Session: 16-03-04: Foundational Framework II

Paper Number: 173853

Standardizing Rheological Measurements of Highly Loaded Ceramic Inks for Additive Manufacturing 

Ceramic additive manufacturing techniques such as vat photopolymerization and direct ink writing can form complex, customizable ceramic structures for a variety of industrial applications, such as in the aerospace, automotive, and biomedical industries. Ceramic vat photopolymerization involves selectively projecting UV light onto photopolymerizable resins, which are highly loaded with ceramic particles, in order to create a desired 3D structure. Ceramic direct ink writing involves extruding a viscoelastic, highly loaded ceramic ink through a deposition nozzle in a layer-by-layer fashion to create a 3D structure. To successfully fabricate these ceramic structures, ceramic precursor inks and resins must be formulated with the proper rheology for the additive manufacturing approach being used. For example, direct ink writing requires the precursor ink to exhibit shear-thinning behavior and have a sufficiently low viscosity to be extruded from the print nozzle and maintain its shape after extrusion. Unfortunately, obtaining reliable and repeatable rheological data from highly loaded ceramic inks can be challenging due to their non-Newtonian and complex time/history-dependent (thixotropy) behavior. Suitable standards (materials and test procedures) for measuring the rheology of dense suspensions filled with small (e.g., submicron) particles do not currently exist.  We are in the process of developing a potential research grade standard test material (RGTM) and rheological measurement protocol that produces repeatable rheological measurements of dense ceramic suspensions. The test material consists of single crystal Al₂O₃ monodispersed particles in a nonaqueous solvent, which was selected to avoid complications related to evaporation during testing. We have investigated the effect of particle concentration, sample loading procedure, and a variety of measurement procedures on the measured rheological characteristics of the test material. Rheological measurements, including creep, flow, and oscillatory tests, were performed on a parallel plate rheometer to determine the yield stress, viscosity as a function of shear rate, and modulus. We additionally investigated test material stability and particle settling over time as well as the effect of these on the rheological measurements. We present our findings on the suitability of this material as a research grade standard test material, as well as the effect of the measured parameters on the reliability of the metrology data. We also present a proposed test protocol that documents best practices and provides a detailed procedure for reproducing process-relevant measurement results. We envision that this protocol, validated through future interlaboratory studies, will facilitate the rheological study of a wide range of highly loaded inks for a variety of ceramic additive manufacturing processes.

Presenting Author: Samuel Hales National Institute of Standards and Technology

Presenting Author Biography: Samuel Hales is a postdoctoral researcher developing methodologies for defect detection in additively manufactured ceramics. His previous research focused on the development, characterization, and application of multimaterial additive manufacturing processes in the creation of novel devices, such as biomedical sensors and plant wearable electronics. He received his B.S. from Brigham Young University and his M.S. and Ph.D. from the University of Utah.

Authors:

Samuel Hales National Institute of Standards and Technology
Russell Maier National Institute of Standards and Technology