State-of-the-Art of Extreme Pressure Lubrication Realized with the

Jan 24, 2017 - The extreme pressure lubrication property of gallium based liquid metal (GBLM) was compared with gear oil and poly-α-olefin in a four-...
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State-of-the-Art of Extreme Pressure Lubrication Realized with the High Thermal Diffusivity of Liquid Metal Haijiang Li,†,§,∥ Pengyi Tian,†,§ Hongyu Lu,† Wenpeng Jia,† Haodong Du,‡ Xiangjun Zhang,† Qunyang Li,*,†,‡ and Yu Tian*,† †

State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, P. R. China AML, CNMM, Department of Engineering Mechanics, Tsinghua University, Beijing, 100084, P. R. China

‡

S Supporting Information *

ABSTRACT: Sliding between two objects under very high load generally involves direct solid−solid contact at molecular/ atomic level, the mechanism of which is far from clearly disclosed yet. Those microscopic solid−solid contacts could easily lead to local melting of rough surfaces. At extreme conditions, this local melting could propagate to the seizure and welding of the entire interface. Traditionally, the microscopic solid−solid contact is alleviated by various lubricants and additives based on their improved mechanical properties. In this work, we realized the state-of-the-art of extreme pressure lubrication by utilizing the high thermal diffusivity of liquid metal, 2 orders of magnitude higher than general organic lubricants. The extreme pressure lubrication property of gallium based liquid metal (GBLM) was compared with gear oil and poly-α-olefin in a four-ball test. The liquid metal lubricates very well at an extremely high load (10 kN, the maximum capability of a four-ball tester) at a rotation speed of 1800 rpm for a duration of several minutes, much better than traditional organic lubricants which typically break down within seconds at a load of a few kN. Our comparative experiments and analysis showed that this superextreme pressure lubrication capability of GBLM was attributed to the synergetic effect of the ultrafast heat dissipation of GBLM and the low friction coefficient of FeGa3 tribo-film. The present work demonstrated a novel way of improving lubrication capability by enhancing the lubricant thermal properties, which might lead to mechanical systems with much higher reliability. KEYWORDS: frictional heat, thermal conductivity, thermal diffusivity, liquid metal, lubricant

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INTRODUCTION

for the excellent commercial extreme pressure gear oil) at a speed of 1800 rpm in a standard four-ball test (FBT).10,12 Extensive research has determined that adiabatic shear instability is the key process in scuffing,13−16 in which an elevated interfacial temperature can decrease the viscosity and hydrodynamic effect of lubricants,9 accelerate the desorption17 and degradation18 of lubricants, and accelerate the oxidation of materials,19 as illustrated in Figure 1a. Such a lubrication state change is accompanied by an increase in the wear scar area. The contact interface is welded together at some critical temperature value, as shown in Figure 1b. Considering the importance of interfacial temperature in scuffing, we turned our eyes from the mechanical properties of the lubricant to its thermal properties, trying to improve the extreme pressure lubrication performance through changing the thermal conductivity and thermal diffusivity of the lubricant. However, the thermal conductivity of traditional organic lubricants is intrinsically limited within a small range, which greatly restricts their heat

Friction is ubiquitous in physical systems and usually involves complex physical processes and chemical reactions.1−4 Among them heat generation is a phenomenon of wide concern. As early as the 1950s, Bowdon and Tabor1 predicted the melting of microscopic rough peaks as a result of frictional heat, which can be observed even at very low loads.5 Although frictional heat is widely used as a manufacturing technology (friction stir welding)6 in the aerospace industry, large-scale adhesion and welding of sliding interfaces (called scuffing) usually lead to catastrophic failures of mechanical systems such as gear boxes or bearings in airplanes, high-speed trains, and wind turbines.7−9 Tremendous effort has been put into preventing scuffing by using various lubricants and extreme pressure additives.10 From traditional lubrication design criteria, the carrying capacity of a lubricant depends mainly on its mechanical properties, especially the viscosity.11 However, even when frictional parts are fully immersed in the most advanced commercial organic lubricants (with extreme pressure additives) based on this criteria, scuffing inevitably occurs under harsh conditions, usually just a few kN load (e.g., about 3 kN © 2017 American Chemical Society

Received: December 9, 2016 Accepted: January 24, 2017 Published: January 24, 2017 5638

DOI: 10.1021/acsami.6b15825 ACS Appl. Mater. Interfaces 2017, 9, 5638−5644

Research Article

ACS Applied Materials & Interfaces

Figure 1. Failure processes of the sliding interface with an increase in frictional heating power. (a) Schematic of evolution of the sliding interface from local to expanded overall melting owing to the increase of frictional heating power. (b) Photographs of evolution of the friction interface from light wear to scuffing (the three balls were welded together) with an increase in frictional heating power. ultrasonicated in a water bath at a temperature above 50 °C. Fourth, to remove the oxide skin of the liquid metal generated during the above three steps, the liquid metal was washed with a magnetic stirring apparatus using a solution of NaOH. After separation of the NaOH solution, fresh GBLM could be obtained. It needs to be noted that the oxide skin could not be removed totally, as GBLM spontaneously forms an oxide skin when exposed to air.29 The gear oil used for comparison was 85W/90 GL-5, a product of SINOPEC Great Wall Lube Oil Group CO. Ltd. Its viscosity grade is SAE 85W/90. It meets the requirements of the industry specifications of API GL-5. Formulated with extreme-pressure additives and antiwear additives, this gear oil is widely used in commercial transmissions, axles, and final drives, where extreme pressures and shock loadings are expected. The poly-α-olefin (PAO) used in experiments shown in Figure 2 and Figure 4d was PAO 15, a product of Shanghai Foxsyn Chemical Technology Co. Ltd. Its viscosity grade is 15. The ball specimens (Shanghai Steel Ball Plant Co. Ltd.) were made of AISI 52100 commercial bearing steel with a diameter of 12.7 mm and an average surface roughness (Ra) of 5 nm (measured by a commercial surface mapping microscope, AMETEK Zygo nexview). 2. Tribological Tests. The lubrication properties of GBLM were evaluated according to standard ASTM D2783-0330 using a commercial FBT machine (MRS-10D, Test Instrument, Co., Ltd. Jinan Shun Mao). Standard FBT ran for 10 s (including the acceleration and deceleration time) at a rotating speed of 1800 rpm. The equivalent sliding speed was 0.691 m/s at the sliding interfaces. The applied load, which was constant for each test, was increased testby-test according to the standard until scuffing occurred. New steel balls and lubricant were used for each test. 3. Estimation of Wear Rate. On the basis of the formulation wear rate = wear volume/(load × sliding distance), the wear volume (V) of the steel balls after FBT was estimated using the following equation:

dissipation capabilities.10 To overcome the disadvantages of traditional organic lubricants, we focused our attention on gallium-based liquid metal (GBLM), which has high thermal diffusivity, low melting point, and high fluidity, biocompatibility, and temperature stability.20,21 GBLM has been emerging as a promising material for applications such as chip cooling and flexible and reconfigurable electronics.21−24 The excellent properties of GBLM also make it a potentially good extreme pressure lubricant. For the application of GBLM lubricant, previous researchers have studied the physical and chemical characteristics of GBLM (including gallium), its lubrication performance as a magnetohydrodynamic lubricant,25 and its usefulness as a lubricant in electricity-carrying contacts,26 journal bearings,27 and other tribological systems requiring good lubricity under certain conditions.28 However, the contact pressure in previously reported applications of GBLM lubricant25−28 was relatively low (mostly less than 1 GPa). The extreme-pressure lubrication performance of GBLM has not been explored to date. The high thermal diffusivity of GBLM led us to believe that it would be useful for exploring the effect of lubricant thermal properties on sliding friction under extreme pressure conditions. Therefore, we carried out comparative tribological tests of GBLM along with commercial gear oil and unformulated base oil using the FBT method in this study.

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MATERIALS AND METHODS

1. Materials. The GBLM used in this study, Ga0.64In0.24Sn0.12, was prepared via the steps described in ref 22. First, small chips or powders of Ga, In, and Sn (mass ratio 64:24:12) were mechanically mixed together. Second, the mixtures were heated above the melting point of Sn of 232 °C, the highest melting point among the three metals. After heating, the three metals melted to form a liquid metal alloy. Third, to ensure that the metals fused sufficiently, the liquid metal was

V= 5639

⎞ ⎛ πH ⎞⎛ 3WSD2 ⎜ ⎟⎜ + H2 ⎟ ⎝ 6 ⎠⎝ 4 ⎠ DOI: 10.1021/acsami.6b15825 ACS Appl. Mater. Interfaces 2017, 9, 5638−5644

Research Article

ACS Applied Materials & Interfaces

Figure 2. Comparing the tribological performance of different lubricants. Rotating speeds were all 1800 rpm. Wear scar diameter (WSD) is defined as the average diameter of wear scars on the stationary balls.30 (a) Schematic of the four-ball test. (b) Friction coefficient and WSD in 10 s FBT of GBLM and gear oil. (c) Experimental results in 150 s FBT. (d) Wear rates of GBLM, gear oil (GO), and PAO (see Materials and Methods for details of wear rate evaluation). The critical scuffing loads of PAO and gear oil were about 1.6 and 3.1 kN, respectively. The test of PAO at 1.2 kN was terminated at 100 s because the wear was too severe to achieve a stable sliding.

Figure 3. Experimental and theoretical analysis of the thermal dissipation capability of GBLM and gear oil 85W/90 GL-5 via FBT. Experimental conditions were the following: LGBLM = 5 kN, nGBLM = 1800 rpm; Lgear oil = 2.5 kN, ngear oil = 1800 rpm. (a) Temperatures measured at point O for GBLM and gear oil at similar frictional power. (b) Finite element simulation of the maximum temperatures at the contact zone of the lower steel balls lubricated with GBLM and gear oil using the experimental friction forces shown in (a). where H = R −

R2 −

D2 4

of the two balls was set to expand with time according to interpolation of the experimentally derived wear scar diameters. As elastohydrodynamic lubrication (EHL) is a common lubrication state for the point contact friction pairs with high speed, a lubricant film was assumed to exist at the sliding interface in the FE model. Since the EHL film thickness is typically around 0.1−1 μm,31 an intermediate film thickness of 0.5 μm was set in the FE model. The frictional forces of GBLM and gear oil lubricated friction pairs versus time were adopted from the experimental results shown in Figure 3a. The model included heat conduction among the balls, holder, and liquid pool as implemented using the transient heat transfer equation. Heat dissipation from the surfaces of the upper ball,

, D is the radius of the steel ball, and WSD

denotes the wear scar diameter. The WSD for each test condition was the average value of three tests. 4. Finite Element Thermal Analysis. To estimate the effects of the thermal dissipation performance of lubricants on local contact zone temperature, a finite element (FE) thermal analysis was performed using COMSOL Multiphysics 5.1. The geometric configurations of the four-ball friction pair, holder, and liquid pool were established according to their actual size of the experiments. The liquid pool was filled with GBLM or gear oil. The contact area of each 5640

DOI: 10.1021/acsami.6b15825 ACS Appl. Mater. Interfaces 2017, 9, 5638−5644

Research Article

ACS Applied Materials & Interfaces holder, and liquid pool to surrounding air by both convection and radiation was also considered. Heat conduction between the steel balls and the lubricant and between the lubricant and the liquid pool was simulated using the fluid heat transfer module. All of the frictional power was assumed to be converted into heat. As the heat was generated on the real contact area, which is only a small portion of the nominal contact area, the effective area for heat generation in our model was set to be the wear area multiplied by a scaling factor k. The value of k was adopted to satisfy the condition that the temperature of lubricant at the center point of the liquid pool (point O) in our finite element model was consistent with the experimental value. The flow field of the lubricant was obtained using the laminar flow module, and the calculated velocity field and pressure of the fluid were used to determine fluid heat transfer. The values of the physical parameters and variables used in the simulation are summarized in Tables S1 and S2 in Supporting Information. 5. Characterization of the Tribo-Film. Observation of the cross section of the subsurface at the contact zone of the steel balls was performed using a scanning electron microscope (SEM, TESCAN LYRA3). Chemical configurations of the tribo-film were determined using an energy-dispersive X-ray spectrometer (EDX, GENESIS XM) and X-ray diffraction (XRD, Rigaku R-Axis Spider). The atomic crystal structure of the tribo-film was observed using a transmission electron microscope (TEM, JEOL 2010F), and the specimen was prepared using a focused ion beam (FIB, TESCAN LYRA3). The hardness of the worn surface at elevated temperatures was measured using a nanoindenter (HysitronTI950). 6. Method of Cleaning Steel Balls after GBLM Lubricated Sliding. Before being washed by NaOH solution, GBLM adhered to the steel surface strongly as the oxide skin enhanced the wettability of GBLM on steel surface. To thoroughly remove the adhering GBLM, the steel balls were immersed in a dilute solution of NaOH (0.25 mol/ L) for 1−2 min at room temperature. Meanwhile, a piece of cloth was used to scrub the steel balls during the immersion time. During the immersion time, NaOH solution reacted with the oxide skin of GBLM.22 After being cleaned by NaOH solution, the steel balls were ultrasonically cleaned in ethanol for 5 min to remove the residual NaOH.

and gear oil, as shown in Figure 2d. The extreme pressure wear rates of PAO and gear oil were characterized at loads sufficiently far from their critical scuffing loads. Wear rates during two periods (namely, 0−10 s (representing the runningin period) and 0−150 s (representing the stable sliding period)) were obtained for each lubricant, as shown in Figure 2d. The wear rate of gear oil at 2 kN was approximately 1 order of magnitude higher than that of GBLM during both periods. The wear rate of PAO at 1.2 kN was about 5 orders of magnitude higher than that of GBLM at the same condition and about 2 orders of magnitude higher than that of GBLM at 10 kN during both periods. Thus, GBLM demonstrated obviously superior antiwear properties to those of both gear oil and PAO. The relation between the excellent thermal properties of GBLM and its superior antiscuffing performance was experimentally and theoretically analyzed as follows. The thermal conductivity (at 25 °C) of GBLM and gear oil was measured to be 25.65 W/(m·K) and 0.15 W/(m·K), respectively. The heat capacity (at 25 °C) was measured to be 2.26 × 106 J/(m3·K) for GBLM and 1.8 × 106 J/(m3·K) for gear oil. The thermal diffusivity (defined as λ/(ρC), where λ, ρ, and C are the thermal conductivity, density, and thermal capacitance, respectively) of GBLM was calculated to be 11.3 mm2/s, 2 orders of magnitude higher than that of gear oil (0.083 mm2/s). This result suggests that GBLM can more efficiently transfer frictional heat from the sliding interface to the bulk lubricant than gear oil. The efficient heat transfer properties of GBLM were experimentally confirmed using FBT. As shown in Figure 3a, a similar frictional force Ff at the rotating speed of 1800 rpm was produced for the GBLM and the gear oil, corresponding to a similar frictional heat power generated at the interfaces. The temperature of the lubricant (TO) during the sliding test was measured at a point just below the lower three balls and above the center point of the liquid pool (point O). As can be seen in Figure 3a, the temperature of GBLM (TO GBLM) began to increase noticeably around 5 s after the test began, which was about 5 s ahead of the noticeable increase of TO Gear oil. Moreover, TO GBLM increased to about 55 °C at a much higher rate than TO Gear oil, indicating that frictional heat generated at the sliding interface was indeed transferred more efficiently into the lubricant by GBLM than by gear oil. Because frictional heat was generated at the sliding interface, the temperature at this interface should be much higher than that at point O. However, owing to the difficulty in experimentally measuring the contact zone temperature during FBT, a finite element simulation was carried out to calculate the temperature distributions around the contact regions lubricated with GBLM and gear oil, respectively (see Materials and Methods for details). By adoption of frictional force versus time as input shown in Figure 3a, the maximum temperature of the contact zone after 60 s testing was calculated to be about 480 °C for GBLM, which is significantly lower than the temperature of 790 °C for gear oil, as shown in Figure 3b. At the beginning of the sliding test, the maximum temperature at the contact zone lubricated with gear oil could be 600 °C higher than the GBLM contact point. This finite element analysis clearly indicated the exceptional thermal dissipation capability of GBLM and partially explained its exceptional antiscuffing capability. In addition to its efficient heat dissipation, the low friction coefficient of GBLM at high loads (Figure 2) is also responsible

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RESULTS AND DISCUSSION The tribological performance of GBLM (Ga0.64In0.24Sn0.12) in a 52100 steel ball sliding contact under FBT was compared with the performance of a typical extreme-pressure lubricant (gear oil 85W/90 GL-5) and unformulated base oil composed of poly-α-olefin (PAO 15). In FBT, a normal load (L) is applied to the upper steel ball, which rotates against the lower three stationary steel balls fully immersed in lubricant, as shown in Figure 2a (see Materials and Methods for details of the FBT method). In this study, the occurrence of scuffing was defined as the overall seizure/welding of the steel balls. As shown in Figure 2b, scuffing did not occur when using gear oil at a load of 2 kN in 10 s standard ASTM D2783-03 FBT30 but occurred at 3.1 kN within around 2 s (the friction coefficient increased dramatically as illustrated by the red dotted line). By contrast, when GBLM was used, scuffing did not occur even when the applied load reached the maximum capability of the tester of 10 kN. For the sphere−sphere contact in FBT, the load of 10 kN would generate a maximum contact stress of 13.7 GPa according to Hertzian contact theory, which was much larger than practical extreme pressure conditions in engineering applications (usually a few GPa).32 Furthermore, in a prolonged 150 s FBT shown in Figure 2c, GBLM produced a low friction coefficient of 0.05−0.06 throughout the 10 kN test, which was about half the friction coefficient of gear oil at 2 kN. The wear rates of the steel balls lubricated by GBLM under extreme pressure were compared with those lubricated by PAO 5641

DOI: 10.1021/acsami.6b15825 ACS Appl. Mater. Interfaces 2017, 9, 5638−5644

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Figure 4. Lubrication behavior of GBLM at different rotating speeds and loads and characterization of the formed tribo-film. (a) Variation of friction coefficient with rotating speed. (b) Variation of friction coefficient with normal load. (c) SEM image of the cross section of the contact zone and chemical composition analysis along the tribo-film depth direction (lower left inset, TEM image of the tribo-film; lower right inset, XRD spectrum of the worn surface of steel). (d) Scuffing initiation time under different conditions (“NO RI” refers to testing without initial running in; “GBLM-RI” refers to testing with initial running using GBLM at a load of 3 kN for 150 s).

data) during the formation of the tribo-film. On the basis of the Fe−Ga phase diagram,35 the melting point of FeGa3 is around 824−860 °C, which is noticeably lower than that of the steel ball (∼1300 °C). Nanoindentation showed that the tribo-film was more compliant than the steel at both room and elevated temperatures (Figure S2). The lower melting point and relatively softer nature of the tribo-film help it to serve as an easy-shear interface layer and result in a low friction coefficient. In addition, GBLM and the tribo-film appeared to have strong mutual adhesion (Figure S3). So it is likely that the tribo-film could effectively bring GBLM into the contact zone as the upper steel ball is rotating, which would also help to reduce friction and wear. The antiscuffing property of the tribo-film was further demonstrated by the following comparative experiments, as shown in Figure 4d. Scuffing occurred within 2 s under 1.6 kN to bare steel balls immersed in PAO. For comparative testing, a FeGa3 tribo-film was intentionally pregrown on the four steel balls by running in at 3 kN and 1800 rpm lubricated with GBLM for 150 s. GBLM was then taken out from the liquid pool of the FBT. After cleaning the steel balls to remove the GBLM thoroughly (see Materials and Methods), PAO was injected into the liquid pool to carry out the next FBT. It was found that the steel balls could endure about 20 s before scuffing at a load of 1.6 kN. When lubricated with GBLM at 10 kN and 1800 rpm, however, the sliding remained smooth even after 260 s, and the temperature of point O reached over 230 °C, where most organic lubricants physically or chemically fail.

for its excellent extreme-pressure lubrication performance. As shown in Figure 4a, under a fixed load of 3.1 kN, the friction coefficient decreased from about 0.17 to 0.08 when the rotating speed was increased from 50 to 1800 rpm. As shown in Figure 4b, the friction coefficient increased from 0.08 to 0.35 when the applied load was decreased from 3.1 to 0.2 kN at n = 1800 rpm, counter to the hydrodynamic effect.33 The friction coefficient appears to depend on the frictional power rather than simply on the load or speed. The stress-assisted and thermally activated growth of a protective tribo-film, which can effectively reduce the friction coefficient,34 might account for the reduced friction of GBLM at high frictional power, as illustrated in the insets of Figure 4a and Figure 4b. Also, the friction coefficient for GBLM increased rapidly at the initial stage, reaching a peak value, and then quickly decreased to a lower steady-state value as shown in Figures 2b, 2c, 4a, and 4b. This was considered to indicate the formation of the tribo-film. The existence of such tribo-film was confirmed by scanning electron microscope (SEM) images of the cross section of the lower steel ball around the contact zone, as shown in Figure 4c, in accordance with previous research involving the use of pure Ga as a lubricant.28 The thickness of the tribo-film increased with load (Figure S1). Energy-dispersive X-ray spectroscopy (EDX) and X-ray powder diffraction (XRD) analysis and transmission electron microscope (TEM) observation (Figure 4c inset) revealed that the tribo-film was primarily composed of FeGa3. The bulk phase of FeGa3 (light region) was mixed with other minor phases (such as α-Fe as indicated by the XRD 5642

DOI: 10.1021/acsami.6b15825 ACS Appl. Mater. Interfaces 2017, 9, 5638−5644

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This demonstrated the ability of GBLM to resist thermal evaporation and chemical decomposition in addition to its exceptional antiscuffing capability. Considering the application of GBLM as an extreme-pressure lubricant, some additional points need to be considered. First, clean GBLM has high surface tension (∼500 mJ/m2 as reported in ref 22), which makes it hardly wet the steel surface. An oxide skin forms spontaneously on GBLM when exposed to air, which significantly reduces its surface tension to near zero22 and makes GBLM easily wet the steel surface.35−36 The enhanced wettability is good for the lubrication performance of GBLM (Figure S4). On the other hand, overoxidized GBLM loses its good liquidity, which is not good for the lubrication performance. An appropriate degree of oxidation of GBLM would be desirable in real applications. Second, it was confirmed that GBLM and steel do not dissolve into each other at 200 °C for 12 h (Figures S5 and S6). This characteristic is helpful for the long-term storage of GBLM at room temperature and its applicability in real mechanical systems. In addition, pure Ga film above its melting temperature showed similar lubrication properties (Figure S7), as would be expected from the nature of the FeGa3 tribo-film. However, the melting temperature of Ga (29.78 °C) is higher than that of GBLM (0−5 °C), which limits its working temperature range.

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CRRC Zhuzhou Electric Co., LTD., Zhuzhou, 412000, P. R. China. Author Contributions §

H.L. and P.T. contributed equally to this work. H.L., P.T., H.L., W.J., and H.D. performed the experiments. H.L., P.T., X.Z., Q.L., and Y.T. conducted the analysis of the data. H.L., P.T., Q.L., and Y.T. prepared the manuscript. Q.L. and Y.T. conceived the project and designed the experiments. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (Grants 51323006, 51425502, 11422218, and 11272177) and the Thousand Young Talents Program of China.

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CONCLUSION GBLM can act as an extreme-pressure lubricant to effectively prevent the welding of sliding interfaces through the synergetic effect of ultrafast heat dissipation and reduced friction coefficient. Its excellent lubrication performance revealed the importance of the thermal properties of the lubricant on its extreme pressure lubrication performance in addition to the mechanical properties. By effective enhancement of the thermal diffusivity and reduction of the friction coefficient, the contact zone temperature could be effectively kept at a safe level to protect friction pairs from scuffing. This study opened a new route for designing excellent high-temperature extremepressure lubricants, which might aid in the design of nextgeneration machines with high-energy density and high reliability.

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REFERENCES

(1) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids, Part I; Clarendon Press: Oxford, U.K., 1950. (2) Dowson, D. History of Tribology; Longman: London, 1979. (3) Fischer, T. E. Tribochemistry. Annu. Rev. Mater. Sci. 1988, 18, 303−323. (4) Tian, Y.; Pesika, N.; Zeng, H.; Rosenberg, K.; Zhao, B.; McGuiggan, P.; Autumn, K.; Israelachvili, J. Adhesion and Friction in Gecko Toe Attachment and Detachment. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 19320−19325. (5) Yagi, K.; Kajita, S.; Izumi, T.; Koyamachi, J.; Tohyama, M.; Saito, K.; Sugimura, J. Simultaneous Synchrotron X-ray Diffraction, NearInfrared, and Visible In Situ Observation of Scuffing Process of Steel in Sliding Contact. Tribol. Lett. 2016, 61, 1−16. (6) Nandan, R.; DebRoy, T.; Bhadeshia, H. K. D. H. Recent Advances in Friction-stir WeldingProcess, Weldment Structure and Properties. Prog. Mater. Sci. 2008, 53, 980−1023. (7) Ludema, K. C. A Review of Scuffing and Running-in of Lubricated Surfaces, with Asperities and Oxides in Perspective. Wear 1984, 100, 315−331. (8) Bowman, W. F.; Stachowiak, G. W. A Review of Scuffing Models. Tribol. Lett. 1996, 2, 113−131. (9) Dyson, A. Scuffing - a Review. Tribol. Int. 1975, 8, 77−87. (10) Straffelini, G. Friction and Wear: Methodologies for Design and Control; Springer International Publishing, 2015. (11) Hamrock, B. J.; Schmid, S. R.; Jacobson, B. O. Fundamentals of Fluid Film Lubrication; CRC Press: Boca Raton, FL, 2004. (12) Khonsari, M. M.; Pascovici, M. D.; Kucinschi, B. V. On the Scuffing Failure of Hydrodynamic Bearings in the Presence of an Abrasive Contaminant. J. Tribol. 1999, 121, 90−96. (13) Hershberger, J.; Ajayi, O. O.; Zhang, J.; Yoon, H.; Fenske, G. R. Evidence of Scuffing Initiation by Adiabatic Shear Instability. Wear 2005, 258, 1471−1478. (14) Ajayi, O. O.; Hersberger, J. G.; Zhang, J.; Yoon, H.; Fenske, G. R. Microstructural Evolution during Scuffing of Hardened 4340 SteelImplication for Scuffing Mechanism. Tribol. Int. 2005, 38, 277−282. (15) Ajayi, O. O.; Lorenzo-Martin, C.; Erck, R. A.; Fenske, G. R. Scuffing Mechanism of Near-Surface Material during Lubricated Severe Sliding Contact. Wear 2011, 271, 1750−1753.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15825. Thickness of the tribo-film under different experimental conditions; hardness of 52100 steel and FeGa3 tribo-film; evidence for strong adhesion between GBLM and the FeGa3 tribo-film; finite element thermal analysis; lubrication property of oxidized GBLM; mutual solubility of Fe and GBLM; lubrication properties of pure gallium; enforced cooling to reduce the wear rate (PDF)

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ABBREVIATIONS GBLM = gallium based liquid metal EP = extreme pressure PAO = poly-α-olefin FBT = four-ball-test WSD = wear scar diameter

AUTHOR INFORMATION

Corresponding Authors

*Q.L.: e-mail, [email protected]. *Y.T.: e-mail, [email protected]. ORCID

Qunyang Li: 0000-0002-6865-3863 Yu Tian: 0000-0001-7742-5611 5643

DOI: 10.1021/acsami.6b15825 ACS Appl. Mater. Interfaces 2017, 9, 5638−5644

Research Article

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DOI: 10.1021/acsami.6b15825 ACS Appl. Mater. Interfaces 2017, 9, 5638−5644