Reducing the process consumption of Tb in grain boundary diffusion of Nd-Fe-B magnets by sputtering deposition instead of adhesive coating

Abstract

Grain boundary diffusion (GBD) can enhance the coercivity of Nd-Fe-B magnets with relatively low consumption of heavy rare earths (HREs). Most of previous investigations on GBD focused on improving the efficiency of diffusion source, but less attention has been paid on reducing the process consumption of HRE. Here, a comparative study on coating methods of adhesive coating and sputtering deposition is carried out to evaluate the surface state of diffused magnet and the process consumption of HRE during surface cleaning. The results suggest that 0.9 wt.% sputtering deposited Tb-Cu alloy can enhance the coercivity of the magnet from 1053 kA/m to 1668 kA/m, due to the formation of high-anisotropy (Nd,Tb)2Fe14B phase and the optimized intergranular phases. To obtain smooth magnet surface for the subsequent surface protection process, the surface of the diffused magnet is polished, which leads to the reductions of magnet thickness and coercivity of ∼40 µm and 25 kA/m, respectively. In comparison, when 0.9 wt.% Tb-Cu source was coated by adhesive coating, although a comparable coercivity of 1663 kA/m can be obtained, the as-diffused magnet shows a rough surface and the polishing process leads to the reductions of magnet thickness and coercivity of ∼140 µm and 81 kA/m, respectively. By fitting the concentration-depth curves based on Fick’s second law, Tb contents in the diffused magnets decrease by 74 % and 13 % after surface cleaning for adhesive coating and sputtering deposition processes, respectively. The results suggest that the process consumption of Tb in GBD of Nd-Fe-B magnets can be greatly saved by sputter deposition.