XPS characterization of oxide layers on FeCo-based nanocrystalline alloys

The main reason for the study of FeSi-based (FINEMET) and FeZr-based (NANOPERM) alloys was their magnetic properties. However, one of the limiting factors in commercial uses of these alloys is the oxidation, which occurs at high service temperature such as 800 °C. Due to this high service temperature, a non-magnetic oxide layer is formed on the surface and the soft magnetic properties remarkably diminish. In this work, it was carried out mass gain measurements in order to identify the oxidation resistance of four different FeCo-based nanocrystalline alloys: Fe38.5Co38.5Nb7Cu1B15, Fe36Co36Nb7Si10B11, Fe36Co36Nb6Cu1Si10B11, and Fe33.5Co33.5Nb7Si15B11. XPS analysis was used in order to identify and quantify the oxides formed during the exposure to an oxidant environment at high temperature. SEM also observed these oxide layers. The main conclusion is that the addition of Si improved the oxidation resistance of the nanocrystalline alloys. This is attributed to the increase of SiO2 amount and decrease of Fe2O3 content on the surface layer, enhancing the surface oxide characteristics.