The high temperature tribological behavior of Si, O containing hydrogenated diamond-like carbon (a-C:H/a-Si:O) coating against an aluminum alloy

Si added DLC coatings may be of value for engineering applications where a low COF and high wear resistance are required for the extended service life, during which the operating temperatures may increase above 200 °C. Hot/warm forming dies, machining tools, or critical sliding components such as the upper compression rings in combustion engines are among the components that operate under demanding conditions. This study examines friction, wear and adhesion behavior of multilayered DLC coatings consisting of a top layer of H-DLC and an inner layer rich in Si and O (a-C:H/a-Si:O) tested against Al-6.5% Si (319 Al) at temperatures above 200 °C where DLC coatings typically show high COF and wear. The a-C:H/a-Si:O coatings maintained the low room temperature (25 °C) steady state COF (μS of 0.17) up to 400 °C with a μS of 0.04 at 200 °C and 0.11 at 400 °C. Low wear rates (W) were observed in the same temperature range with W=2.86×10−5 mm3/Nm at 25 °C and 16.23×10−5 mm3/Nm at 400 °C. Micro-Raman spectroscopy studies of the coating surface revealed a constant ID/IG ratio of 0.4-0.6 between 25 °C and 400 °C. It was suggested that incorporation of Si hindered temperature induced bulk graphitization (due to suppression of sp2 bond formation) of the coating and thus accounted for the low coating wear. Scanning electron microscopy observations indicated formation of a transfer layer on 319 Al contact surface when tested against 319 Al. Cross-sectional transmission electron microscopy of the transfer layer formed on 319 Al at 400 °C revealed that they consisted of amorphous C with traces of SiC, transferred from the a-C:H/a-Si:O and mechanically mixed with Al. X-ray photoelectron spectroscopy of the transfer layer revealed the formation of C-OH, C-H and Si-O bonds. Accordingly, the low steady state friction obtained up to 400 °C could be attributed to H and OH passivation of the C and Si in the coating surface and in the transfer layers.