Stereolithography 3D printing of Si₃N₄ cellular ceramics with ultrahigh strength by using highly viscous paste

Si₃N₄ cellular ceramics with large thickness/length ratio (up to ∼12.0) have been successfully prepared by stereolithography 3D printing with highly viscous (viscosity >10.0 Pa‧s at 30 s⁻¹ shear rate), self-holding pastes. Due to the high viscosity and low curing depth of the paste, structural defects are commonly found in the printed honeycombs or lattices and anisotropic surface roughness is present. The structural defects can be reduced by aligning the lattice orientation at 45° relative to the paste recoating direction to reduce shear stress concentration at the cured part beneath. The resultant honeycombs and simple cubic lattices with density of ∼1.5–1.6 g/cm³ exhibit compressive strength as high as 1.1 GPa and 290.7 MPa, respectively. The specific compressive strength of honeycombs increases from 298.6 to 681.7 MPa‧cm³/g with the ceramic volume fractions increase from 0.19 to 0.52, surpassing that of many other ceramic honeycombs and lattices with similar density reported so far. The controlling factors governing the defect formation, surface finish and mechanical properties of the printed cellular ceramics are discussed thoroughly.