- High-pressure spark plasma sintering of Si 3 N 4 with Y 2 O 3 , Al 2 O 3 and LiF additives was employed to fabricate high quality dense ceramics comprising approximately 92% α-Si 3 N 4 phase and 8% β-Si 3 N 4 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 Si 3 N 4 at a significantly lower sintering temperature (1350 °C). Consequently, virtually no α to β 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 ).