Sol-gel synthesis of carbon based materials reinforced ultra high temperature ceramic composites

This Ph.D. research is based on the development of novel sol-gel techniques for synthesis of nanostructured ultra high temperature ceramics (UHTCs) and subsequent spark plasma sintering (SPS) for densifying the UHTC composites. The liquid nature of the sol-gel process offers advantages such as high purity and ability for mixing and infiltration, and thus it can overcome some shortcomings of the conventional power processing of ceramics. SPS delivers microstructures with good density and fine grain size due to its internal heat generation, high heating rate and short sintering time required. Titanium carbide (TiC) and zirconium diboride (ZrB₂) are selected as the UHTC matrix materials to form composites with different kinds of carbon based materials, including 1D carbon nanotubes (CNTs), 1D chopped carbon fibers, 2D woven and 3D needle-punched carbon fiber fabrics, and 3D graphene networks. One challenge to form the composites by conventional powder processing is the difficulty to achieve a homogeneous mix of ceramic powders and the carbon materials. The aim of this research is to take the advantages of sol-gel processing to synthesize nanostructured carbon reinforced UHTC composites with improved microstructures, mechanical and/or other properties. Graphene/TiC composites were synthesized through sol-gel infiltration of graphene network sponges. The C/Ti ratio of the sol-gel solutions influenced the composition as well as the microstructure of the composites. Graphene sheets could react with titanium dioxide (TiO₂) during the carbothermal reduction, forming a plate-like structure when the carbon content in the sol was low. The samples after SPS sintering showed interesting microstructures of nanostructured TiC surrounded by a network of graphene sheets. The composites were toughened by debonding and bridging of the graphene sheets. It was also proved that the composites exhibited an energy absorption mechanism under impact. CNT/TiC composites were prepared by sol-gel mixing and SPS sintering. The application of sol-gel gave an advantage in homogenous mixing. CNTs were able to toughen the composite by bridging, pullout and crack deflection. The fracture toughness increased with the amount of CNTs, but sacrificing the hardness of the materials. A CNTs containing TiC based sol was coated on ZrB₂ raw powders, forming a homogeneous microstructures after sintering, where most of the TiC grains and CNTs were distributed at the grain boundaries of ZrB₂, forming weak interfaces, which benefited the toughness through crack deflection and impact absorption. The chopped carbon fibers had similar toughening effect to CNTs in the composites. Different types of coatings on the surface of carbon fibers influenced the interface properties between ceramic matrix and fibers, thus affecting the fiber pullout and debonding. Sol-gel infiltration was applied to synthesize ceramic composites with 2D woven and 3D needle-punched carbon fiber fabrics. The results showed that sol-gel infiltration was a much more effective way to uniformly fill the fine pores between fiber filaments than the conventional powder slurry infiltration method. The composites showed improved fracture toughness as a result of fiber pullout, debonding, and crack deflection.