Abstract: The commercial 35M magnet prepared from recycled materials was used as the research object， and a self-made new DyF₃ coating was used to conduct thermal diffusion treatment on the magnets. The results show that the new coating has the characteristics of quick-drying， good adhesion and uniform coating， which effectively improves the stability of thermal diffusion process. The magnet subjected to thermal diffusion treatment at 920℃×10h， has a good performance on the magnetic properties， where the coercive force of the magnet reaches 1710.2kA/m with an increase of 37.9%. Microstructure analysis reveals the diffusion process of Dy element and the mechanism of coercive force improvement of the magnet. It is found that Dy element diffuses along the grain boundary and a Dy-rich shell of(Nd， Dy)₂Fe₁₄B with a higher anisotropy field can be formed around the grains of main phase， which makes the Nd-rich phase of the grain boundary more continuous and clear， resulting in a substantial increase in the coercive force of the magnet.

Key words: NdFeB magnets; grain size; grain boundary diffusion; coercive force; Dy diffusion process