Microstructure-mechanical properties relations in pressureless sintered silicon carbide-titanium diboride composite ceramics

Densification and mechanical properties (hardness, fracture toughness and flexural strength) of the SiC-TiB2 composite were studied. Pressureless sintering experiments were carried out on samples containing 0 to 50 vol % of TiB2 created by an in-situ reaction between TiO 2 and C: 2TiO2+B4C+3C→2TiB 2+4CO↑;It was found that the presence of TiB2 formed by the above reaction serves as an effective obstacle to crack propagation thus increasing both the strength and fracture toughness of SiC while maintaining high a hardness of the sintered samples. Densities higher than 98% TD were achieved depending on both the sintering temperature and heat treatment conditions. From a density viewpoint, the optimum volume fraction of TiB2 was from 12 to 24 vol %. Typical microstructures for samples with this volume fraction of TiB2 consist of TiB2 particles (< 5microm) uniformly dispersed in a matrix of elongated SiC plates. The presence of TiB 2 particles in the matrix of SiC inhibited exaggerated grain growth of the SiC grains and activated additional toughening mechanisms. The subsequent heat treatment of the sintered samples improved mechanical properties. The optimum sintering and heat treatment temperatures were 1940 and 1970°C, respectively. The maximum flexural strength of 593 MPa was obtained in sample with 12 vol % TiB2. A maximum fracture toughness of 6.6 MPa·m 1/2 was measured in samples containing 24 vol % TiB2. While both fracture toughness and strength increased with the presence of TiB 2 particles, hardness on the other hand decreased from ∼18 GPa in samples without TiB2 to 16.4 and 15.9 GPa in samples with 12 and 24 vol % TiB2, respectively. A theoretical analysis was conducted to model the effect of microstructure on the fracture toughness of SiC-TiB 2 composites and was experimentally verified.;Al2O3 and Y2O3 were used as sintering additives to create a liquid phase and promote densification at sintering temperatures ranging from 1820 to 1940°C. The sintered samples were subsequently heat-treated at temperatures ranging from 1850 to 1970°C.