Diamond-like carbon (DLC) nanocomposite films, containing copper (Cu) nanocrystallites, were synthesized and studied. Cu bonds very weakly with carbon, and does not form a carbide phase. Therefore, Cu nanoparticles can be easily formed in a DLC matrix by depositing Cu and carbon together. The mechanical properties of DLC films that contain Cu nanoparticles are interesting since the film toughness may be increased by grain-matrix interface sliding. Hard, tough and stress-free DLC/Cu films were prepared by a sputtering Cu target in an argon/acetylene atmosphere while biasing the substrate with a radio frequency power supply. The residual stress of the film, calculated by Stoney's equation, was as low as 0.7 GPa. The reduced stress and the increased film toughness increased the critical load from 66 N for a conventional DLC film to 80 N for the DLC/Cu film, as measured in a scratch test. However, the DLC/Cu films were slightly less hard than the DLC films. (c) 2004 Elsevier B.V. All rights reserved.