Carbon nitride (CNx) thin films have acquired great attention due to their predicted extreme hardness values. In the present investigation, CNx thin films are synthesized by hot-filament plasma enhanced chemical vapor deposition technique using acetylene (C2H2), ammonia (NH3) and hydrogen (H-2) as feedstock gases. The films are doped with Si by using silane (SiH4) gas along with other gases. Nanoindentation, Fourier transform infrared (FTIR), Rutherford backscattering spectroscopy (RBS) techniques were employed to characterize the, deposited Si-doped CNx films. Nanoindentation revealed that the films were very hard as compared to undoped CNx films, reaching approximate to 26 GPa. The hardness of the films was influenced by the substrate temperature and Si doping percentage. An increase in hardness with the growth temperature and decreasing tendency with increase of SiH4 flow has been observed. From the hardness to elastic modulus ratio, i.e. H/E analysis, it is found that for tribological application low amount of SiH4 flow was more beneficial. FTIR analysis indicates that Si-H, N-H, Si-C and Si-N bonds are present in the Si-doped CNx films. A good correlation of optical property with the mechanical property has been established. The shift of Si-H bands towards higher wave number with the decrease of SiH4 partial pressure is the indication of enhancement of the mechanical properties. RBS measurement reveals that a high amount of nitrogen is incorporated into the film. The typical value of the nitrogen is approximately 40 at.%. The surface morphology and topography of the deposited films have also been characterized by atomic force microscopy. (C) 2003 Elsevier B.V. All rights reserved.