Experimental and calculational analysis about the influence of the grain boundary diffusion depth on the magnetic properties of a sintered Nd-Fe-B magnet

The grain boundary diffusion process (GBDP) has been widely applied to increase the coercivity of Nd-Fe-B magnets. After GBDP with Dy/Tb-rich diffusion sources the thickness of the Dy/Tb-rich shell formed on the epitaxial layer of the 2:14:1 main phase grain decreases from the magnet surface to the center. However, the influence of the Dy/Tb-rich shell gradient distribution on magnetic properties has not been thoroughly studied. In this work, a sintered Nd-Fe-B magnet was subjected to GBDP with Pr60Tb10Cu30 alloy at 860°C for various diffusion times (3h, 6h and 9h). The coercivity improves rapidly from 884kA/m (without GBDP) to 1533kA/m after GBDP of 3h. The coercivity further increases to 1741 kA/m with increased diffusion time to 6h. But only marginal coercivity enhancement (rising to 1803 kA/m) can be obtained by further prolonging the diffusion time to 9h. Microstructure analysis indicates that the long diffusion time leads to the surface grain coarsening, which degrades the diffusion efficiency. Meanwhile, micromagnetic simulation indicates that if the thickness of the Tb-rich shell in magnet center is less than 4nm, the coercivity increases significantly with the enhanced thickness uniformity of the Tb-rich shell. But if the thickness of the Tb-rich shell in magnet center is higher than 4nm, the coercivity cannot be improved effectively by further increasing the thickness uniformity of the Tb-rich shell. The results in this work clarify the mechanism of the magnetic property dependence on the diffusion time and help to optimize the GBDP parameters in the future.