Phase transformations and consolidation of Si₃N₄  ceramics activated with yttrium and silicon oxides in spark plasma sintering

Abstract

The effect of Y₂O₃ and Y₂O₃–SiO₂ additives on the spark plasma consolidation of a-Si₃N₄ was studied. The interaction of the oxides with silicon nitride and their influence on the a→b-phase transformation in silicon nitride during sintering were analyzed. The Y₂O₃SiO₂ sintering activator was shown to significantly intensify consolidation at 1800 °С through the formation of 18–25 vol.% liquid Y₂Si₂O₇ and Si₂N₂O. This significant amount of the liquid phase was responsible for regrouping of nanosized Si₃N₄ grains under pressure. In contrast, consolidation of Si₃N₄–Y₂O₃ was much slower and prolonged exposure to high temperatures led to significant grain growth. Optimization of spark plasma sintering (SPS) parameters (variation in compaction pressure, heating rate, and holding time relative to temperatures of active shrinkage) allowed the total time of high-temperature treatment to be decreased and dense ceramic samples (³ 98 %) to be produced. The phase composition of almost fully dense ceramic samples produced in optimized SPS conditions (1800 °С, 5 min) with a complex Y₂O₃–SiO₂ activator was as follows: 72.4 vol.% b-Si₃N₄, 11.0 vol.% a-Si₃N₄, 14.2 vol.% Si₂N₂O, and 2.4 vol.% γ–Y₂Si₂O₇. The ceramic samples had isotropic 300 nm grains and anisotropic ~2 µm grains. When only yttria was added to silicon nitride, dense ceramic samples were produced at 1800 °С with 20 min holding and consisted of 85 vol.% b-Si₃N₄ and 15  vol.% a-Si₃N₄. The isotropic Si₃N₄ grains were 750 nm in size on average and anisotropic grains were ~3–4 µm in length. The same dense ceramics can be produced by SPS at 1950 °С for only 5 min but contain only b-Si₃N₄: that is, the a→b-phase transformation is complete. The dense b-Si₃N₄ ceramics activated with only Y₂O₃ and with Y₂O₃–SiO₂ activators demonstrate high room-temperature bending strength (~950 MPa), HV10 hardness (~15.3 GPa), and fracture toughness (5.7 MPa · m^1/2).