QMT Features: January 2008
Improving one's image
Better images than SEM - NPL's Advanced Engineered Materials Group uses an Olympus LEXT 3D laser confocal microscope  to explore a wide variety of specimens and structures  down to nano size steps.

The NPL hosted a technical seminar on the Olympus LEXT 3D laser confocal microscope on 22 November 2007. Dr Louise Brown, Higher Research Scientist at the NPL’s Advanced Engineered Materials Group, gave an overview of some of the applications areas the LEXT is used for at the NPL.

An extensive range of specimens and structures are studied within the materials group,  from large metallic objects, indentations and indenter tips, right down to small nano particles.  Extremely versatile, the LEXT microscope is used for large volume analysis through to analysis of features with nano size step heights.

Dr Brown commented that the LEXT provides better images than those obtained from an SEM at the same magnification but with additional benefits such as true colour contrast. “There is no pump down time, no sample preparation required, large specimens can be imaged, electron beam damage to sensitive samples is avoided and charging is not an issue.”
Looking and some of the application areas at the NPL, Dr Brown said, “The LEXT has proved very useful for quickly locating graphene flakes. Graphene is a single planar sheet of sp2 bonded carbon atoms. The optical interference between a SiO2 layer and graphene flakes shifts the perceived colour from purple through blue to white depending on the flake thickness. Using the LEXT we are able to find the thin flakes rapidly by inspecting the wide optical field, unlike using an AFM which, because of the need for large scale scanning, would take a long time. By scanning through the magnifications, the exact location of the graphene on the substrate can be found. The LEXT takes literally minutes to locate thin graphene flakes unlike the AFM which could take up to a day.”

Moving on to carbon nano tubes, Dr Brown described how the LEXT is being used to scan aligned arrays of multi-walled carbon nanotubes with various lengths (up to 10 µm), separated by 5 µm. Separation between the tubes is discernable from the images.

Hardness testing
Vickers indentation is just one of the techniques used for the hardness measurement of materials. Dr Brown outlined how, currently, there is a large uncertainty in the measurements performed using conventional optical techniques for hard metals. “Errors can be in the region of ± 1 µm on each side of the indentation diagonal, giving an overall error of ± 2 µm, thus causing a large variation in hardness value. The high lateral resolution of the LEXT reduces this uncertainty significantly.” An example LEXT image of a Vickers Indentation profile showed that the sides of the indentation bow out slightly. “The indent is plastic deformation, however, there is a small amount of elastic deformation that is restored slowly over time.” Dr. Brown said this would not be observed using conventional techniques and added that   the technique may be useful for measuring residual stress in materials.

Examining wear tracks and indenter tips using the LEXT, an example of an image was shown where the tracks were not smooth. “After examination of the indenter,” commented Dr. Brown, “it was clear that the tip was damaged and hence the scratch uneven.”

The LEXT is very useful in quick examination of nano indenter tips. An example was shown where a routine check of the tip of an AFM indenter showed that there was dirt at the tip. “Previously, an SEM has been used for making the examination, which is a time consuming process.”

Concluding the session on the subject of carbon fibre imaging, Dr Brown said that the key  reason the LEXT is so good for fibre reinforced plastic (FRP) materials is that there is minimal specimen preparation required to give a 3-D image of almost SEM quality. ”It's a quick and easy tool for inspecting FRPs and was particularly useful for imaging fracture surfaces.  For example, the LEXT has been useful for examining size, distribution and failure in carbon fibres.”
E-mail: louise.brown@npl.co.uk




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