Material extrusion of highly-loaded silicon nitride aqueous inks for solid infilled structures

Abstract

The material extrusion technique of direct ink writing (DIW) has garnered recent interest for the ability to create near-net shapes using polymer and/or ceramic based inks using low-cost commercially-available equipment. While most foundational work in the field has shown promise for single-walled structures, the nature of the circular cross-section of extruded ink has led to the reported issue of interfilament voids when multi-walled structures are attempted. In addition, particularly for ceramic-loaded aqueous inks, multi-walled structures have proven difficult to dry post-processing without cracking or void formation. In this study, DIW was used to produce bulk infilled samples of silicon nitride (Si3N4), a high-temperature ceramic of interest for various high-temperature aerospace, automotive, and structural applications. Here, an ink feedstock was developed and modified from an aqueous-based Si3N4 suspension previously developed for room-temperature injection molding. Tailoring the rheological properties for DIW to exhibit low yield stresses (< 100 Pa), and low equilibrium storage moduli (<1000) resulted in little to no interstitial void defects which are common with the infilled aligned print pattern. A drying analysis varying humidity and sample size revealed that bulk infilled samples required a controlled humidity around 90% for the initial drying phase to produce defect free samples. Through careful control of the ink rheology and drying kinetics, mechanical test bars were sintered to high density (∼95%) with four-point flexural strengths greater than 500 MPa (in-line with other reported values for similar sintering conditions). There was no discernable effect of nozzle size (i.e. 1.2 mm or 0.6 mm) on resulting flexural strength, indicating strong interfilament welding. In fact, X-ray CT analysis of fractured mechanical test bars revealed microstructures with minimal large pores (i.e > 100 µm) and no pore channels observed between printed filaments, common issues reported in other works on solid infilled DIW.