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Effect of Graphite and Carbon Nanofiber Additives on the Performance Efficiency of a Gear Pump Driven Hydraulic Circuit Using Ethanol Philip Martorana,† Ilker S. Bayer,*,†,‡ Adam Steele,† and Eric Loth†,‡ Department of Aerospace Engineering, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States, and Department of Mechanical and Aerospace Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22904, United States
We show that fine graphite flake and carbon nanofiber dispersed ethanol solutions can potentially replace conventional hydraulic fluids in gear pump-driven hydraulic circuits operating below 1 MPa gauge pressure. Low-viscosity hydraulic fluids are generally detrimental to pump life. However, both graphite and carbon nanofiber dispersions in ethanol within a concentration range of 195-1500 ppm can sustain hydraulic circuits with increases in pump efficiency and without modifying the viscosity of ethanol. Pump inlet pressure, volumetric flow rate, and electric power consumption data were recorded over a range of pump discharge pressures. Pump power consumption at a given differential pump pressure was found to remain approximately constant for all suspensions. However, increases in both volumetric flow rate and overall pump efficiency were observed when pure ethanol was replaced by the nanostructured carbon/ethanol solutions. The presence of the additives can partially or collectively improve a number of lubrication mechanisms which exist in a gear pump such as boundary, hydrodynamic, hydrostatic, elastohydrodynamic, and mixed-film lubrication. Additionally, we observed that the additives deposited permanently on gear and enclosure surfaces creating low shear strength films which can help reduce friction. Qualitative examination of environmental scanning electron microscope images of colloidal graphite and carbon nanofiber additive morphology before and after extended run periods indicated that graphite retained significant resilience, whereas carbon nanofibers appear to have undergone some scission. 1. Introduction A gear pump uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for hydraulic fluid power applications. Gear pumps are also widely used in chemical installations to pump fluids with a certain viscosity. There are two main variations; external gear pumps which use two external spur gears and internal gear pumps which use an external and an internal spur gear. Fixed displacement gear pumps work on the principle of pumping a constant amount of fluid for each revolution. A gear pump can also be considered as a unique tribo-tester. Dynamics of lubrication within a gear pump is a very complicated process, and the lubricants are generally exposed to very severe operating conditions. Boundary, hydrodynamic, hydrostatic, elastohydrodynamic, and mixed-film lubrication are recognized as the primary lubrication regimes in tribology. Combinations of these lubrication mechanisms may occur within a gear pump depending upon pressure, entrainment velocity, contact geometry, and fluid properties.1 Improving the efficiency of liquid pumps is an area of continually increasing importance in the fluid power industry. The overall efficiency of a pump is a combination of two components: the volumetric efficiency and the torque (or mechanical) efficiency. The latter is a measure of the power lost due to fluid shear and internal friction, while the volumetric efficiency is a measure of the power lost due to fluid compressibility and internal leakage.2 These definitions show that the physicochemical state of the fluid running through a pump is just as important to the efficiency as the physical design of the * To whom correspondence should be addressed. E-mail: ibayer1@ illinois.edu. † University of Illinois at Urbana-Champaign. ‡ University of Virginia.
pump. Viscosity of the hydraulic fluids can play a major role in determining how efficient the pump will perform for a given hydraulic fluid.3 In fact, viscosity of a hydraulic fluid is a major selection criterion for pump performance. Figure 1 schematically shows the relationship between hydraulic pump fluid viscosity and pump operating conditions and related failure modes. For instance, below 50 mm2/s liquid viscosity, reduced equipment life due to high wear rate and overheating is highly probable, whereas above 500 mm2/s, sluggish pumping conditions and cavitation along with the formation of uneven lubrication regions inside the pump result.4 As shown in Figure 2, the volumetric and mechanical efficiencies generally have opposite trends with respect to increasing fluid viscosity. As the fluid viscosity increases, the volumetric efficiency (ηVE) increases due to the reduction in internal leakage but the mechanical efficiency (ηmech) decreases because more power is required to pump the fluid. Thus, there is an optimum viscosity for which the combined influence of internal leakage and friction losses is
Figure 1. Viscosity-dependent operational performance of gear pumps. The hydraulic fluids should be designed to have a viscosity within a 20-50 mm2/s range for optimum pump performance.
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Figure 2. Qualitative representation of the effect of fluid viscosity on the volumetric, mechanical, and overall efficiencies of a gear pump. Reproduced with permission from ref 5. Copyright 2009 University of Minnesota.
nanofibers as colloidal additives in a low-viscosity fluid (ethanol) corresponding to an operating condition below 5 mm2/s hydraulic fluid viscosity, as shown in Figure 1. The additives are used in small enough concentrations (parts per million levels) so that the viscosity of the working fluid is not modified at all times. As such, they are not expected to influence the mechanical efficiency. Graphite has been well-documented9-11 as a solid lubricant and more recently has been used to make nanolubricants.12 In addition, graphite nanofluids have been shown to increase the heat-transfer coefficient in flow systems when used as additives.13 Surface graphitized carbon nanofibers with an average diameter of 150 nm were also chosen since they have unique lubrication properties with additional superior mechanical properties.14 The latter aspect is important as external gear pumps subject additives to a harsh environment; i.e., high molecular-weight polymers may undergo scission. One may thus expect some improvement in wear with these additives. However, to the authors’ knowledge, no previous study has shown that use of additives in small concentration (such that viscosity is unaffected) can alter the volumetric efficiency of a external gear pump. The objective of the study is to investigate the effects of micro-/nanoscale carbon-based additives in a closed hydraulic loop. In particular, the effects of different additive concentrations on external gear pump performance are examined. The additives were used as colloidal suspensions in ethanol with the help of dispersants. Comparisons are presented on the basis of the measured power consumption, measured volumetric flow rates, and calculated overall efficiencies at different concentrations of solid additives (including a baseline case with no additives in the closed hydraulic loop). Possible postrun structural changes in the additives were qualitatively analyzed using pre- and postrun ESEM images of the additives isolated from the colloidal suspensions.
Figure 3. Influence of viscosity on the flow rate for a given pump output pressure.
2. Experimental Methods
minimized so that the overall efficiency (ηOV) of the system will be maximized.5 Bielmeier et al.6 examined the effect of the discharge pressure on the volumetric flow rate of a gear pump for two Newtonian oils with different dynamic viscosities. Their results are summarized in Figure 3 with two fluids, where oil 2 is roughly twice as viscous as oil 1. They found that flow rate decreased in a linear manner as the discharge pressure was increased. This led them to conclude that the decrease in flow rate was due to an increase in internal leakage, consistent with the trend shown in Figure 2. According to Dearn,7 internal leakage occurs in three specific areas within a gear pump: between the gears themselves, between the gears and the cavity plate, and between the mating surfaces of the gears with the end-cap and pump body. If debris and contaminants are contained within the fluid, then over time they may increase internal leakage and reduce volumetric efficiency and pump life, as discussed by Jing et al.8 Thus, in general, the presence of particulate matter in the hydraulic fluid is expected to be detrimental to pump performance. Increasing viscosity can improve wear protection and efficiency. However, as mentioned earlier, increasing viscosity of the hydraulic fluids results in increased power consumption, sluggish performance, cavitation, and frequent occurrence of poorly lubricated regions within the pump (see Figure 1). Therefore, the design of highly efficient new water-based and biobased low-viscosity hydraulic fluids is being considered.4 This study investigates use of fine graphite flakes and carbon
The colloidal graphite stock used in this work was a 22 wt % graphite flake (density ) 2.26 g/cm3, 99+% fixed carbon) dispersion with an average particle size of 0.8-2.0 µm in 200 proof ethanol (Grafo Hydrograf A M2 from Fuchs Lubricant). The suspension contains cellulose acetate as binder and dispersant. As-received graphite stock was diluted with ethanol to obtain three different concentrations of colloidal suspensions, i.e., 1550, 775, and 195 ppm. Highly graphitic, Pyrograf-III, carbon nanofibers (CNFs) were obtained from Applied Sciences Inc. (Dayton, OH, USA). The CNFs are vapor-grown PR-24XT PS grade fibers which were fabricated by pyrolytic stripping of the as-grown fibers to remove polyaromatic hydrocarbon residues of the synthesis process from the nanofiber surface. This surface stripping takes place at around 600 °C without altering the existing carbon nanofiber microstructure. Initially, the CNFs were dispersed in ethanol with the help of a surfactant, sodium dodecyl sulfate (SDS), the concentration of which did not exceed 0.1 wt %. The CNF-ethanol/surfactant mixtures were sonicated using a Sonics VCX 750 ultrasonic processor to form four stable CNF suspensions of 200, 400, 800, and 1600 ppm. Figure 4 shows the viscosity of the graphite and CNF colloidal dispersions in ethanol as a function of concentration. The measurements were conducted using a Brookfiled DVII+PRO desktop viscometer. The additives did not change the viscosity of ethanol for the concentrations studied, as shown in Figure 4. This assures that any changes seen in pump performance as a result of replacing pure ethanol with graphite and/or CNF
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Table 1. Instrument Data instrument
manufacturer
range
accuracy
pressure gauge (vacuum) pressure gauge
Omega
0-30 in Hg
Omega
0-100 psig
volumetric flow meter power meter
Assured Automation Optimum Energy Products
1-30 L/min
(0.25% of full scale (0.25% of full scale (0.5%
0-1875 W
(0.5-2.0%
based cleaning solution and pure ethanol were run successively several times through the hydraulic loop to ensure that particulate residues and debris from the pervious runs were flushed out of the system completely. 3. Results and Discussion
Figure 4. Dynamic viscosity for different fluids and additive concentrations.
Pump inlet and discharge pressures, volumetric flow rate, and power consumption data were recorded as different concentrations of the colloidal suspensions were run through the system. As a baseline without additives, ethanol (dynamic viscosity of 0.9 cP) and mineral oil (dynamic viscosity of 4.5 cP) were chosen as the test fluids, and the results were compared to those of Bielmeier et al.6 Volumetric flow rate change as a function of pump outlet pressure is shown in Figure 6. For both fluids, it can be seen that the flow rate decreased as the pressure increased. This confirms the results of Bielmeier et al.6 (see Figure 3) and is due to increased pressure within gaps in the device. The internal leakage is expected to increase as the pressure drop across the pump increases due to the flow restriction effect caused by the downstream needle valve. The dashed line in Figure 6 marks the maximum flow rate measured in the hydraulic circuit which corresponds to 5.3 L/min. Note that the minimum pump output pressure achievable was 0.1 MPa with the hydraulic loop; hence, this has limited the maximum flow rate to 5.3 L/min. It will be shown later, that the maximum flow rate line in Figure 6 also represents the highest volumetric flow rate recorded for all additive concentrations. To assess the additive performance on mechanical efficiency, the power consumption of the pump was recorded as a function of differential pressure across the pump. The results for pure ethanol and for the graphite and CNF colloidal suspensions can be seen in Figure 7. The results indicate a linear increase in power as the differential pressure across the pump (∆p) increases, consistent with theoretical
Figure 5. (a) Experimental setup and (b) photograph of the gear pump (left) and the internal view of the gear pump (right).
colloidal suspensions are not related to viscous effects. The closed hydraulic loop experimental setup is shown in Figure 5a. A SHURflo self-priming, positive displacement, external rotary gear pump equipped with an internal pressure relief valve drives the hydraulic loop. The pump is driven by a 0.5 horsepower, constant-speed (1725 rpm) electrical motor. The digital vacuum gauge at the intake port has a range of 0-30 in. Hg (referenced to ambient pressure), and the digital pressure gauge at the discharge port has a range of 0-100 psig. A digital volumetric flow meter attached downstream of the discharge port has a range of 1-30 L/min. A precalibrated power meter was used to measure the electrical power consumed by the motor. Additional information about the instrumentation used can be found in Table 1. The discharge pressure was manipulated by using a Deltrol Fluid Products needle valve installed downstream of the discharge port. The experiments were designed so that the lowest concentration graphite or CNF suspensions were run first. At the end of each run, a water-
Figure 6. Influence of viscosity on flow rate for a given differential pump pressure.
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Figure 7. Power as a function of differential pump pressure for (a) graphite solutions and (b) CNF solutions.
Figure 8. Volumetric flow rate as a function of differential pump pressure for (a) graphite solutions and (b) CNF solutions.
trends. Note that for a given pressure drop across the pump, no change in pump power consumption was observed as a function of graphite or CNF concentrations compared to pure ethanol. In other words, the power required at a given ∆p is essentially independent of additive concentration. Therefore, changes in mechanical efficiency due to possible hydraulic fluid viscosity changes can be ruled out. Parts a and b of Figure 8 show volumetric flow rate change as a function of ∆p across the pump for selected graphite and CNF colloidal fluids. These data were plotted to explore the dependence of the maximum volumetric flow rate at a given ∆p on additive concentration. Note that the extrapolated maximum flow line in this figure is the same as the one shown in Figure 6. As Figure 8 shows, for both graphite and CNF additives, as the additive concentration is increased, the volumetric flow rate increases for a given ∆p. This is most clearly seen with the CNF additives in Figure 8b. Though the 775 and 1550 ppm graphite concentrations of Figure 7a produce similar volumetric flow rates for a given ∆p, they still produce increases in flow rate over the 194 ppm graphite concentration and pure ethanol. These results suggest considerable reduction in internal pump leakage. It can be difficult to explain this effect because film thickness variations between gear teeth indicate a complex tribological condition due to variable sliding speed and strong variation in pressure and radius of curvature.15 Both graphite and CNF additives in the hydraulic fluid are forced to confine themselves in these small complex clearances in an unsteady fashion. Furthermore, Wedeven and Bourdoulous16 note that asperity stress, particularly during initial gear pump operation, can cause substantial pump internal leakage. The area of most concern in the gear pumps with respect to lubrication is the region of high sliding near the root and tip of the gear teeth.16 These are the regions where insufficient elastohydrodynamic film thickness is present. In general, good filmforming hydraulic fluids can partially alleviate the resulting problems such as local adhesion, wear, and scuffing. In addition, there have been some improvements in the design of gears from materials with smoother surface finish.17
When graphite surfaces with very small amounts of surfaceadsorbed liquids slide against metal surfaces, the amount of metal debris loss from the surfaces (wear) was shown to be reduced drastically.18 Moreover, it was shown that colloidal graphite deposits on metal surfaces have superior tribological performance compared to conventional colloidal solid lubricants such as MoS2.19 Improved solid lubrication property of sol-gel graphite deposits is believed to be associated with the graphite crystal structure. Within a graphene layer, there are strong covalent bonds (σ bonds), whereas between layers only a weak electron bond (π bond) exists. The tangential frictional force breaks the weak electron bond causing interlayer slip with a low friction coefficient.19 Under the very small gear teeth clearances very high tangential frictional forces are created causing the interlayer slip of graphitic surfaces. In addition, the colloidal graphite and CNF particles in ethanol may fill the asperities (caused by machining or wear on the gear surfaces) thus reducing the available leak paths, i.e., effectively smoothing out the surfaces locally. This hypothesis is consistent with observations by Lu et al.20 whereby dispersed carbon nanotube (CNT)/polymer miniemulsions (suspensions) were found to fill microgaps of the rubbing surfaces as long as the CNT concentrations were kept below 2000 ppm. Thus, the present additives may reduce leakage by reducing the effective clearance though a reduced effective roughness. It should be noted that when pure ethanol was run through the pump after the colloidal suspensions, increased flow rates were still sustained for some time. This further supports the possibility that the additives may adhere to the asperities of the metal surfaces. It is important to note that the reduction in effective clearance does not lead to an increase in power consumption which can accompany low-viscosity fluids (Figure 7). This suggests that other mechanisms associated with improved lubrication are also important in explaining the overall behavior. The results of Lu et al.20 indicated that the carbon nanotube suspensions form a thin selfassembled lubricating film enabling a reduction in friction effect in a four-ball tribo-tester. We hypothesize that the graphite and
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Figure 10. Scanning electron microscope images of graphite (a) as received and (b) postexperiment.
Figure 9. Overall efficiency as a function of differential pump pressure for (a) graphite solutions and (b) CNF solutions.
CNF particles can also self-assemble within gear clearances causing a similar reduction in friction.18 In particular, it can be expected that they form thin permanent lubricating layers penetrating into the surface asperities on gear surfaces as the surfaces mate during pump operation. However, the effectiveness of this effect may depend on the chemical and geometric features of the additives as well as the concentration and the degree of colloidal dispersion in the solutions.18-20 Such differences are manifested in Figure 8a,b. For instance, measured flow rate increases do not seem to correlate linearly with nanostructured carbon additive concentrations. At high concentrations, graphite has little leakage such that the flow rate is near the maximum value over a wider range of pressures than seen for pure ethanol. However, the flow rate drops off rapidly when the differential pressure reaches and exceeds 500 kPa, as seen in Figure 8a. In the case of CNF additives, the increase in flow rate over pure ethanol becomes more pronounced as the pressure rise moves to higher values. This suggests that graphite may be more beneficial as an additive at lower pressures and CNFs may be more effective at higher pressures. Perhaps, one may design novel low-viscosity hydraulic fluids with a combination of conventional and nanoscale colloidal additives for efficient pump operation over a range of pressures. After pressure, volumetric flow rate, and power consumption were recorded so that overall pump efficiency could be calculated. Plots of the overall pump efficiency vs pressure rise for graphite and CNFs can be seen in Figure 9a,b, respectively. Power into the system was assumed to be equal to half the value recorded off the power consumption meter, as recommended by the pump and motor manufacturer. The power output by the pump was calculated by multiplying the volumetric flow rate by ∆p across the pump. It can be seen that higher additive concentrations resulted in higher overall efficiencies. This is particularly evident in Figure 9a. A number of experiments were conducted to find out whether the closed-loop hydraulic system had pressure-dependent hysteresis. This could be investigated by slowly lowering the pressure differential once the maximum ∆p is reached (backward loop) and
measuring volumetric flow rate and power. Figure 9b also shows that the results are essentially independent of whether pressure is moved from low to high or from high to low. In other words, little hysteresis exists between forward and backward loops in this experiment. To qualitatively examine the effect of the confined environment of the gear pump on the graphite and CNFs, environmental scanning electron microscope (ESEM) images of the additives were taken before and after the experiments. Parts a and b of Figure 10 show images of graphite flakes before (a) and after (b) running through the gear pump. The lamellar structure which makes graphite a well-documented lubricant can be seen in this figure. These images were obtained by depositing the graphite/ ethanol suspensions on a glass slide and allowing the ethanol to evaporate. The same lamellar structure appears to have been well-persevered after running through the pump, suggesting that graphite was not degraded during the experiment. Unlike graphite, however, the CNFs were observed to undergo probable scission after running through the pump. Figure 11a shows the microstructure of highly entangled and bundled as-received CNF powder. After sonicating in ethanol with the help of a surfactant to form the colloidal CNF suspensions, CNFs appear to disentangle and show a good degree of dispersion, as shown in Figure 11b. From Figure 11a,b, it can be seen that the prerun images corresponding to the two different types (as-received powder and sonicated) appear similar in structural makeup with the exception of the CNFs in Figure 11b being more dispersed due to sonication. This shows that sonicating the CNFs in solution does not change their structure. However, the CNFs in Figure 11c look thinner than the CNFs in the prerun images and also appear to have a film or a shell surrounding them. The film or shell that surrounds the CNFs in this image could possibly be the residual surfactant (SDS) coating on the graphitic surfaces of the CNFs. The CNFs also appear shorter in length in Figure 11c than in Figure 11a,b. This suggests that possible scission of the CNFs is occurring within the gear pump in time. However, it is still unclear whether the structural changes in CNFs during pump operation have a direct effect on the pump efficiency in the long run. During the pump operation times of this study (∼30 min), the structural changes in CNFs did not seem to reduce the observed increases in pump efficiency. A recent study by Ozkan et al.14 indicates that fractured nanofiber surfaces have the stacked truncated cup structure of the oblique graphene layers. It is possible that when these layers rub against the gear surfaces, additional sites for friction reduction can be created. A detailed investigation on how continuous pump operation affects structural morphology of CNFs is underway in our laboratory. 4. Conclusions Graphite and CNFs have both been shown to positively impact external gear pump performance when used as additives for lowviscosity fluids. The ideal concentration range was found to be
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Acknowledgment We acknowledge the Center for Compact and Efficient Fluid Power and the National Science Foundation for their financial support and the Beckman Institute of Science and Technology of UIUC for their help with SEM imaging. We also thank Mr. Tanil Ozkan and Prof. Ioannis Chasiotis (University of Illinois) for technical assistance. Technical support by Evonik Industries through Steven Herzog is also acknowledged. Literature Cited
Figure 11. Scanning electron microscope images of CNFs (a) as received, (b) from sonicated solution, and (c) postexperiment.
between 400 and 1600 ppm. To the authors’ knowledge, this is the first study to show that nanostructured carbon additives can increase volumetric efficiency without significantly increasing the viscosity of the working fluid, ethanol. Higher concentrations (>400 ppm) of the additives produced more profound positive effects than the lower concentrations. Also, the power consumption was independent of additive concentration, leading to higher overall pump efficiency. However, the manner in which these positive effects are seen differs between graphite and CNFs, and the results suggests that graphite may be more beneficial as an additive at lower pressures and CNFs may be more effective at higher pressures. From a qualitative perspective, graphite does not appear to degrade when run through the gear pump, while scission may be occurring with the CNFs. However, fractured CNFs did not degrade the pump efficiency. This study indicates that nanostructured carbon dispersed biobased low-viscosity fluids can sustain conventional hydraulic circuits and possibly replace hydrocarbonbased hydraulic fluids. It is also worth mentioning that these nanostructured additives can be easily recovered and recycled from such low-viscosity fluids, whereas recovery and/or recycling of additive lubricants from conventional heavy oils can be very costly and unfriendly to the environment.
(1) Murakawa, M.; Komori, T.; Takeuchi, S.; Miyoshi, K. Performance of a rotating gear pair coated with an amorphous carbon film under a lossof-lubrication condition. Surf. Coat. Technol. 1999, 120-121, 646. (2) Manrig, N. Measuring pump efficiency: uncertainty considerations. Trans. ASME 2005, 127, 280. (3) Kozlov, Y. N.; Polyakov, Y. N.; Kozlova, G. I.; Medvedev, V. D.; Faidel, G. I. A study of the operation of a gear pump on highly viscous polymer solutions. Chem. Pet. Eng. 1973, 9, 329. (4) Hall, J. H. Internal gear pumpssYour second choice for thin liquids. World Pumps 2005, 2005 (469), 32. (5) Durfee, W.; Sun, Z. Fluid Power System Dynamics; University of Minnesota: Minneapolis, MN, 2009. (6) Bielmeier, E.; Bartholomae, I.; Neveu, C. D. Measurement of the temporary shear stability of lubricants in a gear pump. Tribotest J. 1999, 193, 193. (7) Dearn, R. The fine art of gear pump selection and operation. World Pumps 2001, 417, 38. (8) Jing, B.; Yang, L.; Lu, S. The research of contaminant abrasion on external gear pump. Proceedings of the 2009 Chinese Control and Decision Conference (CCDC2009); IEEE: Guilin, China, 2009; p 3090. (9) Bhushan, B. Principles and Applications of Tribology; John Wiley & Sons: New York, 1999. (10) Williams, J. Engineering Tribology; Cambridge University Press: Cambridge, U.K., 2005. (11) Blau, P. Friction Science and Technology, 2nd ed.; CRC Press: Boca Raton, FL, 2009. (12) Lee, C.-G.; Hwang, Y.-J.; Choi, Y.-M.; Lee, J.-K.; Choi, C.; Oh, J.-M. A study of the tribological characteristics of graphite nano lubricants. Int. J. Precis. Eng. Manuf. 2009, 10, 85. (13) Yang, Y.; Zhang, Z. G.; Grulke, E. A.; Anderson, W. B.; Wu, G. Heat transfer properties of nanoparticle-in-fluid dispersions (nanofluids) in laminar flow. Int. J. Heat Mass Transfer 2005, 48, 1107. (14) Ozkan, T.; Naraghi, M.; Chasiotis, I. Mechanical properties of vapor grown carbon nanofibers. Carbon 2010, 48, 239. (15) Paffoni, B. Pressure and film thickness in a trochoidal hydrostatic gear pump. Proc. Inst. Mech. Eng., Part G 2003, 217, 179. (16) Wedeven, L. D.; Bourdoulous, R. Hydraulic gear pump failure analysis and tribology simulation. In Hydraulic Failure Analysis Fluids Components and System Effects; Totten, G. E., Wills, D. K., Feldmann, D. G., Eds.; American Society for Testing Materials: West Conshohocken, PA, 2001; p 105. (17) Veronesi, P.; Sola, R.; Poli, G. Electroless Ni coatings for the improvement of wear resistance of bearings for lightweight rotary gear pumps. Int. J. Surf. Sci. Eng. 2008, 2, 190. (18) Buckley, D. H.; Brainard, W. A. Friction and wear of metals in contact with pyrolytic graphite. Carbon 1975, 13, 501. (19) Hai-dou, W.; Bin-shi, X.; Jia-ju, L.; Da-ming, Z. The tribological properties of solid lubrication graphite coatings prepared by a sol-gel method. Carbon 2005, 43, 2013. (20) Lu, H. F.; Fei, B.; Xin, J. H.; Wang, R. H.; Li, L.; Guan, W. C. Synthesis and lubricating performance of a carbon nanotube seeded miniemulsion. Carbon 2007, 45, 936.
ReceiVed for reView April 13, 2010 ReVised manuscript receiVed September 15, 2010 Accepted September 20, 2010 IE100872G
