Towards Additive Manufacturing of Tungsten Carbide Using Renewable Resources
Authors: Islam Monsur , Martinez-Duarte Rodrigo 
Published: February 2020

We present preliminary experiments leading to novel additive manufacturing of carbides. To this end, we shape and heat treat a biopolymer-metal oxide composite featuring Bingham plastic behavior. Extrusion of such composite at room temperature allows for layer-by-layer fabrication; with the rheology of the composite dictating the scale of fabrication, i.e. thin mixtures for millimetric features, and thicker composites for bigger structures. Upon heat treatment to 900 °C, the biopolymer carbonizes. Further increase in temperature leads to a reaction between this carbon and the metal oxide nanoparticles to form metal carbides1. Such manufacturing process could lead to a more energy-efficient route to complex carbides shapes. Renewable biopolymers replace petroleum-based ones as carbon source; and the temperature needed for carbide formation can be drastically reduced due the colloidal proximity of the reactants. Additive manufacturing of the composite that is precursor to carbides could enable complex shapes, which are challenging or impossible when using powder pressing or machining of bulk carbides.
We developed and characterized water-based composites featuring iota-carrageenan (IC) and chitin as mixed with tungsten trioxide (WO3) nanoparticles. The initial target is tungsten carbide (WC). Iota carrageenan powder was mixed with chitin in a weight ratio of 1:4. Thermogravimetric analysis of this mixture yields a 10.85% carbon yield at 900 °C. Thus, WO3 nanoparticles were mixed with this biopolymer powder mixture to achieve a C:WO3molar ratio of 1:8, in accordance to the chemical reaction and providing an excess of carbon. Ultra-pure water was added to the biopolymer-metal oxide nanoparticle mixture and stirred manually to achieve a Bingham plastic composite. The biopolymer composite was loaded in a syringe and manually extruded into 3D spirals shapes of around 10 mm in height. The heat treatment protocol features a ramp of at 5°C/min, nitrogen flow, a dwell of half-hour at 300°C, and a dwell of 3 hours at different temperatures ranging from 750 °C to 1450 °C. Carbonaceous 3D shapes result in all cases. Resultant shapes are of similar geometry to the precursor ones but smaller due to shrinkage.
SEM-EDS and XRD analysis were performed on the carbonaceous samples towards characterizing their composition and geometry at different temperatures. The WO3 reduces to different forms of tungsten oxide and finally to tungsten at 900 °C. With further increase of temperature, the reduced tungsten reacts with carbon to form tungsten carbide. It is observed in Figure 1 that tungsten carbide starts to form above 900 °C. Further increase of temperature increases the crystallinity of the tungsten carbide as WC peaks get higher with temperature as seen from the XRD pattern. Figure 2a and 2b show the grain structure before the carbide formation at 750 °C and 900 °C respectively. It can be observed that the grain size increases with temperature. Figure 2c and 2d show that porous carbide was synthesized at 1025 °C and 1150 °C. The carbides synthesized in this process have grain size ranging from tens of nanometers to 2 µm. From elemental analysis using EDS (Table 1), it is observed that the relative amount of tungsten with respect to carbon increases proportional to temperature. This supports the theory of carbonization of the biopolymer as well as the reduction of WO3with the increasing temperature. In EDS analysis, calcium and potassium were also found to be present as the impurities which may come from the water used for composite formation.
Ongoing work is 1) investigating the effect of time, heating rate and heating environment on carbide formation and composition; 2) further understanding of shrinkage during heat treatment; and 3) implementing automated fabrication using a fused deposition modeling approach.
References:
1. Z. Schnepp, S. C. Wimbush, M. Antonietti, and C. Giordano, Chem. Mater., 22, 5340–5344 (2010).
Table 1: Elemental analysis from EDS

Temp.
Composition (%)

W
C
O
Ca
K

750 °C
39.4
50.7
8.8
0.8
0.2

900 °C
63.7
22.6
10.1
1.6
2.0

1025 °C
85.5
10.4
3.6
0.5
-

1150 °C
91.0
8.6
-
0.4
-

Figure 1