Hard diamond-like carbon (DLC) films have been prepared on PMMA (polymethyl methacrylate), glass, and Si(100) substrates using C2H2-Ar plasma immersion ion processing (PIIP). The composition, structure, and properties of the films were investigated with regard to variation of the deposition parameters. It was found that the modulation of reactive nas composition during FLIP could enhance the formation of DLC films with an increased sp(3) bonding structure, improved surface smoothness, high density and high hardness. An optimal combination of good optical properties and high hardness was highly dependent on the control of hydrogen content in the DLC films. Tribological tests showed that DLC-coated glass and PMMA samples exhibited a reduced friction coefficient and enhanced wear resistance relative to uncoated glass and PMMA materials. The effects of ion energy and gas composition during PIIP deposition on the formation of optically transparent and wear resistant DLC films are discussed.