Low friction and environmentally stable diamond-like carbon (DLC) coatings incorporating silicon, oxygen and fluorine sliding against aluminum

Diamond-like carbon (DLC) coatings exhibit good adhesion mitigating properties when placed in sliding contact against aluminum, but their coefficient of friction (COF) show high sensitivity to environmental conditions. This study examines the tribological properties of a DLC coating containing 20 at.% Si, 12 at.% F, 14 at.% O and 18 at.% H (a-C:H:Si:O:F) tested against a 1100 grade Al in ambient air (39% relative humidity) and vacuum (6.5 × 10− 3 Pa) atmospheres. According to the pin-on-disk type sliding tests performed, this coating exhibited a low steady state COF of 0.08 under both testing atmospheres. Carbonaceous transfer layers that incorporated F, Si and O compounds were formed on the Al contact surface. The composition and microstructure of these transfer layers were investigated using cross-sectional focused ion beam (FIB), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The F concentration was the highest at the Al/transfer layer interface where the formation of an AlF3 compound was observed. The top surfaces of the carbonaceous transfer layers in contact with the a-C:H:Si:O:F coating were rich in Si and O and consisted of loosely packed nano particulates. The low friction achieved with a-C:H:Si:O:F coating was due to the combination of passivation and hydration mechanisms that operated during sliding. The hydration of the SiOSi chains in the coating and transfer layers resulted in the formation of two OH-passivated surfaces at the contact interface that led to low COF.