Lubrication of Si-Based Tribopairs with a Hydrophobic Ionic Liquid: The Multiscale Influence of Water

This work aims to elucidate the role of water on the tribological behavior of silicon-based surfaces lubricated with a hydrophobic ionic liquid (IL), by means of a multi-technique, multi-scale approach. At the nanoscale, the presence of water at the interface was found to promote adhesion between a sharp silicon tip and a silicon substrate, when submerged in the IL. In line with this finding, in the case of samples that had been exposed to humid air, lateral force microscopy at low loads revealed a significant contribution of adhesion to friction. Under dry conditions, a low-to-high-friction-regime transition is observed at low loads, which is reminiscent of the behavior already observed at the nanoscale in previous studies on IL-mediated lubrication. The comparison of friction-vs-load curves from tests carried out under both humid and dry conditions suggests that a similar mechanism of energy dissipation, presumably involving solid-solid contact between sliding counterparts, is established when applied loads are sufficiently high. The macroscopic behavior of a fused silica pin sliding against a Si (100) substrate in a ball-on-disk configuration was investigated over a wide range of sliding speeds. Wear was evaluated by means of both optical microscopy and profilometry. Changes in the surface chemistry and near-surface structure of the contact area following tribotesting were characterized by both Raman and X-ray photoelectron spectroscopies. Macrotribological tests show that, for sufficiently low sliding speeds, the water adsorbed at the solid/IL interface promotes a tribochemical form of wear. However, at high sliding speeds, a regime of wear characterized by extended damage in the form of plastic deformation and fracture dominates, regardless of the presence of water in the IL. Under these conditions, the prevailing mechanism of friction is likely to be related to the welding and rupture of asperity/asperity junctions, and a direct comparison of LFM results might be not possible. In contrast, when in the presence of humid air and at low sliding speed, the absence of plastic deformation in the near-surface region suggests that pressures within the asperity-asperity contacts are in the range of those existing in the LFM experiments described here.