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

摘要

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 degrees 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 Wo to 9.5 wt% and the CaF2 decreases from 2.0 wt% to 0.5 wt%, the conversion rate of the phase transition from alpha-Si3N4 to beta-Si3N4 increases from 71.1% to 81.2% and the largest average aspect ratio of the beta-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.