Effects of duty cycle and water immersion on the composition and friction performance of diamond-like carbon films prepared by the pulsed-DC plasma technique

Diamond-like carbon (DLC) films were prepared in a pulsed-DC discharged CH₄/Ar plasma. Effects of duty cycle (t_on/(t_on + t_off)] × 100%) on the composition and properties of DLC films were investigated. In general, the increased duty cycle led to an up-shift of the G peak position, an increase in the I_D/I_G and sp²/sp³ ratio, and a reduction of the number of C-H bonds and the film hardness, revealing a graphitization tendency with increasing duty cycle. Tribologically, ultralow and steady friction coefficients (0.005 and 0.008) in dry nitrogen atmosphere were obtained for the films prepared under a duty cycle of 50% and 65%. The unique mechanical property and chemical nature brought by the moderate sp²/sp³ ratio and the proper H content were considered to be responsible as the films deposited in this duty cycle range could simultaneously provide the high chemical inertness and the ultrasmooth sliding surfaces required for achieving ultralow friction. In addition, the structure was less vulnerable to water molecules in the case of stewing. The diamond-like nature and the ultralow friction performance were hardly affected even experiencing a 4-month immersion in water.