Superlubricity in presence of DLC-coated surfaces (friction coefficient below 0.01) has already been observed by several authors, but only with hydrogenated DLC (a-C:H) in ultrahigh vacuumfootnote1, in inert gases (nitrogen)footnote2 and in hydrogenfootnote3. Under boundary lubricated condition, surprisingly, such low friction level has not been observed experimentally in the literature. Recently, we have discovered a new specific combination of ta-C (hydrogen-free DLC) coating and ester lubricants providing such extremely low friction under mixed and boundary conditions⁴. The mechanisms of superlow friction was investigated by using both surface sensitive analytical techniques (EV-XPS and ToF-SIMS) and also by using molecular dynamics simulations. We experimentally observed that an ultrathin tribofilm is formed (thickness less than 2 nm) and is responsible for such low friction and wear. This situation is particularly well adapted to computer simulation with MD technique. The conjonction of the two approaches lead us to the conclusion that a OH-terminated carbon surface was formed under friction and provides a new situation of superlubricity under boundary lubrication. This finding is indeed very important for future applications in lubricated systems where low friction and wear are needed in friendly environment.

¹ J. Fontaine et al, Surface and Coatings technology, 146-147, (2001) 286.

² A. Erdemir, Tribology International 37, (2004) 577

³ C. Donnet et al ; Tribology Letters, 9 (2001) 137.⁴ M. Kano et al, Tribology Letters, Vol 18, No 2, (2005) 245.

We report the structure and tribological properties for hydrogen-free amorphous Diamond-Like-Carbon (denoted as ta-C) determined from first principles computer simulations using the ReaxFF reactive force field to create an atomistic model of ta-C. The 3D structure was determined by heating diamond until it melts and then quenching rapidly. These simulations lead to a density of 3.18 g/cc in good agreement with experiments from the JM Martin group in Lyon and the M Kano group at Nissan. This suggests that the simulations lead to a structure similar to that synthesized under appropriate CVD conditions.

The simulations find that the bulk structure has 59% sp³ carbons joined together to form a percolating 3D arrangement to which sp² and sp¹ carbons attach in chains of length 1 to 3. This structure is consistent with SIMS experiments from Lyon. The simulations find a high percent of reactive C at the surface that reacts readily to form a very smooth carbon surface containing OH-terminated groups in agreement with the experiments from Lyon. The simulations find that systems lubricated with glycerol lead to from strong noncovalent interactions between the glycerol and the OH terminated surface that causes one monolayer of the glycerol to stick near the surface for long periods during sliding. Thus all sliding motions are confined within the lubricant film. This may be responsible for the superlubricity observed by Nissan for hydrogen-free amorphous Diamond-Like-Carbon.

Due to its lower density, increasing the use of magnesium based parts brings significant weight reductions to motor vehicles, which in turn means improved fuel economy and less adverse impact on environment. Meanwhile, implementation of appropriate coatings on tool and die surfaces can substantially increase the productivity of the manufacturing processes (e.g. cutting and sheet forming) of Mg based parts as well as improving the surface quality of the finished products. Hence, as a potential tool coating for non-ferrous materials, the tribological behaviour of diamond-like carbon (DLC) coatings against Mg is of interest.

In this study, dry sliding wear behaviour of magnetron sputtered non-hydrogenated DLC coatings deposited on M2 steel discs was investigated against rounded pins (4 mm dia.) of Mg (99.9 wt. %) using a vacuum pin-on-disc tribometer. Tests were performed in air (22 %RH) and in argon under a constant load of 5 N and a sliding speed of 0.12 m/s. In argon, the non-hydrogenated DLC coatings showed low wear rates and low coefficient of friction (COF = 0.05) after a running-in period. Carbonaceous material transfer from the non-hydrogenated DLC to the contact surface of Mg pin was observed in argon. Contrary to the previously reported studies on the tribology of non-hydrogenated DLC coatings against steel or ceramic counterfaces, changing the test atmosphere from argon to ambient air adversely affected the COF and wear rate of non-hydrogenated DLC against Mg. The average COF between non-hydrogenated DLC coating and Mg was 0.44 in air (22 %RH). Formation of oxidized Mg debris, in the forms of plate-like and rounded fine pieces, was observed in ambient air. This presentation will discuss the friction and wear mechanisms of non-hydrogenated DLC coatings against Mg in terms of i) generation of a carbonaceous transfer layer on the Mg counterface, and ii) chemical affinities of the DLC/Mg sliding pair against each other and towards the environment.