Langmuir 1998, 14, 2187-2190
2187
Effects of Surface Ions on the Friction and Adhesion Properties of Mica Lei Xu† and Miquel Salmeron* Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 Received December 12, 1997. In Final Form: February 9, 1998 Friction Force Microscopy studies on ion-exchanged mica surfaces at different humidity levels were performed. Ion exchange was achieved by treating the potassium mica (muscovite) in different solutions to replace K+ surface ions by Ca2+, Mg2+, and H+. It is found that these different surface ions can significantly modify the tribological properties of mica.
Introduction
Experimental Section
Ions can be a very important factor in influencing the tribological properties of solid surfaces. When two surfaces are brought close together, surface ions will introduce electrostatic as well as van der Waals forces. Under liquid, surface charges will produce an electric “double layer”, which will induce repulsive or attractive forces between the two surfaces, depending on the conditions.1 Atomic force microscopy (AFM) is widely used to analyze surface topography down to the nanometer scale. Friction force microscopy (FFM), an offspring of AFM, has been revealed as an important tool to study friction and adhesion properties of materials at nanometer scales.2 Mica appears to be a popular substrate used in many AFM experiments because large atomically flat surfaces can be easily obtained by cleavage. It is well-known that the atomic lattice of the mica surface can be routinely resolved both in air3 and in a vacuum4 by AFM and that the friction on mica decreases with humidity.5 Despite numerous studies, some results remain unexplained. One unexplained result is that potassium ions on the surface have never been observed by AFM. One possible explanation is that these ions are moved around by the tip during sliding, so that the stick-slip period is determined solely by the ordered oxygen network of the substrate. Despite this, the ions should still contribute to the average frictional force, and FFM could be used to study the difference in tribological properties induced by different surface ions. So far, no studies on this subject have been performed. In this paper, we use FFM to study the effects of different ionic species on the tribological properties of mica. These species were introduced by exchange of the naturally present K+ surface ions with H+, Mg2+, and Ca2+. The friction and adhesion forces between a Pt-coated tip and the treated mica surfaces were then measured at different relative humidity (RH) levels.
Mica Preparation. In our experiments, we immerse the freshly cleaved Muscovite mica samples in different solutions. We use deionized water, 0.1 M MgSO4, and saturated CaSO4 solutions to replace K+ with H+, Mg2+, and Ca2+. After the treatment, the mica samples are rinsed thoroughly with deionized water to remove the excess ions, especially the negative ions, and dried with a jet of nitrogen. In the following, we will use the terms K-mica, H-mica, Mg-mica, and Ca-mica to denote the freshly cleaved mica and the corresponding ion-exchanged mica. Friction and Adhesion Measurements. The experimental setup is a homemade AFM inside a humidity-controlled chamber working at room temperature (22-23 °C). Humidity is reduced by flowing dry nitrogen through the chamber and increased by vaporizing deionized water. An RH-20C Omega hygrometer is used to measure the RH with an accuracy of ∼2%. Water molecules will adsorb onto the mica surface to form a film with a thickness that depends on the humidity. The adsorbed water molecules can hydrate surface ions and increase their mobility.7 We use silicon nitride cantilevers (Digital Instruments, Santa Barbara, CA) with a nominal spring constant of 0.58 N/m. The tip and lever are coated with a 50 nm thick layer of Pt. To compare the friction results on different samples and at different RH levels, the same tip is used in all experiments. The radius of this coated tip is ∼50-60 nm. Normal and lateral forces between tip and surface are measured during loading and unloading with an external force. Due to capillary and adhesive forces, the pull-off point in the unloading branch of the friction-load curves appears at negative external loads and varies with humidity. To compare the friction properties of the various surfaces at different humidity levels, we chose the pull-off point as the origin of the total repulsive force between tip and surface. This is based on the assumption that, at the pull-off point, the adhesive and capillary forces nearly exactly compensate the negative external load.
† Permanent address: Shanghai Institute of Nuclear Research, Chinese Academy of Sciences, P.O. Box 800-204, 201800 Shanghai, People’s Republic of China.
(1) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1997; Chapter 12. (2) For a recent review, see: Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (3) Drake, B.; Prater, C. B.; Weissenhorn, A. L.; Gould, S. A. C.; Albrecht, T. R.; Quate, T. R.; Cannell, D. S.; Hansma, H. G.; Hansma, P. K. Science 1989, 243, 1586. (4) Dai, Q.; Vollmer, R.; Carpick, R. W.; Ogletree, D. F.; Salmeron, M. Rev. Sci. Instrum. 1995, 66, 5266. (5) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358.
Results and Discussion Surface Composition. Muscovite mica has a composite layer structure with the typical formula KAl2Si3AlO10(OH)2. Potassium ions balance the negative charges in the AlSiO layer. The cleavage plane is along the potassium layer. Ideally, half of the potassium ions remain on each separated surface. It is also well-known that mica can be treated with different solutions to replace potassium with other ions from the solution.6 After mica was treated by immersion into the appropriate solutions, angledependent X-ray photoelectron spectroscopy (XPS) measurements were performed to compare the atomic concentration of each element. From these measurements, (6) Christenson, H. K. J. Phys. Chem. 1993, 97, 12034. (7) Xu, L.; Lio, A.; Hu, J.; Ogletree, D. F.; Salmeron, M. J. Phys. Chem. 1998, 102, 540.
S0743-7463(97)01365-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/21/1998
2188 Langmuir, Vol. 14, No. 8, 1998
Xu and Salmeron
Figure 1. Friction loops between a Pt-coated tip and an Mgexchanged mica surface at 14 and 55% RH. The stick-slip is periodic at high humidity but random at low humidity. Similar results are found with Ca2+-exchanged mica.
to be presented elsewhere,8 we conclude that the ion exchange process is quick and efficient. After immersion in pure water or in solutions of MgSO4 and CaSO4 for only a few minutes, the potassium ions on the surface are removed completely (within the accuracy of XPS, i.e., 5-10% of a monolayer) and replaced by H+, Mg2+, and Ca2+, respectively. After the rinsing process, no signal indicative of SO42- could be detected by XPS. Another result from our XPS measurements is that the replacement of K+ by Mg2+ and Ca2+ is irreversible; i.e., the divalent cations could not be replaced with either H+ or K+ in subsequent rinses. Friction. The first observation from our friction measurements is that, although atomic resolution is routinely obtained for the K- and H-micas, at low humidity, it is almost impossible to resolve the atomic lattice of the Mg- and Ca-micas. Stick-slip is observed at low humidity (50% RH), the adhesion forces are similar. Note that there is a peak for each curve (at ∼20% RH for K-mica, at ∼50% RH for Ca-mica, and at ∼70% RH for H-mica). An additional decrease of adhesion with RH is also observed for K-mica at ∼ 50% RH.
electrical double layer structure, divalent ions need to hydrate only to half the extent of monovalent ions. At high load, all the friction curves become nearly parallel and have similar slopes. The slope is higher for Ca-mica than for K-mica and H-mica, while the K-mica and H-mica slopes are very similar. Adhesion. The adhesive forces measured from the pulloff point on each sample are presented in Figure 4. It shows that the adhesion force for K-mica is much larger than those for Ca- and H-mica when the RH is
