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

• Published in: Applied physics letters 2021-10, Vol.119 (17)
• Main Authors: Ma, Shuailing, Zhao, Yongsheng, Tang, Ruilian, Yang, Bin, Tao, Qiang, Li, Yan, Cheng, Jiaen, Wang, Yu, Cui, Tian, Zhu, Pinwen
• Format: Article
• Language: English
• Subjects: Additives; Allotropy; Applied physics; Ceramics; Compacts; Density; Diamond pyramid hardness; Grain growth; Grain size; High temperature; Hot pressing; Optical properties; Porosity; Precursors; Silicon nitride; Sintering (powder metallurgy); Synthesis
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/5.0070380

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