Superlubricity of a Mixed Aqueous Solution

A super-low friction coefficient of 0.0028 is measured under a pressure of 300MPa when the friction pair(the silicon nitride ball sliding on the silicate glass)is lubricated by the mixed aqueous solution of glycerol and boric acid.The morphorlogies of the hydroxylated glass plate are observed by an atomic force microscope(AFM)in deionized water,glycerol,boric acid and their mixed aqueous solution.Bonding peaks of the retained liquids adhered on the surface of the sliding track are detected by an infrared spectrum apparatus and a Raman spectrum apparatus.The mechanism of the superlubricity of the glycerol and boric acid mixed aqueous solution is discussed.It is deduced that the formation of the lubricant film has enough strength to support higher loads,the hydration effect offering the super lower shear resistance.

Superlubricity enabled dry transfer of non-encapsulated graphene

Transferring high-quality exfoliated graphene flakes onto different substrates while keeping the graphene free of polymer residues is of great importance, but at the same time very challenging. Currently, the only feasible way is the so-called all-dry "pick-and-lift" method, in which a hexagonal boron nitride(hBN) flake is employed to serve as a stamp to pick up graphene from one substrate and to lift it down onto another substrate. The transferred graphene samples, however,are always covered or encapsulated by hBN flakes, which leads to difficulties in further characterizations. Here, we report an improved "pick-and-lift" method, which allows ultra-clean graphene flakes to be transferred onto a variety of substrates without hBN coverage. Basically, by exploiting the superlubricity at the graphene/hBN stack interface, we are able to remove the top-layer hBN stamp by applying a tangential force and expose the underneath graphene.

Combination of diketone and PAO to achieve macroscale oil-based superlubricity at relative high contact pressures

1-(4-ethylphenyl)-nonane-1,3-dione(0206)is an oil-soluble liquid molecule with rod-like structure.In this study,the chelate(0206-Fe)with octahedral structure was prepared by the reaction of ferric chloride and 1,3-diketone.The experimental results show that when using 0206 and a mixed solution containing 60%0206-Fe and 40%0206(0206-Fe(60%))as lubricants of the steel friction pairs,superlubricity can be achieved(0.007,0.006).But their wear scar diameters(WSD)were very large(532µm,370µm),which resulted in the pressure of only 44.3 and 61.8 MPa in the contact areas of the friction pairs.When 0206-Fe(60%)was mixed with PAO6,it was found that the friction coefficient(COF)decreased with increase of 0206-Fe(60%)in the solution.When the ratio of 0206-Fe(60%)to PAO6 was 8:2(PAO6(20%)),it exhibited better comprehensive tribological properties(232.3 MPa).Subsequent studies have shown that reducing the viscosity of the base oil in the mixed solution helped to reduce COF and increased WSD.Considering the COF,contact pressure,and running-in time,it was found that the mixed lubricant(Oil3(20%))prepared by the base oil with a viscosity of 19.7 mPa·s(Oil3)and 0206-Fe(60%)exhibited the best tribological properties(0.007,161.4 MPa,3,100 s).

Liquid superlubricity of lubricants containing hydroxyl groups and their aqueous solution under rolling/sliding conditions

Macroscale rolling/sliding conditions are in the superlubricity,a little-studied topic so far.The purpose of this paper is to examine the formation of elastohydrodynamic lubrication(EHL)films by water-based lubricants(glycerol and polyethylene glycol(PEG)),providing superlubricous friction.Experiments were carried out on an optical ball-on-disc tribometer under rolling/sliding conditions.The film thickness was measured by the thin film colorimetric interferometry,and the viscosity of liquids was measured by rotational and high-pressure falling body viscometers.The results show that tribochemical reactions are not the mandatory reason for friction to reach the superlubricity level when using the water-based lubricants.The studied liquids themselves are almost Newtonian.With the addition of water,the signs of shear thinning behavior disappear even more.Suitable conditions for this type of lubricant can be predicted using the known Hamrock–Dowson equations.An anomaly in the thickness of the lubricants was observed as an abrupt change at certain conditions.The more PEG there is in the lubricant,the higher the thickness at the beginning of the jump.

Modulation mechanism of electron energy dissipation on superlubricity based on fluorinated 2D ZIFs

Electron energy dissipation is an important energy dissipation pathway that cannot be ignored in friction process.Two-dimensional zeolite imidazole frameworks(2D ZIFs)and fluorine doping strategies give 2D Zn-ZIF and 2D Co-ZIF unique electrical properties,making them ideal materials for studying electron energy dissipation mechanism.In this paper,based on the superlubricity modulation of 2D fluoridated ZIFs,the optimal tribological properties are obtained on the 2D F-Co-ZIF surface,with the friction coefficient as low as 0.0010.Electrical experiments,density functional theory(DFT)simulation,and fluorescence detection are used to explain the mechanism of fluorine doping regulation of tribological properties from the two stages,namely energy transfer and energy release.Specifically,the energy will transfer into the friction system through the generation of electron–hole pairs under an external excitation,and release by radiation and non-radiation energy dissipation channels.Fluorination reduces energy transfer by altering the electronic properties and band structures of ZIFs,and slows down the charge transfer by enhancing the shielding efficiency,thus slowing the non-radiative energy dissipation rate during the energy release stage.Our insights not only help us better understand the role of fluorine doping in improving tribological properties,but also provide a new way to further explore the electron energy dissipation pathway during friction.

Photoinduced superlubricity on TiO_(2) surfaces

Superlubricity control is of great interest in both industry and scientific research,and several methods have been proposed to achieve this goal.In this work,ultraviolet(UV)light was introduced into titanium dioxide(TiO_(2))and silicon nitride(Si_(3)N_(4))tribosystems to accomplish photoinduced superlubricity.The friction coefficients(COFs)between Si_(3)N_(4) balls and TiO_(2) plates in the mixtures of sulfuric acid(H^(2)SO_(4))solution and glycerol solution were obviously reduced,and the system entered the superlubricity region(COF<0.01)after UV illumination at a speed of 56 mm/s.However,the COF was much larger without UV treatment than that with UV treatment.The formation of silica(SiO_(2))layers on the surfaces of Si_(3)N_(4) balls and the elastohydrodynamic effects were determined to be fundamental to the low friction in this experiment,and the enhancement of the combination between the TiO_(2) surface and the hydroxy group of glycerol by UV illumination was the key to the photoinduced superlubricity in this system.These findings showed one method for achieving superlubricity by introducing a light field that could be further applied to special working conditions.

Low friction of superslippery and superlubricity:A review

The issues regarding energy dissipation and component damage caused by the interface friction between a friction pair attract enormous attention to friction reduction.The key-enabling technique to realize friction reduction is the use of lubricants.The lubricants smooth the contact interfaces,achieving an ultralow friction contact,which is called superslippery or superlubricity.At present,superslippery and superlubricity are two isolated research topics.There is a lack of unified definition on superslippery and superlubricity from the viewpoint of tribology.Herein,this review aims at exploring the differences and relations between superslippery and superlubricity from their origin and application scenarios.Meanwhile,the challenges for developing superslippery surface and superlubricity surface are discussed.In addition,perspectives on the interactive development of these two surfaces are presented.We hope that our discussion can provide guidance for designing superslippery or superlubricity surfaces by using varies drag-reduction technologies.

Electric field controlled superlubricity of fullerene-based host–guest assembly

Controlling friction by the electric field is a promising way to improve the tribological performance of a variety of movable mechanical systems.In this work,the assembly structure and microscale superlubricity of a host–guest assembly are effectively controlled by the electric field.With the help of the scanning tunneling microscopy(STM)technique,the host–guest assembly structures constructed by the co-assembly of fullerene derivative(Fluorene-C60)with macrocycles(4B2A and 3B2A)are explicitly characterized.Combined with density functional theory(DFT),the distinct different assembly behaviors of fullerene derivatives are revealed at different probe biases,which is attributed to the molecular polarity of the fullerene derivative.Through the control on the adsorption behavior,the friction coefficient of host–guest assembly is demonstrated to be controllable in the electric field by using atomic force microscopy(AFM).At positive probe bias,the friction coefficient of the host–guest assembly is significantly reduced and achieves superlubricity(μmin=0.0049).The efforts not only help us gain insight into the host–guest assembly mechanism controlled by the electric field,but also promote the further application of fullerene in micro-electro-mechanical systems(MEMS).

Achieving macroscale superlubricity with ultra-short running-in period by using polyethylene glycol-tannic acid complex green lubricant

Superlubricating materials can greatly reduce the energy consumed and economic losses by unnecessary friction.However,a long pre-running-in period is indispensable for achieving superlubricity;this leads to severe wear on the surface of friction pairs and has become one of the important factors in the wear of superlubricating materials.In this study,a polyethylene glycol-tannic acid complex green liquid lubricant(PEG10000-TA)was designed to achieve macroscale superlubricity with an ultrashort running-in period of 9 s under a contact pressure of up to 410 MPa,and the wear rate was only 1.19×10^(–8)mm^(3)·N^(−1)·m^(−1).This is the shortest running-in time required to achieve superlubricity in Si_(3)N_(4)/glass(SiO_(2)).The results show that the strong hydrogen bonds between PEG and TA molecules can significantly reduce the time required for the tribochemical reaction,allowing the lubricating material to reach the state of superlubrication rapidly.Furthermore,the strong hydrogen bond can share a large load while fixing free water molecules in the contact zone to reduce shear interaction.These findings will help advance the use of liquid superlubricity technology in industrial and biomedical.

Adjustable superlubricity system using polyalkylene glycol with various acid aqueous solutions

Polyalkylene glycol(PAG)aqueous solutions have recently been demonstrated to exhibit an ultralow friction coefficient(COF,μ<0.01).However,the prolonged running-in period and low bearing capacity have limited its widespread application.In this study,we determined that the running-in period can be decreased by more than 75%when the pH value of the lubricant is controlled at 3 by introducing various acid solutions.Additionally,less time was required to realize stable superlubricity with inorganic acid at lower pH values.This was mainly attributed to the acceleration effect of hydrogen ions around the contact region.In case of PAG aqueous solution with organic acid,the wear loss between sliding solid surfaces was reduced,and thus the bearing pressure during the superlubricity period was significantly improved from approximately 30 to 160 MPa.Furthermore,the organic acid molecules were considered to form strong hydrogen bonds with PAG macromolecules and solid surfaces.This in turn strengthened the structure of the adsorption layers.The unique effect of different acids in aqueous polymer lubrication can potentially significantly aid in advancing the study of polymer tribology and broadening industrial applications.

Achieving ultrafast superlubricity with layered double hydroxides

Layered double hydroxides(LDHs)have the potential to be superlubricated materials due to their strong adsorption effect and weak internal interaction.However,obtaining stable superlubricity during the ultrafast time(<10 s)is still a challenge.Here,we demonstrated macroscale superlubricity based on LDHs of multiple metal ions at high surface roughness,achieving superlow friction coefficients(0.006)and ultrafast wearing-in time(<7 s),which mainly originated from tribochemical reactions and the formation of nanostructured adsorption layers.Through cross-sectional analysis and density functional theory,we revealed the properties of the protective tribofilm to achieve ultrafast superlubricity.LDHs strongly adsorbed on the surface of the bearing steel,the sliding interface transformed into a heterogeneous interface between the polytetrafluoroethylene and LDH,leading to macroscale superlubricity.These findings demonstrate that tribochemical treatment of surfaces produces tribofilm that effectively reduces wearing-in time and promotes ultralow friction.

How to improve superlubricity performance of diketone at steel interface:Effects of oxygen gas

Achievement of steady and reliable super-low friction at the steel/steel contact interface,one of the most tribological systems applied for mechanical moving parts,is of importance for prolonging machine lifetime and reducing energy consumption.Here we reported that the superlubricity performance of the steel/steel sliding interface lubricated with tiny amounts of diketone solution strongly depends on the oxygen content in surrounding environment.The increase of oxygen not only significantly shortens the initial running-in time but also further reduces the stable coefficient of friction in superlubricity stage due to the enhancement of tribochemical reactions.On the one hand,more severe oxidation wear occurring at higher oxygen content facilitates material removal of the contact interface,lowering the contact pressure and the corresponding initial friction.On the other hand,the growth of iron ions during the shear process in high oxygen environment promotes the formation of chelate which acted as an effective lubricated film chemisorbed at the steel/steel friction interface to further lower the interfacial friction.The results provide a new opportunity to further optimize the tribological performance of diketone superlubricity system,especially towards the lubrication of mechanical engineering materials.

Superlubricity by polyimide-induced alignment

We have investigated the lubrication alignment behavior of point–plane contact and plane–plane contact between the GCr15 steel and polyimide(PI)friction pair using nematic liquid crystals(LCs)as the lubricant.In this system,rubbing orients the macromolecular PI molecules,and the oriented PI molecules induce alignment of the LC molecules in contact with or close to the oriented PI molecules.The LC molecules are aligned in the wear scar grooves of the PI film,and alignment extends to the GCr15-steel counterpart.Alignment of the LC molecules is correlated with the strong interaction force between the PI and LC molecules,the stable coordination structure of the LCs and GCr15 steel,and the weak interaction between the LC molecules.We performed simulations of the pretilt angle of PI and LCs and discussed the relationship between the pretilt angle and the friction properties.Owing to the small pretilt angle between PI and the LCs,the LC molecules orient almost parallel to the PI material,which is beneficial for superlubricity of this type of friction system.

Graphene superlubricity:A review

Superlubricity has drawn substantial attention worldwide while the energy crisis is challenging human beings.Hence,numerous endeavors are bestowed to design materials for superlubricity achievement at multiple scales.Developments in graphene-family materials,such as graphene,graphene oxide,and graphene quantum dots,initiated an epoch for atomically thin solid lubricants.Nevertheless,superlubricity achieved with graphene-family materials still needs fundamental understanding for being applied in engineering in the future.In this review,the fundamental mechanisms for superlubricity that are achieved with graphene-family materials are outlined in detail,and the problems concerning graphene superlubricity and future progress in superlubricity are proposed.This review concludes the fundamental mechanisms for graphene superlubricity and offers guidance for utilizing graphene-family materials in superlubricity systems.

From ultra-low friction to superlubricity state of black phosphorus:Enabled by the critical oxidation and load

Based on the density functional theory(DFT),we investigate the friction properties of inevitable oxidized black phosphorus(o-BP).o-BP with the weaker interlayer adhesion exhibits their great potential as a solid lubricant.At the zero load,the friction property of o-BP is adjusted by its oxidation degree.Expressly,ultra-low friction of P4O2(50%oxidation,O:P=2:4=50%)is obtained,which is attributed to the upper O atoms with lower sliding resistance in the O channel formed by lower layer O atoms.More attractive,we observe superlubricity behavior of o-BP at the critical load/distance due to the flattening potential energy surface(PES).The flattening PES is controlled by the electrostatic role for the high-load(P4O3,O:P=3:4=75%),and by the electrostatic and dispersion roles for the low-load(P4O2).Distinctly,the transform from ultra-low friction to superlubricity state of black phosphorus(BP)can be achieved by critical oxidation and load,which shows an important significance in engineering application.In addition,negative friction behavior of o-BP is a general phenomenon(Z>Z_(min),Z_(min)is the interlayer distances between the outermost P atoms of minimum load.),while its surface-surface model is different from the fold mechanism of the tip-surface model(Z_(0)<Z<Z_(min),Z_(0)is the interlayer distances between the outermost P atoms of equilibrium state.).Thus,this phenomenon cannot be captured due to the jump effect with instability of the atomic force microscopy(AFM)(Z>Z_(min)).In summary,o-BP improves the friction performance and reduces the application limitation,comparing to graphene(Gr),MoS2,and their oxides.

Superlubricity induced by partially oxidized black phosphorus on engineering steel

Macroscale superlubricity has attracted increasing attention owing to its high significance in engineering and economics.We report the superlubricity of engineering materials by the addition of partially oxidized black phosphorus(oBP)in an oleic acid(OA)oil environment.The phosphorus oxides produced by active oxidation exhibit lower friction and quick deposition performance compared to BP particles.The H-bond(–COOH…O–P,or–COOH…O=P)formed between P–O bond(or P=O)and OA molecule could benefit the lubricating state and decrease the possibility of direct contact between rough peaks.The analysis of the worn surface indicates that a three-layer tribofilm consisting of amorphous carbon,BP crystal,and phosphorus oxide forms during the friction,which replaces the shear interface from the steel/steel to carbon–oBP/carbon–oBP layer and enables macroscale superlubricity.

Load and velocity boundaries of oil-based superlubricity using 1,3-diketone

The clarification of the critical operating conditions and the failure mechanism of superlubricity systems is of great significance for seeking appropriate applications in industry.In this work,the superlubricity region of 1,3-diketone oil EPND(1-(4-ethyl phenyl)nonane-1,3-dione)on steel surfaces was identified by performing a series of ball-on-disk rotation friction tests under various normal loads(3.5–64 N)and sliding velocities(100–600 mm/s).The result shows that beyond certain loads or velocities superlubricity failed to be reached due to the following negative effects:(1)Under low load(≤3.5 N),insufficient running-in could not ensure good asperity level conformity between the upper and lower surfaces;(2)the high load(≥64 N)produced excessive wear and big debris;(3)at low velocity(≤100 mm/s),the weak hydrodynamic effect and the generated debris deteriorated the lubrication performance;(4)at high velocity(≥500 mm/s),oil migration occurred and resulted in oil starvation.In order to expand the load and velocity boundaries of the superlubricity region,an optimized running-in method was proposed to avoid the above negative effects.By initially operating a running-in process under a suitable combination of load and velocity(e.g.16 N and 300 mm/s)and then switching to the target certain higher or lower load/velocity(e.g.100 N),the superlubricity region could break through its original boundaries.The result of this work suggests that oil-based superlubricity of 1,3-diketone is a promising solution to friction reduction under suitable operating conditions especially using a well-designed running-in strategy.

Two-dimensional molybdenum carbide(MXene)as an efficient nanoadditive for achieving superlubricity under ultrahigh pressure

In this study,a robust macroscale liquid superlubricity with a coefficient of friction of 0.004 was achieved by introducing molybdenum carbide(Mo_(2)CT_(x))MXene nanoparticles as lubricating additives in a lithium hexafluorophosphate-based ionic liquid at Si_(3)N_(4)-sapphire interfaces.The maximal contact pressure in the superlubricity state could reach 1.42 GPa,which far exceeds the limit of the superlubricity regime in previous studies.The results indicate that a composite tribofilm(mainly containing molybdenum oxide and phosphorus oxide)that formed at the interface by a tribochemical reaction contributed to the excellent antiwear performance.Furthermore,the extremely low shear strength of the tribofilm and the interlayers of Mo_(2)CT_(x)MXene contributed to the superlubricity.This work demonstrates the promising potential of Mo_(2)CT_(x)MXene in improving superlubricity properties,which could accelerate the application of superlubricity in mechanical systems.

Study on the microcosmic superlubricity mechanism of PVPA affected by metal cations

Hydrophilic polymer coatings on artificial implants generate excellent tribological properties.The friction properties of polymer coatings are affected by salt ion factors.Herein,the atomic force microscopy(AFM)was used to show that the superlubricity was achieved between poly(vinylphosphonic acid)(PVPA)-modified Ti6Al4V and polystyrene(PS)microsphere probe lubricated with monovalent salt solutions(LiCl,NaCl,KCl,and CsCl).Considering that adhesion is an important cause of friction changes,the AFM was further utilized to obtain adhesion between friction pairs in different salt solutions.The results indicated that the larger the cation radius in the lubricant,the smaller the adhesion,and the lower the friction coefficient of the PVPA coating.The electrostatic interaction between the PVPA and one-valence cations in lubricants was analyzed by the molecular dynamics(MD)simulation as it was found to be the main influencing factor of the adhesion.Combined analysis results of friction and adhesion indicated that by adjusting the size of cation radius in lubricant,the adhesion between the tribo-pairs can be changed,and eventually the magnitude of friction can be affected.This study opens up a new avenue for analyzing the friction characteristics of hydrophilic polymer coatings from the perspective of intermolecular forces.

Macroscopic superlubricity of potassium hydroxide solution achieved by incorporating in-situ released graphene from friction pairs

Graphene(G),as a typical two-dimensional material,is often used as an additive for liquid lubricants.However,graphene is mostly added to liquid lubricants in a one-time manner in friction;it mainly exists in the form of multilayer agglomerated structures due to theπ–πstacking between graphene sheets,making it unable to fully exert the synergistic lubrication function.Herein,we propose a new macroscopic superlubric system of graphene/potassium hydroxide(G/KOH)solution;and the graphene additive involved is exfoliated in-situ from graphene/epoxy(G/EP)friction pair by friction,continuously providing freshly-peeled graphene into KOH solution and minimizing the adverse effects of graphene agglomeration.Moreover,the in-situ produced graphene additive has thinner thickness and better anti-aggregation ability,which provide more graphene to accommodate OH−,form more stacked sandwich structures of OH−/graphene/OH−between friction pairs(i.e.,equivalent to a moving pulley block with more wheels),and finally realize superlubricity.This study develops a new liquid superlubric system suitable for alkaline environments,and at the same time proposes a new way to gradually release graphene additives in situ,rather than adding them all at once,deepening the understanding to liquid superlubricity mechanism,and paving the experimental foundation for the practical application of macroscopic superlubricity.