Tribological Investigations Using Friction Force Microscopy
- PDF / 7,714,171 Bytes
- 10 Pages / 576 x 777.6 pts Page_size
- 30 Downloads / 202 Views
C.F. Quate, and C. Gerber1 in 1986, a surface analysis technique was introduced, able to probe any surface with a resolution on the atomic scale. Many groups worldwide have used this new technique to investigate surfaces of crystalline and amorphous materials and to examine inorganic and organic systems.2 The design of this real-space analytical tool is based on the scanning tunneling microscope 3 (G. Binnig, H. Rohrer, Nobel Prize, 1986), which scans a very fine tip over the sample surface. The scanning force microscope tip is located on a sensitive cantilever spring. Interactions between the tip and sample cause deflections of the cantilever. These deflections are monitored by a computer providing a two-dimensional image of the sample surface in real space. Depending on the load of the spring, the tip
Sample Figure 1. Bidirectional AFM Method (A). Normal and lateral bending is measured by two sensors: SN (normal) and SL (lateral). Typically, the feedback control holds the normal force constant. In the setup of Mate et al.,4 only the lateral sensor is used.
can adhere to the sample and undergo sliding start-up effects, known in tribology as static friction, and dynamic sliding effects by scanning. CM. Mate and G.M. McClelland took advantage of this situation, modified the scanning force microscope, and measured the lateral instead of the normal movement of the tip.4 Their experiment provided the first friction measurements on the atomic scale. With the development of the laser beamdeflection method (see next section), G. Meyer et al.5 and O. Marti et al.6 produced a single detection setup capable of providing simultaneous information on normal and lateral movement of the tiny cantilever sliding over the sample surface. This article seeks to: • highlight the problems of crosstalk on the topography and torsion signals and present a method that allows one to distinguish between topographically induced and frictionally induced torsion {friction loop), and • describe the first successful applications of the friction force microscope carried out on lubricating systems, such as organic films, also known as boundary lubricants. These boundary lubricant films are either physically or chemically adsorbed on the solid surfaces and their exact microscopic nature remains largely unknown. Self-assembled organic films, such as Langmuir-Blodgett (LB) films, serve as model boundary lubricants. These films are known lubricants, behave in a solidlike manner, adhere well to substrates, and form ordered two-dimensional structures. The scanning force microscope provides a unique opportunity to study friction and wear resulting from a microscopic asperity on these organic compounds. Experimental Details Mate et al.4 modified the atomic force microscope (AFM) to measure the deflection of the cantilever in response to lateral forces (Figure 1). The sideways bending of the cantilever (tungsten wire) was detected using a laterally positioned optical interferometer. With these early measurements of lateral friction forces, it was demonstra
Data Loading...
