Atomic force microscope friction measurements

In this article we investigate atomic force microscope friction (AFM) measurements at the Si02/Si02 interface. We use two approaches which differ in the way in which the surfaces are driven relative to each other in contact. The silicon tip of the AFM is driven at constant velocity or harmonically. The resulting response corresponds to either the tribological or the rheological properties at the contact. Here we bridge the gap between the two approaches which do not always lead to the same conclusions. 

Carpick R W, Agrait N, Ogletree D F and Salmeron M 1996 Measurement of interfacial shear (friction) with an ultrahigh vacuum atomic force microscope J. Vac. Sc/. Technol. B 14 1289... 

Overney R M, Meyer E, Frommer J, Brodbeck D, Luthi R, Flowald L, Guntherodt Fl-J, Fu]ihara M, Takano FI and Gotoh Y 1992 Friction measurements of phase separated thin films with a modified atomic force microscope Nature... 

The atomic force microscope (ATM) provides one approach to the measurement of friction in well defined systems. The ATM allows measurement of friction between a surface and a tip with a radius of the order of 5-10 nm figure C2.9.3 a)). It is the tme realization of a single asperity contact with a flat surface which, in its ultimate fonn, would measure friction between a single atom and a surface. The ATM allows friction measurements on surfaces that are well defined in tenns of both composition and stmcture. It is limited by the fact that the characteristics of the tip itself are often poorly understood. It is very difficult to detennine the radius, stmcture and composition of the tip however, these limitations are being resolved. The AFM has already allowed the spatial resolution of friction forces that exlribit atomic periodicity and chemical specificity [3, K), 13]. 

Figure C2.9.3 Schematic diagrams of the interfaces reaiized by (a) tire atomic force microscope, (b) tire surface forces apparatus and (c) tire quartz crystai microbaiance for achieving fundamentai measurements of friction in weii defined systems.

In 1987 Mate et al. [468] used, for the first time, an atomic force microscope (AFM) to measure friction forces on the nanometer scale (review Ref. [469]). This technique became known as friction force microscopy (FFM) or lateral force microscopy (LFM). To measure friction forces with the AFM, the fast scan direction of the sample is chosen perpendicular to the direction of the cantilever. Friction between the tip and the sample causes the flexible cantilever to twist (Fig. 11.7). This torsion of the cantilever is measured by using a reflected beam of light and a position-sensitive detector in the form of a quadrant arrangement of photodiodes. This new method made it possible for the first time to study friction and lubrication on the nanometer scale. 

While the macroscopic concepts of hardness, adhesion, friction, and slide have evolved over the last two centuries, atomic level understanding of the mechanical properties of surfaces eluded researchers. The discovery of the atomic force microscope in recent years promises to change this state of affairs. Being able to measure forces as small as 10 newton or as large as 10 newton [5] over a very small surface area (few atoms) and by simultaneously providing atomic spatial resolution, this technique permits the study of deformation (elastic and plastic), hardness, and friction on the atomic scale. The buried interface between moving solid surfaces can be studied with spectroscopic techniques on the molecular level. Study of the mechanical properties of interfaces is, again, a frontier research area of surface chemistry. 

Discuss the results of a recent paper that measured friction coefficients using the atomic force microscope (AFM) [60-62]. 

Figure 9.18. (a) Atomic force microscope probe sliding over a smooth surface, (b) Measured friction force- versus normal force fitting Equation (9.16). 

Atomic force microscopes (AFM), whose tips are typically made of silicon or diamond, can probe sur-laces made of practically any material. As the tip of an atomic force microscope is dragged across the sur-lace of a specimen, it is either deflected by or drawn toward the object, depending on whether the atoms in the object are repelled by or attracted to the microscope s tip. By measuring these forces, a magnified representation of the physical structure of the sample can be created. By changing the modes in which these microscopes operate, different properties such as magnetism, friction, and electrical conductivity can be assessed. 

In this study, we clarified these friction-reduction effects under microload conditions by measuring the friction and pull-off forces for two-dimensional asperity arrays on silicon plates. First, two-dimensional asperity arrays were created using a focused ion bean (FIB) system to mill patterns on single-crystal silicon plates. Each silicon plate had several different patterns of equally spaced asperities. Then, the friction and pull-off forces were measured using an atomic force microscope (AFM) that had a square, flat probe. This report describes the geometry effects of creating asperity arrays and the chanical effects of depositing LB films or SAMs on the friction and puU-off forces. 

MEASUREMENTS OF FRICTION AND PULL-OFF FORCES BY AN ATOMIC FORCE MICROSCOPE... 

Atomic Force Microscopy. A homemade beam deflection atomic force microscope equipped with a single tube scanner, an optical deflection scheme, Si3N4 tip-cantilever assemblies (Digital Instruments, CA), and RHK AFM 100 and RHK STM 100 electronics was used to measure the lattice constants and frictional properties of... 

M. Reinstadtler, U. Rabe, V. Scherer, U. Hartmann, A. Goldade, B. Bhushan, and W. Arnold, On the nanoscale measurement of friction using atomic force microscope cantilever torsional resonances, Appl. Phys. Lett. 82, 2604 2606 (2003). 

Instrumentation. Commercial atomic force microscopes Explorer from Topometrix Inc., Nanoscope Illa Digital Instruments Co. Ltd.) which are based on the laser beam deflection detection scheme were used in conjunction with digital oscilloscopes of very stable low frequency (1-20 Hz) trigger system for lateral force (friction) measurements, and dual-phase lock-in amplifiers and function generators for force modulation measurements. Various triangular silicon nitride cantilevers were used. The lateral spring constants were determined with the "blind torsional calibration method discussed in more detail in the Appendix. 

An atomic force microscope is used to stuviscoelastic state at the temperature of experiment. It is shown that, during the preliminary phase of friction and before the transition to the sliding regime, the contact area remains nearly constant. This allows for a determination of the relaxation and of the complex modulus of the material. A good agreement is found between moduli measured by this method and macroscopically determined ones. The position of the transition is seen to scale with the characteristic size of the contact area but it does not depend on the displacement velocity. Finally, a transient stick-slip regime is observed before the sliding steady state is reached. 

Fundamental rmderstanding of friaion on the atomic and nanometer scales is critical for design of micro- and nanoe-lectromechanical systems. There are two powerful instruments in nanotribology the surface force apparatus and the atomic force microscope. AFM experiments allow high-resolution mapping of friction properties between solids on the nanometer scale and usually indude measurements of friction forces by friction force microscopy (FFM) and adhesion by CFM. In FFM, as a sharp tip scans across a surface in the... 

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