UC Berkeley

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.