Magnetic and microstructural characteristics of DyF3/DyHx dip-coated Nd–Fe–B sintered magnets

• Diffusion depth of Dy and core-shell microstructure was optimized.
• Difference in diffusional behavior between DyF3 and DyHx dipping was identified.
• The DyHx dip-coating is more suitable for GBDP than the DyF3 dip-coating.

We investigated the microstructural and magnetic property changes of DyF3/DyHx dip-coated Nd–Fe–B sintered magnets, as a function of the DyF3/DyHx mixing ratio. As the fraction of DyHx in a DyF3/DyHx mixed solution was increased (0 → 50 → 100%), there was a coincident gradual increase in coercivity (14.9 → 15.4 → 16.5 kOe). Similarly, the diffusion depth of Dy from the magnet surface also increased with an increasing fraction of DyHx. In the diffused region, we observed well-developed core–shell microstructures in the DyHx dip-coated magnet, but this was not seen in the case of the DyF3 dip-coated magnet. Diffusion of Dy to the main phase was via lattice diffusion predominantly, rather than through grain boundary diffusion in the DyF3 dip-coated magnet, but the opposite was observed in the DyHx dip-coated magnet, which is believed to be due to the different influences of F− and H+ ions. Therefore, the use of DyHx solution is considered more suitable for developing a core–shell microstructure by encouraging grain boundary diffusion, and creating a Dy-saving effect in the dip-coating process.