Deposition of alpha-WC/a-C nanocomposite thin films by hot-filament CVD

Nanostructured tungsten carbide coatings containing amorphous carbon (a-C) phases are interesting composite materials. The incorporation of the a-C phase simultaneously improves the thermal stabilization of the carbide phase and the coatings' friction coefficient. Such nanocomposite coatings are also electrically conducting with resistivity values comparable to the transition metals, which make them useful for a number of electrochemical and electronic applications. Many deposition techniques have been used for the synthesis of these coatings. However, most of them lead to the formation of complex crystalline structures consisting of more than one carbide phase and varying amorphous contents. The novelty of this work is the formation of WC coatings with controllable film thickness and a-C content, almost fully composed by α-WC phase. Tungsten carbide coatings were deposited on silicon substrates using a hot filament chemical vapor deposition (HFCVD) equipment with hydrogen and methane as the deposition gasses. Two types of nanocomposite coatings were obtained with significant variations of the carbide phase and a-C contents. The results point to the W vaporization time as the main parameter influencing the film thickness. We also conclude that the formation of α-WC phase is the combining result of W filaments vaporization in vacuum and the carbon incorporation at low substrate temperature. The small crystallite size of the carbide grains (5–6nm) could also explain the rapid diffusion of C through the tungsten-containing layer. Preliminary results show that the amount of a-C incorporated in the film is not only dependent on the CH4/H2 ratio but also on the substrate temperature. ►We demonstrate the deposition of α-WC/a-C coatings using an HFCVD reactor. ►With this method the α-WC thickness and the a-C content are easily controllable. ►Tungsten evaporation time is the main parameter influencing the film thickness. ►The a-C content in the coatings is mainly controlled by the CH4/H2 flow ratio.