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 degrees 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 degrees 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 degrees 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.