Nanoceramic composites with duplex microstructure break the strength-toughness tradeoff

There is a strength and fracture toughness tradeoff in nanoceramic composite. The strength varies reciprocally with the grain size whereas the toughness contributed by compressive residual stress increases with the dimension of the second phase. In this work, a novel duplex microstructure with reinforced clusters composing of nanosized grains was proposed and validated using a model system of B_(4)C-TiB_(2) ceramics densified by carbide boronizing. As-obtained ceramics exhibit excellent combined mechanical properties at room temperature, including Vickers hardness, Young's modulus, flexural strength and fracture toughness(by surface crack in flexure method) of 32.1 ± 2.7 GPa, 506.9 ± 2.0 GPa,1175 ± 71 MPa and 5.1 ± 0.4 MPa m^(0.5), respectively. Both strength and toughness are at least ~30 % higher than the counterparts with similar composition but homogenously distributed TiB_(2) grains. Graphite onion was confirmed to be an intermediate product during reactive sintering, it facilitated the grain pullout during fracture and retained the nanometric TiB_(2) grain in the cluster, both of which also contribute the toughening and strengthening mechanisms in the B_(4)C-TiB_(2) ceramics.