The oxidation of SmCo magnets

SmCo alloys form the basis of excellent permanent magnets with potential service applications to 500 degrees C. It is suspected, however, that their oxidation behavior may limit their usefulness but this is a relatively unstudied subject. Two grades of Sm(2)(Co,Fe,Cu,Zr)(17)-based alloys were aged in air at temperatures between 300 and 600 degrees C and the oxidation reaction characterized by post-test scanning electron microscopy examination. Both alloys formed a thin external oxide scale composed of Cu and mixed Fe/Co oxides and an extensive internal oxidation layer consisting of Sm-rich oxide within a CoFe matrix. The depth of the internal oxidation zone was typically an order of magnitude thicker than the surface oxides and increased parabolically with time. This internal oxidation is an important degradation phenomenon because the transformation to CoFe causes a loss of magnetic properties proportional to the volume consumed. The morphologies of the oxidation zones, nature, and mechanisms of various precipitated phases and the differences between the two alloys' oxidation behavior is also discussed. (C) 2008 American Institute of Physics.