Si3N4 ceramics with fine-grained bimodal microstructure and excellent mechanical properties prepared by two-step spark plasma sintering

Abstract

A novel two-step spark plasma sintering (SPS) strategy was employed to prepare more superior Si₃N₄ ceramics, containing MgO and Yb₂O₃ sintering additives, for increasingly stringent application in this study. The Si₃N₄ ceramics with non-densified fine-grained bimodal microstructure consisting of α- and β-Si₃N₄ phases were initially obtained by the first-step sintering under the condition of at high-temperature of 1750 ℃, low-pressure (10 MPa), and short-holding time (2 min). Then, the Si₃N₄ ceramics were then densified by the second-step sintering at low-temperature of 1550 ℃, high-pressure (30 MPa), and long-holding time (10 min). Using the two-step SPS, Si₃N₄ ceramics with fine-grained bimodal microstructure and excellent mechanical properties (Vickers hardness ∼ 20.5 ± 0.3 GPa, fracture toughness ∼ 7.4 ± 0.3 MPa·m^1/2, flexural strength ∼ 856 ± 89 MPa) were obtained. This study provides a newly innovative approach for the development of sintering routes for high performance Si₃N₄ ceramics.