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.