Effect of nano- and micro-sized Si3N4 powder on phase formation, microstructure and properties of β′-SiAlON prepared by spark plasma sintering

The phase formation behavior of β′-SiAlON with the general formula Si6-zAlzOzN8-z was studied comprehensively for z values from 1 to 3 using spark plasma sintering (SPS) as the consolidation technique at synthesis temperatures from 1400 to 1700 °C. The samples were prepared close to the β′-SiAlON composition line: Si3N4 − 4/3(AlN·Al2O3) in the phase diagram using (A) nano-sized amorphous Si3N4 and (B) micro-sized β-Si3N4 precursors. Field-emission scanning electron microscopy (FESEM) was used for microstructural analysis. Most compositions reached almost full density at all SPS temperatures. Compared with the micro-sized β-Si3N4 precursor, the nano-sized amorphous Si3N4 precursor accelerated the reaction kinetics, promoting the formation of dense β′-SiAlON + O′-SiAlON composites after SPS at synthesis temperatures of 1400–1500 °C. This resulted in very high values of Vickers hardness (Hv10) = 18.2–19.2 GPa for the z = 1 composition related to the hardness of the O′-SiAlON component phase. In general, for samples synthesized from nano-sized amorphous Si3N4, which were almost fully dense, containing >95% β′-SiAlON, the hardness values were 13.4–13.8 GPa with a fracture toughness of 3.5–4.6 MPa m1/2. For equivalent samples synthesized from micro-sized β-Si3N4, hardness was in the range 13.9–14.4 GPa with a fracture toughness of 4.3–4.5 MPa.m1/2. These values are comparable with fully dense β′-SiAlONs, usually containing intergranular glass phase which has been sintered by HIP and other processes at much higher temperatures for longer times.