Design strategy of phase and microstructure of Si₃N₄ ceramics with simultaneously high hardness and toughness

Aiming to achieve silicon nitride (Si₃N₄) ceramics with high hardness and high toughness, the relationships among phase composition, microstructure, and mechanical properties of Si₃N₄ ceramics prepared by spark plasma sintering (SPS) at temperatures ranging from 1500 to 1800 ℃ were investigated in this study. Two stages with different phase and microstructure features were observed and summarized. The α–β phase transformation occurs first, and the development and growth of grains lag behind. During the first stage, the average grain size remains basically unchanged, and the hardness maintains at a value of ~20.18±0.26 GPa, despite the β-Si₃N₄ phase fraction increases from 7.67 to 57.34 wt%. Subsequently, the equiaxed grains transform into rod-like grains with a high aspect ratio via the reprecipitation process, resulting in a significant increase in the fracture toughness from 3.36±0.62 to 7.11±0.15 MPa·m^1/2. In the second stage of sintering process, the fraction of β-Si₃N₄ phase increases to 100.00 wt%, and the grain growth also rapidly occurs. Thus, the fracture toughness increases slightly to 7.61±0.42 MPa·m^1/2, but the hardness reduces to 16.80± 0.20 GPa. The current results demonstrate that the phase contents of β-Si₃N₄ and the microstructure shall be carefully tailored to achieve high-performance Si₃N₄ ceramics. Si₃N₄ ceramics with a fine-grained bimodal microstructure, consisting of the main α- and β-phases, can exhibit the optimized combination of hardness and toughness.