In the present paper, we compare the tribological behaviour of Si-free and Si-containing carbon nitride films grown on high speed steel substrates against steel counterfaces. The CNx coatings have been prepared by magnetron sputtering of a carbon target in a N2 atmosphere while the SiCNx films were obtained by the same method, but adding a vapour pressure of Si(CH3)3Cl. In the case of pure CNx, the presence of water molecules in the gas phase produces a negative effect in the tribological response while the Si-containing film is able to maintain a low friction value (0.12) even under humid atmosphere similarly to Si-DLC coatings. To achieve a better understanding of the friction mechanism, both Si-free and Si-containing films were characterised by Energy Filtered Transmission Electron Microscopy (EFTEM), X-Ray Photoelectron Spectroscopy (XPS) and Infrared Spectroscopy (IR). The buffer effect of silicon-doped CNx, decreasing the moisture sensitivity of the friction coefficient, is attributed to the adsorption of water molecules on SiO2 domains formed in the Si-containing films. This adsorbed water may lubricate the contact in humid atmosphere allowing the shear strength to diminish.