Made to Measure…Microscopy

Who are you? Rebecca Tung
What is your role? Undergraduate Medical Engineer on industrial placement in the Microstructural Studies Unit at the University of Surrey
What is your work about? I help to prepare and characterise materials using a variety of techniques, predominantly scanning electron microscopy based techniques.

I beg your pardon? Materials can be examined in a scanning electron microscope (SEM), which produces an electron image, to reveal microstructure and topography. In addition, the chemical composition of the material may be analysed which can help to determine the properties of a material.

What is a SEM? Scanning electron microscopes use a beam of accelerated electrons as a source of ‘illumination’. Electron microscopy offers a much higher spatial resolution than light microscopy and can reveal either microstructure or surface topography. When the beam interacts with the sample surface, some complex physical processes occur. This results in the emission of secondary electrons, backscattered electrons as well as X-rays. The detection of these three signals enables: (i) imaging of the surface topography, (ii) imaging of the microstructure and (iii) measuring the chemical composition. In the micrographs below, the difference between the detection of backscattered electrons and secondary electrons can be seen. Figure 1 shows the same Vickers hardness indent in a brittle material imaged with the two types of emitted electron. The behaviour of the low energy secondary electrons (Figure 1a) and high-energy backscattered electrons (Figure 1b), along with appropriate electron detectors, gives different information about the specimen.

Microscopy

Figure 1 – Comparison of Scanning Electron Microscopy modes: a) secondary electron image (the bright white areas suggest ‘charging’) and b) back scattered electron image.

And? Analysis of a material with a SEM offers not only increased resolution (therefore higher useful magnification) but the benefits of studying any combination of topography, microstructure and chemistry. Modern microscopes have multiple electron detectors that give different combinations and emphasis of information. Other possibilities include variable pressure microscopy which enables wet specimens to be studied and a focused ion beam capability which enables specimens to be sectioned and studied in 3D.

So what? The characterisation of engineering materials is essential for the goal of understanding microstructure-property-processing relationships. The SEM is arguably the most flexible technique that contributes to this goal.

Final Thought: SEM offers a variety of analytical techniques, which makes it a versatile tool for characterisation of materials. The capabilities of the JEOL 7100F SEM, at the University of Surrey, which is my favourite instrument, make it suitable for studying the whole range of engineering materials—it really is an instrument that is Made 2 Measure!