Abstract
The phase formation behavior of beta'-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 degrees C. The samples were prepared close to the beta'-SiAlON composition line: Si3N4 - 4/3 (AlN.Al2O3) in the phase diagram using (A) nano-sized amorphous Si3N4 and (B) micro-sized beta-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 beta-Si3N4 precursor, the nano-sized amorphous Si3N4 precursor accelerated the reaction kinetics, promoting the formation of dense beta'-SiAlON + O'-SiAlON composites after SPS at synthesis temperatures of 1400-1500 degrees C. This resulted in very high values of Vickers hardness (Hv(10)) = 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% beta'-SiAlON, the hardness values were 13.4-13.8 GPa with a fracture toughness of 3.5-4.6 MPa m(1/2). For equivalent samples synthesized from micro-sized beta-Si3N4, hardness was in the range 13.9-14.4 GPa with a fracture toughness of 4.3-4.5 MPa.m(1/2). These values are comparable with fully dense beta'-SiAlONs, usually containing intergranular glass phase which has been sintered by HIP and other processes at much higher temperatures for longer times.