Abstract CNx and SiCN thin films were deposited by microwave plasma assisted chemical vapour deposition on Si, Si/Si3N4 and WC-Co/Si substrates in a N2/CH4 gas mixture. The source of silicon was the substrate itself. Its role in the growth of CNx and SiCN is not well understood yet. To better understand the surface mechanisms involved in the film growth, as well as the role played by the silicon, some films are synthesized on WC-Co cermets and WC-Co recovered by a variable thickness silicon layer. Morphological analyses realized by scanning electron microscopy and transmission electron microscopy show that the obtained films are made of nano-crystalline grains with size ranging between 20 and 70 nm. The X-ray diffraction spectrum and selected area electron diffraction patterns exhibits a signature that could correspond to beta-C3N4, cubic-C3N4 and cubic-SiCN. The films deposited on WC-Co substrates are compared to those, which are synthesized on Si and Si/Si3N4 substrates. We have observed that the deposition of a continuous film was impossible on non-silicon contain (WC-Co) substrates, and only carbon balls were obtained on a such substrate. Otherwise, the deposition becomes possible if a silicon underlayer is deposited on to the WC-Co substrates. The films are close to those synthesized with Si or Si/Si3N4 substrates. In all cases, the growth stops as soon as a thin film is deposited on the substrate and the plasma cannot access to the silicon underlayer. Finally, a growth mechanism showing that silicon takes part of the deposition and improves the nitrogen ratio in the films is proposed. Regarding the energy dispersion X-ray spectroscopy and secondary ion mass spectroscopy results, it seems that the incorporation of nitrogen is activated only by the presence of silicon, which behaves as a catalyst.