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

Abstract

Porous silicon nitride (Si3N4) ceramics have been sintered in a conventional air atmosphere furnace at temperatures between 1500°C and 1700°C, with sintering additions of either 5 or 10 wt% yttria (Y2O3). The use of a protective α‐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 α‐Si3N4 powder bed exhibited sintered densities up to ∼89% of the theoretical, which was notably higher than for similar compositions prepared in a controlled nitrogen atmosphere (i.e., ∼70% of theoretical). Increased densification is proposed to arise from the formation of a thin SiO2 surface layer on individual α‐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 α‐ to β‐Si3N4, when sintering above ∼1550°C. A Si2N2O gradient occurs, with the surface being approximately 80 vol% Si2N2O and the bulk predominantly Si3N4, for samples sintered at 1600°C and 1700°C. Conversely, Si2N2O is not observed in any measurable volume for similar samples sintered in nitrogen.