Impact of relative humidity and deposition parameters on the tribological behavior of soft DLC coatings

When diamond-like carbon coatings are produced with plasma-assisted CVD, a softer but thicker DLC variant with excellent tribological behavior can be achieved. Due to great adhesion and load-bearing capacity, they can be deposited on low-alloy steels, commonly used in mechanical engineering, to reduce energy consumption and extend the lifespan of machine components. In this work, soft a-C:H coatings were deposited over AISI 4140 steel using a modified commercial PA-CVD system, with acetylene as carbon precursor. Two kind of coatings were deposited: Si free (a-C:H) and Si doped (a-C:H:Si). Adhesion, mechanical properties, and film structure were analyzed. Tribological behavior was evaluated with Pin-on-Disk tests using an alumina ball as counterpart and 12 N normal load. At constant room temperature, three different relative humidity conditions were tested: dry, room and wet. The frictional force was registered and the wear scars in the sample and in the counterpart were measured and evaluated by means of confocal, optical, and scanning electron microscopy with EDS. Si-doped coatings showed up to 60 times more wear than Si-free samples, with the later displaying a friction coefficient as low as 0.05, while for the Si-DLC was μ ~ 0.2. This can be caused by a change in the composition of the graphite-like transfer layer produced by the DLC when doping with Si. Both friction coefficient and wear volume increased along with the humidity level for the Si-free DLC, and the opposite trend was observed for the Si-DLC. This can be explained by the oxidation of the DLC in the case of a-C:H and by the stability of a silicon oxide film that is formed and regenerated in accordance with the environment's humidity in the case of a-C:H:Si.