Macroscale superlubricity enabled by hydrated alkali metal ions

Subscriber access provided by UNIV OF DURHAM

Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Macroscale superlubricity enabled by hydrated alkali metal ions Tianyi Han, Chenhui Zhang, and Jianbin Luo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01722 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Macroscale superlubricity enabled by hydrated alkali metal ions Tianyi Han, Chenhui Zhang*, Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China

ABSTRACT: Superlubricity based on hydration lubrication provides a near-frictionless lubrication state for the extreme reduction of friction in aqueous conditions. Nevertheless, how to obtain the hydration superlubricity under macroscale conditions with higher load-carrying capacity still remains a challenge and the mechanisms governing macroscale superlubricity with hydrated ions are still not well comprehended. Here we demonstrate that macroscale superlubricity based on hydrated alkali metal ions (Li+, Na+, K+) can be realized under high contact pressure between the Si3N4 ball and sapphire disc. The ultralow friction coefficients of 0.005 are obtained under average contact pressure up to 0.25 GPa by a universal micro tribometer after a running-in period with acid solutions. The results reveal that running-in stage with acid solutions can not only make the worn region smoother but can generate a silica layer easy to shear which provides excellent boundary lubrication. The hydration superlubricity originates because hydration shells surrounding the alkali metal ions could generate the hydration repulsive force to sustain a large normal load and have a fluid response to shear simultaneously. These findings pave the way to the scale-up of hydration superlubricity and thus to the wide application of new water-based lubricants. KEYWORDS: superlubricity; hydrated ions; hydration lubrication; monovalent salt solutions; silica layer; running-in; hydration repulsion 1

ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

INTRODUCTION Continues efforts to reduce friction and wear have been made since ancient times, and the use of lubricants is one of the effective ways to reduce friction and improve lubrication state.1 Superlubricity known as the state of vanishing friction or no resistance between two contacting and sliding surfaces2 will play a significant role in energy saving, environmental protection and increasing the lifetime of equipment. Considering the limitations of measurement, superlubricity refers to any lubricating state when the sliding friction coefficient is in the 0.001 order of magnitude.3 Since the research on superlubricity appeared, many valuable progresses have been achieved on solid superlubricity3-12 and liquid superlubricity13-20. Over the past decade, superlubricity based on hydration lubrication has been explored, particularly when considering friction between sliding surfaces in aqueous media. And the mechanism of hydration lubrication has emerged as a leading paradigm to understand and explain friction phenomenon and energy dissipation in liquid environments13,

21-22

, whereby the charges are surrounded strongly by

hydration shells. The hydration repulsion of hydrated charges could withstand high contact pressure while overcoming the van der Waals attraction between surfaces and the applied load on contact surfaces without being squeezed out due to the strongly-held hydration layers22. Simultaneously, the hydration layers would remain very rapidly relaxing resulting in a fluid response to shear, in other words, it is easy to shear resulting from the significantly lower shear strength, which provides an excellent boundary lubrication in aqueous environments22-24. Much progress has been made by using a surface force balance (SFB) to explore new hydration lubrication systems and reveal the hydration lubrication mechanism in aqueous 2


Page 2 of 46

Page 3 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

systems. There are mainly four types of hydration lubrication systems which have been discovered until now, including hydrated ions18,

24-26

, hydrated polymer brushes20,

27-28

,

amphiphilic surfactants23, 29-30 and phosphatidylcholine (PC) liposomes or bilayers19, 31-32. As for hydrated alkali metal ions trapped between two mica surfaces which are negatively charged and atomically smooth, the friction coefficient decreases to the order of magnitude of 10-4 ~ 10-3 by sustaining the contact pressure of about 30 MPa.18, 24-25 The friction force between silica surfaces in solutions of LiCl, NaCl, and CsCl salts has been measured using colloid-tipped lateral force microscopy (LFM), which shows that the lubrication property is consistent with the hydration effect of the cations trapped on silica surfaces.26 With respect to hydrated polymer brushes, the friction coefficient reaches the lowest level of 10-4 ~ 10-3 under the contact pressure of more than 15 MPa.20, 27 The friction coefficient of surfactant monolayers between two sliding mica surfaces decreases to the order of 0.001 under the critical pressure below 10 MPa when immersed in water, but its value increases by 100 times or more in dry air or oil.23, 29-30 Extremely ultralow friction coefficients of PC liposomes as low as 10-5 at mean pressures of over 10 MPa are measured between two sliding mica surfaces, and this ultralow friction is attributed to the hydration lubrication due to two hydrated phosphocholine head-group layers exposed by the liposomes rubbing past each other.32-33 However, how to utilize hydration lubrication in technological applications under macroscale conditions with higher load-carrying capacity between solid surfaces with relatively larger surface roughness (compared with the atomically smooth surface of mica) still remains a challenge.

3


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Corresponding to the above question, much effort has been made between some ceramic or glass materials with water or water-based lubricants. The friction coefficient of silicon nitride in water is achieved less than 0.01 by having a suitable running-in process with a pin-on-disc tribometer at the normal load of 1~5 N.14, 34 Furthermore, liquid superlubricity can be achieved between ceramic and sapphire or glass with the lubrication of phosphoric acid, as well as mixtures of acids and polyhydroxy alcohols by using a universal micro tribotester.15-17, 35 These liquid superlubricity is attributed to two factors, for one thing, the running-in process makes the worn region smoother resulting in the contact pressure decreasing, for another, the hydrodynamic effect plays an important role because the viscosity of H3PO4 solution increases to 25 mPa∙s (more than 10 times of the initial viscosity) with the evaporation of water.17, 36 Therefore, in order to obtain superlubricity under macroscale conditions and boundary lubrication state with higher contact pressure, meanwhile, to elucidate the hydration lubrication mechanism under macroscale conditions, we extend our studies on superlubricity based on hydration lubrication. Recently, we have found that the liquid superlubricity based on hydrated alkali metal ions could be achieved even when the maximal contact pressure between two friction surfaces reaches up to 0.25 GPa. For the first time, the hydration superlubricity is achieved between the sapphire disc and silicon nitride ball lubricated by potassium chloride solutions (also other lithium, sodium or potassium salt solutions). Therefore, in the present work, the superlubricity behavior of KCl solutions was studied in detail by a universal micro tribometer to explore conditions for achieving this superlubricity and reveal its mechanism.

4


Page 4 of 46

Page 5 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

MATERIALS AND METHODS The monovalent salts including lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), cesium chloride (CsCl), ammonium chloride (NH4Cl), potassium fluoride (KF), potassium bromide (KBr) and potassium iodide (KI) and alkali of potassium hydroxide (KOH) in the studies were commercially available from Shanghai Aladdin Bio-Chem Technology Co., Ltd and the purity of all these chemicals were 99.99%. Acids including phosphoric acid (H3PO4), hydrochloric acid (HCl) and sulfuric acid (H2SO4) were also commercial products whose purity was greater than 99% (Guaranteed Reagent (GR) or Analytical Reagent (AR)). Pure water used in all cases was purified by water purification system with total organic carbon (TOC) monitoring (GenPure xCAD UV/UF-TOC, Thermo Fisher Scientific), and its conductivity and nominal TOC was 18.2 MΩ·cm and < 2 ppb, respectively. Three different acid solutions (H3PO4, HCl and H2SO4) were prepared by diluting the pure acids to a pH value of 1.5 with ultrapure water. KOH solutions were dissolved by ultrapure water to a pH value of 12.5 and all the monovalent salts used during experiments were dissolved by ultrapure water to a concentration of 50 mmol/L. The friction pairs were composed of a silicon nitride (Si3N4) ball with a diameter of 14.288 mm (purchased from Beijing Sinoma Synthetic Crystal Co., Ltd) and a sapphire disc with a diameter of 25.4 mm and thickness of 3 mm (purchased from Chongqing Zhaohong Technology Co., Ltd). The original surface roughness ( ) of the ball and the disc was 12 nm and 0.2 nm, respectively. Ball-on-disc tribological experiments were conducted with a Universal Micro Tribometer (UMT-5, Bruker) in a rotational mode. Both the silicon nitride ball and the sapphire disc were 5


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ultrasonically cleaned with acetone, ethanol and ultrapure water in turn each for 10 minutes and then dried using compressed nitrogen before the measurements. During the tests, phosphoric acid solution with a pH value of 1.5 was introduced between the ball and the disc at a volume of 100 µL firstly and its friction coefficient was tested for 5 minutes, which was called the running-in period. And then the remaining acid solutions on the friction surfaces were washed out by pure water after the running-in period. Next, one kind of the salt solutions with a volume of 200 µL mentioned above was dropped between the ball and the disc. During the tests, the applied load was 3 N, which corresponded to a maximum initial contact pressure about 700 MPa and an initial contact diameter of about 89 µm according to the Hertz contact theory. The rotation speed of the disc was 180 rpm with a track radius of 5 mm, providing a linear sliding speed of 0.094 m/s. The normal force and frictional force were characterized by a 2D force sensor, and the measurement accuracy of the friction coefficient was ±0.001. All experiments under different conditions were conducted more than five times to ensure the repeatability. The ambient temperature was ~25 ℃ and the relative humidity was 20% - 40% during the whole experiments. The topography and curvature radius or contact radius of the friction surfaces were investigated by non-contact optical three-dimensional interference profilometer (MicroXAM-3D, ADE), before and after friction tests. Moreover, details of the ball’s worn region were characterized by means of scanning electron microscope (SEM, FEI Quanta 200 FEG) under a low vacuum conducted at an energy of 15 ekV, and X-ray photoelectron spectroscopy (XPS, PHI Quantera II, Ulvac-PHI) conducted at an atomic concentration resolution of 0.01% ~ 0.1%. In 6


Page 6 of 46

Page 7 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

addition, atomic force microscopy (AFM, Dimension ICON, Bruker) was used to investigate the topography and details of worn region, operating in ScanAsyst mode under ambient conditions.

RESULTS AND DISCUSSION Superlubricity of potassium chloride (KCl) solutions. The friction coefficients of KCl

solution at a concentration of 50 mmol/L were measured separately with or without running-in by phosphoric acid solution (H3PO4, pH = 1.5) between the Si3N4 ball and sapphire disc, as shown in Fig 1(a). When KCl solution was used directly as the lubricant without running-in by acid solutions, it can be seen that the friction coefficient decreased from 0.3 to 0.024 after a period of 200 s, after which, the friction coefficient was stable at 0.024. Nevertheless, if the original surfaces of the ball and the disc were running-in by H3PO4 solution (pH=1.5) for 300 s first, the friction coefficient decreased to ~0.03. After that, the remaining H3PO4 solution was washed out by ultrapure water, then KCl solution was dropped on the sliding surface and the measurement was continued without changing contact regions on the disc and the ball. Then it can be found that the friction coefficient decreased from 0.03 to 0.005 directly and kept stable till the end of the test. Furthermore, the friction tests with pure water both prior to running in in acid and following such running in were carried out. And it can be found that friction coefficients of pure water and pure water following acid were 0.070 (decreasing from 0.085 to 0.070) and 0.022, respectively. After running-in with acid, the surface roughness of the ball and the contact pressure between the ball and disc decreased (as shown in Fig S1 and Table S1), and the silica layer generated through the acid running-in process was a good boundary lubricant14, 41, which 7


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

made much lower friction coefficient of pure water after running-in with acid, compared with that without acid running-in. Comparing these four results, it can be concluded that the friction coefficient of KCl solution after running-in by acid solution could be reduced to the order of 0.001, which means superlubricity is achieved. Furthermore, as shown in Fig 1(b), it can be found that the ultra-low friction coefficient of 0.005 could be remained for more than 2 hours, corresponding to a sliding distance of about 700 meters. And the friction coefficient is still ultra-low after about 1 hour without pressure and shear, which indicates that the superlubricity state of KCl solution is very stable and can be achieved repeatedly.

Figure 1. Friction coefficient as a function of time under lubrication of 50 mM KCl solution between the Si3N4 ball and sapphire disc. (a) Friction coefficients with the lubrication of KCl solution, H3PO4 solution (pH = 1.5), KCl 8


Page 8 of 46

Page 9 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

solution after running-in by H3PO4 solution, pure water and pure water after running-in by H3PO4 solution. (b) Friction coefficient with time under lubrication of KCl solution for about 2 hours and that with time after waiting for 1 hour. The inset shows the schematic UMT-5 configuration.

Relationship between superlubricity and types of cations or anions. To determine which ions play the key role in achieving this superlubricity, the following experiments were designed. First, investigations between superlubricity and types of cations were conducted. Friction coefficients as a function of time under lubrication of 50 mM chloride salt solutions including lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), cesium chloride (CsCl) and ammonium chloride (NH4Cl) solutions were measured, as shown in Fig 2(a). It can be found that the friction coefficients of CsCl and NH4Cl solutions are 0.014 and 0.025 respectively, which means these two chloride salt solutions couldn’t achieve the superlubricity state, although these two lubricants could attain a low friction. However, it is obvious that the friction coefficients of LiCl, NaCl and KCl solutions are all around 0.005, which represents that these three different alkali metal chloride salt solutions could achieve superlubricity after running-in by H3PO4 solution (pH = 1.5). As shown in Fig 2(b), the friction coefficients of these five different chloride salt solutions were measured more than five times. Apparently, the only difference among these five different chloride salt solutions is the type of the cation, as shown in Fig 2(b). The differences between the hydrated alkali metal ions including the bare ion radii, hydrated radii and hydration number of ions in water are seen clearly in Table 1. It can be found that the lithium ion (Li+), sodium ion (Na+) and potassium ion (K+) can form hydrated ions by combining three or more water molecules. However, each cesium ion (Cs+) and ammonium ion (NH4+) can only combine one or less water molecule, which results in their relatively weak 9


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 46

hydration strength.37 In addition, based on the hydrated radii, hydration number and hydration energy of these ions, it can be seen that the hydration intensity decreases in the sequence Li+ > Na+ > K+ > Cs+ > NH4+, as shown in Table 1. Therefore, it can be concluded that the superlubricity achieved by hydrated alkali metal ions is in high accordance with the hydration strength of these hydrated ions.

Table 1. Properties of the hydrated ions*

bare ionic

hydrated

hydration

hydration energy,

critical hydration

radius, nm

radius, nm

number, ±1

kJ/mol

concentration

Li+

0.068

0.38

5

-510

60 mM

Na+

0.095

0.36

4

-410

10 mM

K+

0.133

0.33

3

-337

0.1 mM

Cs+

0.169

0.33

1

-283

—

NH4+

0.148

0.33

—

—

—

ion

*The hydration number means the number of water molecules in the primary shell. Table compiled from data given by reference.25, 37-40

Next, to investigate the relationship between superlubricity and types of anions, four different potassium salt solutions including potassium fluoride (KF), potassium chloride (KCl), potassium bromide (KBr) and potassium iodide (KI) were used as lubricants. The friction coefficients as a function of time were measured, as shown in Fig 2(c). It is obvious that the friction coefficients were reduced to ~0.03 after a running-in period of 300 s with H3PO4 solution, and then the

10


Page 11 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

friction coefficients of all these potassium salt solutions were reduced to 0.005, namely, all these potassium salt solutions could achieve superlubricity. Fig 2(d) shows the comparison of friction coefficients under lubrication of these four different potassium salt solutions and the inset illustration shows the size of four halogen anions. It should be pointed out that there is no systematic difference between them according to the result of friction coefficients. Therefore, it can be concluded that the superlubricity of the above monovalent salt solutions is mainly determined by cations rather than halogen anions. In addition, we need to point out that the hydration numbers of these four halogen anions are all 0 ~ 2 (±1), which means their hydration strength is relatively weak.37

11


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Friction coefficients under lubrication of 50 mM chloride/potassium salt solutions, (a, b) five different chloride salt solutions (LiCl, NaCl, KCl, CsCl and NH4Cl), and (c, d) four different potassium salt solutions (KF, KCl, KBr and KI) after running-in by H3PO4 solutions (pH = 1.5). (a, c) Filled symbols represent friction coefficients of different salt solutions after running-in by H3PO4 solutions, and half-filled symbols represent friction coefficients of H3PO4 solutions. (b, d) The inset illustration shows the size of these five cations and four halide anions. The error bars represent the standard deviations of these multiple measurements, where the number of measurements per salt solution exceeds five.

Effects of running-in stage on superlubricity. To investigate how the running-in stage influence the superlubricity of KCl solution, friction coefficients of different lubricants used during running-in stage were measured. Fig 3(a) shows friction coefficients versus time under lubrication of KCl solution after running-in by sulfuric acid solution (H2SO4, pH = 1.5) and hydrochloric acid solution (HCl, pH = 1.5). It can be found that the friction coefficients both decrease to ~0.03 after running-in by H2SO4 and HCl solutions for 300 s, which is consistent with the result of running-in by H3PO4 solution. Moreover, friction coefficients of KCl solutions all decrease to 0.004 ~ 0.005 after running-in with H2SO4 and HCl solutions, which is similar with the result after running-in with H3PO4 solution. It can be indicated that all these three acid solutions have the same effect on achieving superlubricity of KCl solution. However, in the cases of running-in with KCl solution (50 mM) and KOH solution (pH = 12.5), friction coefficients of KCl solution become larger to ~0.025 and ~0.030 respectively, as shown in Fig 3(b). Therefore, it can be deduced that the hydrogen ions in acid solutions play the key role in achieving superlubricity of KCl solutions during the running-in stage, which is consistent with the previous result.41 Besides, the inset cartoon illustrates the mechanism of running-in stage with acid solutions, where a silica layer is generated on the worn region of the Si3N4 ball through the tribochemical reaction, and the detailed analysis is in section 3.4. In addition, silicon nitride and 12


Page 12 of 46

Page 13 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

sapphire are negatively charged in aqueous solutions due to the hydroxyl groups ([Al-OH] and [Si-OH]) formed on the surfaces.42-44 Therefore, hydrated potassium ions formed in whole KCl solution would be adsorbed on the surfaces of the Si3N4 ball and sapphire disc, which are formed by water molecules aligning themselves around the potassium ions (K+) due to the polarity of water molecules.37, 40 These results shed light on the macroscale superlubricity mechanism under lubrication of KCl solution achieved by hydrated potassium ions, namely, hydration lubrication of potassium ions.

Figure 3. Friction coefficients as a function of time under lubrication of 50 mM KCl solution after running-in by different materials, including H2SO4 solution (pH =1.5), HCl solution (pH =1.5), KCl solution (50 mM) and KOH solution (pH =12.5). The inset cartoon illustrates the mechanism of running-in stage with acid solutions. The error bars represent standard deviations of more than five measurements. 13


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

To elucidate the effect on superlubricity of the hydrogen ions in acid solutions, the friction coefficients of KCl solution after running-in by H3PO4 solution at different pH values were measured, as shown in Fig 4(a). It can be found that if the pH value of acid solutions is below 3.0, the superlubricity of KCl solution can be achieved after running-in by these acid solutions. However, when the pH value of H3PO4 solution is greater than 3.0, the friction coefficient of KCl solution increases above 0.01, namely, the superlubricity cannot be obtained. Besides, it can be seen that the average contact diameter of the worn region on the ball increases from 124 µm to 242 µm and the average contact pressure between the ball and disc decreases from 292 MPa to 148 MPa with the pH value of H3PO4 solution increasing, as shown in Figs 4(b-g). Figs 4(h-j) present the X-ray photoelectron spectroscopy (XPS) measurements on the Si 2p peaks of the worn region on the Si3N4 ball after running-in by acid solutions with different pH values. The Si 2p peaks for the worn region on the Si3N4 ball (as shown by the black curve) can be deconvoluted into the Si-N (Si3N4, ~101.5 eV, the red line) and Si-O (SiO2, ~102.9 eV, the blue line) peaks according to the peak shape parameters and peak positions of Si-N component and Si-O component.45 And the relative concentration of SiO2 on the worn region of the Si3N4 ball can be calculated from SiO = SiO ⁄SiO + Si N , where SiO and Si N represents the concentration of SiO2 and Si3N4 (which was based on the fitted peak areas of SiO2 and Si3N4), respectively. It can be calculated that the relative concentration of SiO2 on the worn region of the ball after running-in by H3PO4 solutions with pH values of 0.0, 1.5 and 4.5 is 55.1%, 46.0% and 25.7%, respectively. Therefore, it can be concluded that when the 14


Page 14 of 46

Page 15 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

concentration of hydrogen ions in acid solutions is too low (lower than 1 mM, namely pH value greater than 3.0), less silica will be formed on the surface of the Si3N4 ball due to the insufficient tribochemical reaction, resulting in an increase in friction coefficient of KCl solution.

Figure 4. (a) Friction coefficient of KCl solution after running-in by acid solutions with different pH values from 0.0 to 4.5. (b) The contact pressure between Si3N4 ball and sapphire disc, and (c-g) SEM images of wear scar on Si3N4 ball lubricated by KCl solution after running-in by acid solutions with five different pH values. (h-j) The XPS spectra of Si 2p from the worn region on the Si3N4 ball after running-in by acid solutions with pH values of 0.0, 1.5 and 4.5 respectively.

Surface analysis of the contact region. To investigate the lubrication state during the superlubricity and elucidate the superlubricity mechanism, the surface morphologies were 15


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

studied, as shown in Fig 5. Fig 5(a, b) and Fig 5(c, d) show the surface morphologies of the contact area on the Si3N4 ball after running-in by H3PO4 solution and after achieving the superlubricity, respectively. Fig 5(a, b) shows that the contact diameter of the wear scar on the surface of Si3N4 ball is about 150 µm and the average equivalent curvature radius of the wear scar is about 18.8 mm. Therefore, it can be concluded that the wear scar is a curved surface (spherical crown shape) with a curvature radius of 18.8 mm greater than the initial radius (7.144 mm) of the Si3N4 ball, instead of a complete plane, corresponding to the average contact pressure of 254 MPa based on the Hertz contact theory. Fig 5(c, d) shows the contact diameter of the wear scar is also about 150 µm and the average equivalent curvature radius of the wear scar is about 18.9 mm, which is similar to the curvature radius of the wear scar lubricated by H3PO4 solution basically, corresponding to the average contact pressure of 253 MPa. Fig 5(e, f) shows that the surface of the ball inside the worn region ( = 5 nm) is smoother than the surface outside it ( = 12 nm), which means that the surface of the Si3N4 ball becomes smoother after running-in by H3PO4 solution for 300 s. However, Fig 5(f) shows that there are no obvious worn signs on the surface of the sapphire disc after the running-in process.

16


Page 16 of 46

Page 17 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 5. 3D surface morphologies of the contact area on the Si3N4 ball (a, b) after running-in by H3PO4 solution, and (c, d) after the superlubricity is achieved. The inset illustrations of (a) & (c) show the height profile vs width of the contact area of the Si3N4 ball on the position of the black arrows in (b) & (d). The ruler in SEM images of (b) & (d) indicates 50 µm. (e, f) One dimensional morphology of the surfaces on the initial ball, the ball after running-in by H3PO4 solution, the ball and the disc lubricated by KCl solution after running-in by H3PO4 solution.

The previous results shown that a silica layer easy to shear would be formed on the surface of worn region after running-in with acid solutions. In order to further verify the formation of the silica layer on the worn region of the Si3N4 ball, the XPS measurements were carried out on the Si 2p peaks of the worn region to evaluate the components of silica, as shown in Fig 6. The deconvolution of Si 2p spectrum in Fig 6 was the same as that in Fig 4(h-j). Based on the fitted peak areas of SiO2 and Si3N4, it can be calculated that the relative concentration of SiO2 on the worn region of the ball when the superlubricity was achieved was 55.1%, and the relative concentration of SiO2 after running-in by H3PO4, KCl and KOH solutions was 46.0%, 21.7% and 5.9%, respectively, as shown in Fig 6(a-d). As a comparison, the Si3N4 ball after running-in with 17


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

acid solutions was immersed in HF solution (10%) for 20 minutes, in order to remove the silica layer, and then surfaces were analyzed by XPS, as shown in Fig 6(e). It can be found that the relative concentration of SiO2 decreases from 46.0% to 2.1% significantly. Finally, the relative concentration of SiO2 for the original ball without any treatment is 10.1% because there is yttrium oxide (Y2O3) and aluminum oxide (Al2O3) in the sintering aids of Si3N4 ball, as shown in Fig 6(f). Therefore, it can be indicated that the silica layer is more easily generated after running-in by H3PO4 than KCl or KOH solutions.

Figure 6. The XPS spectra of Si 2p from worn region of the silicon nitride ball (a) when superlubricity of hydrated potassium ions is achieved, (b) after running-in by H3PO4 solution, (c) after running-in by KCl solution, (d) after running-in by KOH solution, (e) when soaked by HF solution and (e) the initial surface.

From the above results, it has been shown that only through running-in by acid solutions (H3PO4, HCl, H2SO4) can superlubricity be achieved. In addition, in order to further explore the effect of running-in stage on hydration superlubricity, the friction coefficient of KCl solution 18


Page 18 of 46

Page 19 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

after the ball soaked in HF solution (10%) was measured, as shown in Fig 7 (a). After the running-in period with acid solutions, the ball was immersed in HF solution for 20 minutes, and then KCl solution was added to the system. And it can be found that the friction coefficient is up to ~0.032, which indicates that the superlubricity of KCl solution is failed if the Si3N4 ball is treated with HF solution. Furthermore, when the hydration superlubricity was achieved, the KCl solution was washed out and the ball was immersed in HF solution for 20 minutes, and then KCl solution was added to the system again. It can be seen that the friction coefficient even exceeds 0.045 and increases gradually. Therefore, it can be confirmed that the superlubricity of KCl solution can’t be achieved as long as the Si3N4 ball is treated with HF solution. Besides, it is once reported that Si3N4 would react with water slowly because of the narrow energy gap44, as shown in reaction (1), Si N + 6H O → 3SiO + 4NH ↑

(1)

So, it can be inferred that the reaction was enhanced by the hydrogen ions during the running-in period, and a silica layer was formed on the worn region of the Si3N4 ball, which was consistent with the XPS results in Fig 6. Therefore, the surface of the Si3N4 ball (namely, the silica layer on the Si3N4 ball) is negatively charged in water or KCl solutions due to the hydroxyl groups [Si-OH] on the silica, which could be attributed to the dissociation reactions of the hydroxyl groups as follows,

Si − OH !""# SiO$ + H O

(2)

so the [Si-OH] groups play a major role in the behavior for the negatively charged surface of the Si3N4 ball44, 46. The sapphire disc for A-plane (112'0) has [Al-OH] groups on the surface with a 19


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 46

surface hydroxyl [OH-] density of 14.56 sites/nm2, so the surface on the sapphire disc would also be negatively charged in water.42-43 These analyses clarify why surfaces of the ball and disk are negatively charged, which is further explained by the illustration in Fig 9. Then, when the Si3N4 ball was soaked in HF solution, the dissolution of SiO2 on the worn region into an aqueous HF solution could be described by reaction (3, 4)47, SiO + 4HF → SiF ↑ +2H O

(3)

SiF + 2HF → H SiF*

(4)

Fig 7(b) and Fig 7(c) show AFM images on the worn region of the ball after running-in with H3PO4 and after the ball soaked with HF solution, respectively. It can be seen that there are more pits on the worn region after the ball soaked with HF solution. Therefore, combined with friction coefficients in Fig 7(a), it can be concluded that the silica layers formed through the tribochemical reaction and deposited on the worn region have a negative charge in aqueous solutions, and therefore they trap hydrated potassium counterions which then provide excellent hydration lubrication.

20


Page 21 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 7. (a) Friction coefficients of KCl solution with or without treatment by HF solution between Si3N4 ball and sapphire disc. Different symbols are from different experiments with different conditions, but symbols with same colors represent the same set of experiments between the same ball and disc. (b, c) AFM images of the worn region on Si3N4 ball after running-in with H3PO4 solution and after the ball soaked in HF solution (10%) for 20 minutes. Two figures inside show the height profile of the Si3N4 ball on the position of the blue lines. The maximum depth of pits in (b) is about 5 nm, and the maximum depth of pits in (c) is about 120 nm and there are some pits with the depth of 30 nm.

Analysis for the superlubricity mechanism of hydrated potassium ions. In order to determine the lubrication state when the superlubricity of hydrated potassium ions is achieved and assess the stability of this superlubricity, friction coefficients of KCl solution were measured through changing the disc, the ball and the rotation direction, as shown in Fig 8(a). It can be found that the friction coefficient still remains 0.005 even if the rotation direction is changed, in other words, the KCl solution still stabilize in the region of superlubricity. Besides, when the disc is replaced by a new one or the running track is changed to a new position, the friction 21


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 46

coefficient also remains stable below 0.01. However, the friction coefficient increases significantly when the ball is changed to a new one. Therefore, it can be concluded that the superlubricity could remain stable even if the disc or the rotation direction is changed, then it can be inferred that there’s almost no wear on the sapphire disc. Furthermore, to determine whether the hydrodynamic effect contributes to the realization of the macroscale superlubricity, the film thickness is calculated based on the Hamrock-Dowson theory48,which is based on Reynolds equation35, as shown in formula (5), +,∗ = 2.69

0 ∗1.23 4 ∗1.56 7 ∗1.156

1 − 0.618 $9.:;

(5)

where +,∗ = ℎ, ⁄=> is the dimensionless central film thickness; ? ∗ = @A is the dimensionless material elastic module; B ∗ = C9 D⁄A=> is the dimensionless speed; E ∗ = F ⁄A=> is the dimensionless

load;

G = 1.03J=> ⁄=K L

9.*

G = H⁄I

is

the ellipticity,

which

is

approximately equal

to

. In our experiments, = = => = =K = 18.9 mm is the equivalent radius

of the worn region on the ball when superlubricity is achieved, and @, A, F is the viscosity-pressure coefficient, the effective modulus of elasticity ( 2⁄A = 1 − PQ ⁄AQ + 1 − P ⁄A , and PQ , AQ , P , A is the poisson ratio & elastic modules of the ball & disc, respectively.) and the normal load, respectively. Besides, C9 ≈ 0.9 mPa ∙ s (at 25 ℃) is the dynamic viscosity of the 50 mM KCl solution, and D is the relative velocity between the ball and the disc. According to formula (5), the central film thickness as a function of the relative sliding velocity between the ball and disc is calculated and the calculated result is shown in Fig 8(b). Furthermore, the thickness-roughness ratio λ (the ratio of theoretical central film thickness ℎ, and the equivalent surface roughness σ) is used as the criterion for the lubrication state 22


Page 23 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

division48, which can be expressed by the formula (6), λ = ℎ, ⁄W = ℎ, ⁄XWQ + W

(6)

where WQ and W are the variances of the ball and the disc surface roughness respectively, and in our experiments, WQ = 5.0 nm for the ball and W = 0.2 nm for the disc after friction tests. Generally, if λ ≥ 3, it is full-film lubrication (hydrodynamic lubrication), which means the lubricant has completely separated the surfaces of two friction pairs, and two surfaces is no longer in direct contact. And if 1 ≤ λ ≤ 3, the asperities will contact so it belongs to mixed lubrication, and if λ < 1, it is boundary lubrication. Therefore, the equivalent surface roughness W equals 5 nm (as shown by the purple dotted line in Fig 8(b)) and the film thickness ℎ, is equal to 1.2 nm through calculation in ideal conditions so the ratio λ ≈ 0.23 < 1, which indicates that the friction pairs of Si3N4 ball and sapphire disc run in the boundary lubrication or mixed lubrication regime when superlubricity is achieved, namely, the lubrication state is not full-film lubrication undoubtedly. Besides, it is worth noting that the calculated film thickness was about 1.2 nm, so the hydration effect could exist, which was consistent with Klein’s results25, because they found that strong hydration repulsive force existed in the trapped counterions when the surface separation was less than 2 ~ 3 nm (as shown by the red dotted lines in Fig 8(b))21, 24-25. In other words, these results indicate that the hydrodynamic effect plays a minor role in the macroscale superlubricity achieved by KCl solution. Therefore, there exists other mechanism that leads to the realization of this superlubricity, namely the hydration effect of the cations. Furthermore, to clarify that the lubrication state during the hydration superlubricity is not full-film lubrication, friction coefficients of KCl solution as a function of sliding speed were 23


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

measured, as shown in Fig 8(c). It could be found that the friction coefficient of KCl solution with an applied load of 3 N decreased from 0.063 to 0.002 as the sliding speed increased from 6 mm/s to 880 mm/s. When the sliding speed was larger than 60 mm/s, the superlubricity could be obtained. So, the critical sliding speed (CSS) of KCl solution at the applied load of 3 N was 60 mm/s. When the sliding speed was larger than 0.96 m/s, the solution would be thrown out of the contact area due to the effect of centrifugal force, resulting in the increased friction coefficient. As the applied load was reduced from 3 N to 1 N, the curve of friction coefficient showed similar trend with the sliding speed increasing as the case of 3 N load. It could be found that the critical sliding speed at the applied load of 1 N was 45 mm/s, which was less than the CSS at the load of 3 N. As a comparison, the friction coefficient of pure water (after running-in with acid) as a function of sliding speed was measured. Clearly, the friction coefficient decreased from 0.24 to 0.005 with the sliding speed increasing from 6 mm/s to 630 mm/s. When the sliding speed was larger than 240 mm/s, the superlubricity of water could be achieved because of the hydrodynamic effect, but the critical sliding speed was 240 mm/s, which was larger than that of KCl solution under the same applied load. In addition, the friction coefficients of water over the entire range of sliding speed were larger than that of KCl solution, which further demonstrated that the hydration effect and hydration repulsion of hydrated ions played a significant role in the realization of superlubricity of KCl solution. Therefore, it can be concluded that the hydration effect will play the key role when the sliding speed is less than the CSS (which is 240 mm/s with the applied load of 3 N), and the hydrodynamic effect will dominate the superlubricity if the sliding speed is greater than the CSS. However, the hydration effect still plays an important role 24


Page 24 of 46

Page 25 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

even if the sliding speed is higher than the CSS.

Figure 8. (a) Friction coefficients of KCl solution as a function of time under different conditions, including changing rotation direction, replacing the ball or the disc to a new one, and changing the running track. (b) The central film thickness of KCl solution as a function of the relative sliding velocity between the ball and disc under ideal conditions. The red point represents the film thickness corresponding to the rotation speed (180rpm) in our experiments. (c) Friction coefficients of KCl solution and pure water as a function of sliding speed, The inset shows some of the data on an expanded scale.

According to the aforementioned results and analyses which confirm that the KCl solution can achieve macroscale superlubricity between the silicon nitride ball and the sapphire disc after running-in by acid solutions under boundary lubrication, the superlubricity model for the hydrated alkali metal ions to reveal its mechanism is proposed, as shown in Fig 9. First, during the running-in stage with acid solutions, a considerably smoother contact surface on the worn 25


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

region of the Si3N4 ball (compared with the original surface) could be formed, and the worn region becoming smoother leads to the contact pressure reducing from more than 700 MPa to 250 MPa, which contributes to the superlubricity of hydrated alkali metal ions to some extent. Meanwhile, a silica layer on the worn region is generated through the tribochemical reaction between silicon nitride and acid solutions during the running-in stage, which results in a significantly lower shear strength during the direct contact between the ball and sapphire disc. In other words, the silica layer provides excellent boundary lubrication during the asperity peaks sliding past each other, as shown in Fig 9(a). Next, Fig 9(b) elucidates that the surface of the silicon nitride ball and sapphire disc in KCl solutions is negatively charged, due to the hydroxyl groups ([Al-OH] and [Si-OH]) formed on the surfaces, as also shown by previous reaction (1, 2). Furthermore, when the acid solutions are washed out and the monovalent salt solutions are introduced into this system, the alkali metal ions in the salt solutions would be surrounded by hydration shells to form hydrated ions, which could be confined between the two negatively charged surfaces sliding past each other. More importantly, the hydrated alkali metal ions in the salt solutions have good capacities including not only sustaining a large normal load through the hydration repulsion of the hydrated ions but also a fluid response to shear due to the low viscosity of the monovalent salt solutions similar to the viscosity of water. Therefore, these good capacities could provide excellent boundary lubrication and contribute to the achievement for the macroscale superlubricity of hydrated alkali metal ions significantly, as reveled in Fig 9(c). Finally, based on the superlubricity mechanism shown in Fig 9, the superlubricity achieved by hydrated alkali metal ions could be attributed to forming hydration shells surrounding the alkali 26


Page 26 of 46

Page 27 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

metal ions which could generate the hydration repulsive force to sustain a large normal load and have a fluid response to shear between the silicon nitride ball and the sapphire disc. Simultaneously, the running-in process with acid solutions also plays a significant role in the achievement of this superlubricity, owing to the formation of the silica layer deposited on the worn region which could reduce the shear stress between asperities of the two surfaces. From the superlubricity mechanism and the lubrication model, the superlubricity achieved by hydrated alkali metal ions based on hydration lubrication is first achieved under macroscopic conditions with higher contact pressure (250 MPa, about one order of magnitude higher) than original studies and lager surface roughness (5 nm, about one order of magnitude larger) than mica.25 There is no doubt that the macroscale superlubricity based on hydration lubrication under higher contact pressure or normal load would have great potential for industrial applications.

Figure 9. Schematic illustration of the superlubricity model for hydrated alkali metal ions between the silicon nitride ball and sapphire disc. (a) Relatively macroscopic lubrication models. (b) The reason why the surface of the silicon nitride ball and sapphire disc in KCl solution is negatively charged. (c) The hydration lubrication mechanism schematically when the hydrated alkali metal ions (Li+, Na+, K+) are confined to two negatively charged surfaces sliding past each other. This cartoon is approximately to scale, for ca. 0.7 nm (the diameter of the first hydration shell about the hydrated potassium ion) < D < ca. 3 nm (separation distance where the hydration repulsion is obvious weak relatively).

27


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CONCLUSION The present work demonstrates that the macroscale superlubricity of hydrated alkali metal ions (Li+, Na+, K+) can be achieved between the silicon nitride ball and sapphire disc under a high contact pressure of 250 MPa after running-in with acid solutions (H3PO4, HCl, H2SO4, pH = 0.5~3.0), and the friction coefficients are all around 0.005. For all five alkali metal ions (including Cs+ and NH4+), friction coefficients across confined hydrated ions under lubrication of 50 mM chloride salt solutions are characterized. And the sequence of friction coefficients is as follows, ^_`a ≈ ^bca ≈ ^da < 0.01 < ^efa < ^bag , which is in accordance with the mechanism of hydration lubrication, where a stronger degree of hydration leads to better lubrication and less friction24-25. This result is consistent with the expectation analogous to the concept of the Hofmeister series that lubrication will improve with the extent of ionic hydration49-50. We have also characterized that there is almost no difference for the friction coefficient (all around 0.005) across the four different monovalent halogen anions (F-, Cl-, Br-, I-), which is because the negatively charged surfaces of the friction pairs would absorb more hydrated cations rather than anions. Moreover, the present work has shown that only through running-in with acid solutions can superlubricity of hydrated alkali metal ions be achieved, and the running-in process could play significant roles in two aspects. On the one hand, running-in process makes the surface on the worn region smoother and decreases the contact pressure to 250 MPa (which is a high pressure). On the other hand, the silica layer, which is generated through the tribochemical reaction during the running-in process and negatively charged in aqueous solutions, will trap hydrated potassium counterions which then provide excellent hydration 28


Page 28 of 46

Page 29 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

lubrication to reduce the shear strength between asperities on the surface of friction pairs. In summary, the research proves the feasibility of achieving macroscale superlubricity based on hydration lubrication under high contact pressure of 250 MPa. Moreover, we confirm that the hydration effect rather than the hydrodynamic effect plays a more significant role in this macroscale superlubricity and reveal the mechanism for superlubricity achieved by hydrated alkali metal ions. This research would provide ideas on realizing superlubricity based on hydration lubrication under macroscopic scale and high pressure, meanwhile it is possible to utilize hydration lubrication on industrial applications.

29


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ASSOCIATED CONTENT Supporting Information. 3D surface morphologies of the contact area on the Si3N4 ball after running-in with water and H3PO4 solution, comparison of surface roughness and contact pressure between friction of pure water following acid running-in and without acid running-in.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The work was financially supported by the National Natural Science Foundation of China (51335005, 51527901, 51605351).

30


Page 30 of 46

Page 31 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

REFERENCES

(1) Holmberg, K.; Erdemir, A. Influence of Tribology on Global Energy Consumption, Costs and Emissions. Friction 2017, 5, 263-284. (2) Hirano, M.; Shinjo, K. Superlubricity and Frictional Anisotropy. Wear 1993, 168, 121-125. (3) Martin, J.; Donnet, C.; Le Mogne, T.; Epicier, T. Superlubricity of Molybdenum Disulphide. Phys. Rev. B 1993, 48, 10583. (4) Vu, C. C.; Zhang, S.; Urbakh, M.; Li, Q.; He, Q.-C.; Zheng, Q. Observation of Normal-Force-Independent Superlubricity in Mesoscopic Graphite Contacts. Phys. Rev. B 2016, 94, 081405. (5) Feng, X.; Kwon, S.; Park, J. Y.; Salmeron, M. Superlubric Sliding of Graphene Nanoflakes on Graphene. Acs Nano 2013, 7, 1718-1724. (6) Kawai, S.; Benassi, A.; Gnecco, E.; Söde, H.; Pawlak, R.; Feng, X.; Müllen, K.; Passerone, D.; Pignedoli, C. A.; Ruffieux, P.; Fasel, R.; Meyer, E. Superlubricity of Graphene Nanoribbons on Gold Surfaces. Science 2016, 351, 957-961. (7) Chhowalla, M.; Amaratunga, G. A. Thin Films of Fullerene-like MoS2 Nanoparticles with Ultra-low Friction and Wear. Nature 2000, 407, 164. (8) Berman D.; Deshmukh. S. A.; Sankaranarayanan. S. K. R. S.; Erdemir, A.; Sumant, A. V. Macroscale Superlubricity Enabled by Graphene Nanoscroll Formation. Science. 2015, 348, 1118-1122. (9) Liu, S. W.; Wang, H. P.; Xu, Q.; Ma, T. B.; Yu, G.; Zhang, C.; Geng, D.; Yu, Z.; Zhang, S.; Wang, W.; Hu Y. Z.; Wang H.; Luo, J. Robust Microscale Superlubricity under High Contact Pressure Enabled by Graphene-Coated Microsphere. Nat. Commun. 2017, 8, 14029. (10) Cumings, J.; Zettl, A. Low-friction Nanoscale Linear Bearing Realized From Multiwall Carbon Nanotubes. Science 2000, 289, 602-604. (11) Chen, X.; Kato, T.; Nosaka, M. Origin of Superlubricity in a-C:H:Si Films: A Relation to Film Bonding Structure and Environmental Molecular Characteristic. Acs Appl. Mater. Interfaces 2014, 6, 13389-13405. (12) Chen, X.; Zhang, C.; Kato, T.; Yang, X.-a.; Wu, S.; Wang, R.; Nosaka, M.; Luo, J. Evolution of Tribo-Induced Interfacial Nanostructures Governing Superlubricity in a-C:H and a-C:H:Si Films. Nat. Commun. 2017, 8, 1675. 31


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(13) Raviv, U.; Laurat, P.; Klein, J. Fluidity of Water Confined to Subnanometre Films. Nature 2001, 413, 51-54. (14) Xu, J.; Kato, K. Formation of Tribochemical Layer of Ceramics Sliding in Water and Its Role for Low Friction. Wear 2000, 245, 61-75. (15) Li, J.; Zhang, C.; Luo, J. Superlubricity Behavior with Phosphoric Acid-Water Network Induced by Rubbing. Langmuir 2011, 27, 9413-9417. (16) Li, J.; Zhang, C.; Ma, L.; Liu, Y.; Luo, J. Superlubricity Achieved with Mixtures of Acids and Glycerol. Langmuir 2012, 29, 271-275. (17) Deng, M.; Li, J.; Zhang, C.; Ren, J.; Zhou, N.; Luo, J. Investigation of Running-in Process in Water-Based Lubrication Aimed at Achieving Super-Low Friction. Tribol. Int. 2016, 102, 257-264. (18) Ma, L.; Gaisinskaya-Kipnis, A.; Kampf, N.; Klein, J. Origins of Hydration Lubrication. Nat. Commun. 2015, 6, 6060. (19) Seror, J.; Zhu, L.; Goldberg, R.; Day, A. J.; Klein, J. Supramolecular Synergy in the Boundary Lubrication of Synovial Joints. Nat. Commun. 2015, 6, 6497. (20) Chen, M.; Briscoe, W. H.; Armes, S. P.; Klein, J. Lubrication at Physiological Pressures by Polyzwitterionic Brushes. Science 2009, 323, 1698-1701. (21) Raviv, U.; Klein, J. Fluidity of Bound Hydration Layers. Science 2002, 297, 1540-1543. (22) Klein, J. Hydration Lubrication. Friction 2013, 1, 1-23. (23) Briscoe, W. H.; Titmuss, S.; Tiberg, F.; Thomas, R. K.; Mcgillivray, D. J.; Klein, J. Boundary Lubrication under Water. Nature 2006, 444, 191-194. (24) Gaisinskaya, A.; Ma, L.; Silbert, G.; Sorkin, R.; Tairy, O.; Goldberg, R.; Kampf, N.; Klein, J. Hydration Lubrication: Exploring a New Paradigm. Faraday Discuss. 2012, 156, 217-233. (25) Gaisinskaya-Kipnis, A.; Ma, L.; Kampf, N.; Klein, J. Frictional Dissipation Pathways Mediated by Hydrated Alkali Metal Ions. Langmuir 2016, 32, 4755-4764. (26) Donose. B. C.; Vakarelski. I. U.; Higashitani. K. Silica Surfaces Lubrication by Hydrated Cations Adsorption from Electrolyte Solutions. Langmuir 2005, 21, 1834-1839. (27) Tairy, O.; Kampf, N.; Driver, M. J.; Armes, S. P.; Klein, J. Dense, Highly Hydrated Polymer Brushes via 32


Page 32 of 46

Page 33 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Modified Atom-Transfer-Radical-Polymerization: Structure, Surface Interactions, and Frictional Dissipation. Macromolecules 2015, 48, 140-151. (28) Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J.-F.; Jérôme, R.; Klein, J. Lubrication by Charged Polymers. Nature 2003, 425, 163-165. (29) Silbert, G.; Kampf, N.; Klein, J. Normal and Shear Forces between Charged Solid Surfaces Immersed in Cationic Surfactant Solution: the Role of the Alkyl Chain Length. Langmuir 2014, 30, 5097-5104. (30) Li, J.; Zhang, C.; Cheng, P.; Chen, X.; Wang, W.; Luo, J. AFM Studies on Liquid Superlubricity between Silica Surfaces Achieved with Surfactant Micelles. Langmuir 2016, 32, 5593-5599. (31) Sorkin, R.; Kampf, N.; Zhu, L.; Klein, J. Hydration Lubrication and Shear-Induced Self-Healing of Lipid Bilayer Boundary Lubricants in Phosphatidylcholine Dispersions. Soft Matter 2016, 12, 2773-2784. (32) Goldberg, R.; Schroeder, A.; Silbert, G.; Turjeman, K.; Barenholz, Y.; Klein, J. Boundary Lubricants with Exceptionally Low Friction Coefficients Based on 2D Close-Packed Phosphatidylcholine Liposomes. Adv. Mater. 2011, 23, 3517-3521. (33) Jahn, S.; Klein, J. Hydration Lubrication: the Macromolecular Domain. Macromolecules 2015, 48, 5059-5075. (34) Chen, M.; Kato, K.; Adachi, K. Friction and Wear of Self-Mated SiC and Si3N4 Sliding in Water. Wear 2001, 250, 246-255. (35) Li, J.; Zhang, C.; Luo, J. Superlubricity Achieved with Mixtures of Polyhydroxy Alcohols and Acids. Langmuir 2013, 29, 5239-5245. (36) Deng, M.; Zhang, C.; Li, J.; Ma, L.; Luo, J. Hydrodynamic Effect on the Superlubricity of Phosphoric Acid between Ceramic and Sapphire. Friction 2014, 2, 173-181. (37) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd edition. Academic Press: New York, 2011. (38) Nightingale Jr, E. Phenomenological Theory of Ion Solvation. Effective Radii of Hydrated Ions. J. Phys. Chem. 1959, 63, 1381-1387. (39) Pashley, R. M. DLVO and Hydration Forces between Mica Surfaces in Li+ , Na+ , K+ , and Cs+ Electrolyte Solutions: A Correlation of Double-Layer and Hydration Forces with Surface Cation Exchange Properties. J. Colloid Interface Sci. 1981, 83, 531-546. 33


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(40) Tansel, B.; Sager, J.; Rector, T.; Garland, J.; Strayer, R. F.; Levine, L.; Roberts, M.; Hummerick, M.; Bauer, J. Significance of Hydrated Radius and Hydration Shells on Ionic Permeability during Nanofiltration in Dead End and Cross Flow Modes. Sep. Purif. Technol. 2006, 51, 40-47. (41) Li, J.; Zhang, C.; Sun, L.; Lu, X.; Luo, J. Tribochemistry and Superlubricity Induced by Hydrogen Ions. Langmuir 2012, 28, 15816-15823. (42) Kershner, R. J.; Bullard, J. W.; Cima, M. J. Zeta Potential Orientation Dependence of Sapphire Substrates. Langmuir 2004, 20, 4101-4108. (43) Fitts, J. P.; Shang, X.; Flynn, G. W.; Heinz, T. F.; Eisenthal, K. B. Electrostatic Surface Charge at Aqueous/α-Al2O3 Single-Crystal Interfaces as Probed by Optical Second-Harmonic Generation. J. Phys. Chem. B 2005, 109, 7981-7986. (44) Kulig, M.; Greil, P. Surface Chemistry and Suspension Stability of Oxide-Nitride Powder Mixtures. J. Mater. Sci. 1991, 26, 216-224. (45) Hartung, W.; Rossi, A.; Lee, S.; Spencer, N. D. Aqueous Lubrication of SiC and Si3N4 Ceramics Aided by a Brush-Like Copolymer Additive, Poly(l -lysine)-Graft-Poly(Ethylene Glycol). Tribol. Lett. 2009, 34, 201-210. (46) Bergström, L.; Pugh, R. J. Interfacial Characterization of Silicon Nitride Powders. J. Am. Ceram. Soc. 1989, 72, 103-109. (47) Spierings, G. A. C. M. Wet Chemical Etching of Silicate Glasses in Hydrofluoric Acid Based Solutions. J. Mater. Sci. 1993, 28, 6261-6273. (48) Wen, S.; Huang, P. Principles of tribology. Tsinghua University Press: Beijing, 2012. (49) Cacace, M. G.; Landau, E. M.; Ramsden, J. J. The Hofmeister Series: Salt and Solvent Effects on Interfacial Phenomena. Q. Rev. Biophys. 1997, 30, 241-277. (50) Vlachy, N.; Jagoda-Cwiklik, B.; Vácha, R.; Touraud, D.; Jungwirth, P.; Kunz, W. Hofmeister Series and Specific Interactions of Charged Headgroups with Aqueous Ions. Adv. Colloid Interface Sci. 2009, 146, 42-47.

34


Page 34 of 46

Page 35 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

TOC:

35


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 1. Friction coefficient as a function of time under lubrication of 50 mM KCl solution between the Si3N4 ball and sapphire disc. 99x149mm (300 x 300 DPI)


Page 36 of 46

Page 37 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig 2. Friction coefficients under lubrication of 50 mM chloride/potassium salt solutions, (a, b) five different chloride salt solutions (LiCl, NaCl, KCl, CsCl and NH4Cl), and (c, d) four different potassium salt solutions (KF, KCl, KBr and KI) after running-in by H3PO4 solutions. 156x117mm (300 x 300 DPI)


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 3. Friction coefficients as a function of time under lubrication of 50 mM KCl solution after running-in by different materials, including H2SO4 solution (pH =1.5), HCl solution (pH =1.5), KCl solution (50 mM) and KOH solution (pH =12.5). 87x130mm (300 x 300 DPI)


Page 38 of 46

Page 39 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig 4. (a) Friction coefficient of KCl solution after running-in by acid solutions with different pH values from 0.0 to 4.5. (b) The contact pressure between Si3N4 ball and sapphire disc, and (c-g) SEM images of wear scar on Si3N4 ball lubricated by KCl solution after running-in by acid solutions with five different pH values. (h-j) The XPS spectra of Si 2p from the worn region on the Si3N4 ball after running-in by acid solutions with pH values of 0.0, 1.5 and 4.5 respectively. 170x140mm (300 x 300 DPI)


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 5. 3D surface morphologies of the contact area on the Si3N4 ball (a, b) after running-in by H3PO4 solution, and (c, d) after the superlubricity is achieved. (e, f) One dimensional morphology of the surfaces on the initial ball, the ball after running-in by H3PO4 solution, the ball and the disc lubricated by KCl solution after running-in by H3PO4 solution. 170x93mm (300 x 300 DPI)


Page 40 of 46

Page 41 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig 6. The XPS spectra of Si 2p from worn region of the silicon nitride ball (a) when superlubricity of hydrated potassium ions is achieved, (b) after running-in by H3PO4 solution, (c) after running-in by KCl solution, (d) after running-in by KOH solution, (e) when soaked by HF solution and (e) the initial surface. 170x89mm (300 x 300 DPI)


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 7. (a) Friction coefficients of KCl solution with or without treatment by HF solution between Si3N4 ball and sapphire disc. (b, c) AFM images of the worn region on Si3N4 ball after running-in with H3PO4 solution and after the ball soaked in HF solution (10%) for 20 minutes. 90x112mm (300 x 300 DPI)


Page 42 of 46

Page 43 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig 8. (a) Friction coefficients of KCl solution as a function of time under different conditions, including changing rotation direction, replacing the ball or the disc to a new one, and changing the running track. (b) The central film thickness of KCl solution as a function of the relative sliding velocity between the ball and disc under ideal conditions. (c) Friction coefficients of KCl solution and pure water as a function of sliding speed, The inset shows some of the data on an expanded scale. 144x109mm (300 x 300 DPI)


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 9. Schematic illustration of the superlubricity model for hydrated alkali metal ions between the silicon nitride ball and sapphire disc. 190x79mm (300 x 300 DPI)


Page 44 of 46

Page 45 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

TOC 209x85mm (300 x 300 DPI)


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig S1. 3D surface morphologies of the contact area on the Si3N4 ball after the friction test with pure water and after running-in with H3PO4 solution. 199x91mm (300 x 300 DPI)


Page 46 of 46

Macroscale superlubricity enabled by hydrated alkali metal ions

Recommend Documents