In-situ reaction-sintering of composites with high fracture toughness Si3N4-TiN

To identify the nitridation mechanism of Si compacts and fracture toughening mechanism of insitu synthesized Si3N4-TiN composites, which were made at 1350 degrees C and 1400 degrees C. The composites have been studied by OM, SEM, XRD and TEM. In the sample nitrided at 1350 degrees C for 20 hours, the residual Si particles show an island-like morphology due to the nitridation and contain a high-density of dislocations and microcracks. Cracks and dislocations in the Si particles play an important role to explain the nitridation mechanism of Si compacts. Dense Si3N4-TiN composites from Si and sponge Ti powders were produced by in-situ reaction-bonding and post-sintering under N-2 atmosphere. The fracture toughness of Si3N4-20%TiN composite (nitrided at 1400 degrees C) show a very high value of 12.0MPam(1/2). In the reaction-bonded Si3N4-20%TiN composite (nitrided at 1350 degrees C), Si3N4 grains were composed of alpha- and beta-Si3N4 structures as well as TiN and some amounts of residual Si phase. After post-sintering, alpha-Si3N4 grains were transformed to beta-Si3N4 grains with rod-like shapes. No intermetallic compounds were formed at the interfaces between Si3N4 and TIN grains. The main toughening mechanisms were crack deflection and crack bridging caused by rod-like Si3N4 grains and microcracking due to the dispersion of TiN particles formed during the reaction.