Preparation and properties of Si₃N₄ ceramics via vat photopolymerization using Si₃N₄ powders coated with bowl-like boehmite

It is a significant challenge to fabricate Si₃N₄ ceramics due to the high refractive index (2.1) and UV absorbance (0.8903 at the wavelength of 405 nm) of Si₃N₄ powders. In this study, a method of filling Si₃N₄ powders into bowl-like hollow boehmite microspheres was proposed. And the effects of the boehmite coating content on the properties of Si₃N₄ ceramic slurries, green bodies and sintered samples were systematically investigated. Due to its low refractive index (1.7) and weak UV absorbance (0.2648 at the wavelength of 405 nm), boehmite can serve as a modifier to enhance the photocuring ability of Si₃N₄ ceramic slurries. At the same time, it can also be decomposed into Al₂O₃ at high temperature as a sintering aid of Si₃N₄ ceramics. As the boehmite coating content increased from 2.5 wt % to 10 wt %, the bulk density of Si₃N₄ ceramic samples decreased from 3.05 ± 0.01 g/cm³ to 2.79 ± 0.03 g/cm³, while the porosity increased from 6.84 ± 0.81 % to 13.23 ± 0.93 %. The relative density (93.15 ± 0.02 %) and flexural strength (406.5 ± 18.3 MPa) of sintered Si₃N₄ ceramic samples revealed that both values peaked at 2.5 wt %. This study not only provides a constructive method for modifying Si₃N₄ powders to achieve photopolymerization, but also offers an innovative idea for other non-oxidation ceramics with poor photocuring ability.