Silicon nitride (SiNx) films were prepared by dual ion beam deposition at room temperature. An assisted N-2(+) ion beam (current I-h = 0-45 mA) was directed to bombard the substrate surface to control the N content x, which saturated at x approximate to 1.36 when I-b greater than or equal to 25 mA. The presence of Si-N bonds was indicated by the appearance of a Si 2p photoelectron peak at 101.9 eV and an infrared absorption peak at 850 cm(-1). As x increases from 0 to 1.36, the hardness, elastic modulus and compressive stress increase from 12.2 to 21.5 GPa, 191 to 256 GPa and 0.52 to 1.4 GPa and the friction coefficient against stainless steel ball decreases from 0.65 to 0.37. The optical band gap increases remarkably with a concomitant drop in electrical conductivity (UIT) by more than 107 times. Ion bombardment induces defects and trap states in the mid-gap, such that the transport mechanism is dominated by hopping of charge carriers through the trap states. Consequently, the activation energy of electrical conductivity is much lower than the optical band gap.