Preparation of Ultrafine CVD WC Powders Deposited from WCl6 Gas Mixtures

Ultrafiie WC powders were produced from WCla-C3Hs-H2 gas mixtures in a conventional tubular hot-wall downstream CVD reactor. At reaction temperatures between 1100 and 1550°C powders containing W, WZC, WC and carbon were produced. The overall chemical compositions of the tungsten coinpounds as well as the free carbon contents depended strongly on the reaction temperature and the ratio of the reaction gases introduced. With increased reaction temperature and exposure time the amount of tungsten carbides (WC, W2C) in the deposits increased. At the selected conditions pure WC could be detected by X-ray diffraction. However, excess carbon was always present, while carbon particles could not be observed by SEM in the powder mixtures. This couldbe explained by thin carbon layers on the carbide particles. The WC powder particles were very fine but only particles or agglomerates smaller than 0.5 pm can be observed by SEM. A deposition mechanism based on reduction of the chlorides and their carburization to the carbides is discussed.