Amorphous carbon (a-C) thin films have been prepared by shielded arc ion plating (SAIP) at an argon gas pressure of 1 Pa by applying a substrate bias voltage between 0 and similar to -500 V The chemical structure of the a-C thin films was studied by visible Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The nanohardness and wear resistance of the films were measured with a nanoindenter interfaced with an atomic force microscope (AFM). The friction coefficient of the films was determined with a SUJ2 bearing ball using a ball-on-disc tribo-tester. The maximum nanohardness of 42 GPa was obtained when the film was prepared at a negative substrate bias (V-b) of -100 V This film also showed the highest wear resistance and lowest friction coefficient, although the bearing ball was highly worn down. The nanohardness and wear resistance decreased between - 200 and -500 V The Raman spectroscopy and XPS results showed that the sp(3) fraction increased by applying suitable Vb of -100 V. (C) 2003 Elsevier Science B.V. All rights reserved.