In situ enhancement of toughness of SiC-TiB₂ composites

A process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC-30 wt%, 50 wt%, and 70 wt% TiB₂ composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, relatively equiaxed TiB₂ grains, and yttrium aluminium garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiB₂ powders with the liquid forming additives of Al₂O₃ and Y₂O₃ by hot-pressing at 1850°C and subsequent annealing at 1950°C. The annealing led to the in situ growth of elongated α-SiC grains, due to the β → α phase transformation of SiC, and the coarsening of TiB₂ grains. The fracture toughness of the SiC-50 wt% TiB₂ composites after 6 h annealing was 7.3 MPa m^1/2, approximately 60% higher than that of as-hot-pressed composites (4.5 MPa m^1/2). Bridging and crack deflection by the elongated α-SiC grains and coarse TiB₂ grains appear to account for the increased toughness of the composites.