Preparation of High-Density Si3N4 by a Gas-Pressure Sintering Process

Si3N4 compacts, containing ≅7 wt% of both BeSiN2 and SiO2 as densification aids, can be reproducibly sintered to relative densities >99% by a gas‐pressure sintering process. Nearly all densification takes place via liquid‐phase sintering of transformed β‐Si3N4 grains at T=1800° to 2000°C. Compacts with high density are produced by first sintering to the closed‐pore stage (≅92% relative density) in 2.1 MPa (20 atm) of N2 pressure at 2000°C and then increasing the N2 pressure to 7.1 MPa (70 atm) where rapid densification proceeds at T= 1800° to 2000°C. The experimental density results are interpreted in terms of theoretical arguments concerning the growth (coalescence) of gas‐filled pores and gas solubility effects. Complex chemical reactions apparently occur at high temperatures and are probably responsible for incomplete understanding of some of the experimental data.