Electron probe X-ray Microanalysis (EPMA) is simply a specialised scanning electron microscope SEM fitted with an X-ray detector. This combined instrument is known as an electron microprobe.
As discussed in the XPS explanation, when irradiated with a beam of sufficient energy, atoms will emit characteristic X-rays. These, when detected using an energy dispersive or wavelength dispersive spectrometer, allow a map to be generated showing the distribution of the elements across the surface. Analysis of the size and relative areas of the characteristic peaks allows the composition of each element to be measured. By setting thresholds it is possible to produce size distributions, or Area % figures, for particular compositional features on the surface.
The penetration depth of the beam can be varied by adjusting the beam energy, thus allowing information to be obtained on the variation of chemical composition with depth. Typically, EPMA is able to provide data from depths between 0.1 and 2 mm.
A sample is introduced into the electron microprobe which is then pumped to ultrahigh vacuum. The sample is then imaged using the scanning electron microscope.
By positioning the sample and focussing the electron beam it is possible to determine the surface composition at different positions in the sample using an energy dispersive or wavelength dispersive X-ray spectrometer.
The spectrometer captures the emitted X-rays and analyses these to produce an X-ray spectrum. From the characteristic energy or wavelength peaks it is possible to identify the elements present. Analysis software enables the composition of each element to be determined based on measurements of peak area or peak height.
The microprobe enables images to be produced also for the backscattered X-rays and individual X-rays. By this method composition maps can be produced showing the distribution of specific elements. See the examples below.
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 using an electron microprobe in addition to other techniques after breaking the joint open in a tensile butt test.
The results from other physico-chemical analysis techniques lead to the conclusion that the locus of failure is in a loosely attached oxide layer on the surface of the aluminium alloy. In order to identify the thickness and uniformity of this layer, a sample of the joint was carbon coated and examined in an electron microprobe.
By measuring the aluminium and silicon X-ray intensities from the sample relative to those from elemental standards, it is possible to estimate the thickness of the oxide layer. By repeating the measurements at two primary beam energies it can be confirmed that the elements are confined to a thin surface layer, rather than a uniform distribution throughout the thickness of the adhesive.
Measurements were taken from two areas of the coated sample and at two beam energies, the results of which are shown in Table 1 below:
Table 1 Surface Coverage of Aluminium and Silicon
|
Element |
Area Ref |
Beam Energy (keV) |
K - ratio |
Mass (gm/cm 2 x 10) |
Equivalent Oxide Thickness |
|
Silicon |
1 |
10 |
0.0069 |
1.5 |
8.1 |
|
2 |
10 |
0.0065 |
1.4 |
7.5 | |
|
1 |
7.5 |
0.0127 |
1.5 |
8.6 | |
|
2 |
7.5 |
0.0119 |
1.4 |
8.1 | |
|
Aluminium |
1 |
10 |
0.0055 |
1.2 |
11.6 |
|
2 |
10 |
0.0067 |
1.5 |
14.5 | |
|
1 |
7.5 |
0.0092 |
1.2 |
11.6 | |
|
2 |
7.5 |
0.0123 |
1.6 |
15.5 |
These results indicate that the oxide layer thickness is around 10-20 nm in thickness. Figure 1 below shows the SEM and X-ray images obtained from one of the regions investigated using the microprobe.
All four images are of the same location and are shown at the same magnification. Top left is a conventional scanning electron microscope image of the area in question. From this image debris can be seen on the surface of the adhesive. The back-scatter image, top right, is less easy to interpret, but confirms the existence of debris on the surface of the adhesive. The two X-ray images for aluminium and silicon, bottom left and right respectively, show a relatively uniform distribution of the two elements, other than a few localised hot-spots, which correspond with the debris particles observed in the previous two images.
Scanning Electron Image Backscatter Image
Aluminium X-ray Image Silicon X-ray Image
Figure 1 Electron microprobe images of fracture surface. Tensile butt test of stiffener adhesive joint from Foden truck.
MTS Project 3 Report No 9 Forensic Studies of Adhesive Joints Part 3 – Foden Truck, NPL February 1996
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
