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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2018)

Ceramics have great properties, such as low density, high creep resistance, and high corrosion resistance, but it fractures in a very brittle fashion. This limits the applications of ceramic materials for structural applications due to the potential for catastrophic failure. Many scientists have studied nacre, or the mother-of-pearl portion of abalone shells as an inspiration for creating a tough and strong ceramic material. It has garnered a lot of interest because its fracture toughness is 40x higher (3000x higher in energy terms) than its constituent parts, which is 95 vol% calcium carbonate and 5 vol% biopolymers arranged in a brick-and-mortar structure.

The polymer mortar phase prevents viable applications of this ceramic at higher temperatures, which is critical for applications in aerospace, nuclear materials, and transportation. In lieu of a polymeric mortar, a metallic mortar can improve the high temperature resistance of the material. Unfortunately, capillary forces prevent infiltration of metals into porous ceramics. The thesis presents a “top-down” processing approach and a “bottom-up” processing approach to overcome these capillary forces.

The “top-down” approach uses reactive wetting to infiltrate alumina scaffolds with bulk metallic glass without any added pressure. The results illustrate the effects of infiltration temperatures on the mechanical properties of the materials. The infiltration temperature affects the mechanical properties of the brick-and-mortar materials by changing the interfacial properties of the materials. At higher infiltration temperatures, the interface fails to allow for extrinsic toughening mechanisms, but it leads to significantly lower flexural strength. At lower infiltration temperatures, the material is very brittle and shows will no crack deflection or extrinsic toughening, but the flexural strength is higher.

The alternative (“bottom-up”) approach to synthesize “nacre-like” structures ceramics containing a metallic nickel compliant phase. These materials were fabricated using core-shell alumina/nickel platelets that are aligned using slip casting and rapidly sintered using spark-plasma sintering (SPS). With this process, nacre-like ceramic/metal nanostructures have been fabricated with various microstructural features. By casting NiO-coated platelets along with Ni-coated platelets and very rapid SPS processing, there was limited dewetting of the nickel mortar. As a result, we have produced a “nacre-like” alumina ceramic displaying a resistance-curve toughness up to ~16 MPa·m½ with a flexural strength of ~300 MPa.

    Cover page: Dual Processing Approaches to Fabricate Nacre-like Alumina with a Metallic Compliant Phase for Damage Tolerance
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2019)
    Bouligand structures are widely observed in natural materials; elasmoid fish scales and the exoskeleton of arthropods, such as lobsters, crabs, mantis shrimp and insects, are prime examples. In fish scales, such as those of the Arapaima gigas, the tough inner core beneath the harder surface of the scale displays a Bouligand structure comprising a layered arrangement of collagen fibrils with an orthogonal or twisted staircase (or plywood) architecture. A much rarer variation of this structure, the double-twisted Bouligand structure, has been discovered in the primitive elasmoid scales of the coelacanth fish; this architecture is quite distinct from “modern” elasmoid fish scales yet provides extraordinary resistance to deformation and fracture. Here we examine the toughening mechanisms created by the double-twisted Bouligand structure in comparison to those generated by the more common single Bouligand structures. Specifically, we have developed an orientation-dependent, hyperelastic, phase-field fracture mechanics method to computationally examine the relative fracture toughness of elasmoid fish scales comprising single vs. double-twisted Bouligand structures of fibrils. The model demonstrates the critical role played by the extra inter-bundle fibrils found in coelacanth fish scales in enhancing the toughness of Bouligand-type structures. Synthesis and fracture tests of 3-D printed Bouligand-type materials are presented to support the modeling and complement our understanding of the fracture mechanisms in Bouligand-type structures.
      Cover page: Hyperelastic phase-field fracture mechanics modeling of the toughening induced by Bouligand structures in natural materials
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2019)
      We have successfully developed bulk-metallic glass (BMG) composites reinforced with intertwined dendritic alumina, inspired by the structure of nacre (Abalone shells). We harness the dendritic growth of ice crystals in a thermal gradient to assemble an alumina with dendritic pore architecture, which is then infiltrated with BMG-forming melt to make a bioinspired, micrometer-scale, intertwined BMG-alumina composite. The composite exhibits ultra-high strength with exceptional relative compressive strength, far better than ceramic particulate BMG composites. The method presented here can be further exploited to produce novel BMG composites with excellent mechanical properties.
        Cover page: Synthesis of bioinspired ice-templated bulk metallic glass-alumina composites with intertwined dendritic structure
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2019)
        Bioinspired ceramics with micron-scale ceramic "bricks" bonded by a metallic "mortar" are projected to result in higher strength and toughness ceramics, but their processing is challenging as metals do not typically wet ceramics. To resolve this issue, we made alumina structures using rapid pressureless infiltration of a zirconium-based bulk-metallic glass mortar that reactively wets the surface of freeze-cast alumina preforms. The mechanical properties of the resulting Al2O3 with a glass-forming compliant-phase change with infiltration temperature and ceramic content, leading to a trade-off between flexural strength (varying from 89 to 800 MPa) and fracture toughness (varying from 4 to more than 9 MPa·m½). The high toughness levels are attributed to brick pull-out and crack deflection along the ceramic/metal interfaces. Since these mechanisms are enabled by interfacial failure rather than failure within the metallic mortar, the potential for optimizing these bioinspired materials for damage tolerance has still not been fully realized.
          Cover page: Bioinspired nacre-like alumina with a bulk-metallic glass-forming alloy as a compliant phase
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