Pressureless Sintering and Mechanical Properties of SiC Composites with in-situ Converted TiO2 to TiC
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Densification behaviour and mechanical properties (hardness, fracture toughness and flexural strength) of the SiC-TiC composite system were studied. Pressureless sintering experiments were conducted on samples containing 0 to 30 vol % TiC created through an in-situ reaction between TiO2 and C: TiO2 + 3C -> TiC + 2CO. Sintering of the compacts was carried out in the presence of Al2O3 and Y2O3 sintering additives which promoted densification at sintering temperatures ranging from 1825 to 1925°C. It was determined that the presence of synthesized TiC particles served to effectively toughen the composite through crack deflection, impedance and bridging. An increase in fracture strength and hardness was also observed. Densities in excess of 98 % theoretical density were achieved depending on the sintering conditions and volume fraction of TiC phase. The SiC grain size and morphology was analyzed as a function of TiC volume fraction. The presence of TiC particles in the SiC matrix inhibited the exaggerated grain growth of the SiC grains and activated additional toughening mechanisms. The SiC grains were found to be roughly equiaxed with very fine TiC particles preventing significant elongation. The optimal sintering conditions for room temperature mechanical properties required slow heating through the reaction zone (1300 to 1520°C) followed by a 1 h dwell at 1885°C. At this temperature, the maximum flexural strength of 566 MPa was measured in samples containing 5 vol % TiC. Conversely, a maximum fracture toughness of 5.7 MPa·m0.5 was measured in samples containing 10 vol % TiC sintered at 1900°C. The hardness was shown to increase very little, from ~19.8 GPa in the monolithic SiC samples to 20.1 GPa in samples containing 5 vol % TiC. A theoretical analysis was conducted to model the effect of porosity and grain morphology on the mechanical properties of the SiC matrix and was experimentally verified.