Microstructures, Properties and Fracture Behavior of Solidified TiC-TiB2 Composites with Increasing Content of TiB2

By adjusting the mole ratio of C and B elements in combustion system, solidified TiC-TiB₂ composites with different TiB₂ mole fraction were achieved by combustion synthesis in high-gravity field. XRD, FESEM and EDS results showed that with increasing TiB₂ content, the matrix of TiC-TiB₂ composite ceramics transformed a number of fine TiB₂ platelets from the TiC spherical grains, and fine-grained even ultrafine-grained microstructures were achieved in solidified TiC-50mol% TiB₂ due to eutectic growth under rapid solidification of the ceramic. Properties showed that relative density, Vickers hardness and flexural strength of TiC-50mol%TiB₂ simultaneously reached the maximum values of 21.5 ± 1.5 GPa and 860 ± 35 MPa , whereas TiC-66.7mol%TiB₂ presented the maximum fracture toughness of 13.5 ± 1.5 MPa • m^0.5. FESEM fractography analyses of the ceramics exhibited a mixed mode of transcrystalline fracture of TiC spherical grains and intercrystalline fracture of TiB₂ platelets, and the tendency of intercrystalline fracture was obviously enhanced with increasing TiB₂ content to be 66.7 mol%, resulting in enhanced toughening mechanisms of crack deflection, crack-bridging and pull-out by fine TiB₂ platelets, thus, the highest flexural strength was achieved in TiC-50mol%TiB₂ due to the achievements of both fine-grained microstructures and high fracture toughness in the full-density solidified ceramics.