A wide variety of surface treatments and coatings are applied to metals to enhance their performance, for example, to improve fatigue resistance, increase wear resistance, or improve corrosion or oxidation resistance. Some of these treatments involve diffusion of one or more elements into the metal or alloy followed by post heat treatments. These include the familiar processes of carburizing, nitriding and carbonitriding but also less familiar processes such as ion nitriding, chromizing and boronizing. There are also a variety of coatings that are deposited by electroless or electrolytic means, by physical or chemical vapor deposition, or by thermal or plasma spray. The technological significance of these processes is enormous.
examination of these surface layers is important in the development and
implementation of these technologies. Metallographic preparation of such
specimens is not a trivial matter, as the affected surfaces can be badly damaged
or altered by the use of poor practices. Beyond this, the preparation method
must reveal the true structure so that it can be properly interpreted,
measured, evaluated and documented. Every step of the procedure must be
properly executed if the examination is to yield maximum value.
is almost always required as the first step in the preparation sequence. This
is a crucial step because substantial damage can be produced if it is performed
poorly. The adhesion of some coatings may not be strong, and cutting may
separate the coating from the substrate, especially if the cut begins in the
substrate and ends in the coating. This puts the coating in tension, which is
almost always detrimental. Always perform cutting so that the surface layers
are in compression. Of course, for some components, such as a round shaft,
there is no way to keep the entire coating in compression unless you can rotate
the shaft as it is being cut. This can be done with some precision saws,
depending upon the diameter of the piece being cut. For certain delicate layers
(particularly in failure-analysis studies), it may be necessary to encapsulate
the part in a castable epoxy resin before sectioning. Otherwise, the surface
layers may flake off and be lost.