OPTICAL MICROSCOPE AND METALLOGRAPHIC SECTION

Nikon Eclipse ME600 is an optical microscope equipped with three objectives (20x, 50x and 100x) and an incident illumination system (epi). The microscope can be used in imaging differential interference contrast (DIC), which is an imaging method based on the contrast difference of the samples, and the dark field (DF) mode. With DIC and DF, you can see the details from optically transparent samples that are not visible through ordinary microscopes.

This method is an extension of polarization contrast, and is suitable for viewing of even small differences in the height of the surfaces. A birefringent prism splits the polarized light beam into two partial beams that reach the sample. These two partial beams strike the sample with lateral movement with respect to one another.

Once the partial beams return to the DIC prism and the analyser, they can interfere with each other, forming the final image observed by the operator.

The light emanates from the lighting lamp and collimated at the collection point. Most of the light is then blocked by a high aperture condenser, equipped with a central block that blocks the central ray of the beam. The light is then reflected by a 45° mirror with a circular hole at the centre. In this way, the light travels toward the sample on the outside of the lens. It is then focused on the sample by an interior mirror at the end of the lens housing. The light is deflected by any irregularities on the sample surface, and part of it is passed through the objective lens to the eye of the operator. In this way, not directly (specularly) reflected light is seen by the operator (meaning that a perfectly clean mirror surface is shown in black). Only gradients, provided by the roughness of the sample and the edges are visible in the image.

For oversized samples, it is necessary to proceed with their cutting so that we can allow for the resin encapsulation. This operation is generally performed by means of appropriate power cutters equipped with diamond discs that are capable of performing a clean cut so as to minimize the subsequent step of polishing. It is possible to perform different types of sections (longitudinal, transverse etc.). During this phase, it is necessary to continuously refrigerate the sample in order not to raise the temperature too much resulting in the induction of structural modifications.

The cut sample must be incorporated in a resin, either hot or cold, this is to make the successive polishing and lapping steps possible and facilitate the processing and observation under the microscope.

RESIN EQUIPMENT:

• Cold-curing polymers: polyester or epoxy resins. It is a “casting” which is necessary for the “forms” in which to put the specimens and in which to pour the resin with the hardener already mixed.
• -Thermosetting: Phenolic resin and SEM microscopy. Curing of the powder takes place with supply of heat (≈ 140-180°C) and pressure.

This step consists in polishing the sample through the use of emery cards by decreasing grain size.

This phase is performed by means of appropriate diamond grain size cloths descending (9mm, 3mm, 1mm, 0.5mm). During this phase, it is necessary to lubricate abundantly to remove the particles of the sample which would otherwise scratch the surface to be analysed. The polishing is carried out by always using the rotating rollers lapping machine, but with added a diamond paste (size 6um grains and 1um)

The lapped samples are immersed for a sufficient time in a suitable chemical solution, usually acidic, which selectively corrodes the various structures it highlights. The attack time must be such as to allow the solution to attack the surface of the sample in the best way possible, thus highlighting the desired details; in the case where the attack has been executed for too short or too long a time, it is necessary to repeat the step of lapping and then proceed again with the attack.

The solutions to perform metallographic attack are varied; the choice of one or other depends on the material to be examined and the structures that you want to highlight.

• PRELIMINARY WASHING:
  it is good to precede the attack with the surface degreasing, washing it with a wad of cotton wool impregnated with a very fluid and detergent wash solution.

• CHOICE OF SOLUTION:

• ATTACK AND ITS METHOD                                                                                    The attack can be carried out by immersion. The cold-attack is intended with the reagent at room temperature, while the hot-attack is carried out with the reagent heated to a certain temperature, usually to boiling point. Or more simply may be obtained by wetting of the specular surface of the specimen.
• WASHING
  It stops the action of the attacking reagent placing the surface attached below a jet of running water. A subsequent washing may be carried out with detergent solutions to remove any residual reagent, even in small cavities and along the edges of the crystals preventing the prolongation of its action.
• DRYING
  It is possible to carry out this phase with compressed air flow or with a soft cloth.

The observation of the sample can take place by means of optical microscopy and/or by means of a scanning electron microscope SEM

 If interested in observing the sample with both microscopes, the customer must first notify lab technicians as different encapsulation resins are used.