Efficacy of Magnetron Sputtered Heavy Rare Earth Grain Boundary Diffusion into Nd-Fe-B Permanent Magnets

TIAN Guangke; ZHANG Xi; CUI Guangyu; WANG Yu; XU Yi; XIA Yuan

doi:10.16490/j.cnki.issn.1001-3660.2025.16.019

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Surface Technology ›› 2025, Vol. 54 ›› Issue (16) : 221-230. DOI: 10.16490/j.cnki.issn.1001-3660.2025.16.019

Surface Functionalization

Efficacy of Magnetron Sputtered Heavy Rare Earth Grain Boundary Diffusion into Nd-Fe-B Permanent Magnets

TIAN Guangke^1,*, ZHANG Xi¹, CUI Guangyu¹, WANG Yu², XU Yi³, XIA Yuan³

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Abstract

With the booming development of new energy automobiles and wind power generation, high remanent magnetism, high coercivity and high thermal stability of NdFeB magnets have reached tens of thousands of tons of annual market demands. The grain boundary diffusion process (GBDP) of heavy rare earth elements (HRE) such as Dy and Tb is an important technological means to meet the demand of the above high-performance Nd-Fe-B magnets. However, the utilization of HRE in the present GBDP production lines is still far from being fine enough and there is a lack of quantitative assessment in its efficacy.
In this work, DC magnetron sputtering coating technology is used to deposit a Tb film approximately 10 μm thick on the surface of grade 42SH Nd-Fe-B magnets, and then a high vacuum diffusion furnace is used to carry out grain boundary diffusion on the Tb-coated magnets. After optimizing the best one-step diffusion process parameters of 950 ℃×9 h, the effects of the different process modes of one-step vacuum diffusion with holding temperature at 750, 850 and 950 ℃ for 9 h and gradient vacuum diffusion with holding temperature at each of the three levels of temperature for 3 h on the enhancement of the magnets are investigated. Several quantitative equations such as the HRE infiltration efficiency into the magnet, the actual efficacy per unit of infiltrated HRE on the coercivity enhancement, the actual efficacy per unit of consumed HRE on the coercivity enhancement are defined, the corresponding values under different diffusion modes are calculated and compared. Meanwhile, the distribution characteristics of Tb infiltrating into the magnets under different diffusion modes are analyzed by electron probe microanalyzer (EPMA).
The results show that for one-step diffusion with the same holding time of 9 h, the Tb infiltration efficiency increases from 86% to 91% and 95% when the diffusion temperature increases from 750 ℃ to 850 ℃ and 950 °C, respectively, and the infiltration efficiency of Tb in the gradient diffusion mode is even higher, reaching 96%. In terms of the effect of magnetic properties enhancement by GBDP, the gradient diffusion mode is also the most significant. In the gradient diffusion mode, the coercivity enhancement per unit infiltrated Tb can reach 21.42 kOe/wt.%Tb, and the coercivity enhancement per unit deposited Tb can be 20.5 kOe/wt.%Tb, this means that per kOe of coercivity increase needs at least 0.049wt.% of Tb deposition.
The EPMA composition analysis shows that in the one-step diffusion even at higher temperature of 950 ℃, due to the simultaneous occurrence of Tb atoms diffusing along the grain boundary phases and into the main phase grains, the infiltrated Tb is prone to enrichment at the surface and shallow surface layers of the magnet, and resulting in a lower Tb content in the longitudinal depth of the magnets. But in the gradient diffusion mode, the staged holding at 750 ℃×3 h and 850 ℃×3 h helps the Tb atoms to diffuse firstly along the grain boundaries to the longitudinal position of the magnet, and then, undergoing the third stage of 950 ℃×3 h, forming a core-shell structure (Nd, Tb)₂Fe₁₄B hard magnetic layer with higher magnetic crystal anisotropy field around more main-phase grains located at the deeper position of the magnets, thus the coercivity enhancement in the gradient diffusion mode is the largest.

Key words

Nd-Fe-B permanent magnets / heavy rare earth grain boundary diffusion / magnetic property / microstructure / composition / quantitative assessment

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TIAN Guangke, ZHANG Xi, CUI Guangyu, WANG Yu, XU Yi, XIA Yuan. Efficacy of Magnetron Sputtered Heavy Rare Earth Grain Boundary Diffusion into Nd-Fe-B Permanent Magnets[J]. Surface Technology. 2025, 54(16): 221-230 https://doi.org/10.16490/j.cnki.issn.1001-3660.2025.16.019

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