INVERSE HALL-PETCH BEHAVIOR OF NANOCRYSTALLINE OXIDE CERAMICS FABRICATED BY HIGH-PRESSURE SPS

High pressure (up to 1 GPa) spark plasma sintering (HPSPS) allows to fabricate polycrystalline ceramics at relatively low temperatures with short sintering time. Polycrystalline transparent magnesium aluminate spinel (PMAS) obtained by HPSPS technique displays nano-structure (20-30 nm) and a unique combination of optical and mechanical properties, comparable or even better than the best results reported in literature for a two-stage fabrication process. It was established that by adjusting the sintering temperature and pressure it is possible to tailor the grain size of nanostructured ceramics.
Our recent experimental results regarding fully dense nanocrystalline (20-100 nm) PMAS and yttria stabilized zirconia (YSZ) have raised some fundamental questions related to the mechanical behavior during micro-hardness tests. For the first time, a minimal grain size (roughly >40 nm) where a Hall-Petch relation is valid was established and an inverse Hall-Petch correlation was clearly observed for ceramics with grain size less than 30 and 60 nm for PMAS and YSZ, respectively.
In the present study, a model explaining the effect of the grain size of nanostructured ceramics on the mechanical properties was developed and will be discussed.