Oxidation Kinetics of Nd-Fe-B Permanent Magnets Prepared by Spark Plasma Sintering

An an isotropic Nd-Fe-B magnet was prepared by the spark plasma sintering (SPS) technique. A magnet with the same composition was produced using the conventional sintering method for comparison. The oxidation behavior in oxidizing environments for both magnets was studied. The microstructure and composition of the magnets were investigated using scanning electron microscopy with an energy-dispersive x-ray detector. Compared with the conventional sintered Nd-Fe-B magnet, the SPS magnet possesses a lower oxygen diffusion coefficient, which leads to a lower oxidation rate, a thinner oxidation layer, and lower oxygen Content. Microstructure observation shows that the SPS magnet possesses a unique microstructure due to the special sintering mechanism. In detail, the grain size of the Nd2Fe14B main phase is fine and uniform. Only a little of the Nd-rich phase forms along the grain boundaries of the Nd2Fe14B phase, while most of it agglomerates into triple junctions as small particles. Therefore, the pathways for oxidation propagation are restricted and the intergranular oxidation process through the Nd-rich phase in the magnet is suppressed effectively. As a result, the SPS Nd-Fe-B magnet: possesses better oxidation resistance than that of the conventional magnet.