Carbon nitride (CNx) films with 0-22.8 at. % N were prepared using dual ion beam deposition With increasing N content in this range, more N-containing pyridinelike rings, and single, double, and triple bends between C and N were formed, resulting in polymerization of the film structure. Consequently, the films became more transparent in the infrared region, and the hardness dropped from 24.2 to 12 GPa. Furthermore, nanoscratch tests showed that less debris was produced along the scratch track on the film containing high N content, indicating that the film was less brittle following the polymerization of the film structure. in addition, the minimum (critical) normal load for damaging the films dropped from 7.8 to 4 mN, reflecting that the adhesion between the firms and substrates was weakened, thereby resulting in delamination of the films from the substrates. Finally, the room temperature electrical conductivity was found to be reduced by a factor of 100, consistent with the more pronounced polymerlike nature of the films at high N contents.