Role of oxygen and humidity on the tribo-chemical behaviour of non-hydrogenated diamond-like carbon coatings

Diamond-like carbon (DLC) coatings exhibit low coefficient of friction (COF) against components made of aluminum operating under sliding contact. DLC coatings also mitigate aluminum transfer to tools and dies in machining and forming operations. An impediment to the use of DLC coatings is the wide variation of their COF and adhesion properties with the environmental conditions. This study investigated the friction mechanisms of non-hydrogenated DLC coatings under an oxygen rich environment. Pin-on-disk experiments were conducted against a 319 Al pin under controlled atmospheres. The lowest steady-state COF of 0.09 was achieved in a humid (45%, relative humidity, RH) Ar atmosphere, following an initial running-in period with a COF peak ranging from 0.5 to 0.6. In the 50% O2 + 50% Ar mixture atmosphere with 45% RH, a higher steady-state COF of 0.12 was observed. For the tests conducted in a dry (0% RH) oxygen-rich atmosphere (50% O2 + 50% Ar mixture), a high steady-state COF of 0.35 was recorded. FTIR analyses of the carbon transfer layers formed on aluminum showed that the increase in COF was due to oxidation of surface carbon atoms. When oxygen was combined with humidity, it eliminated the initial high COF observed during the running-in period.