Low-temperature sintered porous Si3N4 ceramics using two fluorides as sintering aid

Porous Si3N4 ceramics with a unique interlocking microstructure have been fabricated by pressureless liquid-phase sintering with YbF3 and CaF2 sintering additives at a relatively low temperature (1550 °C). Phase transition and mass diffusion in the low-temperature sintering process are facilitated by YbF3 and CaF2 producing plenty of liquid phase owing to their extremely low melting points. As the content of YbF3 increases from 8.0 wt% to 9.5 wt% and the CaF2 decreases from 2.0 wt% to 0.5 wt%, the conversion rate of the phase transition from α-Si3N4 to β-Si3N4 increases from 71.1% to 81.2% and the largest average aspect ratio of the β-Si3N4 grains reaches 10.05. At the same time, the porosity declines from 40.4% to 34.9% and the flexural strength increases from 168.5 MPa to 232.1 MPa.