratio of the solid precursors to influence the saturation condition of the process. As such, the ability to tune the nanostructures of boron carbide was achieved by controlling the concentration of gaseous boron oxide in the process with no catalyst involved in the growing process. The phase of the resulting nanostructures were found to be B13C2, which is a much desired phase because its hardness of close to 60 GPa is twice as hard as B4C. Nanoflakes were found to contain high degree of (101)-type twins with their boundaries likely to pass through the center of icosahedra in the structure. Nanoflakes with twinned microstructure are anticipated as a model nanostructure and can provide opportunities to fundamentally explore their mechanistic nature since twins have the potential in inhibiting the crack propagation, leading to toughening the materials.

Synthesis and Properties of Multimetal Oxide Nanopowders via Nano-Explosive Technique

We demonstrate the methodology of engineering the multi-component ceramic nanopowder with precise morphology by ‘nano-blast’ calcinations decomposition of preliminary engineered nanoreactors. Multiple explosions of just melted C3H6N6O6 embedded into preliminary engineered nanoreactors break apart the agglomerates due to the highly energetic impacts of the blast waves. Also, the solid-solubility of one component into the other is enhanced by the extremely high local temperature generated during each nano-explosion in surrounding area. This methodology was applied for production of agglomerate-free nanoaggregates of Gd20Ce80O1.95 with an average size of 42 nm and LaSrGaMgO3-x nanopowder with an average aggregate size of 83 nm.

High-Toughness Tetragonal Zirconia/Alumina Nano-Ceramics

The 0.75 to 3 mol% Y2O3-stabilized tetragonal ZrO2 and Al2O3/Y-TZP nano-composite ceramics with 0.2 to 0.7 wt% of alumina were produced by a colloidal technique and low-temperature sintering. The influence of the resulting density, microstructure, the yttria-stabilizer and the alumina content on toughness was determined. The bulk 2.7Y-TZP ceramic with an average grain size of 110 nm reached fracture toughness of 11.2 MPa·m1/2. A nano-grained alumina/zirconia composite with an average grain size of 92 nm was obtained. Y-TZP ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughness of 13.8 MPa·m1/2 (2Y-TZP), and 14.5 MPa·m1/2 (1.5Y-TZP). Y-TZP/alumina composites with 0.35 wt% of Al2O3 were shown to reach fracture toughness of 15.7 MPa·m1/2 (2Y), 15.3 MPa·m1/2 (1.5Y).

Highly ordered nano-scale structure in nacre of green-lipped mussel Perna canaliculus

The ability of the complex hierarchical structure of nacre to withstand huge work of fracture beyond its constituent materials has generated much interest in its growth mechanism and biomimicry studies. However, the structural features directly involved in the robust mechanical properties have not been successfully identified or emulate in biomimetic materials. In the current article, we present the characterisation of a novel nanostructure framework in nacre of Perna canaliculus after chemical etching. The inner structure of the nacre platelet reveals a crystalline three-dimensional framework of orientated fibrous aragonite crystals, which shows that the hierarchical structure at the nano-scale level is much more complex than researchers previously thought. Fracture analysis shows that the fibrous aragonite framework remains largely intact, which indicates the strengthening nature of the elusive internal structure. The discovery of the nano-scale highly ordered structure could be the key to understanding the toughening mechanism in nacre and could serve as a guide to the future biomimetic design of nacre-like, high strength, tough, composite materials.

Bio-inspired structured boron carbide-boron nitride composite by reactive spark plasma sintering

Nature creates composite materials with complex hierarchical structures that possess impressive mechanical properties enhancement capabilities. An approach to improve mechanical properties of conventional composites is to mimic the biological material structured ‘hard’ core and ‘soft’ matrix system. This would allow the efficient transfer of load stress, dissipation of energy and resistance to cracking in the composite. In the current study, reactive spark plasma sintering (SPS) of boron carbide B4C was carried out in a nitrogen N2 gas environment. The process created a unique core-shell structured material with the potential to form a high impact-resistant composite. Transmission electron microscopy observation of nitrided-B4C revealed the encapsulation of B4C grains by nano-layers of hexagonal-boron nitride (h-BN). The effect of the h-BN contents on hardness were measured using micro- and nano-indentation. Commercially available h-BN was also mechanically mixed and sintered with B4C to compare the effectiveness of nitrided B4C. Results have shown that nitrided B4C has a higher hardness value and the optimum content of h-BN from nitridation was 0.4%wt with the highest nano-indentation hardness of 56.7 GPa. The high hardness was attributed to the h-BN matrix situated between the B4C grain boundaries which provided a transitional region for effective redistribution of the stress in the material.

Nano-Engineering and Catalytic Properties of Zirconia – Noble Metals Composite Powders

The possibility of preparation of Pt–3Y-TZP and Pd–3Y-TZP nano-composites was studied. The sonochemically synthesized Pt (Pd) nano-particles (~2 nm) were impregnated into the zirconia nano-aggregates (20 – 45nm). The complex morphology manipulation technique allowed production of the composite zirconia-based aggregates in which a significant fraction of the Pt or Pd crystallites was embedded into the zirconia dense aggregates. The catalytic properties of composite Pt–3Y-TZP nano-composites were studied and described.

High-Toughness Tetragonal Zirconia and Zirconia/Alumina Nano-Ceramics

The 0.75 to 3 mol% Y2O3-stabilized tetragonal ZrO2 and Al2O3/Y-TZP nano-composite ceramics with 0.2 to 0.7 wt% of alumina were produced by a colloidal technique and low-temperature sintering. The influence of the resulting density, microstructure, the yttria-stabilizer and the alumina content on the hardness and toughness were determined. The bulk 2.7Y-TZP ceramic with an average grain size of 110 nm reached a hardness of 13.6 GPa and fracture toughness of 11.2 MPa·m 1/2 . A nano-grained alumina/zirconia composite with an average grain size of 92 nm was obtained, and the hardness increased to 16.8 GPa. Y-TZP ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughness of 13.8 MPa·m 1/2 (2Y-TZP), and 14.5 MPa·m 1/2 (1.5Y-TZP). Y-TZP/alumina composites with 0.35 wt% of Al2O3 were shown to reach fracture toughness of 15.7 MPa·m 1/2 (2Y), 15.3 MPa·m 1/2 (1.5Y).