Nanocomposite tantalum carbide/amorphous hydrocarbon (TaC/a-C:H) thin film composition, structure, and mechanical properties depend on the direct current bias voltage (Vb) level applied to the substrate during reactive sputter deposition. A set of TaC/a-C:H films was deposited across the range V-b = 0 to -300 V with all other deposition parameters held constant except substrate temperature, which was allowed to reach its steady state during the depositions. Effects of Vb on film composition and structure were explored, including TaC crystallite size and dispersion using X-ray diffraction and high resolution transmission electron microscopy. In addition, the dependency of stress and hardness on Vb was studied with an emphasis on relationships to a-C:H phase structure. (C) 2006 Elsevier B.V. All rights reserved.