Published: September 18th, 2018
A method called negative additive manufacturing is used to produce near fully dense complex shaped boron carbide parts of various length scales. This technique is possible via the formulation of a novel suspension involving resorcinol-formaldehyde as a unique gelling agent that leaves behind a homogenous carbon sintering aid after pyrolysis.
Boron carbide (B4C) is one of the hardest materials in existence. However, this attractive property also limits its machineability into complex shapes for high wear, high hardness, and lightweight material applications such as armors. To overcome this challenge, negative additive manufacturing (AM) is employed to produce complex geometries of boron carbides at various length scales. Negative AM first involves gelcasting a suspension into a 3D-printed plastic mold. The mold is then dissolved away, leaving behind a green body as a negative copy. Resorcinol-formaldehyde (RF) is used as a novel gelling agent because unlike traditional hydrogels, there is little to no shrinkage, which allows for extremely complex molds to be used. Furthermore, this gelling agent can be pyrolyzed to leave behind ~50 wt% carbon, which is a highly effective sintering aid for B4C. Due to this highly homogenous distribution of in situ carbon within the B4C matrix, less than 2% porosity can be achieved after sintering. This protocol highlights in detail the methodology for creating near fully dense boron carbide parts with highly complex geometries.
Boron carbide (B4C), with a Vickers hardness of about 38 GPa, is known as the third hardest commercially available material, behind diamond (~115 GPa) and cubic boron nitride (~48 GPa). This particular property, along with a low density (2.52 g/cm3), makes it attractive for defense applications such as armors1. B4C also has a high melting point, superior wear resistance, and high neutron absorption cross section2,3,4. However, utilization of these favorable mechanical properties typically requires B4C to be s....
CAUTION: Please consult with the safety data sheets (SDS) of all materials, and wear proper protective equipment (PPE) when handling materials before casting and curing. Resorcinol and polyethylene imine are known to be toxic. Formaldehyde is both toxic and carcinogenic20. Preparation of ceramic suspensions should be done in chemical fume hoods or other properly ventilated work environments.
1. Negative Additive Manufacturing
Following the outlined procedure (Figure 1), complex shaped boron carbide parts with carbon (B4C/C) can be sintered up to 97.6 ± 0.4% of theoretical max density with a Vicker's hardness of 23.0 ± 1.8 GPa8. Several possible examples of sintered B4C/C parts are demonstrated (Figure 2). These examples show the fine textural features that can be copied by the gelcasting tech.......
The methodology of negative additive manufacturing described in the protocol allows complex shaped boron carbide parts to be produced at nearly full density after sintering at an optimal temperature of 2290 °C. The first several steps related to preparation and casting are the most critical for generating a high-quality cast with minimal defects. If the viscosity of the suspension is too high, poor mixing will occur. The porosity of the sintered part is also affected since increased viscosity hinders air bubble remo.......
|Boron carbide powder 1250F
|Boron carbide powder 1500F
|Boron carbide powder 3000F
|Polyethylene Imine (PEI)
|Averaged MW ~25,000 by L.S.
|37% by weight; Stabilized with 10-15% Methanol
|ACS Reagent Grade ≥99.5%
|LLNL In-house (Milli-Q)
|Fits 150mL and 300mL Thinky containers
|Acrylonitrile butadiene styrene (ABS) plastic filament
|Taz 6 (3D printer)
|FDM 3D printer
|4% Hydrogen, balanced Argon
|Oven for 80 °C curing
|Quartz tube furnace
|Applied Test Systems, Inc.
|Furnace for 1050 °C carbonization
|Thermal Technology LLC
|Scanning Electron Microscope (SEM)
|Orion 4 Star
|calibrated with buffer standards
|For measurement of viscosity
|X-ray Diffractometer (XRD)
|AX D8 Advanced
|XRD Analysis Software
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