Chenhui Zhang

Superlubricity Behavior with Phosphoric Acid–Water Network Induced by Rubbing

In present work, a superlubricity phenomenon of phosphoric acid (H(3)PO(4)) was found under ambient conditions. An ultralow friction coefficient of about 0.004 between glass/Si(3)N(4) and sapphire/sapphire tribopairs was obtained under the lubrication of a phosphoric acid aqueous solution (pH 1.5) at high contact pressure (the maximum pressure can reach about 1.65 GPa) after a running-in period of about 600 s. The experimental results indicate that the superlow friction state was very stable for more than 3 h. In such a state, solidlike films formed on the two sliding surfaces, which are hydrates of phosphoric acid with a hydrogen-bonded network according to the Raman spectrum. The superlubricity mechanism is mainly attributed to the hydrogen bond effect that forms a hydrated water layer with low shearing strength, and the dipole-dipole effects that form an interfacial Coulomb repulsion force also make some contributions to low friction. This work may help us to introduce a new approach to superlubricity and may lead to the wide application of superlubricity in future technological and biomedical areas.

Superlubricity achieved with mixtures of acids and glycerol.

In this work, superlubricity between glass and Si(3)N(4) surfaces lubricated by mixtures of acid solutions and glycerol solutions has been found by using a traditional tribometer. Ultralow friction coefficients of between 0.004 and 0.006 were obtained after a running-in period. Related experiments indicate that the hydrogen ions in the mixtures play an important role in achieving superlubricity. Moreover, the ultralow friction is also closely related to the pH value of the acid and the concentration of glycerol. According to these results, the possible superlubricity mechanism has been revealed, which is attributed to a fluid-hydrated water layer between the hydrogen-bonded networks of glycerol and water molecules on the positively charged surfaces.

Excellent Lubricating Behavior of Brasenia schreberi Mucilage

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 peoples throats.

Tribochemistry and Superlubricity Induced by Hydrogen Ions

Friction behavior of aqueous solution at macroscale is quite different from that at nanoscale. At macroscale, tribochemistry usually occurs between lubricant and friction surfaces in the running-in process due to a high contact pressure, and most such processes can lead to friction reduction. In the present work, we reported that the hydrogen ions in aqueous solution played an important role in tribochemistry in running-in process (friction reducing process), which could result in the friction coefficient reducing from 0.4 to 0.04 between Si(3)N(4) and glass surfaces at macroscale. It is found that the running-in process and low friction state are closely dependent on the concentration of hydrogen ions in the contact region between the two friction surfaces. The lubrication mechanism is attributed to tribochemical reaction occurring between hydrogen ions and surfaces in the running-in process, which forms an electrical double layer and hydration layer to lower friction force. Finally, the running-in process of H(3)PO(4) (pH = 1.5) was investigated, which could realize superlubricity with an ultralow friction coefficient of about 0.004.

Unipolar Resistive Switching Properties of Diamondlike Carbon-Based RRAM Devices

Resistance random access memory devices based on nanoscale diamondlike carbon (DLC) films are demonstrated. The devices exhibit excellent memory performances such as high on/off-resistance ratio (>; 300), high switching speed ( <; 50 ns), low operation voltage ( <; 1.2 V), low switching power consumption ( <; 16 μW), nondestructive readout, and good reliability. Nanoscale graphitic filament formation and rupture alternately through the field-induced dielectric breakdown and thermal fuse effect, respectively, are proposed to be responsible for the resistance switching. The unipolar switching characteristics shown here suggest that DLC is one of the most promising material candidates for high-density and low-power nonvolatile memory applications.

Superlubricity achieved with mixtures of polyhydroxy alcohols and acids.

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.

Film Forming Characteristics of Oil-in-Water Emulsion with Super-Low Oil Concentration

The oil in water emulsion has been widely used in many fields such as rolling operations. The mechanism and characteristics of film forming have been widely investigated before. However, the mechanism is still dubious and film forming characteristics are seldom discussed under an oil concentration of 0.05%. In this paper, a lubricating film testing apparatus is used to investigate the film forming characteristics and tribological behaviors under different speed of oil-in-water emulsion between a steel ball and a glass disc. By carrying out experiments under an extremely low concentration of oil, some new phenomena are found in our experiments. Oil concentration is even low to 0.0005%. The results indicate that the speed-thickness curves are changed as the condition changes. The effect of droplet size and the stability of emulsion are both considered to be important. The frictional behaviors are investigated under different conditions. The film forming mechanism of oil-in-water emulsion is also discussed by direct observations of emulsion in the contact area. A new viewpoint on the lubrication of emulsion is put forward in this paper.

Analysis of Measurement Inaccuracy in Superlubricity Tests

Although investigations on superlubricity are increasing, the corresponding measurement errors, which have a great effect on the measured friction coefficient, have not been widely discussed. The present article analyzes the origin of friction measurement errors in a rotating ball-on-flat apparatus and shows that these errors play an important role in the relationship between the measured and real friction coefficient. Based on the analysis, two methods were proposed to eliminate the main measurement error. One is obtaining the same friction coefficients in two reverse sliding directions by adjusting the level of the substrate (rotating plane) and the other is averaging the measured friction coefficients in two reverse sliding directions. The designed experiments proved the effectiveness of the two methods in eliminating the measurement error. Such methods are also effective for investigations on superlubricity.

Investigation of the film formation mechanism of oil-in-water (O/W) emulsions

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.

Superlubricity of silicone oil achieved between two surfaces by running-in with acid solution

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.