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

Porous silicon nitride (Si ₃ N ₄ ) 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 (Y ₂ O ₃ ). The use of a protective α‐Si ₃ N ₄ 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‐Si ₃ N ₄ 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 SiO ₂ surface layer on individual α‐Si ₃ N ₄ particles, through passive oxidation, which is expected to promote viscous‐flow densification at lower temperatures. The excess of SiO ₂ , relative to the starting composition, is also reflected in the final phase assembly, with Si ₂ N ₂ O formed in addition to the transformation of α‐ to β‐Si ₃ N ₄ , when sintering above ∼1550°C. A Si ₂ N ₂ O gradient occurs, with the surface being approximately 80 vol% Si ₂ N ₂ O and the bulk predominantly Si ₃ N ₄ , for samples sintered at 1600°C and 1700°C. Conversely, Si ₂ N ₂ O is not observed in any measurable volume for similar samples sintered in nitrogen.