Imagine you need to confirm the structural properties of the metal or materials being used in pipelines, containers or construction materials. What if long-term corrosion is a concern for your project involving buried pipe in moist soil? What if detecting the composition of debris that was filtered out of the material could prevent a maintenance disaster and costly downtime?
Scanning electron microscopy is one of the most powerful methods for evaluating engineering materials and understanding how they perform in real-world applications. The development of the scanning electron microscope (SEM) in the early 1950s brought with it new areas of study in the medical and physical sciences because it allowed for the examination of a great variety of specimens.
Two main advantages of an SEM over a light optical microscope are resolution and depth of field.
As with any microscope, the main objective is resolution or focus for clarity. An optical microscope uses lenses to bend light waves and the lenses are adjusted for focus. In an SEM, electromagnets are used to bend and shape an electron beam, which is used to produce the image on a screen. Think of astigmatism in the eye. The poor shape of the eye causes poor eyesight. However, if the shape is corrected, the quality of sight increases. By using electromagnets, the straightened electron beam can provide greater clarity in the image produced. The resolution of the picture can be used for evaluation purposes and for determining accurate measurements.
SEM is designed for directly studying the surfaces of solid objects. By scanning with an electron beam that has been generated and focused by the operation of the microscope, an image forms in much the same way as the image on a TV. The SEM allows a greater depth of focus than the optical microscope. For this reason, the SEM can produce an image that is a good representation of the three-dimensional sample.
For studying the fractures of a broken material, the depth of field of an SEM is valuable. The SEM provides an image of the peaks and valleys of the fracture. To the untrained eye, this is a cool picture. To the trained eye, the peaks and valleys are the signature of how the fracture occurred or how the material separated. The signature is indicative of the failure mode of the material -- the reason why the material failed. Examples of failure modes include ductile overload, fatigue, stress cracking and hydrogen embrittlement.
An MMT engineer using the SEM's energy dispersive spectrometry capability for material characterization.
The SEM is also used in conjunction with a multitude of tools. These tools by themselves do not have a value. However, once the tool is attached to the SEM, they provide valuable information. Examples of tools include X-ray fluorescence (XRF), energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). XRF is used to determine the elemental composition of a material, EDS is used to determine the relative elemental composition of a material, and EBSD is a microstructural-crystallographic characterization technique commonly used to support corrosion studies.
SEMs are powerful tools used to evaluate materials. The uses for SEMs in industry are many, including failure analysis of materials and material selection to ensure long life of pipelines and equipment.
For more information, visit www.mmtinc.com or call (800) 772-0251.
