This paper reports on the effects of Ar dilution on the structure, bonding and tribological properties of nitrogen-doped amorphous carbon films (a-C:H:N) deposited by r.f. glow discharge from CH4-N-2 and CH4-N-2-Al mixtures on silicon substrates. For different N-2 fractions, film structure and chemical composition were examined using Raman spectroscopy, X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS). The mechanical and tribological properties were investigated by scratch testing and a tribometer in a ball-on-disk configuration. Results indicate that, as argon is introduced in the gas mixture, progressive graphitization of the films occurs and the mass density decreases. The behavior of the adhesive strength, as a function of nitrogen content, achieved with Ar addition is of the same order as those reported for CN, films. However, at a fixed nitrogen partial pressure, films obtained without Ar dilution show better adhesion and a denser sp(3) bonded carbon network.