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Open Access Publications from the University of California

Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2022)

Ultra-incompressible and superhard materials are good candidates for applications in cutting, grinding, polishing tools and coatings. It has been found that transition metal borides, combining highly incompressible transition metals with strong covalently bonded boron network, are potential candidates for machining tools due to good thermal stability and great mechanical properties. In the course of our studies involving a family of metal borides, such as tungsten diboride (WB2), rhenium diboride (ReB2), tungsten tetraborides (WB4) and dodecaborides (MB12, M = metal), and its solid solutions, we demonstrated that hardness could be enhanced through both intrinsic (chemical bonding) and extrinsic (grain boundary) approaches. In this thesis, a complete discussion was made for each diboride-, tetraboride- and dodecaboride- system, aiming to study the deformation mechanism in a lattice specific manner and the intrinsic hardening effects. In addition to intrinsically modify the crystal structure to improve the hardness, extrinsic hardening effect, specifically modifying grain boundaries, was investigated in ReB2 system to understand the size dependence of strength and texture in the nano-system.

    Cover page: High-Pressure Studies of Structural and Mechanical Properties of Superhard Metal Borides
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2024)
    Alloys of tungsten tetraboride (WB4) with the addition of C and Si were prepared by arc-melting of the constituent elements. The phase purity was established by powder X-ray diffraction (PXRD) and surface morphology by scanning electron microscopy (SEM) analysis. Vickers hardness measurements showed hardness enhancement for alloys with a nominal composition of (W0.98Si0.02):11.6B and (W0.95C0.05):11.6B of 52.2 ± 3.0 and 50.5 ± 2.5 GPa, respectively, compared to 41.2 ± 1.4 GPa for pure WB4. (W0.92Zr0.08):11.6B was determined in previous work to have a hardness of 55.9 ± 2.8 GPa. Bulk moduli were calculated following analysis of high-pressure radial diffraction data and were determined to be 329 ± 4 (K0′ = 2) and 390 ± 9 (K0′ = 0.6) GPa for 8 atom % Zr and 5 atom % C-doping, respectively, compared to 326-339 GPa for pure WB4. Computational analysis was used to determine the dopant positions in the crystal structure, and it was found that Zr primarily substitutes W in the 2c position, Si substitutes for the entire B3 trimers, and C inserts in the Bhex-layer. The hardness enhancement in the case of Zr-doping is attributed primarily to extrinsic hardness effects (nanograin morphology), in the case of C─to intrinsic effects (interlayer bond strengthening), and in the intermediate case of Si─to both intrinsic and extrinsic effects (bond strengthening and fine surface morphology).
      Cover page: Hardening in Tungsten Tetraboride with the Addition of Carbon, Zirconium, and Silicon: Intrinsic vs Extrinsic EffectsCreative Commons 'BY-NC-SA' version 4.0 license
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