ME-UY 3811
Scanning Electron Microscopy
1. Objective
● Continuation of the sample preparation and Tensile test lab.
● Become familiar with the concept of electron microscopy.
● Understand the procedures in preparation of samples for electron microscopy.
2. Background
This lab session is basically a demonstration of the scanning electron microscope (SEM) and its applications in failure analysis, fractography and materials research. The SEM is a unique instrument for analyzing surfaces. The instrument is a combination of electron-optical, vacuum, and electronic control devices. The basic components of the SEM are illustrated in Figure 1. Electrons are generated from an electron gun at the top of the column and accelerated in a constant stream down the column. The electrons are directed toward the specimen which is located in the lower part of the column and there are focused by a system of electromagnetic lenses into a small spot. The size of this spot mainly determines the resolution. A general rule is that smaller spot sizes always produce higher resolution images.
Figure 1. Essential Components of a Scanning Electron Microscope
2.1 Mechanism of SEM
In a typical SEM, an electron beam is thermionically emitted from an electron gun fitted with a tungsten filament cathode. Tungsten is normally used in thermionic electron guns because it has the highest melting point and lowest vapor pressure of all metals, thereby allowing it to be electrically heated for electron emission, and because of its low cost. Other types of electron emitters include lanthanum hexaboride (LaB6) cathodes, which can be used in a standard tungsten filament SEM if the vacuum system is upgraded or field emission guns (FEG), which may be of the cold-cathode type using tungsten single crystal emitters or the thermally assisted Schottky type, that use emitters of zirconium oxide.
The electron beam, which typically has an energy ranging from 0.2 keV to 40 keV, is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in diameter. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the x and y axes so that it scans in a raster fashion over a rectangular area of the sample surface.
When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to approximately 5 µm into the surface. The size of the interaction volume depends on the electron's landing energy, the atomic number of the specimen and the specimen's density. The energy exchange between the electron beam and the sample results in the reflection of high-energy electrons by elastic scattering, emission of secondary electrons by inelastic scattering and the emission of electromagnetic radiation, each of which can be detected by specialized detectors. The beam current absorbed by the specimen can also be detected and used to create images of the distribution of specimen current.
Electronic amplifiers of various types are used to amplify the signals, which are displayed as variations in brightness on a computer monitor (or, for vintage models, on a cathode ray tube). Each pixel of computer video memory is synchronized with the position of the beam on the specimen in the microscope, and the resulting image is therefore a distribution map of the intensity of the signal being emitted from the scanned area of the specimen. In older microscopes images may be captured by photography from a high-resolution cathode ray tube, but in modern machines they are digitized and saved as digital images.
Low-temperature SEM magnification series for a snow crystal. The crystals are captured, stored, and sputter-coated with platinum at cryogenic temperatures for imaging.
Figure 2. Mechanisms of emission of secondary electrons(SE), backscattered electrons(BSE), & characteristic X-rays from atoms of the sample
Magnification in a SEM can be controlled over a range of about 6 orders of magnitude from about 10 to 500,000 times. Unlike optical and transmission electron microscopes, image magnification in an SEM is not a function of the power of the objective lens. SEMs may have condenser and objective lenses, but their function is to focus the beam to a spot, and not to image the specimen. Provided the electron gun can generate a beam with sufficiently small diameter, a SEM could in principle work entirely without condenser or objective lenses, although it might not be very versatile or achieve very high resolution.
In an SEM, as in scanning probe microscopy, magnification results from the ratio of the dimensions of the raster on the specimen and the raster on the display device. Assuming that the display screen has a fixed size, higher magnification results from reducing the size of the raster on the specimen, and vice versa. Magnification is therefore controlled by the current supplied to the x, y scanning coils, or the voltage supplied to the x, y deflector plates, and not by objective lens power.
In addition, there are a number of useful signals generated by the interaction of the incident electron beam with the specimen(backscattered electrons, secondary electrons, x-rays, etc.) and devices used to detect or collect the signals. This capability makes the SEM a unique analytical tool. The determination of the mechanism causing fracture is normally accomplished by examination of the fracture surface at high magnification, usually in the scanning electron microscope (SEM).
2.2 Principles of ductile failure analysis
Fracture is described in various ways depending on the behavior. of material under stress upon the mechanism of fracture or even its appearance. Macroscale examination will provide information indicating whether the fracture is ductile or brittle on the macroscale, and it almost always identifies the fracture-initiation site. Ductile fracture is characterized by tearing of metal and significant plastic deformation which is associated with high energy absorption while brittle fracture with lower energy absorption. Figure 3 shows the features of ductile fracture and brittle fracture. Ductile fracture has dimpled, cup and cone fracture appearance.
Figure 3. (a) Ductile cup-and-cone fracture with necking in a tensile, (b) Brittle fracture
If a necked, but not fractured tensile specimen, is sectioned longitudinally, it is apparent that crack initiation started along the centerline of the specimen on a plane macroscopically normal to the applied load, initially growing outward in a radial direction (Figure 3). Failure could then initiate at any point anyplace in the specimen. Once necking initiates in the specimen, the stress distribution is no longer constant along the length or across the cross section. After some growth in the transverse plane, the crack turns and runs on a plane of maximum shear stress. Progressive crack growth leads to the familiar cup-and-cone fracture associated with fracture of ductile cylindrical specimens (Figure 4). The macroscopic appearance of the fracture surface is characterized by a central fibrous zone, a region containing ridge marks and a shear zone. A third feature that also indicates crack growth direction can be described as a river pattern which is formed by a ductile process. However, in addition the term ridged pattern is used to describe surface waviness that is created by microstructural features. Ridge marks point back to the crack-initiation site and are an important feature for determining these sites. Ridge marks are usually visible without magnification.
Figure 4. Schematic of ductile fracture by void coalescence
Figure 5. Ridge pattern is visible on the fracture surface of a material that shows limited ductility during fracture. The marks point back to the crack-initiation site
Figure 6. Dimple rupture. Note the variation in dimple size associated with the variation in crystal size in the dimples
High-magnification examination of the fracture surface reveals dimple shape (Figure 6). Dimple shape can eliminate some possible loading conditions and indicate the direction of crack propagation. For axial loading, the dimples formed around the second phases are circular. For shear and bend loading, they are elongated and open on one end (parabolic shape).
2.3 Principles of brittle failure analysis
Brittle fracture is characterized by rapid crack propagation with low energy release and without significant plastic deformation. Brittle metals experience little or no plastic deformation prior to fracture. The fracture may have a bright granular appearance. Brittle fracture displays either cleavage (transgranular) or intergranular fracture. This depends upon whether the grain boundaries are stronger or weaker than the grains. This type of fracture is associated with nonmetals such as glass, concrete and thermosetting plastics. In metals, brittle fracture occurs mainly when BCC and HCP crystals are present.
It has been proposed that the pattern is developed where there is ‘rapid’ crack propagation. When cracks propagate faster in the interior of a section than at the surface by a brittle mechanism, the result is chevrons (Figure 7). When crack propagation is faster at the surface than at the interior, only one side of the ‘V’is present, and the feature is the set of radial lines. Both ridge patterns formed by ductile processes and radial patterns (brittle) are visible with the naked eye.
Figure 7. Chevrons. The ‘V’ of the chevron points back to the crack-initiation site.
Cleavage in this idealized case occurs on a single macroscale plane, but the fracture plane changes orientation on the microscale as the crack propagates across grain boundaries. Different fracture-surface morphologies are observed depending on the orientation relation ship between two grains. Often, when a propagating cleavage crack crosses a grain boundary, there is usually nucleation on multiple planes in the new grain. These cracks subsequently coalesce as the crack propagates, creating a characteristic feature known as a river pattern (Figure 8).
Coalescence of the multiple cracks “down river” indicates the crack-propagation direction. Microscale river patterns, like macroscale radial and chevron patterns, point back to the crack initiation site. Commercial polycrystalline alloys contain second phases and inclusions of varying shape and deformability as well as lamellar structures. These microstructural constituents provide additional mechanisms of crack initiation and propagation that are not present in single-phase alloys.
Figure 8. River patterns that develop during cleavage fracture. Multiple crack reinitiations occur when the propagating crack crosses a grain boundary. In (a), crack propagation is from 7 o’clock to 2 o’clock. In (b) propagation is from 1 o’clock to 6 o’clock. (c) Schematic showing the effect of a grain boundary to cause reinitiation of the cleavage crack. (a) and (b)
Exposure to elevated temperature implies changes in microstructure with time in service. Figure 9 shows the fracture surface of a set of steel tensile specimens broken at successively higher temperatures. At the lowest temperature, fracture is predominantly by cleavage creating a fine radial pattern as in specimen (a); there is essentially no shear lip, no reduction in area, and no fibrous zone. The specimen in (b) shows a well-developed ridged pattern, a small fibrous zone, and a small shear lip zone. There is still little reduction in area. The specimen in (c) shows a course ridged pattern, plus a greater reduction in area and a larger fibrous zone than the specimen in (b). Finally, in specimen (d) the ridge pattern has disappeared, the reduction in area is large, and the fracture surface consists of a central fibrous region (largest of the four specimens) and a large shear zone.
Figure 8. Fracture surface appearance of steel tensile specimens at increasing temperatures. The fracture surface consists of three zones; an inner fibrous zone nominally perpendicular to the specimen axis, a “radial” zone containing ridge structures, and a shear zone surrounding the radial zone. Depending on the temperature, the size of these zones changes and zones may disappear. There are accompanying changes in the reduction in area. (a) Tested at -160 °C (-256 °F). (b) Temperature not given. (c) 80 °C (176 °F). (d) 160 °C (320 °F).
Brittle fracture can occur in service without prior plastic deformation at the macroscale (although the material may have been plastically deformed during fabrication) so that there is no warning that fracture is imminent. This may result in catastrophic failure. Ductile tensile overload failures typically provide some warning that failure is imminent. Proper maintenance procedures will then cause replacement of the part so that fracture is averted.