In order to prepare nano-TbF3 coating with high quality on the surface of Nd-Fe-B magnets by electrophoretic deposition (EPD) more efficiently, Mg2+ and Cu2+ charging agents are introduced into the electrophoretic suspension and the influence on the electrophoretic deposition is systematically investigated. The results show that the addition of Mg2+ and Cu2+ charging agents can improve the electrophoretic deposition efficiency and coating adhesion of nano-TbF3 powders on sintered Nd-Fe-B magnets. The EPD efficiency increases by 116% with a relative content of Mg2+ as 3%, while it increases by 109% with a relative content of Cu2+ as 5%. Combining the Hamaker equation and diffusion electric double layer theory, the addition of Mg2+ and Cu2+ can change the zeta potential of charged particles, resulting in the improvement of EPD efficiency. The relative content of Mg2+ below 3% and Cu2+ below 5% can increase the thickness of the diffusion electric double layer, the excessive addition of a charging agent will compress the diffusion electric double layer, and thicker diffusion layer represents higher zeta potential. Furthermore, the addition of Mg2+ and Cu2+ charging agents greatly improves the coating adhesion, and the critical load for the cracking of the coating increases to 146.4 mN and 40.2 mN from 17.9 mN, respectively.
Keywords: charging agents; electrophoresis deposition; grain boundary diffusion; sintered Nd-Fe-B magnet.