Friction Coefficient In Water Research Articles

Molecular transport enhancement in pure metallic carbon nanotube porins.

Nanofluidic channels impose extreme confinement on water and ions, giving rise to unusual transport phenomena strongly dependent on the interactions at the channel-wall interface. Yet how the electronic properties of the nanofluidic channels influence transport efficiency remains largely unexplored. Here we measure transport through the inner pores of sub-1 nm metallic and semiconducting carbon nanotube porins. We find that water and proton transport are enhanced in metallic nanotubes over semiconducting nanotubes, whereas ion transport is largely insensitive to the nanotube bandgap value. Molecular simulations using polarizable force fields highlight the contributions of the anisotropic polarizability tensor of the carbon nanotubes to the ion-nanotube interactions and the water friction coefficient. We also describe the origin of the proton transport enhancement in metallic nanotubes using deep neural network molecular dynamics simulations. These results emphasize the complex role of the electronic properties of nanofluidic channels in modulating transport under extreme nanoscale confinement.

Direct modification of polyolefin films by surface-initiated polymerization of a phosphobetaine monomer

The surfaces of polyethylene and polypropylene sheets were modified by grafting with polymeric brushes formed from a phosphobetaine monomer. Pressed sheets of bromo-functionalized polyethylene or polypropylene macroinitiators (PE-MIs and PP-MIs, respectively) were used to initiate the atom-transfer radical polymerization of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) under mild conditions to form a superhydrophilic grafted surface layer. The grafted polyolefin sheets showed excellent wettability and oil-detachment behaviour in water. Even three years after surface grafting had been performed, the PMPC-g-PP sheet retained a water contact angle of less than 10°. Furthermore, the water contact angle of the PMPC-g-PP sheet remained as low as 12° after it had been annealed at 373 K for 60 min under reduced pressure. Dynamic friction tests performed on the PMPC-g-PE sheet by sliding it against a glass ball revealed a marked reduction in the friction coefficient in water at sliding velocities in the range of 10−5 to 10−1 m s−1, possibly as a result of lubrication by water.

Polyelectrolyte brushes: a novel stable lubrication system in aqueous conditions

Surface-initiated controlled radical copolymerizations of 2-dimethylaminoethyl methacrylate (DMAEMA), 2-(methacryloyloxy)ethyl phosphorylcholine (MPC), 2-(methacryloyloxy)ethyltrimethylammonium chloride) (MTAC), and 3-sulfopropyl methacrylate potassium salt (SPMK) were carried out on a silicon wafer and glass ball to prepare polyelectrolyte brushes with excellent water wettability. The frictional coefficient of the polymer brushes was recorded on a ball-on-plate type tribometer by linear reciprocating motion of the brush specimen at a selected velocity of 1.5 x 10(-3) m s-1 under a normal load of 0.49 N applied to the stationary glass ball (d = 10 mm) at 298 K. The poly(DMAEMA-co-MPC) brush partially cross-linked by bis(2-iodoethoxy)ethane maintained a relatively low friction coefficient around 0.13 under humid air (RH > 75%) even after 200 friction cycles. The poly(SPMK) brush revealed an extremely low friction coefficient around 0.01 even after 450 friction cycles. We supposed that the abrasion of the brush was prevented owing to the good affinity of the poly(SPMK) brush for water forming a water lubrication layer, and electrostatic repulsive interactions among the brushes bearing sulfonic acid groups. Furthermore, the poly(SPMK-co-MTAC) brush with a chemically cross-linked structure showed a stable low friction coefficient in water even after 1400 friction cycles under a normal load of 139 MPa, indicating that the cross-linking structure improved the wear resistance of the brush layer.

Tribological properties of hydrophilic polymer brushes under wet conditions

This article demonstrates a water-lubrication system using high-density hydrophilic polymer brushes consisting of 2,3-dehydroxypropyl methacrylate (DHMA), vinyl alcohol, oligo(ethylene glycol)methyl ether methacrylate, 2-(methacryloyloxy)ethyltrimethylammonium chloride (MTAC), 3-sulfopropyl methacrylate potassium salt (SPMK), and 2-methacryloyloxyethyl phosphorylcholine (MPC) prepared by surface-initiated controlled radical polymerization. Macroscopic frictional properties of brush surfaces were characterized by sliding a glass ball probe in water using a ball-on-plate type tribotester under the load of 0.1-0.49 N at the sliding velocity of 10(-5)-10(-1) m s(-1) at 298 K. A poly(DHMA) brush showed a relatively larger friction coefficient in water, whereas the polyelectrolyte brushes, such as poly(SPMK) and poly(MPC), revealed significantly low friction coefficients below 0.02 in water and in humid air conditions. A drastic reduction in the friction coefficient of polyelectrolyte brushes in aqueous solution was observed at around 10(-3)-10(-2) m s(-1) owing to the hydrodynamic lubrication effect, however, an increase in salt concentration in the aqueous solution led to the increase in the friction coefficients of poly(MTAC) and poly(SPMK) brushes. The poly(SPMK) brush showed a stable and low friction coefficient in water even after sliding over 450 friction cycles, indicating a good wear resistance of the brush film.

Precise surface structure control of inorganic solid and metal oxide nanoparticles through surface-initiated radical polymerization

Surface-initiated radical polymerization was carried out in order to modify the surface of inorganic solid and metal oxide nanoparticles. Novel (inorganic nanoparticles/polymer) nanocomposites were prepared through a direct polymer grafting reaction from the surfaces of magnetite (Fe3O4) (d ¼ 10 and 25 nm) and titanium oxide (TiO2) (d ¼ 15 nm) nanoparticles. The initiator for nitroxide-mediated radical polymerization with a phosphoric acid group was chemisorbed onto the nanoparticles and gave controlled polystyrene (PS) and poly(3-vinylpyridine) (P3VP) graft layers on their surfaces. The PS- and P3VP-modified nanoparticles were finely dispersed in organic solvents, whereas protonated P3VP-modified magnetite nanoparticles were dispersed in aqueous phase. The fine dispersion of nanoparticles in the polymer matrix was confirmed by microscopic observation. In order to realize tribological control, atom transfer radical polymerization of (2,2-dimethyl-1,3-dioxolan-4-yl)methyl methacrylate was also carried out from an immobilized initiator on a flat silicon wafer, resulting in a high-density polymer brush that was subsequently converted to a hydrophilic polymer brush consisting of 2,3-dihyroxypropyl methacrylate units. The poly(2,3-dihydroxypropyl methacrylate) brush-immobilized surface showed a low dynamic friction coefficient in water due to the highly stable hydrophilicity.

Synthesis and Frictional Properties of Poly(2,3-dihydroxypropyl methacrylate) Brush Prepared by Surface-initiated Atom Transfer Radical Polymerization

Abstract Surface-initiated atom transfer radical polymerization (ATRP) of (2,2-dimethyl-1,3-dioxolan-4-yl)methyl methacrylate (1) was carried out from an immobilized initiator on a silicon wafer to result in a high-density polymer brush, which was converted to the hydrophilic polymer brush consisting of 2,3-dihyroxypropyl methacrylate (2) units, successively. The poly(2) brush showed low dynamic friction coefficient in water.

Frictional behavior of DLC films in a water environment

Three types of Ar incorporated DLC films were deposited using a thermal electron excited plasma CVD apparatus at bias voltages of −0.5 to −3 kV. Their friction and wear properties were evaluated under water and air environments using a ball-on-plate type reciprocating friction tester. The anti-wear properties of Ar-DLC films in water were superior to that in air; the smallest wear rate in water was approximately 8×10 −9 mm 3/Nm. The reduction in the friction coefficient in water was not clear, compared to that in air. The lowest friction coefficient in water was approximately 0.1. The transferred materials were observed on the wear scar of the mating ball in every experiment, but the transferred materials in water were greater in quantity than those in air. Furthermore, there seemed to be a qualitative difference in transferred materials between water and air environments, based on micro-Raman analysis. The difference in such transfer phenomena seemed to affect both the friction coefficient and wear rates. These friction and wear behaviors suggest that our Ar-DLC films are good candidates for use in rubbing machine parts when in use with water hydraulic systems.

Friction and wear properties in dry, water- and oil-lubricated DLC against alumina and DLC against steel contacts

Diamond-like carbon (DLC) films can be divided into two major categories according to their hydrogen content. These categories have similarities in tribological performance, but the films also behave in a different manner in different tribological conditions. The results of amorphous hydrogenated carbon films (a-C:H) and hydrogen-free hard carbon films (a-C) are reported in this study. The a-C:H films were deposited using the radio frequency (rf) plasma technique, and the hydrogen-free hard carbon films using pulsed vacuum arc. The coatings were characterized and investigated with respect to their tribological performance in dry (50% RH), water-lubricated and oil-lubricated slow sliding conditions (0.004 m s −1). The a-C and a-C:H films had a low friction coefficient in dry sliding conditions (0.15 to 0.22), which was further decreased by 10–40% under boundary lubrication. The a-C:H(Ti) films exhibited good self-lubricating properties (0.10) in dry sliding conditions and the a-C films had the lowest friction coefficient in water- (0.03) and oil-lubricated (0.08) conditions. The hydrogen-free hard carbon films showed excellent wear resistance in dry, water- and oil-lubricated conditions, but hydrogenated a-C:H films suffered from severe wear in aqueous conditions. The performance of a-C:H films could be improved by titanium alloying. In dry sliding conditions, the tribolayer formation of DLC films influenced the friction and wear performance, but in oil-lubricated conditions boundary lubrication layers were formed, which governed the tribological mechanisms in the contact.

Construction and application of solid-lubricant plain bearings : N. A. Spitsin and G. M. Paptsoy, Russian Eng. J., 50 (9) (1970) 16–20; 4 figs., 1 table, 5 refs. (Transl. by P.E.R.A. of Gt. Britain of Vestn. Mashinostr., 50 (9) (1970) 16–20.)

Diamond-like carbon (DLC) films can be divided into two major categories according to their hydrogen content. These categories have similarities in tribological performance, but the films also behave in a different manner in different tribological conditions. The results of amorphous hydrogenated carbon films (a-C:H) and hydrogen-free hard carbon films (a-C) are reported in this study. The a-C:H films were deposited using the radio frequency (rf) plasma technique, and the hydrogen-free hard carbon films using pulsed vacuum arc. The coatings were characterized and investigated with respect to their tribological performance in dry (50% RH), water-lubricated and oil-lubricated slow sliding conditions (0.004 m s−1). The a-C and a-C:H films had a low friction coefficient in dry sliding conditions (0.15 to 0.22), which was further decreased by 10–40% under boundary lubrication. The a-C:H(Ti) films exhibited good self-lubricating properties (0.10) in dry sliding conditions and the a-C films had the lowest friction coefficient in water- (0.03) and oil-lubricated (0.08) conditions. The hydrogen-free hard carbon films showed excellent wear resistance in dry, water- and oil-lubricated conditions, but hydrogenated a-C:H films suffered from severe wear in aqueous conditions. The performance of a-C:H films could be improved by titanium alloying. In dry sliding conditions, the tribolayer formation of DLC films influenced the friction and wear performance, but in oil-lubricated conditions boundary lubrication layers were formed, which governed the tribological mechanisms in the contact.