Impact of Sintering Aid Type and Content on the Mechanical Properties of Digital Light Processing 3D-Printed Si3N4 Ceramics

Digital light processing (DLP) 3D-printed Si₃N₄ ceramics, known for their exceptional performance, offer distinct advantages in meeting the high-strength and complex structural demands of industries such as aerospace, semiconductors, healthcare, automotive, energy, and machinery. However, due to Si₃N₄'s strong chemical stability, low diffusion rate, low self-sintering ability, and high melting point, achieving densification under conventional sintering conditions is challenging. As a result, sintering additives are essential to promote the sintering process, lower the sintering temperature, improve densification, and enhance performance. In this study, 45 vol% Si₃N₄ slurries were prepared using DLP 3D printing technology, incorporating nine different combinations of sintering additives, including aluminum oxide (Al₂O₃), yttrium oxide (Y₂O₃), and aluminum nitride (AlN), in various ratios with Si₃N₄. The slurries were then sintered at 1800 °C for 2 h under a 1 MPa N₂ atmosphere. Additionally, the phase composition, microstructure, grain distribution, and crack propagation of the materials. The results showed that a Si₃N₄ to Al₂O₃ and Y₂O₃ ratio of 95:2.5:2.5 produced elongated β-Si₃N₄ grain structures and enhanced density, achieving a maximum Vickers hardness of 12.88 ± 0.52 GPa. Additionally, the synergistic toughening effect of the rod-like β-Si₃N₄ grains and sintering aids significantly improved the fracture toughness of the Si₃N₄ ceramic matrix, with a flexural strength of 540.63 ± 10.05 MPa and a fracture toughness of 4.92 ± 0.07 MPa·m^1/2. This study lays the foundation for the future application of 3D-printed Si₃N₄ ceramics, optimization of sintering aid combinations at different ratios, and performance enhancement in extreme environments.