Aqueous Lubrication Research Articles

Friction and wear resistance of polytetrafluoroethylene-based composites reinforced with ceramic particles under aqueous environment

Polytetrafluoroethylene (PTFE) exhibits excellent potential under aqueous lubrication due to its low water absorption, low friction, and good corrosion resistance. However, its poor wear resistance limits its application. In this paper, PTFE was reinforced with different kinds of ceramic particles, and the physical properties were tested using various methods. The tribological behavior of PTFE composites sliding against a super duplex stainless steel was investigated under deionized water and seawater lubrication. The worn surface was examined by scanning electron microscopy, energy-dispersive spectroscopy, and 3D topography. All four kinds of ceramic fillers promoted the crystallinity of the PTFE matrix. Particle-reinforced PTFE showed better tribological performance under seawater than under deionized water. The fragility of ceramic particles played an important role in the anti-wear performance of the PTFE composites. Breaking and shedding of Si3N4 or SiO2 particles would generate and aggravate abrasive wear between friction pairs. With the good crush resistance of SiC particles and seawater lubrication effect, the wear rate of the composite decreased by 80% in seawater compared with pure PTFE. The h-BN-filled PTFE showed a good and stable tribological performance under water and seawater because of the low interlaminar shear strength of h-BN.

Macroscale superlubricity achieved between zwitterionic copolymer hydrogel and sapphire in water

Achievement of macroscale superlubricity from the hydration effect could provide a potential application for artificial articular cartilage. In this work, two zwitterionic polymers, 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine methacrylate (SBMA), were used to synthesize a P(MPC-co-SBMA) copolymer hydrogel through the additional polymerization of the alkane carbon-carbon double bond. The incorporation of MPC promoted the water-binding property of copolymer hydrogels as well as improved their anti-compression capability. A superlubricity state with a friction coefficient of approximately 0.002 was achieved when a P(MPC-co-SBMA) copolymer hydrogel hemisphere slid on a sapphire in water, which demonstrated its dependence on sliding velocity, load, and aqueous lubricants. The superlubricity mechanism was mainly attributed to the hydration effect from zwitterionic MPC and SBMA polymer chains, which led to the formation of a uniform hydration layer on the hydrogel surface and the strong adsorption of water molecules on the sapphire surface, which provided additional stabilized hydration layers. These findings might provide insight into the superlubricity mechanism of zwitterionic hydrogels based on the hydration effect, thereby broadening their extensive applications.

A Self‐Assembled Binary Protein Model Explains High‐Performance Salivary Lubrication from Macro to Nanoscale

AbstractSalivary pellicle, a spontaneously formed, intricate architecture in the human oral cavity, is a high‐performance bio‐lubricant that coats and protects biological surfaces with varying elastic modulus against frictional damage. Although salivary lubrication underpins the fundamentals of human feeding and speech, the peculiar molecular mechanism behind such lubrication properties remains elusive. For the first time, this work demonstrates a binary model comprised of salivary proteins, mucin, and lactoferrin (LF), forming an electrostatically driven, multilayered self‐assembly that exhibits a lubrication behavior closely resembling that of human saliva, from macro to nanoscale. The multiscale tribological analysis with applied forces ranging from 1 N to 1 nN, supported by real‐time self‐assembly monitoring on hydrophilic and hydrophobic substrates differentially resolves the distinct roles played by the salivary proteins of this proposed lubricating model. Evidences reveal that hydrated mucin controls the macromolecular viscous lubrication entrapping water molecules in the mucinous network and LF acts as a “molecular glue” between mucin–mucin and mucin–surface, latter aiding boundary lubrication. This study puts forward an unprecedented molecular model that explains the synergistic lubrication by salivary components. These results can aid into the design routes for synthesizing highly efficacious nature‐inspired aqueous lubricants for future biomedical applications and nutritional technologies.

Aqueous Lubrication with the Molecularly Confined Films of Silicone-Based Amphiphilic Block Copolymer Aggregates.

The confined film structures and tribological properties of the dilute aqueous solution of a silicone-based amphiphilic block copolymer, bis-isobutyl poly(ethylene glycol) (PEG)-14/amodimethicone (BIPA) copolymer, between mica surfaces were investigated. The BIPA copolymer existed as positively charged water-soluble aggregates in the solution. The adsorption behavior of the BIPA copolymer aggregates on a mica surface from solution was studied using an atomic force microscope (AFM); the result showed the immediate formation of a uniform adsorbed BIPA copolymer layer, followed by the gradual deposition of BIPA aggregates on the top of the adsorbed layer. Friction measurements were carried out using the surface forces apparatus (SFA) for the confined films of BIPA copolymer solution between mica surfaces, which revealed two different sliding film structures depending on the elapsed time after surface preparation. The sliding film consisting of two adsorbed BIPA copolymer layers was obtained for a relatively short elapsed time (not longer than 3 h), which had an extremely low friction coefficient μ (of the order of 10-5). The sliding film on the following day (elapsed time of approximately 24 h) had the structure of a deposited/kinetically trapped BIPA aggregate layer confined between the opposing adsorbed layers, and the μ values were within the range from 10-4 to 10-3. Our results suggest that the different elapsed time ranges and resulting absence or presence of the intervening layer of trapped aggregates between the absorbed layers determine the tribological properties of the confined films. Molecular friction mechanisms are discussed for the two sliding structures, which give insight into using amphiphilic block copolymer aggregates for a new class of aqueous lubrication system to design extremely low friction interfaces.

Self‐healable polyacrylic acid‐polyacrylamide‐ferric ion dual‐crosslinked hydrogel with good biotribological performance as a load‐bearing surface

ABSTRACTAlthough having been widely investigated, polymer hydrogels still have many defects like poor tribological properties and insufficient durability, hindering their further applications in biomedical fields. In this study, we present a simple method to synthesize polyacrylic acid‐polyacrylamide‐ferric ion (PAA‐PAAm‐Fe3+) dual‐crosslinked hydrogels with self‐healing abilities and “soft‐hard” hydrogel‐polyetheretherketone (PEEK) combined load‐bearing surfaces with low friction coefficients. After analytical characterizations, the results demonstrated that the hydrogels could repair themselves without any external stimuli. Because of the excellent biphasic and aqueous lubrication provided by the hydrogel layer and the load‐bearing capacity provided by the PEEK substrate, the friction coefficient of a load‐bearing surface was as low as 0.048 in water, much lower than a pristine PEEK block or a hydrogel block sample. This work fabricated self‐healable PAA‐PAAm‐Fe3+ hydrogels and low friction bearing surfaces, successfully improving the tribology properties of hydrogels, hopefully promoting their applications as biomedical materials such as articular cartilage. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48499.

Role of nanoparticle materials as water-based lubricant additives for ceramics

Abstract The long running-in time and high wear of water-lubricated ceramics restrict their utility in the tribological field. Aqueous lubricant additives have been added to improve the tribological performance, but the effect of additive materials on the lubrication behavior is still unclear. In this study, nanoparticles of different materials were synthesized and tested for their tribological properties. The wear surfaces were systematically analyzed. The compatibility between the ceramic surface and the nanoparticles is indicated to be one of the key factors for good lubrication behavior. SiO2 nanoparticles performed cooperatively with the ceramic surface and significantly reduced the friction and wear, while ZnO and TiO2 could not form a homogenous protective film, leading to poor tribological performance.

Modelling for water-based liquid lubrication with ultra-low friction coefficient in rough surface point contact

Abstract In this paper, a composite model of average flow Reynolds equation with considering electrical double layer (EDL) effect in roughness surface is proposed to analysis aqueous solution lubrication performance. The numerical simulations are conducted to explain the phenomena of water-based lubrication with ultra-low friction coefficient. The results have showed that: The EDL hydrodynamic and asperity contact will work together to balance the out applied load. The EDL effect has greatly benefited for enhancing film formation and decreasing the friction to obtain the ultralow friction coefficient. To achieve the liquid superlubricity, the sliding velocity, liquid viscosity, surface roughness and EDL effect should be comprehensively considered. The proposed numerical model could be provided a better understanding and design thin film lubrication behavior.

Aqueous Lubrication: Impact of Dispersity and Hydrogen Bonding on the Lubricity of Poly(acrylamide) Brushes (Adv. Mater. Interfaces 14/2019)

Aqueous Lubrication: Impact of Dispersity and Hydrogen Bonding on the Lubricity of Poly(acrylamide) Brushes (Adv. Mater. Interfaces 14/2019)

Evaluating tribological properties of the stearic acid-based organic nanomaterials as additives for aqueous lubricants

Abstract Lubricant additives can effectively enhance tribological properties of water, but most of them are environmentally hazardous. Based on the solubilization principle of surfactants, the water-insoluble stearic acid (SA) can be dissolved in the sodium dodecylbenzene sulfonate (SDBS) aqueous solution to form a milky white liquid (SSA aqueous solutions). This paper presents an evaluation of tribological properties of SSA solution. Compared with the pure water, the SSA aqueous solution is more effective in reducing the friction and wear of the friction pairs. The enhancements are due to the following aspects: (1) on the surfaces of the metals, the SSA aqueous solution can be spread out more easily to form liquid lubricant films; (2) the adsorbed SDBS films can inhibit the corrosion behaviors of the metals; (3) the continuous SSA films can separate the direct rubbing between the friction pairs. The SSA solutions show promising results in reducing friction and wear for various friction pairs under a range of loads and shearing velocities, and can be used as potential lubricants in various processes.

On Lubrication States after a Running-In Process in Aqueous Lubrication

Recently, many studies have reported the ultralow friction coefficient of sliding friction between rigid solid surfaces in aqueous lubrication. A running-in process that goes through high-friction and friction-decreasing regions to a stable ultralow friction region is often required. However, the role of the friction-decreasing region is often ascribed to tribofilm formation in which complexity hindered the quantitative description of the running-in process and the prediction of its subsequent lubrication state. In this work, the frictional energy (Ef) dissipated in the running-in process of a poly(oligo(ethylene glycol) methyl ether acrylate) aqueous lubrication was related to the wear of solid surfaces under different conditions and lubrication states. Experimental results indicated that the high-friction region was in a boundary lubrication state, contributed to most of the wear, and significantly reduced the contact pressure, whereas the friction-decreasing region was in a mixed lubrication state, contributed only to the slight and slow removal of materials, and slightly reduced the contact pressure. Therefore, by establishing relationships among the wear scar diameter, Ef, and the Stribeck curve of the tribological system, the subsequent lubrication state after a running-in process under various working loads and sliding speeds could be quantitatively predicted. The running-in experiments with different aqueous lubrication systems showed good agreement with the prediction of this method. This investigation provides an effective method for the wear and lubrication state prediction after a running-in process, further proving the importance of the Stribeck curve for a lubrication system. This study may also have important implications for the strategy design of the running-in process in various industrial applications.

Tribological behavior of various ceramic materials sliding against CF/PTFE/graphite-filled PEEK under seawater lubrication

In order to select appropriate antifriction and wear resistance material combinations for key frictional pairs in seawater hydraulic components, tribological characteristics of SiC, Si3N4, Al2O3, and ZrO2 ceramics sliding against carbon fiber/polytetrafluoroethylene/graphite-filled polyetheretherketone composite lubricated with seawater were comparatively investigated with a ring-on-ring test rig. The results show that the nonoxide ceramics (Si3N4 and SiC), especially the Si3N4 ceramic, exhibited lower friction coefficients and smaller wear rates than those of oxide ceramics (Al2O3 and ZrO2). And the tribological behaviors of polyetheretherketone/Si3N4 tribopair under dry friction, pure water, and seawater lubrications are further comparatively studied to explore the effect of lubricating medium on the tribological characteristics of Si3N4 ceramic. It is found that the lubricating effect of SiO2 and Si(OH)4 films generated by tribo-chemical reaction between Si3N4 and water is the main factor for the relatively low friction coefficient and wear rate of polyetheretherketone/Si3N4 tribopair under aqueous lubrication. Under seawater lubrication, the Mg(OH)2 and CaCO3 deposition layers caused by the chemical reaction of Mg2+ and Ca2+ in seawater could inhibit the generation of SiO2 and Si(OH)4 films and increase the counterface roughness. As a result, the tribological behaviors of polyetheretherketone/Si3N4 tribopair are worse under seawater lubrication than that of pure water lubrication.

How synergistic aqueous lubrication is mediated by natural and synthetic molecular aggregates

Nature lubricates in aqueous environment, and thus the example of a human synovial joint with its seamless function has been a fascination for scientists since the times of the birth of modern science. Here, inspired by nature, we investigate the mechanistic function of three different types of synergistic molecular aggregates. Firstly, we show how simple phospholipids lubricate hydrophilic model surfaces of silica and how this lubrication is facilitated further by the presence of an anionic polysaccharide, hyaluronan, due to the enhanced surface build-up of lubricant material. Next, we mimic natural polylectrolytesurfactant aggregation by employing a highly positively charged polyelectrolyte and anionic surfactant that strongly associate both in the bulk and at the surfaces by building structured aggregates that lubricate due to hydration lubrication. This occurs despite of the presence of strong attraction between the lubricated surfaces. This is an example of synergistic lubrication due to particular internal structural arrangement of the aggregates. Finally, we investigate the case of synergistic lubrication due to preferential surface ordering of two biological polyelectrolytes, cartilage oligomeric matrix protein and lubricin, that leads to favourable lubrication.

Diprotic Ammonium Succinate Ionic Liquid in Thin Film Aqueous Lubrication and in Graphene Nanolubricant

An ammonium succinate protic ionic liquid (PIL) has been used as additive in water and as base lubricant for graphene dispersions in sapphire-stainless steel lubrication. The tribological performance of a water-based lubricant containing the PIL additive di[bis(2-hydroxyethyl)ammonium] succinate (DSu) in 1 wt% proportion (Water + DSu) has been studied. Both neat DSu and Water + DSu lubricants present a high friction coefficient during the running-in period. Elimination of the running-in high friction period has been achieved with a DSu layer generated on the steel surface by evaporation of water, before the sliding begins. This DSu surface layer reduces the wear rate in two orders of magnitude with respect to full-fluid Water + DSu and to neat DSu lubricants. The high friction running-in period can also be eliminated by the addition of 0.05 wt% few-layers graphene (G) to DSu. The new (DSu + 0.05G) nanolubricant also prevents wear and surface damage on stainless steel. Surface interactions are discussed from contact angles, SEM and TEM microscopies, XPS surface analysis, and Raman microscopy results.

Structural and Morphological Transformations in Aqueous Graphite-Based Lubricant Subjected to Extreme Tribological Interactions of Low-Speed Intermittent Machining

We study morphological and structural changes in graphite flakes present in aqueous graphite-based lubricants when repeatedly subjected to extreme tribological interactions at the tool–chip interface in low speed, high depth of cut intermittent machining, a process involving combined high normal and shear stress. Orthogonal tube turning experiments are conducted on a specially fabricated mild steel workpiece completely submerged in an aqueous graphite solution. Surface profiler, electron microscopy and Raman spectroscopy studies on initial and sheared graphite flakes indicate crushing followed by layer sliding, rolling, edge curling, tearing and exfoliation of layers. Raman spectroscopy studies highlight formation of low amorphous carbon by structural disorder and edge defects formed while crushing and tearing with no new basal defects in the sheared graphite layers. Some regions of the flake thin down to highly disordered few-layer graphene with a thickness reduction from about 30 micrometres to about 1–2 nanometres with an I2D/IG value of around 0.7. When reused back into the tribological region, these morphological changes can significantly influence the continued lubrication process by the generation of few-layer graphene.

A novel comb-typed poly(oligo(ethylene glycol) methylether acrylate) as an excellent aqueous lubricant

HypothesisAqueous lubricants exhibit versatile advantages over oil-based lubricants. However, it still remains a challenge for the aqueous solutions to obtain excellent lubrication properties with high contact pressure on macroscale. ExperimentsIn this work, a comb-typed poly(oligo(ethylene glycol) methylether acrylate) (P(OEGMA)) was successfully synthesized via RAFT polymerization. Rheological, morphological and tribological properties of prepared P(OEGMA) aqueous solutions were characterized via a rheometer, cryo-SEM and ball-on-disk tribometer, respectively. FindingsThe synthesized P(OEGMA) exhibited a uniformly smaller size than that of the commercial linear polyethylene glycol (PEG), leading to reduced viscosities in aqueous solutions. The obtained P(OEGMA) aqueous solutions achieved outstandingly ultralow friction coefficients (μ < 0.01) and a good wear-resistance under high pressure (>300 MPa, two-fold increase than reported in the previous literature). The desirable lubricating performances can be attributed to the well-established running-in period, a good interfacial adsorption property between polymer molecules and solid surfaces, the hydration effect as well as the hydrodynamic effect. The current finding reveals the excellent aqueous lubrication properties possessed by the synthesized comb-typed P(OEGMA), which can broaden the development of aqueous lubricants in practical engineering fields.

Diprotic ammonium palmitate ionic liquid crystal and nanodiamonds in aqueous lubrication. Film thickness and influence of sliding speed

Ionic liquid based lubricants offer potential to reduce the sliding friction and wear of ferrous bearing surfaces such as those used in cutting and machining processes. In this study, two types of promising additives were investigated: (1) diprotic bis(2-hydroxyethyl)ammonium palmitate (DPA) ionic liquid crystal and (2) a 0.1 wt% dispersion of nanodiamonds (ND) in DPA. Both have been used at 1 wt% concentration in aqueous lubricants. Those were Water+DPA, and Water+DPA+ND. The stability of the dispersions has been studied by dynamic light scattering, finding that the interaction between the additives and water changes particle size distribution. Glass-against-steel optical interferometry using water film alone indicated that the film thickness decreased with number of sliding strokes and that in turn increased the friction coefficient. By comparison, Water+DPA and Water+DPA+ND showed lower friction coefficients and increased film thickness. In steel-steel pin-on-disk contacts lubricated only by water, electrical resistance increases at speeds higher than 100 mm/s. DPA and DPA+ND additives displace the electrical resistance increase to lower sliding speed, around 10 mm/s. The additives reduced the coefficients of sliding friction up to 82%, and wear rates of AISI 316L disks by more than two orders of magnitude. The combination of the palmitate ionic liquid crystal and nanodiamonds in Water+DPA+ND gave the lowest wear rates. The wear mechanism and surface analysis associated with these results are discussed.

Effect of Contamination on the Friction and Wear of Carboxylic Acids in Aqueous Lubricants

The friction and wear mechanisms of water-based fluids consisting of various types of carboxylic acids have been systematically investigated using a ball-on-disc tribometer on a super duplex stainless steel under boundary lubricating conditions. Under water-based lubrication, a decrease in friction was found for each of the tested carboxylic acids with a hydrocarbon chain longer than 12 carbon atoms. However, wear reduction was only seen for the carboxylic acids with a hydrocarbon chain longer than 16 carbon atoms or with shorter chain carboxylic acids in concentrations above a critical value. In addition, it was found that when the lubricant is contaminated with bivalent metal ions like Ca2+ or Mg2+, the frictional performance worsened; however, wear improved. Increasing the concentration of bivalent alkaline metal ions reduced the adsorption ability of carboxylic acids to the steel surface leading to higher friction and the presence of Na+ ions facilitated the wear improvement. This is explained by the formation of polyelectrolyte complexes between the metal ions dissolved in the lubricant and the carboxylic acid molecules. To compensate for the detrimental effects in friction, a chelating strategy has been implemented and explained in detail.

Influence of molecular structure on lubrication of aqueous triblock copolymer lubricants between rutile surfaces: An MD approach

The influence of molecular structure on adsorption and lubrication of aqueous triblock copolymer on TiO 2 surfaces has been investigated using a realistic MD model. Reference data obtained from DFT calculations have been employed to develop an interaction potential between the copolymer and TiO 2 surface. The results show that R Pluronics form an anchor–buoy–anchor structure absorbed on TiO 2 surface and L Pluronic for buoy–anchor–buoy structure. The shear of tribo-system influences slightly on the orientation of copolymers and non-slip behavior has been observed at solid-fluid interfaces due to the strong adhesion strength of copolymer on rutile surface. The shear viscosity increases with copolymer concentration and molecular chain length. L Pluronic has a lower shear stress than the R ones. • A new interacting FF between PEO, PPO and TiO 2 surface results in a better prediction of adsorption of copolymer. • The adsorption of copolymer on TiO 2 surface is physisorption with adsorption energy for PPO block larger than PEO. • The adsorption energy per PPO or PEO chain reduces with an increase of surface coverage and in the presence of water. • R Pluronics form an anchor–buoy–anchor structure on the surface while there is a buoy–anchor–buoy one for L types. • The shear affects the orientation of copolymers, and a non-slip behavior has been observed at solid-fluid interfaces. • The shear viscosity increases with the copolymer concentration and chain length. • L Pluronic has a lower shear stress than the R ones.

Promoting Lubricity and Antifouling Properties by Supramolecular-Recognition-Based Surface Grafting

In the supramolecular host-guest system, the host molecule selectively identifies the guest and forms the inclusion complex with the guest molecule. In this study, the physicochemical properties of solid surfaces were regulated by the interfacial supramolecular recognition. The host-guest interaction between β-cyclodextrin and guest molecules, including adamantaneacetic acid, sodium dodecyl sulfonate, and a copolymer of 2-methacryloyloxy-2-methyladamantane and 3-sulfopropyl methacrylate potassium salt, was introduced onto the silicon substrate to construct supramolecular composite surfaces. After the assembly of hydrophilic guest molecules on the host surface, the wettability, aqueous lubrication, and anti-algae cell adhesion properties of the supramolecular composite surfaces were improved. This strategy of host-guest interfacial supramolecular recognition provides a new route to prepare aqueous lubrication and antifouling materials.

Thermo-tribological behaviour of microgels for improved aqueous lubrication for steel/UHMWPE contact

Abstract The thermo-tribological behaviour of 316L stainless steel/ultrahigh molecular weight polyethylene (UHMWPE) contacts under aqueous lubrication was investigated in this work. Thermo-responsive microgels were prepared and used as lubricant additives in aqueous solutions. Tests were conducted using a reciprocating tribometer, over a range of temperatures and microgel concentrations. The microgels were shown to have temperature-dependent friction-reducing and anti-wear properties. The microgels acted as load bearing particles in the water, rolling and sliding between frictional interfaces. A maximum recorded friction coefficient coincided with a phase-transition in the microgels induced at a lubricant temperature of 32 °C. This was indicative of a thermo-tribological sensitivity in the microspheres, whereby particle size and shape changed with temperature, affecting tribological performance. These changes included structural collapse of the microgels due to dehydration at the critical temperature. This characteristic temperature-sensitivity in the microgels offers new strategies for controlling friction and wear of steel/UHMWPE contacts under aqueous lubrication.