Ductility and creep resistance of a silicon nitride ceramic

Abstract

Metals are ductile materials in their annealed state and can usually be readily net-shaped by cold-working. A consequence of cold-working is work-hardening that provides at the end of the forming step a material with improved mechanical resistance. Silicon nitride ceramics have been developed for use at temperatures higher than those allowed by metallic superalloys. However, a drawback of these structural ceramics was the difficulty of making parts with complex geometry and the dimensional tolerances required for mechanical applications. Since the demonstration by Wakai et al. of the superplastic behavior of a covalent crystal composite, Si{sub 3}N{sub 4}/SiC, the availability of commercial nanosized Si{sub 3}N{sub 4} powders has allowed to tailor microstructures that could be deformed by more than 80% in tension. The counterpart of this high ductility is a decrease in creep resistance compared with recent advanced silicon nitride. However, a creep resistance microstructure can be developed by a heat treatment that could be included after the working sage as the final step in the processing schedule.