Toughening of boron carbide composites with hierarchical microstructuring

Boron carbide (B4C) possesses a unique combination of properties including high hardness, low density, wear and corrosion resistance, thermal stability, high neutron absorption, and semi-conductivity, making it a potential candidate for aerospace, nuclear and other applications that involves extreme environment. Its current application however, is limited by its brittleness originated from strong covalent bonding. To toughen boron carbide, past studies implemented toughening mechanisms such as crack deflection and bridging by reinforcement particles, whiskers, and fibers. Recent studies have demonstrated novel deformation mechanisms triggered in nanocrystalline B4C which can potentially enhance its fracture resistance by grain boundary sliding accompanied by nano-pore compression/collapse. In this study, B4C composites with hierarchical microstructure features are fabricated using field assisted sintering (FAST) to combine multiple toughening mechanisms for further fracture toughness enhancement. ‘Soft’ carbon phases in the form of graphite platelets are introduced to promote crack deflection and introduce energy dissipation mechanism by delamination. And sub-micron titanium diboride (TiB2) particles are added to promote crack deflection, bridging, and micro-crack toughening. The fabricated B4C composites exhibit hierarchical microstructure features including micron and sub-micron sized B4C grains, graphite platelets, and TiB2 reinforcements. Fracture toughness enhancements up to 4.16, 4.67, and 4.65 MPa∙m^1/2 (from 2.96 MPa∙m^1/2) are achieved for B4C composites with graphite platelets addition (micro/nano B4C), with TiB2 formation (micro B4C-TiB2), and with both graphite and TiB2 addition (micro/nano B4C-TiB2) respectively. While addition of a second phase can usually lead to hardness degradation, the fabricated micro/nano B4C-TiB2 composites exhibit high fracture toughness while retaining high hardness (31.88 GPa) despite the lower hardness of formed graphite platelets and TiB2 particles. The results demonstrated the ability to obtain high fracture toughness without degradation of other physical properties through multiple toughening mechanisms enabled by the hierarchical microstructure design. When extended to other material systems including silicon carbide and aluminum oxide, this toughening strategy can be applied for matrix toughening and work alongside fiber-reinforcements to achieve further toughness enhancement.