Design of Novel Buffer Layers for Excellent Performance UHTCs; Investigation of Diffusion Mechanisms

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ZrB2 is an ultra-high temperature ceramic (UHTC) possessing a melting point exceeding 3000°C and interesting engineering and physical properties. As such, ZrB2 ceramics are considered potential candidates for space and hypersonic components. The main drawbacks that restrict the employment of ZrB2 ceramics for a wider spectrum of applications are mainly related to its low damage tolerance, poor oxidation resistance and relatively high density. However, the introduction of discontinuous SiC fibers can both increase the toughness of ZrB2 above 6 MPa·m½ and parallel a notably decrease in density. Moreover, the addition of secondary phases such as MoSi2 can further improve the high-temperature strength and, most importantly, the oxidation performance of the boride matrix. The ultimate scope of the project is to obtain a functionally graded (FG) composite made up of a ZrB2-MoSi2 outer scale to provide oxidation and ablation resistance and a progressively SiC fiber-enriched body from top to bottom to guarantee a higher failure tolerance and a lower density of the entire structure. The major obstacle to overcome is the detrimental chemical reaction between Mo-compounds and SiC fibers occurring during sintering, which leads to the microstructural degradation of the fibers and, hence, to the loss of their toughening function.To reach the above mentioned aim, the focus of the project is on the development of a novel ZrB2-based buffer scale able to prevent chemical reactions between SiC fibers and Mo-compounds and on the study of interdiffusion processes across the interface, both during sintering and upon oxidation. It is envisioned to utilize mixtures of ZrB2 and Si3N4, SiCN and HfSiN for the diffusion barrier. The understanding of the chemical reactions across the various layers will be fundamental for the design of a novel UHTC for applications under extreme environments.The work plan foresees the development of various baseline ceramic joints with different composition of the buffer layer and a thorough microstructural characterization of the as-sintered and oxidized specimens by transmission electron microscopy. These analyses, coupled to thermodynamic predictions, will be essential in revealing local diffusion processes and will lead to the definition of an optimized buffer layer composition for the design of a functional UHTC. Thermo-mechanical characterization and arc-jet tests of the optimized composite will be carried out to assess the performance of the novel material.