The oxidation kinetics in air of a commercial NdFeB magnet have been investigated over the temperature range 335-500degreesC The oxide microstructure has been characterized by SEM, XRD and cross-sectional TEM. The results show that the external scale formed consists of an outer layer of Fe2O3 and an inner layer of Fe3O4 but that the principal degradation process is the formation of an extensive zone of internal oxidation. HREM has been used to show that this zone contains NdO particles embedded in an alpha-Fe matrix. These particles are discrete and very small, approximately 2 nm in diameter, and have an amorphous structure. The alpha-Fe matrix has a columnar grain structure with a grain width of approximately 100 nm. It is argued that the high rates of internal oxidation arise because the external-oxide layers are not protective at the oxidation temperature, and oxygen penetrates to the zone front by fast diffusion along the columnar alpha-Fe grain boundaries.
- internal oxidation
- NdFeB magnets