Tribology Transactions, 52: 133-137, 2009 C Society of Tribologists and Lubrication Engineers Copyright
ISSN: 1040-2004 print / 1547-397X online DOI: 10.1080/10402000802192693

Frictional Performances of Activated Carbon and Carbon Blacks as Lubricant Additives SEUNGHYUN BAIK,1 DOKYUNG YOON,1 HYUNG IK LEE,2 JI MAN KIM,2 GYU-SUN LEE,3 and YOUNG-ZE LEE3 1 SKKU Advanced Institute of Nanotechnology(SAINT) Sungkyunkwan University Suwon, Kyunggi-do, 440-746 South Korea 2 Department of Chemistry, Sungkyunkwan University Suwon, Kyunggi-do, 440-746 South Korea 3 School of Mechanical Engineering, Sungkyunkwan University Suwon, Kyunggi-do, 440–746 South Korea

The frictional performances of activated carbon, carbon blacks, mesoporous carbons, and carbon nanotubes were compared as lubricant additives to mineral oils using a ball-on-disk tester at ambient temperature. Well-dispersed, smaller particles with a uniform size were more effective in maintaining the oil gaps and the protecting surfaces. However, the effects of different atomic structures of carbon particles on tribological performances were minimal as long as the particle sizes are of similar order.

rating carbon materials such as diamond, graphite, diamond-like carbon (DLC) materials, fullerene, carbon nanotubes, and mesoporous carbons, which have favorable tribological characteristics (Joly-Pottuz, et al. (6); Lei, et al. (7); Baik, et al. (8), (9)). Among various carbon nanoparticles, carbon nanotubes have received considerable attention because of their unique electrical, optical, and mechanical properties (Tans, et al. (10); Wong, et al. (11); Paul, et al. (12)). Their tensile strength is up to 100 times stronger than that of steel, and their measured Young’s modulus is about 1.5 TPa (Zhan, et al. (13)). Bending is fully reversible up to very large bending angles due to sp2 -sp3 re-hybridization when deformed out of plane (Bahr, et al. (14)). Recent investigations reported improved tribological properties by incorporating carbon nanotubes as additives to base lubricants (Joly-Pottuz, et al. (6); Baik, et al. (8); Chen, et al. (15)). Ordered mesoporous (2–50 nm) carbons, having a unique structure of the hexagonally packed mesostructured carbons, are another promising material in various applications due to their remarkable characteristics, such as high specific surface areas, large pore volumes, chemical inertness, and good mechanical stability (Foley (16); Kyotani (17)). Recently, we reported improved tribological properties of mesoporous carbons as lubricant additives (Baik, et al. (8),(9)). In this study, commercially obtained activated carbon and carbon blacks were used as additives to mineral oils to investigate their effects on tribological performances employing a ballon-disk tester at ambient temperature. Also, their tribological performances were compared with those of multi walled carbon nanotubes (MWNT) and mesoporous carbons (CMK-3) reported by Baik et al. (8)-(9).

KEY WORDS Activated Carbon; Carbon Blacks; Boundary Lubrication; Scuffing

INTRODUCTION Boundary lubrication is one of the lubrication regimes where mechanical components and machine elements operate under the marginally lubricated conditions (Ludema (1)). In particular, boundary lubrication under metal-metal contact conditions has been extensively studied (Yamamoto and Eguchi (2)). Various chemicals have been introduced into the contact region to reduce metallic contacts. In internal combustion engines, the formation of greenhouse gases and limited fuel resources require feasible technological approaches, which can ensure a highly efficient operation of sliding surfaces to provide low fuel consumption, clean exhaust gases, and customer satisfaction (Truhan, et al. (3)). To satisfy these requirements, reduction of friction and wear and prolongation of component wear life are essential (Sheiretov, et al. (4); Lee, et al. (5)). These objectives can be accomplished by incorpo-

EXPERIMENTAL PROCEDURES Preparation of Lubricants with Carbon Additives

Manuscript received January 15, 2008 Manuscript accepted May 7, 2008 Review led by Raj Shah

Three different carbon particles were commercially obtained. Figure 1 shows scanning electron microscope (SEM) images of

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Fig. 2—Schematic diagram of a ball-on-disk sliding wear tester.

Fig. 1—SEM images (a) carbon black type I, (b) carbon black type II, and (c) activated carbon.

two different types of carbon blacks and activated carbon. Carbon black, which was produced by the incomplete combustion of petroleum products, is a form of amorphous carbon with a high surface-to-volume ratio. Carbon black type I has a relatively uniform size with an average diameter of 50 nm, whereas carbon black type II shows a size distribution between 20 nm and 90 nm. Activated carbon is a material derived from charcoal, and the size is on the order of 1 mm (Fig. 1c). These carbon particles were dispersed in mineral oils (1 wt.% each) by ultrasonication for 4 min at 490 W. The carbon particle suspensions in mineral oils were used as lubricants in the boundary lubricated sliding tests.

Boundary Lubricated Sliding Tests A ball-on-disk sliding tester was used to investigate the tribological characteristics of carbon additives, as schematically shown in Fig. 2. A servomotor was used to rotate the shaft with stability. The lower flat disk was mounted on a rotating shaft in an oil bath. A ball was held fixed in a holder, which was clamped to a fixed arm with a transducer, for measuring friction forces. Contact

was achieved by pressing the ball against the flat surface under a normal load by spring force, which reduced the variation of the normal force during sliding tests. The signal from the transducer was stored in a computer at a sampling rate of 5 Hz after digitizing with an analog/digital converter. The stored signal was subsequently converted to the coefficient of friction by a signal processing program. AISI 52100 steel was used for the balls, and the diameter was 10 mm. AISI 1045 steel was used for the disk specimens with a diameter of 60 mm and a thickness of 10 mm. The surface hardness was HV1N 300. The surface topographies of the balls and disk specimens were analyzed using atomic force microscopy (Theromicroscopes). The average surface roughnesses were 4.41 nm for the balls and 11.77 nm for the disk specimens. All specimens were cleaned before the tests using distilled water and acetone in an ultrasonic cleaner. The mineral oil with carbon additives was applied on the top of the disk specimen, and the thickness of the oil layer was about 2 mm. A slow sliding speed of 0.04 m/s (30 rpm) was used in all tests to maintain the condition of boundary lubrication. The load was increased from 100 N up to the onset of scuffing by a step size of 100 N. The test was performed for 3 min at each load, which was enough for the sliding surface to form a protective layer. The test condition was determined to allow a gradual formation of the protective layer on the sliding surface. The catastrophic mode of surface failure is described as scuffing (Ludema (18)). Usually, there is a sudden increase in friction at the onset of scuffing. During the tests, the scuffing failure was detected by an abrupt increase in friction.

RESULTS AND DISCUSSION Repeated pass sliding tests with the ball-on-disk tester were carried out to measure friction forces as well as periods of time up to the onset of scuffing. Figure 3 shows the results of sliding tests performed using pure mineral oil and mineral oils with carbon additives. For reasons of comparison, the tribological characteristics of carbon nanotubes and mesoporous carbons were reproduced from the work of Baik, et al. (8), (9), which were measured using the identical experimental conditions. In the case of tests with pure mineral oil, friction coefficients showed uniform values of about 0.12 and scuffing occurred at the load of 1200 N. The mineral oils with carbon additives showed almost the same values of friction coefficients as those of pure mineral oil. However, the

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Fig. 3—Friction coefficients from sliding tests up to the onset of scuffing. (a) Pure mineral oil, (b) mineral oil with MWNT (Baik, et al. (8)), (c) mineral oil with mesoporous carbon (CMK-3) (Baik, et al. (9)), (d) mineral oil with carbon black type I, (e) mineral oil with carbon black type II, (f) mineral oil with activated carbon.

incorporation of carbon additives significantly increased periods of time up to the onset of scuffing. The loads at the onset of scuffing are summarized in Fig. 4. The scuffing loads significantly increased with carbon additives. The least improvement was observed for activated carbon. The size of activated carbon is on the order of 1 mm, whereas other carbon

Fig. 4—The loads at the onset of scuffing.

particles are much smaller than 1 mm. This indicates that smaller carbon particles are more effective as lubricant additives. Smaller particles might easily exist between two mating metal surfaces. The highest scuffing load was observed for the carbon black type I, which had a relatively uniform size. The results demonstrated that smaller particles with a uniform size were more effective as lubricant additives. Figure 5 shows SEM images of carbon blacks and activated carbon after the repeated pass sliding tests. The structural modification of carbon black type I was minimal during the sliding tests, whereas larger particles of carbon black type II were broken into smaller pieces as shown in Figs. 5a and 5b. Activated carbons were also fractured into smaller particles. This indicates that the structural deformation of smaller carbon particles with a uniform size distribution was minimal. A number of mechanisms have been proposed for the favorable effects of carbon nanotubes, which have cylindrical atomic structures of carbons, on tribological properties as summarized by Baik, et al. (8). Chen, et al. (19) attributed the improvement to the strong mechanical properties and the unique hollow cylindrical structures of nanotubes. Nanotubes can slide or roll between two mating metal surfaces, improving tribological properties. JolyPottuz, et al. (6) related the improvement to the structural deformation of nanotubes in contact surfaces since a high contact pressure was necessary to decrease friction and wear. The destruction of nanotubes was also confirmed by transmission electron

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can lead to the decrease in wear loss. Carbon additives can serve as spacers, preventing the asperities of two mating metals from contacting. We previously showed the SEM images of sliding surfaces before the scuffing failure, proving that carbon additives significantly decreased adhesive and abrasive wear (Baik, et al. (8), (9)). Scuffing times and loads could be increased as a result of less wear. However, the precise mechanism still needs to be explored.

CONCLUSIONS The frictional characteristics of carbon blacks and activated carbon were investigated as lubricant additives to mineral oils using a ball-on-disk tester in boundary lubricated environments. The scuffing times and loads of oils with carbon additives were much longer than those of pure mineral oil, but their frictional forces showed little difference. Carbon blacks with a small and uniform size were the most effective in maintaining oil gaps and protecting surfaces. Also, the structural modification during the sliding tests was minimal. A comparative study with the tribological performances of carbon nanotubes and mesoporous carbons revealed that the effects of different atomic structures of carbon additives were not significant as far as the particle sizes are of similar order. This indicated that the self-lubricating properties of carbon contributed to the improved tribological characteristics of carbonbased lubricant additives.

ACKNOWLEDGEMENTS This work was supported by grant No. R01-2006-000-10085-0 from the Basic Research Program of the Korea Science & Engineering Foundation and by a grant from the Center for Nanoscale Mechatronics & Manufacturing, one of twenty-first century’s frontier research programs supported by the Ministry of Science and Technology, Korea.

REFERENCES Fig. 5—SEM images of (a) carbon black type I, (b) carbon black type II, and (c) activated carbon after the sliding tests.

microscopy. We also observed the sp2 -sp3 transformation of carbon nanotube wall structures and the structural modification of mesoporous carbons, which have spherical geometries, during the sliding tests (Baik, et al. (8), (9)). In order to investigate whether ordered spherical or cylindrical atomic structures of carbon nanoparticles, such as those of mesoporous carbons and carbon nanotubes, are advantageous for lubricating purposes, boundary-lubricated sliding tests were carried out using other carbon particles with amorphous structures; i.e., carbon black type I, carbon black type II, and activated carbon. The scuffing times and loads were significantly improved for all the mineral oils with carbon additives compared to those of the pure mineral oil. Notably, the variation in scuffing times and loads for different carbon particles of similar sizes was not large. This indicates that the self-lubricating properties of carbon contributed to the improved tribological properties rather than the ordered cylindrical or spherical atomic structures of carbon particles. Moreover, the structures of carbon nanotubes and mesoporous carbons were deformed during the sliding tests (Baik, et al. (8), (9)). Chen, et al. (15) also suggested that the self-lubricating properties of carbon

(1) Ludema, K. C. (1984), “A Review of Scuffing and Running-in of Lubricated Surfaces with Asperities and Oxides in Perspective,” Wear, 100, pp 315-331. (2) Yamamoto, T. and Eguchi, M. (2005), “Shear Characteristics of a Boundary Film for a Paper-Based Wet Friction Material: Friction and Real Contact Area Measurement,” Tribology International, 38, pp 327-335. (3) Truhan, J. J., Qu, J. and Blau, P. J. (2005), “A Rig Test to Measure Friction and Wear of Heavy Duty Diesel Engine Piston Rings and Cylinder Liners Using Realistic Lubricants,” Tribology International, 38, pp 211-218. (4) Sheiretov, T., Yoon, H. and Cusano, C. (1998), “Scuffing under Dry Sliding Conditions—Part 1: Experimental Studies,” Tribology Transaction, 41, pp 435-446. (5) Lee, Y. Z. and Kim, B. J. (1999), “The Influence of the Boundary Lubricating Conditions of Three Different Fluids on the Plastic Fatigue Related Mechanisms of Wear and Scuffing,” Wear, 232, pp 116-121. (6) Joly-Pottuz, L., Dassenoy, F., Vacher, B., Martin, J. M. and Mieno, T. (2004), “Ultralow Friction and Wear Behaviour of Ni/Y-Based Single Wall Carbon Nanotubes (SWNTs),” Tribology International, 37, pp 1013-1018. (7) Lei, H., Guan, W. and Luo, J. (2002), “Tribological Behaviour of FullereneStyrene Sulfonic Acid Copolymer as Water-Based Lubricant Additive,” Wear, 252, pp 345-350. (8) Baik, S., Lee, G. S., Yoon, D. K. and Lee, Y. Z. (2006), “Tribological Performance of Multi-Walled Carbon Nanotubes in Mineral Oils under Boundary Lubricated Sliding,” Key Engineering Materials, 321, 3, pp 694-698. (9) Baik, S., Kim, J. M., Lee, G. S., Yoon, D. K., Lee, H. I. and Lee, Y. Z. (2006), “Tribological Performance of Ordered Mesoporous Carbons in Mineral Oils under Boundary Lubricated Sliding,” Key Engineering Materials, 326, 8, pp 353-356. (10) Tans, S. J., Devoret, M. H., Dai, H., Thess, A., Smalley, R. E., Geerligs, L. J. and Dekker, C. (1997), “Individual Single-Wall Carbon Nanotubes as Quantum Wires,” Nature, 386, pp 474-477.

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(11) Wong, E. W., Sheehan, P. E. and Lieber, C. M. (1997), “Nanobeam Mechanics: Elasticity, Strength, and Toughness of Nanorods and Nanotubes,” Science, 277, pp 1971-1975. (12) Paul, W. B., Baik, S., Heller, D. A. and Strano, M. S. (2005), “Near-Infrared Optical Sensors Based on Single-Walled Carbon Nanotubes,” Nature Materials, 4, pp 86-92. (13) Zhan, G. D., Kuntz, J. D., Wan, J. and Mukherjee, A. K. (2003), “Single-Wall Carbon Nanotubes as Attractive Toughening Agents in Alumina-Based Nanocomposites,” Nature Materials, 2, pp 38-42. (14) Bahr, J. L., Yang, J., Kosynkin, D. V., Bronikowski, M. J., Smalley, R. E. and Tour, J. M. (2001), “Functionalization of Carbon Nanotubes by Electrohemical Reduction of Aryl Diazonium Salts: A Bucky Paper Electrode,” J. Am. Chem. Soc., 123, pp 6536-6542.

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(15) Chen, C. S., Chen, X. H., Xu, L. S., Yang, Z. and Li, W. H. (2005), “Modification of Multi-Walled Carbon Nanotubes with Fatty Acid and their Tribological Properties as Lubricant Additives,” Carbon, 43, pp 1660. (16) Foley, H. C. (1995), “Carbogenic Molecular Sieves: Synthesis, Properties and Applications,” Microporous Materials, 4, pp 407-433. (17) Kyotani, T. (2000), “Control of Pore Structure in Carbon,” Carbon, 38, pp 269-286. (18) Ludema, K. C. (1996), Friction, Wear and Lubrication, CRC Press, New York. (19) Chen, W. X., Tu, J. P., Wang, L. Y., Gan, H. Y., Xu, Z. D. and Zhang, X. B. (2003), “Tribological Application of Carbon Nanotubes in a Metal-Based Composite Coating and Composites,” Carbon, 41, pp 215-222.