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Stable graphene oxide-based lyotropic liquid crystals for interfacial lubrication

Friction 2024年第5期12卷 954-967页

作者： Yumei GUO Hanglin LI Jiusheng LI Xiangqiong ZENG Laboratory for Advanced Lubricating Materials Shanghai Advanced Research InstituteChinese Academy of SciencesShanghai201210China University of Chinese Academy of Sciences Beijing100049China

Lyotropic liquid crystals have lubricating properties due to their ordered assembly and fluidity,whose mesogens are often characterized by amphiphilic properties.Despite the attention that graphene oxide(GO)has been studied as a novel amphiphilic lyotropic mesogen this decade,and GO applied as a lubrication additive has been demonstrated in both oil and water-based systems,little research reveals the interfacial lubrication of GO liquid crystals yet.This work reports that GO aqueous dispersion can form lyotropic liquid crystals above a specific critical concentration of 5.00 mg/mL,providing a form of stable water-based lubricant,which can keep stable for several months and can reduce friction by 37.3%and wear by 25.24%.The liquid crystal phase was verified by polarizing microscope and synchrotron radiation small-angle X-ray scattering,and its rheological properties and viscoelasticity were studied by interfacial rheometer.The formation of lyotropic liquid crystals can enhance the stability of GO aqueous dispersions at high density,simultaneously ensuring friction decrease and anti-wear effect.It is attributed to the stable nematic network by the ordered GO sheets.The ordered assembly structure bears vertical shear force,therefore,reducing the wear.It is also assumed that the wide lateral size of graphene oxide promotes the nematic phase thus smoothes the graphene oxide film composed spontaneously under the coincidence of lamellar liquid crystal and 2D layered material.Through this work,the interlayer lubrication of GO was optimized,and the problem of GO dispersion sedimentation was solved by self-assembly.The range of interfacial lubrication of GO aqueous dispersion has been expanded and the synergistic effect is conducive to the environmentally friendly lubricants.

关键词： lyotropic liquid crystal graphene oxide aqueous lubrication tribological behavior

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MoS_(2) reinforced PEEK composite for improved aqueous boundary lubrication

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Friction 2023年第9期11卷 1660-1672页

作者： Xin HOU Pengpeng BAI Jingyang LI Yuanzhe LI Hui CAO Xiangli WEN Yonggang MENG Liran MA Yu TIAN State Key Laboratory of Tribology in Advanced Equipment Department of Mechanical EngineeringTsinghua UniversityBeijing 100084China

Polyether-etherketone(PEEK)is a corrosion-resistant material that has been widely used in aqueous lubrication.However,its anti-wear performance must be improved for its application in the industry.In this study,to improve the anti-wear performance of PEEK for aqueous boundary lubrication,PEEK/MoS_(2)composites were prepared by ball-milling and spark plasma sintering processes.A competitive MoS_(2)mechanism between the low shear strength property and the role of promoting wear debris generation influences the anti-wear performance of PEEK/MoS_(2)composites.Experiments demonstrated that the coefficients of friction(COF)and wear rate of PEEK composite with 0.25 wt%MoS_(2)were significantly reduced 68%and 94%,respectively.Furthermore,this was the first time that a PEEK composite could achieve a COF of less than 0.05 in aqueous boundary lubrication.Its anti-wear performance was verified to be better than that of PEEK/carbon fiber(CF)and Thordon composites.The PEEK/MoS_(2)composite may be a potential material for underwater equipment because of its outstanding anti-wear performance in aqueous boundary lubrication.

关键词： polyether-etherketone(PEEK)composite MoS_(2) anti-wear performance aqueous lubrication boundary lubrication

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aqueous lubrication and surface microstructures of engineering polymer materials (PEEK and PI) when sliding against Si3N4

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Friction 2017年第4期5卷 414-428页

作者： Anying WANG Shuai YAN Bin LIN Xiaofeng ZHANG Xiaoxue ZHOU Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education Tianjin UniversityTianjin 300350China

Polyether-ether-ketone (PEEK) and polyimide (PI) are two kinds of engineering polymer materials widely used as roller bearing cages and rings under extreme environment because of their noise reduction and corrosion resistance properties. The Si3N4 ceramic is the most common ball bearing material. Many current engineering applications of ball bearings require aqueous lubrication. Therefore, this study presents the aqueous lubrication of tribopairs formed by PEEK and PI material sliding against Si3N4 ceramic. Experimental results show that two tribopairs exhibited the similar tribological properties under the dry condition. Water as a lubricant for the PI–Si3N4 tribopair pairs effectively reduces both friction coefficients by 35.5% and wear rates by 32%. The water absorption of PI induces better tribological properties by changing the tribopair surface properties. In addition, the dimples appearing on the PI tribopair surface under water generate additional hydrodynamic lubrication and further improve the friction properties of surface. The PEEK–Si3N4 tribopair shows similar friction coefficients under two kinds of environments. The wear rates under water are approximately more than two times of that under dry sliding. However, water inhibits the appearance of the crush phenomenon and enhances the carrying capacity of the tribopair. Energy dispersive spectroscopy and X-ray diffraction spectra demonstrate no chemical corrosion. The 3D profiler and SEM morphologies illustrate that the transfer film would be formed from the surface of PEEK under water but hindered under dry friction. Overall, the PI–Si3N4 tribopair exhibits better properties than PEEK under water and is promising for future applications in the bearing industry.

关键词： PEEK PI aqueous lubrication dry friction tribological properties ceramic bearing

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Synovial fluid-inspired biomimetic lubricating microspheres:Zwitterionic polyelectrolyte brushes-grafted microgels

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Friction 2023年第6期11卷 938-948页

作者： Guoqiang LIU Yang FENG Xiaohua GAO Zhuo CHEN Nan ZHAO Feng ZHOU Weimin LIU Center of Advanced lubrication and Seal Materials State Key Laboratory of Solidification ProcessingNorthwestern Polytechnical UniversityXi’an710072China State Key Laboratory of Solid lubrication Lanzhou Institute of Chemical PhysicsChinese Academy of SciencesLanzhou730000China

Synovial fluid is made up of various biomacromolecules,including hyaluronic acid,aggrecans,lubricins,and phosphatidylcholine lipid,which are assembled onto the surface of articular cartilage in a gel state.Among them,brush-like biomacromolecules or assemblies have a vital effect on human joint lubrication.Inspired by this,the combination of brush-like molecular structures and gel-like assembly may be an efficient approach for the synthesis of biomimetic lubricating matters.Learning from the lubrication system of human joints,poly(2-methacryloyloxyethyl phosphorylcholine)(PMPC)brushes grafted poly(N-isopropylacrylamide-co-acrylic acid)(poly(NIPAAm-co-AA))microgels,abbreviated as MBs-g-MGs,were synthesized as one kind of biomimetic lubricating additives.It is worth noting that this bionic strategy considered both molecular structure and assembled form,which enabled this hairy microgel to achieve low friction in aqueous medium.Meanwhile,the effective lubrication was still achieved when using MBs-g-MGs at high temperature,indicating that this microgel maintains a good lubricating effect over a wide range of temperature.In addition,this kind of microgel possessed good biocompatibility,which laid the foundation for potential biomedical applications.Looking beyond,these biomimetic microgels may provide an effective lubricating effect for water-based sliding interfaces,especially in biomedical systems.

关键词： biomimetic design zwitterionic polyelectrolyte brushes polymeric microgels aqueous lubrication

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Tribological behavior of thermal- and pH-sensitive microgels under steel/CoCrMo alloy contacts

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Friction 2023年第4期11卷 602-616页

作者： Kequn SUN Yuanhua HU Yinghui DONG Lulu YAO Ruhong SONG Yufu XU Institute of Tribology School of Mechanical EngineeringHefei University of TechnologyHefei 230009China School of Chemistry and Chemical Engineering Hefei University of TechnologyHefei 230009China

The tribological behavior of 316L stainless steel/CoCrMo alloy contacts under aqueous lubrication was investigated in this work.Three types of microgels including poly(N-isopropylacrylamide)(PNIPAM),polyacrylic acid(PAA),and poly(N-isopropylacrylamide-co-acrylic acid)(PNIPAM-co-PAA)were prepared and used as lubricant additives in aqueous solutions.Tribological tests were conducted using a ball-on-disk reciprocating tribometer,over a range of temperatures,pH,and concentrations of the microgels.The PNIPAM-co-PAA microgels were proven to have excellent anti-friction and anti-wear properties,depending on the temperature and pH values of the solutions.The maximum friction coefficient coincided with the lower critical solution temperature of the PNIPAM-co-PAA microgels at 32°C.At the same time,the friction coefficient increased with the increase in pH value of the lubricant solution with the PNIPAM-co-PAA microgels.The unique thermal-and pH-sensitive properties of the PNIPAM-co-PAA microgels provided a new strategy for controlling the friction and wear of steel/CoCrMo alloy contacts under aqueous lubrication.

关键词： poly(N-isopropylacrylamide-co-acrylic acid)(PNIPAM-co-PAA) microgel aqueous lubrication thermal-sensitive pH-sensitive

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Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures

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Friction 2022年第3期10卷 360-373页

作者： Wenrui LIU Rok SIMIČ Yuhong LIU Nicholas DSPENCER State Key Laboratory of Tribology Tsinghua UniversityBeijing 100084China Laboratory for Surface Science and Technology Department of MaterialsETH Zurich8093 ZurichSwitzerland

Polyacrylamide(PAAm)hydrogels with brush-covered or crosslinked surfaces were produced and their tribological behavior was studied over a wide range of sliding speeds for two different contact geometries:sphere-on-flat and flat-pin-on-flat.Irrespective of the contact geometry,the brushy hydrogel surfaces displayed up to an order of magnitude lower coefficients of frictionμ(COF)compared to the crosslinked surfaces,even achieving superlubricity(μ<0.01).In general,a hydrogel sphere showed a lower coefficient of friction than a flat hydrogel pin at a similar contact pressure over the entire range of sliding speeds.However,after normalizing the friction force by the contact area,the shear stress of hydrogels with either crosslinked or brushy surfaces was found to be similar for both contact geometries at low speeds,indicating that hydrogel friction is unaffected by the contact geometry at these speeds.At high sliding speeds,the shear stress was found to be lower for a sphere-on-flat configuration compared to a flat-pin-on-flat configuration.This can be attributed to the larger equivalent hydrodynamic thickness due to the convergent inlet zone ahead of the sphere-on-flat contact,which presumably enhances the water supply in the contact,promotes rehydration,and thus reduces the friction at high sliding speeds compared to that measured for the flat-pin-on-flat contact.

关键词： hydrogels contact geometry aqueous lubrication friction coefficient(COF) superlubricity

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Formulation of lyotropic liquid crystal emulsion based on natural sucrose ester and its tribological behavior as novel lubricant

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Friction 2022年第11期10卷 1879-1892页

作者： Yumei GUO Jiusheng LI Xiaojie ZHOU Yuzhao TANG Xiangqiong ZENG Laboratory for Advanced Lubricating Materials Shanghai Advanced Research InstituteChinese Academy of SciencesShanghai 201210China University of Chinese Academy of Sciences Beijing 100049China National Facility for Protein Science in Shanghai Shanghai Advanced Research InstituteChinese Academy of SciencesShanghai 201210China

The tribological behavior of oil-in-water emulsions formulated with natural lyotropic liquid crystal(LLC)emulsifiers based on natural sucrose ester was studied for the first time.Polarized optical microscopy,synchrotron radiation small-angle X-ray scattering,wide-angle X-ray scattering,and synchrotron radiation infrared microspectroscopy demonstrated that LLC emulsifiers were tightly ordered at the oil–water interface with a distinct nematic texture.The viscosity of emulsion was observed to change over time.Moreover,the zeta potential and laser particle size distribution verified the emulsion’s satisfactory stability.The frictional shearing test proved that the coefficient of friction of the emulsion versus pure oil decreased by 34.2%.The coefficient of friction of the emulsion with liquid crystal decreased 10.1%versus that without liquid crystal.Although liquid crystal emulsion did not exhibit outstanding anti-wear performance compared with pure oil,its wear volume was 29.4%less than the emulsion without liquid crystal.X-ray photoelectron spectroscopy and scanning electron microscope–energy dispersive X-ray spectroscopy(SEM–EDS)proved that the tribo-film of the emulsion with liquid crystal was formed synergistically by the liquid crystal phase with the base oil.The formulation affecting the lubricant quality was further studied by orthogonal experiments.The resulting Stribeck curve behavior suggested that proper composition with a slightly higher viscosity can better reduce friction in both boundary lubrication and mixed lubrication regimes.The lubrication mechanism indicated that the periodically ordered liquid crystal was transported to the sliding asperity in the form of emulsion droplets,which bored the pressure and released the oil to form a tribo-film.This LLC emulsion is environmentally friendly and potentially non-irritant to the skin.Thus,it has promising application prospects as novel water-based and biological lubricants.

关键词： lyotropic liquid crystal(LLC) emulsion aqueous lubrication tribological behavior

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Investigations of the superlubricity of sapphire against ruby under phosphoric acid lubrication

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Friction 2014年第2期2卷 164-172页

作者： Jinjin LI Chenhui ZHANG Mingming DENG Jianbin LUO State Key Laboratory of Tribology Tsinghua UniversityBeijing100084China Shenzhen Key Laboratory of Micro-Nano Manufacturing Research Institute of Tsinghua University in ShenzhenShenzhen518057China

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.

关键词： friction superlubricity sapphire aqueous lubrication

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