Synthesis and hot isostatic pressing of nanocrystalline silicon nitride powders

Nanocrystalline materials have gained tremendous attentions in recent years, as this new material holds great potential for increasing strength and toughness in ceramic structural materials. The properties of nanophase materials are often superior to those of conventional polycrystalline coarse-grained materials. Nanophase materials exhibit increased strength and hardness, enhanced diffusivity, improved toughness, reduced density, a reduced elastic modulus, a higher CTE, increased specific heat, higher electrical resistivity, and lower thermal conductivity compared to conventional coarse-grained polycrystalline materials. In the present study plasma-synthesized nanocrystalline Si3N4 powders followed by compaction of these powders by hot isostatic pressing using nitrogen as the pressurizing medium were investigated. The Si3N4 nanophase powders consisted of particles in the range of 30-100 nm. Al2O3 and Y2O3 were added as sintering additives in the powders, cold pressed at 100 MPa, and hipped at 1750 degrees C. The sintered specimen density was 3.2 g/cm(3), which is 97% of the theroretical density. The average grain size was 100-250 nm, the fracture strength was 1000.0 MPa, and the fracture toughness was 4.8 MPa m(1/2). In this paper, the mechanical properties and microstructure of hipped nanocrystalline Si3N4 specimens are presented and discussed.