Graphene Colloidal Suspensions as High Performance Semi

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Graphene Colloidal Suspensions as High Performance Semi-Synthetic Metal-Working Fluids J. Samuel,*,† J. Rafiee,† P. Dhiman,† Z.-Z. Yu,‡ and N. Koratkar*,† †

Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180-3590, United States ‡ State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: We report the use of graphene as an additive to improve the lubrication and cooling performance of semisynthetic metal-working fluids (MWFs) used in micromachining operations. Microturning experiments were conducted in the presence of MWFs containing varying concentrations of graphene platelets. Graphene-based MWF formulations performed significantly better as compared to conventional MWFs. Moreover, an analysis of the trends in the cutting forces and cutting temperatures, taken in conjunction with the trends in the wetting ability, thermal conductivity, and kinematic viscosity of the modified MWFs, establishes graphene as a superior additive over both single and multiwalled carbon nanotubes. The superior performance of graphene is attributed to the increased wettability of the cutting fluid that allows for penetration of the graphene platelets into the tool-workpiece interface. Once in that interface, the graphene platelets provide efficient lubrication because of the relative sliding of graphene layers within the platelet while they also conduct heat away from the cutting zone.

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icromachining operations such as micromilling, microdrilling, and microturning are currently being used to machine a wide range of precision microparts.1,2 These manufacturing operations are high-strain-rate deformation processes characterized by extreme cutting temperatures and cutting forces that not only affect the life of the cutting tool but also reduce the life cycle of the micropart by inducing thermal damage and residual stresses.2-5 While novel techniques such as atomization-based delivery of microdroplets into the cutting zone have been shown to be successful for micromachining processes,6 the overall performance of such a cutting fluid-delivery system is limited by the cooling and lubrication capacity of the cutting fluid used. Historically, water-based semisynthetic metal working fluids (MWF) have been used as cutting fluids for micromachining applications.7-10 These MWFs consist of an oil base that is dispersed in deionized water by using a surfactant. During the micromachining operation, the lubrication is provided by the oil film whereas the cooling is primarily provided by the evaporation of the water phase. Improving the lubrication efficiency of such an MWF invariably implies increasing the percentage of oil content in the MWF. However, this also increases the viscosity of the MWF, which drastically reduces the droplet delivery efficiency of the atomization system. Thus, there is a critical need to develop new MWF formulations that improve both the lubrication and the cooling performance of MWFs without the expense of increasing its viscosity. r 2011 American Chemical Society

Nanofluids (engineered colloidal suspensions of nanofillers in a base fluid) may provide unique opportunities to enhance MWF performance. The shape and size of the nanofiller additive is expected to significantly affect its suitability for micromachining applications. The characteristic edge radius of cutting tools used in micromachining processes1 is in the 1-2 μm range, and the typical depth-of-cuts encountered are 1000 W/mK) makes them well-suited for use as a performance-enhancing additive for MWFs in micromachining processes. In this study, microturning experiments are conducted Received: November 15, 2010 Revised: January 13, 2011 Published: February 10, 2011 3410

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Figure 1. (A) Transmission electron micrograph of a typical GPL flake indicating lateral dimensions in the micrometer range. (B) High-resolution transmission electron microscopy characterization of the GPL indicating thickness of ∼2 nm. The GPL appears to be comprised of ∼3-4 graphene sheets with an interlater spacing of ∼0.3 nm. Inset shows the measured electron diffraction pattern. (C) X-ray photoelectron spectroscopy of GPL indicating C 1s and O 1s peaks. (D) C 1s core level spectra indicating two fitted peaks that correspond to the nonoxygenated ring C (284.8 eV) and the C in C-OH bonds (286.1 eV).

in the presence of an atomized MWF containing varying percentages of graphene platelets. The cutting performance of graphene-based formulations are also compared with carbon nanotube modified MWFs. Cutting forces and cutting temperatures are used as the machinability measures for comparison purposes. The results are interpreted in light of the trends in the tool contact angle, thermal conductivity, and kinematic viscosity changes that occur with the addition of graphene platelets. Our results indicate that graphene platelets greatly improve both the cooling and the lubrication performance of MWFs without incurring a significant increase in the fluid viscosity. The graphene platelets (GPL) used in our study were synthesized by thermal reduction of graphite oxide as described in detail elsewhere.19 Transmission electron microscopy (TEM) characterization of a typical GPL flake is shown in Figure 1A indicating lateral sheet dimensions of several micrometers. Highresolution TEM (Figure 1B) indicates that the GPL are comprised of ∼3-4 individual graphene sheets within each platelet with an interlayer spacing of ∼0.3 nm. The electron diffraction pattern in the inset confirms the signature of few-layered graphene. To investigate the surface chemistry states, the GPL were characterized using X-ray photoelectron spectroscopy (XPS). Figure 1C shows the broad scan XPS spectra indicating C 1s and O 1s peaks. The peaks in the higher energy range (around ∼750 eV, ∼1000 eV) are attributed to the Auguer peaks of C and O. Elemental analysis gave a carbon-to-oxygen ratio for the GPL as ∼9.1. This indicates that the GPL are not completely reduced and contains residual oxygen. Figure 1D depicts the C 1s

XPS spectra which are split into two distinct peaks (285 eV, 286.1 eV), which are attributed to the nonoxygenated ring C and the C in C-OH bonds, respectively. These pendent oxygen groups render the GPL hydrophilic and compatible with aqueous-based colloidal suspensions.20,21 Figure 2A depicts the schematic layout of the test setup used to perform the micromachining experiments. An atomization-based cutting fluid delivery system is used for transporting the MWF into the cutting zone. It consists of an ultrasonically vibrating piezoelectric transducer that is attached to a cutting-fluid reservoir. The vibrations of the transducer generate atomized droplets that are then carried to a delivery pipe. A coaxial air tube supplies the regulated air velocity needed to transport the atomized droplets into the cutting zone. The atomization-based cutting fluid system described above is attached to a three-axis microscale machine tool capable of performing microturning operations.6 The machine tool is equipped with an NSK electric spindle having a rated speed of ∼50 000 rpm. The linear encoders have a resolution of ∼0.02 μm. Microturning experiments were conducted on a 6 mm diameter rod of 1018 steel using a righthanded cubic-boron nitride microturning tool having an edge radius of ∼2 μm. The cutting speed was maintained at ∼250 m/min. The radial depth-of-cut, feed-per-revolution, and total length of cut were maintained at ∼40 μm, ∼5 μm/revolution, and ∼3 mm, respectively. Semisynthetic cutting fluid Castrol Clearedge 6519 at 12.5% dilution was used as the baseline cutting fluid for this study. GPL loadings of 0.1, 0.2, and 0.5% by weight were added to the 3411

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Table 1. Summary of Test Conditions for the Microturning Operation workpiece

6 mm diameter 1018 steel rod

tool

right-handed cubic-boron nitride cutting tool with an edge radius of 2 μm, rake angle of ∼20°, and a clearance angle of ∼15°

cutting velocity

∼250 m/min

radial-depth-of-cut

∼40 μm

feed-per-revolution cutting fluids tested

∼5 μm (1) baseline cutting fluid: Castrol Clear-edge 6519 (12.5% dilution) (2) baseline þ 0.1% GPL (3) baseline þ 0.2% GPL (4) baseline þ 0.5% GPL (5) baseline þ 0.5% SWCNT (6) baseline þ 0.5% MWCNT

Figure 2. (A) Schematic showing the experimental layout for the microturning operation. (B) Typical time trace for the cutting temperature on the tool during the cut for the baseline (pristine) MWF as well as for various weight fraction of GPL added to the MWF. The GPL additives show significant effect on suppressing the peak temperature of the tool during the cut.

baseline cutting fluid to create three formulations of grapheneenhanced MWFs. Formulations containing 0.5% by weight of single-walled carbon nanotubes (SWCNT) and multiwalled carbon nanotubes (MWCNT) were also prepared. The SWCNT (diameter ∼ 2 nm, length ∼ 10 μm) and the MWCNT (diameter ∼ 20 nm, length ∼ 10 μm) used in this study were provided by Nanocyl. The graphene and nanotube fillers were dispersed in the cutting fluid by ultrasonication (Sonics Vibracell VCX 750 W sonicator) for ∼10 min. Cutting temperatures and forces were the machinability metrics used to compare the performance of the various nanoscale additives. The cutting forces were measured using a Kistler 9018 triaxial load cell by sampling at a frequency of ∼50 kHz. The cutting temperatures were measured using a type-J thermocouple (range 0-750 °C) in conjunction with an Analog Devices 5B47 linearized thermocouple input module. The tip of the thermocouple was attached on the rake face of the tool at a distance of ∼0.8 mm from the cutting edge. Table 1 summarizes the test conditions for the experiments. Figure 2B shows time traces of the temperature measured by the thermocouple during the cut for the baseline MWF as well as formulations of graphene-enhanced MWFs. Incorporation of GPL in the MWF serves to significantly suppress the peak temperature of the tool during the cut. This effect is enhanced with increasing loading fraction of GPL in the colloidal suspension. Another interesting observation is that the cutting

temperature fluctuations with the GPL formulations are smaller than the baseline case. Since the thermocouple was mounted on the rake face of the tool, the temperature profile captures the heat generated at the tool-chip interface, which is primarily a function of the dynamic coefficient of friction at that interface. The fact that the GPL formulations result in lower temperature fluctuations points to a more uniform and lower coefficient of friction at the tool-chip interface as compared to the baseline. This suggests the ability of the GPL to penetrate into the toolchip interface in micromachining processes. Figure 3A and B depicts the trends seen in the rise in cutting temperatures and the resultant cutting forces recorded during the course of the microturning experiments. Addition of graphene to the baseline MWF reduces both the average cutting temperatures and the cutting forces during the cut. The addition of 0.1, 0.2, and 0.5% of graphene is seen to result in ∼5.6, ∼30.5, and ∼42% improvement, respectively, in the cooling performance of the baseline cutting fluid. The performance of 0.5% SWCNT and 0.5% MWCNT solutions is similar and they both lie in between that of the 0.2% and the 0.5% graphene solutions. The cutting force shows an approximately linear decrease as the loading fraction of graphene is increased from 0% to 0.5% by weight. The lowest cutting force is observed for 0.5% GPL with its value being ∼26% lower than that of the baseline cutting fluid. The 0.5% SWCNT solution is seen to result in cutting forces comparable to the 0.1% graphene solution. The performance of 0.5% MWCNT solution lies between that of 0.1% and 0.2% graphene solutions. The error bars in Figure 3 are indicative of fluctuations during the cut. The fact that the fluctuations are reducing with an increase in GPL content indicates a more stable machining operation. In summary, the results reveal that addition of graphene improves both the lubrication and the heat dissipation capabilities of the baseline cutting fluid. The improvements obtained in the cooling performance outweigh those seen in the lubrication performance. The performance of 0.5% GPL solution clearly outperforms that of the 0.5% SWCNT and 0.5% MWCNT formulations. To understand the mechanisms that are underlying the trends seen in the machining responses, the contact angle (with the tool), the thermal conductivity, and the kinematic viscosity of the various MWF formulations were measured. Static contact angle measurements on the tool surface were performed using a Ram
eHart M500 goniometer. A 1 μL volume sessile droplet was placed on the tool surface using an automatic dispenser (VICI Precision 3412

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Figure 3. Trends in the cutting temperatures (A) and the cutting forces (B) encountered for various nanoscale additives. Data is shown for the baseline MWF and for various nanofiller enhanced MWFs. The lowest cutting temperatures and cutting forces were recorded for the ∼0.5% weight GPL-MWF formulation.

Sampling Co., CA, United States). Tests were performed under ambient conditions (∼74 °F), and the axisymmetric drop shape analysis profile (ADSA-P) method was used to estimate the contact angle of the cutting fluid on the tool surface. As can be seen in Figure 4A, the contact angle of the cutting fluid (measured against the tool surface) reduces continuously with an increase in graphene content. When compared against the baseline cutting fluid, an addition of 0.1, 0.2, and 0.5% graphene results in a reduction in contact angle by 43, 52, and 58%, respectively. The contact angles of solutions containing 0.5% SWCNT and 0.5% MWCNT are still higher than those of the 0.1% graphene solution. This indicates that even with the addition of only 0.1% graphene, the cutting fluid has a great propensity to wet the tool. Figure 4B depicts that the thermal conductivity of the cutting fluid increases with an increase in graphene content. The conductivity of the various cutting fluids was measured using a commercial thermal conductivity analyzer (KD2 Pro System, United States). When compared against the baseline cutting fluid, an addition of 0.1, 0.2, and 0.5% graphene results in an increase in thermal conductivity by 2, 3, and 4%, respectively. The thermal conductivity of the 0.5% SWCNT solution is comparable to that of the 0.5% graphene solution. The 0.5% MWCNT solution has the second lowest thermal conductivity with a value that is slightly above that of the baseline

Figure 4. (A) Measured contact angle of the tool surface for the baseline MWF and the nanofiller modified MWFs. (B) Thermal conductivity measurements of the various MWFs used in the study. (C) Data for the kinematic viscosity (units of cSt: centi-Stokes) for the baseline and nanofiller enhanced MWFs. The GPL are superior to MWCNT and SWCNT at increasing the thermal conductivity of the MWF and the wettability of the tool surface with a minimal increase in the viscocity.

cutting fluid. The kinematic viscosity of the various MWF formulations was measured using a capillary type viscometer (Cannon-Fenske, United States). The viscosity is estimated by measuring the efflux time (i.e., time taken by a fixed amount of the fluid to flow past a capillary of known size, under controlled temperature conditions). Figure 4C depicts that an addition of 0.1, 0.2, and 0.5% graphene results in an increase in kinematic viscosity of the MWF by 1, 2, and 3%, respectively. By contrast, the addition of 0.5% by weight of MWCNT and SWCNT results in a nearly 10% increase in the kinematic viscosity over that of the baseline cutting fluid. 3413

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The Journal of Physical Chemistry C The microturning results showed that both the cutting forces and the cutting temperatures reduce with an increase in the graphene content in the MWF. The data for the corresponding contact angles indicates enhanced wettability of the cutting fluid with the addition of graphene. In fact, the contact angle for even the 0.1% graphene formulation is lower than the corresponding values for the 0.5% SWCNT and MWCNT solutions. When wetted by the nanofluid, a coating of nanofillers is expected to form on the surface of the tool.22,23 XPS analysis of the GPL surface chemistry (Figure 1C, D) indicates the presence of oxygen containing functional groups, which in conjunction with the high specific surface area of GPL are expected to greatly enhance the wettability of the tool surface.19 This extreme wettability of the graphene MWF formulations facilitates a more efficient entry of the GPL into the tool-workpiece interface. The reduction in the cutting force implies that the graphene sheets appear to be successfully providing lubrication at the toolworkpiece interface. The fact that for the case of graphene-based MWF formulations there is only a modest 1-3% increase in kinematic viscosity that results in a 10-26% reduction in the magnitude of the cutting forces indicates that lubrication is likely provided by the sliding of the graphene sheets and not by increased viscosity. In the case of regular MWFs, since only the oil film provides the lubrication, an increase in viscosity of the MWF is absolutely necessary to ensure a reduction in cutting forces.6 This increased viscosity poses challenges to the effective delivery of such a cutting fluid using atomization-based techniques. The use of graphene platelet additives opens up the possibility of designing MWFs with improved lubrication efficiency without compromising heavily on the cutting fluid’s viscosity. This has the potential to revolutionize the design of MWFs especially for their use in microscale applications. The improvement seen in terms of the reduction in the cutting temperatures is very significant for the graphene-based MWFs. The low-cutting temperatures for the graphene-based MWF are likely to be the combined effect of both the improved lubrication (resulting in lower heat generation because of friction) and the improved thermal conductivity of the MWF with addition of graphene (resulting in increased heat transfer from the cutting zone to the cutting fluid). Besides, the increased wettability of the graphene formulation implies that a greater surface area of the droplet is in contact with the workpiece. This implies more effective cooling provided by the evaporation of the water phase. Furthermore, the high thermal conductivity of graphene sheets is also expected to help conduct heat away from the cutting zone. The contact angle data reveals that the wettability of solutions containing SWCNT and MWCNT is significantly lower than that of the graphene-based solutions. Therefore, unlike the graphene platelets, the SWCNT and the MWCNT are not likely to penetrate into the tool-workpiece interface as effectively. The MWCNT solution appears to have a slightly higher wettability than the SWCNT solution. This coupled with the lubrication effect provided by the sliding of the MWCNT results in lower cutting force when compared against that of the SWCNT solution. Overall, the performance of 0.2% GPL solution is observed to be comparable to that of 0.5% SWCNT and 0.5% MWCNT solutions.

’ CONCLUSIONS In summary, addition of GPL to a semisynthetic MWF is observed to significantly improve its lubrication and cooling efficiency. This is attributed to the following factors: (1) Increased

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wettability of the graphene modified cutting fluid increases the solid-liquid interfacial contact area enabling enhanced cooling by evaporation of the water phase. The improved wettability also facilitates the penetration of the graphene platelets into the tool-workpiece interface. (2) Effective lubrication provided by the sliding of the graphene sheets within the platelet. (3) Graphene sheets act as heat-sinks to shunt heat away from the cutting zone. We also demonstrate that GPL is a superior additive for MWFs than either SWCNT or MWCNT. On the average, the improvements obtained by the addition of ∼0.2% by weight of GPL are comparable to those obtained by adding ∼0.5% by weight of SWCNT or MWCNT. Finally, MWCNT outperform SWCNT in their ability to provide lubrication likely because of the relative slip of the nested nanotubes within the MWCNT structure. In the 0-0.5% GPL weight fraction range, there seems to be a nearly monotonic decrease in the cutting temperatures and the cutting forces as seen in Figure 3A and B, suggesting that further improvement may be possible by increasing the GPL loading beyond 0.5%. One practical problem that may be encountered at higher concentrations of GPL is that of agglomeration of GPL within the MWF solution. This may adversely affect the relevant properties of the MWF and is likely to result in poor dimensional accuracy of the machined micropart. Currently, the authors are investigating this issue as part of their continued research efforts.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (J.S.) and [email protected] (N.K.).

’ ACKNOWLEDGMENT The authors gratefully acknowledge Professor Shiv G. Kapoor and Professor Richard E. DeVor for the use of their facilities in the Micromachining Processes Laboratory at the University of Illinois, Urbana-Champaign. N.K. acknowledges funding support from the U.S. Office of Naval Research (Award Number: N000140910928) and the U.S. National Science Foundation (Award Number: 0900188). We also thank Professor Diana Borca-Tasciuc’s group at the Rensselaer Polytechnic Institute for their assistance with the thermal conductivity measurements. ’ REFERENCES (1) Liu, X.; DeVor, R. E.; Kapoor, S. G.; Ehmann, K. F. The Mechanics of Machining at the Micro-Scale: Assessment of the Current State of the Science. J. Manuf. Sci. Eng. 2004, 126, 666. (2) Melkote, S. N.; Kai, L. Effect of plastic side flow on surface roughness in micro-turning process. Int. J. Mach. Tool Manuf. 2009, 46, 1778. (3) Tansel, I. N.; Arkan, T. T.; Bao, W. Y.; Mahnedrakar, N.; Shisler, B.; Smith, D.; McCool, M. Tool wear estimation in micro-machining. I. Tool usage-cutting force relationship. Int. J. Mach. Tool Manuf. 2000, 40, 599. (4) Torres, C. D.; Heaney, P. J.; Sumant, A. V.; Hamilton, M. A.; Carpick, E. W.; Pfefferkorn, F. E. Analyzing the Performance of Diamond-coated Micro End Mills. Int. J. Mach. Tool Manuf. 2009, 49, 599. (5) Wang, B.; Lianf, Y. C.; Zhao, Y.; Dong Measurement of the Residual Stress in the Micro Milled Thin-Walled Structures, S. J. Phys.: Conf. Ser. 2006, 48, 1127. (6) Ghai, I; Wentz, J.; DeVor, R. E.; Kapoor, S. G.; Samuel, J. Droplet Behavior on a Rotating Surface for Atomization-based Cutting Fluid Application in Micro-machining. J. Manuf. Sci. Eng. 2010, 132, 0110171. 3414

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