Metallographic Sample Preparation & Optical Microscopy

METALLOGRAPHIC
SAMPLE PREPARATION
AND
OPTICAL MICROSCOPE
2.-3. WEEK
METALLOGRAPHIC SAMPLE
PREPARATION STEPS
1.
CUTTING
2. MOLDING
3. GRINDING
4. POLISHING
5. ETCHING
Metallic clamps can be used for sample molding
MOLDING
• The sample is small,
• Complex shaped
• Contains pores or cracks
• Edge smoothness required
(COATING inspection)
• If standard size is desired
(Automatic polishing devices)molding is done.
In addition, the molded sample provides ease of
cleaning and coding.
The sample can be molded in two ways;
• Hot and pressure molding (bakelite)
• cold molding
Polymeric based material is used in powder
form in molding under hot and pressure.
• The sample surface is placed in
contact with the mold surface and
polymeric powder is poured on it.
• The lid of the mold is closed and fired
at 150-160°C by applying a load.
• Molding parameters (temperature,
pressure, time) vary according to the
chosen polymeric material.
• The hardness and quality of the mold
obtained compared to cold molding is
better
(especially
important
in
automatic preparation)
• Die diameter
tolerances
stays
within
exact
Powders Used: There are two types
Thermoset (Phenolic, Diallyl Phthalate And Epoxy) and Thermoplastic
(Acrylic).
Phenolic: The Oldest Molding Material, Named Bakelite. Wood Chip Filled.
The applied pressure is 29 MPa, the temperature is 150°C and the
processing time is 5-9 minutes depending on the die diameter.
Dially Phthalate: More Expensive than Phenolic and High Abrasion
Resistance. Contains Glass Fiber or Mineral Filling to Increase Hardness
and Strength. 22 MPa, 150°C And 7-12 Minutes
Epoxy: Gives Good Abrasion/Polishing Characteristics. 8 MPa, 150°C And 712 Minutes.
Acrylic: It is the only choice in moldings that are required to be
transparent. Its Wear Rate is Softer Similar to Phenolic.
COLD MOLDING
It is a method that is applied to materials whose properties can
change under pressure and hot molding temperatures and allows
serial sample preparation.
It is suitable for molding under vacuum.
Mold can be prepared in any shape.
It is possible to prepare many samples at once, but the resins used are
more expensive than those in hot molding.
In Cold molding, a mixture of two liquids or a liquid and a powder are
used. One of them is resin (polymeric based) and the other is
hardener.
Resin and hardener should be mixed in appropriate proportions.
Otherwise, the mold will not harden.
• Resins Used: Epoxy, polyester and acrylic.
• Solidification time can range from minutes to hours. While
the solidification time is 10 minutes in acrylic, it is 4-6 hours
in epoxy.
After the molding process;
• The sample must be free of deformation and damage.
• There should be no structural changes in the sample
• A suitable edge smoothness must be provided.
• Must be resistant to common etchants
In many applications, other additions are made to the
polymer powder (in practice, these additions are made by the
manufacturers).
Making such contributions provides the following benefits:
• Conductivity is provided (with conductive metal powders),
• The hardness of the bakelite is increased (it is desired to wear
at the same speed as the sample,
• Edge effect (off sharpness around edges) is reduced
Coding (marking) of samples
In studies with a large number of samples or for archiving, the
necessary notes should be written on the sample with a vibrating
pen.
Mechanical sanding and polishing
On the surface of the sample, there are traces of the tool used for
cutting, and the surface of the sample is slightly deformed during
cutting. Since the sample represents the original structure, the total
deformed layer must be removed. Therefore, after the specimens have
been cut and molded, they are subjected to sanding and polishing for
microscopic examination.
The main purpose of sanding and polishing is to obtain a surface that
reflects light well by reducing surface roughness.
Sanding and polishing processes involve various stages and are done with
the help of abrasives. At each stage, finer abrasives are used than the
abrasives used in the previous stage, thus minimizing the amount of
deformation that each stage creates on the sample surface.
Abrasives;
• It must be hard and maintain its cutting property for a long time.
• It should be sharp and able to cut through hard materials completely
instead of tearing them off.
Sanding (Grinding)
It is done manually or automatically in an aqueous environment.
Abrasives;SiC (most used) is Al2O3, B4C or diamond.
Abrasives are attached to paper (other than diamonds), they can
be discs, plates or tapes
Sandpapers are numbered according to the number of abrasive
particles per unit area. As the particle size decreases, the
number of particles per unit area increases and the number of
the abrasive increases.
Sandpaper grit number
Grain size in microns
80
150
180
240
320
400
600
800
1200
210-177
105-88
88-74
53-45
37-31
31-27
22-18
15-11
6,5-2,5
The process with abrasives up to 150 grit sanding is
called coarse sanding, and the operations with finer
abrasives are called fine sanding.
Sanding can be started from 80 grit after saw cut, 180
grit in abrasive disc, 320 or 400 in wire erosion and
low speed diamond.
Except for automatic devices, when sanding by hand;
• Uniform holding of the sample on the surface of each abrasive,
• Scratches on the surface of the sample should be only that of
used sandpaper,
• These scratches are in one direction,
• Applying the sample to the sandpaper in one direction,
• The operation is carried out under water,
• When changing the sandpaper, thoroughly wash the sample,
hands and sandpaper so that coarse sanding dust is not carried
to the next step and it should be noted that the next sanding is
applied in the 90° vertical direction.
• During sanding, leaving the sample wet for a long time should be
avoided.
Sanding time
The sanding time on a particular abrasive is the time it takes
until the scratches that occurred during the previous
sanding are completely removed. The scratches and
deformation layer formed on the sample surface during
sanding are removed with the next sanding.
Pressure
It should be distributed homogeneously
Low pressures prevent cutting, while high pressures can
cause the abrasive to sink into the material surface and
cause pitting.
Speed
mostly 300 or 600 rpm
However, the pressing force and the rotation speed of the
machine are gradually reduced.
Polishing
It is done with the aim of eliminating the sanding lines after the sanding
process and obtaining a smooth, scratch-free, well reflective surface as
possible.
Polishing is done by abrading the sanded surface by means of abrasive
particles applied to the fabric on a rotating disc. Lubricant is also used to
reduce friction. Abrasive is divided into two groups according to particle
size
Rough polishing; abrasive particle size of 15-3 microns andfine polishing;
abrasive particle size 1 micron and below
Abbresives used;
Alumina (Al2O3), less than diamond chromium oxide (Cr2O3), magnesium
oksit (MgO), iron oksit (Fe2O3), cerium oksit (CeO)
Colloidal silica (SiO2) has an important place in the final polishing stage of
soft materials.
• Alumina is available in paste, solution, or powder form, while
diamond is available in spray, solution, or paste form, while others
are usually available in paste form. If diamond is used, oil-based
lubricants are used, and if alumina and other abrasives are used,
water-type lubricants are used.
• Alumina and diamond are the most commonly used abrasives in
the rough polishing step. Diamond, alumina, colloidal silica,
magnesium oxide, iron oxide, chromium oxide, cerium oxide are
used in the final or fine polishing step.
(Magnesium oxide is used for polishing aluminum and magnesium
alloys, iron oxide and chromium oxide for steel and cast iron,
cerium oxide is used for polishing soft metals and materials with
low melting point)
Abrasives are applied on special polishing fabrics, which are usually
glued or attached to discs made of brass. During polishing,
lubricants such as water and oil are used to prevent heating due
to friction between the sample and the abrasive (fabric).
The polishing fabric should be able to hold the abrasive against the
sample,
Nap (pile) or napless (lint) fabrics are used.
Fabrics with napless or short nap such as canvas, nylon and silk are
preferred for high cutting speed, maximum abrasive contact and
low relief in the rough polishing stage.
In the fine polishing step, pile fabrics such as felt and velvet are
preferred (short, medium or long, if they contain phases of
different hardness, short pile fabrics are preferred)
In the polishing process, the sample is held on the polishing disc
covered with the recommended fabrics and from time to time,
alumina solution (or others) is applied to the polishing fabric.
There are some benefits to moving the sample while holding it. The
sample should be moved in the opposite direction to the rotation
direction of the disc and it should be moved back and forth from
the center of the disc. In this way;
Homogeneous wear of the polishing fabric is ensured by the
homogeneous distribution of the abrasive on the disc surface,
The "comet" appearance caused by oriented polishing, especially
seen in samples containing residue, porosity, and a fine sediment
phase, is avoided.
• After successful polishing, the surface of the
sample looks like a mirror. At the end of the
polishing process, the surface is washed with
detergent water, cleaned with alcohol and dried by
air spraying.
120
240
600
6 µm diamond
320
1 µm diamond
400
Colloidal silica
Electrolytic polishing
Many difficulties are encountered especially in the
mechanical polishing of single-phase and soft materials such
as copper, aluminum, austenitic stainless steel. The main
ones are; It is significant surface deformation due to quick
scratching and excessive load application. Electrolytic
polishing is a more suitable method for polishing such
materials.
An extremely smooth-clean surface is obtained as a result of
the electrolytic polishing process performed in certain
chemical solutions at certain current and voltage in special
electrolytic polishing devices.
An electrolyte reaction cell
with two electrodes (anode
and cathode), consisting of
liquid electrolytic, is used in
electrolytic polishing.
The electrodes are connected
to an external power source
that will cause a reaction
within the cell. And voltage is
applied
for
reaction
formation inside the cell.
When current is passed from
one electrode to the other,
metallic ions move from one
electrode (anode) to the
other (cathode) through the
electrolyte.
➢The cathode must be made of a metal that acts inert to
the electrolyte used. Usually stainless steel is used.
➢Before electropolishing, the sample should be sanded up to
600 sandpaper. It may also be necessary to polish some
materials, such as beryllium and lithium.
➢The sample (anode) to be polished should be in a position
where it can be easily taken from the electrolyte. The
electrical conduction to the sample should be simple and
easily cut off. (To be taken and washed immediately after
polishing)
• In this method, the polishing mechanism is anodic dissolution.
Elimination of the roughness on the sample surface is by
preferential dissolution of the ridges, and the pits between
the ridges are protected from this anodic dissolution.
Because the dissolution rate in these parts is less compared
to the protruding parts.
Since the sample is in the anode state, the (+) charged metal
ions leave the sample surface and go to the cathode. During
the dissolution of the sample in this way, a film layer formed
by metal ions forms just above the sample surface (Anode
Film). The electrical resistance of this film layer is greater
than that of the electrolyte and is of different composition.
The general opinion about the polishing mechanism is that the
film layer on the protruding parts of the surface is thin, the
metal ion concentration is high, and the electrical resistance
is low. Accordingly, under the applied potential, the current
density in the rough parts will be higher than in the pit parts.
If a sufficient amount of potential is applied, the metal will
move from the protruding parts to the electrolyte faster
than from the hollow parts. This situation continues until the
surface is completely flat (shining).
Electrolytic etching;
• After electrolytic polishing of many materials, etching can also be
performed using the same electrolyte. However, for etching some
materials, electrolytes with a different composition than the
electrolyte used in polishing may be required.
• The voltage used during the etching process is one-tenth lower than
the voltage required for polishing.
• Electrolytic polishing and etching process time is very short, it can
be performed in the order of a few minutes or even seconds.
• However; preferential dissolution can occur in multiphase materials.
For Cu metal;
Composition of electrolyte solution used: 10 ml H2SO4+ 90 ml
H2O
Applied voltage: 1-8 V,
Duration: 5-10 sec
The surface obtained as a result of the polishing process is
suitable for examining some material parameters. These;
• Determination of the type of cast irons and classification of
graphite density
• Crack reviews
• Porosity studies
• Non-metallic inclusions (such as MnS, FeS, Al2O3)
• Examination of some coatings
• Some anisotropic metals in polarized light illumination
• But since the polished surfaces reflect light equally, the
details of the structure cannot be observed; to achieve this,
it is necessary to create contrast in the structure; etching is
done for this.
CAST IRON WITH LAMELLAR GRAPHITE
Inclusion
COATING
ETCHING
Contrast is created on the sample surface
With etching;
• Grain and grain size
• Phase and distribution of phases
• Deformation
• Segregation
• Surface treatment and depths (such as cementation,
nitration, decarburization)
• Microstructure details such as coating layers are revealed
Etching can be physical or chemical.
• physical etching; carried out by applying heat or voltage
• chemical etching; The most applied method is the treatment of
polished surfaces with a suitable chemical solution (etching).
Chemical etching is done with a large number of chemical solutions
(etchant, reactive reagents) that vary according to the materials and
the parameters to be examined. Many solutions are available for
etching even a single type of material. The technical staff can also
prepare new solutions using material and chemistry knowledge.
Etching solutions
• Solvent→ water, alcohol, glycerin, glycol or a mixture of
these
• Solute→ organic (such as oxalic, acetic, lactic and citric
acid) and inorganic acids (such as hydrochloric, nitric,
sulfuric acid) and various alkali products
Some common etching solutions
Alloy
Etching Solutions
Steel and Cast Iron
Nital (1-10 ml HNO3+90-99 ml methanol or ethanol)
Pure Ag and alloys
1-5 gr CrO3+100 ml HCl
Ag-Cu alloys
25 ml NH4OH+25 ml Su+50 H2O2
Cr and alloys
Aqua regia (20 ml HNO3+60 ml HCl)
Cr and alloys
Murakami (10 gr K3Fe(CN)6+10 gr KOH+100 ml H2O)
Etching can be performed in several ways:
➢ Dipping
The sample is immersed in the solution with the help of a
stainless tong and waited for the recommended time (watch the
sample surface visually); removed, washed with water, alcohol,
dried. For this method to be applicable, it is particularly
necessary that the molding material is not affected by the
solution.
➢ Swabbing
The cotton dipped in the solution is applied to the polished
surface without pressing it and the color change on the surface
is observed. Matting of the surface indicates complete etching.
At the end of the process, the sample is washed with water,
alcohol and dried.
➢ Dropping
In some cases, a drop or two of solution is dropped onto the
sample surface and the development of the surface is
monitored; The sample is washed, alcoholic and dried.
REFERENCES
•
Doç. Dr. Mediha İpek, Metalografi Dersi Notları, Sakarya Üniversitesi
•
S. Salman, H. Ö. Gülsoy, Metalografi Bilimi, Nobel Yayın, 2014.
•
G.F.V. Voort, ASM Handbook Volume 9: Metallography and Microstructures, ASM International.
•
Callister Jr, W. D., & Rethwisch, D. G. (2015). Callister's Materials Science And Engineering. John Wiley & Sons.