Reduced Graphene Oxide Reinforces Boron Carbide with High-Pressure and High-Temperature Sintering

Introducing a second phase has been an effective way to solve the brittleness of boron carbide (B₄C) for its application. Though reduced graphene oxide (rGO) is an ideal candidate for reinforcing the B₄C duo’s two-dimensional structure and excellent mechanical properties, the toughness is less than 6 MPa·m^1/2, or the hardness is lower than 30 GPa in B₄C–graphene composites. A barrier to enhancing toughness is the weak interface strength between rGO and B₄C, which limits the bridging and pull-out toughening effects of rGO. In this work, internal stress was introduced using a high-pressure and high-temperature (HPHT) method with B₄C–rGO composites. The optimal hardness and toughness values for the B₄C-2 vol% rGO composite reached 30.1 GPa and 8.6 MPa·m^1/2, respectively. The improvement in toughness was 4 times higher than that of pure B₄C. The internal stress in the composite increased gradually from 2.3 GPa to 3.3 GPa with an increase in rGO content from 1 vol% to 3 vol%. Crack deflection, bridging, and rGO pull-out are responsible for the improvement in toughness. Moreover, the high internal stress contributed to the formation of good interface strength by embedding rGO into the B₄C matrix particles, which further enhanced the dissipation of the crack energy during the pull-out process and led to high toughness. This work provides new insights into synthesizing high-toughness B₄C matrix composites.