Energy dispersive X-ray analysis (EDX) is the standard method for obtaining chemical analysis in specimens being examined in electron microscopes. In electron microscopes X-rays are emitted as the electron beam interacts with the material being examined. Each atomic element has discrete characteristic energies for the X-rays providing a basis for chemical analysis.
Chemical analysis of the surface of specimens being examined by SEM or TEM methods. Identification of line of failure in fracture surfaces from failed adhesive bonds.
In EDX the X-rays leaving the surface of the sample are collected by a spectrometer and analysed in terms of energy. From the location and size of the peaks it is possible to obtain a chemical analysis of the near surface of the sample. EDX gives an analysis to a greater depth than surface specific methods such as XPS or Auger electron spectroscopy. Detection of lower atomic number elements can be improved by use of a low absortion window on the detector, for example a berylium window.
The method is useful in forensic analysis to identify the fracture path. The adhesive and underlying metal will give different characteristic spectra. Hence it is possible to identify if the fracture has occurred in the adhesive or passed though the substrate.
EDX is a standard method available in most commercial electron microscopes.
The example shown here comes from an examination in the MTS programme of stiffeners which are adhesively bonded to the doors of Foden trucks. The fracture surface was examined after breaking open in a tensile butt test.
Energy dispersive X-ray spectra were obtained for both surfaces of the joint, over a 500 mm square region. This method of analysing the surfaces is able to penetrate deeper into the material than XPS methods. The results of the analysis on the " metal" surface are shown in Figure 1, which, as might be expected, shows peaks for aluminium and magnesium, the primary components of the alloy. The peak visible for gold results from the sputter coat applied to prevent the samples from charging under the electron beam, and would not originally have been present. The results from the other side of the joint, the " adhesive" side, confirm the results obtained by XPS, in that there is a discernable amount of aluminium on the adhesive surface, identified by the twin peaks of aluminium and chlorine (an indicator for the adhesive), Figure 2.
A further analysis was performed on some crystalline structures identified in the electron microscope study and shown dark in the SEM micrograph in Figure 3. Due to their size, approximately 20um, it had not been possible to analyse these using XPS. The better focusing ability of the energy dispersive X-ray technique allowed their primary chemical components to be assessed, .From this it can be seen that they contain a high percentage of calcium. The calcium crystals appear to be well adhered to the surface of the aluminium. Their presence in clusters over the entire surface of the metal indicates that they were present as a contaminant prior to bonding the joints, probably appearing sometime after rolling.
Metal surface
Figure 1 EDX spectra from metal surface. Fracture surface of adhesively bonded stiffener on door of Foden truck, failed in tensile butt test. Courtesy MTS programme.
“Adhesive” surface
Figure 2 EDX spectra for “adhesive” surface. Fracture surface of adhesively bonded stiffener on door of Foden truck, failed in tensile butt test. Courtesy MTS programme.
Crystalline Structures
Figure 3 EDX spectra for crystalline structures observed on the fracture surface of an adhesively bonded stiffener on door of Foden truck, failed in a tensile butt test. The corresponding SEM micrograph is also shown. Courtesy MTS programme.
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints Part 3 – Foden Truck, NPL February 1996
