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Hydrogel-like structures are responsible for the low friction experienced by our joints when we walk or by our eyelids when we blink. At low loads, hydrogel contacts show extremely low friction that rises with velocity beyond a threshold speed. Here we combine mesoscopic simulations and experiments to test the polymer-relaxation hypothesis for this velocity dependence, where a velocity-dependent regime emerges when the perturbation of interfacial polymer chains occurs faster than their relaxation at high velocity. Our simulations quantitatively match the experimental findings, with a friction coefficient that rises with velocity to some power of order unity in the velocity-dependent regime. We show that the velocity-dependent regime is characterized by reorientation and stretching of polymer chains in the direction of shear, leading to an entropic stress that can be quantitatively related to the shear response. The detailed exponent of the power law in the velocity-dependent regime depends on how chains interact: We observe a power close to 1/2 for chains that can stretch, while pure reorientation leads to a power of unity. These results show that the friction of hydrogel interfaces can be engineered by tuning the morphology of near-surface chains. [ABSTRACT FROM AUTHOR]

Concentrated polymer brushes (CPBs) are promising soft-material coatings for improving tribological properties under severe sliding conditions, even in the macroscopic scale. Therefore, they are expected to be applied to mechanical sliding components. However, the durability of CPBs has remained challenging for industrial applications. Previous studies revealed that applying a groove texture to the CPB substrate is effective in improving the durability of CPBs. In order to achieve further improvement of durability of CPBs, we attempted to apply nano-periodic structures, whereas the groove texture applied in previous studies has widths and depths in micrometres. In this study, the effect of the nano-periodic structure in addition to the groove texture applied to the CPB substrate on the durability of CPB is investigated. The results demonstrate a significant improvement in the durability of CPBs by up to 90% compared with non-textured CPB when an appropriate nano-periodic structure is applied (i.e. a nano-periodic structure oriented parallel to the groove texture). [ABSTRACT FROM AUTHOR]

Coarse-grained molecular dynamics simulations were carried out to investigate the frictional properties of lubricant molecules on nanostructured metal surfaces. The simulation cell consists of a lubricant film enclosed between two metal surfaces. The attractive potential of specific iron atoms on the surface was modified such that the lubricant molecule adsorb preferentially on these atoms. These particular iron atoms were arranged to reproduce a grain boundary surface nanostructure. It is found that below the critical normal stress, the strength of the interaction between Fe and the lubricant molecule has little effect on the friction coefficient. However, the behavior of the lubricant film on the metal surface is sensitive to the interaction strength. Large attraction forces increase the adherence of the lubricant film. [ABSTRACT FROM AUTHOR]

A grafting-to approach is described as a means of fabricating ultrathin, oil-compatible, friction-reducing coatings. The tribological properties of grafted-to coatings of poly(dodecyl methacrylate) (P12MA), prepared by means of a photoactivated perfluorophenylazide adhesion promoter, were compared with those of grafted-from coatings of the same polymer of comparable molecular weight, prepared via surface-initiated, atom-transfer radical polymerisation. It was shown that in a sub-hydrodynamic regime (i.e. boundary/brush lubrication), both coating types can be equivalently effective, exhibiting very low friction coefficients (0.02) against a bare silica countersurface with no detectable wear over the duration of the test (around 1,000 cycles, and maximal Hertzian contact pressure ca 170 MPa as calculated for a bare-bare configuration), providing that sufficiently viscous base lubricants are employed. [ABSTRACT FROM AUTHOR]

We report a highly effective macroscopic, non-aqueous lubrication strategy, employing polymer-brush-based surface modification. Poly(alkyl methacrylates) with hexyl, dodecyl and octadecyl side chains have been prepared on both silicon and iron substrates by means of surface-initiated atom-transfer radical polymerisation. Frictional properties of the three polymers have been evaluated in hexadecane, ethanol and toluene. For the case of the poly(dodecyl methacrylate), the study additionally involved nine base lubricating fluids of different chemical natures, with viscosities ranging from 4 to 2,200 cSt at 20 °C. By constructing a set of Stribeck curves at low values of speed × viscosity (at constant load), it was possible to identify an extended hydrodynamic lubrication regime and, for P12MA-based coatings, a low-friction, brush-lubrication regime, reinforced by time-dependant effects, which appeared to replace the mixed and boundary regimes observed for conventional lubricants. Self-mated poly(dodecyl methacrylate) brushes maintained low coefficient-of-friction values (e.g. 0.012 in hexadecane) over thousands of reciprocating cycles (>100 m total distance) under low speed × viscosity conditions (0.1 cm/s and 4 cSt). [ABSTRACT FROM AUTHOR]

A polymer-brush-based, surface-modification strategy for friction and wear reduction in hard contact under boundary-lubrication conditions is proposed, specifically for a non-aqueous environment. Surface-initiated atom-transfer radical polymerization was employed for the synthesis of three different oil-compatible, hydrophobic polymer brushes based on alkyl methacrylates. This study presents polymerization kinetics, chemical characterization by means of Fourier-transform infrared spectroscopy and surface morphologies observed in atomic force microscopy. The lubrication properties of the anchored polymers were evaluated macroscopically by means of ball-on-disk methods and on the nanonewton scale by lateral force microscopy and showed significant reduction in friction up to contact pressures as high as 460 MPa. The frictional response of surface-grafted polymers is shown to depend strongly on the compatibility of the polymer with the chosen lubricating fluid. [ABSTRACT FROM AUTHOR]

The surface sliding friction of chemically cross-linked poly(dimethyl siloxane) (PDMS) swollen with linear PDMS as an oligomer is investigated. The friction force f increases with the normal pressure P in a power-law relation f∝ Pα, where the exponent α changes in a range of 0-1, depending on the degree of polymerization, Npoly, of the linear PDMS oligomer. When Npoly is in a range of 240-320, a dramatic decrease in friction force is observed at a critical normal pressure, Pc, leading to a very low friction coefficient on the order of 10−3 at high-pressure ranges. The Pc increases with decreasing network size Nnetof the gel and also with increasing polymer length related to Npoly. One possible explanation for this transition phenomenon in friction is that linear PDMS molecules are exuded from the gel network beyond a certain pressure and behave as polymer brushes, which are able to reduce the friction. [ABSTRACT FROM AUTHOR]