The tribological tests were performed using Nitinol 60 alloy pin sliding over GCr15 steel disc in the tribometer system. Four kinds of oils were experimentally investigated as lubrication oils for lubricating Nitinol ...The tribological tests were performed using Nitinol 60 alloy pin sliding over GCr15 steel disc in the tribometer system. Four kinds of oils were experimentally investigated as lubrication oils for lubricating Nitinol 60 alloy in the boundary lubrication regime. The experimental results were compared with a reference dry friction. It was found that Nitinol 60 alloy can be lubricated significantly and has shown remarkable lubrication performance. A superlubricity behavior of Nitinol 60 alloy was observed under castor oil lubrication. An ultra-low coefficient of friction of Nitinol 60 alloy about 0.008 between Nitinol 60 alloy and GCr15 steel was obtained under castor oil lubrication condition after a running-in period. Accordingly, the present study is focused on the lubrication behaviors of castor oil as potential lubrication oil for Nitinol 60 alloy. In the presence of castor oil, coefficient of friction is kept at 0.008 at steady state, corresponding to so-called superlubricity regime （when sliding is then approaching pure rolling）. The mechanism of superlubricity is attributed to the triboformed OH-terminated surfaces from friction-induced dissociation of castor oil and the boundary lubrication films formed on the contact surface due to high polarity and long chain of castor oil allowing strong interactions with the lubricated surfaces.展开更多
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 a...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.展开更多
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 fea...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.展开更多
Superlubricity,a novel lubricity mode ascribing to moirésuperlattice(MSL),has attracted attention in ultra-precise manufacture,microelectronic devices,and national defense areas.Based on incommensurate MSL,nearly...Superlubricity,a novel lubricity mode ascribing to moirésuperlattice(MSL),has attracted attention in ultra-precise manufacture,microelectronic devices,and national defense areas.Based on incommensurate MSL,nearly zero friction can be achieved by eliminating sliding lock-in and offsetting lateral force in principle,and the theoretical foundations are still under extensive investigation.Here,the effects of MSL-induced lattice distortion onπbond and tribological performance in twist MoS_(2)/graphene and MoS_(2)/BN heterointerfaces were studied by first-principles calculations comprehensively.Various contributions of 2pz orbital electron polarization among AA-,AB-,and AC-stacking symmetry areas in different MSL were reflected by band structures to explain the sensitivity ofπbond to MSL.Theπbond perpendicular to the atomic plane depended closely on interfacial distortion,which can not only influence the local distribution of intralayer bond strength but also determine the interlayer charge redistribution.Meanwhile,the interfacial potential energy was changed with the interlayer interaction fluctuation caused by twist angle and atomic stacking modes.Through evaluating the energy barriers and lateral force,MoS_(2)/BN with a twist angle of 20.79°exhibited superlubricity.Moreover,the connection among sliding energy barriers,twist angles,and specific electronic structures has been bridged paving a path to reveal the superlubricity mechanism of two-dimensional materials withπbond.展开更多
Hydrogenated diamond-like carbon(H-DLC)is typically produced as a coating or thin film through plasma-enhanced chemical vapor deposition(PE-CVD).H-DLC is relatively hard and well known to exhibit superlubricity.Is sup...Hydrogenated diamond-like carbon(H-DLC)is typically produced as a coating or thin film through plasma-enhanced chemical vapor deposition(PE-CVD).H-DLC is relatively hard and well known to exhibit superlubricity.Is superlubricity an intrinsic property of H-DLC?This paper argues that H-DLC is not intrinsically superlubricious,but it has an ideal structure that allows transition of the interface region to a superlubricious structure upon frictional shear in proper conditions.Thus,its superlubricity is an extrinsic property.This argument is made by comparing frictional behaviors of three allotropes of carbon materials—graphite,amorphous carbon(a-C),and diamond,and carefully scrutinizing the run-in behavior as well as environment sensitivity of H-DLC friction.The superlubricious structure is generally known to be graphitic,but its exact structure remains elusive and is subject to further study.Nevertheless,accurate knowledge of how superlubricity is induced for H-DLC can guide engineering design to achieve superlubricious behaviors with other carbon materials produced via different synthetic routes.展开更多
Emerging superlubricity provides innovative scientific and engineering solutions for the sustainable future of human beings and nature.Although great progress has been made in the development of novel superlubricity s...Emerging superlubricity provides innovative scientific and engineering solutions for the sustainable future of human beings and nature.Although great progress has been made in the development of novel superlubricity systems and different mechanisms,considerable challenges remain before the engineering development of liquid superlubricity can be realized.Herein,the progress made towards achieving liquid superlubricity has been reviewed with emphasis on the current limitations,potential mechanisms,and future breakthroughs that will be expected to overcome these limitations.The perspectives are highlighted based on rigorous statistics and analyses according to the types of lubricants and materials of friction pairs.This review elucidates the key tribochemical mechanisms and research directions to break through the current limitations and provides constructive ideas for the engineering development of liquid superlubricity in the future,which will enable a sustainable future for human beings and nature.展开更多
The achievement of a superlubric state with vanishing friction and negligible wear has important applications in minimizing energy dissipation and prolonging the service life of moving mechanical systems.However,the s...The achievement of a superlubric state with vanishing friction and negligible wear has important applications in minimizing energy dissipation and prolonging the service life of moving mechanical systems.However,the search for a superlubricious oil system applicable to industrial fields remains a major challenge.In this work,we demonstrate for the first time that precisely employing polyether modification for silicone oil molecules could induce direct superlubricity and superlow wear for engineering steel tribopairs.Superlubricity originates from the fact that polyether-modified silicone oil(PESO)can effectively employ polyether functional groups to interact with friction surfaces,during which a complex tribochemical reaction process can be induced under the catalytic role of friction,where an organic lubricious film composed mainly of carbon,silicon and oxygen can be induced in situ,which can not only effectively passivate friction surfaces but also enable superlubric sliding by virtue of its easy-to-shear nature.Furthermore,iron oxides and chromium oxides could also be confirmed to be distributed within the tribofilm,which is desirable for increasing the load-bearing capability of the tribofilm and toughness.Thus,a remarkable superlubricity of 0.01 without running-in combined with superlow wear was realized at the same time.The results of this work show high promise in promoting the industrial use of oil superlubricity and revolutionizing the development of mechanical systems.展开更多
The mixed lubricant prepared by mixing 1-(4-ethylphenyl)-nonane-1,3-dione(0206)and chelate(0206-Fe)in a ratio of 4:6 had better tribological properties than 0206.In this study,it was found that the mixed lubricant pre...The mixed lubricant prepared by mixing 1-(4-ethylphenyl)-nonane-1,3-dione(0206)and chelate(0206-Fe)in a ratio of 4:6 had better tribological properties than 0206.In this study,it was found that the mixed lubricant prepared by mixing alkane lubricants with 0206-Fe(60%)at a ratio of 2:8 can not only achieve superlubricity,but also reduce the wear scar diameter(WSD)of the friction pairs.The mixed solution prepared by the four polar solutions with 0206-Fe(60%)cannot achieve superlubricity.The results of surface analysis and molecular dynamics(MD)simulation showed that the four polar molecules were preferentially adsorbed on the metal surface,occupying the diketone adsorption sites,and the adsorption layer formed by them cannot produce synergistic lubrication with the chelates.展开更多
Two-dimensional nanomaterials were commonly used as lubrication additives.However,the high demand for the load-bearing capacity limited their industrial applications.This study proposed a new strategy to overcome this...Two-dimensional nanomaterials were commonly used as lubrication additives.However,the high demand for the load-bearing capacity limited their industrial applications.This study proposed a new strategy to overcome this limitation by depositing silver(Ag)nanoparticles on hexagonal boron nitride(hBN)nanosheets via dopamine through the reduction reaction of silver ions,and the Ag modified hBN nanosheets(hBN-Ag)were used as additives in aqueous ethylene glycol solution.The results showed that the superlubricity state with a minimum coefficient of friction(COF)of 0.004 at a maximum contact pressure of 1.20 GPa was achieved without the running-in period.The realization of superlubricity was attributed to the synergic effect of Ag nanoparticles and hBN nanosheets,where Ag nanoparticles were attached to the hBN nanosheets by dopamine,enhancing the load-bearing capacity of the hBN nanosheets.Simultaneously,the Ag modified hBN nanosheets were more easily adsorbed and deposited on the friction interface,generating the tribofilm containing hBN-Ag nanosheets through the tribochemical reaction,which reduced the direct contact of the friction pair and provided a low shear strength to generate extremely low friction.The Ag modified hBN nanosheets induced extremely low friction and wear,contributing to the development of lubricants with high loadbearing pressure and low wear rate.展开更多
Hypericin(C30H16O8)is a naturally occurring substance,an anthraquinone derived from St.John’s wort,possessing outstanding antiviral,antitumor,antibacterial,and antioxidant properties.Today,hypericin is primarily used...Hypericin(C30H16O8)is a naturally occurring substance,an anthraquinone derived from St.John’s wort,possessing outstanding antiviral,antitumor,antibacterial,and antioxidant properties.Today,hypericin is primarily used in medicinal applications.It is a small,flat organic molecule with a graphene-like core surrounded by oxidized functions,suggesting it could act as a graphene precursor in tribological contacts.Therefore,we investigated the lubrication properties of hypericin as an additive in glycerol,used as a base oil.It is well established that glycerol is superlubricious under full and thin film elastohydrodynamic(EHD)lubrication regimes but generally fails with steel under more severe conditions(mixed and boundary regimes).We studied the effect of hypericin added to glycerol for steel-on-steel and steel-on-silicon friction pairs.For the steel-on-steel configuration,results show that hypericin is a strong anti-wear additive due to its antioxidant properties that scavenge OH radicals.Moreover,hypericin is also an efficient friction-reducing agent,providing a steady state and robust ultralow friction coefficient(0.02–0.03).Thus,it outperforms most traditional additive formulations under the same conditions,although it does not achieve superlubricity(coefficient of friction(CoF)<0.01)under more severe conditions.For steel-on-silicon,hypericin significantly extends the superlubricity regime of glycerol to lambda ratios well below unity(low sliding speeds).The mechanism of superlubricity is attributed to the friction-induced formation of graphene layers from hypericin molecules,smoothing friction surfaces,and operating a hybrid liquid–solid superlubricious system.展开更多
Polymers have complex molecular structures that often lead to interchain friction and hinder movement,making it difficult to achieve superlubricity.However,in the field of hydration lubrication,the electronegative int...Polymers have complex molecular structures that often lead to interchain friction and hinder movement,making it difficult to achieve superlubricity.However,in the field of hydration lubrication,the electronegative interface of ceramics readily adsorbs water molecules,creating a protective water film that covers the frictional interface and effectively reduces friction.To achieve hydration lubrication,it is essential to create a continuous lubricating film by selectively enriching specific functional groups of adsorbed water molecules from the polymer solution onto the ceramic surface.By adsorbing a hydrophilic layer composed of polyvinylpyrrolidone with pyrrolidone groups onto a negatively charged Si_(3)N_(4)/sapphire interface,we formed a continuous lubricating film.Research has shown that the interaction between the polymer chain structure of polyvinylpyrrolidone molecules(such as PVP10000)in solution and water molecules could result in excellent superlubricity.When the contact pressure exceeds 198 MPa,the coefficients of friction(COF)can be reduced to 0.004-0.007.Through detailed surface analyses and sophisticated simulations,we uncovered the underlying mechanism involved.The pyrrolidone moieties of polyvinyl pyrrolidone(PVP)formed hydrogen bonds with the Si_(3)N_(4) surface,transforming the initially difficult frictional interface into a PVP/sapphire interface with significantly reduced sliding energy barriers.These findings highlight the vital role of PVP in superlubricity and hydration lubrication and provide a theoretical and experimental basis for the design of materials and lubricants with exceptional lubricating properties.展开更多
At present,achieving oil-based superlubricity depends on lubricating oil viscosity and rubbing pair materials excessively.Therefore,establishing a new route is of scientific significance and application value.Here,a n...At present,achieving oil-based superlubricity depends on lubricating oil viscosity and rubbing pair materials excessively.Therefore,establishing a new route is of scientific significance and application value.Here,a novel strategy,i.e.,the assembly of a dialkyl-dithiophosphoricmodified copper nanoparticle(DDP-Cu NP)lubricating additive and polyetheretherketone(PEEK)-GCr15 friction pairs,is reported to achieve oil-based superlubricity in polyalpha olefin oil(PAO40),a synthetic base oil with high viscosity.To our surprise,when the concentration of DDP-Cu is only 0.25 wt%,the lowest friction coefficient is 0.002,and the wear rate reaches 10^(-9) mm^(3)/(N·m)level(which is lower than that published in the literature by 1-2 orders).Furthermore,such superlubricity occurs without a runningin period and remains during the whole sliding process.These results are attributed mainly to the rolling effect of the spherical Cu NPs,the formation of a high-performance carbon-based tribofilm at the sliding counterface,and the in operando formation of some few(even mono)-layered graphene layers in the lubricant(the tribocatalytic effect of the DDP-Cu NPs).Specifically,DDP-Cu NPs perform in operando tribocatalysis to promote the decomposition of the PEEK matrix and PAO40 and generate some carbon species,including amorphous carbon and even graphene,all of which are responsible for the surprising oil-based superlubricity.The findings will contribute to enriching relevant tribological theory and promoting the exploration and application of oil-based superlubricity.展开更多
The optical manipulation of nanoparticles on superlubricity surfaces was investigated.The research revealed that,due to the near-zero static friction and extremely low dynamic friction at superlubricity interfaces,the...The optical manipulation of nanoparticles on superlubricity surfaces was investigated.The research revealed that,due to the near-zero static friction and extremely low dynamic friction at superlubricity interfaces,the maximum intensity for controlling the optical field can be less than 100 W/cm^(2).The controlled nanoparticle radius can be as small as 5 nm,which is more than one order of magnitude smaller than that of nanoparticles controlled through traditional optical manipulation.Manipulation can be achieved on sub-microsecond to microsecond timescales.Furthermore,the manipulation takes place on solid surfaces and in nonliquid environments,with minimal impact from Brownian motion.By appropriately increasing the dynamic friction,controlling the light intensity,or reducing the pressure,the effects of Brownian motion can be eliminated,allowing for the construction of microstructures with a size as small as 1/75 of the wavelength of light while controlling the light intensity,which is seven orders of magnitude smaller compared to manipulating nanoparticles on traditional surfaces.This enables the control of super-resolution optical microstructures.The optical super-resolution manipulation of nanoparticles on superlubricity surfaces has important applications in fields such as nanofabrication,photolithography,optical metasurfaces,and biochemical analysis.展开更多
Since the term“superlubricity”was put forward at the beginning of 1990s,it has become one of the hottest researches in tribology due to it being close linked to the energy problems.Recently,the International Worksho...Since the term“superlubricity”was put forward at the beginning of 1990s,it has become one of the hottest researches in tribology due to it being close linked to the energy problems.Recently,the International Workshop on“Superlubricity:Fundamental and Applications”was successfully held on 19-20 October 2015 in Beijing,which has attracted many researchers in this field.The recent scientific results in both solid superlubricity and liquid superlubricity have been presented according to these invited wonderful lectures and posters.In the communication,we gave an introduction to the Workshop on Superlubricity,and also summarized the new achievements of superlubricity during recent years according to these reports.Finally,the problems of superlubricity mechanism and the future development direction of superlubricity are discussed.展开更多
Superlubricity refers to a sliding regime in which contacting surfaces move over one another without generating much adhesion or friction[1].From a practical application point of view,this will be the most ideal tribo...Superlubricity refers to a sliding regime in which contacting surfaces move over one another without generating much adhesion or friction[1].From a practical application point of view,this will be the most ideal tribological situation for many moving mechanical systems mainly because friction consumes large amounts of energy and causes greenhouse gas emissions[2].Superlubric sliding can also improve performance and durability of these systems.In this paper,we attempt to provide an overview of how controlled or targeted bulk,surface,or tribochemistry can lead to superlubricity in diamond-like carbon(DLC)films.Specifically,we show that how providing hydrogen into bulk and near surface regions as well as to sliding contact interfaces of DLC films can lead to super-low friction and wear.Incorporation of hydrogen into bulk DLC or near surface regions can be done during deposition or through hydrogen plasma treatment after the deposition.Hydrogen can also be fed into the sliding contact interfaces of DLCs during tribological testing to reduce friction.Due to favorable tribochemical interactions,these interfaces become very rich in hydrogen and thus provide super-low friction after a brief run-in period.Regardless of the method used,when sliding surfaces of DLC films are enriched in hydrogen,they then provide some of the lowest friction coefficients(i.e.,down to 0.001).Time-of-flight secondary ion mass spectrometer(TOF-SIMS)is used to gather evidence on the extent and nature of tribochemical interactions with hydrogen.Based on the tribological and surface analytical findings,we provide a mechanistic model for the critical role of hydrogen on superlubricity of DLC films.展开更多
The topic of superlubricity is attracting considerable interest around the world while humanity is facing an energy crisis.Since various liquid superlubricity systems can be commonly achieved on the macroscale in ambi...The topic of superlubricity is attracting considerable interest around the world while humanity is facing an energy crisis.Since various liquid superlubricity systems can be commonly achieved on the macroscale in ambient conditions,it is considered an effective solution to reduce unnecessary energy and material losses.However,certain practical problems such as low load-bearing pressure,dependence on hydrogen ions,and relatively long running-in processes still limit its widespread application.Two-dimensional(2D)nano-additives with ultrathin longitudinal dimensions can lower the shear resistance between sliding solid surfaces,and thus further optimize the applied conditions.In this review,the latest studies on 2D nano-additives with a combination of various water-based lubricants in the state of superlubricity are reported,typically including black phosphorus(BP),graphene oxide(GO),and layered double hydroxide.During the sliding process,composite lubricants effectively improved the load capacity(up to 600 MPa),reduced wear,and accelerated the running-in period(within 1,000 s)of the liquid superlubricity system.Both macromechanical experiments and microscopic tests are conducted to precisely analyze various interactions at the interfaces of the nano-additives and solid surfaces.These interactions can be described as tribochemical reactions,physical protection,and adsorption enhancement,and improved wear resistance.This review provides better guidance for applying 2D nanomaterials in liquid superlubricity systems.展开更多
In this study,we address the superlubricity behavior of sapphire against ruby(or sapphire against itself)under phosphoric acid solution lubrication.An ultra-low friction coefficient of 0.004 was obtained under a very ...In this study,we address the superlubricity behavior of sapphire against ruby(or sapphire against itself)under phosphoric acid solution lubrication.An ultra-low friction coefficient of 0.004 was obtained under a very high contact pressure,with a virgin contact pressure up to 2.57 GPa.Related experiments have indicated that the load,sliding speed,and humidity of the test environment can affect superlubricity to some degree,so we tested variations in these conditions.When superlubricity appears in this study a thin film is present,consisting of a hydrogen bond network of phosphoric acid and water molecules adsorbed on the two friction surfaces,which accounts for the ultra-low friction.Most significantly,the wear rate of the sapphire and ruby in the friction process is very slow and the superlubricity state is very stable,providing favorable conditions for future technological applications.展开更多
In thin-film lubrication(TFL), generally, the viscosity of the lubricant and its coefficient of friction(Co F) increase. Finding a method to reduce the Co F in TFL is a significant challenge for tribologists. In the p...In thin-film lubrication(TFL), generally, the viscosity of the lubricant and its coefficient of friction(Co F) increase. Finding a method to reduce the Co F in TFL is a significant challenge for tribologists. In the present work, we report a robust superlubricity attained by using polyalkylene glycols(PAGs, polar molecules) and poly-α-olefins(PAOs, nonpolar molecules) as lubricants on steel/steel friction pairs that have been pre-treated by wearing-in with polyethylene glycol aqueous solution(PEG(aq)). A steady superlubricity state with a Co F of 0.0045 for PAG100 and 0.006 for PAO6 could be maintained for at least 1 h. Various affecting factors, including the sliding velocity, normal load, and viscosity of the lubricants, were investigated. Element analysis proved that composite tribochemical layers were deposited on the worn region after the treatment with PEG(aq). These layers were formed by the tribochemical reactions between PEG and steel and composed of various substances including oxides, iron oxides, Fe OOH, and Fe(OH)3, which contributed to the superlubricity. In addition to the tribochemical layers, ordered layers and a fluid layer were formed by the PAGs and PAOs during the superlubricity periods. All the three types of layers contributed to the superlubricity, indicating that it was attained in the TFL regime. Accordingly, a mechanism was proposed for the superlubricity of the PAGs and PAOs in the TFL regime in this work. This study will increase the scientific understanding of the superlubricity in the TFL regime and reveal, in the future, the potential for designing superlubricity systems on steel surfaces for industrial applications.展开更多
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 wea...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.展开更多
In this work,a super-low friction coefficient of 0.003 was found between a silicon nitride ball and a sapphire plate lubricated by phosphoric acid solution.The wear mainly occurred in the running-in period and disappe...In this work,a super-low friction coefficient of 0.003 was found between a silicon nitride ball and a sapphire plate lubricated by phosphoric acid solution.The wear mainly occurred in the running-in period and disappeared after superlubricity was achieved.The friction coefficient was effectively reduced from 0.3 to 0.003 at a constant speed of 0.076 m/s,accompanied by a 12-nm-thickness film.The lubrication regime was indicated to change from boundary lubrication in the running-in period to elastohydrodynamic lubrication in the superlubricity period,which is also supported by the results of the friction coefficient versus sliding speed.In addition,the experimental results showed good agreement with theoretical calculations based on the elastohydrodynamic lubrication theory,suggesting a significant hydrodynamic effect of phosphoric acid on superlubricity.展开更多
基金Project(51305331)supported by the National Natural Science Foundation of ChinaProject(2012M511993)supported by China Postdoctoral Science FoundationProject(TPL1202)supported by the Open Fund Program of the State Key Laboratory of Traction Power,Southwest Jiaotong University,China
文摘The tribological tests were performed using Nitinol 60 alloy pin sliding over GCr15 steel disc in the tribometer system. Four kinds of oils were experimentally investigated as lubrication oils for lubricating Nitinol 60 alloy in the boundary lubrication regime. The experimental results were compared with a reference dry friction. It was found that Nitinol 60 alloy can be lubricated significantly and has shown remarkable lubrication performance. A superlubricity behavior of Nitinol 60 alloy was observed under castor oil lubrication. An ultra-low coefficient of friction of Nitinol 60 alloy about 0.008 between Nitinol 60 alloy and GCr15 steel was obtained under castor oil lubrication condition after a running-in period. Accordingly, the present study is focused on the lubrication behaviors of castor oil as potential lubrication oil for Nitinol 60 alloy. In the presence of castor oil, coefficient of friction is kept at 0.008 at steady state, corresponding to so-called superlubricity regime （when sliding is then approaching pure rolling）. The mechanism of superlubricity is attributed to the triboformed OH-terminated surfaces from friction-induced dissociation of castor oil and the boundary lubrication films formed on the contact surface due to high polarity and long chain of castor oil allowing strong interactions with the lubricated surfaces.
基金Supported by the National Natural Science Foundation of China under Grant Nos 50721004,50727007 and 51075227the Basic Research Program of Shenzhen(0021539012100521066).
文摘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.
基金Project supported by the National Key Research and Development Program of China(Grant No.2016YFA0302001)the National Natural Science Foundation of China(Grant Nos.11574204 and 11774224)
文摘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.
基金financially supported by the National Key Re-search and Development Program of China(Nos.2018YFB0703801 and 2018YFB0703802)the Foundation for Doctor Dissertation of Northwestern Polytechnical University(No.CX2021064)。
文摘Superlubricity,a novel lubricity mode ascribing to moirésuperlattice(MSL),has attracted attention in ultra-precise manufacture,microelectronic devices,and national defense areas.Based on incommensurate MSL,nearly zero friction can be achieved by eliminating sliding lock-in and offsetting lateral force in principle,and the theoretical foundations are still under extensive investigation.Here,the effects of MSL-induced lattice distortion onπbond and tribological performance in twist MoS_(2)/graphene and MoS_(2)/BN heterointerfaces were studied by first-principles calculations comprehensively.Various contributions of 2pz orbital electron polarization among AA-,AB-,and AC-stacking symmetry areas in different MSL were reflected by band structures to explain the sensitivity ofπbond to MSL.Theπbond perpendicular to the atomic plane depended closely on interfacial distortion,which can not only influence the local distribution of intralayer bond strength but also determine the interlayer charge redistribution.Meanwhile,the interfacial potential energy was changed with the interlayer interaction fluctuation caused by twist angle and atomic stacking modes.Through evaluating the energy barriers and lateral force,MoS_(2)/BN with a twist angle of 20.79°exhibited superlubricity.Moreover,the connection among sliding energy barriers,twist angles,and specific electronic structures has been bridged paving a path to reveal the superlubricity mechanism of two-dimensional materials withπbond.
基金This work is a cumulative outcome of a series of projects funded by the Air Force Office of Scientific Research(No.FA9550-08-1-0010)the National Science Foundation(Nos.CMMI-1131128,1727571,1912199,and 2315343).
文摘Hydrogenated diamond-like carbon(H-DLC)is typically produced as a coating or thin film through plasma-enhanced chemical vapor deposition(PE-CVD).H-DLC is relatively hard and well known to exhibit superlubricity.Is superlubricity an intrinsic property of H-DLC?This paper argues that H-DLC is not intrinsically superlubricious,but it has an ideal structure that allows transition of the interface region to a superlubricious structure upon frictional shear in proper conditions.Thus,its superlubricity is an extrinsic property.This argument is made by comparing frictional behaviors of three allotropes of carbon materials—graphite,amorphous carbon(a-C),and diamond,and carefully scrutinizing the run-in behavior as well as environment sensitivity of H-DLC friction.The superlubricious structure is generally known to be graphitic,but its exact structure remains elusive and is subject to further study.Nevertheless,accurate knowledge of how superlubricity is induced for H-DLC can guide engineering design to achieve superlubricious behaviors with other carbon materials produced via different synthetic routes.
基金supported by the National Natural Science Foundation of China(Nos.52222506 and 52405203)the Beijing Natural Science Foundation(Nos.3254033,3222010 and 3234057)the China Postdoctoral Science Foundation(Nos.GZB20230341 and 2024M761643)。
文摘Emerging superlubricity provides innovative scientific and engineering solutions for the sustainable future of human beings and nature.Although great progress has been made in the development of novel superlubricity systems and different mechanisms,considerable challenges remain before the engineering development of liquid superlubricity can be realized.Herein,the progress made towards achieving liquid superlubricity has been reviewed with emphasis on the current limitations,potential mechanisms,and future breakthroughs that will be expected to overcome these limitations.The perspectives are highlighted based on rigorous statistics and analyses according to the types of lubricants and materials of friction pairs.This review elucidates the key tribochemical mechanisms and research directions to break through the current limitations and provides constructive ideas for the engineering development of liquid superlubricity in the future,which will enable a sustainable future for human beings and nature.
基金the Natural Science Basic Research Program of Shaanxi(Program No.2024JC-YBQN-0466)the China Postdoctoral Science Foundation(Grant No.2023M742841)+1 种基金the Guangdong Basic and Applied Basic Research Foundation(Grant No.2022A1515111005)the Natural Science Foundation of Chongqing,China(Grant No.2023NSCQ-MSX3029).
文摘The achievement of a superlubric state with vanishing friction and negligible wear has important applications in minimizing energy dissipation and prolonging the service life of moving mechanical systems.However,the search for a superlubricious oil system applicable to industrial fields remains a major challenge.In this work,we demonstrate for the first time that precisely employing polyether modification for silicone oil molecules could induce direct superlubricity and superlow wear for engineering steel tribopairs.Superlubricity originates from the fact that polyether-modified silicone oil(PESO)can effectively employ polyether functional groups to interact with friction surfaces,during which a complex tribochemical reaction process can be induced under the catalytic role of friction,where an organic lubricious film composed mainly of carbon,silicon and oxygen can be induced in situ,which can not only effectively passivate friction surfaces but also enable superlubric sliding by virtue of its easy-to-shear nature.Furthermore,iron oxides and chromium oxides could also be confirmed to be distributed within the tribofilm,which is desirable for increasing the load-bearing capability of the tribofilm and toughness.Thus,a remarkable superlubricity of 0.01 without running-in combined with superlow wear was realized at the same time.The results of this work show high promise in promoting the industrial use of oil superlubricity and revolutionizing the development of mechanical systems.
基金financially supported by National Key R&D Program of China(Grant No.2020YFA0711003)the National Natural Science Foundation of China(Grant Nos.51925506 and 52305178)the XPLORER PRIZE.
文摘The mixed lubricant prepared by mixing 1-(4-ethylphenyl)-nonane-1,3-dione(0206)and chelate(0206-Fe)in a ratio of 4:6 had better tribological properties than 0206.In this study,it was found that the mixed lubricant prepared by mixing alkane lubricants with 0206-Fe(60%)at a ratio of 2:8 can not only achieve superlubricity,but also reduce the wear scar diameter(WSD)of the friction pairs.The mixed solution prepared by the four polar solutions with 0206-Fe(60%)cannot achieve superlubricity.The results of surface analysis and molecular dynamics(MD)simulation showed that the four polar molecules were preferentially adsorbed on the metal surface,occupying the diketone adsorption sites,and the adsorption layer formed by them cannot produce synergistic lubrication with the chelates.
基金financially supported by the National Key R&D Program of China(No.2020YFA0711003)the National Natural Science Foundation of China(Nos.52175174 and U2268212).
文摘Two-dimensional nanomaterials were commonly used as lubrication additives.However,the high demand for the load-bearing capacity limited their industrial applications.This study proposed a new strategy to overcome this limitation by depositing silver(Ag)nanoparticles on hexagonal boron nitride(hBN)nanosheets via dopamine through the reduction reaction of silver ions,and the Ag modified hBN nanosheets(hBN-Ag)were used as additives in aqueous ethylene glycol solution.The results showed that the superlubricity state with a minimum coefficient of friction(COF)of 0.004 at a maximum contact pressure of 1.20 GPa was achieved without the running-in period.The realization of superlubricity was attributed to the synergic effect of Ag nanoparticles and hBN nanosheets,where Ag nanoparticles were attached to the hBN nanosheets by dopamine,enhancing the load-bearing capacity of the hBN nanosheets.Simultaneously,the Ag modified hBN nanosheets were more easily adsorbed and deposited on the friction interface,generating the tribofilm containing hBN-Ag nanosheets through the tribochemical reaction,which reduced the direct contact of the friction pair and provided a low shear strength to generate extremely low friction.The Ag modified hBN nanosheets induced extremely low friction and wear,contributing to the development of lubricants with high loadbearing pressure and low wear rate.
基金funding from the HORIZON-EIC-2021-PATHFINDEROPEN-0 N.101046693,SSLiP project,funded by the European Union.
文摘Hypericin(C30H16O8)is a naturally occurring substance,an anthraquinone derived from St.John’s wort,possessing outstanding antiviral,antitumor,antibacterial,and antioxidant properties.Today,hypericin is primarily used in medicinal applications.It is a small,flat organic molecule with a graphene-like core surrounded by oxidized functions,suggesting it could act as a graphene precursor in tribological contacts.Therefore,we investigated the lubrication properties of hypericin as an additive in glycerol,used as a base oil.It is well established that glycerol is superlubricious under full and thin film elastohydrodynamic(EHD)lubrication regimes but generally fails with steel under more severe conditions(mixed and boundary regimes).We studied the effect of hypericin added to glycerol for steel-on-steel and steel-on-silicon friction pairs.For the steel-on-steel configuration,results show that hypericin is a strong anti-wear additive due to its antioxidant properties that scavenge OH radicals.Moreover,hypericin is also an efficient friction-reducing agent,providing a steady state and robust ultralow friction coefficient(0.02–0.03).Thus,it outperforms most traditional additive formulations under the same conditions,although it does not achieve superlubricity(coefficient of friction(CoF)<0.01)under more severe conditions.For steel-on-silicon,hypericin significantly extends the superlubricity regime of glycerol to lambda ratios well below unity(low sliding speeds).The mechanism of superlubricity is attributed to the friction-induced formation of graphene layers from hypericin molecules,smoothing friction surfaces,and operating a hybrid liquid–solid superlubricious system.
基金supported by the National Natural Science Foundation of China(Nos.52275203 and 52105194)the China Postdoctoral Science Special Funding Project(No.2022T150353)the Tribology Science Fund of the State Key Laboratory of Tribology in Advanced Equipment(Nos.SKLTKF23A03 and SKLTKF23A04).
文摘Polymers have complex molecular structures that often lead to interchain friction and hinder movement,making it difficult to achieve superlubricity.However,in the field of hydration lubrication,the electronegative interface of ceramics readily adsorbs water molecules,creating a protective water film that covers the frictional interface and effectively reduces friction.To achieve hydration lubrication,it is essential to create a continuous lubricating film by selectively enriching specific functional groups of adsorbed water molecules from the polymer solution onto the ceramic surface.By adsorbing a hydrophilic layer composed of polyvinylpyrrolidone with pyrrolidone groups onto a negatively charged Si_(3)N_(4)/sapphire interface,we formed a continuous lubricating film.Research has shown that the interaction between the polymer chain structure of polyvinylpyrrolidone molecules(such as PVP10000)in solution and water molecules could result in excellent superlubricity.When the contact pressure exceeds 198 MPa,the coefficients of friction(COF)can be reduced to 0.004-0.007.Through detailed surface analyses and sophisticated simulations,we uncovered the underlying mechanism involved.The pyrrolidone moieties of polyvinyl pyrrolidone(PVP)formed hydrogen bonds with the Si_(3)N_(4) surface,transforming the initially difficult frictional interface into a PVP/sapphire interface with significantly reduced sliding energy barriers.These findings highlight the vital role of PVP in superlubricity and hydration lubrication and provide a theoretical and experimental basis for the design of materials and lubricants with exceptional lubricating properties.
基金the National Natural Science Foundation of China(Nos.52475186 and 52305189)the National Key R&D Program of China(No.2023YFB3812104)+1 种基金the National United Engineering Laboratory for Advanced Bearing Tribology(No.202305,Henan University of Science and Technology)the Tribology Science Fund of National and Local Joint Engineering Research Center of Advanced Carbon-Based Ceramics Preparation Technology(No.2024TJZX02,North Minzu University),and the Henan Province Science and Technology Key Project(No.232102230081).
文摘At present,achieving oil-based superlubricity depends on lubricating oil viscosity and rubbing pair materials excessively.Therefore,establishing a new route is of scientific significance and application value.Here,a novel strategy,i.e.,the assembly of a dialkyl-dithiophosphoricmodified copper nanoparticle(DDP-Cu NP)lubricating additive and polyetheretherketone(PEEK)-GCr15 friction pairs,is reported to achieve oil-based superlubricity in polyalpha olefin oil(PAO40),a synthetic base oil with high viscosity.To our surprise,when the concentration of DDP-Cu is only 0.25 wt%,the lowest friction coefficient is 0.002,and the wear rate reaches 10^(-9) mm^(3)/(N·m)level(which is lower than that published in the literature by 1-2 orders).Furthermore,such superlubricity occurs without a runningin period and remains during the whole sliding process.These results are attributed mainly to the rolling effect of the spherical Cu NPs,the formation of a high-performance carbon-based tribofilm at the sliding counterface,and the in operando formation of some few(even mono)-layered graphene layers in the lubricant(the tribocatalytic effect of the DDP-Cu NPs).Specifically,DDP-Cu NPs perform in operando tribocatalysis to promote the decomposition of the PEEK matrix and PAO40 and generate some carbon species,including amorphous carbon and even graphene,all of which are responsible for the surprising oil-based superlubricity.The findings will contribute to enriching relevant tribological theory and promoting the exploration and application of oil-based superlubricity.
基金supported by the National Natural Science Foundation of China(NSFC)(Nos.62174040 and 12174423)the 13th Batch of Outstanding Young Scientific and Technological Talents Project in Guizhou Province(No.[2021]5618)+1 种基金the Science and Technology Projects of Guizhou Provincial(No.ZK[2024]501)the Scientific Research Fund of Guizhou Minzu University(No.GZMUZK[2023]CXTD07).
文摘The optical manipulation of nanoparticles on superlubricity surfaces was investigated.The research revealed that,due to the near-zero static friction and extremely low dynamic friction at superlubricity interfaces,the maximum intensity for controlling the optical field can be less than 100 W/cm^(2).The controlled nanoparticle radius can be as small as 5 nm,which is more than one order of magnitude smaller than that of nanoparticles controlled through traditional optical manipulation.Manipulation can be achieved on sub-microsecond to microsecond timescales.Furthermore,the manipulation takes place on solid surfaces and in nonliquid environments,with minimal impact from Brownian motion.By appropriately increasing the dynamic friction,controlling the light intensity,or reducing the pressure,the effects of Brownian motion can be eliminated,allowing for the construction of microstructures with a size as small as 1/75 of the wavelength of light while controlling the light intensity,which is seven orders of magnitude smaller compared to manipulating nanoparticles on traditional surfaces.This enables the control of super-resolution optical microstructures.The optical super-resolution manipulation of nanoparticles on superlubricity surfaces has important applications in fields such as nanofabrication,photolithography,optical metasurfaces,and biochemical analysis.
基金The work is financially supported by the National Key Basic Research(973)Program of China(No.2013CB934200)the National Natural Science Foundation of China(NSFC,Nos.51405256 and 51335005)China Postdoctoral Science Foundation Funded Project.
文摘Since the term“superlubricity”was put forward at the beginning of 1990s,it has become one of the hottest researches in tribology due to it being close linked to the energy problems.Recently,the International Workshop on“Superlubricity:Fundamental and Applications”was successfully held on 19-20 October 2015 in Beijing,which has attracted many researchers in this field.The recent scientific results in both solid superlubricity and liquid superlubricity have been presented according to these invited wonderful lectures and posters.In the communication,we gave an introduction to the Workshop on Superlubricity,and also summarized the new achievements of superlubricity during recent years according to these reports.Finally,the problems of superlubricity mechanism and the future development direction of superlubricity are discussed.
基金supported by the U.S.Department of Energy,Office of Energy Efficiency and Renewable Energy,under Contract No.DE-AC02-06CH11357。
文摘Superlubricity refers to a sliding regime in which contacting surfaces move over one another without generating much adhesion or friction[1].From a practical application point of view,this will be the most ideal tribological situation for many moving mechanical systems mainly because friction consumes large amounts of energy and causes greenhouse gas emissions[2].Superlubric sliding can also improve performance and durability of these systems.In this paper,we attempt to provide an overview of how controlled or targeted bulk,surface,or tribochemistry can lead to superlubricity in diamond-like carbon(DLC)films.Specifically,we show that how providing hydrogen into bulk and near surface regions as well as to sliding contact interfaces of DLC films can lead to super-low friction and wear.Incorporation of hydrogen into bulk DLC or near surface regions can be done during deposition or through hydrogen plasma treatment after the deposition.Hydrogen can also be fed into the sliding contact interfaces of DLCs during tribological testing to reduce friction.Due to favorable tribochemical interactions,these interfaces become very rich in hydrogen and thus provide super-low friction after a brief run-in period.Regardless of the method used,when sliding surfaces of DLC films are enriched in hydrogen,they then provide some of the lowest friction coefficients(i.e.,down to 0.001).Time-of-flight secondary ion mass spectrometer(TOF-SIMS)is used to gather evidence on the extent and nature of tribochemical interactions with hydrogen.Based on the tribological and surface analytical findings,we provide a mechanistic model for the critical role of hydrogen on superlubricity of DLC films.
基金the National Natural Science Foundation of China(51905294,51527901,and 51875303)the China Postdoc Innovation Talent Support Program(BX20180168)the China Postdoctoral Science Foundation(2019M650654)。
文摘The topic of superlubricity is attracting considerable interest around the world while humanity is facing an energy crisis.Since various liquid superlubricity systems can be commonly achieved on the macroscale in ambient conditions,it is considered an effective solution to reduce unnecessary energy and material losses.However,certain practical problems such as low load-bearing pressure,dependence on hydrogen ions,and relatively long running-in processes still limit its widespread application.Two-dimensional(2D)nano-additives with ultrathin longitudinal dimensions can lower the shear resistance between sliding solid surfaces,and thus further optimize the applied conditions.In this review,the latest studies on 2D nano-additives with a combination of various water-based lubricants in the state of superlubricity are reported,typically including black phosphorus(BP),graphene oxide(GO),and layered double hydroxide.During the sliding process,composite lubricants effectively improved the load capacity(up to 600 MPa),reduced wear,and accelerated the running-in period(within 1,000 s)of the liquid superlubricity system.Both macromechanical experiments and microscopic tests are conducted to precisely analyze various interactions at the interfaces of the nano-additives and solid surfaces.These interactions can be described as tribochemical reactions,physical protection,and adsorption enhancement,and improved wear resistance.This review provides better guidance for applying 2D nanomaterials in liquid superlubricity systems.
基金supported by the National Key Basic Research and Development Program(973)of China(2013CB934200)Foundation for the Supervisor of Beijing Excellent Doctoral Dissertation(20111000305)the National Natural Science Foundation of China(NSFC)(Nos.51321092,51027007).
文摘In this study,we address the superlubricity behavior of sapphire against ruby(or sapphire against itself)under phosphoric acid solution lubrication.An ultra-low friction coefficient of 0.004 was obtained under a very high contact pressure,with a virgin contact pressure up to 2.57 GPa.Related experiments have indicated that the load,sliding speed,and humidity of the test environment can affect superlubricity to some degree,so we tested variations in these conditions.When superlubricity appears in this study a thin film is present,consisting of a hydrogen bond network of phosphoric acid and water molecules adsorbed on the two friction surfaces,which accounts for the ultra-low friction.Most significantly,the wear rate of the sapphire and ruby in the friction process is very slow and the superlubricity state is very stable,providing favorable conditions for future technological applications.
基金financially supported by National Natural Science Foundation of China (Nos. 51775295, 51405256, and 51527901)
文摘In thin-film lubrication(TFL), generally, the viscosity of the lubricant and its coefficient of friction(Co F) increase. Finding a method to reduce the Co F in TFL is a significant challenge for tribologists. In the present work, we report a robust superlubricity attained by using polyalkylene glycols(PAGs, polar molecules) and poly-α-olefins(PAOs, nonpolar molecules) as lubricants on steel/steel friction pairs that have been pre-treated by wearing-in with polyethylene glycol aqueous solution(PEG(aq)). A steady superlubricity state with a Co F of 0.0045 for PAG100 and 0.006 for PAO6 could be maintained for at least 1 h. Various affecting factors, including the sliding velocity, normal load, and viscosity of the lubricants, were investigated. Element analysis proved that composite tribochemical layers were deposited on the worn region after the treatment with PEG(aq). These layers were formed by the tribochemical reactions between PEG and steel and composed of various substances including oxides, iron oxides, Fe OOH, and Fe(OH)3, which contributed to the superlubricity. In addition to the tribochemical layers, ordered layers and a fluid layer were formed by the PAGs and PAOs during the superlubricity periods. All the three types of layers contributed to the superlubricity, indicating that it was attained in the TFL regime. Accordingly, a mechanism was proposed for the superlubricity of the PAGs and PAOs in the TFL regime in this work. This study will increase the scientific understanding of the superlubricity in the TFL regime and reveal, in the future, the potential for designing superlubricity systems on steel surfaces for industrial applications.
基金the National Natural Science Foundation of China(U21A2046,51905518)the Program for Taishan Scholars of Shandong Province(TS20190965)+4 种基金the National Key R&D Program of China(2020YFF0304600)the Innovation Leading Talents Program of Qingdao(19-3-2-23-zhc)in Chinathe Key Research Program of the Chinese Academy of Sciences(XDPB24)the Western Light Project of CAS(xbzg-zdsys-202118)the LICP Cooperation Foundation for Young Scholars(HZJJ21-03)for providing financial support.
文摘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.
基金supported by the National Key Basic Research(973)Program of China(No.2013CB934200)the National Natural Science Foundation of China(Nos.51222507,51335005,51321092).
文摘In this work,a super-low friction coefficient of 0.003 was found between a silicon nitride ball and a sapphire plate lubricated by phosphoric acid solution.The wear mainly occurred in the running-in period and disappeared after superlubricity was achieved.The friction coefficient was effectively reduced from 0.3 to 0.003 at a constant speed of 0.076 m/s,accompanied by a 12-nm-thickness film.The lubrication regime was indicated to change from boundary lubrication in the running-in period to elastohydrodynamic lubrication in the superlubricity period,which is also supported by the results of the friction coefficient versus sliding speed.In addition,the experimental results showed good agreement with theoretical calculations based on the elastohydrodynamic lubrication theory,suggesting a significant hydrodynamic effect of phosphoric acid on superlubricity.
