B ₂ O ₃ ‐Coated Si ₃ N ₄ Expands the Vat‐Photopolymerization Window and Enables High‐Performance Ceramics

ABSTRACT Silicon nitride (Si 3 N 4 ) is attractive for high‐temperature and high‐wear applications, yet vat photopolymerization (VPP) of Si 3 N 4 remains challenging because the powder exhibits a high refractive index and strong absorption near 405 nm, resulting in insufficient cure depth and poor interlayer bonding. This study proposes an aqueous boric‐acid route to form a dehydration‐derived B 2 O 3 surface coating on Si 3 N 4 particles to improve photon utilization during VPP while simultaneously tailoring the transient liquid phase during subsequent sintering. A 30 vol% Si 3 N 4 suspension stabilized by ammonium polyacrylate (PAA–NH 4 ) was modified using four boric acid concentrations, followed by controlled dehydration to obtain coated powders. The coating reduces UV absorbance around 405 nm, and the cure depth increased from 10.51 to 50.93 µm under 20 mW/cm 2 irradiations, enabling stable printing of dense green bodies. After debinding and pressure sintering at 1750°C under 2 MPa N 2 for 2 h, the optimum B 2 O 3 level (∼2.37 wt% in the ceramic powder) delivers 92.22% relative density, 400.19 MPa flexural strength, 7.33 MPa·m 1/2 fracture toughness, 16.44 GPa Vickers hardness, and 34.77 W·m −1 ·K −1 thermal conductivity. The B 2 O 3 surface coating offers a dual lever over light‐curing efficiency and liquid‐phase sintering kinetics, enabling dense Si 3 N 4 with a balanced combination of strength, toughness, hardness, and thermal conductivity. This study provides an effective surface‐modification strategy to expand the process window for VPP‐based additive manufacturing of high‐performance Si 3 N 4 ceramics.