Strategy for co-enhancement of interface adhesion and coercivity of Nd-Fe-B grain boundary diffusion magnet: TbH3 nanopowders used in screen printing

Screen printing technology (SPT) was applied to conduct grain boundary diffusion sintered Nd-Fe-B magnets using TbH₃ and TbF₃ nanopowders as diffusion sources. TbH₃ grain boundary diffusion (HGBD) coating had better adhesion to the magnet surface than TbF₃ grain boundary diffusion (FGBD) coating. When the weight gain ratio was 1.0 wt%, the HGBD magnet achieved a coercivity increment of 11.07 kOe under non-pressurized heat treatment, while effectively controlling the magnet’s residual C and O elements. However, pressure heat treatment was necessary for the FGBD magnet to improve the coercivity due to the poor adhesion between the coating and the magnet surface, resulting in more residual C and O elements inside the magnet. Moreover, the coercivity of the FGBD magnet only increased by 7.96 kOe. Compared to the FGBD magnet, Tb diffused deeper into the HGBD magnet and formed more (Nd, Tb)₂Fe₁₄B-Nd₂Fe₁₄B core–shell structures. The formation of core–shell structures greatly enhanced the nucleation field of the reverse domain, thereby increasing the coercivity. Hence, the HGBD magnet had a higher coercivity increment. In addition, the HGBD magnet possessed better coercivity temperature stability than the original and FGBD magnets. Using TbH₃ nanopowders as a diffusion source for SPT can achieve higher magnetic properties and simplified processes without pressure.