The simultaneous achievement ‐under physiologically high contact pressures‐ of ultra‐low friction, nearly zero surface wear, and long lifetime in the development of human cartilage prosthetics is ...still a big challenge. In this work, inspired by the unique lubrication mechanism of scallion leaves resulting from the synergy of oriented surface micro‐topography and mucus hydration, a novel layered soft hydrogel as cartilage prototype is developed by chemically embedding thick hydrophilic polyelectrolyte brush chains into the sub‐surface of a high strength anisotropic hydrogel bulk. It exhibits an anisotropic polymer network with unique mechanical properties (tensile strength: 8.3 to 23.7 MPa; elastic modulus 20.0 to 30.0 MPa), anisotropic hydrated surface texture, super‐lubricity, and excellent wear resistance. Thydrogel architecture can exhibit low coefficient of friction (COF) less than ≈0.01 under a wide range of contact stresses (0.2 to 2.4 MPa) and maintain cartilage‐like long‐lasting (50k sliding cycles) robust super‐lubricity (COF ≈ 0.006) and nearly‐zero wear under high contact pressure (≈2.4 MPa) condition. Theoretical underpinning reveals how multiscale surface anisotropy, mechanics, and hydration regulate super‐low friction generation. This work provides a novel design paradigm for the fabrication of robust soft materials with extraordinary lubricity as implantable prototypes and coatings.
A novel anisotropic layered lubrication hydrogel (ALLH) is engineered by mimicking the architecture and lubrication mechanism of natural scallion leaf (NSL). The ALLH exhibits low coefficient of friction (COF) less than ≈0.01 under a wide range of contact stresses and demonstrates long‐lasting (50k sliding cycles) robust super‐lubricity (COF ≈ 0.006) along with nearly‐zero wear under high contact pressure (≈2.4 MPa).
Reducing friction is one of the best ways to lower energy consumption and make processes more environmentally friendly. Because of the growing interest in green lubricants, we investigated the ...synergetic effects of graphene quantum dots (GQDs) combined with aqueous glycerol to improve the lubrication performance of self-mated steel contacts in reciprocating sliding motion. As a comparison, the lubrication performance of some other two-dimensional (2D) graphitic materials (graphite, graphene oxide, and graphene nanoplatelets) was also studied. The results demonstrate that the GQDs-based nano-lubricant reduces the running-in period and provides super-low friction at a high contact pressure in the boundary-lubrication regime, with 72% and 53% improvements in anti-friction and anti-wear performance compared with aqueous glycerol. On the other hand, the 2D graphitic materials provide super-low friction in the mixed-lubrication regime due to the dominant polishing effect of the hydroxyl groups. The surface Raman mapping indicates that the 2D graphitic materials were severely damaged by the continuous reciprocating motion owing to their inferior crack resistance. In contrast, the superior deformation resistance of the GQDs helps to develop a tough tribofilm. This lubrication mechanism suggests that internal shearing of graphene layers inside the GQDs significantly reduces the wear and friction during the running-in period, while the in-situ formation of strongly adhered tough tribofilm with more surface coverage contributed to the realization of super-low friction under a high contact pressure. This study demonstrated that GQDs-based green nano-lubricants could provide super-low friction at a high contact pressure in the boundary-lubrication regime, which was so far characteristic mainly for conventional environment-polluting lubricants.
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•Synergism of GQDs and aqeous glycerol is investigated between steel/steel contact.•GQDs provide super-low friction in boundary lubrication regime at a high contact pressure.•GQDs promotes the formations of tough tribofilm with more surface coverage.•2D graphitic materials provide super-low friction in mixed lubrication regime.•Boundary lubrication mechanism of GQDs with aqueous glycerol is proposed.
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•Graphene quantum dots (GQDs) nano-additives in aqueous glycerol provide super-low friction (µ ≈ 0.012) between self-mated steel contacts.•The super-low friction is observed under a ...contact pressure as high as 316.5 MPa in the boundary lubrication regime (rarely observed for steel/steel contacts).•A 98% improvement in wear performance is observed compared to pure aqueous glycerol without GQDs nano-additives.•A more realistic lubrication mechanism is proposed based on the friction-induced structural degradation of GQDs.
Reducing friction is a promising strategy to decrease material losses and energy consumption in industrial systems. However, in aqueous-lubricated steel contacts, the contact pressure rarely exceeds 50 MPa during super-low friction due to excessive wear. This work demonstrates that even in steel/steel contacts, by combining graphene quantum dots (GQDs) with aqueous glycerol, it is possible to maintain super-low friction (µ ≈ 0.012) under a contact pressure as high as 316.5 MPa. Moreover, the use of GQDs improved the wear performance by 98 % compared to pure aqueous glycerol due to the formation of a tribochemical film, resulting from the electrostatic adsorption of GQDs on the positively charged sites on the worn surface. In particular, the exfoliation of graphene sheets within GQDs, the shearing of graphene layers inside the GQDs, and the OH–OH repulsion between the asperities shortens the running-in period and consequently reduces the friction and wear. At the same time, the formation of a chemically adsorbed tribofilm containing friction-induced structurally degraded GQDs protects the surface from wear and facilitates the maintenance of super-low friction at high contact pressures by improving the load-carrying capacity. This study suggests that green nano-lubricants based on GQDs have immense potential in sustainable engineering.
Two-dimensional (2D) lamellar materials are normally capable of rendering super-low friction, wear protection, and adhesion reduction in nanoscale due to their ultralow shear strength between two ...basal plane surfaces. However, high friction at step edges prevents the 2D materials from achieving super-low friction in macroscale applications and eventually leads to failure of lubrication performance. Here, taking graphene as an example, the authors report that not all step edges are detrimental. The armchair (AC) step edges are found to have only a minor topographic effect on friction, while the zigzag (ZZ) edges cause friction two orders of magnitude larger than the basal plane. The AC step edge is less reactive and thus more durable. However, the ZZ structure prevails when step edges are produced mechanically, for example, through mechanical exfoliation or grinding of graphite. The authors found a way to make the high-friction ZZ edge superlubricious by reconstructing the (6,6) hexagon structure to the (5,7) azulene-like structure through thermal annealing in an inert gas environment. This will facilitate the realization of graphene-based superlubricity over a wide range of industrial applications in which avoiding the involvement of step edges is difficult.
This study aims to investigate the potential of double network hydrogel (DN gel) in a low friction mechanical system. In this study, the effect of the thickness of DN gel films on the friction ...property are clarified. An indentation test, contact pressure measurement, and series of friction tests in water using three different thickness (1.5, 3, 6 mm) of DN gel film and a SiC disc were carried out.
Friction of DN gel film sliding against SiC in water can be classified into two types as “Friction region I: friction remains substantially unchanged during the test” and “Friction region II: friction decreases due to run-in”. Friction region transition can be related to the Hersey number region, which consists of viscosity of lubricant, sliding speed and apparent contact pressure. Hersey number larger than 1.5 × 10−10 is a necessary condition for friction region II where different lubrication regimes can be found.
1.5 and 6 mm thick DN gel films have exhibit super low friction (μ < 0.01) under a wide range of friction conditions as friction region II, and especially 1.5 mm thick DN gel film, super low friction is achieved by twice run-in phenomena.
•The double network hydrogel shows super-low friction coefficient (μ < 0.01).•The double network hydrogel film thickness change the friction coefficient tendency.•The type of wear observed on the double network hydrogel is dependent on its friction coefficient tendency.
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•The steel ball-film structure transformed from graphite-like to fullerene-like structure.•The Si substrate-film structure began in FL structure and kept it through the thickness.•The ...FL structural film friction couples could achieve ultra-low friction in open air.•The 3h/Si-3h/Fe ball couple exhibited super-low friction (0.009) and wear life (5.5×105 cycles).
In this study, we prepared hydrogenated amorphous carbon films on steel balls and Si substrates (steel ball- and Si substrate-films) with different deposition time, and discussed their carbon nanostructural evolutions and tribological behaviors. The steel ball-film structure started to be graphite-like structure and then gradually transformed into fullerene-like (FL) structure. The Si substrate-film structure began in FL structure and kept it through the thickness. The difference may be result from the competition between high starting substrate temperature after additional nitriding applied on the steel balls (its supply power is higher than that in the film deposition), and relaxation of compressive stress from energized ion bombardment in film deposition process. The FL structural film friction couples could achieve ultra-low friction in open air. In particular, the Si substrate-film with 3h, against the steel ball-film with 2h and 3h, exhibited super-low friction (∼0.009) and superlong wear life (∼5.5×105 cycles). Our result could widen the superlubricity scope from previously high load and velocity, to middle load and velocity.
In water-based experiments exploring ultralow or super low friction, the implementation of a running-in period before reaching such states is necessary and important. However, the effect of the ...change in contact geometry has not been fully realized. In this paper, a series of running-in tests on a Si
3
N
4
ball and a WC plate have been performed in glycerol–water mixtures with different concentrations. The shape of the wear scars and the chemical compositions of the worn surfaces were characterized in detail. A numerical EHD and mixed lubrication model was established to comprehensively analyze the effects of geometric profiles, surface roughness, and working/lubrication conditions on ultralow or super low sliding friction. The experiment and simulation results of the study have provided an in-depth understanding of the mechanism of super low friction of liquid lubricated sliding point contacts.
Graphical abstract
The hydroxyethyl cellulose (HEC) is a biocompatible and nonperishable natural compound, which makes it an ideal additive for achieving super-low friction in hydration lubrication. In this work, ...ultra-polished quartz glasses were used as friction pairs with a contact area as large as 50.24 mm
2
. The lubrication behavior of HEC solutions was studied and the topology of the friction pairs was investigated. A super-low COF of 0.0012 was achieved under lubrication of 1.00 wt.% HEC. Increasing or decreasing the dosage of HEC will lead to defective lubrication. Dehydration tests have shown that saturated salts compete for the water of the hydrated HEC and destroy the lubricating film, demonstrating the importance of hydration in achieving super-low lubrication. The XPS spectra of quartz and quartz covered with lubricant indicate that there is no chemical interaction between the quartz and HEC during the friction. However, a negative shift in the peak of O 1s lines were detected, suggesting that physical adsorption of the oxygen atoms occurs at the interface between the quartz and the lubricant. The lubrication regime was analyzed by Hamrock-Dowson theory and the ratio
λ
is around 1.7–24.5, which suggested that the lubrication regime was mixed lubrication or hydrodynamic lubrication. According to the experiments and discussions, possible lubrication models are proposed for HEC lubricants with different concentrations, which can contribute to the development of new hydration lubricants with superlubricity properties and are of great importance for scientific exploration and engineering applications.
Self-mated fullerene-like hydrogenated carbon films (FL-C:H films) realized the conversion from low friction (~0.012) to super-low friction (~0.006) state due to the increasing normal load.
The ...structure changes in the conversion and the effect of environments (N2 and humid air) on the structure changes were discussed by the combination of SEM, TEM, Raman and XPS spectra. With the increasing normal load, smaller wear scar area (or contact area) and thicker tribofilm could be observed. At smaller scale (such as nanoscale), not only more perfect graphitic structure can be formed, but newly formed longer graphitic sheets also tended to curve.
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•Self-mated FL-C:H films realize the conversion from low friction to super-low friction.•The conversion accompanies with smaller contact area and thicker tribofilms.•The conversion is related with the formation and curvature of more perfect graphitic structure.
Recently we have developed a novel molecular dynamics program NEW-RYUDO-CR, which can deal with chemical reactions. The developed method has been applied to the study of tribochemical reaction ...dynamics of MoS
2 tribofilm on iron surface. The initially amorphous MoS
2 layer self-organized its structure as result of the tribochemical reactions and formed layered MoS
2 tribofilm. The friction coefficient significantly decreased as the MoS
2 tribofilm was formed. Besides, sliding was observed between sulfur layers of MoS
2 tribofilms which occurred due to repulsive Coulombic interaction forces between sulfur atoms. This indicates that the formation of the layered MoS
2 tribofilm is important to achieve better lubrication properties.
