Scanning, Transmission & Analytical Electron Microscopy 

Electron microscopy has become, over the past thirty years, the foundation of much of materials and failure analysis. It beautifully fills the gap between what we can see with our naked eyes and a stereo microscope and the far higher magnification of the atomic force microscope.

Scanning Electron Microscopy (SEM)

The premier advantage of SEM is its amazing depth of field, which allows the user to see deep into holes or irregular fracture surfaces. Its second most important feature is that most SEMs today also have capabilities to do energy-dispersive x-ray analysis, allowing for determination of the general chemical composition of the sample.

At Matco we normally begin examination of a sample with stereo light microscopy, possibly documenting what we see with digital photography.

One significant problem in SEM work is that all color clues are lost in the SEM, so if color was an important feature of the original sample, documentation of that color distribution needs to be done before putting the sample into the SEM.

In addition, it is not uncommon to lose orientation when magnification is increased. So in many cases, reference marks will be added to the sample prior to photodocumentation.

A final report will thus gradually carry the client from what he saw in the sample as-submitted into the magnification range at which the evidence is interpretable for his samples, providing an easily understood explanation of “root cause of failure” or other problem solution.

Matco’s SEM instrumentation is fully digital, so that reports can be immediately issued on CD. In many instances clients who come into the lab to work with us leave at the end of the day with their data already in hand.

Transmission Electron Microscopy (TEM)                   

Materials for TEM must be thin enough allow electrons to pass through the sample, much like light does in conventional transmitted-light microscopy.  TEM samples are commonly on the order of 1000 Angstroms (100 nanometers) thick. Special sophisticated techniques must be used to prepare samples to these thicknesses including both ulra-microtomy and FIB (focused ion beam). Because the wavelength of electrons is much smaller than that of light, the resolution limit in Transmission Electron Microscopy is three or four orders of magnitude lower than that for a light microscope  (About 1 Angstrom versus 1 micrometer). Thus, TEMs can reveal the finest details of internal structure – at the limit today as small as individual atoms.

Phase determination, defect and precipitate orientation and compositional analysis are typical desired outcomes of conventional metallurgical TEM experiments. Microstructural characterization can be carried out using various combinations of imaging and electron diffraction techniques. A 200 kV TEM available at the University of Pittsburgh can provide high resolution lattice imaging in TEM mode or annular-dark field (DF) lattice imaging in STEM (scanning) mode. Bright Field (BF) and Dark Field (DF) imaging are performed in TEM and STEM modes, as is phase contrast imaging. Analytical TEM attachments for Energy Dispersive X-ray Spectroscopy (EDS) (Z ≥ 11 can be detected) and electron energy-loss spectroscopy (EELS) in both point & chemistry mapping modes (Z>11 are detectable) are available for compositional and chemical characterization from areas as small as ~15nm. Atomic structures can be determined by electron diffraction in Selected Area, CBED (convergent beam), and nanodiffraction modes. Thus, TEM is a complementary tool to conventional methods such as XRD and scanning electron microscopy (SEM).

Special Testing and Analysis

Matco’s senior staff have been involved with electron microscopy and other electron-beam-based surface analysis techniques(AES & XPS) since the very beginnings of the technology, providing a wealth of experience in sample preparation and observation for the clients.

                                  Dr. Zee                                Dr. Gibbon

                                  Dr. Bayer                             Dr. Xu

                                 Joe. Turek                             W. Gretz, PE