A discussion on the formation mechanism of tungsten carbides during mechanical milling of CaWO4[sbnd]Mg[sbnd]C mixtures

Abstract

Synthesis mechanism of tungsten carbides via mechanically-induced self-propagating reaction (MSR) was investigated. Scheelite (CaWO4) is the main ore of tungsten and was used as precursor of tungsten in this study. To investigate the effect of magnesium and carbon contents on the milling products various mixtures were prepared according to the following equation: CaWO4 +x Mg + (4 - x) C = WC + x MgO + (3 - x) CO (g) + CaO. The value of x was varied from 0.5 to 3. It was seen that the reaction is gradual when x≤2 while the reaction occurred in MSR mode at higher x values. CO gas was detected in vial for samples in MSR mode that was due to participation of carbon in reduction reaction of scheelite. Therefore W2C phase was formed because of carbon deficiency. WC was not formed because of high reaction temperature and carbon deficiency. In the second group of experiments milling of CaWO4 - 3 Mg - (1 + y) C mixtures were investigated. The elimination of carbon deficiency was the aim of these experiments. It was seen that metallic tungsten was converted to W2C. Addition of more carbon lead to the formation of WC at y=0.8−1. WC disappeared by addition of more carbon due to decrease of adiabatic temperature and interruption of atomic diffusion.