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Negative Additive Manufacturing of Complex Shaped Boron Carbides

Published: September 18th, 2018



1Lawrence Livermore National Laboratory

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....

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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

  1. Preparation of a 120 mL .......

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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.......

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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.......

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This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM release LLNL-JRNL-750634.


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Name Company Catalog Number Comments
Boron carbide powder 1250F Tetrabor Ceramics Lot 211M419 >96% purity
Boron carbide powder 1500F Tetrabor Ceramics Lot 209M102/9 >96% purity
Boron carbide powder 3000F Tetrabor Ceramics Lot 111m53/9  >96% purity
Polyethylene Imine (PEI) Sigma Aldrich MKBP3417V Averaged MW ~25,000 by L.S. 
Resorcinol Sigma Aldrich MKBG6751V BioXtra, ≥99%
Formaldehyde Fisher Scientific F79-1 37% by weight; Stabilized with 10-15% Methanol
Acetic Acid Sigma Aldrich SKU 695092 Glacial ≥99.7%
Acetone Sigma Aldrich SKU 179124 ACS Reagent Grade ≥99.5%
Water LLNL In-house (Milli-Q)
Planetary Mixer Thinky AR-250 Fits 150mL and 300mL Thinky containers
Acrylonitrile butadiene styrene (ABS) plastic filament eSUN Natural color
Taz 6 (3D printer) Lulzbot FDM 3D printer
4%H2/96%Ar gas Air Gas UHP 4% Hydrogen, balanced Argon
Helium gas Air Gas UHP Helium
Heating oven Neytech Vulcan 9493308 Oven for 80 °C curing
Quartz tube furnace Applied Test Systems, Inc.  LEA 05-000075 Furnace for 1050 °C carbonization
Graphite furnace Thermal Technology LLC Sintering furnace
Scanning Electron Microscope (SEM) Jeol JSM-7401F
pH meter Thermo Scientific Orion 4 Star calibrated with buffer standards
Rheometer TA Instrument AR2000ex For measurement of viscosity
X-ray Diffractometer (XRD) Bruker AX D8 Advanced
Analytical balance Mettler Toledo XS104
Bruker EVA  XRD Analysis Software

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