The Formation Mechanism Investigations of Nano-Tungsten Carbide Powder.

Abstract

Formation mechanism of synthesizing nanoparticle tungsten carbide (WC) was studied. WC was synthesized by carbothermal hydrogen reduction (CHR) method under various reaction temperatures for holding different post-treatment time in 20% (v/v) CH₄/H₂. The phase transformation mechanism of gaining WC was investigated, by combining CHR with X-ray diffraction (XRD) and temperature programmed reduction mass spectroscopy (TPR-MS). The results show that pure phase of WC has been obtained by CHR after isothermal heat treatment for 24 hours at 750 °C and 12 hours at 950 °C, respectively. These results indicated that two key parameters of higher temperature and longer isothermal heat treatment time are necessary for synthesizing pure phase of WC powder. In order to find out the phase transformation mechanism of tungsten trioxide (WO₃) to WC, the reduction and carburization process among the temperature range from 600 °C to 1000 °C for holding 3 hours at the final temperature were studied. It was shown that at 600 °C, WO₃ was reduced to WO₂, and from 600 °C to 750 °C, WO₂ was reduced to metallic tungsten (W). Moreover, at the temperature range from 750 °C to 900 °C, the mixture phases of tungsten carbide (WC), metallic tungsten (W), or/and tungsten sub-carbide (W₂C) were formed without any oxides species, which indicated that tungsten carbides (WC and W₂C) phases appeared because the oxides phase was thoroughly reduced. However, the occurrence of carburization process was still limited due to the presence of oxygen in the solid. Because of the formed CO and CO₂ there was not enough activated methane reacting with metallic tungsten, so the phase of WC and W₂C were both formed simultaneously, but the reaction of forming WC was the main reaction in the whole carburization process. Moreover, the TPR-MS and XRD results indicated that, WC was formed at lower temperature (750 °C) by the reduced metallic W, which was produced form W₂C in the gas mixture for holding a long time, while at a higher temperature (950 °C), WC was formed form W₂C with the mixture gas directly.