The effects of oxygen and humidity on friction and wear of diamond-like carbon films

Diamond-like carbon (DLC) films were prepared on Si(100) wafers by plasma-assisted chemical vapor deposition at a pressure of 900 mTorr. Their wear rates and friction coefficients against a silicon nitride ball were measured in a pin-on-disk tribometer in argon and air with varying relative humidities. In 50% relative humidity, the measured wear rates of the ball and DLC were of the order of 10-8 mm3 N-1 m-1 and 10-7 mm3 N-1 m-1 respectively. In dry argon, dry air and 100% humid air, the wear rates of DLC were 10-9, 10-9 and 10-8 mm3 N-1 m-1, but that of the ball was below the detection limit. The measured friction coefficients were 0.06 in dry argon, 0.08 in 50% humid air and 0.09 in 100% humid argon, and around 0.2 in 50% humid argon, dry and 100% in humid air. In dry argon, the contact area of the ball was covered with material transferred from the DLC film during sliding; the low friction coefficient and wear rate measured in dry argon are attributed to this material. In dry and humid air, surface layers of DLC were oxidized by a tribochemical reaction forming a C=O bond. They covered the contact area of both the DLC film and the ball. This film increased friction coefficients, but it acted as a protective coating when its thickness was sufficient to prevent direct contact of the DLC film against the ball in 100% humid air. (orig.)