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Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy

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Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy Chem. Rev. 1997, 97, 1163−1194 1163 Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy Robert W. Carpick Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Physics, University of California at Berkeley, Berkeley, California 94720 Miquel Salmeron* Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received October 3, 1996 (Revised Manuscript Received March 31, 1997) Contents I. Introduction I. Introduction 1163 A few years after the invention of the scanning II. Technical Aspects 1165 tunneling microscope (STM), the atomic force micro- 1 A. Force Sensing 1165 scope (AFM) was developed. Instead of measuring B. Force Calibration 1166 tunneling current, a new physical quantity could be investigated with atomic-scale resolution: the force C. Probe Tip Characterization 1167 between a small tip and a chosen sample surface. D. AFM Operation Modes 1167 Sample conductivity was no longer a requirement, 1. Normal Force Measurements 1168 and so whole new classes of important materials, 2. Lateral Force Measurements 1168 namely insulators and large band-gap semiconduc- 3. Force Modulation Techniques 1169 tors, were brought into the realm of atomic-scale 4. Force-Controlled Instruments 1170 scanning probe measurements. The initial operation 5. Lateral Stiffness Measurements 1170 mode measured the vertical topography of a surface III. Nanotribology with Other Instruments 1171 by maintaining a constant repulsive contact force A. The Surface Forces Apparatus (SFA) 1171 between tip and sample during scanning, akin to a B. The Quartz-Crystal Microbalance (QCM) 1172 simple record stylus. However, since its inception, IV. Bare Interfaces 1172 a myriad of new operation modes have been devel- oped which can measure, often simultaneously, vari- A. Friction Measurements with Well-Defined 1172 Samples ous sample (and even tip) properties. Perhaps the most notable extension so far of AFM capabilities was 1. Atomic-Scale Stick−Slip Behavior 1172 the realization that lateral forces between the tip and 2. Properties of a Single Asperity: Interfacial 1175 sample could also be measured.2 It was thus recog- Friction nized that the atomic-scale origins of friction could 3. Frictional Anisotropy 1179 be probed with this technique, usually described as 4. Chemical Effects 1180 friction force microscopy (FFM). This, along with B. Wear and Plastic Deformation 1181 other types of AFM measurements, has established 1. AFM Measurements 1181 the AFM as an important tool in the emerging field 2. Atomic-Scale Metallic Contacts 1183 of nanotribology; the study of the atomic-scale ingre- V. Model Lubricated Interfaces 1184 dients of interactions between surfaces in relative A. The Structure of SAM and LB Films 1184 motion, such as friction, adhesion, lubrication, and 1. Thiols 1184 wear.3-8 This paper reviews progress and recent 2. Silanes 1185 results obtained with AFM and other closely related B. Friction, Adhesion, and Chemical Identity 1185 techniques in the field of nanotribology, and attempts to point out many of the unresolved questions that C. Friction, Adhesion, and Elastic Properties 1185 remain. D. Molecular Structure of Films Under Moderate 1186 The corresponding study of friction, adhesion, and Pressure (<100 MPa) so forth from a macroscopic and practical perspective E. Molecular Structure of Films Under High 1188 is known, of course, as tribology and while this term Pressure (>1 GPa) was coined in the 1960s, these ideas have been 1. Large Tip Radii (R > 1000 Å) 1188 pursued for centuries.9 Most of this work has been 2. Sharp Tip Radii (R < 1000 Å) 1188 performed in the domain of engineering because of VI. Conclusions 1190 the obvious practical importance: reducing energetic VII. Acknowledgments 1191 losses due to friction (e.g., helping a car engine to VIII. References 1191 work efficiently), utilizing friction as an operation mechanism (e.g., car brakes), reducing material losses due to wear (e.g., longer-lasting tires), and optimizing * Author to whom correspondence should be sent. Phone: (510) lubricants (e.g., increasing the efficiency and lifetime 486-6230. Fax: (510) 486-4995. E-mail: [email protected]. of engine parts) are important issues for a wide range S0009-2665(96)00068-4 CCC: $28.00 © 1997 American Chemical Society 1164 Chemical Reviews, 1997, Vol. 97, No. 4 Carpick and Salmeron research.11 With the advent of small devices trig- gered in part by the tremendous development of silicon microfabrication techniques,12 novel problems appear that require knowledge at the nanometer scale. For example, the whole technology of informa- tion storage as exemplified by the case of computer hard disks with coatings and lubricants that protect the stored information, with dimensions that are measured in nanometers.13 Micrometer-sized actua- tors, sensors, and motors are another example of novel technology requiring such knowledge for per- formance optimization.14 Despite the heavy volume of work performed so far,6,8,11,15-17 there does not exist a fundamental understanding of tribological processes (although a Robert W. Carpick is a Ph.D. candidate in the Department of Physics, University of California at Berkeley. He received his B.Sc. in Physics great deal of practical successes have occurred). It from the University of Toronto in 1991 and was awarded a 1967 Science would be fair to say that friction itself is one of the & Engineering Scholarship from the Natural Sciences and Engineering most common, yet least understood, physical phe- Research Council of Canada for graduate work. His doctoral research nomena. The main reason for this lack of under- in the group of Dr. Salmeron at Lawrence Berkeley National Laboratory standing is that probing the atomic processes taking has focused on atomic-scale tribology using ultrahigh vacuum atomic force place at a buried interface is an inherently difficult microscopy and surface science techniques. He has probed relations between atomic-scale friction in the elastic regime and continuum contact task. The main conclusion of the earlier observa- mechanics. He was the recipient of the 1995 Russell & Sigurd Varian tions, as most high school physics students learn, is Fellowship from the American Vacuum Society in recognition of his thesis that friction between a pair of surfaces is proportional work and has been awarded a Postdoctoral Fellowship from the Natural to load (normal force). Furthermore, friction is Sciences and Engineering Research Council of Canada. evidently independent of the apparent area of contact and only weakly dependent on the relative sliding velocity. This linear or nearly linear dependence of friction upon load is a result of complex phenomena at the interface, particularly multiple asperity con- tact,18,19 adhesion-induced deformation,15 and plowing of the surfaces by wear particles during sliding.20 While these observations are important, they fail to explain the behavior at the atomic scale and cannot be used for predictive analysis. At the fundamental level, friction, adhesion, and wear need to be understood in terms of chemical bonding and of the elementary processes that are involved in the excitation and dissipation of energy modes. Several mechanisms have been proposed that Miquel Salmeron is a Senior Staff Scientist and Principal Investigator in we shall enumerate briefly here and discuss in more the Materials Sciences Division of the Lawrence Berkeley National detail later, in relation to specific examples discussed Laboratory. He graduated from the University of Barcelona in 1967. He throughout this review. One is due to coupling to obtained a Master’s Degree in solid-state physics from the Universite´ Paul the substrate (and tip) electron density that causes Sabatier, Toulouse, France, in 1970, and a Ph.D. in physics at the a drag force, similar to that causing an increase of Universidad Autonoma de Madrid, Spain, in 1975. After spending two electrical resistance by the presence of surfaces in years as a postdoctoral fellow in Berkeley studying reactive scattering of 21-25 molecular beams of hydrogen molecules on platinum, he returned to Spain thin films. Another is the excitation of surface in 1977. There he was an investigator in the Spanish Research Council phonon modes in atomic stick-slip events that will (CSIC) and Professor of Physics at the Universidad Autonoma de Madrid. be discussed below. Delocalization of the excited In 1984 he assumed his current position where he has since developed phonons by coupling to other phonon modes through STM and AFM techniques and applied them to studies of surface structure, anharmonic effects and transport of the energy away chemisorption, semiconductors and tribology. He has been recently from the excited volume leads to efficient energy elected to fellowship in the American Physical Society and received the 26 awards for Sustained Outstanding Research in 1996 and for Outstanding dissipation. At high applied forces, wear processes Scientific Accomplishment in 1995 in the Materials Chemistry Category leading to rupture of many atomic bonds, displace- as part of the U.S. Department of Energy Materials Sciences Research ment and creation of dislocations and debris particles, Competition. He serves on the Editorial Advisory Board of Tribology are important and are part of the wide topic of plastic Letters. He has taught at the University of Barcelona in Spain, at the deformation of materials. As we shall see later,
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