Mounting of Specimens
The primary purpose of mounting specimens is for convenience in handling specimens of difficult shapes or sizes during the subsequent steps of preparation and examination. A secondary purpose is to protect and preserve extreme edges or surfaces defects during prepartion. Specimens also may require mounting to accommodate various types of automatic devices used in laboratories or to facilitate placement on the microscope stage.

Small specimens generally require mounting so that the specimen is supported in a stable medium for grinding and polishing. The medium chosen can be either a cold curing resin or a hot mounting compound.

Characteristics of the mounting material include:

Good abrasion characteristics and sufficient hardness such that the edges of the sample are protected, i.e., the rate at which abrasion takes place should be even across the face of the mount and the specimen.

Stable and adherent to sample. This is important. If the mounting material has poor adhesion or high shrinkage, gaps may open up between the mounting material and the sample surface. When this happens, it is very difficult to prevent cross-contamination of one abrasive to another, causing heavy scratching in the finished section. Also any friable surface layers (oxide layers etc.) should be held adhered to the surface and not pulled off.

Proper curing - insufficient time and temperature can lead to partially cured specimen mounts. Under these conditions the properties of the mounting material are not properly developed and the material may be loose and powdery. Generally, if the material is improperly cured, the hardness and abrasion characteristics are poor and the material is adversely affected by etches and solvents. Further, the characteristics under vacuum are very poor with out-gassing a major problem. If the mounting stage is suspected to be at fault, it is best to break the sample out and start again.

Stable in vacuum - no out-gassing or vapour to cause contamination. This is articularly important for high magnification work, long map acquisition times and microscopes with high vacuum requirement.

The mounting operation accomplishes three important functions (1) it protects the specimen edge and maintains the integrity of a materials surface features (2) fills voids in porous materials and (3) improves handling of irregular shaped samples, especially for automated specimen preparation. The majority of metallographic specimen mounting is done by encapsulating the specimen into a compression mounting compound (thermosets - phenolics, epoxies, diallyl phthalates or thermoplastics - acrylics), casting into ambient castable mounting resins (acrylic resins, epoxy resins, and polyester resins), and gluing with a thermoplastic glues.
An added benefit of mounting is the ease with which a mounted specimen can be identified by name, alloy number, or laboratory code number for storage by scribing the surface of the mount without damage to the specimen.

Mount Size and Shape

As the size of the specimen increases, so does the difficulty of keeping the specimen surface area flat during grinding and polishing. A saving in the time required for the preparation of one large metallographic specimen may be realized by sectioning the specimen into two or more smaller specimens. A specimen having an area of approximately 1/4 sq in. is perhaps the most suitable; the maximum area should be limited to about 4 sq in. if possible. Thickness of the mount should be sufficient to enable the operator to hold the mount firmly during grinding and polishing and thereby to pervent a rocking motion and to maintain a flat surface. Circular mounts are commonly 1 to 2 in. in diameter and are the most easily handled. The length-to-width ratio of rectangular mounts should be limited to approximately 2 to 1 to facilitate handling.

Mounting Methods

The method of mounting should in no way be injurious the microstructure of the specimen. Mechanical deformation and the heat are the most likely sources of injurious effects. The mounting medium and the the specimen should be compatible with respect to hardness and abrasion resistance. A great difference in hardness or abrasion resistance between mounting media and specimen promotes differential polishing characteristics, relief, and poor edge preservation. The mounting medium should be chemically resistant to the polishing and etching solutions required for the development of the microstructure of the specimen.

Clamp Mounting

Clamps are used most often for mounting thin sheets of metal when preparing metallographic cross sections. Several specimens can be clamped conveniently in sandwich form. The two clamp plates are frequently made from 1/4 in. thick steel; in general, the hardness of the clamp should be approximate or exceed the hardness of the specimen. The clamp plates are cut longer and wider than specimens to be clamped. Then two holes are drilled and tapped in the face of one clamp plate outboard of the specimen area; corresponding holes are drilled in the other clamp plate. Machine bolts are inserted through these latter holes and into the tapped holes; the clamp plates with the specimen or specimens are drawn tightly up means of these bolts. Sometimes, a third bolt positioned near the top of the clamp midway between the ends is useful for maintaining a uniform vertical separation between the clamp plates.

Clamp mounting affords a means of rapid mounting, and of very good edge preservation by virtue of the initimate contact between specimens. On the other hand, hairline separations between specimens occour frequently and entrap abrasive particles or liquid solutions during prepa- ration. Sometimes, the particle and liquids can be removed by soaking the mount in alcohol an then thoroughly drying it. If this cannot be done, the liquid eventually seeps out and stains the polished surface, and often obscures the true microstructure after etching. One solution to this difficulty is the insertion of one thickness of transparent plastic wrapping film at each interface. (The plastic must be one that is inert to alcohol and etchants). Under clamping pressure, the plastic flows readily and seals all hair-line separations. Since the film is only a fraction of a mil thick, specimen edges are preserved by adjoining specimens or clmap edges. Alternatively, soft, thin sheets of metal of the same type as that be examined can be used instead of the plastic film, or the mount can be vacuum impregnated.

Compression Mounting

Compression mounting, the most common mounting method, involves molding around the specimen by heat and pressure such molding materials as bakelite diallyl phthalate resins, and acrylic resins. Bakelite and diallylic resins are thermosetting, and acrlyic resins are thermoplastic. Both thermosetting and thermoplastic materials require heat and pressure during the molding cycle, but after curing, mounts made of thermosetting materials may be ejected from the mold at maximum temperature. Thermoplastic materials remain molten at the maximum molding temperature and must cool under pressure before ejection.

Mounting presses equipped with molding tools and a heater are necessary for compression mounting. Readily available molding tools for mounts having diameters of 1, 1 1/4 and 1 1/2 in. consist of a holow cylinder of hardened steel, a base plug, and a plunger. A specimen to be mounted is placed on the base plug, which is inserted in one end of the cylinder. The cylinder is nearly filled with molding material in powder form, and the plunger is inserted into open end of the cylinder. A cylindrical heater is placed around the mold assembly, which has been positioned between the platens of the mounting press. After the prescribed pressure has been exerted and maintained on the plunger to compress the molding material until it and the mold assembly have been heated to the proper temperature, the finished mount may be ejected from the mould by forcing the plunger entirely through the mold cylinder.

Not all materials or specimens can be mounted in termosetting or thermoplastic mounting mediums. The heating cycle may cause changes in the microstructure, or the pressure may cause delicate specimens to collapse or deform. The size of selected specimen may be to large to be accepted by the availaible mold sizes. These difficulties are usually overcome by cold mounting.

For metals, compression mounting is widely used. Phenolics are popular because they are low cost, whereas the diallyl phthalates and epoxy resins find applications where edge retention and harder mounts are required. The acrylic compression mounting compounds are used because they have excellent clarity.

Compression Mounting Resin Properties

Note

Phenolics

Acrylics

Epoxy
(glass filled)

Diallyl Phthalates

Cost

Low

Moderate

Moderate

Moderate

Ease of use

Excellent

Moderate

Good

Good

Availability of colors

Yes

No

No

No

Cycle times

Excellent

Moderate

Good

Good

Edge retention

Fair

Good

Excellent

Excellent

Clarity

None

Excellent

None

None

Hardness

Low

Good

High

High


Resin

Phenolic

Acrylic

Epoxy

Diallyl Phthalate

Form

Granular

Powder

Granular

Granular

Specific gravity
(gm/cm3)

1.4

0.95

1.75-2.05

1.7-1.9

Colors

Black, Red, Green

Clear

Black

Blue

Shrinkage (compression)(in/in)

0.006

-

0.001-0.003

0.001-0.003

Coefficient of Linear Thermal Expansion
(in/in/C 10-6)

50

-

28

19

Chemical resistance

Glycol, petrochemicals, solvents, some acids and bases

Alcohol, dilute acids & alkalies, and oxidizers

Solvents, acids, alkalies

Solvents, acids, alkalies

Molding temperature

150-165C
(300-330F)

-

143-177C
(290-350F)

160-177C
(320-350F)

Molding pressure

21-28 Mpa
(3050-4000 psi)

-

17-28 MPA
(2500-4000 psi)

24-41 MPA
(3500-6000 psi)

Hardness

-

Rockwell M63

Barcol 72

-

Curing time
(1/2" mount @ temp.)
and pressure)

90-120 seconds

2-4 minutes

90-120 seconds

90-120 seconds


Cold Mounting

Cold mounting requires no pressure and little heat, and is a means of mounting large numbers of specimes more rapidly than by compression mounting.

Materials for cold mounting are classified as polyesters, epoxides and acrylics. Polyesters are transparent and usually water clear; epoxides are almost transparent and straw color; acrylics are opaque. Cold mounting materials of all three classifications are two component systems that consist of resin and a hardener; both the resin and the hardener can be liquid, both can be solids, or one can be liquid and the other a solid. Mixing of the resin and the hardener produces exothermic polymerization, and therefore this operation is crytical in producing a satisfactory cure and limiting the temperature to a permissible level. The temperature rise may reduced at the expense of longer curing time.

Cold mounting is a casting method, because each of the three classifications of cold mounting materials is liquid after the resin and hardener are mixed (two-solid systems are melted before mixing). The casting molds can be of any size or shape desired. For round molds, either bakelite ring forms, or ring sections cut from plastic or metal tubes or pipes are suitable. The mold material may become part of the mount in the form of an outher shell, or mold release agents may be used to permit the mount the mount to be ejected from the mold. Rectangular molds are formed readily by wrapping heavy-duty aluminium foil around wood blocks of the desired size. The aluminium foil can be removed from the mount by peeling it away, grinding it off, or using a mold release agent. Molds any size or shape can be prepared from silicone rubber materials. The flexibility of silicone rubber molds allows cured cold mounts to be removed easily.

Epoxy resins are the most widely used cold mounting materials. The are hard and adhere tenaciosly to most metalurgical, mineral and ceramic specimen. They also exhibit lower volume shinkage then either polyesters or acrylics and are very useful for impregating porous structures or cracks by vacuum method. Epoxy resin mounts amy be cured in a low-temperature or placed in a low temperature oven for fast curing, depending on the mixture ratio of resin to hardener.

Polyester resins have greater volume shrinkage the epoxies. They provide water-clear or slightly colored transparent mounts, which strip readily from glass casting surfaces and metal molds.

Acrylic materials are fast curing, and the mixing and casting process for the acrylics is quick and simple. The fast curing rate results from the relatively high rate heat evolution during exothermic ploymerization, but some control of the exothermal temperature rise can be accomplished by varying the sizes of the specimen and the mount. Stripping acrylic mounts from metal od glass molds is not difficult.

Castable mouting resins are commonly used for electronic and ceramic materials. Castable mounting resins are recommended for brittle and porous materials. These mounting compounds are typically two component systems (1-resin and 1-hardener). Typical curing times range from minutes to hours with the faster curing resins producing higher exothermic temperature which causes the mounting material to shrink away from the edge during curing. For example, the Acrylic Cold Mounting Resins cure in less than 10 minutes and Epoxy Castable Resins cure in approximately 4-6 hours. Note that the Epoxy Castable Resin curing cycle can be enhanced by adding an external energy source such as heat or microwave energy. It is recommended that the room temperature be less than 85 F to avoid overheating and uncontrollable curing of the mounting compound.

Castable Mounting Resin Properties

Note

EPOXY

ACRYLIC

POLYCAST Resin

Type

Epoxy resin and hardener

Acrylic resin and powder

Polyester resin and hardener

Peak Temperature

82 F

80 F

100 F

Shore D Hardness

82

80

76

Cure Time

6-8 hours

5-8 minutes

6-8 hours

Comments

Moderate hardness, low shrinkage, transparent

Very fast curt, translucent, some shrinkage

Transparent, clear


Conductive Mounting

For specimens requiring metallographic preparation by electrolytic techniques, an electrically conductive mount affords a convinient means of completing the electrical circuit through the specimen; merely an electrical contact with the mount, rather than with specimen, is required. Most conductive mounting materials are mixtures of a metal, usually copper or iron powder, and thermosetting or thermoplastic molding materials. During compression mounting the metal powder particles are compacted sufficiently to provide electrical countinuity throughout the mount. An equally convenient method is to attach a copper wire to the back of the specimen and to formit an a helix to stand upright in the mounting press mold with its top in contact with the center of plunger. After ejection of the mount the free end of the helix may be dug out of the mount for electrical connection.

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