Property Technique Synopsis

As already noted, any single technique cannot provide all the information needed to completely categorize a coating material. For phase identification, x-ray diffraction is the primary technique used, although electron diffraction can also be used. The volume of material sampled by the x-ray technique is much larger, and a more representative picture of the complete coating phase composition is produced. However, very thin coatings or phases with very small volume fractions may be difficult to detect with x-rays, in which case electron diffraction would be preferred.

Coating thickness and surface morphology can be determined from fracture or polished cross sections using either light or scanning electron microscopy. Some estimates of grain size or porosity can also be made in these cases, if an appropriate sample preparation method is used. Etching may be necessary before polished cross sections can reveal grain or phase boundaries. For coatings that are less than 10 pm (400 pin.), scanning electron microscopy is necessary to determine the structure in cross section. For films that are considerably less than 1 pm (40 pin.) in thickness, only transmission electron microscopy is capable of giving cross-sectional images that can be used to determine grain size, phase distribution, or porosity. This is particularly true when interfacial structures are to be examined.

The importance of good specimen preparation for all structural analysis techniques cannot be overemphasized. In many cases, apparently featureless fracture or polished cross sections have been taken to indicate amorphous or single-phase coating. Subsequent etching or diffraction analysis has shown the presence of grain or phase boundaries.

It is also important to correlate the structure of coatings with their composition, which can vary through the coating thickness. The article "Surface and Interface Analysis of Coatings and Thin Films" in this Section of the Volume provides information on coating analysis.

Surface and Interface Analysis of Coatings and Thin Films

S. Hofmann, Max-Planck-Institut für Metallforschung, Institut für Werkstoffwissenschaft, Stuttgart, Germany

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