Dependence of magnetic performance on coating method in grain boundary diffusion processed Nd-Fe-B sintered magnets

In this study, commercial sintered Nd–Fe–B magnets were coated with a grain-boundary-diffusion HRE source (TbH_x) using two different approaches, and the resulting variations in the magnetic properties were systematically investigated. The coercivity increased significantly from 12.8 to 24.4 kOe and to 26.8 kOe by enabling TbH_x diffusion through dip and spray coating, respectively. Slight diffusion-induced decreases in remanence were observed for the dip- and spray-coated magnets, from 14.5 to 14.0 kG and to 14.3 kG, respectively. Microstructural analysis indicated that the spray-coated magnet exhibited performance superior to that of its dip-coated counterpart; this was likely due to the mitigation of the low-efficiency diffusion from the side-wall coating, in which the direction of diffusion—which was perpendicular to the alignment orientation of the magnet—promoted diffusion through the grain interior rather than along the grain boundaries.