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Deformation Processes for the Next Generation Ceramics
In addition to lower specific weight, ceramics have a number of attractive properties for application as components in various heavy vehicle systems. However, many heavy vehicle components require ceramics with complex shapes as well as function. Furthermore the demanding service conditions necessitate improved mechanical reliability and fracture resistance. Underlying the performance and use of ceramics is their tendency to fail in a brittle fashion except at temperatures generally well above 1000°C. Complex shape forming processes for ceramics are relegated to approaches based on powder processing as a result of the excessive temperatures for hot forming. On the other hand, metallic alloys can be hot worked at temperatures below 1000°C to not only form the desired shape but also to enhance their properties. The ductility (> 20% plastic strain) of metallic alloys at temperatures below 500°C is the source of their 10- to 100-fold greater fracture toughness as compared to that of ceramics. In ceramics systems, combined experimental and modeling studies will address approaches to reduce the temperature (1) required for the high permanent strains (> 50%) to be able to hot form them and (2) to attain plastic strains of 1 to 5% to enhance their fracture resistance and reliability at lower temperatures. The hot forming study will extend the findings of super plastic (strains > 100%) behavior observed in some ceramics with grain sizes (G) of > 0.1 µm to nanocrystalline (G < < 0.1 µm) ceramics. Recent studies reveal that material transport by diffusion can be greatly enhanced at temperature well below 1000°C by forming nano-grain sized materials. This will then be combined with studies to modify the microstructure of hot-formed ceramic to develop the properties needed for specific applications. Baseline studies are currently establishing the temperature dependence of the deformation response of zirconia ceramics with grains sizes >100 nm but < 500 nm. These studies will be extended to < 100 nm grain sized ceramics supplied by Subtask 4.24 (Synthesis of Nanocrystalline Ceramics). This will include (A) nanostructured ceramics to explore for the softening effects observed nanocrystalline metals, and (B) the application of external factors (e.g., electric fields) that have been shown to enhance deformation processes in ceramics whose bonding have some ionic character.
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