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

DELFIN, F.; BRUHL, S. P.; DOMMARCO, R. C.; FORSICH, CH; HEIM, D.

SURFACE AND COATINGS TECHNOLOGY

ELSEVIER SCIENCE SA

Lugar: Amsterdam; Año: 2023 vol. 474

0257-8972

When diamond-like carbon coatings are produced with plasma-assisted CVD, a softer but thicker DLC variantwith excellent tribological behavior can be achieved. Due to great adhesion and load-bearing capacity, they canbe deposited on low-alloy steels, commonly used in mechanical engineering, to reduce energy consumption andextend the lifespan of machine components.In this work, soft a-C:H coatings were deposited over AISI 4140 steel using a modified commercial PA-CVDsystem, with acetylene as carbon precursor. Two kind of coatings were deposited: Si free (a-C:H) and Sidoped (a-C:H:Si). Adhesion, mechanical properties, and film structure were analyzed. Tribological behavior wasevaluated with Pin-on-Disk tests using an alumina ball as counterpart and 12 N normal load. At constant roomtemperature, three different relative humidity conditions were tested: dry, room and wet. The frictional force wasregistered and the wear scars in the sample and in the counterpart were measured and evaluated by means ofconfocal, 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 frictioncoefficient as low as 0.05, while for the Si-DLC was μ ~ 0.2. This can be caused by a change in the composition ofthe graphite-like transfer layer produced by the DLC when doping with Si. Both friction coefficient and wearvolume increased along with the humidity level for the Si-free DLC, and the opposite trend was observed for theSi-DLC. This can be explained by the oxidation of the DLC in the case of a-C:H and by the stability of a siliconoxide film that is formed and regenerated in accordance with the environment´s humidity in the case of a-C:H:Si.