Sintering Behavior and Microstructure Development of Porous Silicon Nitride Ceramics Prepared in an Air Atmosphere Furnace

Porous silicon nitride (Si3N4) ceramics have been sintered in a conventional air atmosphere furnace at temperatures between 1500 degrees C and 1700 degrees C, with sintering additions of either 5 or 10 wt% yttria (Y2O3). The use of a protective alpha-Si3N4 powder bed helps to prevent significant oxidation of the samples during sintering in air, keeping oxidation weight gains below 4 wt% (typically < 2.5 wt%). This compares with more typical weight losses when sintering in nitrogen. Samples prepared with a protective alpha-Si3N4 powder bed exhibited sintered densities up to similar to 89% of the theoretical, which was notably higher than for similar compositions prepared in a controlled nitrogen atmosphere (i.e., similar to 70% of theoretical). Increased densification is proposed to arise from the formation of a thin SiO2 surface layer on individual alpha-Si3N4 particles, through passive oxidation, which is expected to promote viscous-flow densification at lower temperatures. The excess of SiO2, relative to the starting composition, is also reflected in the final phase assembly, with Si2N2O formed in addition to the transformation of alpha- to beta-Si3N4, when sintering above similar to 1550 degrees C. A Si2N2O gradient occurs, with the surface being approximately 80 vol% Si2N2O and the bulk predominantly Si3N4, for samples sintered at 1600 degrees C and 1700 degrees C. Conversely, Si2N2O is not observed in any measurable volume for similar samples sintered in nitrogen.