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Injection molding of boron carbide (B4C) slurries affords the production of complex-shaped personal armor. To injection mold, however, requires preparation of a well dispersed, flowable suspension with >45 vol% B4C loadings to reduce porosity that must be removed during sintering. In the present study, the preparation of highly-loaded B4C suspensions is investigated using zeta potential and rheological measurements, varying dispersant type, molecular weight, and amount. Of those dispersants investigated, polyethylenimine (PEI) with a molecular weight of 25,000 g/mol was found to produce suspensions with up to 56 vol% B4C and the requisite rheological properties suitable for injection molding. A PEI concentration of 1.83 mg/m2 was established as the appropriate to produce highly-loaded B4C suspensions. The effect of a prior B4C powder treatment (ethanol washed or attrition milled) on rheological properties of the suspensions was also investigated. The PEI was completely burned out in argon, nitrogen, and air at 450 °C.

Direct ink writing, a type of additive manufacturing, has been used to fabricate near-net shaped boron carbide (B 4 C) specimens at room temperature with aqueous suspensions. Suspensions with B 4 C solids loading of 48–56 vol.% were dispersed with polyethlenimine (PEI, 25,000 and 750,000 g/mol) and exhibited yield-pseudoplastic behavior. Specimens with filament layer shape retention were produced with suspensions with 50–56 vol.% B 4 C and yield stresses ≥43 Pa and equilibrium storage moduli ≥700 Pa. No residual porosity or cracking between deposited layers was observed in any samples. However, warpage was observed in some green body specimens and was minimized through use of a low molecular weight polymer and reduction of the B 4 C solids loading. Optimal specimens with high filament layer shape retention and no warpage were produced with suspensions containing 54 vol.% B 4 C and 25k g/mol PEI.

Aqueous suspensions of zirconium diboride (ZrB2), boron carbide (B4C) and tungsten carbide (WC) with dispersant and water-soluble polyvinylpyrrolidone (PVP) were investigated for processing by room-temperature injection molding, a novel, environmentally benign ceramic processing method. B4C and WC were used as sintering aids, and the as-received powders were attrition milled to reduce particle size to promote full densification of ZrB2 specimens by pressureless sintering. Zeta potential measurements of individual ZrB2, B4C and WC powders and of powder mixtures revealed that maximum stability was achieved in aqueous solutions of attrition milled powder mixtures dispersed using an ammonium polyacrylate dispersant. A maximum powder loading of 49 vol% with ≤5 vol% PVP was attained for ZrB2/B4C/WC suspensions with dispersant. Although exhibiting a time-dependent rheological response determined by parallel-plate rheometry, suspensions containing 49 vol% powders and ≤3 vol% PVP, as well as suspensions of 46 vol% powders and ≤4 vol% PVP, were flowable under the conditions of the process. ZrB2 rings prepared by room-temperature injection molding were machinable prior to binder removal and exhibited maximum brown densities of 56% true density (TD). Sintered densities were >98%TD with ~20% linear shrinkage. Scanning electron microscopy revealed an average grain size of 7.3±2.8 µm, and chemical analysis confirmed that no undesirable oxide phases remained in the sintered ZrB2 specimens. Aqueous ZrB2-based suspensions containing B4C and WC sintering aids and PVP were effectively processed via room-temperature injection molding to yield dense ZrB2 rings after binder burnout and pressureless sintering.