Magnetic and microstructural characteristics of a DyF₃ dip-coated Nd-Fe-B sintered magnet

We investigated the microstructural and magnetic property changes of DyF₃ dip-coated Nd-Fe-B sintered magnets as a function of the first post-sintering annealing (PSA) temperature and clarified the optimum first PSA temperature for the dip-coating process. Coercivity increased (from 26.7 to 29.06 kOe) with increasing first PSA temperature (from 700 to 900°C) owing to the increased diffusion depth of the Dy atoms. In the diffused region, the formation of the rare earth-rich phase (Nd-Dy-O) was suppressed and the core-shell microstructure was developed as a result of the DyF₃ dip-coating process. However, coercivity decreased when the samples were annealed at 950°C because h-Nd₂O₃ phases were formed in the Cu-rich triple junction phase and the grain boundary phase. By contrast, the C-Nd₂O₃ phase, which is closely related to coercivity enhancement after second PSA, was formed in the magnet annealed at 900°C. The result fulfilled our basic aim of utilizing the DyF₃ dip-coating process in order to enhance the coercivity of Nd-Fe-B sintered magnets without remanence reduction.