Low friction and wear behaviour of non-hydrogenated DLC (a-C) sliding against fluorinated tetrahedral amorphous carbon (ta-C-F) at elevated temperatures

The low coefficient of friction (COF) of sp2 rich non-hydrogenated diamond-like carbon (a-C) coatings make them suitable for tribological applications including machining of lightweight alloy castings and for certain components in internal combustion engines. Retaining the low COF of a-C at temperatures >100 °C is however a challenge that has limited further industrial applications of these coatings. This study examines the high temperature stability of a non-hydrogenated a-C coating (H < 2 at.%) sliding against a sp3 rich fluorinated tetrahedral amorphous carbon (ta-C-F) and tool steel counterfaces using ball-on-disk type experiments. It was shown that a-C coated balls run against ta-C-F maintained a low steady state COF that varied between 0.1 and 0.2 up to 300 °C while a-C sliding against uncoated tool steel resulted in high COF values accompanied with high wear rates of a-C coating. Transfer layers were formed on the a-C coatings' contact surfaces sliding against ta-C-F and these layers consisting of fluorine incorporating carbonaceous material remained stable up to 300 °C. The results suggested that a low COF in this tribological system can be achieved at elevated temperatures as a result of passivation of the surface carbon atoms.