Extended Hardness Limit in Bulk Nanoceramics.
Mechanical strengthening by grain refinement is a method whereby a material's strength and hardness can be increased by decreasing the average crystallite grain size. The empirical Hall-Petch relationship mathematically describes grain boundary strengthening and provides guidance for a straightforward way to produce stronger materials. While the phenomenon has been widely explored in nanocrystalline metals, the difficulty associated with fabricating high-quality dense nanocrystalline ceramics has left unanswered the question of the validity and extent of the relationship in ceramics. Prior studies suggest the occurrence of an inverse Hall-Petch response in ceramics with grain sizes.
