Published: Jan 2012
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ELECTRON MICROSCOPES ARE FREQUENTLY USED in the analysis of coatings and their components because of their high resolution and multifunctional capabilities. The two common types are the scanning electron microscope (SEM) and the transmission electron microscope (TEM). In various operating modes and with specialized detectors, these instruments can be used to examine surface texture and internal structure, determine particle size and shape, provide elemental analysis, and reveal crystallographic orientation and identity. The resolving power of electron microscopes is orders of magnitude greater than that of light-optical microscopes, partly due to the shorter electromagnetic wavelength (higher energy) of electron illumination. Modern SEMs with field-emission electron guns have the ability to resolve better than 1 nm between objects and commercial TEMs can resolve better than 0.2 nm. However, the actual resolution obtained is highly dependent on the nature and preparation of the sample. Contrast develops in SEM by electrons emitted at or near the surfaces of bulk specimens and, therefore, topography and composition are examined. Contrast develops in TEM by electrons transmitted through thin specimens and, therefore, variations in structure and composition are examined. Due to the improved resolution of modern SEMs, TEMs are no longer needed to see coating components 50 nm or smaller. In this chapter, SEM will receive more attention than TEM for this reason, along with its greater availability and documented coatings applications. Types of signals generated in a sample by a vertically incident electron beam are illustrated in Fig. 1. An SEM typically has detectors for secondary electrons (SEs), back-scattered electrons (BSEs), and X rays positioned above the surface of bulk specimens. The TEM collects elastically and inelastically scattered electrons, and sometimes X rays from thin (ca. 100 nm) specimens. Hybrid TEMs have some SEM capabilities. They may collect BSEs and SEs from the incident-beam side of thin specimens, a variety of transmitted electron signals from the opposite side of the sample, and X rays from both sides. These are known as scanning transmission electron microscopes (STEMs) or analytical electron microscopes. Their greatest strength is their ability to focus a fine electron probe on very thin samples, achieving high spatial resolution in elemental and structural analysis.
Rothbard, David R.
Bureau of Engraving and PrintingInstitute of Paper Science and Technology, WashingtonAtlanta, DCGA
Sheehan, John G.
Bristol-Myers Squibb Company, Garden City, NY