This paper reports on the tribological behaviour of CN x thin ®lms deposited in a dual-ion-beam-sputtering system using Ar + to deposit graphite and low energy (<100 eV) N 2 ions to irradiate the ®lm during growth. Both the stability and the tribological properties of the ®lms have been found to depend on the nitrogen content of the ®lm and the type of bonding (i.e., sp, sp 2 and sp 3 ) as well as on the thickness of the ®lms, which on the other hand can be controlled by the deposition conditions. In general, for a sputtering rate of the graphite of about 0.25 A Ê /s (i.e. 500 eV Ar + energy, 1 mA/cm 2 current density) the best behavior is obtained for assisting conditions in the range of 55±70 eV for the energy and 0.043±0.070 mA/cm 2 current densities of the N 2 ions. In this case the ®lms appear to be dominated by C±N and C.N bonds and show promising tribological properties. On the contrary, increasing the energy of the assisting ions leads to an increase of the nitrogen content mainly in the form of nitrile groups, as well as to a signi®cant reduction of the ®lm thickness due to re-sputtering of the growing ®lm.