This article reports on the composition, microhardness, cohesion, adhesion, and friction coefficients of Si-diamondlike carbon coatings on M50 steel and silicon substrates prepared at room temperature by the ion beam assisted deposition process, using energetic Ar+ ions and tetraphenyl-tetramethyl-trisiloxane (Type 704 Dow-Coming silicone oil) as a precursor material. The thickness, microhardness, and friction coefficients of these films were investigated. The cohesion and adhesion failure loads of the films were also evaluated by automatic scratch testing in combination with optical and scanning electron microscopy observation of the scratch. Rutherford backscattering spectroscopy analysis showed that film stoichiometry is insensitive to deposition parameters including the relative arrival ratios of Ar+ ions and deposited silicon-hydrocarbon molecules. Knoop microhardness values (at 25-g load) were found to vary from 800 to 1600 kg/mm2, and the unlubricated friction coefficients ranged from 0.21 to 0.27, depending on deposition parameters.