Techniques to measure friction

The standard application of the QCM is to monitor the adsorption ofthin films on surfaces via the induced frequency shift [948]. It was demonstrated by Kim and coworkers that the QCM can also detect the sliding of atomic monolayers on metal surfaces [949,950]. The slippage of adsorbed layers on the QCM leads to a damping 

Classical, macroscopic devices to measure friction forces under well-defined loads are called tribometers. To determine the dynamic friction coefficient, the most direct experiment is to slide one surface over the other using a defined load and measure the required drag force. Static friction coefficients can be measured by inclined plane tribometers, where the inclination angle of a plane is increased until a block on top of it starts to slide. There are numerous types of tribometers. One of the most common configurations is the pin-on-disk tribometer (Fig. 11.6). In the pin-on-disk tribometer, friction is measured between a pin and a rotating disk. The end of the pin can be flat or spherical. The load on the pin is controlled. The pin is mounted on a stiff lever and the friction force is determined by measuring the deflection of the lever. Wear coefficients can be calculated from the volume of material lost from the pin during the experiment. 

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. 

In 1988 a modified surface forces apparatus (SFA) was introduced [470,471] to analyze friction. The principle of operation of the SFA has already been introduced in Section 6.4. The modified version allowed a relative shearing of the two mica surfaces. In the SFA, the substrate has to have an atomically flat, transparent surface. In most cases mica is used to fulfill these requirements. Although there is a strong limitation in the choice of materials, due to the high resolution in the vertical direction, the SFA has become an important tool to study the friction and lubrication properties of molecularly thin films. 

Another tool used to study friction on the molecular scale is the quartz crystal microbalance (QCM) introduced in Section 9.4.1. The QCM has been used to monitor the adsorption of thin films on surfaces via the induced frequency shift [385], In the years since 1986, Krim and coworkers could show that the slippage of adsorbed layers on the QCM leads to a damping of the oscillator [472], This damping is reflected as a decrease in the quality factor Q of the oscillator. From the change in Q, a characteristic time constant rs, the so-called slip-time, can be derived. This corresponds to the time for the moving object s speed to fall to 1 /e, i.e. 

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In the first part of this chapter, we discuss macroscopic fiiction phenomena. The second part will focus on the field of nanotribology that has emerged with the invention of corresponding experimental techniques to measure friction on the nanoscale. 

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