TREATMENT OF BULK GROUP-VI TRANSITION-METAL CARBIDES WITH HYDROGEN AND OXYGEN

Starting from bulk carbides, the removal by hydrogen of free carbon, o xygen from passivation and from carbidic carbon was studied in tempera ture-programmed experiments. Treatment of WC1+xOy with flowing hydroge n shows that the production of methane is maximum at about 700 degrees C, above which it decreases. However, at the final pretreatment tempe rature, a residual partial pressure of methane is still observed, but this residual methane pressure decreases with temperature and disappea rs. After a few hours' treatment in hydrogen the final solid correspon ds approximately to the stoichiometry WC without extra carbon. Such a treatment was monitored by X-ray diffraction in a diffractometer allow ing temperature-programmed experiments from room temperature up to 900 degrees C. It was shown that no metallic tungsten appears below 700 d egrees C. The amount of metal formed during treatment at 750 degrees C is very small, and above 750 degrees C it increases with temperature. However, even after a two hour treatment at 800 degrees C, followed b y treatment at 900 degrees C, large proportions of carbide are maintai ned. No W2C formation is detected. W2C, after passivation, is very sen sitive to hydrogen treatment, since a thermogravimetric experiment sho wed that, after a five hour treatment in hydrogen at 600 degrees C, W2 C was completely decarburized into W. W2C seems relatively stable at 4 00 degrees C. A treatment of Mo2C containing large amounts of free car bon in hydrogen at 700 degrees C leads to MoC0.47, that is, to clean M o2C. On the other hand, Cr3C2 behaves differently in hydrogen since so me decarburization occurs before complete elimination of excess free c arbon. By temperature-programmed X-ray diffraction in a O-2 (2%)-N-2 m ixture from room temperature to 500 degrees C, it is shown that oxidat ion of WC to WO3 starts at 400 degrees C and is very fast at 500 degre es C. No oxycarbides or intermediary oxides such as WO2 were detected.