Transparent β-Si₃N₄ and γ-Si₃N₄ compacts synthesized with mixed-size precursor under high pressure and high temperature

Transparent polycrystalline ceramics exhibit improved mechanical and optical properties. However, synthesizing transparent without additives is nontrivial. Herein, we report the synthesis of two (β-Si3N4 γ-Si3N4) under high pressure temperature from a pure Si3N4 precursor with nano-/micro-dual grain sizes. Synthesized β-Si3N4 exhibited significantly enhanced Vickers hardness reaching 24.2 GPa (at 10 N load) when transparency was achieved. nano-grained γ-Si3N4 37.3 GPa. These are highest values reported for these phases at load. Density microstructure measurements suggest that specimens correlate both size porosity/density. The negligible amount pores accounts superior allotropes. As higher pressures can effectively suppress growth minimize between grains, high-pressure sintering demonstrated as an effective way to realize highly dense ceramics.