Synthesis and characterisation of tungsten carbide

Abstract

The supply of tungsten carbide is very limited, which has resulted in EU categorizing tungsten as a critical raw material. Instead of substituting tungsten entirely, this paper has looked into the possibility of using SiC as a much cheaper bulk material together with some amount of tungsten carbide, to achieve properties similar to cemented tungsten carbide.

Tungsten carbide (WC) and tungsten oxide (WO3) supported on β-silicon carbide (SiC) were prepared with 20 and 50 weight percentage loading based on elemental tungsten (W). The Pechini method was used to coat the SiC supports with ammonium metatungstate (AMT). These samples were then calcined in nitrogen to yield tungsten oxide coated on the SiC. To produce tungsten carbide coated supports, various carburization schedules with different carburizing agents (CH4/H2, CO or CO/H2) were employed. Analysis of the XRD spectra after calcination showed a definite formation of tungsten oxide on the support, believed to consist primarily of the hexagonal WO3. Analysis of XRD spectra after carburization revealed the formation of different tungsten carbides depending on the carburization atmosphere. Samples carburized in pure CO, or a CO/H2 atmosphere were found unable to produce pure WC phase under the studied conditions. However, by reducing WO3 to W in hydrogen prior to CO carburization, WC could be observed. Carburization in CH4/H2 gas mixtures (2-20% CH4) showed that an increase in CH4 content increased the carburization rate. SEM imaging revealed that the morphology and structure of the final WC coating relied heavily on the morphology of the initial precursor layer.

Based on a literature study and earlier work, the overall process of carburization was found to be very complex. The tungsten carbide phases and crystallinity were found to be highly dependent on the carburizing gas and temperature schedule. Higher loading and temperature was found to increase the sintering of the tungsten carbide crystals.

Several of the tungsten carbide coated on SiC samples were tested to see if a metal-alloy could wet them. By comparing the results, it was evident that the coating had a distinct effect on the wetting properties. The uncoated sample was not wetted at all while the coated samples were more readily wetted. The metal alloy was found to completely wet one sample that was held at 1200°C for 20 minutes. XRD analysis on this sample found the SiC to react with the metal alloy constituents, and graphite-like carbon was observed on the surface by SEM/EDX imaging.