Friction measurements were carried out for a poly(dimethylsiloxane) (PDMS) melt (Mw ≈ 80000) confined between hydrophobic surfaces using the surface forces apparatus. The PDMS films were prepared by two different procedures: i) compression of a droplet into a hard-wall state (compressed system); ii) adhesive contact of two thin films cast on each substrate from solution (cast system). The dynamic thicknesses were 1.4 nm for the compressed system and 2.0 nm for the cast system. Despite the large thickness, the friction of the cast system was larger than that of the compressed system. Large thicknesses generally give low friction; the unusual result suggests that the confined structures may be different between the two systems. The PDMS molecules in both systems lay parallel to surfaces, but the extent of ordering could be much higher for the compressed system. The compressed film has a layer structure and slipping mainly occurs between the layers, resulting in the low friction. On the contrary, the cast system should have a disordered structure; molecules may interdigitate to each other and possibly form bridges across the sliding surfaces, which could induce large friction. The effect of film the preparation procedures on molecular ordering is discussed.
Silica nano-particles that have the average diameter of 7 nm were added to water, and their effects on friction were studied for Si3N4 self-mated sliding couples by a ball-on-disk tribotester under boundary lubricated conditions. By adding 0.01 or 0.1 wt % of silica particles to water, the running-in distance was reduced significantly in both cases at the sliding velocity of 0.21 m/s and resulted in low and stable friction (μ < 0.02). The excessive amount of silica (0.1 wt %) caused high and unstable friction (μ > 0.05) under low velocity conditions (v ≤ 0.12 m/s). The optimum concentration of the silica for the low and stable friction was 0.01 wt % among those tested in this study, which is the closest to the solubility limit of silica to water.
Lubricant additive technology to improve lubricant performances is an important issue for ionic liquids to be applied practically. Effects of carboxylic acids with different carbon chain length in ionic liquid on tribological properties were examined. Solubility of carboxylic acids was dependent on the chain length of carboxylic acids and imidazolium cations. The carboxylic acids reduced friction and wear in comparison with additive free one. It was found that friction reducing properties of carboxylic acid depended on their chain length. Interestingly, additive response of ionic liquids was found to be superior to those for conventional ester oil as base oil. Surface images obtained by an optical microscope clearly show that the additive depressed chemical wear by ionic liquids especially at low load. Surface analysis with EPMA supported films of carboxylic acids as a cause of improvement of friction property. Therefore, the mechanism of carboxylic acids was considered to provide adsorbed film which accompanied with low friction and anti-wear properties.
Reduction of friction at very low normal loads and for very small contact areas is important for the development of micro devices. High performance hard coatings such as diamond-like carbon (DLC) films and improved surfaces with nanostructures are being investigated with respect to their friction reduction properties. Previously, difficulties associated with the production of nano-scale periodic structures in DLC films have prohibited the study of such films. In the present study, the friction properties of the DLC film with periodic structures were investigated at the nano-scale using an atomic force microscope (AFM). These periodic structures were generated on the surface of the DLC film by means of a femtosecond (fs) laser having the fluence near the ablation threshold. Friction tests were carried out under normal loads ranging from 20 nN to 130 nN, and the frictional directions were 0º, 45º and 90º (relative to the line along which the periodic structures were created). The lateral force of the DLC film with the periodic structures was lower than that of the film without the periodic structures. We have concluded that decreases in adhesive forces produce significant decreases in lateral forces for the same normal loads.