Analysis of structural and chemical inhomogeneity of thin films developed on ferrite grains by color etching with Beraha-I type etchant with spectroscopic ellipsometry and XPS

The structural and chemical homogeneity of the developed thin film upon color etching a low-carbon steel specimen with Beraha-I type color etchant was investigated by spectroscopic ellipsometry and electron backscatter diffraction examinations. The obtained layer thickness maps showed a good correlation with the crystallographic orientation of the individual ferrite grains, corresponding well with previous studies that found a relation between etching and layer build-up speeds and the <100>, <111> directions. However, the refractive index map also showed a dependence on the grain orientation, which contradicts previous models that treat the developed interfering layer as a homogenous material with a constant refractive index and chemical composition. Scanning ellipsometry and X-ray photoelectron spectroscopy measurements confirmed that the chemical composition and refractive index of the developed layer are inhomogeneous both along the surface and the thickness of the film. It was shown that the developed layer consists of mainly oxides and sulfides and that the oxygen content decreases, while the Fe content increases along the normal direction from the surface of the film, in good agreement with the increasing refractive index. The observed differences can be related to the different etching speeds of the ferrite grains (Fe dissolution rate), depending on their orientation.