Oxidation kinetics of Nd–Fe–B permanent magnets with modified microstructure

Oxidation kinetics of Nd–Fe–B permanent magnets prepared by both spark plasma sintering (SPS) and conventional sintering techniques has been studied at elevated temperature in dry air environment. Compared with the conventional sintered magnet, the SPS magnet possesses much lower oxidation rate, which leads to less reduction of magnetic properties of the SPS magnet than that of the conventional sintered magnet. Microstructure observation shows that the SPS magnet possesses unique microstructure due to the special sintering mechanism of SPS technique. In detail, the grain size of the Nd₂Fe₁₄B main phase is fine and uniform, only a few Nd-rich phases form along the grain boundaries of Nd₂Fe₁₄B phase, while most of them agglomerate into triple junctions as small particles. Such microstructure modification restrains effectively the intergranular corrosion along Nd-rich phase in the SPS magnet. As a result, the SPS magnet possesses excellent corrosion resistance at elevated temperature.