New strategies for preparing NanoSized silicon nitride ceramics

We report the preparation of nanosized silicon nitride (Si3N4) ceramics via high-energy mechanical milling and subsequent spark plasma sintering. A starting powder mixture consisting of ultratine beta-Si3N4 and sintering additives of 5-mol% Y2O3 and 2-mo1% Al2O3 was prepared by high-energy mechanical milling. After milling, the powder mixture was mostly transformed into a non-equilibrium amorphous phase containing a large quantity of well-dispersed nanocrystalline beta-Si3N4 particles. This powder precursor was then consolidated by spark plasma sintering at a temperature as low as 1600 degrees C for 5 min at a heating rate of 300 degrees C/min. The fully densified sample consisted of homogeneous nano-Si3N4 grains with an average diameter of about 70 nm, which led to noticeable high-temperature ductility and elevated hardness.