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

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 degrees C and 500 degrees 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 (mu(s)) decreased from 0.36 at 25 degrees C to 0.11 at 400 degrees C. The reduction in friction was attributed to sliding-induced sp(3) to sp(2) 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 mu(s) of 0.13 was observed at 400 degrees C compared to 0.39 at 25 degrees C The maximum value of running-in coefficient of friction (mu(R)) for ta-C-F tested at 400 degrees C was 0.38, lower than the mu(R) of ta-C (0.65). Both coatings maintained low wear at elevated temperatures but at 500 degrees 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 degrees C. It was suggested that a sp(2) carbonaceous transfer layer passivated by F atoms was responsible for the further decrease in mu(s) and mu(R) of ta-C-F at elevated temperatures (200 degrees C-400 degrees C) compared to ta-C. (C) 2015 Elsevier B.V. All rights reserved.