Development of non-rare earth grain boundary diffusion process for Nd-Fe-B permanent magnets

Nd-Fe-B based rare earth permanent magnets are essential for a variety of devices. The performances of the magnetic materials are directly related to their microstructure. Recent researches have shown that the magnetic properties are very much dependent on the grain boundary structure. As a result, grain boundary diffusion (GBD) process has become an effective approach for improving the coercivity and reducing the heavy RE content of Nd-Fe-B magnets since the beginning of this century. In this paper, the development of GBD process has been introduced. Aiming to continuously reduce the material cost, GBD process has undergone three stages, as shown in Fig.1. In the early stage, heavy rare earth elements Dy or Tb and their compounds like Dy2O3, Dy-H, Dy-F were employed as the diffusion media. The magnetic properties, especially coercivity, of sintered NdFeB magnets and melt spun Nd-Fe-B powders were significantly enhanced. In the second stage, eutectic compounds RE-TM (RE=Nd, Pr, etc.; TM=Cu, Ni, etc.) were used for GBD. These compounds do not contain expensive rare earth elements Dy and Tb, and thus further reduced the material cost. The coercivities of both sintered magnets and hot deformed magnets have been greatly improved. Recently, we reported the enhancements of the magnetic properties and corrosion resistance of Nd-Fe-B magnets by a non-RE compound diffusion process [1], which can be regarded as the third stage of GBD. The details of non-RE GBD process and our very recent work will be presented in this paper. For the details, metal oxides were firstly employed as the diffusion media. The Tb, Dy-free sintered Nd-Fe-B magnets were coated with an oxide layer by magnetron sputtering, followed by solid diffusion heat treatment. With the successful diffusion of oxide into the magnet, the coercivity of the magnets has increased significantly and the maximum energy product was also enhanced without a significant decrease in remanence. The underlying mechanisms for these enhancements have been analyzed. The microstructural investigations show that the oxide entered mainly into the intergranular regions and modified the composition and structure of the grain boundary phase. The intergranular oxide phases observed in the oxide diffused magnet also contribute to the improved temperature stability and corrosion resistance of the magnet. Two types of oxides including MgO and ZnO have been discussed in this paper and both are effective in improving the coercivity of sintered Nd-Fe-B magnet. One example is shown in Fig.2. Our further studies indicate that not only oxides but also the low melting point alloys can work as the grain boundary diffusion medium for improving the coercivity of RE-lean Nd-Fe-B magnets. This novel GBD approach may overcome the limitations of conventional grain boundary diffusion, which requires heavy rare earth or rare earth compound, and has significance in further minimizing the use of rare earth resources.