Coercivity enhancement of sintered Nd–Fe–B magnets by efficiently diffusing DyF3 based on electrophoretic deposition

• The optimum diffusion temperature and time for the DyF3 EPD-coated magnets were clarified.
• A significant increment in coercivity of more than 6.5 kOe was achieved with less than 1.2 wt.% Dy.
• Dy concentrations variation versus distance from the magnet surface was accurately characterized.
• The increment of the coercivity was closely related to the concentration and distribution of Dy.
• The Dy-rich core–shell microstructure was well developed in the DyF3 EPD-coated magnets.

Using electrophoretic deposition (EPD) method, we coated DyF3 powder onto the sintered Nd–Fe–B magnets, investigated the changes in magnetic and microstructural properties of DyF3 EPD-coated magnets and clarified the optimum diffusion temperature and time. Through controlling the process parameters (such as temperature and time), the coercivity was increased from 16.1 to 22.8 kOe with a slight reduction in remanence after the diffusion. A significant increment in coercivity of more than 6.5 kOe was achieved with less than 1.2 wt.% Dy in the Dy-free original magnets. Microstructure analysis suggested that the increase of the coercivity was closely related to the concentration and the distribution of Dy. At 100 μm from the surface of the magnet, Dy concentration gradually increased with the diffusion temperature and time, which corresponds well with the change of coercivity. During the diffusion, Dy favors to substitute for Nd to form the (Nd, Dy)2Fe14B core–shell phases in the outer region of Nd2Fe14B grains. The higher magnetocrystalline anisotropy of the core–shell phase and the improved decoupling by the continuous grain boundary phase contribute to the coercivity enhancement.