Reducing Friction of Diamond-Like Carbon Film in Sliding through Fluorine Doping

A hydrogenated Diamond-Like Carbon (DLC) film possesses an ultra-low coefficient of friction in a low-humidity environment and higher as the humidity increases. This is due to the presence of water molecules in the atmospheric environment that is physically adsorbed and forming a thick layer on the surfaces that inhibits the growth of carbonaceous transfer film. One viable solution for this problem is to dope fluorine into the hydrogenated DLC film to decrease its surface energy. Therefore, this study varies carbon tetrafluoride feed as a doping source on the hydrogenated DLC film produced via plasma-enhanced chemical vapor deposition on 304 stainless steel substrates. The film hardness, carbon hybridization, and surface hydrophobicity were evaluated using nano-indentation testing, Raman spectroscopy, and contact angle measurement respectively. The coefficient of friction was analyzed by utilizing a ballon-disc tribometer at a controlled temperature and humidity. The findings suggest that with the increase of the fluorine, the film hardness decreased as the sp3/sp2 carbon ratio decreased because weaker C-F bonds are substituting the strong C=C bonds. The increase in fluorine was also observed to produce a more hydrophobic surface. The ball-on-disc test analysis shows that the coefficient of friction was significantly reduced as the C-F bond increased which enables the prevention of carbonaceous transfer film through the adsorption of the water molecule. © 2023 Japanese Society of Tribologists.