Scanning electron microscopy (SEM) at low voltage (LVSEM) provides a highly effective means of determining the morphology of carbon based and low conductivity samples, such as adhesive surfaces.
In Scanning Electron Microscopy (SEM) the specimen is placed in a vacuum chamber, and a fine electron beam scanned over the surface of interest. The resulting secondary electrons leaving the surface are detected and used to build up an image. Modern SEMs can accommodate quite large samples, typically 100mm wide or greater. If a low voltage is used the method may be referred to as Low Voltage Scanning Electron microscopy (LVSEM).
Electron microscopy offers higher resolution than is possible with optical microscopy, and a greater depth of field. Whilst non-conducting samples can be examined using LVSEM, in practice better results are obtained when a thin conducting layer of carbon or gold is vacuum deposited on the samples. This layer is sufficiently thin and uniform that it does not interfere with the surface topology being investigated.
In common with many electron microscopes, the LVSEM is fitted with an energy dispersive spectrometer, which allows elemental identification.
For example the Hitachi S-800 microscope used in forensic investigations in the MTS programme, has a high brightness field emission gun which gives excellent imaging for accelerating voltages from 500V, with 20 nm secondary electron image resolution, up to 25 kV, with 2.5 nm resolution. The low voltage capability allows carbon based material to be examined, with a minimum of beam induced damage.
SEM was one of the analysis tools used in the MTS programme to interpret the results from Tensile butt testing. The aim was to look at the integrity of adhesive bonding of stiffeners to the doors of Foden trucks. In this design stiffeners are attached to the skin using a toughened, two part acrylic adhesive, originally Permabond F241. The tests were carried out on an Instron hydraulic test bed, using a Zwick extensometer, modified to give zero gauge length.
After the mechanical testing the fracture surfaces of the joints were observed to determine the locus of failure. Initial observations with the naked eye indicated that one surface of the joint was generally free of adhesive, whilst the other surface had a thick film of adhesive on its surface, suggesting an interfacial mode of failure. In the discussion below these two sides will be referred to as " metal" and " adhesive" sides respectively.
Figure 1 Visual examination of fracture surface
Closer inspection was then carried out using a Hitachi S-800 low voltage electron microscope, at a beam energy of 3 keV. Even at the low energy used, it was necessary to vapour deposit a thin layer of gold onto the specimens, to prevent their charging under the electron microscope beam. This film is sufficiently uniform and thin that it does not show up on the microscope images at the magnifications used. The resulting secondary electron microscope images are shown in Figure 2 below.
Figure 2 SEM micrographs of the fracture surfaces of the aluminium stiffener, adhesive bonds after tensile butt testing
The lower magnification image of the aluminium surface, left, clearly shows the regular linear markings resulting from the rolling process. Also visible on this image are a number of crystalline structures, approximately 20 microns in size, which appear to be well attached to the aluminium surface. These appear to occur in clusters, approximately 1 mm in diameter. Closer inspection of the aluminium surface, right, shows debris retained on the surface after failure, and in a number of places, areas where some of the surface aluminium has been pulled away with the adhesive layer.
x70 x5000
Figure 3 Mating adhesive surface to the surfaces shown in Figure 2 above.
Figure 3 above shows the mating adhesive surface. At the lower magnification a degree of air entrainment in the adhesive can be observed, which would be expected given the nature of the adhesive and the assembly process. Also visible are the matching linear imprints from the rolled surface of the aluminium to which it was bonded. Higher magnification reveals that the adhesive surface has a brittle layer on it, which has cracked as the sample was loaded. Other than this the adhesive surface is relatively featureless.
SEM examination of the fracture surfaces was used to understand issues to do with the adhesive bonding of shoe soles. Figure 4 left, shows a view of the sole surface, at a magnification of x70. At this level of magnification a large number of localised damage sites were visible, as shown in the photomicrograph. From the image it is apparent that some of the PVC surface has fractured when the joint failed.
Figure 4 SEM micrographs of fractured shoe sole. Left micrograph of shoe sole; right micrograph of fracture surface adhesive side
Figure 4, shows the surface of the adhesive, at the same magnification. This clearly shows the root site of one of the pulled fibrils of adhesive. This can be compared with the surface topology resulting from a region at the extreme edge of the joint, where the adhesive was not in contact with other material, shown below in Figure 5. In this image the relatively smooth surface of the adhesive can be seen, covering the fibrous roughened surface of the prepared leather
Figure 5 SEM micrographs of fractured shoe sole. Top left micrograph of Shoe Sole; top right Micrograph of Fracture Surface (Adhesive Side); bottom micrograph of adhesive surface
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints Part 3 – Foden Truck, NPL February 1996
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints. Part 4 Footwear
MTS Project 3 Environmental Durability of Adhesive Bonds Report No. 9
Forensic Studies of Adhesive Joints. Part 1 - General Introduction and Conclusions AE Bond, February 1996
The Role of Surface Analysis in Adhesive Bonding. SJBull, BA Bellamy, HE Bishop, JF Watts and D Brewis. MTS Project 4 Report No 3 May 1995
