Creep Resistant Silicon Nitride Ceramics- Approaches of Microstructural Design

Abstract

The use of silicon nitride ceramics for high-temperature applications is still restricted due to creep, oxidation, and limited lifetime. In the present work, several microstructural design tools, such as the careful adjustment of the densification additive, the addition of a nanosized SiC dispersion, and thermal treatments, were applied to develop materials of improved creep and oxidation resistance. These measures lead to materials which were found to be among the most creepresistant Si3N4 ceramics ever tested. Deformation was characterized by a steady-state creep regime, a small stress exponent around 2, and a cavitation that was substantially less than reported for other Si3N4 ceramics. These findings are taken as evidence that cavitation-induced deformation can be suppressed successfully. Strong hints were found that the improvements in creep resistance are caused by chemical effects rather than by microstructural influences. Both thermal treatments and the presence of the nanosized SiC phase in the microstructure are thought to affect the amount and composition of the intergranular phases, leading to an enhanced grain-boundary viscosity.