Quaternary Si-B-C-N films were prepared using reactive direct current magnetron co-sputtering of silicon, boron and carbon from a single C-Si-B target in nitrogen-argon gas mixtures at substrate temperatures of 180-350 degrees C. We focused on complex relationships between process parameters, elemental composition, bonding structure, and mechanical, tribological and optical properties of the deposited materials. The films, typically 1.0-2.4 mu m thick, are found to be amorphous with a density of around 2.4 g cm(-3). They exhibit a high hardness (up to 36 GPa) and a good adhesion to Si(100) substrates at a low compressive stress (1.0-1.6 GPa). A friction coefficient against Al2O3, ranging from 0.85 to 0.34, and a low surface roughness of approximately 1 nm were accompanied by the corresponding refractive index ranging from 1.8 to 2.2 (at 550 nm) and an extinction coefficient between 2 x 10(-4) and 0.3. The Si-B-C-N films appear attractive for both tribological and optoelectronic applications. (c) 2008 Elsevier B.V. All rights reserved.