In order to have better control over material properties, it is necessary to understand their nano-structure and to relate it with the macro-properties. The development of the Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) allowed the ability to see and manipulate atoms, molecules and nano-sized objects, making it possible to explore the structure of materials at a scale of 40 to
50 nm. Most publications on characterization of pavement-related make use of either the SEM or AFM. In the following sections the basic principles of use of the two techniques are briefly discussed, followed by references to existing work where the applications of these characterization methods have lead to an improved understanding of the properties and behavior of pavement materials.
The SEM is typically used for imaging surface and subsurface microstructure of samples. An accelerating voltage of between 1 and 30 kV is typically used. It images the sample by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography. A schematic diagram of the SEM is shown in Fig. 7 [Kelsall et al 2004].
3.3.3 Atomic Force Microscopy
The AFM consists of a cantilever with a sharp probe that is used to scan the sample surface. When the probe is brought into proximity of a sample surface, forces between the probe and the sample lead to a deflection of the cantilever according to Hooke's law. Forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces etc. A schematic diagram of the AFM is shown in Fig. 8 [Kelsall et al 2004].
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