Langmuir 2001, 17, 6953-6960
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Influence of Vapor Condensation on the Adhesion and Friction of Carbon-Carbon Nanocontacts M. J. Adams,† B. J. Briscoe,* J. Y. C. Law,‡ P. F. Luckham, and D. R. Williams Department of Chemical Engineering, Imperial College, London SW7 2BY, U.K. Received March 12, 2001 The adhesion and friction between two orthogonally arranged carbon fibers has been measured in undersaturated vapor pressures of decane, n-propanol, and water. An analysis, which is described, of the frictional data allowed the normal adhesive force under sliding conditions to be deduced. Contact angle measurements and adsorption studies showed that both decane and n-propanol wetted the fibers and also their vapors exhibited typical BET adsorption isotherms. It was also found that water did not wet these fibers and that the adsorption isotherm could not be described by the BET equation. Equilibrium thermodynamic theory predicts that the two wetting fluids should significantly attenuate the autoadhesion. The converse was observed and is ascribed to the actions of a combination of two factors. First, it is argued that the high contact pressures (ca.109 Pa) at the carbon interface, which were developed even under the adhesive loads alone, resulted in the adsorbates being excluded or displaced from the contact region. Second, the crack propagation velocity during the interfacial separation process was very fast relative to the rate of vapor transport and hence the rate of adsorption at the crack tip. A similar effect is observed with environmental stress cracking at high crack propagation velocities. An increase in the adhesion at high relative vapor pressures of the n-propanol and water is considered to correspond to the formation of capillary bridges. The rate at which this process occurs appears to be enhanced under sliding conditions due to an accumulation of the adsorbates in the moving contact. The capillary bridges formed in saturated decane vapor were highly unstable which may be related to the relatively weak adsorption characteristics.
Introduction Numerous theoretical models have been developed that seek to describe the autoadhesion between curved elastic bodies arising from short-range interaction forces such as the van der Waals kind. The DMT model1 is regarded as being applicable to the cases of relatively small hard bodies with low surface free energies while the JKR model2 is appropriate for relatively large soft bodies with high surface free energies. More recently it has been recognized that these are the two limiting cases and a unified treatment has been developed.3,4 It was based upon a fracture mechanics formulation, and the theory has been extended to describe the friction at autoadhesive contacts for which there is a corresponding increase in the friction. A similar effect is observed for the adhesive force arising from capillary bridges, for example, with rigid disk drives and read/write head sliders.5-7 Such bridges originate from the presence of a thin layer of liquid lubricant and, at high relative humidities, generated by moisture condensation. The JKR and DMT models have been generalized to account for the influence of a liquid meniscus on the autoadhesion.8,9 The presence of a partially or perfectly * Corresponding author. † Visiting Professor from Unilever Research Port Sunlight. ‡ Present address: Hunsonburg Enterprises Co. Ltd., 19/F V. Heun Building, 138 Queen’s Road, Central Hong Kong. (1) Derjaguin, B. V.; Muller, V. M.; Toporov, Y. P. J. Colloid Interface Sci. 1975, 53, 314. (2) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London 1971, A324, 301. (3) Johnson, K. L. Langmuir 1996, 12, 4510. (4) Kim, K.-S.; McMeeking, R. M.; Johnson, K. L. J. Mech. Phys. Solids 1998, 46, 243. (5) Mate, C. M. J. Appl. Phys. 1992, 72, 3084. (6) Gao, C.; Tian, X. F.; Bhushan, B. Tribol. Trans. 1995, 38, 201. (7) Gao, C. Appl. Phys. Lett. 1997, 71, 1801. (8) Fogden, A.; White, L. R. J. Colloid Interface Sci. 1990, 138, 414. (9) Maugis, D.; Gauther-Manuel, B. J. Adhes. Sci. Technol. 1994, 8, 1311.
wetting liquid will also act to partially or completely attenuate the autoadhesion. The reported adhesion and friction of poly(ethylene terephthalate) (PET) fibers in various liquids is such an example.10 The results could be described using the DMT model by taking account of the reduction in the thermodynamic work of adhesion. The effect of moisture vapor condensation and also that of organic liquids on the autoadhesion of smooth mica contacts has been carefully characterized using a surface force balance (SFA).11 The Kelvin analysis appears to be a valid description for capillary bridges whose radii are as little as the order of nanometers. At high vapor pressures, the reduced Laplace pressure accounts for the normal forces developed in such contacts.12 The wetting and capillary effects observed in the above experimental studies on autoadhesive contacts seem to be adequately described using established equilibrium theories. However, such quasi-static treatments may not be appropriate for describing contacts in which the time required for a capillary bridge to form is long compared with the contact time prior to a normal separation or during a quasi-continuous sliding process. For continuous sliding, the predominant contact time may be usefully defined as the ratio of the contact diameter and the sliding velocity while, for intermittent sliding, the contact time depends on the duration of the stick phase. Typically, equilibrium capillary bridges may take many hours to develop an equilibrium configuration.13 For the systems studied in the current work, it is shown that the contact time is apparently significantly shorter than that required for such a steady-state vapor adsorption. The wetting dynamics that control the behavior of capillary bridges (10) Adams, M. J.; Briscoe, B. J.; Kremnitzer, S. L. In Microscopic Aspects of Adhesion and Lubrication; Georges, J. M., Ed.; Elsevier Scientific Publishing Co.: Amsterdam, 1982; p 405. (11) Fisher, L. R.; Israelachvilli, J. N. Colloids Surf. 1981, 3, 303. (12) Christenson, H. K. J. Colloid Interface Sci. 1988, 121, 170. (13) Bhushan, B.; Dugger, M. T. J. Tribol. 1990, 112, 217.
10.1021/la0103719 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/04/2001
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between solids has been studied, for example in the case of heterogeneous surfaces, using the SFA with a sinusoidal modulation.14 Also, direct evidence of such time effects upon the frictional forces has been provided by lateral force microscopy (LFM) under environmental conditions in which the relative humidity was varied;15 the dynamics of the tip motion led to complex and apparently inexplicable behavior. As mentioned above, the normal separation and sliding of an autoadhesive contact (a contact with no or little directly applied normal force) may be envisaged as a mode I or a mixed mode II/III fracture process, respectively.3,4 The transport and adsorption of vapor near the crack tip must be relatively fast compared to the crack propagation velocity (sliding velocity) to reduce the interfacial energy. Experimental studies of the environmental stress cracking of initially coherent solids indicates that the rate of transport and adsorption of vapor near a crack tip may significantly influence the measured interfacial energy.16,17However, there is a lower threshold of the critical crack velocity above which the fracture behavior approaches that which is observed in a vacuum. An autoadhesive contact, in the presence of a liquid, may be expected to respond in a similar manner provided that the contact pressure is sufficient to exclude the formation of adsorption layers from within the contact zone. The present paper explores the likelihood of this prospect by examining the influence of potential environmentally ratelimited vapor-solid interaction processes upon the autoadhesion and the friction of solid-solid contacts. For this purpose, carbon fibers in undersaturated vapors of decane, n-propanol, and water, which have different wetting and/or adsorption characteristics, have been studied. A procedure for measuring the autoadhesion and friction between carbon fibers in an inert environment has been described previously18 and involves an orthogonal contact configuration between the fibers. An advantage of carbon fibers, for the present purpose, is that they are relatively inert and hence the effect of the adsorption of the fluids will be limited to a modification of their interfacial free energy rather than their bulk mechanical properties. Furthermore, it will be shown that the high stiffness and small size of carbon fibers results in relatively high contact pressures at values of the attractive forces generated by capillary bridges and autoadhesion; thus, there is a reasonable prospect that only solid-solid interactions prevail in the contact zone between the fibers in an unloaded state. Additionally, the intrinsic elasticity of the fibers enables their use as effective simple beam transducers. Experimental Section Materials. Pyrolyzed polyacrylonitrile (PAN) carbon fibers were supplied by BP Research Ltd and were used without further treatment. The main structural elements of PAN fibers are graphitic ribbons that are oriented approximately in the direction of the fiber axis.19 These ribbons are formed from graphite crystallites and are high in carbon content (>99%). Scanning electron-microscopy of fibers (with a 2 nm gold coating) used in the current study revealed that they have a cylindrical geometry (14) Crassous, J.; Charlaix, E.; Loubet, J.-L. Phys. Rev. Lett. 1997, 78, 2425. (15) Piner, R. D.; Mirkin, C. A. Langmuir 1997, 13, 6864. (16) Wiederhorn, S. M. In Mechanical and Thermal Properties of Ceramics; Wachtman, J. B., Ed., NBS Special Publication Vol. 303; NBS: Washington, DC, 1969; p 217. (17) Freiman, S. W. J. Am. Ceram. Soc. 1975, 58, 339. (18) Roselman, I. C.; Tabor, D. J. Phys. D: Appl. Phys. 1976, 9, 2517. (19) Peebles, L. H. In Carbon Fibres: Formation, Structure and Properties; CRC Press: Boca Raton, FL, 1995; p 43.
Adams et al.
Figure 1. SEM Image of fiber surface showing smooth surface features.
Figure 2. High-magnification SEM Image of fiber surface showing surface striations. with a mean diameter of 7.1 ( 0.5 µm (Figure 1). At high magnifications (>104), fine longitudinal striations were observed with a width of ca. 60 nm (Figure 2). These features are a residual fine structure derived from the precursor material. Decane (>98% purity), n-propanol (>99% purity), and methylene iodide (>98% purity) were obtained from Aldrich, Dorset, U.K., and the water was deionized. Fiber Modulus. Young’s modulus of the fibers was determined to be ca. 470 GPa. The fiber stiffness was determined using a bending method based on a micromotion stage and a Cahn D-200 electro-microbalance (Cahn). Fibers were carefully clamped at one end to form a simple single ended fiber cantilever. The cantilever was then mounted onto a x-y computer-controlled micro stage system (Photon Control, Cambridge, U.K.) with a position resolution of 0.2 µm in both x and y directions over a total travel of 25 mm. This fiber positioning ability allowed the fiber to be loaded against the load cell for various beam lengths and for various normal beam displacements. The microbalance used a high precision load cell to measure the flexural force with a resolution of 0.1 µg. By determination of the flexural force as a function of fiber cantilever length, reduced Young’s modulus was explicitly determined for a given fiber diameter. Mechanical creep was not detected during these studies which is a useful advantage for the experimental procedures described here. Surface Energy. The contact angles for the carbon fibers with selected liquids were measured using a microbalance-based wetting force (Wilhelmy) determination.20 Advancing contact angles were determined for single fibers which were at least 20 mm long using a Cahn DCA-322 (Cahn) dynamic contact angle analyzer. The fibers were attached using an epoxy resin (CibaGeigy, U.K.) to a small Nichrome wire hook which was in turn attached to the wetting balance. The average advancing wetting forces was determined at an imposed interface velocity of 20 µm/s over a fiber length of 10 mm. Fresh fibers used for each new liquid determination. The computed contact angles for carbon fibers with decane, n-propanol, methylene iodide, and water were
