Superplastic deformation of nano-sized silicon nitride ceramics

The superplastic deformation of nano-sized (~68 nm) Si3N4 ceramics has been studied in compression over a wide range of strain rates and temperatures.The experimental results revealed, for the first time, a transition in the stress exponent (n) at each temperature. The n values decreased from ~2 to ~1 with increasing stress. The activation energy was also different for the two regions, decreasing from 858.2 kJ/mol in the n ~2 region to 571.8 kJ/mol in the n ~1 region. Concurrent grain growth occurred during deformation tests;; however, grain growth was not accompanied by obvious strain hardening due to the strong texture developed. Assessment of possible rate controlling deformation mechanism indicated that in the n ~2 region,superplastic behavior was considered to be controlled by the interface-reaction controlled solution-precipitation process with two-dimensional nucleation, whereas deformation in the n ~1 region was controlled by a grain boundary sliding process accommodated by a diffusion-controlled solution-precipitation.