An extended hardness limit in bulk nanoceramics

Abstract

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 <100nm. This paper demonstrates a novel integrated approach, comprised of nanopowder processing and high-pressure, low-temperature sintering to fabricate bulk, fully dense and high-purity nanocrystalline ceramics with unprecedentedly small nanometer-sized grains. Using magnesium aluminate spinel as an archetypal hard ceramic, the hardness of this transparent ceramic armor is shown to rigorously follow the Hall–Petch relationship down to grain sizes of 28nm. Consequentially, the nanocrystalline spinel ceramics are shown to exhibit a 50% increase in hardness over a corresponding order of magnitude reduction in grain size without a decline in density or fracture resistance. Additionally, the produced nanocrystalline ceramics have an optical transparency near theoretical. Reaching an exceptional hardness of 20.2GPa at 28nm, the behavior shows no evidence supporting an inverse Hall–Petch effect.