Nanocrystalline diamond/amorphous carbon (NCD/a-C) composite thin films have been deposited by microwave plasma chemical vapour deposition from methane-rich CH4/N-2 mixtures. The films have been thoroughly characterized with respect to basic properties such as growth rates, morphology and structure, composition, crystallinity, and bonding environment. They consist of diamond nanocrystals with diameters of 3-5 nm, which are embedded in an amorphous carbon matrix. Further studies are aimed at application relevant properties. I/V and Hall measurements showed that the films are p-type conductive with a resistitivity of 0.14 Omega cm, a carrier concentration of 1.9 x 10(17) cm(-3), and a carrier mobility of 250 cm(2)/Vs. Reflection, scattering and ellipsometric measurements revealed a refractive index of 1.95 - 2.1 in the visible region and an rather high extinction coefficient of about 0. 14 at 400 nm. The films possess a hardness of ca. 40 GPa and a Young's modulus of ca. 390 GPa. Nano tribo test and nano scratch tests proved a low friction coefficient, and a strong protective effect and good adhesion on silicon substrates. First biomedical tests showed that the films are not cytotoxic but bioinert. Finally, the deposition of multilayers nano/ polycrystalline diamond with improved properties is demonstrated. (c) 2005 Elsevier B.V. All rights reserved.