msc_thesis/metallurgy.org

* Introduction

Metallography has been described as both a science and an art. Traditionally,
metallography has been the study of the microscopic structure of metals and
alloys using optical metallographs, electron microscopes or other surface
analysis equipment. More recently, as materials have evolved, metallography
has expanded to incorporate materials ranging from electronics to sporting
good composites. By analyzing a material’s microstructure, its performance
and reliability can be better understood. Thus metallography is used in
materials development, incoming inspection, production and manufacturing
control, and for failure analysis; in other words, product reliability.

Metallography or microstructural analysis includes, but is not limited to, the
following types of analysis:
• Grain size
• Porosity and voids
• Phase analysis
• Dendritic growth
• Cracks and other defects
• Corrosion analysis
• Intergranular attack (IGA)
• Coating thickness and integrity
• Inclusion size, shape and distribution
• Weld and heat-affected zones (HAZ)
• Distribution and orientation of composite fillers
• Graphite nodularity
• Recast
• Carburizing thickness
• Decarburization
• Nitriding thickness
• Intergranular fracturing
• HAZ Sensitization
• Flow-line Stres


Grain Size
For metals and ceramics, grain size is perhaps the most significant
metallographic measurement because it can be directly related to the
mechanical properties of the material. Although grain size is actually a 3-
dimensional property, it is measured from a 2-dimensional cross section of the
material. Common grain size measurements include grains per unit area/
volume, average diameter or grain size number. Determination of the grain
size number can be calculated or compared to standardized grain size charts.
Modern image analysis algorithms are very useful for determining grain size.

Twin Boundaries
Twin boundaries occur when two crystals mirror each other. For some
materials, twinning occurs due to work hardening at low temperatures. To
correctly determine the grain size in these types of materials, the twin
boundaries need to be removed from the calculation

Porosity and Voids
Holes or gaps in a material can generally be classified as either porosity or
voids. Porosity can also refer to holes resulting from the sintering of metal or
ceramic powders or due to casting shrinkage issues. Voids are generally a
result of entrapped air and are common in wrapped or injection molded
materials such as polymer matrix composites (PMC’s).

Cracks
Defects such as cracking can lead to catastrophic failure of a material.
Metallography is often used in failure analysis to determine why a material
broke, however, cross sectional analysis is also a very useful technique to
evaluate manufacturing issues which may cause these defects.

Phases
Metal alloys can exhibit different phase (homogenous) regions depending
upon composition and cooling rates. Of interest to the metallographer might
be the distribution, size and shape of these phases. For composite materials,
identification and characteristics of the filler would also be of interest.

Dendrites
By slowly solidifying a molten alloy, it is possible to form a treelike dendritic
structure. Dendrites initially grow as primary arms and depending upon the
cooling rate, composition and agitation, secondary arms grow outward from
the primary arms. Likewise, tertiary arms grow outward from the secondary
arms. Metallographic analysis of this structure would consist of
characterizing the dendrite spacing.

Corrosion
The effects of corrosion can be evaluated by metallographic analysis
techniques in order to determine both the root cause as well as the potential
remedies.

Intergranular Attack
Intergranular corrosion (IGC), also termed intergranular attack (IGA), is a
form of nonuniform corrosion. Corrosion is initiated by inhomogeneities in
the metal and is more pronounced at the grain boundaries when the corrosion
-inhibiting compound becomes depleted. For example, chromium is added to
nickel alloys and austenitic stainless steels to provide corrosion resistance. If
the chromium becomes depleted through the formation of chromium carbide
at the grain boundaries (this process is called sensitization), intergranular
corrosion can occur.

Coating Thickness
Coatings are used to improve the surface properties of materials. Coatings can
improve temperature resistance (plasma coating), increase hardness
(anodizing), provide corrosion protection (galvanized coatings), increase wear
resistance, and provide better thermal expansion adherence for dielectric/
metal interfaces. Metallographic analysis can provide useful information
regarding coating thickness, density, uniformity and the presence of any
defects.

Inclusions
Inclusions are foreign particles that contaminate the metal surface during
rolling or other metal forming processes. Common inclusion particles
include oxides, sulfides or silicates. Inclusions can be characterized by their
shape, size and distribution.


1 9
Weld Analysis
Welding is a process for joining two separate pieces of metal. The most
common welding processes produce localized melting at the areas to be
joined, this fused area is referred to as the bead and has a cast-like structure.
The area or zone adjacent to the bead is also of interest and is known as the
HAZ (heat affected zone). Typically the welded area will have a different
microstructure and therefore different physical and mechanical properties as
compared to the original metals. Analysis can also include evaluating cracks
and interdiffusion of the base metals within the welded area

Solder Joint Integrity
For electronic components, the integrity of the solder joints is very important
for characterizing the reliability of electronic components.

Composites
Composites are engineered materials which contain fillers in a matrix.
Common fillers include ceramic or graphite particles and carbon or ceramic
fibers. These fillers are encased, or cast, into a polymer, metal, or ceramic
matrix. Metallographic analysis of composites includes analyzing the
orientation and distribution of these fillers, voids and any other defects.

Graphite Nodularity
Cast irons are typically characterized by their nodularity (ductile cast iron) or
by their graphite flakes (gray cast iron). Since gray cast irons can eventually
fail due to brittle fracture, ductile nodular cast irons are the preferred
structure. To produce ductile cast irons, magnesium or cerium are added to
the iron melt prior to solidification. Cross-sectional analysis is used to
characterize the melt prior to pouring the entire batch.

Recast
The recast layer is made up of molten metal particles that have been
redeposited onto the surface of the workpiece. Both the HAZ (heat affected
zone) and recast layer can also contain microcracks which could cause stress
failures in critical components.

Carburizing
The most common heat treating process for hardening ferrous alloys is known
as carburizing. The carburizing process involves diffusing carbon into ferrous
alloys at elevated temperatures. By quenching the metal immediately after
carburizing, the surface layer can be hardened. Metallographic analysis, along
with microhardness testing, can reveal details regarding the case hardness and
its depth.

Decarburization
Decarburization is a defect which can occur when carbon is lost at the surface
of a steel when it is heated to high temperatures, especially in hydrogen
atmospheres. This loss of carbon can reduce both the ductility and strength of
the steel. It can also result in hydrogen embrittlement of the steel.

Nitriding
Nitriding is a process for producing a very hard case on strong, tough steels.
The process includes heating the steel at 500-540°C (930-1000°F) in an
ammonia atmosphere for about 50 hours. No additional quenching or heat
treating is required. The Vickers hardness is about 1100 and the case depth is
about 0.4 mm. Nitriding can also improve the steel’s corrosion resistance.

Intergranular Fracture
Intergranular cracking or fracturing is a fracture that occurs along the grain
boundaries of a material. An intergranular fracture can result from improper
heat treating, inclusions or second-phase particles located at grain boundaries,
and high cyclic loading.

Weld Sensitization
Sensitization is a condition where the chromium as an alloy becomes depleted
through the formation of chromium carbide at the grain boundaries. For
welding, sensitization occurs due to slow heating and cooling through a
temperature range specific to the alloy being welded. For example, 300 series
stainless steels form chromium carbide precipitates at the grain boundaries in
the range of 425-475°C.

Flow Line Stress
Flow stress is the stress required to keep a metal flowing or deforming. the
direction of the flow is important.

* Abrasive Sectioning

ABRASIVE SECTIONING
The first step in preparing a specimen for metallographic or microstructural
analysis is to locate the area of interest. Sectioning or cutting is the most
common technique for revealing the area of interest. Proper sectioning has the
following characteristics:
DESIRABLE EFFECTS:
- Flat and cut close to the area of interest
- Minimal microstructural damage

UNDESIRABLE EFFECTS:
- Smeared (plastically deformed) metal
- Heat affected zones (burning during cutting)
- Excessive subsurface damage (cracking in ceramics)
- Damage to secondary phases (e.g. graphite flakes, nodules or grain pull-out)

The goal of any cutting operation is to maximize the desirable effects, while
minimizing the undesirable effects.
Sectioning can be categorized as either abrasive cutting or precision wafer
cutting. Abrasive cutting is generally used for metal specimens and is
accomplished with silicon carbide or alumina abrasives in either a resin or
resin-rubber bond. Proper blade selection is required to minimize burning and
heat generation during cutting, which degrades both the specimen surface as
well as the abrasive blades cutting efficiency. Wafer cutting is achieved with
very thin precision blades. The most common wafering blades are rim-pressed
abrasive blades, in which the abrasive is located along the edge or rim of the
blade. Precision wafering blades most commonly use diamond abrasives,
however, cubic boron nitride (CBN) is also used for cutting samples that react
to dull diamond (e.g. high carbon, heat treated steels cut more effectively with
CBN as compared to diamond). Wafer cutting is especially useful for cutting
electronic materials, ceramics and minerals, bone, composites and even some
metallic materials.

2.1 ABRASIVE BLADE SELECTION GUIDELINES
Selecting the correct abrasive blade is dependent upon the design of the cut-off
machine and, to a large extent, the operator preference. Abrasive blades are
generally characterized by their abrasive type, bond type and hardness.
Determining the correct blade is dependent upon the material or metal hardness
and whether it is a ferrous or a nonferrous metal. In practice, it often comes
down to odor and blade life. Resin/rubber blades smell more because the
rubber will burn slightly during cutting, however resin/rubber blades do not
wear as fast and therefore last longer. On the other hand, resin blades are more
versatile and do not produce a burnt rubber odor, but they do break down
faster. Resin blades also provide a modestly better cut because the cutting
abrasive is continually renewed and thus produces a cleaner cut.
Also note that the traditional “older” technology for producing abrasive blades
resulted in very specialized resin/rubber blades. Finding the proper resin/
rubber hardness, abrasive size, and blade thickness to match the sample
properties and the cutting machine parameter required a lot of testing and
experimentation. Thus, in the past, resin/rubber blades had been more popular
in the US market; however, in more recent years as resins have improved, there
has been more of a trend towards resin bonded abrasives. Conversely, resin
bonded blades have typically been more widely used in the European and
Asian markets for quite some time.

 ABRASIVE CUTTING PROCESS DESCRIPTION
Abrasive sectioning has primarily been used for sectioning ductile materials.
Examples include metals, plastics, polymer matrix composites, metal matrix
composites, plastics and rubbers. The proper selection of an abrasive blade
requires an understanding of the relationship between the abrasive particle,
abrasive bonding and the specimen properties.
Abrasive Type - Today's high performance abrasive blades use alumina or
silicon carbide abrasives. Alumina is a moderately hard and relatively tough
abrasive which makes it ideal for cutting ferrous metals. Silicon carbide is a
very hard abrasive which fractures and cleaves very easily. Thus, silicon
carbide is a self-sharpening abrasive and is more commonly used for cutting
nonferrous metals.
Bonding Material - The hardness and wear characteristics of the sample
determine which resin system is best-suited for abrasive cutting. In general,
the optimum bonding material is one that breaks down at the same rate as the
abrasive dulls; thus, exposing new abrasives for the most efficient and effective
cutting operation.

RECOMMENDED CUTTING PROCEDURES
- Select the appropriate abrasive blade.
- Secure specimen. Improper clamping may result in blade and/or specimen
damage.
- Check coolant level and replace when low or excessively dirty. Note abrasive
blades break down during cutting and thus produce a significant amount of
debris.
- Allow the abrasive blade to reach its operating speed before beginning the
cut.
- A steady force or light pulsing action will produce the best cuts and minimize
blade wear characteristics, as well as maintain sample integrity (no burning).
- When sectioning materials with coatings, orient the specimen so that the
blade is cutting into the coating and exiting out of the base material, thereby
keeping the coating in compression.

 CUTTING FLUIDS
Lubrication and swarf removal during abrasive cutting and diamond wafer
cutting are required in order to minimize damage to the specimen. For some
older abrasive cutters, the proper cutting fluid can also have the added benefit
of coating cast iron bases and the fixtures in order to reduce or eliminate
corrosion.
TIP: Most metallographic abrasive cutters have a hood, which can produce a
corrosive humidity chamber when not in use. In order to reduce these
corrosive effects, keep the hood open when not in use.
Abrasive Cutting Fluid - The ideal cutting fluid for abrasive cutting is one that
removes the cutting swarf and degraded abrasive blade material. It should have
a relatively high flash point because of the sparks produced during abrasive
sectioning.

ABRASIVE SECTIONING TROUBLESHOOTING
The most common problems with abrasive cutting include broken abrasive
blades and cracked or burnt samples.

* Precision Wafer Sectioning
Precision wafer cutting is used for sectioning very delicate samples or for
sectioning a sample to a very precise location. Precision wafering saws
typically have micrometers for precise alignment and positioning of the
sample, and have variable loading and cutting speed control (see Figure 3-1).

WAFERING BLADE CHARACTERISTICS
In order to minimize cutting damage, precision wafer cutting most frequently
uses diamond wafering blades, however, for some materials the use of cubic
boron nitride (CBN) is more efficient. In addition, optimal wafer cutting is
accomplished by maximizing the abrasive concentration and abrasive size, as
well as choosing the most appropriate cutting speed and load. Table III
provides some general guidelines and parameters for precision sectioning a
variety of materials.

WAFERING BLADE CHARACTERISTICS
In order to minimize cutting damage, precision wafer cutting most frequently
uses diamond wafering blades, however, for some materials the use of cubic
boron nitride (CBN) is more efficient. In addition, optimal wafer cutting is
accomplished by maximizing the abrasive concentration and abrasive size, as
well as choosing the most appropriate cutting speed and load. Table III
provides some general guidelines and parameters for precision sectioning a
variety of materials.

In some cases, precision cutting requires a coarser grit wafering blade. Usually
the coarsest standard blade uses 120 grit abrasive particles. For metallographic
applications, coarse abrasives are mostly associated with electroplated blades
(Figure 3-3a). The main characteristic of coarse electroplated blades is that the
abrasive has a much higher, or rougher, profile. The advantage of this higher
profile is that the blade does not “gum up” when cutting softer materials such
as bone, plastics and rubbery materials.

Although less common, thin resin-rubber abrasive blades can be used for cutting
on precision wafering saws (Figure 3-3b). For cutting with abrasive blades on
precision wafer saws, set the speed of the saw to at least 3500 rpm. Note that
abrasive blades create significantly more debris which requires changing out of
the cutting fluid more frequently.

Perhaps the most important parameter for precision sectioning is the abrasive
size. Similar to grinding and polishing, finer abrasives produce less damage.
For extremely brittle materials, finer abrasives are required to minimize and
manage the damage produced during sectioning. Sectioning with a fine
abrasive wafering blade is often the only way that a specimen can be cut so that
the final polished specimen represents the true microstructure. Examples
include: silicon computer chips, gallium arsenide, brittle glasses, ceramic
composites, and boron-graphite composites. Figures 3-4a and 3-4b compare
the effects of cutting with a fine grit blade vs. a standard medium grit blade for
sectioning a boron graphite golf shaft. As can be seen, the fine grit blade
produces significantly less damage to the boron fibers.

The second most important blade characteristic is the abrasive concentration
because it directly affects the load which is applied during cutting. For example,
brittle materials such as ceramics require higher effective loads to efficiently
section; whereas, ductile materials such as metals require a higher abrasive
concentration in order to have more cutting points. The result is that low
concentration blades are recommended for sectioning hard brittle materials such
as ceramics and high concentration blades are recommended for ductile materials
containing a large fraction of metal or plastic.

The wafering blade bonding matrix can also significantly affect a blade’s
cutting performance. Metal pressed wafering blades require periodic dressing
in order to maintain performance. A common misconception is that the cutting
rates for these blades decrease because the diamond or abrasive is being
"pulled out" of the blade. In reality, the metal bond is primarily smearing over
the abrasive and "blinding" the cutting edge of the abrasive. With periodic
dressing, using a ceramic abrasive encased in a relatively soft matrix
(Figure 3-5), this smeared material is removed and the cutting rate restored.
Figure 3-6 shows the effect of dressing a standard grit, low concentration
diamond blade for cutting a very hard material such as silicon nitride. Without
dressing the blade, the cut rate significantly decreases after each subsequent
cut. After dressing the blade, the sample once again cuts like a new blade.
Note it is highly recommended that a dressing fixture be used for conditioning
or dressing the wafering blades in order to reduce the risk of breaking or
chipping the wafering blades (Figure 3-7). Blade dressing is also accomplished
at low speeds (<300 rpm) and at light loads (<200 grams).

 CUTTING PARAMETERS
Most wafer cutting is done at speeds between 50 rpm and 5000 rpm with loads
varying from 10-1000 grams. Generally, harder specimens are cut at higher
loads and speeds (e.g. ceramics and minerals) and more brittle specimens are
cut at lower loads and speeds (e.g. electronic silicon substrates) (see Table IV).
It is interesting to note that the cutting efficiency for sectioning hard/tough
ceramics improves at higher speeds and higher loads. Figure 3.8 compares the
resulting surface finish for sectioning partially stabilized zirconia at a low
speed/low load (Figure 3-8a) vs. cutting at a higher load/higher speed (Figure
3-8b). As can be seen, partially stabilized zirconia has less fracturing and grain
pull out after sectioning at higher speeds and loads. This observation may
seem counter intuitive, however for sectioning hard/tough ceramics, high
cutting speeds and loads result in producing a crack that propagates in the
direction of the cut instead of laterally into the specimen.


For wafer cutting it is recommended that a cutting fluid be used. The
characteristics of a good cutting fluid include:
- Removes and suspends the cutting swarf
- Lubricates the blade and sample
- Reduces corrosion of the sample, blade and cutting machine parts

  In general, cutting fluids are either water-based or oil-based (Figure 3-9). Water-
based cutting fluids are the most common because they are easier to clean;
whereas, oil-based cutting fluids typically provide more lubrication.

 RECOMMENDED WAFER CUTTING PROCEDURES
- Prior to cutting the sample, condition or dress the wafering blade with the
appropriate dressing stick.
- Clamp the specimen sufficiently so that the sample does not shift during
cutting. If appropriate, clamp both sides of the specimen in order to
eliminate the cutting burr which can form at the end of the cut.
- For brittle materials clamp the specimen with a rubber pad to absorb vibration
from the cutting operation.
- Begin the cut with a lower force in order to set the blade cutting kerf.
- Orient the specimen so that it is cut through the smallest cross section.
- For samples with coatings, keep the coatings in compression by sectioning
through the coating and into the substrate material.
- Use largest appropriate blade flanges to prevent the blade from wobbling or
flexing during cutting.
- Reduce the force toward the end of the cut for brittle specimens
- Use the appropriate cutting fluid.

* Specimen Mounting

SPECIMEN MOUNTING
The primary reasons for specimen mounting are to better hold the part to be
ground and polished, and to provide protection to the edges of the specimen.
Secondarily, mounted specimens are easier to fixture into automated machines
or to hold manually. The orientation of the specimen can also be more easily
controlled by fixturing it and then setting it in place via mounting.
Metallographic mounting is accomplished by casting the specimen into a
castable plastic material or by compression mounting the plastic under pressure
and temperature.

4.1 CASTABLE MOUNTING
Castable resins are monomer resins which utilize a catalyst or hardener for
polymerization. Polymerization results in cross-linking of the polymer to form
a relatively hard mount. Castable resins also have the advantage of
simultaneously mounting multiple samples at one time for increased
throughput. A number of resin systems (Figure 4-1) are used for
metallographic mounting and include:
- Epoxy resins
- Acrylic (castable) resins
- Polyester (clear) resins

Epoxy Resins
The most common and best performing castable resins are epoxy based (Figure
4-2). Epoxy resins are typically two-part systems consisting of a resin and a
catalyst (hardener). Mixing ratio's vary from ten-parts resin with one-part
hardener to five-parts resin with one-part by weight of hardener. The
advantages of mounting with epoxy resins include:
- Low shrinkage
- Relatively clear
- Relatively low exotherms
- Excellent adhesion
- Excellent chemical resistance
- Good hardness
- Relatively inexpensive


Epoxy curing times are dependent upon a number of variables including:
- Volume of mounting resin (larger mounts cure faster).
- Thermal mass of specimen (larger specimens absorb heat and therefore
require longer curing time).
- Specimen material properties.
- Initial resin temperature (higher temperatures cure faster).
- Ambient temperature (higher temperatures cure faster).
- Relative humidity and shelf life (absorption of water degrades resin and
shortens shelf life).
- Mounting molds (plastic, phenolic rings and rubber absorb heat differently).
As a general rule, curing times can vary from 30 minutes to 2 hours for fast
curing epoxies up to 24 hours for slower curing epoxies. For metallographic
epoxies to grind properly, the hardness needs to be at least a Shore D80. Note
that epoxy resins typically will continue to harden over a longer period of time
(maximum hardness, Shore D90).
In some cases, the curing time and temperature may need to be controlled to
compensate for the above variables. For example, an 8-hour resin system can
be cured in 30-45 minutes by preheating the resin to approximately 120°F
(50°C) prior to mixing and then curing at room temperature. This procedure
initiates the catalytic reaction sooner; however, this may also increase the
maximum exotherm temperature.

TIP: Preheat the specimen to initiate the epoxy resin curing at the surface of
the mount and thus have the epoxy shrink towards the sample for better edge
retention.
Conversely, the resin curing cycle can be slowed or reduced by decreasing the
curing temperature by forcing air over the curing mounts (fume hood or fan),
placing the mounts into a water bath, or curing in a refrigerator. In these cases,
care must be taken to not stop the reaction; however if this does occur or the
resin is too soft after curing, heating it to 100-120°F for several hours should
push the reaction to completion and the mount should be hard after cooling to
room temperature.
Table VII lists the relative properties for several metallographic epoxy resin
systems.

Acrylic Castable Resins
Castable acrylics are easy to use and are very robust (Figure 4-3). The main
advantage of mounting with castable acrylics is the fast curing time.
Depending upon the mixing ratio, castable acrylic mounts are typically ready to
use within 8-15 minutes. Also unlike epoxy resins, the ratio of the various
acrylic parts (powder to liquid) can be altered by up to 25% with no adverse
effect to the final properties of the mount. This is because both the liquid and
powder are acrylics with various additives and curing agents. By varying the
ratio of the liquid to powder, the curing time and viscosity can be altered.
Note: the powder contains a catalyst that reacts with the liquid hardener to start
the curing process. Fillers are added to increase hardness and to reduce
shrinkage

Characteristics of Castable Acrylics (see Table VIII) includes:
- Rapid mounting
- Very repeatable and consistent mounts
- Moderate shrinkage
- Good hardness
- Semi-transparent
- High odor
TIP: Acrylics can be submerged into a water bath during curing. This
reduces the exotherm heat and thus reduces the shrinkage of the mount at the
specimen interface. A secondary advantage is that the water absorbs the odor.

Polyester Castable Resins
Polyesters are typically used when a very clear mount is required. Polyester
resins are also useful for mounting parts for display. In this case, the part
appears suspended in the plastic. The procedure for molding samples for
display is to first determine the mixing ratio of the resin to hardener (catalyst).
This ratio is variable depending upon the mass of the casting (Table IX)

For larger volumes, the amount of hardener needs to be reduced significantly.
The procedure for suspending the sample in the mount is to pour an initial layer
and allow it to pot or gel (do not let it fully cure). The object or specimen is
then placed on the initial rubbery polyester layer and another layer of the liquid
polyester is poured. Multiple layers can be poured in this fashion if required.
Characteristics of Polyester include:
- Very clear (water clear)
- High odor
- Best resin system for making large castings
Polyester resins are similar to acrylics and can be submerged into water during
the curing cycle in order to reduce the exotherm temperature and shrinkage.

CASTABLE MOUNTING PROCEDURES
- Clean and thoroughly dry specimens to remove cutting and handling residues.
- Remove debris from molding cups.
- Apply thin coat of mold release compound to molding cup.
- Center specimen in molding cup.
- Accurately measure resin and hardener.
- Mix thoroughly (gentle mixing to avoid producing excessive air bubbles).
- To reduce air bubbles, pull a vacuum on the specimen before pouring the
resin. After pouring the resin over the specimen, cure at room pressure or
apply pressure in an autoclave chamber.
TIP: Before mixing, preheat resin, hardener and specimen to 85°F (30°C) to
expedite curing cycle
Note: this will also increase maximum exotherm temperature.

Vacuum/Pressure Mounting
Vacuum impregnation is a very useful technique used to fill in pores or voids
prior to specimen preparation. It is highly useful for thermal spray coatings
and other porous samples.
The most effective technique is to pour the resin under vacuum and/or apply
pressure during the curing cycle (advantages - better infiltration of pores and
cracks, more transparent mounts, and fewer air bubbles) (see Figure 4-5).

For porous or cracked specimens, the resin can aid in supporting these features.
Filling these voids can be difficult depending upon their size, with the smaller
voids being much more difficult to impregnate than larger voids. This arises
mainly because of the compressibility and volume of air within the void. By
applying a vacuum to the specimen and pouring while under vacuum the total
pressure of this air can be reduced significantly. Subsequent curing at increased
pressures will force (or push) the resin into the voids. Note that the vacuum
time on both the resin and specimen should be kept to a minimum in order to
minimize degassing of the resin.

PV = nRT (gas law)
P - Pressure
V - Volume
T - Temperature
V(bubble size) = nRT
P
Thus in order to decrease the air bubble size, impregnate at low pressures and
cure at higher pressures.
Recommended Procedure:
1. Place mold and sample into impregnation chamber
2. Mix castable mounting resin
3. Place cover on chamber and pull vacuum
4. Pour resin into mount
5. Slowly increase the pressure
6. Allow the mount to cure at room pressure or apply an external pressure.
TIP: Do not pull vacuum for more than 60 seconds. Extended vacuum causes
the dissolved gases in the liquid resin to degass and bubble (similar to opening
up a carbonated beverage bottle).
TIP: To reduce the curing time, preheat resin, hardener and specimen to 85°F
(30°C).
Note: this will also increase maximum exotherm.
TIP: Slight preheating of the epoxy will also reduce the viscosity of the resin
and allow it to flow better.

CASTABLE MOUNTING MISCELLANEOUS
Figures 4-6 to 4-8 show a variety of accessories used with castable mounting,
ranging from mounting molds and mounting clips to mixing cups and storage
containers. Table X provides a description of each.

CASTABLE MOUNTING TROUBLESHOOTING
In general, acrylics are the easiest and most robust castable mounting materials
to use. Epoxies are very useful; however, complete mixing and the proper
resin-to-hardener ratio is very important. Polyesters, especially for larger
casting, may require some trial and error testing prior to mounting one-of-a-
kind samples.

COMPRESSION MOUNTING
Compression mounting is a very useful mounting technique which can provide
better specimen edge retention compared to castable mounting resins.
Compression mounting resins are available in different colors and with various
fillers to improve hardness or conductivity (Figure 4-9).
Several compression mounting characteristics include:
- Convenient means to hold the specimen
- Provides a standard format to mount multiple specimens
- Protects edges
- Provides proper specimen orientation
- Provides the ability to label and store the specimens

  Compression mounts are quick and easy to produce, requiring several minutes
to cure at the appropriate mounting temperature. Most of the time required
occurs during the heating and cooling cycles. When choosing a compression
mounting machine, the most important features include its maximum heating
temperature and how intimately the heater and water cooler are connected to
the mold assembly. The better compression mounting machines have heaters
which can reach temperatures of at least 250-300°C ( 480-575°F). For faster
turn around time, water cooling is essential (see Figure 4-10).

The primary compression mounting resins include:
- Phenolic Resins (standard colors are black, red and green) (see Figure 4-11)
- Acrylic Resins (clear)
- Diallyl Phthalate Resins (blue and black) (Figure 4-12)
- Epoxy Resins (glass-filled) (Figure 4-13)
- Conductive Resins (phenolics with copper or graphite filler) (see Figure 4-14)

  COMPRESSION MOUNTING RESIN PROPERTIES
There are a variety of compression mounting materials. The two main classes
of compression mounting materials are thermoset and thermoplastics.
Thermoset resins require heat and pressure to cross-link the polymer and the
reaction is irreversible. Thermoplastic, on the other hand, can theoretically be
remelted. Table XIIa provides a relative comparison of the most common
compression mounting resins, and Table XIIb provides more specific
information for the various compression mounting resins.
TIP: Compression mounting at higher then the recommended minimum
temperature generally improves the properties of the mount.
TIP: A useful tip for marking or identifying a specimen is to mold the label
inside of the mount (Figure 4-15). If the entire mount is an acrylic, just place
the label on top the mount and cover it with a little acrylic powder. To label
other compression mounting resins, add a thin layer of acrylic over the base
mounting material and then position the label on this layer. Finish off the
mount with another layer of acrylic.

Phenolics
In general, phenolics are used because of their relatively low cost. In addition,
phenolics are available in a variety of colors (Figure 4-16).
Figure 4-16 Phenolic resins are available in a variety of colors.
TIP: Use different color phenolics to color code jobs, specimen types, or for
different testing dates. For example, changing the phenolic color each month
will show which samples or jobs are getting old.
TIP: If the color dye in the mount bleeds out when rinsing with an alcohol,
this is an indication that the mount was not cured either at a high enough
temperature or for the proper length of time (see Figure 4-17).

Acrylics
The main application for compression mounting acrylics is for their excellent
clarity. This is particularly important for locating a specific feature within the
specimen mount (Figure 4-18).


TIP: A common problem, know as the “cotton ball” effect, can occur with
thermoplastic resins if they are not heated and held for a sufficiently long
enough time to completely melt the plastic. For acrylic resins, the unmelted
resin takes the appearance of a cotton ball in the middle of the mount. To
correct this problem, simply put the mount back into the mounting press and
either increase the time or temperature of the press. Eventually this will
eliminate the “cotton ball” (see Figure 4-19).

Epoxies / Diallyl Phthalates
Glass-filled epoxies and diallyl phthalates are compression mounting resins
used to provide a harder mounting support edge next to the specimen (see
Figure 4-20). These resins are commonly used to support the edges of
coatings, heat treated samples and other specimens requiring better flatness.
Figure 4-21 shows the polished interface between a glass-filled epoxy and
tungsten carbide specimen. Note that there is no noticeable gap between the
specimen and the mounting material, therefore showing that glass-filled
epoxies provide excellent support to the specimen edge even for extremely
hard specimens.


TIP: Epoxies (glass-filled) and diallyl phthalates are significantly more
expensive then phenolic and acrylics. In order to reduce the cost of these
mounts, they can be layered with a lower cost mounting compound such as a
phenolic. The technique is to place a sufficiently thick enough layer of the glass-filled epoxy or diallyl phthalate around the specimen in order to
compensate for any grinding loss. The rest of the mount can then be supported
with a lower cost compression mounting compound such as a phenolic. Red
phenolics are used frequently for this technique (Figure 4-22).

Specialized Compression Mounting Resins
With the addition of fillers such as graphite or copper, the compression
mounting compounds can be made conductive (Figure 4-23). Conductive
mounts are used in scanning electron microscopes (SEM) to prevent the
specimen from building up a charge. Conductive mounts are also used for
specimens requiring electrolytic etching or polishing.

COMPRESSION MOUNTING PROCEDURES
- Clean specimens to remove cutting and handling residues
- Remove debris from mold assembly
- Apply thin coat of mold release compound to mold assembly
6 5
- Raise mold ram to up position
- Center 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
TIP: Preheat resin and sample to 95°F (35°C) to expedite the initial heating
process and for increasing throughput.

* Abrasive Grinding

ABRASIVE GRINDING
In most cases, the specimen surface and subsurface are damaged after cutting
and sectioning. The depth or degree of damage is very dependent on how the
material was cut. The purpose of abrasive grinding is to remove this damage
and to restore the microstructural integrity of the specimen for accurate
analysis. It is also important to realize that it is possible to create more damage
in grinding than in sectioning. In other words, it is better to properly cut the
sample as close as possible to the area of interest using the correct abrasive or
wafering blades as opposed to grinding with very coarse abrasives. For
metallographic specimen preparation, silicon carbide, zirconia, alumina and
diamond are the most commonly used abrasives (Figure 5-1)

Proper abrasive grinding is dependent to various degrees upon the following
parameters:
- Abrasive type
- Abrasive bond
- Grinding speeds
- Grinding loads
- Lubrication

  ABRASIVES USED FOR GRINDING
The following description offers a more detailed explanation of these abrasive
grinding variables. Perhaps the most significant variable is the abrasive and
how it interacts with the specimen. The properties of the more commonly used
abrasives for metallographic cutting, grinding and polishing are shown in Table
XIV.

Silicon Carbide
Silicon carbide (SiC) is a manufactured abrasive produced by a high
temperature reaction between silica and carbon. It has a hexagonal-
rhombohedral crystal structure and has a hardness of approximately 2500 HK.
It is an ideal abrasive for cutting and grinding because of its high hardness and
sharp edges. It is also somewhat brittle, and therefore it cleaves easily to
produce sharp new edges (self-sharpening). SiC is an excellent abrasive for
maximizing cutting rates while minimizing surface and subsurface damage. For
metallographic preparation, SiC abrasives are used in abrasive blades and in
coated abrasive grinding papers ranging from very coarse 60 grit to very fine
1200 (P4000) grit abrasive sizes.
Bonded or coated abrasive papers of SiC (Figure 5-2) are designed so that the
abrasive will have a large number of cutting points (negative abrasive rank
angle). This is achieved by aligning the abrasive particles approximately
Normal to the backing. Note: coated abrasives are not quite coplanar, however
SiC papers, produce excellent cut rates (stock removal) and produce minimal
damage.

Grinding with SiC grinding papers is the most common and repeatable process
for obtaining consistent stock removal for rough grinding of metals. SiC
abrasives are sized or classified by grit size, where the smaller grit number
represents coarser abrasive sizes.
Also note that the European grading system is slightly different than the U.S.
grading system. Simply put, both systems are related to the number of
openings in a metal mesh screen. The primary difference is when the size of
the openings approaches the size of the metal wire. For the European grading
system, the size of the wire is not taken into account, whereas, the ANSI or
U.S. grit size compensates for the wire size. Thus for the finer grit sizes, the
European numbers can be significantly larger. Proper classification or
identification of the European grading system should include the letter “P” in
front of the grit number.

As can be seen in Table XVI, the surface roughness significantly decreases
when grinding with finer silicon carbide papers. In particular, there is a large
improvement in surface roughness using finer than 600 (P1200) grit grinding
papers. It should be noted that the process used to manufacture metallographic
papers ranging from 60 grit to 600 (P1200) grit is done by coating the abrasive
on the grinding paper by a process known as electrostatic discharge. For
electrostatic discharge, the abrasive is charged by passing it over a high-voltage
wire. This process charges the abrasive particles and orients them so that the
sharp edge of the abrasive is facing up. These charged abrasives are then coated
onto a paper backed adhesive and cured in an oven.
For the finer abrasives ranging from 800 (P2400) to 1200 (P4000) grit,
metallographic SiC abrasive papers are produced by a completely different
manufacturing process. For these finer abrasive sizes, the manufacturing
process is accomplished with a slurry coat process. For slurry coating, the
abrasive is mixed into an epoxy binder to form a slurry. This slurry is then
uniformly spread onto the paper backing using a knife blade. The resulting
abrasive exposure is much lower for slurry coating than by electrostatic
deposition. The result is that metallographic fine grit papers produce a much
finer surface finish as compared to industrial or other commercially
manufactured fine grit abrasive papers.

Grinding characteristics of silicon carbide abrasives
Grinding with SiC abrasives produces very repeatable and consistent results.
In general, grinding papers are typically used once and thrown away, thus they
do not change with time as is the case for alternative abrasive grinding surfaces
such as diamond impregnated grinding surfaces.
The following figures show the effects of grinding with SiC and the effects that
abrasive size, applied load, and grinding times have on the performance of SiC
grinding papers.

Figure 5-4 shows the effect that abrasive size has on stock removal for a 1-inch
diameter tool steel specimen. It is not surprising that coarser silicon carbide
abrasives remove more material; however, as previously noted, there is a
significant drop in removal rates between 600 (P1200) grit and 800 (P2400) grit
grinding papers. Charts such as these can also be used to determine how large a
step can be made between grit sizes and still remove sufficient material in order
to eliminate the damage from the previous step.
Figure 5-5 shows how the silicon carbide paper breaks down and loses its cut
rate over time. For the tool steel specimen, removal rates drop to half within a
couple of minutes. This chart also compares the results of grinding at 10 lbs
per specimen vs. 5 lbs per specimen. Interestingly, at higher loads, the initial
grinding rate is greater; however, after the initial minute of grinding, there is no
advantage to grinding rates at higher forces.

Figure 5-6 illustrates the optimum removal for silicon carbide papers for a 1-
inch diameter tool steel specimen at approximately 15 lbs force. Note that
most procedures are written for forces of 5 lbs per sample. There are two
reasons why lower forces are suggested: (a) the optimum load has not been
previously studied and (b) many automated machines can only apply a
maximum of 60-90 lbs force due to air compressor limitations.


Alumina
Figure 5-7 Calcined alumina abrasive.
Alumina is a naturally occurring mineral (Bauxite) (see Figure 5-7). It exists in
either the softer gamma (Mohs 8) or harder alpha (Mohs 9) phase. Alumina
abrasives are used primarily as final polishing abrasives because of their high
hardness and durability. Unlike SiC abrasives, alumina is readily classified or
sized to submicron or colloidal particles (< 1 micron).
Note that larger coated or bonded grit size papers of alumina are also
commercially available. Alumina grinding papers are an excellent alternative
to grinding with SiC abrasives, primarily because initial grinding can be
obtained with a much finer abrasive. Therefore reducing the number of
grinding steps. For example, for planarization of most metals one 600 (P1200)
grit alumina paper can replace grinding with 240 (P220), 320 (P360), 400
(P800) and 600 (P1200) grit SiC papers. Alumina abrasives also do not fracture
as easily as SiC so they produce less embedded abrasives in soft materials.



Diamond
Diamond is the hardest material known to man (Mohs 10, 8000 HV). It has a
cubic crystal structure, and is available either as a natural or an artificial
product. Although diamond would be ideal for coarse grinding, its price makes
it a very cost-prohibitive coarse grinding material for anything except hard
ceramics and glass (see Figure 5-8).
7 5
Figure 5-8 Blocky monocrystalline diamond.
For metallographic applications, both monocrystalline and polycrystalline
diamond are used, however polycrystalline diamond has a number of advantages
over monocrystalline diamond, especially for the finer micron sizes. These
advantages include:
- Higher cutting rates
- Very uniform surface finish
- More uniform particle size distribution
- Higher removal rates (self-sharpening abrasives)
- Harder/tougher particles
- Blocky shaped
- Hexagonal microcrystallites (equally hard in all directions)
- Extremely rough surface (more cutting points)
- Surface area 300% greater than monocrystalline diamond
- No abrasion-resistant directionality (abrasion independent of particle
orientation)

Figure 5-9 shows polycrystalline diamond has a higher cut rate as compared to
monocrystalline diamond for sizes up to 15 micron. For coarser diamond the
cut rates do not differ significantly between polycrystalline and
monocrystalline diamond.
In addition to higher cut rates, polycrystalline diamond also produces a finer
surface finish. From Figure 5-10, the surface roughness, Ra, for rough
polishing a low carbon steel with a 3 micron diamond was 0.03 micron for
polycrystalline diamond and 0.09 micron for monocrystalline diamond. As
demonstrated by the Rq value (0.012 micron for monocrystalline diamond,
0.04 micron polycrystalline diamond), the average depth of the scratches is also
much deeper for monocrystalline diamond as compared to the PC diamond.

Higher magnification characterization of polycrystalline and monocrystalline
diamond shows that polycrystalline diamond has a rougher surface with a
larger number of smaller cutting points (Figure 5-11). Polycrystalline diamond
also has higher friability due to its ability to cleave along these microcrystalline
planes. In general, higher-friability diamonds produce better surface finishes.

Zircon
Zircon, or zirconium silicate, is another less common abrasive used for coarse
grinding (Figure 5-12). It is a very tough abrasive, so it lasts longer, however it
is generally not as hard or sharp, and thus requires higher pressures to be
effective. Typically 60 or 120 grit sizes have been found to be the most useful
grain sizes for metallographic grinding with zircon.


ABRASIVE BONDING
5.2.1 Fixed Abrasive Grinding
For fixed abrasive (two-body) grinding disks or surfaces, the abrasive is rigidly
held in place (Figure 5-13). Common bonding materials include:
- Nickel plating
- Polymer / epoxy resins
- Soft lapping plates (tin, zinc or lead alloys)

 The characteristic features for grinding with fixed abrasives are high, or
aggressive, removal rates with the potential for significant surface and
7 9
subsurface damage. Common fixed abrasive grinding surfaces are bonded
diamond disks, silicon carbide / alumina papers and lapping films.
Application (Fixed-abrasive Grinding)
- Start with the finest abrasive possible (typically 240 (P220) or 320 (P360)
grit abrasive paper for SiC papers, 600 (P1200) grit alumina paper, or 30-45
micron diamond).
Note: Only use coarser grits or larger abrasives for very heavy stock removal
and be careful about the additional damage produced.
- Apply lubricant to abrasive surface. Water is the most common lubricant;
however, light oils can be used for water-sensitive samples.
- Clean specimens and holder thoroughly before proceeding to the next finer
abrasive step.
5.2.2 Free Abrasive Grinding
For free abrasive (three-body) grinding, the abrasive is not rigidly held in place
and is allowed to freely move between the specimen and the working plate
(Figure 5-14). This abrasive action leads to very non-aggressive removal with
the flatness of the specimen matching that of the base lapping plate surface.
Free abrasive grinding is commonly used for lapping hard materials on hard
lapping surfaces such as cast iron. This is not a very common metallographic
specimen preparation technique.


Semi-fixed Abrasive Grinding
Semi-fixed abrasive grinding is a hybrid process which uses a rough, or an
interrupted, grinding surface (Figure 5-15). The abrasive is applied in the same
fashion as free abrasive grinding, however the abrasive can become
temporarily fixed in the interrupted surface, thereby providing a more
aggressive grinding action.
8 0
Figure 5-15 Abrasive is temporarily held in place for semi-fixed abrasive grinding.
The characteristic grinding features of semi-fixed abrasive grinding includes:
- Good removal rates
- Medium abrasive exposure (less damaging)
- Excellent for grinding/polishing brittle materials
- Rechargeable abrasive
Application (CERMESH metal mesh cloth) (see Figure 5-16)
- Apply CERMESH metal mesh cloth to flat base surface
- Pre-charge CERMESH metal mesh cloth with DIAMAT polycrystalline
diamond
- To avoid tearing the cloth, begin initial grinding at 50% of the force to
planarize the specimen(s) with the metal mesh cloth
- Ramp-up force gradually
- Add abrasive as required
- Rinse CERMESH metal mesh cloth with water at the end of the grinding
cycle to remove grinding swarf debris
Figure 5-16 CERMESH interrupted metal mesh disk for semi-fixed abrasive grinding.
TIP: To apply adhesive-backed abrasives, peel back protective paper at one
corner and align it with the working wheel surface. Continue to pull the
backing liner with one hand while applying the paper/film with the other hand

 ROUGH GRINDING PARAMETERS
Successful grinding is also a function of the following parameters:
1. Grinding pressure
2. Relative velocities and grinding direction
3. Machine considerations
The machining parameters which affect the preparation of metallographic
specimens include grinding/polishing pressure, relative velocity distribution
between the specimen and grinding surface, and the direction of grinding/
polishing action relative to the specimen. In general, grinding removal rates
are described by Preston’s Law. This relationship states that removal rates are
proportional to the grinding velocity and applied pressure.
PRESTON’S LAW
Removal Rate = kPV
k - Preston’s constant
P - Polishing pressure
V - Polishing velocity
5.3.1 Grinding Pressure
Grinding/polishing pressure is dependent upon the applied force (pounds or
Newtons) and the area of the specimen and mounting material. Pressure is
defined as the Force/Area (psi, N/m2 or Pa). For specimens significantly harder
than the mounting compound, the pressure is better defined as the force divided
by the specimen surface area. Thus, for large hard specimens, a higher
grinding/polishing pressure increases stock removal rates, however, higher
pressure can also increase the amount of surface and subsurface damage.
Note: Increasing the grinding force can extend the life of the SiC grinding
papers as the abrasive grains dull and cut rates decrease.
Higher grinding/polishing pressures can also generate additional frictional heat
which may actually be beneficial for the chemical mechanical polishing (CMP)
of ceramics, minerals and composites. Likewise for extremely friable
specimens such as nodular cast iron, higher pressures and lower relative
velocity distributions can aid in retaining inclusions and secondary phases.

Relative Velocity
Current grinding/polishing machines are designed so that the specimens are
mounted in a disk holder and machined on an abrasive grinding disk surface.
This disk-on-disk rotation allows for a variable velocity distribution depending
upon the specimen head speed relative to the abrasive wheel base speed (see
Figure 5-17).
Figure 5-17 Automated polishers using disk on disk rotation.
For disk-on-disk rotation, the relative direction of the specimen disk and the
grinding disk are defined as operating in either the complementary direction
(same rotation) or contra direction (opposite rotation) (see Figure 5-18).

For high stock removal, a slower head speed relative to a higher base speed
produces the most aggressive grinding/polishing operation (Figure 5-19a). As
can be seen from Figure 5-19, the relative velocity is very high at the outside
edge of the working wheel when the specimen and abrasive are traveling in the
opposite, or contra, direction. Conversely, at the inside diameter of the
abrasive working wheel, where the specimen is traveling in the same direction
as the abrasive, the relative velocities cancel each other and are at a minimum.
This “hammering” action of contra rotation produces very aggressive grinding
rates and can possibly damage the brittle components, inclusions or the more
sensitive features of the specimen. Another drawback to high velocity
distributions is that the abrasive (especially SiC papers) may not breakdown
uniformly. This effect can result in nonuniform removal across the specimen
surface.

Operating the specimen power head in the same direction and at the same rpm
as the abrasive working wheel produces a condition having a minimal velocity
distribution (Figure 5-19b). This condition is known as grinding / polishing in
the complementary direction, and provides the best condition for retaining
inclusions and brittle phases, as well as obtaining a uniform finish and flatness
across the entire specimen. The main disadvantage of operating at the same
speeds in the complementary direction is that stock removal rates are relatively
low.

In practice for most common materials, matching the head and base speed at as
high a speed as possible is the best condition for obtaining a uniform and flat
surface which also minimizes damage to the critical features of the
microstructure. Matching the head and base speed is also more critical when
coarse grinding is accomplished with individual specimen loading using semi-
automated machines. For example, grinding with 180 grit or coarser paper in
the contra direction (-200 rpm head/ 200 rpm base) will result in a wedge being
ground across the sample when using individually loaded pistons (see
illustration in Figure 5-20). Conversely, if the head is run at 200 rpm and the
base is run at 200 rpm in the same direction, the sample remains squarer.

Note: for certain materials where chemical mechanical polishing (CMP) is
recommended, high velocity distributions can provide some frictional heat
which can enhance the chemical polishing action. For CMP polishing, high
speeds and high relative velocity distributions can be useful as long as brittle
phases are not present (e.g. monolithic ceramics such as silicon nitride and
alumina). Figure 5-21 shows the relative guideline charts for planar grinding
various classes of materials.

The orientation of the specimen can also have a significant impact on the
preparation results, especially for specimens with coatings. In general, when
grinding and polishing materials with coatings, the coating component should
be kept in compression. In other words, the direction of the abrasive should be
through the coating and into the substrate.

Machine Considerations
There are a number of considerations that need to be considered when
determining the type of machine to be used for metallographic specimen
preparation. A few of the more significant include:
1. Size of the sample
2. Material properties
3. How the sample was cut or sectioned
4. Flatness required
5. Stock removal requirements or limitations
6. Number of samples

   Sample Size Limitation: As a general rule, the largest specimen size that can be
effectively ground or polished is approximately 1/3 the diameter of the working
wheel. This limitation is due to the changing velocities which occur as the
sample crosses the center of the working wheel (change in the direction of
grinding).
Note: Grinding samples too large for the working wheel can create significant
safety issues which can result in personal injury. DO NOT grind samples
larger then 1/3 the diameter of the working wheel!
Material Properties: The specimen preparation procedure and thus the
equipment design depends upon the properties of the material. The two basic
material properties which dictate the grinding/ polishing procedure are the
hardness and ductility (brittleness) of the specimen. In general, machines with
variable speed working wheels and variable speed polishing heads utilizing
variable force are recommended for metallographic specimen preparation.
Cutting or Sectioning Damage: As a guideline, it is better to reduce the initial
cutting/ sectioning damage by using the recommended abrasive blade, abrasive
size and cutting conditions. By reducing sectioning damage, finer initial
grinding abrasives can be used. This is very important for manual and
individual specimen automated polishing machines where if not carefully
controlled can result in a non-square surface/mounts.
Note: For individual specimen preparation, variable speed power heads are
recommended in order to match the speed of the polishing base with the
polishing head to reduce damage and to produce square mounts.
Flatness Required: For specimens requiring equal stock removal or a high
degree of flatness the grinding machine must be able to hold the specimen flat.
The best way to maintain flatness is to mount multiple samples into a fixed or
central specimen holder. The advantage of central pressure polishing is that the
sample is held in a larger diameter fixed plane. The disadvantages include (1)
a minimum of three samples is required to establish the plane, (2) samples
cannot be removed and remounted without having to re-planarize the specimens
and (3) controlling the removal rate across multiple samples can be very
difficult. It is also possible to use individual specimen preparation and obtain
flat specimens; however, in order to achieve flat specimens with this polishing
mode the speed of the specimen head needs to match the speed of the polishing
wheel rotating in the same direction.

Stock Removal Limitations: In some cases, a materials removal needs to be
monitored and perhaps controlled. One such example is the specimen
preparation of a heat treated part for testing surface hardness. For this
requirement the surface needs to be flat and relatively smooth; however, the
amount of stock removal is limited. Specimen preparation for these types of
samples requires an individual specimen preparation machine which has the
ability to produce flat or uniform stock removal. This requires that the head
and base speed be run at the same speed and in the same direction. A machine
with both a variable speed head and a variable speed working wheel provides
for the most efficient specimen preparation techniques.
Material removal: It is also important to be able to measure the stock removal
in order to ensure that the sample is not overground. A very simple way to do
this is to mount one or more steel ball bearings in the mount with the sample.
Using either a low powered microscope or the filars on a microhardness tester
the measured segment diameter of the ball bearing can be used to calculate the
amount of material removed (Figure 5-23). 1/8 or 1/4-inch diameter steel ball
bearings are commonly used for this application.

PLANAR GRINDING (ROUGH GRINDING)
The best time-tested methods for rough grinding metals, plastics, rubber and
softer composite materials are to use alumina or SiC abrasive grinding papers.
Other techniques have been used; however, they typically are very expensive
and require too much maintenance for these types of materials. Alumina and
SiC abrasive papers fit into a class of grinding known as fixed abrasive
grinding.
Number of Samples: The required number of samples that are polished also
determines the size of the polishing machines and power head, as well as,
whether central or individual specimen preparation is more efficient. Unless,
there are other overriding requirements, central polishing pressure on 12-inch
diameter polishing machines are recommended for high volume samples. For
low volume samples, 8-inch diameter machines using individual specimen
preparation heads is acceptable as long as the head and base speeds are set
properly as previously described.
5.4.1 Soft Nonferrous Metals
It is recommended that the initial grinding of soft nonferrous metals be done
with 600 (P1200) grit alumina abrasive paper followed by 800 (P2400) and
1200 (P4000) grit SiC papers. Since these materials are relatively soft and can
embed fractured abrasives, initial grinding with alumina is generally sufficient
for minimizing initial deformation while maintaining good removal rates.
5.4.2 Soft Ferrous Metals
Soft ferrous metals are relatively easy to grind with the depth of deformation
being a major consideration. Using 240 (P220) grit SiC abrasives provides a
good initial start, with the subsequent use of 320 (P360) , 400 (P800), 600
(P1200), 800 (P2400) and 1200 (P4000) grit SiC papers. Planar grinding
starting with 360 (P500) or 600 (P1200) grit alumina grinding papers can also
be effective and reduce the number of grinding steps.
5.4.3 Hard Ferrous Metals
Harder ferrous metals require more aggressive abrasives to achieve adequate
material removal. Thus, coarse SiC abrasives (120 or 180 grit) are
recommended for stock removal requirements. Once planarity and the area of
interest are obtained, a standard 240 (P220) , 320 (P360), 400 (P800) and 600
(P1200) grit series is recommended.

Super Alloys and Hard Nonferrous Alloys
Hard nonferrous metals such as titanium are relatively easy to grind with SiC
papers. Depending upon the initial condition of the specimen, grinding with
240 (P220), 320 (P360), 400 (P800), 600 (P1200), 800 (P2400) and 1200
(P4000) grit papers will produce excellent results.
5.4.5 Ceramics
Engineered ceramics are extremely hard, corrosion-resistant and brittle
materials. They fracture easily, producing both surface and subsurface damage.
Proper grinding minimizes both of these forms of damage. This requires the
application of a semi-fixed abrasive. The use of a metal mesh cloth
(CERMESH cloth) with an applied abrasive accomplishes these goals. The
abrasive size is also important because very coarse abrasives will quickly
remove material but can seriously damage the specimen. For ceramics,
consideration of the damage produced at each preparation step is critical to
minimizing the preparation time.
5.4.6 Composites
Composite materials are perhaps the most difficult specimens to prepare
because of their wide range of mechanical and chemical properties. For
example, a metal matrix composite (MMC) such as silicon carbide ceramic
particles in an aluminum metal matrix is a difficult specimen to prepare. This
composite contains extremely hard/brittle ceramic particles dispersed in a
relatively soft/ductile metal matrix. As a general rule, initial grinding should
focus on planarization of the metal and grinding to the area of interest. The
secondary grinding steps require focusing on the ceramic particles and
typically require the use of diamond and CMP polishing.

PLANAR GRINDING TROUBLESHOOTING
The most common problems associated with planar grinding result from using
the incorrect abrasive type or size (see Table XVII).

PRECISION GRINDING WITH LAPPING FILMS
The use of lapping films range from polishing semiconductor dies, fiber optics,
optical components, ceramic capacitors, computer hard drive read-write heads,
ceramic seals, etc. The main characteristic of lapping films is that they produce
very flat surfaces, especially across materials having a wide range of hardness.

Lapping films consist of a polyester backing (typically 3 mils) on which the
abrasive is rigidly fixed with an epoxy binder (Figures 5-25 & 5-26). The most
commonly used abrasives for lapping films include diamond, silicon carbide,
alumina, and to a lesser extent ceria and colloidal silica.

Diamond Lapping Films
Diamond lapping films are uniformly coated
abrasives using precisely graded diamond. A
very flexible adhesive bonding agent is used
which tenaciously holds the abrasive particles
to the backing. This adhesive is designed to
resist cracking or peeling. It is applied to a
tough, durable, tear-resistant polyester plastic
film which is waterproof and resistant to many
solvents.
Diamond lapping films provide less polishing
relief than intermediate rough polishing cloths
and remove material in a fixed diamond
grinding mode. Diamond lapping films are
exceptional abrasives for preparing
microelectronic materials for SEM and TEM
analysis.
Diamond lapping films range in particle sizes
from 0.1 micron up to 60 micron, with rough
grinding lapping films typically ranging from
15 micron to 60 micron (Figure 5-26).
Diamond Lapping Films Applications
For plain backed diamond lapping films:
- Wet the flat lap plate with water or a water/
surfactant solution (or use a receiver disk)
- Place the plain backed lapping film on
surface
- Roll or press out any entrapped air bubbles
- Apply necessary lubricant
- Begin polishing with lower force to avoid
tearing film
- Increase grinding force gradually
- Clean specimen and film for final 10-15
seconds of polishing cycle
- Clean and dry specimens

  For PSA backed diamond lapping films:
- Place the PSA backed lapping film on the
surface
- Apply necessary lubricant
- Begin polishing with lower force to avoid
tearing film
- Increase grinding force gradually
- Clean specimen and film for final 10-15
seconds of polishing cycle
- Clean and dry specimens
5.6.2 Silicon Carbide Lapping Films
Silicon carbide lapping films are similar to SiC grinding papers; however, the
abrasive is applied to a polyester backing instead of paper. The advantages of a
polyester backing are that it produces flatter surface finishes and less polishing
round-off compared to paper backed abrasives.
Silicon Carbide Lapping Films Applications
For plain backed silicon carbide lapping films:
- Wet the flat lap plate with water or a water/ surfactant solution (or use a
receiver disk)
- Place the plain backed lapping film on the surface
- Roll out or press out any entrapped air bubbles
- Apply necessary lubricant
- Begin polishing with lower force to avoid tearing film
- Clean specimen and film for final 10-15 seconds of polishing cycle
- Clean and dry specimens
For PSA backed silicon carbide lapping films
-Place the PSA backed lapping film on the surface
-Apply necessary lubricant
-Begin polishing with lower force to avoid tearing film
-Clean specimen and film for final 10-15 seconds of polishing cycle
-Clean and dry specimens

Alumina Lapping Films
Alumina lapping films consist of an aluminum oxide, which is a naturally
occurring material (Bauxite). It exists in either the softer gamma (mohs 8) or
harder alpha (mohs 9) phase. Alumina abrasives are used primarily as final
polishing abrasives because of their high hardness and durability. Alumina
abrasives are also available in a wide range of particles sizes for lapping films,
ranging from 0.05 micron up to 60 grit. The typical range for rough lapping is
12 micron and coarser. Lapping films are also color coded in order to better
distinguish the abrasive size (Figure 5-27).
Figure 5-27 Color coding for alumina lapping films.
Alumina Lapping Films Applications
For plain backed alumina lapping films:
- Wet the flat lap plate with water or a water/surfactant solution (or use a
receiver disk)
- Place the plain backed lapping film on the surface
- Roll out or press out any entrapped air bubbles
- Apply necessary lubricant
- Begin polishing with lower force to avoid tearing film
- Increase grinding force gradually
- Clean specimen and film for final 10-15 seconds of polishing cycle
- Clean and dry specimens.
For PSA backed alumina lapping films:
- Place the PSA backed lapping film on the surface
- Apply necessary lubricant
- Begin polishing with lower force to avoid tearing film
- Increase grinding force gradually
- Clean specimen and film for final 10-15 seconds of polishing cycle
- Clean and dry specimens.

  ROUGH POLISHING
The most critical metallographic preparation step is rough polishing. At this
step, the remaining surface and subsurface damage needs to be removed. After
this stage, the true microstructure of the material should be restored (inclusions,
brittle phases, voids, porosity, etc.) with exception of a few surface
imperfections, which can be subsequently removed at the final polishing stage.
Rough polishing is most commonly accomplished with woven, low napped
(napless) polishing pads paired with abrasive slurries such as diamond or
alumina (Figure 5-25). The primary objective for rough polishing with woven
polishing pads is to maintain the flatness across the specimen surface,
especially if the specimen has both hard and soft phases, coatings or other
critical features.
Note: For cases where flatness is absolutely critical, lapping films may be a
better alternative

Rough Polishing Abrasives
Rough polishing abrasives typically range from 15 micron down to 1 micron,
with alumina and diamond suspensions or fixed lapping films representing the
majority of the abrasive applications. For relatively soft materials, alumina
powders, suspensions or slurries are widely used. Note: Alumina is also a
relatively inexpensive abrasive, compared to diamond.
On the other hand, diamond has either a blocky (monocrystalline) structure or a
spherical nodular (polycrystalline) structure. For rough polishing,
polycrystalline diamond typically produces higher cut rates compared to
monocrystalline diamond and, in general, produces a better surface finish (see
section 5.1.3).
5.8.2 Rough Polishing Pads
For rough polishing operations that use alumina or diamond slurries, the correct
choice for the polishing pad surface is very critical. As already indicated, low
napped polishing pads are recommended for rough polishing. Low napped
polishing pads include woven, urethane coated fibers and porous urethane
pads. Although, to a certain extent, determining the correct polishing pad is
based on empirical trial and error experimentation, a number of properties
which affect the polishing pad characteristics include:
1. Resin fibers (hardness, density, size, count, chemistry)
2. Type of weave
3. Compressibility of the pad
4. Porosity or polishing pad surface area
5. Wetability of the abrasive suspension with the pad
Following are examples of common polishing pads:




* ISO 4287

Geometric Product Specifications - Surface texture: Profile Method
https://www.youtube.com/watch?v=Ay7Vzw0U-uI

** Amplitude Parameters

| Parameter | Description                                         |
|-----------+-----------------------------------------------------|
| Rt        | total height of the profile                         |
| Rp        | maximum profile peak height                         |
| Rv        | maximum profile valley depth                        |
| Rz        | maximum height of the profile                       |
| Ra        | arithmetic mean deviation of the assessed profile   |
| Rq        | root mean square deviation of the assessed profile  |
| Rsk       | skewness of the assessed profile                    |
| Rku       | kurtosis of the assessed profile                    |
| Rc        | mean height of profile elements                     |

Rt, total height of the profile:
height between the deepest valley and the highest peak on the evaluation length.

Rp, maximum profile peak height:
height of the highest peak from the mean line, defined on the sampling length.

Rv, maximum profile valley depth:
depth of the deepest valley from the mean line, defined on the sampling length.

Rz, maximum height of the profile:
defined on the sampling length: this parameter is frequently used to check whether the profile has protruding peaks that might affect static or sliding contact function.

Ra, arithmetic mean deviation of the assessed profile:
defined on the sampling length. Ra is used as a global evaluation of the roughness amplitude on a profile. It does not say anything on the spatial frequency of the irregularities or the shape of the profile. Ra is meaningful for random surface roughness (stochastic) machined with tools that do not leave marks on the surface, such as sand blasting, milling, polishing

Rq, root mean square deviation of the assessed profile:
corresponds to the standard deviation of the height distribution, defined on the sampling length. Rq provides the same information as Ra.

Rsk, skewness of the assessed profile:
asymmetry of the height distribution, defined on the sampling length. This parameter is important as it gives information on the morphology of the surface texture. Positive values correspond to high peaks spread on a regular surface (distribution skewed towards bottom) while negative values are found on surfaces with pores and scratches (distribution skewed towards top). It is therefore interesting when contact or lubrication functions are required. However, this parameter does not give any information on the absolute height of the profile, contrary to Ra.

Rku, kurtosis of the assessed profile:
sharpness of the height distribution, defined on the sampling length.


Rc, mean height of profile elements:
defined on the evaluation length. This parameter can be calculated on surfaces having texture cells or grains. It is similar to the motif parameter R found in ISO 12085 and, in that sense, it should be considered as a feature parameter (see ISO 25178).

** Spatial parameters


| Parameter | Description                      |
|-----------+----------------------------------|
| RSm       | mean spacing of profile elements |

RSm, mean spacing of profile elements, defined on the evaluation length.
This parameter is interesting on surfaces having periodic or pseudo-periodic motifs, such as turned or structured surfaces. In these cases, RSm will approximate their spacing. RSm is meaningless on random surface texture.

** Hybrid parameters


| Parameter | Description                                    |
|-----------+------------------------------------------------|
| Rdq       | root mean square slope of the assessed profile |
| RPc       | peak count number.                             |

Rdq, root mean square slope of the assessed profile, defined on the sampling length.
Rdq is a first approach to surface complexity. A low value is found on smooth surfaces while higher values can be found on rough surfaces having microroughness.

RPc, peak count number.
Was introduced in Amendment 1. Provides the density of peaks per unit of length.

** Functional parameters

| Parameter | Description                                         |
|-----------+-----------------------------------------------------|
| Rmr       | material ratio at a given depth.                    |
| Rdc       | profile section height between two material ratios. |


Rmr, material ratio at a given depth.
This parameter gives the percentage of material cut at a given depth from the top of the profile. The reference may also be taken from the center line or another reference height (c0).

Rdc, profile section height between two material ratios.
A stable value of roughness height can be evaluated using Rdc(2%-98%) if outliers are present on the surface. This calculation excludes the highest peaks that will be worn out and the deepest valleys that will be filled in. In the automotive industry, the material height removed during running-in can be assessed with Rdc(1%-33%) and the void volume used for lubricant pockets can be assessed with Rdc(25%-99%).


* Microscopes

Scanning Electron Microscopes show you the surface of your sample at high magnifications ~100000X
Transmission Electron Microscope show you

If you're not satisfied with your personal brain microscope at ten million magnification (all brains are not equal, after all), you need to buy a so-called "High Resolution Transmission Electron Microscope", or HRTEM for short, if you want to be able to keep up with your peers.
    The mighty transmission electron microscopes are not to be confused with the lowly "Scanning Electron Microscopes" or SEM. Those can only show you the surface of your specimen at magnifications rarely exceeding a few hundred thousand. With a HRTEM you can actually look inside the specimen at magnifications exceeding 10 million.
If you want very high magnification or resolution but are satisfied to see just the surface of your iron, you may want to get a "Scanning Tunneling Microscope" or STM.
      Microscopes are not exactly cheap. A simple run-of-the-mill optical microscope, the kind of microscope all those movie actors pretending to be weird scientists with a German accent are always looking into, slightly frowning because they don't have the faintest idea of what they are doing, will only set you back 50.000 $ to 100.000 $. Those cheapies will also be of no use for what we are about to do.
You need to cough up at least 3 Million $ for a decent HRTEM if you're not satisfied with your brain microscope. A decent SEM will be around 600.000 $. Simple STM's you may get already at a bargain price of a few 100.000 $; better ones will take a million or so

** Optical Microscopy

Optical microscopy using metallographic microscopes is a widely used technique for analyzing metallographic specimens. The typical magnification range for optical microscopes is 50 to 1000X, however higher magnifications are possible with specialized 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 microstructural features, including the following illumination techniques: brightfield, darkfield, polarized light, 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.

Definitions
- Brightfield
  an image condition where the background is light and the features are dark (high angle of illumination)
- Darkfield
  an image condition where the background is dark and the features are bright (low angle of illumination)
- Depth of Field
  the distance or depth at which the specimen surface will be in focus
- Empty Magnification
  the magnification limit where no additional information is obtained; increasing magnification beyond this limit only magnifies existing features
- 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 characterized 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 (see Figure 9-2).

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. The main types of optical filters include:

- Neutral Density Filters reduce the illumination intensity without affecting the color temperature
- Green Monochromatic Filters
  produce a single wavelength of light to ensure a sharp focus on black and white film
- Blue Color Correction Filters
  allow the operator to use daylight film with tungsten illumination and vice versa.
- Color Compensating Filters
  used to compensate for minor color temperature differences between the film and the illumination source

*** Illumination types
**** Brightfield illumination
Brightfield (B.F.) illumination is the most common illumination technique for
metallographic analysis. The light path for B.F. illumination is from the
source, through the objective, reflected off the surface, returning through the
objective, and back to the eyepiece or camera. This type of illumination
produces a bright background for flat surfaces, with the non-flat features
(pores, edges, etched grain boundaries) being darker as light is reflected back
at a different angle.

**** Darkfield illumination
Darkfield (D.F.) illumination is a lesser known but powerful illumination
technique. The light path for D.F. illumination is from the source, down the
outside of the objective, reflected off the surface, returned through the
objective and back to the eyepiece or camera. This type of illumination
produces a dark background for flat surfaces, with the non-flat features
(pores, edges, etched grain boundaries) being brighter as light is reflected at
an angle back into the objective.

**** DIFFERENTIAL INTERFERENCE CONTRAST
Differential Interference Contrast (DIC) is a very useful illumination technique for providing enhanced specimen features. DIC uses a Normarski prism along with a polarizer in the 90° crossed positions. Essentially, two light beams are made to coincide at the focal plane of the objective, thus rendering height differences more visible as variations in color.

* METALLOGRAPHIC IMAGE ANALYSIS
Quantifying and documenting a materials microstructure can provide very
useful information for process development, quality control and failure
analysis applications. Stereological techniques are used to analyze and
characterize 3-dimensional microstructural features from 2-dimensional
images or planar specimen cross sections.
The most common stereological analysis includes: point counting, length,
area and volume measurements; although, for automated image analysis,
counting picture points has recently been added. The following list of
measurements or calculations are used for determining a number of
metallographic features:

A = average area of inclusions or particles, (μm2 )
A A = area fraction of the inclusion or constituent
148
A i = area of the detected feature
A T = measurement area (field area, mm 2 )
H T = total project length in the hot-working direction of an inclusion or
constituent in the field, microns
L = average length in the hot-working direction of the inclusion or
constituent, (μm)
L T = true length of scan lines, pixel lines, or grid lines (number of lines
times the length of the lines divided by the magnification), mm
n = the number of fields measured
N A =number of inclusions or constituents of a given type per unit area,
mm 2
N i = number of inclusions or constituent particles or the number of
feature interceptions, in the field
N L = number of interceptions of inclusions or constituent particles per unit
length (mm) of scan lines, pixel lines, or grit lines
PP i = the number of detected picture points
PP T = total number of picture points in the field area
s = standard deviation
t = a multiplier related to the number of fields examined and used in
conjunction with the standard deviation of the measurements to
determine the 95% CI
V V = volume fraction
X = mean of a measurement
X i = an individual measurement
∑X = the sum of all of a particular measurement over n-fields
∑X2 = sum of all of the squares of a particular measurement over n-fields
λ = mean free path (μm) of an inclusion or constituent type
perpendicular to the hot-working direction

95% CI = 95% confidence interval
% RA = relative accuracy, %
For stereological measurements:
Volume fraction = V V = A A = A i /A T = PP i /PP T
Number per unit area (inclusions) = N A =N i / A T
Average length of each inclusion

Average area of each inclusion or particle = A = A A / N A
Mean free path or the mean edge-to-edge distance between inclusions (oxide
and sulfide) or particle types, perpendicular to the hot-working axis:
λ = (1-A A )/ N L
Several commonly used metallographic quantification procedures include the
following:
- Grain size (ASTM E112, E930, E1181 and E1382)
- Phase analysis (ASTM E562, E1245)
- Nodularity (ASTM A247)
- Porosity (ASTM 562)
- Inclusion (ASTM E45, E1245)
- Decarburization (ASTM E1077)
- Coating thickness (ASTM B487)
- Weld analysis

** Grain size (ASTM E112, E930, E1181)
A grain is defined as the individual crystal in a polycrystalline material.
Although grain size is a 3-dimensional feature, it is measured from a 2-
dimensional cross section of the material. ASTM (American Society for
Testing Materials) provides a number of internationally recognized standards
for measuring and classifying a materials grain size.
- ASTM E112 - Standard Test Methods for Determining Average Grain Size (31)
- ASTM 930 - Standard Test Methods for Estimating the Largest Grain Observed in a Metallographic Section (ALA Grain Size) (32)
- ASTM E1181 - Standard Test Methods for Characterizing Duplex Grain Sizes (33)
- ASTM E1382 - Standard Test Methods for Determining Average Grain Size Using Semiautomatic and Automatic Image Analysis (34)

ASTM E112 describes several procedures for measuring grain size, including the Comparison procedure, Planimetric (Jeffries) procedure, and general Intercept procedures. The Comparison procedure is useful for completely recrystallized materials with equiaxed grains and uses a set of standardized charts that can be obtained or purchased from ASTM. These charts are used to compare the etched specimens microstructure, at the same magnification, to the appropriate comparison chart (31).

For the Planimetric method, a rectangle or circle having a known area (5000 mm 2 ) is placed over a micrograph of the etched specimen and the number of full grains are counted and the number of grains that intersect the circumference of the area are counted and multiplied by 1/2, this gives the total number of grains. This number is then multiplied by the Jeffries multiplier which is based on the magnification (note: proper magnification requires at least 50 grains).

The Intercept procedure is recommended for all structures which do not have uniform equiaxed grain structure. The Heyn Lineal Intercept Procedure (31) counts the number of grain boundary intercepts along a straight line. Another intersect technique utilities a circular test line.
Note: for determining grain size, twin boundaries should be removed from the calculation.

ASTM E930 is used to measure the grain size for materials with very large grain structures when there are not enough grains to use ASTM E112. For example, galvanized coatings can have very large grain structures (32). This standard determines the largest observed grain in the sample, often referred to ALA (as large as) grain size. The methods used to determine the ALA grain size include measuring the largest grain with a caliper or by photographing the largest grain at the highest magnification which shows the entire grain.

For the caliper method the largest diameter and the largest diameter perpendicular to this line are measured. These two numbers are multiplied
together and then multiplied by 0.785 to give an elliptical area. This number is divided by the square of the magnification to give the grain size at a magnification of 1X. Using the appropriate ASTM table, the ASTM grain size number can be determined. Another techniques uses a photograph with an ASTM overlay. The number of grid intersections are counted and converted to grain size number.

ASTM E1181 is the standard used for characterizing grain sizes for materials which have two or more distinctive grains sizes (33).

ASTM E1382 is the standard which covers the procedures for automatically determining the mean grain size, the distribution of grain intercept length, or grain areas. The primary issue for semi-automatic and automatic image nalysis is proper specimen preparation, including proper grinding, polishing and etching. The resulting microstructure should fully and uniformly reveal the grain boundaries (34).

** Phase Analysis (ASTM E562, E1245)
Phases are defined as physically homogenous and distinct constituents of the material. Phase analysis can be characterized and measured using area or volume fraction measurements per ASTM E562 (Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count) (35) or ASTM E1245 (Standard Practice for Determining the Inclusion or Second-Phase Constituent Content of Metals by Automatic Image Analysis) (36). Common measurements used in phase analysis include: length, area, number, volume fraction, mean free path, number of detected picture points, and 95% CI – confidence interval.
Examples where phase analysis are used include stereological measurements that describe the amount, number, size, and spacing of the indigenous inclusions (sulfides, oxides and silicates) in steel, porosity, and the analysis of any discrete second-phase constituent in the material.

** Nodularity (ASTM A247)
Nodularity describes the type and distribution of graphite in cast irons.
ASTM A247 (Standard Test Method for Evaluating the Microstructure of Graphite in Iron Castings) (37) is used to classify and characterize the graphite for all iron-carbon alloys containing graphite particles. This method can be applied to gray irons, malleable irons, and ductile (nodular) irons. Quantification of cast irons can be described with three classifications:
- graphite form (Roman number I through VII),
- graphite distribution (letter A-E), and
- graphite size (1-largest to 8-smallest).

Types I-VI are for nodular cast iron and Type VII would be for the graphite flakes in gray cast irons.
Classification of the graphite is typically accomplished by comparison with ASTM Plate I for the type, ASTM Plate II for the distribution of the graphite, and ASTM Plate III would reference the size of the graphite.

** Porosity (ASTM E562, E1245)
Porosity are voids in the material caused by entrapped air and incomplete or poor sintering. Porosity can be measured as a volume fraction, either manually using ASTM E562 (35) or with automated image analysis using ASTM E1245. ASTM E562 (Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count) (35) is a point counting method using a clear plastic test grid or an eyepiece reticle with a regular array of test points overlaid on the image. The number of test point falling within the phase or constituent of interest are counted and divided by the total number of grid points.

** Inclusion rating (ASTM E45)
Inclusions are particles of foreign material that are insoluble in the metal or materials matrix. For steels, common inclusions include oxides, sulfides or silicates; however, any foreign substance can be classified as an inclusion.
ASTM E45 (Standard Test Methods for Determining the Inclusion Content of Steel) (38) is used to characterize the type, size and severity of the inclusions in wrought steel. ASTM E45 describes the JK-type inclusion rating system. The JK-type inclusion rating system first characterizes the type of inclusion (Type A-D):
- Type A-sulfide type
- Type B-alumina type
- Type C-silicate type
- Type D-globular oxide type

Type A and C are very similar in size and shape, however Type A-Sulfide are light gray which Type S-Silicate are black when viewed under Brightfield
illumination.

Type B stringers consist of a number (at least three) round or angular oxide particles with aspect ratios less than 2 that are aligned nearly parallel to the deformation axis.
The second characterization parameter is thickness: designated H-heavy, T-thin.
The third characterization parameter is “Severity Level” and are partitioned based on the number or length of the particles present in a 0.50 mm2 field of view (38).

** Decarburization (ASTM E1077)
Decarburization is the loss of carbon at the metals surface due to chemical reaction(s) with the contacting media. Decarburization can over time significantly change the surface properties of the metal.

ASTM E1077 (Standard Test Methods for Estimating the Depth of Decarburization of Steel Specimens) provides the guidelines for estimating the average or greatest depth of decarburization in hardened or non-hardened steel products (39). Metallographic analysis of a properly polished and etched sample is considered an acceptable technique for determining decarburization for heated-treated, spherodize-annealed, cold-worked, as-hot rolled, as-forged, annealed, or normalized steel specimens. The depth of decarburization can be determined by the observed changes in the microstructural cross-section due to changes in the carbon content.

ASTM defines the following terms (39):
- Average depth of decarburization – the mean value of 5 or more measurements of the total depth of decarburization.
- Average free-ferrite depth – the mean value of 5 or more measurements of the depth of complete decarburization
- Complete decarburization – loss of carbon content at the surface of a steel specimen to a level below the solubility limit of carbon in ferrite so that only ferrite is present.
- Partial decarburization – loss of carbon content at the surface of a steel specimen to a level less than the bulk carbon content of the unaffected interior by greater then the room temperature solubility limit of carbon in ferrite. The partial decarburization zone would contain both ferrite and pearlite.
- Total depth of decarburization – the perpendicular distance from the specimen surface to that location in the interior where the bulk carbon content is reached; that is, the sum of the depths of complete and partial decarburization. For heat-treated specimens, the presence of non-martensitic structures in the partially decarburized zone is used to estimate the total depth of decarburization.
- Maximum depth of decarburization – the largest measured value of the total depth of decarburization.

** Coating thickness (ASTM B487)
Measurement of coating thickness is very important for characterizing the
performance of many materials. Such coatings can have very important wear,
heat resistance, and corrosion resistant properties. ASTM B487 (Standard
Test Method for Measurement of Metal and Oxide Coating Thickness by
Microscopical Examination of Cross Section) describes the recommended
acceptance testing procedures for measuring coating thickness.
As with other metallographic analysis, proper specimen preparation is
required to obtain a meaningful quantitative number. In general, the
specimens need to be mounted, polished and etched so that the cross section
is perpendicular to the coating as to avoid any geometrical errors in
measuring the coating thickness. It is important that the surface be flat across
the entire sample so that the boundaries are sharply defined. The cross
section should also be prepared to eliminate deformation, smearing and other
polishing artifacts.

** Weld analysis
Metallographic cross sectional analysis of welded components are listed in a
number of SAE and AWS standards; however, no specific general standard is
presently known. A number of common measurements include:
-Distance from the foot of the fillet to the center of the face (or throat)
-Distance from the root of the joint to the junction between the exposed
surface of the weld and the base metal (leg)
-Angles and the root penetration
-Depth of HAZ (heat affected zone)
-Area of HAZ
-Joint penetration
-Phase counting, etc.



* Crystals

Many of the properties of metals come straight from the fact that metals are crystals with crystal defects. By crystal, I mean solid materials with an orderly arrangement of atoms, not the glasses and trinkets that are often called crystals.

Most of the rocks around us and every metal is a crystal. So how come nobody noticed this?
    Because the overwhelming majority of natural crystals are not showy single crystals with an easily recognizable "crystal" shape, but poly crystals, an agglomeration of small crystals stuck together haphazardly. On top of that, these crystals are as full of defects as a politician of BS.
The figure below illustrates that much better than many words:


[[download:20230719-150413_screenshot.png]]

** Point defects

There are four kinds of zero-dimensional defects.

[[download:20230720-132730_screenshot.png]]

Why do these crystals have defects? Any crystal can and will make vacancies because a lil bit of disorder makes the crysta happy. So with increasing temperatures, crystals have increasing disorder, and this can result in vacancies and atoms squeezing into places they're not supposed to be, giving you vacanies and self-interstitials. Most crystals prefer vacancies, not self-interstitials, so you can ignore the effect of inter-stitials. Somehow, silicon self-interstitials are important tho; something about the lack of vacancies making self-interstitials actually important.


When we talk about carbon in iron, we usually consider concentrations between 0 % and 1,5 %. Those are always weight percent (written wt % if you want to be precise) if not otherwise stated

This makes a lot of sense in material engineering because you can easily make desired mixes by weighing the ingredients.
In materials science , however, we look at atoms. In order to assess foreign atoms in a host crystal, we need their concentration in atom percent (at %). In other words, we want to know how many percent of all the atoms present are carbon atoms?
There is no easy way to switch from one to the other; consult the link for how it's done.

If we take, for example, the "magical" 6.7 wt % of carbon and express it in atom percent, we get 6.7 wt %=25 at % . A quarter of all atoms then are carbon atoms. Why 6.7 wt % of carbon of carbon is "magical" we will see later—but you are welcome to make a guess, now that you know it corresponds to a mix of iron : carbon=3 : 1 in terms of atoms. Hint: one could write that Fe3C.
What a schematic iron crystal would look like with 1.75 at % (≈ 0.3 wt %) or 5 at % (≈1 wt %), respectively, of carbon atoms dissolved in the volume of the iron, is schematically shown below.
Now that we have played around a bit with an extrinsic defect like carbon in iron, the same question as above for the intrinsic defects comes up:
Why should a crystal contain those extrinsic defects, and if there is some reason, how many should it be?
    In an ideal crystal world the answers to those questions would be exactly the same as for intrinsic point defects:

    Extrinsic defects increase disorder, and a certain number of all the other 90 or so elements should be part of any nirvana-seeking crystal.
    The proper "nirvana" number of extrinsic defects depends, among other things, strongly on the temperature. Generally, the concentration should increase with increasing temperature.


    Carbon (C), nitrogen (N), and hydrogen (H) are always dissolved as interstitial impurity atoms in iron.
Phosphorus (P), Sulfur (S), Manganese (Mn), Nickel (Ni), Chromium (Cr), Vanadium (V) and most other atoms are always dissolved as substitutional impurity atoms in iron.
Oxygen (O) and boron (B) can be dissolved in both ways.
All of them will be imprisoned in precipitates if they exceed a certain specific concentration






* Alloying Elements


Carbon (C)
- Increases hardness
- Increases resistance to wear and abrasion

Manganese (Mn)
- Increases strength, hardness, and wear resistance
- Increases response to heat treatment
- Acts as deoxidizer

Silicon (Si)
- Increases elastic limit
- Improves resistance
- Improves electrical behavior

Sulfur (S)
- Improves machinability

Phosphorus (P)
- Improves machinability

Chromium (Cr)
- Increases strength and hardness
- Increases resistance to wear
- Increases response to heat treatment
- Increases hardenability
- Improves resistance to oxidation and corrosion
- Increases creep strength

Nickel (Ni)
- Increases strength
- Increases hardenability
- Improves room-temperature and low-temperature toughness
- Improves resistance to oxidation and corrosion

Molybdenum (Mo)
- Increases strength and hardness
- Increases hardenability
- Increases resistance to softening
- Improves resistance to pitting corrosion

Tungsten (W)
- Increases strength and hardness
- Increases hot hardness
- Increases wear resistance
- Increases creep strength

Vanadium (V)
- Acts as grain refiner
- Acts as deoxidizer
- Increases strength
- Increases wear resistance
- Improves creep resistance

Titanium (Ti)
- Increases wear resistance
- Takes care of sensitization and intergranular corrosion in stainless steel
- Helps in improving high-temperature strength

Niobium (Nb)
- Takes care of sensitization and intergranular corrosion in stainless steel
- Helps in improving high-temperature strength

Copper (Cu)
- Improves resistance to atmospheric corrosion
- Improves machinability

Lead (Pb)
- Improves machinability

Aluminum (Al)
- Acts as deoxidizer
- Acts as grain refiner
- Improves resistance to scaling
- Important element in nitriding steels

Boron (B)
- Improves depth of hardening

* Personal Safety

Wear appropriate eye protection (safety goggles or safety glasses – see MSDS sheets).
- Wear approved rubber gloves and laboratory coats or aprons.
- Mix etchant and etch with adequate and appropriate ventilation. A fume hood is generally recommended.
- FIRST AID – Review MSDS sheets for specific medical instructions.
SKIN – In case of contact, immediately flush skin with plenty of water for at least 15 minutes while removing contaminated clothing and shoes. Get medical attention immediately. Wash clothing before reuse. Thoroughly clean shoes before reuse.
EYES – Immediately flush eyes with plenty of water for at least 15 minutes, lifting lower and upper eyelids occasionally. Get medical attention immediately.
INHALATION – Remove to fresh air immediately. If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Get medical attention immediately.
INGESTION: Refer to MSDS sheets for specific guidelines. Never give anything by mouth to an unconscious person. Get medical attention immediately

** Mixing acids and bases
+ Acids and Bases: ALWAYS mix concentrated acids and bases into water to prevent excessive heat generation.
+ Monitor temperature of mixture to prevent overheating.
+ Mix in well ventilated area.
+ Use recommended personal safety equipment.

** Disposal
When appropriate, dilute all concentrated chemicals prior to disposal. If
regulations allow disposal to sewer, use a substantial amount of running water
and slowly add etchant to flow. Continue to purge drain thoroughly with
water. Follow all Local, State and Federal Disposal Guidelines.