Microscopy of Coatings
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Rädlein, E. 2006. Microscopy of Coatings. Encyclopedia of Analytical Chemistry. .
- Published Online: 15 SEP 2006
Fundamental research on films and the development and optimization of coatings are not possible without high-resolution imaging. This article describes the advantages and disadvantages of the newly introduced scanning probe microscopy (SPM) in comparison with well-established microscopy techniques with light and particles [e.g. scanning electron microscopy (SEM) and transmission electron microscopy (TEM)], which are limited by the diffraction limit, i.e. 200 nm for conventional light microscopy and 4 nm for SEM. This limit does not hold for SPM, which uses near-field interactions for imaging. Thus, even simple atoms or molecules can be resolved on well-ordered structures by scanning tunneling microscopy (STM) or atomic force microscopy (AFM). Both possess an extremely good height sensitivity of 0.02 nm.1 In the constant-height operation mode, AFM can detect vertical forces as low as 10−15–10−4 N. Atomic resolution is not always possible or not always necessary for many practical applications; a lateral resolution of a few nanometers is sufficient. Besides the capability of achieving this value with ease, simple sample preparation, versatile environmental conditions and observation of reactions in situ are advantages of STM and AFM.
Especially AFM has proved to be a useful technique for investigating coatings, because it is independent of conductivity and provides topographic data together with information on structure, local variations of mechanical properties and surface alterations in different media. However, some artifacts and restrictions have to be considered which can differ considerably from customary electron micrographs with comparable resolutions, such as onset overswing and tip convolution.
Microscopy helps in solving coating problems according to substrate quality, nucleation, wettability, thickness, roughness, microstructure, crystal phases (texture, orientation), lateral and vertical homogeneity, density, porosity, interfaces, adhesion, hardness, stress, scratch and abrasion resistance, tribology, wear and corrosion. Practical examples of microscopy results on these questions are presented together with suitable preparation methods and interpretation of artifacts.