Curing Behavior and Stacking Stability of Photocurable Si₃N₄ Slurries for DLP 3D Printing

Digital light processing (DLP) 3D printing has emerged as a promising technique for fabricating highprecision ceramic components, including biomedical implants. However, achieving high-strength silicon nitride (Si₃N₄) spinal implants via DLP remains challenging due to slurry formulation constraints. This study optimizes the formulation of a photocurable Si₃N₄ slurry to enable DLP-based printing of spinal implant components. The effects of monomer composition and photoinitiator content on slurry viscosity and stability were evaluated. Slurries with only monomer A exhibited post-printing warpage, while those with only monomer B showed rapid polymerization shrinkage, causing delamination. In contrast, a 1:1 weight ratio blend of monomers A and B with 3 wt% photoinitiator (M-AB-3) maintained viscosity stability within 3% over 24 h and showed excellent shape fidelity. The flexural strength of the green body increased to 122 MPa when 8 wt% oligomer A was added, while viscosity rose to ~9,100 cP. A low-viscosity oligomer B was added to reduce viscosity. A formulation with 4 wt% oligomer A and 4 wt% oligomer B achieved optimal performance, with ~7,000 cP viscosity and 154 MPa green strength. Based on this optimized resin, ceramic solid loading was increased stepwise, and 66 wt% was determined to be the maximum printable content. Printed specimens were sintered under nitrogen (1800 ℃, 1 MPa), resulting in a relative density exceeding 99%, flexural strength of 1,070 MPa, Vickers hardness of 1,670 HV, and fracture toughness of 7.3 MPa·m1/2. These results confirm that the developed slurry enables stable high-solid loading and yields high-performance Si₃N₄ components suitable for spinal implant applications.