By Ebatco
Have some unknown samples? We can help you to determine what they are and where they came from. X-ray Energy Dispersive Spectroscopy (EDS) is a semi-quantitative x-ray technique that can identify and measure chemical composition in SEM analysis. Figure 1 shows the principle of EDS. The SEM focuses an electron beam on the sample surface. The electrons knock core-shell electrons out of atoms inside the sample. To fill this vacancy, a higher energy electron from the atom will fall down to take its place, and the difference in energy between the two states is emitted as an x-ray. The generated x-rays possess energies unique to the element which are dependent upon the atomic number (Z) and the orbital transition involved. Measuring the spectrum of emitted x-ray allows for chemical characterization of the sample. Figure 1. A representation of the principle behind EDS. This project, an automotive emblem of unknown composition was analyzed by SEM/EDS. EDS can measure the composition at a single point, along a line, or map an area. In this case, the chemical composition was analyzed by EDS area scanning following ASTM E1508 – 12a using an accelerating voltage of 15 kV, take-off angle of 35º, and sample tilt of 0º. The test was performed using a standardless method with P/B-ZAF matrix correction. Figure 2 shows a typical microstructure and the resulting EDS spectrum. Five randomly selected areas at 200X magnification were analyzed by EDS. Figure 2. Typical microstructure and EDS spectrum result of unknown sample Through EDS, we found that this sample is mainly composed of Zn with Al as an alloying element. The elemental concentrations were calculated by difference, and the K lines were selected to do the quantitative analysis. Table 1 lists the individual and average EDS results. The average concentrations of Zn and Al are 93.8 and 6.2 wt% respectively, which is a common composition for zinc die castings. Zn alloys have excellent finishing characteristics for plating, and chromate treatments. It is low cost, has excellent thin wall capabilities, and possesses high strength and hardness. EDS can also do high resolution composite element mapping. Through the element map, we can see a detailed overview of where the different elements are located. Figure 3 clearly indicates that there were three layers of coating on the emblem. The innermost layer in direct contact with the Zn-Al substrate was identified as Cu. The middle layer was Ni, and the outermost layer was Cr. This coating was likely prepared by an electroplating process where the Cu layer facilitated the formation of a good Ni coating. This is a common processing step for electroplating Ni onto cast metal parts. The thicknesses of the Ni and Cr coatings are approximately 20 and 2 µm, respectively. Figure 3. Element mapping images of unknown sample’s coating.
