Stabilizing Highly Loaded Silicon Nitride Aqueous Suspensions Using Comb Polymer Concrete Superplasticizers

Abstract

The stabilization of highly loaded silicon nitride suspensions will afford the processing of complex and near‐net shaped parts using methods such as injection molding or direct write additive manufacturing. In this study, aqueous silicon nitride suspensions up to 45 vol% solids loading were dispersed using commercially available comb‐type copolymer. These copolymers are used as superplasticizers in the concrete industry and are referred to as water‐reducing admixtures (WRAs). Four different WRA dispersants were examined and chemical analysis determined that each was made up of a sodium salt of polyacrylic acid (PAA‐Na) backbone with neutral polyethylene oxide (PEO) side chains that afford steric stabilization. The general structures of the WRAs were compared to each other by measuring the relative areas of their prominent FTIR peaks and calculating a PAA‐Na/PEO peak ratio. Suspensions were made with as‐received silicon nitride powders with 5 wt% aluminum oxide and 5 wt% yttrium oxide added as sintering aids. Three of the four WRA dispersants studied were able to produce suspensions with 43 vol% solids loading and 5 vol% polymer dispersant, while exhibiting a yield‐pseudoplastic behavior up to 30 s−1. At higher solids loading (45–47 vol%), a shift to shear‐thickening behavior was observed at a critical shear rate for these WRAs. Those WRAs with a lower PAA‐Na/PEO peak ratio displayed better stabilization and diminished shear‐thickening behavior. The vol% of the dispersant was optimized producing yield‐pseudoplastic suspensions containing 45 vol% solids loading with yield stresses less than 75 Pa, no shear‐thickening behavior, and viscosities less than 75 Pa·s for shear rates in the range of 1–30 s−1.

Design of Optimal Organic Materials System for Ceramic Suspension‐Based Additive Manufacturing

“…To our best knowledge, 51 vol% silicon nitride suspension is the highest ceramic content suspension that is utilized in silicon nitride additive manufacturing. Generally, the silicon nitride suspension applied to various additive manufacturing are in a range of 30–45 vol% . With photocurable HDDA monomer, the 51 vol% silicon nitride suspension is successfully prepared with reasonable viscosity (≈2.5 Pa·s) to be applied for DLP 3D printing in a large shear rate window around 10 s −1 (Figure a,b) .…”

“…The intrinsic characteristics of silicon nitride hamper formation of high solid loading suspension and retard application of silicon nitride in DLP. In literatures, the silicon nitride suspensions applied to various ceramic processing are generally in a lower solid loading range around 10–45 vol%, with silicon nitride powder having a particle size of 0.5–1 μm . Therefore, rational material design is mandatory to secure rheological processability of a highly loaded photocurable ceramic suspension for DLP additive manufacturing applications.…”

Granulation of Silicon Nitride Powders by Spray Drying: A Review

“…However, more silica results in lower mechanical properties at elevated temperatures of the sintered bodies, since they contain more intergranular quartz glass [84][85][86]. Another method for stabilizing ceramic powders is to use polyelectrolyte dispersants, consisting of low molecular weight water-soluble polymers, which, being adsorbed on silicon nitride particles, prevent powder particles from sticking together due to electrostatic repulsions [87,88].…”

Surface modification of ultrafine silicon nitride powders by calcination

“…Ultrafine powder is prone to form agglomerates during this ceramic formation process due to its high surface energy, leading to the formation of defects in the green body which are detrimental to the mechanical properties and reliability of the final product. Thus, the fabrication of Si 3 N 4 ceramic components with improved strength and highly reliable mechanical properties depends essentially on the uniform dispersion of ultrafine ceramic powders . An approach to realize this control is through colloidal processing, during which the surface chemistry of the particle is of great importance.…”