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
The continuous reduction in sulfur content of fuels would lead to diesel fuel with poor lubricity which could re- sult in engine pump failure. In the present work, fatty acids were adopted as lubricity additives to lo...The continuous reduction in sulfur content of fuels would lead to diesel fuel with poor lubricity which could re- sult in engine pump failure. In the present work, fatty acids were adopted as lubricity additives to low-sulfur diesel fuel. It was attempted to correlate the molecular structures of fatty acids, such as carbon chain length, degree of saturation and hy- droxylation, to their lubricity enhancement, which was evaluated by the High-Frequency Reciprocating Rig （HFRR） meth- od. The efficiency order was supported by the density functional theory （DFT） calculations and the molecular dynamics （MD） simulations. The lubricity enhancing properties of fatty acids are mainly determined by the cohesive energy of adsorbed films furmed on iron surface. The greater the cohesive energy, the more efficiently the fatty acid would enhance the lubricity of low-sulfur diesel fuel.展开更多
In this work,fatty acid and its derivatives were adopted as lubricity additives for low sulfur diesel.Tribological evaluation obtained from the High-Frequency Reciprocating Rig(HFRR)apparatus showed that the lubricati...In this work,fatty acid and its derivatives were adopted as lubricity additives for low sulfur diesel.Tribological evaluation obtained from the High-Frequency Reciprocating Rig(HFRR)apparatus showed that the lubricating performance of the additives increased in the following order:stearic acid>glycol monopalmitate>stearyl alcohol>ethyl palmitate>cetyl ethyl ether.The adsorption behavior of the additives on Fe(110)surface and Fe2O3(001)surface was investigated by molecular dynamics(MD)simulations to verify their lubricity performance.The results suggested that adsorption energies of the additives on Fe(110)surface are determined by the van der Waals forces,while adsorptions on Fe2O3(001)surface are significantly attributed to the electrostatic attractive forces.Higher values of adsorption energy of the additives on Fe2O3(001)surface indicate that the additive has more efficient lubricity enhancing properties.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
Since the 1990 s, the kerosene fuel(code: JP-8) had been applied in the ground equipment provided with direct injection compression ignition engines in the U.S. Army, resulting in increased occurrence of injection pum...Since the 1990 s, the kerosene fuel(code: JP-8) had been applied in the ground equipment provided with direct injection compression ignition engines in the U.S. Army, resulting in increased occurrence of injection pump failures. Anti-wear additives must be used in the single fuel due to its poor lubricity. In the present work, lubricity improvers were selected on the basis of molecular simulation theoretically and these agents were evaluated to improve the lubricity of jet fuel using the high frequency reciprocating rig(HFRR) apparatus and the ball-on-cylinder lubricity evaluator(BOCLE). It was revealed that dimer acid with higher value of adsorption energy on the Fe(110) plane surface had more efficient lubricity promoting properties than that of naphthenic acid. The experimental results suggested that the dimer acid had a better tribological behavior compared with that of naphthenic acid used as lubricity improver of jet fuel. And addition of anti-wear additives at a dosage of 15 μg/g was able to promote the lubricity of jet fuel to a required level on BOCLE, while a higher concentration over 80 μg/g was needed to improve the lubricity to a demanded value of diesel on HFRR.展开更多
The lubricities of fifteen rare earth compounds of lanthanum, praseodymium, samarium, europium and gadolinium are evaluated on a four ball tester and compared with that of ZDDP(Zinc Dialkyldithiophosphate). Results s...The lubricities of fifteen rare earth compounds of lanthanum, praseodymium, samarium, europium and gadolinium are evaluated on a four ball tester and compared with that of ZDDP(Zinc Dialkyldithiophosphate). Results show that some of rare earth compounds provide much better lubricities than ZDDP, and the hexagonal type of rare earth metal compounds exhibit better lubricites than the body centered cubic type of metal compounds.展开更多
Graphite nanoplatelets were prepared by a novel magnetic-grinding method using self-made equipments. Under a variant magnetic field, magnetic needles collided at a high rotating speed and exfoliated pristine graphite ...Graphite nanoplatelets were prepared by a novel magnetic-grinding method using self-made equipments. Under a variant magnetic field, magnetic needles collided at a high rotating speed and exfoliated pristine graphite into graphite nanoplatelets with high efficiency. The obtained graphite nanoplatelets are highly crystalline, and the thickness is less than 10 nm. Moreover, the surface area could reached 738.1 m^2/g with a grinding time of 4 h. Silanized graphite nanoplatelets can disperse well in SG 15W-40 engine oil and serve as lubricant additive. Tribological results indicate that the friction coefficient and wear-scar of the friction pairs are lower than 76% and 41%, respectively, by adding 1.5‰(mass fraction) of silanized graphite nanoplatelets. Notably, the functionalized graphite nanoplatelets can realize large-scale production and commercial application.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
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
Lithography-based techniques for cross-scale nanopore array fabrication are expensive,and the design of nanoporous structures relative to wettability or lubricity is intricate.This study presents a low-cost strategy i...Lithography-based techniques for cross-scale nanopore array fabrication are expensive,and the design of nanoporous structures relative to wettability or lubricity is intricate.This study presents a low-cost strategy integrating the anodization and pore-widening processes to successfully fabricate anodic aluminum oxide(AAO)nanoporous structures with pore diameters ranging from 40 to 330 nm and interpore distances ranging from 60 to 335 nm.This study reveals the synergistic regulation mechanism of pore characteristics on surface wettability and lubricity.Results showed that both pore diameter and interpore distance increase with applied voltage,and a minimum pore diameter of 40 nm achieves a low voltage(i.e.,40 V)and short anodization time.The pore-widening process significantly enhances pore uniformity.The AAO surface exhibits excellent lubricating properties with a minimum coefficient of friction of 0.05 after pore widening.In addition,the wettability of AAO can be precisely controlled by adjusting pore diameter and porosity.A small pore diameter of 40 nm exhibits hydrophobicity(contact angle of approximately 100°).The pore-widening process helps enhance the hydrophobicity of the AAO surface,where an appropriate pore diameter of 100 nm exhibits near superhydrophobicity(contact angle of approximately 141°).However,a large pore diameter of 330 nm exhibits superhydrophilicity(contact angle of approximately 4°).This study provides valuable insights into understanding the interaction between the structural characteristics and the surface properties of AAO,contributing to the development of high-performance nanopore array templates.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
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
The transition from atomic stick-slip to continuous sliding has been observed in a number of ways.If extended contacts are moved in different directions,so-called structural lubricity is observed when the two surface ...The transition from atomic stick-slip to continuous sliding has been observed in a number of ways.If extended contacts are moved in different directions,so-called structural lubricity is observed when the two surface lattices are non-matching.Alternatively,a“superlubric”state of motion can be achieved if the normal force is reduced below a certain threshold,the temperature is increased,or the contact is actuated mechanically.These processes have been partially demonstrated using atomic force microscopy,and they can be theoretically understood by proper modifications of the Prandtl−Tomlinson model.展开更多
Artificial joint cartilage materials are central to arthroplasty for the treatment of osteoarthritis.Hydrogels are highly promising materials for fabricating artificial cartilage owing to their excellent biocompatibil...Artificial joint cartilage materials are central to arthroplasty for the treatment of osteoarthritis.Hydrogels are highly promising materials for fabricating artificial cartilage owing to their excellent biocompatibility and lubricity.Inspired by natural articular cartilage,in this study,we designed a modification strategy to enhance the lubricity of double-network(DN)hydrogels.Specifically,two lubricating substances,nonionic surfactant Tween 80 and hydrogenated soybean phosphatidylcholine(HSPC),were incorporated into a DN hydrogel.Lubricity-enhanced DN hydrogel exhibited superlubricity through the synergistic effect of Tween 80 and HSPC,with a low coefficient of friction of 0.008,which remained stable after 6 h of continuous tribological testing.In addition,the mechanical properties of lubricity-enhanced DN hydrogel were greater than those of unmodified DN hydrogel,with a 29%increase in fracture strain and a 1.7-fold increase in toughness.Tween 80 micelles reinforced the physically cross-linked network through hydrogen bonding with the DN hydrogel,whereas HSPC vesicles encapsulated in the polymer network served as reinforcement nodes to enhance the chemically cross-linked network.As a result,lubricity-enhanced DN hydrogel exhibited both excellent lubricity and mechanical properties.This study demonstrates an innovative way to design hydrogels exhibiting both superlubricity and excellent mechanical properties,broadening the applications of DN hydrogels in the field of artificial joint cartilage.展开更多
The term“structural lubricity”denotes a fundamental concept where the friction between two atomically flat surfaces is reduced due to lattice mismatch at the interface.Under favorable circumstances,its effect may ca...The term“structural lubricity”denotes a fundamental concept where the friction between two atomically flat surfaces is reduced due to lattice mismatch at the interface.Under favorable circumstances,its effect may cause a contact to experience ultra-low friction,which is why it is also referred to as“superlubricity”.While the basic principle is intriguingly simple,the experimental analysis of structural lubricity has been challenging.One of the main reasons for this predicament is that the tool most frequently used in nanotribology,the friction force microscope,is not well suited to analyse the friction of extended nanocontacts.To overcome this deficiency,substantial efforts have been directed in recent years towards establishing nanoparticle manipulation techniques,where the friction of nanoparticles sliding on a substrate is measured,as an alternative approach to nanotribological research.By choosing appropriate nanoparticles and substrates,interfaces exhibiting the characteristics needed for the occurrence of structural lubricity can be created.As a consequence,nanoparticle manipulation experiments such as in this review represent a unique opportunity to study the physical conditions and processes necessary to establish structural lubricity,thereby opening a path to exploit this effect in technological applications.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
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
To reduce their fuel related logistic burden,North Atlantic Treaty Organization(NATO)Armed Forces are advancing the use of a single fuel for both aircraft and ground equipment.To this end,F-34(the commercial equivalen...To reduce their fuel related logistic burden,North Atlantic Treaty Organization(NATO)Armed Forces are advancing the use of a single fuel for both aircraft and ground equipment.To this end,F-34(the commercial equivalent is Jet A-1)is replacing distillate diesel fuel in many applications.However,tests conducted with this kerosene type on high frequency reciprocating rig showed that this type of fuel causes unacceptable wear.This excessive wear is caused by the poor lubricity of aviation fuel.In order to make this type of fuel compatible with direct injection compression engines,seven di-carboxylic acid esters have tested to improve the lubricity of kerosene.Tribological results showed that all esters tested in this series of experiments seem to be suitable for increasing the kerosene lubricity to a satisfactory level.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
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 Fundamental Research Funds for the Central Universities of China(11CX06036A)
文摘The continuous reduction in sulfur content of fuels would lead to diesel fuel with poor lubricity which could re- sult in engine pump failure. In the present work, fatty acids were adopted as lubricity additives to low-sulfur diesel fuel. It was attempted to correlate the molecular structures of fatty acids, such as carbon chain length, degree of saturation and hy- droxylation, to their lubricity enhancement, which was evaluated by the High-Frequency Reciprocating Rig （HFRR） meth- od. The efficiency order was supported by the density functional theory （DFT） calculations and the molecular dynamics （MD） simulations. The lubricity enhancing properties of fatty acids are mainly determined by the cohesive energy of adsorbed films furmed on iron surface. The greater the cohesive energy, the more efficiently the fatty acid would enhance the lubricity of low-sulfur diesel fuel.
基金financially supported by "the Fundamental Research Funds for the Central Universities,China"(11CX06036A)
文摘In this work,fatty acid and its derivatives were adopted as lubricity additives for low sulfur diesel.Tribological evaluation obtained from the High-Frequency Reciprocating Rig(HFRR)apparatus showed that the lubricating performance of the additives increased in the following order:stearic acid>glycol monopalmitate>stearyl alcohol>ethyl palmitate>cetyl ethyl ether.The adsorption behavior of the additives on Fe(110)surface and Fe2O3(001)surface was investigated by molecular dynamics(MD)simulations to verify their lubricity performance.The results suggested that adsorption energies of the additives on Fe(110)surface are determined by the van der Waals forces,while adsorptions on Fe2O3(001)surface are significantly attributed to the electrostatic attractive forces.Higher values of adsorption energy of the additives on Fe2O3(001)surface indicate that the additive has more efficient lubricity enhancing properties.
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.
文摘Since the 1990 s, the kerosene fuel(code: JP-8) had been applied in the ground equipment provided with direct injection compression ignition engines in the U.S. Army, resulting in increased occurrence of injection pump failures. Anti-wear additives must be used in the single fuel due to its poor lubricity. In the present work, lubricity improvers were selected on the basis of molecular simulation theoretically and these agents were evaluated to improve the lubricity of jet fuel using the high frequency reciprocating rig(HFRR) apparatus and the ball-on-cylinder lubricity evaluator(BOCLE). It was revealed that dimer acid with higher value of adsorption energy on the Fe(110) plane surface had more efficient lubricity promoting properties than that of naphthenic acid. The experimental results suggested that the dimer acid had a better tribological behavior compared with that of naphthenic acid used as lubricity improver of jet fuel. And addition of anti-wear additives at a dosage of 15 μg/g was able to promote the lubricity of jet fuel to a required level on BOCLE, while a higher concentration over 80 μg/g was needed to improve the lubricity to a demanded value of diesel on HFRR.
文摘The lubricities of fifteen rare earth compounds of lanthanum, praseodymium, samarium, europium and gadolinium are evaluated on a four ball tester and compared with that of ZDDP(Zinc Dialkyldithiophosphate). Results show that some of rare earth compounds provide much better lubricities than ZDDP, and the hexagonal type of rare earth metal compounds exhibit better lubricites than the body centered cubic type of metal compounds.
基金Project(ZR2011BL005)supported by the Natural Science Foundation of Shandong Province,China
文摘Graphite nanoplatelets were prepared by a novel magnetic-grinding method using self-made equipments. Under a variant magnetic field, magnetic needles collided at a high rotating speed and exfoliated pristine graphite into graphite nanoplatelets with high efficiency. The obtained graphite nanoplatelets are highly crystalline, and the thickness is less than 10 nm. Moreover, the surface area could reached 738.1 m^2/g with a grinding time of 4 h. Silanized graphite nanoplatelets can disperse well in SG 15W-40 engine oil and serve as lubricant additive. Tribological results indicate that the friction coefficient and wear-scar of the friction pairs are lower than 76% and 41%, respectively, by adding 1.5‰(mass fraction) of silanized graphite nanoplatelets. Notably, the functionalized graphite nanoplatelets can realize large-scale production and commercial application.
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.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support from the National Natural Science Foundation of China(Grant Nos.52405447 and 52275299)the Key Research and Development Program of Jiangxi Province in China(Grant No.20232BBE50011)+1 种基金the National Key Research and Development Program of China(Grant No.2021YFB1716200)the Research Funds for Leading Talents Program(Grant Nos.048000514123686 and 048000514123581).
文摘Lithography-based techniques for cross-scale nanopore array fabrication are expensive,and the design of nanoporous structures relative to wettability or lubricity is intricate.This study presents a low-cost strategy integrating the anodization and pore-widening processes to successfully fabricate anodic aluminum oxide(AAO)nanoporous structures with pore diameters ranging from 40 to 330 nm and interpore distances ranging from 60 to 335 nm.This study reveals the synergistic regulation mechanism of pore characteristics on surface wettability and lubricity.Results showed that both pore diameter and interpore distance increase with applied voltage,and a minimum pore diameter of 40 nm achieves a low voltage(i.e.,40 V)and short anodization time.The pore-widening process significantly enhances pore uniformity.The AAO surface exhibits excellent lubricating properties with a minimum coefficient of friction of 0.05 after pore widening.In addition,the wettability of AAO can be precisely controlled by adjusting pore diameter and porosity.A small pore diameter of 40 nm exhibits hydrophobicity(contact angle of approximately 100°).The pore-widening process helps enhance the hydrophobicity of the AAO surface,where an appropriate pore diameter of 100 nm exhibits near superhydrophobicity(contact angle of approximately 141°).However,a large pore diameter of 330 nm exhibits superhydrophilicity(contact angle of approximately 4°).This study provides valuable insights into understanding the interaction between the structural characteristics and the surface properties of AAO,contributing to the development of high-performance nanopore array templates.
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E.M.acknowledges financial support by the Swiss National Science Foundation(SNF)the Commission for Technology and Innovation(CTI),COST Action MP1303 and the Swiss Nanoscience Institute(SNI).E.G.acknowledges the Spanish Ministry of Economy and Competitiveness(MINECO)Project MAT2012-26312.
文摘The transition from atomic stick-slip to continuous sliding has been observed in a number of ways.If extended contacts are moved in different directions,so-called structural lubricity is observed when the two surface lattices are non-matching.Alternatively,a“superlubric”state of motion can be achieved if the normal force is reduced below a certain threshold,the temperature is increased,or the contact is actuated mechanically.These processes have been partially demonstrated using atomic force microscopy,and they can be theoretically understood by proper modifications of the Prandtl−Tomlinson model.
基金supported by the National Natural Science Foundation of China(52350323)the Guangdong Basic Research Foundation(2024A1515012406)+1 种基金the Guangdong Applied Basic Research Foundation(2023A1515110565)the Tribology Science Fund of State Key Laboratory of Tribology in Advanced Equipment(SKLTKF22B13).
文摘Artificial joint cartilage materials are central to arthroplasty for the treatment of osteoarthritis.Hydrogels are highly promising materials for fabricating artificial cartilage owing to their excellent biocompatibility and lubricity.Inspired by natural articular cartilage,in this study,we designed a modification strategy to enhance the lubricity of double-network(DN)hydrogels.Specifically,two lubricating substances,nonionic surfactant Tween 80 and hydrogenated soybean phosphatidylcholine(HSPC),were incorporated into a DN hydrogel.Lubricity-enhanced DN hydrogel exhibited superlubricity through the synergistic effect of Tween 80 and HSPC,with a low coefficient of friction of 0.008,which remained stable after 6 h of continuous tribological testing.In addition,the mechanical properties of lubricity-enhanced DN hydrogel were greater than those of unmodified DN hydrogel,with a 29%increase in fracture strain and a 1.7-fold increase in toughness.Tween 80 micelles reinforced the physically cross-linked network through hydrogen bonding with the DN hydrogel,whereas HSPC vesicles encapsulated in the polymer network served as reinforcement nodes to enhance the chemically cross-linked network.As a result,lubricity-enhanced DN hydrogel exhibited both excellent lubricity and mechanical properties.This study demonstrates an innovative way to design hydrogels exhibiting both superlubricity and excellent mechanical properties,broadening the applications of DN hydrogels in the field of artificial joint cartilage.
基金Financial support was provided by the DFG(Project SCHI 619/8-1)the EUROCORES program FANAS of the European Science Foundation,and the EC 6th framework program(Grant No.ERAS-CT-2003-980409)U.S.acknowledges primary financial support by the National Science Foundation through the Yale Materials Research Science and Engineering Center(Grant No.MRSEC DMR-1119826).
文摘The term“structural lubricity”denotes a fundamental concept where the friction between two atomically flat surfaces is reduced due to lattice mismatch at the interface.Under favorable circumstances,its effect may cause a contact to experience ultra-low friction,which is why it is also referred to as“superlubricity”.While the basic principle is intriguingly simple,the experimental analysis of structural lubricity has been challenging.One of the main reasons for this predicament is that the tool most frequently used in nanotribology,the friction force microscope,is not well suited to analyse the friction of extended nanocontacts.To overcome this deficiency,substantial efforts have been directed in recent years towards establishing nanoparticle manipulation techniques,where the friction of nanoparticles sliding on a substrate is measured,as an alternative approach to nanotribological research.By choosing appropriate nanoparticles and substrates,interfaces exhibiting the characteristics needed for the occurrence of structural lubricity can be created.As a consequence,nanoparticle manipulation experiments such as in this review represent a unique opportunity to study the physical conditions and processes necessary to establish structural lubricity,thereby opening a path to exploit this effect in technological applications.
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.
文摘To reduce their fuel related logistic burden,North Atlantic Treaty Organization(NATO)Armed Forces are advancing the use of a single fuel for both aircraft and ground equipment.To this end,F-34(the commercial equivalent is Jet A-1)is replacing distillate diesel fuel in many applications.However,tests conducted with this kerosene type on high frequency reciprocating rig showed that this type of fuel causes unacceptable wear.This excessive wear is caused by the poor lubricity of aviation fuel.In order to make this type of fuel compatible with direct injection compression engines,seven di-carboxylic acid esters have tested to improve the lubricity of kerosene.Tribological results showed that all esters tested in this series of experiments seem to be suitable for increasing the kerosene lubricity to a satisfactory level.
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.
