High-pressure spark plasma sintering of silicon nitride with LiF additive

High-pressure spark plasma sintering of Si3N4 with Y2O3, Al2O3 and LiF additives was employed to fabricate high quality dense ceramics comprising approximately 92% alpha-Si3N4 phase and 8% beta-Si3N4 phase. The relatively high pressure applied (up to 650 MPa) had a substantial effect on densification by enhancing particle rearrangement, making it possible to obtain dense Si3N4 at a significantly lower sintering temperature (1350 degrees C). Consequently, virtually no alpha to beta phase transformation transpired during the liquid phase sintering process. The LiF additive had an indispensable influence on the densification process by lowering the viscous glass formation temperature, which also contributed to enhanced particle rearrangement. The nearly fully dense samples (theoretical density >= 99%) obtained displayed a good combination of mechanical properties, namely elastic modulus (304-316 GPa), hardness (1720-1780 HV2) and fracture toughness (6.0 MPa m(1/2)).