Microstructure, mechanical properties and sintering mechanism of pressureless-sintered porous Si3N4 ceramics with YbF3-MgF2 composite sintering aids

Abstract Porous Si3N4 ceramics were fabricated by pressureless sintering at 1550 °C for 2 h with YbF3-MgF2 composite sintering aids, systematically investigated for the effect of sintering aids addition on the microstructure and mechanical properties, further explored the formation process and sintering mechanism. The α-β phase transition rate of all samples approached 100% and MgF2 showed a positive effect on the elongating of β-Si3N4 crystal grains in morphology. The apparent porosity of porous Si3N4 ceramics ranged from 34% to 39%. In addition, when 2.5 wt% YbF3 was replaced by the same amount of MgF2, the flexural strength sharply increased to 298 MPa, an increase of ~49%, while the porosity increased to 37%, an increase of ~15%. With MgF2 content increasing from 0 to 7.5 wt%, the structure factor reached a minimum of 3.23 when MgF2 addition was 2.5 wt%, which demonstrated unparalleled superiority in mechanical properties compared with previous studies due to a more preferred form of morphology of connected rod-like β-Si3N4 crystals network. Besides, our research indicated that MgF2 addition could develop higher aspect ratio of β-Si3N4 grains and had obviously positive effect on the elongating characteristic of β-Si3N4 grains, which was related to the activation energy variation for nucleation on the different facets.