Scanning Electron Microscopy (SEM)

Ebatco NAT Lab has recently added a brand-new, state-of-the-art, low vacuum scanning electron microscope (SEM) to its nano analytical tool set. The JEOL 6610LV can produce high resolution images under either secondary electron mode or a backscattered electron mode for surface topography and composition analysis. The SEM is expected to further expand NAT Lab’s portfolio offerings in surface morphology, metallurgy, particle sizing, surface roughness and micro/nano dimensional measurement areas.

In principle, the SEM generates a primary focused electron beam in a vacuum chamber and raster scans the beam over a sample surface. The incident primary electrons interact with the atoms in the surface of the sample. Some of the primary electrons excite the sample to emit secondary electrons while some are being scattered back. A secondary electron detector or a backscattered electron detector collects and converts the secondary electrons or the back scattered electrons into high resolution images.

Secondary electrons are emitted by atoms near the surface of a sample material when their electrons become excited and have sufficient energy to escape the sample surface. Secondary electrons carry information about morphology and surface topography of the sample. The contrast is dominated by the so-called edge effect: more secondary electron can leave the sample at edges for reduced material retardation and adsorption and thus can lead to increased brightness.
A highly sensitive backscatter electron detector captures composition, topography, and shadow images. Elements of higher atomic number will backscatter electrons more strongly than those of lower atomic numbers. Backscatter imaging allows for improved image contrast for compositional images as well as topographical images. Therefore, backscatter detectors are used to detect areas of varying chemical compositions. Backscattered electron imaging of nonconductive, uncoated samples shows composition by contrast; higher atomic number regions are brighter, while lower atomic number regions are darker.

The most advantageous features of the JEOL 6610LV SEM are the low voltage and low vacuum operation capabilities. The low voltage operation has the advantage of reducing the interaction volume within the specimen, thus increasing the surface specificity of the technique. It gives greater visibility of surface marks, ultra-thin film and contaminations. By operation at low voltage, the charge buildup on non-conductive surface can be minimized or eliminated. It allows high resolution imaging of non-conductive samples without pretreatment. This feature is particularly useful for samples whose surface can’t be changed or modified, such as museum, forensic and biological samples. The low-vacuum (LV) capability allows for viewing the samples which are wet, oily, outgas excessively. It could be used for freeze-dried samples of hydrated surface with basic structural integrity. The low-vacuum operation capability is also great for working with nonconductive materials by eliminating electric charge built up on non-conductive sample surfaces through chamber gas ionization. With appropriate combination of voltage and vacuum, the JEOL 6610LV has versatility and high resolution across the magnification range of 5x-300,000x, delivering amazing clarity, revealing the finest structures of conducting, non-conducting and biological samples.

This JEOL 6610LV SEM is equipped with a large specimen chamber that allows for analysis of a wide variety of samples with sizes and shapes up to 80 mm in height and 200 mm in diameter. Besides, the SEM is also equipped with a convenient eucentric 5-axis motorized stage with automatic tilt and rotation. This stage is very useful for keeping the area of interest in the field of view, particularly for analyzing areas that are offset from the point of rotation.

Typical Experimental Results

Calibration grid

Failure Analysis: upper row, cross-section of a fractured copper wire; lower row, cross-section of a fractured steel wire

Pollen samples imaged without conductive coating: left, yellow sweet clover imaged at 2.5kV; right: goldenrod imaged at low vacuum 110Pa

Coral surface imaged without conductive coating at high voltage (30kV, left) and low voltage (1.5kV, right)

Fresh Goldenrod flower imaged without conductive coating: petals (left, 30 Pa); pollens on stigma (right, 70Pa)

Zinc oxide particles with sub-micron particle sizes

Gold particles imaged at 300,000x showing the nano meter resolution of the SEM

Applications

For more information please read our application notes:
Low-Cycle Failure Analysis of Steel Wire, PDF
SEM/EDS Analysis of Bicentennial Penny Patina, PDF
SEM/EDS Analysis on Scratch Failure of PTFE Coated Stainless Steel Guide Wire, PDF

Instruments: JEOL 6610 LV Scanning Electron Microscope

Instrument Key Specifications

Fracture Failure Analysis of Steel Wire

Failure analysis is of utmost importance in many industrial applications, such as semiconductor, packaging, transportation, manufacturing and biomedical devices. Knowing how and why a material failed is essential to ensuring a product’s reliability. Scanning Electron Microscopy (SEM) is one of the most powerful tools in failure analysis. By revealing the information about the microstructure at the fracture surface, one can derive lots of information about the failure mechanisms.

Material failure below its yield stress limit is often called fatigue failure. It occurs when the sample undergoes cyclic loading. Local microscopic cracks may develop at a certain stress threshold below its yield strength. Over a period of time, these cracks propagate, coalescence and eventually cause a fracture. A typical metal fatigue failure develops in three stages: crack initiation, crack propagation and final fracture. Each stage has its own characteristic morphology. The crack initiation site is located at the most stress-concentrated areas. The crack propagation shows typical striation lines, perpendicular to the direction of crack propagation and formed by each loading cycle. The final fracture area is the rapid fracture region, which typically shows a rough fracture surface. A dependable design against fatigue-failure requires thorough understanding on the failure mechanisms and knowledge in structural engineering, mechanical engineering, and materials science and engineering.

Figure 1 illustrates an SEM image obtained by Ebatco NAT Lab’s JEOL 6610LV for fracture failure analysis of a steel wire failed under cyclic loading. Bending of the wire back and forth has caused both sides of the wire to undergo compression and tension forces, and the final wire rupture. Arrows marked with 1, 2 and 3 indicate the typical three stages of fatigue failure. Arrows marked with 1 indicate crack initiation regions. Arrows marked with 2 indicate crack propagation regions. Arrows marked with 3 are the final fracture regions.

Figure 2 presents higher magnification images of the red rectangles shown in Figure 1. The image below at left shows the area close to the crack initiation region. In this image, the striations as typical fatigue characteristics caused by crack propagation can be easily seen. The image below at right illustrates typical dimple structures from ductile fracture in the final fracture region. The dimples are formed due to high local plastic deformation at final rupture of the wire.

SEM/EDS Analysis of Bicentennial Penny Patina

The process and nature of corrosion is of utmost importance across a wide array of fields. Knowing the properties of the corroded material is a crucial step in gaining an understanding as to why and how the corrosion occurred. Corrosion can take on various forms and behaviors even within a small area of a given material, so being able to examine a specific point on the sample is greatly useful in the analysis of the corrosion.

One method for analyzing corrosion is by means of a SEM (Scanning Electron Microscopy) equipped with EDS (Energy Dispersive X-ray Spectroscopy). Working in tandem, SEM and EDS analyses can reveal a tremendous amount of useful information on corrosion processes and mechanisms, as well as material anti-corrosion properties. With the SEM system, micrographs can be taken for morphological inspection to understand how the corrosion surface is forming and changing. Pits, cracks, fractures and other microscopically observable characteristics of the corroded materials are useful to visualize what may have happened. In addition to SEM observations, the EDS system can further assist in identifying and quantifying the chemical compositions of the micro areas of interest by measuring the characteristic X-rays produced by atoms that are present in the lattice of the material when excited by electron bombardment. To obtain quick and accurate results, the EDS system in use operates using a peak to background ZAF algorithm analysis technique. This method utilizes the Bremsstrahlung X-rays created to calculate fundamental peak to background ratios which are then used to analyze the detected peak characteristic X-rays from the sample. This analysis procedure provides accurate results without the need for calibration using reference standards.

As an example to illustrate the above points, a corroded penny of 1976 was briefly studied in our JEOL JSM-6610LV SEM with Bruker QUANTAX 200 EDS system. A SEM micrographic image was taken on a green colored area and EDS compositional spectra were taken at several points as indicated by the numbers in Figure 1. The elements found in the corroded points are listed in Table 1. Figure 2 shows a typical EDS spectrum of the corroded area.

From the SEM image, it is obvious that the corroded area has characteristics of severe metal corrosion: uneven, porous and powdery microstructure, and cracks on the surface caused by corrosion stresses. The EDS compositional analysis has indicated that three main elements in the majority of locations are copper, oxygen, and carbon. It is suspected that these elements are existing in the forms of copper (I) carbonate or copper (II) carbonate as the results of penny corrosion. Some small amounts of zinc, the alloying material used in pennies produced between 1962 and 1982, such as the 1976 penny we examined, were also detected in some of the locations. In addition, all analyzed locations show some amounts of chlorine. This could be evidence of copper or zinc chlorides present due to corrosion. Furthermore, the presence of sodium suggests some amount of sodium chloride, likely due to sweat deposited on the surface when handling of the penny.

SEM/EDS Analysis on Scratch Failure of PTFE Coated Stainless Steel Guide Wire

Coatings are used on a wide variety of substrates such as metals, alloys, semiconductors, polymers, biomedical devices for decorative or functional purposes. The adhesion behaviors of coatings are essential to their applications. Scratch test is one of the broadly used, fast, and effective methods to evaluate coating adhesion properties. During a scratch test, a stylus or scratch tip gradually penetrates into a coating under a progressive load while it also moves across the coating sample. The normal load at which the coating fails due to delamination or other separation mechanisms is called the critical load of interfacial adhesion failure. The critical load of interfacial adhesion failure is related to the practical adhesion strength of the coating to the substrate. One complementary technique for analyzing scratch failure of coating is Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS). Working in tandem, SEM and EDS analyses can reveal a tremendous amount of useful information on scratch failure processes and mechanisms, as well as material anti-scratch properties. With the SEM system, micrographs can be taken for morphological inspection in order to understand how the scratch surface is forming and changing. The SEM micrographs of the scratch surfaces can reveal much more details as a result of SEM’s larger depth of field, higher resolution and greater magnification than the optical microscope available on a scratch tester. In addition to SEM observations, the EDS system can further assist in identifying and quantifying the chemical compositions of the micro areas of interest by measuring the characteristic X-rays produced by atoms that are present in the coating and substrate materials.

PTFE coated stainless steel guide wires are popular in many medical applications. PTFE coatings are applied to the wire surface for smooth surface finish, reduced friction, increased lubricity and durability of the guide wire. Obviously the PTFE coating adhesion to the guide wire is critical not only for the desired functionalities but also for the health and safety of the patient to whom the guide wire is to be used. An undesired issue would be flaking of the coating material due to adhesion problems, which could lead to blockage of a passage or clogging of blood vessels. Figure 1 is an SEM image of a PTFE coated stainless steel guide wire after scratch test. Blue color box Zone 1 includes scratch before interfacial adhesion failure, transitional Zone 2 in which adhesion failure occurred, steady scratch Zone 3 and scratch end Zone 4. The bright scratch track in Zones 2-4 indicates non-existence of the PTFE coating and exposure of the stainless steel substrate.

Figure 2 shows the elemental profiles of Zone 1 with the EDS line scanning across the entire scratch. The EDS compositional analysis has verified the morphological interpretations of the SEM image; the coating has in deed delaminated in Zone 2 where element Fe from stainless steel substrate increases and C and F from the PTFE coating decreases. The zoom-in SEM images and EDS elemental Hypermaps of the Zones 2-4 presented in Figure 3 provides further details of the elemental distributions of the coating and substrate materials along the scratch.