Effect of Al₂O₃ coating on the properties of Si₃N₄ ceramics prepared by vat photopolymerization

The preparation of Si₃N₄ ceramics by vat photopolymerization (VPP) has motivated increasing research interest. However, it is challenging to prepare Si₃N₄ ceramics by VPP due to the high UV-light absorbance and refractive index of powder. In this paper, a method for Al₂O₃-coated Si₃N₄ powder was proposed. Combined with the boehmite-coated and high-temperature treatment, Al₂O₃ was successfully coated on the surface of Si₃N₄ powder. The effect of Al₂O₃ content on the properties of Si₃N₄ powders, slurry and the sintered Si₃N₄ samples were investigated. The Al₂O₃ coating layer not only improves the curing forming ability of Si₃N₄ slurries, but also can directly act as one of the sintering aids of Si₃N₄ ceramics. The bulk density of the samples decreases from 3.04 ± 0.02 g/cm³ to 2.97 ± 0.01 g/cm³ with the increase of coating content, while the porosity increases from 5.38 ± 0.89 % to 7.54 ± 0.63 %. The sample of 5 wt % Al₂O₃ coating content has the maximum flexural strength of 474.28 ± 16.38 MPa and the highest relative density of 94.62 ± 0.89 %. This work can not only obtain a great modification effect, but also promote the dense sintering of Si₃N₄ ceramics during subsequent stages, which provides a constructive method for modifying Si₃N₄ powders to achieve photopolymerization.