Tungsten carbide thin films were deposited on (100) Si single crystal substrates by reactive RF sputtering from a tungsten target in Ar-CH4 mixture. Under 1% of methane, the formation of a mixture of nanocrystalline WC1-x presenting a partial (200) texture and W2C phases was observed. The film deposited under 2% of CH4 shows a maximum hardness of 22 GPa (a value comparable to the bulk WC reference material), these coatings also exhibit the lowest friction coefficient of 0.1. For coatings performed with CH4 percentage higher than 3%, a progressive amorphization of the layers was observed. At 5% of CH4 concentration, the films become essentially amorphous. Simultaneously the hardness decreases continuously to achieve a minimum value of 13 GPa. (c) 2005 Elsevier B.V. All rights reserved.