CHARACTERIZATION OF HIGH-TEMPERATURE OXIDATION OF ND-FE-B MAGNETS

The long term high temperature oxidation properties of a Nd16.4Fe75.7B 7.9 commercial sintered magnet were investigated in pure oxygen atmosp here up to 500 degrees C and in air between 350 and 600 degrees C. In pure oxygen atmosphere, three exothermic reactions occur, correspondin g to the oxidation of the Nd-rich intergranular regions, the Nd2Fe14B matrix phase, and the alpha-Fe phase that forms during the oxidation o f Nd2Fe14B. In air, the oxidation of Nd2Fe14B was investigated further . Instead of the oxidation proceeding along the grain boundaries, the Nd2Fe14B matrix dissociates to form an adherent grey surface layer whi ch grows transgranularly into the magnet. This is probably due to a re action occurring in the Nd-rich regions which prevents fast path oxyge n diffusion along the grain boundaries. The main reaction is the disso ciation of the Nd2Fe14B matrix into alpha-Fe nanocrystals which contai n small precipitates of oxides of Nd. The products of this reaction fo rm the adherent grey layer which grows transgranularly into the magnet . The activation energy and the diffusivity pre-exponential factor for this reaction were found to be 114 kJ mol(-1) and 0.7 mm(2) s(-1) res pectively. After further oxidation of the dissociated grey layer, alph a-Fe is oxidised to form alpha-Fe2O3 and finally, from about 600 degre es C, some of the alpha-Fe2O3 reacts with the small precipitates of ox ides of Nd to form FeNdO3.