Due to a unique combination of properties including high hardness, low density, chemical and thermal stability, semi-conductivity, and high neutron absorption, boron carbide (B C) is a potential candidate for various applications involving extreme environment. However, B C’s current application is limited because of its low fracture toughness. In this study, a hierarchical microstructure design with features including TiB grains and graphite platelets was used to toughen B C by simultaneously utilizing multiple toughening mechanisms including crack deflection, bridging, and micro-crack toughening. Using field-assisted sintering technology (FAST), B C composites with dense and hierarchical microstructure were fabricated. Previously, the fracture toughness of fabricated B C composites was measured at micro-scale using micro- indentation to have up to 56% improvement. In this work, the B C composites’ fracture toughness was characterized at macro-scale using four-point bending methods and compared with previous results obtained at micro-scale. Micromechanics modeling of fracture behaviors for B C-TiB composites was also performed to evaluate the contributions from experimentally observed toughening mechanisms. From four-point bending tests, B C composites reinforced with both TiB grains (~15 vol%) and graphite platelets (~8.7 vol%) exhibited the highest fracture toughness enhancement from 2.38 to 3.65 MPa·m1/2. The measured values were lower than those obtained using micro- indentation but maintained the general trends. The discrepancy between the indentation and four-point bending test results originated from the complex deformation behaviors triggered by the high contact load during indentation tests. Through micromechanics modeling, introduced thermal residual stress due to thermal expansion mismatch between B C and TiB , and weak interphases at B C-TiB boundaries were identified as the main causes for experimentally observed toughness enhancement. These results proved the effectiveness of hierarchical microstructure designs for B4C toughening and can provide reference for the future design of B4C composites with optimized microstructures for further fracture toughness enhancement.
