High temperature tribological behavior of tetrahedral amorphous carbon (ta-C) and fluorinated ta-C coatings against aluminum alloys

• Tribological behavior of ta-C and ta-C–F vs. 319 Al between 25 and 500 °C was studied.
• Low coefficient of friction of 0.11 (ta-C) and 0.13 (ta-C–F) was observed at 400 °C.
• ta-C and ta-C-F maintained low wear rates up to ≥ 400 °C.
• F in ta-C–F further reduced running-in friction compared to ta-C at 200–400 °C.

Friction and wear behavior of tetrahedral amorphous carbon (ta-C) and fluorinated ta-C (ta-C–F) coatings were studied in the temperature range between 25 °C and 500 °C. Pin-on-disk type tests conducted on ta-C against Al–6.5% Si (319 Al) showed that the steady-state coefficient of friction (μs) decreased from 0.36 at 25 °C to 0.11 at 400 °C. The reduction in friction was attributed to sliding-induced sp3 to sp2 transformation (graphitization) of the coating and formation of oxygen rich transfer layer as revealed by X-ray photoelectron and micro-Raman spectroscopy. The coefficient of friction of ta-C–F also decreased with an increase in the test temperature. A low μs of 0.13 was observed at 400 °C compared to 0.39 at 25 °C. The maximum value of running-in coefficient of friction (μR) for ta-C–F tested at 400 °C was 0.38, lower than the μR of ta-C (0.65). Both coatings maintained low wear at elevated temperatures but at 500 °C they showed high wear and high coefficient of friction and did not reach a steady state friction regime. The atomic % of F transferred to the Al surface increased with an increase in the test temperature up to 400 °C. It was suggested that a sp2 carbonaceous transfer layer passivated by F atoms was responsible for the further decrease in μs and μR of ta-C–F at elevated temperatures (200 °C–400 °C) compared to ta-C.