Controllable Superlubricity of Glycerol Solution via Environment

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Controllable Superlubricity of Glycerol Solution via Environment Humidity Zhe Chen, Yuhong Liu, Shaohua Zhang, and Jianbin Luo* State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P. R. China ABSTRACT: 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 humiditydependent 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. addition, Martin’s group found30−32 that polyhydric alcohols, including glycerol and myo-inositol, can reduce friction between friction pairs made of steel or DLC. While these studies have provided useful information proving that hydrogen-bond network consisting of glycerol and water molecules plays a key role in reducing friction, the conformation of the network, which is able to bear normal load, is yet to be elucidated. Moreover, the environmental factors, including relative humidity and temperature, were not considered seriously. In the paper, taking the effect of humidity into consideration, the lubricating ability of glycerol solution was further investigated. The relationship between humidity and final concentration of glycerol solutions was presented. Under various relative humidity, glycerol solution was used as lubricating medium between steel surfaces and the friction coefficient was measured. It was found that the friction coefficient can be controlled via adjusting surrounding humidity and a stable superlubricity state can be achieved under a certain range of humidity. The mechanism of controllability, including the mechanism of ultralow friction coefficient, was discussed according to several experimental results based on the hydrogen-bond patterns of glycerol/water mixtures. Given the excellent lubrication property and controllability as well as the water-basement, noncorrosiveness, and environmental friendliness, an application of this lubrication system for watercrafts was suggested to prevent contamination caused by lubricant leaking.

1. INTRODUCTION Energy, which is the basis of every activity in our life, has become more and more precious. However, it is estimated1 that the annual cost of energy in highly industrialized nations is as much as 5% of their gross national products (GDP) due to friction. It is obvious that reducing friction to as low as possible is essential for saving energy. Since the concept of superlubricity was proposed in 1990, it has been developed very fast in recent times.2−6 Owing to its significance to our daily life and the environment, many researchers have investigated into superlubricity and obtained ultralow friction coefficients with molybdenum disulfide,7,8 diamond-like carbon (DLC) film,9−11 highly oriented pyrolytic graphite (HOPG),12,13 CNx film,14,15 polymer brushes,16−18 polysaccharide mucilage from plants,19,20 mixture of polyhydroxy alcohol and acids,21−24 and so on. Meanwhile, the realization of controllable friction state or easy switch between superlubricity and nonsuperlubricity also has great practical value.25 Glycerol, also known as glycerin, is a trihydric alcohol, acknowledged as the most widely distributed polyhydric alcohol in nature, combined in fats and other lipids essential to life processes.26 Glycerol is strongly hygroscopic; even when the humidity in the air is very low, anhydrous glycerol will still absorb moisture from the surrounding air.27 Additionally, glycerol solution of any concentration, when exposed to air, will emit or absorb moisture until the concentration balances with the humidity of the air.27 Because of this specific characteristic, the glycerol solution is a perfect choice for humectant.28,29 Meanwhile, it is discovered that glycerol is an excellent lubricant. In our previous work,22,23 the mixture of glycerol and acids can lead to ultralow friction coefficient between a glass disk and a Si3N4 ball. However, the existence of acid severely limits the application of this accomplishment in industry, where iron and steel are of paramount importance. In © 2013 American Chemical Society

Received: June 26, 2013 Revised: August 27, 2013 Published: August 28, 2013 11924

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2. MATERIALS AND METHODS

coefficient, the whole test can be divided into four periods (shown in Figure 1). At the very start, the friction coefficient keeps high and undulating for about 350 s. In the second period, the friction coefficient stops fluttering and keeps on decreasing smoothly. The third period begins from about the 590th second, when the friction coefficient falls below 0.01 (entering superlubricity region), to about the 1050th second. During this period, the friction coefficient first continues to decrease until it reaches 0.006 and then begins to increase and reaches 0.01 at the end of the period. In the last period, the friction coefficient stabilizes at around 0.01 in the rest time of the test. In the whole experimental process, superlubricity lasts for about 460 s. After the test, it is found that the whole lubrication system loses a weight of 20.85 mg. Since water in the solution is the only substance with significant volatility, it can be deduced that, during the test, there is a process of water evaporation before the liquid−vapor equilibrium is reached. Given that the friction coefficient is changing dramatically first and then becomes stable, it is quite reasonable to speculate that there is a relationship between the water content and the friction coefficient. Further, it is assumed that the friction coefficient is dependent on the content of water in the solution, and the schematic of the supposed water content as a function of time is displayed in Figure 1. In the first three periods of the test, the solution is volatilizing moisture continually and friction coefficient changes with the water content in lubricating solution. In the last period, the water in the solution is in equilibrium with the moisture in the air. The solution concentration becomes steady and so does the friction coefficient. The third period of the test, i.e. the superlubricity period, aroused our great interest. According to the assumption raised above, it needs more water than that in the final solution under the humidity of 30% RH to achieve superlubricity. Therefore, it is further assumed that there exists a range of concentration and superlubricity can be achieved when the solution concentration is within this range. As mentioned in the Introduction, the final concentration of glycerol solution is dependent on environment humidity.27 In order to raise the water content in the balanced solution to achieve ultralow friction coefficient, the humidity should be turned up. To prove the assumption and further investigate the effect of humidity on friction coefficient as well, the environment humidity was not only turned up but also turned down. The friction coefficient between steel surfaces lubricated by 30 μL of 30 wt % glycerol solution under different relative humidity ranging from 10% RH to 80% RH in the steps of 10% RH was obtained and is shown in Figure 2. The results show that the friction coefficient is directly associated with relative humidity. Generally, the higher the relative humidity, the slower the friction coefficient decreases. When the relative humidity is 60% RH or lower, the friction coefficient will settle before the test of 3600 s ends. The time for friction coefficient to stabilize and the final value of friction coefficient has been displayed in Figure 3. It is obvious that the final friction coefficient differs with each other and it takes longer time for friction coefficient to stabilize under higher relative humidity, which is in accordance with the water evaporation speed. Therefore, it is proved that the friction coefficient is closely linked to the amount of water in the solution. When the relative humidity is less than 40% RH, the friction coefficient will rebound after reaching its minimum,

The glycerol used in tests is a commercial product with its purity no less than 99.0%, and the water is deionized. The tests were performed on a rotational ball-on-disk tribotester (UMT-3, CETR), which is equipped with an environment chamber, in which the relative humidity can be controlled or remain constant. The ball is made of AISI 52100, with its diameter of 10 mm and its surface roughness (root-mean-square deviation of the profile σ) of 23.8 nm. The material of the disk is also AISI 52100, and it is polished with its σ of 8.4 nm. Before all the tribological experiments, the balls and disks were ultrasonically cleaned in petroleum ether and acetone for 15 min, respectively. Then 30 μL of glycerol solution was dropped on the disk for lubricating. The ball was loaded with 300g, which can generate a maximum contact pressure of 667.5 MPa. The rotation speed of the disk was 360 rpm, and the radius of the friction track was 4 mm, which means the linear sliding velocity was approximately 0.15 m/s. Glycerol solution with its concentration of 30 wt % was tested under the condition of various relative humidity ranging from 10% RH to 80% RH and room temperature (20 °C). The precision of the friction force sensor is 0.25g, which is accurate enough for the measurement of a friction coefficient in the order of millesimal under a load of 300g.21 Furthermore, the tribotester was well adjusted before tests ensuring the obtained friction coefficient is authentic.33 The surface roughness of both ball and disk was measured by a surface mapping microscope (ADE PHASE SHIFT MicroXAM) with a 50× lens. The wear scar on the ball and the wear track on the disk were observed by optical microscope (Olympus BX60) and scanning electron microscope (FEI Quanta 200 FEG) after the sliding surfaces were cleaned up. The characteristic of steel surface before and after rubbing was analyzed by Raman spectra and water contact angle. Raman spectra with the resolution of 1.5 cm−1 were taken under ambient conditions by using a Raman spectroscope (HORIBA Jobin Yvon HR800). The spectrometer used the 514 nm line of an argon ion laser at 12 mW of power on the sample, which was focused on the solid surface with a specimen depth of 1 μm. The scattered radiation was analyzed using a 600 grooves/mm grating with a liquid-nitrogencooled 1024 × 256 pixel array CCD detector. The contact angle was measured three times by an optical contact angle measurement (Powereach JC2000A) with a drop (2 μL) of deionized water placed on the surface. The bulk viscosity of glycerol solution was measured with cone-and-plate mode at 20 °C with a standard rheometer (Anton Paar Physica MCR301).

3. RESULTS AND DISCUSSION The friction coefficient as a function of time, between steel surfaces lubricated with 30 μL glycerol solution (30 wt %) under the environment relative humidity of 30% RH, is displayed in Figure 1. According to the state of the friction

Figure 1. Friction coefficient between steel surfaces lubricated with 30 μL of 30 wt % glycerol solution under environment relative humidity of 30% RH. 11925

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Figure 2. Friction coefficient obtained with 30 wt % glycerol solution under several environment humidity values. Superlubricity is achieved at 40% RH and 50% RH.

Figure 4. A drop of deionized water (2 μL) placed on two different areas of the disk. (a) Contact angle of 16.5° in the area covered with solution during the test. (b) Contact angle of 50° in the area covered with nothing during the test.

contact angle in the area covered with glycerol solution, shown in Figure 4a, is 16.5°. While in the other area, shown in Figure 4b, the contact angle is about 50°. The variety of hydrophilicity of different areas demonstrates that a hydrophilic film exists on the area where glycerol solution contacted before. As is reported,34 when exposed to air, a nanosized oxyhydroxide film, which is FeOOH, will naturally grow on the surface of iron. It is quite understandable that the FeOOH film will absorb a thin film of glycerol making the surface much more hydrophilic. Raman spectroscopy was used to check whether there was chemical reaction happened between the lubrication and friction pair during the process of rubbing. As shown in Figure 5, compared to the spectrum of virgin 30 wt % glycerol

Figure 3. Time for friction coefficient to stabilize and final friction coefficient after a test of 3600 s under different relative humidity.

which is below 0.01, and stabilizes above or near 0.01. When the relative humidity is 40% RH or 50% RH, the friction coefficient falls below 0.01, fluctuates within a very narrow range around 0.006, and does not rise anymore, which means stable superlubricity is achieved. When the humidity is 60% RH or higher, the friction coefficient maintains over 0.01 during the whole test. For the humidity of 60% RH, it takes about 3300 s for friction coefficient to stabilize, and the final value of friction coefficient is about 0.012. For the humidity of 70% RH, water in the solution evaporates so slowly that the friction coefficient is not completely settled in 3600 s but tends to stabilize at about 0.016 in the end of the test. For the humidity of 80% RH, the friction coefficient still changes remarkably in the final stage of the test, and the value of friction coefficient is the highest among all the tests. Limited by the resolution of the humidity control system, the lubricating experiments with fine environment humidity variety, which is changed with smaller steps, were not performed. Nevertheless, it is reasonable to draw the conclusion that when the humidity is between about 40% RH and 50% RH, superlubricity will appear and maintain through the end of the experiment. After tests, it is found that the solution stuck closely to the steel surfaces, and it is thought that a hydrophilic film has formed on the steel surface during the test. To approve the conjecture, the disk was washed with deionized water to remove the remaining solution and dried. At this moment, the disk can be divided into two areas according to whether the area was covered with solution during the test. On each of these two areas, a drop of deionized water (2 μL) was placed, which is shown in Figure 4, and the contact angle was measured. The

Figure 5. Raman spectrum surface analysis (in wear track, outside wear track and virgin 30 wt % glycerol solution).

solution, there is no distinct peak displacement in the spectrum of the tribofilm in the wear track. The only notable difference is that the ratio between glycerol to water increases a lot after test. Moreover, the spectra inside wear track and outside wear track are almost the same. Therefore, it can be concluded that no obvious chemical reaction takes place. The experimental 11926

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analysis from Joly-Pottuz et al.31 also supports this conclusion. By the means of secondary ion mass spectroscopy (SIMS), no high mass molecule is detected, indicating that no polymerization occurs. In order to ascertain the conclusion that no chemical reaction happens during the experiments, AISI 440, which is known as stainless bearing steel and is much more difficult to react with oxygen or acids than AISI 52100, was used as friction pairs. The experimental phenomenon (see Figure 6) obtained with AISI 440 is almost the same with that of AISI 52100.

Figure 7. (a) Optical and (b) SEM image of wear scar on the ball and (c) optical and (d) SEM image of wear track on the disk.

wear surface is 5.9 nm. Meanwhile, the σ of the wear track on the disk is 16.1 nm. At the beginning of the test, the friction pairs have not worn. According to Hamrock−Dowson theory of elastohydrodynamic lubrication (EHL) of point contacts, the minimum film thickness can be calculated by the formula35

Figure 6. Friction coefficient between pairs made of AISI 440 under the humidity of 10% RH and 40% RH.

As mentioned in Introduction, the concentration of glycerol solution will be affected by environment humidity. It is obvious that viscosity of solution depends on the concentration. It has been revealed that only within a certain range of humidity ultralow friction coefficient will appear and stay. The correspondence between relative humidity and balanced concentration of glycerol solution at 25 °C is displayed in Table 1.27 Since the effect of temperature on the final

Hmin = 3.63

glycerol wt fraction at 25 °C27 (%)

glycerol mol fraction (XG) at 25 °C

bulk viscosity at 20 °C (cP)

10 20 30 40 50 60 70 80

95 92 89 84 79 72 64 51

0.79 0.69 0.61 0.51 0.42 0.33 0.26 0.17

368 224 139 68 43.8 24 10.7 5.8

(1)

where Hmin = hmin/R, U = ηV/E′R, G = αE′, W = F/E′R , hmin is the minimum film thickness, R is the radius of the ball, η is the bulk viscosity of lubricating medium, V is the averaged linear velocity of ball and disk, E′ is the effective elastic modulus, F is the normal load, α (≈1.5 GPa−1 36,37) is the viscosity-pressure coefficient, and k (≈1) is the ellipticity parameter. The lubrication regime can be distinguished by the method of using the ratio of theoretical minimum film thickness to the combined surface roughness, and the ratio can be calculated by the formula38,39 2

Table 1. Relative Humidity (RH) over Glycerol−Water Mixtures and Corresponding Viscosity27 RH (%)

U 0.68G 0.49 (1 − e−0.68k ) W 0.073

λ=

hmin = σ

hmin σ12 + σ2 2

(2)

where hmin is the theoretical minimum film thickness, σ is the combined surface roughness, and σ1 and σ2 are the surface roughness of the ball and disk, respectively.40 The lubrication regime is EHL or HDL if the ratio λ is larger than 3, mixed lubrication if the ratio ranges from 1 to 3, and boundary lubrication when the ratio is smaller than 1.5 At the beginning of the test, the initial concentration of the lubricating solution is 30 wt % and its bulk viscosity is 2.42 cP, so the film thickness (hmin) is 0.89 nm. Given that the combined roughness (σ) equals 25.24 nm, λ is about 0.035, meaning that the lubrication of the initial stage is in the regime of boundary lubrication, which corresponds to the wear in the test. At the final stage of the test, as shown in Figure 7a, a circle flat surface, whose diameter is about 310 μm, is created due to wear, which makes the Hamrock−Dowson film thickness formula unfitted to the situation. However, an approximate

concentration is very limited27 and little heat is generated from such small friction, the data measured at 25 °C are appropriate for our research. Furthermore, the bulk viscosity of solution with different concentration was measured with coneand-plate mode at 20 °C with a standard rheometer (Anton Paar Physica MCR301) and is presented in Table 1. To assess the effect of wear of friction pairs, after the test under the relative humidity of 50% RH, the surface of both friction pairs were observed by optical microscope (Olympus BX60) and scanning electron microscope (FEI Quanta 200 FEG) when the remained solution is cleaned, and they are shown in Figure 7. For the ball, it is measured that the diameter of the wear scar is about 310 μm and the roughness (σ) of the 11927

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method is proposed to estimate the film thickness at the final stage. In EHL theory, the deformation of ball follows the Hertz contact theory, which can be expressed by the formula40 a=

⎛ 3RF ⎞1/3 ⎜ ⎟ ⎝ 2E′ ⎠

molecules covering the layer of hydrogen-bonded construction, which is very difficult to be pushed out of the contact area, just like a water layer on ice surface.46 In this case, the sliding surfaces are separated by the hydrated layer of water, which is quite easy to shear and results in ultralow friction coefficient.47 The lubrication model is proposed in Figure 8. The

(3)

where a is the radius of Hertz contact area, R is the radius of the ball, F is the normal load, and E′ is the effective elastic modulus. According to the Hertz contact theory, under the same load, the contact area enlarges as the radius of the ball increases. Therefore, the wear scar can be regarded as the Hertz elastic deformation of an equivalent ball with a larger size under the original load, and the radius of the equivalent ball can be calculated by the formula R′ =

⎛ a′ ⎞ 3 ⎜ ⎟ ⎝a⎠

(4)

where R′ is the radius of the equivalent ball and a′ is the radius of the wear scar on the ball. For the test in the condition of 50% RH, the final viscosity of the solution is 43.8 cP (Table 1) and a′ is 155 μm. Substituting eq 4 into eq 1, the approximate value of the film thickness is 30.03 nm. Meanwhile, the combined roughness (σ) of the sliding surfaces is 17.15 nm. Thus, λ is about 1.81, which indicates that the lubrication of the final stage, which is also the superlubricity stage, is in the regime of mixed lubrication. As mentioned above, glycerol is an alcohol with three hydroxyl groups, via which it is constructed with water through hydrogen bonds.41,42 Since no distinct chemical reaction is detected and the possibility of fully flooded elastohydrodynamic lubrication has been excluded, hydrogen bond may be an important factor for such a low friction coefficient. To further investigate into the hydrogen bond patterns in the solution, the amount of glycerol and water is measured in moles, which is also shown in Table 1. When the humidity is higher than 60% RH, the final XG is less than 0.33. According to the research of Dashnau et al.42 and Oleinikova et al.,43 there exists spanning networks of hydration water in the solution, which makes the solution thin and vulnerable to endure normal stress. Therefore, the friction coefficient obtained at this humidity region is high and unstable. As the humidity is in the range of about 40% RH to 50% RH, the final XG is between around 0.33 and 0.61. At this point, it is reported42 that the ratio of water molecules to glycerol molecules is so small that the percolation nature of the mixture has been disrupted and glycerol−water interactions and glycerol-glycerol interactions have become the predominant hydrogen-bonding interactions in the solvation. Additionally, it is found that as glycerol concentration increases, glycerol− water hydrogen bonds become progressively linear, strengthening of the interaction among molecules.42 Moreover, the singledirection relative movement of the friction pairs makes molecules in the solution arrange in order as well, which further enhances the molecule interaction.44,45 Consequently, on each side of the sliding surfaces, based on the FeOOH layer, glycerol molecules and water molecules forms a hydrogenbonded layer, which is firm enough to bear normal load, protecting the solid surfaces from contacting with each other. The ultralow friction coefficient demonstrates that there must be a layer, which is very easy to shear, between the friction pairs. Thus, it is inferred that there is a hydrated layer of water

Figure 8. Schematic illustration of the lubrication model and molecular schematic presentation of hydrogen-bonded layer of glycerol and water. Molecular schematic representation of hydrogen bond construction and schematic model of the hydration layer.

superlubricity is attributed to the hydrated layer of water between the hydrogen-bonded layers, which consist of glycerol and water molecules and strong enough to bear load, absorbed to the FeOOH layer on each side of the solid surfaces. As provided by Table 1, the viscosity of glycerol solution grows almost exponentially as relative humidity decreasing. When the humidity is lower than 30% RH, the lubricating solution has become so thick that the film thickness (h) is bigger than 72.4 nm and λ is larger than 3, entering the region of elastohydrodynamic lubrication. Furthermore, the amount of water in the solution is so little that shear occurs directly between glycerol molecules conquering much greater viscous force than that of water, resulting in relative high friction coefficient. With the aim to testify the lubrication model, two experiments were performed. After superlubricity was stable with proper humidity, a different amount of deionized water was added into lubricating liquid, and the results are presented in Figure 9. It is found that the friction coefficient rises abruptly by nearly 1 order of magnitude at the moment that water is added into the solution, indicating that the firm construction is totally destroyed by the added water. As time went on, the friction gradually recedes and finally stabilizes in superlubricity region. The inset of Figure 9 shows that the superlubricity restoring time is almost proportional to the volume of added water, which accords with the process of water evaporation. From this experiment, it can arrive to the conclusion that the firm hydrogen-bonded layer will always recover from excrescent water as long as the environment humidity is suitable. 11928

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Figure 11. Superlubricity and nonsuperlubricity can be switched by adjusting the humidity ranging from 15% RH to 40% RH.

Figure 9. Friction coefficient changes with excess water of different volume. The inset shows the time superlubricity needs to restore after adding excess water.

4. CONCLUSIONS In summary, our study has revealed that the friction coefficient between steel pairs lubricated with glycerol solution can be controlled via relative humidity. Superlubricity can be achieved when the humidity is in the range around 40% RH to 50% RH. It means that the lubrication state can be switched between superlubricity and nonsuperlubricity by adjusting humidity. A model of this lubrication system has been proposed. The mechanism of the controllability, including the origin of such low friction, is mainly attributed to the hydrogen bond patterns and water content in the lubricating solution, which depend on the environment humidity. This study enriches the research about water-based superlubricity and has a potential application in practical industrialization.

Another experiment was designed to reveal the deep correlation between the relative humidity and the friction coefficient. After the friction coefficient remained steady in the humidity of 10% RH, humidity was altered from 10% RH to 70% RH and then back to 10% RH. Figure 10 illustrates the

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AUTHOR INFORMATION

Corresponding Author

*Tel 8610-62781385; e-mail [email protected] (J.L.). Notes

The authors declare no competing financial interest.

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Figure 10. Friction coefficient changes as environment relative humidity and superlubricity is achieved under some range of humidity.

ACKNOWLEDGMENTS The work is financially supported by the National Natural Science Foundation of China (NSFC) (51027007), the National Key Basic Research Program of China (2013CB934200), and the Foundation for the Supervisor of Beijing Excellent Doctoral Dissertation (20111000305).

apparent response of the friction coefficient to the changing humidity and superlubricity will take place for a while during both the humidity rising process and falling process. It is necessary to note that there is about 8 cm of distance between the friction pairs and the humidity sensor, and there exists a delay for the sensor to response especially when humidity is changing. Therefore, Figure 10 just provides the trend of the humidity, not the exact humidity value of the air around the friction pairs. This experimental result gives further evidence that the firm hydrogen-bonded layer will be disrupted with inapposite humidity and recover when the humidity is appropriate. Finally, the results obtained above manifest that the friction coefficient can be controlled via the relative humidity and superlubricity can be achieved with proper humidity. In other words, the lubrication state can be switched between superlubricity and nonsuperlubricity by adjusting humidity. Considering friction coefficient is unstable and wear is large with thin lubricating solution under high humidity, the lower range of humidity is chosen to control friction (see Figure 11). With the controllability, the newly found lubrication system is of great potential for practical application.

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REFERENCES

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dx.doi.org/10.1021/la402422h | Langmuir 2013, 29, 11924−11930