Discrimination between γ′-Fe4N and ɛ-Fe2–3N iron nitride compounds using PCA of Auger electron spectra

AES offers high spatial resolution that is useful when analysing cross sections of nitrided steels. To discriminate between different iron nitride compounds in such an analysis, so-called factor analysis (FA) can be applied. This requires acquisition of Auger electron spectra which are associated with either the γ-Fe 4 N or the e-Fe 2-3 N phases as principal components. In this study, the N KLL and Fe LMM Auger electron spectral lines of γ'-Fe 4 N and e-Fe 2-3 N phases were acquired in great detail from the pure γ'-Fe 4 N and e-Fe 2-3 N phases obtained by gaseous nitriding of pure iron. To this end, two different samples of known composition were analysed. Firstly, Auger electron spectra were recorded from a nitrided layer with large γ'-Fe 4 N grains (∼30 μm) on top of an iron substrate. Secondly, Auger electron spectra were recorded from an e-Fe 2-3 N outer layer (∼6 μm) with 26 at.% N, on a γ'-Fe 4 N layer (∼4 μm) with 20 at.% N, created on top of an iron substrate. The latter sample was also used for FA. It is shown that principal component analysis of a series of N KLL Auger electron spectra recorded from the iron nitride double layer could be represented by three contributions. These principal components are associated with γ'-Fe 4 N, e-Fe 2-3 N and the background. On the other hand, when a background subtraction was applied to the acquired N KLL Auger electron spectra, it was not possible to discriminate between the two iron nitride phases.