This dissertation presents an investigation into the processing, microstructure, and mechanical behavior of ZrB2-based composites for ultra-high-temperature applications. Various forms of SiC including nano-sized particles, micron-sized particles, and continuous fibers were used as reinforcement.;Three major investigations were conducted in this dissertation. First, the effect of incorporating nano-sized SiC particles into ZrB2 was investigated. Spark plasma sintering was used to consolidate nano-sized SiC/ZrB2 composite. The detailed microstructure of the composite was analyzed using transmission electron microscope. Micropillar compression test was also conducted. It was found that incorporation of nano-sized SiC effectively hindered the grain growth of ZrB2. The second study focused on the ternary ZrC-ZrB2-SiC ceramics. The fully densed ceramics were prepared by spark plasma sintering. Elastic modulus, hardness, fracture toughness, thermal conductivity, and electrical conductivity of ternary ZrC-ZrB2-SiC ceramics were measured. It was found that the fracture toughness of ternary ZrC-ZrB2-SiC ceramics is comparable to that of the ZrB2 ceramics and ZrB2-SiC ceramics. In addition, micropillar compression tests revealed information about typical longitudinal cracking behavior and generation of stacking faults. The third part of this dissertation focused on studying the effect of incorporating continuous SiC fibers on the microstructure and properties of ZrB2. The composite was consolidated by conventional hot pressing method. The chemical reaction between fiber and matrix materials was not observed based on thermodynamic calculation and TEM microstructural analysis. The fracture toughness of composite was measured to be four times higher than that of matrix materials. However, extensive matrix cracking was observed due to mismatch in thermal expansion coefficient between the fiber and matrix.;Finally, the challenge and future research needs in developing ultra-high-temperature ceramics are discussed.
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