The effect of humidity on the lubrication property of glycerol solution between steel surfaces has been investigated in this paper. A stable superlubricity with a friction coefficient about 0.006 has been found under the relative humidity between around 40% RH and 50% RH. Especially, it is noted that the lubrication state can be switched between superlubricity and nonsuperlubricity by adjusting humidity, which is attributed to the humidity-dependent hydrogen-bonding pattern in the solution. The mechanism of such superlubricity is attributed to the hydrated layer of water between the surface layers, which is formed by hydrogen-bonded glycerol and water molecules and strong enough to bear load, absorbed on each side of the solid surfaces. The work has potential applications, providing a simple and environment-friendly way to accomplish controllable superlubrication between steel pairs, which are commonly used in industry.

Layered double hydroxides (LDHs) are a class of naturally occurring inorganic minerals that are composed of divalent and trivalent metal cations. In this study, three different sized NiAl-LDH nanoplatelets were synthesized by varying crystallization time during the microemulsification process. The layered structure and three-dimensional size of nanoplatelets were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As lubricant additives, their tribological properties in base oil were evaluated by use of a ball-on-disk reciprocating tribometer under three different loads: 50, 100, and 150 N (which created peak Hertz pressures of 1.74, 2.16, and 2.47 GPa). Under contact pressures of up 2.16 GPa, not only did the coefficient of friction (COF) decrease by about 10% after nano-LDHs were added but also the wear performance improved substantially. These improvements resulted from a protective tribolayer formation on the contact interface, as revealed by detailed surface and structure analytical studies. In particular, cross-sectional TEM images revealed that the larger size nanoplatelets (NiAl-24h), rather than the smaller ones (NiAl-6h) showed the best and most stable tribological performance. This was mainly because of their higher degree of crystallinity, which in turn resulted in the formation of a tribofilm with far superior mechanical properties during sliding. Owing to the simple synthetic method and superior tribological properties as oil-based additives, nano-LDHs hold great potential for use in demanding industrial applications in the future.Keywords: LDHs; nanoadditive; oil-based; tribological properties;

In the present work, an excellent biological lubricant extracted from an aquatic plant called Brasenia schreberi (B.s) is reported. With a rotary cylinder-on-ring tribometer, the lubrication properties of the B.s mucilage between quartz glass surfaces have been investigated under different rotation velocity, and an ultralow friction coefficient between 0.004 and 0.006 is obtained. It is observed that the ultralow friction coefficient is independent of the rotation speed, when it is less than 0.1 m/s. SEM images indicate that the mucilage surrounding B.s is composed of polysaccharide gels with a layered structure, which are called nanosheets in the following work. Moreover, it can be deduced that the liquid superlubricity is closely related to the B.s mucilage layer absorbed on the quartz glass surface by hydrogen bonds and the superlubricity behavior only occurs when the adsorption layer stably forms between the quartz glass surface and the B.s mucilage. It is also found that superlubricity is closely dependent upon the sheet structure of the B.s mucilage and water molecules in the mucilage. According to these results, a layered nanosheets lubrication mechanism has been revealed, i.e., the ultralow friction coefficient is due to the adsorption layer of polysaccharide on the quartz glass surface and the hydration layers of water molecules bonded on the polysaccharide nanosheets between the sliding surfaces.

Recently, studies on graphene-based lubrication additives have been widely researched, but few refer to their preparation by thermal reduction which shows potential in not only significantly lowering the mass-production cost, but also the simple, nonchemical process. In this study, mild thermal reduction of graphene oxide (MRGO) has been achieved by high temperature (700 °C) treatment and the product used as a lubrication additive. It shows a relatively ordered lamellar structure and a certain level of oxygen by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis, and exhibits excellent tribological properties as a lubrication additive. The friction coefficient can be reduced by as much as 30% and the rubbing surfaces display few scratches at a lower additive concentration (0.5 wt%) compared with that of base oil (Poly Alpha Olefins Type 6: PAO 6) without MRGO additive under the same friction conditions. Based on the advantages of green, low-cost and simple synthesis operation, the MRGO offers significant potential application as a lubrication additive.

•Central film thicknesses of oil-in-water emulsions (1% to 40% oil percentage) were measured up to 30 m/s.•The film forming is mainly based on oil phase in emulsions which is determined by the competitive between oil and water.•Three important factors are the interfacial tension between solid and liquid, the high speed, and the nano gap.•The ratio hmin/hc shows the change of local oil concentration in the contact.The film forming mechanism of oil-in-water (O/W) emulsions has been investigated at high speeds in the present paper. The lubricant reservoir formed in nano-confined space between a smooth plate disc and a highly polished ball could be observed via the microscope and the film thickness was calculated by the technique of relative optical interference intensity (ROII). The film formation mechanism and influencing factors including the oil percentage, the high speed and the width of nano-gap were discussed.

Bimodal atomic force microscopy (AFM), where the first two flexural eigenmodes of the cantilever are simultaneously excited, is an extension of dynamic AFM to enhance the contrast of multicomponent materials. The effects of the operating parameters for both modes are explored by imaging a polymer blend of polystyrene (PS) and low-density polyethylene (LDPE) the in attractive and repulsive regimes. In this study, the phase and amplitude contrasts of the individual eigenmodes for bimodal AFM are quantitatively calculated using a statistical expression. Meaningfully, the higher free amplitudes of the second mode, which were chosen beyond the traditional values, show significantly better compositional contrast, particularly for the phase of the second mode. The phase contrast of the second mode increases by 3–10 times compared with that of the traditional results. The relationship between the contrast and amplitude ratio of the first mode is also researched experimentally in the bimodal mode. Virial and energy dissipation power for the corresponding modes are employed to explain the enhanced phase contrast physically. The meaningful results obtained are applicable to optimize the operating parameters of image contrast and study the surface morphology and properties of soft, even biological materials.

Higher mode and bimodal atomic force microscopy (AFM) are two recently developed imaging modes of dynamic AFM for improving resolution. In higher mode, the higher flexural mode of the cantilever instead of the traditional fundamental eigenmode is excited. In bimodal mode, two flexural modes of the cantilever are simultaneously excited for obtaining more information about the properties of the material. The first three flexural modes for higher mode and superposition of two excitation signals for bimodal mode are explored and compared by imaging a polymer blend of polystyrene (PS) and low density polyethylene (LDPE). The effects of different operating conditions of the two imaging modes are researched to improve image contrast and material discrimination. Dissipated power and virial are employed to explain the origin of contrast for the complex and highly nonlinear dynamical tip-sample interfacial system. Amplitude and phase contrasts of each single mode and bimodal mode are calculated by Ashman's D statistical equation. It is found that higher single modes with small free amplitudes show enhanced phase contrast. The bimodal of the first and the third modes gains a clear advantage over the bimodal of the first and second modes for phase and amplitude image contrasts. In addition, the best contrast of bimodal imaging occurs when it is a combination of a large free amplitude for the first mode and a small free amplitude for the third mode.

•Surface composition and structure due to running-in process favor super-low friction.•Running-in provides condition for elastohydrodynamic lubrication to reduce friction.•Silica layer formed in running-in process provides excellent boundary lubrication.•Hydrogen ions adsorbed on the friction surfaces contribute to super-low friction.In this work, effects of running-in process on achieving super-low friction between silicon nitride ball and sapphire disc under lubrication of mixtures of sulfuric acid and glycerin, was investigated. The results revealed that sulfuric acid plays the key role in the running-in process. Owing to this acid, a smooth worn region was generated on the silicon nitride ball through chemical-mechanical abrasion, which provided favorable conditions for super-low friction under elastohydrodynamic lubrication. Meantime, a silica layer formed on part of the worn region through tribochemical reaction during running-in process, contributing to the realization of super-low friction under boundary and mixed lubrication. Moreover, the adsorption of hydrogen ions on friction surfaces generated a repulsive double-layer force that also contributed to super-low friction.

•The lubricating and anti-wear properties of some water-based additives on titanium alloys are investigated.•PPE solution can decrease friction coefficient and wear rate to the titanium alloys–tungsten carbide tribo-pair.•The lubricating mechanism of PPE was investigated with XPS analysis.•PPE is a candidate additive in developing cutting fluid of titanium alloys.This work investigated the lubricating and anti-wear properties of some water-based additives on titanium alloys. It was found that nonylphenol polyoxyethylene ether phosphate ester (PPE) solution can decrease friction coefficient and wear rate. The morphology of worn surface was observed and the mechanism was investigated. The adsorption of phosphate to titanium through P–O–Ti covalent bond plays the most important role. This steady film prevents direct contact of the ball with the disc, thus a relatively low friction coefficient and wear rate occurring. This research indicates that PPE is a candidate additive in developing cutting fluid of titanium alloys.

•Nanoparticles improving the properties of Electro-hydrostatic-actuator is proposed.•The surface roughness and temperature influence on the additives are investigated.•MoS2 shows better potential for the application than multilayer graphene as additives.Recently, we have proposed an Electro-hydrostatic-actuator (EHA) based on a novel linear oscillating motor, which permits nanoparticle additives, such as layered graphene and MoS2, using in hydraulic oil. Especially under a super smooth (Ra~5 nm) surface unlike past studies, the tribological properties of these additives is sent more highlight. In addition, oil temperature is introduced into the friction process. We show that the friction coefficient can be decreased to as low as 0.04 when adding MoS2 nanaosheets for the chemical absorption and forming “available solid lubricant” under smooth surface and high temperature while multilayer graphene performs relatively unstable tribological properties because of agglomerates and crystal defects. It can be concluded that MoS2 nanosheets possess better potential value for the application.

•Superlubricity (COF ≈ 0.006) is achieved between steel surfaces with nanodiamonds glycerol colloid solution.•The wear volume is significantly reduced due to the rolling effect of nanodiamonds.•It is the first time that colloid solution is introduced into the field of liquid superlubricity.In this paper, nanodiamonds glycerol colloidal solution is investigated as a lubricant between steel ball and disk, and it is compared with glycerol solution. Although stable superlubricity (coefficient of friction ≈ 0.006) can be achieved with both solutions after a running-in period, the colloidal solution causes much less wear than the glycerol solution. With the content of the nanodiamonds at only 0.01 wt%, the colloidal solution can lead to a reduction of 32.5% in the diameter of the wear scar on the ball, resulting in an increase of 119.3% in the contact pressure between the rubbing surfaces during the superlubricity period. Therefore, a novel superlubricity system based on nanodiamonds glycerol colloidal solution is discovered. Through theoretic calculations, the novel superlubricity system is determined to be in the mixed lubrication regime, but not far from the hydrodynamic lubrication regime. It is believed that the ultralow COF is attributed to the hydrodynamic effect and the hydrogen bond layer. And the reduced wear derives from the rolling effect of the nanodiamonds. This work enriches the field of water-based superlubricity by firstly introducing nanoparticles into this area and has prosperous potential applications.

Thin film lubrication (TFL), a lubrication regime that fills the gap between boundary lubrication (BL) and elastohydrodynamic lubrication (EHL) regimes, was proposed 20 years ago. Since it was first recorded in the literature, TFL has gained substantial interest and has been advanced in the fields of theoretical and experimental research. Following the revelation of the TFL phenomenon and its central ideas, many studies have been conducted. This paper attempts to systematically review the major developments in terms of both the history and the advances in TFL. It begins with the description and definition of TFL, followed by the state-of-art studies on experimental technologies and their applications. Future prospects of relevant studies and applications are also discussed.

By using atomic force microscopy (AFM), we showed that the liquid superlubricity with a superlow friction coefficient of 0.0007 can be achieved between two silica surfaces lubricated by hexadecyltrimethylammonium bromide (C16TAB) solution. There exists a critical load that the lubrication state translates from superlow friction to high friction reversibly. To analyze the superlow friction mechanism and the factors influencing the critical load, we used AFM to measure the structure of adsorbed C16TAB molecules and the normal force between two silica surfaces. Experimental results indicate that the C16TAB molecules are firmly adsorbed on the two silica surfaces by electrostatic interaction, forming cylinder-like micelles. Meanwhile, the positively charged headgroups exposed to solution produce the hydration and double layer repulsion to bear the applied load. By controlling the concentration of C16TAB solution, it is confirmed that the critical load of superlow friction is determined by the maximal normal force produced by the hydration layer. Finally, the superlow friction mechanism was proposed that the adsorbed micellar layer forms the hydration layer, making the two friction surfaces be in the repulsive region and meanwhile providing excellent fluidity without adhesion between micelles.

In this paper, we showed that the superlubricity of silicone oil can be achieved between friction surfaces (Si3N4/glass) by running-in with an acid solution. The friction coefficient of silicone oil can be reduced to about 0.004, which is only one-thirtieth of its original value (μ = 0.13). Experimental results indicate that the formation of a circular plane under the action of hydrogen ions on the worn region of the ball is closely linked to superlubricity, while the topography of the track on the glass substrate has no obvious effect on the superlubricity. In addition, the tribochemical reaction between Si3N4 and water can lead to the reduction of the friction coefficient in mixed lubrication, but it has no effect on the friction coefficient in hydrodynamic lubrication. According to these results, a superlubricity mechanism was proposed, in which the two friction surfaces form a micro-slope plain bearing, owing to the effect of hydrogen ions, and the silicone oil forms a hydrodynamic film at a certain speed.

In the present work, we show that the liquid superlubricity (μ < 0.01) of water-based lubricants can be achieved between sapphire/sapphire even when the average contact pressure is higher than 100 MPa, while the superlubricity of oil-based lubricants cannot be achieved with the same contact pressure. However, when the pressure is reduced to 50 MPa, the friction coefficient of oil-based lubricants can translate from non-superlubricity (μ > 0.01) to superlubricity while the friction coefficient of water-based lubricants is always in the superlubricity region. The calculated friction results indicate that the liquid superlubricity is closely linked to pressure and the pressure–viscosity coefficient. When the pressure is high, the pressure–viscosity coefficient has to be as small as possible to achieve superlubricity, but when the pressure is low, superlubricity can be achieved with a wide range of pressure–viscosity coefficients. Finally, the liquid superlubricity region dependent on pressure and the pressure–viscosity coefficient were established, which are useful for us to design liquid superlubricity systems.

•This paper presents a review of important research progresses on the charged lubrication.•Review the lubrication and friction behaviors of different lubrication states under charged conditions.•Review representative lubrication instabilities at the charged interface.•Review two typical applications pertaining to charged lubrication, i.e., charged bearings and electric contact lubrication systems.Understanding on the lubrication properties under charged conditions is not only one of the important fundamental research directions in the lubrication field, but also is of great significance in some newly-emerging applications. This paper presents a review of important research progresses on charged lubrication in recent years. First, the lubrication and friction behaviors in different lubrication states under charged conditions, as well as representative lubrication instabilities at charged interfaces are reviewed. Subsequently, two relevant applications, i.e., charged bearings and electric contact lubrication systems, are introduced. Finally, a brief summary and future outlook of the researches on charged lubrication are given.

•EHL film thicknesses were measured up to 30 m/s under oil−air lubrication.•The minimum film thicknesses decrease linearly against speed in starved regime.•A defined parameter η shows 30 times higher under oil−air lubrication at 30 m/s.•The roles of micron-order oil droplets were verified and discussed.The film forming behavior has been investigated under oil−air lubrication, compared with that under oil−jet lubrication in present work. Images of microscopic oil reservoir and interference were obtained up to 30 m/s. A parameter η describing the oil supply effects is 30 times higher and the film thickness reduces in starved regime much slower under oil−air lubrication compared with that under oil−jet lubrication. The contribution of micron-order oil droplets on film forming is discussed. More micron-order oil droplets can spread onto the disc while less of them are driven away by centrifugal effects. As a result, the oil supply efficiency of oil−air lubrication is improved. The high pressure compressed air can blow off the oil and intensify the film oscillation.

•Chemical mechanical polishing technique was used to prepare the ultra-smooth surface of AISI 52100 steel.•The effects of pH, complexing agent such as glycine, H2O2 and benzotriazole on the polishing performance of AISI 52100 steel were investigated.•The polishing mechanism of AISI 52100 steel was analyzed and proposed.•An ultra-smooth surface of AISI 52100 steel with the surface roughness Ra value of 1.8 nm can be achieved by the developed two-step polishing process.AISI 52100 steel has been widely used in the mechanical industry due to its excellent mechanical properties and high availability. In some cases, an ultra-smooth surface of AISI 52100 steel is needed and is even indispensable for the satisfactory performance of devices. In this paper, chemical mechanical polishing technique was employed to prepare the ultra-smooth surface of AISI 52100 steel. Colloidal silica was used as the abrasive. The effects of pH, complexing agent such as glycine, H2O2 and benzotriazole (BTA) on the polishing performance were studied. It is revealed that, with the increase of pH, the static etching rate (SER) and the material removal rate (MRR) are both gradually reduced, and the post-CMP surface roughness Ra decreases. This is attributed to the fact that compact and passive iron oxides, especially Fe(III) oxides, gradually form on the top surface. At pH 4.00, in the presence of glycine, and with the increase of the H2O2 concentration, the SER is further suppressed, and the surface roughness Ra gradually decreases; the MRR initially dramatically increases due to the fact that, with the addition of small amount of H2O2, the porous iron oxide layer with relatively low mechanical strength can be rapidly formed on the surface. Moreover, glycine intensifies the chemical dissolution by chelating iron ions, especially Fe(II) ions, and thereby the mechanical strength of the oxide layer further weakens. Then, after reaching the peak value, the MRR gradually decreases when the H2O2 concentration further increases since the compactness of the oxide layer gradually increases. With the increase of the BTA concentration, the MRR is gradually suppressed and the surface roughness Ra decreases due to the formation of Fe-BTA passivating film on the top surface. Finally, a two-step polishing process was developed. The polishing results show that, within 20 min, a rough surface of AISI 52100 steel with the Ra value of 188 nm can be polished into an ultra-smooth surface with the Ra value of 1.8 nm.

In the present study, the pH dependence of liquid superlubricity between Si3N4 and glass achieved with phosphoric acid solution was investigated. It is seen that the superlubricity can be achieved only when the pH value is in the range of 0.75–2. To reveal the mechanism, the evolutions of confined solutions with different pH values between two friction surfaces were investigated by an online observation. It was seen that the superlubricity appeared when the confined solution between the two friction surfaces forms a starvation state. When the pH is in the range of 0.75–1.75, the starvation state can be formed as long as the running-in period is end. When the pH is in the range of 1.75–2, the starvation state can be formed by adding a certain amount of phosphoric acid molecules in the contact region, which leads to the transformation of an unstable friction state to a superlubricity state. When the pH is in the range of 0–0.75, the superlubricity cannot be obtained, no matter how the test conditions are changed because of the high contact pressure and lack of time for the tribochemical reaction to take place between the friction surfaces and hydrogen ions. When pH is greater than 2, the value of friction stays high because the amount of hydrogen ions adsorbed on the friction surfaces are not sufficient to make the surfaces positively charged.

In the present work, an excellent biological lubricant extracted from an aquatic plant called Brasenia schreberi (B.s) is reported. With a rotary cylinder-on-ring tribometer, the lubrication properties of the B.s mucilage between quartz glass surfaces have been investigated under different rotation velocity, and an ultralow friction coefficient between 0.004 and 0.006 is obtained. It is observed that the ultralow friction coefficient is independent of the rotation speed, when it is less than 0.1 m/s. SEM images indicate that the mucilage surrounding B.s is composed of polysaccharide gels with a layered structure, which are called nanosheets in the following work. Moreover, it can be deduced that the liquid superlubricity is closely related to the B.s mucilage layer absorbed on the quartz glass surface by hydrogen bonds and the superlubricity behavior only occurs when the adsorption layer stably forms between the quartz glass surface and the B.s mucilage. It is also found that superlubricity is closely dependent upon the sheet structure of the B.s mucilage and water molecules in the mucilage. According to these results, a layered nanosheets lubrication mechanism has been revealed, i.e., the ultralow friction coefficient is due to the adsorption layer of polysaccharide on the quartz glass surface and the hydration layers of water molecules bonded on the polysaccharide nanosheets between the sliding surfaces.

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.

The lubrication behavior of starved elastohydrodynamic contacts at high speeds was investigated in this study. A new ball-on-disc test rig with the ability to measure traction force at high speeds up to 100 m/s and lubrication film thickness at speeds up to 42 m/s was built. The relative optical interference intensity technique was used to measure the film thickness. The experimental results show that the film thickness decreased rapidly and asymmetrically when the speed exceeded a critical speed under the starved lubrication condition. Starvation is governed by the amount of lubricant available both in the inlet region and on the side of the oil reservoir. The shape of the oil reservoir becomes asymmetric and the amount of oil gradually reduces against the speed at high speeds because of the centrifugal effects, under which the oil on the outer side of the oil reservoir will be thrown away and the oil on the inner side of the oil reservoir will be compressed. The balance of oil supply and oil loss due to centrifugal force determines the starvation behavior.

AISI 1045 steel has been widely used as the substrate for thin film deposition. In some cases, an ultra-smooth surface of AISI 1045 steel is needed and is even indispensible for the satisfactory deposition of thin film. In this paper, chemical mechanical polishing technique was employed to prepare the ultra-smooth surface of AISI 1045 steel. The effects of pH and H2O2 on the polishing performance of AISI 1045 steel were investigated. It is revealed that, with the increase of pH, the material removal rate (MRR) and the static etching rate (SER) of AISI 1045 steel gradually decrease due to the formation of passive iron oxides on the top surface, and thus the surface quality gradually improves. At pH 4.00, with the addition of H2O2, the SER of AISI 1045 steel is further suppressed; while the MRR of AISI 1045 steel first dramatically increases due to the formation of porous iron oxides with relatively low mechanical strength on the surface when the H2O2 concentration increases from 0 to 0.01 wt%, and then decreases since the porous iron oxides gradually grow compact when the H2O2 concentration further increases. The increase of the compactness of the iron oxides might be attributed to the crystallization of γ-FeOOH into α-FeOOH and even into α-Fe2O3 and the resulting polymerization of the amorphous iron oxides.

An ultrathin layer is investigated for its potential application of replacing conventional diffusion barriers and promoting interface adhesion for nanoelectric circuits with porous ultralow dielectrics. The porous ultralow dielectric (k ≈ 2.5) substrate is silanized by 3-aminopropyltrimethoxysilane (APTMS) to form the nanoadhesive layer by performing oxygen plasma modification and tailoring the silanization conditions appropriately. The high primary amine content is obtained in favor of strong interaction between amino groups and copper. And the results of leakage current measurements of metal-oxide-semiconductor capacitor structure demonstrate that the aminosilanization nanoadhesive layer can block copper diffusion effectively and guarantee the performance of devices. Furthermore, the results of four-point bending tests indicate that the nanoadhesive layer with monolayer structure can provide the satisfactory interface toughness up to 6.7 ± 0.5 J/m2 for Cu/ultralow-k interface. Additionally, an annealing-enhanced interface toughness effect occurs because of the formation of Cu–N bonding and siloxane bridges below 500 °C. However, the interface is weakened on account of the oxidization of amines and copper as well as the breaking of Cu–N bonding above 500 °C. It is also found that APTMS nanoadhesive layer with multilayer structure provides relatively low interface toughness compared with monolayer structure, which is mainly correlated to the breaking of interlayer hydrogen bonding.Keywords: adhesive; aminosilanization; diffusion barrier; interface adhesion; low dielectric; oxygen plasma;

In the present work, we show that the superlubricity can be achieved when the polyhydroxy alcohol solutions are mixed with acid solutions. The lowest friction coefficients between 0.003 and 0.006 are obtained on a traditional tribometer with a high pressure under the lubrication of these mixtures. Experimental results indicate that the superlubricity mechanism is in accordance with that under the lubrication of the mixture of glycerol and acid solutions in the study by Li et al. (Li, J. J.; Zhang, C. H.; Ma, L. R.; Liu, Y. H.; Luo, J. B.Superlubricity achieved with mixtures of acids and glycerol. Langmuir 2013, 29, 271−275). It is also found that the superlubricity is closely dependent upon the concentration of polyhydroxy alcohol and the number of hydroxyl groups in the molecular structure of polyhydroxy alcohol. However, the number of carbon atoms and the arrangement of hydroxyl groups in the molecular structure almost have no effect on superlubricity.

The effect of humidity on the lubrication property of glycerol solution between steel surfaces has been investigated in this paper. A stable superlubricity with a friction coefficient about 0.006 has been found under the relative humidity between around 40% RH and 50% RH. Especially, it is noted that the lubrication state can be switched between superlubricity and nonsuperlubricity by adjusting humidity, which is attributed to the humidity-dependent hydrogen-bonding pattern in the solution. The mechanism of such superlubricity is attributed to the hydrated layer of water between the surface layers, which is formed by hydrogen-bonded glycerol and water molecules and strong enough to bear load, absorbed on each side of the solid surfaces. The work has potential applications, providing a simple and environment-friendly way to accomplish controllable superlubrication between steel pairs, which are commonly used in industry.

Superlubricity is a new area in tribology, in which the slide friction coefficient is about 1/1000 to 1/100 of the general ones. Since the concept of superlubricity was proposed, it has attracted more and more attentions from researchers in fields of tribology, physics, chemistry, materials, etc. Many significant progresses have been made during the last two decades in experimental studies on superlubricity. In the present work, the recent advancements in solid superlubricity and liquid superlubricity are reviewed and the lubricating mechanisms of different superlubricity systems are discussed. Finally, the problems on the superlubricity mechanism and the development of superlubricity in the future are addressed.

The present work reports an excellent lubrication property of an aquatic plant called Brasenia schreberi (BS). To investigate the lubrication characteristics of the BS mucilage, a novel measuring system is designed, and an ultralow friction coefficient about 0.005 between the mucilage and glass surface has been obtained. It is found that the ultralow friction is closely related to the structure of mucilage and water molecules in the mucilage. The microstructure analysis indicates that the mucilage surrounding BS forms a kind of polysaccharide gel with many nanosheets. A possible lubrication mechanism is proposed that the formation of hydration layers among these polymer nanosheets with plenty of bonded water molecules causes the ultralow friction. The excellent lubrication property has a potential application for reducing the friction between a glossy pill coated with such layer of mucilage and people’s throats.

To reveal the film formation mechanism of oil-in-water (O/W) emulsions, the thicknesses of nanofilms of an aqueous paraffin oil emulsion, stabilized with the nonionic Tween 80 and Span 80 surfactants, were measured in confinement between two solid surfaces, by the use of the relative optical interference intensity (ROII) approach. Such films’ thicknesses were found to be sensitive to the rolling speed. In contrast to a single-phase oil lubricant, which as an elastohydrodynamic film had a thickness that always increased with speed when there was a sufficient supply of new material, the film formation of oil-in-water emulsions normally has a hill-like appearance (the film undergoes a collapse to a relatively low thickness at a critical speed after the evident rise in thickness with increasing rolling speed). The critical speeds for film formation of O/W emulsions with various emulsifiers and oil concentrations were focused on to gain an insight into the film formation mechanism of oil-in-water emulsions. Droplets can be observed to concentrate and break up before the contact at a low speed, which induces an oily pool. The oily pool seemed to act as the provider of the lubrication of the contact during the rolling process. The re-emulsification effect was employed to explain the collapse of the film thickness as the speed exceeded the critical value. A theoretical model was proposed to describe the re-emulsification effect, which established a relationship between the critical speed and the concentrations of either the oil or the emulsifiers.

The effects of an external electric field (EEF) on dielectric liquid menisci formed in a small gap between a smooth plate and a high precision steel ball have been investigated. It was found that thin spreading films were pulled out and moved away from the menisci of low permittivity dielectric liquids after exposure to the EEF. During the initial period of spreading, a strong “electric wind” due to the gas discharge in the vicinity of the three-phase contact line (TCL) of the liquid meniscus was observed. However, such electrospreading phenomena were absent for the menisci of conductive liquids which were commonly used in classical electrowetting studies. It is deduced that the spreading of liquid menisci under EEFs is driven by the thermocapillary force near the TCL.

Understanding the structures and dynamics of the confined liquids in nonequilibrium states becomes increasingly important for many practical applications, e.g., nanolubrication and micro/nanofabrication. In the present paper, the effect of external electric fields (EEFs) on several normal straight-chain aliphatic liquids nanoconfined in a ball-plate configuration has been investigated by measuring the dynamic film thickness with the thin film interferometry and calculating the effective viscosity. The results indicate that the EEF effect on promoting the formation of interfacial ordered regions due to dipole orientation and hence affecting the effective viscosity of the confined liquid film is dependent on the type of the headgroup of the molecule. The dipole orientation in the n-alkanol film with a shorter alkyl chain has a larger probability to be affected by EEFs due to the smaller dispersion force and the larger ionic concentration.

The consequence of bearings exposed to shaft voltage is a very important tribological problem, especially with the increasing use of variable-frequency drives (VFDs) to control alternate current (AC) motors. The emerging behavior of gas micro-bubbles and the film forming characteristics between base oil (liquid paraffin) films with and without ZDDP additive under an external electric field (EEF) in a nanogap have been compared. Experimental results indicated that the micro-bubble emerging intensity increases slightly when the additive is involved in the base oil. The magnitude of the electric current flowing through the lubricant film closely related to the intensity of the micro-bubble emerging. No obvious difference in the film thickness can be found between the liquid paraffin films with and without ZDDP additive. The influence of the EEF on the film thickness of the liquid paraffin with the additive is more significant.

The problem of the solidlike transition of fluids in a nanogap has drawn much fundamental and practical attention. Here, we directly observed the disappearance of the fluidity of liquids confined within a gap with a surface separation of >10 nm under an EF in a ball-plate system, which is called the “freezing” of liquids. The flow of the nanoconfined liquid became very weak as the EF intensity was increased to a critical value and was correlated with the liquid polarity and the film thickness. It is deduced that the EF can induce more liquid molecules to be aligned to form more ordered layers in the nanogap.

The effect of an external electric field (EEF) on water-based thin films has been investigated. Some micro-bubbles emerging around the edge of the Hertz contact region in the films of deioned water and polyethylene glycols (PEG) aqueous solutions have been observed. A higher EEF intensity is needed at which the micro-bubbles begin to emerge after the EEF is applied on the liquid film, which is defined as the threshold EEF intensity. The threshold EEF intensity increases with the molecular weight of PEG solution at lower molecular weight. There is a maximum value when the molecular weight reaches 10,000 Da, beyond which the threshold EEF intensity tends to decrease. The threshold EEF intensity also increases with the concentration of PEG solution. Micro-bubble emerging at negative EEF is easier than at positive EEF. The micro-bubble emerging in the film of deioned water is sensitive to the variation of EEF intensity, and disappears eventually as time progresses.