Background: Boron carbide is light-weight, is thermally stable, has high hardness/stiffness, and is multi-functional (semiconducting, thermoelectric, and high neutron absorption cross-section). Boron carbide has been of interest for applications in extreme environments, including turbine engines, protection armor against impact, heat, and radiation, but such application is currently limited due to its brittleness and low sinterability. The toughening of ceramics has been investigated for many years as a light-weight, thermally/chemically stable alternative to structural materials. Among many methods, ceramic micro-fibers implementation has been effective, and further toughening is expected though engineering of matrices, specifically by implementing intentionally weak interphases to provide locally controlled deformation and thus energy dissipation within matrices. For example, in the past we experimentally studied the potentials of nano-porosity introduction into ceramics on deformation behaviors, by indenting on a model system of anodic aluminum oxide. Normally, porosity in ceramics is regarded as the defect, but we identified that, when pore size is below 100 nm, nano-pores deform in a controlled manner (collapse or shear band, see Figure 3a), contributing to fracture toughness increase. Meanwhile, introduction of nano-porosity resulted in stiffness and hardness decrease.
Our study: In our lab, a hierarchical micro-structure was designed to increase toughness without compromising stiffness and hardness about a model system of boron carbide (see Figure 3b). The grain sizes were decreased to microns, and a tougher secondary phase (titanium boride) was added to increase stiffness and hardness. “Soft” interphases were introduced to enhance fracture toughness by crack deflection, nanopore compression, and grain boundary sliding. Field assisted sintering technology (FAST) was selected as a fabrication method as FAST enables consolidation in short time at lower temperatures with minimum grain size growth. This engineered boron carbide samples were characterized for their hardness and fracture toughness using micro-indentation, and their micro-structures were inspected using electron microscopy before and after indentation to evaluate the deformation and fracture behaviors. So far, some fabricated samples exhibited maintained hardness and increased fracture toughness by ~50%.
Potential applications: Our current study can work alongside other toughening mechanisms such as fiber/whisker toughening to provide additional fracture toughness enhancement. The resulting higher toughness will enable ceramic materials to find more structural applications, especially those involving extreme environment. These applications include but are not limited to turbine engine shrouds and blades, thermal barrier coatings, leading edge of hypersonic aircraft, protective armor, etc. Currently, we are also working on reducing the sintering temperature of ceramics through in-situ reaction. Such process will lower the requirements for ceramic sintering process which often requires very high temperature.
Figure 3. Ceramic toughening by hierarchical structuring.
Funding: “Multi-Functional Nano-Porous Ceramics,” Office of Naval Research, June 1, 2017 – May 30, 2020, Grant N000141712361.
Collaborators:
- Dr. Jogender Singh, Applied Research Laboratory, Department of Materials Science and Engineering, the Penn State University
- Dr. Enrique Gomez, Department of Chemical Engineering, the Penn State University