A trade-off between the diffusion depth, the thickness of the Tb-rich shell and the surface grain coarsening during the grain boundary diffusion of sintered Nd-Ce-Fe-B magnets

The grain boundary diffusion process (GBDP) has become one of the main methods to enhance the coercivity of Nd-Ce-Fe-B magnets. In this study, we examined how the magnetic properties of sintered Nd-Ce-Fe-B magnets are influenced by the combined impacts of diffusion depth, Tb-rich shell thickness, and surface grain coarsening after conducting grain boundary diffusion. There exists a trade-off between achieving a desired diffusion depth and avoiding excessive surface grain coarsening. To examine this trade-off, samples with varying diffusion depths were prepared through controlled diffusion time. Results revealed that compared to the original annealed magnets, the coercivity increments of the magnets diffused for 1 h and 3 h were 148 kA/m and 290 kA/m, respectively, while the coercivity of the magnet diffused for 9 h remained nearly the same as that diffused for 3 h. Microstructural analysis indicated that surface grain coarsening intensified with increasing diffusion time, leading to a reduction in the surface diffusion channels, thereby diminishing diffusion efficiency. In addition, strong mutual diffusion was observed between the magnet and the diffusion source. Furthermore, micromagnetic simulation studies revealed that severe surface grain coarsening limits the enhancement of coercivity even with increased depth of diffusion and thickness of the Tb-rich shell layer. This study offers valuable insights into the correlation between diffusion depth, Tb-rich shell thickness, surface grain coarsening, and the ultimate magnetic properties in sintered Nd-Ce-Fe-B magnets after GBDP, providing guidance for enhancing the efficiency of GBDP.