How to Mount a Metallographic Specimen for Examination
Mounting specimens is an important step in the metallographic examination process. It provides convenience in handling specimens of difficult shapes or sizes during preparation and examination, as well as protecting and preserving extreme edges or surface defects. In addition, it allows for the use of various laboratory devices and the identification of the specimen through scribing the surface of the mount. Here, we’ll discuss the characteristics of the mounting material, the purpose of the mounting operation, and the types of mountings.
Characteristics of the Mounting Material
When selecting a mounting material, it’s important to consider the characteristics such as abrasion resistance, adhesion, hardness, and curing stability. Good abrasion characteristics and sufficient hardness protect the edges of the sample. The mounting material should also be stable and adherent to the sample so that gaps don’t open up between the mounting material and the sample surface. Additionally, the material should not be adversely affected by etches and solvents, and should be stable in vacuum with no out-gassing.
Purpose of Mounting
The primary purpose of mounting a specimen is for convenience in handling specimens of difficult shapes or sizes during preparation and examination. It also protects and preserves extreme edges or surface defects, and accommodates various types of devices used in laboratories or facilitates placement on the microscope stage. Additionally, it allows for the identification of the specimen by name, alloy number, or laboratory code number for storage.
Types of Mountings
The majority of metallographic specimen mounting is done by encapsulating the specimen into a compression mounting compound, casting into ambient castable mounting resins, and gluing with a thermoplastic glue. The mounting operation accomplishes three important functions: it protects the specimen edge and maintains the integrity of a material’s surface features, fills voids in porous materials, and improves handling of irregular shaped samples.
Mounting is an important step in the metallographic examination process. It provides convenience in handling specimens of difficult shapes or sizes during preparation and examination, as well as protecting and preserving extreme edges or surface defects. When selecting a mounting material, it’s important to consider characteristics such as abrasion resistance, adhesion, hardness, and curing stability. The majority of metallographic specimen mounting is done by encapsulating the specimen into a compression mounting compound, casting into ambient castable mounting resins, and gluing with a thermoplastic glue. The purpose of mounting is to provide convenience, protect and preserve, accommodate various types of devices or facilitate placement, and allow for the identification of the specimen.
Metallographic Specimen Preparation: Mount Size and Shape, Mounting Methods
Metallographic specimen preparation is a process that involves grinding and polishing the surface of a specimen to prepare it for microscopic examination. This process is essential for studying the microstructure of a specimen and determining its properties. Proper specimen preparation is critical for obtaining reliable and accurate results. In this article, we will discuss mount size and shape, as well as different mounting methods for metallographic specimens.
Mount Size and Shape
The size and shape of the mount are important factors in specimen preparation. As the size of the specimen increases, it becomes more difficult to keep the surface area flat during grinding and polishing. It is often more efficient to cut the specimen into smaller sections. The maximum area should be limited to about 4 sq. in. if possible. For circular mounts, they should be 1 to 2 in. in diameter. For rectangular mounts, the length-to-width ratio should be limited to approximately 2 to 1.
Mounting Methods
The method of mounting should not damage the microstructure of the specimen. The mounting medium and the specimen should have similar hardness and abrasion resistance. Incompatible materials can lead to poor edge preservation, differential polishing characteristics, and relief. There are several different mounting methods used for metallographic specimens, such as clamp mounting, compression hot mounting, cold mounting, and conductive mounting. Cold sample mounting and hot sample mounting are two types of conductive mounting.
Clamp Mounting
Clamp mounting is the most widely used method for metallographic specimen preparation. This method involves clamping the specimen in a mold with a mounting medium. The mounting medium must be compatible with the specimen and should not cause any damage to the microstructure.
Compression Hot Mounting
This method involves placing the specimen in a mold that is filled with a mounting medium. The mold is then placed in a furnace and heated to a certain temperature. As the temperature increases, the mounting medium expands and creates a hermetic seal around the specimen.
Cold Mounting
Cold mounting involves placing the specimen in a mold that is filled with a cold mounting medium. This method is used when the specimen is too fragile to withstand the heat required for compression hot mounting.
Conductive Mounting
Conductive mounting is used for specimens that require electrical or thermal conductivity. This method involves placing the specimen in a mold that is filled with a conductive medium. The mold is then placed in a furnace and heated to a certain temperature. The heat causes the medium to expand and create a hermetic seal around the specimen. There are two types of conductive mounting: cold sample mounting and hot sample mounting.
Conclusion
Metallographic specimen preparation is an essential part of studying the microstructure of a specimen and determining its properties. Proper specimen preparation is critical for obtaining reliable and accurate results. Mount size and shape, as well as different mounting methods, are important factors in specimen preparation. This article has provided an overview of mount size and shape, as well as different mounting methods for metallographic specimens.
Preparing Metallographic Cross Sections with Clamp Mounting
Cross sectional analysis is an essential tool for examining the microstructure of metals. In order to prepare a metallographic cross section, the specimen must be mounted securely, without damaging the edge of the sample. Clamp mounting is a popular method for doing this due to its ease of use and excellent edge preservation.
What is Clamp Mounting?
Clamp mounting involves clamping two plates together with the specimen sandwiched between them. The two plates are usually made of 1/4 inch thick steel and must be harder than the specimen material. Holes are drilled into the plates and machine bolts are inserted through them to draw the plates together tightly. A third bolt midway between the ends of the clamp is often used to maintain a uniform separation between the plates.
Advantages of Clamp Mounting
Clamp mounting is a fast and reliable method of preparing a metallographic cross section. The specimen edges are well preserved due to the intimate contact between the specimen and the clamp plates. However, the clamping pressure can sometimes cause hairline separations between specimens which can trap abrasive particles or liquid solutions.
Solutions for Hairline Separations
One way to prevent this issue is to insert a thickness of transparent plastic wrapping film between the specimens. This will seal the hairline separations and preserve the specimen edges. Additionally, soft, thin metal sheets of the same type as the specimen can be used instead of plastic film, or the mount can be vacuum impregnated.
Compression Mounting: The Most Common Method for Specimen Mounting
Mounting specimens for microscopic observation is a necessary step for many scientific and industrial applications. Compression mounting is the most widely used method, and involves molding around the specimen with heat and pressure, using materials such as Bakelite, diallyl phthalate resins, and acrylic resins. With the right tools and equipment, it is possible to mount most specimens with this method.
What is Compression Mounting?
Compression mounting is a method of mounting specimens by heating and pressing molding material around the specimen. This creates a mount that is strong, durable, and can be used in a variety of applications. For example, compression mounting of metals is widely used, while phenolics are popular due to their low cost. Diallyl phthalates and epoxy resins are used where edge retention and harder mounts are desired, and acrylic compression mounting compounds are used when clarity is a priority.
What Equipment is Needed for Compression Mounting?
Compression mounting requires a mounting press equipped with molding tools and a heater. Readily available molding tools for mounts with diameters of 1, 1 1/4 and 1 1/2 inches consist of a hollow cylinder of hardened steel, a base plug, and a plunger. The specimen to be mounted is placed on the base plug, which is inserted into one end of the cylinder. The cylinder is filled with molding material in powder form, and the plunger is then inserted into the open end of the cylinder. A cylindrical heater is then placed around the mold assembly, which is positioned between the platens of the mounting press.
How Does Compression Mounting Work?
Once the prescribed pressure has been applied and maintained on the plunger to compress the molding material until it and the mold assembly is heated to the required temperature, the finished mount can be ejected from the mold by forcing the plunger entirely through the mold cylinder. Thermosetting and thermoplastic materials are both used for compression mounting, however thermoplastic materials remain molten at the maximum molding temperature, and must cool under pressure before ejection.
Are There Any Limitations with Compression Mounting?
Unfortunately, not all materials or specimens can be mounted in thermosetting 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 the selected specimen may also be too large to be accepted by the available mold sizes. These difficulties can usually be overcome by cold mounting.
Cold Mounting: A Comprehensive Guide
Cold mounting is a casting process used to mount large numbers of specimens quickly and easily. It requires no pressure and little heat, and is a popular choice for mounting a wide variety of materials. This guide covers the three main classifications of cold mounting materials – polyesters, epoxides, and acrylics – and provides information on how to use them correctly for optimal results.
What Are Cold Mounting Materials?
Cold mounting materials are two-component systems consisting of a resin and a hardener. The resin and hardener can both be liquid or one can be liquid and the other a solid. When the resin and hardener are mixed, an exothermic polymerization occurs, producing heat. It is important to control this heat to ensure the temperature does not rise too high and damage the specimen.
Different Types of Cold Mounting Materials
The three main classifications of cold mounting materials are polyesters, epoxides, and acrylics.
Polyester Resins: Transparent and usually water clear, polyester resins are popular for their low volume shrinkage. They can be used to impregnate porous structures or cracks by vacuum method.
Epoxides: Epoxides are almost transparent and straw colored. They are hard and adhere tenaciously to most metallurgical, mineral, and ceramic specimens.
Acrylics: Acrylics are opaque and fast-curing. They can be used for electronic and ceramic materials, or for brittle and porous materials.
How to Use Cold Mounting Materials
When using cold mounting materials, it is important to pay attention to the mixing and casting process. The mold material may become part of the mount in the form of an outer shell, or mold release agents may be used to permit the mount to be ejected from the mold. For round molds, Bakelite ring forms or ring sections cut from plastic or metal tubes or pipes can be used. Rectangular molds can be formed by wrapping heavy-duty aluminum foil around wood blocks.
Epoxy resins are the most widely used cold mounting materials and can be cured in a low-temperature oven 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 than epoxies, and acrylics have a fast curing rate.
Tips for Cold Mounting
When cold mounting, it is important to keep the temperature below 85°F to avoid overheating and uncontrollable curing of the mounting compound. Additionally, it is wise to use a mold release agent to ensure the mount can be ejected from the mold. Finally, for faster curing, an external energy source such as heat or microwave energy can be used.
Cold mounting is a simple, fast, and reliable method for mounting large numbers of specimens. By understanding the different types of cold mounting materials and how to use them correctly, you can ensure optimal results.
The Benefits of Conductive Mounting for Metallographic Preparation
Metallography is the study of the structure and composition of metals, which is done by examining thin sections of metal samples under the microscope. To prepare such samples for examination, a variety of mounting methods are available, including conductive mounting. This method offers several advantages over other mounting methods, making it a popular choice among metallographers.
What is Conductive Mounting?
Conductive mounting is a method of mounting a specimen for metallographic examination that uses an electrically conductive material to complete the electrical circuit. This material is typically a mixture of a metal such as copper or iron powder, and a thermosetting or thermoplastic molding material. The metal powder particles are compacted sufficiently to provide electrical continuity throughout the mount, and a copper wire can also be attached to the back of the specimen to create the necessary electrical circuit.
Advantages of Conductive Mounting
Conductive mounting offers several distinct advantages over other mounting methods. Here are some of the key benefits of this method:
Convenience: Conductive mounting is a convenient and straightforward method for metallographic preparation. It requires minimal setup and can be completed quickly with the right materials.
Electrical Continuity: This method provides electrical continuity throughout the mount, which is necessary for electrolytic techniques such as electrochemical etching.
Protection: Conductive mounts offer excellent protection for delicate specimens. The mounting material is designed to provide a cushion that absorbs shock and prevents damage to the sample.
Improved Resolution: Conductive mounts allow for improved resolution, as they reduce the amount of electrical noise that can interfere with the examination.
Cold Sample Mounting
Epoxy resin types generally have the best characteristics with respect to hardness and
shrinkage. However, epoxy resins tend to be slower curing and adequate time should be
allowed to ensure that the material is fully cured before proceeding. Epoxies often take
a considerable period of time after initial ‘setting’ to develop full hardness. It is not
generally possible to make cold curing resins conductive suitable for SEM examination.
Hot Mounting: A Guide to Improved Specimen Preservation
Hot mounting is a popular technique used to preserve specimens for analysis and examination. This method utilizes a thermosetting compound and a mounting press to apply both heat and high pressure to the specimen. The result is a hard mount that can be created quickly and efficiently. However, not all specimens are suitable for hot mounting due to the temperature and pressure that it requires.
What is Hot Mounting?
Hot mounting is a process that utilizes a thermosetting compound and a mounting press to apply both heat and pressure to the specimen. The heat applied generally ranges between 120°C and 200°C, while the pressure can reach up to 50 kN. The result is a hard mount that is created in a relatively short period of time.
Advantages of Hot Mounting
Hot mounting is an effective technique for preserving specimens for analysis and examination. It is a fast and efficient process, and the mounts created are generally very hard and durable. Hot mounting is also suitable for most specimens, including those that are delicate, soft or have a low melting point.
Disadvantages of Hot Mounting
Not all specimens are suitable for hot mounting. Some materials may be too sensitive to withstand the temperature and pressure required. Additionally, the materials used for hot mounting may not be as durable or long-lasting as those used for cold setting.
Protecting Delicate Samples
When hot mounting delicate specimens, techniques can be used to protect the sample from the effects of pressure. This can include placing the sample under a supporting structure within the molding cavity, which can protect the sample from the initial pressure applied. Additionally, hydrostatic pressure can be applied to most specimens without causing any damage.
Conductive Mounts
Conductive mounts are available that are suitable for use with a scanning electron microscope (SEM). These mounts are not as hard or durable as epoxy hot set compounds, but they can still provide good adhesion and hardness. If the edges of the specimen are not important, then nonconductive mounting materials can be used.
Conclusion
Hot mounting is an effective technique for preserving specimens for analysis and examination. While it is generally suitable for most specimens, it may not be appropriate for delicate, soft or low melting point specimens. Additionally, the materials used for hot mounting may not be as durable or long-lasting as those used for cold setting. Care must be taken to protect delicate samples from the effects of temperature and pressure, and conductive mounts may be required for SEM examination.
Hot Mounting vs Cold Sample Mounting
When it comes to mounting a specimen for metallographic examination, there are two main methods to choose from: hot mounting and cold sample mounting. Hot mounting involves using a thermosetting material that is cured in a mounting press under both heat and pressure. This method produces hard mounts quickly, but it may not be suitable for delicate, soft or low melting point specimens. Cold sample mounting, on the other hand, uses a non-conductive material that is cured at room temperature. This method is slower and produces less durable mounts, but it is better for very soft or thermally sensitive materials.
Conclusion
Conductive mounting is a popular choice for metallographic preparation, as it offers several advantages over other mounting methods. It is convenient and provides electrical continuity throughout the mount, as well as excellent protection for delicate specimens. However, it may not be suitable for all specimens, so it is important to consider the type of sample being examined before deciding which method to use.