Transmission electron microscopy (TEM) is a very high resolution microscopical technique widely used in the study of material structure.
TEM methods have been applied in forensic studies to look at adherend, adhesive and interfacial structures.
High resolution examination in transmission of the interface between the adherend and adhesive
A thin foil a few nm thick taken from the sample is required, typically 4-5mm in diameter. The thin foil is normally prepared by metallographic and electrolytic methods. This preparation is designed to penetrate the sample, producing a very thin region , a few nm thick, at the edge of the hole suitable for electron transmission.
Non metallic materials such as adhesives require a different preparation route. In forensic studies on adhesives microtoming is more typically used to prepare the the TEM foil as this is less likely to affect or contaminate the sample.
The specimen is then examined in a transmission electron microscope (TEM) under ultra high vacuum. An electron beam is passed through the sample and the transmitted electrons collected by a detector to produce a high resolution image of the sample. Diffraction patterns may also be recorded.
Energy or wavelength dispersive detectors in the TEM can be used to collect spectra from scattered electrons to provide chemical analysis of local areas of the material from identification and measurement of the characteristic peaks. Such energy dispersive analysis is normally automated in modern TEM detector systems.
TEM was one of the analytical methods used in the DTI MTS programme in examination of the airframe from an old Comet aircraft. The sectioning details and an example TEM micrograph of the interfacial region are shown in Figure 1 and Figure 2 below.
Transmission electron microscopy was used to examine the aluminium/adhesive interface of the Comet airframe by cutting a thin section of the joint at a shallow angle using an ultramicrotome as shown in Figure 1.
The transmission electron micrograph of this section, Figure 2 shows a columnar structure between the aluminium and the adhesive. This surface morphology is typical of the cell formation of the aluminium oxide during anodising and appears to confirm the use of a chromic anodising pretreatment prior to bonding.
Figure 1 Schematic showing section taken through interface of adhesively bonded section of Comet airframe for TEM examination. Courtesy DTI MTS Programme
Figure 2 TEM through section at interface of adhesively bonded section of Comet airframe. Courtesy DTI MTS Programme
DTI MTS Project 3 Environmental Durability of Adhesive Bonds Report No 9,
Forensic Studies of Adhesive Joints. Part 2 - Bonded Aircraft Structure.
E8 Long term adhesive performance De Haviland Comet
