Amorphous silicon-containing diamond-like carbon (Si-DLC) coatings were deposited by Ar+ ion beam-assisted physical vapor deposition of tetraphenyl-tetramethyl-trisiloxane (704 Dow Coming diffusion pump oil). The steel substrates studied included AISI 4130, 17-7 PH, 440-C, and 4340 (bare and nitride-precoated) specimens. DLC coating thicknesses ranged from 1.8 to 4.31 mu. Deposition rates increased with increasing beam current density and varied with the steel substrate composition. Nanoindentation measurements of the hardness and elastic modulus at two different depths yielded values of 9-10 GPa and 99-128 GPa, respectively. Film cohesion and adhesion failure loads increased with increasing underlying Layer hardness, chromium content in the substrate, or the presence of a titanium nitride precoat. The friction coefficient of a diamond stylus against the coating surface decreased and wear resistance increased with nitride precoating.