Pressure-induced nano-crystallization and high hardness of optically transparent α-Si3N4 ceramics

Transparent silicon nitride ceramics with good optical and mechanical properties are promising ceramics for scientific and industrial window materials with a long service life. The optical transparency and mechanical strength will be substantially enhanced in dense nano-polycrystalline monoliths. However, the synthesis of nano-polycrystalline α-Si₃N₄ has not been realized due to the limitations of conventional sintering techniques. Here, nano-polycrystalline α-Si₃N₄ was prepared by direct conversion of micron-grain silicon nitride without additives under high-pressure conditions of 5 GPa and a limited temperature range 1650°C–1700°C. The micron-sized grains undergo grain refinement and recrystallization to form uniform nano-grains under high pressure and high temperature. Furthermore, transparent α-Si₃N₄ samples exhibit the highest Vickers hardness of 26.7 GPa, which is far higher than that of specimens with or without additives used in other sintering methods (e.g., SPS, and HP). According to the Hall-Petch and Taylor dislocation hardening effects, the refined nano-grains, coherent grain boundaries, and high dislocation density lead to high hardness. Moreover, the high density, nanoscale grains, and fine grain boundaries are ascribed to the improvement of transparency. Ultrahigh-pressure sintering induces grain refinement, grain coherency, and increased dislocation in silicon nitrides, thus providing a promising method for preparing advanced transparent ceramic windows in the future.