From Nanoscience: Friction and Rheology on the Nanometer Scale

8.1 Material-specific contrast of friction force microscopy

The ability of FFM to be material specific is of general interest for the whole field of scanning probe microscopy (SPM), where contrast mechanisms are searched that give information in addition to topography. However, lateral forces are not independent of topography, but are influenced by local gradients (for more details see chapter IV). In order to exclude these topography effects, the sample should be flat. A surface that consists of atomically smooth terraces is ideal. The topography effect is restricted to the step regions, whereas a pure friction contrast originates from the flat terraces. On rough surfaces the non-dissipative part of the lateral forces due to the topography effect I (see chapter IV) can be separated by subtracting back and forward scan. However, the topography effect II due to changes of long-range forces and contact area is more difficult to be separated.

8.1.1 Langmuir-Blodgett films

Langmuir-Blodgett (LB) films are found to be ideal samples for FFM. Any number of layers can be transferred. Using smooth substrates, such as silicon wafers or mica, films of high quality can be prepared. An example is given in Fig. 8.2 where two bilayers of Cd-arachidate were transferred onto a silicon wafer 2. From macroscopic experiments it is known, that Cd-arachidate is a good model system for boundary lubrication [3]. Novotny et al. [4] performed pin-on-disc measurements on coated Si-wafers and found that wear rates could be lowered by a factor of 10 [5].

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