Densification kinetics of silicon nitride hot pressed with ceria additions

Abstract

The densification kinetics of silicon nitride containing (1–20 wt%) CeO 2 hot pressed in a nitrogen atmosphere were studied at T = 1823 – 1973 K and P a = 14.7–39.2 MN/m 2. Densification occurs in the presence of an essentially cerium silicate liquid phase that partially crystallizes during cooling to Ce 4.67 (SiO 4) 3O and to occasional minor amounts of Ce 2 O 3.2Si 3N 4, Ce 4Si 2O 7N 2 and CeSiO 2N. Liquid phase aided grainboundary sliding accompanied by particle fragmentation and limited solution seems to govern the rearrangement stage. Solution of nitride particles in - or diffusion through - the liquid phase appear as the operative rate limiting mechanisms of the solution reprecipitation process at low or, respectively, high applied stress and liquid phase content. Viscous flow of the grain boundary glassy phase via a grainboundary sliding mechanism in suggested as the rate limiting step for densification in the final stage. The apparent activation energies for the three stages were approx. 290, 540 and 700 KJ mol −1, respectively.