Effect of test atmosphere on the tribological behaviour of the tetrahedral amorphous carbon (ta-C) and fluorinated ta-C (ta-C-F) coatings against steel

Tetrahedral amorphous carbon based coatings generally exhibit a low coefficient of friction (COF) while sliding against steel and aluminum, but their tribological properties are sensitive to the test environment. In this study, tribological behaviour of tetrahedral amorphous carbon (ta-C) and fluorinated ta-C (ta-C-F) coatings were investigated under different atmospheres including ambient air (53% relative humidity, RH) and dry N2 (3% RH) against a steel counterface. The ta-C showed a period of low COF of 0.27 in ambient air and a C transfer layer was formed on the counterface, but the COF increased with sliding duration to ~ 0.4 due to transfer of Fe2O3 debris to ta-C surface. A high COF of ~ 0.60 with high fluctuations occurred in dry N2 as no carbon transfer layers were formed on the counterface. The ta-C-F coating exhibited a steady-state behaviour with low COF (μs) values under both ambient (μs = 0.25) and dry N2 (μs = 0.23) atmospheres. Formation of an F containing carbonaceous transfer layer on the counterface provided low COF for ta-C-F.