Negative Additive Manufacturing of Complex Shaped Boron Carbides

Summary

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.

Abstract

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.

Introduction

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 section^2,3,4. However, utilization of these favorable mechanical properties typically requires B₄C to be s....

Protocol

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 two-part suspension
   NOTE: A two-part suspension will be prepared to help prolong the shelf life of the suspensions before casting. One....

Results

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 technique. This is.......

Discussion

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.......

Materials

List of materials used in this article

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