17 - Instrumental and computational methods in nanotribology

Summary

In this chapter we introduce the methods conventionally used to explore friction on the nanoscale. The leading position among the instrumental setups is held by the atomic force microscope. Here we will briefly illustrate the type of forces sensed by this instrument and its basic modes of application. Other experimental techniques in nanotribology are the surface force apparatus, the quartz crystal microbalance and also, to some extent, scanning tunneling microscopy and transmission electron microscopy. Virtual experiments rely on molecular dynamics simulations. A short introduction to this method will be followed by a series of numerical results reproducing friction and wear measurements at the atomic level.

Atomic force microscopy

In a typical atomic force microscope (AFM) [24] a sharp micro-fabricated tip is scanned over a surface. Standard AFM tips are made of silicon or silicon nitride, but tips can be also coated to allow a large variety of material combinations. The probing tip is attached to a cantilever force sensor, the sensitivity of which can be well below 1 nN. Images of the surface topography are recorded by controlling the tip–sample distance in order to maintain a constant (normal) force. This is made possible by using piezoresistive cantilevers, or, most commonly, by a light beam reflected from the back side of the cantilever into a photodetector, which allows one to monitor the cantilever bending (Fig. 17.1). The lateral force between tip and surface is responsible for the cantilever torsion and can be measured if the photodetector is equipped with four quadrants. If this is the case the AFM can be used as a friction force microscope (FFM), see Appendix A. The design of a home-built AFM, optimized for friction measurements in ultra-high vacuum (UHV), is shown in Fig. 17.2.

The tip–sample force can be related not only to the static bending or torsion of the cantilever.