Scanning electron microscopy for surface investigations

When surfaces of materials and components are exposed to plasma they can change their properties. Depending on the material and load these changes occur in the nanometre or micrometre range.  The electron microscope can make this visible.

 

Determining these changes is indispensable for evaluating many investigations, e.g. ion beam analysis. In addition, structures below the surface are also investigated, e.g. grain size, pore formation, interfaces and layer thicknesses and their homogeneity. Topical questions, for example, are:

 

How does blistering arise in tungsten?

When a hydrogen plasma acts on a surface, hydrogen is implanted in the wall material close to the surface. Part of it migrates to the surface and escapes into the plasma, while the rest diffuses into the deep. If the hydrogen impinges on flaws or grain boundaries in the crystal lattice, it may adhere to these. Single hydrogen atoms arriving here reduce the mechanical stability between the grains. More hydrogen leads to molecule formation, thus producing hydrogen gas. The tiny cavities along the grain boundaries are filled up to very high pressures. Ultimately, many grain interfaces give way mechanically. Further filling of the cavity spaces with hydrogen gas then gives rise to the observed blistering.

Scanning electron microscope picture of a tungsten sample exposed to hydrogen plasma.
Left: Surface viewed at a slant; right: cutting with the focussed ion beam makes the cross-section surface visible; the arrow indicates the direction of the plastic deformation of the material.

By cutting open individual blisters with a focussed ion beam it was shown that some of the blisters are located below the implantation depth of the hydrogen, the latter being typically up to 100 nm. Also, the corresponding cavity always runs parallel to the grain boundaries; the hydrogen gas in the blisters reaches a pressure of a few hundred bar.

Why do spiky structures form on steel surfaces?

Steel could afford advantages as wall material in future fusion devices and is therefore becoming increasingly prominent in fusion research. One of the aspects investigated is erosion, this being the main disadvantage of steel exposed to plasma. A variety of surface morphologies were observed. At temperatures of a few hundred degrees Celsius these form where the sample surfaces are bombarded with hydrogen.

The spiky structures observed attain heights appreciably exceeding the erosion depths measured. Furthermore, the alloy elements of the steel have a distribution different to that in the original material. These fascinating formations are due to the increased mobility of the atoms.

Scanning electron microscope picture of a steel sample: at a temperature of 530°C it was exposed to a hydrogen plasma. The originally polished surface is covered with a fibrous or spiky structure (picture width: 5.12 µm).

Scanning electron microscope picture of a steel sample exposed at 670°C to a hydrogen plasma. The originally polished surface shows a spiky structure with individual particles on top (picture width: 12.8 µm).

Further examples of electron microscope investigations are featured, for instance, under the topics:

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