Insights of Direct Ink Writing parameters towards dense silicon nitride manufacturing

This study investigates the influence of printing parameters such as infill patterning (0° and 90°), nozzle diameter (0.41, 0.33, 0.25 and 0.15 mm) and printing speed (5–25 mm.s −1 ), on the printability of Si 3 N 4 parts by Direct Ink Writing (DIW). Firstly, a fixed paste composition was used to evaluate the effect of infill direction, nozzle diameter and post processing (cold isostatic pressing) in the final density, flexural strength and hardness of printed samples. Then, a map was constructed by relating printability with the nozzle diameter and the printing speed. Lastly, the paste composition was adjusted, by controlling the concentration of additives and solids, to improve the printability for small tip diameters (<0.41 mm). Average sintered relative densities of ≈97 % and ≈550 MPa of flexural strength were attained for samples printed with 0.33 mm tip nozzle diameter, using 0° infill direction, reaching ≈99 % and ≈600 MPa with cold isostatic pressing as post processing. Further decrease in nozzle diameter size to 0.25 mm resulted in high variability for both densification and flexural strength, because of the high quantity of macrodefects arising from the printing process, highlighting the need to adjust the rheological properties of the ink for a proper extrudability. Increasing the amount of carboxymethyl cellulose in the paste with 37 vol% of solids, increases its stiffness and consequently, larger shear forces are required to induce flow, without phase separation.