Simultaneous application of Dy–X (X = F or H) powder doping and dip-coating processes to Nd–Fe–B sintered magnets

We investigated the microstructural and magnetic property changes of Nd–Fe–B sintered magnets with the application of simultaneous processing via powder doping and dip-coating, using DyF3 and/or DyH2. The simultaneous use of DyF3 powder doping and DyH2 dip-coating has a synergistic effect of improving the magnet’s coercivity (Hc) while minimizing the reduction of remanence (Br). DyF3 powder doping suppresses the formation of the rare-earth segregated oxide phase (RE-rich phase, Dy–Nd–O or Nd–O), dramatically improving the grain boundary diffusion depth of Dy (from 250 to 600 μm) during the grain boundary diffusion process (GBDP). The RE-rich phase in Nd–Fe–B magnets is a major obstacle for the grain boundary diffusion of Dy because of the high surface energy of the RE-rich phase and the consumption by the non-stoichiometric RE-rich phase of diffused Dy to form a stoichiometric (Nd, Dy)2O3 phase. The slight reduction of the relative density of the as-sintered DyF3-doped magnet (from 98.1% to 97.5%) may have also helped to improve the grain boundary diffusion depth of Dy during the GBDP when the magnets were dip-coated with DyH2.