msc_thesis/Archive/metallurgy.org

• Introduction
• Abrasive Sectioning
• Precision Wafer Sectioning
• Specimen Mounting
• Abrasive Grinding
• ISO 4287
  • Amplitude Parameters
  • Spatial parameters
  • Hybrid parameters
  • Functional parameters
• Microscopes
  • Optical Microscopy
    • Optical Filters
    • Illumination types
      • Brightfield illumination
      • Darkfield illumination
      • DIFFERENTIAL INTERFERENCE CONTRAST
• METALLOGRAPHIC IMAGE ANALYSIS
  • Grain size (ASTM E112, E930, E1181)
  • Phase Analysis (ASTM E562, E1245)
  • Nodularity (ASTM A247)
  • Porosity (ASTM E562, E1245)
  • Inclusion rating (ASTM E45)
  • Decarburization (ASTM E1077)
  • Coating thickness (ASTM B487)
  • Weld analysis
• Crystals
  • Point defects
• Alloying Elements
• Personal Safety
  • Mixing acids and bases
  • Disposal

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.