Warpage-free Si₃N₄ slurry strategy for vat photopolymerization

Si₃N₄ ceramics are promising wave-transparent materials with excellent mechanical and dielectric properties. Vat photopolymerization (VPP) 3D printing provides a strategy for preparing ceramics with controllable complex structures. However, the difficulty in solidifying the slurry due to partial ultraviolet (UV) light absorption and high refractive index by the Si₃N₄ particles during the VPP process severely hampers the molding of Si₃N₄ ceramics. Higher laser power has to be used to increase the curing depth, which generates large internal stresses and leads to the warping of the samples. This paper presents a method to solve the warpage problem during VPP-3D printing using tributyl citrate as a plasticizer. The plasticizer can weaken the force between polymer molecular chains and reduce the internal stress of the green body. The warpage decreases gradually with the increase of tributyl citrate content, and the warpage decreases to 0 when the plasticizer content reaches 30 wt.% at high laser powers from 600 mW to 750 mW. Samples with different layer thicknesses were printed and the optimum thickness of 40 μm was obtained, at which the sintered Si₃N₄ samples possess a unique combination of mechanical properties, including a bending strength of 338.29±12.08 MPa, a fracture toughness of 6.94±0.11 MPa·m^1/2 for the loading direction perpendicular and 5.37±0.99 MPa·m^1/2 for the loading direction parallel to the build surface, respectively. The dielectric constant of all the samples is maintained in the range of 5.462 to 6.414. This work is expected to guide vat photopolymerization and the preparation of complex Si₃N₄ ceramic components.