Frictional force microscope FFM

In order to study microscale friction and wear, scientists have developed the friction force microscope (FFM), nanoindentation and nanoscratch tester which serve as excellent tools in micro tribological research [1,6-9]. In this chapter, we first compare the differences between macro and micro friction and wear, and then introduce some results of our research group on microscale friction and wear of ordered films, thin solid films, and multilayers. 

Figure 2.17. Sketch of a friction force microscope (FFM) with a beam-deflection detection scheme. Cantilever movements are monitored by a laser beam with a four-quadrant photodiode. The topography (T) is measured simultaneously with the lateral forces (L). Irreversible lateral forces are by definition frictional forces. (From Ovemey, 1995.)...

A modified type of AFM, called a friction-force microscope (FFM), has been described." " This method was shown to be able to distinguish between fluorocarbons and hydrocarbons on a phase-separated LB film. This shows that although AFM has proven useful for imaging organic thin films on an atomic scale, the technique can also provide useful information about the composition of molecules, as well as their conformation. 

Scanning Electron Microscopy (SEM) Transmission Electron Microscopy (TEM) Scanning Tunneling Microscopy (STM) Atomic Force Microscopy (AFM) Frictional Force Microscope (FFM). 

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. 

Figure 11.7 Working principle of the lateral or friction force microscope (LFM or FFM).

Figure 11.9 Friction force microscope pictures (a, b) of a graphite(OOOl) surface as obtained experimentally with FFM and results of simulations (c, d) of the stick-slip friction using a two-dimensional equivalent of the Tomlinson model. The friction force parallel to the scan direction (a, c) and the lateral force perpendicular to the scan direction (b, d) are shown. The scan size is 20 Ax 20 A. Pictures taken from Ref. [481] with kind permission from R. Wiesendanger.

The three major new atomic-scale experimental methods developed in the last decade are the quartz crystal microbalance (QCM) [2 4], atomic and friction force microscopes (AFM/FFM) [5,6], and the surface force apparatus (SEA) [7,7a,8]. These new tools reveal complementary information about tribology at the nanometer scale. The QCM measures dissipation as an adsorbed him of submonolayer to several monolayer thickness slides over a substrate. AFM and FFM explore the interactions between a surface and a tip whose radius of curvature is 10 100 nm [9]. The number of atoms in the contact ranges from a few to a few thousand. Larger radii of curvature and contacts have been examined by gluing spheres to an AFM cantilever [10,11]. SEA experiments measure shear forces in even larger-diameter ( 10 pm) contacts, but with angstrom-scale control of the thickness of lubricating hlms. 

The phase separation of SAMs has been characterized so far by various analytical techniques such as XPS, IR, ellipsom-etry, and scanning probe microscopes (SPMs). SPMs among them, in particular, have sufficient lateral resolution and therefore appear the most promising for direct imaging of nanometer scale patterned SAMs [281]. The analysis is required for characterizing the spatial distribution of molecules on the 1-100-nm scale [281]. STM is a powerful tool that meets the requirement [281-283]. Friction force microscopy (FFM) is also useful for the purpose [284-286]. Although studies were conducted using L-B films. 

The lateral forces result in a torsion of the cantilever, and this can be measured using the same optical detection system that measures vertical deflection (see Fig. 2.8). This signal from the tip-specimen interaction leads to another operational mode, the lateral or frictional force microscope (LFM or FFM) [142, 143]. In LFM the cantilever is designed to be... 

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... 

FFM = LFM friction force (lateral force) microscopy/microscope 3.7d... 

Quantitative measurements of nano-scale frictional properties of pure and mixed SAMs on Au (111) were achieved by in situ normal and lateral force calibration of AFM/FFM. For pure SAMs, the friction coefficients for the same alkanethiol system but with different tips, differ by less than 15%, indicating the reliability of nano-scale frictional and normal force measurements using a scanning force microscope. The friction coefficient increases as the chain length decreases as also found previously by other workers. Tip-based molecular dynamics simulations were carried out to interpret the chain length dependence on frictional properties of alkanethiols. Simulation results show that AFM/FFM tip penetrates deeper into films formed by shorter chain SAMs, causing higher friction. 

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