OPTICAL MICROSCOPE AND METALLOGRAPHIC SECTION

OPTICAL MICROSCOPE AND METALLOGRAPHIC SECTION

OUR LABORATORY IS EQUIPPED WITH VARIOUS EQUIPMENT AND CAN MEET VARIOUS REQUIREMENTS OF CHEMICAL ANALYSIS AND METALLOGRAPHIC ANALYSIS.
FOR THE SELECTED INSTRUMENT, WE ILLUSTRATE THE FEATURES IN DETAIL, THE OPERABLE FUNCTIONALITY AND ANALYSIS, IN COLLABORATION WITH
Tecnologie Superficiali Srl.

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What is an optical microscope and what is it used for?

The optical microscope is a tool that allows you to see objects that are not seen with the naked eye, through magnification (up to 1000x).
In many cases it is strongly recommended to combine analysis by light microscopy with that using a scanning electron microscope (SEM), which can provide more detailed information on the morphology, surface elemental composition (in weight or atomic) or different stratigraphic levels (for metallographic sections in a suitable resin).

The technology complies with ISO 1463 measurement methods.

If interested in both forms of analysis, the customer must first notify lab technicians as different encapsulation resins are used.

Our laboratory is equipped with a Nikon Eclipse ME600 optical microscope.

 

What can we analyse at Metalcoating?

  • Non-destructive and non-invasive analysis of the sample or its metallographic section
  • Analysis of whole or section of the sample, such as a resin or as a result of encapsulation, lapping and polishing with diamond paste (metallographic section)
  • Superficial morphological studies at low and high magnification
  • Study of the physical structure of the materials and / or of their state of preservation: presence of cracks, defects, alteration of residues, imperfections, evaluation of miniatures or small details
  • Measurement of depth of porosity, holes, cracks
  • 3D reconstruction of a surface or of a particular shape

 

For embedded samples in metallographic sections and polished by lapping, the following is also possible:

  • Calculation of deposit thickness and consequent deposition rate measurement
  • Assessment of the degree of adhesion between the surface coating and the substrate or base metal
  • Assessment of the homogeneity of the coating with particular attention to the presence of cracks or other imperfections which are microscopic in nature (fractures protrusions, bulges, levelling of the events associated with the base metal, etc…)

Principle of operation

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.

Digital interference contrast (DIC)

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 dark field (DF)

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.

Metallographic section

PHASES:

  1. OBTAINING THE SAMPLE AND ITS CUTTING  (the latter not always necessary)    sezione metallografica
  2. ENCAPSULATION OF THE SAMPLE
  3. SMOOTHING
  4. LAPPING
  5. METALLOGRAPHIC ATTACK (phase not always necessary)
  6. MICROSCOPE OBSERVATION

CUTTING OF THE SAMPLE

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.

ENCAPSULATION OF THE SAMPLE

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.

SMOOTHING

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

LAPPING

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)

METALLOGRAPHIC ATTACK

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:

 

DENOMINATION USE
PICRAL For all steels and ordinary or weakly bound cast irons. It highlights fine structures. Highlights the various structural components and the arrangement of the grains.
NITAL For all steels and ordinary or weakly bound cast irons. It highlights the different structural constituents and defines the outline of the grains.
GLYCERINE AQUA REGIA (VILELLA) For stainless steels and cast irons which can’t be attacked with nital and picral
SODIUM PICRATE
For hypereutectoid steels (steels with carbon contents greater than those of eutectoid around 0.77% by weight of carbon). For alloy steels species and tungsten. For high-speed steel for cast irons. Cementite colours and complex carbides
FERROCYANIDE POTASSIUM ALKALINE (MURAKAMI) For alloy steels species and tungsten. For high-speed steel for steels especially resistant to heat and corrosion and for all stainless steels. For ordinary cast iron, grey cast iron and for it cast iron alloys. Colours carbides and tungsten of high speed steel
CUPRIC CHLORIDE (KALLING) For alloyed steels for chromium steels (in particular for Cr> 5%)
UNIQUE (COGNE) Any type of attack
HF 0,5% Aluminium alloys
PERCHLORIDE IRON Copper alloys. Highlights the limits of grains and beta crystals
  • 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.

MICROSCOPE OBSERVATION

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.

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