Grain boundary modification and properties enhancement of sintered Nd-Fe-B magnets by ZnO solid diffusion

Abstract

A grain boundary diffusion (GBD) process using rare earth-free compound as the diffusion source is employed to enhance coercivity of sintered Nd-Fe-B magnets. Typically, a ZnO layer was coated on the Dy-free sintered Nd-Fe-B magnets by sputtering deposition. After a solid diffusion heat treatment, the coercivity and corrosion resistance have been significantly improved. For a 4 mm-thick magnet, the diffusion of ZnO led to a maximum increase of intrinsic coercivity from 1085 to 1290 kA/m. This coercivity enhancement is much higher than those obtained in the previous investigations by GBD of non-RE compounds or alloys. Based on the microstructure analysis, the coercivity improvement is induced by the modifications of the composition and structure of grain boundary phase, which is slightly different from the conventional rare earth-based GBD. The formation of oxides at the triple junction of grains is the main reason for the enhanced corrosion resistance of diffused magnets. The present work thus demonstrated that ZnO can be employed as the GBD source for both coercivity enhancement and corrosion resistance improvement, which are important for preparing low-cost and high-performance Nd-Fe-B permanent magnets.