Handbook of Modern Coating Technologies

Specimen preparation

There is a whole spectrum of combinations of specimens and analyses types that can be done through XRD. Specimens may be in the form of single crystals, polycrystalline materi­als, powder conglomerates, thin films, biomolecules, and so on. Commensurate with the title of this handbook, the present section provides an outline of specimen preparation proce­dures for polycrystalline metallic materials in uncoated or coated condition. Further detail or procedures pertaining to other types of specimens can be found in relevant literature [27,80].

XRD is inherently a nondestructive technique and does not require much in terms of specimen preparation. A primary requirement for XRD is that the specimen surface should be flat with good finish—unless there is specific interest in carrying out analysis under the given surface conditions—namely, for specimens coated with some thin film. Besides, speci­men preparation also depends on the location of interest, that is, whether the surface or bulk of the specimen is to be characterized. The latter would require sectioning and essen­tially render the technique as destructive. Sometimes, sectioning could also be required for extracting specimen from components of large dimensions that cannot be accommodated on the stage of standard laboratory equipment. It might be advisable to consider use of portable diffraction equipment under such situations; though there would obviously be some compromise in terms of precision and capability.

It is clear from the above discussion that some surface-finishing, flattening, or sectioning is often required while preparing specimens for XRD analysis. Most of these operations can be readily done using saws, surface grinders, slitting wheels or angle grinders, and so on (refer Fig. 3—19). Soil materials, such as aluminum should only be sectioned using some hard saw or microtome, so as to avoid abrasive particles from getting embedded into the specimen. While performing above operations, care needs to be exercised at all times not to

Specimen

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FIGURE 3-19 Specimen extraction for X-ray diffraction analysis.

let the specimen heat-up or hold it too tight in the machine's mount—especially for speci­mens meant for residual stress measurements.

Any sectioning or abrasive machining operation will leave a plastically deformed layer, which can be up to a few micrometers in thickness—often extending beyond the zone up to which X-rays can penetrate into the material. Hence, it is important to limit the thickness of plastically deformed layer in the first place and its subsequent removal through some less abu­sive processes, such as manual polishing and eventually, through electrochemical polishing or chemical etching (namely, using Nital etch for ferrous materials). The amount of material removed through chemical etching can be determined by measuring thickness of specimens before and after etching. In general, chemical dissolution of 2—10 pm layer from the surface should be sufficient to remove plastically deformed layer. To prevent extensive roughening of surface during the course of electropolishing or etching, it might help to prepare the specimen through a cyclic process of etching to remove 1 or 2 pm of material, followed by light polishing on a fine-grit emery paper, making sure to end the cycle with etching.

Coated specimens should not be subjected to any of the above specimen preparation steps. For specimens coated with crystalline films up to several tens of nanometers thick, or amorphous coatings having thickness up to of a few micrometers, X-rays can easily penetrate through the coating to produce diffraction from the underlying substrate, while signal from the film would be much weaker, unless grazing-incidence method is used.

However, in case the coated specimen does not possess a flat surface, one can either opt for X-ray characterization using a smaller cross-section X-ray beam, or make corrections in the diffraction data (possible for regular, namely, cylindrical specimens) or plan to fabricate a flat specimen using the same material, processing and coating deposition, by which the actual component had been prepared.

While mounting the specimen thus prepared onto the XRD equipment, its flat surface must be made coplanar with the plane on which the machine is calibrated to make measure­ments. This task can be simplified by using a specimen-holder (refer Fig. 3—20), which allows mounting by pressing the specimens against clay-dough, so that their flat surface becomes coplanar with the top face of the specimen-holder.