Nanoscale sliding friction phenomena at the interface of diamond-like carbon and tungsten
Macroscopic tribometry is linked to classical atomistic simulations in order to improve understanding of the nanoscale interfacial processes during sliding of hydrogenated DLC (a-C:H) against a metal (W) in dry and lubricated conditions. Experimentally, using an online tribometer, wear and roughness measurements are performed after each sliding cycle, which are then correlated with the frictional resistance. Ex situ analysis is also performed on the worn surfaces (i.e. plates and counterfaces) using X-ray photoelectron spectroscopy, Auger electron spectroscopy and cross-sectional transmission electron microscopy imaging of the near-surface region. Then, in order to elucidate the atomistic level processes that contribute to the observed microstructural evolution in the experiments, classical molecular dynamics are performed, employing a bond order potential for the tungsten-carbon-hydrogen system. Macroscopic tribometry shows that dry sliding of a-C:H against W results in higher frictional resistance and significantly more material transfer compared with lubricated conditions. Similarly, the molecular dynamic simulations exhibit higher average shear stresses and clear material transfer for dry conditions compared with simulations with hexadecane as a lubricant. In the lubricated simulations, the lower shear stress and the absence of a material transfer are attributed to hexadecane monolayers that are partially tethered to the a-C:H surface and significantly reduce adhesion and mechanical mixing between the sliding partners.