Shock‐Compacted Si 3 N 4 Nanocrystalline Ceramics: Mechanisms of Consolidation and of Transition from α‐ to β‐form

Ultrafine powder of α‐Si 3 N 4 several tens of nanometers in size was successfully consolidated using a shock‐compaction technique under 40 GPa. The bulk density of the most dense portion attained 99% of the theoretical value, and the Vick‐ers microhardness was 2300 kg/mm. Consolidation was achieved through both densificarion by plastic deformation of constituent grains and interparticle bonding by surface melting. A transition process to β‐Si 3 N 4 was observed by transmission electron microscopy: Surface melting of the individual grains propagated into the center as temperature increased, and a large mass formed. A precursor of β–Si3N4 then emerged from the mass and finally grew into a β‐Si 3 N 4 crystal.