Tribological behavior of fullerene styrene sulfonic acid copolymer as water-based lubricant additive

Transcription

1 Wear 252 (2002) Tribological behavior of fullerene styrene sulfonic acid copolymer as water-based lubricant additive Hong Lei a,b,, Wenchao Guan b, Jianbin Luo a a The State Key Laboratory of Tribology, Tsinghua University, Beijing , China b Huazhong University of Science and Technology, Wuhan , China Received 16 July 2001; received in revised form 29 October 2001; accepted 11 December 2001 Abstract A novel fullerene styrene sulfonic acid copolymer was synthesized. The tribological properties of base stock (2 wt.% triethanolamine aqueous solution) containing the fullerene copolymer were measured using a four-ball tribotester. Experiments indicated that the anti-wear performance and load-carrying capacity of the base stock were raised and its friction coefficient was decreased. Then, the surface of worn scar was investigated by using electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). Finally, its lubrication mechanism was deduced preliminarily Published by Elsevier Science B.V. Keywords: Fullerene copolymer; Nanometer ball; Tribology 1. Introduction The lubricating properties of fullerene C 60 have been speculated since the time of its discovery because of its unique spherical shape with cage diameter of 0.71 nm, high load-bearing capacity, low surface energy, high chemical stability, weak intermolecular and strong intramolecular bonding [1 4], and it was expected to act as tiny ball bearing [5,6]. Theoretical simulations found that C 60 molecule was able to roll between graphite sheets, as well as, between hydrogen-terminated surfaces of diamond under relatively low load [7]. The tribological properties of fullerene particles, as an additive to liquid lubricants, were studied by a few groups [8,9]. The results indicated that the presence of fullerene was able to increase the load-carrying ability and decrease the friction coefficient and wear. However, in these experiments fullerene particles were dispersed in liquid lubricants by some physical or mechanical ways (such as solid grinding, solvent evaporation, ultrasonic, etc.) since they are only soluble in few non-polar solvents (such as benzene, toluene, carbon disulfide, etc.), so fullerene particles were present in the form of molecular cluster or super-corpuscle [8,9]. Up to this date, the tribological properties of soluble fullerene and its derivatives have seldom been studied. In this paper, a novel water-soluble fullerene styrene sulfonic acid copolymer was synthesized, and the frictional behav- Corresponding author. Fax: address: hong lei2000@sohu.com (H. Lei). ior of soluble fullerene copolymer containing the element sulfur was studied for the first time. 2. Experimental methods 2.1. Preparation and characterization of fullerene styrene sulfonic acid copolymer The fullerene styrene copolymer was synthesized by typical free radical polymerization, and then it was sulfonated by H 2 SO 4 solution containing Ag 2 SO 4 as catalyst. The product was a brown solid. The copolymer was characterized by FTIR, UV, VPO and elemental analysis. The results indicate that the copolymer molecule contains C wt.%, H 8.85 wt.% and S 11.4 wt.%, with an average molecular mass of The molecular structure of the copolymer may be star polymer with fullerene as core and a grafting of one or several polystyrene sulfonic acid chain segments. Morphology and size of the fullerene copolymer in water were measured using a JEM 100CXII model transmission electron microscope (TEM) with a voltage of 200 kv Measurement of tribology properties Since pure water has poor lubricity and bad corrosioninhibiting property, as well as, since triethanolamine is widely used in the metal-working fluids as a multifunctional additive, 2 wt.% of triethanolamine aqueous solution /02/$ see front matter 2002 Published by Elsevier Science B.V. PII: S (01)

2 346 H. Lei et al. / Wear 252 (2002) was chosen as base stock. The fullerene styrene sulfonic acid copolymer prepared above was used as lubricant additive in the base stock. The tribological measurements were carried out by using an MQ-800 four-ball tribotester at a rotational speed of 1450 rpm and at a temperature of 20 C. The maximum nonseized load was obtained by GB , similar to ASTMD2783; wear scar diameter was measured under a load of 200 N and a test duration of 30 min; friction coefficient was measured under a load of 200 N and a test duration of 10 s. The stainless steel balls used in the tests were made of GCr15(AISIE52100) bearing steel with the surface HRC hardness and m of surface roughness Ra. In order to study the effect of fullerene in the fullerene styrene sulfonic acid copolymer, the same concentration of the polystyrene sulfonic acid reference as additive was measured under the same conditions Analyses of the worn surfaces A JXA-8800R electron probe microanalysis (EPMA) was used to study the rubbing surface morphology and surface distribution of elements, at a voltage of 25 kv. X-ray photoelectron spectroscopy (XPS) was conducted on a KRATOS XSAM 800 electron spectrometer using the Mg K line with pass energy of 12.5 kv 18 ma. The binding energy of C 1s (284.6 ev) was used as reference. 3. Experimental results and discussion 3.1. Morphology and size of the fullerene styrene sulfonic acid copolymer in water The synthesized fullerene copolymer is soluble in water, giving a clear brown solution. TEM analysis indicates that the copolymer presents an ideal spherical shape in water with a diameter of about 3 40 nm, as shown in Fig. 1. For comparison, the appearance of the polystyrene sulfonic acid in water is lamellar shape. The physical structure of the fullerene copolymer nanometer tiny balls may be described as follows: the core is very hard fullerene, the shell is polystyrene sulfonic acid, which may be relatively soft but very elastic Effect of the fullerene copolymer content on maximum nonseized load, wear scar diameter and friction coefficient The maximum nonseized load (P B value) represents the load-carrying capacity of the lubricant. The P B value was measured and the results are given in Fig. 2(a). The P B value of the base stock is 130 N, and the base stock with the fullerene copolymer showed higher maximum nonseized load than the base stock. In other words, the fullerene copolymer could strengthen the load-carrying capacity of the base stock. When the fullerene copolymer content reaches 0.5 wt.%, the P B value is maximum. Then excessive fullerene copolymer resulted in a decrease of load-carrying capacity of the base stock. The wear scar diameter data are given in Fig. 2(b). It is seen that the addition of the fullerene copolymer can decrease the wear scar diameter of the base stock. When the fullerene copolymer content reaches 0.5 wt.%, the wear scar diameter is minimum. But excessive fullerene copolymer exhibits a larger wear scar diameter than the base stock. Decrease in load-carrying capacity and wear resistance at excessive additive may be attributed to the corrosive wear, since the fullerene styrene sulfonic acid copolymer additive is a strong acid and may react with the surface of the metal. The friction coefficient measurements were carried out and the results are given in Fig. 2(c). It is indicated that with the increasing of the additive content, the friction coefficient reduces rapidly from of base stock to minimum of that containing 2 wt.% fullerene copolymer. The reduction in friction coefficient implies that the fullerene copolymer has the possibility to cause microcosmic rolling effect between two rubbing surfaces as nanometer tiny ball Effect of load on wear scar diameter and friction coefficient Fig. 1. Morphology of nanometer balls of the fullerene copolymer in water. Amplification: 36,000. The dependence of wear scar diameter on load is shown in Fig. 3(a). Under testing loads, the wear scar diameter of base stock with 0.5 wt.% fullerene styrene sulfonic acid copolymer is smaller than that with 0.5 wt.% polystyrene sulfonic acid. It means that, the presence of fullerene can strengthen the wear resistance of base stock. Fig. 3(b) shows that, during the testing loads, the friction coefficient of base stock with 0.5 wt.% fullerene styrene sulfonic acid copolymer is lower than that with 0.5 wt.% polystyrene sulfonic acid, especially at the load of 200 N, the difference in friction coefficient between the two additives is larger, which may be attributed to the lower load-carrying capacity

3 H. Lei et al. / Wear 252 (2002) Fig. 3. Effect of load on (a) wear scar diameter and (b) friction coefficient. (1) Base stock with 0.5 wt.% polystyrene sulfonic acid; (2) base stock with 0.5 wt.% fullerene styrene sulfonic acid copolymer. Fig. 2. Effect of the fullerene copolymer content on (a) maximum nonseized load; (b) wear scar diameter and (c) friction coefficient. of the polystyrene sulfonic acid. The direct touch between the two friction surfaces then occurred, which in turn led to a rapid rise in friction coefficient Effect of rubbing time on wear scar diameter The dependence of friction time on wear scar diameter is shown in Fig. 4. At the beginning of 10 min, the wear scar diameter of base stock with 0.5 wt.% fullerene styrene Fig. 4. Effect of friction time on wear scar diameter. (1) Base stock with 0.5 wt.% polystyrene sulfonic acid; (2) base stock with 0.5 wt.% fullerene styrene sulfonic acid copolymer.

4 348 H. Lei et al. / Wear 252 (2002) Analyses of the worn surfaces Fig. 5. Morphology of the wear scar lubricated by base stock with 0.5 wt.% polystyrene sulfonic acid. The worn surface in four-ball machine testing, which was obtained under a load of 200 N and a testing time of 30 min, was observed by EPMA. The wear scar of base stock with 0.5 wt.% polystyrene sulfonic acid and that with 0.5 wt.% fullerene styrene sulfonic acid copolymer are shown in Figs. 5 and 6, respectively. It indicated that the wear scar obtained with the fullerene copolymer additive is obviously smaller and exhibits mild scratches, but in comparison, larger wear scar and sharp tracks were observed in the presence of base stock with polystyrene sulfonic acid additive. In other words, the fullerene styrene sulfonic acid copolymer can improve microcosmic wear condition. Fig. 7 shows that the elements carbon and sulfur are well-distributed on the above worn surface lubricated by base stock with the fullerene styrene sulfonic acid copolymer, implying that the fullerene copolymer forms a continuous film on the rubbing surface. The XPS spectra were recorded to determine the chemical valency of the sulfur on the worn scar. The results are illustrated in Fig. 8. For comparison, the spectra of S 2p for pure fullerene styrene sulfonic acid copolymer is also indicated, where the binding energy of S 2p occurs at ev, corresponding to the sulfur as sulfate. However, the binding energy of S 2p on the surface of the worn scar shifts to ev. The results indicate that FeS 2 exists on the worn surface, showing that the fullerene copolymer reacts with the surface of the metal during the friction process. Fig. 6. Morphology of the wear scar lubricated by base stock with 0.5 wt.% fullerene styrene sulfonic acid copolymer. sulfonic acid copolymer is slightly smaller than that with 0.5 wt.% polystyrene sulfonic acid. After 10 min rubbing, the difference in wear scar diameter becomes larger, which indicates further that the presence of fullerene can strengthen the anti-wear performance of the base stock. 4. Acting mechanism of novel fullerene copolymer additive Based on the above tribological measurements and worn surface analyses, the lubrication mechanism of the fullerene styrene sulfonic acid copolymer additive can be deduced. (1) The fullerene copolymer plays the role of a solid lubricant. Since fullerene has very high load-carrying capacity and the fullerene copolymer is nanometer tiny Fig. 7. Mg K surfacial distribution of the elements carbon and sulfur within the wear scar surface of the fullerene copolymer.

5 H. Lei et al. / Wear 252 (2002) Fig. 8. XPS analyses of wear scar and pure additive. S 2p photoelectron spectrum of (a) fullerene copolymer; (b) wear scar lubricated by fullerene copolymer. balls with core-shell structure, which can penetrate into rubbing surfaces and deposit there, it is reasonable to speculate that the fullerene copolymer nanometer balls may be more effective than its corresponding homopolymers to support and isolate two relative motion surfaces, and therefore, the load-carrying capacity and anti-wear performance of the base stock were improved. Moreover the nanometer balls are expected to roll between two relative motion surfaces to reduce the friction coefficient. (2) Element S in the fullerene copolymer reacts with the surface to produce FeS 2 inorganic lubrication film on the metal surface which can also improve the extreme pressure and load-carrying capacity. Nevertheless, considering the interactions between the fullerene copolymer and the metal surfaces as well as triethanolamine additive, the acting mechanism may be more complicated. More works are needed to be done on these in the future. 5. Conclusions A novel fullerene styrene sulfonic acid copolymer was prepared. It is completely soluble in water, yielding a clear brown solution. TEM analysis shows that it presents an ideal spherical shape in water with a diameter ranging from 3 to 40 nm. As a lubricant additive in base stock (2 wt.% triethanolamine aqueous solution), it can improve the wear resistance, load-carrying capacity and anti-friction ability

6 350 H. Lei et al. / Wear 252 (2002) of base stock. Excessive additive was disadvantageous for the wear resistance and load-carrying capacity. Based on the results of EPMA and XPS analyses, the lubrication mechanism of the fullerene copolymer was deduced. References [1] W. Kratschmer, L.D. Lamb, K. Fostiropoaulos, Solid C 60 :anew form of carbon, Nature 347 (1990) 354. [2] H.W. Kroto, J.R. Heath, S.C. O Brien, C 60 : Buckminsterfullerene, Nature 318 (1985) 162. [3] B. Feng, Relation between the structure of C 60 and its lubricity: a review, Lubric. Sci. 9 (2) (1997) [4] B. Bhushan, B.K. Gupta, G.W. Van Cleef, Fullerene (C 60 ) films for solid lubrication, Tribol. Trans. 36 (1993) 573. [5] P.J. Blau, C.E. Haberlin, An investigation of the microfrictional behavior of C 60 particle layers on aluminum, Thin Solid Films 219 (1992) 129. [6] B.K. Gupta, B. Bhushan, C. Capp, Materials characterization and effect of purity and ion implantation on the friction and wear of sublimed fullerene films, J. Mater. Res. 9 (1994) [7] C.T. White, J.W. Mintmire, R.C. Mowrey, Buckminsterfullerenes, VCH Press, New York, 1993, p [8] B.K. Gupta, B. Bharat, Fullerene particles as an additive to liquid lubricants and greases for low friction and wear, Lubric. Eng. 50 (1994) 524. [9] F.Y. Yan, Z.S. Jin, X.S. Zhang, Study on tribological behavior of C 60 /C 70 as an oil additive, Tribology 1 (1993) 59 (in Chinese).