High α-β phase transition and properties of YbF3-added porous Si3N4 ceramics obtained by low temperature pressureless sintering

In this paper, we used YbF3 as a sintering additive to get a high α-β phase transition in porous Si3N4 ceramics. The mechanism of YbF3 as sintering additives as well as the relationship between microstructure and mechanical properties have been investigated in detail. In addition, we used pressureless sintering to lower the temperature to 1550 °C. YbF3 makes α-Si3N4 completely transform to β-Si3N4, whereas only 41.1% β-Si3N4 could be obtained with Yb2O3. This process yielded ceramics with more flexural strength and increased fracture toughness using less energy. In addition, using YbF3 substituted for part Yb2O3 could promote sintering behaviors of Si3N4 ceramics at low temperature to increase α-β phase transition rate and improve the properties of silicon nitride ceramics significantly. In particular, when we used YbF3-Yb2O3 as additives, we obtained a flexural strength of 269.87 MPa and a fracture toughness of 4.59 MPa·m1/2.