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Light or optical microscopy is the primary means for scientists and engineers to examine the microstructure of materials. The history of using a light microscope for microstructural examination of materials can be traced back to the 1880s. Since then, light microscopy has been widely used by metallurgists to examine metallic materials. Light microscopy for metallurgists became a special field named metallography. The basic techniques developed in metallography are not only used for examining metals, but also are used for examining ceramics and polymers. In this chapter, light microscopy is introduced as a basic tool for microstructural examination of materials including metals, ceramics, and polymers.

1.1 Optical Principles

1.1.1 Image Formation

Reviewing the optical principles of microscopes should be the first step to understanding light microscopy. The optical principles of microscopes include image formation, magnification, and resolution. Image formation can be illustrated by the behavior of a light path in a compound light microscope as shown in Figure 1.1. A specimen (object) is placed at position A where it is between one and two focal lengths from an objective lens. Light rays from the object first converge at the objective lens and are then focused at position B to form a magnified inverted image. The light rays from the image are further converged by the second lens (projector lens) to form a final magnified image of an object at C.

Figure 1.1 Principles of magnification in a microscope.

The light path shown in Figure 1.1 generates the real image at C on a screen or camera film, which is not what we see with our eyes. Only a real image can be formed on a screen and photographed. When we examine microstructure with our eyes, the light path in a microscope goes through an eyepiece instead of projector lens to form a virtual image on the human eye retina, as shown in Figure 1.2. The virtual image is inverted with respect to the object. The virtual image is often adjusted to be located as the minimum distance of eye focus, which is conventionally taken as 250 mm from the eyepiece. A modern microscope is commonly equipped with a device to switch from eyepiece to projector lens for either recording images on photographic film or sending images to a computer screen.

Figure 1.2 Schematic path of light in a microscope with an eyepiece. The virtual image is reviewed by a human eye composed of the eye lens and retina.

Advanced microscopes made since 1980 have a more complicated optical arrangement called “infinity-corrected” optics. The objective lens of these microscopes generates parallel beams from a point on the object. A tube lens is added between the objective and eyepiece to focus the parallel beams to form an image on a plane, which is further viewed and enlarged by the eyepiece.

The magnification of a microscope can be calculated by linear optics, which tells us the magnification of a convergent lens, M:

where f is the focal length of the lens and v is the distance between the image and lens. A higher magnification lens has a shorter focal length, as indicated by Eq. (1.1). The total magnification of a compound microscope as shown in Figure 1.1 should be the magnification of the objective lens multiplied by that of the projector lens.

When an eyepiece is used, the total magnification should be the objective lens magnification multiplied by the eyepiece magnification.

1.1.2 Resolution

We naturally ask whether there is any limitation for magnification in light microscopes because Eq. (1.2) suggests there is no limitation. However, meaningful magnification of a light microscope is limited by its resolution. Resolution refers to the minimum distance between two points at which they can be visibly distinguished as two points. The resolution of a microscope is theoretically controlled by the diffraction of light.

Light diffraction controlling the resolution of microscope can be illustrated with the images of two self-luminous point objects. When the point object is magnified, its image is a central spot (the Airy disk) surrounded by a series of diffraction rings (Figure 1.3), not a single spot. To distinguish between two such point objects separated by a short distance, the Airy disks should not severely overlap each other. Thus, controlling the size of the Airy disk is the key to controlling resolution. The size of the Airy disk (d) is related to the wavelength of light (λ) and the angle of light coming into the lens. The resolution of a microscope (R) is defined as the minimum distance between two Airy disks that can be distinguished (Figure 1.4). Resolution is a function of microscope parameters as shown in the following equation:

where μ is the refractive index of the medium between the object and objective lens and α is the half-angle of the cone of light entering the objective lens (Figure 1.5). The product, μ sin α, is called the numerical aperture (NA).

Figure 1.3 A self-luminous point object and the light-intensity distribution along a line passing through its center.

Figure 1.4 Intensity distribution of two airy disks with a distance d/2. I₁ indicates the maximum intensity of each point and I₂ represents the overlap intensity.

Figure 1.5 The cone of light entering an objective lens showing α is the half-angle.

According to Eq. (1.3), to achieve higher resolution we should use shorter-wavelength light and larger NA. The shortest wavelength of visible light is about 400 nm, while the NA of the lens depends on α and the medium between the lens and object. Two media between object and objective lens are commonly used: either air for which μ = 1, or oil for which μ ≈ 1.5. Thus, the maximum value of NA is about 1.5. We estimate the best resolution of a light microscope from Eq. (1.3) as about 0.2 µm.

1.1.2.1 Effective Magnification

Magnification is meaningful only in so far as the human eye can see the features resolved by the microscope. Meaningful magnification is the magnification that is sufficient to allow the eyes to see the microscopic features resolved by the microscope. A microscope should enlarge features to about 0.2 mm, the resolution level of the human eye. This means that the microscope resolution multiplying the effective magnification should be equal to the eye resolution. Thus, the effective magnification of a light microscope should approximately be M_eff = 0.2 ÷ 0.2 × 10³ = 1.0 × 10³.

A higher magnification than the effective magnification only makes the image bigger, may make eyes more comfortable during observation, but does not provide more detail in an image.

1.1.2.2 Brightness and Contrast

To make a microscale object in a material specimen visible, high magnification is not sufficient. A microscope should also generate sufficient brightness and contrast of light from the object. Brightness refers to the intensity of light. In a transmission light microscope the brightness is related to the numerical aperture (NA) and magnification (M).

These relationships indicate that the brightness decreases rapidly with increasing magnification, and controlling NA is not only important for resolution but also for brightness, particularly in a reflected-light microscope.

Contrast is defined as the relative change in light intensity (I) between an object and its background.

Visibility requires that the contrast of an object exceeds a critical value called the contrast threshold. The contrast threshold of an object is not constant for all images but varies with image brightness. In bright light, the threshold can be as low as about 3%, while in dim light the threshold is greater than 200%.

1.1.3 Depth of Field

Depth of field is an important concept when photographing an image. It refers to the range of position for an object in which image sharpness does not change. As illustrated in Figure 1.6, an object image is only accurately in focus when the object lies in a plane within a certain distance from the objective lens. The image is out of focus when the object lies either closer to or farther from the lens. Since the diffraction effect limits the resolution R, it does not make any difference to the sharpness of the image if the object is within the range of D_f shown in Figure 1.6. Thus, the depth of field can be calculated.

Figure 1.6 Geometric relation among the depth of field (D_f), the half-angle entering the objective lens (α), and the size of the Airy disk (d).

Equation (1.7) indicates that a large depth of field and high resolution cannot be obtained simultaneously; thus, a larger D_f means a larger R and worse resolution. We may reduce angle α to obtain a better depth of field only at the expense of resolution. For a light microscope, α is around 45° and the depth of field is about the same as its resolution.

We should not confuse depth of field with depth of focus. Depth of focus refers to the range of image plane positions at which the image can be viewed without appearing out of focus for a fixed position of the object. In other words, it is the range of screen positions in which and images can be projected in focus. The depth of focus is M² times larger than the depth of field.

1.1.4 Aberrations

The aforementioned calculations of resolution and depth of field are based on the assumptions that all components of the microscope are perfect, and that light rays from any point on an object focus on a correspondingly unique point in the image. Unfortunately, this is almost impossible due to image distortions by the lens called lens aberrations. Some aberrations affect the whole field of the image (chromatic and spherical aberrations), while others affect only off-axis points of the image (astigmatism and curvature of field). The true resolution and depth of field can be severely diminished by lens aberrations. Thus, it is important for us to have a basic knowledge of aberrations in optical lenses.

Chromatic aberration is caused by the variation in the refractive index of the lens in the range of light wavelengths (light dispersion). The refractive index of lens glass is greater for shorter wavelengths (for example, blue) than for longer wavelengths (for example, red). Thus, the degree of light deflection by a lens depends on the wavelength of light. Because a range of wavelengths is present in ordinary light (white light), light cannot be focused at a single point. This phenomenon is illustrated in Figure 1.7.

Figure 1.7 Paths of rays in white light illustrating chromatic aberration.

Spherical aberration is caused by the spherical curvature of a lens. Light rays from a point on the object on the optical axis enter a lens at different angles and cannot be focused at a single point, as shown in Figure 1.8. The portion of the lens farthest from the optical axis brings the rays to a focus nearer the lens than does the central portion of the lens.

Figure 1.8 Spherical aberration.

Astigmatism results when the rays passing through vertical diameters of the lens are not focused on the same image plane as rays passing through horizontal diameters, as shown in Figure 1.9. In this case, the image of a point becomes an elliptical streak at either side of the best focal plane. Astigmatism can be severe in a lens with asymmetric curvature.

Figure 1.9 Astigmatism is an off-axis aberration.

Curvature of field is an off-axis aberration. It occurs because the focal plane of an image is not flat but has a concave spherical surface, as shown in Figure 1.10. This aberration is especially troublesome with a high magnification lens with a short focal length. It may cause unsatisfactory photography.

Figure 1.10 Curvature of field is an off-axis aberration.

There are a number of ways to compensate for and/or reduce lens aberrations. For example, combining lenses with different shapes and refractive indices corrects chromatic and spherical aberrations. Selecting single-wavelength illumination by the use of filters helps eliminate chromatic aberrations. We expect that the extent to which lens aberrations have been corrected is reflected in the cost of the lens. It is a reason that we see huge price variation in microscopes.

1.2 Instrumentation

A light microscope for examining material microstructure can use either transmitted or reflected light for illumination. Reflected-light microscopes are the most commonly used for metallography, while transmitted-light microscopes are typically used to examine transparent or semitransparent materials, such as certain types of polymers. Figure 1.11 illustrates the structure of a light microscope for materials examination.

Figure 1.11 An Olympus light microscope used for material examination. The microscope includes transmission- and reflection-illumination systems.

(This image is courtesy of Olympus Corporation.)

1.2.1 Illumination System

The illumination system of a microscope provides visible light by which a specimen is observed. There are three main types of electric lamps used in light microscopes:

Tungsten bulbs provide light of a continuous wavelength spectrum from about 300 to 1500 nm. Their color temperature of the light, which is important for color photography, is relatively low. Low color temperature implies warmer (more yellow–red) light while high color temperature implies colder (more blue) light. Tungsten–halogen bulbs, like ordinary tungsten bulbs, provide a continuous spectrum. Their light is brighter and the color temperature is significantly higher than ordinary tungsten bulbs. The high filament temperature of tungsten–halogen bulbs, however, needs a heat filter in the light path and good ventilation. Gas-discharge tubes filled with pressurized mercury or xenon vapor provide extremely high brightness. The more commonly used tubes are filled with mercury, of which the arc has a discontinuous spectrum. Xenon has a continuous spectrum and very high color temperature. As with tungsten–halogen bulbs, cooling is required for gas-discharge tubes.

In a modern microscope, the illumination system is composed of a light lamp (commonly a tungsten–halogen bulb), a collector lens and a condenser lens to provide integral illumination; such a system is known as the Köhler system. The main feature of the Köhler system is that the light from the filament of a lamp is first focused at the front focal plane of the condenser lens by a collector lens. Then, the condenser lens collects the light diverging from the source and directs it at a small area of the specimen be examined. The Köhler system provides uniform intensity of illumination on the area of specimen. The system generates two sets of conjugate focal planes along the optic axis of a microscope as shown in Figure 1.12. One set of focal planes is for illuminating rays; these are known as the conjugate aperture planes. Another set comprises the image-forming rays called the conjugate field planes. During normal microscope operation, we see only the image-forming rays through the eyepiece. We can use the illuminating rays to check the alignment of the optical system of the microscope.

Figure 1.12 Two sets of conjugate focal planes in the Köhler system illustrated in a transmitted-light microscope. Image-forming rays focus on the field planes and illuminating rays focus on the aperture planes. The far left-hand and far right-hand parts of the diagram illustrate the images formed by image-forming rays and illuminating rays, respectively.

There are two important controllable diaphragms in the Köhler system: the field diaphragm and the aperture diaphragm. The field diaphragm is placed at a focal plane for the image-formation rays. Its function is to alter the diameter of the illuminated area of the specimen. When the condenser is focused and centered, we see a sharp image of the field diaphragm with the image of specimen (Figure 1.13). The field diaphragm restricts the area of view and blocks scattering light that could cause glare and image degradation if they entered the objective lens and eyepiece. The aperture diaphragm is placed at a focus plane of the illuminating rays. Its function is to control α, and thus affect the image resolution and depth of field (Sections 1.1.2 and 1.1.3). We cannot see the aperture diaphragm with the image of specimen. Figure 1.14 illustrates the influence of the aperture diaphragm on the image of a specimen.

Figure 1.13 Image of the field diaphragm with an image of the specimen. Magnification 100×.

Figure 1.14 Effect of aperture diaphragm on specimen image when: (a) the aperture is large and (b) the aperture is small. Magnification 500×.

The main difference between transmitted-light and reflected-light microscopes is the illumination system. The Köhler system of reflected light illumination (epi-illumination) is illustrated in Figure 1.15 in which a relay lens is included. The illuminating rays are reflected by a semitransparent reflector to illuminate the specimen through an objective lens. There is no difference in how reflected and transmitted-light microscopes direct light rays after the rays leave the specimen. There may be a difference in the relative position of the field and aperture diaphragms (Figure 1.12). However, the field diaphragm is always on the focal plane of the image-forming rays while the aperture diaphragm is on a focal plane of the illuminating rays.

Figure 1.15 Illumination system of a reflected-light microscope with illuminating rays.

Light filters are often included in the light path of illumination systems, even though they are not shown in Figures 1.12 and 1.15. Light filters are used to control the wavelengths and intensity of illumination in microscopes in order to achieve optimum visual examination for photomicrography. Neutral density (ND) filters can regulate light intensity without changing wavelength. Colored filters and interference filters are used to isolate specific colors or bands of wavelength. The colored filters are commonly used to produce a broad band of color, while the interference filters offer sharply defined bandwidths. Colored filters are used to match the color temperature of the light to that required by photographic films. Selected filters can also increase contrast between specimen areas with different colors. Heat filters absorb much of the infrared radiation that causes heating of specimen when a tungsten–halogen bulb is used as light source.

1.2.2 Objective Lens and Eyepiece

The objective lens is the most important optical component of a light microscope. The magnification of the objective lens determines the total magnification of the microscope because eyepieces commonly have a fixed magnification of 10×. The objective lens generates the primary image of the specimen, and its resolution determines the final resolution of the image. The numerical aperture (NA) of the objective lens varies from 0.16 to 1.40, depending on the type of lens. A lens with a high magnification has a higher NA. The highest NA for a dry lens (where the medium between the lens and specimen is air) is about 0.95. Further increase in NA can be achieved by using a lens immersed in an oil medium. The oil-immersion lens is often used for examining microstructure greater than 1000× magnification.

Classification of the objective lens is based on its aberration-correction capabilities, mainly chromatic aberration. The following lenses are shown from low to high capability.

The achromatic lens corrects chromatic aberration for two wavelengths (red and blue). It requires green illumination to achieve satisfactory results for visual observation of black and white photography. The semiachromatic lens improves correction of chromatic aberration. Its NA is larger than that of an achromatic lens with the same magnification and produces a brighter image and higher resolution of detail. The apochromatic lens provides the highest degree of aberration correction. It almost completely eliminates chromatic aberration. It also provides correction of spherical aberration for two colors. Its NA is even larger than that of a semiachromatic lens. Improvement in quality requires a substantial increase in the complexity of the lens structure, and costs. For example, an apochromatic lens may contain 12 or more optical elements.

The characteristics of an objective lens are engraved on the barrel as shown in Figure 1.16. Engraved markings may include the following abbreviations.

Figure 1.16 Engraved markings on the barrel of an objective lens.

The eyepiece is used to view the real primary image formed by the objective lens. In some cases it also completes the correction of aberrations. The eyepiece allows a glass disc with an etched graticule to be inserted into the optical path. The graticule serves as a micrometer for measurement. The eyepiece has either a helical thread or a sliding mount as a focusing mechanism. Importantly, the focusing mechanism of an eyepiece provides a “parfocal” adjustment of the optics so that the same focal plane examined by the eye will be in focus on the film plane of the camera mounted on the microscope. Thus, focusing the eyepiece is a necessary step before photographing images in a microscope.

We can summarize the methods for achieving optimum resolution and depth of field in light microscopy. While both resolution and depth of field are crucial for achieving high-quality images, one often is achieved at the expense of the other. Thus, compromises must be made while using good judgment.

1.2.2.1 Steps for Optimum Resolution

1.2.2.2 Steps to Improve Depth of Field

1.3 Specimen Preparation

The microstructure of a material can only be viewed in a light microscope after a specimen has been properly prepared. Metallurgists have developed extensive techniques and accumulated knowledge of metal specimen preparations for over a century. In principle, we can use these techniques to examine not only metallic materials but also ceramics and polymers; in practice, certain modifications are needed and a certain degree of caution must be exercised. The main steps of specimen preparation for light microscopy include the following.

1.3.1 Sectioning

Sectioning serves two purposes: generating a cross section of the specimen to be examined; and reducing the size of a specimen to be placed on a stage of a light microscope, or reducing the size of a specimen to be embedded in mounting media for further preparation processes. The main methods of sectioning are abrasive cutting, electric discharge machining, and microtomy that is mainly for polymer specimens.

1.3.1.1 Cutting

Abrasive cutting is the most commonly used method for sectioning materials. Specimens are sectioned by a thin rotating disc in which abrasive particles are supported by suitable media. The abrasive cutoff machine is commonly used for sectioning a large sample. The machine sections the sample with a rapidly rotating wheel made of an abrasive material, such as silicon carbide, and bonding materials such as resin and rubber. The wheels are consumed in the sectioning process. Abrasive cutting requires cooling media in order to reduce friction heat. Friction heat can damage specimens and generate artifacts in the microstructure. Commonly used cooling media consist of water-soluble oil and rust-inhibiting chemicals. The abrasive cutoff machine can section large specimens quickly but with poor precision.

More precise cutting can be achieved by a diamond saw or electric discharge machine (EDM) (Figure 1.17). The diamond saw is a precision abrasive cutting machine. It sections specimens with a cutting wheel made of tiny diamond particles bonded to a metallic substrate. A cooling medium is also necessary for diamond saw cutting. Electrically conductive materials can be sectioned by an EDM. Cutting is accomplished by an electric discharge between an electrode and the specimen submerged in a dielectric fluid. EDM is particularly useful for materials that are difficult to section by abrasive cutting. EDM may produce significant changes at the machined surface because the electric discharge melts the solid in the cutting path. The solidified material along a machining path must be carefully removed during further preparation processes.

Figure 1.17 Specimen sectioning by: (a) wire cutting with electric discharging and (b) diamond saw sectioning.

1.3.1.2 Microtomy

Microtomy refers to sectioning materials with a knife. It is a common technique in biological specimen preparation. It is also used to prepare soft materials such as polymers and soft metals. Tool steel, tungsten carbide, glass, and diamond are used as knife materials. A similar technique, ultramicrotomy, is widely used for the preparation of biological and polymer specimens in transmission electron microscopy. This topic is discussed in Chapter 3.

1.3.2 Mounting

Mounting refers to embedding specimens in mounting materials (commonly thermosetting polymers) to give them a regular shape for further processing. Mounting is not necessary for bulky specimens, but it is required for specimens that are too small or oddly shaped to be handled or when the edge of a specimen needs to be examined in transverse section. Mounting is popular now because most automatic grinding and polishing machines require specimens to have a cylindrical shape. There are two main types of mounting techniques: hot mounting and cold mounting.

Hot mounting uses a hot-press equipment as shown in Figure 1.18. A specimen is placed in the cylinder of a press and embedded in polymeric powder. The surface to be examined faces the bottom of the cylinder. Then, the specimen and powder are heated at about 150 °C under constant pressure for tens of minutes. Heat and pressure enable the powder to bond with the specimen to form a cylinder. Phenolic (bakelite) is the most widely used polymeric powder for hot mounting. Hot mounting is suitable for most metal specimens. However, if the microstructure of the material changes at the mounting temperature, cold mounting should be used.

Figure 1.18 Internal arrangement of a hot mounting press.

In cold mounting, a polymer resin, commonly epoxy, is used to cast a mold with the specimen at ambient temperature. Figure 1.19a shows a typical mold and specimens for cold mounting. Figure 1.19b demonstrates the casting of epoxy resin into the mold in which the specimen surface to be examined is facing the bottom. A cold mounting medium has two constituents: a fluid resin and a powder hardener. The resin and hardener should be carefully mixed in proportion following the instructions provided. Curing times for mounting materials vary from tens of minutes to several hours, depending on the resin type. Figure 1.20 shows the specimens after being cold mounted in various resins.

Figure 1.19 Cold mounting of specimens: (a) place specimens on the bottom of molds supported by clamps and (b) cast resin into the mold.

(Reproduced with permission of Struers A/S.)

Figure 1.20 Cold mounted specimens: (a) mounted with polyester; (b) mounted with acrylic; and (c) mounted with acrylic and mineral fillers.

(Reproduced with permission of Struers A/S.)

An important issue in the selection of a mounting material is hardness compatibility with the specimen. Generally, plastics used for embedding are not as hard as the specimen, particularly when the specimens are of metallic or ceramic. Too great a difference in hardness can cause inhomogeneous grinding and polishing, which in turn may generate a rough, rather than sharp edge on the specimen. A solution to this problem is to embed metal beads with a specimen to ensure that the grinding surface has a more uniform hardness.

There are a number of other mounting techniques available but they are less widely used. The simplest is mechanical clamping, in which a thin sheet of the specimen is clamped in place with a mechanical device. Adhesive mounting is glueing a specimen to a large holder. Vacuum impregnation is a useful mounting method for porous specimens and ceramics. It removes air from the pores, crevices, and cracks of specimens, and then replaces such empty space in the specimen with epoxy resin. First, a specimen is ground with grit paper to flatten the surface to be examined. The specimen is placed with the surface uppermost inside the mold in a vacuum chamber. Then, the chamber is evacuated for several minutes before filling the mold with epoxy. The vacuum is maintained for a few minutes and then air is allowed to enter the chamber for a curing period.

1.3.3 Grinding and Polishing

Grinding refers to flattening the surface to be examined and removing any damage caused by sectioning. The specimen surface to be examined is abraded using a graded sequence of abrasives, starting with a coarse grit. Commonly, abrasives (such as silicon carbide) are bonded to abrasive paper. Abrasive paper is graded according to particle size of abrasives such as 120-, 240-, 320-, 400-, and 600-grit paper. The starting grit size depends on the surface roughness and depth of damage from sectioning. Usually, the starting grade is 240 or 320 grit after sectioning with a diamond saw or EDM. Both hand grinding and machine grinding are commonly used.

1.3.3.1 Grinding

We can perform hand grinding with a simple device in which four belts of abrasive paper (240-, 320-, 400-, and 600-grit) are mounted in parallel as shown in Figure 1.21. Running water is supplied to cool specimen surfaces during hand grinding. Grinding produces damage that must be minimized by subsequent grinding with finer abrasives. The procedure is illustrated in Figure 1.22. In particular, two procedures must be followed to ensure optimal results. First, specimens are rinsed with running water to remove surface debris before switching grinding belts; and secondly, specimens are rotated 90° from the previous orientation. Rotation ensures that grinding damage generated by a coarse grit is completely removed by a subsequent finer grit. Thus, at the end of any grinding step, the only grinding damage present must be from that grinding step. Damage from the final grinding step is removed by polishing.

Figure 1.21 Hand grinding using a simple hand grinding device.

(Reproduced with permission of Buehler Ltd.)

Figure 1.22 Hand grinding procedure.

Automatic grinding machines have become very popular because they reduce tedious work and are able to grind multiple specimens simultaneously. Also, machine grinding produces more consistent results. A disc of abrasive paper is held on the surface of a motor-driven wheel. Specimens are fixed on a holder that also rotates during grinding. Modern grinding machines control the speeds of the grinding wheel and the specimen holder independently. The direction of rotation of the holder and the compressive force between specimen and grinding wheel can also be altered. The machines usually use running water as the cooling medium during grinding to avoid friction heat and to remove loose abrasives that are produced continuously during grinding.

1.3.3.2 Polishing

Polishing is the last step in producing a flat, scratch-free surface. After being ground to a 600-grit finish, the specimen should be further polished to remove all visible scratches from grinding. Effects of grinding and polishing a specimen surface are shown in Figure 1.23. Polishing generates a mirror-like finish on the specimen surface to be examined. Polishing is commonly conducted by placing the specimen surface against a rotating wheel either by hand or by a motor-driven specimen holder (Figure 1.24). Abrasives for polishing are usually diamond paste, alumina, or other metal-oxide slurries. Polishing includes coarse and fine polishing. Coarse polishing uses abrasives with a grit size in the range from 3 to 30 µm; 6-µm diamond paste is the most popular. The abrasive size for fine polishing is usually less than 1 µm. Alumina slurries provide a wide range of abrasive size, ranging down to 0.05 µm.

Figure 1.23 Sample of specimen surfaces after grinding and polishing with abrasives of different grits and size.

Figure 1.24 Polishing on a rotating wheel with a mechanical sample holder.

(Reproduced with permission of Struers A/S.)

A polishing cloth covers a polishing wheel to hold the abrasives against the specimen during polishing. Polishing cloths must not contain any foreign matter that may scratch specimen surfaces. Polishing cloths should also be able to retain abrasives so that abrasives are not easily thrown out from the wheel. For coarse polishing, canvas, nylon, and silk are commonly used as they have little or no nap. For fine polishing, medium- or high-nap cloths are often used; one popular type consists of densely packed, vertical synthetic fibers.

When hand polishing using a polishing wheel, we should not push the specimen too hard against the wheel as excessive force will generate plastic deformation in the top layer of a polished surface. We should also rotate the specimen against the rotation direction of wheel. Without rotation, artifacts of comet tailing will appear on the polished surfaces as shown in Figure 1.25. After each polishing step, the surface should be cleaned in running water with cotton or tissue, followed by alcohol or hot-air drying. Alcohol provides fast drying of surfaces without staining.

Figure 1.25 Comet tailing generated by polishing on specimen surface: (a) bright-field image and (b) Nomarski contrast image.

(Reproduced with permission of Struers A/S.)

Electrolytic polishing is an alternative method of polishing metallic materials. A metal specimen serves as the anode in an electrochemical cell containing an appropriate electrolyte. The surface is smoothed and brightened by the anodic reaction in an electrochemical cell when the correct combination of bath temperature, voltage, current density, and time are used. The advantage of this method over conventional polishing is that there is no chance of plastic deformation during the polishing surface. Plastic deformation in the surface layer of specimens can be generated by compression and shear forces arising from conventional polishing methods. Plastic deformation from polishing may generate artifacts in microstructures of materials.

The aforementioned methods of specimen preparation, except microtomy, are regarded as an important part of metallography. These methods are also used for nonmetallic materials such as ceramics, composites, and polymers. However, various precautions must be taken in consideration of each material's particular characteristics. For example, ceramic materials are brittle. To avoid fracture they should be mounted and sectioned with a slow-speed diamond saw. Composite materials may exhibit significant differences in mechanical properties between the reinforcement and matrix. These specimens require light pressure and copious cooling during grinding and polishing. Polymeric materials can be examined by either reflected or transmitted-light microscopes. For reflected-light microscopy, specimen preparation is similar to that of metals. For transmitted-light microscopy, a thin-section is required. Both surfaces of the thin section should be ground and polished. This double-sided grinding and polishing can be done by mounting the specimen in epoxy, preparing one surface, mounting that polished surface on a glass slide, and finally grinding and polishing the other side.

1.3.4 Etching

Chemical etching is a method to generate contrast between microstructural features in specimen surfaces. Etching is a controlled corrosion process by electrolytic action between surface areas with differences in electrochemical potential. Electrolytic activity results from local physical or chemical heterogeneities that render some microstructural features anodic and others cathodic under specific etching conditions. During etching, chemicals (etchants) selectively dissolve certain areas of the specimen surface because such areas exhibit different electrochemical potentials and will serve as the anode in an electrochemical reaction on the specimen surface. For example, grain boundaries in polycrystalline materials are more severely attacked by etchant, and thus are revealed by light microscopy because they reflect light differently, as illustrated in Figure 1.26a and appear as the dark lines shown in Figure 1.26b. Also, grains are etched at different rates because of differences in grain orientation (certain crystallographic planes are more subject to etching), resulting in crystal faceting. Thus, the grains show different brightness. Etching a specimen that has a multiphase microstructure will result in selective dissolution of the phases.

Figure 1.26 Contrast generated by etching grain boundaries in light microscope: (a) reflection from different parts of a surface (Reproduced with permission from Ref. [3]. © 2006 John Wiley & Sons Inc.) and (b) micrograph of grain boundaries that appear as dark lines.

(Contribution of the National Institute of Standards and Technology.)

Many chemical etchants are mixtures of acids with a solvent such as water. Acids oxidize atoms of a specimen surface and change them to cations. Electrons released from atoms of specimen surfaces are combined with hydrogen to form hydrogen gas. For more noble materials, etchants must contain oxidizers (such as nitric acid, chromic acid, iron chloride, and peroxides). Oxidizers release oxygen, which accepts electrons from atoms of the specimen surface. Table 1.1 lists some commonly used etchants, their compositions and applications.

Etching can simply be performed by immersion or swabbing. For immersion etching, the specimen is immersed in a suitable etchant solution for several seconds to several minutes, and then rinsed with running water. The specimen should be gently agitated to eliminate adherent air bubbles during immersion. For swab etching, the polished surface of a specimen is wiped with a soft cotton swab saturated with etchant. Etching can also be assisted with direct electric current, similar to an electrolytic polishing, using the specimen as an anode and an insoluble material (such as platinum) as the cathode in an electrochemical cell filled with electrolyte. The electrochemical reaction on the anode produces selective etching on the specimen surface. Since electrochemical etching is a chemical reaction, besides choosing a suitable etchant and electrolyte, temperature and time are the key parameters to avoiding underetching and overetching of specimens.

We may also use the method of tint etching to produce color contrast in microstructures. Tint etchants, usually acidic, are able to deposit a thin (40–500 nm) film such as an oxide or sulfide on specimen surfaces. Tint etching requires a very high-quality polished surface for best results. Tint etching can also be done by heat tinting, a process by which a specimen is heated to a relatively low temperature in air. As it warms, the polished surface is oxidized. The oxidation rate varies with the phase and chemical composition of the specimen. Thus, differences in the thickness of oxidation films on surfaces generate variations in color. Interference colors are obtained once the film reaches a certain thickness. The effectiveness of heat tinting depends on the material of specimens: it is effective for alloy steels and other nonferrous metals and carbides, but not for carbon or low-alloy steels.

1.4 Imaging Modes

The differences in properties of the light waves reflected from microscopic objects enable us to observe these objects by light microscopy. The light wave changes in either amplitude or phase when it interacts with an object as illustrated in Figure 1.27. The eye can only appreciate amplitude and wavelength differences in light waves, not their phase difference. The most commonly used examination modes, bright-field and dark-field imaging, are based on contrast due to differences in wave amplitudes. The wave phase differences have to be converted to amplitude differences through special optical arrangements such as in the examination modes of phase contrast, polarized light, and Nomarski contrast. This section introduces commonly used modes of light microscopy for materials characterization.

Figure 1.27 (a) Reference wave; (b) amplitude difference; and (c) phase difference generated by objects.

1.4.1 Bright-Field and Dark-Field Imaging

Bright-field imaging is the predominant mode for examining microstructure. Dark-field imaging is also widely used to obtain an image with higher contrast than in bright-field imaging. Figure 1.28 illustrates the difference in optical arrangement between these modes in transmitted illumination. In bright-field imaging, the specimen is evenly illuminated by a light source. Dark-field imaging requires that the specimen is illuminated by oblique light rays. There is a central stop in the light path to block the central portion of light rays from illuminating the specimen directly. Thus, the angle of the light rays illuminating the specimen is so large that light from the specimen cannot enter the objective lens unless it is scattered by microscopic objects. The dark field in reflected illumination is also realized using a central stop in the light path (Figure 1.29Figure 1.30Figure 1.30

Materials	Composition^a	Procedure
Al and alloys	Keller's reagent	Immerse 10–20 s
	2.5 ml HNO₃, 1.5 ml HCl
	1.0 ml HF, 95 ml water
Fe and steels	Nital	Immerse few seconds to 1 min
Fe and steels	1–10 ml HNO₃ in 90–99 ml methanol Picral	Immerse few seconds to 1 min
Stainless steels	4–10 g picric acid, 100 ml ethanol Vilella's Reagent	Immerse for up to 1 min
	1 g picric acid, 5 ml HCl, 100 ml ethanal
Cu and alloys	2 g K₂Cr₂O₇, 8 ml H₂SO₄, 4 drops HCl, 100 ml water	Add the HCl before using; immerse 3–60 s
Ti and alloys	10 ml HF, 5 ml HNO₃, 85 ml water	Swab 3–20 s
Mg and alloys	1 ml HNO₃, 75 ml ethylene glycol, 25 ml water	Swab 3–120 s
Zn and alloys	Palmerton's Reagent	Immerse up to 3 min; rinse in 20% aq. CrO₃
	40 g CrO₃, 3 g Na₂SO₄, 200 ml water
Co and alloys	15 ml HNO₃, 15 ml acetic acid, 60 ml HCl, 15 ml water	Age 1 h before use; immerse for up to 30 s
Ni and alloys	50 ml HNO₃, 50 ml acetic acid	Immerse or swab 5–30 s; use hood, do not store
Al₂O₃	15 ml water, 85 ml H₃PO₄	Boil 1–5 min
CaO and MgO	Concentrated HCl	Immerse 3 s to a few minutes
CeO₂, SrTiO₃, Al₂O₃, and ZrO–ZrC	20 ml water, 20 ml HNO₃, 10 ml HF	Immerse up to 15 min
Si₃N₄	Molten KOH	Immerse 1–8 min
SiC	10 g NaOH, 10 g K₃Fe(CN)₆ in 100 ml water at 110 °C	Boil to dryness
Polyethylene (PE)	Xylene	Immerse 1 min at 70 °C
Poly(acrylonitrile butadiene styrene) (ABS); high-impact polystyrene (HIPS); and poly(phenylene oxide) (PPO)	400 ml H₂SO₄, 130 ml H₃PO₄, 125 ml water, 20 g CrO₃	Immerse 15–180 s
Polypropylene (PP)	6 M CrO₃	Immerse 96 h at 70 °C
Phenol formaldehyde	75 ml dimethyl sulfoxide, 25 ml HNO₃	Immerse 4 h at 75–80 °C