A dual direct current and radio frequency (dc-rf) plasma system was used to deposit hydrogenated amorphous carbon (a-C:H) films from methane plasma. It has the advantages of separately controlling ion density and ion energy by rf power and dc bias, respectively, over conventional simply capacitive-coupled rf-PECVD system. Thus the a-C:H films were obtained at different applied dc biases using CH4 plus H-2 as the feedstock. The structural, mechanical and tribological properties of the resulting a-C:H films were investigated as a function of the applied dc bias voltage in the range 0-500 V. The results showed that the structure and properties of a-C:H films strongly depended on the applied dc bias voltage. With an increase in the applied dc bias voltage, the deposition rate and the bonded hydrogen content decreased, and the hardness and sp(3) content (according to Raman spectra) were shown to reach their maximum values simultaneously at an applied dc bias voltage of 100 V. At the same time, the smallest friction coefficient and the best wear-resistance were obtained. (C) 2004 Elsevier B.V. All rights reserved.