Preparation of Metallographic Specimen

Source:  Preparation of Metallographic Specimen    Tag:  low angle xrd
Micro sections for the microscopically examination of metals are samples of the piece to be examined, prepared with one plane surface polished free from scratches.

Sampling:

            Taking the sample for the micro section, like the taking of the sample for chemical analysis, is the most important step in the entire examination, for on this depend the value and integrity of the conclusions drawn. The sample chosen must be truly characteristic of the specimen it represents, its size and location depending on the size and nature of the specimen and the purpose of the examination.

Mounting:

Small samples are generally mounted in plastic for convenience in handling and to protect the edges of the specimen being prepared. Compression-type molding is commonly applied to encase specimens in 1 to 1.5 inch diameter plugs of a hard polymer. Compression molding materials are classified as either thermosetting or thermoplastic:
  • Bakelite is a low cost, relatively hard thermosetting polymer that is commonly utilised.
  • Expensive Thermoplastics are utilised when transparency is required.

Mounting involves following procedure:
  • Clean specimens to remove cutting and handling residues
  • Remove debris from mold assembly
  • Apply thin coat of mold release compound to mold assembly
  • Raise mold ram to up position
  • Centre specimen on ram
  • Lower ram assembly
  • Pour predetermined amount of resin into mold
  • Clean and remove any excess resin from around the mold assembly threads
  • Lock mold assembly cover
  • Slowly raise ram into up position
  • Apply recommended heat and maintain pressure for specified period of time
  • Cool to near room temperature
  • Remove mounted specimen
  • Clean mold and ram assembly


Coarse Grinding:

In view of the perfection required in an ideally prepared metallographic sample, it is essential that each preparation stage be carefully performed. The specimen must:
  • Be free from scratches, stains and others imperfections which tend to mark the surface.
  • Retain non-metallic inclusions.
  • Reveal no evidence of chipping due to brittle inter metallic compounds and phases.
  • Be free from all traces of disturbed metal.

It involves following procedure:
  • Turn on the Motor, then the water, adjust the flow to obtain a good film of water. Too much water will cause a spray when it contacts the sample.
  • Beginning with the 120 grit belt and using both hands to hold the specimen, carefully place the sample face onto the exposed area of the belt being careful not to contact the rotating surface with a sharp edge of the specimen or your hand.
  • Applying moderate pressure evenly, move the sample left-and-right across the belt surface to obtain uniform grinding. Use both hands to hold the specimen; unsecured specimens can "Catch an Edge and FLY"!
  • Lift the sample from the wheel periodically to determine the progress of grinding but do not rotate the sample. The 120 grit stage is complete when all the lines scratched in by the grinder are parallel on the specimen surface. If any line or scratch is not in the same direction as the other lines, continue grinding until all of the lines are parallel.
  • When all of the sample's scratches are parallel, carefully wash all of the debris from sample using tap water and dry the specimen immediately using a paper towel or pressurised air to avoid corrosion.

Fine Grinding:

Medium and Fine Grinding of metallurgical samples are closely allied with the Coarse Grinding which precedes them. Each stage of metallographic sample preparation must be carefully performed; the entire process is designed to produce a scratch free surface by employing a series of successively finer abrasives. Failure to be careful in any stage will result in an unsatisfactory sample.
            It involves following Procedure:
"The sample MUST be washed thoroughly before proceeding from one fine grinding stage to the next!!!"
  • Manual Fine Grinding is performed by drawing the specimens in one direction across the surface of the water lubricated abrasive paper. (Back to front is recommended) Use of backward and forward motion is less desirable because there is a tendency to rock the sample, producing a curved rather than a flat surface.
  • Begin with the lowest grade abrasive paper (100 grit) and proceed to the highest (1500 grit).
  • To monitor progress, each fine grinding step should be performed in a direction off-angle with respect to the previous step.
  • Fine Grinding should be continued until the previous stage's scratches are gone, using a few extra strokes to assure complete scratch removal.
  • Sufficient water must be applied to provide lubrication and flush away the removal products. Too much water will result in a hydroplaning action where the sample rides on a film of water, thereby reducing the effectiveness of the abrasive. Use the valve at the top of the roll-grinder to increase and decrease the quantity of water needed or desired.
  • The specimen should be carefully rinsed after each step of Fine Grinding; debris from one step must not contaminate the next step! Rinse the specimen very thoroughly before proceeding to polishing.

Mechanical Polishing:

Polishing involves the use of abrasives, suspended in a water solution, on a cloth-covered electrically powered wheel. Diamond abrasives provide the best, and most expensive, compounds utilised in polishing; standard sized aluminium oxide powders are applied for general use purposes. Following the final 1500 grit fine-grinding stage, the sample MUST be washed and carefully dried before proceeding to the first polishing stage! At the polishing stages, even hard dust particles in the air which settles on the polishing cloth can cause unwanted scratching of the specimen! Careful washing of the specimen and the operator's hands must be carried out prior to each stage of polishing! Beginning with 25-micron suspended aluminium oxide particles (suspended in water) on a Nylon-cloth, the final fine-grinding surface layer resulting from the previous grinding procedure should be completely removed with a rotation rate of 150-200 rpm.
            It involves following procedure:
  • Make sure your specimen and hands have been thoroughly cleaned before Polishing!
  • Begin with the 25-micron, nylon cloth polishing station!
  • Turn the water on adjusting to less than one drop per second!
  • Apply a small amount of the aluminium oxide abrasive solution to the polishing cloth.
  • When polishing the specimen, hold it with both hands, apply a moderate amount of pressure, and don't let it go. The Rough Polishing Stages (5-25 microns) should take between 1 and 2 minutes each! If you let go of the specimen it may fly, harm you or others in the laboratory and become damaged forcing you to start over again with coarse grinding - hold it tight and be careful!
  • Do not contaminate the polishing wheel; cover the wheel when it's not in use!
  • Before proceeding to the next polishing stage, wash and dry both the specimen and your hands thoroughly then rinse the specimen.
  • Repeat steps 4 through 8 for the 5-micron stage.
  • Proceed to the Final Polishing Station (1 micron) when all of the 25 micron marks are removed at the 5 micron stage.
  • Hold the specimen with both hands and apply a SMALL amount of pressure, and Don't Let Go!
  • The Final Polishing Stage (1-micron) should take between 1/2 and 1 minute!
  • Be very careful that you do not contaminate the polishing wheel! Cover the wheel when it's not in use!!!
  • Before proceeding to Etching, wash and dry both the specimen and your hands thoroughly then rinse the specimen with distilled water.

Etching:

Microscopic examination of a properly polished, unetched specimen will reveal only a few structural features such as inclusions and cracks or other physical imperfections. Etching is used to highlight, and sometimes identify, micro structural features or phases present. Even in a carefully prepared sample, a surface layer of disturbed metal, resulting from the final polishing stage, is always present and must be removed. Etchants are usually dilute acid or dilute alkalies in water, alcohol or some other solvent. Etching occurs when the acid or base is placed on the specimen surface because of the difference in rate of attack of the various phases present and their orientation. The etching process is usually accomplished by merely applying the appropriate solution to the specimen surface for several seconds to several minutes. Nital, a Nitric Acid - Alcohol mixture, is the etchant commonly utilised with common irons and steels. Nital is dripped onto the specimen using an eye-dropper or cotton swab.
            It involves following Procedure:
  • Place the specimen on the table under the Fume Hood with the polished surface up.
  • Turn on the Fume Hood.
  • Without touching the specimen surface, clean the surface with alcohol and let it dry using the hot air gun. Do not let anything but the alcohol touch the specimen surface!
  • Using the Eye-Dropper, apply a few drops of Etchant to the specimen surface covering the entire metallic surface of the specimen.
  • After about 20 to 30 seconds, rinse the Etchant into the sink with water and quickly rinse the specimen with alcohol, but do not touch the surface!
  • Use the Hot Air Gun to dry the sample.
  • Proceed to Microscopic Examination; if further etching is required you may return and proceed through steps 1 through 6 varying the time in step 5 depending on the results.
  • If the specimen has many scratches and marks or the micro structure cannot be seen after several etches, return to fine grinding and go back through the necessary steps.

MICROSCOPY:

Optical microscopy using metallographic microscopes is a widely used technique for analysing metallographic specimens. The typical magnification range for optical microscopes is 50 to 1000X, however higher magnifications are possible with specialised oil immersion lenses. The standard resolution for optical microscopes using air immersion lenses is between 0.5 to 10 micron. Optical microscopes use a number of different optical techniques to reveal specific micro structural features oblique (stereo) and differential interference contrast (DIC). Scanning electron microscopy is also used for metallographic analysis and has a resolution ranging from Angstroms to microns.
Numerical Aperture (N.A.) – measure of objective lens light-gathering ability (also determines the quality of the lens)
Resolution – the distance at which two individual features can be seen as individual objects
Working Distance – the distance between the objective lens and the specimen surface when the image is in focus  
Resolution and Numerical Aperture (N.A.):
The most important components of the optical microscope are its objective lenses. The quality of these lens ability to gather light is characterised by the numerical aperture (N.A.)
                                                            N.A. = μ sin θ
Where:
μ - Refractive index of the medium in front of the objective (μ = 1 for air)
θ - The half-angle subtended by the objective in front of the objective at the specimen.

Resolving Power = (2 * N.A.)/λ
            λ = wavelength of light used
            λ = 0.54 micron – green light
            λ = 0.1 Angstrom – electron beam
            Limit of Resolution = λ/ (2 * N.A.)
Total magnification = objective mag. * eyepiece mag. * tube factor mag.
Optical Filters:
Optical filters are used to enhance the definition of the specimen image, especially for photographic film.

TYPES OF MICROSCOPE:

Optical microscopes can be broadly categorised into two types, upright and inverted, based on the positions of the light source and the objective.
Upright Microscope
  • Upright microscopes are constructed with the tip of the objective pointing downward so as to view the specimen from above.
  • Light is directed on the specimen from below.
  • This type of microscope is suitable for viewing prepared slides.

Inverted Microscope
  • Inverted microscopes are constructed with the tip of the objective pointing upward so as to view the specimen from below.
  • The objective is underneath the stage and light is directed on the specimen from above.
  • This type of microscope is suitable for viewing culture vessels.