Effects of environmental conditions on fluorinated diamond-like carbon tribology

The effects of environmental conditions and fluorine content on friction of diamond-like carbon (DLC) films are determined and discussed. Relative humidity (RH) has a considerable effect on friction values, modifying them up to a factor 2. Two regions of RH are found, depending on its effect over friction. From 20% to 60% of RH, friction decreases probably due to a better substitution of superficial atoms by hydrogen. From 60% to 80% friction remains stable and equal for almost all samples. A possible explanation would be the formation of a physisorbed layer of water on the surface, which would mask the relatively small chemical and structural variations of the films. Moreover, the different components of surface free energy (SFE) are determined using Van Oss–Chaudury–Good model. The results clearly indicate the importance of Lifshitz–Van der Waals component on SFE and the small correction that supposes acid-base interactions. According to this model, the studied DLC is a monopolar basic material. The basic component is reduced when F content is increased.