William J. Costakis Jr. (1) (2), Connor Wyckoff (1) (3), Andrew Schlup (1) (3), Margeaux Wallace (5), Tyriek Craigs (4), Elizabeth Malek (1) (4), Ashley Hilmas (1), Lisa Rueschhoff (1)
SSRN. (24 November 2022). DOI: http://dx.doi.org/10.2139/ssrn.4285344

Keywords

Material extrusion, Direct ink writing, ceramic slurry, silicon nitride, bulk infilled ceramics, mechanical properties

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 (less than 100Pa), and low equilibrium storage moduli (less than 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 (among the highest reported for additively manufactured Si3N4 densified via pressureless sintering). 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.

How Our Software Was Used

Deep learning based segmentation methods was performed using Dragonfly in order to quantify the porosity within the scans.

Author Affiliation

(1) Materials and Manufacturing Directorate, Air Force Research Laboratory, WPAFB, OH 45433
(2) National Research Council Research Associate Program, Washington, D.C., 20001
(3) UES, Inc., Dayton, OH 45432
(4) Strategic Ohio Council for Higher Education, Beavercreek, OH 45440
(5) GE Research, General Electric, Niskayuna NY 12309